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Graphene Synthesis and Assembly for Graphene Actuators

Permanent Link: http://ufdc.ufl.edu/UFE0044805/00001

Material Information

Title: Graphene Synthesis and Assembly for Graphene Actuators
Physical Description: 1 online resource (44 p.)
Language: english
Creator: Chen, Yichen
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: actuator -- electromechanical -- graphene
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this thesis, we provide an improved method to synthesize graphene and also introduce the assembly of a new kind of graphene actuators. Graphene is an allotrope of carbon with a unique one-atom thick structure. It exhibits many exciting properties such as excellent electrical/thermal conductivity, high surface area, great mechanical strength and inherent flexibility. The synthesis of graphene contained two steps. First graphite oxide was synthesized from nature graphite by a modified Hummer method. Then we exfoliated the graphite oxide into GO using ultra-sonication and reduced GO to graphene by chemical reduction. After using flow-filtration method, we were able to assemble conductive graphene paper, which consisted of parallel graphene sheets. After studying the properties of graphene paper, we developed electromechanical actuators based on two strips of graphene paper with an intermediate dielectric layer. The actuation mechanism of graphene actuators was most likely due to swelling of electrodes originating from dopant intercalations.  Graphene actuator strips in the size of 1 mm by 15 mm were assembled. The displacement of actuators under the repeated potential steps between -2 and 2 volts in 1 M NaCl solution was determined to be around 1.2 mm when 10 mm of graphene actuator was immersed in electrolyte. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s reached almost the same displacement. The actuation didn’t die out until140 cycles.
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 Yichen Chen.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Jiang, Peng.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044805:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044805/00001

Material Information

Title: Graphene Synthesis and Assembly for Graphene Actuators
Physical Description: 1 online resource (44 p.)
Language: english
Creator: Chen, Yichen
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: actuator -- electromechanical -- graphene
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this thesis, we provide an improved method to synthesize graphene and also introduce the assembly of a new kind of graphene actuators. Graphene is an allotrope of carbon with a unique one-atom thick structure. It exhibits many exciting properties such as excellent electrical/thermal conductivity, high surface area, great mechanical strength and inherent flexibility. The synthesis of graphene contained two steps. First graphite oxide was synthesized from nature graphite by a modified Hummer method. Then we exfoliated the graphite oxide into GO using ultra-sonication and reduced GO to graphene by chemical reduction. After using flow-filtration method, we were able to assemble conductive graphene paper, which consisted of parallel graphene sheets. After studying the properties of graphene paper, we developed electromechanical actuators based on two strips of graphene paper with an intermediate dielectric layer. The actuation mechanism of graphene actuators was most likely due to swelling of electrodes originating from dopant intercalations.  Graphene actuator strips in the size of 1 mm by 15 mm were assembled. The displacement of actuators under the repeated potential steps between -2 and 2 volts in 1 M NaCl solution was determined to be around 1.2 mm when 10 mm of graphene actuator was immersed in electrolyte. Actuations of a graphene actuator operated by cyclic voltammetry at a scan rate of 50 mV/s reached almost the same displacement. The actuation didn’t die out until140 cycles.
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 Yichen Chen.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Jiang, Peng.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044805:00001


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1 GRAPHENE SYNTHESIS AND ASSEMBLY FOR GRAPHENE ACTUATORS By YICHEN CHEN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Yichen C hen

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3 To my family and friends

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4 ACKNOWLEDGMENTS I would like to thank my advisor, Dr. Peng Jiang for offering me the opportunity to join this project and giving me numerous of precious advice for my research. I would a lso like to thank my fellow lab mates who have assisted me with my work. I want to take this opportunity to acknowled ge former graduate students W ei han Hu ang and Tzung hua L in, my research partner Ta ching Wu and all the students in Dr. Peng J. Ziegler for his valuable inputs and guidance. Last but not least, I want to specially thank my parents for teaching me the value of life and for always be ing there when I need them

