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Surface plasmon resonant enhancement of photocatalysis
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Content
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
ii
Acknowledgements
Firstly, I would like to thank my supervisor, Prof. Stephen B. Cronin for his
invaluable help and guidance. I greatly appreciate all of his time, hard work,
encouragements, suggestions, and funding he invested in me. I would also like to thank
Prof. Hanna Reisler, my advisory committee member, and Prof. Curt Wittig for all of
their help during my PhD study. Particularly, I couldn’t survive my first semester at USC
without their help and support. Thank Prof. Stephan Haas for being my outside
committee member.
I’m thankful to all the members of Cronin research group, Zuwei Liu, Jesse R.
Theiss, Mehmet Aykol, Prathamesh Pavaskar, Wei-Hsuan Hung, Chia-Chi Chang, Chun-
Chung Chen, David Valley, I-Kai Hus, Rohan Dhall, Shun-Wen Chang, Mohammed
Amer, Jing Qiu, Zhen Li, and Guangtong Zeng. Especially, thanks to Zuwei Liu for his
help in experimental setup; thanks to Jesse R. Theiss for helping me evaporate Au thin
films; thanks to Prathamesh Pavaskar for his help in simulations; thanks to Mehmet
Aykol for helping me fix instrument problems. In addition, I also want to thank Prof.
Surya Prakash and his postdocs, Dr. Alain Goeppert and Dr. Bo Yang for their help in
GC measurements.
Much thanks to the staffs in Chemistry and EEP departments. Thanks to
Angelique Miller and Kim Reid for ordering chemicals. Thanks to Michele Dea for
managing my financial support. Thanks to Katie McKissick for preparing all the
paperwork.
Finally, I’d like to thank my parents for their constant love and support.
iii
Table of Contents
Acknowledgements ............................................................................................................. ii
List of Tables ...................................................................................................................... v
List of Figures .................................................................................................................... vi
Abstract .............................................................................................................................. xi
Chapter 1: A Review of Metal Nanoparticle Enhanced Photocatalysis ............................. 1
1.1 Introduction .............................................................................................................. 1
1.2 Surface Plasmon Resonance .................................................................................... 4
1.3 Basic Principles ........................................................................................................ 6
1.3.1 Dye Photodegradation ...................................................................................... 6
1.3.2 Photocatalytic Water Splitting ......................................................................... 7
1.3.3 Photocatalytic Reduction of CO
2
................................................................... 11
1.4 Factors Limiting the Photocatalytic Efficiency...................................................... 13
1.4.1 Metal Loading ................................................................................................ 14
1.4.2 Surface Area ................................................................................................... 15
1.4.3 Matching the Solar Spectrum ......................................................................... 17
1.4.4 Oxidation of Ag .............................................................................................. 17
1.4.5 Metal Nanoparticle Size ................................................................................. 18
1.5 Mechanisms ........................................................................................................... 19
1.5.1 Enhancement Mechanism under UV Illumination ......................................... 20
1.5.2 Enhancement Mechanisms under Visible Illumination ................................. 23
Chapter 2: Plasmonic Enhancement of Photocatalytic Decomposition of Methyl
Orange under Visible Light .................................................................................. 28
2.1 Introduction ............................................................................................................ 29
2.2 Experimental .......................................................................................................... 31
2.3 Results and Discussion ........................................................................................... 32
2.4 Conclusions ............................................................................................................ 40
Chapter 3: Photocatalytic Conversion of CO
2
to Hydrocarbon Fuels via Plasmon-
Enhanced Absorption and Metallic Interband Transitions ................................... 41
3.1 Introduction ............................................................................................................ 42
3.2 Experimental .......................................................................................................... 45
3.3 Results and Discussion ........................................................................................... 50
3.4 Conclusions ............................................................................................................ 68
Chapter 4: Plasmon Resonant Enhancement of Dye Sensitized Solar Cells .................... 69
4.1 Introduction ............................................................................................................ 70
iv
4.2 Experimental .......................................................................................................... 72
4.3 Results and Discussion ........................................................................................... 76
4.4 Conclusions ............................................................................................................ 87
Chapter 5: Exploring the Effect of Doping in TiO
2
.......................................................... 89
5.1 Sol-gel TiO
2
........................................................................................................... 89
5.2 Ion Implantation ..................................................................................................... 91
5.3 N-doping by Plasma Ion Implantation (PII) .......................................................... 92
5.4 H-doping ................................................................................................................ 94
5.5 C-doping ................................................................................................................. 96
5.6 Photocatalytic Characterization ............................................................................. 97
5.7 Conclusions ............................................................................................................ 99
Chapter 6: Photocatalytic Activity of Several Semiconductors ...................................... 100
6.1 Spray Pyrolysis TiO
2
............................................................................................ 100
6.2 Thermally Oxidized TiO
2
..................................................................................... 106
6.3 ALD TiO
2
............................................................................................................. 107
6.4 Ga
1-x
As
x
N ............................................................................................................. 108
6.5 Fe
2
O
3
.................................................................................................................... 113
6.6 Nb-doped SrTiO
3
.................................................................................................. 118
Bibliography ................................................................................................................... 120
v
List of Tables
Table 3-1. Overall quantum efficiency (%) of reduction of CO
2
and H
2
O on TiO
2
,
Au, and Au/TiO
2
catalysts under UV and visible irradiation. .............................. 63
Table 4-1. Summary of photovoltaic device performances. ............................................. 85
Table 4-2. Comparison of photovoltaic device performances of Au
nanoparticle/dye/TiO
2
configuration #2 with and without the second
annealing. .............................................................................................................. 86
Table 5-1. Conditions of Cr ion implantation for samples #1-4. ...................................... 92
Table 6-1. Summary of solar cell performances under solar simulator illumination.
............................................................................................................................. 106
Table 6-2. Growth and material parameters of Ga
1-x
As
x
N samples. .............................. 109
Table 6-3. Quantum efficiencies of TiO
2
and Ga
1-x
As
x
N-photocatalyzed methane
generation with and without plasmon enhancement under 532 nm laser
illumination. ........................................................................................................ 113
vi
List of Figures
Figure 1-1. Schematic illustration of semiconductor photocatalysis. ................................. 2
Figure 1-2. Energy band diagrams for various semiconductors. ........................................ 3
Figure 1-3. (a) Schematic illustration of plasmon oscillations. (b) Simulation of
the electrical field near Au nanoparticle. ................................................................ 6
Figure 1-4. Energy band diagrams of several semiconductors plotted together with
the redox potentials to form HO
2
· and OH· radicals. .............................................. 7
Figure 1-5. Schematic diagram of a photoelectrochemical cell for photocatalytic
water splitting.......................................................................................................... 8
Figure 1-6. The redox potentials of hydrogen and oxygen evolution at different
pH values together with the valence band and conduction band energies of
semiconductors. .................................................................................................... 11
Figure 1-7. Schematic illustration of a single-particle photocatalytic reaction for
the reduction of CO
2
. ............................................................................................ 13
Figure 1-8. Schematic diagram illustrating the charge separation mechanism. ............... 22
Figure 1-9. Schematic illustration of a proposed charge transfer mechanism.
(Reprinted with permission from Ref [158] Copyright (2005) American
Chemical Society.) ................................................................................................ 24
Figure 1-10. Schematic diagrams of the local electric field enhancement
mechanism (left) and the charge transfer mechanism (right). ............................. 26
Figure 1-11. Electric field enhancement contour around Ag/SiO
2
/CdS multilayer
nanocomposites with different thickness ratios (a) R
1
/R
2
/R
3
= 20/30/50 nm;
(b) R
1
/R
2
/R
3
= 17/25/50 nm; and (c) R
1
/R
2
/R
3
= 15/20/50 nm. (Reprinted
with permission from Ref [38] Copyright (2011) Elsevier.) ................................. 27
Figure 2-1. UV-Vis absorption spectra of TiO
2
with and without gold
nanoparticles. ........................................................................................................ 33
Figure 2-2. UV-Vis spectra of MO aqueous solution before (black) and after (red)
1h UV illumination using (a) TiO
2
and (b) Au nanoparticle/TiO
2
photocatalysts. ....................................................................................................... 34
vii
Figure 2-3. UV-Vis spectra of MO aqueous solution before (black) and after (red)
1h 532 nm laser illumination using (a) TiO
2
and (b) Au/TiO
2
as
photocatalysts. ....................................................................................................... 35
Figure 2-4. (a) SEM image of a 5 nm thick Au island film deposited on anodic
TiO
2
. (b-d) Electric field intensities calculated at the interface of Au –
TiO
2
using the FDTD method. .............................................................................. 38
Figure 3-1. Schematic diagrams of three types of photocatalysts. ................................... 47
Figure 3-2. (a) Raman spectrum and (b) XRD profile of sol-gel TiO
2
. ............................ 48
Figure 3-3. SEM image of 5 nm Au thin film after the second annealing. ....................... 49
Figure 3-4. Schematic diagram of experimental setup. .................................................... 50
Figure 3-5. (a) Photocatalytic product yields (after 15 h of visible irradiation) on
three different catalytic surfaces. (b) Energy band alignment of anatase
TiO
2
, Au, and the relevant redox potentials of CO
2
and H
2
O under visible
illumination. .......................................................................................................... 52
Figure 3-6. UV-vis absorption spectra of TiO
2
with and without gold
nanoparticles and gold nanoparticles deposited on glass by electron-beam
evaporation. ........................................................................................................... 53
Figure 3-7. (a) Photocatalytic product yields (after 15 h of 254 nm UV irradiation)
on three different catalytic surfaces. (b) Energy band alignment of anatase
TiO
2
, Au, and the relevant redox potentials of CO
2
and H
2
O under 254 nm
UV illumination. ................................................................................................... 57
Figure 3-8. X-ray photoelectron spectra (XPS) of Au/TiO
2
sample before and
after reactions. ....................................................................................................... 58
Figure 3-9. Photocatalytic product yields of 5 nm Pt on glass and a Cu foil
compared with that of 5 nm Au on glass. ............................................................. 59
Figure 3-10. (a) Photocatalytic product yields (after 15 h of 365 nm UV
irradiation) on three different catalytic surfaces. (b) Energy band
alignment of anatase TiO
2
, Au, and the relevant redox potentials of CO
2
and H
2
O under 365 nm UV illumination. ............................................................. 61
Figure 3-11. (a) TEM image of a 5 nm thick Au island film deposited on TiO
2
. (b-
c) Electric field intensity at the interface of Au – TiO
2
calculated using
FDTD. (d) Voltage and (e) Current density across the Au – TiO
2
interface. ....... 67
viii
Figure 4-1. Schematic diagrams of three different Au nanoparticle/dye/TiO
2
configurations. ...................................................................................................... 74
Figure 4-2. (a) SEM image of 5 nm Au thin film before the second annealing. (b)
SEM image of 5 nm Au thin film after the second annealing. (c) SEM
image of TiO
2
film with a mesoporous structure. ................................................. 75
Figure 4-3. Short-circuit photocurrents of DSSCs with three different Au
nanoparticle/dye/TiO
2
configurations. .................................................................. 77
Figure 4-4. Short-circuit photocurrents of DSSCs with bare TiO
2
, Au
nanoparticles embedded in TiO
2
without dye molecules, and 5 nm Au thin
film without the second annealing deposited between the TiO
2
layer and
the dye monolayer as working electrodes. ............................................................ 78
Figure 4-5. (a) Photocurrent spectra of DSSCs with different working electrodes.
(b) Absorption spectra of different working electrodes. ....................................... 81
Figure 4-6. (a) Absorption spectra of bare TiO
2
and Au nanoparticles embedded
in TiO
2
without dye molecules. (b) Photocurrent spectra of DSSCs with
bare TiO
2
and Au nanoparticles embedded in TiO
2
without dye molecules
as working electrodes. ........................................................................................... 83
Figure 4-7. I-V characteristics of DSSCs with different working electrodes. .................. 85
Figure 4-8. FDTD calculated enhancement factor plotted as a function of
wavelength for embedded nanoparticles and Au nanoparticles on top of
TiO
2
with respect to the conventional DSSC. ....................................................... 87
Figure 5-1. (a) Raman spectrum, (b) XRD profile, and (c) SEM image of sol-gel
TiO
2
. ...................................................................................................................... 90
Figure 5-2. UV-vis spectra of sol-gel TiO
2
thin films before and after Cr ion
implantation. ......................................................................................................... 92
Figure 5-3. UV-vis absorption spectra of sol-gel TiO
2
thin films before and after
N ion implantation before and after annealing in O
2
. ........................................... 93
Figure 5-4. (a) UV-vis absorption spectra and (b) photograph of sol-gel TiO
2
thin
films before and after H-doping at different temperatures. .................................. 94
Figure 5-5. (a) Schematic diagram of the hot-probe measurement. V oltage vs. time
curve of (b) standard silicon n-type semiconductor and (c) H-doped TiO
2
in the hot-probe measurement. .............................................................................. 95
ix
Figure 5-6. (a) UV-vis absorption spectra and (b) photograph of sol-gel TiO
2
thin
films before and after C-doping at different temperatures. V oltage vs. Time
curve of (c) standard p-type silicon semiconductor and (d) C-doped TiO
2
in the hot-probe measurement. .............................................................................. 97
Figure 5-7. The photoelectrochemical cell for photocatalytic water splitting. ................. 98
Figure 5-8. Log of H
2
signal intensity measured by mass spectrometer for H-
doped and C-doped TiO
2
-catalyzed water splitting reactions. .............................. 99
Figure 6-1. I-V characteristics of three types of Au/TiO
2
photocatalysts in the
water splitting reactions. ..................................................................................... 101
Figure 6-2. Short-circuit photocurrent with (a) 532 nm laser (b) UV light on and
off. ....................................................................................................................... 102
Figure 6-3. I-V characteristics of (a) embedded Au nanoparticles in TiO
2
and (b)
evaporated Au nanoparticles on TiO
2
. (c) Short-circuit photocurrent under
532 nm laser irradiation. (d) Dye N719 absorption spectrum superimposed
over the solar spectrum A.M. 1.5. ....................................................................... 104
Figure 6-4. I-V characteristics of three types of Au/TiO
2
DSSCs under simulated
sun light. .............................................................................................................. 105
Figure 6-5. I-V characteristics for photocatalytic water splitting of (a) thermal
oxidized TiO
2
compared with that of (b) ATO and (c) sol-gel TiO
2
. ................. 107
Figure 6-6. Short-circuit photocurrent of ALD TiO
2
under (a) UV and (b) 532 nm
wavelength laser irradiation. ............................................................................... 108
Figure 6-7. UV-vis absorption spectra of Ga
1-x
As
x
N samples. ....................................... 109
Figure 6-8. I-V characteristics of Ga
1-x
As
x
N samples for photocatalytic water
splitting. .............................................................................................................. 111
Figure 6-9. Gas chromatograph data for Ga
1-x
As
x
N-photocatalized methane
generation under (a) UV and (b) under 532 nm laser irradiation. ...................... 112
Figure 6-10. UV-vis absorption spectra of Fe
2
O
3
prepared by three different
methods. .............................................................................................................. 114
Figure 6-11. UV-vis absorption spectra indicating the concentration change of
methyl orange after (a) UV and (b) 532 nm laser illumination. ......................... 115
x
Figure 6-12. GC data indicating the relative amount of products generated in
Fe
2
O
3
-photocatalyzed reduction of CO
2
with H
2
O under UV and 532 nm
laser illumination. ............................................................................................... 116
Figure 6-13. Short-circuit photocurrent of bare Fe
2
O
3
-catalyzed water splitting
under (a) UV and (b) 532 nm laser irradiation. .................................................. 117
Figure 6-14. I-V characteristics of photocatalytic water splitting for Fe
2
O
3
samples
prepared by (a) electrochemical oxidation and (b) thermal oxidation. ............... 117
Figure 6-15. (a) Short-circuit photocurrent of Nb-doped SrTiO
3
-catalyzed water
splitting under 532 nm laser irradiation. (b) I-V characteristics of Nb-
doped SrTiO
3
for photocatalytic water splitting. ................................................. 119
xi
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, TiO
2
, 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 TiO
2
’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
TiO
2
films by exploiting the extremely large plasmon resonance of Au nanoparticles with
strongly catalytic TiO
2
. 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 TiO
2
, 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 CO
2
to hydrocarbon fuels via
xii
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 CO
2
. 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 TiO
2
. While the plasmonic Au nanoparticles enhance the
photocatalytic activity of TiO
2
in the visible range, they result in a reduction in the
photocatalytic activity under UV exposure, due to the reduction in TiO
2
surface area
exposed to the aqueous solution. Finite-difference time-domain (FDTD) simulations of
these Au nanoparticle/TiO
2
photocatalysts show that the enhanced photocatalytic activity
is due to the large plasmonic enhancement of the incident electromagnetic fields, which
xiii
increases the electron–hole pair generation rate at the TiO
2
surface, and hence the
photodecomposition rate of methyl orange. This enhancement mechanism relies on the
presence of defect states in the TiO
2
, which enables sub-bandgap absorption. The near-
field optical enhancement of the Au nanoparticles couples light efficiently to the surface
of the TiO
2
, making its photocatalytic performance robust to defects.
In Chapter Three, a systematic study of the mechanisms of Au nanoparticle/TiO
2
-
catalyzed photoreduction of CO
2
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 C
2
H
6
, CH
3
OH, and
HCHO. This occurs because the energy of the d band excited electrons lies above the
redox potentials of the additional reaction products CO
2
/C
2
H
6
, CO
2
/CH
3
OH, and
CO
2
/HCHO. We model the plasmon excitation at the Au nanoparticle-TiO
2
interface
using FDTD simulations, which provides a rigorous analysis of the electric fields and
charge at the Au nanoparticle-TiO
2
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
xiv
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 TiO
2
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/TiO
2
configurations, which corroborate the
enhancement observed experimentally.
In Chapter Five, the effect of doping in photocatalysis is explored. TiO
2
is doped
by ion implantation, plasma ion implantation (PII), and annealing in H
2
, CH
4
. The p- or
n-type carriers of these H, C, and N-doped TiO
2
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.
xv
In Chapter Six, the photocatalytic activities of several other semiconductors,
including Fe
2
O
3
, GaN, Nb-doped SrTiO
3
, spray pyrolysis TiO
2
, and thermally oxidized
TiO
2
are investigated. This study serves to evaluate alternative semiconductor
photocatalysts with potentially higher photocatalytic efficiencies.
1
Chapter 1: A Review of Metal Nanoparticle Enhanced
Photocatalysis
1.1 Introduction
Solar energy is a clean, renewable, carbon neutral, and abundant energy
alternative to fossil fuels. In addition to direct solar-to-electric conversion (as in a solar
cell), photocatalysis provides an alternative method for storing the solar energy in
chemical bonds that can be released later without producing harmful byproducts.[106]
Photocatalytic water splitting, reduction of CO
2
into fuels, and degradation of organic
wastes to purify water and air have been well studied.[18, 102, 133] Many
semiconductors serve as good photocatalysts due to their electronic band structures. In
photocatalysis, incident light with energy matching or exceeding the band gap is
absorbed by semiconductors, creating electron-hole pairs. The photo-generated electrons
enter the conduction band, leaving the holes in the valence band. The energy separation
between the electron-hole pairs can be used to drive redox reactions (e.g., water splitting
or reduction of carbon dioxide), as shown in Figure 1-1. Ideally, semiconductors for
photocatalysis should have a band gap small enough to absorb the visible light in the
solar spectrum with the conduction band lying higher in energy (or more negative on the
NHE (normal hydrogen electrode) scale) than the potential of the reduction half reaction
and the valence band lying lower in energy (or more positive) than the potential of the
oxidation half reaction.[103] The energy diagrams of several semiconductors are plotted
in Figure 1-2. together with the redox potentials of the two half reactions for water
2
splitting. The difference in potential between the vacuum level and the NHE is usually
taken to be -4.5 eV.[162] Several semiconductors have band gap energies sufficient for
driving a wide range of desired chemical reactions, for example, TiO
2
, WO
3
, SrTiO
3
,
Fe
2
O
3
, Cu
2
O, GaP, and PbO. However, TiO
2
has proven to be the most suitable for
photocatalysis since it is biologically and chemically inert, photostable, inexpensive, and
non-toxic.[18, 61] Some other semiconductors’ band gaps are also in the visible or near
UV range, for example, ZnO, CdS, Fe
2
O
3
. However, ZnO does not have long-term
stability in aqueous solutions, metal sulfides suffer photoanodic corrosion, and Fe
2
O
3
is
prone to photocathodic corrosion.[18] For these reasons, these semiconductors are not as
popular as TiO
2
in the photocatalytic studies.
