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Electric Fields on Gibbsite Nanoplatelet Assemblies, Nanopyramid Sers Substrates and Graphene Actuators

Permanent Link: http://ufdc.ufl.edu/UFE0041857/00001

Material Information

Title: Electric Fields on Gibbsite Nanoplatelet Assemblies, Nanopyramid Sers Substrates and Graphene Actuators
Physical Description: 1 online resource (142 p.)
Language: english
Creator: Lin, Tzung-Hua
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: actuator, biomimetics, electrophoresis, graphene, nanocomposites, nanoplatelets, nanopyramids, nanostructure, sers
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This dissertation focuses on the exertion of electric fields to assemble gibbsite nanoplatelets along with various polymers to mimic the intricate brick-and-mortar nanostructure found in abalone shells. A simple electrophoretic (co-)deposition technology that enables rapid production of large-area polymer nanocomposites with layered structures was studied. Addition of binders and assembling of surface-roughened gibbsite nanoplatelets were also studied. The tensile strength and the stiffness of these biomimetic nanocomposites were significantly improved when compared to pure polymer films. The exertion of electric fields to conduct electrochemical SERS on nanostructured substrates that were templated from self-assembled colloidal silica arrays as well as to drive graphene-based actuators that were made by flow-directed assembly of one-atom-thick graphene sheets were also studied. Periodic arrays of nanopyramids with nanoscale sharp tips and high tip density demonstrated an enhancement on the order of 10^6. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s were able to last up to 140 cycles without significant degradation.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Tzung-Hua Lin.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Jiang, Peng.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041857:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041857/00001

Material Information

Title: Electric Fields on Gibbsite Nanoplatelet Assemblies, Nanopyramid Sers Substrates and Graphene Actuators
Physical Description: 1 online resource (142 p.)
Language: english
Creator: Lin, Tzung-Hua
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: actuator, biomimetics, electrophoresis, graphene, nanocomposites, nanoplatelets, nanopyramids, nanostructure, sers
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This dissertation focuses on the exertion of electric fields to assemble gibbsite nanoplatelets along with various polymers to mimic the intricate brick-and-mortar nanostructure found in abalone shells. A simple electrophoretic (co-)deposition technology that enables rapid production of large-area polymer nanocomposites with layered structures was studied. Addition of binders and assembling of surface-roughened gibbsite nanoplatelets were also studied. The tensile strength and the stiffness of these biomimetic nanocomposites were significantly improved when compared to pure polymer films. The exertion of electric fields to conduct electrochemical SERS on nanostructured substrates that were templated from self-assembled colloidal silica arrays as well as to drive graphene-based actuators that were made by flow-directed assembly of one-atom-thick graphene sheets were also studied. Periodic arrays of nanopyramids with nanoscale sharp tips and high tip density demonstrated an enhancement on the order of 10^6. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s were able to last up to 140 cycles without significant degradation.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Tzung-Hua Lin.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Jiang, Peng.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041857:00001


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ELECTRIC FIELDS ON GIBBSITE NANOPLATELET ASSEMBLIES,
NANOPYRAMID SERS SUBSTRATES AND GRAPHENE ACTUATORS




















By

TZUNG-HUA LIN


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2010

































2010 Tzung-Hua Lin

































To my Mom, Yue-Shia Wu









ACKNOWLEDGMENTS

I would like to thank Dr. Jiang for his advice over the past few years and people in

Jiang's group for their help, especially Wei-Han Huang for providing gibbsite

nanoplatelets and In-Kook Jun for mechanical tests.









TABLE OF CONTENTS

page

A C KNOW LEDG M ENTS ......... ............... ............................................. ............... 4

LIST O F TABLES .............. ............................................................................. 8

LIST O F F IG U R E S .................................................... 9

LIST OF ABBREVIATIONS..................... .......... .............................. 14

A BST RA C T ............... ... ..... ......................................................... ...... 16

CHAPTER

1 INTR O D U CT IO N ............................................................................................. 17

2 BIOINSPIRED ASSEMBLY OF GIBBSITE NANOPLATELETS BY ELECTRIC
F IE LD ............................................................................................. 19

B background .............................. .............. ...... 19
Learning from Nature ..................... ........................ 19
Bottom-up Self-assembly .................. .................... ..... 19
Electrodeposition ................................................... 20
G ibbsite N anoplatelets ...................... ....... ......... .. ............................ 2 1
E xpe rim e nta l ........................................... 2 2
Materials and Substrates.......................... ..... ........ ........ 22
Instrumentation......................... .....................22
Synthesis of Gibbsite Nanoplatelets ............. .... ............... .......... 23
Surface Modification of Gibbsite Nanoplatelets ............................. 24
EPD of Gibbsite Nanoplatelets ...... ............. .......... ......... 25
ETPTA-Gibbsite Nanocomposites ............................ ............... 25
Mechanical Test ..................... .................................. 26
Results and Discussion.......................................... ............... 26
Gibbsite Characterization .............. ........ .................. ... 26
EPD of Gibbsite Nanoplatelets ...... ............. .......... ......... 27
Polymer-Gibbsite Nanocomposites .......................... .. ...... ... .... ......... 29
TGA and Tensile Strength of Gibbsite-based Nanocomposites ...................... 30
S u m m a ry ............. ......... .. .. ......... ......... ......... .......................................... 3 2

3 ELECTROPHORETIC CO-DEPOSITION OF POLYMER-GIBBSITE
COMPOSITES ................. ........ .... .......... 46

B a c k g ro u n d ...................................................... ............... 4 6
E x p e rim e n ta l...................... .................................................................... 4 6
E P D of N a no p late lets ......... .............. ............................ ... ............... 4 6
EPD of PVA-Gibbsite nanoplatelets ..................... ............... 47









EPD of PEI-Gibbsite nanoplatelets .... ............................ .. ... ............ 47
Results and Discussion...................................... ......... 47
Electrophoretic Co-deposition of PVA-Gibbsite............................................ 47
Electrophoretic Co-deposition of PVA-Gibbsite............................................ 49
S um m ary ..................................................... ................ ......... .. 53

4 ELECTROPHORETIC ASSEMBLY OF SURFACE-ROUGHENED GIBBSITES.... 62

B background .............................. .............. ...... 62
Experim ental.................................................................. 63
Coating of Gibbsite Nanoplatelets with Silica ............ ................................ 63
EPD of Nanoplatelets ............................................... 64
EPD of SCG nanoplatelets.......................................... 64
EPD of PEI-SCG nanoplatelets....................... ..................... 64
Results and Discussion.......................................... ............... 65
SCG Nanoplatelets........................................... ............... 65
EPD of SCG Nanoplatelets ...... .... ............ ..................... ....... ........ 66
PEI-SCG Colloidal Stability ........... ........ ....... .......... ............ 67
EPD of PEI-SCG Nanoplatelets ...... .... ............ .................... ............... 68
ETPTA-PEI-SCG Nanocomposites ............... ............ ...... ....... ........... 70
Summary .............. .................. ........ ................ 72

5 ELECTROCHEMICAL SERS AT PERIODIC METALLIC NANOPYRAMID
ARRAYS .................................. ..... ........ .................... .............. 80

B background .............................. .............. ...... 80
Surface Plasmon ...... ......... ........ .................. 80
Extraordinary Optical Transmission....................... ..... ............... 81
Surface-enhanced Raman Scattering ................................... .................. ..... 82
Substrates for Surface-enhanced Raman Scattering ............ ............... 83
Experim ental............................ ... .............. ................... 85
Preparation of Electrochemical SERS-active Gold Nanopyramid Arrays ......... 85
Electrochemical Surface-enhanced Raman Scattering .............. ............ 86
Cyclic Voltammetry Measurements ...................... ......... ....... ............... 87
Electromagnetic Modeling of Raman Enhancement................ ............... 87
R results and D iscussion................................................................................ 88
Colloidal Templating Process for Nanopyramid Array Fabrication ................ 88
Electrochemical SERS Spectra of Pyridine on Gold Nanopyramid Arrays....... 89
Electrode Effects ......... ......... .. ............. .................. 91
Electromagnetic Modeling ............ .............. .......... ............ 93
S u m m a ry ................................................................................... ............................ 9 4

6 GRAPHENE PAPER ACTUATORS ...... ...................... .............. 105

Background...................................... ............... 105
Experim ental................................. ........ ....... 106
Materials and Methods ... .. ...................................... .... ........... 106


6









G raphene papers ...................... ....... ......... .. ............................ 106
Prior to Hummers' method ..................... .................... ....... .. 107
Hum mers' method...................................................... 107
Exfoliation and reduction of GO .......... ......... .. .. ..... ...... ........... 108
Graphene actuators ...... ............. ....................... 108
Results and Discussion............................ ...............109
GO and Graphene Dispersions ............... ......... ........... ........ ..... 109
Graphene Papers ................................. ................. 110
Graphene Actuators ................................................. 111
Sum m ary ............... ........................................ ............ ................. 114

7 C O N C LU S IO N S ..................................................................... 13 1

LIST O F REFERENCES .. ................................. ........................................... 134

BIOGRAPHICAL SKETCH ...... .................................... 142









LIST OF TABLES


Table page

5-1 Assignment of SERS peaks for pyridine adsorbed on gold nanopyramid
e le c tro d e .............................................. ....... .......... ...... 9 6









LIST OF FIGURES


Figure page

2-1 (A) Image of an abalone shell. (B) SEM image of fracture surface of
aragonitic portion of abalone nacre. (C) TEM image of the nacre cross
s e c tio n .............. ......... ................................................................ 3 4

2-2 A model of biocomposites. (A) A schematic diagram of staggered mineral
crystals embedded in protein matrix. (B) A simplified model showing the load-
carrying structure of the mineral-protein composites. Most of the load is
carried by the mineral platelets whereas the protein transfers load via the
high shear zones between m ineral platelets................... .................................. 35

2-3 (A) Schematic of cathodic electrophoretic deposition (EPD) and electrolytic
deposition (ELD). (B) Thickness of coatings deposited using ELD and EPD..... 36

2-4 (A) Lattice structure of Gibbsite. (B) Hexagon-shape of Gibbsite and
corresponding isoelectric points. ............ ............ ......... ............... 37

2-5 Schematic illustrations of (A) an electrophoretic cell and (B) deposit after
E P D ................. ......... ........... ................ ........................................ 3 8

2-6 TEM image of gibbsite nanoplatelets. The inset shows the electron diffraction
patterns obtained from a single nanoplatelet ................ ............. ............... 39

2-7 Electrophoretic assembly of gibbsite nanoplatelets. (A) Photograph of a free-
standing gibbsite film. (B) Top-view SEM image of the sample in (A). (C)
Cross-sectional view of the same sample. ................................ .................. 40

2-8 XRD patterns of the gibbsite film in Figure 2-7A ................ .... ... ............. 41

2-9 Thickness dependence of the electroplated films on the concentration of
colloidal gibbsite suspensions. The thickness standard deviation for all
sam ples is ca. 10% ............. ........... .. ............. ............ ...... ......... 41

2-10 Free-standing gibbsite-ETPTA nanocomposite. (A) Photograph of a
transparent film. (B) Cross-sectional SEM image of the same nanocomposite
film ............. ......... .. .. ......... .. .. .............................................. 4 2

2-11 Normal-incidence transmission spectrum of the sample in Figure 2-10A......... 43

2-12 XRD patterns of the nanocomposite sample in Figure 2-10A .......................... 43

2-13 TGA of the ETPTA-Gibbsite nanocomposite as shown in Figure 2-1 0A........... 44

2-14 Tensile stress versus strain curves for plain ETPTA film, ETPTA-Gibbsite
nanocomposite, and ETPTA-TPM-Gibbsite nanocomposite. ......................... 44









2-15 Tensile strength of composites as functions of volume fraction and aspect
ratio of gibbsite nanoplatelets ...................... .. ...................... ...... ......... 45

3-1 Electrodeposited PVA-Gibbsite nanocomposite. (A) Photograph of a
composite film on an ITO electrode. (B) Top-view SEM image of the sample
in (A). (C) Cross-sectional SEM image of the sample in (A). (D) Magnified
cross-sectional im age............................................................... ............. 54

3-2 XRD patterns of an electrodeposited PVA-gibbsite composite on ITO
e le ctro d e .................................................... ......... ..... .. .......... 5 5

3-3 Deposit weight on ITO electrode versus electrophoretic duration................... 55

3-4 TGA of the nanocomposite sample as shown in Figure 3-1A .......................... 56

3-5 Particle size distribution of nanoplatelet suspensions at different PEI/gibbsite
weight ratio. (A) R = 0, (B) R = 0.03, (C) R = 0.075, and (D) R =0.75. ............... 57

3-6 Electrophoretic mobility and corresponding zeta-potential of nanoplatelets at
different PEI/gibbsite weight ratio. ....................... ...... .......... .... .............. 58

3-7 SEM images of PEI-Gibbsite nanocomposite. (A) Top-view image, (B)
magnified top-view image, (C) cross-sectional image, and (D) magnified
cross-sectional im age ........................................ ................ .............. 59

3-8 XRD patterns of an electrodeposited PEI-Gibbsite nanocomposite on Au
e le ctro d e ............. ......... .. .. ......... .. .. ........ .................................. 6 0

3-9 TGA of an electrodeposited PEI-Gibbsite nanocomposite ............................... 60

3-10 Reduced modulus of pure gibbsite and PEI-Gibbsite nanocomposite
m measured by nanoindentation. ............................... ............. ............... 61

4-1 Cross section of abalone nacre showing the detailed structure at the lamellae
boundaries. Arrows highlight locations where the nano-asperities interpose. .... 74

4-2 (A) TEM image of acid-leached SCG nanoplatelets. The arrows point to a
silica shell with a thickness of ca. 10nm. (B) Photograph of an
electrodeposited SCG film on a gold electrode................................................. 74

4-3 Zeta-potential of PEI-SCG nanoplatelets with different amount of PEI
addition. The inset shows the molecular structure of PEI...... ....................... 75

4-4 SEM images of electrodeposited PEI-SCG nanocomposite. (A) Top-view
image. (B) Magnified top-view image. (C) Cross-sectional image. (D)
Magnified cross-sectional image. .............. .... .......................... ............ 76









4-5 XRD patterns of a PEI-SCG nanocomposite on an ITO electrode. Blue
arrows point to the characteristic peaks of ITO. The inset shows a table with
major lattice planes of gibbsite. ............ ....... ............ ............ ............... 77

4-6 Photographs of (A) ETPTA-PEI-SCG and PEI-SCG deposits on ITO
electrodes ........................... ............ .......... .............. ..........78

4-7 Normal-incidence transmission spectra of ETPTA-PEI-SCG nanocomposite,
ETPTA-PEI-SCG nanocomposite on an ITO electrode, and PEI-SCG deposit
on an ITO electrode ............ .......... ................. ................. .............. 78

4-8 SEM images of an ETPTA-PEI-SCG nanocomposite on an ITO electrode. (A)
Cross-sectional image. (B) Magnified cross-sectional image. Red and black
arrows in (A) point to a thin wetting layer of ETPTA and the ITO electrode,
respectively.............................................................................. 79

4-9 Tensile stress vs. strain curves for plain ETPTA film, ETPTA-Gibbsite
nanocomposite, and ETPTA-PEI-SCG nanocomposite............... ........... 79

5-1 Surface plasmons propagate along a metal/dielectric interface ...................... 97

5-2 Extraordinary transmittance at normal incidence for a square array of holes.
The area covered by holes is only 11% while the normalized-to-area
transm ittance of lights is 130% .......... ............. ............. .. ............... 97

5-3 Schematic SERS process in which light is Raman scattered by a molecule on
the surface .............. ....... .......... ............ .. ....................... ... 98

5-4 Schematic illustration of electrochemical SERS set up. .............. ............... ... 98

5-5 Schematic illustration of the templating procedures for fabricating gold
nanopyramid array by using spin-coated monolayer colloidal crystal as
te m p la te ............. ......... .. .. ......... .. .. ........ ................................... 9 9

5-6 Tilted (350) SEM images of a gold nanopyramid array electrode prior to (A)
and after (B) electrochemical SERS experiments. As templates, 320 nm
silica spheres were uses. ....... ..... ......... ........................ ..... .............. 100

5-7 Electrochemical SER spectra recorded on a gold nanopyramid array
supported by a conductive carbon disk and a copper tape (red) and a flat
gold control sample on silicon (black) in 0.1 M NaCI solution containing 0.05
M pyridine ............. ............... .......................... ......... 101

5-8 Electrochemical SER spectra recorded on a gold nanopyramid array
supported by a conductive carbon disk and a copper tape in 0.1 M NaCI
solution containing 0.05 M pyridine. .......... ... ......... ...... ....... .......... 101









5-9 The gold electrode potential was swept from -1.0 V (top) to +1.0 V (middle)
and then back to -1.0 V (bottom ). ............... .............. ............ .... ........... 102

5-10 Cyclic voltammograms of a conductive carbon tape, a conductive copper
tape, a gold nanopyramid array supported by a carbon tape, and a gold
nanopyramid array supported by a carbon disk and a copper tape in 0.1 M
NaCI. ............. .................... .......... ........ ......... .......... 102

5-11 Electrochemical SER spectra obtained on a gold nanopyramid array
supported by a conductive carbon tape in 0.1 M NaCI solution containing
0.05 M pyridine. The gold electrode potential was swept from -1.0 to 0.2 V.
The spectra were taken using a 785 nm diode laser at 48 pW with an
integration tim e of 10 s. .......... .......... ..................... ............... 103

5-12 (A) Modeled Raman enhancement factor around two gold nanopyramids with
base length of 320 nm and nanotips radius of curvature of 5 nm at A = 785
nm. (B) Simulated maximum SERS enhancement factor (Gmax) vs. number of
tips of the templated nanopyramid array with the same structural parameters
as (A )................ .... .. ...... .................................................. 104

6-1 M other of all graphitic form s. ............ .................. ....... .. ............... 116

6-2 Schematic illustration of an additional oxidation prior to Hummer's method..... 117

6-3 Schematic illustration of Hummers' method for GO preparation ....... .......... 118

6-4 Schematic illustration of preparation of graphene papers.............................. 119

6-5 Images of colloidal dispersions of (A) GO and (B) graphene............................ 120

6-6 TEM image of graphene sheets ................................ 120

6-7 (A) Tapping-mode AFM image of graphene sheets with (B) height profiles B1
and B2 taken along the lines in (A). The sample was prepared by drop-
casting diluted graphene dispersion onto a mica substrate............................ 121

6-8 (A) Top and (B) bottom side images of a free-standing graphene paper made
by vacuum filtration of graphene dispersion through an Anodisc membrane. .. 122

6-9 SEM images of a graphene paper. (A)Top-view SEM image, (B) bottom-view
SEM image and (C) cross-sectional SEM image................... ........ .......... 123

6-10 (A) Top and (B) bottom side images of a free-standing graphene paper made
by vacuum filtration of GO dispersion through an Anodisc membrane ........... 124

6-11 SEM images of a GO paper. (A)Top-view SEM image, (B) bottom-view SEM
image and (C) cross-sectional SEM image. ............ ...... .................. 125









6-12 Tensile stress versus strain curve for a free-standing graphene and GO
p a pe r. ......... ..... ............. ................................... ........................... 12 6

6-13 Schematic illustrations of a graphene actuator. (A) Front-view of the actuator,
(B) side-view of the actuator and (C) apparatus used for displacement
m easurem e nt ......... ..................................... .. ................ ........... 12 7

6-14 (A) Cyclic voltammograms of a graphene strip at various scan rates in 1 M
NaCI solution. A saturated calomel electrode was used as the reference
electrode and a platinum wire was used as the counter electrode. The
superficial active area was 0.2 cm2 and the weight of graphene paper
immersed was 0.12 mg. (B) A plot of steady state currents in (A) versus
corresponding scan rates. The slop in (B) is 0.006 F. ....................... ........ 128

6-15 (A) Cross-sectional images of a graphene actuator under eight successive
potential steps with a total of four cycles (-2/2 V repeatedly). (B)
Displacements of the actuator tip in (A) under repeated potential steps........... 129

6-16 (A) Two-electrode cyclic voltammograms of a graphene actuator operated
between -2 and 2 volts in 1 M NaCI solution with a scan rate of 50 mV/s. (B)
Corresponding displacements of the actuator in (A) as a function of cycle
num ber. .................. ............ ...... .............. .............. ................... 130









LIST OF ABBREVIATIONS

AFM Atomic force microscopy

AgFON Silver film over nanosphere

ALD Atomic layer deposition

DIW Deionized water

ELD Electrolytic deposition

EPD Electrophoretic deposition

ETPTA Ethoxylated trimethylolpropane triacrylate

FEM Finite-element-method

FIB Focused ion beam

GO Graphite oxide

IEP Isoelectric point

ITO Indium tin oxide

LBL Layer-by-layer

MFON Metal film over nanosphere

PAH Poly(allylamine hydrochloride)

PDMS Polydimethylsiloxane

PDDA Poly(diallyldimethylammonium chloride)

PEI Polyethylenimine

PML Perfect matched layers

PVA Polyvinyl alcohol

PVP Polyvinylpyrrolidone

SAED Selected area electron diffraction

SCG Silica-coated-gibbsite

SEM Scanning electron microscopy









SERS Surface-enhanced Raman scattering

SP Surface plasmon

SPR Surface plasmon resonance

TEM Transmission electron microscopy

TEOS Tetraethyl orthosilicate

TERS Tip-enhanced Raman scattering

TGA Thermogravimetric analysis

TPM 3-(trimethoxysilyl)propyl methacrylate

vol. Volume

wt. Weight

XRD X-ray diffraction









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

ELECTRIC FIELDS ON GIBBSITE NANOPLATELET ASSEMBLIES,
NANOPYRAMID SERS SUBSTRATES AND GRAPHENE ACTUATORS

By

Tzung-Hua Lin

August 2010

Chair: Peng Jiang
Major: Chemical Engineering

This dissertation focuses on the exertion of electric fields to assemble gibbsite

nanoplatelets along with various polymers to mimic the intricate brick-and-mortar

nanostructure found in abalone shells. A simple electrophoretic (co-)deposition

technology that enables rapid production of large-area polymer nanocomposites with

layered structures was studied. Addition of binders and assembling of surface-

roughened gibbsite nanoplatelets were also studied. The tensile strength and the

stiffness of these biomimetic nanocomposites were significantly improved when

compared to pure polymer films.

The exertion of electric fields to conduct electrochemical SERS on nanostructured

substrates that were templated from self-assembled colloidal silica arrays as well as to

drive graphene-based actuators that were made by flow-directed assembly of one-

atom-thick graphene sheets were also studied. Periodic arrays of nanopyramids with

nanoscale sharp tips and high tip density demonstrated an enhancement on the order of

106. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of

50 mV/s were able to last up to 140 cycles without significant degradation.









CHAPTER 1
INTRODUCTION

Electrodeposition technologies that enable the creation of inorganic-organic

nanocomposites with oriented layered nanostructures were studied to mimic the

intricate brick-and-mortar nanostructure found in the nacreous layer of abalone shells.

To resolve the colloidal aggregation issue faced by using nanoclays as building blocks,

electrostatically stabilized gibbsite nanoplatelets with high-aspect ratio were employed

as a model system. Electrophoretic deposition (EPD) of gibbsite nanoplatelets will be

discussed in Chapter 2. The interstitials between the assembled nanoplatelets were

infiltrated with polymer to form optically transparent nanocomposites. The tensile

strength and the stiffness of these biomimetic composites were significantly improved

when compared to pure polymer films.

To avoid the infiltration process, a novel electrophoretic co-deposition technology

for rapid production of polymer-gibbsite nanocomposites in a single step was also

studied and will be discussed in Chapter 3. Furthermore, it is found that the

arrangement of nano-asperities interposing between neighboring lamellae plays a

crucial role in determining the inter-lamellae slip and the resulting mechanical properties

of the natural composites. In order to help understand the surface roughness effect,

gibbsite nanoplatelets were surface-coated with rough silica and results will be shown in

Chapter 4. The current bottom-up technology enables scalable production of large-area

nanocomposites with ordered layered structure that have potential applications ranging

from gas-barrier films for optoelectronic devices to light-weight reinforced materials.

Other than assembly of gibbsite nanoplatelets by EPD, applying electric fields on

surface-enhanced Raman scattering (SERS) substrates and graphene-based actuators









were also studied. In Chapter 5, a better way to generate periodically tailored structures

of SERS electrodes for chemical and biochemical analysis is provided. A spin-coating

technique that combines the simplicity and cost benefits of bottom-up self-assembly

with the scalability and compatibility of standard top-down microfabrication in creating a

large variety of nanostructured materials has been demonstrated. The methodology is

based on shear-aligning concentrated colloidal suspensions using standard spin-coating

equipment. The shear flow generated during the spin-coating process coupled with

interparticle interaction induces the formation of wafer-scale, non-close-packed colloidal

crystals with adjustable thickness ranging from monolayer to hundreds of layers. These

self-assembled colloidal arrays can be used as structural templates to make metallic

nanostructures. Periodic arrays of nanopyramids with nanoscale sharp tips and high tip

density can enhance the local electromagnetic field in the vicinity of the nanotips,

resulting in high SERS enhancement (on the order of 106). The effects of the applied

electrode potential and the electrode redox reactions on the SERS enhancement were

investigated.

In Chapter 6, electromechanical actuators based on sheets of graphene papers

will be discussed. Graphite oxide (GO) sheets were generated by exfoliation of highly

oxygenated graphite in water and then chemically reduced to one-atom-thick graphene

sheets. The graphene sheet dispersions were further filtrated to produce graphene

papers. The actuators were operated under repeated potential steps and cyclic

voltammetry. Cycling stability of the graphene-based actuators was also discussed.









CHAPTER 2
BIOINSPIRED ASSEMBLY OF GIBBSITE NANOPLATELETS BY ELECTRIC FIELD

Background

Learning from Nature

The spontaneous organization of nonspherical colloids has attracted great recent

interest due to the wide range of potential applications of the resulting assemblies in

photonic crystals (1-4), metamaterials (5), surface-enhanced Raman scattering sensors

(6), and reinforced nanocomposites (7-9), as well as fundamental studies of liquid

crystal phase transitions (10-13) and particle packing (14-16). Among a large variety of

nonspherical colloids, platelet particles are particularly interesting as they enable the

bottom-up assembly of layered nanocomposites that mimic the nacreous layer of

mollusk shells (17-19). The intricate brick-and-mortar nanostructure (as in Figure 2-1)

found in nacre, which consists of ~95 vol.% of brittle aragonite platelets and ~5 vol.% of

soft biological macromolecules (20-23), makes the shells exceptionally tough and stiff

with a tensile stress of around 100 MPa (19, 23) because most of the load can be

carried by the mineral platelets whereas the protein transfers load via the high shear

zones between mineral platelets, as shown in Figure 2-2 (18). The unusual combination

of the mechanical strength, toughness, and stiffness in these natural inorganic-organic

composites has inspired scientists to create artificial nanocomposites that mimic the

mechanical design principles found in nature (7-9).

Bottom-up Self-assembly

Bottom-up self-assembly of nonspherical colloidal building blocks is of great

interest for the development of new materials with potential applications in

optoelectronics, photonics, magnetics, catalysis, and mechanics (1, 2, 15, 19, 24-28).









Layer-by-layer (LBL) assembly of inorganic nanoplatelets (e.g., nanoclays) and

polyelectrolytes has recently been demonstrated as an efficient methodology in making

reinforced polymer nanocomposites (7, 8). Ice-templated crystallization, gravitational

sedimentation, centrifugation, spin-coating, and dip-coating have also been employed to

align inorganic nanosheets to form nacre-like assemblies (28-31). However, these

techniques are either time-consuming or require multiple steps to infiltrate the inorganic

assemblies with polymer to make nanocomposites. For example, LBL assembly is a

relative slow process and hundreds of bilayers need to be deposited to form composites

with micrometer-scale thickness. Additionally, the significant agglomeration of

commonly used clay nanoplatelets hampers the formation of highly aligned structures

and thus impairs the mechanical properties of the resulting nanocomposites.

Electrodeposition

Electrodeposition is widely used for the deposition of thin films and coatings.

Electrophoretic deposition (EPD) and electrolytic deposition (ELD) are the two

commonly used processes, as shown in Figure 2-3. EPD is carried out based on the

use of ceramic particles in suspensions and enables the preparation of thick ceramic

films while ELD uses solutions of metal salts and is an important tool for the formation of

nanostructured think films (32). Electrophoresis is a well-established technology in

assembling spherical colloids into highly ordered colloidal crystals (33-35). In this

methodology, charged colloids are attracted by electrical force toward the counter

electrode and then deposited on the electrode surface by particle coagulation (32).

Electrodeposition is a simple, inexpensive, and scalable technology that enables rapid

production of thick films over large areas. Electrophoretic co-deposition of colloids and

polymer is also possible for the formation of nanocomposites in a single step. In









addition, deposition of metals and conducting polymers in the interstitials of colloids is

easily achieved by electrophoresis. This will significantly expand the available materials

for the fabrication of layered nanocomposites. Electrophoretic assembly of nanoclays

has previously been tested, but the entrapment of non-platy particles caused by the

agglomeration of nanoclays deteriorates the layered structure (36).

Gibbsite Nanoplatelets

Various synthetic methods have been developed to make fairly monodispersed

colloidal platelets with high stability in suspensions (37-40). For instance, uniform

gibbsite (AI(OH)3) nanoplatelets with well defined hexagon-shape can be synthesized

by hydrolysis of Al(OH2)63+ at 85C (37, 41). The aspect ratio of the synthesized gibbsite

nanoplatelets (~ 10) is close to that of natural aragonite platelets in nacre (20). The

diameter and thickness of the gibbsite nanoplatelets can be controlled by seeded

growth (42). The gibbsite structure is a stacking of AI-OH layers and each A13+ is

surrounded by six hydroxyl groups, as shown in Figure 2-4. The reaction of surface

hydroxyl groups with water makes the nanoplatelets highly charged in water and

alcoholic suspensions. The surface hydroxyl groups also facilitate the chemical

modification of the particle surface to render different functionality (41). By using

gibbsite nanoplatelet as a model system, Lekkerkerker et al. have extensively exploited

the liquid crystal phase transition in suspensions of plate-like particles (10-12, 43). Opal-

like columnar gibbsite colloidal crystals have also been demonstrated by forced

sedimentation (13, 44).

We used electrostatically stabilized gibbsite nanoplatelets with well-defined shape

and size as a model system to explore the oriented assembly of plate-like colloids by

electrophoresis. A simple spin-coating process were developed to infiltrate the









interstitials between the assembled nanosheets to form artificial nacreous

nanocomposites. The resulting self-standing films were transparent and exhibited

significantly improved mechanical properties over those of pure polymer. We also

chemically functionalized the surface of the gibbsite nanoplatelets to facilitate the

formation of covalent linkage between the ceramic platelets and the polymer matrix.

This further reinforced these biomimetic nanocomposites.

Experimental

Materials and Substrates

All solvents and chemicals were of reagent quality and were used without further

purification. Ultrapure water (18.2 MQ cm-1) was used directly from a Barnstead water

system. Ethanol (200 proof) was purchased from Pharmaco Products. Hydrochloric acid

(37%), aluminum sec-butoxide (95%), and aluminum isopropoxide (98%) were obtained

from Aldrich. Ethoxylated trimethylolpropane triacrylate (ETPTA) monomer was

provided by Sartomer (Exton, PA). The photoinitiator, Darocur 1173 (2-hydroxy-2-

methyl-1-phenyl-l-propanone), was obtained from Ciba Specialty Chemicals. Two-part

polydimethylsiloxane (PDMS, Sylgard 184) was provided by Dow Corning. Indium tin

oxide (ITO) coated glass substrates with sheet resistance of 8 Q were purchased from

Delta Technologies. Silicon wafers (test grade, n type, (100)) were purchased from

University Wafer.

Instrumentation

An EG&G Model 273A potentiostat/galvanostat was used for EPD. Scanning

electron microscopy (SEM) was carried out on a JEOL 6335F FEG-SEM. A thin layer of

gold was sputtered onto the samples prior to imaging. Transmission electron

microscopy (TEM) and selected area electron diffraction (SAED) were performed on a









JEOL TEM 2010F. Atomic force microscopy (AFM) was conducted on a Digital

Instruments Dimension 3100 unit. X-ray diffraction (XRD) spectra of the electroplated

gibbsite films were obtained with Philips APD-3720 equipment. A Cu Kai(A = 1.54049 A)

radiation was scanned from 100 to 700 with a scan rate of 2.40/min. Thermogravimetric

analysis (TGA) was carried out in air with a Perkin-Elmer thermogravimetric analyzer

and a platinum crucible between 20 and 8000C at a heating rate of 5C/min. The zeta-

potential of gibbsite nanoplatelets was measured by a Brookhaven ZetaPlus Analyzer

(Brookhaven Instrument Corporation). An Ocean Optics HR4000 high resolution fiber

optic UV-vis-near-IR spectrometer was used for optical transmission measurement. A

standard spin-coater (WS-400B-6NPP-Lite spin processor, Laurell) was used to spin-

coat ETPTA monomer. The polymerization of ETPTA was carried out on a pulsed UV

curing system (RC 742, Xenon).

Synthesis of Gibbsite Nanoplatelets

The gibbsite nanoplatelets were synthesized by following a preparation method as

described in the reference (37). To 1 L of ultrapure water were added hydrochloric acid

(0.09 M), aluminum sec-butoxide (0.08 M), and aluminum isopropoxide (0.08 M). The

mixture was stirred for 10 days and then heated in a polyethylene bottle in a water bath

at 85 oC for 72 h. After cooling to room temperature, dispersions of gibbsite

nanoplatelets were centrifuged at 3500g for 6 h, and the sediments were redispersed in

deionized water. For completely removing the unreacted reactants and concentrating

the nanoplatelets, this process was repeated for five times. Detailed processes are

listed as below:

1. Clean a 1000 ml glass flask by soaking in saturated KOH in isopropanol for 24 h.

2. Rinse the flask with DIW, then fill with 2% HF, and storage overnight.









3. Rinse the flask with DIW several times and then dry in an oven.

4. Draw 20 ml of aluminum tri-sec-butoxide (97%) with a syringe. This process might
take 10 to 20 mins.

5. Meanwhile, put a stirring bar in the flask and add about 800 ml of DIW.

6. Stir at medium speed and then add 7.4 ml of HCI (37%) into the flask.

7. Add 16.3 g of aluminum isopropoxide (98%).

8. Inject the above 20 ml of aluminum tri-sec-butoxide with a syringe.

9. Fill the flask to 1000 ml with DIW.

10. Increase stirring speed.

11. Age for 10 days till all the particles dissolves.

12. Distribute the solution into several plastic bottles and then put the bottles in
isotemp water bath and heat at 85C for 72 h.

