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SURFACE PLASMON RESONANT ENHANCEMENT OF PHOTOCATALYSIS
by
Wenbo Hou
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
August 2012
Copyright 2012 Wenbo Hou
Object Description
| Title | Surface plasmon resonant enhancement of photocatalysis |
| Author | Hou, Wenbo |
| Author email | wenbohou@usc.edu;whwenbohou@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts And Sciences |
| Date defended/completed | 2012-06-04 |
| Date submitted | 2012-07-09 |
| Date approved | 2012-07-09 |
| Restricted until | 2012-07-09 |
| Date published | 2012-07-09 |
| Advisor (committee chair) | Steve, Cronin B. |
| Advisor (committee member) |
Reisler, Hanna Haas, Stephan |
| Abstract | Light absorbed by semiconductors creates electron-hole pairs that are separated in energy by the bandgap of the material. This energy separation can be used to drive electrons in a circuit (solar cells) or to drive electrochemical redox reactions (photocatalysis). These two types of solar energy conversion (photon-to-electric and photon-to-chemical energy conversion) are completely analogous and face similar challenges in achieving high conversion efficiencies. The main factor limiting solar cell and photocatalyst efficiencies is the inherent mismatch between the absorption spectra of semiconductors and the solar spectrum. For example, TiO₂, as one of the most promising semiconductors for solar cells and photocatalysis, does not absorb light in the visible region of the electromagnetic spectrum. Because of TiO₂’s short wavelength cutoff, there are very few solar photons (~4%) that can be used to drive this photocatalyst. In my dissertation, I demonstrate a new mechanism for inducing increased amounts of charge in TiO₂ films by exploiting the extremely large plasmon resonance of Au nanoparticles with strongly catalytic TiO₂. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields, which can be used to increase sub-bandgap electron-hole pair generation in semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors, like TiO₂, can be extended into the visible region of the electromagnetic spectrum. ❧ My dissertation includes three major applications of plasmon-enhanced photocatalysis: plasmonic enhancement of photocatalytic decomposition of methyl orange under visible light, photocatalytic conversion of CO₂ to hydrocarbon fuels via plasmon-enhanced absorption and metallic interband transitions, and plasmon resonant enhancement of dye sensitized solar cells. After these applications, the effect of doping in plasmon-enhanced photocatalysis is discussed and the photocatalytic activities of several other semiconductors are evaluated. ❧ In Chapter One, metal nanoparticle enhanced photocatalysis is reviewed. This chapter starts with a brief introduction of the surface plasmon resonance phenomenon and basic principles of photocatalytic reactions, including degradation of organic wastes, water splitting, and reduction of CO₂. This is followed by a summary of a recent burst of papers in this field. A particular emphasis is given to the factors limiting photocatalytic conversion efficiencies and the plasmon enhancement mechanisms by which surface plasmon resonance of noble metal nanoparticles can influence the photocatalytic activity of nearby semiconductors. ❧ In Chapter Two, the application of plasmon resonant enhancement to increase the photocatalytic decomposition of methyl orange under visible light is demonstrated. A 9-fold improvement in the photocatalytic decomposition rate of methyl orange is observed using a photocatalyst consisting of strongly plasmonic Au nanoparticles deposited on top of strongly catalytic TiO₂. While the plasmonic Au nanoparticles enhance the photocatalytic activity of TiO₂ in the visible range, they result in a reduction in the photocatalytic activity under UV exposure, due to the reduction in TiO₂ surface area exposed to the aqueous solution. Finite-difference time-domain (FDTD) simulations of these Au nanoparticle/TiO₂ photocatalysts show that the enhanced photocatalytic activity is due to the large plasmonic enhancement of the incident electromagnetic fields, which increases the electron–hole pair generation rate at the TiO₂ surface, and hence the photodecomposition rate of methyl orange. This enhancement mechanism relies on the presence of defect states in the TiO₂, which enables sub-bandgap absorption. The near-field optical enhancement of the Au nanoparticles couples light efficiently to the surface of the TiO₂, making its photocatalytic performance robust to defects. ❧ In Chapter Three, a systematic study of the mechanisms of Au nanoparticle/TiO₂-catalyzed photoreduction of CO₂ and water vapor is carried out over a wide range of wavelengths. When the photon energy matches the plasmon resonance of the Au nanoparticles (free carrier absorption), which is in the visible range (532 nm), a 24-fold enhancement in the photocatalytic activity is observed because of the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles as discussed in Chapter Two. When the photon energy is high enough to excite d band electronic transitions in the Au, in the UV range (254 nm), a different mechanism occurs resulting in the production of additional reaction products, including C₂H₆, CH₃OH, and HCHO. This occurs because the energy of the d band excited electrons lies above the redox potentials of the additional reaction products CO₂/C₂H₆, CO₂/CH₃OH, and CO₂/HCHO. We model the plasmon excitation at the Au nanoparticle-TiO₂ interface using FDTD simulations, which provides a rigorous analysis of the electric fields and charge at the Au nanoparticle-TiO₂ interface. ❧ In Chapter Four, the application of plasmonic enhancement to improve the efficiency of dye sensitized solar cells (DSSCs) is explored. By comparing the performance of DSSCs with and without Au nanoparticles, a 2.4-fold enhancement in the photoconversion efficiency is demonstrated. Enhancement in the photocurrent extends over the wavelength range from 460 nm to 730 nm. The underlying mechanism of enhancement is investigated by comparing samples with different geometries, including nanoparticles deposited on top of and embedded in the TiO₂ electrode, as well as samples with the light absorbing dye molecule deposited on top of and underneath the Au nanoparticles. The mechanism of enhancement is attributed to the local electromagnetic response of the plasmonic nanoparticles, which couples light very effectively from the far field to the near field at the absorbing dye molecule monolayer, thereby increasing the local electron–hole pair (or exciton) generation rate significantly. The UV-vis absorption spectra and photocurrent spectra provide further information regarding the energy transfer between the plasmonic nanoparticles and the light absorbing dye molecules. Based on scanning electron microscope images, we perform electromagnetic simulations of these different Au nanoparticle/dye/TiO₂ configurations, which corroborate the enhancement observed experimentally. ❧ In Chapter Five, the effect of doping in photocatalysis is explored. TiO₂ is doped by ion implantation, plasma ion implantation (PII), and annealing in H₂, CH₄. The p- or n-type carriers of these H, C, and N-doped TiO₂ films are measured using hot-probe measurements. The p- or n-type carriers then are correlated with the photocatalytic performance, which is measured in the photocatalytic water splitting system. This study serves to establish the validity of the plasmonic enhancement mechanism proposed in the previous chapters. ❧ In Chapter Six, the photocatalytic activities of several other semiconductors, including Fe₂O₃, GaN, Nb-doped SrTiO₃, spray pyrolysis TiO₂, and thermally oxidized TiO₂ are investigated. This study serves to evaluate alternative semiconductor photocatalysts with potentially higher photocatalytic efficiencies. |
| Keyword | photocatalysis; plasmon; plasmonic; semiconductor; Au nanoparticles |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m |
| Rights | Hou, Wenbo |
| Access conditions | The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given. |
| Repository name | University of Southern California Digital Library |
| Repository address | USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA |
| Repository email | cisadmin@usc.edu |
| Archival file | uscthesesreloadpub_Volume4/etd-HouWenbo-926.pdf |
Description
| Title | Page 1 |
| Full text | SURFACE PLASMON RESONANT ENHANCEMENT OF PHOTOCATALYSIS by Wenbo Hou A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY) August 2012 Copyright 2012 Wenbo Hou |
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