Figure 1-1. Schematic illustration of semiconductor photocatalysis.
3
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
H
2
O/O
2
2.8 eV
2.6 eV
3.2 eV
1.7 eV
2.25 eV
1.1 eV
2.8 eV
2.3 eV
2.4 eV
1.6 eV
3.2 eV
Rutile
TiO
2
Anatase
TiO
2
CuO
Cu
2
O
Fe
2
O
3
WO
3
Si
GaP
CdSe
SrTiO
3
V
2
O
5
PbO
3.0 eV
Vacuum
NHE
H
2
/H
2
O
7
6
5
4
3
2
1
0
-1
-2
Figure 1-2. Energy band diagrams for various semiconductors.
The two different phases of TiO
2
most commonly used in photocatalysis are
anatase and rutile. The crystal structure of anatase and rutile can be distinguished by the
distortion of each octahedron and by the assembly pattern of the octahedral chain.[103]
These differences in lattice structures result in different electronic band structures such
that rutile exhibits a lower band gap (~3.0 eV) in comparison to anatase (~3.2 eV) and
can thus be excited by irradiation at longer wavelengths. However, anatase TiO
2
generally appears to be more photoactive and practical than rutile TiO
2
. While TiO
2
is
one of the most promising photocatalysts,[155, 170, 172] it does not absorb light in the
visible region of the electromagnetic spectrum. Because of TiO
2
’s short wavelength
cutoff, there are very few solar photons (~4%) that can be used to drive this photocatalyst.
Several attempts have been made to extend the cutoff wavelength of this catalyst,
4
including coupling with narrow band gap semiconductors,[95] metal ion/nonmetal ion
doping,[95, 134, 139, 169, 184] and surface sensitization by organic dyes or metal
complexes.[95] In recent years, improving photocatalytic activity of TiO
2
under visible
illumination has been achieved by depositing plasmonic noble metal nanoparticles
(mainly gold and silver) on the TiO
2
surfaces. Au and Ag nanoparticles strongly absorb
visible light due to the localized surface plasmon resonance derived from the collective
oscillation of free electrons.[141]
In this chapter, I exclusively review the recent process of surface plasmon
resonance-enhanced photocatalysis. I focus on three examples: (1) surface plasmon
resonance-enhanced photocatalytic degradation of organic wastes; (2) surface plasmon
resonance-enhanced photocatalytic water splitting; and (3) surface plasmon resonance-
enhanced photocatalytic reduction of CO
2
to form hydrocarbon fuels. To provide a
background for this review, I start 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 CO
2
. This is followed by
a summary of a recent burst of papers in this field. I will focus this discussion on 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.
1.2 Surface Plasmon Resonance
Plasmons are the collective oscillation of free charge in a conducting material.
Light below the plasma frequency is reflected because the electrons in the metal screen
5
the electric field of the light. Light above the plasma frequency is transmitted because the
electrons cannot respond fast enough to screen it. Surface plasmons are oscillations
confined to the surfaces of conducting materials and interact strongly with light. A
resonance in the absorption occurs at the plasmon frequency when the real part of the
dielectric function goes to zero. Irradiating metal nanoparticles with light at their plasmon
frequency generates intense electric fields at the surface of the nanoparticles, as shown in
Figure 1-3. The frequency of this resonance can be tuned by varying the nanoparticle size,
shape, material, and proximity to other nanoparticles[12]. For example, the plasmon
resonance of silver, which lies in the UV, can be shifted into the visible range by making
the nanoparticle size very small. Similarly, the plasmon resonance of gold in the visible
range can be brought into the infrared wavelength range by minimizing the nanoparticle
size. Nurmikko and others have fabricated arrays of nanoparticles with different spacing
using electron beam lithography[12, 145]. Their optical measurements show that the
plasmon resonance increases asymptotically as the particles are brought closer together.
This was corroborated by the calculations of Schatz et al. using an interacting dipole
model that showed the plasmonic resonance to be 10
3
times stronger between two nearly
touching nanoparticles[190]. The intense electric fields produced near plasmon resonant
metallic nanoparticles are currently utilized in surface enhanced Raman spectroscopy
(SERS) to produce enhancement factors as high as 10
14
.[86, 87, 93] Numerical
simulations have predicted SERS enhancement factors up to 10
10
.[119, 122]
6
Figure 1-3. (a) Schematic illustration of plasmon oscillations. (b) Simulation of the
electrical field near Au nanoparticle.
1.3 Basic Principles
1.3.1 Dye Photodegradation
The environmental application of semiconductor photocatalysis is photocatalytic
degradation of organic pollutants for water and air purification. In photocatalytic
degradation, photogenerated electrons reduce adsorbed oxygen (O
2
) to form superoxide
(HO
2
·) radicals and photogenerated holes react with H
2
O to form OH· radicals.[127]
Hydroxyl radicals subsequently oxidize the organic pollutants resulting in mineralization
and complete degradation.[18, 127] The overall process can be summarized by the
following reaction equation:[112]
Organic pollutant + O
2
In addition to decomposing organic wastes, these radicals can also kill bacteria in the
water. Figure 1-4 plots the conduction and valence band positions for a variety of
different semiconductors together with the redox potentials for the H
2
O/OH· and
O
2
/HO
2
· couples at pH= 0.
semiconductor
ultra-bangap light
CO
2
+H
2
O+ mineral
(a)
(b)
7
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
ZnO
3.2 eV
H
2
O/OH·
2.8 eV
2.6 eV
3.2 eV
1.7 eV
2.25 eV
1.1 eV
2.8 eV
2.3 eV
2.4 eV
1.6 eV
3.2 eV
Rutile
TiO
2
Anatase
TiO
2
CuO
Cu
2
O
Fe
2
O
3
WO
3
Si
GaP
CdSe
SrTiO
3
V
2
O
5
PbO
3.0 eV
Vacuum
NHE
O
2
/HO
2
·
7
6
5
4
3
2
1
0
-1
-2
Figure 1-4. Energy band diagrams of several semiconductors plotted together with the
redox potentials to form HO
2
· and OH· radicals.
1.3.2 Photocatalytic Water Splitting
One of the most important applications of semiconductor photocatalysis in solar
energy conversion is photocatalytic water splitting, that is the production of hydrogen (H
2
)
and oxygen (O
2
) from water by the direct conversion of solar energy. With oil and other
nonrenewable fuels becoming increasingly depleted and expensive, hydrogen fuel
presents a promising alternative, which burns cleanly without producing greenhouse
gases or having many adverse effects on the atmosphere. Photocatalytic water splitting
has been of great interest since the early 1970s, after the first demonstration by Fujishima
and Honda under ultraviolet radiation.[43] This reaction can be carried out in a
photochemical cell, as shown in Figure 1-5. When the semiconductor photoelectrode is in
8
contact with an electrolyte solution (water in this case), a space charge layer is formed at
the interface between the semiconductor electrode and the electrolyte solution in order to
establish the thermodynamic equilibration. When the semiconductor photoelectrode is
irradiated with the light above the band gap, electron-hole pairs are generated and
separated in the space charge layer. For n-type semiconductors, the majority carrier
charge is electrons. Therefore, the photogenerated electrons move to the external circuit
and hydrogen is formed at the surface of the counter electrode, while the photogenerated
holes move toward the interface of the semiconductor electrode and the electrolyte
solution driving the evolution of oxygen gas. The reverse processes take place at a p-type
semiconductor electrode.[42]
Figure 1-5. Schematic diagram of a photoelectrochemical cell for photocatalytic water
splitting.
As mentioned above, the valence band energy of the semiconductor electrode
should be lower (or more positive on the electrochemical (NHE) scale) than the oxygen
evolution potential so that the photogenerated holes can be accepted by water to produce
9
oxygen. Similarly, the conduction band energy should be higher (or more negative) than
the hydrogen evolution potential in order for the photogenerated electrons to flow to the
acceptors to reduce the water into hydrogen without an applied potential. The standard
reduction potentials (E
o
) of oxygen and hydrogen evolution are as follows:[192]
Reduction: 4H
2
O + 4e
-
→2H
2
(g) + 4OH
-
E
o
red
=-0.83v (1-1)
Oxidation: 2H
2
O + 4h
+
→ O
2
(g) + 4H
+
E
o
ox
=1.23v (1-2)
Standard reduction potentials are measured under standard conditions of 25°C, 1 M
concentration for each ion participating in the reaction, a partial pressure of 1 atm for
each gas that is part of the reaction, and metals in their pure state.[192] By comparing
these redox potentials with the energies of the valence and conduction bands of the
semiconductors, as indicated in Figure 1-6, we can see that a few semiconductors have
both the conduction band energy lying higher than the hydrogen evolution potential and
the valence band energy below the oxygen evolution potential. However, these standard
potentials given in Equation (1-1) and (1-2), assume an anode chamber with 1 M H
+
and
a cathode chamber with 1 M OH
-
. The actual reduction potentials are related to the actual
concentrations of H
+
and OH
-
ions, or pH. The actual reduction potentials at a specific pH
can be calculated using Nernst Equation. Equation (1-3) is the original form of Nernst
Equation.
ln
o ox
red
C RT
EE
nF C
(1-3)
10
Where E is the actual reduction potential, E
o
is the standard reduction potential, R is the
gas constant, T is the absolute temperature, F is faraday constant, n is the number of
electrons needed in the reaction, and C
ox
and C
red
are concentrations of oxidation species
and reduction species, respectively. For water splitting reaction, the Nernst Equation can
be simplified into the following relation, since only H
+
and OH
-
ions are involved in the
reaction:
E = E
o
±0.059pH (1-4)
where “+” is used for the reduction half reaction and “-” is used for the oxidation half
reaction. Therefore, the actual reduction potentials can be adjusted by pH. For example,
the actual reduction potential of hydrogen evolution is -0.42 V and the reduction potential
of oxygen evolution is 0.82 V in pure water, where [H
+
] = [OH
-
] = 10
-7
M. Figure 1-6
shows the reduction potentials at various pH values. At pH=0, the redox potential needed
to drive the reduction of hydrogen from water is 0.0 V vs. NHE, and oxidation of oxygen
from water is 1.23 V vs. NHE (oxidation reaction). Several semiconductors, such as
TiO
2
, CdSe, and SrTiO
3
, can drive photocatalytic water splitting by light without an
applied potential. However, semiconductors are not stable at pH=0, even though it is
favorable to produce hydrogen. 1M KOH or NaOH is usually used as the electrolyte for
photocatalytic water splitting.[16, 165] Whereas, neutral solutions at pH=6.3~6.5 are also
the common electrolyte for this reaction.[129] The conduction band energies of TiO
2
,
CdSe, and SrTiO
3
are lower than the reduction potential to evolve hydrogen in neutral or
basic solutions. This problem can be solved by applying an external bias or using an acid
solution as the catholyte and a neutral or basic solution as the anolyte. To date, most
11
reports of hydrogen evolution from photocatalytic water splitting is achieved under an
additional voltage.
-8
-7
-6
-5
-4
-3
pH=0 H+/H
2
pH=0
pH=7
pH=7
anatase rutile
H
2
O/O
2
2.8 eV
3.2 eV
1.7 eV
2.25 eV
1.1 eV
2.8 eV
2.3 eV
2.4 eV
1.6 eV
3.2 eV
TiO
2
TiO
2
CuO
Cu
2
O
Fe
2
O
3
WO
3
Si
GaP
CdSe
SrTiO
3
PbO
3.0 eV
Vacuum (eV)
NHE (Volts)
H
2
O/H
2
pH=14
3
2
1
0
-1
Figure 1-6. The redox potentials of hydrogen and oxygen evolution at different pH
values together with the valence band and conduction band energies of semiconductors.
1.3.3 Photocatalytic Reduction of CO
2
Another important application of semiconductor photocatalysis in the area of solar
energy conversion is the photochemical reduction of carbon dioxide. Carbon dioxide, a
well-known an atmospheric pollutant, can be reduced with water to form organic raw
materials or fuels under sunlight. This reaction can reduce CO
2
from the atmosphere and
store solar energy in the form of chemical bonds, simultaneously. Electrochemical
reduction of carbon dioxide usually requires a large overpotential and produces formic
acid or CO as the major reduction product. In the photoelectrochemical reduction of
carbon dioxide, the energy of the incident photon is used to reduce the overpotential of
CO
2
reduction. Therefore, no external energy other than solar energy is required. In
12
addition, semiconductor electrodes are favorable to form highly reduced products, such
as methanol and methane. This is due to the particular band structure of semiconductors,
particularly the potential of the conduction band edge relative to the reduction potential
of carbon dioxide for different reduction products. The reduction potentials for the
reduction half reactions in an aqueous solution of pH 7 are as follows, assuming the
carbon dioxide is reduced directly to give products of interest:[157]
CO
2
(g) + 8H
+
+ 8e
-
→ CH
4
(g) + 2H
2
O E
o
= -0.24V (1-5)
CO
2
(g) + 4H
+
+ 4e
-
→ HCHO (aq) + H
2
O E
o
= -0.48V (1-6)
CO
2
(g) + 6H
+
+ 6e
-
→ CH
3
OH (aq) + H
2
O E
o
= -0.38V (1-7)
CO
2
(g) + 2H
+
+ 2e
-
→ HCOOH (aq) E
o
= -0.61V (1-8)
CO
2
(g) + 2H
+
+ 2e
-
→ CO (g) + H
2
O E
o
= -0.52V (1-9)
2CO
2
(g) + 2H
+
+ 2e
-
→ H
2
C
2
O
4
(aq) E
o
= -0.90V (1-10)
where the standard redox potentials are given in Volts versus NHE at pH 7.0, and (g) and
(aq) denote the gaseous state and aqueous solution, respectively. The conduction band
edge of the semiconductor electrodes should be more negative than the reduction
potentials of carbon dioxide to facilitate energetically favorable electron transfer from the
conduction band of the semiconductor to the CO
2
molecule. This means that it is
energetically favorable to form reduction products with more positive standard redox
potentials.
13
In 1979, Inoue et al. first demonstrated the photoelectrocatalytic reduction of
carbon dioxide to produce formic acid, formaldehyde, methyl alcohol, and methane using
semiconductor powders suspended in water as catalysts.[71] These semiconductor
particles act as tiny photoelectrolysis cells with the entire electrochemical cycle occurring
on the surface of each particle, as illustrated in Figure 1-7. No electrodes and supporting
electrolyte are required in this system. Since both oxidation and reduction reactions occur
on the same particle, it is impossible to separate them. The reoxidation of reduced CO
2
species occurs easily, resulting in low energy conversion efficiencies. However, the
experimental setup of this semiconductor suspension system is much simpler than that of
a photoelectrochemical cell using electrodes. Also fast reaction rates are expected due to
the huge surface area of semiconductor particles.
Figure 1-7. Schematic illustration of a single-particle photocatalytic reaction for the
reduction of CO
2
.
1.4 Factors Limiting the Photocatalytic Efficiency
While photocatalysis has a great potential for environmental and solar energy
conversion applications, low photocatalytic efficiencies in the solar spectral range have
14
made most applications unfeasible. In the recent decade, a large number of papers have
reported that visible light photocatalysis has been improved by integrating metal
nanoparticles with the semiconductor photocatalyst. For example, Yu et al. reported Ag-
TiO
2
nanocomposite plasmonic photocatalysts with high visible-light photocatalytic
activity in the photocatalytic degradation of methyl orange.[181] Zaleska et al.
established that the decomposition rate constant of phenol increased from 0.001 min
-1
to
0.07 min
-1
after integrating Ag nanoparticles with TiO
2
using a microemulsion
method.[189] From these references, the factors affecting the photocatalytic efficiency
are summarized as follows.
1.4.1 Metal Loading
Some groups reported that an optimum amount for metal nanoparticles should be
integrated into semiconductors in order to obtain the maximum plasmon
enhancement.[10, 26, 40, 76, 141] Falaras et al. found titania photocatalysts with a gold
surface coverage of 0.8 μg cm-2 to be the most active.[10] They also showed a 3-times
improvement in the photodegradation of methyl orange by photodeposition of an
optimum amount of silver nanoparticles on titania Degussa P-25 thin films. The
photocatalytic performance was found to decrease when the concentration of Ag
nanoparticles was higher than this optimum concentration.[40] Gao et al. found that 2-4
wt% Au loading corresponded to the maximum conversion efficiency.[26] Ji et al.
showed the photocatalytic activity of titanosilicate ETS-10 for the degradation of
methylene blue can be increased by a factor of 10 with an intermediate Ag loading level.
The photocatalytic activity was found to decrease with more Ag loading level as the
15
surface area loss dominates the activity of high Ag-loading catalysts.[76] Garcia et al.
found that the most active visible light Au/TiO
2
photocatalyst for water splitting
contained 0.2 wt% gold,[141] and Keller et al. found that Au/TiO
2
photocatalyst with a
0.3 wt% Au loading was the most efficient for H
2
production from water splitting.[132]
1.4.2 Surface Area
The surface area of the semiconductor plays a key role in plasmonic
photocatalysts. Even though integrating plasmonic metal nanoparticles enhances the
photocatalytic activity of semiconductors, it reduces the surface area of semiconductor
available for photocatalysis. In order to obtain the maximum enhancement, the surface
area loss by covering the semiconductor with metal nanoparticles has to be minimized.
Several ways have been used to reduce the surface area loss when plasmonic metal
nanoparticles are introduced into the photocatalytic system. The mesoporous structure
TiO
2
has been used to enlarge the semiconductor surface area.[5, 107, 114, 132, 176] The
small metal nanoparticles can then be deposited into the mesopores of the TiO
2
preventing it from covering a significant fraction of the TiO
2
surface area. Keller’s group
proved that the porosity of the TiO
2
photocatalyst is crucial to the enhancement of H
2
evolution from water splitting.[132] Garcia et al. were able to convert mesoporous titania,
which is completely transparent in the visible wavelength range into an efficient
photocatalyst for the visible-light decontamination of Soman by adding Au nanoparticles
into it.[5] Cho et al. obtained a more than 14× increase in the photocatalytic activity of
Ag-loaded mesoporous TiO
2
for methyl orange degradation with respect to that of P-25
TiO
2
under visible irradiation.[176] Quan et al. showed that the rate constant of 2,4-
16
dichlorophenol degradation under visible light irradiation is increased by 2.3 times when
Au nanoparticles are uniformly dispersed into porous TiO
2
.[107] Mohamed et al.
observed that Ag nanoparticles can shift the absorption maximum of TiO
2
to the visible
wavelength range and that the resulting nano-silver/mesoporous titania photocatalysts
exhibit a higher efficiency for the photodegradation of herbicide pollutant 2,4-
dichlorophenoxyacetic acid under visible irradiation than bare TiO
2
.[114] In order to
increase the photocatalytic surface area, Zhu’s group integrated Ag nanoparticles with an
alumina nanofiber membranes with pore size of 10 nm.[80] Core-shell metal-
semiconductor structures have also been used to maximize the photocatalytically active
the surface area.[108, 115, 140, 143, 161] In these structures, Ag or Au metal
nanoparticles are embedded in semiconductor shells. Similarly, Yamashita et al.
demonstrated have enhanced catalytic activity of core/shell Ag-SiO
2
nanoparticles for
photooxidation of dye molecules.[115] Yu’s group reported that Au/TiO
2
core-shell
catalysts with higher photocatalytic activity for the degradation of acetaldehyde under
visible illumination than bare P-25 TiO
2
and Au deposited on P-25 TiO
2
.[143] They also
observed enhanced photocatalytic activity of Au-SnO
2
and Ag-SnO
2
core-shell
nanocomposites in the photooxidation of acetaldehyde.[161] Imae et al. demonstrated
enhanced photodegradation of phenol by Ag-TiO
2
core-shell nanoparticles, while the
photocatalytic activity is decreased in TiO
2
-Ag core-shell nanoparticles due to the TiO
2
surface area lost by covering with Ag.[140] El-Sayed et al. observed a more than 4-fold
increase in the degradation rate of methylene blue for Cu
2
O-Au nanoframes with
17
different Cu
2
O layer thickness than Cu
2
O, with the photodegradation rate increasing with
Cu
2
O thickness.[108].