13. Wash particles by centrifuging at 3500g for 6 h.

14. Dump supernatant and add DIW.

15. Re-disperse particles.

16. Repeat processes 16 to 18 for 5 times.

Surface Modification of Gibbsite Nanoplatelets

Gibbsite nanoplatelets were surface-modified with 3-(trimethoxysilyl)propyl

methacrylate (TPM) (45). Prior to adding gibbsite nanoplatelets, 10 ml of TPM was

mixed with 100 ml of a water-methanol solution (water/methanol volume ratio of 3:1) for

1 h to fully hydrolyze TPM. Surface modification was then accomplished by adding 100

ml of gibbsite dispersion (ca. 1 vol.% aqueous solution) into the hydrolyzed TPM

solution. The suspension was stirred at 40C for 30 min. The modified nanoplatelets

were washed by repeated centrifugation-redispersion cycles with pure ethanol and

finally concentrated to a stock suspension of 0.045 and 0.035 (g/g) in ethanol.









EPD of Gibbsite Nanoplatelets

EPD of gibbsite nanoplatelets was performed in a sandwich cell placed

horizontally, as shown in Figure 2-5A. The bottom and the top of the cell were an ITO

working electrode and a gold counter electrode, respectively. The gold electrode was

prepared by sputtering deposition of 20 nm of chromium and 200 nm of gold on a (100)

silicon wafer. PDMS was used as a spacer to get an active area of 1.5 x 1.5 cm2 and a

cell gap of 2.2 mm. Aqueous suspensions of gibbsite nanoplatelets with different weight

percentage were used. Ethanol (200 proof) was added into the suspensions to make

the volumetric ratio of ethanol to the aqueous suspension to be 2. A constant voltage of

-2.5 V (ITO vs. Au) was applied for 30 min to deposit the positively charged gibbsite

nanoplatelets onto the ITO cathode. After deposition, the electroplated gibbsite films (as

in Figure 2-5B) were rinsed with 200-proof ethanol and then dried with compressed air.

ETPTA-Gibbsite Nanocomposites

After the oriented assembly, ETPTA-Gibbsite nanocomposites can then be made

by filling the interstitials between the aligned nanoplatelets with photocurable

monomers, followed by photopolymerization. We chose a nonvolatile monomer ETPTA

to form the nanocomposites. A free standing gibbsite was first put on a PDMS

substrate. The ETPTA monomer with 1% photoinitiator was then dropped on the

gibbsite film. The sample was then put under vacuum for 10 min in order to remove as

many trapped air bubbles as possible. After 10 min, the gibbsite film became

transparent and then was spin-coated at 4000 rpm for 1 min to remove excess

monomer solution and then polymerized by exposure to ultraviolet radiation.









Mechanical Test

For tensile strength measurement, three types of thin films (ETPTA, ETPTA-

Gibbsite, and ETPTA-TPM-modified gibbsite) were tested using an Instron model 1122

load frame upgraded with an MTS ReNew system and equipped with a 500 g load cell

at a crosshead speed of 0.5 mm/min. Testing samples with widths of 1.5 mm and

thickness ranging from 30 to 80 pm were adhered on homemade sample holders with a

20 mm gap using polyurethane monomer as an adhesive and then UV-cured. The

thickness of the tested samples was measured by cross-sectional SEM to calculate the

final tensile strength.

Results and Discussion

Gibbsite Characterization

Figure 2-6 shows a typical TEM image of purified gibbsite nanoplatelets. The

particles are hexagonally shaped and are relatively uniform in size. The diameter of the

nanoplatelets is measured to be 188 40 nm by averaging over 100 particles from the

TEM micrographs. TEM images also reveal that the nanoplatelets tend to align parallel

to the surface of TEM grids. AFM experiments show the platelet thickness ranges from

10 to 15 nm. The purified gibbsite nanoplatelets are electrostatically stabilized, and the

zeta-potential of the colloids in deionized water is measured to be +40.5 2.3 mV by

fitting experimental data using Smoluchowski's model. The high surface charge makes

the nanoplatelets stable in aqueous and alcoholic dispersions, and aggregated particles

are rarely seen in TEM images. The SAED patterns from a single platelet as shown in

the inset of Figure 2-6 indicate that the as-made gibbsite nanoplatelets are single-

crystal.









EPD of Gibbsite Nanoplatelets

The EPD of positively charged gibbsite nanoplatelets is carried out using a

parallel-plate sandwich cell, which consists of an ITO working electrode, a gold counter

electrode, and a PDMS spacer (-2.2 mm thick). The bath solution is gibbsite

nanoplatelets dispersed in a water-ethanol mixture with volumetric ratio of 1:2. The

volume fraction of gibbsite particles is adjusted to -1%. Ethanol is added to the

aqueous dispersions to reduce the dielectric constant of the solvent and thus reduce the

electrical double-layer thickness of the particles to promote colloidal coagulation on the

ITO electrode. Without ethanol, no particle deposits are adhered on the working

electrode after disassembling the electrical cell. The addition of ethanol also facilitates

reduction of cracking and porosity in the electrophoretically deposited films. The applied

electric field strength is -1100 V/m. The electrophoretic velocity of the gibbsite

nanoplatelets is estimated to be -7.5 pm/s by using the Smoluchowski's equation:


UE E


where e is the dielectric constant of the solution, so is the permittivity of the vacuum, P is

the solution viscosity, and E is the applied electric field strength. For a 2.2 mm thick

sandwich cell, the estimated time to deposit most particles on the ITO electrode is about

5 min, agreeing with our experimental observation. Besides parallel-plate geometry,

electrodes can also be vertically inserted into the colloidal baths to conduct the EPD. As

the gravitational sedimentation of the gibbsite nanoplatelets during the electrophoretic

process is negligible, uniform deposits on the electrodes are obtained. After EPD, the

gibbsite deposits on the ITO cathode are washed with ethanol and then dried with

compressed air. The deposits can be easily peeled off from the ITO surface by using a









sharp razor blade, resulting in the formation of self-standing films as shown in Figure 2-

7A. The film is opaque and brittle, and the side facing the ITO cathode is smoother than

the side facing the suspension. The size of the resulting films is solely determined by

the dimensions of the ITO electrode. Figure 2-7A depicts a sample with 1.6 x 0.6 inch2

size deposited on a 2 x 1 inch2 ITO electrode. Figure 2-7B shows a top-view SEM

image of the suspension side of the sample in Figure 2-7A. The hexagonal gibbsite

nanoplatelets are densely packed and aligned parallel to the electrode surface. The

alignment of gibbsite nanoplatelets is further confirmed by the layered structure as

shown in the cross-sectional SEM image of Figure 2-7C. Another convincing evidence

of the orientated deposition comes from the XRD patterns shown in Figure 2-8. Only

(002) and (004) peaks are observed in the XRD spectrum. As the crystallographic c-axis

of single-crystal gibbsite is normal to the platelet surfaces, the (002) and (004)

reflections are from gibbsite platelets oriented parallel to the electrode surface (46).

Analysis of the half-height width of the (002) peak with the Scherrer equation yields an

average platelet thickness of 15.1 nm, agreeing with AFM measurement.

The oriented deposition of gibbsite nanoplatelets in a direct-current (dc) electric

field can be understood by considering the charge distribution on the gibbsite surfaces.

Early study shows the isoelectric point (IEP) of the edges (pH = 7) differs from that on

the faces (pH = 10) (37). The pH of the suspension in the electrophoretic experiments is

close to 7, resulting in positively charged surfaces and almost neutral edges. Therefore,

the applied electric field exerts a force only on the surfaces of the gibbsite platelets, and

Brownian motion could provide sufficient torque to reorient perpendicular particles to

face the ITO electrode. Once close to the electrode, the gibbsite nanoplatelets will be









forced to align parallel to the electrode surface as this orientation is more energetically

favorable than the perpendicular one. Similar to the evaporation-induced alignment of

gibbsite nanoplatelets on TEM grids (Figure 2-6), further evidence shows that capillary

force during solvent evaporation is sufficient to orient gibbsite particles into layered

assemblies. However, the rapid and uniform deposition of nanoplatelets over large

areas is the major advantage of the electrodeposition technology over evaporation and

gravitational sedimentation-induced assembly. If the duration of the electrophoretic

process is long enough, almost all gibbsite platelets can be deposited on the ITO

electrode. The thickness of the deposits is then linearly proportional to the particle

volume fraction of the suspension as shown in Figure 2-9.

Polymer-Gibbsite Nanocomposites

After the oriented assembly, polymer-Gibbsite nanocomposites can then be made

by filling the interstitials between the aligned nanoplatelets with photocurable

monomers, followed by photopolymerization. We chose a nonvolatile monomer ETPTA

to form the nanocomposites. The monomer with 1% photoinitiator is spin-coated at 4000

rpm for 1 min to infiltrate the electroplated gibbsite film and then polymerized by

exposure to ultraviolet radiation. The resulting nanocomposite film becomes highly

transparent (Figure 2-10A) as a result of the matching of the refractive index between

the gibbsite platelets and the polymer matrix. The cross-sectional SEM image in Figure

2-10B shows that the nanocomposite retains the layered structure of the original

electroplated gibbsite film, and thin wetting layers of ETPTA (-1 pm thick) are observed

on the surfaces of the film. The normal-incidence transmission measurement as shown

in Figure 2-11 shows that the free-standing nanocomposite film exhibits high

transmittance (>80%) for most of the visible wavelengths. As the reflection (R) from an









interface between two materials with refractive indices of nl and n2 is governed by

Fresnel's equation R = [(nl n2)/(n1 + n2)]2, we can estimate the normal-incidence

reflection from each air-nanocomposite interface to be about 4%. Thus, the optical

scattering and absorption caused by the nanocomposite itself is approximately 10%.

This suggests that the polymer matrix has infiltrated most interstitial spaces between the

aligned gibbsite nanoplatelets. The oriented arrangement of the nanoplatelets is also

maintained throughout the polymer infiltration process as confirmed by the distinctive

(002) and (004) peaks of the XRD spectrum shown in Figure 2-12.

TGA and Tensile Strength of Gibbsite-based Nanocomposites

The ceramic weight fraction in the ETPTA-Gibbsite nanocomposite film is

determined by TGA as shown in Figure 2-13. From the TGA curve and the

corresponding weight loss rate, it is apparent that two thermal degradation processes

occur. One happens at -250C and corresponds to the degradation of the polymer

matrix, while another occurs at -350C and is due to the decomposition reaction of

gibbsite: 2AI(OH)3 -- A1203 + 3H20. On the basis of the residue mass percentage

(45.65%) and assuming the ash is solely A1203, we can estimate the weight fraction of

gibbsite nanoplatelets in the original nanocomposite film to be -0.70. Considering the

density of gibbsite (-2.4 g/cm3) and ETPTA (-1.0 g/cm3), the volume fraction of

gibbsite nanoplatelets in the nanocomposites is approximately 0.50. The complete

infiltration of ETPTA between the electroplated gibbsite platelets is further confirmed by

the selective dissolution of gibbsite in a 2% hydrochloric acid aqueous solution. This

results in the formation of a self-standing porous membrane with stacked hexagon-

shaped pores, which are a negative replica of the assembled gibbsite platelets.









The mechanical properties of the biomimetic polymer nanocomposites are

evaluated by tensile tests. We compare the tensile strength for three types of thin films,

including pure ETPTA, ETPTA-Gibbsite, and ETPTA-TPM-modified Gibbsite. The

surface hydroxyl groups of gibbsite nanoplatelets can be easily modified by reacting

with TPM through the well-established silane coupling reaction (45). This results in the

formation of surface-modified particles with dangling acrylate bonds that can be cross-

linked with the acrylate-based ETPTA matrix. The colloidal stability and the surface

charge of the resulting nanoplatelets are not affected by this surface modification

process as confirmed by TEM and zeta-potential measurement. Figure 2-14 shows the

tensile stress versus strain curves for the above three types of films. The ETPTA-

Gibbsite nanocomposite displays -2-time higher strength and -3-time higher modulus

when compared with pure ETPTA polymer. Even more remarkable improvement occurs

when TPM-Gibbsite platelets are cross-linked with the ETPTA matrix. We observe -4-

time higher strength and nearly 1 order of magnitude higher modulus than pure

polymer. This agrees with early studies that reveal the crucial role played by the

covalent linkage between the ceramic fillers and the organic matrix in determining the

mechanical properties of the artificial nacreous composites (8). We also conduct a

simple calculation to evaluate if the measured mechanical properties of the ETPTA-

Gibbsite nanocomposites are reasonable. For a polymer matrix having a yield shear

strength ,y and strong bonding to the gibbsite nanoplatelet surface (e.g., TPM-modified

gibbsites), the tensile strength of the composite (ac) can be calculated using the volume

fraction of nanoplatelet (V,), the nanoplatelet aspect ratio (s), and the tensile strength of

the nanoplatelet (ap) and of the polymer matrix (am), as (9)









a.g = aVp up + (1 Vp)u

For the gibbsite nanoplatelet which has a relatively small aspect ratio (s 12-18), the

factor R in the above equation can be estimated as


2ap

From the above TGA analysis, the volume fraction of gibbsite nanoplatelets in the

polymer nanocomposite is -0.50. If we take s = 15, the first equation can then be

simplified as

ag = 3.75Ty + 0.5cSm

For acrylate-based polymer (like ETPTA), the yield shear strength should be close to its

tensile strength. The final equation can further be simplified as ac 4.25am. This

indicates that the strength of the nanocomposite is about fourfold of the strength of the

polymer matrix, agreeing with our experimental results. Tensile strength of composites

estimated by the above method can be further plotted as functions of volume fraction

and aspect ratio of gibbsite nanoplatelets, as shown in Figure 2-15. The dot in Figure 2-

15 is our experimental result while the net surface is from the proposed model.

Summary

In summary, we have developed a simple and rapid electrodeposition technology

for assembling gibbsite nanoplatelets into large-area, self-standing films. These

nanosheets with high aspect ratio are preferentially aligned parallel to the electrode

surface. The interstitials between the assembled nanoplatelets can be infiltrated with

polymer to form optically transparent nanocomposites. The tensile strength and the

stiffness of these biomimetic composites are significantly improved when compared to









pure polymer films. The current electrodeposition technology is a quite general

approach to achieve oriented deposition of platelet-like particles with various aspect

ratios. Preliminary results show that silica-coated gibbsite nanoplatelets, hollow silica

nanoplatelets, and zeolite platelets can also be aligned by EPD. The technology is also

promising for developing layered metal-ceramic and conducting polymer-ceramic

nanocomposites that may exhibit improved mechanical and electrical properties but are

not easily available by other bottom-up technologies.














































Figure 2-1. (A) Image of an abalone shell. (B) SEM image of fracture surface of
aragonitic portion of abalone nacre. (C) TEM image of the nacre cross section
(21).











34









A


Mineral platelet







Protein matrix


Tension zones
of protein






"High shear /ones
of protein


Figure 2-2. A model of biocomposites. (a) A schematic diagram of staggered mineral
crystals embedded in protein matrix. (b) A simplified model showing the load-
carrying structure of the mineral-protein composites. Most of the load is
carried by the mineral platelets whereas the protein transfers load via the high
shear zones between mineral platelets (18).































EPD


I I I I


10- 10-2 10-1 100 101 102 103 10
Coating Thickness, im


Figure 2-3. (A) Schematic of cathodic electrophoretic deposition (EPD) and electrolytic
deposition (ELD). (B) Thickness of coatings deposited using ELD and EPD
(32).


EPD ELD

S Charged ceramic particles
][ Ions or complexes


Electrodeposition

VL


ELD


a


. <-


I I-





















OH-


B
IEP: pH ~ 10

D




IEP: pH ~ 7

Figure 2-4. (A) Lattice structure of Gibbsite. (B) Hexagon-shape of Gibbsite and
corresponding isoelectric points (41).














w -


Gibbsite I
Nanoplatelets
ddWW


ITO orAu


(B)

Figure 2-5. Schematic illustrations of (A) an electrophoretic cell and (B) deposit after
EPD.


- -----------r`~~ ~~~~~~~--------------------


M


v ,.-- B

-`---------------- ---- ---




















1f~


AI


4


."
;rr;t

:, o


.. .


Figure 2-6. TEM image of gibbsite nanoplatelets. The inset shows the electron
diffraction patterns obtained from a single nanoplatelet.


L *"


















































Figure 2-7. Electrophoretic assembly of gibbsite nanoplatelets. (A) Photograph of a
free-standing gibbsite film. (B) Top-view SEM image of the sample in (A). (C)
Cross-sectional view of the same sample.






40













10000


8000


6000


4000


2000


10 20 30 40 50 60 70
29

Figure 2-8. XRD patterns of the gibbsite film in Figure 2-7A.


0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6


Volume Fraction (%)

Figure 2-9. Thickness dependence of the electroplated films on the concentration of
colloidal gibbsite suspensions. The thickness standard deviation for all
samples is ca. 10%.


(002)













(004)








A


UFWIHOn
t fm-iOwfn* Ger Mok 1


Figure 2-10. Free-standing gibbsite-ETPTA nanocomposite. (A) Photograph of a
transparent film. (B) Cross-sectional SEM image of the same nanocomposite
film.
































400 500 600 700 800
Wavelength (nm)


Figure 2-11. Normal-incidence transmission spectrum of the sample in Figure 2-10A.


8000



6000



4000



2000


10 20 30 40 50 60 70
29


Figure 2-12. XRD patterns of the nanocomposite sample in Figure 2-10A.


















0.5





40

-0.5
20



0 ** I I I I -1.0
100 200 300 400 500 600 700

Temperature (C)



Figure 2-13. TGA of the ETPTA-Gibbsite nanocomposite as shown in Figure 2-10A.


Strain


Figure 2-14. Tensile stress versus strain curves for plain ETPTA film, ETPTA-Gibbsite
nanocomposite, and ETPTA-TPM-Gibbsite nanocomposite.


- ETPTA-TPM-Gibbsite
- ETPTA-Gibbsite
- ETPTA
















1,000


0
8 100


0 10






o/01/7r 0.1 1,000
eractioo 10 100
gibbsite, 0.01 1 Aspect ratio, s



Figure 2-15. Tensile strength of composites as functions of volume fraction and aspect
ratio of gibbsite nanoplatelets.









CHAPTER 3
ELECTROPHORETIC CO-DEPOSITION OF POLYMER-GIBBSITE COMPOSITES

Background

In Chapter 2, we have demonstrated a simple and rapid electrodeposition

technology for assembling gibbsite nanoplatelets into large-area, self-standing films.

The interstitials between the assembled nanoplatelets can then be infiltrated with

polymer to form optically transparent nanocomposites. In Chapter 3, we will show a

simple electro-co-deposition technology that enables the creation of inorganic-organic

nanocomposites with oriented layered nanostructures in a single step. The

electrodeposited inorganic-organic nanocomposite films are optically transparent and

flexible, even though the weight fraction of the brittle inorganic phase is higher than

80%.

We will also show the assembly of nanocomposites with similar organic/inorganic

weight ratio and ordered multilayered structure as nacres by using the proposed co-

deposition method. The novelty of the technology is that the positively charged

nanoplatelets and polyelectrolytes are both electrophoretically attracted by the applied

direct-current electric field and then simultaneously deposited on the cathode to form

ordered nanocomposites. The mechanical properties of these biomimetic

nanocomposites and the colloidal stability of the nanoplatelet-polyelectrolyte dispersions

have also been investigated.

Experimental

EPD of Nanoplatelets

EPD of nanoplatelets is performed in a water-ethanol mixture in a sandwich cell

placed horizontally. The bottom and the top of the cell are either an ITO or a gold









electrode. The gold electrode is prepared by sputtering deposition of 20 nm of titanium

and 200 nm of gold on a glass slide. PDMS is used as a spacer to get an active area of

1.5x1.5 cm2 and a cell gap of 2.2 mm.

EPD of PVA-Gibbsite nanoplatelets

To prepare the electrophoretic bath solution, 1 ml of 5 wt.% polyvinyl alcohol

(PVA, Mw 89,000 ~ 98,000, Sigma-Aldrich) aqueous solution was firstly mixed with 9 ml

of 2 wt.% gibbsite nanoplatelet solution. Twenty milliliters of 200-proof ethanol was then

added into the above suspension. The bottom and the top of the cell were an ITO

working electrode and a gold counter electrode, respectively. A constant voltage of -2.5

V (ITO vs. Au electrode) was applied to deposit gibbsite nanoplatelets on the ITO

cathode. After the EPD, the as-deposited PVA-gibbsite film was dried in an oven at

800C.

EPD of PEI-Gibbsite nanoplatelets

Electrophoretic bath solution was prepared by mixing 2 ml of 0.3 wt.%

polyethylenimine (PEI, Mw ~750,000, Sigma-Aldrich) aqueous solution, 3 ml of 2.0 wt.%

gibbsite nanoplatelet aqueous solution, and 10 ml of 200-proof ethanol. Gold electrodes

were served as both working and counter electrodes. Constant current of 0.3 mA was

applied for 15 min to deposit gibbsite nanoplatelets and PEI on the bottom gold

cathode. After EPD, the as-deposited PEI-gibbsite nanocomposite was dried in air.

Results and Discussion

Electrophoretic Co-deposition of PVA-Gibbsite

As the synthesized gibbsite platelets have positively charged surfaces and almost

electrically neutral edges due to their different isoelectric point (pH 10 and 7,

respectively), they tend to re-orient in the electric field with their surfaces facing the ITO









electrode. The high-molecular weight PVA (Mw 89,000-98,000) is neutrally charged in

the electrophoretic bath. They can be absorbed on the surfaces of gibbsite

nanoplatelets and function as water-soluble binders to cement electrodeposited ceramic

particles together. Ethanol (~50% of total volume) is also added to the aqueous colloidal

suspensions to reduce the dielectric constant of the solvent, and thus reduce the

electrical double-layer thickness of the particles to further promote colloidal coagulation

on the ITO cathode.

Figure 3-1A shows a photograph of a PVA-Gibbsite nanocomposite formed on an

ITO cathode. The film can be easily peeled off from the electrode surface by using a

sharp razor blade. The resulting self-standing film is flexible and transparent. Optical

transmission measurement at normal-incidence (not shown here) shows the film

exhibits 60-80% transmittance for most of the visible wavelengths. Top-view SEM

image in Figure 3-1B illustrates the gibbsite nanoplatelets are preferentially oriented

with their crystallographic c-axis perpendicular to the electrode surface. It is very rare to

find edge-on platelets.

The ordered layered structure is clearly evident from the cross-sectional SEM

images as shown in Figure 3-1C and D. The oriented assembly of high-aspect ratio

gibbsite nanoplatelets is further confirmed by XRD. Figure 3-2 displays a XRD spectrum

of an electrodeposited PVA-Gibbsite nanocomposite on an ITO electrode. The

diffraction peaks from (222), (400), (441), and (662) planes of the ITO substrate are

clearly appeared. Other than ITO diffraction peaks, we only observe (002) and (004)

peaks from gibbsite single crystals. As the crystallographic c-axis of single-crystalline

gibbsite is normal to the platelet surfaces, the (002) and (004) reflection are from









gibbsite platelets oriented parallel to the electrode surface. This strongly supports the

macroscopic alignment of gibbsite nanoplatelets in the electrophoretically deposited

nanocomposites. Analysis of the half-height width of the (002) and (004) peaks with the

Scherrer equation (46) yields an average platelet thickness of 10.3 nm, agreeing with

cross-sectional SEM measurement.

The current EPD technology enables large-scale assembly of ordered

nanocomposite films in a very short time. Figure 3-3 shows the relationship between the

measured weight of deposits on ITO cathode and the electrophoretic duration. A weight

plateau is reached in ca. 8 min. Experimental observation shows almost all gibbsite

nanoplatelets have already been deposited in this time interval and the electrophoretic

bath changes from turbid to clear. The electrophoretic velocity of gibbsite nanoplatelets

is estimated to be ~8.2 pm/s by using the Smoluchowski's equation For a 2.16 mm thick

sandwich cell, the estimated time to deposit all particles on the ITO electrode is ca. 5

min, reasonably agreeing with the experimental observation.

The weight fraction of the inorganic phase in the electrodeposited nanocomposites

can be determined by TGA. Figure 3-4 shows the TGA curve and the corresponding

weight loss rate for the nanocomposite film as shown in Figure 3-1A. An apparent

thermal degradation process occurs at ~250C that corresponds to the degradation of

the PVA matrix and the decomposition reaction of gibbsite. Based on the residue mass

percentage (53.96%) and assuming the ash is solely A1203, we can estimate the weight

fraction of gibbsite nanoplatelets in the original nanocomposite film to be 0.825.

Electrophoretic Co-deposition of PVA-Gibbsite

PEI, which is a weak polyelectrolyte and contains amine groups, is positively

charged under the electrophoretic conditions. The gibbsite nanoplatelets with a small









amount of PEI are well dispersed in a water-ethanol mixture solution due to the

electrostatic repulsion between particles. However, adding a larger amount of PEI leads

to the agglomeration of gibbsite nanoplatelets. To allow the electrophoresis at a

controlled deposition rate, as well as the formation of ordered layered structure, gibbsite

nanoplatelets must be stabilized in suspensions. We therefore study the influence of the

PEI concentration on the stability of gibbsite by measuring particle size distribution and

zeta-potential.

To prepare the testing solution, (6 n) ml of 2.0 wt.% gibbsite solution is mixed

with n ml of 0.3 wt.% PEI aqueous solution, where n = 0-5. The weight ratio (PEI to

gibbsite, R) is calculated as (n x 0.3)/[(6 n) x 2]. Fig. 3-5 shows the size distribution of

gibbsite nanoplatelets at different R values measured by laser diffraction. The average

diameter of the as-synthesized gibbsite nanoplatelets (R = 0) is 150 nm (Figure 3-5A),

which is smaller than that observed from TEM images in Chapter 2. The random

mismatch of the surface of nanoplatelets to the incident laser beam reduces the

effective diffraction area, resulting in a smaller average diameter. Figure 3-5B shows

that no significant change in the particle size distribution is observed when a small

amount of PEI is added (R = 0.03). However, further increasing of PEI concentration, as

shown in Figure 3-5C and D (R = 0.075 and 0.75, respectively), leads to a larger particle

diameter resulting from the flocculation of nanoplatelets. The flocculation at high

polyelectrolyte concentration can be explained by the increase in ionic strength, which

leads to the decrease in the electrical double-layer thickness and the instability of the

colloids (47). Depletion flocculation also plays an important role. At a high polymer

concentration, the polymer concentration gradient between the inter-particle gap and









the remainder of the solution generates an osmotic pressure difference, forcing solvent

flows out of the gap until particles flocculate (48).

Electrophoretic mobility and zeta-potential of nanoplatelets in PEI-Gibbsite

suspensions with different R values are shown in Figure 3-6. Zeta-potential is obtained

by fitting experimental data using Smoluchowski's model. The increase of the

electrophoretic mobility and zeta-potential when a small amount of PEI is added (R from

0 to 0.03) is due to the contribution of highly charged PEI that possesses a zeta-

potential of ~ +60 mV in water at neutral pH. Further increasing of PEI concentration

results in the decreasing of electrophoretic mobility and zeta-potential due to the particle

flocculation as shown in Figure 3-5.

As gibbsite nanoplatelets have positively charged surfaces (IEP ~10) and almost

neutral edges (IEP ~7) under the electrophoretic conditions (pH ~7), the electrical force

tends to re-orient the nanoplatelets to face the cathode. The positively charged PEI

molecules are also electrophoretically migrated toward the cathode together with

gibbsite and simultaneously sandwiched between nanoplatelets, forming PEI-Gibbsite

nanocomposite. The addition of ethanol reduces the effective dielectric constant of the

solvent, promoting particle coagulation by suppressing the electrical double-layer

thickness of the nanoplatelets. The high pH near the cathode due to cathodic reactions

also helps to coagulate nanoplatelets, as well as neutralize the protonated PEI

macromolecules. Top-view SEM images in Figure 3-7A and B show that the

electrodeposited nanoplatelets are preferentially oriented with their crystallographic c-

axis perpendicular to the electrode surface. The hexagonal shape and the size of the

platelets can be clearly seen in Figure 3-7B. Cross-sectional SEM images showed in









Figure 3-7C and D provide further evidence of the ordered layered structure. XRD

spectrum of the PEI-Gibbsite nanocomposite on an Au electrode is shown in Figure 3-8.

The diffraction peak from the (002) plane of gibbsite single crystals is clearly appeared.

Comparing to our previous results, which show diffraction peaks from both (002) and

(004) planes of gibbsite crystals, the weaker diffraction peak from (004) plane is

overlapped with the strong diffraction peak of Au. The (004) diffraction peak can be

clearly seen by simply replacing Au electrode with Pt (not shown here). As the (002)

and (004) diffraction are originated from gibbsite platelets oriented parallel to the

electrode surface, the oriented assembly of high-aspect-ratio nanoplatelets is further

confirmed.

TGA is carried out to determine the weight fraction of the organic phase in the

nanocomposites shown in Figure 3-9. An apparent thermal degradation process occurs

at ~2500C that corresponds to the degradation of the polymer matrix and the

decomposition reaction of gibbsite. Based on the residual mass percentage (63.7%)

and assuming the ash contains only A1203, the weight fraction of PEI in the

nanocomposite film is estimated to be ~0.03, which is close to the organic content of

natural nacre consisting of less than 5 wt.% of soft biological macromolecules (19).

The mechanical properties of the electrodeposited nanocomposites are evaluated

using nanoindentation. In a nanoindentation test, a diamond Berkovich indenter is

forced perpendicularly into the coating surface. The load-displacement profile is then

used to calculate the reduced modulus, Er, using the Oliver-Pharr method (49). Figure

3-10 shows the Er as a function of contact depth obtained from the nanoindentation

tests. The observed Er is in the range of 2.20-5.17 GPa, which is comparable to those of









ordered artificial nacres prepared by centrifugal deposition (31) and LBL assembly (50).

The decrease in Er with increasing contact depth may be related to the indentation size

effects. The size effects are explained as a result of deformation, which originates

mainly from crack propagation for ceramics, and factors such as surface roughness,

interaction between inorganic and organic phases, and other structural details of the

coatings (51, 52). The Er of PEI-Gibbsite nanocomposite is ~0.4 GPa lower than that of

pure gibbsite coating, showing the effect of the soft PEI layers in between the hard

gibbsite nanoplatelets (53).

Summary

We have developed a scalable EPD technology for rapid production of nacre-like

inorganic-organic nanocomposites in a single step. The applied direct-current electric

field enables the preferential alignment of gibbsite nanoplatelets and the co-deposition

of non-ionic-type polymer between the inorganic nanosheets. The resulting self-

standing nanocomposite films contain high-weight percentage of inorganic platelets, but

are still optically transparent and flexible. The co-deposition technology is readily

applicable to many cationic polyelectrolytes, such as poly(diallyldimethylammonium

chloride) (PDDA) and poly(allylamine hydrochloride) (PAH), gibbsite nanoplatelets with

larger size, and even silica-coated gibbsite particles.

































Figure 3-1. Electrodeposited PVA-Gibbsite nanocomposite. (A) Photograph of a
composite film on an ITO electrode. (B) Top-view SEM image of the sample
in (A). (C) Cross-sectional SEM image of the sample in (A). (D) Magnified
cross-sectional image.














4000
(002)

Gibbsite
Indium Tin Oxide
3000




S2000

S(222) (400) (441) (622)

1000




0(04)
10 20 30 40 50 60 70
26


Figure 3-2. XRD patterns of an electrodeposited PVA-gibbsite composite on ITO
electrode.


u.u
0 2 4 6 8 10 12 14 16

Deposition Time (min)


Figure 3-3. Deposit weight on ITO electrode versus electrophoretic duration.


U
U U
U

U



U

U













100



80



60-



40



20



0
0 I I I I I I I
0 100 200 300 400 500 600 700

Temperature (oC)



Figure 3-4. TGA of the nanocomposite sample as shown in Figure 3-1A.


1.0




0.5




0.0


-I


-0.5


I -1.0
800












20



15







5 -




0.1 1 10 100 1000
Particle Diameter (1iT)
8
C





4



2



ai rl _____i ^ ri ^ -J ____ L ____


01 1 10


100 1000


E
2


Particle Diameter (Jmn)


B

15 .











4B I
0.1 1 10 1 1 000
Particle Diameter (prn)
3.5
D
3.0

2.5

2.0

1.5

1.0

B.5


0.1 1 10 100 1000
Particle Diameter (pJn)


Figure 3-5. Particle size distribution of nanoplatelet suspensions at different
PEI/gibbsite weight ratio. (A) R = 0, (B) R = 0.03, (C) R = 0.075, and (D) R
=0.75.












6 70
0 mobility
o zeta potential -60
,5
50 N


a Q 40 0



o 4 20 0
o30




10
-21 10
2 I I I I I 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0

PEI/Gibbsite, R (wt/wt)



Figure 3-6. Electrophoretic mobility and corresponding zeta-potential of nanoplatelets
at different PEI/gibbsite weight ratio.


































124m 1 _ir
Figure 3-7. SEM images of PEI-Gibbsite nanocomposite. (A) Top-view image, (B)
magnified top-view image, (C) cross-sectional image, and (D) magnified
cross-sectional image.














80000




60000



40000



20000


10 20 30 40
29


10 20 30 40 50 60 70


2000

Au 1600
1200

800
400
0







(002)

.A i ,


Figure 3-8. XRD patterns of an electrodeposited PEI-Gibbsite nanocomposite on Au
electrode.


-Figure 3-9. TGA
0)















Figure 3-9. TGA


110


100


90


80


70


60


50


40
0 100 200 300 400 500 600 700 801

Temperature (C)


of an electrodeposited PEI-Gibbsite nanocomposite.


