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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.

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Permanent Link: http://ufdc.ufl.edu/UFE0044212/00001

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Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.
Physical Description: Book
Language: english
Creator: Sweeny, Brendan C
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

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

Notes

Statement of Responsibility: by Brendan C Sweeny.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Wei, Wei (David).
Electronic Access: INACCESSIBLE UNTIL 2014-05-31

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044212:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044212/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-05-31.
Physical Description: Book
Language: english
Creator: Sweeny, Brendan C
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

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

Notes

Statement of Responsibility: by Brendan C Sweeny.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: Wei, Wei (David).
Electronic Access: INACCESSIBLE UNTIL 2014-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044212:00001


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1 A STUDY OF PLASMON MEDIATED ELECTRON TRANSFER ON THE SINGLE NANOSTRUCTURE LEVEL By BRENDAN CHARLES SWEENY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS F OR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Brendan Charles Sweeny

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3 To my family

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4 ACKNOWLEDGMENTS There are numerous people who have supported me over the course of my academic career Although it would be impossibl e to acknowledge them all individually I am extremely grateful for their advice and guidance At the University of Florida, I would like to thank my advisor Dr. W. David Wei for his research support and guidance Additionally, I would like to thank my gra du ate committee for their advice I would like to acknowledge my fellow group members for their assistance, valuable input and friendship. I would especially like to thank Dr. Wenxin Niu for his research mentoring and guidance. Additionally, I would like t o express my thanks to Yi Chung Wang and Jingjing Qiu for their assistance with instrumentatio n and data collection. I want to acknowledge group members, Joe Duchene, John Abendroth, Dr. Kun Qian, Jeremy Graham, Elias Munoz and Frances Ooi for their helpfu l suggestions, feedback and stimulating conversation. Finally, I wou ld also like to thank my family Elizabeth, Gary, Andrew and Al for their encouragement and support It has been invaluable throughout my life.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 6 LIST OF ABBREVIATIONS ................................ ................................ ............................. 7 ABSTRACT ................................ ................................ ................................ ..................... 8 CHAPTER 1 BACKGROUND AND SIGNIFICANCE ................................ ................................ ... 10 Introduction ................................ ................................ ................................ ............. 10 Semiconductor Photocatalysis ................................ ................................ ................ 11 Metal Semiconductor Heterostructures ................................ ................................ ... 14 2 LOCALIZED SURFACE PLASMON RESONANCE ................................ ................ 17 The Localized Surface Plasmon Resonance Phenomenon ................................ .... 17 Roles of Plasmonic Materials in Photocatalytic Heterostructures ........................... 19 Direct Charge Injection Mechanism ................................ ................................ 19 Local Electric Field Enhancement Mechanism ................................ ................. 20 3 EVIDENCE FOR PLASMON MEDIATED ELECTRON TRANSFER ...................... 24 Hydrothermal Technique ................................ ................................ ......................... 24 Sodium Titanate Nanotube Synthesis ................................ .............................. 25 Anatase TiO 2 Nanorod Synthesis ................................ ................................ ..... 25 Photodeposition of Metal Nanoparticles ................................ ................................ .. 26 Au TiO 2 Heterostructure Synthesis ................................ ................................ ... 26 Plasmon Mediated Photodeposition of Pt Nanostructures on Au TiO 2 ............. 27 Results a nd Discussion ................................ ................................ ........................... 27 4 CONCLUSION ................................ ................................ ................................ ........ 41 LIST OF REFERENCES ................................ ................................ ............................... 42 BIOGRAPHI CAL SKETCH ................................ ................................ ............................ 46

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6 LIST OF FIGURES Figure page 1 1 A band structure comparison of common semiconductors relative to the vacuum energy level and the stan dard hydrogen electrode at pH 1. .................. 16 1 2 A band representation of a Schottky junction.. ................................ ................... 16 2 1 A conceptual representation of ele ctron cloud oscillation due to LSPR. ............. 22 2 2 The extinction characteristics of LSPR.. ................................ ............................. 22 2 3 The optical properties of silver nan ostructures of different shapes. Images and spectra courtesy of John Abendroth. ................................ ........................... 23 2 4 Suggested electron injection mechanism.. ................................ ......................... 23 3 1 SEM images comparing Au and Pt photodeposition.. ................................ ......... 33 3 2 X ray diffraction pattern for anatase phase TiO 2 ................................ ................ 34 3 3 Extincti on Spectra of TiO 2 nanorods (black), Au TiO 2 heterostructures (red), and Au TiO 2 Pt heterostructures (blue).. ................................ ............................ 34 3 4 Experimental design for photochemical reduction and oxidation reactions. ....... 35 3 5 Schematic of plasmon mediated photodeposition. ................................ ............. 35 3 6 TEM and HRTEM images of plasmon mediated deposition.. ............................. 36 3 7 TEM images of TiO 2 in the presence of H 2 PtCl 6 and methanol following visible light irradiation.. ................................ ................................ ....................... 37 3 8 TEM images of surfactant less go ld nanoparticles after visible light irradiation in the presence of H 2 PtCl 6 .. ................................ ................................ ................ 37 3 9 Location specific EDS spectra.. ................................ ................................ .......... 38 3 10 Cha racterization of Au TiO 2 Pd.. ................................ ................................ ........ 39 3 11 Electron microscopy images of Au ZnO and Au ZnO Pt heterostructures.. ....... 40