1.4.3 Matching the Solar Spectrum
Matching the spectral enhancement of the metal nanoparticles with the spectral
absorption of the semiconductor photocatalyst and the spectrum of the illumination
source (i.e., the solar spectrum) is also important for reaching maximum plasmon
enhancement. Linic et al. showed that Ag cube-modified TiO
2
has higher photocatalytic
activity for methylene blue photolysis than Au sphere-modified TiO
2
because the surface
plasmon resonance of Ag cubes overlaps with the illumination light source spectrum
better than that of Au spheres. They also showed that the overlap of the semiconductor
absorbance spectrum and the spectrum of the illumination source is also important. N-
doped TiO
2
works better than undoped TiO
2
since the absorbance spectrum of N-TiO
2
matches the illumination source spectrum (centered at 450 nm) better than that of TiO
2
due to defect states in the bandgap created by N-doping.[70]
1.4.4 Oxidation of Ag
When Ag nanoparticles are used to improve the photocatalytic activity of
semiconductors, the oxidation process of Ag has to be considered, since Ag is very
readily oxidized. Awazu et al. have shown evidence that the oxidation of Ag
0
nanoparticles into Ag
+
decreases the photocatalytic activity of Ag-TiO
2
photocatalysts. In
order to prevent oxidation of Ag by direct contact with TiO
2
, they used a silica shell to
protect the Ag. The photodecomposition of methylene blue is enhanced by a factor of 7
using this silica-protected Ag/TiO
2
plasmonic photocatalyst.[14] On the other hand, Yu et
18
al believe that the presence of Ag
+
accelerates the photooxidation of organic waste,
resulting in the increased photocatalytic activity of plasmonic photocatalysts.[185]
1.4.5 Metal Nanoparticle Size
The size of the metal nanoparticles also affects the photoactivity of plasmonic
photocatalysts. First, the plasmon frequency of metal nanoparticles is largely determined
by their size. Kowalska et al. reported fifteen commercial TiO
2
powders could absorb
visible light with an absorption peak between 530-600 nm after modifying with gold
nanoparticles, where the size and shape of the Au nanoparticles determined the
absorption range.[92] Liu et al. reported dramatic enhancement in the photocatalytic
activity of KNbO
3
loaded with Au nanoparticles for rhodamine B degradation under
visible illumination, with the photoreactivity increasing with the Au nanoparticle size
from 5 nm to 10 nm.[97] The data of Kim et al. also indicated that a visible light active
photocatalyst composed of silica-titania core-shell (SiO
2
-TiO
2
) nanostructures decorated
with 15 nm Au nanoparticles displays the optimum catalytic efficiency for methylene
blue, methyl orange, and p-nitrophenol degradation since 15 nm Au nanoparticles couple
the visible light (> 420 nm) most efficiently.[89] Gupta et al. found that 1.5-5 nm gold-
containing titania nanotubes display higher activity for photooxidation of acetaldehyde
compared to bare TiO
2
nanotubes and also the commercial TiO
2
catalyst, Degussa P-25.
However, this enhancement is lost completely with larger size Au clusters (10-70
nm).[110] Garcia et al. indicated that small Au nanoparticles (1.87 nm) favor the
photocatalytic activity of Au/TiO
2
.[141] Di Vece et al. conjectured that the photocatalytic
activity of TiO
2
is quenched by 30 nm Ag nanoparticles, since the plasmon resonance
19
energy of Ag nanoparticles of this size is comparable to the band gap of TiO
2
.[36]
However, this is not consistent with the experimental results from other groups.
In addition, the conversion efficiency of plasmon-enhanced photocatalysts can
also depend on other factors, including catalyst concentration, metal/semiconductor
nanostructure geometry, electrolyte, pH, and etc. Mohamed et al. have suggested that the
optimum catalyst concentration (i.e., the ratio of catalyst to reagent) must be determined
in order to avoid excess catalyst and ensure total absorption of the efficient photons.
Excess photocatalyst loading causes the scattering of light and reduction of light
penetration into the solution.[41] Kim et al. observed 30% enhanced photocatalytic
degradation of methylene blue for ordered arrays of Ag/TiO
2
nanodots compared with
pure TiO
2
nanoparticles.[79] Sun et al. reported that the photocatalytic reaction of 4-
aminothiophenol only takes place under neutral and alkaline conditions (pH=7 and 10)
and can not occur at pH=3.[147]
In practice, easy removal of catalyst particles from the solution is also important.
Dai et al. prepared a novel plasmonic photocatalyst based on Ag-AgI/Fe
3
O
4
@SiO
2
nanoparticles, which show excellent photocatalytic activity for the degradation of
rhodamine B and 4-chlorophenol under visible illumination. This plasmon enhanced
photocatalyst can be easily recovered due to its paramagnetic property.[51]
1.5 Mechanisms
Understanding the surface plasmon enhancement mechanism is useful in the
design of plasmonic photocatalysts with high photoconversion efficiencies. In this section,
I review several mechanisms proposed for surface plasmon-enhanced photocatalysis.
20
Since the enhancement mechanisms under UV illumination and visible illumination are
different, I will discuss them separately with a particular emphasis on the enhancement
mechanisms under visible illumination.
1.5.1 Enhancement Mechanism under UV Illumination
The main reasons why the photocatalytic activity is enhanced after integrating
metal nanoparticles under UV illumination are attributable to the interband transitions of
metals[25, 188] and electron-hole pair separation at the metal-semiconductor (i.e.,
Schottky) junction.[22, 74, 78, 90, 104, 152, 171, 180, 182] Metals are often thought of
as simply having electronic states filled up to a Fermi energy corresponding to the work
function of the metal. However, like any crystalline material, metals have higher lying
electron bands that are normally unoccupied.[48, 187] In noble metals, the d-electron
bands lie below the Fermi level (E
F
). Interband transitions from the d-band to an empty
sp state above E
F
can occur during the optical absorption process.[166] In Au, the lowest
energy of interband excitation occurs at the X-point in the Brillouin zone, at an energy of
2.5 eV.[30, 77] Since the energy of UV light exceeds the minimum energy required for
interband transitions in metals, and therefore is able to excite electrons from the filled
valence band to the unfilled conduction band, photoexcited electron-hole pairs in metals
can also participate in redox reactions, resulting in higher photocatalytic activities and/or
more products. Zhu et al. observed photocatalytically driven dye photodecomposition,
phenol degradation, and benzyl alcohol oxidation under UV illumination arising from the
electronic interband transitions in Ag[25] and reduction of nitroaromatic compounds
driven by the interband excitation of electrons from 5d to 6sp in Au.[188]
21
When the energy of the incident light is below the minimum energy required for
interband transitions in metals, metals can act as a reservoir for photogenerated electrons
in semiconductors. Under UV illumination, the photoexcited electrons in the
semiconductors can transfer from the conduction band to the metal, as shown
schematically in Figure 1-8. The presence of metals inhibits the electron-hole pair
recombination in semiconductors and therefore enhances the efficiency of photocatalytic
reactions.[78] Yu et al. observed a 6.3 ×enhancement in the photocatalytic degradation of
methyl orange under UV illumination after adding Ag to TiO
2
. They attributed this
enhancement to the photogenerated electron accumulation on Ag.[182] Liu et al.
demonstrated that improved photocatalytic hydrogen production under UV irradiation
after loading Ag into SrTiO
3
. Here, the enhancement of photocatalytic activity of the Ag-
SrTiO
3
complex was attributed to the Ag nanoparticles, which inhibit the recombination
of electron-hole pairs.[104] Kamat’s group also attributed an enhancement in the
photocurrent under UV illumination after modification of TiO
2
with Au or Ag
nanoparticles to the metal nanoparticles’ ability to capture and store the photogenerated
electrons.[22, 152] Koci et al. reported that the yield of methane and methanol from the
photocatalytic reduction of CO
2
increases when modifying the TiO
2
by Ag incorporation
under 254 nm illumination. The Shottky barrier at the Ag-TiO
2
interface spatially
separates electrons and holes, thereby decreasing the probability of their recombination
and increasing their lifetime.[90] Kudo et al. observed the photocatalytic improvement of
titanate, niobate, and tantalite photocatalysts for water splitting by the loading of gold
particles to enhance the charge separation under UV illumination.[74] Kojima et al.’s
22
observation that Ag, Pt, and Au enhance the photocatalytic performance of TiO
2
for
methylene blue photodegradation was attributed to the photogenerated electrons being
trapped by these metals, causing high efficiency of charge separation under 365 nm
illumination.[180] In this study, Pt worked as good as Ag and Au for separating electrons
and holes even though Pt is not a strongly plasmonic metal as Au and Ag. Zhao et al.
showed that loading Ag on TiO
2
greatly decreases its photoluminescence intensity due to
the charge transfer from TiO
2
to Ag, which provides a direct measurement of the charge
separation of photoinduced electrons and holes.[171]
Figure 1-8. Schematic diagram illustrating the charge separation mechanism.
In the discussion above, the photocatalytic enhancement under UV illumination is
not related to the surface plasmon resonance of the metal nanoparticles. Instead, the
enhanced photocatalytic process is achieved through interband transitions in metals
and/or charge separation at the metal-semiconductor interface, neither of which requires a
nanometric morphology. As such, the proposed enhancement mechanisms under UV
illumination are not only applicable to Au- and Ag-modified photocatalysts. Other metals,
including Pt and Cu can also be used to improve the photocatalytic activity of
23
semiconductors.[78, 152] In addition, the energy of the UV light source is significantly
higher than the surface plasmon resonance. Strictly speaking, the enhancement under UV
illumination is not associated with plasmonic phenomena.
1.5.2 Enhancement Mechanisms under Visible Illumination
Two main mechanisms have been discussed regarding plasmonic enhancement of
photocatalysis under visible illumination: the charge transfer mechanism and the local
electric field mechanism. Back in 2004, Tatsuma’s group proposed a charge transfer
mechanism to explain their experimentally observed photon-to-current conversion
efficiency (IPCE) enhancement under visible light illumination upon loading Au or Ag
nanoparticles into TiO
2
films.[2, 158, 159] In their proposed charge transfer mechanism,
the plasmon resonance excites electrons in Au or Ag, which are then transferred to the
conduction band of the adjacent TiO
2
, as shown in Figure 1-9. This charge transfer
mechanism is similar to that of a dye-sensitized solar cell.[120] Subsequently, several
other groups used this mechanism to explain enhanced photocatalytic water splitting,[141]
methyl orange decomposition,[181] and photooxidation of formaldehyde[156] observed
under visible illumination. Furube et al. claimed a direct observation of plasmon-induced
electron transfer from 10 nm gold nanodots to TiO
2
nanoparticles using femtosecond
transient absorption spectroscopy with an IR probe.[44] In 2011, Moskovits et al. further
explicated this charge transfer mechanism, in which the surface plasmon decay produces
electron-hole pairs in the gold and hot electrons produced in the decay of localized
surface-plasmon polaritons excited in gold nanoparticles are directly inject into TiO
2
by
quantum tunneling.[116]
24
Figure 1-9. Schematic illustration of a proposed charge transfer mechanism. (Reprinted
with permission from Ref [158] Copyright (2005) American Chemical Society.)
While this charge transfer mechanism has been cited by many groups, surface
plasmons consist of the collective oscillation of charge bound to a metal surface, and
therefore have no HOMO-LUMO or valence band-conduction band energy separation
associated with them, as indicated in Figure 1-9. Instead, both the electrons and holes in
the plasmon excitation lie at the Fermi energy of the metal. If plasmon-excited electrons
can transfer from Au to TiO
2
, they should also be able to transfer to the reagents to drive
the photocatalytic reactions without semiconductors. In this case, metal nanoparticles
themselves can be used as plasmonic photocatalysts, which is not the case. Another
mechanism proposed for plasmon enhanced photocatalysis is based on the local electric
field enhancement associated with plasmonic nanoparticles. In this mechanism, the
25
plasmon enhancement was attributed to the strong electric fields produced by the surface
plasmon resonance of Au or Ag nanoparticles. The collective oscillations of conduction
electrons in metal nanoparticles resonate with the electromagnetic field of the incident
light, which results in a significant enhancement of the local electromagnetic fields at the
metal-semiconductor interface. The schematic diagrams in Figure 1-10 illustrate the
difference between the local electric field mechanism and the charge transfer mechanism.
Based on the results of finite-difference time-domain (FDTD) electromagnetic
simulations, local “hot spots”, in which the electric field intensity can reach as much as
1000 times that of the incident electric field at the TiO
2
surface, are seen in regions
between nearly touching Au nanoparticles. This is a well-known phenomenon,
corroborated by the calculations of several research groups [98, 191]. In these “hot spot”
regions, the electron-hole pair generation rate is 1000 times that of the incident
electromagnetic field. Thus, an increased amount of photo-induced charge is generated
locally in the TiO
2
due to the local field enhancement of the plasmonic nanoparticles.
This enhancement also relies on the presence of defect states in the TiO
2
, which are
needed in order to enable light absorption below the band gap of the semiconductor.
Mizeikis et al. performed simulations of optical field enhancement in a system consisting
of spherical and hemispherical noble metal nanoparticles on a smooth titania surface
using the FDTD technique. Large near-field enhancement factors up to 10
4
were obtained
at the metal/titania interface in their simulations.[113] Several other groups have also
adopted the local electric field enhancement mechanism.[38, 70, 102, 107, 160, 163, 188]
Quan et al. ascribed a 2.3-fold faster 2,4-dichlorophenol degradation catalyzed by Au
26
nanoparticle/TiO
2
to the enhanced light harvesting caused by the surface plasmon
resonance.[107] Linic’s group also attributed the enhanced photocatalytic water splitting
and methylene blue decomposition to the enhancement in the intensity of electric fields
compared with the field intensity of the incoming photon flux.[70, 102] Xuan et al.
investigated enhanced electron-hole pair generation rates in CdS by the enhanced near-
field amplitudes of surface plasmon resonance on the Ag surface in a Ag/SiO
2
/CdS
multilayer nanocomposite. They demonstrated that the thickness ratio of Ag, SiO
2
, CdS
layers plays an important role in the near-field enhancement, as shown in Figure 1-11.[38]
Ishihara et al. also indicated that surface plasmon resonance-induced electric field and
photocatalytic activity of Au/SiO
2
/CdS for photocatalytic water splitting greatly depend
on the distance between CdS and Au nanoparticles.[160]
Figure 1-10. Schematic diagrams of the local electric field enhancement mechanism (left)
and the charge transfer mechanism (right).
27
Figure 1-11. Electric field enhancement contour around Ag/SiO
2
/CdS multilayer
nanocomposites with different thickness ratios (a) R
1
/R
2
/R
3
= 20/30/50 nm; (b) R
1
/R
2
/R
3
=
17/25/50 nm; and (c) R
1
/R
2
/R
3
= 15/20/50 nm. (Reprinted with permission from Ref [38]
Copyright (2011) Elsevier.)
28
Chapter 2: Plasmonic Enhancement of Photocatalytic
Decomposition of Methyl Orange under Visible Light
The contents of this chapter were recently published as a full paper in the Journal
of Catalysis, and is reprinted from: Journal of Catalysis, 277, Wenbo Hou, Zuwei Liu,
Prathamesh Pavaskar, Wei Hsuan Hung, Stephen B. Cronin, Plasmonic enhancement of
photocatalytic decomposition of methyl orange under visible light, p 149-153, Copyright
2011, with permission from Elsevier. In addition, all co-authors have given their
permission for use of this paper in this dissertation.
29
By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO
2
,
we observe enhanced photocatalytic decomposition of methyl orange under visible
illumination. Irradiating Au nanoparticles at their plasmon resonance frequency creates
intense electric fields, which can be used to increase electron-hole pair generation rate in
semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors,
like TiO
2
, can be extended into the visible region of the electromagnetic spectrum. Here,
we report a 9-fold improvement in the photocatalytic decomposition rate of methyl
orange driven by a photocatalyst consisting of strongly plasmonic Au nanoparticles
deposited on top of strongly catalytic TiO
2
. Finite-difference time-domain (FDTD)
simulations indicate that the improvement in photocatalytic activity in the visible range
can be attributed to the electric field enhancement near the metal nanoparticles. The
intense local fields produced by the surface plasmons couple light efficiently to the
surface of the TiO
2
. This enhancement mechanism is particularly effective because of
TiO
2
’s short exciton diffusion length, which would otherwise limit its photocatalytic
efficiency. Our electromagnetic simulations of this process suggest that enhancement
factors many times larger than this are possible if this mechanism can be optimized.
2.1 Introduction
Photocatalytic decomposition of organic molecules has been of great interest for
the removal of pollutants from water and air. Semiconductor photocatalysts (e.g., TiO
2
,
ZnO, SnO, In
2
O
3
) have been shown to effectively catalyze many chemical reactions,
including reduction of aqueous CO
2
,[2, 9, 56, 72] CO oxidation,[33, 175] water
splitting,[111, 132, 148] and decomposition of pollutants.[82, 155, 169] While TiO
2
is
30
one of the most promising photocatalysts for these purposes,[155, 170, 172] it does not
absorb light in the visible region of the electromagnetic spectrum. Because of TiO
2
’s
short wavelength cutoff, there are very few solar photons (~4%) that can be used to drive
this photocatalyst. Several attempts have been made to extend the cutoff wavelength of
this catalyst, including doping [134, 139, 169] and defect creation.[153, 154] However,
these have only extended the absorption edge of TiO
2
to approximately 420 nm.[134, 139,
169, 177] Therefore, most of the solar spectrum is still unable to drive this photocatalyst.
Dye-sensitized solar cells have enabled the photovoltaic response of semiconductors to
be extended to longer wavelengths by the direct transfer of charge from the dye
molecules to the conduction band of the semiconductor.[49] The degradation of organic
dye molecules has been studied under UV illumination of Au nanoparticle/TiO
2
composites previously by Falaras’ and Kojima’s groups.[10, 179, 180] The improvement
in photocatalytic efficiency of TiO
2
with gold nanoparticles was attributed to high
efficiency charge separation between the Au and TiO
2
. In addition to photocatalytic
enhancement under UV illumination, improved photocatalytic activity of Au
nanoparticle/TiO
2
under visible irradiation has been also reported.[92, 158, 159]
According to these previous works, a charge transfer mechanism occurs whereby the
plasmon resonance excites electrons in Au, which are then transferred to the conduction
band of the adjacent TiO
2
. This proposed charge transfer mechanism is similar to that of
a dye-sensitized solar cell.[120] However, the band energies for electrons and holes are
very different for the metal-semiconductor interface, and no rigorous model for this
process has been put forth in the context of plasmonics or catalysis.[92] Furthermore, the
31
energy band alignment of anatase TiO
2
with respect to the work function of Au is
energetically unfavorable for the direct transfer of electrons from Au to TiO
2
.