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 6 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 RESEARCH BACKGROUND ................................ ................................ ................. 11 1.1 Graphite ................................ ................................ ................................ ............ 11 1.2 Graphite Oxide ................................ ................................ ................................ .. 12 1.3 Graphene ................................ ................................ ................................ .......... 12 1.4 Graphene Oxide ................................ ................................ ................................ 13 1.5 Methods to Produce Graphene ................................ ................................ ......... 14 1.5.1 Micro mechanical cleavage ................................ ................................ ..... 14 1.5.2 Drawing method 22 ................................ ................................ .................... 14 1.5.3 Epitaxial growth on SiC 24 ................................ ................................ ......... 14 1.5.4 Graphene oxide reduction 25 ................................ ................................ ..... 15 2 SYNTHESIS OF GRAPHENE ................................ ................................ ................ 18 2.1 Synthesis of Graphite oxide ................................ ................................ .............. 18 2.2 Exfoliate Graphite Oxide into Graphene Oxide ................................ ................. 20 2.3 Synthesis of Graphene from Graphene Oxide ................................ .................. 21 2.4 Results and Discussion ................................ ................................ ..................... 22 3 ASSEMBLY OF GRAPHENE SHEET ................................ ................................ ..... 27 3.1 Experimental ................................ ................................ ................................ ..... 27 3.2 Results and Discussions ................................ ................................ ................... 28 4 GRAPHENE ACTUATORS ................................ ................................ ..................... 33 4.1 Introduction ................................ ................................ ................................ ....... 33 4.2 Experiment ................................ ................................ ................................ ........ 34 4.3 Results and Discussions ................................ ................................ ................... 35 LIST OF REFERENCES ................................ ................................ ............................... 40 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 44

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6 LIST OF FIGURES Figure page 1 1 Structure of Graphite Oxide ................................ ................................ ................. 15 1 2 Structure of Graphite Oxide. ................................ ................................ ................ 16 1 3 Structure of Graphene ................................ ................................ ......................... 16 1 4 Different structure formed by graphene ................................ ............................... 17 1 5 micro mechanical cleavage produce graphene by break the Van der Waals force between the monolayer carbon plane inside graphite. ............................... 17 2 1 Filtration system for rinsing graphite oxide products. ................................ .......... 24 2 2 Graphite Oxide after one night drying in the oven ................................ ............... 2 4 2 3 Graphite oxide solution after diluted ................................ ................................ .... 25 2 4 Graphite oxide aqueous colloids in a Petri Dish ................................ .................. 25 2 5 0.5 mg/mL Graphene oxide solution in a centrifuge tube ................................ ... 25 2 6 Graphene solution ................................ ................................ .............................. 26 2 7 Color difference of graphite oxide solution and graphene solution ..................... 26 2 8 Aggregation in the graphene solution ................................ ................................ 26 3 1 Filtration system for graphene actuator ................................ .............................. 30 3 2 Ideal graphene paper ................................ ................................ ......................... 30 3 3 SEM picture of the surface of A) GO paper B) Graphene paper ........................ 31 3 4 SEM picture of cross section of A) GO paper B) graphene paper. ..................... 31 3 5 Graphene paper with particle on the surface ................................ ...................... 32 3 6 Cracked and wrinkled graphene paper ................................ .............................. 32 4 1 Graphene actuator with Au electrodes on both side of the graphene strips. A) front view. B) side view. ................................ ................................ ...................... 37 4 2 Actuator partly immersed in the Nacl solution holding by the rectangular glass tank. ................................ ................................ ................................ .......... 37

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7 4 3 Actuations of a graphene actuator operated by potential step method. A) cross sectional images of a graphene actuator. B) Displacements of actuator tip. ................................ ................................ ................................ ....................... 38 4 4 Actuations of a graphene actuator operated by cyclic volatmettry method. A) Two electrode cyclic valtammogram. B) Corresponding displacements of the actuator ................................ ................................ ................................ .............. 39

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8 LIST OF ABBREVIATION S CMG Chemically modified graphene CNT Carbon Nano Tube DI Deionized GO Graphene Oxide MWCNT Multi walled Carbon Nano Tube

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science GRAPHEN E SYNTHESIS AND ASSEMBLY FOR GRAPHENE ACTUATORS By Yichen Chen De cember 2012 Chair: Peng Jiang Major: Chemical Engineering In this thesis, we provide a n improved method to synthesize graphene and also introduce the assembly of a new kind of graphene actuator s Graphene is an allotrope of carbon with a unique one atom thick structure. It exhibits many exciting properties such as excellent electrical/thermal conductivity high surface area, great mechanical strength and inherent fl exibility. The synt hesis of graphene contained two steps. First g raphite oxide was synthesized from nature graphite by a modified Hummer method. Then we exfoliated the graphite oxide into GO using ultra sonication and reduce d GO to graphene by chemical reduction Aft er using flow filtration method we were able to assemble conductiv e graphene paper, which consisted of parallel graphene sheets. After studying the properties of graphene paper, we developed electromechanical actua tors based on two strips of grap hene paper with a n intermediate dielectric layer. The actuation mechanism of graphene actuators was most likely due to swelling of electrodes originating from dopant intercalations. Graphene actuato r strips in the size of 1 mm by 15 mm were assembled The displacement of actuators under the repeated potential steps between 2 and 2 volts in 1 M NaCl solution was det ermined to be around 1.2 mm when 10 mm of graphene actuator was immersed in electrolyte.