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7 LIST OF ABBREVIATION S CB Conduction Band EDS E nergy Dispersive X Ray Spectroscopy eV Electronvolt fs Femtosecond HRTEM High Resolution Transmission Electron Microscopy LSPR Localized Surface Plasmon Resonance Micrometer Microliter nm Nanometer SHE Standard Hydrogen Electrode SEM Scanning Electro n Microscopy TEM Transmission Electron Microscopy UV Ultraviolet VB Valence Band XRD X Ray Diffraction

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8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Deg ree of Master of Science A STUDY OF PLASMON MEDIATED ELECTRON TRANSFER ON THE SINGLE NANOSTRUCTURE LEVEL By Brendan Charles Sweeny May 2012 Chair: Wei David Wei Major: Chemistry This thesis focuses on work done to demonstrate the direct electron transf er from gold (Au) nanostructures to titanium dioxide (TiO 2 ) nanorods. Au TiO 2 nanostructures with direct physical contact were prepared using a photochemical deposition synthesis method. The plasmon mediated charge transfer proces s was observed by the phot ochemical reduction of metal ions onto the surface of the heterostructure by the controlled excitation of the localized surface plasmon resonance (LSPR) of the gold nanostructures. Using transmission electron microscopy (TEM) and scanning electron microsco py (SEM) it was observed that the separated electrons have a relatively long diffusion distance and lifetime. The results of the photochemical reduction provide a visual map of the sites where reduction of Pt ions occurs and fundamental insight into the L SPR enhancement mechan isms at the single nanostructure level. The results of this study show promise for the use of plasmonic materials in heterostructures for the generation of chemical fuels. In many semiconductor photovoltaic and photocatalytic devices, recombination chemical instability and weak visible absorption remain prominent cause s of device inefficiency. By developing a fundamental understanding of the LSPR enhancement mechanism as well as the

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9 charge separation and transport properties at the Au TiO 2 interface more effective photovoltaic and photocatalytic devices can be engineered. Additionally, t he results of our work also allow for the versatile design o f photocatalysts for specific photoreduction and photo oxidation reactions using multiple semiconductor and metal species

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10 CHAPTER 1 BACKGROUND AND SIGNI FICANCE Introduction Global climate change is a growing environmental issue that will continue to prompt the reevaluation of how humanity as a whole consumes natural resources. Hydrocarbon fu els are an important energy source because of their high energy density and pr esent day abundance However, the issues associated with using hydrocarbon fuels are well documented and include the broad negative environm ental impac t and finite availability In the year 2008 more than 80% of the total amount of energy consumed in the United States came from fossil fuels 1 and the emission of carbon dioxide, a well known green house gas, has increased by 14 % between 1990 and 2008 2 G lo bal temperatures have shown the current decade to be the warmest decade on record in addition to a constant increase in the annual mean temperature over the past century 3 T he approach to issue s resulting from the overuse of hydrocarbon based fuels involves the development of methods that make use of renewable energy resources on a large scale to meet the increased global energy need Hydrogen is a clean and efficient form of energy with applic ation toward fuel cell technologies that have the potential to replace a signif icant portion of the energy requirement currently satisfied by fossil fuels A s an energy source hydrogen is hindered by the current i ndustrial manufacturing process, for 95% o f the hydrogen produced in the United States is produced through steam reformation 4 ( Equation 1 1 ). This process forms hydrogen from the reaction of hot steam with methane and can be further refined through the water gas shift ( Equation 1 2 ) CH 4 + H 2 2 (1 1)

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11 CO + H 2 2 + H 2 (1 2) These process es require a high energy input, which is accommodated by hydrocarbon sources If hydrogen is expected to be a viable clean energy option, it must be produced using renewable sources of energy in a p rocess that seeks to e liminate or limits environmentally harmful emissions. Solar radiation is a promising and abundant renewable energy resource suitable for this purpose The amount of energy per year that reaches the earth in the form of solar radiation is approximately 3x10 24 J oules 5 which far surpasses the approximately 5x10 20 Joules that was consumed by the global population in 2008 6 However, m aking efficient use of this energy source remains an exceptional challenge. Semiconductor Photocatalysis There are multiple ways to harness solar energy for the production of hydrogen gas. B iomass re formation 7 8 and photocatalysis 7 9 10 are just a couple of the directions currently being explored Semiconductor based photocatalysts have been studied for light harvesting applications since their water splitting activity was first reported by Honda and Fujishima in the 1970s 9 Semiconductors represent a promising class of material for phot ocatalysis because they generally exhibit discrete band gaps This allows for absorption of inciden t photons with energy larger than the energy difference between the valence band (VB) maximum and conduction band (CB) minimum In traditional semic onductor photocatalysis there are three main processes : absorption diffusion and surface transfer 11 Specifically, incident photons can generate excitons in the semiconduct or through the process of exciting VB electrons to the CB, leaving behind a charge vacancy, or hole in a process known as charge separation. If charge separation occurs sufficiently close to an interface with t he electrolyte electrons and