Here, we demonstrate photocatalytic enhancement of TiO
2
under visible
illumination by depositing Au nanoparticles on the TiO
2
surface. The enhancement can
be accounted for based purely on the classical electric field enhancement near the Au
nanoparticle surfaces. Finite-difference time-domain (FDTD) calculations of the
electromagnetic response of the Au nanoparticles provide a quantitative prediction of the
photocatalytic enhancement factor, which is in good agreement with our experimental
values. This model is based on the near-field optical enhancement provided by the Au
nanoparticles, and does not require direct electron transfer between the materials. Several
research groups have reported enhanced light absorption and/or photocurrents in solar
cells using a similar plasmonic coupling mechanism. Here, we utilize the plasmonic field
enhancement to improve TiO
2
photocatalysis in the visible wavelength range.
2.2 Experimental
TiO
2
was fabricated by the electrochemical oxidation of titanium foils using an
ethylene glycol electrolyte containing 0.25 wt% NH
4
F at an anodization potential of 30
V.[50] The resulting material is commonly referred to as anodic titanium oxide (ATO). A
gold film with a nominal thickness of 5 nm was then evaporated on the surface of the
TiO
2
. Thin Au evaporated films (~5 nm) are known to form island-like growth, which
serve as good substrates for surface enhanced Raman spectroscopy (SERS) and other
plasmonic phenomena.[94, 164] Absorption spectra of the bare TiO
2
and Au
nanoparticle/TiO
2
films were recorded on a Perkin-Elmer Lambda 950 UV/Vis/NIR with
32
an integrating sphere detector. The photocatalytic activity was tested using methyl orange
(MO) photodegradation as the model reaction. The decay in absorbance of the MO
aqueous solution at 460 nm was monitored by Varian Cary 50 UV-Vis spectrophotometer
after 1h exposure to UV (365 nm, mercury lamp with a bandpass filter centered near 365
nm, 0.02 W) or green laser (532 nm, 0.2 W) irradiation.
2.3 Results and Discussion
Figure 2-1 shows the spherically integrated UV-Vis absorption spectra of TiO
2
with and without gold nanoparticles. The spectrum taken for an undoped TiO
2
film
prepared by the solgel method (solid black curve) shows transparency for wavelengths
above 370 nm, which corresponds to the bandgap of anatase TiO
2
.[17, 138] However, the
anodic TiO
2
film (red solid curve) shows significant absorption at longer wavelengths
due to N- and F-defects produced during the anodization process that create electronic
states in the bandgap.[100, 177] The absorption spectrum taken from anodic TiO
2
with
gold nanoparticles (blue dashed curve) exhibits a peak in the absorption around 546 nm,
corresponding to the plasmon resonance of the Au nanoparticles, although the absorption
of this film is quite broad due to its inhomogeneity.[37]
33
300 400 500 600 700 800
Optical Absorption
Wavelength (nm)
Solgel TiO
2
thin film
Anodic TiO
2
Anodic TiO
2
with Au
Figure 2-1. UV-Vis absorption spectra of TiO
2
with and without gold nanoparticles.
Figure 2-2 shows the photocatalytic degradation of MO achieved under UV
irradiation. Here, the absorption spectra taken before and after irradiating with UV light
(365 nm) are used to quantify the relative MO concentration and, hence, the
photocatalytic decomposition rate. After 1h of UV illumination, the absorbance of the
MO aqueous solution and, hence, concentration, is observed to drop by 23% for bare
TiO
2
(Figure 2-2a), but only by 10% for the Au nanoparticle/TiO
2
sample
(Figure 2-2b).
Therefore, the addition of gold nanoparticles results in more than a 2-fold reduction in the
photodecomposition rate due to the reduction in the active TiO
2
surface area. This
reduction in active TiO
2
surface area can be seen in Figure 2-4a, as the gold nanoparticle
film covers a significant fraction of the TiO
2
surface, preventing it from coming into
direct contact with the aqueous solution to be photocatalyzed. In this photochemical
process, the photogenerated electrons and holes react with H
2
O and O
2
in the MO
aqueous solution to produce highly active oxidizing species, which in turn result in the
34
photodecomposition of MO into inorganic final products (SO
4
2-
, NO
3
-
, NH
4
+
, CO
2
and
H
2
O).[10, 92, 155, 172, 179]
300 400 500 600
0h
1h
Absorbance
Wavelength (nm)
TiO
2
@ UV
(a)
Figure 2-2. UV-Vis spectra of MO aqueous solution before (black) and after (red) 1h UV
illumination using (a) TiO
2
and (b) Au nanoparticle/TiO
2
photocatalysts.
Figure 2-3 shows the MO absorption spectra taken before and after irradiating
anodic TiO
2
with and without Au nanoparticles with visible light (532 nm laser). For bare
TiO
2
(Figure 2-3a), the absorbance (or concentration) of the MO solution is only
observed to drop by 1.4% after 1h of illumination. However, with the addition of gold
nanoparticles a 13% reduction in the MO absorbance is observed due to the plasmon-
enhanced photocatalytic decomposition mechanism, as described below. This
corresponds to a more than 9-fold enhancement in the photocatalytic activity. This 9-fold
enhancement is not evident in the bulk optical absorption spectra in Figure 2-1 because
the improvement in photocatalytic activity is mainly due to the local near-field
enhancement, which is not reflected in the bulk UV-vis absorption spectra. In addition,
the bulk UV-vis spectra contain absorption processes that do not contribute to
photocatalysis, such as recombination centers due to impurities.
300 400 500 600
Au/TiO
2
@ UV
Absorbance
Wavelength (nm)
0h
1h
(b)
35
300 400 500 600
0h
1h
TiO
2
@ Vis
Absorbance
Wavelength (nm)
(a)
Figure 2-3. UV-Vis spectra of MO aqueous solution before (black) and after (red) 1h 532
nm laser illumination using (a) TiO
2
and (b) Au/TiO
2
as photocatalysts.
In order to understand the increase in photocatalytic activity under visible
illumination (Figure 2-3) and the reduction in photocatalytic activity under UV
irradiation (Figure 2-2) with the addition of Au nanoparticles, we perform FDTD
numerical simulations of the electromagnetic response of these plasmonic/catalytic
nanostructures.[151] Figure 2-4a shows a scanning electron microscope (SEM) image of
a gold nanoparticle-island film deposited on top of anodic TiO
2
. In this SEM image, the
light grey regions are gold nanoparticles and the dark regions are in the interstitial space
in between (underlying substrate alone). The electromagnetic response of these Au
nanoparticle/TiO
2
composites are shown in Figures 2-4b-d. The white lines in Figures 2-
4b and 2-4c outline the gold regions, based on the Au nanoparticle geometries from the
SEM image in Figure 2-4a. By comparing these Figures, a one-to-one correspondence
can be seen between the shapes traced out by the white lines in Figure 2-4b and light grey
regions in Figure 2-4a. Local “hot spots” can be seen in regions between nearly touching
Au nanoparticles. This is a well-known phenomenon, corroborated by the calculations of
300 400 500 600
0h
1h
Au/TiO
2
@ Vis
Absorbance
Wavelength (nm)
(b)
36
several research groups.[98, 191] The importance of the local fields can be seen in Figure
2-4d, which shows a cross-sectional plot of the electric fields in one of these hot spot
regions. Here, the electric field intensity at the TiO
2
surface reaches 1000 times that of
the incident electric field. Thus, the photoabsorption (and hence electron-hole pair
generation) rate is 1000 times higher than that of the normal incident light. Furthermore,
because this field is confined within a few nm of the TiO
2
surface, a majority of the
plasmon-induced electron-hole pairs diffuse to the photocatalytic surface and contribute
to the catalytic process. This is not the case for the normal, non-enhanced fields, which
produce electron-holes pairs too far below the TiO
2
surface to contribute to
photocatalysis.
Based on the results of the FDTD electromagnetic simulations shown in Figure 2-
4, we can calculate the expected photocatalytic enhancement due to this surface plasmon
resonance phenomenon. Since the photon absorption rate is proportional to the electric
field squared (|E|
2
), integrated over the volume of the catalyst, the photocatalytic
enhancement factor is given by
||
|
|
(2-1)
Here, we integrate in z only from the TiO
2
surface (z = 0) to one exciton diffusion length
below the surface (z = -10nm). Performing this integral over the whole area of the film
that was simulated (400 nm x 300 nm) yields in an enhancement factor of 12X, which is
close to the 9-fold enhancement observed experimentally. It should be noted, however,
that this random distribution of Au nano-islands is far from optimized. If, instead, we
only integrate over one individual “hot spot” region, as shown in Figure 2-4c, the
37
expected enhancement factor is 190X. Therefore, if the geometry of this plasmonic film
could be optimized,[125] enhancement factors many times larger than this could be
achieved.
As can be seen from Figure 2-4b, the electromagnetic response of the plasmonic
film is dominated by a few localized hot spots. Therefore, a significant fraction of the
plasmonic surface area is not utilized. In addition, chemically, there is a significant
reduction in the TiO
2
surface area directly in contact with the aqueous solution, due to the
presence of the Au nanoparticle film. Remarkably, we still observe a net improvement in
the photocatalytic activity with the addition of Au nanoparticles, despite these two
disadvantageous factors. The reason for this remarkably robust enhancement lies in the
short exciton diffusion lengths of this anodic TiO
2
. This enhancement relies on the
presence of defect states in the TiO
2
, which enable sub-bandgap absorption. The near-
field optical enhancement provided by the Au nanoparticles is well-suited to this defect-
rich material, which possess very short exciton diffusion lengths.[105, 142] Therefore,
virtually all of the photogenerated charge excited by these plasmon-enhanced fields
contributes to the photocatalytic reaction. There is a tradeoff, however, with doping.
Doping (or defects) are needed in order to enable light absorption below the bandgap,
however, these dopants result in very short exciton diffusion lengths, which ultimately
spoil the photocatalytic performance. The plasmon enhancement mechanism that we have
demonstrated here provides a way around this, by focusing light into the near-field at the
TiO
2
/photocatalytic surface, thus, making it more robust to defects.
38
Figure 2-4. (a) SEM image of a 5 nm thick Au island film deposited on anodic TiO
2
. (b-d)
Electric field intensities calculated at the interface of Au – TiO
2
using the FDTD method.
39
The photocatalytic activity of this Au nanoparticle/TiO
2
composite under UV
(Figure 2-2b) and visible (Figure 2-3b) illumination are comparable. However, under UV
illumination, the photon absorption mechanism is quite different from that under visible
illumination. UV light is absorbed by direct interband transitions in the TiO
2
semiconductor. Under visible light, however, charge is excited to and/or from defect
states in the band gap of the TiO
2
. Therefore, a direct comparison of UV and visible
photocatalytic activity is not meaningful.
As a control experiment, we performed the photodegradation of MO aqueous
solution under UV and green laser irradiation without any photocatalysts. Here, we
observed no MO photodecomposition even after 24 hours UV or green laser irradiation.
Another control reaction was carried out by irradiating Au nanoparticles alone in solution
without TiO
2
. These Au nanoparticles were prepared according to the previous work of
Hou et al. [64] Even after 4h of green laser irradiation with a power 3 times higher than
those used in Figure 2-3, no MO decomposition was observed. Thus, the presence of a
semiconductor, such as TiO
2
, is necessary in order to create electron-hole pairs, which
drive the photodecomposition process. According to the previously proposed charge
transfer mechanism,[158] visible light has enough energy to create electron-hole pairs on
the Au surface capable of decomposing MO. However, if this were true, we would
observe MO photodecomposition using only Au nanoparticles as the photocatalyst, which
is not the case. The photocatalytic activity of the Au nanoparticle/TiO
2
photocatalyst was
also tested under 633 nm and 785 nm laser illumination, which are below the plasmon
resonance energy of Au nanoparticles. After 4 h illumination, no MO decomposition was
40
observed. Therefore, in order for sufficient electric fields to be achieved, the laser energy
must match the plasmon resonance frequency of the nanoparticles.
2.4 Conclusions
In conclusion, we demonstrate plasmon resonant enhancement of the
photocatalytic decomposition of methyl orange under visible light exposure by
integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO
2
. While the
plasmonic Au nanoparticles enhance the photocatalytic activity of TiO
2
in the visible
range, they result in a reduction in the photocatalytic activity under UV exposure, due to
the reduction in TiO
2
surface area exposed to the aqueous solution. Finite-difference
time-domain simulations of these Au nanoparticle/TiO
2
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 TiO
2
surface, and hence the photodecomposition rate of methyl orange. This
enhancement mechanism relies on the presence of defect states in the TiO
2
, which
enables sub-bandgap absorption. The near-field optical enhancement of the Au
nanoparticles couples light efficiently to the surface of the TiO
2
, making its
photocatalytic performance robust to defects.
41
Chapter 3: Photocatalytic Conversion of CO
2
to Hydrocarbon
Fuels via Plasmon-Enhanced Absorption and Metallic
Interband Transitions
The contents of this chapter were recently published as a full paper in the ACS
Catalysis, and is reprinted from: ACS Catalysis, 1, Wenbo Hou, Wei Hsuan Hung,
Prathamesh Pavaskar, Alain Goeppert, Mehmet Aykol, and Stephen B. Cronin,
Photocatalytic conversion of CO
2
to hydrocarbon fuels via plasmon-enhanced absorption
and metallic interband transitions, p 929-936, Copyright 2011, with permission from
ACS. In addition, all co-authors have given their permission for use of this paper in this
dissertation.
42
A systematic study of the mechanisms of Au nanoparticle/TiO
2
-catalyzed
photoreduction of CO
2
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), we observe a 24-fold
enhancement in the photocatalytic activity due to the intense local electromagnetic fields
created by the surface plasmons of the Au nanoparticles. These intense electromagnetic
fields enhance sub-bandgap absorption in the TiO
2
, thereby enhancing the photocatalytic
activity in the visible range. 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 C
2
H
6
, CH
3
OH, and
HCHO. This occurs because the energy of the d band excited electrons lies above the
redox potentials of the additional reaction products CO
2
/C
2
H
6
, CO
2
/CH
3
OH, and
CO
2
/HCHO. We model the plasmon excitation at the Au nanoparticle-TiO
2
interface
using finite difference time domain (FDTD) simulations, which provides a rigorous
analysis of the electric fields and charge at the Au nanoparticle-TiO
2
interface.
3.1 Introduction
Photocatalytic conversion of carbon dioxide into hydrocarbons is of great interest
for its potential to convert an abundant greenhouse gas to useful hydrocarbon fuels. In
1979, Inoue et al. first demonstrated the photoelectrocatalytic reduction of aqueous
carbon dioxide to produce formic acid, formaldehyde, methyl alcohol, and methane using
semiconducting photocatalytic powders, including TiO
2
, ZnO, CdS, GaP, SiC, and
WO
3
.[71] In addition, Halmann reported formic acid production from aqueous CO
2
at the
43
p-type GaP photocathode in an electrochemical photocell[53] and oxide semiconductors
in a photochemical solar collector.[55] Hemminger and coworkers demonstrated
photosynthetic reduction of carbon dioxide in water vapor to form methane on SrTiO
3
crystalline surfaces without any externally applied potential and in the absence of a liquid
electrolyte.[60]
TiO
2
is one of the most promising photocatalysts for carbon dioxide reduction;
however, it does not absorb light in the visible region of the electromagnetic spectrum.
Because of TiO
2
’s short wavelength cutoff, only a small fraction of solar photons (~4%)
can be used to drive this photocatalyst. The resulting low photocatalytic yield of TiO
2
is
perhaps its main disadvantage for the photocatalytic conversion of carbon dioxide into
hydrocarbons. Several attempts have been made to increase its yield.[1] For example,
copper-,[1, 8, 174] copper oxide-,[173, 178] silver-,[91, 174] and ruthenium dioxide-
doped[54] TiO
2
have resulted in increased yields. Previously, our group and several
others have reported plasmon resonant enhancement of dye photodegradation,[63]
oxidation of CO[67] and organic compounds,[92] and photoelectrochemical
reactions.[158, 159]
However, the mechanism for this increased photocatalytic activity is
controversial. Tatsuma’s group and others treat the plasmon excitation in the metals as
having an energy separation between the electrons and holes, which enables electron
transfer from the Au nanoparticles to the adjacent TiO
2
.[92, 158, 159] However, surface
plasmons consist of the collective oscillation of charge bound to the Au surface, and
therefore have no HOMO-LUMO or valence band-conduction band energy separation
associated with them. It is well known that a Schottky barrier is formed at metal-
44
semiconductor junctions. In this paper, we provide a rigorous analysis of this charge
transfer process by calculating the electron transfer from the plasmon excitation in the Au
nanoparticle to the TiO
2
semiconductor using the electric potentials calculated from
numerical electromagnetic simulations together with the ideal diode equation for a
Au/TiO
2
Schottky junction.
Noble metal nanoparticles combined with semiconductors have been widely
studied for improved charge separation of photogenerated electron-hole pairs, thus
enhancing the overall photocatalysis of semiconductors under UV illumination.[10, 32,
150, 168, 179, 180] Noble metal nanoparticles alone are also potential photocatalysts,
since they also can absorb UV light via interband transitions.[25] However, the
photocatalytic activity of noble metal nanoparticles themselves has not been
acknowledged as significant. It is worth mentioning that Haruta and co-workers have
previously carried out extensive studies of Au nanoparticles on metal oxide surfaces.[57-
59, 135, 136] In their work, Au nanoparticle/metal oxide composites were used to
catalyze thermally-driven oxidation reactions at low temperatures, rather than
photocatalytic processes. Zhu et al. have observed photocatalytically driven dye
photodecomposition, phenol degradation, and benzyl alcohol oxidation under UV
illumination arising from the electronic interband transitions in Ag.[25] Under visible
illumination, however, no phenol degradation or benzyl alcohol oxidation was observed.
Photodecomposition of organic dye molecules is observed under visible light, and was
attributed to surface plasmon resonance. However, the details of this mechanism were not
discussed, whether it was a charge-, thermal-, or field-mediated process. Besides Ag, Au
45
also has interband electronic transitions in the UV range,[4, 39, 48] and thus Au
nanoparticles themselves can also contribute to the photocatalytic activity of
metal/semiconductor composites under UV radiation.
In this paper, we study the mechanisms of Au nanoparticle/TiO
2
-catalyzed
photoreduction of aqueous CO
2
under four different excitation wavelengths (two in the
visible range and two in the UV range), which enables us to separate processes associated
with the plasmon resonance (visible range) from those associated with electronic
transitions in the Au itself (UV range). The mechanisms for increased photocatalytic
activity are studied systematically by monitoring various reaction products, which have
different reduction potentials. We model the plasmon excitation at the Au nanoparticle-
TiO
2
interface using finite difference time domain (FDTD) simulations, which provides a
rigorous analysis of the electric fields and charge at the Au nanoparticle-TiO
2
interface.
Our interband transition hypothesis/model provides an alternative explanation for the
enhancement role of noble metal nanoparticles in TiO
2
photocatalysis purely based on the
relative energies of the electrons and holes in the solid materials with respect to the redox
potentials of the reaction products.
3.2 Experimental
In this work, three basic sample types are fabricated and characterized: 1) bare
TiO
2
, 2.) Au nanoparticles deposited on top of TiO
2
, and 3.) bare Au nanoparticles, as
depicted in Figure 3-1. Anatase titania thin films are prepared in our lab by the sol-gel
process and follow the general recipe of acid catalyzing dilute titanium ethoxide in
ethanol.[3] The solution is then mixed with surfactant (P123) and stirred for several hours
46
until a sol forms. Substrates of glass or quartz are spin-coated to achieve the desired film
thickness of 400 nm. The substrates are then positioned horizontally and dried at room
temperature in air for 24 h, thereby allowing most of the solvents and hydrochloric acid
to evaporate and the surfactant to self-organize. The dried films are then annealed at 400
o
C in air for 4 h to improve their crystallinity and drive off any remaining solvents and
surfactant. Raman and XRD spectra of the resulting TiO
2
are given in Figure 3-2, which
shows that anatase TiO
2
is obtained. A thin film of gold is deposited on the TiO
2
surface
in vacuum using electron beam evaporation, while the film thickness is monitored with a
crystal oscillator. A 5 nm deposition of gold is not thick enough to form a continuous
film and, instead, produces an island-like morphology that is known to be strongly
plasmonic.[31, 94] A high resolution transmission electron microscope (TEM) image of a
5 nm Au film is shown in Figure 3-11a. Subsequent annealing of this island-like film at
400
o
C in air for 1 h produces more spherical Au nanoparticles, as shown in Figure 3-3.