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10 Actuations of a graphene actuator operated by cyclic voltammetry at a s can rate of 50 mV/s 140 cycles.

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11 CHAPTER 1 RESEARCH BACKGROUND 1.1 Graphite Graphite is an allotrope of carbon. It is a low density, non expensive material and microcrystalline grap hite has been commonly used in pencil manufacturing for years. Graphite has high electrical and thermal conductivity 1 The conductivity of graphite depends on its structure. A single crystal of graphite has an electric a l resistivity from 4 to 6x10 5 O hm cm 2 3 which has the same order of the conductivity of a poor metal. However, polycrystalline graphite has a much higher resistivity due to the crystal boundaries. Graphite has a multi layer structure and each layer contains hexagonal lattice arrays. As shown in Figure 1 1 g raphite is made of pa rallel layers of graphene whic h has hexagonal rings 4 Each yellow sphere stands for a carbon atom and each blue line stands for the covalent bond between carbon atoms. The dotted line represents the weak V an der Waals forces between the par allel hexagonal rings that allow sliding movement. The angle between the two covalent bonds is 120. The space between the lattice planes of graphite is 3.37 and every hexagonal lattice in the same plane has a distance of 1. 42 between each othe r 5 Each layer of graphite is of sp 2 carbon atoms. There are three kinds of natural graphite: flake graphite, vein graphite and microcrystalline graphite. Flake graphite is the most common source of graphite used in chemical reacti on s It is a natural mineral that is purified to remove heter o atomic contamin ation. 6 Vein graphite structure has high electrical conductivity because of its high crystall inity Microcrystalline graphite is also called amorphous. Besides natural graphite, there are also synthetic graphite such as primary synthetic graphite and

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12 graphite fiber. brushes, refrac to ry materials lubricants, friction materials and cathode material in zinc carbon and lithium ion batteries. 1.2 Graphite Oxide Graphite Oxide is a compound of carb on, oxygen and hydrogen. It is nearly amorphous and has the stru cture of graphene oxide wh ich is very hydrophilic 7 As shown in the Figure 1 2 the structure of graphite oxide contains two kinds of regi ons, t he aromatic region with unoxidized benzene ring s and the aliphatic six membered rings 8 The size s of both regions depend on the degree of oxidation of graphite oxide. The inter layer distance between the graphene oxide sheets of graphite increases reversibly with increasing humidity in the environment and it varies f rom 6 to 12 Graphite oxide is also the middle product in the process of graphene synthesis since graphite oxide can be fully exfoliate d into graphene oxide sheet when it is in the form of aqueous colloidal suspensions. It can be seen that g raphite oxi de sheets carry a negative charge when they are dispersed in water. Due to the electrostatic repulsion between graphite oxide sheets, graphite oxide can form a stab le aqueous suspension 7 1.3 Graphene Similar to graphite, graphene is an allotropy of carbon. Graphene has a two dimensional structure packed in a honeycomb crystal lattice 9 ,1 0 As shown in Figure 1 3 graphene is a one atom t hick carbon layer of sp 2 bonded carbon atoms containing hexagonal latt ice arrays. The yellow dots r epresent the carbon atoms and the blue lines r epresent the bond s between two carbon atoms. The length of the carbon carbon bond in graphene is about 0.142 nm On 2010 Andre Geim and Konstantin Novoselov at the University of Manchester were

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13 experiments regarding the two Graphene has a high value of ( ~1,100 Gp a ) 2 11 great fracture strength (125GPa) 2 good thermal conductivity (~5.000 Wm 1 K 1 ) 3 excellent mobility of charge carrie r s (200,000 cm 2 V 1 S 1 ) 4 12 and high specific surface area ( calculated value, 2,630 m 2 g 1 ) 5 In addition graphene also has several transport phenomena (the electron transport is described by the re lativistic like Dirac equation ) such as quantum Hall effect All t hose significant properties of graphene and chemically modified graphene (CMG) hold great promise for potential application in the area of energy storage materials 13 materials, polymer composites, liquid crystal devices and mecha nic al resonators 7 Figure 1 4 is a n image of 2 D graphene forming different dimensional carbon materials. As shown in the Figure graphene can be wrapped up into 0 D buck y ball rolled into 1 D nanotube or stacked into 3 D graphite 9 Although the monolayer of nano structure provides graphene with both chemical and chemical stability, the properties of graphene still de pend on the way it is aligned 1 4 F or example, electrons move fast er in the 2 D graphene structure with a mobility exceeding 15,000 m 2 V 1 s 1 while graphene nanoribbon shows s emi conductivity when it has narrow width and smooth edges 15 The most c ommon method for producing graphene sheet s in large quantities is by using a chemical process that involve s graphite oxidation, exfoliation and reduction. 1.4 Graphene Oxide Graphene oxide ( GO) i s an oxygen rich monolayer nano material. It is provided by the controlled oxidat ion of graphite 16 GO can be prepared by ex foliating or heat expanding of graphite oxide. GO are hydrophilic since the hydroxyl (OH) groups are bonded to the surface of the GO nanosheets 17 therefore GO is the precursor material