50 60 70













6
Gibbsite
o PEI-Gibbsite
0
5-



(9
O 4



00




2 i i i I I
400 500 600 700 800 900 1000

hc (nm)

Figure 3-10. Reduced modulus of pure gibbsite and PEI-Gibbsite nanocomposite
measured by nanoindentation.









CHAPTER 4
ELECTROPHORETIC ASSEMBLY OF SURFACE-ROUGHENED GIBBSITES

Background

In Chapter 2, we have demonstrated that ordered assemblies of single-crystalline

gibbsite nanoplatelets can be achieved by EPD and polymer-gibbsite nanocomposites

are obtained by subsequent polymer infiltration. In Chapter 3, we have also shown that

the co-deposition of gibbsite nanoplatelets and either non-ionic-type polymers or

polyelectrolytes can be achieved by EPD. However, there is a major difference between

the natural platelets found in nacres and the synthetic gibbsite nanoplatelets i.e., the

natural aragonite platelets are rough while the single-crystalline gibbsite nanoplatelets

have smooth surface (37, 54). It is found that the arrangement of nano-asperities

interposing (as in Figure 4-1) between neighboring lamellae plays a crucial role in

determining the inter-lamellae slip and the resulting mechanical properties of the natural

composites (55, 56). This is because the stress at which the inelastic deformation

proceeds is governed by these nano-sized asperities on the surface of the aragonite

tablets (56).

To help understand the surface roughness effect, we intend to increase the

surface roughness of synthetic nanoplatelets by coating gibbsite particles with rough

silica to mimic the asperity of natural aragonite. EPD of these surface-roughened

nanoplatelets is carried out to form artificial nacreous coatings. Unfortunately, the

shrinkage of sol-gel silica during the drying process after electrodeposition results in

severe cracking. Similar drying-induced crack formation is also a detrimental factor that

affects the crystalline quality of silica colloidal crystals prepared by the convective self-

assembly technology (57, 58). To resolve this cracking issue, here we report a new









EPD approach to form biomimetic monolithic multilayer by reversing the surface charge

of silica-coated-gibbsite (SCG) nanoplatelets using adsorbed polyelectrolytes. Different

kinds of polymer nanocomposites are prepared and their mechanical properties are

evaluated by tensile tests. The resulting self-standing films are transparent and exhibit

significantly improved mechanical properties over those of pure polymer.

Experimental

Coating of Gibbsite Nanoplatelets with Silica

Purified gibbsite nanoplatelets are coated with a thin layer of silica by following a

procedure consisting of two steps: adsorption of polyvinylpyrrolidone (PVP) and growth

of silica shell in ethanol via Stober method (59). PVP (Mw ~40,000) is first dissolved in

deionized water by ultrasonication and vigorous stirring. Subsequently, 200 ml of

gibbsite nanoplatelet aqueous suspension (1 wt.%) is mixed with 300 ml of PVP solution

(10 wt.%). The mixture is then stirred for 1 day to ensure the complete adsorption of

PVP on the gibbsite surface. To transfer PVP-coated gibbsite nanoplatelets into

ethanol, the mixture is centrifuged and the sediment is redispersed in ethanol. This

process is repeated for three times for the complete replacement of water with ethanol.

The final volume of the PVP-modified gibbsite nanoplatelet suspension is adjusted to

500 ml. The suspension is then mixed with 33 ml of ammonium hydroxide (14.8 N) and

1 ml of tetraethyl orthosilicate (TEOS, 99+%) for the growth of silica shell. After 4-6 h of

stirring, dispersions of SCG nanoplatelets are centrifuged and the sediments are

redispersed in deionized water. For TEM imaging, 1 ml of HCI (37%) and 10 ml of SCG

nanoplatelets (~0.1 wt.%) are mixed and stirred for 2 days to remove the gibbsite core.









EPD of Nanoplatelets

EPD of nanoplatelets (SCG and PEI-SCG) is performed in a water-ethanol mixture

in a sandwich cell placed horizontally. The bottom and the top of the cell are either an

ITO or a gold electrode. PDMS spacer is used to get an active area of 1.5 x 1.5 cm2 and

a cell gap of 2.2 mm.

EPD of SCG nanoplatelets

The bath solution is SCG nanoplatelets dispersed in a water-ethanol mixture. 200-

proof ethanol is added into 2 wt.% of aqueous suspensions of SCG nanoplatelets to

make the volumetric ratio of ethanol to the aqueous suspension to be two. A constant

voltage of 3.5 V (Au vs. ITO) is applied for 20 min to deposit the negatively charged

SCG nanoplatelets onto the bottom gold anode. An ITO electrode is used as the top

counter cathode to enable the in situ observation of the EPD process.

EPD of PEI-SCG nanoplatelets

The electrophoretic bath solution for depositing PEI-SCG nanoplatelets is

prepared by mixing 9 ml of 1.5 wt.% SCG nanoplatelet aqueous suspension with 1 ml of

1.5 wt.% PEI aqueous solution. The bath solution is ultrasonicated for 30 min to

minimize the agglomeration of SCG nanoplatelets. Positively charged PEI

macromolecules are adsorbed on the negatively charged surface of SCG nanoplatelets

due to Coulombic attraction, forming positively charged PEI-SCG nanoplatelets. A

constant voltage of -2.5 V (ITO vs. Au) is applied for 20 min to deposit the positively

charged PEI-SCG nanoplatelets onto the bottom ITO cathode. A gold electrode is

served as the top counter anode.









Results and Discussion


SCG Nanoplatelets

SCG nanoplatelets are synthesized by coating a thin shell of sol-gel silica on

gibbsite nanoplatelet cores (59). The synthetic process consists of two steps: adsorption

of PVP and the subsequent growth of silica shell via Stober method. The amphiphilic

PVP macromolecule acts as a coupling agent. It can be adsorbed onto a broad range of

colloids and stabilizes them in water and various nonaqueous solvents (e.g., ethanol).

The PVP-modified gibbsite nanoplatelets can be directly dispersed in ethanol for the

subsequent growth of silica shell. The thickness of the silica shell can be easily

controlled by adjusting the sol-gel reaction conditions (e.g., precursor concentration and

reaction time) (59). To confirm the formation of silica shell, the gibbsite core of SCG

nanoplatelets can be selectively removed by a hydrochloric acid wash (41). Figure 4-2A

shows a typical TEM image of hollow silica nanoplatelets after selectively leaching out

gibbsite cores. Dark edges in the TEM image reveal that these nanoplatelets are hollow.

The arrows indicate a silica shell with a thickness of ca. 10 nm. The regular hexagonal

shape and the thickness uniformity of silica shells are clearly evident from the TEM

image. Additionally, by comparing the TEM image with that of gibbsite nanoplatelets in

Figure 2-5, it is evident that the sol-gel-derived silica shells are much rougher than the

single-crystalline gibbsite nanoplatelets. Moreover, the silica coating reverses the

surface charge of gibbsite colloids and the zeta-potential of SCG nanoplatelets in

ethanol is measured to be -38.1 1.6 mV by using Smoluchowski's model. This further

confirms that the surface of gibbsite nanoplatelets has been coated with silica which is

negatively charged at neutral pH (60).









EPD of SCG Nanoplatelets

EPD of negatively charged SCG nanoplatelets is carried out in a water-ethanol

mixture using a parallel-plate sandwich cell, which consists of a gold working anode on

the bottom, an ITO cathode on the top, and a PDMS spacer (~2.2 mm thick). Ethanol is

used to reduce the dielectric constant of the solvent and therefore decrease the

electrical double-layer thickness of the SCG nanoplatelets to promote colloidal

coagulation on the gold electrode. Deionized water is added to the suspension for

bringing about the following anodic reaction:

2H20 -- 02 + 4H+ + 4e- E = -1.229V

which leads to a local pH decrease at the electrode surface. Since the zeta-potential of

silica reduces when the solution pH decreases (60), the above anodic reaction can thus

lower the surface charge of negatively charged SCG nanoplatelets to further assist

colloidal coagulation. A photograph of an electrodeposited SCG film on a gold electrode

is shown in Figure 4-2B. In sharp contrast to the electrodeposited gibbsite films which

are monolithic and crack free, cracks can be easily formed on the SCG film during the

drying process after electrodeposition. We attribute the formation of cracks to the

excess stress induced by the shrinkage of sol-gel silica shell of SCG nanoplatelets. It is

well-known that the drying of Stober silica spheres during the convective self-assembly

process leads to cracks in the resulting colloidal crystal films (57, 58). To eliminate

shrinkage-induced cracks, one common approach is to sinter sol-gel silica at high

temperature (>500C) (58). However, this approach cannot be used for SCG

nanoplatelets as gibbsite cores will be thermally decomposed at ~300C as addressed

in Chapter 2.









PEI-SCG Colloidal Stability

To resolve the cracking issue, we pursue a new approach by adding PEI to the

electrophoretic bath in order to increase the adherence and strength of the

electrodeposited films. PEI, which is a weak polyelectrolyte with the molecular structure

shown in the inset of Figure 4-3 (pKa ~10.5), is positively charged under the

electrophoretic conditions (pH ~6) due to the possessing of multiple amine functional

groups. It also acts as a particle binder because it adsorbs strongly onto silica at a wide

range of pH values (48). Therefore, positively charged PEI macromolecules are

adsorbed on the negatively charged surface of SCG nanoplatelets via Coulombic

attraction, forming positively charged PEI-SCG nanoplatelets. However, adding a large

amount of PEI leads to the flocculation of SCG nanoplatelets.

To investigate the colloidal stability of PEI-SCG nanoplatelets in suspensions as

well as the reversal of surface charge, the influence of the PEI concentration is studied

by measuring zeta-potential of colloidal suspensions with different amount of PEI. The

testing solution is prepared by mixing 4.5 g of 0.075 wt.% SCG dispersion in ethanol

with 0.5 g of PEI aqueous solution at different concentration, CPEI (wt.%). The mixture is

ultrasonicated for 30 min to minimize the agglomeration of SCG nanoplatelets. The

specific surface area of SCG nanoplatelets is calculated to be ~57 m2/g by using the

geometry determined from experiments and the density of gibbsite, 2.2 g/cm3. The

amount of PEI addition is calculated as [(0.5 x CPEI)/(4.5 x 0.075 x 57)] x 103 mg/2.

The zeta-potential of PEI-SCG nanoplatelets as a function of the amount of PEI addition

is shown in Figure 4-3. At zero PEI addition, the zeta-potential of SCG nanoplatelets is

measured to be -38.1 1.6 mV, showing the surface of colloids are negatively charged









and silica is coated on gibbsite cores. With the increasing of the amount of PEI, zeta-

potential of SCG nanoplatelets initially reduces and becomes less negative. Further

addition of PEI triggers the reversal of zeta-potential when the PEI addition is larger

than 0.5 mg/m2. A zeta-potential plateau is reached at around +28 mV when 2 mg/m2 of

PEI is added. The formation of the plateau is caused by the saturation of adsorbed PEI

macromolecules on SCG nanoplatelets. When a larger amount of PEI is added, PEI-

SCG nanoplatelets tend to flocculate and zeta-potential decreases. The agglomeration

of nanoplatelets at high PEI addition can be explained by depletion flocculation. At a

high polymer concentration, the concentration gradient between the inter-particle gap

and the remainder of the solution generates an osmotic pressure difference, forcing

solvent flows out of the gap until particles flocculate (48). Flocculation of gibbsite

nanoplatelets and the decreasing of zeta-potential are observed when a high

concentration of PEI (26 mg/m2) is added to SCG dispersion. To get stable SCG

nanoplatelets for EPD, the amount of PEI addition is thus controlled in the range of the

zeta-potential plateau at 1.54 mg/m2.

EPD of PEI-SCG Nanoplatelets

The cathodic electrodeposition of PEI-SCG nanoplatelets is performed using a

parallel-plate cell with electric field strength of 1100 V/m. The positively charged PEI-

SCG nanoplatelets are attracted toward the bottom ITO cathode by the electrical force.

Gravity only plays a minor role as the sedimentation speed of PEI-SCG particles is

much slower than the electrophoretic mobility. Top-view SEM images in Figure 4-4A

and B show that most of the electrodeposited PEI-SCG nanoplatelets are aligned

parallel to the ITO electrode surface, though a few nanoplatelets are found to orient

perpendicularly as shown by the arrows in Figure 4-4B. Compared to electrodeposited









films consisting of pure gibbsite nanoplatelets, the preferential alignment of PEI-SCG

nanoplatelets is slightly deteriorated. The layered structure and the slight worsening of

the oriented deposition of PEI-SCG nanoplatelets are further confirmed by the cross-

sectional SEM images in Figure 4-4C and D and the XRD patterns in Figure 4-5. The

diffraction peaks showing (222), (400), (441), and (622) planes are from the ITO

electrode. Other than ITO diffraction peaks, we observe mainly (002) and (004) peaks

from gibbsite single crystals. As the crystallographic c-axis of single-crystalline gibbsite

is normal to the platelet surface, the (002) and (004) diffraction peaks are from PEI-SCG

nanoplatelets oriented parallel to the electrode surface (46). Low-intensity diffraction

peaks, such as those from the (023) and (024) lattice planes at 45.4780 and 52.2190,

respectively, can also be seen from the XRD spectrum. This indicates that small amount

of nanoplatelets are not aligned parallel to the electrode surface, agreeing with our SEM

observation.

Unlike gibbsite nanoplatelets that have positive charges on their surfaces (IEP

~10) and almost neutral edges (IEP ~7) under the electrophoretic conditions (pH ~7),

PEI-SCG nanoplatelets have positive charges on both surfaces and edges because of

the uniform coverage of silica shell and adsorbed PEI macromolecules. The difference

in the spatial distribution of surface charges distinguishes the resulting arrangement of

electrodeposited PEI-SCG nanoplatelets from that of gibbsite nanoplatelets. For the

latter, the electrical force tends to re-orient the nanoplatelets in the electrophoretic bath

to face the cathode. The deposited gibbsite nanoplatelets are thus densely packed with

their crystallographic c-axis normal to the electrode surface. By contrast, no electric-

field-induced re-orientation is occurred for PEI-SCG nanoplatelets because of the









uniform distribution of surface charge. Nevertheless, most of the PEI-SCG nanoplatelets

are still preferentially aligned to the electrode surface because this orientation is more

energetically favorable than the perpendicular one. Another reason for the imperfect

alignment of PEI-SCG nanoplatelets may come from the bridging flocculation

mechanism, which allows the formation of polymer bridges between neighboring

particles (48). Since the PEI macromolecules used are highly branched with high

molecular weight (Mw ~750,000) and large numbers of amine groups, they are easily

attached to several SCG nanoplatelets. However, this attachment could be random and

the nanoplatelets in the resulting aggregates might not be aligned.

ETPTA-PEI-SCG Nanocomposites

After electrodeposition, polymer nanocomposites can be made by filling the

interstitials between the PEI-SCG nanoplatelets with photocurable monomers, followed

by photopolymerization. A non-volatile monomer, ETPTA, is chosen to form the ETPTA-

PEI-SCG nanocomposite. The monomer with 1 wt.% of photoinitiator is first added on a

PEI-SCG film on an ITO electrode and the sample is then kept under vacuum for a few

hours to promote the monomer penetration. After the sample becomes transparent, it is

spin-coated at 4000 rpm for 1 min to remove the excess monomer. Exposure to

ultraviolet radiation is then carried out to polymerize ETPTA monomer.

The resulting ETPTA-PEI-SCG nanocomposite film is highly transparent (as in

Figure 4-6) due to the matching of the refractive index between the PEI-SCG

nanoplatelets and the polymer matrix. The normal-incidence transmission spectra in

Figure 4-7 indicate that the ETPTA-PEI-SCG nanocomposite on an ITO electrode

(ETPTA-PEI-SCG/ITO) exhibits high transmittance (>80%) for most of the visible

wavelengths. This suggests that most interstitial spaces between the PEI-SCG









nanoplatelets have been infiltrated by the polymer. Free-standing ETPTA-PEI-SCG

nanocomposites can be obtained by soaking ETPTA-PEI-SCG/ITO in 1 M of sodium

hydroxide solution for several hours. Higher transmittance is achieved for the ETPTA-

PEI-SCG nanocomposite due to the removal of the ITO electrode. Compared to the

high transmittance of the ETPTA-PEI-SCG nanocomposite, the PEI-SCG film on an ITO

electrode (PEI-SCG/ITO) (Figure 4-6B) shows a transmittance less than 10% for most

of the visible spectrum.

The cross-sectional SEM images in Figure 4-8 show that the layered structure of

the original PEI-SCG film is retained and a thin wetting layer of ETPTA (~2.5 pm thick)

is observed on the surface of the nanocomposite. The red and black arrows in Figure 4-

8A indicate the ETPTA wetting layer and the ITO electrode, respectively. The

mechanical properties of the biomimetic polymer nanocomposites are evaluated by

tensile tests. The tensile strength for plain ETPTA, ETPTA-gibbsite nanocomposite, and

ETPTA-PEI-SCG nanocomposite are tested and the results are shown in Figure 4-9.

Compared with pure ETPTA polymer, the ETPTA-gibbsite nanocomposite shows ~2-

time higher strength and ~3-time higher modulus. For the ETPTA-PEI-SCG

nanocomposite, even higher tensile strength than that of the ETPTA-Gibbsite film can

be achieved. This is due to the presence of the PEI macromolecule, which acts as a

binder by strongly adsorbing on the negatively charged surface of SCG nanoplatelets

via Coulombic attraction. Its highly branched molecular structure also enables the

interlock with cross-linked ETPTA backbone. Early studies reveal that the interfacial

bonding between the ceramic fillers and the organic matrix is crucial in determining the

mechanical properties of the artificial nacreous composites (8, 9). The strong ionic









bonding between the PEI macromolecules and the SCG nanoplatelets along with the

natural elasticity of PEI macromolecules make the ETPTA-PEI-SCG nanocomposites

have 3 to 5-time higher strain than those of pure ETPTA and ETPTA-Gibbsite

nanocomposites. The higher strain could also come from the surface roughness of PEI-

SCG nanoplatelets due to the silica coating and the rotation of misaligned SCG

nanoplatelets under an applied tensile load. A rough estimation based on the area

under the tensile stress-strain curve indicates that the energy needed to rupture the

ETPTA-PEI-SCG nanocomposite is nearly one-order-of-magnitude and 6-time higher

than those required to break pure ETPTA polymer and ETPTA-Gibbsite nanocomposite,

respectively.

A simple calculation based on the shear lag model as in Chapter 2 is carried out to

validate the measured mechanical properties. Since the volume of the adsorbed PEI on

the SCG nanoplatelets is quite small, we can simply use the volume fraction of ETPTA

to calculate tensile strength of the nanocomposite. From our previous TGA of ETPTA-

gibbsite nanocomposites prepared by the same spin-coating technique as reported

here, the volume fraction of ETPTA in the polymer nanocomposite is ~0.50. Therefore

the tensile strength of the nanocomposite can be estimate to be about 2.75am, agreeing

with our experimental results.

Summary

In summary, we have developed a simple and scalable EPD technology for

assembling surface-roughened inorganic nanoplatelets into organized multilayer. The

adsorption of polyelectrolyte macromolecules on the surface of nanoplatelets can

reverse the surface charge and simultaneously eliminate the cracks induced by the

shrinkage of the sol-gel silica shell of the surface-roughened nanoplatelets during









drying. We expect this approach could be applicable to the convective self-assembly of

spherical colloidal silica particles to facilitate the formation of crack-free colloidal

photonic crystals.






















Figure 4-1. Cross section of abalone nacre showing the detailed structure at the
lamellae boundaries. Arrows highlight locations where the nano-asperities
interpose (56).











100nm 3 mm


Figure 4-2. (A) TEM image of acid-leached SCG nanoplatelets. The arrows point to a
silica shell with a thickness of ca. 10nm. (B) Photograph of an
electrodeposited SCG film on a gold electrode.












40



20 -

E P

0
H H2
0 N N 2 H

S-20 H H
Sn
N |
a H2N N N NH2
-40


-1 0 1 2 25 26 27
PEI addition (mg/m2)



Figure 4-3. Zeta-potential of PEI-SCG nanoplatelets with different amount of PEI
addition. The inset shows the molecular structure of PEI.

































Figure 4-4. SEM images of electrodeposited PEI-SCG nanocomposite. (A) Top-view
image. (B) Magnified top-view image. (C) Cross-sectional image. (D)
Magnified cross-sectional image.


Eiii~-~i~e~PI 4~m












3500


3000 2- e Intensity III I 2 Intensity III, I
18.297 100 002 41.725 27 312
2500 20.316 70 110 44.203 40 313
S20.565 50 200 45.478 28 023
C( 2000 26.918 30 112 52.219 30 024
37.113 15 004 54.470 30 314
U)
c 1500


S1000 (222)(400) (441) (622)


500 I 1 ( 1
500


10 20 30 40 50 60 70
20



Figure 4-5. XRD patterns of a PEI-SCG nanocomposite on an ITO electrode. Blue
arrows point to the characteristic peaks of ITO. The inset shows a table with
major lattice planes of gibbsite.

























Figure 4-6. Photographs of (A) ETPTA-PEI-SCG and PEI-SCG deposits on ITO
electrodes.


60 -


40 1-


20 1-


0 1
40


0


500


600


Wavelength (nm)


Figure 4-7. Normal-incidence transmission spectra of ETPTA-PEI-SCG
nanocomposite, ETPTA-PEI-SCG nanocomposite on an ITO electrode, and
PEI-SCG deposit on an ITO electrode.


-- ETPTA-PEI-SCG


----- ETPTA-PEI-SCG/ITO
... PEI-SCG/ITO


.......... I .......... 1 I I










I" IJ.


Figure 4-8. SEM images of an ETPTA-PEI-SCG nanocomposite on an ITO electrode.
(A) Cross-sectional image. (B) Magnified cross-sectional image. Red and
black arrows in (A) point to a thin wetting layer of ETPTA and the ITO
electrode, respectively.


01
0.00


0.02 0.04 0.06 0.08


0.10


Strain


Figure 4-9. Tensile stress vs. strain curves for plain ETPTA film, ETPTA-Gibbsite
nanocomposite, and ETPTA-PEI-SCG nanocomposite.









CHAPTER 5
ELECTROCHEMICAL SERS AT PERIODIC METALLIC NANOPYRAMID ARRAYS

Background

Surface Plasmon

Surface plasmons (SPs) are electromagnetic waves that propagate along a

metal/dielectric interface (61). These are incident light waves that are trapped on the

interface because of their interaction with free electrons in metal, leading to a strong

concentration of electromagnetic energy at the interface. In this interaction, the free

electrons oscillate in response to the incident light waves, as shown in Figure 5-1. The

resonant interaction between the surface charge oscillation and the electromagnetic

field of the light forms the SP and gives rise to its unique properties. Typical metals

used to support surface plasmons are gold and silver (62, 63), but metals such as

copper (64), titanium (65), or chromium (66, 67) can also sustain surface plasmon

generation.

There are two consequences of the coupling to surface plasmons. One is that in

contrast to the propagation of SPs along the interface, the field normal to the metal

surface decays exponentially from the interface, resulting in non-radiative nature of SPs

and preventing power from losing. The other is that the interaction between the free

electron and the electromagnetic field results in the momentum of SP, hks, being

greater than that of incident photons, hko This can be obtained by solving Maxwell's

equations under appropriate boundary conditions and gives


k p =ko k 0
E. E









where Ed and e are the permittivity of the metal and the dielectric material, respectively.

The difference in momentum of incident light and SPs must be compensated in order

to generate SPs. Typically there are three ways to bridge the momentum. The first one

requires the use of prism to enhance the momentum of the incident light. The second

one exploits surface defects to scatter incident light. The third one is to generate

periodically patterned nanostructures, such as sub-wavelength nanohole arrays.

Extraordinary Optical Transmission

One of the important applications of SP is the extraordinary optical transmission

through sub-wavelength hole arrays (68-71), as shown in Figure 5-2. The desire to

control photons in a manner analogous to the control of electrons has made SP

significant. According to the standard aperture theory, the transmission of a sub-

wavelength aperture is extremely low and proportional to the fourth power of the ratio of

its diameter and light wavelength while apertures are smaller than the wavelength of the

incident photon, resulting in a main constraint in manipulation of light (72). However, it

has been found that arrays of films perforated with periodic sub-wavelength holes allow

unusually high transmission of light at wavelengths larger than the array period due to

surface plasmon resonance (SPR), showing extraordinary optical transmission. The

transmission of light through sub-wavelength hole arrays made in a metal film can be

orders of magnitude larger than expected from standard aperture theory. Experiments

have provided evidence that these unusual optical properties are as a result of the

coupling of light with SPs on the surface of the periodically patterned metal film.

Considerable interest in the optical properties of the periodic arrays of sub-

wavelength apertures in metallic thin films has raised due to their potential numerous

applications in photonic circuits, light manipulation, sub-wavelength photolithography









and optical modulators. In the past, absorption of SPs by metal was a significant

problem that SPs were not considered for photonic elements because the SP

propagation length was smaller than the size of components. It has been demonstrated

that coherent spatial SP propagation lengths are a few pm and ultrafast decay of the SP

polarization occurs on a 10 fs time scale (73). However, this view has been changed

due to advances in nanotechnology. Recent SP-based components are significantly

smaller than the propagation length. This opens up the way to integrate several SP-

based devices into circuits before propagation losses. Typically focused ion beam (FIB)

milling is used to fabricate sub-wavelength holes (74-76) but it is a time consuming,

expansive and low throughput process and the size of substrate is limited.

Surface-enhanced Raman Scattering

Since SP waves travel on the boundary of the metal and the external medium, the

adsorption of molecules to the metal surface greatly changes the oscillation and

therefore can be also used as sensors. SPR technique is of great importance for

monitoring binding events in biological systems. Typically, reflection geometry

(Kretschmann configuration) is required to excite SP through prism coupling. SPR

techniques provide good sensitivity at the submonolayer level. However, because the

principle of operation establishes on the oscillation change of SP resonances, specificity

at the molecular level is poor. SP-enhanced spectroscopic methods therefore become a

powerful tool for chemical and biochemical analysis, providing better molecular

specificity.

SERS is the most commonly used SP-mediated molecular spectroscopic method,

as shown in Figure 5-3 (77). The enhanced Raman signal provides a molecular

fingerprint due to its narrower bandwidth. SERS is a noninvasive technique that enables









the detection and characterization of both small organic and big biological molecules at

very low concentrations, or even at the single-molecule level (78-81). This opens up

exciting new opportunities for the sensitive and selective detection of analytes that are

commonly encountered in the analysis of chemical warfare agents, biological products,

food regulation, water quality control, and environmental monitoring. Electrochemical

SERS is an important branch of SERS studies and has attracted great scientific and

technological interest as it enables in situ investigation of adsorption and reaction at

electrochemical interfaces, promising for developing fundamental understanding and

control of fuel cells, metal corrosion, semiconductor processing, electrocatalysis

processes, and electroanalysis (82-85). Electrochemically roughened metal surfaces

have been extensively exploited as electrodes for electrochemical SERS (82, 86-89).

However, the relatively low SERS enhancement (on the order of 104), the poor

reproducibility of SERS enhancement (intensity variation by a factor of -10 across a

sample surface), and the electrochemical instability at high cathodic potentials are major

drawbacks for these roughened electrodes. Therefore, how to generate reproducible

SERS substrates that provide high enhancement factor is of great importance.

Substrates for Surface-enhanced Raman Scattering

Bottom-up colloidal self-assembly and templating nanofabrication provide an

inexpensive and simple-to-implement alternative to the electrochemical roughening

process in creating nanostructured SERS electrodes (90-97). Metal film over

nanosphere (MFON) electrodes prepared by vapor deposition of a SERS-active metal

(Au or Ag) over a self-assembled nanosphere monolayer have been demonstrated to

exhibit improved stability and reproducibility for electrochemical SERS experiments









(98). Rapid detection of an Anthrax biomarker was achieved using SERS on silver film

over nanosphere (AgFON) substrates (99). Atomic layer deposition (ALD) is used to

deposit a sub-1-nm alumina layer on AgFON substrates to improve Anthrax biomarker

detection (100). In comparison to the bare AgFON substrates, the ALD-modified AgFON

substrates show higher sensitive and better stability. Sculpted electrochemical SERS-

active electrodes with regular hexagonal arrays of sphere segment nanovoids, which

show reproducible and high (1.5 x 105) surface enhancement, have been replicated

from colloidal crystal templates via electrodeposition of coinage metals in particle

interstitials (101). Unfortunately, most of the current bottom-up approaches suffer from

low throughput and incompatibility with standard microfabrication, thereby impeding the

cost efficiency and scale-up of these unconventional methodologies in generating SERS

active electrodes.

Inspired by tip-enhanced Raman scattering (TERS) (92, 102-104), we have

recently developed a simple yet scalable colloidal templating technique for producing

wafer-scale gold nanopyramid arrays with nanoscale tips and high tip density (6 x 108

tips cm-2) (105). These periodic arrays of nanopyramids can enhance the local

electromagnetic field in the vicinity of the sharp nanotips, resulting in strong surface

enhancement for Raman scattering from benzenethiol molecules absorbed on the gold

surfaces. Here we demonstrate that these templated nanopyramid arrays can be

utilized as electrodes for achieving high SERS enhancement. The resulting SERS

intensity can be adjusted by tuning the applied electrode potential and the

electrochemical reactions on the electrode.









Experimental

Preparation of Electrochemical SERS-active Gold Nanopyramid Arrays

The synthesis and purification of monodispersed silica microspheres with 320 nm

diameter in 200-proof ethanol were performed according to a reference (106). Detailed

procedures are as below:

1. Clean all glassware as the procedures in Chapter 2.

2. Put a stirring bar in a 1000 ml flask.

3. Enclose the flask with a septum and weight the flask

4. Enclose a bottle of EtOH with a septum.

5. Draw EtOH from the bottle with a 60 ml syringe.

6. Put a 0.2 um PTFE hydrophobic filter on the syringe and inject EtOH into the flask.

7. Repeat 4-5 until EtOH reaches 665 ml (525g, density = 0.789).

8. Take off the filter and draw sufficient amount of DIW.

9. Put a 0.2 um hydrophilic filter on the syringe and inject 55.7g of DIW into the flask.

10. Take off the hydrophilic filter and draw sufficient amount of ammonia hydroxide.

11. Put the hydrophilic filter on and inject 25.7ml (23.1 g, density = 0.8988) of ammonia
hydroxide.

12. Strongly stir on hot plate and then inject 50 ml of DISTILLATED TEOS as quickly
as possible and vigorously shake the flask.

13. Aging for 8 hours on a stirring plate at a stirring rate around 7~8.

The purified silica colloids were concentrated by centrifugation and redispersed in

ETPTA using a vortex mixer (Fisher). To this 1 wt.% Darocur 1173 was added as

photoinitiator. The final particle volume fraction was adjusted to ~20%. The colloidal

suspension was dispensed on a silicon wafer (testgrade, n type, (100), Wafernet) which

had been primed by 3-acryloxypropyl trichlorosilane (Gelest). The established spin-









coating process was then utilized to generate monolayer colloidal crystal embedded in

ETPTA monomer using a standard spin coater (WS-400B-6NPP-Lite Spin Processor,

Laurell). The ETPTA monomer was photopolymerized for 4 s using a pulsed UV curing

system (RC 742, Xenon). The polymerized ETPTA matrix was then removed by oxygen

plasma etching operated at 40 mTorr pressure, 40 sccm flow rate, and 100 W for 2 min

on a Unaxis Shuttlelock RIE/ICP reactive-ion etcher. The released silica particles were

utilized as shadow masks during electron-beam deposition of 30 nm thick chromium

using a Denton DV-502A EB evaporator with a typical deposition rate of 2 A/s. The

templating silica particles could then be removed by rubbing the wafer with a cleanroom

Q-tip under flowing deionized water, resulting in the formation of chromium nanohole

arrays on the (100) silicon wafer. The wafer was wet etched at 60C for 4 min in a

freshly prepared solution containing 62.5 g of KOH, 50 ml of anhydrous 2-propanol, and

200 ml of ultrapure water to create inverted pyramids in silicon. After dissolving the

chromium layer in CR-7 etchant (Transene), 500 nm thick of gold was deposited on the

silicon template at a deposition rate of ~5 A/s with a Kurt J. Lesker CMS-18 Multitarget

Sputter. The gold layer could finally be peeled of from the wafer surface with a

conductive double-sided carbon disk (SPI Supplies), yielding an electrochemical SERS-

active nanopyramid array in gold. The templated gold nanopyramid arrays were

examined using a SEM prior to and after the electrochemical SERS experiments.

Electrochemical Surface-enhanced Raman Scattering

The electrochemical setup used to conduct the electrochemical SERS

experiments was constructed as shown in Figure 5-4. A glass slide (Corning, 2.5 x 4.0

cm) was used as the substrate. On top of the slide were a conducting copper tape (3M,

1.2 x 4.0 cm) and then the conductive carbon disk (diameter of 1.2 cm) with the









templated gold nanopyramid array on its top side. Insulating tape obtained from Furan

Co. was used to cover the rest of the copper tape. Platinum wire purchased from

Sigma-Aldrich was used as the counter electrode. An aqueous solution consisting of 0.1

M NaCI and 0.05 M pyridine was used as the electrolyte. A flat gold film deposited by

the same sputtering process as described above was used as the control sample for

SERS measurements. The voltage (Au vs. Pt electrodes) was controlled by an EG&G

Model 273A potentiostat (Princeton Applied Research). All Raman spectra were

recorded on a Renishaw Raman microscope using a 785 nm diode laser at 48 pW with

an integration time of 10 s.

Cyclic Voltammetry Measurements

Two-electrode cyclic voltammetry was used to characterize electrodes in 0.1 M

NaCI solution with or without 0.05 M pyridine, including electrodes that had only

conductive carbon or copper tape on the glass slide and electrodes that had a gold

nanopyramid array on carbon tape with or without copper tape between the carbon tape

and the glass slide substrate. The active area of each electrode was controlled at 1 cm2.

Platinum wire was used as both counter and reference electrodes. The voltage was

scanned between -1.0 and 1.0 V with a scan rate of 50 mV/s by using the EG&G

potentiostat.