Thin Au evaporated films (~5 nm) are known to form island-like growth, which serve as
good substrates for surface enhanced Raman spectroscopy (SERS) and other plasmonic
phenomena.[24, 85, 94, 164] In order to make bare Au nanoparticles on an inactive
support, a gold film with a nominal thickness of 5 nm is evaporated on the surface of the
glass.
47
Figure 3-1. Schematic diagrams of three types of photocatalysts.
Bare TiO
2
Au/TiO
2
Bare Au nanoparticles
TiO
2
Au nanoparticles
48
100 200 300 400 500 600 700 800
Raman Shift (cm
-1
)
Anatase TiO
2
20 30 40 50 60
Intensity
2Theta
A(101)
R(110)
A(004)
A(113)
A(200)
R(211)
A(211)
Figure 3-2. (a) Raman spectrum and (b) XRD profile of sol-gel TiO
2
.
(a)
(b)
49
Figure 3-3. SEM image of 5 nm Au thin film after the second annealing.
Absorption spectra of the bare TiO
2
and Au nanoparticle/TiO
2
films are recorded
on a Perkin-Elmer Lambda 950 UV/Vis/NIR spectrophotometer with an integrating
sphere detector. The photocatalytic reduction of CO
2
and H
2
O are carried out in a sealed
51.6 ml stainless steel reactor with a quartz window for the three basic sample types
described above. The photocatalytic films are placed on the catalyst holder, which is on
the bottom of the reactor. A schematic diagram of the experimental setup is shown in
Figure 3-4. The reactor is first purged with CO
2
saturated water vapor for 1 h before
closing the system. The reactor is then illuminated with either UV light (254 nm 20
mW/cm
2
or 365 nm 20 mW/cm
2
UV lamp) or visible light (532 nm 350 mW/cm
2
green
laser) for 15 h at 75
o
C. The irradiated surface area is limited by the surface area of the
photocatalysts (10 cm
2
). Reaction products are analyzed using a Varian gas
chromatograph (GC) equipped with TCD (with a detection limit of 100 ng for CO
2
) and
FID (with a detection limit of 50 pg for small organic molecules) detectors. The GC is
calibrated by a series of gas samples with known amounts of CH
4
, CH
3
OH, HCHO, and
C
2
H
6
. 300 µL of gas (products and unreacted reagents) is sampled after 15 h of
50
illumination for each reaction. Since only 300 µL of unreacted reagents and products are
sampled and tested using GC, the yields are calculated by normalizing to the full volume
of the reactor (51. 6 ml).
Figure 3-4. Schematic diagram of experimental setup.
3.3 Results and Discussion
Figure 3-5a shows the product yields of the photoreduction of aqueous CO
2
expressed per 1 m
2
of catalyst surface area after 15 h visible (532 nm laser) illumination.
Here, methane is the only product detected by the GC for the three basic sample types,
bare sol-gel TiO
2
, Au nanoparticle/TiO
2
, and bare Au nanoparticles. These reactions can
be understood by comparing the conduction and valence band energies of TiO
2
with the
reduction potentials of CO
2
for the three reduction products CH
4
, HCHO, and CH
3
OH, as
shown in Figure 3-5b.
1, 4
Since the conduction band of TiO
2
lies above the reduction
potential of CO
2
/CH
4
,[49] it is energetically favorable for electrons from the conduction
band of TiO
2
to transfer to CO
2
to initiate the reduction of CO
2
with H
2
O producing
CH
4
.[11] Methane is the only favorable product since the reduction potentials of
CO
2
/HCHO and CO
2
/CH
3
OH lie above the conduction band of TiO
2
.[60, 71] For the bare
51
TiO
2
-catalytzed reduction, only a small amount of methane is detected by GC since the
energy of the 532 nm wavelength light (2.41 eV) is significantly lower than the bandgap
of TiO
2
(3.2 eV). The yield is finite, yet small (0.93 µmol/m
2
-cat.), because of electronic
transitions excited to and from defect states in the bandgap of TiO
2
. On the other hand,
the yield of Au nanoparticle/TiO
2
-catalyzed reduction is 22.4 µmol/m
2
-cat., a 24-fold
enhancement over the bare constituent materials. This enhancement in sub-bandgap
absorption/photocatalysis is consistent with our previous work,[63, 106] wherein the
intense local fields produced by the plasmonic nanoparticles couple light very effectively
from the far-field to the near-field, short-lived defect states at the TiO
2
surface. As a
control experiment, bare Au nanoparticles without TiO
2
were also tested and found to
exhibit a negligible photocatalytic yield (Figure 3-5a), indicating the importance of the
TiO
2
surface in this catalytic process. This result agrees well with our previous
studies.[63, 106] Under visible illumination, electron-hole pairs are generated by the sub-
band transitions in TiO
2
, instead of in Au. Plasmon-excited electrons in Au nanoparticles
are not be able to transfer to the either TiO
2
or the reagents.
(
52
Figure 3-5. (a) Photocatalytic product yields (after 15 h of visible irradiation) on three
different catalytic surfaces. (b) Energy band alignment of anatase TiO
2
, Au, and the
relevant redox potentials of CO
2
and H
2
O under visible illumination.
The UV-vis absorption spectrum taken for a bare TiO
2
film prepared by the sol-
gel method (as shown in Figure 3-6) shows transparency for wavelengths above 387 nm,
which corresponds to the bandgap of anatase TiO
2
(3.2 eV). In our previous work
0
5
10
15
20
25
=532 nm
Product Yield ( mol/m
2
-cat.)
CH
4
TiO
2
Au/TiO
2
Au
-8
-7
-6
-5
-4
-3
3.2 eV
CO
2
/CH
3
OH -0.32 V
CO
2
/HCHO -0.40 V
Anatase TiO
2
H
2
O/O
2
0.82 V
CO
2
/CH
4
-0.244 V
Vacuum (eV) NHE (Volts)
3
2
1
0
-1
-2
Fermi level of Au
(
(a)
(b)
53
involving anodic TiO
2
(ATO),[106] however, this defect concentration was much higher
due to N- and F-impurities produced during the anodization process,[100] which resulted
in an obvious UV-vis absorption. While the UV-vis absorption spectra of the bare sol-gel
TiO
2
shows no apparent absorption below the bandgap, finite sub-bandgap absorption
does occur due to a small concentration of defect states in the bandgap due to a Ti
4+
stoichiometry deficiency,[45] thus, enabling electron-hole pair generation at 532 nm. In
our previous water-splitting photocatalysis work, no photocurrent was observed for the
sol-gel prepared TiO
2
because of the high resistance of the sol-gel film, which decreases
the water splitting photocurrent. This high resistance, however, does not affect the CH
4
photocatalytic reaction, since there is no electrochemical circuit. In this present work,
ATO was also tested, and was found to produce CH
4
under visible (532 nm) illumination
(19.7 µmol/m
2
-cat.). Furthermore, photocatalytic enhancement by 17% was observed
with the addition of gold nanoparticles (5 nm film) on the ATO (23.1 µmol/m
2
-cat.).
300 400 500 600 700 800
Absorbance
Wavelength (nm)
Sol-gel TiO
2
thin film
Sol-gel Au/TiO
2
thin film
5 nm Au thin film on glass
Figure 3-6. UV-vis absorption spectra of TiO
2
with and without gold nanoparticles and
gold nanoparticles deposited on glass by electron-beam evaporation.
54
In order to understand the mechanism of enhanced hydrocarbon production by the
photoreduction of CO
2
and H
2
O, we also characterized this reaction under UV irradiation.
Figure 3-7 shows the product yields of the photoreduction of aqueous CO
2
expressed per
1 m
2
of catalyst surface area after 15 h UV (254 nm mercury lamp) illumination. For the
bare TiO
2
catalyst, methane is the only product detected by GC. However, we observe
additional reaction products, including ethane, formaldehyde, and methanol, for the Au
nanoparticle/TiO
2
-catalyzed reactions. Interestingly, we also observe these same four
reaction products upon illumination of bare Au nanoparticles deposited on glass,
indicating that the reaction is now taking place on the Au surface.
Again, we are able to understand these results by comparing the conduction and
valence band energies of TiO
2
and Au with the reduction potentials of CO
2
for the
reduction products CH
4
, HCHO, and CH
3
OH, as shown in Figure 3-7b. While the
conduction band of TiO
2
lies above the reduction potential of CO
2
/CH
4
, it is below the
reduction potentials of CO
2
/HCHO and CO
2
/CH
3
OH. As a result, methane is the only
product for the bare TiO
2
-catalytzed reaction. Metals are often thought of as simply
having electronic states filled up to a Fermi energy corresponding to the work function of
the metal. However, like any crystalline material, metals have higher lying electron bands
that are normally unoccupied.[48, 187] In noble metals, the d-electron bands lie below
the Fermi level (E
F
). Interband transitions from the d-band to an empty sp state above E
F
can occur during the optical absorption process.[166] In Au, the first interband excitation
occurs at the X-point in the Brillouin zone, at an energy of 2.5 eV.[30, 77] The energy of
the 254 nm wavelength light (4.88 eV) exceeds the minimum energy required for
55
interband transitions in Au and therefore is able to excite electrons from its d band to the
conduction band, which lies above the conduction band of TiO
2
and the reduction
potentials of CO
2
/CH
4
, CO
2
/CH
3
OH, and CO
2
/HCHO, as shown in Figure 3-7b. These
highly energetic electrons are then able to drive all of these products of the reduction of
CO
2
and H
2
O. Thus, methane, ethane, formaldehyde, and methanol are observed in the
Au nanoparticle/TiO
2
- and bare Au-catalyzed reactions under 254 nm UV illumination.
In this process, both redox half reactions occur at the Au surface. The excited electrons
are given to CO
2
to form reduction products, while the holes drive the other half reaction
to form O
2
. X-ray photoelectron spectra (XPS) (Figure 3-8) show the binding energies of
Au
4f7/2
in the Au/TiO
2
sample before and after reaction at 84.3 and 84.2 eV, which are
significantly different from Au
+
4f7/2
(85.2 eV) and Au
3+
4f7/2
(86.7 eV) but similar to
Au
0
4f7/2
(84.0 eV).[149] If the reduction half-reactions occur on the Au surface and the
oxidation half-reaction on the TiO
2
surface, the Au should lose electrons and leave holes
on its surface. This would shift the electron binding energies of Au to the higher energies,
corresponding to oxidized states after the reaction. However, we observe no change in the
XPS binding energies, indicating that the oxidation state of the Au surface remains the
same before and after the reaction. Therefore, this confirms that both half reactions occur
on the Au surface. The slight enhancement (~2X) in the product yields of Au/TiO
2
over
bare Au likely arises from charge transfer from the highly energetic excited electrons in
the Au to the adjacent TiO
2
, and from charge generated by the direct absorption of UV
light in the TiO
2
. In this scenario, reactions take place on both the TiO
2
and Au surfaces.
We would like to point out that at 254 nm, which is significantly above the plasmon
56
resonance of the gold, there is no local field enhancement produced by the Au
nanoparticles. The catalytic activity of a 30 nm evaporated Au continuous thin film
deposited on glass without TiO
2
was also tested under UV illumination and was found to
produce the same yields for methane, ethane, formaldehyde, and methanol as the 5 nm
Au thin film without TiO
2
. This photocatalytic process under 254 nm UV illumination is
driven solely by the interband transitions in Au, which do not require a nanometric
morphology. A 5 nm film of gold is not thick enough to form a continuous film. Instead,
this produces a film of gold islands with an average size of approximately 20 nm. These
films are known to be strongly plasmonic, and have been utilized for surface enhanced
Raman spectroscopy (SERS) for many years.[31, 94] The island-like morphology of a 5
nm Au film is shown in the transmission electron microscope (TEM) image of Figure 3-
11a. The 30 nm Au thin film forms a continuous bulk Au layer, not Au nanoparticles, and
serves as a control sample to separate and identify effects associated with bulk Au (i.e.,
interband transitions) rather than plasmon resonance.
57
(a)
0
50
100
150
200
250
300
=254 nm
Product Yield ( mol/m
2
-cat.)
CH
4
C
2
H
6
HCHO
CH
3
OH
TiO
2
Au/TiO
2
Au
(b)
-8
-7
-6
-5
-4
-3
4.88 eV
3.2 eV
CO
2
/CH
3
OH -0.32 V
CO
2
/HCHO -0.40 V
Anatase TiO
2
H
2
O/O
2
0.82 V
CO
2
/CH
4
-0.244 V
Vacuum (eV) NHE (Volts)
3
2
1
0
-1
-2
d band of Au
Fermi level of Au
Figure 3-7. (a) Photocatalytic product yields (after 15 h of 254 nm UV irradiation) on
three different catalytic surfaces. (b) Energy band alignment of anatase TiO
2
, Au, and the
relevant redox potentials of CO
2
and H
2
O under 254 nm UV illumination.
58
80 82 84 86 88 90 92
87.8 eV
Au
4f5/2
Au
4f7/2
Binding Energy (eV)
before reaction
after reaction
84.2 eV
Figure 3-8. X-ray photoelectron spectra (XPS) of Au/TiO
2
sample before and after
reactions.
Several earlier studies by Gupta’s group showed that different photo-oxidation
products were caused by distinct free-radicals or ion-radical species formed at TiO
2
surfaces and Au/TiO
2
interfaces due to different incident radiation energy.[13, 109]
However, here we observe different products with different catalysts. In addition, we
expect the effects of localized plasmonic heating to be negligibly small in this system.
Previously, we observed plasmonic heating for incident light intensities above 1.4×10
6
W/cm
2
.[68] In the work presented here, the incident light intensity is 0.35 W/cm
2
, which
is seven orders of magnitude below that which is expected to produce significant heating.
Unfortunately, we are unable to identify radical species in our experimental setup,
however, our results can be understood purely based on the relative energies of the
electrons and holes in the solid materials with respect to the redox potentials of the
reaction products. This model does not depend on or give information about intermediate
59
radical species, but provides an alternative framework with which to understand the
Au/TiO
2
catalytic system.
The interband transition mechanism proposed under UV illumination is not only
applicable to gold-catalyzed photoreactions. In addition, the catalytic activities of a 5 nm
Pt thin film deposited on glass and a Cu foil were also tested. The yields for methane,
ethane, formaldehyde, and methanol of the 5 nm Pt thin film and Cu foil are similar to
that of the 5 nm Au thin film, as shown in Figure 3-9.
0
20
40
60
80
100
120
=254 nm
Product Yield ( mol/m
2
-cat.)
CH
4
C
2
H
6
HCHO
CH
3
OH
Au Pt Cu
Figure 3-9. Photocatalytic product yields of 5 nm Pt on glass and a Cu foil compared
with that of 5 nm Au on glass.
We also characterized this reaction under 365 nm UV illumination for the same
three basic sample types (Figure 3-10). Under 365 nm UV illumination, no products were
formed for the 5 nm Au thin film alone and only methane was formed for TiO
2
and Au
nanoparticle/TiO
2
-catalyzed reactions. The reason no products are formed on the bare Au
nanoparticles surface is that the energy of the 365 nm light (3.4 eV) is not high enough to
60
excite electrons beyond the redox potentials of the CO
2
/CH
4
, CO
2
/HCHO, CO
2
/CH
3
OH
reduction potentials, as shown in Figure 3-10b. The electrons excited by interband
transitions in the Au also lie below the conduction band of TiO
2
, and, therefore, no
electrons are transferred from the Au to the TiO
2
. These results under UV illumination
further confirm that the plasmonic enhancement of photocatalysis under visible
illumination is a result of the strong electric fields created by the surface plasmons of the
Au nanoparticles rather than direct transfer of the charge. The methane formed in the
TiO
2
and Au nanoparticle/TiO
2
-catalyzed reactions is due to direct absorption in the TiO
2
,
since the energy of the 365 nm light is above the bandgap of TiO
2
It should be noted that
this 365 nm light lies above the plasmonic modes in the system; therefore, no plasmonic
enhancement occurs in the Au nanoparticle-TiO
2
catalyst sample.
61
(a)
0
5
10
15
20
Product Yield ( mol/m
2
-cat.)
CH
4
TiO
2
Au/TiO
2
Au
=365 nm
(b)
-8
-7
-6
-5
-4
-3
3.4 eV
3.2 eV
CO
2
/CH
3
OH -0.32 V
CO
2
/HCHO -0.40 V
Anatase TiO
2
H
2
O/O
2
0.82 V
CO
2
/CH
4
-0.244 V
Vacuum (eV) NHE (Volts)
3
2
1
0
-1
-2
d band of Au
Fermi level of Au
Figure 3-10. (a) Photocatalytic product yields (after 15 h of 365 nm UV irradiation) on
three different catalytic surfaces. (b) Energy band alignment of anatase TiO
2
, Au, and the
relevant redox potentials of CO
2
and H
2
O under 365 nm UV illumination.
62
As another control experiment, we performed bare TiO
2
and Au/TiO
2
-catalyzed
photoreduction of CO
2
and H
2
O under 633 nm laser irradiation, which is well below the
bandgap energy of TiO
2
and the Au interband transition energy. No products were
observed after 15 h irradiation. In our previous study, ATO was used as the photocatalyst
instead of sol-gel TiO
2
.[106] The concentration of defect states with energies below the
bandgap of ATO are much larger than that in the sol-gel prepared TiO
2
(as shown in the
UV-vis spectra, Figure 3-6). Therefore, no products were observed under 633 nm
illumination for sol-gel samples.
The quantum efficiencies of our samples are calculated using Equation 3-1 and
are summarized in Table 3-1. Here, n is the number of moles of electrons required to
produce one mole of reduction product from CO
2
, which includes CH
4
, C
2
H
6
, CH
3
OH
and HCHO. The total photon flux irradiating the catalytic surface is determined from the
incident light intensity and exposure time. For the Au/TiO
2
-catalyzed reaction, the
quantum efficiency is roughly equal to the sum of the quantum efficiencies of the TiO
2
-
catalyzed reaction and Au nanoparticles alone-catalyzed reaction under UV illumination.
This confirms that the mechanism under UV illumination includes interband transitions
in both TiO
2
and Au. The quantum efficiency under visible light irradiation is 3 orders of
magnitude smaller than that under UV irradiation, since the high energy photons can
drive both sub-bandgap transitions and interband transitions. Low energy photons, on the
other hand, can only drive sub-bandgap transitions in TiO
2
, which have absorption cross-
sections that are significantly smaller than interband transitions. Here, we observe a 24-
63
fold plasmonic enhancement, similar to enhancement factors reported in our previous
papers for other reaction systems.[63, 106]
Quantum efficiency (%) =
100% (3-1)
As a control experiment, the CO
2
and H
2
O vapor reactants were irradiated without
any photocatalyst. Here, no products were detected under 254 nm or 532 nm wavelength
irradiation. In addition, we performed TiO
2
and Au/TiO
2
-catalyzed reduction of CO
2
and
H
2
O at 400
o
C without illumination and observed no reaction.
Table 3-1. Overall quantum efficiency (%) of reduction of CO
2
and H
2
O on TiO
2
, Au,
and Au/TiO
2
catalysts under UV and visible irradiation.
Light Source TiO
2
Au/TiO
2
Au
UV (20 mW/cm
2
, 254 nm) 8.9×10
-3
2.3×10
-2
8.8×10
-3
UV(20 mW/cm
2
, 365 nm) 6.5×10
-4
6.9×10
-4
0
Visible (350 mW/cm
2
, 532 nm) 8.8×10
-7
2.1×10
-5
0
In order to obtain a more rigorous understanding of the mechanism of catalytic
enhancement, we perform numerical simulations of the charge and electric potential
distributions at the Au nanoparticle-TiO
2
interface under plasmon resonance excitation,
as shown in Figure 3-11. These simulations are based on the transmission electron
microscope (TEM) image of the Au nanoparticle-TiO
2
surface, shown in Figure 3-11a,
which is used to define the spatial extent of the Au nanoparticle islands in our simulation.