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14 for synthesis of hydropho bic graphene 18 GO is considered as a new type of macromolecul ar form of carbon. In most case s graphene nanosheets are hydrophilic and this property can be enhanced by react ing it with allylamine, graphene nanosheets can also be changed into hydrophobic via functionalizing its surface with phenylisocynate ( C 6 H 5 HCO) through the solvothermal synthesis process 19 The reaction is 1.5 Methods to Produce Graphene 1.5 1 Micro mechanical cleavage Micro mechanical cleavage is a physical way to produce graphene 20 It can produce one atom t hick graphene structure s by repeated exfoliation of small mass of high oriented pyrolytic graphite 21 Figure 1 5 shows that micro mechanical cleavage produce s graphene by break ing the Van der Waals force between the mon olayer carbon planes inside the graphite. 1.5 2 Drawing m ethod 22 On 2004, a research group in Manchester University provided graphene by rubbing the pencil again st an oxidized s ilicon wafer, turning the thick grap hite flakes of the pencil into monol ayer carbon structure graphene 23 The reason that oxidized silicon wafer s were used instead of paper was that oxidized silicon wafer s reflected a rainbow of colors which enhanced the interface contrast between the thin layer of graphene and the wafer 22 1.5 3 Epitaxial g rowth on SiC 24 Graphitization of silicon carbide (SiC) by Si subli mation during high temperatu re (>1100C) vacuum annealing reduce s it to graphene. Silicon carbide is a material with

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15 high resistivity and it is already available in the f orm of large diameter wafers. Ep itaxial growth on SiC is the most common way to produc e graphene based electronics. However, e pitaxial growth on SiC can lead to problems of graphene growth disorder 24 1.5 4 Graphene oxide r eduction 25 There are f our steps to make graphene nano sheet s from graphite flakes: 1. Prepare graphite oxide by using Hummer method for starting graphite 26 2. Exfoliate graphite oxide into hydrophilic graphene oxide by ultrasonic vibration. 27 3. Reduce hydrophobic graphene oxide into hydrophilic graphene by reacting with hydrazin e 28 Figure 1 1 Structure of Graphite Oxide

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16 Figure 1 2 Structure of Graphite Oxide Figure 1 3 Structure of Graphene

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17 Figure 1 4 Different structure formed by graphene Figure 1 5. M icro mechanical cleavage produce graphene by break the Van der Waals force between the monolayer carbon plane inside graphite.

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18 CHAPTER 2 SYNTHESIS OF GRAPHENE 2.1 Synthesis of Graphite oxide The aim of this process is to transform graphite flakes into graphite oxide. We measured 1 g of powdered graphite (Grade 3243, Asbury Graphite Mills Inc, Khtanning, Pa) and left it in a Petri dish. When we took the graphite from its container, we checked and made sure the graphite that it was in powder form Using a graduated cylinder, we measured 3 mL of H 2 SO 4 (95% 98%, Sigma Aldrich Inc, St. Louis, MO) and put it in a 50 mL beaker, adding 1 g of K 2 S 2 O 8 (99+%. Sigma Aldrich Inc, St. Louis, MO) and 1 g of P 2 O 5 (>=98%, Sigma Aldrich Inc, St. Louis, MO) into the beaker. At the same time we set up a water bath system and heated the water. When the water temperature reac hed 80 C, we placed the beaker with all the chemicals into the water bath and stirred all the reactants with a glass stick for 15 minutes. Afterwards, we took the beaker out of the water bath and let it cool down in room temperature for 6 hours. After the beaker s temperature equilibrated to room temperature we took out the black mud like products from the beaker. This reaction is part of the modified Hummers method which involves an additional oxidation step prior to the real Hummers method. In order to remove the acid residue from the product, we built a vacuum filter and put the product we got on top of the filter paper. Figure 2 1 shows the equipment setup for this step, t he pipe on the left opening side of the Erlenmeyer flask was connected to an air vacuum pump By adding water into the product and let ting the vacuum to drain the water away through the filter, we ensure that the product gets rinse d thoroughly. When the PH strips we put on the outlet of the filter shows that the PH of the liq uid is betw een 6 and 7, it