Electromagnetic Modeling of Raman Enhancement

In the finite-element-method (FEM) model, the gold nanopyramid array was

supposed to be placed horizontally so that the interface between the substrate and the

medium (water) was parallel to the xz-plane while the nanopyramids were along the y-

axis. FEM was employed under a COMSOL Multiphysics environment to obtain

numerical solutions of Maxwell's equations for each substrate (water and gold). In order









to obtain high-resolution numerical solutions, the computational domain needs to be

bounded and the boundary conditions should be well-defined. To this end, the "perfect

matched layers" (PML) boundary approach was utilized for the simulation (47).

Artificially constructed 10 boundary layers were around the medium (water) and the

scatter (gold) domains. The electric and magnetic conductivities of each boundary layer

could be set artificially so that little or no electromagnetic radiation would be reflected

back into the domain of scatter. TO simulate electromagnetic fields in the newly

augmented domain, Maxwell's equations in all subdomains were solved. For the outer

boundaries of the PML layers, a low-reflection boundary condition (47) was provided to

minimize residual reflection and attenuate the wave quickly within the layers. After

solving Maxwell's equations together with the above boundary conditions, the two-

dimensional electric field could be used to calculate the Raman enhancement factor as

G(Xy) = log(I E,4)

where E(x,y) was the electric field amplitude at location (x,y) and Eo was the incident

electric field amplitude (107). The maximum value of the Raman enhancement could be

obtained over the medium (water) domain.

Results and Discussion

Colloidal Templating Process for Nanopyramid Array Fabrication

The schematic illustration of the colloidal templating process for fabricating gold

nanopyramid array electrodes is shown in Figure 5-5. The established spin-coating

technique is first applied to shear-align submicrometer-sized silica particles into ordered

colloidal monolayers. In contrast with previous colloidal self-assembly approaches, spin-

coating enables rapid production of colloidal crystal templates with wafer-scale area (up









to 8 in. diameter). Though the particles are not touching each other, they do exhibit

long-range hexagonal ordering. After removing the polymer matrix surrounding silica

particles by brief oxygen plasma etch process, the nontouching silica particles can be

used as shadow masks during physical vapor deposition of chromium to create periodic

nanohole arrays, which are then utilized as etching masks to make inverted silicon

pyramidal pits by anisotropic KOH wet etch. Wafer-scale gold nanopyramid arrays with

sharp tips can finally be replicated by sputtering a thin layer of gold on the silicon

templates, followed by a simple adhesive peeling process. By simply controlling the size

of the templating silica spheres and the anisotropic wet etch conditions (e.g.,

temperature, duration, and etchant concentration), the dimensions of the templated

nanopyramids, such as base length, depth, and separation, can be adjusted. Figure 5-

6A shows a tilted SEM image of an array of gold nanopyramids templated from 320 nm

silica spheres. The long-range hexagonal ordering of nanopyramids is clearly evident

from the image. Magnified SEM images show that most of the pyramidal tips have a

radius of curvature of r < 10 nm.

Electrochemical SERS Spectra of Pyridine on Gold Nanopyramid Arrays

The electrochemical SERS measurements are carried out using a 0.05 M pyridine

aqueous solution with 0.1 M NaCI as a background electrolyte. Figure 5-7 shows a

comparison of SER spectra obtained at -1.0 V on the gold nanopyramid array electrode

and a flat gold control electrode prepared by the same sputtering process. The

nanopyramid electrode exhibits a strong Raman scattering signal, while the featureless

gold control sample does not show distinctive SERS peaks at the same experimental

conditions. The control sample has been prepared in the same sputtering batch as the









nanopyramid electrode and therefore has a similar surface roughness. The peak

positions and the relative amplitude of the peaks obtained at the nanopyramid

electrodes agree well with those in the literature for pyridine adsorbed on roughened

gold disk electrodes (108, 109), but are significantly different from those obtained at

sculpted gold nanovoid array electrodes (101). The assignment of the spectral peaks is

shown in Table 5-1 (110). From Table 5-1, it is clear that almost all the enhanced

vibrational modes are associated with the in-plane perturbations, indicating that the

adsorbed pyridine molecules are bonded perpendicular to the metal surface via their

nitrogen lone pairs (97, 98, 108). Another evidence of the end-on configuration of the

adsorbed molecules comes from the two peaks at 1013 and 1037 cm-1, which

correspond to the ring breathing mode and the ring mode (V12) and occur at frequencies

close to those obtained for pyridine in solution (87, 108). By contrast, for flat-adsorbed

pyridine molecules, the frequencies of the ring modes are expected to decrease when

compared to those of the "free" molecules in the liquid state, due to the interaction of the

nr-electrons of the ring with the electrode surface (111).

Figure 5-8 shows the SER spectra recorded for adsorbed pyridine as a function of

electrode potential applied on the gold nanopyramid array electrode (vs. a platinum

counter electrode). It is clearly evident that stronger SERS enhancement occurs at

higher negative potentials when the potential is swept from +1.0 to -1.0 V. Similar

SERS intensity dependence on the applied electrode potential has previously been

reported on Au(210) single-crystal electrodes (108) and AgFON electrodes (98). The

maximum surface enhancement factor at -1.0 V is estimated to be -2.7 x 106 using the

method described in the literature by comparing the Raman intensity for the peak at









1013 cm-1 obtained for a solution and at the nanopyramid electrode and assuming a

surface coverage of 0.40 nmol cm-2 for pyridine on gold and a surface roughness of 3.0

(84). This enhancement is more than 1 order of magnitude higher than that obtained at

other nanostructured electrodes (98, 101). Figure 5-9 shows the SER spectra of

pyridine adsorbed on a gold nanopyramid electrode when the potential is swept from

-1.0 V (top spectrum) to +1.0 V (middle spectrum) and then back to -1.0 V (bottom

spectrum). The peak amplitude is greatly reduced when the potential is swept from -1.0

to +1.0 V and the 1013 cm- peak is shifted to 1018 cm-1. When the potential is cycled

from +1.0 V back to -1.0 V, the SERS signal is even stronger than the original spectrum

obtained at -1.0 V and the peak at 1013 cm- reaches the detection limit of the Raman

spectrometer. Further potential cycling experiments show that the high SERS

enhancement at -1.0 V can be consistently achieved for at least five cycles and then

starts to decrease for more sweeps.

Electrode Effects

The experimental results shown in Figures 5-8 and 5-9 are contradictory to those

obtained at sculpted nanovoid arrays, where higher SERS intensity is observed at more

positive potentials (101). To help understand this contradiction, we conducted two-

electrode cyclic voltammetry measurements to evaluate potential redox reactions on

nanopyramid electrodes in 0.1 M NaCI solution with or without 0.05 M pyridine. As

shown by the dashed curve in Figure 5-10, the nanopyramid electrode that consists of a

gold nanopyramid array on an adhesive carbon disk and a conductive copper tape

exhibits apparent redox activities when the electrode potential is cycled between -1.0

and +1.0 V. This is caused by the electrochemical reactions on the conductive copper









tape which is used as a conducting wire to connect the gold nanopyramid array to the

potentiostat and is partially exposed to the electrolyte solution. A similar cyclic

voltammogram is obtained when pure copper tape is used as the electrode as shown by

the thin solid curve in Figure 5-10. Since the applied cyclic electrode potentials are

below the electrolytic potential of water ( 1.23 V) (112), we believe that the anodic

reaction on the conductive copper tape is Cu -* Cu2 + 2e-. By contrast, when pure

conducting carbon tape (thick solid curve) and gold pyramid array on carbon tape

(dotted curve) are used as electrodes, no apparent redox reactions are observed.

Similar cyclic voltammetry results are obtained when the electrolyte solution contains

0.1 M NaCI and 0.05 M pyridine.

We speculate that the electrochemical reactions on the conductive copper tape

are responsible for the observed SERS intensity-electrode potential contradiction

between the templated nanopyramid array and sculpted nanovoid array electrodes. It is

well-known that pyridine can easily conjugate with Cu2+ ions to form a positively

charged complex, [Cu(py)4]2+ (107), which can be electrophoretically attracted by the

cathode, while being repelled from the anode. This could lead to a higher concentration

of pyridine on the cathode surface and therefore results in higher SERS intensity at

more negative potentials. To verify this speculation, we conducted the same

electrochemical SERS experiments with a gold nanopyramid array supported only by an

electrochemically inert conductive carbon tape (see the cyclic voltammetry results in

Figure 5-10). The experimental results as shown in Figure 5-11 exhibit the same SERS

intensity-electrode potential relationship as observed on sculpted nanovoid array

electrodes (i.e., higher SERS intensity occurs at more positive potentials). The relatively









low SERS enhancement could be due to the reduced sharpness of the nanotips of the

nanopyramid array which was templated from an inverted silicon mold that had been

used multiple times.

Electromagnetic Modeling

We believe the electromagnetic enhancement caused by the significant

concentration of the electromagnetic field in the vicinity of the sharp nanotips is the

dominating mechanism for the observed SERS enhancement at nanopyramid

electrodes. To verify this hypothesis, we conduct finite element electromagnetic

modeling using COMSOL Multiphysics software to calculate the electric field amplitude

distribution and the corresponding Raman enhancement factors surrounding arrays of

nanopyramids (113). Since the periodic nanostructure is symmetric, it is reasonable to

construct a simplified two-dimensional (2-D) model which can be considered as sections

through a three-dimensional nanopyramid array at the point of maximum enhancement

(Figure 5-12). To numerically solve the 2-D Maxwell's equations, the "perfect matched

layers" (PML) boundaries method is utilized for the simulation (114). The widely used

optical constants for gold (115) are employed to conduct the electromagnetic modeling,

and the surrounding medium is water.

Figure 5-12A shows the simulated distribution of the SERS enhancement factor

around two adjacent nanopyramids with base length of 320 nm, interpyramid distance of

1.414 x 320 nm (106), and nanotip radius of curvature of 5 nm. The height of

nanopyramids is determined by the base length as wet-etched silicon pyramids have

characteristic 54.70 side walls (116). The simulation results show that the significant

enhancement of the electromagnetic field and the maximum SERS enhancement (1047)

happen at the vertices of the nanotips, and are favorably comparable to other numerical









simulations for nanotips and nanorings (92, 117, 118). The spatial distribution of the

electromagnetic "hot spots" of the two triangles is asymmetric. This is caused by the

electromagnetic interaction between neighboring nanotips. Figure 5-12B shows that

larger arrays with more nanotips but the same structural parameters result in higher

enhancement and the maximal enhancement factor reaches a plateau (Gmax 107.5)

when the array has more than 12 tips. This indicates that the electromagnetic coupling

between adjacent scatters played a critical role in determining the electric field

amplitude distribution and the corresponding Raman enhancement factors surrounding

arrays of nanopyramids. Indeed, the calculated Gmax at the nanotip apex could be even

higher if the sharp edges and facets of the nanopyramids are considered in a more

realistic three-dimensional (3-D) model instead of the current 2-D model. The very small

effective area occupied by the sharp nanotips (electromagnetic hot spots) could be the

reason for the significant difference between the simulated Gmax and the experimental

enhancement factor. A recent experimental study shows that a very small percentage of

molecules (0.0063%) in the hottest spots contribute 24% to the overall SERS intensity

(119). We believe that the surface roughness of the templated nanopyramid electrodes

plays only a minor role in the observed SERS enhancement. Indeed, the SEM image in

Figure 5-6B shows that the surface roughness of the nanopyramid electrode does not

change much after the electrochemical SERS experiment.

Summary

In summary, we have developed a bottom-up approach for fabricating periodic

arrays of gold nanopyramids with nanoscale sharp tips. These nanotips can significantly

enhance the local electromagnetic field at the tip apex, resulting in more than 1 order of

magnitude higher SERS enhancement than other nanostructured electrodes. We have









also found that the redox reactions occurring near the nanopyramid electrode play a

crucial role in determining the dependence of SERS enhancement on the applied

electrode potential. The current templating technology is scalable and compatible with

standard microfabrication, enabling large-scale production of SERS-active electrodes

for in situ electrochemical studies and sensitive electroanalysis.









Table 5-1. Assignment of SERS peaks for pyridine adsorbed on gold nanopyramid
electrode
Label SERS peak (cm-1) Vibration mode Vibration type
a 634 V6a Symmetric
b 650 V6b Asymmetric
c 699
d 1013 vi Ring breathing
e 1037 V12 C-H in-plane deformation
f 1068 v18a C-H in-plane deformation
g 1216 V9a C-H in-plane deformation
h 1600 vs Ring stretching















Dielectric




+++ ++--- + ---
Metal


Figure 5-1. Surface plasmons propagate along a metal/dielectric interface (61).


15





10

0
.0

E

5


0 L-
400


500 600 700 800 900 1000
Wavelength (nm)


Figure 5-2. Extraordinary transmittance at normal incidence for a square array of holes.
The area covered by holes is only 11% while the normalized-to-area
transmittance of lights is 130% (69).





















Figure 5-3. Schematic SERS process in which light is Raman scattered by a molecule
on the surface (77).


Glass Substrate


Figure 5-4. Schematic illustration of electrochemical SERS set up.









Silica in monomer


Spin coat; UV-cure



SRIE



SSpuntter Cr


Wet etch; Cr etch


SSputter Au



Peel offAu
'
^myilf~j


Figure 5-5. Schematic illustration of the templating procedures for fabricating gold
nanopyramid array by using spin-coated monolayer colloidal crystal as
template (105).


Remove Silica



















500 nm

w~a~


A 500 nm

Figure 5-6. Tilted (350) SEM images of a gold nanopyramid array electrode prior to (A)
and after (B) electrochemical SERS experiments. As templates, 320 nm silica
spheres were uses.


100



























I ........ I I ...I I I
600 800 1000 1200 1400 1600
Raman Shift (cm1)

Figure 5-7. Electrochemical SER spectra recorded on a gold nanopyramid array
supported by a conductive carbon disk and a copper tape (red) and a flat gold
control sample on silicon (black) in 0.1 M NaCI solution containing 0.05 M
pyridine.


600 800 1000 1200
Raman Shift (cm1)


1400 1600


Figure 5-8. Electrochemical SER spectra recorded on a gold nanopyramid array
supported by a conductive carbon disk and a copper tape in 0.1 M NaCI
solution containing 0.05 M pyridine.


101





























400 600 800 1000 1200 1400 1600 1800
Raman Shift (cm1)


Figure 5-9. The gold electrode potential was swept from
and then back to -1.0 V (bottom).


-0.6 I-
-1.0


1.0 V (top) to +1.0 V (middle)


-0.5 0.0 0.5
Potential (V) vs. Pt


Figure 5-10. Cyclic voltammograms of a conductive carbon tape, a conductive copper
tape, a gold nanopyramid array supported by a carbon tape, and a gold
nanopyramid array supported by a carbon disk and a copper tape in 0.1 M
NaCI.


102












40000


30000

cU)
C
o
0 20000
C
C9
E
0 -0.2 V
10000


-1.0 V
0
500 1000 1500 2000 2500 3000
Raman Shift (cm1)



Figure 5-11. Electrochemical SER spectra obtained on a gold nanopyramid array
supported by a conductive carbon tape in 0.1 M NaCI solution containing 0.05
M pyridine. The gold electrode potential was swept from -1.0 to 0.2 V. The
spectra were taken using a 785 nm diode laser at 48 pW with an integration
time of 10 s.


103

















6

S

4


-5 -4 -3 -2 -1 0
x (Jim)


1 2 3 4 5


5-

0-

5-

0-

5

0-

5-

n5I


30 35


4.

4

3.5







'1.5

1



0.5

-0.5


Figure 5-12. (A) Modeled Raman enhancement factor around two gold nanopyramids
with base length of 320 nm and nanotips radius of curvature of 5 nm at A =
785 nm. (B) Simulated maximum SERS enhancement factor (Gmax) vs.
number of tips of the templated nanopyramid array with the same structural
parameters as (A).


104


5 10 15 20 25

Number of Tips









CHAPTER 6
GRAPHENE PAPER ACTUATORS

Background

Graphene, a two-dimensional honeycomb building block for graphitic materials of

other dimensionalities (120, 121), has been extensively studied due to its extraordinary

qualities in electrical, thermal, and mechanical aspects that provide itself opportunities

in fields such as electronics (122, 123), composites (124-126), sensors (127, 128) and

capacitors (129). It can be wrapped up into OD fullerenes, rolled into 1D nanotubes or

stacked into 3D graphite, as shown in Figure 6-1 (121). Besides these fields, graphene

is also a strong candidate in making actuators that can convert electrical energy into

mechanical energy because of its high surface area for double-layer charging and the

above excellent properties. The direct conversion of energy through materials is of great

importance for applications in robotics, prosthetic devices, optical displays as well as

micro pumps (130). As an actuator material, graphene has the following advantages: it

is electrically conductive with high electron mobility of 15,000 cm2V-ls-1; it has high

intrinsic stress of 130 GPa and it is light-weight (121, 131). All the extraordinary

properties mentioned above make graphene attractive for use in making actuators.

Electromechanical actuators based on sheets of single-walled carbon nanotubes

were reported (132), showing stresses higher than natural muscle and strains higher

than ferroelectric materials. The quantum chemical-based expansion without ion

intercalation is believed as the mechanism that causes actuation. Actuation of graphite

oxide (GO)/graphene bilayer papers were also reported (133). Instead of driving by an

applied voltage as the carbon nanotube actuator, the bilayer actuator was driven by

variation of humidity and temperature and therefore has poor controllability. Here we


105









report electromechanical actuators based on two strips of graphene papers with an

intermediate dielectric layer. Graphene papers were produced by flow-directed

assembly (134) of one-atom-thick graphene sheets, which were obtained from

chemically reduced GO sheets. GO sheets were generated by exfoliation of highly

oxygenated graphite in water. The actuators were operated under repeated potential

steps and cyclic voltammetry. Cycling stability of the graphene-based actuators was

also explored.

Experimental

Materials and Methods

Graphene papers

Graphene papers were basically made by following a procedure presented by Li

and colleagues (135, 136). Schematic illustrations of graphene paper preparation are

shown in Figure 6-2, 6-3, and 6-4. Simply put, graphite oxide was first synthesized from

natural graphite by a modified Hummers method that consists of an additional oxidation

prior to the typical Hummers method (137, 138). As-synthesized graphite oxide was

subjected to multiple cycles of centrifugation and re-dispersion until no supernatant

formed after centrifugation and then suspended in ultrapure water (18.2 MQ cm1,

Barnstead water system) to give a viscous, inhomogeneous, brown suspension with a

concentration of 0.6 wt.% and then stored. To prepare colloidal dispersion of GO, the

stored graphite oxide suspension was diluted with ultrapure water to 0.05 wt.%, followed

by exfoliation carried out with ultrasonication for 60 min. The dispersion was then

subjected to centrifugation at 3,000 r.p.m. for 30 min to remove unexfoliated graphite

oxide and subsequent dialysis for several hours to remove residual salts and acids,

resulting in a clear, homogeneous and brownish dispersion as shown in Figure 6-5A.


106









Chemical reduction (136) of GO to graphene was conducted by mixing 3 ml of GO

dispersion with 3 ml of ultrapure water, 21 pl of ammonia solution (14.8 N, Fisher

Chemical) and 3 pl of hydrazine (35 wt.% solution in water, Sigma Aldrich) in a glass

vial with a hot water bath for 1 h to give a homogeneous and black dispersion as shown

in Figure 6-5B. The graphene paper was then made by vacuum filtration of the

graphene dispersion through an Anodisc membrane (25 mm in diameter, 0.02 pm pore

size, Whatman) (134). The resulting deposit was then air dried and peeled from the

membrane to give a free-standing graphene paper. The graphene paper was cut into 2

mm by 15 mm strips by a razor blade for mechanical testing and making actuators.

Detailed procedures for making graphene dispersions are listed as below:

Prior to Hummers' method (additional oxidation, as in Figure 6-2)

1. Grind graphite flakes into powder.

2. Put graphite powder (1g) into an 800C solution of concentrated H2SO4 (1.5 ml),
K2S208 (0.5 g) and P205 (0.5 g). Use a 250 ml flask. No stirring bar required.

3. Thermally isolate and allow the dark blue mixture to cool to room temperature over
a period of 6 h.

4. Carefully dilute the mixture with DIW.

5. Filter and wash the mixture with vacuum filtration until the rinse water pH becomes
neutral.

6. Dry the product in air at ambient temperature overnight.

Hummers' method (as in Figure 6-3)

7. Put this pre-oxidized graphite into concentrated H2SO4 (23 ml) that have been
cooled to 0C in an ice-bath with vigorous stirring.

8. Gradually add KMnO4 (3 g) so that the temperature of the mixture is not allowed to
reach 200C.

9. Remove the ice-bath, bring the temperature to 350C and stir for 2 h. (As the
reaction progressed, the mixture gradually thickened with a diminishing in
effervescence. The mixture was brownish grey in color at the end of the reaction.)


107









10. Slowly stir DIW (46 ml) into the paste. (Violent effervescence occurs and
temperature increases to 98C.)

11. Keep the diluted brown suspension at this temperature for 15 minutes.

12. Further dilute the suspension with WARM DIW (140 ml).

13. Slowly treat the suspension with 30% H202 (2.5 ml) to reduce the residual
permanganate and manganese dioxide to colorless soluble manganese sulfate.
Upon treatment with the peroxide, the suspension turned bright yellow.

14. Filter and wash the suspension with WARM 1:10 HCI solution (250 ml) to remove
metal ions, resulting in a yellowish-brown filter cake. (The filtering has to be
conducted while the suspension was still warm to avoid precipitation of slightly
soluble salt of mellitic acid formed as a side reaction.)

15. Wash the filter cake with DIW by multiple centrifugation/re-dispersion steps until
no supernatant forms after centrifugation.

16. Suspend the GO product in DIW to give a viscous, brown dispersion, which is
stable for a period of years.

Exfoliation and reduction of GO (as in Figure 6-4)

17. Exfoliation can be achieved by dilution of the GO dispersion with DIW, followed by
sonication and dialysis.

18. Prepare GO dispersion (0.05 wt. %) by sonication and use dialysis to completely
remove metal ions and acids. (For 30 ml 0.05 wt. % dispersion, take 2.5 ml of 6
mg/ml solution and then dilute to 30 ml.)

19. Mix the above dispersion (3 ml), DIW (3 ml) and 28 wt. % NH40H (21 pl) into a 20
ml glass vial.

20. Add 35 wt.% hydrazine (3 pl) into the vial and shake vigorously.

21. Put the vial in a 950C water bath for 1h.

Graphene actuators

Graphene actuators were made in a relatively simple way as presented by

Baughman and colleagues (132). Schematic illustrations of an actuator and apparatus

used are shown in Figure 6-11. The actuator was made by laminating two strips of

graphene papers with an intermediate larger of strip of Scotch Double Stick Tape (1 mm


108









wider and longer than graphene strips), resulting in a sandwich structure as shown in

Figure 6-11A and B. Two pieces of Au/Cr/glass electrodes, made by sputter deposition

of 20 nm of Cr and 200 nm of Au, were then attached on opposite sides of the upper

end of the actuator with a clamp, facing to the graphene strips with the Au surface but

not touching each other. A voltage could therefore be applied across the two Au

electrodes without breaking the graphene strips. Actuation was carried out by

immersing 10 mm of the actuator in 1 M NaCI solution held in a homemade rectangular

glass tank as shown in Figure 6-11C. A graph paper was attached to one side of the

glass tank as background for displacement measurement, which was done with the aid

of videotaping the motion of the actuator under applied voltages. Displacements of the

actuator tip were then calculated via the graph paper.

Results and Discussion

GO and Graphene Dispersions

Colloidal dispersions of GO were prepared by exfoliation of graphite oxide

synthesized by a modified Hummers method, resulting in a homogeneous and brownish

dispersion as shown in Figure 6-5A. Zeta-potential of the GO sheets in deionized water

was measured to be -42.27+1.33 mV by fitting experimental data using Smoluchowski's

model. The sign and magnitude reveal that the GO sheets are highly negatively charged

and electrostatically stabilized, resulting from phenolic hydroxyl and carboxylic acid

groups formed during oxidation of graphite powders. Chemical reduction of GO to

graphene was carried out by following a procedure presented by Li and co-workers. In

this procedure, GO dispersions are adjusted to a low concentration of 0.025 wt.%.

Ammonia is then added to GO dispersions in order to maximize the surface charge of

GO sheets and therefore further stabilize the dispersions. After that, chemical reduction


109









is carried out by adding a small amount of hydrazine, resulting in a homogeneous and

black dispersion. A graphene dispersion made by this method is shown in Figure 6-5B.

Zeta-potential of this dispersion was measured to be around -2 mV. The low magnitude

shows that graphene sheets in the dispersion are weakly charged as a result of

unreduced carboxylic acid groups.

A TEM image of graphene sheets made by the above method is shown in Figure

6-6 in which graphene sheets are observed to be rested on TEM grids, folded and

wrinkled. An AFM image of graphene sheets and height profiles are shown in Figure 6-

7A and B, respectively. The height profile in Figure 6-7B1 shows that size of graphene

sheets can be as large as 600 nm with a thickness of 0.6 nm, resulting in an aspect

ratio of 1000. This high aspect ratio is beneficial in making organic-inorganic biomimetic

nanocomposites. The height profile in Figure 6-7B2 consists of one and two layers of

graphene sheets.

Graphene Papers

Because of the low zeta-potential, dispersions of graphene prepared by this

procedure showed visible agglomeration after 2 days. Therefore, vacuum filtration

through an Anodisc membrane of graphene dispersions was conducted immediately

after chemical reduction to avoid severe aggregation which hinders mechanical

properties of graphene papers. Top and bottom side images of a free-standing

graphene paper made by vacuum filtration are shown in Figure 6-8A and B,

respectively. The graphene paper was air dried and peeled directly from the membrane.

As discovered by Li and co-workers, the graphene paper exhibits a metallic texture that

demonstrates its smoothness.


110









The top-view and bottom-view SEM images in Figure 6-9A and B demonstrate that

both surfaces are relatively smooth and graphene sheets are aligned parallel to the

Anodisc membrane. The cross-sectional SEM image in Figure 6-9C provide further

evidence that the graphene sheets are aligned and stacked parallel to each other to

from a layered structure. It has been proposed that the layered structure is formed

because rising in sheets concentration during filtration causes increasing in sheet-to-

sheet interactions that make the sheets tend to align parallel to each other to reduce

total energy of the system. Unlike graphene papers, GO papers made by the vacuum

filtration method keeps a layered structure but do not exhibit smooth surfaces, as shown

in Figure 6-10 and 6-11. Instead, SEM images of GO papers in Figure 6-11A and B

show many humps one after another on both top and bottom surfaces. We believe this

is caused by high surface charge of GO sheets (-42.271.33 mV) that make GO sheets

repel each other rather than adhere together during the increasing in sheet-to-sheet

interactions and result in rumpled surfaces. Tensile strengths of graphene papers and

GO papers tested are about 140 MPa and 110 MPa, respectively, as shown in Figure 6-

12, and are close to the strength reported.

Graphene Actuators

Graphene actuators were made in a relatively simple way by laminating two strips

of graphene papers with an intermediate layer of larger strip of double-sided tape,

resulting in a sandwich structure. Schematic illustrations of an actuator and apparatus

used are shown in Figure 6-13. Detailed information is in materials and methods

session. In order to estimate capacitance of graphene papers, cyclic voltammetry

measurements under different scan rates were carried out on a single strip of graphene


111









paper immersed in 1 M NaCI solution. Dimensions of the strip were 2 mm in width and

15 mm in length. The length of the strip immersed was 10 mm. One side of the strip was

covered with an insulating tape, resulting in a superficial active area of 0.2 cm2. Since

2.5 cm2 of graphene paper can be made by filtration of 6 ml of graphene dispersion

containing 1.5 mg of graphene, the weight of graphene immersed in solution therefore

becomes 0.12 mg. After several cycles to reach maximum degree of wetting, cyclic

voltammograms of the graphene strip under various scan rates are shown in Figure 6-

14A where a saturated calomel electrode is used as the reference electrode and a

platinum wire is used as the counter electrode. The capacitance of graphene papers

can therefore be estimated by calculating the ratio of steady state current to scan rate or

by plotting steady state currents versus scan rates as shown in Figure 6-14B that gives

a capacitance of 0.006 F or 50F/g, which is three-time higher than a nanotube paper,

demonstrating high surface area of graphene sheets (132).

Actuations of a graphene actuator operated by potential step method between -2

and 2 volts in 1 M NaCI solution are shown in Figure 6-15. Voltages reported here are

with respect to the graphene strip on the right hand side. Few cycles of operations were

also performed before hand to ensure maximum wetting of the actuator. Positions and

displacements of the actuator tip were then recorded and calculated via the graph paper

with the initial position of the actuator tip as the origin. Figure 6-15A shows cross-

sectional images of a graphene actuator under eight successive potential steps with a

total of four cycles (-2/2 V repeatedly) in which the actuator moves to the right when a

positive voltage is applied and to the left when a negative voltage is applied. The

moving direction of the graphene actuator is actually opposite to that of the carbon


112









nanotube actuators proposed by Baughman and colleagues with respect to the direction

of an applied voltage. The actuation mechanism of carbon nanotube actuators is based

on quantum chemical-based expansion that causes dimensional changes in covalently

bonded directions and injection of electrons results in bond expansion (132). As a result,

carbon nanotube actuators bend to the anode. Meanwhile, anions and cations in

electrolyte are moved into the anode and cathode, respectively, to compensate the

injected charges. These dopant intercalations can actually result in swelling electrodes

and extent of swelling depends on the size of dopants (130, 139, 140). Since chloride

ions are larger than sodium ions, actuation due to dopant intercalations more likely bend

to the cathode and is opposite to that due to quantum chemical-based expansion. For

our graphene actuators, we found that dopant intercalations are required and important.

From Figure 6-15A, since the graphene actuator bend to the cathode, we conclude that

actuation due to dopant intercalation suppresses that due to quantum chemical-based

expansion, resulting in a movement opposite to carbon nanotube actuators.

Displacements of the actuator tip in Figure 6-15A under repeated potential steps

between -2 and 2 volts in 1 M NaCI solution are calculated and shown in Figure 6-15B.

From this simple measurement, the displacement is found to be around 1.2 mm with 10

mm of graphene actuators immersed in 1 M NaCI electrolyte.

Actuations of a graphene actuator operated by cyclic voltammetry method

between -2 and 2 volts in 1 M NaCI solution with a scan rate of 50 mV/s are shown in

Figure 6-16. The left side of the graphene actuator was set as the working electrode

while the right side was set at both counter and reference electrodes. No pre-cycling

was conducted before recording. The very beginning cycles of the cyclic


113









voltammograms of the graphene actuator are shown in Figure 6-16A. The increasing in

the current at this stage demonstrates that wetting of the actuator occurs at beginning

along with dopant intercalations. After few cycles of operations, the current becomes

steady, corresponding to the finish of the wetting process. The displacement measured

is shown in Figure 6-16B. At the beginning, almost no displacement is observed. The

extent of displacement increases along with the increasing in cyclic numbers and then

reaches a steady value of around 1.2 mm, which is almost the same as actuators

operated by potential steps. Figure 15B also demonstrates that the graphene actuator

can be cycled up to 140 cycles.

Summary

We demonstrate that electromechanical graphene actuators can be made by

laminating two strips of graphene papers with an intermediate dielectric layer. Graphene

papers were produced by flow-directed assembly of graphene sheets, which were

obtained from chemically reduced GO sheets. GO sheets were generated by exfoliation

of highly oxygenated graphite in water. Capacitance of graphene papers was estimated

to be 0.006F or 50F/g by cyclic voltammetry and is almost three-time higher than carbon

nanotube papers. We also found that the actuation mechanism of graphene actuators is

most likely due to swelling of electrodes originating from dopant intercalations. The

displacements of actuators under repeated potential steps between -2 and 2 volts in 1

M NaCI solution was determined to be around 1.2 mm with 10 mm of graphene

actuators immersed in electrolyte. Actuations of a graphene actuator operated by cyclic

voltammetry method between -2 and 2 volts in 1 M NaCI solution with a scan rate of 50

mV/s were also carried out. Wetting of the graphene actuator was found during the first


114









few cycles. The extent of displacement increases along with the increasing in cyclic

numbers and then reaches a steady value of around 1.2 mm, which is almost the same

as actuators operated by potential steps. Actuation of graphene actuators was able to

last up to 140 cycles without significant degradation.


115


















'C''


, .- f .-'



o .
* ,- ,- i- '- ", --. "
r r-" i- ','- -~ e- "' r- ~ ,-" "





,i
4


. .


tto

" a.


CL
L
CE-~~
t
x~ -c- L-i
F


m..
C-~-


Figure 6-1. Mother of all graphitic forms (121).


116


" c:tr~t,
[rlr- ~
iL
r








Ig
graphite
powder
'


1.5 ml H2SO,
0.5 g KS208O
0.5 g P2O5


6 hrs DIW



K---


Figure 6-2. Schematic illustration of an additional oxidation prior to Hummer's method.





















117


,j









3 3gKMn04
pre-oxidized
graphite
powder


23 ml H2SO,
-- Q1 _


Q_-5


46 ml DIW
'i


I 2 hrs
HC I '
f WARM 1:10'
HCI solution


fa


A
15 mins

(1) 14 ml
WARM DIW
a (2) 2.5 ml
30% H202


dispersion N



centrifugation


rpm-~


6 mg/ml
GO solution


Figure 6-3. Schematic illustration of Hummers' method for GO preparation.


118


,4,








dialysis


ultra-
sonication
M 30 mins


6 mg/ml
GO solution


0.05 wt%
GO solution


3 ml 0.05 wt% GO
3 ml DIW
21 ulNH4OH
3 ul 35 wt% hydrazine


6




L


a'


0 I.--


Figure 6-4. Schematic illustration of preparation of graphene papers.


119


























Figure 6-5. Images of colloidal dispersions of (A) GO and (B) graphene.


Figure 6-6. TEM image of graphene sheets.


120


(A)


(B)


























1.0


0.5


0 200 400 600 800 1000
2
B2
1

0
0 100 200 300 400 500

Figure 6-7. (A) Tapping-mode AFM image of graphene sheets with (B) height profiles
B1 and B2 taken along the lines in (A). The sample was prepared by drop-
casting diluted graphene dispersion onto a mica substrate.