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12 holes may diffu se to the surface of the semiconductor and react as part of a reduction or oxidation half reaction producing hydrogen and oxygen gas, respectively 12 Several cond itions need to be met in order for a semiconductor to be su itable for photocatalytic generation of chemical fuels O ne of the more important conditions to be concerned with is the energy levels and redox potentials of the CB minimum and VB maximum. Followi ng electron hole pair generation and charge diffusion, surface reactions will be allowed based on the relative redox potentials of the CB a nd VB and the ad sorbed reactant 13 14 In addition to the redox potentials of electrons occupying energy levels in the CB and charge vacancies in the VB t he band gap of the semiconductor is also an important parameter for ph otocatalysis. Specifically, a semiconductor capable of absorbing incident light in th e visible range would have a band gap in the range of approximately 1.77 to 2.95 eV (corresponding to w avelengths from 700 to 420 nm). In our work the band gap will be co nsidered in order to separate the absorption ranges of the substituent materials in the heterostructure which will allow for the careful control over which materials are involved in the absorption process Even though a suitable band gap and CB and VB re dox potentials are requisite for photocatalysis other factors, such as photo chemical stability, charge carrier mobility, lattice defects and surface structure, must also be considered 12 Titanium dioxide (TiO 2 ) is a commonly studied n type metal oxide semiconductor for photovoltaics 5 15 and photocatalysis 10 16 What makes TiO 2 attractive for photocatalysis are the positions of the CB minimum and VB maximum. The VB max imum for TiO 2 is around 2.9 V versus standard hydrogen electrode ( SHE ) (Figure 1 1), wher eas the CB minimum is 0.3 V versus SHE 5 both at a pH of 1 These energy

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13 levels allow for the reduction (0 V vs. SHE ) and oxidation (1.23 V vs. SHE ) of water. In addition, TiO 2 and other wide band gap semiconductors are less susceptible to photocorrosion 5 15 However, it is the absorption range that limits TiO 2 as a photocatalytic material. The two commonly studied phases of TiO 2 are rutile and anatase, which have band gap s of 3.0 and 3.2 eV, respectively 17 Because of this wide band gap, TiO 2 has a strong absorption in the UV range but does not absorb visible range photons which make up over 40% of the solar emission spectrum 18 Improving the visible range absorption of TiO 2 would take advantage of a much larger portion of the solar emission range. Even for semiconductors that absorb in the visible range and have appropriate CB and VB pot entials, t he quick recombination of genera ted electron hole pairs is one of the primary reasons for low efficiencies in photocatalysis where the semiconductor is the light absorber 5 11 14 19 In order for a semiconductor to absorb a significant portion of incident photons, it must be (i.e. the optical path length of incident photons must be long) but for efficient use of separated charge carriers, the diffusion distance must be minimized. In relatively thick devices, this can lead to an increase in the probability of the recombinat ion of separated electrons and holes when electron hole pairs are generated in the same material. Minimizing the electron hole pair recombination process is critical to the development of efficient semiconductor photocatalysts. Various strategies have been employed to improve the absorption characteristics of semiconductors in photocatalysis and decrease the probability of recombination including heterostructure formation. In terms of photocatalysis heterostructure

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14 formation is the coordination of multipl e discrete materials in order to utilize the resulting electronic and chemical properties to improve the limitations of the individual components (e.g. unsuitable band gap, inappropriate redox potentials and high pr obability of recombination ) The use of h eterostructure materials is important to minimizing the probability of electron hole pair recombination by providing alternate charge transfer pathways typically across the interface of two materials. Ultimately, this allows for the reduction and oxidatio n half reactions to occur in different regions of the heterostructure. Metal Semiconductor Heterostructures R esearch concerning photocatalytic heterostructures commonly use nanostructures made up of c onductive metals such as Au or platinum (Pt) coordinat ed to a semiconductor species 10 11 20 The interface of these two materials represents the most important regio n of the entire heterostructure and is of pivotal importance to our work. The electronic properties of the substituent materials determine the type of interface that is formed, which can promote or hi nder efficient electron transfer. A Schottky barrier is a type of rectifying potential barrier formed at the interface between a metal and an n type semiconductor. Assuming an ideal interface, without Fermi pinning due to semiconductor surface states, a me tal with a high work function relative to the work function of the semiconductor can induce band bending and a potential energy barrier in the semiconductor 21 The band bending occurs bec ause of the initial transfer of electrons from higher energy states in the semiconductor to lower energy states in the Au Shown in Figure 1 2 is an example of a Au and TiO 2 interface. The work function for bulk Au ( m ) is approximately 5.1 eV relative to the vacuum energy level. The a natase form of TiO 2 has a band gap of 3.2 eV with an electron

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15 affinity ( s ) close to 4.21 eV 22 Being a n n type semiconductor, the Fermi energy level (illustrated as the semiconductor work function, s ) is energetically closer to the CB minimum than the VB maximum. Upon the initial interfacing of the two materials, electrons occupying energy states in the CB near the interface of TiO 2 can tunnel to the high work function Au nanostructure, fill ing avai lable energy states near the Au Fermi energy in order to reach thermodynamic equilibrium 21 This results in the formation of a depletion layer in the TiO 2 that is characterized by the net positive charge near the Au TiO 2 i nterface with the nanostructure, thereby forming a potential energy barrier known as the Schottky barrier The potential energy barrier that an electron must overcome to transfer from metal to semiconductor is given by Eq uation 1 3 (1 3) In the case of a Au TiO 2 interface, the Schottky barrier height ( bn ) has been shown to be around 1 eV 23 24 This type of interface is important when considering the potential enhancement mechanisms in photocatalytic heterostructures and is critical for plasmon mediated charge transfer of electrons.