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19 contain acid residue anymore and therefore we can stop adding more water. The next step is to dry out the water from the product. We put the blac k mud like product into a 200 mL beaker and then we left it in a lab drying oven (Isotemp vacuum oven, model 281 A) with a medium high temperature (Level 6, about 85C) over night. Figure 2 2 show s t he graphite inside the beaker was partially oxidized and it formed a uniform fine powder with a dark metallic grey co lor. The next morning we picked up the product from the oven and started to run the Hummers method to make graphite oxide. The f irst thing we did was to build a 0C ice bath by adding ice cube s into a water bath to offset the heat released from the Hummers reaction. Afterwards we added 26 mL 98% H 2 SO 4 with a stir bar followed by 3 g of KMnO 4 into the beaker really slowly, leaving it stirring under 35C for 2 hours. Two hours later we added 46 mL of wate r into the graphite oxide and this gave the product a bright brown color Soon another 140 mL of water and 4 mL ( the range is 2 5 mL according to the recipe) of H 2 O 2 we re added into the solution to stop the reaction. By now we finished the Hummers method a nd the next step is to purify the products. Instead of using DI water, we used 250 mL 1/ 10 HCl solution ( using 25 mL HCl (32% 38% solution. F isher Scientific, Fair lawn, NJ ) and 225 mL DI water under 50 60 C) to wash the extra ion residue from the graphite oxide. After adding the 1/10 HCl solution, we put the solution in a centrifuge (IEC centra cl2, Thermo Electron Corporation) and kept the bottom aqueous colloids while pouring away the clear solution on the top of the centrifuge tube after every c entrifuging. After finishing the first round of centrifuging we add ed 15 mL of DI water to each tube and ra n the same cen trifuge steps another 2

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2 0 times. At this stage we have successfully synthesized the graphite oxide solution from graphite. Figure 2 5 sh ows a sample of graphite oxide solution with dark brown color. We also obtained the weight percentage data by leaving a graphite oxide colloid sample in a Petri dish and letting it dry out overnight in the lab oven. The formula used to calculate the wt % is We needed to dilute graphite oxide colloid s into 30 mL of graphite oxide solution with a concentration of 1 mg/ mL with the aim of making graphite oxide paper and graphene paper later. So we prepared the amount of graphite oxide aqueous colloid s and the DI water we needed based on the wt% calculated for the aqueous colloid graphite oxide. Shortly afterwards, we mixed them together in a large glass vial and use d ultrasonic v ibration to make the graphite oxide homogenously disperse d in water. 2. 2 Exfoliate Graphite Oxide into Graphene Oxide We used ultrasonic vibration to exfoliate the multi layered structure of graphene oxide. The parameters we set for the ultrasonic vibrati on in the lab was level 9 and we let it run for one hour while protecting the glass vial in a room temperature water bath. Once the vibration was done, we took out the graphen oxide solution and separated it into two centrifuge tubes. The solution then got centrifuged under 3900 rpm for 30 minutes 29 By that time we had already obtained the uniform graphite oxide solution with a concentrati on of 1 mg/ mL the last step involved the extraction of the extra ions generat ed from the rea ction. Therefore we cut a adequate length of ion exchange membrane ( Dialysis tubng,#21 152 4, Fisher Scientific) for 30 mL graphite oxide solution and withdrew up the clean solution from the centrifuged solution into the

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21 m embrane. After sealing both ends of the membrane with c lips, we placed it in a big bowl of water with a stir bar and let it dialysis in DI water overnight. As shown in Figure 2 5 graphene oxide solution had a similar carbon black color as the graphite oxide c olloids. T he graphene oxide was well dispersed and there were no graphene oxide fla k es that could be seen in the solution Before we used the graphene oxide solution, we let the solution go through the ultrasonic vibration again for 5 minutes to make sure there was no aggregated graphene oxide turned into graphite oxide in the solution. To prevent aggregation of graphene oxide from occurring during the following filtration process, we took out 10 mL of the 1mg/ mL graphite oxide solution and diluted it with another 10 mL of DI water. After dilution the GO solution showed a light yellow color as shown in Figure 2 3. Later on, the same 0.5 mL / mg graphene oxide solution was used to prepare both graphene a ctuator and graphene paper 2.3 Synthesis of Graphene from Graphen e Oxide During this process we reduced the 0.5 mg/ mL graphene oxide solution into 0.25 mg/ mL graphene solution. 20 mL 0.5 mg/ mL graphene oxide solution was used in this process Before we used it in this process, we let the solution go through the ultraso nic vibration to eliminate the possible aggregation of graphene oxide. Then we took 5 mL from the same solution and put it in a small glass vial. Later we mixed it with another 5 mL of DI water and we got 10 mL of graphene oxide solution with a concentrati on of 0.25 mg/ mL Since 10 mL solution was only enough for one piece of graphene paper and we wanted two piece s every time, we diluted another 5 mL of 0.5 mg/ mL graphene oxide solution in another sma ll glass vial. After the dilution, we brought another clean glass vial and injected 2 mL of NH 4 OH into it. Using a 1 200 L syringe, we added 35

