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Title Plasmon-Induced Water Splitting under Visible Light Irradiation using Gold Nanostructured Strontium Titanate SingleCrystal Substrate

Author(s) 鐘, 玉磬

Citation 北海道大学. 博士(情報科学) 甲第11973号

Issue Date 2015-09-25

DOI 10.14943/doctoral.k11973

Doc URL http://hdl.handle.net/2115/59961

Type theses (doctoral)

File Information Yuqing_Zhong.pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

Plasmon-Induced Water Splitting under Visible Light Irradiation

using Gold Nanostructured Strontium Titanate Single Crystal

Substrate

Thesis by

Yuqing Zhong

In Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

RESEARCH INSTITUTE FOR ELECTRONIC SCIENCE

HOKKAIDO UNIVERSITY

SAPPORO, JAPAN

2015

Dedication

DEDICATED

TO

MY PAPERENTS

i

Table of Contents

Acknowledgements .............................................................................................................. iv

Thesis Abstract ..................................................................................................................... vi

Chapter 1 .............................................................................................................................. 1

Introduction .......................................................................................................................... 1

1.1 Background and Motivation .................................................................................... 1

1.2 Origin of Photoelectrochemical (PEC) Water Splitting .......................................... 4

1.3 PEC Water Splitting over Semiconductor Photocatalysts ....................................... 8

1.3.1 Basic Principles of PEC Water Splitting ....................................................... 8

1.3.2 Common Materials for PEC Water Splitting ............................................... 11

1.3.3 Development of Visible Light Driven Water Splitting ................................ 15

1.3.4 Plasmon-enhanced Water Splitting .............................................................. 20

1.4 Evaluation of PEC Water Splitting ........................................................................ 24

1.4.1 Stoichiometry of H2 and O2 Evolution ........................................................ 24

1.4.2 Time Course of H2 and O2 Evolution .......................................................... 24

1.4.3 Turnover Number ........................................................................................ 24

1.4.4 Quantum Yield ............................................................................................. 26

1.4.5 Photoresponse .............................................................................................. 27

1.5 Current Scenario and Challenges .......................................................................... 28

1.6 Aim and outline of this thesis ................................................................................ 30

1.7 References ............................................................................................................. 31

Chapter 2 ............................................................................................................................ 44

Plasmon-Induced Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal

Substrate ............................................................................................................................. 44

2.1 Abstract .................................................................................................................. 44

2.2 Introduction ........................................................................................................... 45

ii

2.3 Experimental details .............................................................................................. 47

2.3.1 Niobium doped strontium titanate single crystals ....................................... 47

2.3.2 Preparation of Au-NPs on a SrTiO3 substrate ............................................. 50

2.3.3 Construction of the water splitting device ................................................... 51

2.3.4 Quantitative determination of hydrogen evolution ..................................... 53

2.3.5 Quantitative determination of oxygen evolution ......................................... 54

2.4 Results and discussion ........................................................................................... 57

2.4.1 Characterization of Au-NPs ......................................................................... 57

2.4.2 Numerical simulations of Au-NPs on SrTiO3.............................................. 60

2.4.3 Influence of irradiation wavelength on H2 and O2 generation .................... 62

2.4.4 Influence of pH value combinations on H2 and O2 generation ................... 64

2.4.5 Mechanism for the plasmon-assisted water splitting reaction ..................... 67

2.4.6 Stability of the plasmon-assisted water splitting system ............................. 70

2.5 Conclusions ........................................................................................................... 71

2.6 References ............................................................................................................. 72

Chapter 3 ............................................................................................................................ 76

Co-catalyst Effects on Hydrogen Evolution in the Plasmon-Induced Water Splitting

System ................................................................................................................................ 76

3.1 Abstract .................................................................................................................. 76

3.2 Introduction ........................................................................................................... 78

3.3 Experimental details .............................................................................................. 82

3.3.1 Preparation of noble metal and metal oxide co-catalyst .............................. 82

3.3.2 Electrochemical measurements ................................................................... 82

3.4 Results and discussion ........................................................................................... 84

3.4.1 Elimination of the Schottky barrier effect ................................................... 84

3.4.2 Characteristics of the Ru and Rh species thin film on Pt board .................. 89

3.4.3 Water splitting performance using various H2 evolution co-catalyst .......... 93

3.4.4 Dependence of co-catalyst thickness on water splitting activity ................. 98

iii

3.5 Conclusions ......................................................................................................... 100

3.6 References ........................................................................................................... 101

Chapter 4 .......................................................................................................................... 104

Plasmon-Induced Water Splitting Utilizing Heterojunction Synergistic Effect between

SrTiO3 and Rutile TiO2 ..................................................................................................... 104

4.1 Abstract ................................................................................................................ 104

4.2 Introduction ......................................................................................................... 105

4.3 Experimental details ............................................................................................ 107

4.3.1 Preparation and characterization of the TiO2 thin film .............................. 107

4.4 Results and discussion ......................................................................................... 109

4.4.1 Dependence of Pt co-catalyst thickness on water splitting activity .......... 109

4.4.2 Characteristics of the TiO2 thin film .......................................................... 113

4.4.3 Water splitting performance by TiO2 with different crystalline phases ..... 115

4.5 Conclusions ......................................................................................................... 118

4.6 References ........................................................................................................... 119

Chapter 5 .......................................................................................................................... 122

Summary and Future Perspectives ................................................................................... 122

5.1 Summary ............................................................................................................. 122

5.2 Future Perspectives .............................................................................................. 124

List of Publications ........................................................................................................... 126

iv

Acknowledgements

First and foremost, I would like to express my special appreciation and thanks to my

supervisor, Prof. Hiroaki Misawa, he has been a tremendous mentor for me. I would like

to thank him for encouraging my research and for allowing me to grow as a research

scientist. His advice on both research as well as on my career have been priceless. I am

also deeply grateful for the continuous support, insight and patience of Prof. Kosei Ueno.

He is a good teacher who encouraged and expected us to think more independently about

our experiments and results. I would also like to thank Prof. Tomoya Oshikiri for his

scientific advice and many insightful discussions and suggestions. Thanks to their careful

and friendly supervision, ideas and comments I could learn a lot in these years. Without

their constant trust and, sometimes, gentle prodding, this thesis would not have been

completed. I will forever be thankful to my former college research advisor, Prof. Hongbo

Sun. He has been helpful in providing advice many times during my graduate school

career. I still think fondly of my time as an undergraduate student in his lab. His

recommendation played a vital role in my application for the doctoral program in

Hokkaido University. He was the reason why I decided to go to pursue a career in

scientific research. His enthusiasm and love for research is contagious.

I would like to give special thanks to Ms. Yumiko Yamaguchi, Ms. Yukiko Sugawara

and Ms. Rumiko Kobayashi, who helped me with many application procedures. I am also

very grateful to Ms. Yuko Mori, who offered the constant practical help which is essential

for the smooth running of any project. She kindly assisted me with my experiment and

offered her invaluable expertise on technical skills to me. The improvement of my

Japanese was also attributed to the interesting discussion with her. The Research Institute

for Electronic Science has been a wonderful community in which to grow up as a research

student, the personal and professional relationships forged in these years will stay with me

forever. Sincere thanks go to Dr. Xu Shi, who generously introduced me to the

v

mechanisms of photochemistry and also helped me to solve problems in the experiments

when I first came to this laboratory. The discussion with him was meaningful and

enjoyable. Special thanks also go to Mr. Han Yu, who helped me in study and life. We had

so much memorable experience in Japan together.

I would also like to express my deep gratitude to other laboratory members, Dr. Quan

Sun, Dr. Hiyori Uehara, Dr. Jie Li, Dr. Youzhuan Zhang, Dr. Yu Teng, Dr. Olivier Lecarme,

Ms. Narisu, Mr. Jingchun Guo, Mr. Xiaolong Yang, Mr. Ahamed Shalan, Mr. Takaya

Tokiwa, Mr. Sho Nozawa, Mr. Keisuke Nakamura, Mr. Daisuke Aoyo, Mr. Hidenori Asahi,

Mr. Masamitsu Muto, Mr. Yuki Kotake, Mr. Yuki Matsuzuka and Mr. Yoshiomi Kamata.

We’ve all been there for one another and have taught ourselves and each other many tools

and issues of experimental method. I know that I could always ask them for advice and

opinions on lab related issues. They are wonderful and generous colleagues and partners.

It has been a pleasure working with you. I appreciate help of Mr. Ko Onishi and Ms.

Naomi Kawai from the research support department for facilities maintenance and their

meticulous guidance. I would also like to thank all of my friends who supported me and

incented me to strive towards my goal.

I am indebted to the China Scholarship Council (CSC) for their generous funding,

which removed financial concerns from my decision to embark on this journey.

As a final note, I owe special thanks to my family. Words cannot express how

grateful I am to my parents for all of the sacrifices that they have made for me. I dedicate

this thesis to them.

vi

Thesis Abstract

As a potential solution to the energy crisis, H2 production from water via solar

irradiation has attracted global attention over the last few decades, because the chemical

energy in H2 can be stored for long periods. Numerous attempts have been made since the

photoelectrochemical (PEC) water splitting based on an n-type TiO2 photoanode was

discovered. However, because of the wide band gap of the semiconductor materials

adopted, only UV light can be used to directly drive the water splitting reaction. The UV

irradiation merely accounts for 5% of the solar spectrum. Visible and infrared light, which

take the major part, are still underutilized. Therefore, it is of significance to extend the

light-energy conversion to longer wavelengths, especially to the visible and infrared

regions. Common materials used for water splitting are particulate. They are undesired

because extra energy is required to separate H2 and O2, which evolved in the same reaction

chamber. In addition, an external electric bias is necessary in PEC water splitting. To

overcome these problems, a plasmon-induced water splitting system under visible light

irradiation using gold nanostructured strontium titanate (SrTiO3) single crystal is presented

in this study. Gold nanoparticles (Au-NPs) with particle size of several tens of nanometer

were employed for SrTiO3 surface decoration, which enhanced the SrTiO3 photorespons in

visible wavelength region due to the localized surface plasmon resonance. It is proposed

that the simultaneous evolution of H2 and O2 can be realized without an electric wire

connecting the anode and the cathode and with space conserved for light irradiation on the

anode side through the use of two sides of the same semiconductor substrate.

In order to fabricate Au-NPs, a 3-nm Au thin film was sputtered on SrTiO3 surface

and annealed in nitrogen atmosphere at 800°C. After annealing, the gold films

transformed to a discontinuous nanoparticle structure. The Au-NPs exhibited

characteristic plasmon resonance band with a peak at ca. 630 nm. Numerical simulation

shows that the electromagnetic field intensity is greatly enhanced around the interface

between gold and SrTiO3 substrate. The stoichiometric evolution of H2 and O2 was clearly

vii

demonstrated. The efficiency action spectrum closely corresponds to the plasmon

resonance spectrum, indicating that the plasmon-induced charge separation at the

Au/SrTiO3 interface promotes water oxidation and the subsequent reduction of a proton on

the backside of the SrTiO3 substrate. The photo energy derived from visible light

irradiation is successfully converted to chemical energy, which is stored as the form of H2

gas. As a substitution of the conventional external electric bias, the chemical bias used to

promote water splitting is significantly reduced to 0.23 V by plasmonic effects (Chapter

2).

The co-catalyst effect on hydrogen evolution in the plasmon-induced water splitting

is discussed in detail. A Pt board was stick on the SrTiO3 substrate via In-Ga alloy to

obtain ohmic contact. The water spitting yield with this SrTiO3/Pt composite substrate was

remarkably enhanced. Subsequently, the Pt board was decorated with a noble metal or

metal oxide as a co-catalyst for H2 evolution. Enhanced evolution of H2 and O2 with

stoichiometric ratio was demonstrated with all kinds of co-catalysts. Rhodium thin layer

exhibited relatively high performance as a 3-fold increment compared with the absence of

any co-catalyst. The separate evolution of H2 and O2 was successfully realized, so as to

suppress the undesirable reverse reactions of water splitting. A rational evaluation method

of the co-catalyst is thus developed based on the elimination Schottky barrier (Chapter 3).

A facile plasmon-enhanced water splitting system using gold nanostructured

semiconductor heterojunction is described. With a larger potential gradient in the energy

band, the electron transfer can be facilitated by using semiconductors with different band

positions. The substrate composed of single crystal SrTiO3 with the rutile TiO2 thin film

heterojunction exhibited enhanced water spitting activity compared with the absence of

TiO2 thin film because the back electron transfer reaction might be prevented due to the

synergistic effect. The effect on the H2 evolution efficiency of the thickness of the Pt

co-catalyst film was also explored (Chapter 4).

Herein, the plasmon-enhanced water splitting was demonstrated in visible

wavelength with gold nanoparticles loaded SrTiO3. The H2 and O2 generation efficiency is

viii

highly dependent on the pH values of the solution environments. It is expected that the

plasmon resonant wavelength of Au nanostructures can be tuned by simply changing their

shape and/or size to cover a large part of the solar spectrum. It indicates the possibility of

constructing an artificial photosynthesis system with low energy consumption. The results

of these studies warrant future research to evaluate the mechanism of plasmon-enhanced

water splitting and provide a facility approach for further design of the plasmon-enhanced

solar energy conversion devices.

Keywords: electrochemistry, artificial photosynthesis, strontium titanate (SrTiO3),

localized surface plasmons, photochemistry, water splitting, nanostructures.

1

Chapter 1

Introduction

1.1 Background and Motivation

The dramatic increase of the world population in the last century raises the question of

how long the resources of our planet will last. The world population has been estimated to

be 7.209 billion by December 6th, 2014 and it is expected to increase even more rapidly

according to the World Population Report of the United Nations Population Fund.[1]

At the same time the world energy consumption, mostly based on fossil fuels such as

oil, coal and gas is also increasing.[2]

These resources are created by a very slow

geological process and their quantity is therefore limited. As a finite resource, it is

reasonable to be expected to reach a limitation and decline in the production afterwards.

The oil and natural gas have been predicted to last for a number of decades by several

estimations, while the coal could last up to a few hundred years, although there is no

agreement on the exact numbers. Most of these analyses are based on conventional oil

storage, but the numbers will be different if new oil resources can be found and extracted.

Currently, oil companies are investing in unconventional oil, extracted it with techniques

different from the traditional oil well method and from places that can not be reached with

the present technology. This oil is definitely more expensive than the conventional one.

The world economy will be seriously damaged if the production of oil starts to decrease

rapidly. Therefore an effective solution of this problem by means of the introduction of

new technologies must be activated well ahead of schedule since the time scale for

introducing new technologies to our society could be very long.

The use of fossil fuels is also relevant to a number of other issues. For instance, when

fossil fuels are consumed and burnt, CO2 is emitted into the atmosphere. This gas will

remain in the atmosphere for many years because it can not be fixed by nature rapidly. The

2

carbon cycle will be broken consequently. Human activities have been confirmed to give

rise to the increase of the CO2 concentration in the atmosphere. The CO2 is supposed to

bring about some environmental problems because it is a green house gas. The average

temperature in the globe will be increased along with the CO2 amount in the atmosphere.

The warming of our planet will probably result in the melting of the arctic ice and glaciers

and finally changes of climate with negative influence of all the life forms. Actually, the

global temperature has already been observed increasing recently. The solution for the

decrease of the CO2 production needs to be mastered as quickly as possible.[3]

Figure 1.1 Total World Energy Consumption by Source 2013

So far, there has been some CO2 sequestration method proposed by several

researchers. However, these solutions all involve capturing and trapping the emitted CO2

underground. Therefore, they are not feasible solutions for long term since they cannot

close the carbon cycle, but merely a postponement of the situation as a result.

When we pursuit a civilized society with the energy sources that are especially

sustainable and renewable to substitute the fossil fuels, the solution has eventually been

the sun that always seen shining in the sky over our heads. As a yellow dwarf star, the sun

has a diameter of about 109 times compared with the one of the earth. The nuclear fusion

of hydrogen into helium takes place in the sun and thus produces energy. This kind of

energy diffuses into the surrounding outer space as the form of kinetic energy of light

radiation or particles after generated in the core of the sun. It can be considered as a source

3

of energy that will never exhaust on human time scale, because the sun is supposed to

keep shining for billions more years.

In Figure 1.2, the solar irradiance is plotted as a function of wavelength for standard

condition (air mass 1.5, i.e., AM 1.5). The average flux of solar irradiation reaching the

earth surface is 174.7 W/m2 taking the scattering and absorption by the atmosphere and

the clouds into consideration. The tremendous potential of the solar energy is obvious

because the human average power consumption worldwide is about 15 TW every year. If

we could build a photoelectric conversion device with a solar energy conversion efficiency

of 10%, only 0.2% of the earth surface area needs to be covered with such kind of devices

in order to obtain the energy of 15 TW as mentioned.

Nevertheless, the solar irradiation is not a source of energy that is constantly available.

Because the day and night alters and it also highly dependent on the weather conditions,

such as sunny and cloudy day, etc. Hence, this kind of energy has to be stored as a readily

useable form. Whereas for the purpose of making large amount of energy stored,

electricity has never been an ideal choice. Superior performance could be obtained by

chemical energy with a higher energy density. In such scenario, the energy is stored in

chemical bonds and could be released whenever necessary by a simple chemical reaction,

combustion in the case of fossil fuels for instance.

Figure 1.2 The radiation energy in solar spectrum as a function of wavelength.

4

1.2 Origin of Photoelectrochemical (PEC) Water Splitting

Innovations in both science and technology are inspired by the increasing demand for

renewable energy. Being considered as one of the most potential clean energy carriers,

hydrogen possesses the unique characters. It has a very high energy density. The enthalpy

of combustion is 286 kJ/mol. And since the only product after the burning of hydrogen is

water, without any other pollutant or green house gas, it is environmental friendly.

Furthermore, hydrogen also has some important industrial applications, e.g., fuel cell

industry and industrial ammonia synthesis. Up until now, the production of hydrogen is

mainly by means of biomass or fossil fuels reforming reactions.[4]

The commercial

hydrogen gas is produced by the steam reforming of natural gas (methane) in industry.

CH4 + 2H2O 4H2 + CO2 ΔG0

= 131 kJ mol-1

(1-1)

If we take energy and environmental issues into consideration, the present technique

is defective and even unacceptable. Firstly, very high temperature (700-1000°C) is

necessary during this process since it is an intensive endothermic reaction. Secondly, this

reaction has to consume the non-renewable fossil fuels and there is green house gas CO2

emission as a byproduct in the process. These drawbacks during the industrial production

process hinder the extensive application of H2. As a consequence, the production of H2 via

environmental-friendly and renewable methods has been come an urgent issue.

Water is a perfect source of hydrogen from the view point of saving fossil fuels and to

avoid the undesired CO2 emissions. However, external energy driving force, e.g.,

electrical power, is needed in order to split water into H2 and O2. The process is a general

knowledge as the electrolysis of water.

H2O(g) 1/2O2(g) + H2(g) ΔH0 = 241.8 kJ mol

-1 (1-2)

An inexpensive and clean energy substitution that is capable to supply the power for

our planet could be expected as the form of H2 if we can make use of a renewable source,

such as solar energy, as the energy input.[5]

Some hydrogen production methods from

5

water using solar light have been proposed. One of them is electrolysis of water by a solar

cell. Besides, promising strategies have also been provided through photocatalytic or (PEC)

water splitting to achieve this purpose.[6-8]

Honda and Fujishima demonstrated the photoelectrochemical water splitting for the

first time under ultraviolet (UV) irradiation using n-type single-crystal TiO2

photoe1ectrode in 1972.[9, 10]

Under UV irradiation, electrons and holes are generated in TiO2, as shown in Figure

1.3. With some external electric bias, water can be reduced by the photo-induced electrons

to generate H2 on the Pt counter electrode. On the other hand, O2 will be formed on the

TiO2 photoelectrode via the water oxidation by the remained holes. During this reaction

course, the UV light harvesting and the electron generation for water reduction are

through the agency of the TiO2 semiconductor. It plays a key role in the conversion of the

irradiation energy into the chemical energy, which is stored in H2, by splitting the water

molecules. This kind of functional semiconductor is thus commonly referred to as

photocatalyst in this research field. In the last few decades, a great deal of research on the

photocatalytic water splitting based on the semiconductor materials have been proceed

after the pioneer work of Honda and Fujishima.

Figure 1.3 The sketch map of Honda-Fujishima effect-water splitting using a TiO2

photoelectrode.

6

Among the various kinds of semiconductors, only a few of them are capable of

performing the photocatalytic water splitting reaction. As is known to all, there is a band

gap separating the conduction band (CB) and the valance band (VB) in semiconductors.

The electrons can be excited from CB to VB and leaving holes in the VB if the

semiconductor is irradiated by an incident light with the energy larger than its band gap.

Then H2 and O2 will be subsequently formed through the water reduction by the electrons

and the water oxidation by the holes respectively to complete the overall water splitting.

Theoretically, in order to ensure the water splitting reactions to carry out normally,

there are three key issues. For the thermodynamically aware, the electrons and holes

generated by the incident light excitation should have the appropriate redox potentials so

as to perform the reduction and the oxidation reactions of the water molecules. In other

words, the bottom position of the conduction band needs to be more negative than the

redox potential of H+/H2 (0 V vs. NHE), and meanwhile the top position of the valence

band needs to be more positive than the redox potential of O2/H2O (1.23 V vs. NHE), as

shown in Figure 1.4. As a consequence, only the semiconductors with a band gap larger

than 1.23 eV and with proper positions of CB and VB are candidates for water splitting.

Figure 1.4 Fundamental steps for water splitting reaction by solid-state photocatalysts.

Based on the above mentioned analysis, it seems that the ideal options of the materials

for water splitting are semiconductors with relatively large band gaps. To date, a lot of

7

such semiconductors have been investigated for PEC water splitting, such as TiO2,[11]

ZrO2,[12]

K4Nb6O17 [13, 14]

et al. Nevertheless, a severe drawback of the large band gap

semiconductors is that they can only absorb the UV light. Whereas taking the efficiency

issue into consideration, the energy composition of the solar spectrum (AM 1.5) is

constituted by ca. 2% ultraviolet light, 47% visible light (corresponding to the wavelength

region from 380 nm to 780 nm) and 51% infrared light. A major part of the solar energy

will thus be wasted if we use the large band gap semiconductors for water splitting.

Therefore, many new types of photocatalysts that respond to visible light are currently

under investigation. Besides these basic requirements and efficiency problems, the

stability of the semiconductor materials in aqueous solutions under irradiation of light is

another important issue. Due to their appropriate energy band positions, several

chalcogenide semiconductors have been considered as suitable photocatalysts for water

splitting that respond to visible light irradiation. Even though some of these chalcogenide

semiconductors, CdS for example,[15, 16]

is capable of splitting water very efficiently under

visible light irradiation,[17]

the CdS itself will also be oxidized by the photogenerated holes

during the water splitting reaction process. A dramatic decrease of the water splitting

efficiency is inevitable as the result of the decomposition of the catalytic material CdS.

Therefore, for most chalcogenide photocatalysts, this is a critical defect. At present, a

great challenge still remains for the development of semiconductors materials that are

stable and feasible for photoelectrochemically splitting water very efficiently under the

solar irradiation.

8

1.3 PEC Water Splitting over Semiconductor Photocatalysts

1.3.1 Basic Principles of PEC Water Splitting

Only the deep ultraviolet light with a wavelength shorter than 190nm can be used to

split water directly, since the visible light cannot be absorbed.[18]

As mentioned in the

former section, at least 1.23 V bias is required to promote the electrolysis of water. The

bias voltage at this amount is corresponding to an irradiation light wavelength of about

1000 nm. Therefore, the utilization of the visible region of the solar spectrum for water

splitting could be expected theoretically, in condition that the irradiation is used very

effectively. The original work of H2 generation from water splitting was done by Honda

and Fujishima in 1970s. As shown in Figure 1.5, the typical PEC water splitting cell they

have used is composed of a photoanode of n type TiO2 semiconductor and a Pt counter

electrode.

Figure 1.5 Photoelectrochemical water splitting cell based on n type semiconductor

TiO2 photoelectrode.[19]

The surface region of the TiO2 photoelectrode is charged, as the result of the charge

transfer at the interface between the electrolyte and TiO2, since they contact. The charges

move from the TiO2 to the electrolyte, a surface charge area is thus formed and the energy

9

band of the TiO2 bends upwards, as depicted in the figure. The potential barrier resulted

by this energy band bending is referred to as Helmholtz barrier. The property of the

semiconductor and the characteristic of the aqueous electrolyte both affect this barrier. At

the interface between the electrolyte and semiconductor, this Helmholtz barrier could

drive the separation between electrons and holes. Furthermore, the transportation of

chargers at the bending band region would also be facilitated. Electron and hole pairs will

be generated in the TiO2 when it is irradiated by light with an energy lager than its

bandgap value. The electrons excited by light irradiation will transfer to the counter

electrode and generate H2 evolution by reducing water, whereas the holes will remain at

the TiO2 surface and generate O2 evolution via water oxidation.[20, 21]

The reactions on the

two electrodes can be described by equations as follow:

Photanode: H2O + 2h+ 2H

+ + 1/2O2 E

0ox = -1.23 V (1-3)

Cathode: 2H+ +2e

- H2 E

0red = 0 V (1-4)

In principle, to drive the solar water splitting, a minimum amount bias voltage of

1.23V is generally required between the two electrodes. The bandgap of the

semiconductors used in the water splitting reaction is usually larger than 1.23V

accordingly. Nevertheless, the essential conditions for a water splitting system to function

stably include but are not limited to a suitable bandgap value. An external electric bias is

necessary to drive the water splitting reaction in many situations, even using a

semiconductor with a bandgap larger than 1.23 eV, such as Fe2O3 and WO3. Actually, the

external bias needed is highly dependent on the energy band structure of the

semiconductors.

There are three major steps in a typical photocatalytic reaction, as shown in figure 1.6.

They all have strong impact on the photocatalytic capability of a photocatalyst.

10

Figure 1.6 Processes involved in photocatalytic overall water splitting on a heterogeneous

photocatalyst.