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16 Figure 1 1. A b a nd structure c omparison of common semiconductors relative to the vacuum energy level and the standard hydrogen electrode at pH 1. Figure 1 2. A band representation of a Schottky junction. A) Relative band structure of TiO 2 and Au before contact, where m s and X s represent the work function of the metal, the work function of the semiconductor and the semiconductor electron affinity, respectively. B) A Schottky barrier formed at the Au TiO 2 interface with the potential energy barriers bn and bi for electrons transferring from the metal and semiconductor, respectively.

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17 CHAPTER 2 LOCALIZED SURFACE PL ASMON RESONANCE The Localized Surface Plasmon Resonance Phenomenon An emer ging field that has potential application to heterostructure photocatalysis is t he field of plasmonics. Plasmonics is the study of light matter interactions in noble metal nano structures. LSPR occurs in nanoscale noble metal structures that are smaller than the wavelength of incident photons 25 Furthermore, this pheno menon is characterized by the collective oscillation of conduction electrons in a metal that occurs when the incident light is resonant with the natural oscillation frequency of the surface electrons (Figure 2 1) LSPR manifests itself at the surface of the particle as an intense, spatially non homogenous electric field which concentrates the incident light flux into a tight volume immediate ly surrounding the nanoparticle 26 28 This phenomenon can be observed experimentally as the sharp absorption of a narrow wavelength range of incident photons as shown in Figure 2 2 Here the LSPR absorp tion peaks at 550 nm with Au interband transitions absorbing at shorter wavelengths 29 30 The spectra l character of LSPR can be described mathematically through the Mie which describe the extinction cross section of electromagnetic radiation by a metallic sphere 27 31 When the nanoparticles are significantly smaller than the wavelength of incident light, the solutions can be obtained using the quasi static approximation and the extinction spectrum of the nanoparticles is dominated by absorption The extinction cross section ( C ext ) for a material is dependent function ( r and i respectively ), the dielect ric function of the surrounding material ( m ) as well as the

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18 wavelength ( ) of the incident photons and the size of the metal sphere ( R ) (Equation 2 1) 25 32 (2 1) From a materials perspective, t he impo rtant variables for the manipulation of LSPR are the dielectric components and shape of the metal nanostructure since it is often th e case that the other parameters, such as the dielectric constant of the su rrounding material, are fixed As the denominator of the second term in Equation 2 1 goes to zero the value of ( C ext ) increases rapidly, satisfying the resonan ce condition. In orde r to fulfill this requirement, the value of r must be similar to 2 m 27 In addition the value of i which describes the LSPR damping process, m ust be close to zero The unique and attractive property of LSPR is that it is highly tunable. Depen ding on the metal composition the resonant photon wavelength required to induce this type of response can vary, with nanostructures of Au, Ag and Cu exhibi ting a plasm onic response to vis ible light 27 In addition to na nostructure composition, the LSPR extinction properties can be modified by the size, shape and surrounding di electric medium As demonstrated in literature 25 33 and in our lab (Figure 2 3) the plasmonic response in silver nanoprisms can be altered simply by modifying the edge length of the nanostructure As the edge length increases, the LSPR extinction peak shifts to shorter wavelengths. Since a strong portion of the solar emission spectrum is made up of ultraviolet and visible range photons, designing a method to incorporate plasmonic materials with a carefully controlled range of sizes and shapes could not only extend the absorption of a photocatalytic heterostructur e into the visible range, but conceivably take advantage of the majorit y of the solar emission range

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19 Roles of Plasmonic Materials in Photocatalytic Heterostructures In addition to the electronic properties introduced at the interface of a metal and a semi conductor, t he visible range absorption by plasmonic materials has been experimentally demonstrated to enhanc e the photocatalytic activity compared to the semiconductor alone 20 26 28 34 37 However, the exact mechanism through which this enhancement occu rs following the absorption by the plasmonic material has been the subject of debate which our investigations aim to clarify In recent literature, one way that this enhancement mechanism has been suggested to occur is through the direct electron inject ion from the plasmonic metal to the semiconductor. 20 36 38 41 Direct Charge Injection Mechanism In the direct electron injection mechanism, t he transfer process is initiated by the absorption of incident photons by the plasmonic metal in direct contact with a semiconductor The incident photon s can excite the conduction electrons to excited su rface plasmon states (Figure 2 4 A ). F ollowing excitation, these electrons can relax through dephasing and subsequent electron electron scattering and electron phonon scattering 31 35 42 43 If electrons at the top of the re sulting energy distribution are energetic enough, they may be able to relax to the semiconductor conduction band (Figure 2 4 B) 35 If an electron is injected into the semiconductor CB, it can diffuse to a suitable reaction site and unde rg o a r edox reaction with an adsorbed reactant molecule ( Figure 2 4 C ) In this suggested mechanism, the Schottky barrier plays a very important role by hindering the relaxation of injected CB electrons back to the plasmonic nanostructure 44 This essentially traps the separated el ectrons in the semiconductor CB and pr ovides a physical separation between CB electrons and charge vacan cies on Au.