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22 L of NH 4 OH(Fisher Scientific, Fair lawn, NJ) into each graphene oxide solution vial and shook both vials by hand to ensure good mixin g Then we switched to a 1 200 L syringe to add 5 L of Hydrazine (35 wt%, solution in water. Sigma Aldrich Inc, St. Louis, MO ). For this step we mixed the hydrazine with the solution by shaking both vials for 5 minutes. We then prepared a 98 C water ba th and let both vials get heated up in the water bath for 1 hour. As shown in Figure 2 6 and 2 7 c ompared to the graphite oxide solution, graphene solution seemed to be darker. The graphene solution was homogenous and had almost the same density as pure water. 2.4 Results and Discussion The concentration of graphite ox ide colloids was 6.773 g/cm 3 Figure 2 4 was a picture of a GO colloid sample in a Petri dish. We could tell that GO colloids has a l arge s urface tension. During the graphene oxide reductio n process, we found serious aggregation of graphene in the solution after reacting in 9 8 C water bath for one hour. We made two assumptions based on that fact: the first was that the 0.5 mg/ mL graphene oxide solution we used to manufacture graphene had ag grega ted or small graphene oxide fla k e s inside it; the second assumption was that there were extra ions in the graphene oxide solution when we ran the reduction. Figure 2 8 shows the poor dispersion of graphene in aggregated graphene solution. In order to avoid the aggregation, we sonicated the graphene oxide solution fo r 20 minutes before we reduction step. In addition, we sonicated the graphite oxide solution n the aggregations. Therefore we started to think about the possibility that the graphene solution was contaminated by some extra ions before or after the reduction step. The first

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23 contaminant source we thought about was water; therefore we ran the whole process p ossibility was that the ions were introduced into the solution after redu ction, which could be caused by having excess hydrazine or NH 4 OH in graphene solution. This reminded us about the fact that there might not be enough graphene oxide to reduce during the reaction. After the centrifuging step fo r the dialysis of the 1mg/ mL graphite solut ion, we removed all the flakes present that might contain aggregated graphite from the bottom of the centrifuge tube. Although the amount of hydrazine and NH 4 OH was designed to reduce the exactly 10 mL of 0.25 mg/ mL graphene oxide solution, the concent ration was calculated based on data we got from the graphite oxide collides and d the concentration of graphite oxide or graphene oxide solution after that. There were chances that the solute in the solution might decrease; for example, the graphite oxide fla k e s left in the bottom of th e centrifuge tube before we ran dialysis, the solution we lost while transfer ing it from one container to another.

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24 Figure 2 1 Filtration system for rinsing graphite oxide products. Figure 2 2. Graphite Oxide after one night drying in the ove n

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25 Figure 2 3 Graphite oxide solution after diluted Figure 2 4. Graphite oxide aqueous colloids in a Petri Dish Figure 2 5. 0.5 mg/ mL Graphene oxide solution in a centrifuge tube

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26 Figure 2 6. Graphene solution Figure 2 7. Color difference of graphite oxide solution and graphene solution Figure 2 8. Aggregation in the graphene solution

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27 CHAPTER 3 ASSEMBLY OF GRAPHENE SHEET 3.1 Experimental During this process we manufactured conductive graphene paper from the homogenous g raphene solution we made from the last step. We built a filtration system with a water circulation pump that provided a low pressure difference. 30 The same system was also used to man ufacture graphene actuators Figure 3 1 is the equipment setup for the filtration. The right open ing on the Erlenmeyer flask can help attract water down from the solution by providing a pressure difference between the inside of t he flask and the solution on top. The pipe connects the open end of the flask with the water circulation pump installed in the air cabinet. For the filter, we used a anodisc me mbrane filter (25 mm diameter, 0.2 micrometer porosity. Whatman Inc, Maidstone,England) instead of regular filter paper to gain a more s mooth graphene paper surface 31 After setting up the filtration system, we used a pipette to tak e 8 mL 0.25 mL / mg graphene solution and dropped it carefully on to the filtration membrane. W e observed the status of the filter system and made sure that it was stable. After six hours the graphene solution became a cake slightly attached to the membrane. In order to remove the graphene cake from the filtration system, we took off the clip and used slight air flow to dry th e graphene inside the pipe. After that we used a razor to l ift up the membrane slightly and the graphene oxide cake slid from the pipe with the membrane. We use d another old membrane to cover the naked side of the graphene cake and put a weight on top of it to reduce the moistness inside the cake. After one night we came back and remove d the weight and the graphen e cake beca me a thin, conductive graphite paper with a smooth surface.