(i) Electron hole pairs generation in photocatalysts by means of photon absorption. If

the incident irradiation has an energy larger than that of the semiconductor bandgap, the

photons from the incident light can be absorbed and electron hole pairs will be generated

consequently. The electrons will transfer to the conduction band of the semiconductor

after the excitation by the incident photons. On the other hand, the holes will remain at the

valence band. The electrons and holes will reduce and oxidize water molecules to form H2

and O2 generation respectively to accomplish the overall water splitting.

(ii) The separation and migration of the electron hole pairs generated by

photoexcitation. Generally, before the electron hole pairs participate in the photocatalytic

reactions, they need to separate and transfer to the reaction sites on the photocatalysts

surface. During this process, the crystal structure, crystallinity and the particle size of the

semiconductor material show a great influence. Defects often function as a charge trap and

also a recombination center for the electrons and holes generated by photoexcitation.

Therefore, they are undesired in the semiconductor used as a photocatalytic material since

11

the activity will sure be ruined. The photocatalyst with relatively higher crystallinity takes

advantage for charge separation and migration in this respect since the number of defects

would be smaller. The photocatalyst particle size should be small in order to obtain higher

efficiency. The reason is that the surface area and the number of the reactive sites are

larger for smaller particles. Furthermore, the distance that an electron or a hole has to

transfer to the surface reaction sites is shorter so as to reduce the recombinations.

(iii) The chemical reactions on the semiconductor surface. This step is influenced by

the active sites and the area of the surface. A large part of the electrons and holes

generated in the above steps will be lost in the recombination instead of participate in the

water splitting, in spite of the fact that they possess enough potentials thermodynamically

for the reaction. To address this issue, many kinds of co-catalysts, such as Pt,[22]

Pd,[23]

Ru,[24]

NiO,[25]

RuO2,[26]

and Cr/Rh oxides,[27]

are decorated on the surface of

photocatalysts. The function of these co-catalysts is generally considered to be as the

active sites, where the photocatalytic reactions take place. Except for the formation of the

active sites on the semiconductor surface, increasing the surface area of photocatalysts, e.g.

fabrication of some nanostructures on the photocatalysts, is another effective method to

enhance the photocatalytic activity.

These three steps are all essential for photocatalytic water splitting reaction. Hence, an

ideal photocatalyst should possess suitable energy band structure, bulk properties and

surface characters. Until now, the photocatalytic water splitting research still remains in

the laboratory level. This means that some mechanisms are not yet clearly understood.

And visible light driven photocatalysts with acceptable efficiency have not been

developed. A lot of efforts have been made to explore new type of photocatalysts and to

functionalize or modify photocatalysts that already exist.

1.3.2 Common Materials for PEC Water Splitting

Some essential requirements have to be fulfilled in the semiconductor materials used

for the water splitting reaction so as to obtain a considerable efficiency. Generally, the

12

semiconductors should have conduction band and valence band straddling the redox

potentials of E0(H

+/H2) and E

0(O2/H2O).

[28] By this means, the water splitting reaction can

be promoted by the photogenerated electrons and holes with enough electrochemical

potential in the absence of any external bias. Moreover, the semiconductors should be

capable of absorbing solar irradiation efficiently and also stable for a long term during the

water splitting process. The absorption ability for solar irradiance is affected by the

semiconductor bandgap. It greatly influences the solar to hydrogen conversion efficiency.

The solar energy spectrum on the basis of irradiation energy and photon numbers is shown

in Figure 1.7. The UV irradiation only takes a quite small part of the whole solar spectrum

as can be seen in the figure.

Figure 1.7 Number of photons in solar spectrum, versus photon energy. The optical

band gaps of common metal oxide photocatalysts are indicated for comparison.[29]

The minimum bandgap for a semiconductor used in PEC water splitting is 1.23 eV in

theory. However, because of the energy loss in the PEC water splitting process, the

photons that could be utilized practically, usually have energy larger than 1.23 eV. And

the solar to hydrogen conversion efficiency is still much smaller than we expected. There

are several reasons that can induce this energy loss, such as the polarization within the

13

PEC, the recombination between electrons and holes, and the potential loss caused by the

resistance of electrodes and contact. The energy loss has been estimated to be about 0.8

eV. The practical energy limit of the photons feasible for PEC water splitting is thus

determined to be about 2.0 eV. The great effort that has been devoted into the research of

visible light respond PEC water splitting is also based on this concern.

Many researchers have focused on the investigation of new type semiconductor

materials with preferable performance for PEC water splitting ever since the innovative

work with a TiO2 photoelectrode succeeded. So far, numerous materials have been

developed for the PEC water splitting, including metal oxides such as TiO2,[30-42]

Fe2O3,

ZnO,[43-50]

Nb2O5, BiVO4,[51-64]

ZrO2, WO3,[65-72]

metal nitrides such as Ta3N5,[73-76]

metal

phosphides such as GaP,[77, 78]

metal titanate such as SrTiO3, BaTiO3, metal tantalate such

as KTaO3, and metal oxynitrides such as TaON[79-82]

. These materials are all used as

photoelectrodes. They can be classified as either photoanodes or photocathodes according

to the different reactions occur on their surfaces. As described in the former section, there

are three main steps in the PEC water splitting: solar irradiation absorption, charge

separation and migration, and surface chemical reactions.[83]

The intrinsic properties of the

catalytic materials, e.g. energy band structure, bandgap value, crystallinity, electronic

conductivity, and particle morphology, have a significant influence on these processes.

The energy band structures of typical catalytic semiconductor materials and their

corresponding bandgap values are shown in Figure 1.8.[84]

The normal hydrogen electrode

(NHE) at pH=0 and vacuum energy level are both employed. For metal oxides, only the

ones with bandgaps large enough, such as SrTiO3, ZrO2 and ZnO, can straddle the both

redox potentials of water. However, the light harvesting ability will be limited if the

bandgap is too large. And the PEC water splitting efficiency will also be seriously affected.

Whereas in the case of metal oxides with relatively small bandgap, such as WO3 and

Fe2O3, external bias is necessary to promote the water splitting reaction since the

conduction band position is not more negative than the reduction potential of water.

Furthermore, the Fe2O3 has a very poor electrical conductivity, the recombination between

14

electrons and holes is thus severe because of its limited carrier diffusion length. CdSe and

CdS seem to have both the suitable CB and VB positions and small bandgap values.

However, they are not stable during the water splitting process. They can be readily

oxidized to metal oxides and the ideal energy band structure would no longer exist.[85, 86]

Figure 1.8 The bandgap and band structure potentials of typical semiconductors relative

to the redox potential of water; the potentials are versus normal hydrogen electrode (NHE)

at pH=0 and vacuum potential.[84]

For metal nitrides and metal oxynitrides, in despite of that the band structure and

electronic property of catalytic materials can be modified by the incorporation of nitrogen,

the nitride materials will finally lose their activity upon the irradiation because of

photodecomposition. A considerable percentage of the solar irradiation can be absorbed

by silicon, because its bandgap is quite small (1.1 eV). However, silicon has a valence

band that is not more positive than the water oxidation potential. An external bias is thus

inevitable. It can be supplied by making hybrid devices, e.g., a hybrid system of PEC

device and dye sensitized solar cell.[87-90]

Nevertheless, because of the large surface

overpotentials and surface oxidation, silicon is also unstable under irradiation.[91]

The

solutions of the issues in catalytic semiconductors about light harvesting, energy band

structures, electronic property, and chemical durability, have been systematically

15

investigated. Some strategies have been proposed such as nanostructure decoration and

chemical modification, e.g., element doping, design of semiconductor surface

heterojunction, and co-catalyst modification.

1.3.3 Development of Visible Light Driven Water Splitting

With the purpose of water splitting in the absence of any external bias, semiconductor

materials with flat band potential more negative than the H+/H2 potential, such as SrTiO3

or KTaO3, are preferred. However, as shown in Figure 1.9, these materials are usually

unable to absorb visible light because of their large bandgaps.[92]

The solar energy

conversion efficiency is very low as a result.

Figure 1.9 Photoelectrolysis of water using a SrTiO3 photoanode. Due to the position of

conduction and valence band edges hydrogen and oxygen are evolved without need of

external bias.[93]

So far, visible light driven photocatalytic materials have been investigated extensively.

For UV respond metal oxide photocatalysts, as can be seen in Figure 1.8, the bottoms of

16

the conduction bands are slightly more negative than 0 V, whereas the tops of the valence

bands are more positive than 3 V (vs. NHE at pH 0). As a consequence, visible light can

not be absorbed due to the large bandgap. However, these metal oxide photocatalysts are

capable of oxidizing water since there is a considerable difference between the top of the

valence band and the oxidation potential of water (1.23 V vs. NHE). Hence, it would be

of significance if we can maintain the conduction band position and meanwhile improve

the band structure. Figure 1.10 shows the main techniques to utilize visible light by a

metal oxide with a large bandgap. They are summarized accordingly as follows.

Figure 1.10 Primary approaches of energy band engineering for a metal oxide to

obtain the visible light response.

(i) Doping with transition-metal ions. One of the most common used methods for the

crystal structure modification of a metal oxide is to dope it with some other elements. In

general, the dopant will substitute a lattice point of the crystalline host material.

Consequently, it forms a donor or acceptor level in the forbidden band of the

semiconductor by the dopant which is a transition-metal ion. These doping levels

functions as a visible absorption center. Unfortunately, the dopant usually serves as

discrete energy levels instead of an energy band. The efficient transfer of the electrons and

holes will be seriously affected as the result.[94]

Furthermore, when we dope the host metal

oxide with another element, the charge balance is destroyed in many circumstances.

17

Vacancies that function as recombination centers can be readily generated. Therefore, the

doping method is not an ideal solution for the metal oxide material used as visible light

driven photocatalyst to some extent.

(ii) Valence band control using the p orbitals of an anion or the s orbitals of metal ion.

In despite of some successful examples about the above mentioned doping method, the

adverse effect of the defects working as recombination centers is still serious. Moreover,

the discrete doping levels formed by the dopants hinder the carrier transfer. In contrast, it

would be preferable for the metal oxide to have a continuous valence band formed by

atomic orbitals of the constituent elements. A lot of researchers have reported this kind of

special photocatalysts with a valence band-control.[95]

Figure 1.11 Basic principle of visible light driven water splitting based on spectral

sensitization depicted by energy band alignment

(iii) Spectral sensitization. In the early 1970s, H. Gerischer demonstrated the spectral

sensitization of semiconductor particles with large bandgap.[96]

Since then, in order to

realize the H2 generation by visible light driven water splitting, a great effort has been

made to the development of this strategy.[97-104]

The main process of the spectral sensitized

water splitting is shown in Figure 1.11. Organic dyes and inorganic semiconductors with a

narrow bandgap can be employed as sensitizers. Their bandgap should be small enough to

18

absorb the visible light. In addition, their excited-state potential needs to be higher than

the conduction band of the metal oxide to facilitated electron injection. The electrons

injected are involved in the water reduction to generate H2 at a co-catalyst such as Pt.

While the sensitizers are compensate with electrons from donor molecules. Water is the

ultimate electron donor in a system without the sacrifice reagent. However, sacrificial

reagent such as alcohols, organic acids, and hydrocarbons, are commonly used to avoid

the side reactions such as H2-O2 recombination. In this situation, the electron transfer from

the sensitizer to co-catalyst, e.g. Pt, is mediated by the semiconductor. There is a

competition between the water reduction and the electron back transfer to the oxidized

sensitizer. Therefore, the H2 evolution activity is highly dependent on the semiconductor

material, the sensitizer and the co-catalyst in this system.[105-115]

To date, a number of

visible light driven photosynthesis systems have achieved apparent quantum efficiency as

high as several tens of percent for H2 generation from water using spectral sensitizers with

suitable electron donor.

Figure 1.12 Overview of water splitting on Z-scheme photocatalysis system with an

iodate (IO3-) and iodide (I

-) ion redox couple.

[28]

19

Another main approach for achieving visible light driven water splitting is to apply a

two-step photoexcitation process using two different photocatalysts, as depicted in Figure

1.12. This strategy is known as the Z-scheme water splitting.[116-124]

It was inspired by

natural photosynthesis through which the green plants convert solar energy into chemical

energy. In such a two-step system, the water splitting process is divided into two stages:

one for H2 generation and the other for O2 generation. A solution containing the shuttle

redox couple (Red/Ox in the figure) will connect them. At the H2 evolution photocatalyst,

H2 evolves via water reduction by the photoexcited electrons, while the holes in its

valence band oxidize the reductant (Red) to oxidant (Ox). The oxidant is subsequently

reduced back to the reductant by photoexcited electrons generated at the O2 evolution

photocatalyst, where the holes oxidize water to evolve O2. The energy requirement for

photocatalysis is substantially reduced by this system. The utilization of visible irradiance

for water splitting has thus become more efficient. This means that the semiconductor with

the capability of either reducing or oxidizing water can be used in overall water splitting.

For instance, some visible light active oxides such as SnO2 can be used as O2 evolution

photocatalysts in condition that they can reduce certain oxidant to a reductant. Similarly,

non-oxide photocatalysts such as sulfides, oxynitrides and dyes, can be used as H2

evolution photocatalysts in condition that they can oxidize certain reductant to an oxidant.

Therefore, many kinds of semiconductor materials can be utilized in the Z-scheme water

splitting despite that they are unqualified in one-step system. Furthermore, separate

evolution of H2 and O2 can be realized in Z-scheme system via a separator by which only

the redox mediators can be transferred. It is theoretically impossible to obtain separate

generation of H2 and O2 in a conventional one-step system because H2 and O2 are

simultaneously evolved at small semiconductor particles. However, there is a critical

drawback of the Z-scheme systems that the number of photons required in a two-step

photoexcitation system is twice that for a one-step system to achieve overall water

splitting.

20

1.3.4 Plasmon-enhanced Water Splitting

Plasmon is a collective oscillation of the free electron gas density in a conductive

material, as shown in Figure 1.13. Light below the plasma frequency will be reflected

since the electrons in the metal screen its electric field. Light above the plasma frequency

will be transmitted since the electrons respond too slowly to screen it. Surface plasmons

are coherent delocalized electron oscillations confined to the surfaces of conductive

materials. The interaction between surface plasmons and light is very strong. If the real

part of the dielectric function becomes zero, at the plasmon frequency, there will be a

resonance in the absorption. Intense electric fields can be generated on the surface of the

metal nanoparticles when they are irradiated by light at their plasmon frequency. The size,

shape and material of the nanoparticle and its distance from other nanoparticles all have

significant impact on the frequency of this resonance.[125-128]

For instance, if we fabricate

the silver nanoparticle to a size small enough, its plasmon resonance can be shifted from

the UV region into the visible range. Similarly, the regulation of the gold plasmon

resonance from the visible region to the infrared region is also possible via the

nanoparticle size miniaturization.

Figure 1.13 Schematic of plasmon oscillation for a sphere, showing the displacement of

the conduction electron charge cloud relative to the nuclei.[129]

Improved photocatalytic water splitting under visible irradiation has been

demonstrated by a number of researchers via plasmonic metal nanoparticles deposition on

the top of anatase TiO2. In such a scenario, the H2 and O2 generation is detected by gas

chromatography with mass spectrometry, and the photocurrents are monitored via a

21

potentiostat. Thomann et al. have observed the enhanced photocurrent of water splitting

using Au/Fe2O3 as the photocatalyst. The maximum enhancement was as high as 11-fold.

An interesting phenomenon was also discovered that the position of the gold nanoparticles

in the Fe2O3 film had a great impact on the photocurrent enhancement peak position and

curve shape.[130]

Enhanced photocatalytic water splitting was also reported by Torimoto et

al. using Au core-SiO2 shell particles decorated with CdS nanoparticles.[131]

As shown in

Figure 1.14, Liu et al. deposited Au nanoparticles on the top of catalytic TiO2 as

photoanode. They have demonstrated photocurrents through the water splitting reaction

with the enhancement of 5-fold and 66-fold at the visible irradiation wavelength of 532

nm and 633 nm, respectively.[132]

Figure 1.14 (a) Schematic diagram of the photoelectrochemical measurement setup. (b)

Photocurrent of anodic TiO2 with and without Au nanoparticles irradiated with λ = 633 nm

light for 22 s.[132]

Ingram et al. reported a visible light driven photocurrent using nitrogen-doped TiO2

decorated by Ag nanocubes with a 10-fold enhancement compared with in the absence of

Ag nanocubes.[133]

Duan et al. also observed enhancement of light absorption by Ag

nanoparticles on CdS with a SiO2 intermediate layer in between. This intermediate layer

can be used to regulate the plasmon resonance peak effectively.[134]

Some researchers have

focused on the investigation of the optimum weight percentage of Au nanoparticles in

TiO2 instead of their nanoparticle geometry.[135, 136]

However, the optimum Au weight

22

percentage distinguished from each other in the case of photocatalytic semiconductor with

different morphology. It can not lead to a profound comprehension of the physical

mechanism of the water splitting enhancement. Despite of the considerable enhancement

excited by the plasmon resonance, the overall solar to energy conversion efficiencies are

still quite low especially in the visible wavelength region.

Figure 1.15 Plasmon-assisted photocurrent generation and water oxidation from visible to

near-infrared wavelength using a Au-nanorods/TiO2 Electrode.[137]

Nishijima et al. reported the plasmon-enhanced photocurrent generation and water

oxidation by visible and near infrared light irradiation, as shown in Figure 1.15.[137-140]

They fabricated gold nanorods array on the surface of TiO2 photoelectrode. Stoichiometric

evolution of oxygen was demonstrated via water oxidation through the irradiation of this

photocurrent generation system. The electrons are supplied by water molecules, since

there is no electron donor except a potassium perchlorate aqueous solution, which was

employed as an electrolyte solution. The plasmonic optical antenna effect of the gold

nanorods promoted the stoichiometric evolution of oxygen and hydrogen peroxide via the

four- or two-electron oxidation process of water molecules, respectively. After the electron

transfer from gold to TiO2, it is speculated that the water oxidation recovered effectively

23

via the multiple holes generated by plasmon-induced charge excitation. This photoelectric

conversion system as proposed is considered to be a promising approach for artificial

photosynthesis using visible and near-infrared light.

24

1.4 Evaluation of PEC Water Splitting

To date, hundreds of materials have been investigated as photocatalytsts, and

thousands of corresponding applications have already been published in academic journals

concerning the modification and improvement of the existing photocatalysts so as to

enhance the photocatalytic activity. Thereupon, the evaluation of the capability of these

photocatalysts is necessary to be within a unified standard for both theoretical research

and practical application.

1.4.1 Stoichiometry of H2 and O2 Evolution

For a water splitting process without any sacrificial reagent employed, both H2 and O2

should be formed in a stoichiometric ratio, i.e., the amount of the H2 generation should be

twice that of the O2 generation. Occasionally, the H2 evolution can be observed while the

O2 evolution is undetectable. In this situation, the amount of H2 evolution is usually very

small even compared with the amount of the photocatalyst. It is thus not confirmed if such

kind of reaction is a photocatalytic process and it is crucial to clarify that this is not a

sacrificial reaction.

1.4.2 Time Course of H2 and O2 Evolution

In a typical water splitting reaction, the amounts of the H2 and O2 evolution should

increase with the irradiation time. The amount of evolved H2 and O2 over per unit

irradiation time is one of the most important indicators for the assessment of the

photocatalytic activity. The long term stability of a photocatalyst is usually investigated by

repeated experiments. After the water splitting reaction for a long time, there should be no

or negligible decrease in the activity for a stable photocatalyst.

1.4.3 Turnover Number

In the research field of catalysis, turnover number (TON) is an important index for the

25

character of many catalytic materials. In photocatalysis, TON is also indispensable to

evaluate a catalyst. The quantity of the H2 and O2 evolved should overwhelm that of the

photocatalysts employed. Otherwise, the process could be a chemical reaction instead of a

photocatalytic reaction. There still remains a possibility that the H2 and O2 evolution are

generated from the chemical reactions between water and semiconductor materials, or

from the self-decomposition of the semiconductors. None of these is an essentially

catalytic process.

In general, turnover number is defined as the number of reacted molecules per active

site.

TON = number of reacted molecule

number of active sites (1-5)

Nevertheless, precise determination of the number of active sites on a photocatalyst

surface is impossible in the case of heterogeneous photocatalysis. Hence, an alternative

method is proposed for practical application. The number of photocatalyst molecules is

employed in above equation instead of the number of active sites. The equation is thus

modified as follows:

TON = number of reacted electrons

number of molecules in a photocatalyt (1-6)

The number of reacted electrons can be calculated from the number of molecules of

H2 or O2 gas evolution. Since the number of all the photocatalyst molecules is definitely

larger than that of the active sites on a certain photocatalyst surface, the TON value

calculated by the above equation is actually smaller than the real value. Sometimes the

photocatalytic activity is normalized by the weight of the photocatalyst employed, e.g.,

mmol h-1

g-1

. However, that is somewhat inaccurate since the photocatalytic activity tends

to be saturated with the weight of photocatalyst if the amount of photocatalyst used is

large enough for a certain experimental condition. The optimum quantity of certain

photocatalyst needs to be determined for each experimental system individually. In this

situation, the photocatalytic activity mainly depends on the photon absorption by the

26

photocatalyst unless the irradiation is too strong.

1.4.4 Quantum Yield

Since photocatalytic water splitting has become a worldwide concern, numerous

photocatalysts have been investigated using different experimental apparatus with distinct

parameters by different laboratories. Even for the same photocatalysts, the activities will

show a considerable difference, such as the evolution time course of H2 or O2 derived

from the measurements, if they are measured with different experimental setups or in

different conditions. Therefore, such kinds of activities are unacceptable for comparison in

order to evaluate the efficiencies of the photocatalysts.

At present, quantum yield is generally acknowledged as a standard for the evaluation

of the efficiencies of various photocatalysts reported by different research groups. The

definition of the quantum yield is depicted as the follow equation:

Quantum yield = number of reacted electrons

number of absorbed photons (1-7)

The influence of different light sources can be excluded by using this quantum yield

value for the assessment. However, the number of photons absorbed by the photocatalysts

is very difficult to measure in practical terms. Therefore, the evaluation of the

performance of photocatalysts is commonly indicated with an extended term “external

quantum yield” or “apparent quantum yield”, abbreviated to EQY or AQY respectively.[19]

AQY % = number of reacted electrons

number of incident photons x 100 (1-8)

Spectroradiometer can be used for the measurement of the number of incident photons.

Due to the light reflection and scattering of the solid semiconductors, the number of

absorbed photons is definitely smaller than that of incident photons. Therefore, the AQY

is usually lower than the real quantum yield.

It should be noted that the quantum yield discussed above is different from the solar

energy conversion efficiency which is frequently used in the assessment of solar cells.

27

Solar energy conversion % = Output energy as H2

Energy of incident solar light x 100 (1-9)

Since the activities of most photocatalysts are too low to measure, there is only a few

photocatalysts exhibiting passable solar energy conversion efficiency until now. However,

for the end of solar hydrogen production, the photocatalytic water splitting should finally

be evaluated in terms of the solar energy conversion efficiency.

1.4.5 Photoresponse

When a photocatalyst is irradiated by light with energy larger than its band gap, water

splitting should occur. The action spectrum is very important for the investigation of the

photoresponse, particularly for a photocatalyst that responds to visible light. During the

measurement of the action spectrum, monochromatic light is generally obtained using

band-path and interference filters. Even if a material is capable for visible light absorption,

it does not always have the photocatalytic activity induced by the excitation of the visible

light absorption. The onset wavelength can be determined when using cut-off filters to

observe the photoresponse. In dark condition with constant stirring, water splitting on

some metal oxides can proceed by mechanocatalysis instead of photocatalysis.[141, 142]

Therefore, for the purpose of confirming the photocatalytic reaction and excluding the

possibility of the mechanocatalytic water splitting, some control experiments such as in

the absence of any photocatalyst or any irradiation should be performed.

28

1.5 Current Scenario and Challenges

Photocatalytic water splitting offers an attractive potential solution for the

production of environmentally clean energy carriers obtained from renewable material

sources and abundant solar energy.[143, 144]

The design and synthesis of complex water

splitting strategy, which functions at high apparent quantum yield and long term stability

for H2 and O2 evolution, with low activation barrier and moderate overpotential, remain

major challenges in this field. A key issue is to devise an apparatus that exhibits superior

capabilities. It should be able to efficiently separate H2 and O2 evolution with a high rate

and sustain consecutive gas generation for many rounds of photocatalytic cycles.

Numerous schemes have been established by many researchers. However, some

severe problems still remain on despite of the progress made in recent years, including:

(1) Only the UV light of the solar spectrum is utilized, whereas the visible and

infrared irradiations which take a major part of the solar spectrum are underutilized.

(2) An external electric bias is constantly required throughout the water splitting

process, which increases complexity of the water splitting system.

(3) Additional energy consumption will be necessary for the separation of the

evolved H2 and O2 gas after the water splitting process. For the purpose of ensure the

security during the transportation and storage, the purification of the H2 gas is inevitable.

(4) In the case of the water splitting reaction using particulate semiconductor

materials, undesired backward reaction, such as H2-O2 recombination and/or O2

photoreduction, would readily be involved due to the adjacent reaction sites for H2 and O2

evolution.

(5) For the conventional water splitting systems, there is a schottky barrier between

the semiconductor material and the co-catalyst, hindering electron transfer. The water

splitting activity is thus restricted.

(6) There is an absence of an effective evaluation method of the various kinds of

co-catalysts. The most suitable H2 evolution co-catalyst for particular water splitting

reaction environment is also suspensive.

29

(7) The charge carrier migration in the semiconductor is not efficient enough. The

recombination between electrons and holes drastically retards the water splitting reaction.

These issues hamper the exploitation in water splitting reaction system assemblies and

photocatalytic devices for fuel generation. If these challenges can be met, PEC water

splitting may emerge as an economically viable and truly renewable pathway towards the

clean H2 production.

30

1.6 Aim and outline of this thesis

All warning signs in regard to the energy crisis and global warming force us to

consider about the solution to utilize energy which derives from natural resources.

Therefore, the principal aim of this thesis is to make use of renewable energy such as solar

energy and convert it into chemical energy stored in H2 gas by PEC water splitting process.