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20 It is expected that this enhancement mechanism will only occur with direct contact between metal and semiconductor In a femtosecond transient absorption study conducted by Furube et. al. it was suggested that electron transfer from Au to TiO 2 can occur in 50 120 fs 35 which is on the same time scale as the electron electron scattering relaxation process (~100 fs) 31 This was supported experimentally i n a recent work by Silva et. al. where it was demonstrated tha t Au nanoparticles directly deposited on P25 TiO 2 (multicrystalline TiO 2 consisting of approximately 80% anatase and 20% rutile phases) increased the volume of hydrogen evolved under visible light conditions compared to P25 alone 20 A potential advantage to this mechanism is that the interfacial properties of the two materials the Schottky barrier, is used to prolong the charge separation lifetime in the h eterostr ucture, rather than relying on the electronic structure and the transport propert ies of the semiconductor alone This leads to the generation of electrons near the surface which is an advantage over absorption in the bulk semiconductor due to the short mi nority carrier diffusion length of some semiconductors (TiO 2 ~10 nm) 26 From an engi neering perspective, this means that the band gap energy of the semiconductor i s a less important consideration since it is not involved in the abso rption process. Instead, t he key parameter s are the Fermi level of the semiconductor and the CB band position in relation to the metal Fermi level Local Electric Field Enhancement Mechan ism In contrast to the direct injection mechanism, a local field enhancement mechanism has been proposed in a number of recent studies 26 34 45 including systems where a thin dielectric material is used to separate the semiconductor and plasmonic nanostructure 34 46 This mechanis m is supported as an additional mechanism i n recent

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21 work published by Li u et. al 26 In this mechanism the excitation of the LSPR modes of the plasmonic nanostructure results in a strong local electric field This field can be orders of magnitude more intense than the incident optical field 26 47 This enhanced field is spatially non homogenous and decays exponentially with distance from the nanostructure 48 When the semiconductor is brought into close proximity with the excited plasmonic nanostructure, the intense electric field is suggested to induce further electron hole pair generation based on the semiconductor absorption rate being proportio nal to the square of the electric field intensity 26 | E | 2 However, this enhancement can only occur when the energy of incident photons is sufficient to facilitate an interband transition in the semiconductor and also excite the plasmonic material, which occurs when the absorption ranges of the semiconductor and plasmonic material overlap 28 When this condition is met the increased local electric field due to LSPR has been suggested to increase the exciton generation in a semiconductor by a few orders of magnitude 26 Similar to direct injection an intense spatially non homogenous local field in close proximity to the semiconductor leads to electron hole pairs being generated close to the surface and to the site of reaction. In our work w e focus on the charge transfer mechanism and eliminate the potential for the local field enhancement mechanism to contribute to electron hole pair generation. This is accomplished using wavelengths of light that can be absorbed by the plasmonic material but not by the semiconductor. The charge transfer between materials is observed on th e single nanostructure level by a photodeposition method.

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22 Figure 2 1. A conceptual representation of electron cloud oscillation due to LSPR. Figure 2 2 The extinction ch aracteristics of LSPR A) Octahedral Au nanoparticle s (sca le bar: 50 nm). B) LSP R extinction spectrum for octahedral Au nanoparticles

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23 Figure 2 3 The optical properties of silver nanostructures of different shapes. A) Silver nanoprisms with an average edge length of approximately 40 nm (scale bar: 300 nm) B) LSPR abs orption due t o silver nanostructures of various edge lengths Images and spectra c ourtesy of John Abendroth. Figure 2 4 Suggested electron injection mechanism. A ) Absorpt ion by the plasmonic material. B ) Relaxation to the semiconductor conduction band. C ) Diffusion to the site of reaction.

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24 CHAPTER 3 EVIDENCE FOR PLASMON MEDIATED ELECTRON TR ANSFER There have been numerous published methods for fabricating metal semiconductor heterostructures. Some of these include the initial synthesis of the semiconductor followed b y the deposition of the noble metal species. Examples of this are composite mixtures 20 26 38 49 50 lithography techniques 37 40 51 52 and hydrothermal synthesis 53 55 Even though Au is a strong light absorber, it is not well understood how Au contributes to the enhanced visible light response of Au TiO 2 at the single nanostructure level. For composite mixtures of Au TiO 2 it has been argued by Tian et. al. that direct electron transfer occurs between Au and TiO 2 38 Conversely, Liu and Hou et. al. have argued that the photocatalytic improvement in a Au TiO 2 composite mixture was due to a local field enhancement from the Au nanostructures and not direct charge transfer 26 In our study, the focus is on the visible light absorption by the plasmonic metal and the subsequent electron hole pair generation at the interface due to direct charge transfer from Au to TiO 2 A combination of hydrothermal and photodeposition m ethods were used to synthesize Au TiO 2 heterostructure s with direct physical contact. Through the incorporation of a plasmon mediated photod eposition method the availability of the separated elect rons for redox reactions was demonstrated in the Au TiO 2 system The plasmon mediated photodeposition of Pt nanostructures was used to observe charge transfer and diffusion of separated electrons on individual Au TiO 2 heterostructures. Hydrothermal Technique TiO 2 nanorods were synthesized via a hydrothermal technique based on the technique developed by Li and Xu 55 The process involved the synthesis of an initial