64
As discussed in previous publications, the electromagnetic response of the film (Figures
3-11b and 3-11c) is dominated by local hot spots where the electric field intensity reaches
1000 times that of the incident electric field at the TiO
2
surface.[20, 63, 106, 130] This
means that the electron-hole pair generation rate is 1000 times that of the incident
electromagnetic field. Thus, an increased amount of charge is induced locally in the TiO
2
due to the local field enhancement of the plasmonic nanoparticles. This estimate is based
on our electromagnetic simulations, which was in good agreement with our previous
experimental results on water spitting and methyl blue decomposition.[63, 106] While we
have no way of measuring the electron-hole pair generation rate in an isolated plasmonic
hot spot (~2nm x 2nm), our previous work measuring plasmon-enhanced water splitting
provides perhaps the most direct measurement of this,[106] since every two electrons
produces one hydrogen molecule.
In our previous work, we assumed that the observed catalytic enhancement was
based purely on the classical electric field enhancement near the metal nanoparticle
surfaces, and that no direct transfer of charge was occurring between the plasmonic Au
nanoparticles and the TiO
2
semiconductor. However, this mechanism has been discussed
widely in previous literature[69, 92, 118, 158, 159] and is of particular interest for sub-
bandgap absorption, providing an alternate mechanism that does not require defects. Here,
we explore the possibility of direct electron transfer from the plasmonically excited Au to
the adjacent TiO
2
using a thermionic emission model to describe the electron transport
across the Schottky barrier, based on the potential differences calculated in the FDTD
simulation. By adopting this FDTD-circuit hybrid model, this metal-semiconductor
65
interface should behave like a diode, allowing charge to flow in only one direction. The
dielectric function of Au was modeled after the data given in Palik's handbook.[123]
TiO
2
is modeled as a dielectric with a refractive index of 2.48. The potential difference
across the interface is calculated by the formula
∑
where E
z
is the z-component of the electric field calculated using the FDTD simulation,
and dz is the grid spacing, which in our calculations is 0.2 nm. The current density across
the Au nanoparticle-TiO
2
interface is given by the ideal diode equation:
1 ,
where J
s
is the reverse saturation current density given by
.
Here, m* is the effective mass of the electron and
b
is the potential barrier across the
interface. Figure 3-11d shows the voltage across the interface, as calculated from our
FDTD simulation using Equation 3-2. Figure 3-11e shows the corresponding electric
current density calculated using Equation 3-3. While the equation above clearly indicates
diode behavior and suggests a net charge transfer, we can see from the current plot
(Figure 3-11e), that the positive and negative charge transfer (currents) across the
interface are equal and, thus, cancel each other. If we integrate the current density over
the whole area in the simulation, the net current flow is zero. The reason for this perfect
cancelation of positive and negative currents is the very small potential differences across
the interface (10
-5
V), which are well within the ohmic regime of this so-called “diode”.
(3-3)
(3-4)
(3-2)
66
In the analysis above, we have calculated the electron transfer from the plasmon
excitation in the Au nanoparticle to the TiO
2
semiconductor using the voltages calculated
from FDTD simulations together with the ideal diode equation for a Au/TiO
2
Schottky
junction. We found that there is no net flow of electrons because the Au/TiO
2
junction
does not exhibit rectifying behavior at these small voltages. As a result, charge flows in
both directions during each cycle of the plasmon excitation, which cancel each other and
produce no net current.
We have calculated the electric fields produced in vacuum and in aqueous
solution. The peak local plasmon-induced electric fields are 140 V/m in a H
2
O dielectric
environment, while in vacuum they are 120 V/m. Therefore, we expect the presence of
H
2
O molecules to have a minimal effect on the electron-hole pair generation rate. The
primary effect of the H
2
O environment is to red shift the plasmon resonance wavelength
by about 25 nm.
67
Figure 3-11. (a) TEM image of a 5 nm thick Au island film deposited on TiO
2
. (b-c)
Electric field intensity at the interface of Au – TiO
2
calculated using FDTD. (d) Voltage
and (e) Current density across the Au – TiO
2
interface.
68
3.4 Conclusions
In summary, we observe plasmonic enhancement of the photocatalytic reduction
of CO
2
with H
2
O under visible illumination when Au nanoparticles are deposited on top
of TiO
2
. This enhancement is due to the strong electric fields created by the surface
plasmon resonance of the Au nanoparticles, which excite electron-hole pairs locally in
the TiO
2
at a rate several orders of magnitude higher than the normal incident light. We
demonstrate that the plasmon-excited electrons in the Au nanoparticles cannot transfer
directly from the Au to the TiO
2
. Only when the photon energy is high enough to excite
the d band electrons of Au to a conduction band that lies above the conduction band of
TiO
2
, does direct charge transfer occur between these two materials. When the incident
photon energy is high enough (254 nm UV), an additional mechanism involving the
interband electric transitions in Au produces a number of additional photocatalytic
reaction products. In this wavelength range, both the excited electrons in Au and TiO
2
contribute to the reduction of CO
2
with H
2
O vapor.
69
Chapter 4: Plasmon Resonant Enhancement of Dye Sensitized
Solar Cells
The contents of this chapter were recently published as a full paper in the Energy
& Environmental Science, and is reprinted from: Energy & Environmental Science, 4
(11), Wenbo Hou, Prathamesh Pavaskar, Zuwei Liu, Jesse Theiss, Mehmet Aykol and
Stephen B. Cronin, Plasmon Resonant Enhancement of Dye Sensitized Solar Cells, p
4650-4655, Copyright 2011, with permission from Royal Society of Chemistry (RSC). In
addition, all co-authors have given their permission for use of this paper in this
dissertation.
70
We report an improvement in the efficiency of dye sensitized solar cells (DSSCs)
by exploiting the plasmonic resonance of Au nanoparticles. By comparing the
performance of DSSCs with and without Au nanoparticles, we demonstrate a 2.4-fold
enhancement in the photoconversion efficiency. 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 TiO
2
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/TiO
2
configurations,
which corroborate the enhancement observed experimentally.
4.1 Introduction
Since the dye-sensitized solar cell (DSSC) was first demonstrated by O'Regan and
Grätzel in 1991,[121] scientists have been working on improving its performance.
Several methods, including novel sensitizers,[23, 46, 47, 84, 128, 167] electrolytes,[15,
21, 73] and semiconductors,[29, 62, 183] have been utilized to enhance the power
71
conversion efficiency of DSSCs during the past two decades. However, no significant
improvements in efficiency have been made, and the best efficiency is still below 12%.
The plasmon resonance of noble metal nanoparticles has become an important area of
research with applications in surface-enhanced Raman spectroscopy,[85] nonlinear
optical process,[24, 34] sub-wavelength photolithography,[144] optical antennas,[88] and
TiO
2
photocatalysis.[63, 106] For photocatalysis, the large electric fields produced near
the surface of metal nanoparticles can be used to accelerate sub-bandgap electron-hole
pair generation in TiO
2
catalysts, thereby enhancing the photoconversion efficiency in the
visible wavelength range.[63, 106] Several attempts have been made to employ noble
metal nanoparticles to increase the efficiency of DSSCs.[27, 28, 96, 126, 146] In these
previous studies, the enhanced efficiency of the DSSC was attributed to the Schottky
barrier formed at the metal-semiconductor interface.[27, 28, 96, 146] In other works, a
decreased efficiency was observed with the addition of metal nanoparticles, and
attributed to the decreased surface area of the underlying semiconductor in direct contact
with the absorbing dye molecules.[126] To date, metal nanoparticle plasmon enhanced
DSSCs are is still open study.
In this paper, different geometric configurations of Au nanoparticles/dye/TiO
2
are
investigated as the working electrode in a DSSC. The relative performances of these
geometric configurations enable us to identify and rule out several key aspects of the
underlying enhancement mechanism, such as charge transfer, annealing, and surface area.
Comparing the UV-vis absorption with the spectral response of the photocurrent provides
further information regarding the energy transfer between the plasmonic nanoparticles
72
and the absorbing dye molecules. Electromagnetic simulations of these geometric
configurations (based on electron microscope images) corroborate the enhancement
observed experimentally.
4.2 Experimental
Three basic working electrode configurations with different geometric structures
are fabricated and characterized: #1) a monolayer of Ru dye N719 deposited on top of Au
nanoparticles embedded in a TiO
2
film, #2) a monolayer of Ru dye N719 on top of an
evaporated 5 nm Au-island thin film deposited on the TiO
2
layer, #3) an evaporated 5 nm
Au-island thin film deposited on top of the dye monolayer and the TiO
2
layer. Schematic
illustrations of these three sample configurations are given in Figure 4-1. The
conventional DSSC with a monolayer of Ru dye N719 on top of TiO
2
is also prepared as
a control. Anatase titania films are prepared in our lab by the sol-gel process and follow
the general recipe of acid catalyzing dilute titanium ethoxide in ethanol.[3] The solution
is then mixed with surfactant (P123) and stirred for several hours until a sol forms. ITO
substrates are spin-coated to achieve the desired film thickness. The substrates are then
positioned horizontally and dried at room temperature in air for 24 h, thereby allowing
most of the solvents and hydrochloric acid to evaporate and the surfactant to self-
organize. The dried films are then annealed at 400
o
C for 4 h in air to improve their
crystallinity and drive off any remaining solvents and surfactant. For Sample #1, Au
nanoparticles are embedded in the TiO
2
films using a modified recipe described by Li et
al.[99] In this recipe, P 123, titanium ethoxide, and HAuCl
4
are mixed in the ethanol at
room temperature and spin-coated on ITO substrates. After aging at 40
o
C for 24 h and
73
100
o
C for 12 h, the gels are calcined at 350
o
C for 4 h in air using a heating rate of 0.5
o
C/min. In order to prepare Sample #2, a gold film with a nominal thickness of 5 nm is
evaporated on the top of the sol-gel TiO
2
film. A second annealing process is carried out
by placing the sample into a furnace and calcining in air at 400
o
C for 1 h. The 5 nm
evaporated Au films are not thick enough to form a continuous film of Au, and instead
form island-like growth, as shown in Figure 4-2a. The second annealing process makes
these islands more spherical, as shown in Figure 4-2b. These thin Au films serve as good
substrates for surface enhanced Raman spectroscopy (SERS) and other plasmonic
phenomena.[94, 164] The conventional DSSC and Samples #1 and #2 are then sensitized
by immersing into a 0.3 mM solution of N719 dye in ethanol. All samples are sensitized
for 48 h. In order to make Sample #3, a gold film with a nominal thickness of 5 nm is
evaporated on top of the dye surface of the conventional DSSC. Absorption spectra of all
working electrodes are recorded on a Perkin-Elmer Lambda 950 UV/Vis/NIR
spectrophotometer with an integrating sphere detector. For the counter electrode, a 10 nm
film of Pt is deposited by electron-beam evaporation onto ITO. The working electrode
and the counter electrode are separated by a polypropylene spacer approximately 20 µm
thick and bonded using binder clips. The internal space of the cells is filled with
electrolyte (Iodolyte AN-50 from Solaronix) and sealed with hot-melt sealing foil
(Meltonix 1170-25 from Solaronix). The open-circuit photovoltage, short-circuit
photocurrent, and I-V characteristics of these solar cells are measured using a digital
multimeter (Keithley 2400). A fixed wavelength green laser (60 mW, 532 nm) is
employed to illuminate the solar cells. In addition, the spectral response of the
74
photocurrent is measured using a Fianium supercontinuum white light laser in
conjunction with a Princeton Instruments double grating monochromator.
#1
#2
#3
TiO
2
; Au nanoparticles; Dye N719
Figure 4-1. Schematic diagrams of three different Au nanoparticle/dye/TiO
2
configurations.
75
Figure 4-2. (a) SEM image of 5 nm Au thin film before the second annealing. (b) SEM
image of 5 nm Au thin film after the second annealing. (c) SEM image of TiO
2
film with
a mesoporous structure.
76
4.3 Results and Discussion
Figure 4-3 shows the short-circuit photocurrent during a 28-second laser exposure
at a wavelength of 532 nm. Under illumination, the highest photocurrent is produced by
working electrode #1 (dye monolayer deposited on top of embedded Au
nanoparticles/TiO
2
). Comparing this with the photocurrent produced by the conventional
DSSC, the photocurrent increased by a factor of 2. This enhancement is due to the intense
local electromagnetic fields produced by the plasmonic nanoparticles, which couple light
very effectively from the far-field to the near-field of the dye molecule monolayer. Under
these intense local fields, the exciton generation rate in the dye molecule monolayer
increases significantly, thereby improving the photocurrent. The photocurrents produced
by #2 and #3, however, are lower than that of the conventional DSSC, even with the
addition of Au nanoparticles. The decrease in photocurrent, here, is attributed to the
following facts. (1) For #2, the 5 nm Au thin film is deposited between the TiO
2
film and
the dye monolayer. The Au thin film, therefore, decreases the active surface area of TiO
2
in direct contact with the dye molecules. This decrease also demonstrates that charge is
not transferring from the dye to the Au or from the Au to the TiO
2
. That is, in order for
this solar cell to function properly, the charge must be created in the dye layer and then
transferred to the TiO
2
and surrounding electrolyte solution. Even though the plasmon
enhancement partially compensates for the decrease in photocurrent caused by the
reduced contact area between the dye and TiO
2
, the photocurrent produced by #2 is still
lower than that of the conventional DSSC. As a control experiment, we also prepared a
sample with a 5 nm Au thin film without the second annealing process between the TiO
2
77
layer and the dye monolayer. The photocurrent produced by this control sample (0.34 mA)
is lower than that produced by #2 (0.86 mA), as shown in Figure 4-4. This is because the
Au islands in the 5 nm film tend to cluster and form more spherical nanoparticles after
annealing, as shown in Figure 4-2b, and therefore do not cover as much of the TiO
2
surface area. (2) For #3, the 5 nm Au thin film lies on the top of the dye monolayer. For
this geometric structure, less dye can absorb the light since the Au thin film covers about
40% surface area of the dye (see Figure 4-2b). Therefore, while Samples #1, #2, and #3
all consist of Au nanoparticles/dye/TiO
2
working electrodes, the effectiveness of the
plasmon-enhancement depends on the specific geometric configuration of these three
constituent materials.
Figure 4-3. Short-circuit photocurrents of DSSCs with three different Au
nanoparticle/dye/TiO
2
configurations.
20 40 60
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Photocurrent (mA)
Time (s)
#1-Embedded Au NPs
Conventional DSSC
#3-Au NPs on top
#2-Au NPs in between
78
0 20 40 60 80 100 120 140
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Photocurrent (mA)
Time (s)
Bare TiO
2
Au NPs embedded in TiO
2
Au NPs in between without annealing
Figure 4-4. Short-circuit photocurrents of DSSCs with bare TiO
2
, Au nanoparticles
embedded in TiO
2
without dye molecules, and 5 nm Au thin film without the second
annealing deposited between the TiO
2
layer and the dye monolayer as working electrodes.
Anatase TiO
2
prepared by the acid catalyzing sol-gel process has a hexagonal
close packed mesoporous structure, as shown in Figure 4-2c.[3] In the plasmon enhanced
configuration (#1), the Au nanoparticles sit in the pores of the TiO
2
. These embedded Au
nanoparticles not only create intense local fields, but also maintain most of the contact
area between the dye and the TiO
2
. The photocurrents of bare TiO
2
and Au nanoparticles
embedded in TiO
2
are also measured as a control. No photocurrents are observed for
these two samples, as expected, since no dye molecule is deposited. This result further
confirms that the plasmon excited electrons are not able to transfer directly from the Au
nanoparticles to the TiO
2
, consistent with our previous work on plasmon-enhanced
photocatalysis.[63, 106] In contrast to our previous work, however, doping and defects in
79
the TiO
2
do not affect the DSSC performance, since the electron-hole pairs are generated
in the dye rather than the TiO
2
.[63, 106]
Figure 4-5a shows the photocurrent spectra of DSSCs with the four different
working electrodes. Sample #1 shows a large enhancement in photocurrent with respect
to the conventional DSSC over most of the visible wavelength range. The inset in Figure
4-5a shows the photocurrent enhancement ratio of Sample #1 with respect to the
conventional DSSC. We observe enhancement from 460 nm all the way out to the near
infrared (730 nm). This enhancement ratio peaks at 6.5 at wavelengths of 613 nm. The
peak photocurrents for #1 (red dashed curve), and #2 (blue dotted curve) are redshifted
with respect to the conventional DSSC (dark solid curve). This redshift is caused by the
plasmonic enhancement, which has a maximum around 566 nm, as shown in Figure 4-6a.
The overall photocurrents of samples #2 and #3 are lower than the conventional DSSC
for the same reasons given above, namely the decreased effective area of the dye
molecule monolayer.
In order to understand the performance of these solar cells, spherically integrated
UV-vis absorption spectra of the four working electrodes are measured, and are shown in
Figure 4-5b. The absorption of the conventional DSSC (black solid curve) peaks around
510 nm due to the absorption of the dye. With the addition of Au nanoparticles, this
absorption increases further and redshifts to 530 nm (red dashed curve) because the
plasmonic Au nanoparticles couple the light more effectively in this region of the
electromagnetic spectrum and improve the light absorption of the dye. The absorption of
#2 and #3 are lower than the absorption of the conventional DSSC due to the decrease in
80
dye-coated surface area. The absorption of #1 and #3 are bigger than that of #2, since the
plasmonic Au nanoparticles absorb light most effectively in #1 and #3 due to their
geometric configuration. It is for this reason that #1 can create the highest photocurrent.
However, in #3, the dye layer is covered by the Au thin film. Consequently,
approximately 40% of the dye surface area is covered by the Au nanoparticles and cannot
absorb the incident light effectively. The photocurrent produced by #3 is still lower than
that of the conventional DSSC. Note that the UV-vis absorption of #1 and the
conventional DSSC are comparable, while the photocurrent of #1 is much greater than
the conventional DSSC. This is because the plasmonic near-field absorption dominates
the photocurrent behavior, while bulk (far-field) absorption dominates the UV-vis
response.
81
Figure 4-5. (a) Photocurrent spectra of DSSCs with different working electrodes. (b)
Absorption spectra of different working electrodes.
450 500 550 600 650 700 750 800
0
5
10
15
20
25
Conventional
#1-embedded Au NPs
#2-Au NPs in between
#3-Au NPs on top
Photocurrent ( A)
Wavelength (nm)
400 500 600 700 800
Absorbance
Wavelength (nm)
Conventional
#1-Embedded Au NPs
#2-Au NPs in between
#3-Au NPs on top
450 500 550 600 650 700 750 800
0
1
2
3
4
5
6
7
PC Enhancement Ratio
Wavelength (nm)
82
Two control samples were also measured, a bare TiO
2
film prepared by the sol-gel
method and Au nanoparticles embedded in TiO
2
with no dye molecule. Figure 4-6 shows
the UV-vis spectra and photocurrent spectra taken for these two control samples. The
bare TiO
2
UV-vis spectrum shows transparency for wavelengths above 450 nm.
Correspondingly, no photocurrent is observed for this sample under visible illumination,
as shown in Figure 4-6b. The absorption spectrum taken from Au nanoparticles
embedded in TiO
2
(red dashed curve in Figure 4-6a) exhibits a peak in the absorption
around 566 nm, corresponding to the plasmon resonance of the Au nanoparticles,
although the absorption of this film is quite broad due to its inhomogeneity.[37] While
this sample absorbs light in the visible range, it does not produce a photocurrent (Figure
4-6b) since no dye molecule is present to initiate electron-hole generation.