121







































Figure 6-8. (A) Top and (B) bottom side images of a free-standing graphene paper
made by vacuum filtration of graphene dispersion through an Anodisc
membrane.


122

















































Figure 6-9. SEM images of a graphene paper. (A)Top-view SEM image, (B) bottom-
view SEM image and (C) cross-sectional SEM image.







123






































Figure 6-10. (A) Top and (B) bottom side images of a free-standing graphene paper
made by vacuum filtration of GO dispersion through an Anodisc membrane.


124


















































Figure 6-11. SEM images of a GO paper. (A)Top-view SEM image, (B) bottom-view
SEM image and (C) cross-sectional SEM image.


125


20 I.rm

C













160


-m 120
a-

*c

C- 80 -

U,


40 -




0
I-

0 ,I


0.000 0.005 0.010 0.015 0.020 0.025
Strain



Figure 6-12. Tensile stress versus strain curve for a free-standing graphene and GO
paper.


126












WE


Au electrode


Sdouble-sided
Scotch tape


m -m... .i..m.m.-- ..- .-.-


(c)

Figure 6-13. Schematic illustrations of a graphene actuator. (A) Front-view of the
actuator, (B) side-view of the actuator and (C) apparatus used for
displacement measurement.


127


riI


CE/RE


(A)


(B)


I I I I I I I I M l I I I I I I I n


z

















0.1 20 mVls

10 mVls
S5 mVs //
0.0



-0.1



-0.2 I I I I
-0.2 0.0 0.2 0.4 0.6
Potential (V vs. SCE)


(A)


0.14

0.12

0.10

E 0.08 -
4,-
S 0.06

0.04

0.02

0.00 i i
0 5 10 15 20 25
Scan Rate (mV/s)

(B)
Figure 6-14. (A) Cyclic voltammograms of a graphene strip at various scan rates in 1 M
NaCI solution. A saturated calomel electrode was used as the reference
electrode and a platinum wire was used as the counter electrode. The
superficial active area was 0.2 cm2 and the weight of graphene paper
immersed was 0.12 mg. (B) A plot of steady state currents in (A) versus
corresponding scan rates. The slop in (B) is 0.006 F.


128






























1.6

o o
1.2 o
o
E 0.8 0
E 0 0

0.4
E 00
a 0.0 0 0
u. o




-0.8 o o
0 o

-1.2




(B)

Figure 6-15. (A) Cross-sectional images of a graphene actuator under eight successive
potential steps with a total of four cycles (-2/2 V repeatedly). (B)
Displacements of the actuator tip in (A) under repeated potential steps.


129
















0.4 1- 10 cycles


0.2
E
E 0.0


O -0.2


-0.4-


-0.6 I I I I
-2 -1 0 1 2
Voltage (V)


(A)


1.6



S 1.2- o o
E '
E /

0.8
E
W I
Cu
0.
W 0.4
0


0.0o -


0 20 40 60 80 100 120 140
Cycle Number


(B)

Figure 6-16. (A) Two-electrode cyclic voltammograms of a graphene actuator operated
between -2 and 2 volts in 1 M NaCI solution with a scan rate of 50 mV/s. (B)
Corresponding displacements of the actuator in (A) as a function of cycle
number.


130









CHAPTER 7
CONCLUSIONS

A simple electrodeposition technology that enables rapid production of large-area

polymer nanocomposites with layered structures that mimic the nacreous layer of

mollusk shells was studied. Uniform, electrostatically stabilized gibbsite nanoplatelets

with high aspect ratio were preferentially oriented parallel to the electrode surface when

an external direct current electric field was applied. The electroplated ceramic films had

uniform thickness, and the thickness could be controlled by adjusting the nanoplatelet

concentration of the electroplating baths. Homogeneous, optically transparent

nanocomposites were obtained when the interstitials between the aligned nanosheets

were infiltrated with polymer. The resulting ceramic-polymer nanocomposites exhibited

four-time higher tensile strength and nearly 1 order of magnitude higher modulus than

pure polymer films. The covalent linkage between the nanoplatelets and the polymer

matrix plays an important role in determining the mechanical properties of these

biomimetic nanocomposites.

Electrophoretic co-deposition of polymer-gibbsite nanocomposites was also

demonstrated. The electrodeposited PVA-Gibbsite nanocomposite films were optically

transparent and flexible, even though the weight fraction of the brittle inorganic phase

was higher than 80%. The electrophoretic co-deposition assembly of positively charged

gibbsite nanoplatelets and cationic PEI polyelectrolytes into ordered multilayer in a

single step was performed. The resulting nanocomposite had similar organic/inorganic

weight ratio and ordered brick-and-mortar nanostructure as natural nacres.

Nanoindentation tests showed that this nanocomposites exhibited similar hardness and

reduced modulus as those of pure gibbsite coatings.


131









Assembling of surface-roughened inorganic nanoplatelets into ordered multilayer

that mimic the asperities interposing nanostructure found in the nacreous layer of

mollusk shells was investigated. A thin layer of sol-gel silica was coated on smooth

gibbsite nanoplatelets in order to increase the surface roughness to mimic the asperity

of aragonite platelets found in nacres. To avoid the severe cracking caused by the

shrinkage of sol-gel silica during drying, polyelectrolyte PEI was used to reverse the

surface charge of silica-coated-gibbsite nanoplatelets and increased the adherence and

strength of the electrodeposited films. Polymer nanocomposites could then be made by

infiltrating the interstitials of the aligned nanoplatelet multilayer with photocurable

monomer followed by photopolymerization. The resulting self-standing films were highly

transparent and exhibited nearly three-time higher tensile strength and one-order-of-

magnitude higher toughness than those of pure polymer. The measured tensile strength

agrees with that predicted by a simple shear lag model.

A simple and scalable colloidal templating nanofabrication technology for

generating periodic metallic nanopyramid arrays as electrodes for electrochemical

surface-enhanced Raman spectroscopy (SERS) was conducted. These periodic arrays

of nanopyramids with nanoscale sharp tips and high tip density could enhance the local

electromagnetic field in the vicinity of the nanotips, resulting in high SERS enhancement

(on the order of 106). The effects of the applied electrode potential and the electrode

redox reactions on the SERS enhancement were investigated. Finite element

electromagnetic modeling was also developed to simulate the electric field amplitude

distribution and the corresponding Raman enhancement factors surrounding arrays of

nanopyramids.


132









Actuators that can convert electrical energy into mechanical energy are of great

importance for applications in many fields. Development of novel actuators requires

materials that have excellent electrical, thermal, and mechanical properties. Graphene

has been studied over years and in known to have extraordinary qualities. We studied

electromechanical actuators based on two strips of graphene papers with an

intermediate dielectric layer. The actuation mechanism of graphene actuators was most

likely due to swelling of electrodes originating from dopant intercalations. The swelling-

induced bending actually suppressed that due to quantum chemical-based expansion,

resulting in a bending direction opposite to that of carbon nanotube actuators.

Capacitance of graphene papers was estimated to be 0.006F or 50F/g by cyclic

voltammetry and was almost three times higher than carbon nanotube papers. The

displacements of actuators under repeated potential steps between -2 and 2 volts in 1

M NaCI solution was determined to be around 1.2 mm with 10 mm of graphene

actuators immersed in electrolyte. Actuations of a graphene actuator operated by cyclic

voltammetry at a scan rate of 50 mV/s were able to last up to 140 cycles without

significant degradation.


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BIOGRAPHICAL SKETCH

Tzung-Hua Lin received his Bachelor of Science in chemical engineering from the

National Taiwan University in 2003. He continued his studies at the National Taiwan

University and received a Master of Science in 2005. He joined the Department of

Chemical Engineering at the University of Florida in August 2007.


142





PAGE 1

1 ELECTRIC FIELDS ON GIBBSITE NANOPLATELET ASSEMBLIES, NANOPYRAMID SERS SUBSTRATES AND GRAPHENE ACTUATORS By TZUNG HUA LIN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Tzung Hua Lin

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3 To my Mom, Yue Shia Wu

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4 ACKNOWLEDGMENTS I would like to thank Dr. Jiang for his advice over the past few years and people in especially Wei Han Huang for providing gibbsite nanoplatelets and In Kook Jun for mechanical tests

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATI ONS ................................ ................................ ........................... 14 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 17 2 BIOINSPI RED ASSEMBLY OF GIBBSITE NANOPLATELETS BY ELECTRIC FIELD ................................ ................................ ................................ ...................... 19 Background ................................ ................................ ................................ ............. 19 Learning from Nature ................................ ................................ ....................... 19 Bottom up Self assembly ................................ ................................ ................. 19 Electrod eposition ................................ ................................ .............................. 20 Gibbsite Nanoplatelets ................................ ................................ ..................... 21 Experimental ................................ ................................ ................................ ........... 22 Materials and Substrates ................................ ................................ .................. 22 Instrumentation ................................ ................................ ................................ 22 Synthesis of Gibbsite Nanoplatelets ................................ ................................ 23 Surface Modification of Gibbsite Nanoplatelets ................................ ................ 24 EPD of Gibbsite Nanoplatelets ................................ ................................ ......... 25 ET PTA Gibbsite Nanocomposites ................................ ................................ .... 25 Mechanical Test ................................ ................................ ............................... 26 Results and Discussion ................................ ................................ ........................... 26 Gibbsite Characterization ................................ ................................ ................. 26 EPD of Gibbsite Nanoplatelets ................................ ................................ ......... 27 Polymer Gibbsite Nanocomposites ................................ ................................ .. 29 TGA and Tensile Strength of Gibbsite based Nanocomposites ....................... 30 Summary ................................ ................................ ................................ ................ 32 3 ELECTROPHORETIC CO DEP OSITION OF POLYMER GIBBSITE COMPOSITES ................................ ................................ ................................ ........ 46 Background ................................ ................................ ................................ ............. 46 Experimental ................................ ................................ ................................ ........... 46 EPD of Nanoplatelets ................................ ................................ ....................... 46 EPD of PVA Gibbsite nanoplatelets ................................ ........................... 47

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6 EPD of PEI Gibbsite nanoplatelets ................................ ............................ 47 Results and Discussion ................................ ................................ ........................... 47 Electrophoretic Co deposition of PVA Gibbsite ................................ ................ 47 Electrophoretic Co deposition of PVA Gibbsite ................................ ................ 49 Summary ................................ ................................ ................................ ................ 53 4 ELECTROPHORETIC ASSEMBLY OF SURFACE ROUGHENED GIBBSITES .... 62 Background ................................ ................................ ................................ ............. 62 Experimental ................................ ................................ ................................ ........... 63 Coating of Gibbsite Nanoplatelets with Silica ................................ ................... 63 EPD of Nanoplatelets ................................ ................................ ....................... 64 EPD of SCG n anoplatelets ................................ ................................ ......... 64 EPD of PEI SCG nanoplatelets ................................ ................................ .. 64 Results and Discussion ................................ ................................ ........................... 65 SCG Nanoplatelets ................................ ................................ ........................... 65 EPD of SCG Nanoplatelets ................................ ................................ .............. 66 PEI SCG Colloidal Stability ................................ ................................ .............. 67 EPD of PEI SCG Nanoplatelets ................................ ................................ ....... 68 ETPTA PEI SCG Nanocomposites ................................ ................................ .. 70 Summary ................................ ................................ ................................ ................ 72 5 ELECTROCHEMICAL SERS A T PERIODIC METALLIC NANOPYRAMID ARRAYS ................................ ................................ ................................ ................. 80 Background ................................ ................................ ................................ ............. 80 Surface Plasmon ................................ ................................ .............................. 80 Extraordinary Op tical Transmission ................................ ................................ .. 81 Surface enhanced Raman Scattering ................................ .............................. 82 Substrates for Surface enhanced Raman Scattering ................................ ....... 83 Experimental ................................ ................................ ................................ ........... 85 Preparation of Elect rochemical SERS active Gold Nanopyramid Arrays ......... 85 Electrochemical Surface enhanced Raman Scattering ................................ .... 86 Cyclic Voltammetry Measurements ................................ ................................ .. 87 Electromagnetic Modeling of Raman Enhancement ................................ ......... 87 Results and Discussion ................................ ................................ ........................... 88 Colloidal Templating Process for Nanopyramid Array Fabrication ................... 88 Electrochemical SERS Spectra of Pyridine on Gold Nanopyramid Arrays ....... 89 Electrode Effects ................................ ................................ .............................. 91 Electromagnetic Modeling ................................ ................................ ................ 93 Summary ................................ ................................ ................................ ................ 94 6 GRAPHENE PAPER ACTUATORS ................................ ................................ ..... 105 Background ................................ ................................ ................................ ........... 105 Experimental ................................ ................................ ................................ ......... 106 Materials and Methods ................................ ................................ ................... 106

PAGE 7

7 Graphene papers ................................ ................................ ..................... 106 ................................ ................................ ....... 107 Hummer ................................ ................................ .................... 107 Exfoliation and reduction of GO ................................ ............................... 108 Graphene actuators ................................ ................................ ................. 108 Results and Discussion ................................ ................................ ......................... 109 GO and Graphene Dispersions ................................ ................................ ...... 10 9 Grap hene Papers ................................ ................................ ........................... 110 Graphene Actuators ................................ ................................ ....................... 111 Summary ................................ ................................ ................................ .............. 114 7 CONCLUSIONS ................................ ................................ ................................ ... 131 LIST OF REFERENCES ................................ ................................ ............................. 134 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 142

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8 LIST OF TABLES Table page 5 1 Assignment of SERS peaks for pyridine adsorbed on gold nanopyramid electrode ................................ ................................ ................................ ............. 96

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9 LIST OF FIGURES Figure page 2 1 (A) Image of an abalone shell. (B) SEM image of fra cture surface of aragonitic portion of abalone nacre. (C) TEM image of the nacre cross section. ................................ ................................ ................................ ............... 34 2 2 A model of biocomposites. ( A ) A schematic diagram of staggered mineral crystals embedded in protein matrix. ( B carrying structure of the mineral protein composites. Most of the load is carried by the mineral platelets whereas the protein transfers load via the high shear zones between mineral platelets. ................................ ...................... 35 2 3 (A) Schematic of cathodic electrophoretic deposition (EPD) and elect rolytic deposition (ELD). (B) Thickness of coatings deposited using ELD and EPD. .... 36 2 4 (A) Lattice structure of Gibbsite. (B) Hexag on shape of Gibbsite and corresponding isoelectric points. ................................ ................................ ........ 37 2 5 Schematic illustrations of (A) an electrophoretic cell and (B) deposit after EPD. ................................ ................................ ................................ ................... 38 2 6 TEM image of gibbsite nanoplatelets. The inset shows the electron diffraction patterns obtained from a sin gle nanoplatelet. ................................ ..................... 39 2 7 Electrophoretic assembly of gibbsite nanoplatelets. (A) Photograph of a free standing gibbsite film. (B) T op view SEM image of the sample in (A). (C) Cross sectional view of the same sample. ................................ ......................... 40 2 8 XRD patterns of the gibbsite film in Fig ure 2 7A. ................................ ................ 41 2 9 Thickness dependence of the electroplated films on the concentration of colloidal gibbsite suspensions. The thickness standard deviation for all samples is ca. 10%. ................................ ................................ ............................ 41 2 10 Free standing gibbsite ETPTA nanocomposite. (A) Photograph of a tra nsparent film. (B) Cross sectional SEM image of the same nanocomposite film. ................................ ................................ ................................ ..................... 42 2 11 Normal incidence transmission spectrum of the s ample in Figure 2 10A. .......... 43 2 12 XRD patterns of the nanocomposite sample in Figure 2 10A. ............................ 43 2 13 TGA of the ETPTA Gibbsite nanocomposite as shown in Figure 2 10A. ............ 44 2 14 Tensile stress versus strain curves for plain ETPTA film, ETPTA Gibbsite nanocomposite, and ETPTA TPM Gibbsite nanocomposite. ............................. 44

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10 2 15 Tensile strength of composites as functions of volume fraction and aspect ratio of gibbsite nanoplatelets. ................................ ................................ ............ 45 3 1 Electrodeposited PVA Gibbsite nanocomposite. (A) Photograph of a composite film on an ITO electrode. (B) Top view SEM image of the sample in (A). (C) Cross sectional SEM image of the sample in (A). (D) Magnified cross secti onal image. ................................ ................................ ........................ 54 3 2 XRD patterns of an electrodeposited PVA gibbsite composite on ITO electrode. ................................ ................................ ................................ ............ 55 3 3 Deposit weight on ITO electrode versus electrophoretic duration. ...................... 55 3 4 TGA of the nanocomposite sample as shown in Figure 3 1A. ............................ 56 3 5 Particle size distribution of nanoplatelet suspensions at different PEI/gibbsite weight ratio. (A) R = 0, (B) R = 0.03, (C) R = 0.075, and (D) R =0.75. ............... 57 3 6 Electrophoretic mobility and corresponding zeta potential of nanoplatelets at different PEI/gibbsite weight ratio. ................................ ................................ ...... 58 3 7 SEM images of PEI Gibbsite nanocomposite. (A) Top view image, (B) magnified top view image, (C) cross sectional image, and ( D) magnified cross sectional image. ................................ ................................ ........................ 59 3 8 XRD patterns of an electrodeposited PEI Gibbsite nanocomposite on Au electrode. ................................ ................................ ................................ ............ 60 3 9 TGA of an electrodeposited PEI Gibbsite nanocomposite. ................................ 60 3 10 Reduced modulus of pure gibbsite and PEI Gibbsite nanocomposite measured by nanoindentation. ................................ ................................ ........... 61 4 1 Cross section of abalone nacre showing the detailed structure at the lamellae boundaries. Arrows highlight locations where the nano asperities interpose. .... 74 4 2 (A) TEM image of acid leached SCG nanoplatelets. The arrows point to a silica shell with a thickness of ca. 10nm. (B) Photograph of an electrodeposited SCG film on a gold electrode. ................................ .................. 74 4 3 Zeta potential of PEI SCG nanoplatelets with different amount of PEI addition. The inset shows the molecular structure of PEI. ................................ .. 75 4 4 SEM images of electrodeposited PEI SCG nanocomposite. (A) Top view image. (B) Magnified top view image. (C) Cross sectional image. (D) Magnified cross sectional image. ................................ ................................ ....... 76

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11 4 5 XRD patterns of a PEI SCG nanocomposite on an ITO electrode. Blue arrows point to the characteristic peaks of ITO. The inset shows a table with major lattice planes of gibbsite. ................................ ................................ .......... 77 4 6 Photographs of (A) ETPTA PEI SCG and PEI SCG deposits on ITO electrodes. ................................ ................................ ................................ .......... 78 4 7 Normal incidence transmission spectra of ETPTA P EI SCG nanocomposite, ETPTA PEI SCG nanocomposite on an ITO electrode, and PEI SCG deposit on an ITO electrode. ................................ ................................ ........................... 78 4 8 SEM images of an ETPTA PEI SCG nanocomposite on an ITO electrode. (A) Cross sectional image. (B) Magnified cross sectional image. Red and black arrows in (A) point to a thin wetting layer of ETPTA and the ITO electrode, respectively. ................................ ................................ ................................ ........ 79 4 9 Tensile stress vs. strain curves for plain ETPTA film, ETPTA Gibbsite nanocomposite, and ETPTA PEI SCG nanocomposite. ................................ ..... 79 5 1 Surface plasmons propagate along a metal/dielectric interface. ........................ 97 5 2 Extraordinary transmittance at normal incidence for a square array of holes. The area covered by holes is only 11% while the normalized to area transmittance of lights is 130%. ................................ ................................ .......... 97 5 3 Schematic SERS process in which light is Raman scattered by a molecule on the surface. ................................ ................................ ................................ ......... 98 5 4 Schematic illustration of electrochemical SERS set up. ................................ ..... 98 5 5 Schematic illustration of the templating procedures for fabricating gold nanopyramid array by using spin coated monolayer colloidal crystal as template. ................................ ................................ ................................ ............. 99 5 6 Tilted (35 ) SEM images of a gold nanopyramid array electrode prior to (A) and after (B) electrochemical SERS experiments. As templates, 320 nm silica spheres were uses. ................................ ................................ ................. 100 5 7 Electrochemical SER spectra recorded on a gold nanopyramid array supported by a conductive carbon disk and a copper tape (red) and a flat gold control sample on silicon (black) in 0.1 M NaCl solution containing 0.05 M pyridine. ................................ ................................ ................................ ........ 101 5 8 Electrochemical SER spectra recorded on a gold nanopyramid array supported by a conductive carbon disk and a copper tape in 0.1 M NaCl solution containing 0.05 M pyridine. ................................ ................................ 101

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12 5 9 The gold electrode potential was swept from 1.0 V (top) to +1.0 V (middle) and then back to 1.0 V (bottom). ................................ ................................ .... 102 5 10 Cyclic voltammograms of a conductive carbon tape, a conductive copper tape, a gold nanopyramid array supported by a carbon tape, and a gold nanopyramid arr ay supported by a carbon disk and a copper tape in 0.1 M NaCl. ................................ ................................ ................................ ................ 102 5 11 Electrochemical SER spectra obtained on a gold nanopyramid array supported by a conductive carbon tape in 0.1 M NaCl solution containing 0.05 M pyridine. The gold electrode potential was swept from 1.0 to 0.2 V. The spectra were taken using a 785 nm diode laser at 48 W with an integration time of 10 s. ................................ ................................ .................... 103 5 12 (A) Modeled Raman enhancement factor around two gold nanopyramids with base length of 320 nm and nanotips radius of curvature of 5 nm at = 785 nm. (B) Simulated maximum SERS enhancement factor (G max ) vs. number of tips of the templated nanopyramid array with the same structural parameters as (A). ................................ ................................ ................................ ............... 104 6 1 Mother of all graphitic forms. ................................ ................................ ............ 116 6 2 .... 117 6 3 ...................... 118 6 4 Schematic illustration of preparation of graphene papers. ................................ 119 6 5 Images of colloidal dispersions of (A) GO and (B) graphene. ........................... 120 6 6 TEM image of graphene sheets. ................................ ................................ ....... 120 6 7 (A) Tapping mode AFM image of graphene sheets with (B) height profiles B1 and B2 taken along the lines in (A). The sample was prepared by drop casting dilut ed graphene dispersion onto a mica substrate. ............................. 121 6 8 (A) Top and (B) bottom side images of a free standing graphene paper made by v acuum filtration of graphene dispersion through an Anodisc membrane. .. 122 6 9 SEM images of a graphene paper. (A)Top view SEM image, (B) bottom view SEM image and (C) cross sectional SEM image. ................................ ............. 123 6 10 (A) Top and (B) bottom side images of a free standing graphene paper made by vacuum filtration of GO dispersion through an Anodisc membrane. ............ 124 6 11 SEM images of a GO paper. (A)Top vi ew SEM image, (B) bottom view SEM image and (C) cross sectional SEM image. ................................ ..................... 125

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13 6 12 Tensile stress versus strain curve for a free s tanding graphene and GO paper. ................................ ................................ ................................ ............... 126 6 13 Schematic illustrations of a graphene actuator (A) Front view of the actuator, (B) side view of the actuator and (C) apparatus used for displacement measurement. ................................ ................................ ................................ ... 127 6 14 (A) Cyclic voltammograms of a graphene stri p at various scan rates in 1 M NaCl solution. A saturated calomel electrode was used as the reference electrode and a platinum wire was used as the counter electrode. The superficial active area was 0.2 cm 2 and the weight of graphene paper immersed was 0.1 2 mg. (B) A plot of steady state currents in (A) versus corresponding scan rates. The slop in (B) is 0.006 F. ................................ ...... 128 6 15 (A) Cross sec tional images of a graphene actuator under eight successive potential steps with a total of four cycles ( 2/2 V repeatedly). (B) Displacements of the actuator tip in (A) under repeated potential steps. .......... 129 6 16 (A) Two electrode cyclic voltammograms of a graphene actuator operated between 2 and 2 volts in 1 M NaCl solution with a scan rate of 50 mV/s. (B) Corresp onding displacements of the actuator in (A) as a function of cycle number. ................................ ................................ ................................ ............ 130

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14 LIST OF ABBREVIATION S AFM Atomic force microscopy AgFON Silver film over nanos phere ALD Atomic layer deposition DIW Deionized water ELD Electrolytic deposition EPD Electrophoretic deposition ETPTA Ethoxylated trimethylolpropane triacrylate FEM Finite element method FIB Focused ion beam GO Graphite oxide IEP Isoelectric point ITO Indium tin oxide LBL Layer by layer MFON Metal film over nanosphere PAH Poly(allylamine hydrochloride) PDMS Polydimethylsiloxane PDDA Poly(diallyldimethylammonium chloride) PEI Polyethylenimine PML Perfect matched layers PVA Polyvinyl alcohol PVP Polyvinylpyrrolidone SAED Selected area electron diffraction SCG Silica coated gibbsite SEM Scanning electron microscopy

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15 SERS Surface enhanced Raman scattering SP Surface plasmon SPR Surface plasmon resonance TEM Transmission electron microscopy TEOS Tetra ethyl orthosilicate TERS Tip enhanced Raman scattering TGA Thermogravimetric analysis TPM 3 (trimethoxysilyl)propyl methacrylate vol. Volume wt. Weight XRD X ray diffraction

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16 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ELECTRIC FIELDS ON GIBBSITE NANOPLATELET ASSEMBLIES, NANOPYRAMID SERS SUBSTRATES AND GRAPHENE ACTUATORS By Tzung Hua Lin August 2010 Chair: Peng Jiang Major: Chemical Engineering This dissertation focuses on the exertion of electric fields to assemble gibbsite nanoplatelets along with various polymers to mimic the intricate brick and mortar nanostructure found in abalone shells A simple electrophoretic (co )deposition technology that enables rapid production of large area polymer nanocomposites with layered structures was studied. Addition of binders and assembling of surface roughened gibbsite nanoplatelets were also studied. The tensile strength and the stiffness of these biomimetic nanocomposites were significantly improved when compared to pure polymer films. The exertion of electric fields to conduct electrochemical SERS on nanostructured substrates that were templated from self assembled colloidal silica arrays as well as to drive graphene based actuators that were made by flow directed assembly of one atom thick graphene sheets were also studied. Periodic arrays of nanopyramids with nanoscale sharp tips and high tip densit y demonstrated an enhancement on the order of 10 6 Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s were able to last up to 140 cycles without significant degradation.

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17 CHAPTER 1 INTRODUCTION Electrodeposition technologies that enable the creation of inorganic o rganic nanocomposites with oriented layered nanostructures were studied to mimic the intricate brick and mortar nanostructure found in the nacreous layer of abalone shells. To resolve the colloidal aggregation issue faced by using nanoclays as building blocks, electrostatically stabilized gibbsite nanoplatelets with high aspect ratio were employed a s a model system. Electrophoretic deposition (EPD) of gibbsite nanoplatelets will be discussed in Chapter 2. The interstitials between the assembled nanoplatelets were infiltrated with polymer to form optically transparent nanocomposites. The tensile stren gth and the stiffness of these biomimetic composites were significantly improved when compared to pure polymer films. To avoid the infiltration process, a novel electrophoretic co deposition technology for rapid production of polymer gibbsite nanocomposit es in a single step was also studied and will be discussed in Chapter 3. Furthermore, it is found that the arrangement of nano asperities interposing between neighboring lamellae plays a crucial role in determining the inter lamellae slip and the resulting mechanical properties of the natural composites. In order to help understand the surface roughness effect, gibbsite nanoplatelets were surface coated with rough silica and results will be shown in Chapter 4. The current bottom up technology enables scalab le production of large area nanocomposites with ordered layered structure that have potential applications ranging from gas barrier films for optoelectronic devices to light weight reinforced materials. Other than assembly of gibbsite nanoplatelets by EPD, applying electric fields on surface enhanced Raman scattering (SERS) substrates and graphene based actuators

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18 were also studied. In Chapter 5, a better way to generate periodically tailored structures of SERS electrodes for chemical and biochemical analysi s is provided A spin coating technique that combines the simplicity and cost benefits of bottom up self assembly with the scalability and compatibility of standard top down microfabrication in creating a large vari ety of nanostructured materials has been demonstrated The methodology is based on shear aligning concentrated colloidal suspensions using standard spin coating equipment. The shear flow generated during the spin coating process coupled with interparticle interaction induces the formation of wafer scale, non close packed colloidal crystals with adjustable thickness ranging from monolayer to hundreds of layers. T hese self assembled colloidal arrays can be used as structural templates to make metalli c nanostructures. P eriodic arrays of nanopyram ids with nanoscale sharp tips and high tip density can enhance the local electromagnetic field in the vicinity of the nanotips, resulting in high SERS enhancement (on the order of 10 6 ). The effect s of the applied electrode potential and the electrode redox reactions on the SERS enhancement were investigated. In Chapter 6, electromechanical actuators based on sheets of graphene papers will be discussed. Graphite oxide (GO) sheets were generated by exfoliation of highly oxygenated graphite in water and then chemically reduced to one atom thick graphene sheets. The graphene sheet dispersions were further filtrated to produce graphene papers. The actuators were operated under repeated potential steps and cyclic voltammetry. Cycling stability of the graphene bas ed actuators was also discussed.

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19 CHAPTER 2 BIOINSPIRED ASSEMBLY OF GIBBSITE NANOPLAT ELETS BY ELECTRIC FI ELD Background Learning from Nature The spontaneous organization of nonspherical colloids has attracted great recent interest due to the wid e range of potential applications of the resulting assemblies in photonic crystals ( 1 4 ), metamaterials ( 5 ), surface enhanced Raman scattering sensors ( 6 ), and reinforced nanocomposites ( 7 9 ), as well as fundamental studies of liquid crystal phase transiti ons ( 10 13 ) and particle packing ( 14 16 ). Among a large variety of nonspherical colloids, platelet particles are particularly interesting as they enable the bottom up assembly of layered nanocomposites that mimic the nacreous layer of mollusk shells ( 17 19 ). The intricate brick and mortar nanostructure (as in Figure 2 1) found in nacre, which consists of ~95 vol.% of brittle aragonite platelets and ~5 vol.% of soft biological macromolecules ( 20 23 ), makes the shells exceptionally tough and stiff with a tens ile stress of around 100 MPa ( 19, 23 ) because most of the load can be carried by the mineral platelets whereas the protein transfers load via the high shear zones between mineral platelets, as shown in Figure 2 2 ( 18 ). The unusual combination of the mechanical strength, toughness, and stiffness in these natural inorganic organic mechanical design principles found in nature ( 7 9 ). Bottom up Self assembly Bottom up self assembly of nonspherical colloidal building blocks is of great interest for the development of new materials with potential applications in optoelectronics, photonics, magnetics, catalysis, and mechanics ( 1, 2, 15, 19, 24 28 ).

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20 Layer by layer (LBL) assembl y of inorganic nanoplatelets (e.g., nanoclays) and polyelectrolytes has recently been demonstrated as an efficient methodology in making r einforced polymer nanocomposite s ( 7, 8 ) Ice templated crystallization g ravitational sedimentation, centrifugation, s pin coating, and dip coating have also been employed to align inorganic nanosheets to form nacre like asse mblies ( 28 31 ) However, these techniques are either time consuming or require multiple steps to infiltrate the inorganic assemblies with polymer to m ake nanocomposites. For example, LBL assembly is a relative slow process and hundreds of bilayers need to be deposited to form composites with micrometer scale thickness. Additionally, the significant agglomeration of commonly used clay nanoplatelets hampe rs the formation of highly aligned structures and thus impairs the mechanical properties of the resulting nanocomposites Electrodeposition Electrodeposition is widely used for the deposition of thin films and coatings. Electrophoretic deposition (EPD) and electrolytic deposition (ELD) are the two commonly used processes, as shown in Figure 2 3. EPD is carried out based on the use of ceramic particles in suspensions and enables the preparation of thick ceramic films while ELD uses solutions of metal salts a nd is an important tool for the formation of nanostructured think films ( 32 ). Electrophoresis is a well established technology in assembling spherical colloids into highly ordered colloidal crystals ( 33 35 ) In this methodology, charged colloids are attrac ted by electrical force toward the counter electrode and then deposited on the electrode surface by particle coagulation ( 32 ) Electrodeposition is a simple, inexpensive, and scalable technology that enables rapid production of thick films over large areas Electrophoretic co deposition of colloids and polymer is also possible for the formation of nanocomposites in a single step. In

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21 addition, deposition of metals and conducting polymers in the interstitials of colloids is easily achieved by electrophoresis. This will significantly expand the available materials for the fabrication of layered nanocomposites. Electrophoretic assembly of nanoclays has previously been tested, but the entrapment of non platy particles caused by the agglomeration of nanoclays dete riorates the layered structure ( 36 ) Gibbsite Nanoplatelets Various synthetic methods have been developed to make fairly monodisperse d colloidal platelets with high stability in suspensions ( 37 40 ) For instance, uniform gibbsite (Al(OH) 3 ) nanoplatelets with well defined hexagon shape can be synthesized by hydrolysis of Al(OH 2 ) 6 3+ at 85 C ( 37, 41 ) The aspect ratio of the synthesized gibbsite nanoplatelets (~ 10) is close to that of natural aragonite platelets in nacre ( 20 ) The diameter and thickness of the gibbsite nanoplatelets can be controlled by seeded growth ( 42 ) The gibbsite structure is a stacking of Al OH layers and each Al 3+ is su rrounded by six hydroxyl groups, as shown in Figure 2 4. The reaction of surface hydroxyl groups with water makes th e nanoplatelets highly charged in water and alcoholic suspensions. The surface hydroxyl groups also facilitate the chemical modification of the particle surface to render different functionality ( 41 ) By using gibbsite nanoplatelet as a model system, Lekke rkerker et al. have extensively exploited the liquid crystal phase transition in suspensions of plate like particles ( 10 12, 43 ) Opal like columnar gibbsite colloidal crystals have also been demonstrated by forced sedimentation ( 13, 44 ) We used electrostatically stabilized gibbsite nanoplatelets with well defined shape and size as a model system to explore the oriented assembly of plate like colloids by electrophoresis. A simple spin

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22 interst nanocomposites. The resulting self standing films were transparent and exhibited significantly improved mechanical properties over those of pure polymer. We also chemically functionalized t he surface of the gibbsite nanoplatelets to facilitate the formation of covalent linkage between the ceramic platelets and the polymer matrix. This further reinforced these biomimetic nanocomposites. Experimental Materials and Substrates All solvents and c hemicals were of reagent quality and were used without further 1 ) was used directly from a Barnstead water system. Ethanol (200 proof) was purchased from Pharmaco Products. Hydrochloric acid (37%), aluminum sec but oxide (95%), and aluminum isopropoxide (98%) were obtained from Aldrich. Ethoxylated trimethylolpropane triacrylate (ETPTA) monomer was provided by Sartomer (Exton, PA). The photoinitiator, Darocur 1173 (2 hydroxy 2 methyl 1 phenyl 1 propanone), was obtain ed from Ciba Specialty Chemicals. Two part polydimethylsiloxane (PDMS, Sylgard 184) was provided by Dow Corning. Indium tin Delta Technologies. Silicon wafers (test grade, n type, (100)) were purchased from University Wafer. Instrumentation An EG& G Model 273A potentiostat/galvanostat was used for EPD. Scanning electron microscopy (SEM) was carried out on a JEOL 6335F FEG SEM. A thin layer of gold was sputtered onto the samples prior to imaging. Transmission electron microscopy (TEM) and selected ar ea electron diffraction (SAED) were performed on a

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23 JEOL TEM 2010F. Atomic force microscopy (AFM) was conducted on a Digital Instruments Dimension 3100 unit. X ray diffraction (XRD) spectra of the electroplated gibbsite films were obtained with Philips APD 3720 equipment. A Cu K 1 radiation was scanned from 10 to 70 with a scan rate of 2.4/min. Thermogravimetric analysis (TGA) was carried out in air with a Perkin Elmer thermogravimetric analyzer and a platinum crucible between 20 and 800C at a heating rate of 5 C/min. The zeta potential of gibbsite nanoplatelets was measured by a Brookhaven ZetaPlus Analyzer (Brookhaven Instrument Corporation). An Ocean Optics HR4000 high resolution fiber optic UV vis near IR spectrometer was used for optic al transmission measurement. A standard spin coater (WS 400B 6NPP Lite spin processor, Laurell) was used to spin coat ETPTA monomer. The polymerization of ETPTA was carried out on a pulsed UV curing system (RC 742, Xenon). Synthesis of Gibbsite Nanoplatele ts The gibbsite nanoplatelets were synthesized by following a preparation method as described in the reference ( 37 ). To 1 L of ultrapure water were added hydrochloric acid (0.09 M), aluminum sec butoxide (0.08 M), and aluminum isopropoxide (0.08 M). The mi xture was stirred for 10 days and then heated in a polyethylene bottle in a water bath at 85 C for 72 h. After cooling to room temperature, dispersions of gibbsite nanoplatelets were centrifuged at 3500g for 6 h, and the sediments were redispersed in deio nized water. For completely removing the unreacted reactants and concentrating the nanoplatelets, this process was repeated for five times. Detailed processes are listed as below: 1. Clean a 1000 ml glass flask by soaking in saturated KOH in isopropanol fo r 24 h 2. Rinse the flask with DIW, then fill with 2% HF and storage overnight.