The body of thesis consists of six chapters including the present chapter 1. This chapter

briefly describes the backgrounds of the study, literature review, mechanism of the water

splitting reaction and the objectives of the study. Chapter 2 introduces the apparatus

designed for hydrogen production using water splitting and experimental details about the

quantitative determination of hydrogen and oxygen evolution. Chapter 3 reports the

plasmon-assisted water splitting using two sides of the same SrTiO3 single-crystal

substrate. The properties of the gold nanoparticles deposited on the SrTiO3 single-crystal

substrates were characterized. The influence of irradiation wavelength and pH value

combinations on H2 and O2 generation were summarized and analyzed. Chapter 4 is

devoted to the co-catalyst effects on hydrogen evolution in the plasmon-induced water

splitting system. Schottky barrier effect between the semiconductor and the co-catalyst

was eliminated. An effective evaluation method of various co-catalysts was proposed.

Chapter 5 presents the plasmon-enhanced water splitting utilizing heterojunction

synergistic effect between SrTiO3 and rutile TiO2. Enhanced water splitting activity was

ascribed to the larger potential gradient between the front and back side of the composite

substrate due to the energy band position difference between SrTiO3 and rutile TiO2. At

the end of the thesis, chapter 6 summarizes the results described in chapter 3-5. This

chapter also describes how this project leads to a future perspective on water splitting

systems for solar fuels generation. Several possibilities are proposed as the further

extension and development of this work.

31

1.7 References

[1]. Gerland, P., Raftery, A. E., Sevcikova, H., Li, N., Gu, D. A., Spoorenberg, T., Alkema, L., Fosdick, B.

K., Chunn, J., Lalic, N., Bay, G., Buettner, T., Heilig, G. K. and Wilmoth, J., World population stabilization

unlikely this century. Science 2014, 346 (6206), 234-237.

[2]. Akhmat, G., Zaman, K., Shukui, T. and Sajjad, F., Does energy consumption contribute to climate

change? Evidence from major regions of the world. Renewable & Sustainable Energy Reviews 2014, 36

123-134.

[3]. Houghton, J., Global warming. Reports on Progress in Physics 2005, 68 (6), 1343-1403.

[4]. Cortright, R. D., Davda, R. R. and Dumesic, J. A., Hydrogen from catalytic reforming of

biomass-derived hydrocarbons in liquid water. Nature 2002, 418 (6901), 964-967.

[5]. Lewis, N. S. and Nocera, D. G., Powering the planet: Chemical challenges in solar energy utilization.

Proceedings of the National Academy of Sciences 2006, 103 (43), 15729-15735.

[6]. Osterloh, F. E., Inorganic materials as catalysts for photochemical splitting of water. Chemistry of

Materials 2008, 20 (1), 35-54.

[7]. Walter, M. G., Warren, E. L., McKone, J. R., Boettcher, S. W., Mi, Q. X., Santori, E. A. and Lewis, N.

S., Solar Water Splitting Cells. Chemical Reviews 2010, 110 (11), 6446-6473.

[8]. Chen, X. B., Shen, S. H., Guo, L. J. and Mao, S. S., Semiconductor-based Photocatalytic Hydrogen

Generation. Chemical Reviews 2010, 110 (11), 6503-6570.

[9]. Fujishima, A. and Honda, K., Electrochemical Evidence for the Mechanism of the Primary Stage of

Photosynthesis. Bulletin of The Chemical Society of Japan 1971, 44 (4), 1148-1150.

[10]. Fujishima, A. and Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238 (5358), 37-38.

[11]. Fujishima, A., Zhang, X. and Tryk, D. A., Heterogeneous photocatalysis: From water photolysis to

applications in environmental cleanup. International Journal of Hydrogen Energy 2007, 32 (14), 2664-2672.

[12]. Sayama, K. and Arakawa, H., Photocatalytic decomposition of water and photocatalytic reduction of

carbon dioxide over zirconia catalyst. The Journal of Physical Chemistry 1993, 97 (3), 531-533.

[13]. Domen, K., Kudo, A., Shibata, M., Tanaka, A., Maruya, K.-I. and Onishi, T., Novel photocatalysts,

ion-exchanged K4Nb6O17, with a layer structure. Journal of the Chemical Society, Chemical

Communications 1986, (23), 1706-1707.

32

[14]. Domen, K., Kudo, A., Shinozaki, A., Tanaka, A., Maruya, K.-i. and Onishi, T., Photodecomposition of

water and hydrogen evolution from aqueous methanol solution over novel niobate photocatalysts. Journal of

the Chemical Society, Chemical Communications 1986, (4), 356-357.

[15]. Matsumura, M., Saho, Y. and Tsubomura, H., Photocatalytic hydrogen production from solutions of

sulfite using platinized cadmium sulfide powder. The Journal of Physical Chemistry 1983, 87 (20),

3807-3808.

[16]. Reber, J. F. and Rusek, M., Photochemical hydrogen production with platinized suspensions of

cadmium sulfide and cadmium zinc sulfide modified by silver sulfide. The Journal of Physical Chemistry

1986, 90 (5), 824-834.

[17]. Yan, H., Yang, J., Ma, G., Wu, G., Zong, X., Lei, Z., Shi, J. and Li, C., Visible-light-driven hydrogen

production with extremely high quantum efficiency on Pt–PdS/CdS photocatalyst. Journal of Catalysis 2009,

266 (2), 165-168.

[18]. Coehn, A., Studien über photochemische Gleichgewichte. IV. Das Lichtgleichgewicht

Knallgas-Wasserdampf. Berichte der deutschen chemischen Gesellschaft 1910, 43 (1), 880-884.

[19]. Kudo, A. and Miseki, Y., Heterogeneous photocatalyst materials for water splitting. Chemical Society

Reviews 2009, 38 (1), 253-278.

[20]. Wang, G., Ling, Y. and Li, Y., Oxygen-deficient metal oxide nanostructures for photoelectrochemical

water oxidation and other applications. Nanoscale 2012, 4 (21), 6682-6691.

[21]. Li, Y. and Zhang, J. Z., Hydrogen generation from photoelectrochemical water splitting based on

nanomaterials. Laser & Photonics Reviews 2010, 4 (4), 517-528.

[22]. Sato, S. and White, J. M., Photo-Decomposition of Water over P-TiO2 Catalysts. Chemical Physics

Letters 1980, 72 (1), 83-86.

[23]. Yamaguti, K. and Sato, S., Photolysis of water over metallized powdered titanium dioxide. Journal of

the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1985, 81 (5),

1237-1246.

[24]. Hitoki, G., Takata, T., Kondo, J. N., Hara, M., Kobayashi, H. and Domen, K., An oxynitride, TaON, as

an efficient water oxidation photocatalyst under visible light irradiation (lambda ≤ 500 nm). Chemical

Communications 2002, (16), 1698-1699.

[25]. Kato, H., Asakura, K. and Kudo, A., Highly efficient water splitting into H2 and O2 over

lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. Journal of The

American Chemical Society 2003, 125 (10), 3082-3089.

33

[26]. Sato, J., Saito, N., Nishiyama, H. and Inoue, Y., New Photocatalyst Group for Water Decomposition of

RuO2-Loaded p-Block Metal (In, Sn, and Sb) Oxides with d10 Configuration. The Journal of Physical

Chemistry B 2001, 105 (26), 6061-6063.

[27]. Maeda, K., Teramura, K., Lu, D., Takata, T., Saito, N., Inoue, Y. and Domen, K., Photocatalyst

releasing hydrogen from water. Nature 2006, 440 (7082), 295-295.

[28]. Abe, R., Recent progress on photocatalytic and photoelectrochemical water splitting under visible light

irradiation. Journal of Photochemistry and Photobiology C-Photochemistry Reviews 2010, 11 (4), 179-209.

[29]. Bak, T., Nowotny, J., Rekas, M. and Sorrell, C. C., Photo-electrochemical hydrogen generation from

water using solar energy. Materials-related aspects. International Journal of Hydrogen Energy 2002, 27 (10),

991-1022.

[30]. Park, J. H., Kim, S. and Bard, A. J., Novel Carbon-Doped TiO2 Nanotube Arrays with High Aspect

Ratios for Efficient Solar Water Splitting. Nano letters 2005, 6 (1), 24-28.

[31]. Lu, N., Quan, X., Li, J., Chen, S., Yu, H. and Chen, G., Fabrication of Boron-Doped TiO2 Nanotube

Array Electrode and Investigation of Its Photoelectrochemical Capability. The Journal of Physical Chemistry

C 2007, 111 (32), 11836-11842.

[32]. Rockafellow, E. M., Stewart, L. K. and Jenks, W. S., Is sulfur-doped TiO2 an effective visible light

photocatalyst for remediation? Applied Catalysis B: Environmental 2009, 91 (1–2), 554-562.

[33]. Park, Y., Kim, W., Park, H., Tachikawa, T., Majima, T. and Choi, W., Carbon-doped TiO2 photocatalyst

synthesized without using an external carbon precursor and the visible light activity. Applied Catalysis B:

Environmental 2009, 91 (1–2), 355-361.

[34]. Wang, G. M., Wang, H. Y., Ling, Y. C., Tang, Y. C., Yang, X. Y., Fitzmorris, R. C., Wang, C. C., Zhang,

J. Z. and Li, Y., Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano

letters 2011, 11 (7), 3026-3033.

[35]. Hensel, J., Wang, G., Li, Y. and Zhang, J. Z., Synergistic Effect of CdSe Quantum Dot Sensitization

and Nitrogen Doping of TiO2 Nanostructures for Photoelectrochemical Solar Hydrogen Generation. Nano

Lett 2010, 10 (2), 478-483.

[36]. Wang, H., Wang, G., Ling, Y., Lepert, M., Wang, C., Zhang, J. Z. and Li, Y., Photoelectrochemical

study of oxygen deficient TiO2 nanowire arrays with CdS quantum dot sensitization. Nanoscale 2012, 4 (5),

1463-1466.

[37]. Wang, G., Ling, Y., Lu, X., Wang, H., Qian, F., Tong, Y. and Li, Y., Solar driven hydrogen releasing

from urea and human urine. Energy & Environmental Science 2012, 5 (8), 8215-8219.

34

[38]. Hoang, S., Guo, S., Hahn, N. T., Bard, A. J. and Mullins, C. B., Visible Light Driven

Photoelectrochemical Water Oxidation on Nitrogen-Modified TiO2 Nanowires. Nano Lett 2011, 12 (1),

26-32.

[39]. Hoang, S., Berglund, S. P., Hahn, N. T., Bard, A. J. and Mullins, C. B., Enhancing Visible Light

Photo-oxidation of Water with TiO2 Nanowire Arrays via Cotreatment with H2 and NH3: Synergistic Effects

between Ti3+

and N. Journal of the American Chemical Society 2012, 134 (8), 3659-3662.

[40]. Hwang, Y. J., Hahn, C., Liu, B. and Yang, P., Photoelectrochemical Properties of TiO2 Nanowire

Arrays: A Study of the Dependence on Length and Atomic Layer Deposition Coating. Acs Nano 2012, 6 (6),

5060-5069.

[41]. Xu, M., Da, P., Wu, H., Zhao, D. and Zheng, G., Controlled Sn-Doping in TiO2 Nanowire Photoanodes

with Enhanced Photoelectrochemical Conversion. Nano Lett 2012, 12 (3), 1503-1508.

[42]. Hwang, Y. J., Boukai, A. and Yang, P., High Density n-Si/n-TiO2 Core/Shell Nanowire Arrays with

Enhanced Photoactivity. Nano Lett 2008, 9 (1), 410-415.

[43]. Cooper, J. K., Ling, Y., Longo, C., Li, Y. and Zhang, J. Z., Effects of Hydrogen Treatment and Air

Annealing on Ultrafast Charge Carrier Dynamics in ZnO Nanowires Under in Situ Photoelectrochemical

Conditions. The Journal of Physical Chemistry C 2012, 116 (33), 17360-17368.

[44]. Yang, X. Y., Wolcott, A., Wang, G. M., Sobo, A., Fitzmorris, R. C., Qian, F., Zhang, J. Z. and Li, Y.,

Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett 2009, 9 (6),

2331-2336.

[45]. Wang, G. M., Yang, X. Y., Qian, F., Zhang, J. Z. and Li, Y., Double-Sided CdS and CdSe Quantum Dot

Co-Sensitized ZnO Nanowire Arrays for Photoelectrochemical Hydrogen Generation. Nano Lett 2010, 10

(3), 1088-1092.

[46]. Qiu, Y. C., Yan, K. Y., Deng, H. and Yang, S. H., Secondary Branching and Nitrogen Doping of ZnO

Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting. Nano Lett 2012,

12 (1), 407-413.

[47]. Shi, J., Starr, M. B., Xiang, H., Hara, Y., Anderson, M. A., Seo, J. H., Ma, Z. Q. and Wang, X. D.,

Interface Engineering by Piezoelectric Potential in ZnO-Based Photoelectrochemical Anode. Nano Lett 2011,

11 (12), 5587-5593.

[48]. Xie, S., Lu, X., Zhai, T., Li, W., Yu, M., Liang, C. and Tong, Y., Enhanced photoactivity and stability

of carbon and nitrogen co-treated ZnO nanorod arrays for photoelectrochemical water splitting. Journal of

Materials Chemistry 2012, 22 (28), 14272-14275.

35

[49]. Li, H. X., Cheng, C. W., Li, X. L., Liu, J. P., Guan, C., Tay, Y. Y. and Fan, H. J., Composition-Graded

ZnxCd1-xSe@ZnO Core-Shell Nanowire Array Electrodes for Photoelectrochemical Hydrogen Generation.

Journal of Physical Chemistry C 2012, 116 (5), 3802-3807.

[50]. Lin, Y. G., Hsu, Y. K., Chen, Y. C., Chen, L. C., Chen, S. Y. and Chen, K. H., Visible-light-driven

photocatalytic carbon-doped porous ZnO nanoarchitectures for solar water-splitting. Nanoscale 2012, 4 (20),

6515-6519.

[51]. Sayama, K., Nomura, A., Arai, T., Sugita, T., Abe, R., Yanagida, M., Oi, T., Iwasaki, Y., Abe, Y. and

Sugihara, H., Photoelectrochemical decomposition of water into H2 and O2 on porous BiVO4 thin-film

electrodes under visible light and significant effect of Ag ion treatment. Journal of Physical Chemistry B

2006, 110 (23), 11352-11360.

[52]. Ng, Y. H., Iwase, A., Kudo, A. and Amal, R., Reducing Graphene Oxide on a Visible-Light BiVO4

Photocatalyst for an Enhanced Photoelectrochemical Water Splitting. The Journal of Physical Chemistry

Letters 2010, 1 (17), 2607-2612.

[53]. Long, M. C., Cai, W. M. and Kisch, H., Visible light induced photoelectrochemical properties of

n-BiVO4 and n-BiVO4/p-Co3O4. Journal of Physical Chemistry C 2008, 112 (2), 548-554.

[54]. Chatchai, P., Murakami, Y., Kishioka, S. Y., Nosaka, A. Y. and Nosaka, Y., Efficient photocatalytic

activity of water oxidation over WO3/BiVO4 composite under visible light irradiation. Electrochimica Acta

2009, 54 (3), 1147-1152.

[55]. Su, J. Z., Guo, L. J., Bao, N. Z. and Grimes, C. A., Nanostructured WO3/BiVO4 Heterojunction Films

for Efficient Photoelectrochemical Water Splitting. Nano Lett 2011, 11 (5), 1928-1933.

[56]. Iwase, A. and Kudo, A., Photoelectrochemical water splitting using visible-light-responsive BiVO4

fine particles prepared in an aqueous acetic acid solution. Journal of Materials Chemistry 2010, 20 (35),

7536-7542.

[57]. Zhong, D. K., Choi, S. and Gamelin, D. R., Near-Complete Suppression of Surface Recombination in

Solar Photoelectrolysis by "Co-Pi" Catalyst-Modified W:BiVO4. Journal of the American Chemical Society

2011, 133 (45), 18370-18377.

[58]. Pilli, S. K., Furtak, T. E., Brown, L. D., Deutsch, T. G., Turner, J. A. and Herring, A. M.,

Cobalt-phosphate (Co-Pi) catalyst modified Mo-doped BiVO4 photoelectrodes for solar water oxidation.

Energy & Environmental Science 2011, 4 (12), 5028-5034.

[59]. Berglund, S. P., Flaherty, D. W., Hahn, N. T., Bard, A. J. and Mullins, C. B., Photoelectrochemical

Oxidation of Water Using Nanostructured BiVO4 Films. Journal of Physical Chemistry C 2011, 115 (9),

3794-3802.

36

[60]. Ye, H., Lee, J., Jang, J. S. and Bard, A. J., Rapid Screening of BiVO4-Based Photocatalysts by

Scanning Electrochemical Microscopy (SECM) and Studies of Their Photoelectrochemical Properties.

Journal of Physical Chemistry C 2010, 114 (31), 13322-13328.

[61]. Liang, Y. Q., Tsubota, T., Mooij, L. P. A. and van de Krol, R., Highly Improved Quantum Efficiencies

for Thin Film BiVO4 Photoanodes. Journal of Physical Chemistry C 2011, 115 (35), 17594-17598.

[62]. Luo, W. J., Yang, Z. S., Li, Z. S., Zhang, J. Y., Liu, J. G., Zhao, Z. Y., Wang, Z. Q., Yan, S. C., Yu, T.

and Zou, Z. G., Solar hydrogen generation from seawater with a modified BiVO4 photoanode. Energy &

Environmental Science 2011, 4 (10), 4046-4051.

[63]. Abdi, F. F. and van de Krol, R., Nature and Light Dependence of Bulk Recombination in

Co-Pi-Catalyzed BiVO4 Photoanodes. Journal of Physical Chemistry C 2012, 116 (17), 9398-9404.

[64]. Berglund, S. P., Rettie, A. J. E., Hoang, S. and Mullins, C. B., Incorporation of Mo and W into

nanostructured BiVO4 films for efficient photoelectrochemical water oxidation. Physical Chemistry

Chemical Physics 2012, 14 (19), 7065-7075.

[65]. Wang, G. M., Ling, Y. C., Wang, H. Y., Yang, X. Y., Wang, C. C., Zhang, J. Z. and Li, Y.,

Hydrogen-treated WO3 nanoflakes show enhanced photostability. Energy & Environmental Science 2012, 5

(3), 6180-6187.

[66]. Zhang, X. H., Lu, X. H., Shen, Y. Q., Han, J. B., Yuan, L. Y., Gong, L., Xu, Z., Bai, X. D., Wei, M.,

Tong, Y. X., Gao, Y. H., Chen, J., Zhou, J. and Wang, Z. L., Three-dimensional WO3 nanostructures on

carbon paper: photoelectrochemical property and visible light driven photocatalysis. Chemical

Communications 2011, 47 (20), 5804-5806.

[67]. Qin, D. D., Tao, C. L., Friesen, S. A., Wang, T. H., Varghese, O. K., Bao, N. Z., Yang, Z. Y., Mallouk, T.

E. and Grimes, C. A., Dense layers of vertically oriented WO3 crystals as anodes for photoelectrochemical

water oxidation. Chemical Communications 2012, 48 (5), 729-731.

[68]. Liu, X., Wang, F. Y. and Wang, Q., Nanostructure-based WO3 photoanodes for photoelectrochemical

water splitting. Physical Chemistry Chemical Physics 2012, 14 (22), 7894-7911.

[69]. Esposito, D. V., Forest, R. V., Chang, Y. C., Gaillard, N., McCandless, B. E., Hou, S. Y., Lee, K. H.,

Birkmire, R. W. and Chen, J. G. G., Photoelectrochemical reforming of glucose for hydrogen production

using a WO3-based tandem cell device. Energy & Environmental Science 2012, 5 (10), 9091-9099.

[70]. Cristino, V., Caramori, S., Argazzi, R., Meda, L., Marra, G. L. and Bignozzi, C. A., Efficient

Photoelectrochemical Water Splitting by Anodically Grown WO3 Electrodes. Langmuir 2011, 27 (11),

7276-7284.

[71]. Hill, J. C. and Choi, K. S., Effect of Electrolytes on the Selectivity and Stability of n-type WO3

37

Photoelectrodes for Use in Solar Water Oxidation. Journal of Physical Chemistry C 2012, 116 (14),

7612-7620.

[72]. Kronawitter, C. X., Vayssieres, L., Shen, S. H., Guo, L. J., Wheeler, D. A., Zhang, J. Z., Antoun, B. R.

and Mao, S. S., A perspective on solar-driven water splitting with all-oxide hetero-nanostructures. Energy &

Environmental Science 2011, 4 (10), 3889-3899.

[73]. Feng, X. J., LaTempa, T. J., Basham, J. I., Mor, G. K., Varghese, O. K. and Grimes, C. A., Ta3N5

Nanotube Arrays for Visible Light Water Photoelectrolysis. Nano Lett 2010, 10 (3), 948-952.

[74]. Cong, Y. Q., Park, H. S., Wang, S. J., Dang, H. X., Fan, F. R. F., Mullins, C. B. and Bard, A. J.,

Synthesis of Ta3N5 Nanotube Arrays Modified with Electrocatalysts for Photoelectrochemical Water

Oxidation. Journal of Physical Chemistry C 2012, 116 (27), 14541-14550.

[75]. Yang, Y., Ling, Y. C., Wang, G. M., Lu, X. H., Tong, Y. X. and Li, Y., Growth of gallium nitride and

indium nitride nanowires on conductive and flexible carbon cloth substrates. Nanoscale 2013, 5 (5),

1820-1824.

[76]. Li, Y. B., Takata, T., Cha, D., Takanabe, K., Minegishi, T., Kubota, J. and Domen, K., Vertically

Aligned Ta3N5 Nanorod Arrays for Solar-Driven Photoelectrochemical Water Splitting. Adv Mater 2013, 25

(1), 125-131.

[77]. Sun, J. W., Liu, C. and Yang, P. D., Surfactant-Free, Large-Scale, Solution-Liquid-Solid Growth of

Gallium Phosphide Nanowires and Their Use for Visible-Light-Driven Hydrogen Production from Water

Reduction. Journal of the American Chemical Society 2011, 133 (48), 19306-19309.

[78]. Liu, C., Sun, J. W., Tang, J. Y. and Yang, P. D., Zn-Doped p-Type Gallium Phosphide Nanowire

Photocathodes from a Surfactant-Free Solution Synthesis. Nano Lett 2012, 12 (10), 5407-5411.

[79]. Maeda, K., Higashi, M., Siritanaratkul, B., Abe, R. and Domen, K., SrNbO2N as a Water-Splitting

Photoanode with a Wide Visible-Light Absorption Band. Journal of the American Chemical Society 2011,

133 (32), 12334-12337.

[80]. Banerjee, S., Mohapatra, S. K. and Misra, M., Synthesis of TaON nanotube arrays by

sonoelectrochemical anodization followed by nitridation: a novel catalyst for photoelectrochemical

hydrogen generation from water. Chemical Communications 2009, (46), 7137-7139.

[81]. Higashi, M., Domen, K. and Abe, R., Highly Stable Water Splitting on Oxynitride TaON Photoanode

System under Visible Light Irradiation. Journal Of The American Chemical Society 2012, 134 (16),

6968-6971.

[82]. Higashi, M., Domen, K. and Abe, R., Fabrication of efficient TaON and Ta3N5 photoanodes for water

splitting under visible light irradiation. Energy & Environmental Science 2011, 4 (10), 4138-4147.

38

[83]. Tachibana, Y., Vayssieres, L. and Durrant, J. R., Artificial photosynthesis for solar water-splitting.

Nature Photonics 2012, 6 (8), 511-518.

[84]. Jing, D. W., Guo, L. J., Zhao, L. A., Zhang, X. M., Liu, H. A., Li, M. T., Shen, S. H., Liu, G. J., Hu, X.

W., Zhang, X. H., Zhang, K., Ma, L. J. and Guo, P. H., Efficient solar hydrogen production by photocatalytic

water splitting: From fundamental study to pilot demonstration. International Journal Of Hydrogen Energy

2010, 35 (13), 7087-7097.

[85]. Liu, L., Wang, G., Li, Y., Li, Y. and Zhang, J., CdSe quantum dot-sensitized Au/TiO2 hybrid

mesoporous films and their enhanced photoelectrochemical performance. Nano Research 2011, 4 (3),

249-258.

[86]. Larsen, G. K., Fitzmorris, B. C., Longo, C., Zhang, J. Z. and Zhao, Y., Nanostructured homogenous

CdSe-TiO2 composite visible light photoanodes fabricated by oblique angle codeposition. Journal of

Materials Chemistry 2012, 22 (28), 14205-14218.

[87]. Shin, K., Yoo, J.-B. and Park, J. H., Photoelectrochemical cell/dye-sensitized solar cell tandem water

splitting systems with transparent and vertically aligned quantum dot sensitized TiO2 nanorod arrays.

Journal of Power Sources 2013, 225 (0), 263-268.

[88]. Swierk, J. R. and Mallouk, T. E., Design and development of photoanodes for water-splitting

dye-sensitized photoelectrochemical cells. Chem Soc Rev 2013, 42 (6), 2357-2387.

[89]. Khaselev, O. and Turner, J. A., A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen

Production via Water Splitting. Science 1998, 280 (5362), 425-427.

[90]. Brillet, J., Yum, J. H., Cornuz, M., Hisatomi, T., Solarska, R., Augustynski, J., Graetzel, M. and Sivula,

K., Highly efficient water splitting by a dual-absorber tandem cell. Nature Photonics 2012, 6 (12), 823-827.

[91]. Chen, Y. W., Prange, J. D., Dühnen, S., Park, Y., Gunji, M., Chidsey, C. E. D. and McIntyre, P. C.,

Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation. Nat Mater 2011, 10

(7), 539-544.

[92]. Kumar, A., Santangelo, P. G. and Lewis, N. S., Electrolysis of water at strontium titanate (SrTiO3)

photoelectrodes: distinguishing between the statistical and stochastic formalisms for electron-transfer

processes in fuel-forming photoelectrochemical systems. The Journal of Physical Chemistry 1992, 96 (2),

834-842.

[93]. Nozik, A. J. and Memming, R., Physical Chemistry of Semiconductor−Liquid Interfaces. The Journal

of Physical Chemistry 1996, 100 (31), 13061-13078.