83
400 500 600 700 800
Absorbance
Wavelength (nm)
Au NPs embedded in TiO
2
Bare TiO
2
(a)
450 500 550 600 650 700 750 800
-1.0
-0.5
0.0
0.5
1.0
Photocurrent ( A)
Wavelength (nm)
Bare TiO
2
Au NPs embedded in TiO
2
(b)
Figure 4-6. (a) Absorption spectra of bare TiO
2
and Au nanoparticles embedded in TiO
2
without dye molecules. (b) Photocurrent spectra of DSSCs with bare TiO
2
and Au
nanoparticles embedded in TiO
2
without dye molecules as working electrodes.
84
Figure 4-7 shows the typical I-V curves for the four solar cells under 532 nm
wavelength laser illumination. Table 1 summarizes this data, listing the open-circuit
photovoltages, short-circuit photocurrents, fill factors, and overall power conversion
efficiencies. The equations 4-1 and 4-2 are used to calculate fill factors and
photoconversion efficiencies. The DSSC made of #1 gives the highest performance of
2.28% conversion efficiency. It should be noted, however, that these samples are
intended for proof-of-principle purposes, and have not been optimized for overall
photoconversion efficiency. We believe the photoconversion efficiencies will be much
higher after optimization, for example, thicker TiO
2
layer. The conventional DSSC made
of TiO
2
without Au nanoparticles gives an efficiency of 0.94%. The performance of this
DSSC is improved by 2.4 times due to the plasmonic enhancement. However, devices #2
and #3 show lower performances than the conventional DSSC either due to the reduced
effective area of TiO
2
in contact with the dye molecules (#2) or the decreased dye area
exposed to the light (#3) by Au nanoparticle layer, as discussed above. In addition, we
found that the second annealing process is very important to form strongly plasmonic Au
nanoparticles in Sample #2. As a control experiment, the efficiency of device #2 without
the second annealing process is also measured (0.14% in Table 4-2). This is more than 4
times smaller than the efficiency of device #2 (0.60%).
85
-1.0 -0.5 0.0 0.5 1.0
-2
0
2
4
6
Current (mA)
Voltage (V)
Conventional
#1-embedded Au NPs
#2-Au NPs in between
#3-Au NPs on top
Figure 4-7. I-V characteristics of DSSCs with different working electrodes.
Table 4-1. Summary of photovoltaic device performances.
Working Electrode V
oc
(V) I
sc
(mA) FF(%) η (%)
Conventional 0.72 1.48 53 0.94
#1 0.83 3.04 54 2.28
#2 0.73 0.86 58 0.60
#3 0.61 1.26 46 0.58
The power conversion efficiency η of the solar cells is determined by
(%) 100
oc sc
in
VI FF
PS
(4-1)
where V
oc
is the open-circuit photovoltage, I
sc
is the short-circuit photocurrent, and P
in
S is
the incident laser power times the working electrode area (60 mW). The fill factor FF is
given by
86
mm
oc sc
VI
FF
VI
(4-2)
where V
m
and I
m
are the voltage and the current at the maximum output power point,
respectively.
Table 4-2. Comparison of photovoltaic device performances of Au nanoparticle/dye/TiO
2
configuration #2 with and without the second annealing.
Working Electrode V
oc
(V) I
sc
(mA) FF(%) η (%)
Without annealing 0.48 0.34 52 0.14
With annealing 0.73 0.86 58 0.60
Figure 4-8 shows the calculated enhancement factors due to Au nanoparticles.
These results were calculated using full three-dimensional finite difference time domain
(FDTD) simulations of the different Au nanoparticle geometries studied experimentally.
The enhancement factors were calculated as ratios of the absorption cross-sections in the
sol-gel TiO
2
with and without Au nanoparticles. The dielectric function used for Au was
derived from the data by Palik and Ghosh.[124] The dielectric constant of the sol-gel
TiO
2
was calculated using the Maxwell Garnett effective medium theory due to its
mesoporous geometry.[117] The enhancement factor for the embedded Au nanoparticles
in Figure 4-8 shows a well defined peak with an enhancement of 9, which is of the order
of the experimental value of 6.5. There is little to no enhancement in the case of Au
nanoparticles deposited on top of the TiO
2
, which also agrees well with the
experimentally observed results shown in Figure 4-5. Here, it should be noted that the
absorption was calculated over a 100 nm thick volume of TiO
2
, whereas in our previous
87
article it was calculated over only a 10 nm thick volume. This more accurately reflects
the porous sample topography of the dye-TiO
2
interface. The lack of enhancement
calculated in the case of Au nanoparticles deposited on top of TiO
2
is due to the local
electric fields produced by the nanoparticles, which do not penetrate deep enough to
provide a net enhancement over this volume.
450 500 550 600 650 700 750 800
0
1
2
4
6
8
10
Embedded
Au on top
Enhancement Factor
Wavelength (nm)
Figure 4-8. FDTD calculated enhancement factor plotted as a function of wavelength for
embedded nanoparticles and Au nanoparticles on top of TiO
2
with respect to the
conventional DSSC.
4.4 Conclusions
In summary, a 2.4-fold plasmonic enhancement in the overall conversion
efficiency of DSSCs under visible illumination is observed when Au nanoparticles are
embedded in TiO
2
. This enhancement is dependent on the geometric configuration of the
Au nanoparticles/dye/TiO
2
working electrode. In particular, the embedded Au
nanoparticles show an enhancement in the photocurrent over the wavelength range from
88
460 nm to 730 nm. Conversely, depositing Au nanoparticles on top of the TiO
2
results in
a decreased efficiency by reducing the effective area of TiO
2
in contact with dye or by
reducing the dye surface area exposed to the light. This efficiency decrease also reveals
that direct charge transfer from Au to TiO
2
does not occur, and that the plasmonic
enhancement results solely from the local electromagnetic fields. Finally, electromagnetic
simulations agree well with the experimental observations, further corroborating the
mechanism of plasmonic enhancement in these DSSCs.
89
Chapter 5: Exploring the Effect of Doping in TiO
2
As discussed in previous chapters, doping plays a vital role in plasmon-enhanced
photocatalysis. In particular, doping of metal oxides can significantly improve their
photocatalytic properties in the visible wavelength range. In this chapter, we study the
effects of dopant species and concentration on the photocatalytic activity of TiO
2
. The
effects of doping on TiO
2
represent a vast body of literature. In particular, we focus on
the conditions for successful C-, N-, H-, Cr- doping and the effects of dopant species on
H
2
evolution in photocatalytic water splitting.
5.1 Sol-gel TiO
2
Sol-gel TiO
2
was used for this study, since the sol-gel process produces TiO
2
in
the anatase phase with few defect states. Anatase titania films were prepared in our lab by
the sol-gel process, following the general recipe of acid catalyzing dilute titanium
ethoxide in ethanol.[3] Briefly, the procedure can be described as follows. 0.5 g of
Pluronic P123 was dissolved in 6 g of ethanol (200 proof) and magnetically stirred for 1 h.
Separately, 2.1g of titanium ethoxide was plunged into 1.6 g of concentrated hydrochloric
acid using a syringe needle, and the mixture was stirred vigorously for 10 min, resulting
in a clear solution. Care was taken to prevent the ethoxide from coming into contact with
air. The two mixtures were then mixed together and stirred for 3 h. Substrates were spin-
coated at 4000 rpm for 1 min to achieve the desired film thickness of 100 nm. The
substrates were then positioned horizontally and dried at room temperature in air for 24 h,
thereby allowing most of the solvents and hydrochloric acid to evaporate and the
surfactant to self-organize. The dried film was heated with a rate of 1
o
C /min in stagnant
90
air to 400
o
C where it was kept for 4 h before cooling to improve their crystallinity and
drive off any remaining solvents and surfactant. Figure 5-1 shows the Raman spectrum,
XRD profile, and SEM image of sol-gel TiO
2
. Lastly, the TiO
2
thin films were doped by
Cr, C, N, and H, as discussed below.
20 30 40 50 60
Intensity
2Theta
A(101)
R(110)
A(004)
A(113)
A(200)
R(211)
A(211)
100 200 300 400 500 600 700 800
Raman Shift (cm
-1
)
Anatase TiO
2
Figure 5-1. (a) Raman spectrum, (b) XRD profile, and (c) SEM image of sol-gel TiO
2
.
(a)
(b)
91
5.2 Ion Implantation
Anpo’s group have reported that ion implantation doping of TiO
2
, followed by
calcinations in oxygen, have resulted in dramatic increases in the photocatalytic activity
in the visible range (400~600 nm).[6, 7, 75, 186] In our study, Cr ion implantation was
carried out at the Michigan Ion Beam Laboratory (MIBL) at the University of Michigan,
which is part of the National Nanotechnology Infrastructure Network (NNIN). This
facility offers heavy ion implantation (e.g., Cr) at energies up to 200kV with high
fluences. Table 5-1 summarizes the conditions for Cr ion implantation that were explored
in the work presented here. Figure 5-2 shows UV-Vis absorption spectra for Cr ion
implanted samples. A slight shift in the absorption edge and increased absorption
between 400~ 550 nm can be seen in the absorbance spectra of samples #3 and #4, which
were doped at an ion energy of 200 keV and the fluence higher than 1×10
15
ion/cm
2
. Also,
samples #1, 2 remained transparent, while samples #3, 4 became yellow and green color
after Cr ion implantation. Several papers have shown that calcination in oxygen after ion
implantation increases the visible light absorption of TiO
2
.[6, 7, 75, 186] However, in our
study, we found that calcinations in oxygen after the ion implantation are not necessary.
Samples #3 and #4 were annealed in O
2
at 477
o
C for 4h. After annealing, the resistance
of both samples became infinite, even though the absorption spectra did not change.
92
200 300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Abs
Wavelength (nm)
Before Ion Implantation
#1 After Ion Implantation
#2 After Ion Implantation
200 300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Abs
Wavelength (nm)
Before Ion Implantation
#3 After Ion Implantation
#4 After Ion Implantation
Figure 5-2. UV-vis spectra of sol-gel TiO
2
thin films before and after Cr ion implantation.
Table 5-1. Conditions of Cr ion implantation for samples #1-4.
Sample # Fluence Ion Energy Resistance
#1
8×10
11
ion/cm
2
180 keV infinite
#2
1.6×10
13
ion/cm
2
180 keV infinite
#3
1×10
15
ion/cm
2
200 keV 0.5 M Ω
#4
1×10
16
ion/cm
2
200 keV 25 k Ω
5.3 N-doping by Plasma Ion Implantation (PII)
Nitrogen ion implantation was carried out at the University of Saskatchewan in
Canada in the laboratory of Prof. Michael Bradley. Plasma Ion implantation (PII) is a
surface modification technique of doping semiconductor surfaces by high voltage
acceleration of plasma ions from a conformal sheath, so as to implant the semiconductor
with suitable dopants, especially at low dosages.[19, 35, 131] The implantation
conditions are as follows:
93
Gas Parameters: N
2
feedstock gas, 10 sccm input flow rate, Pressure = 5.8 mTorr
(base pressure = 5 ×10
-6
Torr)
Plasma RF Parameters: P
forward
= 200 Watt, P
ref
< 4 Watt
Pulse Parameters: 38.1 pulses per second, pulse duration = 26.8 microseconds, 1.5
×10
5
total pulses delivered (~ 60 minutes total plasma exposure time)
Implantation voltage = 4kV
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
Abs
Wavelength (nm)
Before doping
After doping
After annealing
Figure 5-3. UV-vis absorption spectra of sol-gel TiO
2
thin films before and after N ion
implantation before and after annealing in O
2
.
Figure 5-3 shows UV-vis absorption spectra of a 100 nm thick sol-gel TiO
2
thin
film before and after N ion implantation using PII. Here, N-doping does not change the
visible absorption of the TiO
2
thin film, most likely due to the low dosage of N-doping.
This is not surprising given the implantation voltage (4 kV) and fluence (10
10
ion/cm
2
),
which are much lower than that of the Cr ion implantation (200 kV, 10
15
~10
16
ion/cm
2
).
Post implantation annealing at 500
o
C for 3 h did not improve the activation of dopants or
94
the visible light absorption. In the future studies, higher fluences and thinner films are
recommended.
5.4 H-doping
In addition to ion implantation, we have explored several other methods of doping
known to improve the photocatalytic properties of TiO
2
. Annealing TiO
2
in H
2
gas
creates oxygen vacancies, which dope the material n-type[137]. In the work presented
here, sol-gel TiO
2
thin films were annealed in H
2
with a flow rate of 200 sccm at 350, 450,
and 550
o
C for 30 minutes. Figure 5-4a shows the UV-vis absorption spectra of sol-gel
TiO
2
thin films before and after H-doping at various temperatures. Here, the absorption
edge of TiO
2
thin films significantly red-shifts after annealing in H
2
. Increased visible
absorption is observed with the increased annealing temperature. The color of the TiO
2
thin films turns from clear to yellowish or brown after H-doping, as shown in Figure 5-4b.
300 400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Abs
Wavelength (nm)
Undoped
350
o
C
450
o
C
550
o
C
Figure 5-4. (a) UV-vis absorption spectra and (b) photograph of sol-gel TiO
2
thin films
before and after H-doping at different temperatures.
The hot-probe measurement technique is a method of quickly determining
whether a semiconductor is n-type or p-type. Figure 5-5a schematically illustrates the
(a)
(b)
95
experimental setup for the hot-probe measurement. Here, a voltmeter is attached to the
sample, and a heat source, such as a soldering iron, is placed on one of the leads. The heat
source will cause charge carriers (electrons in n-type, holes in p-type) to diffuse away
from the hot probe, creating voltage difference. For example, if the heat source is placed
on the positive lead of a voltmeter attached to an n-type semiconductor, a positive voltage
reading will result, as the area around the heat source/positive lead becomes positively
charged. Figure 5-5c shows the voltage versus time curve of H-doped TiO
2
thin film.
Compared with the voltage versus time curve of the standard n-type semiconductor
(Figure 5-5b), H-doped TiO
2
is n-type, which is consistent with literature.
Figure 5-5. (a) Schematic diagram of the hot-probe measurement. Voltage vs. time curve
of (b) standard silicon n-type semiconductor and (c) H-doped TiO
2
in the hot-probe
measurement.
(a)
(b)
(c)
96
5.5 C-doping
Annealing TiO
2
in CH
4
at 400
o
C is known to create carbon impurities that dope
TiO
2
n-type.[52, 81] TiO
2
thin films were annealed in CH
4
at a flow rate of 200 sccm at
300, 400, and 600
o
C for 1h. Here, the annealing temperature plays a key role in the C-
doping, as shown in Figure 5-6a. Only the samples annealed at 400
o
C show a clear
improvement in the visible light absorption. The absorption spectra of the samples
annealed at 300 and 600
o
C remain the same as that of the undoped TiO
2
. The color
change of C-doped TiO
2
can be seen in Figure 5-6b. The color of the sample annealed at
400
o
C turns yellowish, while the color of the samples annealed at 300 and 600
o
C
remains transparent. The color change confirms that the optical absorption of TiO
2
can be
shifted into visible range after annealing in CH
4
at 400
o
C. Other annealing temperatures
do not dope TiO
2
by carbon successfully. Hot-probe measurements indicate that C-doped
TiO
2
is p-type, as indicated in Figure 5-6c and 5-6d, in contrast to previous papers, which
report C-doping of TiO
2
to be n-type.[52, 81]
97
300 400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Abs
Wavelength (nm)
Undoped
300
o
C
400
o
C
400
o
C
600
o
C
Figure 5-6. (a) UV-vis absorption spectra and (b) photograph of sol-gel TiO
2
thin films
before and after C-doping at different temperatures. Voltage vs. Time curve of (c)
standard p-type silicon semiconductor and (d) C-doped TiO
2
in the hot-probe
measurement.
5.6 Photocatalytic Characterization
The photocatalytic activity of H-doped and C-doped TiO
2
thin films on FTO was
tested in the photocatalytic water splitting reaction. The reaction was carried out in a
photoelectrochemical cell, as shown in Figure 5-7. This photoelectrochemical cell has a
quartz window on one side, which allows the UV light (254 nm) to go through. A three-
electrode potentiostat-system is used in this cell with the TiO
2
as the working electrode, a
(a) (b)
(c) (d)
98
Ag/AgCl electrode as the reference electrode, and a graphite electrode as the counter
electrode. The resulting photocurrent was measured using a poteniostat, and the gasses
produced were monitored using a mass spectrometer. After 3h UV illumination with an
overpotential of 0.8 eV, the produced H
2
on either the working electrode side or the
counter electrode side was measured using mass spectrometer. From Figure 5-8, we can
see a large amount of H
2
was formed on the counter electrode side, as expected for H-
doped TiO
2
(n-type). A small amount of H
2
also was measured on the working electrode
side due to gas diffusion through the membrane between the working electrode side and
the counter electrode side of the photoelectrochemical cell. A control experiment was
carried out to confirm that the gasses can permeate the membrane if the pressures are not
equal on both sides. For C-doped TiO
2
(p-type)-catalyzed reaction, no H
2
was observed
on either the working electrode or the counter electrode. These photocatalytic tests
demonstrate that the H-doped TiO
2
(n-type) performs better than the C-doped TiO
2
(p-
type).
Figure 5-7. The photoelectrochemical cell for photocatalytic water splitting.
99
0 100 200 300 400
-9.5
-9.0
-8.5
-8.0
-7.5
Log (H
2
Signal)
Time (s)
H-doped (counter)
H-doped (working)
C-doped (counter)
C-doped (working)
Figure 5-8. Log of H
2
signal intensity measured by mass spectrometer for H-doped and
C-doped TiO
2
-catalyzed water splitting reactions.
5.7 Conclusions
This TiO
2
doping study has shown that Cr ion implantation can successfully dope
TiO
2
, resulting in slight visible light absorption. However, nitrogen plasma ion
implantation did not change the UV-vis absorption spectrum of TiO
2
due to the low ion
implantation voltage and fluence. In addition, annealing sol-gel TiO
2
in H
2
or CH
4
can
successfully dope TiO
2
, also enabling visible light absorption. H-doped TiO
2
is a better
photocatalyst for photocatalytic water splitting reaction than C-doped TiO
2
, since n-type
semiconductors perform better than p-type semiconductors given the mobility of
electrons are better than holes.
100
Chapter 6: Photocatalytic Activity of Several Semiconductors
In Chapters Two, Three, and Four, the surface plasmon-enhanced photocatalytic
activity of TiO
2
has been systematically studied. We also investigated the plasmon-
enhanced photocatalytic activities of some other semiconductors, including Fe
2
O
3
, GaN,
Nb-doped SrTiO
3
,
and TiO
2
, which were prepared by various methods. In this Chapter,
these results will be summarized.
6.1 Spray Pyrolysis TiO
2
In addition to anodic titanium dioxide (ATO) and sol-gel TiO
2
, TiO
2
made by
thermal oxidation, atomic layer deposition (ALD) and spray pyrolysis (in collaboration
with Prof. Hai Wang’s group at USC) were also investigated. We used three different
methods of incorporating Au nanoparticles into TiO
2
made by spray pyrolysis: depositing
pre-made Au nanoparticle from solution on top of TiO
2
, adding Au nanoparticles during
the “densification process” and evaporating Au nanoparticles on top of TiO
2
using
electron beam deposition. Figure 6-1 demonstrates the I-V characteristics of these three
types of Au/TiO
2
in water splitting reactions under 532 nm illumination. Figure 6-2a
shows the short-circuit photocurrent under 532 nm laser irradiation. The short-circuit
photocurrents, and I-V characteristics of these three types of Au/TiO
2
are measured using
a two-electrode system with graphite as the counter electrode and a digital multimeter
(Keithley 2400). A fixed wavelength green laser (60 mW, 532 nm) is employed to
illuminate the photoelectrochemical cells. No photocurrents were generated for all three
types of Au/TiO
2
under 532 nm laser illumination, as shown in Figure 6-1 and 6-2a. The
poor performance of these photoelectrochemical cells is attributed to the low
101
concentration of doping in the TiO
2
made by spray pyrolysis. Little visible light can be
absorbed by the sub-bandgap absorption, which is a result of the low doping levels. The
short-circuit photocurrents of spray pyrolysis TiO
2
without Au nanoparticles under UV
irradiation (254 nm and 365 nm) are shown in Figure 6-2b. Once the irradiation light
energy is above the bandgap of TiO
2
, the photocurrent is generated and water splitting
reaction is induced.