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24 3 Rinse the flask with DIW several times and then dry in an oven. 4 Draw 20 ml of aluminum tri sec butoxide (97%) with a syringe. This process might take 10 to 20 mins. 5. Meanwhile, put a stirring bar in the flask and add about 800 ml of DIW. 6. Stir at medium speed and then add 7.4 ml of HCl (37%) into the flask. 7. Add 16.3 g of aluminum isopropoxide (98%). 8. Inject the above 20 ml of aluminum tri sec butoxide with a s yringe. 9. Fill the flask to 1000 ml with DIW. 10. Increase stirring speed. 11. Age for 10 days till all the particles dissolves. 12. Distribute the solution into several plastic bottles and then put the bottles in isotemp water bath and heat at 85 C for 7 2 h 13. Wash particles by centrifuging at 3500g for 6 h. 14. Dump supernatant and add DIW. 15. Re disperse particles. 16. Repeat processes 16 to 18 for 5 times. Surface Modification of Gibbsite Nanoplatelets Gibbsite nanoplatelets were surface modifie d with 3 (trimethoxysilyl)propyl methacrylate (TPM) ( 45 ). Prior to adding gibbsite nanoplatelets, 10 ml of TPM was mixed with 100 ml of a water methanol solution (water/methanol volume ratio of 3:1) for 1 h to fully hydrolyze TPM. Surface modification was then accomplished by adding 100 ml of gibbsite dispersion (ca. 1 vol.% aqueous solution) into the hydrolyzed TPM solution. The suspension was stirred at 40C for 30 min. The modified nanoplatelets were washed by repeated centrifugation redispersion cycles with pure ethanol and finally concentrated to a stock suspension of 0.045 and 0.035 (g/g) in ethanol.

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25 EPD of Gibbsite Nanoplatelets EPD of gibbsite nanoplatelets was performed in a sandwich cell placed horizontally as shown in Figure 2 5A The bottom and the top of the cell were an ITO working electrode and a gold counter electrode, respectively. The gold electrode was prepared by sputtering deposition of 20 nm of chromium and 200 nm of gold on a (100) silicon wafer. PDMS was used as a spacer to get an act ive area of 1.5 1.5 cm 2 and a cell gap of 2.2 mm. Aqueous suspensions of gibbsite nanoplatelets with different weight percentage were used. Ethanol (200 proof) was added into the suspensions to make the volumetric ratio of ethanol to the aqueous suspensi on to be 2. A constant voltage of 2.5 V (ITO vs. Au) was applied for 30 min to deposit the positively charged gibbsite nanoplatelets onto the ITO cathode. After deposition, the electroplated gibbsite films (as in Figure 2 5B) were rinsed with 200 proof et hanol and then dried with compressed air. ETPTA Gibbsite Nanocomposites After the oriented assembly, ETPTA Gibbsite nanocomposites can then be made by filling the interstitials between the aligned nanoplatelets with photocurable monomers, followed by photo polymerization. We chose a nonvolatile monomer ETPTA to form the nanocomposites. A free standing gibbsite was first put on a PDMS substrate. The ETPTA monomer with 1% photoinitiator was then dropped on the gibbsite film. The sample was then put under vacuu m for 10 min in order to remove as many trapped air bubbles as possible. After 10 min, the gibbsite film became transparent and then was spin coated at 4000 rpm for 1 min to remove excess monomer solution and then polymerized by exposure to ultraviolet rad iation.

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26 Mechanical Test For tensile strength measurement, three types of thin films (ETPTA, ETPTA Gibbsite, and ETPTA TPM modified gibbsite) were tested using an Instron model 1122 load frame upgraded with an MTS ReNew system and equipped with a 500 g load cell at a crosshead speed of 0.5 mm/min. Testing samples with widths of 1.5 mm and thickness ranging from 30 to 80 m were adhered on homemade sample holders with a 20 mm gap using polyurethane monomer as an adhesive and then UV cured. The thickness of th e tested samples was measured by cross sectional SEM to calculate the final tensile strength. Results and Discussion Gibbsite Characterization Figure 2 6 shows a typical TEM image of purified gibbsite nanoplatelets. The particles are hexagonally shaped and are relatively uniform in size. The diameter of the nanoplatelets is measured to be 188 40 nm by averaging over 100 particles from the TEM micrographs. TEM images also reveal that the nanoplatelets tend to align parallel to the surface of TEM grids. AFM experiments show the platelet thickness ranges from 10 to 15 nm. The purified gibbsite nanoplatelets are electrostatically stabilized, and the zeta potential of the colloids in deionized water is measured to be +40.5 2.3 mV by fitting experimental data the nanoplatelets stable in aqueous and alcoholic dispersions, and aggregated particles are rarely seen in TEM images. The SAED patterns from a single platelet as shown in the inset of Figure 2 6 in dicate that the as made gibbsite nanoplatelets are single crystal.

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27 EPD of Gibbsite Nanoplatelets The EPD of positively charged gibbsite nanoplatelets is carried out using a parallel plate sandwich cell, which consists of an ITO working electrode, a gold c ounter electrode, and a PDMS spacer ( 2.2 mm thick). The bath solution is gibbsite nanoplatelets dispersed in a water ethanol mixture with volumetric ratio of 1:2. The volume fraction of gibbsite particles is adjusted to 1%. Ethanol is added to the aqueou s dispersions to reduce the dielectric constant of the solvent and thus reduce the electrical double layer thickness of the particles to promote colloidal coagulation on the ITO electrode. Without ethanol, no particle deposits are adhered on the working el ectrode after disassembling the electrical cell. The addition of ethanol also facilitates reduction of cracking and porosity in the electrophoretically deposited films. The applied electric field strength is 1100 V/m. The electrophoretic velocity of the g ibbsite nanoplatelets is estimated to be equation: where is the dielectric constant of the solution, 0 is the permittivity of the vacuum, is the solution viscosity, and E is the applied electric field strength. For a 2.2 mm thick sandwich cell, the estimated time to deposit most particles on the ITO electrode is about 5 min, agreeing with our experimental observation. Besides parallel plate geometry, electrodes can also be vertically inserted into the colloidal b aths to conduct the EPD. As the gravitational sedimentation of the gibbsite nanoplatelets during the electrophoretic process is negligible, uniform deposits on the electrodes are obtained. After EPD, the gibbsite deposits on the ITO cathode are washed with ethanol and then dried with compressed air. The deposits can be easily peeled off from the ITO surface by using a

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28 sharp razor blade, resulting in the formation of self standing films as shown in Figure 2 7 A. The film is opaque and brittle, and the side fa cing the ITO cathode is smoother than the side facing the suspension. The size of the resulting films is solely determined by the dimensions of the ITO electrode. Figure 2 7 A depicts a sample with 1.6 0.6 inch 2 size deposited on a 2 1 inch 2 ITO electro de. Figure 2 7 B shows a top view SEM image of the suspension side of the sample in Figure 2 7 A. The hexagonal gibbsite nanoplatelets are densely packed and aligned parallel to the electrode surface. The alignment of gibbsite nanoplatelets is further confirmed by the layered structure as shown in the cross sectional SEM image of Figure 2 7 C. Anothe r convincing evidence of the orientated deposition comes from the XRD patterns shown in Figure 2 8 Only (002) and (004) peaks are observed in the XRD spectrum. As the crystallographic c axis of single crystal gibbsite is normal to the platelet surfaces, t he (002) and (004) reflections are from gibbsite platelets oriented parallel to the electrode surface ( 46 ). Analysis of the half height width of the (002) peak with the Scherrer equation yields an average platelet thickness of 15.1 nm, agreeing with AFM me asurement. The oriented deposition of gibbsite nanoplatelets in a direct current (dc) electric field can be understood by considering the charge distribution on the gibbsite surfaces. ers from that on ( 37 ). The pH of the suspension in the electrophoretic experiments is close to 7, resulting in positively charged surfaces and almost neutral edges. Therefore, the applied electric field exerts a force only on the surfac es of the gibbsite platelets, and Brownian motion could provide sufficient torque to reorient perpendicular particles to face the ITO electrode. Once close to the electrode, the gibbsite nanoplatelets will be

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29 forced to align parallel to the electrode surfa ce as this orientation is more energetically favorable than the perpendicular one. Similar to the evaporation induced alignment of gibbsite nanoplatelets on TEM grids (Figure 2 6 ), further evidence shows that capillary force during solvent evaporation is s ufficient to orient gibbsite particles into layered assemblies. However, the rapid and uniform deposition of nanoplatelets over large areas is the major advantage of the electrodeposition technology over evaporation and gravitational sedimentation induced assembly. If the duration of the electrophoretic process is long enough, almost all gibbsite platelets can be deposited on the ITO electrode. The thickness of the deposits is then linearly proportional to the particle volume fraction of the suspension as s hown in Figure 2 9 Polymer Gibbsite Nanocomposites After the oriented assembly, polymer Gibbsite nanocomposites can then be made by filling the interstitials between the aligned nanoplatelets with photocurable monomers, followed by photopolymerization. We chose a nonvolatile monomer ETPTA to form the nanocomposites. The monomer with 1% photoinitiator is spin coated at 4000 exposure to ultraviolet radiation. The resulting nan ocomposite film becomes highly transparent (Figure 2 10 A) as a result of the matching of the refractive index between the gibbsite platelets and the polymer matrix. The cross sectional SEM image in Figure 2 10 B shows that the nanocomposite retains the laye red structure of the original electroplated gibbsite film, and thin wetting layers of ETPTA ( 1 m thick) are observed on the surfaces of the film. The normal incidence transmission measurement as shown in Figure 2 1 1 shows that the free standing nanocompo site film exhibits high transmittance (>80%) for most of the visible wavelengths. As the reflection (R) from an

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30 interface between two materials with refractive indices of n 1 and n 2 is governed by 1 n 2 )/(n 1 + n 2 )] 2 we can estima te the normal incidence reflection from each air nanocomposite interface to be about 4%. Thus, the optical scattering and absorption caused by the nanocomposite itself is approximately 10%. This suggests that the polymer matrix has infiltrated most interst itial spaces between the aligned gibbsite nanoplatelets. The oriented arrangement of the nanoplatelets is also maintained throughout the polymer infiltration process as confirmed by the distinctive (002) and (004) peaks of the XRD spectrum shown in Figure 2 1 2 TGA and Tensile Strength of Gibbsite based Nanocomposites The ceramic weight fraction in the ETPTA Gibbsite nanocomposite film is determined by TGA as shown in Figure 2 1 3 From the TGA curve and the corresponding weight loss rate, it is apparent th at two thermal degradation processes occur. One happens at 250C and corresponds to the degradation of the polymer matrix, while another occurs at 350C and is due to the decomposition reaction of gibbsite: 2Al(OH) 3 Al 2 O 3 + 3H 2 O. On the basis of the re sidue mass percentage (45.65%) and assuming the ash is solely Al 2 O 3 we can estimate the weight fraction of gibbsite nanoplatelets in the original nanocomposite film to be 0.70. Considering the density of gibbsite ( 2.4 g/cm 3 ) and ETPTA ( 1.0 g/cm 3 ), the volume fraction of gibbsite nanoplatelets in the nanocomposites is approximately 0.50. The complete infiltration of ETPTA between the electroplated gibbsite platelets is further confirmed by the selective dissolution of gibbsite in a 2% hydrochloric a cid aqueous solution. This results in the formation of a self standing porous membrane with stacked hexagon shaped pores, which are a negative replica of the assembled gibbsite platelets.

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31 The mechanical properties of the biomimetic polymer nanocomposites are evaluated by tensile tests. We compare the tensile strength for three types of thin films, including pure ETPTA, ETPTA Gibbsite, and ETPTA TPM modified Gibbsite. The surface hydroxyl groups of gibbsite nanoplatelets can be easily modified by reacting w ith TPM through the well established silane coupling reaction ( 45 ). This results in the formation of surface modified particles with dangling acrylate bonds that can be cross linked with the acrylate based ETPTA matrix. The colloidal stability and the surf ace charge of the resulting nanoplatelets are not affected by this surface modification process as confirmed by TEM and zeta potential measurement. Figure 2 1 4 shows the tensile stress versus strain curves for the above three types of films. The ETPTA Gibb site nanocomposite displays 2 time higher strength and 3 time higher modulus when compared with pure ETPTA polymer. Even more remarkable improvement occurs when TPM Gibbsite platelets are cross linked with the ETPTA matrix. We observe 4 time higher stre ngth and nearly 1 order of magnitude higher modulus than pure polymer. This agrees with early studies that reveal the crucial role played by the covalent linkage between the ceramic fillers and the organic matrix in determining the mechanical properties of ( 8 ). We also conduct a simple calculation to evaluate if the measured mechanical properties of the ETPTA Gibbsite nanocomposites are reasonable. For a polymer matrix having a yield shear strength y and strong bonding to the gibbsite nanoplatelet surface (e.g., TPM modified gibbsites), the tensile strength of the composite ( c ) can be calculated using the volume fraction of nanoplatelet ( V p ), the nanoplatelet aspect ratio ( s ), and the tensile strength of the nanoplatelet ( p ) and of the polymer matrix ( m ), as ( 9 )

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32 For the gibbsite nanoplatelet which has a relatively small aspect ratio (s 12 18), the factor R in the above equation can be estimated as From the above TGA analysis, the volume fraction of gibbs ite nanoplatelets in the polymer nanocomposite is 0.50. If we take s = 15, the first equation can then be For acrylate based polymer (like ETPTA), the yield shear strength should be close to its c 4.25 m This indicates that the strength of the nanocomposite is about fourfold of the strength o f the polymer matrix, agreeing with our experimental results. Tensile strength of composites estimated by the above method can be further plotted as functions of volume fraction and aspect ratio of gibbsite nanoplatelets, as shown in Figure 2 1 5 The dot i n Figure 2 1 5 is our experimental result while the net surface is from the proposed model. Summary In summary, we have developed a simple and rapid electrodeposition technology for assembling gibbsite nanoplatelets into large area, self standing films. Th ese nanosheets with high aspect ratio are preferentially aligned parallel to the electrode surface. The interstitials between the assembled nanoplatelets can be infiltrated with polymer to form optically transparent nanocomposites. The tensile strength and the stiffness of these biomimetic composites are significantly improved when compared to

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33 pure polymer films. The current electrodeposition technology is a quite general approach to achieve oriented deposition of platelet like particles with various aspect ratios. Preliminary results show that silica coated gibbsite nanoplatelets, hollow silica nanoplatelets, and zeolite platelets can also be aligned by EPD. The technology is also promising for developing layered metal ceramic and conducting polymer ceramic nanocomposites that may exhibit improved mechanical and electrical properties but are not easily available by other bottom up technologies.

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34 Figure 2 1. (A) I mage of an abalone shell. (B) SEM image of fracture surface of aragonitic portion of abalone n acre. (C) TEM image of the nacre cross section ( 21 ).

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35 Figure 2 2. A model of biocomposites. (a) A schematic diagram of staggered mineral carrying structure of the mineral protein composites. Most of the load is carried by the mineral platelets whereas the protein transfers load via the high shear zones between mineral platelets ( 18 ).

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36 Figure 2 3. (A) Schematic of cathodic electrophoretic deposition (EPD) and e lectrolytic deposition (ELD). (B) Thickness of coatings deposited using ELD and EPD ( 32 ).

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37 Figure 2 4. (A) Lattice structure of Gibbsite. (B) Hexagon shape of Gibbsite and corresponding isoelectric points ( 41 ).

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38 (A) (B) Figure 2 5. Schematic illustrations of (A) an electrophoretic cell and (B) deposit after EPD.

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39 Figure 2 6 TEM image of gibbsite nanoplatelets. The inset shows the electron diffraction patterns obtained from a single nanoplatelet.

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40 Figure 2 7 Electrophoretic assembly of gibbsite nanoplatelets. (A) Photograph of a free standing gibbsite film. (B) Top view SEM image of the sample in (A). (C) Cross sectional view of the same sample.

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41 Figure 2 8 XRD patterns of the gib bsite film in Figure 2 7 A. Figure 2 9 Thickness dependence of the electroplated films on the concentration of colloidal gibbsite suspensions. The thickness standard deviation for all samples is ca. 10%.

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42 Figure 2 10 Free s tanding gibbsite ETPTA nanocomposite. (A) Photograph of a transparent film. (B) Cross sectional SEM image of the same nanocomposite film.

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43 Figure 2 1 1 Normal incidence transmission spectrum of the sample in Figure 2 10 A. Figure 2 1 2 XRD patterns of the nanocomposite sample in Figure 2 10 A.

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44 Figure 2 1 3 TGA of the ETPTA Gibbsite nanocomposite as shown in Figure 2 10A Figure 2 1 4 Tensile stress versus strain curves for plain ETPTA film, ETPTA Gibbsite nanocomposite, and ETPTA TPM Gibbsite nanocomposite.

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45 Figure 2 1 5 Tensile strength of composites as functions of volume fraction and aspect ratio of gibbsite nanoplatelets.

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46 CH APTER 3 ELECTROPHORETIC CO DEPOSITION OF POLYME R GIBBSITE COMPOSITES Background In Chapter 2, we have demonstrated a simple and rapid electrodeposition technology for assembling gibbsite nanoplatelets into large area, self standing films. The interstitials between the assembled nanoplatelets can then be infiltrated with polymer to form opticall y transparent nanocomposites. In C hapter 3 we will show a simple electro co deposition technology that enables the creation of inorganic organic nanocomposites with oriented layered nanostructures in a single step. The electrodeposited inorganic organic n anocomposite films are optically transparent and flexible, even though the weight fraction of the brittle inorganic phase is higher than 80%. We will also show the assembly of nanocomposites with similar organic/inorganic weight ratio and ordered multilay ered structure as nacres by using the proposed co deposition method. The novelty of the technology is that the positively charged nanoplatelets and polyelectrolytes are both electrophoretically attracted by the applied direct current electric field and the n simultaneously deposited on the cathode to form ordered nanocomposites. The mechanical properties of these biomimetic nanocomposites and the colloidal stability of the nanoplatelet polyelectrolyte dispersions have also been investigated. Experimental EPD of Nanoplatelets EPD of nanoplatelets is performed in a water ethanol mixture in a sandwich cell placed horizontally. The bot tom and the top of the cell are either an ITO or a gold

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47 electrode. The gold electrode is prepared by sputtering deposition of 20 n m of titanium and 200 nm of gold on a glass slide PDMS is used as a spacer to get an active area of 1 .51.5 cm 2 and a cell gap of 2. 2 mm. EPD of PVA Gibbsite nanoplatelets To prepare the electrophoretic bath solution, 1 ml of 5 wt.% polyvinyl alcohol (PVA, Mw 89,000 ~ 98,000, Sigma Aldrich) aqueous solution was firstly mixed with 9 ml of 2 wt.% gibbsite nanoplatelet solution. Twenty milliliters of 200 proof ethanol was then added into the above suspension. The bottom and th e top of the cell were an ITO working electrode and a gold counter electrode, respectively. A constant voltage of 2.5 V (ITO vs. Au electrode) was applied to deposit gibbsite nanoplatelets on the ITO cathode. After the EPD, the as deposited PVA gibbsite f ilm was dried in an oven at 80 C. EPD of PEI Gibbsite nanoplatelets Electrophoretic bath solution was prepared by mixing 2 ml of 0.3 wt.% polyethylenimine (PEI, Mw ~750,000, Sigma Aldrich) aqueous solution, 3 ml of 2.0 wt.% gibbsite nanoplatelet aqueous so lution, and 10 ml of 200 proof ethanol. Gold electrodes were served as both working and counter electrodes. Constant current of 0.3 mA was applied for 15 min to deposit gibbsite nanoplatelets and PEI on the bottom gold cathode. After EPD, the as deposited PEI gibbsite nanocomposite was dried in air. Results and Discussion Electrophoretic Co deposition of PVA Gibbsite As the synthesized gibbsite platelets have positively charged surfaces and almost electrically neutral edges due to their different isoelect ric point (pH 10 and 7, respectively), they tend to re orient in the electric field with their surfaces facing the ITO

PAGE 48

48 electrode. The high molecular weight PVA (Mw 89,000 98,000) is neutrally charged in the electrophoretic bath. They can be absorbed on the surfaces of gibbsite nanoplatelets and function as water soluble binders to cement electrodeposited ceramic particles together. Ethanol (~50% of total volume) is also added to the aqueous colloidal suspensions to reduce the dielectric constant of the solv ent, and thus reduce the electrical double layer thickness of the particles to further promote colloidal coagulation on the ITO cathode. Figure 3 1A shows a photograph of a PVA Gibbsite nanocomposite formed on an ITO cathode. The film can be easily peeled off from the electrode surface by using a sharp razor blade. The resulting self standing film is flexible and transparent. Optical transmission measurement at normal incidence (not shown here) shows the film exhibits 60 80% transmittance for most of the v isible wavelengths. Top view SEM image in Figure 3 1B illustrates the gibbsite nanoplatelets are preferentially oriented with their crystallographic c axis perpendicular to the electrode surface. It is very rare to find edge on platelets. The ordered laye red structure is clearly evident from the cross sectional SEM images as shown in Figure 3 1C and D. The oriented assembly of high aspect ratio gibbsite nanoplatelets is further confirmed by XRD. Figure 3 2 displays a XRD spectrum of an electrodeposited PVA Gibbsite nanocomposite on an ITO electrode. The diffraction peaks from (222), (400), (441), and (662) planes of the ITO substrate are clearly appeared. Other than ITO diffraction peaks, we only observe (002) and (004) peaks from gibbsite single crystals. As the crystallographic c axis of single crystalline gibbsite is normal to the platelet surfaces, the (002) and (004) reflection are from

PAGE 49

49 gibbsite platelets oriented parallel to the electrode surface. This strongly supports the macroscopic alignment of gib bsite nanoplatelets in the electrophoretically deposited nanocomposites. Analysis of the half height width of the (002) and (004) peaks with the Scherrer equation ( 46 ) yields an average platelet thickness of 10.3 nm, agreeing with cross sectional SEM measu rement. The current EPD technology enables large scale assembly of ordered nanocomposite films in a very short time. Figure 3 3 shows the relationship between the measured weight of deposits on ITO cathode and the electrophoretic duration. A weight platea u is reached in ca. 8 min. Experimental observation shows almost all gibbsite nanoplatelets have already been deposited in this time interval and the electrophoretic bath changes from turbid to clear. The electrophoretic velocity of gibbsite nanoplatelets is estimated to be ~8.2 sandwich cell, the estimated time to deposit all particles on the ITO electrode is ca. 5 min, reasonably agreeing with the experimental observation. The weight fraction of the inorganic phase in the electrodeposited nanocomposites can be determined by TGA. Figure 3 4 shows the TGA curve and the corresponding weight loss rate for the nanocomposite film as shown in Figure 3 1A. An apparent thermal degradation process occurs at ~250 C that corresponds to the degradation of the PVA matrix and the decomposition reaction of gibbsite. Based on the residue mass percentage (53.96%) and assuming the ash is solely Al 2 O 3 we can estimate the weight fraction of gibbsite nanoplatelets i n the original nanocomposite film to be 0.825. Electrophoretic Co deposition of PVA Gibbsite PEI, which is a weak polyelectrolyte and contains amine groups, is positively charged under the electrophoretic conditions. The gibbsite nanoplatelets with a small

PAGE 50

50 amount of PEI are well dispersed in a water ethanol mixture solution due to the electrostatic repulsion between particles. However, adding a larger amount of PEI leads to the agglomeration of gibbsite nanoplatelets. To allow the electrophoresis at a contr olled deposition rate, as well as the formation of ordered layered structure, gibbsite nanoplatelets must be stabilized in suspensions. We therefore study the influence of the PEI concentration on the stability of gibbsite by measuring particle size distri bution and zeta potential. To prepare the testing solution, (6 n) ml of 2.0 wt.% gibbsite solution is mixed with n ml of 0.3 wt.% PEI aqueous solution, where n = 0 5. The weight ratio (PEI to gibbsite, R) is calculated as (n 0.3)/[(6 n) 2]. Fig. 3 5 shows the size distribution of gibbsite nanoplatelets at different R values measured by laser diffraction. The average diameter of the as synthesized gibbsite nanoplatelets (R = 0) is 150 nm (Figure 3 5A), which is smaller than that observed from TEM im ages in Chapter 2. The random mismatch of the surface of nanoplatelets to the incident laser beam reduces the effective diffraction area, resulting in a smaller average diameter. Figure 3 5B shows that no significant change in the particle size distributio n is observed when a small amount of PEI is added (R = 0.03). However, further increasing of PEI concentration, as shown in Figure 3 5C and D (R = 0.075 and 0.75, respectively), leads to a larger particle diameter resulting from the flocculation of nanopla telets. The flocculation at high polyelectrolyte concentration can be explained by the increase in ionic strength, which leads to the decrease in the electrical double layer thickness and the instability of the colloids ( 47 ). Depletion flocculation also pl ays an important role. At a high polymer concentration, the polymer concentration gradient between the inter particle gap and

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51 the remainder of the solution generates an osmotic pressure difference, forcing solvent flows out of the gap until particles flocc ulate ( 48 ). Electrophoretic mobility and zeta potential of nanoplatelets in PEI Gibbsite suspensions with different R values are shown in Figure 3 6. Zeta potential is obtained e lectrophoretic mobility and zeta potential when a small amount of PEI is added (R from 0 to 0.03) is due to the contribution of highly charged PEI that possesses a zeta potential of ~ +60 mV in water at neutral pH. Further increasing of PEI concentration r esults in the decreasing of electrophoretic mobility and zeta potential due to the particle flocculation as shown in Figure 3 5. As gibbsite nanoplatelets have positively charged surfaces (IEP ~10) and almost neutral edges (IEP ~7) under the electrophoret ic conditions (pH ~7), the electrical force tends to re orient the nanoplatelets to face the cathode. The positively charged PEI molecules are also electrophoretically migrated toward the cathode together with gibbsite and simultaneously sandwiched between nanoplatelets, forming PEI Gibbsite nanocomposite. The addition of ethanol reduces the effective dielectric constant of the solvent, promoting particle coagulation by suppressing the electrical double layer thickness of the nanoplatelets. The high pH near the cathode due to cathodic reactions also helps to coagulate nanoplatelets, as well as neutralize the protonated PEI macromolecules. Top view SEM images in Figure 3 7A and B show that the electrodeposited nanoplatelets are preferentially oriented with th eir crystallographic c axis perpendicular to the electrode surface. The hexagonal shape and the size of the platelets can be clearly seen in Figure 3 7B. Cross sectional SEM images showed in

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52 Figure 3 7C and D provide further evidence of the ordered layered structure. XRD spectrum of the PEI Gibbsite nanocomposite on an Au electrode is shown in Figure 3 8. The diffraction peak from the (002) plane of gibbsite single crystals is clearly appeared. Comparing to our previous results, which show diffraction peaks from both (002) and (004) planes of gibbsite crystals, the weaker diffraction peak from (004) plane is overlapped with the strong diffraction peak of Au. The (004) diffraction peak can be clearly seen by simply replacing Au electrode with Pt (not shown here). As the (002) and (004) diffraction are originated from gibbsite platelets oriented p arallel to the electrode surface, the oriented assembly of high aspect ratio nanoplatelets is further confirmed. TGA is carried out to determine the weight fraction of the organic phase in the nanocomposites shown in Figure 3 9. An apparent thermal degrad ation process occurs at ~250 C that corresponds to the degradation of the polymer matrix and the decomposition reaction of gibbsite. Based on the residual mass percentage (63.7%) and assuming the ash contains only Al 2 O 3 the weight fraction of PEI in the n anocomposite film is estimated to be ~0.03, which is close to the organic content of natural nacre consisting of less than 5 wt.% of soft biological macromolecules ( 19 ). The mechanical properties of the electrodeposited nanocomposites are evaluated using nanoindentation. In a nanoindentation test, a diamond Berkovich indenter is forced perpendicularly into the coating surface. The load displacement profile is then used to calculate the reduced modulus, E r using the Oliver Pharr method ( 49 ). Figure 3 10 sh ows the E r as a function of contact depth obtained from the nanoindentation tests. The observed E r is in the range of 2.20 5.17 GPa, which is comparable to those of

PAGE 53

53 ( 31 ) and LBL assembly ( 50 ). The decrease in E r with increasing contact depth may be related to the indentation size effects. The size effects are explained as a result of deformation, which originates mainly from crack propagation for ceramics, and factors such as surface roughness, int eraction between inorganic and organic phases, and other structural details of the coatings ( 51, 52 ). The E r of PEI Gibbsite nanocomposite is ~0.4 GPa lower than that of pure gibbsite coating, showing the effect of the soft PEI layers in between the hard g ibbsite nanoplatelets ( 53 ). Summary We have developed a scalable EPD technology for rapid production of nacre like inorganic organic nanocomposites in a single step. The applied direct current electric field enables the preferential alignment of gibbsite n anoplatelets and the co deposition of non ionic type polymer between the inorganic nanosheets. The resulting self standing nanocomposite films contain high weight percentage of inorganic platelets, but are still optically transparent and fl exible. The co d eposition technology is readily applicable to many cationic polyelectrolytes, such as poly(diallyldimethylammonium chloride) (PDDA) and poly(allylamine hydrochloride) (PAH), gibbsite nanoplatelets with larger size, and even silica coated gibbsite particles

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54 Figure 3 1. Electrodeposited PVA Gibbsite nanocomposite. (A) Photograph of a composite film on an ITO electrode. (B) Top view SEM image of the sample in (A). (C) Cross sectional SEM image of the sample in (A). (D) Magnified cross sectional image.

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55 Figure 3 2. XRD patterns of an electrodeposited PVA gibbsite composite on ITO electrode. Figure 3 3. Deposit weight on ITO electrode versus electrophoretic duration.

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56 Fi gure 3 4. TGA of the nanocomposite sample as shown in Figure 3 1A.

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57 Figure 3 5. Particle size distribution of nanoplatelet suspensions at different PEI/gibbsite weight ratio. (A) R = 0, (B) R = 0.03, (C) R = 0.075, and (D) R =0.75.

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58 Figure 3 6. Electrophoretic mobility and corresponding zeta potential of nanoplatelets at different PEI/gibbsite weight ratio.

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59 Figure 3 7. SEM images of PEI Gibbsite nanocomposite. (A) Top view image, (B) magnified top view image, (C) cross sectional image, and (D) magnified cross sectional image.

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60 Figure 3 8. XRD patterns of an electrodeposited PEI Gibbsite nanocomposite on Au electrode. Figure 3 9. TGA of an electrodeposited PEI Gibbsite nanocomposite.

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61 Figure 3 10. Reduced modulus of pure gibbsite and PEI Gibbsite nanocomposite measured by nanoindentation.