[94]. Herrmann, J.-M., Disdier, J. and Pichat, P., Effect of chromium doping on the electrical and catalytic

39

properties of powder titania under UV and visible illumination. Chemical Physics Letters 1984, 108 (6),

618-622.

[95]. Yoshimura, J., Ebina, Y., Kondo, J., Domen, K. and Tanaka, A., Visible light-induced photocatalytic

behavior of a layered perovskite-type rubidium lead niobate, RbPb2Nb3O10. The Journal of Physical

Chemistry 1993, 97 (9), 1970-1973.

[96]. Gerischer, H., Electrochemical Techniques For The Study Of Photosensitization. Photochemistry and

Photobiology 1972, 16 (4), 243-260.

[97]. Kim, Y. I., Salim, S., Huq, M. J. and Mallouk, T. E., Visible-light photolysis of hydrogen iodide using

sensitized layered semiconductor particles. Journal of The American Chemical Society 1991, 113 (25),

9561-9563.

[98]. Saupe, G. B., Mallouk, T. E., Kim, W. and Schmehl, R. H., Visible Light Photolysis of Hydrogen

Iodide Using Sensitized Layered Metal Oxide Semiconductors:  The Role of Surface Chemical Modification

in Controlling Back Electron Transfer Reactions. The Journal of Physical Chemistry B 1997, 101 (14),

2508-2513.

[99]. Abe, R., Hara, K., Sayama, K., Domen, K. and Arakawa, H., Steady hydrogen evolution from water on

Eosin Y-fixed TiO2 photocatalyst using a silane-coupling reagent under visible light irradiation. Journal of

Photochemistry and Photobiology A: Chemistry 2000, 137 (1), 63-69.

[100]. Ikeda, S., Abe, C., Torimoto, T. and Ohtani, B., Photochemical hydrogen evolution from aqueous

triethanolamine solutions sensitized by binaphthol-modified titanium(IV) oxide under visible-light

irradiation. Journal of Photochemistry and Photobiology A: Chemistry 2003, 160 (1–2), 61-67.

[101]. Bae, E., Choi, W., Park, J., Shin, H. S., Kim, S. B. and Lee, J. S., Effects of Surface Anchoring

Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO2 on Visible Light Reactivity

in Aqueous Suspensions. The Journal of Physical Chemistry B 2004, 108 (37), 14093-14101.

[102]. Park, H. and Choi, W., Visible-Light-Sensitized Production of Hydrogen Using Perfluorosulfonate

Polymer-Coated TiO2 Nanoparticles:  An Alternative Approach to Sensitizer Anchoring. Langmuir 2006, 22

(6), 2906-2911.

[103]. Li, Q., Jin, Z., Peng, Z., Li, Y., Li, S. and Lu, G., High-Efficient Photocatalytic Hydrogen Evolution

on Eosin Y-Sensitized Ti−MCM41 Zeolite under Visible-Light Irradiation. The Journal of Physical Chemistry

C 2007, 111 (23), 8237-8241.

[104]. Kim, W., Tachikawa, T., Majima, T. and Choi, W., Photocatalysis of Dye-Sensitized TiO2

Nanoparticles with Thin Overcoat of Al2O3: Enhanced Activity for H2 Production and Dechlorination of

CCl4. The Journal of Physical Chemistry C 2009, 113 (24), 10603-10609.

40

[105]. Houlding, V. H. and Gratzel, M., Photochemical hydrogen generation by visible light. Sensitization of

titanium dioxide particles by surface complexation with 8-hydroxyquinoline. Journal of The American

Chemical Society 1983, 105 (17), 5695-5696.

[106]. Shimidzu, T., Iyoda, T. and Koide, Y., An advanced visible-light-induced water reduction with

dye-sensitized semiconductor powder catalyst. Journal of The American Chemical Society 1985, 107 (1),

35-41.

[107]. Furlong, D. N., Wells, D. and Sasse, W. H. F., Colloidal semiconductors in systems for the sacrificial

photolysis of water: sensitization of titanium dioxide by adsorption of ruthenium complexes. The Journal of

Physical Chemistry 1986, 90 (6), 1107-1115.

[108]. Kim, Y. I., Atherton, S. J., Brigham, E. S. and Mallouk, T. E., Sensitized layered metal oxide

semiconductor particles for photochemical hydrogen evolution from nonsacrificial electron donors. The

Journal of Physical Chemistry 1993, 97 (45), 11802-11810.

[109]. Hirano, K., Suzuki, E., Ishikawa, A., Moroi, T., Shiroishi, H. and Kaneko, M., Sensitization of TiO2

particles by dyes to achieve H2 evolution by visible light. Journal of Photochemistry and Photobiology A:

Chemistry 2000, 136 (3), 157-161.

[110]. Abe, R., Sayama, K. and Arakawa, H., Dye-sensitized photocatalysts for efficient hydrogen

production from aqueous I− solution under visible light irradiation. Journal of Photochemistry and

Photobiology A: Chemistry 2004, 166 (1–3), 115-122.

[111]. Bae, E. and Choi, W., Effect of the Anchoring Group (Carboxylate vs Phosphonate) in

Ru-Complex-Sensitized TiO2 on Hydrogen Production under Visible Light. The Journal of Physical

Chemistry B 2006, 110 (30), 14792-14799.

[112]. Li, Q., Chen, L. and Lu, G., Visible-Light-Induced Photocatalytic Hydrogen Generation on

Dye-Sensitized Multiwalled Carbon Nanotube/Pt Catalyst. The Journal of Physical Chemistry C 2007, 111

(30), 11494-11499.

[113]. Maeda, K., Eguchi, M., Youngblood, W. J. and Mallouk, T. E., Niobium Oxide Nanoscrolls as

Building Blocks for Dye-Sensitized Hydrogen Production from Water under Visible Light Irradiation.

Chemistry of Materials 2008, 20 (21), 6770-6778.

[114]. Maeda, K., Eguchi, M., Lee, S.-H. A., Youngblood, W. J., Hata, H. and Mallouk, T. E., Photocatalytic

Hydrogen Evolution from Hexaniobate Nanoscrolls and Calcium Niobate Nanosheets Sensitized by

Ruthenium(II) Bipyridyl Complexes. The Journal of Physical Chemistry C 2009, 113 (18), 7962-7969.

[115]. Maeda, K., Eguchi, M., Youngblood, W. J. and Mallouk, T. E., Calcium Niobate Nanosheets Prepared

by the Polymerized Complex Method as Catalytic Materials for Photochemical Hydrogen Evolution.

Chemistry of Materials 2009, 21 (15), 3611-3617.

41

[116]. Sayama, K., Mukasa, K., Abe, R., Abe, Y. and Arakawa, H., A new photocatalytic water splitting

system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis. Journal of

Photochemistry and Photobiology A: Chemistry 2002, 148 (1–3), 71-77.

[117]. Kato, H., Hori, M., Konta, R., Shimodaira, Y. and Kudo, A., Construction of Z-scheme Type

Heterogeneous Photocatalysis Systems for Water Splitting into H2 and O2 under Visible Light Irradiation.

Chemistry Letters 2004, 33 (10), 1348-1349.

[118]. Tada, H., Mitsui, T., Kiyonaga, T., Akita, T. and Tanaka, K., All-solid-state Z-scheme in CdS-Au-TiO2

three-component nanojunction system. Nat Mater 2006, 5 (10), 782-786.

[119]. Kato, H., Sasaki, Y., Iwase, A. and Kudo, A., Role of iron ion electron mediator on photocatalytic

overall water splitting under visible light irradiation using Z-scheme systems. Bulletin of The Chemical

Society of Japan 2007, 80 (12), 2457-2464.

[120]. Higashi, M., Abe, R., Ishikawa, A., Takata, T., Ohtani, B. and Domen, K., Z-scheme overall water

splitting on modified-TaON photocatalysts under visible light (lambda < 500 nm). Chemistry Letters 2008,

37 (2), 138-139.

[121]. Sasaki, Y., Iwase, A., Kato, H. and Kudo, A., The effect of co-catalyst for Z-scheme photocatalysis

systems with an Fe3+

/Fe2+

electron mediator on overall water splitting under visible light irradiation. Journal

of Catalysis 2008, 259 (1), 133-137.

[122]. Iwase, A., Ng, Y. H., Ishiguro, Y., Kudo, A. and Amal, R., Reduced Graphene Oxide as a Solid-State

Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. Journal of The American

Chemical Society 2011, 133 (29), 11054-11057.

[123]. Sasaki, Y., Kato, H. and Kudo, A., [Co(bpy)3]3+/2+

and [Co(phen)3]3+/2+

Electron Mediators for Overall

Water Splitting under Sunlight Irradiation Using Z-Scheme Photocatalyst System. Journal of The American

Chemical Society 2013, 135 (14), 5441-5449.

[124]. Kobayashi, R., Tanigawa, S., Takashima, T., Ohtani, B. and Irie, H., Silver-Inserted Heterojunction

Photocatalysts for Z-Scheme Overall Pure-Water Splitting under Visible-Light Irradiation. The Journal of

Physical Chemistry C 2014, 118 (39), 22450-22456.

[125]. Mock, J. J., Barbic, M., Smith, D. R., Schultz, D. A. and Schultz, S., Shape effects in plasmon

resonance of individual colloidal silver nanoparticles. The Journal of Chemical Physics 2002, 116 (15),

6755-6759.

[126]. Atay, T., Song, J.-H. and Nurmikko, A. V., Strongly Interacting Plasmon Nanoparticle Pairs:  From

Dipole−Dipole Interaction to Conductively Coupled Regime. Nano letters 2004, 4 (9), 1627-1631.

42

[127]. Kolwas, K., Derkachova, A. and Shopa, M., Size characteristics of surface plasmons and their

manifestation in scattering properties of metal particles. Journal of Quantitative Spectroscopy and Radiative

Transfer 2009, 110 (14–16), 1490-1501.

[128]. Kim, K.-H., Husakou, A. and Herrmann, J., Linear and nonlinear optical characteristics of composites

containing metal nanoparticles with different sizes and shapes. Optics Express 2010, 18 (7), 7488-7496.

[129]. Kelly, K. L., Coronado, E., Zhao, L. L. and Schatz, G. C., The optical properties of metal

nanoparticles: The influence of size, shape, and dielectric environment. Journal of Physical Chemistry B

2003, 107 (3), 668-677.

[130]. Thomann, I., Pinaud, B. A., Chen, Z. B., Clemens, B. M., Jaramillo, T. F. and Brongersma, M. L.,

Plasmon Enhanced Solar-to-Fuel Energy Conversion. Nano letters 2011, 11 (8), 3440-3446.

[131]. Torimoto, T., Horibe, H., Kameyama, T., Okazaki, K.-i., Ikeda, S., Matsumura, M., Ishikawa, A. and

Ishihara, H., Plasmon-Enhanced Photocatalytic Activity of Cadmium Sulfide Nanoparticle Immobilized on

Silica-Coated Gold Particles. The Journal of Physical Chemistry Letters 2011, 2 (16), 2057-2062.

[132]. Liu, Z. W., Hou, W. B., Pavaskar, P., Aykol, M. and Cronin, S. B., Plasmon Resonant Enhancement of

Photocatalytic Water Splitting Under Visible Illumination. Nano letters 2011, 11 (3), 1111-1116.

[133]. Ingram, D. B. and Linic, S., Water Splitting on Composite Plasmonic-Metal/Semiconductor

Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the

Semiconductor Surface. Journal of The American Chemical Society 2011, 133 (14), 5202-5205.

[134]. Duan, H. and Xuan, Y., Enhancement of light absorption of cadmium sulfide nanoparticle at specific

wave band by plasmon resonance shifts. Physica E: Low-dimensional Systems and Nanostructures 2011, 43

(8), 1475-1480.

[135]. Rosseler, O., Shankar, M. V., Du, M. K.-L., Schmidlin, L., Keller, N. and Keller, V., Solar light

photocatalytic hydrogen production from water over Pt and Au/TiO2(anatase/rutile) photocatalysts: Influence

of noble metal and porogen promotion. Journal of Catalysis 2010, 269 (1), 179-190.

[136]. Silva, C. G., Juarez, R., Marino, T., Molinari, R. and Garcia, H., Influence of Excitation Wavelength

(UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the

Generation of Hydrogen or Oxygen from Water. Journal of The American Chemical Society 2011, 133 (3),

595-602.

[137]. Nishijima, Y., Ueno, K., Kotake, Y., Murakoshi, K., Inoue, H. and Misawa, H., Near-Infrared

Plasmon-Assisted Water Oxidation. Journal of Physical Chemistry Letters 2012, 3 (10), 1248-1252.

[138]. Nishijima, Y., Ueno, K., Yokota, Y., Murakoshi, K. and Misawa, H., Plasmon-Assisted Photocurrent

Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. The Journal of

43

Physical Chemistry Letters 2010, 1 (13), 2031-2036.

[139]. Ueno, K. and Misawa, H., Plasmon-enhanced photocurrent generation and water oxidation from

visible to near-infrared wavelengths. Npg Asia Materials 2013, 5 e61.

[140]. Ueno, K. and Misawa, H., Surface plasmon-enhanced photochemical reactions. Journal of

Photochemistry and Photobiology C-Photochemistry Reviews 2013, 15 31-52.

[141]. Domen, K., Kondo, J. N., Hara, M. and Takata, T., Photo- and Mechano-Catalytic Overall Water

Splitting Reactions to Form Hydrogen and Oxygen on Heterogeneous Catalysts. Bulletin of The Chemical

Society of Japan 2000, 73 (6), 1307-1331.

[142]. Kawai, T. and Sakata, T., Photocatalytic Hydrogen Production From Water By The Decomposition Of

Poly-Vinylchloride, Protein, Algae, Dead Insects, and Excrement. Chemistry Letters 1981, 10 (1), 81-84.

[143]. Mullins, C. S. and Pecoraro, V. L., Reflections on Small Molecule Manganese Models that Seek to

Mimic Photosynthetic Water Oxidation Chemistry. Coordination Chemistry Reviews 2008, 252 (3-4),

416-443.

[144]. Youngblood, W. J., Lee, S.-H. A., Maeda, K. and Mallouk, T. E., Visible Light Water Splitting Using

Dye-Sensitized Oxide Semiconductors. Accounts of Chemical Research 2009, 42 (12), 1966-1973.

44

Chapter 2

Plasmon-Induced Water Splitting Using Two Sides of the Same

SrTiO3 Single-Crystal Substrate

2.1 Abstract

A plasmon-induced water splitting system that operates under visible light

irradiation is successfully developed. This system is based on the utilization of both sides

of the same SrTiO3 single-crystal substrate. The most important feature of this system is

that both sides of a strontium titanate single-crystal substrate are used without an

electrochemical apparatus. The water splitting system contains two solution chambers to

separate H2 and O2 evolution. To promote water splitting, a chemical bias was applied by

the regulation of the pH values at the two chambers. The quantity of H2 evolved from the

surface of platinum, which was used as a reduction co-catalyst, was twice the quantity of

O2 evolved from an Au nanostructured surface. Thus, the stoichiometric evolution of H2

and O2 was successfully demonstrated. The hydrogen-evolution action spectrum closely

corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced

charge separation at the Au/SrTiO3 interface promotes water oxidation and the subsequent

reduction of a proton on the backside of the SrTiO3 substrate. It is further elucidated that

the chemical bias is significantly reduced by plasmonic effects, which indicates the

possibility of constructing an artificial photosynthesis system with low energy

consumption. Plasmon-induced water splitting occurred even with a minimum chemical

bias of 0.23 V due to the plasmonic effects based on the efficient oxidation of water and

the use of platinum as a co-catalyst for reduction.

45

2.2 Introduction

To address the global energy issues, the development of a system that converts solar

energy to not only electrical but also chemical energy that can be stored for long periods is

critical.[1-3]

Honda and Fujishima have developed photoelectrochemical water splitting

systems that use a TiO2 photoelectrode irradiated by ultraviolet light.[4, 5]

However, only

approximately 5% of the solar irradiance observed on the Earth’s surface is composed of

ultraviolet radiation (<400 nm), whereas visible light (400-800 nm) and infrared light

(>800 nm) represent 50% and 45%, respectively. Therefore, the extension of light-energy

conversion to longer wavelengths, especially to the visible and near-infrared regions, is

important.[6-8]

The conversion of visible light can be achieved by several methods,

including the use of a Z-scheme electron transfer process,[9, 10]

TiO2 with the addition of

certain dopants,[11-14]

and semiconductor nanoparticles with a bandgap wavelength in the

visible region.[15-18]

To date, semiconductor particles have been used to induce H2 and O2

evolution through the response of such photocatalysts to visible light.[19, 20]

One of the

disadvantages of the use of semiconductor particles in a photocatalytic water splitting

system that requires energy to separate H2 from O2 is that H2 and O2 are evolved in the

same reaction chamber.[21-23]

Recently, the evolution of H2 and O2 in different reaction

chambers has been studied using photoelectrodes fabricated by sintering semiconductor

particles.[24]

Except for the Z-scheme reaction system, such systems do not require cathode

irradiation because hydrogen evolution occurs via a dark reaction. Therefore, the

optimization of a reaction cell design that can effectively utilize light for water splitting is

expected. In this chapter, it is proposed that the simultaneous evolution of H2 and O2 can

be realized without an electric wire connecting the anode and the cathode and with space

conserved for light irradiation on the anode side through the use of two sides of the same

semiconductor substrate. It is a plasmon-induced water splitting system that responds to

visible light, which is based on gold nanoparticles (Au-NPs) loaded onto SrTiO3

single-crystal substrates. The successful separation of H2 and O2 evolution in different

chambers using both sides of the same substrate is also demonstrated. The

46

plasmon-induced charge separation between Au-NPs and oxide semiconductor interfaces

studied here can act as a co-catalyst in water oxidation, which is considered to be a

bottleneck process in water splitting. Thus, this system is promising for artificial

photosynthesis and is expected to be deployed in the future.

47

2.3 Experimental details

2.3.1 Niobium doped strontium titanate single crystals

N-type SrTiO3 has been proven to be efficient anodes for photochemical water

splitting under UV light irradiation. While the large band gaps of SrTiO3 (3.2 eV) resulted

in their low conversion efficiencies of solar energy, thus hindering their practical

applications. For the effective usage of solar energy, many efforts have been made to

develop the visible light responding photoanodes. Recently, the band gaps of n-TiO2 were

effectively reduced by replacing oxygen with nitrogen or carbon to elevate the potential at

the top of the valence band.[11, 25]

Figure 2.1 Energy band structures of TiO2 and SrTiO3

Our group has already successfully observed photocurrent generation and O2

evolution with TiO2 substrate under certain electric bias. Whereas for overall water

splitting, H2 evolution with accurate quantity is also very important. Furthermore, it is

highly desired to realize the overall water splitting without any electric bias. The

conduction band (CB) electrochemical potential of TiO2 is 0 V vs reversible hydrogen

48

electrode (RHE), which is the same as the redox potential of the proton reduction. As a

result, it would be quite difficult for the TiO2 to reduce protons to generate H2 without any

electric bias. While as shown in Figure 2.1, the CB position of the strontium titanate

(SrTiO3) is -0.2 V vs RHE, more negative than the redox potential of the proton reduction,

which makes it take advantage at the H2 evolution. Therefore, the SrTiO3 was chosen as

the catalytic material in this study.

At room temperature, SrTiO3 crystallizes in the ABO3 cubic perovskite structure

where A and B are cations. The lattice constant of SrTiO3 is 0.3905 nm. Its crystal

structure is shown in Figure 2.2. The Ti4+

ions are sixfold coordinated by O2- ions, whereas

each of the Sr2+

ions is surrounded by four TiO6 octahedra. Therefore, each Sr2+

ion is

coordinated by 12 O2- ions. Within the TiO6 octahedra, while a hybridization of the O-2p

states with the Ti-3d states leads to a pronounced covalent bonding, Sr2+

and O2- ions

exhibit ionic bonding character.[26]

Hence, SrTiO3 has mixed ionic-covalent bonding

properties. This nature of chemical bonding leads to a unique structure, which make it a

model electronic material.

Figure 2.2 Crystalline structures of SrTiO3

Figure 2.3 shows the transmittance spectra of the Nb-SrTiO3 (0.05 wt% and 0.5 wt%)

49

single crystals. The transmittance spectra of these single crystals were measured by using

an ultraviolet–visible Spectrometer (Shimadzu UV-3100, Japan). It is obvious that in the

region of UV light absorption, the samples almost show the same absorption edge (about

3.2 eV), corresponding to the electronic excitation from the O 2p state to the Ti 3d state.[27]

In the region of the longer wavelength, the Nb-doped SrTiO3 single crystals show strong

absorption. This phenomenon should partly be ascribed to the optical absorption of free

carriers in the semiconductor. With the larger concentration of carriers, Nb-SrTiO3 (0.5

wt%) shows stronger optical absorption of free carriers in the region of long wavelength

than Nb:SrTiO3 (0.05 wt%). The main features of Figure 2.3 are strong visible light

absorption around 514 nm (c.a. 2.4 eV) for Nb-SrTiO3 (0.05 wt%) and Nb-SrTiO3 (0.5

wt%) single crystals. The enlargement of the transmittance spectrum for Nb-SrTiO3 (0.5

wt%) single crystal is shown in the Figure 2.3 (b). It means that there are intermediate

states within the band gap of Nb-SrTiO3 single crystals.

Taking these aspects into consideration, the SrTiO3 single crystals doped with Nb by

0.05 wt% was chosen as the semiconductor photocatalytic material in this study.

Figure 2.3 (a) The transmittance spectra of the semiconductor single-crystal Nb-SrTiO3

with different doping percentages (0.05 wt% and 0.5 wt%). (b) An enlarged view of (a) in

which the transmittance spectra of 0.5 wt% doped Nb-SrTiO3 is present.

50

2.3.2 Preparation of Au-NPs on a SrTiO3 substrate

Single-crystal strontium titanate (SrTiO3, 0.05 wt% niobium doped, 10 × 10 × 0.5

mm3, Furuuchi Chemical Co.) with a (110) surface was used as a semiconductor substrate

for water splitting. The SrTiO3 substrate as purchased was sequentially rinsed with

acetone, methanol, and deionized water in an ultrasonic bath for 5 min and was dried

under a flow of pure nitrogen. The substrate was subsequently exposed to ozone for 5 min

by excimer lamp (PC-01-H, N-Cobo Co.) irradiation under an O2 atmosphere in order to

remove the organic matter stuck to the surface. A thin gold film (3 nm) was deposited onto

the front side of the SrTiO3 by helicon sputtering (MPS-4000, ULVAC Co.) at a

deposition rate of 1 Å/s and was annealed at a temperature of 800°C for 1 h in a nitrogen

atmosphere to form the Au-NPs onto the SrTiO3 surface.[28]

The Au-NPs on the SrTiO3

surface were observed by field-emission scanning electron microscopy (FE-SEM,

JSM-6700FT, JEOL). The maximum resolution attainable at an electron acceleration

voltage of 15 kV was 1 nm. The extinction spectrum of the Au-NPs was monitored using a

UV/vis spectrometer (UV-3100, Shimadzu Co.).

Figure 2.4 The gold nanoparticles loaded SrTiO3 substrate fabrication and platinum board

decoration process.

51

2.3.3 Construction of the water splitting device

An In-Ga alloy (4:1 weight ratio) paste was applied to the backside of the

Nb-SrTiO3 substrate to form Ohmic contacts. Subsequently, a Pt board (10 × 10 × 0.2

mm3, 99.98%, Nilaco Corporation) was adhered to the backside of the substrate via Ag

paste (D-550, Fujikura Kasei Co.). A schematic illustration of the water splitting system

based on the Au-NP-loaded Nb-SrTiO3 substrate is shown in Figure 2.5. The water

splitting device contained sealed reaction cells with two solution compartments separated

by the SrTiO3 substrate. The Au-NP-loaded surface served as the anode on the front side

of the photoelectrode, which was irradiated by visible light to induce O2 evolution,

whereas a Pt board served as the cathode for H2 evolution on the backside. In the front

chamber, O2 evolution is expected to follow the plasmon-induced charge separation and

the subsequent water oxidation. The evolution of H2 is expected in the back chamber

because of the reduction of protons by photogenerated electrons injected into the

conduction band of Nb-SrTiO3 at the Pt surface, which was used as a reduction

co-catalyst.[29]

To adjust the chemical bias between the H2- and O2-evolution chambers,

the pH was regulated using arbitrary concentrations of hydrochloric acid (HCl) and

potassium hydroxide (KOH) aqueous solutions. The volumes of H2- and O2-evolution

chambers are 600 and 230 µL, respectively. To maintain the charge balance between the

two chambers, a salt bridge with 2 wt% agar was used. The pH values of the solutions

were examined using a pH meter (AS600, AS ONE Co).

Figure 2.5 A schematic illustration of the water splitting device using Au-NPs loaded

Nb-SrTiO3 photoelectrode.

52

In order to keep the charge and ion balance between the two reaction chambers, a

conventional salt bridge was used. The 100 mL potassium chloride (KCl) aqueous solution,

which contained 30 g KCl (nearly saturated condition), was dissolved by agar powder

with 2 wt%.[30]

The solution was then injected into a colorimetric tube, and heated in water

bath to keep the temperature steady at 95℃ , so as to prevent the solution from

solidification. A silica capillary tube with an inner diameter of 0.2 mm was inserted into

the agar solution afterwards. Due to the capillary effect, the agar solution successfully

entered into the silica capillary tube.[31]

Subsequently, the tube was taken out of the agar

solution. After the refrigeration of the agar solution in the tube to the room temperature, a

salt bridge was acquired. The tips of the salt bridge were inserted into the two reaction

chambers respectively during the water splitting process. During the water splitting

process, protons transferred from the front side to the back side of the reaction system

through this salt bridge in order to keep the charge balance between the two reaction

chambers. The salt bridge has the same function as a proton exchange membrane, e.g.

nafion film.[32, 33]

Figure 2.6 The installation for the fabrication of the salt bridge and its assembly to

the water splitting reaction cell.