-1.0 -0.5 0.0 0.5 1.0
-0.010
-0.005
0.000
0.005
Current (A)
Voltage (V)
dark
GL
Au Solution on top TiO
2
-1.0 -0.5 0.0 0.5 1.0
-0.010
-0.005
0.000
0.005
0.010
Current (A)
Voltage (V)
dark
GL
Densified TiO
2
with Au NPs
-1.0 -0.5 0.0 0.5 1.0
-0.010
-0.005
0.000
0.005
0.010
Current (A)
Voltage (V)
dark
GL
Evaporated 5 nm Au on TiO
2
Figure 6-1. I-V characteristics of three types of Au/TiO
2
photocatalysts in the water
splitting reactions.
102
0 20 40 60 80 100 120 140
-20
-10
0
10
20
30
Laser off
Laser on
Laser off
Laser on
Photocurrent ( A)
Time (s)
evaporated Au
densify Au
Au solution
Laser off
0 50 100 150 200
0
5
10
15
20
365 nm
Photocurrent ( A)
Time (s)
254 nm
Figure 6-2. Short-circuit photocurrent with (a) 532 nm laser (b) UV light on and off.
The photoactivity of these Au/TiO
2
samples made by spray pyrolysis was also
studied in dye-sensitized solar cells (DSSCs). After adding a monolayer of Ru dye N719
into these samples, the photocurrents increased significantly under 532 nm laser
(a)
(b)
103
illumination, as shown in Figure 6-3. In these solar cells, the Ru dye N719 is the visible
light absorber. The plot in Figure 6-3d shows the dye absorption spectrum superimposed
over the solar spectrum A.M. 1.5. When Au nanoparticles are embedded in TiO
2
, the
photocurrent increases compared with that of bare TiO
2
without Au nanoparticles (Figure
6-3c). This enhancement is due to the intense local electromagnetic fields produced by
the plasmonic nanoparticles, which couple light very effectively from the far-field to the
near-field of the dye molecule monolayer. Under these intense local fields, the exciton
generation rate in the dye molecule monolayer increases significantly, thereby improving
the photocurrent. However, when Au nanoparticles are evaporated on top of TiO
2
, the
photocurrent decreases compared with that of bare TiO
2
without Au nanoparticles (Figure
6-3c). This decrease is due to the decreased active surface area of TiO
2
in direct contact
with the dye molecules, since the Au nanoparticles lie between the TiO
2
film and the dye
monolayer. These results agree well with what we observed for sol-gel TiO
2
, as discussed
in Chapter Four.
104
-1.0 -0.5 0.0 0.5 1.0
-4
-2
0
2
4
6
8
10
Current (mA)
Voltage (V)
Dark
532 nm
-1.0 -0.5 0.0 0.5 1.0
-4
-2
0
2
4
6
8
10
Current (mA)
Voltage (V)
Dark
532 nm
0 1020 3040 5060 70
0
2
4
6
8
10
12
14
16
Photocurrent (mA)
Time (s)
Embedded Au
Evaporated Au
Bare TiO
2
300 400 500 600 700 800
0.0
0.5
1.0
1.5
Absorbance (AU)
Wavelength (nm)
Solar spectrum
N719
Figure 6-3. I-V characteristics of (a) embedded Au nanoparticles in TiO
2
and (b)
evaporated Au nanoparticles on TiO
2
. (c) Short-circuit photocurrent under 532 nm laser
irradiation. (d) Dye N719 absorption spectrum superimposed over the solar spectrum
A.M. 1.5.
Figure 6-4 shows the I-V characteristics of three types of Au/TiO
2
DSSCs under
simulated sun light. Table 6-1 summarizes the open-circuit photovoltages, short-circuit
photocurrents, fill factors, and overall power conversion efficiencies of these samples.
Even though the short-circuit photocurrent under 532 nm laser illumination is increased
by embedding Au into TiO
2
, the performance of Au/TiO
2
solar cells decreases under
simulated sun light irradiation. The reason for this is that the energy of the 532 nm laser
Embedded Au NPs in TiO
2
Evaporated Au NPs on TiO
2
(a) (b)
(c) (d)
105
matches the plasmon frequency of Au nanoparticles. Therefore, the plasmon
enhancement factor reaches a maximum value under 532 nm laser illumination. The
simulated sun light includes a wide range of wavelengths. Under illumination of light
away from the plasmon resonance frequency, the available TiO
2
surface area limits the
performance of these solar cells. Since the Au nanoparticles decrease the active surface
area of TiO
2
in direct contact with the dye molecules, the performance of Au/TiO
2
solar
cells decreases under illumination of light away from the plasmon resonance frequency.
As a result, the overall performance of Au/TiO
2
dye sensitized solar cells is worse than
that of TiO
2
dye sensitized solar cells under the simulated sun light irradiation.
0.80.6 0.40.2 0.0
-1
0
1
2
3
4
Current (mA)
Voltage (V)
No Au
Au solution
Evaprated Au
densified with Au
Figure 6-4. I-V characteristics of three types of Au/TiO
2
DSSCs under simulated sun
light.
106
Table 6-1. Summary of solar cell performances under solar simulator illumination.
Anode Efficiency (%) Isc (mA) Voc (V) FF
No Au 6.165 3.3671 0.7571 48.9696
Au solution 3.6143 1.6012 0.7916 51.3264
Dens. Au 3.4152 2.0243 0.7402 50.1428
Evap. Au 1.2417 0.4781 0.7021 53.0428
6.2 Thermally Oxidized TiO
2
The photocatalytic activity of TiO
2
made by thermal oxidation was also tested in
photocatalytic water splitting. In this work, Ti foils were placed into a furnace and
oxidized in air at 400
o
C for 4h. A three-electrode potentiostat was used with the TiO
2
as
the working electrode, a Ag/AgCl electrode as the reference electrode, and a graphite
electrode as the counter electrode. The resulting I-V characteristics measured using the
poteniostat are shown in Figure 6-5. For thermally oxidized TiO
2
, the onset overpotential
is -0.4 V vs. Ag/AgCl under 254 nm UV illumination (Figure 6-5a). The dark current and
photocurrent almost overlap for ATO due to the high impurity concentration in the ATO
(Figure 6-6b). No photocurrent is observed for sol-gel TiO
2
under UV illumination, as a
result of the high resistance of the undoped sol-gel samples (Figure 6-5c).
107
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Photocurrent (mA)
Potential (V) vs. Ag/AgCl
Dark
254 nm
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-1.2
-0.8
-0.4
0.0
0.4
Current (mA)
Potential (V) vs. Ag/AgCl
Dark
254 nm
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-0.3
-0.2
-0.1
0.0
Photocurrent (mA)
Potential (V) vs. Ag/AgCl
Dark
254 nm
Figure 6-5. I-V characteristics for photocatalytic water splitting of (a) thermal oxidized
TiO
2
compared with that of (b) ATO and (c) sol-gel TiO
2
.
6.3 ALD TiO
2
The changes in photocurrent of TiO
2
made by ALD in photocatalytic water
splitting under UV and 532 nm laser irradiation are shown in Figure 6-6. The short-
circuit photocurrents are measured using a two-electrode system digital multimeter
(Keithley 2400) with graphite as the counter electrode. The photocurrent generated under
254 nm UV illumination is larger than that generated under 365 nm UV illumination
Thermal oxidized TiO
2
ATO
Sol-gel TiO
2
(a) (b)
(c)
108
(Figure 6-6a), since the optical absorption of ALD TiO
2
is larger at 254 nm than at 365
nm. Almost no photocurrent is observed under 532 nm laser illumination, as shown in
Figure 6-6b. The poor performance of ALD TiO
2
under 532 nm laser irradiation is
attributed to the low concentration of defects and doping in ALD TiO
2
. Little visible light
can be absorbed by the sub-bandgap transitions, which result from defects and doping
levels.
0 20 40 60 80 100 120 140
0
2
4
6
8
10
365 nm UV on
Photocurrent ( A)
Time (s)
254 nm UV on
20 40 60 80 100
-0.005
0.000
0.005
0.010
0.015
Laser off Laser off
Photocurrent ( A)
Time (s)
Laser on
Figure 6-6. Short-circuit photocurrent of ALD TiO
2
under (a) UV and (b) 532 nm
wavelength laser irradiation.
6.4 Ga
1-x
As
x
N
Ga
1-x
As
x
N samples were grown by using metalorganic chemical vapor deposition
(MOCVD) by our collaborator Prof. Kin Man Yu from Lawrence-Berkeley National
Laboratory. The growth parameters of these five Ga
1-x
As
x
N samples are listed in Table 6-
2. The UV-vis spectra of these samples, shown in Figure 6-7, agree well with the
absorption edges listed in the Table 6-2, except for sample SN347.
(a) (b)
109
300 400 500 600 700 800
-1
0
1
2
3
4
Absorbance (AU)
Wavelength (nm)
284
290
338
347
349
Figure 6-7. UV-vis absorption spectra of Ga
1-x
As
x
N samples.
Table 6-2. Growth and material parameters of Ga
1-x
As
x
N samples.
The photocatalytic activity of these Ga
1-x
As
x
N samples were tested in
photocatalytic water splitting (Figure 6-8), and methane generation (Figure 6-9). These
SN-284 SN-290 SN338 SN347 SN349
Substrate Sapphire Sapphire sapphire glass glass
GaN
Growth T
(mV))
5mV 5mV 10.5mV 5mV 5mV
Ga (Torr)
~1.5 ×10
-7
~7.1× 10
-8
~1.2 ×10
-7
~1.2×10
-7
~1.2×10
-7
As (Torr)
~7.5× 10
-6
~7.2× 10
-6
~7.5× 10
-6
~7.4 ×10
-6
~7.4 ×10
-6
t (hr) 2hr 2hr 2hr 2hr 2hr
Film thickness
(nm)
~500 ~200 ~280 ~430 ~360
Average As
content, x
0.45 0.36 0.26 ~0.45 ~0.5
XRD amorphous amorphous
Weak broad
GaN peak
amorphous amorphous
Absorption
edge (eV)
1.62 1.8 2.2 1.89 1.87
110
samples did not produce any measurable water splitting because of their high resistivity.
However, we were able to detect CH
4
and C
2
H
6
by the photocatalytic reduction of CO
2
with water, which does not require high conductivity. Here, we observed CH
4
and C
2
H
6
under both UV and visible illumination. After evaporating Au nanoparticles on top of the
Ga
1-x
As
x
N, a slight plasmonic enhancement for methane formation was observed, as
shown in Figure 6-9b. The enhancement factor normalized by the catalyst surface area is
1.86.
111
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Current ( A)
Voltage (V)
Dark
532 nm
SN284
-1.0 -0.5 0.0 0.5 1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Current ( A)
Voltage (V)
Dark
532 nm
SN290
-1.0 -0.5 0.0 0.5 1.0
-10
-8
-6
-4
-2
0
2
4
6
Current ( A)
Voltage (V)
Dark
532 nm
SN338
-1.0 -0.5 0.0 0.5 1.0
-100
-50
0
50
Current ( A)
Voltage (V)
Dark
532 nm
SN347
-1.0 -0.5 0.0 0.5 1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Current ( A)
Voltage (V)
Dark
532 nm
SN349
Figure 6-8. I-V characteristics of Ga
1-x
As
x
N samples for photocatalytic water splitting.
112
2.4 2.5 2.6 2.7 2.8 2.9 3.0
0
2000
4000
6000
8000
GC Signal
Retention Time (min)
365 nm
254 nm
CH
4
C
2
H
6
2.32.4 2.52.6 2.72.8
0
1000
2000
3000
4000
GC Signal
Retention Time (min)
No Au
With Au
CH
4
C
2
H
6
Figure 6-9. Gas chromatograph data for Ga
1-x
As
x
N-photocatalized methane generation
under (a) UV and (b) under 532 nm laser irradiation.
The quantum efficiencies of these Ga
1-x
As
x
N samples for the methane generation
reaction under 532 nm laser irradiation are summarized in Table 6-3. The equation used
for the quantum efficiency calculation has been discussed in Chapter Three (Equation 3-
(a)
(b)
113
1). Compared with the quantum efficiency of TiO
2
, the quantum efficiency of Ga
1-x
As
x
N
is three orders of magnitude higher due to their smaller bandgaps. Under 532 nm laser
illumination, interband transitions are induced in Ga
1-x
As
x
N, while only sub-bandgap
transitions are possible in TiO
2
. In general, sub-bandgap transitions have significantly
smaller absorption cross-sections than interband transitions.
Table 6-3. Quantum efficiencies of TiO
2
and Ga
1-x
As
x
N-photocatalyzed methane
generation with and without plasmon enhancement under 532 nm laser illumination.
Sample Ga
1-x
As
x
N
Ga
1-x
As
x
N +
Au
TiO
2
TiO
2
+ Au
QE (%)
3.8×10
-4
6.0×10
-4
8.8×10
-7
2.1×10
-5
6.5 Fe
2
O
3
Fe
2
O
3
(i.e., hematite) is another popular semiconductor used for photocatalysis.
Fe
2
O
3
used in our study was prepared by the sol-gel process, electrochemical oxidation of
Fe thin films, thermal oxidation of Fe foils, or powder bought from Sigma-Aldrich. In the
sol-gel process, an iron hydroxide was precipitated by adding an aqueous ammonium
hydroxide solution into an aqueous solution of ferric nitrate. After the powder was
separated, it was peptized in acetic acid to constitute the sol. Fe
2
O
3
thin films then are
deposited by spin-coating this sol on glass or ITO at 1000 rpm for 1 min. Annealing these
thin films at 500
o
C for 2h to ensure the hematite phase.[101] Figure 6-10 shows the UV-
vis absorption spectra of three types of Fe
2
O
3
. The absorption cutoff wavelength is about
600 nm, which agrees well with the expected bandgap of 2.2 eV.
114
300 400 500 600 700 800
0.0
0.2
0.4
0.6
Absorbance (AU)
Wavelength (nm)
Commercial Power
Sol-gel
Electrochemical oxidization
Figure 6-10. UV-vis absorption spectra of Fe
2
O
3
prepared by three different methods.
The photocatalytic activity of the sol-gel Fe
2
O
3
for methyl orange
photodegradation is shown in Figure 6-11. After 4 h under 365 nm UV illumination,
about 15% of the methyl orange is decomposed. After 4h under 532 nm laser irradiation,
however, no methyl orange degradation is observed, even though the sol-gel Fe
2
O
3
does
have absorption under 532 nm irradiation. This is due to the valence band of Fe
2
O
3
below
the potential to form OH· radicals, which can’t drive this reaction.
115
300 400 500 600
0.0
0.2
0.4
0.6
0.8
Absorbance (AU)
Wavelength (nm)
Before irradiation
After irradiation
365 nm UV
300 400 500 600
0.0
0.2
0.4
0.6
0.8
Absorbance (AU)
Wavelength (nm)
Before irradiation
After irradiation
532 nm green laser
Figure 6-11. UV-vis absorption spectra indicating the concentration change of methyl
orange after (a) UV and (b) 532 nm laser illumination.
Figure 6-12 shows GC data for different products in photocatalytic reduction of
CO
2
reaction. No products were observed for either bare sol-gel Fe
2
O
3
-catalyzed or
Au/Fe
2
O
3
-catalyzed reactions under 532 nm laser illumination. However, for Au/Fe
2
O
3
-
catalyzed reaction, CH
4
, C
2
H
6
, HCHO, and CH
3
OH were produced under 254 nm
irradiation. Based on our previous study (Chapter Three), these reactions are driven by
the excited electrons in Au instead of Fe
2
O
3
.[65]
(a)
(b)
116
2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8
GC Signal (arb. units)
Retention Time (min)
254 nm with Au
532 nm without Au
532 nm with Au
CH
4
C
2
H
6
HCHO
CH
3
OH
Figure 6-12. GC data indicating the relative amount of products generated in Fe
2
O
3
-
photocatalyzed reduction of CO
2
with H
2
O under UV and 532 nm laser illumination.
The photocatalytic activity of Fe
2
O
3
for photocatalytic water splitting was also
investigated. Figure 6-13 shows the short-circuit photocurrent of Fe
2
O
3
prepared by
electrochemical oxidation under UV and 532 nm irradiation. Under 254 nm UV
illumination, no photocurrent (i.e., water splitting) was generated. Under 365 nm UV and
532 nm laser illumination a very small photocurrent was observed. Figure 6-14a shows
the I-V charactoristics of electrochemically oxidized Fe
2
O
3
-catalyzed water splitting
reaction. The onset overpotential is about 0.8 V (vs. RHE). However, a dark current was
also observed above 0.8V, possibly due to impurities in electrochemically oxidized Fe
2
O
3
,
which cause corrosive side reactions without illumination. Fe
2
O
3
prepared by the thermal
oxidation of Fe foils does not yield successful photocatalytic water splitting. Figure 6-14b
shows the I-V curves of the thermal oxidized Fe
2
O
3
,
showing that the current is almost
zero, even at high overpotentials.
117
20 40 60 80 100
7.0
7.5
8.0
8.5
9.0
365 nm
UV on
Photocurrent (nA)
Time (s)
254 nm
UV on
20 40 60 80 100 120 140 160
18
19
20
21
22
Photocurrent (nA)
Time (s)
532 nm
laser on
Figure 6-13. Short-circuit photocurrent of bare Fe
2
O
3
-catalyzed water splitting under (a)
UV and (b) 532 nm laser irradiation.
0.6 0.8 1.0 1.2 1.4
0.0
0.1
0.2
0.3
0.4
0.5
Current (mA)
Potential (V) vs. RHE
Dark
365 nm
254 nm
0.6 0.8 1.0 1.2 1.4
-0.1
0.0
Photocurrent (mA)
Potential (V) vs. RHE
Dark
254 nm
365 nm
Figure 6-14. I-V characteristics of photocatalytic water splitting for Fe
2
O
3
samples
prepared by (a) electrochemical oxidation and (b) thermal oxidation.
From these studies, we can see the photocatalytic activity of Fe
2
O
3
is much worse
than TiO
2
. The poor photocatalytic activity of Fe
2
O
3
is mainly due to low conductivity
and high electron-hole pair recombination rates.[66, 83]
Thermal oxidation Electrochemical oxidation
(a)
(b)
(a) (b)
118
6.6 Nb-doped SrTiO
3
Nb-doped SrTiO
3
was prepared by our collaborator Prof. Xiaoxing Xi at the
Temple University. Figure 6-15a shows the Nb-doped SrTiO
3
-photocatalyzed water
splitting photocurrent measured while switching the 532 nm green laser on and off. When
the 532 nm laser is turned on, a small change in the current was observed. Finite dark
short-circuit current indicates an unintentional corrosion reaction. However, no
photocurrent was observed in the I-V characteristics (Figure 6-15b). The low
photocatalytic activity of Nb-doped SrTiO
3
is attributed to the small surface area and
poor conductivity of the sample.
119
0 50 100 150 200 250 300
1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
Short-Circuit Current ( A)
Time (s)
Nb-doped SrTiO
3
on off
-0.4 -0.2 0.0 0.2 0.4
-20
-10
0
10
20
Current ( A)
Voltage (V)
Dark
532 nm
Figure 6-15. (a) Short-circuit photocurrent of Nb-doped SrTiO
3
-catalyzed water splitting
under 532 nm laser irradiation. (b) I-V characteristics of Nb-doped SrTiO
3
for
photocatalytic water splitting.
(a)
(b)
120
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Abstract (if available)
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.
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Hou, Wenbo
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Core Title
Surface plasmon resonant enhancement of photocatalysis
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College of Letters, Arts and Sciences
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Doctor of Philosophy
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Chemistry
Degree Conferral Date
2012-08
Publication Date
07/09/2012
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06/04/2012
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