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62 CHAPTER 4 ELECTROPHORETIC ASSE MBLY OF SURFACE ROUGHENED GIBBSIT ES Background In Chapter 2, we have demonstrated that ordered assemblies of single crystalline gibbsite nanoplatelets can be achieved by EPD and polymer gibbsite nanocomposites are obtained by subsequent polymer infiltration. In Chapter 3, we have also sh own that the co deposition of gibbsite nanoplatelets and either non ionic type polymers or polyelectrolytes can be achieved by EPD. However, there is a major difference between the natural platelets found in nacres and the synthetic gibbsite nanoplatelets i.e., the natural aragonite platelets are rough while the single crystalline gibbsite nanoplatelets have smooth surface ( 37, 54 ). It is found that the arrangement of nano asperities interposing (as in Figure 4 1) between neighboring lamellae plays a cruc ial role in determining the inter lamellae slip and the resulting mechanical properties of the natural composites ( 55, 56 ). This is because the stress at which the inelastic deformation proceeds is governed by these nano sized asperities on the surface of the aragonite tablets ( 56 ). To help understand the surface roughness effect, we intend to increase the surface roughness of synthetic nanoplatelets by coating gibbsite particles with rough silica to mimic the asperity of natural aragonite. EPD of these su rface roughened shrinkage of sol gel silica during the drying process after electrodeposition results in severe cracking. Similar drying induced crack formation is also a detrimental factor that affects the crystalline quality of silica colloidal crystals prepared by the convective self assembly technology ( 57, 58 ). To resolve this cracking issue, here we report a new

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63 EPD approach to form biomimetic monolithic multilayer by reversing the surface charge of silica coated gibbsite (SCG) nanoplatelets using adsorbed polyelectrolytes. Different kinds of polymer nanocomposites are prepared and their mechanical properties are evaluated by tensile tests. The resulting self standing films are transparent and exhibit significantly improved mechanical properties over those of pure polymer. Experimental Coating of Gibbsite Nanoplatelets with Silica Purified gibbsite nanoplatelets are coated with a thin layer of silica by following a proc edure consisting of two steps: adsorption of polyvinylpyrrolidone (PVP) and growth of silica shell in ethanol via Stber method ( 59 ). PVP (Mw ~40,000) is first dissolved in deionized water by ultrasonication and vigorous stirring. Subsequently, 200 ml of g ibbsite nanoplatelet aqueous suspension (1 wt.%) is mixed with 300 ml of PVP solution (10 wt.%). The mixture is then stirred for 1 day to ensure the complete adsorption of PVP on the gibbsite surface. To transfer PVP coated gibbsite nanoplatelets into etha nol, the mixture is centrifuged and the sediment is redispersed in ethanol. This process is repeated for three times for the complete replacement of water with ethanol. The final volume of the PVP modified gibbsite nanoplatelet suspension is adjusted to 50 0 ml The suspension is then mixed with 33 ml of ammonium hydroxide (14.8 N) and 1 ml of tetraethyl orthosilicate (TEOS, 99+%) for the growth of silica shell. After 4 6 h of stirring, dispersions of SCG nanoplatelets are centrifuged and the sediments are redispersed in deionized water. For TEM imaging, 1 ml of HCl (37%) and 10 ml of SCG na noplatelets (~0.1 wt.%) are mixed and stirred for 2 days to remove the gibbsite core.

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64 EPD of Nanoplatelets EPD of nanoplatelets (SCG and PEI SCG) is performed in a water ethanol mixture in a sandwich cell placed horizontally. The bottom and the top of the cell are either an ITO or a gold electrode. PDMS spacer is used to get an active area of 1.5 1.5 cm 2 and a cell gap of 2.2 mm. EPD of SCG nanoplatelets The bath solution is SCG nanoplatelets dispersed in a water ethanol mixture. 200 proof ethanol is added into 2 wt.% of aqueous suspensions of SCG nanoplatelets to make the volumetric ratio of ethanol to the aqueous suspension to be two. A constant voltage of 3.5 V (Au vs. ITO) is applied for 20 min to deposit the negatively charged SCG nanoplatelets on to the bottom gold anode. An ITO electrode is used as the top counter cathode to enable the in situ observation of the EPD process. EPD of PEI SCG nanoplatelets The electrophoretic bath solution for depositing PEI SCG nanoplatelets is prepared by mixing 9 ml of 1.5 wt.% SCG nanoplatelet aqueous suspension with 1 ml of 1.5 wt.% PEI aqueous solution. The bath solution is ultrasonicated for 30 min to minimize the agglomeration of SCG nanoplatelets. Positively charged PEI macromolecules are adsorbed on the nega tively charged surface of SCG nanoplatelets due to Coulombic attraction, forming positively charged PEI SCG nanoplatelets. A constant voltage of 2.5 V (ITO vs. Au) is applied for 20 min to deposit the positively charged PEI SCG nanoplatelets onto the bott om ITO cathode. A gold electrode is served as the top counter anode.

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65 Results and Discussion SCG Nanoplatelets SCG nanoplatelets are synthesized by coating a thin shell of sol gel silica on gibbsite nanoplatelet cores ( 59 ). The synthetic process consists of two steps: adsorption of PVP and the subsequent growth of silica shell via Stber method. The amphiphilic PVP macromolecule acts as a coupling agent. It can be adsorbed onto a broad range of colloids and stabilizes them in water and various nonaqueous sol vents (e.g., ethanol). The PVP modified gibbsite nanoplatelets can be directly dispersed in ethanol for the subsequent growth of silica shell. The thickness of the silica shell can be easily controlled by adjusting the sol gel reaction conditions (e.g., pr ecursor concentration and reaction time) ( 59 ). To confirm the formation of silica shell, the gibbsite core of SCG nanoplatelets can be selectively removed by a hydrochloric acid wash ( 41 ). Figure 4 2A shows a typical TEM image of hollow silica nanoplatelet s after selectively leaching out gibbsite cores. Dark edges in the TEM image reveal that these nanoplatelets are hollow. The arrows indicate a silica shell with a thickness of ca. 10 nm. The regular hexagonal shape and the thickness uniformity of silica sh ells are clearly evident from the TEM image. Additionally, by comparing the TEM image with that of gibbsite nanoplatelets in Figure 2 5, it is evident that the sol gel derived silica shells are much rougher than the single crystalline gibbsite nanoplatelet s. Moreover, the silica coating reverses the surface charge of gibbsite colloids and the zeta potential of SCG nanoplatelets in ethanol is measured to be confirms that the surface of gibbsite nanop latelets has been coated with silica which is negatively charged at neutral pH ( 60 ).

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66 EPD of SCG Nanoplatelets EPD of negatively charged SCG nanoplatelets is carried out in a water ethanol mixture using a parallel plate sandwich cell, which consists of a go ld working anode on the bottom, an ITO cathode on the top, and a PDMS spacer (~2.2 mm thick). Ethanol is used to reduce the dielectric constant of the solvent and therefore decrease the electrical double layer thickness of the SCG nanoplatelets to promote colloidal coagulation on the gold electrode. Deionized water is added to the suspension for bringing about the following anodic reaction: 2H 2 O O 2 + 4H + + 4e E 0 = 1.229V which leads to a local pH decrease at the electrode surface. Since the zeta pote ntial of silica reduces when the solution pH decreases ( 60 ), the above anodic reaction can thus lower the surface charge of negatively charged SCG nanoplatelets to further assist colloidal coagulation. A photograph of an electrodeposited SCG film on a gold electrode is shown in Figure 4 2B. In sharp contrast to the electrodeposited gibbsite films which are monolithic and crack free, cracks can be easily formed on the SCG film during the drying process after electrodeposition. We attribute the formation of c racks to the excess stress induced by the shrinkage of sol gel silica shell of SCG nanoplatelets. It is well known that the drying of Stber silica spheres during the convective self assembly process leads to cracks in the resulting colloidal crystal films ( 57, 58 ). To eliminate shrinkage induced cracks, one common approach is to sinter sol gel silica at high temperature (>500 C) ( 58 ). However, this approach cannot be used for SCG nanoplatelets as gibbsite cores will be thermally decomposed at ~300 C as add ressed in C hapter 2

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67 PEI SCG Colloidal Stability To resolve the cracking issue, we pursue a new approach by adding PEI to the electrophoretic bath in order to increase the adherence and strength of the electrodeposited films. PEI, which is a weak polyelect rolyte with the molecular structure shown in the inset of Figure 4 3 (pK a ~10.5), is positively charged under the electrophoretic conditions (pH ~6) due to the possessing of multiple amine functional groups. It also acts as a particle binder because it adsorbs strongly onto silica at a wide range of pH values ( 48 ). Therefore, p ositively charged PEI macromolecules are adsorbed on the negatively charged surface of SCG nanoplatelets via Coulombic attraction, forming positively charged PEI SCG nanoplatelets. However, adding a large amount of PEI leads to the flocculation of SCG nano platelets. To investigate the colloidal stability of PEI SCG nanoplatelets in suspensions as well as the reversal of surface charge, the influence of the PEI concentration is studied by measuring zeta potential of colloidal suspensions with different amou nt of PEI. The testing solution is prepared by mixing 4.5 g of 0.075 wt.% SCG dispersion in ethanol with 0.5 g of PEI aqueous solution at different concentration, C PEI (wt.%). The mixture is ultrasonicated for 30 min to minimize the agglomeration of SCG na noplatelets. The specific surface area of SCG nanoplatelets is calculated to be ~57 m 2 /g by using the geometry determined from experiments and the density of gibbsite, 2.2 g/cm 3 The amount of PEI addition is calculated as [(0.5 C PEI )/(4.5 0.075 57)] 10 3 mg/m 2 The zeta potential of PEI SCG nanoplatelets as a function of the amount of PEI addition is shown in Figure 4 3. At zero PEI addition, the zeta potential of SCG nanoplatelets is measured to be 38.1 1.6 mV, showing the surface of colloids ar e negatively charged

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68 and silica is coated on gibbsite cores. With the increasing of the amount of PEI, zeta potential of SCG nanoplatelets initially reduces and becomes less negative. Further addition of PEI triggers the reversal of zeta potential when the PEI addition is larger than 0.5 mg/m 2 A zeta potential plateau is reached at around +28 mV when 2 mg/m 2 of PEI is added. The formation of the plateau is caused by the saturation of adsorbed PEI macromolecules on SCG nanoplatelets. When a larger amount of PEI is added, PEI SCG nanoplatelets tend to flocculate and zeta potential decreases. The agglomeration of nanoplatelets at high PEI addition can be explained by depletion flocculation. At a high polymer concentration, the concentration gradient between th e inter particle gap and the remainder of the solution generates an osmotic pressure difference, forcing solvent flows out of the gap until particles flocculate ( 48 ). Flocculation of gibbsite nanoplatelets and the decreasing of zeta potential are observed when a high concentration of PEI (26 mg/m 2 ) is added to SCG dispersion. To get stable SCG nanoplatelets for EPD, the amount of PEI addition is thus controlled in the range of the zeta potential plateau at 1.54 mg/m 2 EPD of PEI SCG Nanoplatelets The cathod ic electrodeposition of PEI SCG nanoplatelets is performed using a parallel plate cell with electric field strength of 1100 V/m. The positively charged PEI SCG nanoplatelets are attracted toward the bottom ITO cathode by the electrical force. Gravity only plays a minor role as the sedimentation speed of PEI SCG particles is much slower than the electrophoretic mobility. Top view SEM images in Figure 4 4A and B show that most of the electrodeposited PEI SCG nanoplatelets are aligned parallel to the ITO elect rode surface, though a few nanoplatelets are found to orient perpendicularly as shown by the arrows in Figure 4 4B. Compared to electrodeposited

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69 films consisting of pure gibbsite nanoplatelets, the preferential alignment of PEI SCG nanoplatelets is slightl y deteriorated. The layered structure and the slight worsening of the oriented deposition of PEI SCG nanoplatelets are further confirmed by the cross sectional SEM images in Figure 4 4C and D and the XRD patterns in Figure 4 5. The diffraction peaks showin g (222), (400), (441), and (622) planes are from the ITO electrode. Other than ITO diffraction peaks, we observe mainly (002) and (004) peaks from gibbsite single crystals. As the crystallographic c axis of single crystalline gibbsite is normal to the plat elet surface, the (002) and (004) diffraction peaks are from PEI SCG nanoplatelets oriented parallel to the electrode surface ( 46 ). Low intensity diffraction peaks, such as those from the (023) and (024) lattice planes at 45.478 and 52.219 respectively, can also be seen from the XRD spectrum. This indicates that small amount of nanoplatelets are not aligned parallel to the electrode surface, agreeing with our SEM observation. Unlike gibbsite nanoplatelets that have positive charges on their surfaces (IE P ~10) and almost neutral edges (IEP ~7) under the electrophoretic conditions (pH ~7), PEI SCG nanoplatelets have positive charges on both surfaces and edges because of the uniform coverage of silica shell and adsorbed PEI macromolecules. The difference in the spatial distribution of surface charges distinguishes the resulting arrangement of electrodeposited PEI SCG nanoplatelets from that of gibbsite nanoplatelets. For the latter, the electrical force tends to re orient the nanoplatelets in the electrophor etic bath to face the cathode. The deposited gibbsite nanoplatelets are thus densely packed with their crystallographic c axis normal to the electrode surface. By contrast, no electric field induced re orientation is occurred for PEI SCG nanoplatelets beca use of the

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70 uniform distribution of surface charge. Nevertheless, most of the PEI SCG nanoplatelets are still preferentially aligned to the electrode surface because this orientation is more energetically favorable than the perpendicular one. Another reason for the imperfect alignment of PEI SCG nanoplatelets may come from the bridging flocculation mechanism, which allows the formation of polymer bridges between neighboring particles ( 48 ). Since the PEI macromolecules used are highly branched with high molec ular weight (Mw ~750,000) and large numbers of amine groups, they are easily attached to several SCG nanoplatelets. However, this attachment could be random and the nanoplatelets in the resulting aggregates might not be aligned. ETPTA PEI SCG Nanocomposite s After electrodeposition, polymer nanocomposites can be made by filling the interstitials between the PEI SCG nanoplatelets with photocurable monomers, followed by photopolymerization. A non volatile monomer, ETPTA, is chosen to form the ETPTA PEI SCG nan ocomposite. The monomer with 1 wt.% of photoinitiator is first added on a PEI SCG film on an ITO electrode and the sample is then kept under vacuum for a few hours to promote the monomer penetration. After the sample becomes transparent, it is spin coated at 4000 rpm for 1 min to remove the excess monomer. Exposure to ultraviolet radiation is then carried out to polymerize ETPTA monomer. The resulting ETPTA PEI SCG nanocomposite film is highly transparent (as in Figure 4 6) due to the matching of the refra ctive index between the PEI SCG nanoplatelets and the polymer matrix. The normal incidence transmission spectra in Figure 4 7 indicate that the ETPTA PEI SCG nanocomposite on an ITO electrode (ETPTA PEI SCG/ITO) exhibits high transmittance (>80%) for most of the visible wavelengths. This suggests that most interstitial spaces between the PEI SCG

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71 nanoplatelets have been infiltrated by the polymer. Free standing ETPTA PEI SCG nanocomposites can be obtained by soaking ETPTA PEI SCG/ITO in 1 M of sodium hydroxi de solution for several hours. Higher transmittance is achieved for the ETPTA PEI SCG nanocomposite due to the removal of the ITO electrode. Compared to the high transmittance of the ETPTA PEI SCG nanocomposite, the PEI SCG film on an ITO electrode (PEI SC G/ITO) (Figure 4 6B) shows a transmittance less than 10% for most of the visible spectrum. The cross sectional SEM images in Figure 4 8 show that the layered structure of the original PEI SCG film is retained and a thin wetting layer of ETPTA (~2.5 m thi ck) is observed on the surface of the nanocomposite. The red and black arrows in Figure 4 8A indicate the ETPTA wetting layer and the ITO electrode, respectively. The mechanical properties of the biomimetic polymer nanocomposites are evaluated by tensile t ests. The tensile strength for plain ETPTA, ETPTA gibbsite nanocomposite, and ETPTA PEI SCG nanocomposite are tested and the results are shown in Figure 4 9. Compared with pure ETPTA polymer, the ETPTA gibbsite nanocomposite shows ~2 time higher strength a nd ~3 time higher modulus. For the ETPTA PEI SCG nanocomposite, even higher tensile strength than that of the ETPTA Gibbsite film can be achieved. This is due to the presence of the PEI macromolecule, which acts as a binder by strongly adsorbing on the neg atively charged surface of SCG nanoplatelets via Coulombic attraction. Its highly branched molecular structure also enables the interlock with cross linked ETPTA backbone. Early studies reveal that the interfacial bonding between the ceramic fillers and th e organic matrix is crucial in determining the ( 8, 9 ). The strong ionic

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72 bonding between the PEI macromolecules and the SCG nanoplatelets along with the natural elasticity of PEI macromolecules make the ETPTA PEI SCG nanocomposites have 3 to 5 time higher strain than those of pure ETPTA and ETPTA Gibbsite nanocomposites. The higher strain could also come from the surface roughness of PEI SCG nanoplatelets due to the silica coating and the rotation of misaligned SCG nanoplatelets under an applied tensile load. A rough estimation based on the area under the tensile stress strain curve indicates that the energy needed to rupture the ETPTA PEI SCG nanocomposite is nearly one order of magnitude and 6 time higher than those required to break pure ETPTA polymer and ETPTA Gibbsite nanocomposite, respectively. A simple calculation based on the shear lag model as in Chapter 2 is carried out to validate the measured mechanical properties. Since the volume of the adsorbed PEI on the SCG nanoplatelets is quite small, we can simply use the volume fraction of ETPTA to calculate tensile strength of the nanocomposite. From our previous TGA of ETPTA gibbsite nanocomposites prepared by the same spin coating technique as reported here, the volume fraction of ETPTA in the polymer nanocomposite is ~0.50. Therefore the tensile strength of the nanocomposite can be estimate to be about 2.75 m agreeing with our experimental results. Summary In summary, we have developed a simpl e and scalable EPD technology for assembling surface roughened inorganic nanoplatelets into organized multilayer. The adsorption of polyelectrolyte macromolecules on the surface of nanoplatelets can reverse the surface charge and simultaneously eliminate t he cracks induced by the shrinkage of the sol gel silica shell of the surface roughened nanoplatelets during

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73 drying. We expect this approach could be applicable to the convective self assembly of spherical colloidal silica particles to facilitate the forma tion of crack free colloidal photonic crystals.

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74 Figure 4 1. Cross section of abalone nacre showing the detailed structure at the lamellae boundaries. Arrows highlight locations where the nano asperities interpose ( 56 ). Figure 4 2. (A) TEM image of acid leached SCG nanoplatelets. The arrows point to a silica shell with a thickness of ca. 10nm. (B) Photograph of an electrodeposited SCG film on a gold electrode.

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75 Figure 4 3. Zeta potential of PEI SCG nanoplatelets wi th different amount of PEI addition. The inset shows the molecular structure of PEI.

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76 Figure 4 4. SEM images of electrodeposited PEI SCG nanocomposite. (A) Top view image. (B) Magnified top view image. (C) Cross sectional image. (D) Magnified cross sectional image.

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77 Figure 4 5. XRD patterns of a PEI SCG nanocomposite on an ITO electrode. Blue arrows point to the characteristic peaks of ITO. The inset shows a table with major lattice planes of gibbsite.

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78 Figure 4 6. Photographs of (A) ETPTA PEI SCG and PEI SCG deposits on ITO electrodes. Figure 4 7. Normal incidence transmission spectra of ETPTA PEI SCG nanocomposite, ETPTA PEI SCG nanocomposite on an ITO electrode, and PEI SCG depos it on an ITO electrode.

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79 Figure 4 8. SEM images of an ETPTA PEI SCG nanocomposite on an ITO electrode. (A) Cross sectional image. (B) Magnified cross sectional image. Red and black arrows in (A) point to a thin wetting layer of ETPTA and the ITO elect rode, respectively. Figure 4 9. Tensile stress vs. strain curves for plain ETPTA film, ETPTA Gibbsite nanocomposite, and ETPTA PEI SCG nanocomposite.

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80 CHAPTER 5 ELECTROCHEMICAL SERS AT PERIODIC METALLIC NA NOPYRAMID ARRAYS Background Surface Plasmon Surface plasmons ( SPs ) are electromagnetic waves that propagate along a metal/dielectric interface ( 61 ) These are incident light waves that are trapped on the interface because of their interaction with free electrons in metal, leading to a strong concentration of electromagnetic energy at the interface. In this interaction, the free electrons oscillate in response to the incident light waves as shown in Figure 5 1 The resonant interaction between the surface charge oscillation and the electromagnetic field of the light forms the SP and gives rise to its unique properties. T ypical metals used to support surface plasmons are gold and silver ( 62, 63 ) but metals such as copper ( 64 ) titanium ( 65 ) or chromium ( 66, 67 ) can also sustain surface plasmon generation. There are two consequences of the coupling to surface plasmons. One is that in contrast to the propagation of SPs along the interface, the field normal to the metal surface decays exponentially from the interface, resulting in non radiative nature of SPs and preventing power from losing The other is that the interaction between the free electron and the electromagnetic field results in the momentum of SP, sp being greater than that of incident photons, 0 This can be obtained by solving Maxwell s equations under appropriate boundary conditions and gives

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81 where d and m are the permittivity of the metal and the dielectric material, respectively. The difference in momentums of incident light and SP s must be compensated in order to generate SPs. Typically there are three ways to bridge the momentums. The first one requires the use of prism to enhance the momentum of the incident light. T he second one exploits surface defects to scatter incident light. The third one is to generate periodically patterned nanostructures, such as sub wavelength nanohole arrays. Extraordinary Op tical Transmission One of the important applications of SP is the extraordinary optical transmission through sub wavelength hole arrays ( 68 71 ), as shown in Figure 5 2 The desire to control photons in a manner analogous to the control of electrons has made SP significant. According to the standard aperture theory, the transmission of a sub wavelength aperture is extremely low and proportional to the fourth power of the r atio of its diameter and light wavelength while apertures are smaller than the wavelength of the incident photon, resulting in a main constraint in manipulation of light ( 72 ) However, it has been found that arrays of films perforated with periodic sub wav elength holes allow unusually high transmission of light at wavelengths larger than the array period due to surface plasmon resonance ( SPR ) showing extraordinary optical transmission. The transmission of light through sub wavelength hole arrays made in a metal film can be orders of magnitude larger than expected from standard aperture theory Experiments have provided evidence that these unusual optical properties are as a result of the coupling of light with S P s on the surface of the periodically patterned metal film Considerable interest in the optical properties of the periodic arrays of sub wavelength apertures in metallic thin films has raised due to their potential numerous applications in photonic circui ts, light manipulation, sub wavelength photolithography

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82 and optical modulators. In the past, absorption of SP s by metal was a significant problem that SP s were not considered for photonic elements because the SP propagation length was smaller than the size of components. It has been demonstrated that coherent spatial SP propagation lengths are a few m and ultrafast decay of the SP polarization occurs on a 10 fs time scale ( 73 ) However, this view has been changed due to advances in nanotechnology. Recent S P based components are significantly smaller than the propagation length. This opens up the way to integrate several SP based devices into circuits before propagation losses. Typically focused ion beam (FIB) milling is used to fabricate sub wavelength hole s ( 74 76 ) but it is a time consuming expansive and low throughput process and the size of substrate is limited. Surface enhanced Raman Scattering Since SP waves travel on the boundary of the metal and the external medium, the adsorption of molecules to th e metal surface greatly change s the oscillation and therefore can be also used as sensors SPR technique is of great importance for monitoring binding events in biological systems. Typically, reflection geometry (Kretschmann configuration) is required to e xcite SP through prism coupling. SPR techniques provide good sensitivity at the submonolayer level. However, because the principle of operation establishes on the oscillation change of SP resonances, specificity at the molecular level is poor. SP enhanced spectroscopic methods therefore become a powerful tool for chemical and biochemical analysis, providing better molecular specificity. SERS is the most commonly used SP mediated molecular spectroscopic method as shown in Figure 5 3 ( 77 ) The enhanced Raman signal provides a molecular fingerprint due to its narrower bandwidth. SERS is a noninvasive technique that enables

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83 the detection and characterization of both small organic and big biological molecules at very low concentrations, or even at the single mol ecule level ( 78 81 ) This opens up exciting new opportunities for the sensitive and selective detection of analytes that are commonly encountered in the analysis of chemical warfare agents, biological products, food regulation, water quality control, and e nvironmental monitoring. Electrochemical SERS is an important branch of SERS studies and has attracted great scientific and technological interest as it enables in situ investigation of adsorption and reaction at electrochemical interfaces, promising for d eveloping fundamental understanding and control of fuel cells, metal corrosion, semiconductor processing, electrocatalysis processes and electroanalysis ( 82 85 ) Electrochemically roughened metal surfaces have been extensively exploited as electrodes for electrochemical SERS ( 82, 86 89 ) However, the relatively low SERS enhancement (on the order of 10 4 ), the poor reproducibility of SERS enhancement (intensity variation by a factor of 10 across a sample surface), and the electrochemical instability at high cathodic potentials are major drawbacks for these roughened electrodes. Therefore, how to generate reproducible SERS substrates that provide high enhancement factor is of great importance. Substrates for Surface enhanced Raman Scattering B ottom up colloid al self assembly and templating nanofabrication provide an inexpensive and simple to implement alternative to the electrochemical roughening process in creating nanostructured SERS electrodes ( 90 97 ) M etal film over nanosphere (MFON) electrodes prepared b y vapor deposition of a SERS active metal (Au or Ag) over a self assembled nanosphere monolayer have been demonstrated to exhibit improved stability and reproducibility for electrochemical SERS experiments

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84 ( 98 ) Rapid detection of an Anthrax biomarker was achieved using SERS on silver film over nanosphere (AgFON) substrates ( 99 ) Atomic layer deposition (ALD) is used to deposit a sub 1 nm alumina layer on AgFON substrates to improve Anthrax biomarker detection ( 100 ) In comparison to the bare AgFON substrat es, the ALD modified AgFON substrates show higher sensitive and better stability. Sculpted electrochemical SERS active electrodes with regular hexagonal arrays of sphere segment nanovoids, which show reproducible and high (1.5 10 5 ) surface enhancement, have been replicated from colloidal crystal templates via electrodeposition of coinage metals in particle interstitials ( 101 ) Unfortunately, most of the current bottom up approaches suffer from low throughput and incompatibility with standard microfabrica tion thereby impeding the cost efficiency and scale up of these unconventional methodologies in generating SERS active electrodes. Inspired by tip enhanced Raman scattering (TERS) ( 92, 102 104 ) we have recently developed a simple yet scalable colloidal templating technique for producing wafer scale gold nanopyramid arrays with nanoscale tips and high tip density (6 10 8 tips cm 2 ) ( 105 ) These periodic arrays of nanopyramids can enhance the local electromagnetic field in the vicinity of the sharp nanoti ps, resulting in strong surface enhancement for Raman scattering from benzenethiol molecules absorbed on the gold surfaces. Here we demonstrate that these templated nanopyramid arrays can be utilized as electrodes for achieving high SERS enhancement. The r esulting SERS intensity can be adjusted by tuning the applied electrode potential and the electrochemical reactions on the electrode.

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85 Experimental Preparation of Electrochemical SERS active Gold Nanopyramid Arrays The synthesis and purification of monodisp ersed silica microspheres with 320 nm diameter in 200 proof ethanol were performed according to a reference ( 106 ). Detailed procedures are as below: 1. Clean all glassware as the procedures in Chapter 2. 2. Put a stirring bar in a 1000 ml flask. 3. Enclose the flask with a septum and weight the flask 4. Enclose a bottle of EtOH with a septum. 5. Draw EtOH from the bottle with a 60 ml syringe. 6. Put a 0.2 um PTFE hydrophobic filter on the syringe and inject EtOH into the flask. 7. Repeat 4~5 until EtOH reac hes 665 ml (525g, density = 0.789). 8. Take off the filter and draw sufficient amount of DIW. 9. Put a 0.2 um hydrophilic filter on the syringe and inject 55.7g of DIW into the flask. 10. Take off the hydrophilic filter and draw sufficient amount of ammoni a hydroxide. 11. Put the hydrophilic filter on and inject 25.7ml (23.1g, density = 0.8988) of ammonia hydroxide. 12. Strongly stir on hot plate and then inject 50 ml of DISTILLATED TEOS as quickly as possible and vigorously shake the flask. 13. Aging for 8 hours on a stirring plate at a stirring rate around 7~8. The purified silica colloids were concentrated by centrifugation and redispersed in ETPTA using a vortex mixer (Fisher). To this 1 wt.% Darocur 1173 was added as photoinitiator. The final particle v olume fraction was adjusted to ~20%. The colloidal suspension was dispensed on a silicon wafer (testgrade, n type, (100), Wafernet) which had been primed by 3 acryloxypropyl trichlorosilane (Gelest). The established spin

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86 coating process was then utilized t o generate monolayer colloidal crystal embedded in ETPTA monomer using a standard spin coater (WS 400B 6NPP Lite Spin Processor, Laurell). The ETPTA monomer was photopolymerized for 4 s using a pulsed UV curing system (RC 742, Xenon). The polymerized ETPTA matrix was then removed by oxygen plasma etching operated at 40 mTorr pressure, 40 sccm flow rate, and 100 W for 2 min on a Unaxis Shuttlelock RIE/ICP reactive ion etcher. The released silica particles were utilized as shadow masks during electron beam de position of 30 nm thick chromium using a Denton DV 502A EB evaporator with a typical deposition rate of 2 /s. The templating silica particles could then be removed by rubbing the wafer with a cleanroom Q tip under flowing deionized water, resulting in the formation of chromium nanohole arrays on the (100) silicon wafer. The wafer was wet etched at 60C for 4 min in a freshly prepared solution containing 62.5 g of KOH, 50 ml of anhydrous 2 propanol, and 200 ml of ultrapure water to create inverted pyramids in silicon. After dissolving the chromium layer in CR 7 etchant (Transene), 500 nm thick of gold was deposited on the silicon template at a deposition rate of ~5 /s with a Kurt J. Lesker CMS 18 Multitarget S putter. The gold layer could finally be peeled of from the wafer surface with a conductive double sided carbon disk (SPI Supplies), yielding an electrochemical SERS active nanopyramid array in gold. The templated gold nanopyramid arrays were examined using a SEM prior to and after the electrochemical SERS experiments. Electrochemical Surface enhanced Raman Scattering The electrochemical setup used to conduct the electrochemical SERS experiments was constructed as shown in Figure 5 4 A glass slide (Corning, 2.5 4.0 cm) was used as the substrate. On top of the slide were a conducting copper tape (3M, 1.2 4.0 cm) and then the conductive carbon disk (diameter of 1.2 cm) with the

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87 templated gold nanopyramid array on its top side. Insu lating tape obtained from Furan Co. was used to cover the rest of the copper tape. Platinum wire purchased from Sigma Aldrich was used as the counter electrode. An aqueous solution consisting of 0.1 M NaCl and 0.05 M pyridine was used as the electrolyte. A flat gold film deposited by the same sputtering process as described above was used as the control sample for SERS measurements. The voltage (Au vs. Pt electrodes) was controlled by an EG&G Model 273A potentiostat (Princeton Applied Research). All Raman s pectra were recorded on a Renishaw Raman microscope using a 785 nm diode laser at 48 W with an integration time of 10 s. Cyclic Voltammetry Measurements Two electrode cyclic voltammetry was used to characterize electrodes in 0.1 M NaCl solution with or w ithout 0.05 M pyridine, including electrodes that had only conductive carbon or copper tape on the glass slide and electrodes that had a gold nanopyramid array on carbon tape with or without copper tape between the carbon tape and the glass slide substrate The active area of each electrode was controlled at 1 cm 2 Platinum wire was used as both counter and reference electrodes. The voltage was scanned between 1.0 and 1.0 V with a scan rate of 50 mV/s by using the EG&G potentiostat. Electromagnetic Modelin g of Raman Enhancement In the finite element method (FEM) model, the gold nanopyramid array was supposed to be placed horizontally so that the interface between the substrate and the medium (water) was parallel to the xz plane while the nanopyramids were a long the y axis. FEM was employed under a COMSOL Multiphysics environment to obtain

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88 to obtain high resolution numerical solutions, the computational domain needs to be bounded and the boundary conditions should be well ( 47 ). Artificially constructed 10 boundary layers were around the medium (water) and the scatter (gold) domains. The electric and magnetic conductivities of each boundary layer could be set artificially so that little or no electromagnetic radiation would be reflected back into the domain of scatter. TO simulate electromagnetic fields in the newly aug boundaries of the PML layers, a low reflection boundary condition ( 47 ) was provided to minimize residual reflection and attenuate the wave quickly within the layers. After solv dimensional electric field could be used to calculate the Raman enhancement factor as where E ( x,y ) was the electric field amplitude at location ( x,y ) and E 0 was the incident electric field amplitude ( 107 ). The maximum value of the Raman enhancement could be obtained over the medium (water) domain. Results and Discussion Colloidal Templating Process for Nanopyramid Array Fabrication The schematic illustration of the c olloidal templating process for fabricating gold nanopyramid array electrodes is shown in Figure 5 5 The established spin coating technique is first applied to shear align submicrometer sized silica particles into ordered colloidal monolayers. In contrast with previous colloidal self assembly approaches, spin coating enables rapid production of colloidal crystal templates with wafer scale area (up

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89 to 8 in. diameter). Though the particles are not touching each other, they do exhibit long range hexagonal ord ering. After removing the polymer matrix surrounding silica particles by brief oxygen plasma etch process, the nontouching silica particles can be used as shadow masks during physical vapor deposition of chromium to create periodic nanohole arrays, which a re then utilized as etching masks to make inverted silicon pyramidal pits by anisotropic KOH wet etch. Wafer scale gold nanopyramid arrays with sharp tips can finally be replicated by sputtering a thin layer of gold on the silicon templates, followed by a simple adhesive peeling process. By simply controlling the size of the templating silica spheres and the anisotropic wet etch conditions (e.g., temperature, duration, and etchant concentration), the dimensions of the templated nanopyramids, such as base le ngth, depth, a nd separation, can be adjusted. Figure 5 6 A shows a tilted SEM image of an array of gold nanopyramids templated from 320 nm silica spheres. The long range hexagonal ordering of nanopyramids is clearly evident from the image. Magnified SEM ima ges show that most of the pyramidal tips have a radius of curvature of r < 10 nm. Electrochemical SERS Spectra of Pyridine on Gold Nanopyramid Arrays The electrochemical SERS measurements are carried out using a 0.05 M pyridine aqueous solution with 0.1 M NaCl as a background electrolyte. Figure 5 7 shows a comparison of SER spectra obtained at 1.0 V on the gold nanopyramid array electrode and a flat g old control electrode prepared by the same sputtering process. The nanopyramid electrode exhibits a strong Raman scattering signal, while the featureless gold control sample does not show distinctive SERS peaks at the same experimental conditions. The cont rol sample has been p repared in the same sputtering batch as the