53

2.3.4 Quantitative determination of hydrogen evolution

The amount of evolved H2 was quantitatively determined by a gas chromatograph

equipped with a thermal conductivity detector (GC-TCD 2014, Shimadzu Co.). The area

counts of the peaks and the identity of the gas were analyzed from the calibrated carrier

times. The HCl aqueous solution was bubbled with N2 for 60 minutes before each

experiment in order to remove the residual O2 in the solution. N2 gas was injected into the

H2 evolution side, as reference gases for the yield calculations. The GC-TCD was

equipped with a 6.0-m packed column (Shincarbon ST 50/80, Shimadzu Co.), and the

carrier gas was Ar with a flow rate of 30 mL/min. The gas evolved (10.0 μL) from the

backside chamber was injected. The column and sample injection temperatures were

120°C and 200°C, respectively. The detector temperature was set to 200°C.

The calibration curve of H2 and the reference gas N2 is shown in the Figure 2.8.

According to this calibration, the H2 detection limit of our GC-TCD device is 5.6 × 10-5

mol dm-3

or less. Since the volume of the sample injected into the GC-TCD is 10 μL, the

H2 detection limit of our GC-TCD device is estimated to be 0.56 nmol.

Figure 2.7 Gas chromatogram of typical sample extracted from the H2 evolution side

after the water splitting process.

54

Figure 2.8 The calibration curve of H2 using N2 as a reference gas.

2.3.5 Quantitative determination of oxygen evolution

The amount of evolved O2 was quantitatively determined by gas

chromatography-mass spectroscopy (GC-MS 2010-plus, Shimadzu Co.). The area counts

of the peaks and the identity of the gas were analyzed from the calibrated carrier times.

The KOH aqueous solution on the O2 evolution side of the cell was labeled with 18

O-water

(17.4 atom % isotopic purity) in order to quantify the oxygen evolution. The KOH

aqueous solution was bubbled with Ar for 60 minutes before each experiment in order to

remove the residual O2 in the solution. Ar gas was injected into the O2 evolution side, as

reference gas for the yield calculations. The GC-MS used to separate and determine the O2

was equipped with an electron ionization (EI) source and a 30.0 m RT-Msieve 5A column

(RESTEK); He at a flow rate of 42 mL/min was used as the carrier gas. The gas evolved

(5.0 μL) from the front side of the photoelectrode was injected in the split mode, with a

split ratio of 30, and the selected ion monitoring (SIM) mode was used for the detection of

O2. The column and sample injection temperatures were maintained at room temperature.

The EI ionization source temperature was set to 200°C.

The mass-to-charge ratio (m/z) of 28, 32, 34, 36, and 40 were determined by the gas

chromatography-mass spectrometry (GC-MS) spectra. A typical GC-MS chromatogram of

55

the sample from the water splitting device after visible light irradiation is shown in Figure

2.9. The (m/z) of 28, 32, 34, 36, and 40 are corresponding to 28

N2, 32

O2, 34

O2, 36

O2 & 36

Ar,

and 40

Ar gas chromatogram respectively. The area of gas chromatogram was used for the

calculation of gas concentration.

The peak of the 36

O2 was submersed in that of the 36

Ar, which was an isotope of the

reference gas used for quantitative determination. Because it was difficult to distinguish

these peaks from each other, the 36

O2 was not used to calculate the amount of O2 gas

evolution. In order to prevent the contamination from the air during the measurement, 32

O2

was not used in the calculation either. Thus, the 34

O2 gas chromatogram was used for

quantitative determination of the oxygen evolution, which was then compared with the

natural abundance ratio. The calibration curve of 34

O2 using 40

Ar as a reference gas is

shown in Figure 2.10. According to this calibration, the 34

O2 detection limit of our GC-MS

device is 6.0 × 10-6

mol dm-3

or less. Since the volume of the sample injected into the

GC-MS is 5 μL, the 34

O2 detection limit of our GC-MS device is estimated to be 0.03

nmol.

The mass-to-charge ratio (m/z) of 34 was determined from the gas

chromatography-mass spectrometry (GC-MS) spectra. The gas of m/z 34 is a kind of

oxygen isotope gas, which contains one 16

O and one 18

O atom. The gas chromatogram

intensity of the m/z of 34 increased with irradiation time.

Figure 2.9 (a) A typical GC-MS chromatogram of the sample from the water splitting

56

device after the anode side chamber was irradiated with visible light. Intensity was

normalized to m/z 40. (b) A zoomed-in view of the area in which argon and oxygen peaks

are present.

Figure 2.10 The calibration curve of 34

O2 using 40

Ar as a reference gas.

57

2.4 Results and discussion

2.4.1 Characterization of Au-NPs

A top view scanning electron microscope image of the Au-NPs on Nb-SrTiO3 single

crystal substrates was used for statistical analysis of the Au-NPs particle size distribution

based on the usage of free software ImageJ (http://rsb.info.nih.gov/ij/). With prolonged

annealing, the gold thin film grows in shape to give a more ordered structure as reported

previously.[34]

The scanning electron microscope image and Au-NPs particle size

histograms distribution of Au-NPs/SrTiO3 were shown in the Figure 2.11.

Figure 2.11 The scanning electron microscope top view image (a) and the particle size

histograms distributions (b) of Au-NPs on Nb-SrTiO3 photoelectrode after annealing at

800°C in a nitrogen atmosphere. The scale bars represent 100 nm.

The SEM images confirmed that the gold films transformed to a discontinuous

nanoparticle structure after annealing at 800°C in a nitrogen atmosphere, and the

interparticle spacing was comparable to the nanoparticle size. The diameter of the Au-NPs

distributes nearly symmetrically with the average size at around 52 nm, and the standard

deviation for the structural size was estimated to be 19 nm. The surface area coverage of

gold nanoparticles is approximately 26 % on the SrTiO3. Typical scanning transmission

58

electron microscope (STEM) images of Au-NPs cross-section and top view were shown in

Figure 2.12. The shape of Au-NPs on SrTiO3 was spherical segmental structure with

average height of about 32.7 nm. The Au-NPs tightly contact with TiO2 surface.

Figure 2.12 The transmission electron microscope cross section view with different

magnification (a), (b) and top view (c) images of Au-NPs on Nb-SrTiO3 photoelectrode

after annealing at 800°C in a nitrogen atmosphere. The scale bars represent 100 nm in all

images.

Typical XPS spectra characterization of Au-NPs on SrTiO3 substrate after annealing

at 800°C in a nitrogen atmosphere is shown in Figure 2.13. The Au 4f5/2 and 4f7/2 peaks

were centered at approximately 87.5 eV and 83.8 eV, which are in good accordance with

the standard values reported.[35]

The X-ray diffraction patterns of Au-NPs on SrTiO3 after

annealing at 800°C in a nitrogen atmosphere is shown in Figure 2.14. The peak that

represents the gold (111) crystal facet was clearly observed, indicating the crystalline

nature of the gold nanoparticles. Figure 2.15 shows the extinction spectrum of the Au-NPs

on the Nb-SrTiO3 substrate. A localized surface plasmon resonance (LSPR) band peaking

at a wavelength of 630 nm is clearly observed.

59

Figure 2.13 Partial XPS spectra characterization of Au-NPs on SrTiO3 after

annealing at 800°C in a nitrogen atmosphere.

Figure 2.14 X-ray diffraction patterns of Au-NPs on SrTiO3 after annealing at

800°C in a nitrogen atmosphere.

100 95 90 85 80 75 70

Au 4f7/2

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Au 4f5/2

20 30 40 50 60 70 80

0

50000

100000

150000

200000

250000STO (220)STO (110)

Counts

2θ (degree)

Au-NP on STO

Au (111)

60

Figure 2.15 Extinction spectrum of the Au-NPs on Nb-SrTiO3 after annealing at

800°C in a nitrogen atmosphere.

2.4.2 Numerical simulations of Au-NPs on SrTiO3

The extraordinarily powerful localized electromagnetic field that generated by

localized surface plasmon resonance may have a strong impact on the water splitting

activity enhancement. Nevertheless, the experimental measurement of the localized

electromagnetic field enhancement is very difficult. Therefore, the computational

simulation of the near-field intensity enhanced by the gold nanoparticles is an efficient

method to address this issue.

The finite difference time domain (FDTD) simulation is a numerical analysis

technique which is used for modeling the computational electrodynamics. This method is

one of the many categories of the general class of grid-based differential time-domain

numerical modeling methods. As a versatile modeling technique, FDTD is used to solve

Maxwell's equations and provide animated displays of the electromagnetic field

movement through the model. An increasing number of researches are using FDTD

technique to investigate optical properties of certain noble metal nanostructures in recent

years because the electric field intensity pattern is readily available in real space and the

algorithm is independent of the particular structure to be solved. Additionally, noble metal

450 500 550 600 650 700 750 800 850

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Extin

ctio

n

Wavelength (nm)

61

nanostructures have drawn considerable attention because of the existence of surface

plasmon resonance, which give rise to resonant effects and field enhancement leading to

many interesting applications.

To obtain a more rigorous understanding of the mechanism of water splitting

activity enhancement, the FDTD simulation method (Lumerical) was thus adopted to

simulate the electric field intensity distribution of gold nanostructures on SrTiO3 substrate

under the LSPR excitation. These simulations are based on the transmission electron

microscope (TEM) image of the Au nanoparticle and SrTiO3 surface, as shown in Figure

2.12, which is used to define the spatial extent of the Au nanoparticle islands in our

simulation. To calculate the field distribution, the following parameters were used: the

refractive index of the SrTiO3 substrate was set as 2.4, the optical constants for gold was

taken from Johnson and Christy, and the mesh accuracy was set at 0.5 nm.

Maps of the near-field electromagnetic intensity distribution patterns localized

around gold nanostructures on SrTiO3 at the maximum wavelength of the extinction

spectra were obtained from the FDTD simulation method in both the x-z and x-y planes,

as shown in Figure 2.16. For field enhancement, hemispherical shaped gold nanostructures

with diameters of 52 nm were used under periodic boundary conditions. In the x-z plane,

the intensity detector is located at the sphere center of the gold nanostructure and overlaps

the x axis. In the x-y plane, the detector is located at the interface between the gold

nanostructure and the SrTiO3 substrate. Notably, from the simulation, the electromagnetic

field intensity is greatly enhanced around the common boundary between gold, SrTiO3

substrate, and aqueous solutions, with the highest enhancement factor of nearly 500,

compared with the incident electric field at the SrTiO3 surface. This electromagnetic

response is dominated by local hot spots. This means that the electron hole pair generation

rate is 500 times that of the incident electromagnetic field. Thus, an increased amount of

charge is induced locally in the SrTiO3 because of the local field enhancement of the

plasmonic gold nanoparticles. Such kind of highly localized electromagnetic field was

supposed to play a key role for the electron excitation and the subsequent charge

62

separation. While there is no way to measure the electronhole pair generation rate in an

isolated plasmonic hot spot, the present work measuring plasmon-enhanced water splitting

provides perhaps the most direct measurement of this, since every two electrons produce

one hydrogen molecule.

Figure 2.16 FDTD simulation showing the electric field intensity distribution around the

gold nanostructure in the x-z (a) and x-y (b) planes. The dashed rectangle represents the

periphery of the gold nanostructure. The scale bar shows the enhancement factor.

2.4.3 Influence of irradiation wavelength on H2 and O2 generation

Figure 2.17 (a) depicts the irradiation time dependence of H2 and O2 evolution in the

backside and front-side chambers, respectively. Experimental about the quantitative

determination of H2 and O2 evolution was described in the supporting information. The

pH value was adjusted to 1 on the H2-evolution side, whereas the pH value on the

O2-evolution side was adjusted to 13 (i.e., the chemical bias was equivalent to 710 mV). A

xenon light spectrally filtered over the wavelength range of 450 nm to 850 nm was used to

irradiate the Au-NPs and induce LSPR. Under the irradiation conditions, the evolution of

H2 and O2 increased linearly with increasing irradiation time. As for the time dependence

of H2 and O2 evolutions, linearity was maintained in irradiation time, and it was not a

quantity of evolution to the extent that pH changes. Actually, it has been confirmed that

63

the pH value did not change before or after the irradiation. In a separate experiment, it has

also been confirmed that neither H2 nor O2 was evolved from a Nb-SrTiO3 substrate

without Au-NPs under the same irradiation conditions. No production of H2 or O2 was

observed in the absence of irradiation for an extended period (6 h), even when the

Au-NP-loaded Nb-SrTiO3 substrate was used. Figure 2.17 (a) shows that the quantity of

H2 evolved from the surface of Pt, which was used as a reduction co-catalyst, is twice the

quantity of O2 evolved from the Au nanostructured surface. Therefore, the stoichiometric

evolution of H2 and O2 was clearly demonstrated in the water splitting system based on the

Au-NP-loaded Nb-SrTiO3 single-crystal substrate.

Figure 2.17 (a) An irradiation time dependence of H2 (red circle) and O2 (blue circle)

evolution obtained in back and front side chamber, respectively. (b) The action spectrum

of apparent quantum efficiency with several wavelength regions using a histogram. The

solid line indicates LSPR band already shown in Figure 2.15.

The histogram in Figure 2.17 (b) shows the action spectrum of apparent quantum

efficiency (AQE). The pH conditions used were a pH of 1 for the H2-evolution side and a

pH of 13 for the O2-evolution side. Sectionalized yields of the AQE dependent on the

irradiation wavelength was measured with the increment as 100nm. In the 600 ± 50 nm,

700 ± 50 nm, and 800 ± 50 nm wavelength regions, the evolution efficiency (mol/h) of H2

is in approximate agreement with the segmented integral area of the the LSPR band of the

64

extinction spectrum indicated by the solid line in Figure 2.17 (b), whereas the evolution

efficiency of H2 in the 500 ± 50 nm wavelength region is higher than that indicated by the

spectrum of the Au-NPs on the Nb-SrTiO3. This result indicates that H2 and O2 evolution

was induced not only by LSPR excitation but also by the direct excitation of the interband

transition from the d-bands to the sp-conduction band of gold in the 500 ± 50 nm

wavelength region. Thus, the H2-evolution efficiency in the shorter-wavelength region is

higher than that in the longer-wavelength region. Notably, the water splitting is closely

related to the LSPR excitation because the AQE is strongly dependent on the LSPR band.

In the present system, the number of active sites or molecules of the photocatalyst is

impossible to be estimated. Therefore, turnover number is unavailable as an index to

evaluate the performance in this study. In addition, since the UV irradiation was cut off in

order to investigate the visible response, solar energy conversion efficiency is also

unaccessible in this research.

2.4.4 Influence of pH value combinations on H2 and O2 generation

To further understand this system in detail, a pH-dependence experiment was

performed to investigate the effect of the chemical bias on the water splitting. In the

experiment, a xenon light spectrally filtered over the wavelength range of 550 nm to 650

nm with an intensity of 0.32 W/cm2 was used for the LSPR excitation. Figure 2.18 (a)

indicates the representative irradiation time dependence of H2 and O2 evolution at the pH

combination of 1 and 13. The evolution of both H2 and O2 increases linearly with

increasing irradiation time, and the quantity of H2 evolved is twice that of O2 evolved,

which is analogous to the results in Figure 2.17 (a). Figure 2.18 (b) shows the pH

dependence of the H2 and O2 evolution efficiency. The pH value on the O2-evolution side

was fixed at 13, as indicated by the blue background in Figure 2.18 (b), which is the

condition that preferentially induces water oxidation. The evolution efficiency of H2 is

twice that of O2 at the observed pH value. However, the evolution efficiency of both H2

and O2 gradually decreased with increasing pH value on the H2-evolution side, and no

65

production of H2 or O2 was observed at a pH value of 4. Therefore, a chemical bias was

required in the water splitting in this system of at least 0.59 V, which corresponds to the

difference between pH 3 and pH 13. However, a pH value of 1 was fixed on the

H2-evolution side, as indicated by the red background in Figure 2.18 (b), which is the

condition that preferentially induces H2 evolution. Similarly, the evolution efficiency of

H2 was twice that of O2 at the observed pH value. In contrast, the evolution efficiency of

both H2 and O2 gradually decreased with decreasing pH value on the O2-evolution side.

However, the evolution of both H2 and O2 was still observed, even at a pH of 6.8 using a

pure water. In addition, neither H2 nor O2 evolution was observed at a pH of 6. Therefore,

in this system, a chemical bias was required in the water splitting of at least 0.34 V, which

corresponds to the difference between pH 1 and pH 6.8. Therefore, the chemical bias

required for the water splitting changes as the fixed pH value in the chamber changes.

When the pH value at the cathode was fixed to 1 and the pH at the anode was

increased from 6.8 to 10, hydrogen was evolved due to the reduction of protons at the

cathode, whereas oxygen was evolved due to the oxidation of water molecules via a

four-electron reaction. Therefore, the water splitting reaction did not exhibit a significant

pH dependence. When the pH at the anode was increased from 11 to 13.5, oxygen was

evolved due to the oxidation of water molecules and OH- ions. The amount of evolved

gases increased gradually as the pH was increased at the anode. Similarly, when the pH at

the anode was fixed at 13 and the pH at the cathode was gradually increased from 1, the

quantity of the evolved gases decreased because the proton concentration at the cathode

decreased with increasing pH values. Neither gas was detectable at pH 4 on the cathode

side. On the basis of these results, it is hypothesized that hydrogen evolution was the

rate-limiting process of the present water splitting reaction. Because water splitting did not

proceed when the pH at the anode was 6, the minimum chemical bias in this system was

estimated to be 230 mV, which corresponds to the combination of pH 3 (cathode side) and

pH 6.8 (anode side). Although water oxidation is the rate-limiting process in a typical

water splitting system, it is concluded that the water splitting proceeded at a relatively low

66

chemical bias of 230 mV because the plasmon-induced charge separation efficiently

oxidized the water.

Figure 2.18 (a) An irradiation time dependence of H2 (red circle) and O2 (blue circle)

evolution at the pH combination of 1 and 13 under the conditions that the xenon light

spectrally filtered to the wavelength from 550 nm to 650 nm with an intensity of 0.32

W/cm2 was irradiated. (b) The pH dependence of the H2 (closed circle and square) and O2

(open circle and square) evolution efficiency under the conditions that the pH value in the

H2 evolution side is fixed at 1 as plotted on red background and the pH value in the O2

evolution side is fixed at 13 as plotted on blue background, respectively.

To further explore the limiting pH conditions for H2 and O2 evolution, the pH values

on the H2- and O2-evolution sides were fixed at 3 and 6.8, respectively, which are the

minimum acidic and alkaline conditions observed in the previous experiments to promote

evolution of H2 and O2, respectively. The irradiation time dependence of H2 and O2

evolution is shown in Figure 2.19(a). The results confirm that the evolution of both H2 and

O2 increased approximately linearly with increasing irradiation time and that the

efficiency of H2 evolution was twice that of O2 evolution. The pH dependence is shown in

Figure 2.19(b), which is summarized as being analogous to Figure 2.18(b). The evolution

efficiency of both H2 and O2 gradually decreased with increasing pH on the H2-evolution

side, and no production of H2 or O2 was observed at pH 4. When the pH value was set to 4

on the H2-evolution side, the water splitting did not proceed at either pH value on the

67

O2-evolution side. This result indicates that the reaction rate of hydrogen evolution is the

rate-determining process in water splitting. In addition, the absolute efficiency is

significantly lower than the previous data because of the insufficient chemical bias applied

to that system. In the case where the pH value on the H2-evolution side was fixed, water

splitting was induced, even at a pH combination of 3 and 6.8, which corresponds to a

minimum chemical bias of 0.23 V in this plasmon-assisted water splitting system.[36]

To

the best of the authors’ knowledge, this chemical bias is the smallest value reported for a

water splitting system driven by visible-light irradiation.

Figure 2.19 (a) An irradiation time dependence of H2 (red circle) and O2 (blue circle)

evolution at the pH combination of 3 and 6.8. (b) The pH dependence of the H2 (closed

circle and square) and O2 (open circle and square) evolution efficiency under the

conditions that the pH value in the H2 evolution side is fixed at 3 as plotted on yellow

background and the pH value in the O2 evolution side is fixed at 6.8 as plotted on green

background, respectively.

2.4.5 Mechanism for the plasmon-assisted water splitting reaction

The pH dependence experiments clearly demonstrated that the pH dependence of

water splitting exhibits an asymmetrical response. The mechanism for water oxidation is

considered to rely on the adsorption of OH- onto the surface of SrTiO3, close to the

Au-NPs. In addition, the efficiency of O2 evolution is strongly dependent on the

68

concentration of OH-, as defined by the pH value, because the adsorbed hydroxyl ions

must be oxidized by multiple electron holes to evolve oxygen.[37]

However, under neutral

conditions, such as pH 6.8, the surface concentration of OH- is lower than that under

alkaline conditions. Water molecules must dissociate to form adsorbed OH- on the surface

of SrTiO3, thereby resulting in the observation that the reaction rate of water oxidation is

lower than that of OH- oxidation. Notably, the effective water-splitting process proceeds

under neutral conditions in the present system, which suggests that water molecules could

be directly oxidized by the plasmon-induced water splitting system. Thus, the efficient

oxidation of water and OH- also requires highly concentrated electron holes at a local site

because multiple electron transfer processes are required with two water molecules or four

OH- groups. The possibility exists that the LSPR generates multiple holes at the

plasmonically enhanced optical near-field because of an efficient charge separation and

that the multiple holes trapped at the surface states of SrTiO3 near the hot site might be

stored at a local site of the SrTiO3. The stored multiple holes confined at a local site of the

SrTiO3 may be able to accelerate the oxidation of water or OH- and the subsequent

evolution of oxygen.

Figure 2.20 A schematic diagram of the hot sites near the interface between the

Nb-SrTiO3 substrate and the Au-NPs.

Figure 2.21 illustrates the energy diagram of the plasmon-induced water splitting

system based on the Au-NP-loaded SrTiO3 photoelectrode at the pH combination of 3 and

6.8. It is hypothesized that the plasmon resonance is first induced by visible and

69

near-infrared light, and the enhanced optical near-field excites an electron from the gold

nanoparticles or from the surface-state electron of the Nb-SrTiO3. The excited electron is

transferred to the conduction band of the Nb-SrTiO3. The electron that arrived at the back

of the substrate reduces a proton on the Pt surface and hydrogen is evolved from the

cathode. Conversely, the hole left behind at the Au/SrTiO3/water interface oxidizes water

or OH- via multi-electron transfer, and oxygen is evolved from the anode.

[38] Thus,

plasmon-induced water splitting was clearly demonstrated, and the efficient water

oxidation due to the plasmon effect reduced the chemical bias by 0.23 V.

Figure 2.21 Energy diagram of the plasmon-induced water splitting system using Au-NPs

loaded SrTiO3 substrate at the pH combination of 3 and 6.8. The flat band potential of

SrTiO3 at pH 6.8 was estimated as -0.601 V vs. SHE because the potential of the

conduction band of SrTiO3 is known to be -0.2 V vs. SHE at pH 0. The difference in the

potential of 0.401 V is due to pH difference.

The water oxidation appears to proceed efficiently because the hole does not remain

in the Au-NPs. Moreover, multiple holes are trapped in the surface states of Nb-SrTiO3 in

a spatially selective nanospace near the Au-NPs/Nb-SrTiO3/water interface, thus allowing

for oxidization of the water molecules. Therefore, it can be concluded that the

plasmon-induced charge separation behaves like a co-catalyst for oxygen evolution. It is

70

reasoned that the generated holes did not remain in the Au-NPs because the Fermi levels

(Ef) of Nb-SrTiO3 and Au coincide when the Au-NP contacts the surface of Nb-SrTiO3. It

is known that Ef exists slightly below the conduction band energy of Nb-SrTiO3. Because

the generated holes might easily migrate to the Ef, the holes will lose the capability of

oxidizing water in a thermodynamically favored manner because the potential difference

between the flat band potential and Ef is approximately 0.9 V when Au is in contact with

TiO2. The main characteristic of the water splitting system is that hydrogen and oxygen

are produced separately and can be removed from the different electrolysis chambers.

Additionally, only the Au-NP side needs to be irradiated, which reduces the area required

for light irradiation because the reduction of the proton is a dark reaction.

2.4.6 Stability of the plasmon-assisted water splitting system

The water splitting experiment with a longer time irradiation was carefully

performed to check its stability. The irradiation time dependence of H2 and O2 evolution

with a pH combination of 3 and 6.8 is shown in Figure 2.22. The linear relationship of

both H2 and O2 evolution was obviously observed even with an irradiation of 48 hours.

Thus, a stable plasmon-assisted water splitting system was constructed.

Figure 2.22 The irradiation time dependence of H2 and O2 evolution with a pH

combination of 3 and 6.8. The data from 1 to 6 as an irradiation time was taken from

Figure 2.18 (a).

71

2.5 Conclusions

In conclusion, a stable and simple plasmon-induced water splitting system without

an external electrochemical apparatus was successfully developed by using two sides of

the same SrTiO3 single-crystal substrate. The reaction cell involved no electric bias.

Instead, the pH value regulation is adopted. Namely, the electric potential has been

successfully substituted with chemical potential in the photocatalytic water splitting

reaction. The huge advantage using plasmonic Au nanostructures is stable over long time

unlike molecular sensitizers and its plasmon resonant wavelength can be tuned by simply

changing their shape and/or size to cover a large part of the solar spectrum. A

stoichiometric evolution of H2 and O2 was simultaneously obtained from two separate

solution chambers. The quantity of the evolved gases linearly increased with the

irradiation time. Solar energy has been successfully converted into chemical energy and

stored in the form of hydrogen gas. The separate evolution of H2 and O2 is expected to

suppress the recombination of the two gases. Even the irradiation of the unprecedented

long wavelength from 750 nm to 850 nm succeeded in water splitting. The mechanism for

the water splitting device occurs through plasmon-induced charge separation at the

Au-NP/SrTiO3 interface, thereby promoting water oxidation and subsequent reduction of a

proton on the backside of the SrTiO3 substrate. The H2 and O2 generation efficiency is

highly dependent on the pH values of the solution environments. The pH dependence

experiments revealed that the chemical bias is substantially reduced by plasmonic effects

up to 0.23 V because of efficient water oxidation. Although the apparent quantum

efficiency is still low (4.4 × 10-4

% at 600 nm), these results indicate the possibility of

constructing an efficient artificial photosynthesis system that responds to visible and

near-infrared light and does not need the chemical bias through the combination of an

efficient co-catalyst for H2 evolution on the backside of a SrTiO3 substrate as well as

understanding the detailed mechanism.