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25 sodium titanate nanotube precursor (Na 2 x H x Ti 3 O 7 ) followed by the formation of TiO 2 nanorods. These TiO 2 nanorods were then used as the support for the plasmonic gold nanoparticles. Sodium Titanate Nanotube Synthesis To synthesize s odium titanate nanotubes 2 g of P25 TiO 2 powder was dispersed in an 80 mL solution of 10 M NaOH Once thoroughly mixed the solution was transferred to a 100 mL capacity Teflon line d stainless steel autoclave The autoclave was placed into a n oven heated to 120 C for 24 hours and allowed to cool naturally. W hite sodium titanate precipitates were produced These precipitate s were isolated and washed thoro ughly through centrifugatio n and dispersion cycles until the pH reached approximately 10.5. Anatase TiO 2 Nanorod Synthesis The sod ium titanate nanotube precursor was used without further modification To transform the sodium titanate nanotube precursor int o TiO 2 nanorods, 2 g of the wet sodium titanate precipitate were redispersed in 80 mL of deionized water. This solution was transferred to a 100 mL Teflon lined stainless steel autoclave and placed into an oven heated to 140 C for 24 hours and allowed to cool naturally when finished. The white nanorods were isolated from solution by centrifugation and redispersed in 50 mL of water for further use. The TiO 2 nanorods were uniformly grown and approximately 500 nm in length and 100 nm wide as observed in the scanning electron microscopy (S EM) images (Figure 3 1 A and B). The X ray diffraction (XRD) patterns confirm ed the presence of the tetragonal anatase phase of TiO 2 (Figure 3 2).

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26 Photodeposition of Metal Nanoparticles The TiO 2 nanorods from the above proced ure were used in the formation of a plasmonic photocatalyst made u p of Au nanoparticles and TiO 2 nanorods The photodeposition method takes advantage of the photocatalytic properties of the semiconductor 38 In this procedure, we rely on the reduction potential of the TiO 2 conduction band electrons ( 0.3 V vs. SHE ) excited by incident UV photons to reduce free gold chlorid e ions [AuCl 4 ] in solution to metallic gold, Au 0 on the surface of the TiO 2 nanorods (Equation 3 1 ) [ Au Cl 4 ] + 3 e Au 0 + 4Cl (1.0 V vs. SHE ) 56 (3 1) This method is used as an alternative to chemical reduction procedures because of the surfactant free nature of the resulting heterostructure This method creates a direct contact between the metal and semico nductor that is required for the Schottky barrier interface and facilitates direct charge transfer Au TiO 2 Heterostructure Synthesis In order to photocatalytically reduce Au onto the surface of TiO 2 0.25 mL of the presynthesized TiO 2 nanorod solution was added to 48 mL water in a 100 mL beaker. To this, 100 L of a 10 mM HAuCl 4 solution and 2 mL of methanol were added. The beaker was irradiated using a 500 W Hg (Xe) lamp with a beam turning dich roic mirror (280 400 nm) for 5 minutes under constant magne tic stirring. The sample solution turned color, from cloudy white to pink as a result of the nucleation and growth of Au nanostructures and the resulting LSPR absorption (Figure 3 3) In order to separate the Au TiO 2 heterostr uctures from the precursor sol ution the mixture was centrifuged and redispersed in 50 mL water.

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27 Plasmon Mediated Photodeposition of Pt Nanostructures on Au TiO 2 Using 10 mL of the Au TiO 2 heterostructure suspension, 20 L of 10 mM H 2 PtCl 6 solution and 0.4 mL of methanol were added to a round bottom flask. In addition, 40 L of 0.2 M NaOH was added to increase the colloidal stability of the Au TiO 2 heterostructures and decrease aggregation which allows for a more uniform photodeposition of the Pt nanostructures Once mixed, the flask w as evacuated and back filled with argon three times. Because oxygen species tend to adsorb on the surface of the TiO 2 is aqueous solution, t his degassing procedure was necessary in order to prevent any dissolved oxygen from scavenging the surface trapped e lectrons 14 57 The dissolved [ Pt Cl 6 ] 2 ions were reduced to Pt metal nanostructures on the surface o f the TiO 2 nano rods by a three hour irradiation with a 300 W Xe lamp 435 nm longpass filter and power density of 0.500 mW/cm 2 (Figure 3 4 ) Temperature measurements were taken to ensure that heat from the lamp did not play a role in the deposition Indeed the maximum temperature of the reaction did not exceed 32 C confirming that the deposition was not due to the decomposition of the Pt precursor The resulting Au TiO 2 Pt heterostructures were separated from the growth solution by centrifugation and red ispersed in water for characterization. Results and Discussion The preceding photodeposition synthesis allows for the in situ growth of Au nanoparticles on TiO 2 nanorods The surfactant less photodeposition method results in a direct contact between Au and TiO 2 which can support LSPR induced charge transfer from Au to TiO 2 under visible light conditions. According to the scanning electron microscope (SEM) images (Figure 3 1 C and D ) the Au deposited on TiO 2 was approximately 15 nm in diameter. Following m ultiple centrifugation and sonication