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28 3.2 Results and Discussions The id eal product of graphene paper should be a metallic black thin memb rane with mirror smooth surface as shown in Figure 3 2. Compared to GO paper, graphene paper shou ld not have any aggregated carbon struc ture on the surface. From Figure 3 3 (a), we could see that there were lots o f aggregations on the surface of GO paper thus the surface was not smooth. Figure 3 3 (b) is a SEM image of graphene paper we made showed that there was almost no aggregation on the surface ( From the cross section SEM image Figure 3 4 (a), we could obser ve that GO paper had multiple layers of carbon structure and they were connected with each o ther. Figure 3 4(b) shows that the graphene paper we made consist ed of a monolayer of carbon structure that is parallel to each other However we mainly found two k inds of defects in our graphene paper products. The first kind of defect is non smooth surface. Figure 3 5 was a SEM picture of our graphene paper sample with some tiny particle s scattered on the surface. There were two possibilities for the presence of t he particle s : first possibility was that the particle s had dust on them that got into the so lution during the previous step, the second possibility was that the black particle was graphene aggregated into graphite during the overnight filtration step. In both cases it resulted in a reduction of the conductivity of the graphene paper because both dust and graphite particle s are nonconductive. The other kind of defect present w as cracks or wrinkled surface s that could be easil y seen in Figure 3 6. The crack was usually due to the re being less than enough graphene solution added into the filtration system. The cracks on the surface also reduce the conductivity of the graphene paper. The wrinkled surfac e was because we dried the graphene cake too soon thus resulting in the shrinkage of the gr aphene paper surface Based on our measurement, most of the ideal graphe ne paper s

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29 synthesized had a resistance close to 200 some even reached a excellent c onductivity with a resistance cl ose to 100 ; while the non ideal graphene paper usually had a higher resistance ranged from 210 to 480 Therefore it was very important to ensure smooth surface for the graphene paper because most of the defects w ill influence its performance with the graphene actuator.

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30 Figure 3 1. Filtrat ion system for graphene actuator Figure 3 2. Ideal graphene paper

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31 Figure 3 3 SEM picture of the surface of A ) GO paper B ) Graphene paper Figure 3 4. SEM picture of cro ss section of A ) GO pape r B ) graphene paper.

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32 Figure 3 5 Graphene paper with particle on the surface Figure 3 6 Cra cked and wrinkled graphene paper

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33 CHAPTE R 4 GRAPHENE ACTUATORS 4.1 Introduction Actuator is the equipment tha t could mechanically respond to external change in temperature, light, humidity and current 32 Reversible actuators have variou s applications in sensors, artificial muscle, switches and memory chips 33 Graphene actuators focused on developing the electromechan ical feature of the material. Graphene has a unique two dimensional structure with a thickness of one atom. It is zero band gap semi metallic conductor material 34 which exhi bi ts excellent electrical /thermal conductivity 35 high surface area, great mechanical strength and inherent flexibility 36 Compared to carbon nano tube( CNT) material, graphene provides a la rger surface area (theoretically 2630 m 2 g 1 ) 37 Besides that, single layer graphene has a steady strength of conductivity of 6000 S cm 1 38 Because of its natural inversion symmetry graphene is not intrinsically piezoelectric. In order to induce piezoelectricity for graphene, the inversion symmetry need s to be broken by limit ing the adsorption of atoms on the surface o f graphene only on one side 39 Research has already been done on graphene based and CNT based actuators. A multi walled CNTs( MWCNT ) /GO bilayer actuator was built using sol ution filtration method. Since the OH an d COOH groups on the GO side were sensitive to humid ity whi t, the actuator curled in different directions depending on the environment humidity 40 However, the high cost of CNTs synthes is as well as the non linear control of the actuators with humidity limited the potential application of this process There was also research done on single walled CNT