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90 nanopyramid electrode and therefore has a similar surface roughness. The peak positions and the relative amplitude of the peaks obtained at the nanopyramid electrodes agree well with those in the literature for pyridine adsorbed on roughened gold disk electrodes ( 108, 109 ) but are significantly different from those obtained at sculpted gold nanovoid array electrodes ( 101 ) The assignment of the spectral peaks is shown in Table 5 1 ( 110 ) From Table 5 1, it is clear that almost all the enhanced vibrational modes are associated with the in plane perturbations, indicating that the adsorbed pyridine molecules are bonded perpendicular to the metal surface via their nitrogen lone pairs ( 97, 98, 108 ) Another evidence of the end on configuration of the adsorbed molecules c omes from the two peaks at 1013 and 1037 cm 1 which correspond to the ring breathing mode and the ring mode ( 12 ) and occur at frequencies close to those obtained for pyridine in solution ( 87, 108 ) By contrast, for flat adsorbed pyridine molecules, the frequencies of the ring modes are expected to decrease when due to the interaction of the elect rons of the ring with the electrode surface ( 111 ) Figure 5 8 shows the SER spectra recorded for adsorbed pyridine as a function of electrode potential applied on the gold nanopyramid array electrode (vs a platinum counter electrode). It is clearly evident that stronger SERS enhancement occurs at higher negative potentials when the potential is swept from +1.0 to 1.0 V. Similar SERS intensity dependence on the applied electrode potential has previously been reported on Au(210) single crystal electrodes ( 108 ) and AgFON electro des ( 98 ) The maximum surface enhancement factor at 1.0 V is estimated to be 2.7 10 6 using the method described in the literature by comparing the Raman intensity for the peak at

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91 1013 cm 1 obtained for a solution and at the nanopyramid electrode and assuming a surface coverage of 0.40 nmol cm 2 for pyridine on gold and a surface roughness of 3.0 ( 84 ) This enhancement is more than 1 order of magnitude higher than that obtained at other nanostructured electrodes ( 98, 101 ) Figure 5 9 shows the SER spec tra of pyridine adsorbed on a gold nanopyramid electrode when the potential is swept from 1.0 V (top spectrum) to +1.0 V (middle spectrum) and then back to 1.0 V (bottom spectrum). The peak amplitude is greatly reduced when the potential is swept from 1 .0 to +1.0 V and the 1013 cm 1 peak is shifted to 1018 cm 1 When the potential is cycled from +1.0 V back to 1.0 V, the SERS signal is even stronger than the original spectrum obtained at 1.0 V and the peak at 1013 cm 1 reaches the detection limit of the Raman spectrometer. Further potential cycling experiments show that the high SERS enhancement at 1.0 V can be consistently achieved for at least five cycles and then starts to decrease for more sweeps. Electrode Effects The experimental results shown in Figures 5 8 and 5 9 are contradictory to those obtained at sculpted nanovoid arrays, where higher SERS intensity is observed at more positive potentials ( 101 ) To help understand this contradiction, we conducted two elect rode cyclic voltammetry measurements to evaluate potential redox reactions on nanopyramid electrodes in 0.1 M NaCl solution with or without 0.05 M pyridine. As shown by the dashed curve in Figure 5 10 the nanopyramid electrode that consists of a gold nano pyramid array on an adhesive carbon disk and a conductive copper tape exhibits apparent redox activities when the electrode potential is cycled between 1.0 and +1.0 V. This is caused by the electrochemical reactions on the conductive copper

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92 tape which is used as a conducting wire to connect the gold nanopyramid array to the potentiostat and is partially exposed to the electrolyte solution. A similar cyclic voltammogram is obtained when pure copper tape is used as the electrode as shown by the thin solid cu rve in Figure 5 10 Since the applied cyclic electrode potentials are below the electrolytic potential of water ( 1.23 V) ( 112 ) we believe that the anodic reaction on the conductive copper tape is Cu Cu 2+ + 2e By contrast, when pure conducting carbon tape ( thick solid curve) and gold pyramid array on carbon tape ( dotted curve) are used as electrodes, no apparent redox reactions are observed. Similar cyclic voltammetry results are obtained when the electrolyte solution contains 0.1 M NaCl and 0.05 M pyr idine. We speculate that the electrochemical reactions on the conductive copper tape are responsible for the observed SERS intensity electrode potential contradiction between the templated nanopyramid array and sculpted nanovoid array electrodes. It is wel l known that pyridine can easily conjugate with Cu 2+ ions to form a positively charged complex, [Cu(py) 4 ] 2+ ( 107 ), which can be electrophoretically attracted by the cathode, while being repelled from the anode. This could lead to a higher concentration of pyridine on the cathode surface and therefore results in higher SERS intensity at more negative potentials. To verify this speculation, we conducted the same electrochemical SERS experiments with a gold nanopyramid array supported only by an electrochemica lly inert conductive carbon tape (see the cyclic voltammetry results in Figure 5 10 ). The experimental results as shown in Figure 5 1 1 exhibit the same SERS intensity electrode potential relationship as observed on sculpted nanovoid array electrodes (i.e., higher SERS intensity occurs at more positive potentials). The relatively

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93 low SERS enhancement could be due to the reduced sharpness of the nanotips of the nanopyramid array which was templated from an inverted silicon mold that had been used multiple tim es. Electromagnetic Modeling We believe the electromagnetic enhancement caused by the significant concentration of the electromagnetic field in the vicinity of the sharp nanotips is the dominating mechanism for the observed SERS enhancement at nanopyramid electrodes. To verify this hypothesis, we conduct finite element electromagnetic modeling using COMSOL Multiphysics software to calculate the electric field amplitude distribution and the corresponding Raman enhancement factors surrounding arrays of nanopy ramids ( 113 ). Since the periodic nanostructure is symmetric, it is reasonable to construct a simplified two dimensional (2 D) model which can be considered as sections through a three dimensional nanopyramid array at the point of maximum enhancement (Figur e 5 1 2 ). To numerically solve the 2 ( 114 ). The widely used optical constants for gold ( 115 ) are employed to conduct the electromagnetic modeling, and the surrounding medium is water. Figure 5 1 2 A shows the simulated distribution of the SERS enhancement factor around two adjacent nanopyramids with base length of 320 nm, interpyramid distance of 1.414 320 nm ( 106 ), and nanotip radius of curvature of 5 nm. The height of nanopyramids is determined by the base length as wet etched silicon pyramids have characteristic 54.7 side walls ( 116 ). The simulation results show that the significant enhancement of the electromagnetic field and the maximum SERS enhanc ement (10 4.7 ) happen at the vertices of the nanotips, and are favorably comparable to other numerical

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94 simulations for nanotips and nanorings ( 92, 117, 118 ). The spatial distribution of the s is caused by the electromagnetic interaction between neighboring nanotips. Figure 5 1 2 B shows that larger arrays with more nanotips but the same structural parameters result in higher enhancement and the maximal enhancement factor reaches a plateau (G max ~10 7.5 ) when the array has more than 12 tips. This indicates that the electromagnetic coupling between adjacent scatters played a critical role in determining the electric field amplitude distribution and the corresponding Raman enhancement factors surrou nding arrays of nanopyramids. Indeed, the calculated G max at the nanotip apex could be even higher if the sharp edges and facets of the nanopyramids are considered in a more realistic three dimensional (3 D) model instead of the current 2 D model. The very small effective area occupied by the sharp nanotips (electromagnetic hot spots) could be the reason for the significant difference between the simulated G max and the experimental enhancement factor. A recent experimental study shows that a very small perc entage of molecules (0.0063%) in the hottest spots contribute 24% to the overall SERS intensity ( 119 ). We believe that the surface roughness of the templated nanopyramid electrodes plays only a minor role in the observed SERS enhancement. Indeed, the SEM i mage in Figure 5 6 B shows that the surface roughness of the nanopyramid electrode does not change much after the electrochemical SERS experiment. Summary In summary we have developed a bottom up approach for fabricating periodic arrays of gold nanopyramids with nanoscale sharp tips. These nanotips can significantly enhance the local electromagnetic field at the tip apex, resulting in more than 1 order of magnitude higher SERS enhancement than other nanostructured electrodes. We have

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95 also found t hat the redox reactions occurring near the nanopyramid electrode play a crucial role in determining the dependence of SERS enhancement on the applied electrode potential. The current templating technology is scalable and compatible with standard microfabri cation, enabling large scale production of SERS active electrodes for in situ electrochemical studies and sensitive electroanalysis.

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96 Table 5 1. Assignment of SERS peaks for pyridine adsorbed on gold nanopyramid electrode Label SERS peak (cm 1 ) Vibration mode Vibration type a 634 6a Symmetric b 650 6b Asymmetric c 699 d 1013 1 Ring breathing e 1037 12 C H in plane deformation f 1068 18a C H in plane deformation g 1216 9a C H in plane deformation h 1600 8 Ring stretching

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97 Figure 5 1. Surface plasmons propagate along a metal/dielectric interface ( 61 ). Figure 5 2. Extraordinary transmittance at normal incidence for a square array of holes. The area covered by holes is only 11% while the normalized to area transmittance of lights is 130% ( 69 ).

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98 Figure 5 3. Schematic SERS process in which light is Raman scattered by a molecule on the surface ( 77 ). Figure 5 4. Schematic illustration of electrochemical SERS set up.

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99 Figure 5 5. Schematic illustration of the templating procedures for fabricating gold nanopyramid array by using spin coated monolayer colloidal crystal as template ( 105 ).

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100 Figure 5 6 Tilted (35 ) SEM images of a gold nanopyramid array electrode prior to (A) and after (B) el ectrochemical SERS experiments. As templates, 320 nm silica spheres were uses.

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101 Figure 5 7 Electrochemical SER spectra recorded on a gold nanopyramid array supported by a conductive carbon disk and a copper tape (red) and a fla t gold control sample on silicon (black) in 0.1 M NaCl solution containing 0.05 M pyridine. Figure 5 8 Electrochemical SER spectra recorded on a gold nanopyramid array supported by a conductive carbon disk and a copper tape in 0.1 M NaCl solution containing 0.05 M pyridine.

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102 Figure 5 9 The gold electrode potential was swept from 1.0 V (top) to +1.0 V (middle) and then back to 1.0 V (bottom). Figure 5 10 Cyclic voltammog rams of a conductive carbon tape, a conductive copper tape, a gold nanopyramid array supported by a carbon tape, and a gold nanopyramid array supported by a carbon disk and a copper tape in 0.1 M NaCl

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103 Figure 5 1 1 Electrochemi cal SER spectra obtained on a gold nanopyramid array supported by a conductive carbon tape in 0.1 M NaCl solution containing 0.05 M pyridine. The gold electrode potential was swept from 1.0 to 0.2 V. The spectra were taken using a 785 nm diode laser at 48 W with an integration time of 10 s.

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104 Figure 5 1 2 (A) Modeled Raman enhancement factor around two gold nanopyramids with base length of 320 nm and nanotips radius of curvature of 5 nm at = 785 nm. (B) Simulated maximum SERS enhancement factor (G max ) vs. number of tips of the templated nanopyramid array with the same structural parameters as (A).

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105 CHAPTER 6 GRAPHENE PAPER ACTUA TORS Background Graphene, a two dimensional honeycomb building block for graphitic materials of other dimensionalit ies ( 120, 121 ), has been extensively studied due to its extraordinary qualities in electrical, thermal, and mechanical aspects that provide itself opportunities in fields such as electronics ( 122, 123 ), composites ( 124 126 ), sensors ( 127, 128 ) and capacito rs ( 129 ). It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite, as shown in Figure 6 1 ( 121 ). Besides these fields, graphene is also a strong candidate in making actuators that can convert electrical energy into mec hanical energy because of its high surface area for double layer charging and the above excellent properties. The direct conversion of energy through materials is of great importance for applications in robotics, prosthetic devices, optical displays as wel l as micro pumps ( 130 ). As an actuator material, graphene has the following advantages: it is electrically conductive with high electron mobility of 15,000 cm 2 V s ; it has high intrinsic stress of 130 GPa and it is light weight ( 121, 131 ). All the extra ordinary properties mentioned above make graphene attractive for use in making actuators. Electromechanical actuators based on sheets of single walled carbon nanotubes were reported ( 132 ), showing stresses higher than natural muscle and strains higher than ferroelectric materials. The quantum chemical based expansion without ion intercalation is believed as the mechanism that causes actuation. Actuation of graphite oxide (GO)/graphene bilayer papers were also reported ( 133 ). Instead of driving by an applied voltage as the carbon nanotube actuator, the bilayer actuator was driven by variation of humidity and temperature and therefore has poor controllability. Here we

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106 report electromechanical actuators based on two strips of graphene papers with an intermediat e dielectric layer. Graphene papers were produced by flow directed assembly ( 134 ) of one atom thick graphene sheets, which were obtained from chemically reduced GO sheets. GO sheets were generated by exfoliation of highly oxygenated graphite in water. The actuators were operated under repeated potential steps and cyclic voltammetry. Cycling stability of the graphene based actuators was also explored. Experimental Materials and Methods Graphene papers Graphene papers were basically made by following a proced ure presented by Li and colleagues ( 135, 136 ). Schematic illustrations of graphene paper preparation are shown in Figure 6 2, 6 3, and 6 4. Simply put, g raphite oxide was first synthesized from natural graphite by a modified Hummers method that consists of an additional oxidation prior to the typical Hummers method ( 137, 138 ). As synthesized graphite oxide was subjected to multiple cycles of centrifugation and re dispersion until no supernatant formed after centrifugation and then suspended in ultrapure wat er (18.2 M cm 1 Barnstead water system) to give a viscous, inhomogeneous, brown suspension with a concentration of 0.6 wt.% and then stored. To prepare colloidal dispersion of GO, the stored graphite oxide suspension was diluted with ultrapure water to 0.05 wt.%, followed by exfoliation carried out with ultrasonication for 60 min. The dispersion was then subjected to centrifugation at 3,000 r.p.m. for 30 min to remove unexfoliated graphite oxide and subsequent dialysis for several hours to remove residua l salts and acids, resulting in a clear, homogeneous and brownish dispersion as shown in Figure 6 5 A.

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107 Chemical reduction ( 136 ) of GO to graphene was conducted by mixing 3 ml of GO dispersion with 3 ml of ultrapure water, 21 l of ammonia solution (14.8 N, Fisher Chemical) and 3 l of hydrazine (35 wt.% solution in water, Sigma Aldrich) in a glass vial with a hot water bath for 1 h to give a homogeneous and black dispersion as shown in Figure 6 5 B. The graphene paper was then made by vacuum filtration of the graphene dispersion through an Anodisc membrane (25 mm in diameter, 0.02 m pore size, Whatman) ( 134 ). The resulting deposit was then air dried and peeled from the membrane to give a free standing graphene paper. The graphene paper was cut into 2 mm by 15 mm strips by a razor blade for mechanical testing and making actuators. Detailed procedures for making graphene dispersions are listed as below: 2) 1. Grind graphite flakes into powder. 2. Put graphite powder (1g) into an 80C solution of concentrated H 2 SO 4 (1.5 ml), K 2 S 2 O8 (0.5 g) and P 2 O 5 (0.5 g). Use a 250 ml flask. No stirring bar required. 3. Thermally isolate and allow the dark blue mixture to cool to room temperature over a period of 6 h 4. Carefully dilute the mixture with DIW. 5. Filter and wash the mixture with vacuum filtration until the rinse water pH becomes neutral. 6. Dry the product in air at ambient temperature overnight. 3) 7. Put this pre oxidi zed graphite into concentrated H 2 SO 4 (23 ml) that have been cooled to 0C in an ice bath with vigorous stirring. 8. Gradually add KMnO 4 (3 g) so that the temperature of the mixture is not allowed to reach 20C. 9. Remove the ice bath, bring the temperature to 35C and stir for 2 h. (As the reaction progressed, the mixture gradually thickened with a diminishing in effervescence. The mixture was brownish grey in color at the end of the reaction.)

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108 10. Slowly stir DIW (46 ml) into the paste. (Violent effervesce nce occurs and temperature increases to 98C.) 11. Keep the diluted brown suspension at this temperature for 15 minutes. 12. Further dilute the suspension with WARM DIW (140 ml). 13. Slowly treat the suspension with 30% H 2 O 2 (2.5 ml) to reduce the residua l permanganate and manganese dioxide to colorless soluble manganese sulfate. Upon treatment with the peroxide, the suspension turned bright yellow. 14. Filter and wash the suspension with WARM 1:10 HCl solution (250 ml) to remove metal ions, resulting in a yellowish brown filter cake. (The filtering has to be conducted while the suspension was still warm to avoid precipitation of slightly soluble salt of mellitic acid formed as a side reaction.) 15. Wash the filter cake with DIW by multiple centrifugation/r e dispersion steps until no supernatant forms after centrifugation. 16. Suspend the GO product in DIW to give a viscous, brown dispersion, which is stable for a period of years. Exfoliation and reduction of GO (as in Figure 6 4) 17. Exfoliation can be achi eved by dilution of the GO dispersion with DIW, followed by sonication and dialysis. 18. Prepare GO dispersion (0.05 wt. %) by sonication and use dialysis to completely remove metal ions and acids. (For 30 ml 0.05 wt. % dispersion, take 2.5 ml of 6 mg/ml s olution and then dilute to 30 ml.) 19. Mix the above dispersion (3 ml), DIW (3 ml) and 28 wt. % NH4OH (21 l) into a 20 ml glass vial. 20. Add 35 wt. % hydrazine (3 l) into the vial and shake vigorously. 21. Put the vial in a 95C water bath for 1h. Graphe ne actuators Graphene actuators were made in a relatively simple way as presented by Baughman and colleagues ( 132 ). Schematic illustrations of an actuator and apparatus used are shown in Figure 6 11 The actuator was made by laminating two strips of graphene papers with an intermediate larger of strip of Scotch Double Stick Tape (1 mm

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109 wider and longer than graphene strips), resulting in a sandwich structure as shown in Figure 6 11 A and B. Two pieces of Au/Cr/glass electrodes, made by sputter deposition of 20 nm of Cr and 200 nm of Au, were then attached on opposite sides of the upper end of the actuator with a clamp, facing to the graphene strips with the Au surface but not touching each other. A volt age could therefore be applied across the two Au electrodes without breaking the graphene strips. Actuation was carried out by immersing 10 mm of the actuator in 1 M NaCl solution held in a homemade rectangular glass tank as shown in Figure 6 11 C. A graph paper was attached to one side of the glass tank as background for displacement measurement, which was done with the aid of videotaping the motion of the actuator under applied voltages. Displacements of the actuator tip were then calculated via the graph paper. Results and Discussion GO and Graphene Dispersions Colloidal dispersions of GO were prepared by exfoliation of graphite oxide synthesized by a modified Hummers method, resulting in a homogeneous and brownish dispersion as shown in Figure 6 5 A. Z eta potential of the GO sheets in deionized water was measured to be 42.27 1 .3 3 model The sign and magnitude reveal that the GO sheets are highly negatively charged and e lectrostatically stabilized, re sulting from phenolic hydroxyl and carboxylic acid groups formed during oxidation of graphite powders. Chemical reduction of GO to graphene was carried out by following a procedure presented by Li and co workers. In this procedure, GO dispersions are adjus ted to a low concentration of 0.025 wt.%. Ammonia is then added to GO dispersions in order to maximize the surface charge of GO sheets and therefore further stabilize the dispersions. After that, chemical reduction

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110 is carried out by adding a small amount o f hydrazine, resulting in a homogeneous and black dispersion. A graphene dispersion made by this method is shown in Figure 6 5 B. Zeta potential of this dispersion was measured to be around 2 mV. The low magnitude shows that graphene sheets in the dispersi on are weakly charged as a result of unreduced carboxylic acid groups. A TEM image of graphene sheets made by the above method is shown in Figure 6 6 in which graphene sheets are observed to be rest ed on TEM grids, folded and wrinkled. An A FM image of gra phene sheets and height profiles are shown in Fig ure 6 7 A and B, respectively. The height profile in Fig ure 6 7 B1 shows that size of graphene sheets can be as large as 600 nm with a thickness of 0.6 nm, resulting in an aspect ratio of 1000. This high aspec t ratio is beneficial in making organic inorganic biomimetic nanocomposites. The height profile in Fig ure 6 7 B2 consists of one and two layers of graphene sheets. Graphene Papers Because of the low zeta potential, dispersions of graphene prepared by this procedure showed visible agglomeration after 2 days. Therefore, vacuum filtration through an Anodisc membrane of graphene dispersions was conducted immediately after chemical reduction to avoid severe aggregation which hinders mechanical properties of grap hene papers. Top and bottom side images of a free standing graphene paper made by vacuum filtration are shown in Figure 6 8 A and B, respectively. The graphene paper was air dried and peeled directly from the membrane. As discovered by Li and co workers, the graphene paper exhibits a metallic texture that demonstrates its smoothness.

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111 The top view and bottom view SEM images in Fig ure 6 9 A and B demonstrate that both surfaces are relatively smooth and graphene sheets are aligned parallel to the Anodisc membrane. The cross sectional SEM image in Fig ure 6 9 C provide further evidence that the graphene sheets are aligned and stacked parallel to each other to from a layered structure. It has been proposed that the layered struc ture is formed because rising in sheets concentration during filtration causes increasing in sheet to sheet interactions that make the sheets tend to align parallel to each other to reduce total energy of the system. Unlike graphene papers, GO papers made by the vacuum filtration method keeps a layered structure but do not exhibit smooth surfaces as shown in Figure 6 10 and 6 11 Instead, SEM images of GO papers in Figure 6 11A and B show many humps one after another o n both top and bottom surfaces We bel ieve this is caused by high surface charge of GO sheets ( 42.27 1.33 mV ) that make GO sheets repel each other rather than adhere together during the increasing in sheet to sheet interactions and result in rumpled surfaces. Tensile strength s of graphene pap ers and GO papers tested are about 140 MPa and 110 MPa, respectively as shown in Fig ure 6 12 and are close to the strength reported. Graphene Actuators Graphene actuators were made in a relatively simple way by laminating two strips of graphene papers w ith an intermediate layer of larger strip of double sided tape resulting in a sandwich structure Schematic illustrations of an actuator and apparatus used are shown in Figure 6 13 Detailed information is in materials and methods session. In order to est imate capacitance of graphene papers, cyclic voltammetry measurements under different scan rates were carried out on a single strip of graphene

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112 paper immersed in 1 M NaCl solution. Dimensions of the strip were 2 mm in width and 15 mm in length. The length of the strip immersed was 10 mm. One side of the strip was covered with an insulating tape, resulting in a superficial active area of 0.2 cm 2 Since 2.5 cm 2 o f graphene paper can be made by filtration of 6 ml of graphene dispersion containing 1.5 mg of graphene, the weight of graphene immersed in solution therefore becomes 0.12 mg. After several cycles to reach maximum degree of wetting, cyclic voltammograms of the graphene strip under various scan rates are shown in Fig ure 6 14 A where a saturated calomel electrode is used as the reference electrode and a platinum wire is used as the counter electrode. The capacitance of graphene papers can therefore be estimate d by calculating the ratio of steady state current to scan rate or by plotting steady state currents versus scan rates as shown in Fig ure 6 14 B that gives a capacitance of 0.006 F or 50F/g, which is three time higher than a nanotube paper, demonstrating hi gh surface area of graphene sheets ( 132 ) Actuations of a graphene actuator operated by potential step method between 2 and 2 volts in 1 M NaCl solution are shown in Fig ure 6 1 5 Voltages reported here are with respect to the graphene strip on the right h and side. Few cycles of operations were also performed before hand to ensure maximum wetting of the actuator. Positions and d isplacements of the actuator tip were then recorded and calculated via the graph paper with the initial position of the actuator ti p as the origin Figure 6 1 5 A shows cross sectional images of a graphene actuator under eight successive potential steps with a total of four cycles ( 2/2 V repeatedly) in which the actuator moves to the right when a positive voltage is applied and to the left when a negative voltage is applied. The moving direction of the graphene actuator is actually opposite to that of the carbon

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113 nanotube actuators proposed by Baughman and colleagues with respect to the direction of an applied voltage. The actuation mech anism of carbon nanotube actuators is based on quantum chemical based expansion that causes dimensional changes in covalently bonded directions and injection of electrons results in bond expansion ( 132 ) As a result, carbon nanotube actuators bend to the a node. Meanwhile, anions and cations in electrolyte are moved into the anode and cathode, respectively, to compensate the injected charges. These dopant intercalations can actually result in swelling electrodes and extent of swelling depends on the size of dopants ( 130, 139, 140 ). Since chloride ions are larger than sodium ions, actuation due to dopant intercalations more likely bend to the cathode and is opposite to that due to quantum chemical based expansion. For our graphene actuators, we found that dopa nt intercalations are required and important. From Figure 6 1 5 A, since the graphene actuator bend to the cathode, we conclude that actuation due to dopant intercalation suppresses that due to quantum chemical based expansion, resulting in a movement opposite to carbon nanotube actuators. Displacements of the actuator tip in Figure 6 1 5 A under repeated potential steps between 2 and 2 volts in 1 M NaCl solution are calculated and shown in Fig ure 6 1 5 B. From this simple measurement, the displacement is found to be around 1.2 mm with 10 mm of graphene actuators immersed in 1 M NaCl electrolyte. Actuations of a graphene actuator operated by cyclic voltammetry method between 2 and 2 volts in 1 M NaCl solution with a scan rate of 50 mV/s are shown in Fig ure 6 1 6 The left side of the graphene actuator was set as the working electrode while the right side was set at both counter and reference electrodes. No pre cycling was conducted before recording. The very beginning cycles of the cyclic

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114 voltammograms of th e graphene actuator are shown in Figure 6 1 6 A. The increasing in the current at this stage demonstrates that wetting of the actuator occurs at beginning along with dopant intercalations. After f ew cycles of operations the current becomes steady, correspon ding to the finish of the wetting process. The displacement measured is shown in Figure 6 1 6 B. At the beginning, almost no displacement is observed. The extent of displacement increases along with the increasing in cyclic numbers and then reaches a steady value of around 1.2 mm, which is almost the same as actuators operated by potential steps. Figure 1 5 B also demonstrates that the graphene actuator can be cycled up to 140 cycles. Summary We demonstrate that electromechanical graphene actuators can be made by laminating two strips of graphene papers with an intermediate dielectric layer. Graphene papers were produced by flow directed assembly of graphene sheets, which were obtained from chemically reduced GO sheets. GO sheets were generated by exfoliation of highly oxygenated graphite in water. C apacitance of graphene papers was estimated to be 0.006F or 50F/g by cyclic voltammetry and is almost three time higher than carbon nanotube papers. We also found that the actuation mechanism of graphene actuators is most likely due to swelling of electrodes originating from dopant intercalations. The displacements of actuators under repeated potential steps between 2 and 2 volts in 1 M NaCl solution was determined to be around 1.2 mm with 10 mm of graphene actuators immersed in electrolyte. Actuations of a graphene actuator operated by cyclic voltammetry method between 2 and 2 volts in 1 M NaCl solution with a scan rate of 50 mV/s were also carried out. Wetting of the graphene actuator was found during the first

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115 few cycles. The extent of displacement increases along with the increasing in cyclic numbers and then reaches a steady value of around 1.2 mm, which is almost the same as actuators operated by potential steps. Actuation of graphene actuators was able to last u p to 140 cycles without significant degradation.

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116 Figure 6 1. Mother of all graphitic forms ( 121 ).

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117 Figure 6

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118 Figure 6 method for GO preparation.

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119 Figure 6 4. Schematic illustration of preparation of graphene papers.

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120 Figure 6 5 Images of colloidal dispersions of (A) GO and (B) graphene. Figure 6 6 TEM image of graphene sheets.

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121 Figure 6 7 (A) Tapping mode AFM image of graphene sheets with (B) height profiles B1 and B2 taken along the lines in (A). The sample was prepared by drop casting diluted graphene dispersion onto a mica substrate.

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122 Figure 6 8 (A) Top and (B) bottom side images of a free stan ding graphene paper made by vacuum filtration of graphene dispersion through an Anodisc membrane.

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123 Figure 6 9 SEM images of a graphene paper. (A)Top view SEM image, (B) bottom view SEM image and (C) cross sectional SEM image.

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124 Figure 6 10. (A) Top and (B) bottom side images of a free standing graphene paper made by vacuum filtration of GO dispersion through an Anodisc membrane.

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125 Figure 6 11. SEM images of a GO paper. (A)Top view SEM image, (B) bottom view SEM image and (C) cross sectional SEM image.

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126 Figure 6 12 Tensile stress versus strain curve for a free standing graphene and GO paper.

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127 Figure 6 13 Schematic illustrations of a graphene actuator (A) Front view of the actuator, (B) side view of the actuator and (C) apparatus used for displacement measurement.

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128 (A) (B) Figure 6 14 (A) Cyclic voltammograms of a graphene strip at various scan rates in 1 M NaCl solution. A saturated calomel electrode was used as the reference electrode and a platinum wire was used as the counter electrode. The superficial active area was 0.2 cm 2 and the weight of graphene paper immersed was 0.12 mg. (B) A plot of steady state currents in (A) versus correspond ing scan rates. The slop in (B) is 0.006 F.

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129 (A) (B) Figure 6 1 5 (A) Cross sectional images of a graphene actuator under eight successive potential steps with a total of four cycles ( 2/2 V repeatedly). (B) Displacements of the actuator tip in (A) under repeated potential steps

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130 (A) (B) Figure 6 1 6 (A) Two electrode cyclic voltammograms of a graphene actuator operated between 2 and 2 volts in 1 M NaCl solution with a scan rate of 50 mV/s (B) Corresponding displacements of the actuator in (A) as a function of cycle number.

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131 CHAPTER 7 CONCLUSIONS A simple electrodeposition technology that enables rapid production of la rge area polymer nanocomposites with layered structures that mimic the nacreous layer of mollusk shells was studied. Uniform, electrostatically stabilized gibbsite nanoplatelets with high aspect ratio were preferentially oriented parallel to the electrode surface when an external direct current electric field was applied. The electroplated ceramic films had uniform thickness, and the thickness could be controlled by adjusting the nanoplatelet concentration of the electroplating baths. Homogeneous, optically transparent nanocomposites were obtained when the interstitials between the aligned nanosheets were infiltrated with polymer. The resulting ceramic polymer nanocomposites exhibited four time higher tensile strength and nearly 1 order of magnitude higher m odulus than pure polymer films. The covalent linkage between the nanoplatelets and the polymer matrix plays an important role in determining the mechanical properties of these biomimetic nanocomposites. Electrophoretic co deposition of polymer gibbsite na nocomposites was also demonstrated. The electrodeposited PVA Gibbsite nanocomposite films were optically transparent and flexible, even though the weight fraction of the brittle inorganic phase was higher than 80%. The electrophoretic co deposition assembl y of positively charged gibbsite nanoplatelets and cationic PEI polyelectrolytes into ordered multilayer in a single step was performed. The resulting nanocomposite had similar organic/inorganic weight ratio and ordered brick and mortar nanostructure as na tural nacres. Nanoindentation tests showed that this nanocomposites exhibited similar hardness and reduced modulus as those of pure gibbsite coatings.

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132 Assembling of surface roughened inorganic nanoplatelets into ordered multilayer that mimic the asperities interposing nanostructure found in the nacreous layer of mollusk shells was investigated. A thin layer of sol gel silica was coated on smooth gibbsite nanoplatelets in order to increase the surface roughness to mimic the asperity of aragonite platelets fo und in nacres. To avoid the severe cracking caused by the shrinkage of sol gel silica during drying, polyelectrolyte PEI was used to reverse the surface charge of silica coated gibbsite nanoplatelets and increased the adherence and strength of the electrod eposited films. Polymer nanocomposites could then be made by infiltrating the interstitials of the aligned nanoplatelet multilayer with photocurable monomer followed by photopolymerization. The resulting self standing films were highly transparent and exhi bited nearly three time higher tensile strength and one order of magnitude higher toughness than those of pure polymer. The measured tensile strength agrees with that predicted by a simple shear lag model. A simple and scalable colloidal templating nanofab rication technology for generating periodic metallic nanopyramid arrays as electrodes for electrochemical surface enhanced Raman spectroscopy (SERS) was conducted. These periodic arrays of nanopyramids with nanoscale sharp tips and high tip density could e nhance the local electromagnetic field in the vicinity of the nanotips, resulting in high SERS enhancement (on the order of 10 6 ). The effects of the applied electrode potential and the electrode redox reactions on the SERS enhancement were investigated. Fi nite element electromagnetic modeling was also developed to simulate the electric field amplitude distribution and the corresponding Raman enhancement factors surrounding arrays of nanopyramids.

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133 Actuators that can c onvert electrical energy into mechanical energy are of great importance for applications in many fields. Development of novel actuators requires materials that have excellent electrical, thermal, and mechanical properties. Graphene has been studie d over ye ars and in known to have extraordinary qualities. W e studied electromechanical actuators based on two strips of graphene papers with an intermediate dielectric layer. T he actuation mechanism of graphene actuators was most likely due to swelling of electrod es originating from dopant intercalations. The swelling induced bending actually suppressed that due to quantum chemical based expansion, resulting in a bending direction opposite to that of carbon nanotube actuators. C apacitance of graphene papers was est imated to be 0.006F or 50F/g by cyclic voltammetry and was almost three times higher than carbon nanotube papers. The displacements of actuators under repeated potential steps between 2 and 2 volts in 1 M NaCl solution was determined to be around 1.2 mm w ith 10 mm of graphene actuators immersed in electrolyte. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s were able to last up to 140 cycles without significant degradation.

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142 BIOGRAPHICAL SKETCH Tzung Hua Lin received his Bachelor of Science in c hemical e ngineering from the National Taiwan University in 2003. He continued his studies at the National Taiwan University and received a Master of Science in 2005. He joined the Department of Chemical Engineering at the University of Florida in August 2007.