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28 cycles, the Au nanostructures remain firmly adhered confirming strong physical contact between Au nanoparticles and TiO 2 nanorods In the extinction spectrum (Figure 3 3 ) the LSPR extinction peak near 550 nm is prese nt in addition to the strong band gap absorption of the TiO 2 at 320 nm. The deposit ion of Pt was chosen to provide a visual demonstration of the LSPR mediated charge transfer from Au nanoparticles to the TiO 2 CB (Figure 3 5 ) In addition to using Pt as an indicator of separated electrons, the effect of using Pt as an electron sink has been demonstrated in literature to reduce the recombination process and increase the photocatalytic activity 10 11 58 59 I t is expected to play a similar role in this system by preventing the back transfer of electrons to TiO 2 and the recombination with separated charge vacan cies on the Au nanostructures. To demonstrate the plasmon mediated charge transfer and prevent the ba nd gap excitation of TiO 2 a 435 nm longpass filter was chosen It was observed following the irradiation of Au TiO 2 with visible light that the nucleation and growth of Pt nanoparticles can occur with the reduction of [ Pt Cl 6 ] 4 ions in solution (Equation 3 2 and 3 3 ). [PtCl 6 ] 4 + 2e [PtCl 4 ] 2 + 2Cl (0.68 V vs. SHE ) 56 (3 2) [PtCl 4 ] 2 + 2 e Pt 0 + 4Cl (0.76 V vs. SHE ) 56 (3 3 ) After three hours, a large amount of uniformly distributed Pt nanoparticles are observable under SEM (Figu re 3 1 E and F ). The Au and Pt nanostructures are clearly visible and show a relatively uniform distribution on the surface of the TiO 2 in the TEM images (Figure 3 6 A, B and C ). The h igh resolution transmission electron microscopy (HRTEM) analysis confirm s the presence of Au TiO 2 and Pt by resolving and indexing

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29 the {101} plane of TiO 2 (0.35 nm), the {110} plane of Au (0.14 nm), and the {111} plane of Pt (0.23 nm) (Figure 3 6 D ). In order to analyze the function of each component in the heterostructure an d exclude the possibility of electron hole pair generation due to the substituents alone a series of control experiments were conducted. When TiO 2 nanostructures were irradiated in the presence of H 2 PtCl 6 under otherwise identical experiment al conditions (>435 nm light 3 hours ), no Pt deposition or free nucleation was observed (Figure 3 7 ) This further suggests that the LSPR excitation of Au is required to facilitate charge separation under visible light conditions and that there is no band gap excitati on of the TiO 2 during the Pt photodeposition. These results also explain the observable extinction of the TiO 2 that extends into the visible range as scattering (Figure 3 3 ) In order to exclude the possibility that Pt can be nucleated by surfactant less A u nanostructures through excitation of the LSPR or interband transitions in the absence of TiO 2 Au nanostructures were photodeposited onto the surface of silica (SiO 2 ) This procedure was chosen because it is ideal to use surfactant free Au nanoparticles in order to avoid any influen ce on the reduction of Pt due to chemical interaction with the surfactant To perform this control reaction, 1 mL of a 100 mM silica bead solution and 10 mM HAuCl 4 solution were combined with 2 mL of methanol and 48 mL of water This solution was irradiated by a 500 W Hg (Xe) lamp with a beam turning dichroic mirror (280 nm 240 nm) for 10 min utes to nucleate Au nanostructures based on a photoreduction mechanism (SiO 2 band gap, 8.9 eV) No Pt deposition was observed on bare Au particles nucleated on silica or Au nanostructures that had detached from the silica support (Figure 3 8 ) This suggests that it is not only Au that is required for

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30 electron hole pair generation and Pt deposition under visible light conditions, but that th e Au TiO 2 interface fac ilitates the charge separation. It is expected that the nucleation and growth occurs within close proximity to where the Pt ions are reduced b y the separated electrons injected from the Au to the semiconductor As discussed earlier a Schottky barrier of ~1 eV 23 24 is expected to form at the Au TiO 2 interface. After the visible light exci tatio n of the Au nanoparticles either through the interband transition or LSPR mode an electron with sufficient energy can overcome this barrier and transfer to the TiO 2 conduction band leaving behind a charge vacan cy on Au 20 The second purpose of the Schottky barrier in this system is to create a potential well for injected conduction band electrons and decrease the probability of back transfer to Au These se parated electrons can diffuse to the surface of the TiO 2 where they can reduce the Pt salt in solution to Pt metal. In this reaction, methanol acts as a sacrificial reagent and can quickly become oxidized by the Au nanostructures 14 In general, t he fast recombination of generated electron hole pairs hinders the photocatalytic activity in numerous systems 13 44 60 61 This LSPR mediated photodeposition method provides a visual map of el ectron transfer and diffusion through the TiO 2 semiconductor lattice on the single nanostructure level According to Figures 3 1 and 3 6 the Pt nanostructures show a nearly uniform distribution and lengths of up to 100 nm between nearby Au nanostructures. This observation suggests a relatively long diffusion length and consequently, a long electron lifetime. In addition to the uniform distribution of Pt nanostructures on the surface of the TiO 2 the energy di spersive X ray spectroscopy (EDS ) analysis (Figu re 3 9 ) suggests some Pt deposition on the Au nanostructure by monitoring of the L and L X ray transition s Because no Pt was

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31 observed with bare TiO 2 under visible light conditions, it is suggested that the observed Pt deposition on Au is a result of ele ctron back transfer from the TiO 2 CB to Au nanoparticles However, it is worth noting that because Pt nanostructures were observed on Au, it means that the back transfer did not always result in electron hole recombination This suggests that the oxidation of methanol is a faster reaction compared to the electron hole pair recombination process. In ad dition to the insights into plasmon mediated charge transfer provided by this Au TiO 2 Pt system, plasmon mediated charge transfer is a useful synthesis method to fabricate a variety of heterostructures by substituting different redox reactions. For example, t he separated electron hole pairs can be monitored by the plasmon mediated phot odeposition of palladium from [PdCl 4 ] 2 ions in solution (Figure 3 10 ). In ord er to demonstrate further the application of the plasmon mediated charge separation mechanism, Au ZnO (Figure 3 11 A and B ) was used as a heterostructure support for the plasmon mediated p hotodeposition of Pt The needle shaped ZnO rods used are several mi crometers in length and a few hundred nanometers in width. The photodeposition of Au onto ZnO was prepared using a similar photodeposition procedure as the Au TiO 2 heterostructures and yielded a uniform deposition of Au nanostructures on the ZnO surface. U sing this heterostructure resulted in the uniform deposition of Pt nanoparticles (Figure 3 11 C) in the same manner as the Au TiO 2 Pt photodeposition. The synthesis presented herein provides a visual map of separated electron hole pairs across a metal semi conductor interface when the heterostructure is irradiated by visible light. T he direct observation of Pt nanoparticles on a single nanostructure level