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34 actuator that showed electromechanical material feature s. 41,42 The actua tion came from chemical based expansion via the non faradaic charging/discharging 43 In order to improve the actuation performance and also lower the cost, w e assembled the electromechanical graphene actuators. The actuators were made with two paper thin graphene strips adhered on both side of a dielectric layer. We app l ied cyclic voltammetry on the actuators and also we treat ed the actuators with r epeated po tential steps. Cyclic stability of the graphene based actuators was also explored. 4.2 Experiment The actuators were mad e in sandwich structure with a strip of Scotch Double Stick Tap e as intervening dielectric between two graphene strips. After getting g raphene sheet from pervious steps, we cut paper like graphene discs into 1 mm by 15 mm strips. The graphene sheets were cut by a razor blade and usually one piece of graphene sheet can be divi ded into four graphene strips with the required size. The d ouble sided tape strips were prepared by cutting the Scotch Double Stick Tape into 1 mm wide s trips with a length a little over 15 mm. For each actuator we attached two pieces of Au/C r/glass electrodes pointing opposite direction on the edge of both graphene strips. The electrodes were made by sputter deposition of 20 nm of Cr and 200 nm of Au and each were connected with a clip for applying voltage. Both of the electrodes play an important role in its actuation performance Figure 4 1 (A) and (B) show detail ed imagi es of the experiment setting. Figure 4 2 shows that during the experiment there was a bout 10 mm of the actuators immersed in the rectangular glass tank, which was full of 1 M N aCl solution. First we used potential steps ( 2/ 2 V) on actuators and recorded the actuation. Th en we used cyclic voltammetry, applying voltage between 2 and 2 volts with a scan rate of 50 mV/s throug h the electrode. By videotaping the

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35 actuators, we could observe the bending of the actuators in the solution corresponding t o the voltage change A K urt J. Lesker CMS 18 Multitarget Sputter was used for the sputtering deposition of Cr and Au. Scanning electron microscopy (SEM) was carried out on a JEOL 6335F FEG SEM. A thin layer of carbon was sputtered onto the samples prior t o imaging. An EG&G Model 273A potentiostat/galvanostat (Princeton Applied Research) was used for potential step and cyclic voltammetry operations. 4.3 Results and Discussions We immersed 10 mm of graphene actuators in 1 M NaCl solution and then applied repeatedly potential steps between 2 and 2 volts. Before we recorded the actuation perfor mance, we operated several step s to make sure the actuator reaches maximum wetting. We also recorded the whole process and calculated the displacements of tip based on its position on the grid paper on the back of the water tank. The as origin. Figure 4 3 (A) showed that the actuator moved to the right when a positive charge was applied and to the left wh en a negative charge was applied during the eight successive potential steps with a total of four cy cles( 2/ 2 V repeatedly). Based on the fact that the actuator always bended to the cathode during the experiment, we believed what happened was anions (Cl ) and cations ( Na + ) in the electrolyte (NaCl) were moved to the anode and cathode Since chloride ions were larger than sodium ions in size, the dopant intercalations resulted in the bending of actuators. Figure 4 3 (B) showed that under the same operati on as (A), Then we operated actuators by using the cyclic voltammetry method between 2 and 2 volts in 1 M NaCl solution with a scan rate of 50 mV/s. This time we started

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36 recording the data withou t waiting for the wetting of the ac tuator. Figure 4 4 (A) showed the first several cycles of cyclic voltammograms of the graphene actuator and the current in this stage increased with time, which demonstrated that the wetting of the actuator occurred with dopant intercalations at the beginning. After several cycles the change of the current started to maintain a steady state. From Figure 4 4 (B), we could see that the actuation performance was also influenced by the wetting time. We could barely observe d i splacement the cycle number increased, the extent of displacement also increased until it reached a steady value around 1.2 mm, which almost matched the actuation performance under potential ste ps. After maintaining steady actuation performance for a while, actuators die out after about 140 cycles.

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37 Figure 4 1 Graphene actuator with Au electrodes on both side of the graphene strips. A) front view. B ) side view. Figure 4 2. Actuator partly immersed in the Nacl solution holding by the rectangular glass tank.

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38 Figure 4 3 Actuations of a graphene actuator operated by potential step method A) cross sectional images of a graphene actuator B) Displacements of actuator ti p

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39 (A) (B) Figure 4 4. Actuations of a graphene actuator operated by cyclic volatmettry method A) Two electrode cyclic valtammogram B) Corresponding displacements of the actuator

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44 BIOGRAPHICAL SKETCH Yichen Chen was born in 1988 in Shanghai, China. On 2006 she started her undergraduate in East China University of Science and Technology (ECUST), Shanghai. Three years later, she left the city that she had been living for twenty one years and went to the United States as an exchange student at Lamar University, Beaumont, Texas. She completed her exchange study one year later then recei ved her Bachelor of Science in chemical e ngineering from ECUST. On that same year, she was accepted by University of Florida as a gra duate student and received a Master with thesis degree in the fall of 2012