72

2.6 References

[1]. Gust, D., Moore, T. A. and Moore, A. L., Mimicking Photosynthetic Solar Energy Transduction.

Accounts of Chemical Research 2000, 34 (1), 40-48.

[2]. Kamat, P. V., Meeting the clean energy demand: Nanostructure architectures for solar energy

conversion. Journal of Physical Chemistry C 2007, 111 (7), 2834-2860.

[3]. Linic, S., Christopher, P. and Ingram, D. B., Plasmonic-metal nanostructures for efficient conversion of

solar to chemical energy. Nature Materials 2011, 10 (12), 911-921.

[4]. Fujishima, A. and Honda, K., Electrochemical Evidence for the Mechanism of the Primary Stage of

Photosynthesis. Bulletin of The Chemical Society of Japan 1971, 44 (4), 1148-1150.

[5]. Fujishima, A. and Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238 (5358), 37-38.

[6]. Graetzel, M., Artificial photosynthesis: water cleavage into hydrogen and oxygen by visible light.

Accounts of Chemical Research 1981, 14 (12), 376-384.

[7]. Vlachopoulos, N., Liska, P., Augustynski, J. and Graetzel, M., Very efficient visible light energy

harvesting and conversion by spectral sensitization of high surface area polycrystalline titanium dioxide

films. Journal of The American Chemical Society 1988, 110 (4), 1216-1220.

[8]. Rensmo, H., Keis, K., Lindström, H., Södergren, S., Solbrand, A., Hagfeldt, A., Lindquist, S. E., Wang,

L. N. and Muhammed, M., High Light-to-Energy Conversion Efficiencies for Solar Cells Based on

Nanostructured ZnO Electrodes. The Journal of Physical Chemistry B 1997, 101 (14), 2598-2601.

[9]. Liu, G., Wang, L., Yang, H. G., Cheng, H.-M. and Lu, G. Q., Titania-based photocatalysts-crystal

growth, doping and heterostructuring. Journal of Materials Chemistry 2010, 20 (5), 831-843.

[10]. Kudo, A., Kato, H. and Tsuji, I., Strategies for the development of visible-light-driven photocatalysts

for water splitting. Chemistry Letters 2004, 33 (12), 1534-1539.

[11]. Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K. and Taga, Y., Visible-light photocatalysis in

nitrogen-doped titanium oxides. Science 2001, 293 (5528), 269-271.

[12]. Ohno, T., Mitsui, T. and Matsumura, M., Photocatalytic Activity of S-doped TiO2 Photocatalyst under

Visible Light. Chemistry Letters 2003, 32 (4), 364-365.

[13]. Umebayashi, T., Yamaki, T., Itoh, H. and Asai, K., Analysis of electronic structures of 3d transition

metal-doped TiO2 based on band calculations. Journal of Physics and Chemistry of Solids 2002, 63 (10),

73

1909-1920.

[14]. Zakrzewska, K., Radecka, M. and Rekas, M., Effect of Nb, Cr, Sn additions on gas sensing properties

of TiO2 thin films. Thin Solid Films 1997, 310 (1–2), 161-166.

[15]. Maeda, K. and Domen, K., New non-oxide photocatalysts designed for overall water splitting under

visible light. Journal of Physical Chemistry C 2007, 111 (22), 7851-7861.

[16]. Youngblood, W. J., Lee, S.-H. A., Kobayashi, Y., Hernandez-Pagan, E. A., Hoertz, P. G., Moore, T. A.,

Moore, A. L., Gust, D. and Mallouk, T. E., Photoassisted Overall Water Splitting in a Visible

Light-Absorbing Dye-Sensitized Photoelectrochemical Cell. Journal of The American Chemical Society

2009, 131 (3), 926-927.

[17]. Higashi, M., Domen, K. and Abe, R., Fabrication of an Efficient BaTaO2N Photoanode Harvesting a

Wide Range of Visible Light for Water Splitting. Journal of The American Chemical Society 2013, 135 (28),

10238-10241.

[18]. Maeda, K., Takata, T., Hara, M., Saito, N., Inoue, Y., Kobayashi, H. and Domen, K., GaN:ZnO Solid

Solution as a Photocatalyst for Visible-Light-Driven Overall Water Splitting. Journal of The American

Chemical Society 2005, 127 (23), 8286-8287.

[19]. Ishikawa, A., Takata, T., Kondo, J. N., Hara, M., Kobayashi, H. and Domen, K., Oxysulfide

Sm2Ti2S2O5 as a Stable Photocatalyst for Water Oxidation and Reduction under Visible Light Irradiation (λ

≤650 nm). Journal of The American Chemical Society 2002, 124 (45), 13547-13553.

[20]. Kudo, A., Omori, K. and Kato, H., A Novel Aqueous Process for Preparation of Crystal

Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and

Its Photocatalytic and Photophysical Properties. Journal of The American Chemical Society 1999, 121 (49),

11459-11467.

[21]. Perkins, C. and Weimer, A. W., Likely near-term solar-thermal water splitting technologies.

International Journal of Hydrogen Energy 2004, 29 (15), 1587-1599.

[22]. Minggu, L. J., Daud, W. R. W. and Kassim, M. B., An overview of photocells and photoreactors for

photoelectrochemical water splitting. International Journal of Hydrogen Energy 2010, 35 (11), 5233-5244.

[23]. Maeda, K. and Domen, K., Photocatalytic Water Splitting: Recent Progress and Future Challenges.

The Journal of Physical Chemistry Letters 2010, 1 (18), 2655-2661.

[24]. Selli, E., Chiarello, G. L., Quartarone, E., Mustarelli, P., Rossetti, I. and Forni, L., A photocatalytic

water splitting device for separate hydrogen and oxygen evolution. Chemical Communications 2007, (47),

5022-5024.

74

[25]. Khan, S. U. M., Al-Shahry, M. and Ingler, W. B., Efficient Photochemical Water Splitting by a

Chemically Modified n-TiO2. Science 2002, 297 (5590), 2243-2245.

[26]. Leapman, R. D., Grunes, L. A. and Fejes, P. L., Study of the L23 edges in the 3d transition metals and

their oxides by electron-energy-loss spectroscopy with comparisons to theory. Physical Review B 1982, 26

(2), 614-635.

[27]. Guo, X. G., Chen, X. S., Sun, Y. L., Sun, L. Z., Zhou, X. H. and Lu, W., Electronic band structure of

Nb doped SrTiO3 from first principles calculation. Physics Letters A 2003, 317 (5–6), 501-506.

[28]. Shi, X., Ueno, K., Takabayashi, N. and Misawa, H., Plasmon-Enhanced Photocurrent Generation and

Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode. The Journal of Physical

Chemistry C 2013, 117 (6), 2494-2499.

[29]. Wang, W.-N., An, W.-J., Ramalingam, B., Mukherjee, S., Niedzwiedzki, D. M., Gangopadhyay, S. and

Biswas, P., Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine

Pt Nanoparticles on TiO2 Single Crystals. Journal of The American Chemical Society 2012, 134 (27),

11276-11281.

[30]. Veneman, P. L. M. and Pickering, E. W., Salt bridge for field redox potential measurements.

Communications In Soil Science and Plant Analysis 1983, 14 (8), 669-677.

[31]. Kong, J., Shen, C.-J., Xin, P., Song, Z., Li, L., Barry, D. A., Jeng, D. S., Stagnitti, F., Lockington, D. A.

and Parlange, J. Y., Capillary effect on water table fluctuations in unconfined aquifers. Water Resources

Research 2013, 49 (5), 3064-3069.

[32]. Adjemian, K. T., Lee, S. J., Srinivasan, S., Benziger, J. and Bocarsly, A. B., Silicon Oxide Nafion

Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140°C. Journal of The

Electrochemical Society 2002, 149 (3), A256-A261.

[33]. Chae, K. J., Choi, M., Ajayi, F. F., Park, W., Chang, I. S. and Kim, I. S., Mass transport through a

proton exchange membrane (Nafion) in microbial fuel cells. Energy & Fuels 2008, 22 (1), 169-176.

[34]. Ming, T., Suntivich, J., May, K. J., Stoerzinger, K. A., Kim, D. H. and Shao-Horn, Y., Visible Light

Photo-oxidation in Au Nanoparticle Sensitized SrTiO3:Nb Photoanode. Journal of Physical Chemistry C

2013, 117 (30), 15532-15539.

[35]. Nam, S. H., Kim, M. H., Hyun, J. S., Kim, Y. D. and Boo, J. H., XPS Analysis by Exclusion of

a-Carbon Layer on Silicon Carbide Nanowires by a Gold Catalyst-Supported Metal-Organic Chemical Vapor

Deposition Method. Journal of Nanoscience and Nanotechnology 2010, 10 (4), 2741-2745.

[36]. Thimsen, E., Le Formal, F., Gratzel, M. and Warren, S. C., Influence of Plasmonic Au Nanoparticles

on the Photoactivity of Fe2O3 Electrodes for Water Splitting. Nano letters 2010, 11 (1), 35-43.

75

[37]. Diaz-Morales, O., Calle-Vallejo, F., de Munck, C. and Koper, M. T. M., Electrochemical water

splitting by gold: evidence for an oxide decomposition mechanism. Chemical Science 2013, 4 (6),

2334-2343.

[38]. Pillai, S., Catchpole, K. R., Trupke, T. and Green, M. A., Surface plasmon enhanced silicon solar cells.

Journal of Applied Physics 2007, 101 (9), 093105.

76

Chapter 3

Co-catalyst Effects on Hydrogen Evolution in the

Plasmon-Induced Water Splitting System

3.1 Abstract

Recently great progress has been achieved in the development of

photoelectrochemical (PEC) water splitting. Especially, water splitting systems that

respond to visible light have received considerable attention in the past decade. The water

splitting systems that respond to visible light have been developed according to metal ion

doping to semiconductor, Z-scheme process, and dye or plasmon sensitization. Although

the progress in plasmon-induced water splitting system made in recent few years, the

water splitting efficiency is still low. It is well known that loading proper co-catalysts on

semiconductors (as light harvesters) can significantly enhance the activities of PEC water

splitting reactions. Therefore, it is of significance to establish a system with rational

design to precisely evaluate the capacity of different kinds of co-catalysts. A lot of

studies were performed using n-type semiconductor particles system to elucidate

co-catalyst effects so far. In these studies, however, it is difficult to evaluate only the

catalytic activity of H2 and O2 evolutions as following reasons; 1) An electron injected

into the conduction band of semiconductor is hard to transfer to the H2 evolution

co-catalyst because Schottky barrier can be readily formed when a metallic co-catalyst

for H2 evolution contacts the n-type semiconductor, hindering the performances of the

co-catalyst from being comprehended. 2) There is a possibility that a reverse reaction of

water splitting occurs and these co-catalysts also promote reduction of the evolved O2. In

order to tackle these factors, a facile plasmon-induced water splitting system using gold

nanoparticles (Au-NPs) loaded strontium titanate (SrTiO3) single crystal substrate was

77

developed. In this chapter, the co-catalyst effect on hydrogen evolution in the

plasmon-induced water splitting was investagated. To eliminate the adverse effect of the

Schottky barrier, a Pt board was stick on the hydrogen (H2) evolution side of the SrTiO3

substrate via In-Ga alloy to obtain ohmic contact between semiconductor and metal or

metal oxide co-catalysts. The water spitting yield with this SrTiO3/Pt composite substrate

was remarkably enhanced as compared to that with the Pt thin film without In-Ga Alloy

as H2 evolution co-catalyst. The importance of the ohmic contact for the H2 evolution in

the plasmon-induced water splitting system was thus successfully demonstrated.

Subsequently, the Pt board was decorated with a noble metal or their corresponding metal

oxide as a co-catalyst for H2 evolution. Enhanced evolution of H2 and O2 with

stoichiometric ratio was obviously demonstrated with all kinds of co-catalysts. As the

result, among the co-catalyst materials examined, rhodium thin layer with a thickness of

2 or 3 nm deposited on platinum board exhibited relatively high performance as a 3-fold

increment compared with the absence of any metal or metal oxide co-catalyst thin layer

for this application. Furthermore, the evolution of H2 and O2 were successfully separated

according to using two sides of the same SrTiO3 substrate. The separate evolution of H2

and O2 was supposed to suppress the undesirable reverse reactions of water splitting such

as H2–O2 recombination and/or photoreduction of O2.

78

3.2 Introduction

In order to realize a substitution for the fossil fuels, the development of clean,

environmental friendly, and sustainable energy sources has drawn considerable attention

by many researchers. Currently, it has become common sense that solar energy will play a

key role in the development of new energy sources since it is abundant, clean and

especially renewable. Hydrogen has attracted significant interest as a renewable energy

carrier offering both high-energy capacity and minimal environmental impact. As a

sustainable approach for new energy sources, photocatalytic hydrogen production utilizing

solar energy is a promising strategy.[1]

Photoelectric conversion and the photoelectrochemical water splitting has been of

great interest since the early 1970s after the first demonstration using a titanium dioxide

(TiO2) photoelectrode under ultraviolet radiation by Fujishima and Honda.[2]

Nevertheless,

one drawback of TiO2 is that it can only be excited by ultraviolet light, i.e., with

wavelengths shorter than ca. 400 nm. Therefore, only a very small portion of solar

radiation (3–5%) can be utilized to drive such chemical reactions. Thus, extension of its

absorption wavelength range to the visible region becomes an important issue. Numerous

attempts have been made afterwards to extend the cutoff wavelength of the water splitting

system. To date, utilization of visible light has been attained by doping,[3]

defect creation,[4]

and organic dye sensitization.[5]

Thereinto, plasmon-induced water splitting realized by

noble metal nanoparticles decorated on catalytic semiconductor materials possesses

unique advantages, as has been reported by some researchers.[6, 7]

Previously, our research

group successfully demonstrated plasmon-enhanced photocurrent generation and water

oxidation through irradiation with visible and near-infrared light using a TiO2 single

crystal photoelectrode loaded with Au-NPs.[8]

The solar to chemical energy conversion efficiency is still low and far from practical

application up until now, although many materials have been found to be capable of

performing the water splitting reaction. The reason might be that there are three crucial

steps for the water splitting reaction which is considered to be not yet efficient: solar light

79

harvesting, charge separation and transportation, and the catalytic reduction and oxidation

reactions. Among these problems, in this chapter, the co-catalyst effects concerning with

the multiple electron transfer process is discussed.

As a thermodynamically uphill reaction, water splitting requires the transfer of

multiple electrons, making it one of the most challenging reactions in chemistry. It is well

known that adequate co-catalysts loaded semiconductors significantly enhance the

activities of the water splitting. For semiconductor-based PEC systems, loading oxidation

and/or reduction co-catalysts can facilitate oxidation and reduction reactions by providing

the active sites and reaction sites while suppressing the charge recombination and reverse

reactions. The important roles of co-catalysts in PEC water splitting reactions have been

under investigation for a long time. Domen et al. have reported multiple co-catalysts to

enhance the water-splitting activity, including co-catalysts for the reduction of water to

form H2 such as Pt, Rh, and co-catalysts for the oxidation of water to form O2 such as IrO2,

Mn3O4, RuO2.[9, 10]

Based on the former work, it is already known that the photocatalytic

activity is highly dependent on the kind, size and morphology of the co-catalysts.

Nevertheless, the intrinsic properties of many kinds of co-catalyst and their interaction

mechanisms with catalytic semiconductor materials are still not so clear at the present time.

Thus, it is of special significance to evaluate the capacity of various co-catalysts and to

acquire an in-depth understanding of their function in the process of the PEC water

splitting reactions. However, in most conventional photocatalyst architectures, it is

difficult to evaluate co-catalyst effect for hydrogen (H2) evolution correctly in the water

splitting system using semiconductor particles as following reasons. As for H2 evolution, it

becomes problem that Schottky barrier formed by loading metal on the surface of

semiconductor makes electron transfer rate from the conduction band of semiconductor to

the metallic co-catalyst slow as result in increasing the possibility to induce the back

electron transfer reaction. Namely, it is not understandable because not only co-catalyst

effects but also Schottky barrier effect is included in the experimental results. In addition,

it also becomes problem that a reverse reaction of water splitting such as H2–O2

80

recombination to form H2O and photoreduction of O2 simultaneously induced in the

semiconductor particles system because the co-catalysts also promote the reduction of the

evolved O2. Therefore, the evaluation of co-catalyst effect for H2 evolution might not be

simple. In the previous chapter, a facile artificial photosynthesis system using the In/Ga

alloy to eliminate the Schottky barrier was described.[11]

The evolution of H2 and O2 has

been successfully separated according to using two sides of the same SrTiO3 substrate

which work as cathode and anode, respectively.

Liao et al. reported that cobalt oxide (CoO) nanoparticles can carry out overall water

splitting with a solar-to-hydrogen efficiency of around 5%.[12]

However, the H2 and O2

were evolved at the same site of the identical nanoparticle material. Hence, the undesired

recombination between H2 and O2 was thus inevitable. Luo et al. described a low-cost

water splitting cell combining a solution-processed perovskite tandem solar cell and a

bifunctional earth-abundant catalyst with a solar-to-hydrogen efficiency of 12.3%.[13]

Despite the exceptionally high efficiency, external electric bias was constantly required in

the water splitting process.

Herein, it is proposed that the stoichiometric evolution of H2 and O2 can be realized

by using two sides of the same semiconductor substrate without any external electric bias.

Chemical potential was employed by adjusting the pH value difference between the two

reaction chambers, in order to substitute the conventional electrical potential as a driving

force to promote the water splitting reaction.

In this chapter, the plasmon-induced water splitting system that responds to visible

light using Au-NPs loaded SrTiO3 single crystal substrate is presented. Electron transfer

from the semiconductor material to the cathode was facilitated by the Ohmic contact,

which was formed by In/Ga alloy paste. Noble metal and metal oxide deposited on Pt

board as H2 evolution co-catalyst were characterized and the photocatalytic activity of

which were evaluated. With the O2 evolution side architecture remained the same

(Au-NPs loaded SrTiO3), both the evolution yields of H2 and O2 were improved by all

kinds of co-catalysts deposited on the H2 evolution side. In this chapter, the performance

81

of H2 evolution was explored using noble metal such as Rhodium (Rh) and Ruthenium

(Ru) or their metal oxide as a co-catalyst in the plasmon-induced water splitting system,

among which the Rh showed the best enhancement as 3-fold. It is thus speculated that the

co-catalysts accelerated the electron transfer and its reaction with protons to form H2

evolution. The recombination of electrons and holes was reduced accordingly, resulted in

enhanced yields of both H2 and O2 evolution. Furthermore, the evaluation of various kinds

of co-catalysts with the effects of Schottky barrier excluded became realizable since the

successfully formation of the ohmic contact between the semiconductor material and the

H2 evolution co-catalyst. Moreover, H2 and O2 evolutions were performed in different

chambers with cathode and anode, respectively. Au-NPs side works as anode for O2

evolution and Pt board with metal or metal oxide co-catalyst side works cathode for H2

evolution. Therefore, O2 and H2 were evolved from the surface and the back of the same

SrTiO3 substrate according to separate the evolved H2 and O2 as the result. The separation

of H2 and O2 evolution in different chambers was supposed to inhibit the backward

reactions so as to enhance the water splitting efficiency. According to eliminate the

Schottky barrier effect and avoid a reverse reaction of water splitting, it becomes possible

to evaluate co-catalyst effects on H2 evolution.

82

3.3 Experimental details

3.3.1 Preparation of noble metal and metal oxide co-catalyst

Pt thin film was sputtered on the back side of the SrTiO3 using Super Fine Coater

ESC-101 (ELIONIX, Japan) with a deposition rate of 0.2 Å/s. The Ru and Rh species thin

films were deposited onto the Pt substrate (10 × 10 × 0.2 mm3, 99.98%, Nilaco

Corporation) by a co-sputtering system (ACS-4000-C3-HS, ULVAC Co.), and the distance

from the target to the substrate was maintained at 150 mm. The targets were ruthenium

and rhodium metal with a purity of 99.95%, respectively. For the Ru and Rh oxide thin

film deposition, a mixture of Ar and O2 flow was maintained during the sputtering process.

Whereas for the metallic Ru and Rh fabrication, an Ar gas flow with a rate of 29 sccm was

induced in the absence of O2 flow. The deposition pressure was kept at 2.9×10−1

Pa. The

substrate was without intentional heating during the deposition processes. The Pt board

with noble metal or metal oxide thin film on the surface was adhered to the backside of the

Nb-SrTiO3 substrate via Ag paste (D-550, Fujikura Kasei). The X-ray photoemission

spectrum (XPS) characterization was performed by X-ray photoemission spectrometer

(JPS-9200, JEOL, Japan). The center part with 3 mm × 3 mm area of the substrate surface

was used for XPS characterization. The AlKa line was used as the X-ray source. The C 1s

signal was used as an internal reference (284.6 eV). The cross sectional structure and the

elemental distribution of the thin films on the Pt board were analyzed by using

transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS)

on JEOL JEM-ARM 200F with aberration correction.

3.3.2 Electrochemical measurements

Bare Pt board and Pt boards with 2 nm Ru or Rh thin film prepared by the sputtering

process were used as the working electrodes. Ag paste (D-550, Fujikura Kasei) film was

pasted on the back of the Pt substrate and connected to the electrochemical analyzer

(ALS/CH Instruments 852C, ALS) with a lead wire. A platinum wire and a saturated

83

calomel electrode (SCE) were used as the counter electrode and reference electrode,

respectively. The electrochemical properties of the electrodes were measured in an

aqueous solution of 0.05 M K2SO4 adjusted to pH 1 with H2SO4 with continuous N2

bubbling.

84

3.4 Results and discussion

3.4.1 Elimination of the Schottky barrier effect

In this study, a 0.05 wt% niobium (Nb)-doped strontium titanate (Nb-SrTiO3) single

crystal with a bad gap of 3.2 eV was employed as a semiconductor photoelectrode for

efficient water splitting. The water splitting system comprises two reaction chambers

separated by the Au-NPs loaded Nb-SrTiO3 substrate, as shown in Figure 3.1. In order to

facilitate the water splitting reaction, a chemical bias was applied between the H2- and

O2-evolution chambers. Specifically, the pH was regulated using hydrochloric acid (HCl)

and potassium hydroxide (KOH) aqueous solutions with a concentration of both 0.1 M.

Hence, the pH value in the front side chamber (O2 evolution side) was regulated at 13

while that in the back side chamber (H2 evolution side) was set at 1 with alkaline and

acidic aqueous solutions, respectively. According to the Nernst equation, the relationship

between the redox potential (U0) and the pH value at 25ºC is given by:

U 0

= U 0

(pH = 0) − 0.059 × pH (3-1)

Therefore, the chemical bias applied between the two chambers was estimated to be 0.71

V.[14]

Figure 3.1 A schematic diagram illustrating the water splitting device using the

Au-NP-loaded Nb-SrTiO3 photoelectrode.

85

A φ6 mm2 area of the gold-nanostructured Nb-SrTiO3 substrate was irradiated by a

xenon lamp with a light intensity of 0.32 W/cm2 and a wavelength spectrally filtered from

550 nm to 650 nm as a visible light source in this experiment. The incident photon flux

was measured using spectroradiometer (MSR-7000N, Optoresearch Co.). Under the

irradiation of visible light to Au-NPs, plasmon-induced charge separation between

Au-NPs and Nb-SrTiO3, and subsequent water oxidation are supposed to take place at the

front side chamber, so as to promote the O2 evolution. Whereas in the back side chamber,

H2 evolution is expected on account of the reduction of protons by photogenerated

electrons injected into the conduction band of Nb-SrTiO3 at Pt surface used as a cathode

material. A conventional salt bridge with 2% of agar in weight ratio was employed to

maintain the charge balance between these compartments. It was clearly confirmed that

the pH value did not change before and after the irradiation in each experiment, the salt

bridge was thus verified to be effective.

Figure 3.2 A schematic illustration of the photoelectrode architecture for water splitting

using a Pt board (a) and a Pt thin film (b) as the hydrogen evolution site, and the

irradiation time dependence of H2 (solid circles) and O2 (open circles) evolution (c) by

86

which respectively. The red and blue colors correspond to the Pt board and Pt thin film,

respectively.

In order to demonstrate the effect of the Schottky barrier to the water splitting

activity in the present system, substrates with different cathode structure were fabricated.

Their architectures and the H2 and O2 evolution time dependence are shown in Figure 3.2

(a) and (b), respectively. For the contrastive structure, Pt thin film with a thickness of 1

nm was sputtered on the back side of the Nb-SrTiO3 substrate, as a substitute for the Pt

board.

From the separate experiment, it has been confirmed that water splitting efficiency

was best with a Pt thickness of 1 nm when Pt thin film was deposited by sputtering

directly on the SrTiO3 substrate. The averaged thickness of Pt film was controlled by

sputtering time and determined by an absolute calibration method. Therefore, the film

might be discontinuous because the film thickness was set to less than a grain size of 2 nm

in this Pt sputtering process.

The XPS spectra of the back side Nb-SrTiO3 surface before and after Pt nanoparticles

deposition are shown in Figure 3.3. Characterization of Pt nanoparticles was conducted by

X-ray photoemission spectrum (XPS, JPC-9010MC, JEOL). The AlKa line was used as

the X-ray source. The C 1s signal was used as an internal reference (284.6 eV).

Under the irradiated conditions, the evolution of H2 and O2 is linearly increased with

an irradiation time by both kinds of the structures. Stoichiometric evolution of H2 and O2

(2:1) was observed in both cases. Note that the H2 and O2 evolution by the photoelectrode

with Pt thin film are only half that of the one with Pt board stick on the Nb-SrTiO3 via

In/Ga alloy. Without Au-NPs decoration, it has been confirmed that neither H2 nor O2 was

evolved from a Nb-SrTiO3 substrate under the same irradiation conditions. No production

of H2 or O2 was observed by either photoelectrode architecture in the absence of

irradiation for up to 6 hours.

87

Figure 3.3 Partial XPS spectra of the Nb-SrTiO3 loaded with Pt nanoparticles. A

comparison of before and after the Pt nanoparticle deposition on the back side surface.