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32 demonstrates the availability of transferred electrons to reduce adsorbed species Ultimately, the resul ts of this study provide fundamental insight into the direct charge transfer LSPR mechanism

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33 Figure 3 1 SEM image s comparing Au and Pt photodeposition A B) H ydrothermally synthesized TiO 2 nanorods. C D ) Photodeposited gold nanostructures on TiO 2 nano rods. E F ) Au TiO 2 Pt prepared through a plasmon mediated photodeposition. Scale bars: A, C and E: 500 nm; B, D and F: 200 nm.

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34 Figure 3 2. X ray diffraction pattern for anatase phase TiO 2 (tetragonal, I 4 1 / amd JCPDS 21 1272). Figure 3 3 Extinction Spe ctra. TiO 2 nanorods (black), Au TiO 2 heterostructures (red), and Au TiO 2 Pt heterostructures (blue). Dashed line represents the spectral location of the 435 nm longpass filter.

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35 Figure 3 4 Experimental design for photochemical reduction and oxidation re actions. Figure 3 5 Schematic of plasmon mediated photodeposition.

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36 Figure 3 6 TEM and HRTEM images of plasmon medi ated deposition. A C) TEM images of Au TiO 2 Pt heterostructures (scale bars: 500 nm, 100 nm, 50 nm) D) HRTEM image of heterostructure with lattice spacings (scale bar: 5 nm ) Pt nanoparticles are circled

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37 Figure 3 7 TEM images of TiO 2 in the presence of H 2 PtCl 6 and methanol following visible light irradiation. A) 500 nm scale bar. B) 100 nm scale bar. Figure 3 8 TEM images of surf actant less gold nanoparticles after visible light irradiation in the presence of H 2 PtCl 6 A and C) SiO 2 supported gold nanostructures .B and D) F ree gold nanostructures Scale bar s : 50 nm.

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38 Figure 3 9 Location specific EDS spectra. A) TEM image of Au TiO 2 Pt heterostructure. B) Spectrum 1 gold nanostructure. C) Spectrum 2, platinum nanostructure. D) Spectrum 3, TiO 2 surface. Scale bar: 20 nm.

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39 Figure 3 10 Characterization of Au TiO 2 Pd. A B) SEM images of Au TiO 2 Pd heterostructures with scale bars of 200 nm and 100 nm, respectively. C) TEM image of Au TiO 2 Pd heterostructure s (scale bar: 50 nm). D E) EDS specified locations in the TEM image, corresponding to Au (D) and Pd (E)

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40 Figure 3 11 Electron microscopy images of Au ZnO and Au ZnO Pt heterost ructures. A) SEM image of Au ZnO scale bar : 1 m. B) SEM image of Au ZnO s cale bar : 100 nm. C) TEM image of Au ZnO Pt scale bar : 50 nm.

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41 CHAPTER 4 CONCLUSION As the adverse effects resulting from the use of hydrocarbon fuels continue to prompt a reevaluation of the way fuel is harvested and consumed strategies to make use of renewable energy resources will remain active subjects of research. As a result, the field of plasmonics is a field that is becoming more active for applications in light harvesting. E xtend ing the absorption of heterostructure m at erials into the visible range, increasing the probability of charge separation, and limiting the possibility of recombination are a few of the advantages of using plasmonic materials in heterostructure photocatalysis We have demonstrated plasmon mediate d charge transfer on a single nanostructure level. In a Au TiO 2 heterostructure, the absorption of visible light by the Au nanostructure can lead to the efficient charge transfer to the TiO 2 semiconductor This is observed by the significant deposition of Pt nanoparticles on the TiO 2 surface. The transferred electrons are suggested to have a relat ively long diffusion distance within the TiO 2 semiconductor and are expected to improve the visible light activity of semiconductor based heterostructures for chem ical fuel production. Furthermore, it has been shown that this versatile method can be adapted to similar sem iconductor and metal materials.

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46 BIOGRAPHICAL SKETCH Brendan Sweeny was born in Burlington, Vermont in 1987 to Elizabeth and Gary Sweeny. He grew up in Willis t on, Vermont and attended Champl ain Valley Union High School. Due to the engaging science courses and enthusiasm of his teachers and peers it was in high school that his interest in science and chemistry began to develop. Outside of academics, his time was spent p laying baseball, snowboarding and enjoying the company of family and friends. Brendan enrolled a t Saint Anse lm College in 2005 to pursue a b s degree in chemistry and graduated in 2009 After enrolling at the University of Flori da, Brendan began working under the direction of Dr. W. David Wei and with the incorporation of plasmonic materials in photocatalysis. Following his time at UF, he intends to pursue a career in renewable energy research.