Figure 3.4 depicts the energy band alignment of the composite substrate structure. It

is hypothesized that an excited electron is transferred to the conduction band of SrTiO3

immediately following the inter- or intraband transition of the Au-NPs induced by the

plasmonically enhanced optical near-field or by hot electron transfer, thus leaving an

electron hole trapped at the surface states of the SrTiO3 near the Au/SrTiO3/water

interface. The trapped holes can subsequently induce the oxidation of hydroxyl ion or

water molecules efficiently via multi electron transfer.[15]

There is a potential gradient

between the two sides of the Nb-SrTiO3 substrate because a chemical bias was applied by

pH regulations. Under the influence of this potential gradient, the photogenerated

electrons injected into the conduction band of SrTiO3 transferred to the back side and

subsequently induced the reduction of proton at the Pt surface. With the Pt nanoparticles

on the back side of the Nb-SrTiO3, there is an undesired Schottky barrier between the

Nb-SrTiO3 and Pt, which hindered the electrons from transferring to the cathode, resulting

in the relatively lower H2 and O2 evolution efficiency. Whereas using In/Ga alloy to stick

the Pt board with the Nb-SrTiO3 substrate, an ohmic contact is very important for the H2

evolution in the plasmon-induced water splitting system, so as to facilitate the electron

transfer.

88

Figure 3.4 Energy band alignment of the plasmon-induced water splitting system with the

Au-NPs loaded SrTiO3 substrate using Pt nanoparticles (a) and a Pt board (b) as the

hydrogen evolution site respectively. UFB and U0 show the flat band potential of SrTiO3

and the redox potentials versus SHE (standard hydrogen electrode), respectively. The

symbols [−] and [+] indicate electrons and holes.

89

3.4.2 Characteristics of the Ru and Rh species thin film on Pt board

A water splitting system is commonly composed of a semiconductor material and

co-catalysts for H2 and O2 evolution, respectively. The photocatalytic activity of the

system is highly dependent on the co-catalysts and their properties. Hence, the evaluation

of different co-catalysts becomes a very important issue.[16, 17]

Based on the as mentioned

composite structure, the Schottky barrier between the co-catalyst and the semiconductor

could be eliminated, giving out the possibility to verify the performance of various kinds

of co-catalysts.

The ruthenium (Ru)[18, 19]

and rhodium (Rh)[20, 21]

species, which have been

considered to be excellent co-catalysts for H2 evolution, were employed in the present

study. A thin film of these two metals with a thickness of 2 or 3 nm was deposited by

sputtering on the Pt board and served as a co-catalyst for H2 evolution. By precisely

controlling the oxygen flow during the sputtering process, oxide of Ru and Rh species

were also prepared, respectively. The cross sectional view of a transmission electron

microscope (TEM) image of Rh thin film with a thickness of 3 nm deposited on the Pt

board by sputtering is shown in Figure 3.5(a) and its corresponding energy dispersive

X-ray spectrometry (EDS) image is shown in Figure 3.5(b). It was obviously confirmed

that a continuous Rh thin film with a thickness of 3 nm was formed on the Pt board

surface. On the other hand, the cross sectional view of TEM image of rhodium oxide with

a thickness of 4 nm and its corresponding EDS image are shown in Figure 3.5(c) and

3.5(d), respectively. Although images of the film with a thickness of 4 nm were

demonstrated here instead of 2 or 3 nm film for easier understanding by TEM observation,

importantly, rougher surface is clearly observed in the case of rhodium oxide film as

compared to the Rh film. The top view SEM images for Pt boards partially covered by

these thin films are shown in Figure 3.6. Due to the difference in the conductivity, all the

thin films show a relatively dark color compared with the bare Pt board.

90

Figure 3.5 Cross sectional view of transmission electron microscope images for (a) Rh, (c)

RhOx, (e) Ru and (g) RuOx, and their corresponding energy dispersive X-ray

spectrometry (EDS) images (b) Rh, (d) RhOx, (f) Ru and (h) RuOx thin films deposited on

the Pt board by sputtering. The signal of Rh/Ru and Pt in the EDS images is indicated by

green and blue colors, respectively. The scale bars represent 5 nm in the all images.

91

Figure 3.6 The scanning electron microscope images for (a) Ru, (b) RuOx, (c) Rh, and (d)

RhOx thin film deposited on the Pt board by compact sputtering. The scale bars represent

1 μm in the all images.

The XPS spectra of the metal and oxide of Ru and Rh are shown in Figure 3.7.

Without O2 flow during the sputtering process, the peak positions of the Ru species are at

461.3 and 483.5 eV for Ru 3p3/2 and 3p1/2, the peak positions of the Rh species are at 306.5

and 311.3 eV for Rh 3d5/2 and 3d3/2, which are all in good coincidence with the standard

values of the metallic species.[22]

Whereas with O2 flow during the sputtering process, the

as-deposited Ru and Rh species are clearly oxidized, but not as much as a RuO2 or Rh2O3

bulk reference. (462.7 and 484.8 eV correspond to the Ru4+

oxide RuO2, 308.4 and 313.2

eV correspond to the Rh3+

oxide Rh2O3). Accordingly, these oxides will be referred to as

RuOx and RhOx hereafter, respectively. Figure 3.8 depicts the thickness dependence of

partial XPS spectra for RuOx thin film deposited on the Pt board. With an increased RuOx

thickness, the corresponding XPS peak area ratio between Ru 3p and Pt 4f increase almost

linearly.

92

Figure 3.7 XPS spectra for (a) Ru 3p and (b) Rh 3d thin films sputtered with (red solid

line) and without (black solid line) O2 flow on the Pt board. The black and the red dashed

lines indicate the standard metal and oxidation state peak positions of Ru and Rh,

respectively.

Figure 3.8 (a) The thickness dependence of partial XPS spectra for RuOx thin film

deposited on the Pt board, the black, red, blue and green lines indicate the RuOx thickness

of 1, 2, 3 and 4 nm, respectively. (b) The corresponding XPS peak area ratio between Ru

3p and Pt 4f.

93

3.4.3 Water splitting performance using various H2 evolution co-catalyst

Water splitting was conducted with all kinds of the co-catalysts and with a bare Pt

board as a control experiment. The schematic illustration of the photoelectrode

architecture is shown in Figure 3.9(a). Figure 3.9(b) depicts the irradiation time

dependence of photocatalytic H2 and O2 evolution over 2 nm Rh on the Pt board.

Stoichiometric evolution of H2 and O2 (2:1) was still observed in the case of Rh

co-catalyst. This result indicated that noble metal deposited on Pt does not involve any

side reactions and works well as a co-catalyst for H2 evolution. Figure 3.9(b) also shows

the result of recycling reactions to verify the stability of the water splitting system.

Significantly, the reaction proceeded with no noticeable deactivation in the 4 rounds

recycling reactions with an irradiation time sums up to more than 80 h, demonstrating the

good stability of the present architecture. With the reaction time approached 6 hours, the

photocatalytic activity tends to be saturated. The increase of pressure due to the evolved

gas in the system seemed to suppress further evolution of H2 and O2. Hence, various kinds

of noble metal and metal oxide were used as H2 evolution co-catalyst for comparison and

the corresponding rates of H2 and O2 evolution is shown in Figure 3.9(c). The evolution of

both H2 and O2 is linearly increased with an irradiation time in all cases. The quantity of

H2 evolved from the surface of all kinds of reduction co-catalyst is twice that of O2

evolved from the Au nanostructured surface. Therefore, stoichiometric evolution of H2

and O2 was obviously demonstrated by various co-catalysts.

Compared with the bare Pt board, the H2 evolution rate has been enhanced by all

co-catalysts to varying degrees. These co-catalysts are supposed to facilitate the electron

transfer and its reaction with the protons in the acidic solution environment. The primary

role of the noble-metal or metal oxide thin film on the Pt board is to offer active sites for

H2 evolution reaction.[23]

Thus, all above-mentioned results indicate that the

Au-NPs/Nb-SrTiO3/In-Ga alloy/Pt/co-catalyst architecture is a promising candidate as

photocatalysts. Similar tendency was also observed using co-catalyst film with a thickness

of 3 nm.

94

Figure 3.9 (a) A schematic illustration of the photoelectrode architecture for the overall

water splitting using a Pt board decorated with various kinds of co-catalysts as the

hydrogen evolution site. (b) The recycling irradiation time dependence of H2 and O2

evolution by the present water splitting system using Rh deposited on the Pt board as the

H2 evolution co-catalyst with a thickness of 2 nm. (c) A comparison for the rates of H2

(pattern filled bars) and O2 evolution (open bars) by the water splitting system using a Pt

board decorated with various kinds of co-catalyst respectively.

The stability of the noble metal and metal oxide co-catalysts under visible light

irradiation was confirmed using XPS. Each of the Pt boards with co-catalysts after 24 h

light irradiation was collected and served for the measurements. Neither Ru 3p, Ru 3d, Rh

3p nor Rh 3d XPS spectra was changed after the irradiation by all kinds of the co-catalysts

(Figure 3.10). Therefore, the present noble metal or metal oxide on the Pt board was quite

stable under the present experimental conditions.

95

Figure 3.10 XPS spectra of Ru 3p and 3d for (a) Ru, (b) RuOx and Rh 3p and 3d for (c)

Rh, (d) RhOx thin film on the Pt board before (black) and after (red) water splitting

reaction for 24 h.

With the purpose of further confirming the function of the ohmic contact, as a

comparison, a RuOx thin film with a thickness of 2 nm used as the H2 evolution

co-catalyst was deposited directly on the back side of the Nb-SrTiO3 substrate in the

absence of the In/Ga alloy and Pt board. The composite substrate structure and the

corresponding time dependence of H2 and O2 evolution are shown in Figure 3.11. It was

confirmed that the water splitting yield markedly declined without the In/Ga alloy and Pt

board.

96

Figure 3.11 A schematic illustration of the photoelectrode architecture for water splitting

using RuOx thin film with a thickness of 2 nm deposited on the Pt board (a) and directly

on the Nb-SrTiO3 substrate (b) as the hydrogen evolution site, and the irradiation time

dependence of H2 (solid circles) and O2 (open circles) evolution (c) by which respectively.

The red and blue colors correspond to with and without the Pt board, respectively.

As have been reported by many researchers, some noble metals and metal oxides

deposited on a photocatalyst work as co-catalysts for H2 evolution even in this

plasmon-induced water splitting. Ideally, the reaction occurring on such co-catalysts

should be hydrogen evolution. In some cases, however, undesirable reverse reactions, such

as H2–O2 recombination to form H2O and photoreduction of O2, occasionally take place

on the surface of such co-catalysts, thereby hindering the forward reaction. To solve these

problems, Maeda et al. reported that using Cr2O3 shell, which is permeable to protons and

hydrogen but not oxygen, photodeposited on noble-metals or metal oxides to prevent the

reverse reactions.[24-26]

With the unique two chamber structure, the water splitting system

97

presented in this study is also capable to suppress the reverse reactions by the separate

evolution of H2 and O2. Furthermore, certain metal oxides, RuO2 for instance, can work as

both a water reduction and oxidation site during the water-splitting reaction.[27, 28]

The

separation of the H2 evolution co-catalyst from O2 evolution co-catalyst in two different

chambers with different solution environment provides a new approach to investigate

individual co-catalyst, excluding the influence from the other.

Cyclic voltammograms of the bare and Ru-, Rh-coated Pt electrodes obtained with

continuous N2 bubbling are shown in Figure 3.12. The currents indicating proton

adsorption/desorption and hydrogen evolution were observed at from -0.2 to -0.1 V and

-0.3 V, respectively, on all cases. For the Rh-coated Pt electrode, the current showed an

enhanced slope compared with the bare Pt board when the voltage tends to -0.3V,

indicating the H2 evolution had been kinetically accelerated by the Rh decoration.

However, the onset potential exhibited a negative shift with Rh thin film, revealing that it

is thermodynamically adverse to the proton reduction. Whereas in the case of the Ru

coating, on the contrary, the proton reduction and the H2 evolution had an advantage

thermodynamically, yet they suffered kinetically. Therefore, the higher yield of the H2

evolution obtained by the Rh as a co-catalyst is presumably ascribed to the interplay

between the above-mentioned two aspects. Furthermore, the work function of Rh (4.98 eV)

is higher than that of Ru (4.71 eV). The electron transfer from Pt (with a work function of

5.65 eV) to the H2 evolution co-catalyst might be thus preferable in case of Rh compared

with Ru. The same tendency has been reported previously.[29]

98

Figure 3.12 Cyclic voltammograms for bare and 2 nm Ru- and Rh-coated Pt electrodes in

0.05 M K2SO4 aqueous solution adjusted to pH 1 with H2SO4 under N2 bubbling (scan rate,

50 mV/s).

The relatively higher yield of the H2 evolution obtained by the Rh than the Ru as a

co-catalyst is presumably ascribed to the difference of the free energy of hydrogen

adsorption ∆GH. It was reported that this quantity is highly concerned with hydrogen

evolution activity for a wide variety of metals and alloys. Actually, it is reported that the

absolute value of ∆GH of Rh is lower than that of Ru from DFT calculations. This means

that the H2 evolution is preferable in the case of Rh co-catalysts as compared to Ru one.

As the result, rhodium thin layer with a thickness of 2 or 3 nm deposited on platinum

board exhibited relatively high performance as a 3-fold increment compared with the

absence of a metal or metal oxide co-catalyst thin layer. Furthermore, the co-catalyst

effects for H2 evolution in the plasmon-induced water splitting system have been

successfully evaluated according to eliminate the Schottky barrier effect and separate two

reaction chambers for hydrogen and oxygen evolutions to avoid a reverse reaction of

water splitting.

3.4.4 Dependence of co-catalyst thickness on water splitting activity

Among all the co-catalysts examined, Rh showed the highest yield. Therefore, Rh

was selected for the further investigation for the co-catalyst thickness dependence of the

H2 and O2 evolution efficiency. As depicted in Figure 3.13, stoichiometric evolution of H2

and O2 was clearly observed with Rh co-catalyst by all thicknesses. With the increment of

the Rh thickness, the rates of H2 and O2 evolution both increased up to 2 nm Rh thin film

thickness, beyond 3 nm they leveled off markedly and remained almost constant. A 3-fold

enhancement of the water splitting yield was obtained by the optimal Rh thickness,

compared with the absence of any co-catalyst. The relatively low H2 yield of the films

with a small quantity of Rh depositions is due to insufficient sites present to trap electrons.

99

On the other hand, the decrease in activity at higher Rh loadings is probably related to

excess coverage of the Rh nanoparticles deposited on the Pt board. Such excess loading

can cause an inner-filter effect, contributing to a decrease in activity.[30]

Figure 3.13 The rate of H2 and O2 evolution by the present water splitting system using

Rh thin film deposited on the Pt board as a H2 evolution co-catalyst with different

thickness.

100

3.5 Conclusions

In summary, a novel plasmon-induced water splitting system without an external

electrochemical apparatus was successfully developed. Using semiconductor single crystal

substrate to separate the two chambers for H2 and O2 evolution, the presented water

splitting system features a stable and simple architectural design. The separation of the

simultaneous evolution of H2 and O2 is supposed to suppress the recombination of the two

gases. The mechanism for the water splitting device is considered by plasmon-induced

charge separation at Au-NPs/SrTiO3 interface, thereby promoting water oxidation and

subsequent reduction of proton at the SrTiO3 backside. As a result of the field

enhancement by the LSPR near the Au-NPs, photoresponse has been expanded to the

visible region. In the present study, efforts have been focused on the co-catalyst effect on

H2 evolution in the plasmon-induced water splitting. A plasmon-induced water splitting

with noble-metal or metal oxide as a co-catalyst for H2 evolution without the Schottky

barrier effect and a reverse reaction of water splitting was successfully developed. The

Schottky barrier between the Pt cathode and the semiconductor material was successfully

removed by employing the In/Ga alloy paste. Thereupon, the accessible evaluation of

different kinds of co-catalysts would be possible due to the ohmic contact. With

noble-metal or metal oxide decorated on the Pt board as a H2 evolution co-catalyst,

stoichiometric evolution of H2 and O2 was observed in all cases, the yields of water

splitting has been enhanced to different levels. Among all the co-catalysts tested, metallic

Rh gives the greatest improvement of the efficiency as 3-fold compared with the absence

of any co-catalyst. The activity also shows a thickness dependence on the co-catalyst. In

addition, the plasmon-induced water splitting system with a noble metal as a co-catalyst is

highly stable with an irradiation time sums up to more than 80 h. It is of significance to

understand and to evaluate the roles of co-catalysts in photocatalytic reactions. The

present study has paved a new avenue of utilizing various co-catalysts in the design of

new and efficient systems for plasmon-induced water splitting.

101

3.6 References

[1]. Bard, A. J. and Fox, M. A., Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen.

Accounts of Chemical Research 1995, 28 (3), 141-145.

[2]. Fujishima, A. and Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238 (5358), 37-38.

[3]. Chen, X. and Burda, C., The Electronic Origin of the Visible-Light Absorption Properties of C-, N- and

S-Doped TiO2 Nanomaterials. Journal of The American Chemical Society 2008, 130 (15), 5018-5019.

[4]. Petrik, N. G., Zhang, Z., Du, Y., Dohnálek, Z., Lyubinetsky, I. and Kimmel, G. A., Chemical Reactivity

of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption. The Journal of

Physical Chemistry C 2009, 113 (28), 12407-12411.

[5]. Grätzel, M., Dye-sensitized solar cells. Journal of Photochemistry and Photobiology C: Photochemistry

Reviews 2003, 4 (2), 145-153.

[6]. Chen, H. M., Chen, C. K., Chen, C.-J., Cheng, L.-C., Wu, P. C., Cheng, B. H., Ho, Y. Z., Tseng, M. L.,

Hsu, Y.-Y., Chan, T.-S., Lee, J.-F., Liu, R.-S. and Tsai, D. P., Plasmon Inducing Effects for Enhanced

Photoelectrochemical Water Splitting: X-ray Absorption Approach to Electronic Structures. Acs Nano 2012,

6 (8), 7362-7372.

[7]. Warren, S. C. and Thimsen, E., Plasmonic solar water splitting. Energy & Environmental Science 2012,

5 (1), 5133-5146.

[8]. Shi, X., Ueno, K., Oshikiri, T. and Misawa, H., Improvement of Plasmon-Enhanced Photocurrent

Generation by Interference of TiO2 Thin Film. Journal of Physical Chemistry C 2013, 117 (47),

24733-24739.

[9]. Wang, D., Hisatomi, T., Takata, T., Pan, C., Katayama, M., Kubota, J. and Domen, K., Core/Shell

Photocatalyst with Spatially Separated Co-Catalysts for Efficient Reduction and Oxidation of Water.

Angewandte Chemie International Edition 2013, 52 (43), 11252-11256.

[10]. Ma, S. S. K., Maeda, K., Abe, R. and Domen, K., Visible-light-driven nonsacrificial water oxidation

over tungsten trioxide powder modified with two different cocatalysts. Energy & Environmental Science

2012, 5 (8), 8390-8397.

[11]. Zhong, Y., Ueno, K., Mori, Y., Shi, X., Oshikiri, T., Murakoshi, K., Inoue, H. and Misawa, H.,

Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate:

Conversion of Visible Light to Chemical Energy. Angewandte Chemie International Edition 2014, 53 (39),

10350-10354.

102

[12]. Liao, L. B., Zhang, Q. H., Su, Z. H., Zhao, Z. Z., Wang, Y. N., Li, Y., Lu, X. X., Wei, D. G., Feng, G.

Y., Yu, Q. K., Cai, X. J., Zhao, J. M., Ren, Z. F., Fang, H., Robles-Hernandez, F., Baldelli, S. and Bao, J. M.,

Efficient solar water-splitting using a nanocrystalline CoO photocatalyst. Nature Nanotechnology 2014, 9 (1),

69-73.

[13]. Luo, J., Im, J.-H., Mayer, M. T., Schreier, M., Nazeeruddin, M. K., Park, N.-G., Tilley, S. D., Fan, H. J.

and Grätzel, M., Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant

catalysts. Science 2014, 345 (6204), 1593-1596.

[14]. Zaban, A., Ferrere, S., Sprague, J. and Gregg, B. A., pH-Dependent Redox Potential Induced in a

Sensitizing Dye by Adsorption onto TiO2. The Journal of Physical Chemistry B 1997, 101 (1), 55-57.

[15]. Thomann, I., Pinaud, B. A., Chen, Z. B., Clemens, B. M., Jaramillo, T. F. and Brongersma, M. L.,

Plasmon Enhanced Solar-to-Fuel Energy Conversion. Nano letters 2011, 11 (8), 3440-3446.

[16]. Maeda, K., Xiong, A. K., Yoshinaga, T., Ikeda, T., Sakamoto, N., Hisatomi, T., Takashima, M., Lu, D.

L., Kanehara, M., Setoyama, T., Teranishi, T. and Domen, K., Photocatalytic Overall Water Splitting

Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution under Visible Light.

Angewandte Chemie-international Edition 2010, 49 (24), 4096-4099.

[17]. Yang, J., Wang, D., Han, H. and Li, C., Roles of Cocatalysts in Photocatalysis and

Photoelectrocatalysis. Accounts of Chemical Research 2013, 46 (8), 1900-1909.

[18]. Sato, J., Saito, N., Yamada, Y., Maeda, K., Takata, T., Kondo, J. N., Hara, M., Kobayashi, H., Domen,

K. and Inoue, Y., RuO2-Loaded β-Ge3N4 as a Non-Oxide Photocatalyst for Overall Water Splitting. Journal

of The American Chemical Society 2005, 127 (12), 4150-4151.

[19]. Liu, H., Yuan, J., Shangguan, W. and Teraoka, Y., Visible-Light-Responding BiYWO6 Solid Solution

for Stoichiometric Photocatalytic Water Splitting. The Journal of Physical Chemistry C 2008, 112 (23),

8521-8523.

[20]. Hata, H., Kobayashi, Y., Bojan, V., Youngblood, W. J. and Mallouk, T. E., Direct Deposition of

Trivalent Rhodium Hydroxide Nanoparticles onto a Semiconducting Layered Calcium Niobate for

Photocatalytic Hydrogen Evolution. Nano letters 2008, 8 (3), 794-799.

[21]. Maeda, K., Masuda, H. and Domen, K., Effect of electrolyte addition on activity of (Ga1−xZnx)(N1−xOx)

photocatalyst for overall water splitting under visible light. Catalysis Today 2009, 147 (3–4), 173-178.

[22]. Kezilebieke, S., Ali, M., Shadeke, B. and Gunnella, R., Magnetic properties of ultrathin Ni81Fe19 films

with Ta and Ru capping layers. Journal of Physics-condensed Matter 2013, 25 (47), 476003.

[23]. Maeda, K., Lu, D. and Domen, K., Direct Water Splitting into Hydrogen and Oxygen under Visible

103

Light by using Modified TaON Photocatalysts with d0 Electronic Configuration. Chemistry – A European

Journal 2013, 19 (16), 4986-4991.

[24]. Sakamoto, N., Ohtsuka, H., Ikeda, T., Maeda, K., Lu, D., Kanehara, M., Teramura, K., Teranishi, T.

and Domen, K., Highly dispersed noble-metal/chromia (core/shell) nanoparticles as efficient hydrogen

evolution promoters for photocatalytic overall water splitting under visible light. Nanoscale 2009, 1 (1),

106-109.

[25]. Maeda, K., Sakamoto, N., Ikeda, T., Ohtsuka, H., Xiong, A., Lu, D., Kanehara, M., Teranishi, T. and

Domen, K., Preparation of Core–Shell-Structured Nanoparticles (with a Noble-Metal or Metal Oxide Core

and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light. Chemistry – A

European Journal 2010, 16 (26), 7750-7759.

[26]. Yoshida, M., Takanabe, K., Maeda, K., Ishikawa, A., Kubota, J., Sakata, Y., Ikezawa, Y. and Domen,

K., Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall

Water Splitting Studied by Model Electrodes. The Journal of Physical Chemistry C 2009, 113 (23),

10151-10157.

[27]. Lee, Y., Teramura, K., Hara, M. and Domen, K., Modification of (Zn1+xGe)(N2Ox) Solid Solution as a

Visible Light Driven Photocatalyst for Overall Water Splitting. Chemistry Of Materials 2007, 19 (8),

2120-2127.

[28]. Maeda, K., Abe, R. and Domen, K., Role and Function of Ruthenium Species as Promoters with

TaON-Based Photocatalysts for Oxygen Evolution in Two-Step Water Splitting under Visible Light. The

Journal of Physical Chemistry C 2011, 115 (7), 3057-3064.

[29]. Kitano, M., Tsujimaru, K. and Anpo, M., Decomposition of water in the separate evolution of

hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: Effect of the work

function of the substrates on the yield of the reaction. Applied Catalysis A-general 2006, 314 (2), 179-183.

[30]. Bao, N., Shen, L., Takata, T. and Domen, K., Self-Templated Synthesis of Nanoporous CdS

Nanostructures for Highly Efficient Photocatalytic Hydrogen Production under Visible Light. Chemistry of

Materials 2007, 20 (1), 110-117.

104

Chapter 4

Plasmon-Induced Water Splitting Utilizing Heterojunction

Synergistic Effect between SrTiO3 and Rutile TiO2

4.1 Abstract

A facile plasmon-enhanced water splitting system using gold nanostructured

strontium titanate (SrTiO3) single crystal photoelectrode with titanium dioxide (TiO2) thin

film heterojunction was developed. Simultaneous evolution of hydrogen (H2) and oxygen

(O2) in stoichiometric ratio was successfully observed in separate reaction chambers under

visible light irradiation. The substrate composed of single crystal SrTiO3 with the rutile

TiO2 thin film heterojunction exhibited enhanced water spitting activity compared with the

absence of TiO2 thin film because the back electron transfer reaction might be prevented

due to the synergistic effect. A thickness dependence of the Pt co-catalyst on the water

splitting activity is also discussed.

105

4.2 Introduction

The human demand for energy is increasing dramatically in recent years, while the

fossil fuel is diminishing and may exhaust in the next few decades. Moreover, we have

long been puzzled by environment problems caused by the usage of fossil fuel, e.g.

greenhouse gas emissions. Therefore, it is crucial to exploit a new energy source to replace

the traditional one. Among all the novel energy considered and explored, solar energy

draws much attention since it is clean, abundant, widespread, and inexhaustible. Because

fuel cells can convert hydrogen to electrical energy with high efficiency, hydrogen serves

as an important chemical energy storage material. The use of hydrogen has been restricted

to industrial applications until now, it is currently being expanded for use as a new energy

source.

The construction of a light-energy conversion system that uses sunlight for

renewable energy with high efficiency is required for the realization of a low-carbon

society. Artificial photosynthesis, which stores solar energy as a stable chemical and

enables energy use when required, is an important subject. It has attracted attention as a

method for storing useful chemical energy, as the form of hydrogen, without any

environmental impact. Because the required components for hydrogen generation are only

water and sunlight, these energy storage products can be generated using reactants that are

virtually infinitely available on Earth. The construction of an artificial photosynthesis

system that is sensitive to light energy from ultraviolet to near-infrared wavelengths is

indispensable for enhancing the solar energy conversion efficiency of artificial

photosynthesis.

H2 evolution via a photoelectrochemical water splitting using solar light irradiation

has been extensively studied ever since the pioneering investigation of Honda and

Fujishima in 1971.[1, 2]

Since then, water splitting systems that respond to visible light

have been developed according to doping the semiconductor materials with metal ion,[3, 4]

utilizing a Z-scheme process with a combination of two kinds of visible wavelength

semiconductors via redox mediator,[5]

and using dye or plasmon sensitization.[6]

106

A plasmon-assisted water splitting system using two sides of the same SrTiO3

single crystal substrate with Au-NPs on the front side and a platinum (Pt) board with

ohmic contact on the back side was successfully demonstrated in the previous chapter. In

this system, it is considered that an excited electron is transferred to the conduction band

of SrTiO3 following the inter- or intraband transition of the Au-NPs induced by the

plasmonically enhanced optical near-field. Thus the electron migrates to the back side of

the SrTiO3 substrate and reduces protons on the Pt co-catalyst surface for H2 evolution. On

the other hand, a hole in the Au-NPs might be trapped on the surface state of the SrTiO3

near the Au/SrTiO3/water interface, and the trapped holes can subsequently induce the

oxidation of hydroxyl ions or water molecules efficiently via multi electron transfer for O2

evolution.[7]

On despite of the progress made in recent few years, the relatively low efficiency

still hinders the application of the plasmon-assisted water splitting system. One of the

conventional problems lies in the rapid recombination between electron and hole after the

charge separation. The unidirectional transportation of electrons utilizing two types of

semiconductors with different conduction band energies is thus preferable for use in

photoelectrochemical water splitting systems so as to prevent the backward electron

transfer.

It is known that a mixture of anatase/rutile TiO2 materials have been reported to

exhibit advanced performance for photocatalytic reactions.[8-11]

It is considered that the

mechanism was ascribed to the different energy band structures between anatase and rutile

TiO2. It is commonly known that SrTiO3 shares similar energy band structure with anatase

TiO2.[12]

Hence, it is of significance to elucidate the synergistic effect of the combination

of SrTiO3 and rutile or anatase TiO2 on the plasmon-induced water splitting system. In this

chapter, an improvement of plasmon-enhanced water splitting using Au-NPs loaded

SrTiO3 single crystal photoelectrode with a TiO2 thin film heterojunction is explored. The

H2 evolution co-catalyst thickness dependence of the water splitting yields is investigated.

The role of semiconductor materials with different energy band structure is also discussed.

107

4.3 Experimental details

Single-crystal strontium titanate (SrTiO3, 0.05wt% niobium doped, 10 × 10 × 0.5

mm3, Furuuchi Chemical Co.) with a (110) surface was used as a semiconductor substrate

for water splitting in this study. A thin gold film with a thickness of 3 nm was deposited

onto the front side of the SrTiO3 by helicon sputtering (MPS-4000, ULVAC Co.) at a

deposition rate of 1 Å/s. The substrate was subsequently annealed at a temperature of

800°C for 1 h in a nitrogen atmosphere. The thin film was transformed to discontinuous

gold nanoparticles (Au-NPs) after annealing. On the other hand, as a H2 evolution

co-catalyst, a Pt thin film was subsequently sputtered by an ESC-101 Super Fine Coater

(ELIONIX Co.) on the back side of the SrTiO3 substrate.

4.3.1 Preparation and characterization of the TiO2 thin film

Atomic layer deposition (ALD) is a method for depositing thin films conformally on

substrates with uniform thickness. It is a self-limiting process in which the film thickness

can be accurately controlled by reaction cycles. (Figure 4.1 shows a reaction cycle for a

layer of TiO2 growth.) The low deposition temperature and purging procedure make ALD

advantageous in preventing substrates from other unnecessary side reactions. It is

therefore very useful for growing oxides (e.g. TiO2 and Al2O3), nitrides (e.g. TiN and TaN),

and high-k dielectrics for electronic applications. This method was adopted to grow

heterojunction nanomaterials and fabricate electronic devices.

Titanium dioxide thin films with different thicknesses were deposited onto the

SrTiO3 substrate using a commercial hot-wall flow-type ALD reactor (SUNALETM R

series Atomic Layer Deposition reactors, Picosun, Espoo, Finland). The deposition

procedure involved alternating exposure of TiCl4 and deionized water vapor at a process

temperature of 300 °C with N2 as a precursor carrier and purge gas at a pressure of 1.6 kPa.

The pulse and purge times for the precursors were 0.1 and 4 s, respectively. The

deposition rate of TiO2 on the SrTiO3 substrate was estimated to be 0.41 Å per cycle. To

108

fabricate uniform, one sided deposition TiO2 films, the SrTiO3 substrate was tightly

attached to a smooth cover glass, which was washed with deionized water in an ultrasonic

bath for 5 min. The TiO2 film thickness was estimated from the interference oscillations in

the visible spectra using an optical spectroscopic reflectometer (F20, Filmetrics, U.S.A.)

with a lower bound accuracy of ±2 nm, and the difference between film thicknesses is ±4

nm.

The reaction process is

1. (-OH) (s) + TiCl4 (g) → (-O-)TiCl3 (s) + HCl (g) (adsorption) (4-1)

2. (-O-)TiCl3 (s) + 2H2O (g) → (-O-)TiO(OH) (s) + 3HCl (g) (reaction) (4-2)

Figure 4.1 Reaction cycle for the growth of one layer of TiO2 on the SrTiO3 substrate by

atomic layer deposition.

The X-ray diffraction spectra of the TiO2 thin films were recorded with a RIGAKU

RINT-2000/PC using Cu Kα radiation and a scanning speed of 2° (2θ)/min within the 2θ

range from 20 to 80°. Characterization of the TiO2 thin film was also conducted by X-ray

photoemission spectrum (XPS, JPC-9010MC, JEOL). The AlKa line was used as the

X-ray source. The C 1s signal was used as an internal reference (284.6 eV).

109

4.4 Results and discussion

4.4.1 Dependence of Pt co-catalyst thickness on water splitting activity

The plasmon-induced water splitting system was constructed as a sealed reaction cell

with two chambers, which were separated by the SrTiO3 substrate as shown in Figure

4.2.[7, 13]

The Au-NPs decorated surface was on the front side, which was irradiated by

visible light, whereas the surface with Pt thin film was on the back side. The pH values for

the front and back side chambers were adjusted to 13 and 1 using aqueous potassium

hydroxide (KOH) and hydrochloric acid (HCl) solutions respectively. These two reactions

were supposed to take place in the front and back side respectively:

4h++4OH

- → O2 + 2H2O (4-3)

2e- + 2H

+ → H2 (4-4)

The acidic and alkaline conditions were engaged as a chemical driving force to

promote these two reactions.[14-17]

To maintain the charge balance between the two

chambers, a conventional salt bridge with 2wt% agar was employed. The pH values for

both chambers were confirmed to be unchanged before and after each irradiation

experiment.

Figure 4.2 (a) A schematic illustration of a plasmon-induced water splitting system using

an Au-NPs loaded SrTiO3 substrate with a rutile TiO2 layer.

110

As shown in Figure 4.3, plasmonically-enhanced optical near-field promotes a hot

electron transfer from Au-NPs to the conduction band of SrTiO3, and namely

plasmon-induced charge separation is induced as the result.[18, 19]

The Pt on the back side

of the SrTiO3 is supposed to act as an electron trap site and co-catalyst for the H2 evolution.

The holes remaining in the Au-NPs, which might be trapped in the surface states of SrTiO3

near the Au/SrTiO3/water interface, induce the oxidation of water molecules and hydroxyl

ions to evolve O2.[20, 21]

According to the extinction spectrum of the Au-NPs on the SrTiO3 substrate, which

clearly show a localized surface plasmon resonance band. The samples were irradiated by

a xenon light spectrally filtered over the wavelength range of 550 nm to 650 nm with an

intensity of 0.32 W/cm2.

Figure 4.3 Energy band diagram of the plasmon-induced water splitting system using an

Au-NPs loaded SrTiO3 substrate with and without the rutile TiO2 layer. The symbols [−]

and [+] indicate electrons and holes, respectively.

First of all, the effect on the H2 evolution efficiency of the thickness of the Pt

co-catalyst film was explored in the plasmon-induced water splitting system. Samples

111

with different Pt thickness were fabricated, and their performance of the water splitting

was evaluated.

In this experiment, the average thickness of the Pt film was controlled by sputtering

time and was determined by an absolute calibration method. Therefore, there is a

possibility that the film may become discontinuous if the film thickness was set to less

than the grain size in Pt sputtering.[22]

Figure 4.4(a) depicts the irradiation time

dependence of the H2 and O2 evolution by the water splitting system with a Pt thickness

of 1 nm as a typical example. The evolution of both H2 and O2 increased almost linearly

with the extended irradiation time. Furthermore, the amount of evolved H2 was

approximately twice that of the evolved O2, demonstrating a stoichiometric ratio from

successful overall water splitting in all Pt thickness cases. In a separate experiment, it

was confirmed that neither H2 nor O2 was evolved from a SrTiO3 substrate without

Au-NPs under the same irradiation conditions. No production of H2 or O2 was observed

in the absence of irradiation over an extended period as long as 6 h, even with the

Au-NPs loaded on the SrTiO3 substrates. The water splitting activity by various Pt

co-catalyst loading amount, represented by the H2 evolution rates, is summarized in

Figure 4.4(b). The samples with Pt co-catalyst decoration all exhibited superior water

splitting performance compared with that in the absence of any co-catalyst. The activity

increased with Pt loading to a maximum at a thickness of 1 nm, above which the activity

decreased. The relatively lower H2 evolution efficiency with less Pt depositions is

considered to be attributed to the lower density of Pt particle grains based on the

insufficient sites for electron trapping because that the grain size in Pt sputtering is

estimated to be ∼2 nm using this apparatus, and a significantly thinner film is expected to

be discontinuous. While on the other hand, with increased Pt depositions, the active

surface sites and the surface area of the Pt will also decrease because the film gradually

becomes continuous, and hence resulted in lower water splitting performance. The Pt thin

film thickness was thus fixed as 1 nm in the following experiments.

112

Figure 4.4 (a) The irradiation time dependence of H2 and O2 evolution by the water

splitting system with a Pt co-catalyst thickness of 1 nm. (b) The Pt co-catalyst thickness

dependence of the H2 evolution rate.

The function of the Pt co-catalyst is speculated to be twofold. Firstly, it is capable to

trap the electrons transferred from the semiconductor substrate. The recombination

between electrons and holes is thus retarded. Secondly, the following three reactions are

supposed to take place in sequence during the H2 evolution process at the back side

chamber of the water splitting system:

Volmer reaction: e- + H

+ + Pt → Pt-H (4-5)

Heyrovsky reaction: e- + H

+ + Pt-H → H2 + Pt (4-6)

Tafel reaction: Pt-H + Pt-H → H2 + 2Pt (4-7)

The Pt-H represents the hydrogen atoms adsorbed on the Pt surface. The Volmer

reaction, the initial discharge of protons to give adsorbed hydrogen atom, is followed by

combination of an adsorbed hydrogen atom with a proton in the solution and an electron

to give H2 (Heyrovsky reaction) or the combination of two adsorbed hydrogen atoms

(Tafel reaction). Generally, with Pt as the cathode material, Tafel reaction would be the

rate determining step on account of its very small overpotential for the H2 evolution.

Whereas in the present study, it has been demonstrated that the proton concentration

greatly affects the H2 evolution. Detailed investigation is currently in process.

113

The Pt thin film works as a catalyst providing reaction site for the electrons and

protons to generate hydrogen radicals. Instead of diffusing into the solution, these

hydrogen radicals adsorbed at the surface of the Pt co-catalyst. Subsequently, two

hydrogen radicals combined and formed one hydrogen molecule evolution. Normally, the

backward reaction of the hydrogen radical formation would also occur. However, the

presence of the Pt functioned as a catalytic center effectively hindering this backward

reaction.

4.4.2 Characteristics of the TiO2 thin film

A TiO2 thin film with a thickness of 300 nm was deposited on the back side of a

SrTiO3 substrate by atomic layer deposition (SUNALETM R series, Picosun Oy.) prior to

the Pt decoration. The samples were subsequently annealed at different temperatures for 1

hour. The XRD investigation, as shown in Figure 4.5, indicates that the crystallographic

phase of the TiO2 thin film is anatase-type after annealing at 800°C, whereas it

transformed to rutile-type by increasing the annealing temperature to 1000°C.[23-26]

Figure 4.5 X-ray diffraction patterns of bare SrTiO3 (black line) and SrTiO3 substrates

with a TiO2 thin film annealed at 800°C (red line) and 1000°C (blue line), respectively.

114

Red letters and blue letters indicate facets derived from anatase and rutile TiO2,

respectively.

The surface element composition and chemical state of the sample were further

confirmed by XPS results, and the corresponding experimental results are shown in Figure

4.6. The Ti 2p XPS spectrum of the TiO2 thin film on the SrTiO3 substrate shows two

major peaks with binding energies at 464.9 eV and 459.1 eV, corresponding to Ti 2p3/2

and Ti 2p1/2, respectively, which are the typical characteristic of a TiO2 phase. The

spectrum of O 1s of the TiO2 thin film on the SrTiO3 substrate is observed with a peak at

binding energy located at 530.4 eV. This energy peak corresponds to the coordination of

oxygen in Ti–O–Ti. Furthermore, the Sr 3d peaks were detected only on the surface of

bare SrTiO3 substrate in the absence of the TiO2 deposition. Therefore, the thin film

fabricated by atomic layer deposition on the SrTiO3 substrate was thus confirmed to be

TiO2.

115

Figure 4.6 XPS survey spectra of SrTiO3 substrate with and without the TiO2 thin film by

atomic layer deposition: (a) Ti 2p, (b) O 1s, (c) Sr 3d.

4.4.3 Water splitting performance by TiO2 with different crystalline phases

Figure 4.7(a) demonstrates the H2 and O2 evolution rates of water splitting systems

with anatase or rutile TiO2 layers were comparable to that without any TiO2 layer between

the SrTiO3 substrate and the Pt co-catalyst. The activity was increased with rutile layer

decoration. The enhancement of the water spitting activity using a rutile-TiO2 layer is

estimated to be 25%. This might be due to the conduction band of rutile, which is 0.2 eV

lower than that of SrTiO3. As illustrated in Figure 4.3, because of the pH difference, there

is a potential gradient between the O2 evolution and H2 evolution sides of the substrate,

under the influence of which the excited electrons are transferred from the Au-NPs to the

Pt co-catalyst, so as to accomplish the water splitting reaction. With the lower conduction

band position at the H2 evolution side, the electron transfer is supposed to be further

facilitated due to the greater conduction band slope as result in the prevention of back

electron transfer, despite H2 evolution being thermodynamically disfavored by the forming

of the rutile TiO2 layer.

On the other hand, in the case of the water splitting system with the TiO2 as

anatase-type, whose conduction band energy is almost same as that of SrTiO3, the activity

decreased compared to that without the TiO2 layer. The reason is speculated to be that the

recombination between electron and hole is aggravated in this anatase layer, on account of

the inevitable formation of the lattice defects during the atomic layer deposition process.

These defects could serve as a recombination center for electrons and holes.[27]

It is

noteworthy that in all cases of the composite substrate architectures described above for

water splitting, the front side where the oxidation reaction took place for O2 evolution

remained unchanged, i.e., Au-NPs/SrTiO3. However, the activities varied remarkably, with

H2 and O2 evolution maintaining a stoichiometric ratio, as the semiconductor material or

116

the morphology of the H2 evolution co-catalyst changed. These aspects are thus

considered to play a key role in the water splitting reaction.

In addition, the water splitting experiment using the present system with a pH

combination of 9 and 1 in the front and back chambers was explored. The H2 and O2

evolution rates are shown in the following Figure 4.7(b). With a reduced chemical bias as

compared to that with a pH combination of 13 and 1, the water splitting activities

decreased in all cases with a stoichiometric ratio between H2 and O2 evolution maintained.

However, the enhancement of the water spitting activity using a rutile-TiO2 under layer

with a pH combination of 9 and 1 is estimated to be 56 %, remarkably higher than that

with a pH combination of 13 and 1 (25 %). This result supports the speculated mechanism

that the rutile-TiO2 under layer enhances a charge separation by forming larger energy

band bending. Whereas the water spitting activity using the TiO2 under layer as

anatase-type with a lower chemical bias is further decreased compared with that without

any TiO2 layer between the SrTiO3 substrate and the Pt co-catalyst.

Figure 4.7 H2 and O2 evolution rates of water splitting systems with anatase or rutile

layers compared with that without a TiO2 layer. The pH value in the H2 evolution side is

fixed at 1. The pH value in the O2 evolution side is fixed at 13 (a) and 9 (b) respectively.

The pattern filled bars and open bars in the graph indicate the rate of H2 and O2 evolutions,

respectively.

117

The lattice constant of the anatase TiO2 is rather closer to that of the SrTiO3 than the

rutile TiO2. In the case of the thin film deposition on the single crystal SrTiO3 surface, it

has been reported that the anatase TiO2 is preferred to be formed as compared to the rutile

TiO2.[28]

The transition temperature of TiO2 thin film from anatase to rutile phase is as

high as 1000℃ in this experiment, indicating the good stability of the anatase TiO2 on

SrTiO3 compared with the rutile TiO2. Thus, from the view point of the crystallinity, the

water spitting activity of the system with an anatase TiO2 under layer should be better than

that with a rutile TiO2 under layer. However, in despite of these considerations, the rutile

TiO2 under layer showed superior water spitting activity based on our experiment results.

This enhanced water spitting activity is thus speculated to be ascribed to the enhanced

charge separation by forming larger energy band bending with the rutile-TiO2 under layer.

118

4.5 Conclusions

In summary, an almost stoichiometric evolution of H2 and O2 at separate sites have

been successfully demonstrated by decorating noble metal nanoparticles and thin films on

the two sides of the same single crystal SrTiO3 substrate. The quantity of the evolved H2

and O2 increased linearly with the irradiation time. The activity of the water splitting was

found to be highly dependent on the loading amount of the Pt, which was used as a H2

evolution co-catalyst. The optimal thickness of the Pt thin film was determined to be 1 nm.

The electron transfer from the oxidation reaction site to the reduction reaction site was

facilitated by the synergistic effect of SrTiO3 and rutile TiO2, due to the difference

between their energy band structures. The water splitting efficiency was thus enhanced to

a certain degree.

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4.6 References

[1]. Fujishima, A. and Honda, K., Electrochemical Evidence for the Mechanism of the Primary Stage of

Photosynthesis. Bulletin of The Chemical Society of Japan 1971, 44 (4), 1148-1150.

[2]. Fujishima, A. and Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238 (5358), 37-38.

[3]. Leung, D. Y. C., Fu, X., Wang, C., Ni, M., Leung, M. K. H., Wang, X. and Fu, X., Hydrogen Production

over Titania-Based Photocatalysts. ChemSusChem 2010, 3 (6), 681-694.

[4]. Ji, P., Takeuchi, M., Cuong, T.-M., Zhang, J., Matsuoka, M. and Anpo, M., Recent advances in visible

light-responsive titanium oxide-based photocatalysts. Research on Chemical Intermediates 2010, 36 (4),

327-347.

[5]. Higashi, M., Abe, R., Ishikawa, A., Takata, T., Ohtani, B. and Domen, K., Z-scheme overall water

splitting on modified-TaON photocatalysts under visible light (lambda < 500 nm). Chemistry Letters 2008,

37 (2), 138-139.

[6]. Kim, W., Tachikawa, T., Majima, T. and Choi, W., Photocatalysis of Dye-Sensitized TiO2 Nanoparticles

with Thin Overcoat of Al2O3: Enhanced Activity for H2 Production and Dechlorination of CCl4. The Journal

of Physical Chemistry C 2009, 113 (24), 10603-10609.

[7]. Zhong, Y., Ueno, K., Mori, Y., Shi, X., Oshikiri, T., Murakoshi, K., Inoue, H. and Misawa, H.,

Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate:

Conversion of Visible Light to Chemical Energy. Angewandte Chemie International Edition 2014, 53 (39),

10350-10354.

[8]. Abe, R., Sayama, K., Domen, K. and Arakawa, H., A new type of water splitting system composed of

two different TiO2 photocatalysts (anatase, rutile) and a IO3−

/I− shuttle redox mediator. Chemical Physics

Letters 2001, 344 (3–4), 339-344.

[9]. Kho, Y. K., Iwase, A., Teoh, W. Y., Madler, L., Kudo, A. and Amal, R., Photocatalytic H2 Evolution

over TiO2 Nanoparticles. The Synergistic Effect of Anatase and Rutile. The Journal of Physical Chemistry C

2010, 114 (6), 2821-2829.

[10]. Tsukamoto, D., Shiraishi, Y., Sugano, Y., Ichikawa, S., Tanaka, S. and Hirai, T., Gold Nanoparticles

Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic

Oxidation. Journal of The American Chemical Society 2012, 134 (14), 6309-6315.

[11]. Connelly, K., Wahab, A. K. and Idriss, H., Photoreaction of Au/TiO2 for hydrogen production from

renewables: a review on the synergistic effect between anatase and rutile phases of TiO2. Materials for

120

Renewable and Sustainable Energy 2012, 1 (1), 1-12.

[12]. Hou, W. B. and Cronin, S. B., A Review of Surface Plasmon Resonance-Enhanced Photocatalysis.

Advanced Functional Materials 2013, 23 (13), 1612-1619.

[13]. Oshikiri, T., Ueno, K. and Misawa, H., Plasmon-Induced Ammonia Synthesis through Nitrogen

Photofixation with Visible Light Irradiation. Angewandte Chemie International Edition 2014, 53 (37),

9802-9805.

[14]. Kitano, M., Takeuchi, M., Matsuoka, M., Thomas, J. A. and Anpo, M., Photocatalytic water splitting

using Pt-loaded visible light-responsive TiO2 thin film photocatalysts. Catalysis Today 2007, 120 (2),

133-138.

[15]. Matsuoka, M., Kitano, M., Takeuchi, M., Tsujimaru, K., Anpo, M. and Thomas, J. M., Photocatalysis

for new energy production - Recent advances in photocatalytic water splitting reactions for hydrogen

production. Catalysis Today 2007, 122 (1-2), 51-61.

[16]. Kitano, M., Iyatani, K., Tsujimaru, K., Matsuoka, M., Takeuchi, M., Ueshima, M., Thomas, J. M. and

Anpo, M., The effect of chemical etching by HF solution on the photocatalytic activity of visible

light-responsive TiO2 thin films for solar water splitting. Topics In Catalysis 2008, 49 (1-2), 24-31.

[17]. Kitano, M., Tsujimaru, K. and Anpo, M., Hydrogen production using highly active titanium

oxide-based photocatalysts. Topics In Catalysis 2008, 49 (1-2), 4-17.

[18]. Nishijima, Y., Ueno, K., Yokota, Y., Murakoshi, K. and Misawa, H., Plasmon-Assisted Photocurrent

Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. The Journal of

Physical Chemistry Letters 2010, 1 (13), 2031-2036.

[19]. Ueno, K. and Misawa, H., Plasmon-enhanced photocurrent generation and water oxidation from

visible to near-infrared wavelengths. Npg Asia Materials 2013, 5 e61.

[20]. Nishijima, Y., Ueno, K., Kotake, Y., Murakoshi, K., Inoue, H. and Misawa, H., Near-Infrared

Plasmon-Assisted Water Oxidation. Journal of Physical Chemistry Letters 2012, 3 (10), 1248-1252.

[21]. Shi, X., Ueno, K., Takabayashi, N. and Misawa, H., Plasmon-Enhanced Photocurrent Generation and

Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode. The Journal of Physical

Chemistry C 2013, 117 (6), 2494-2499.

[22]. Wang, W.-N., An, W.-J., Ramalingam, B., Mukherjee, S., Niedzwiedzki, D. M., Gangopadhyay, S. and

Biswas, P., Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine

Pt Nanoparticles on TiO2 Single Crystals. Journal of The American Chemical Society 2012, 134 (27),

11276-11281.

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[23]. Hanaor, D. A. H. and Sorrell, C. C., Review of the anatase to rutile phase transformation. Journal of

Materials Science 2011, 46 (4), 855-874.

[24]. Pavasupree, S., Suzuki, Y., Yoshikawa, S. and Kawahata, R., Synthesis of titanate, TiO2 (B), and

anatase TiO2 nanofibers from natural rutile sand. Journal of Solid State Chemistry 2005, 178 (10),

3110-3116.

[25]. Santos, J. G. d., Ogasawara, T. and Corrêa, R. A., Synthesis of mesoporous titania in rutile phase with

pore-stable structure. Brazilian Journal of Chemical Engineering 2009, 26 555-561.

[26]. Kumar, A., Jose, R., Fujihara, K., Wang, J. and Ramakrishna, S., Structural and Optical Properties of

Electrospun TiO2 Nanofibers. Chemistry of Materials 2007, 19 (26), 6536-6542.

[27]. Chen, X. B., Shen, S. H., Guo, L. J. and Mao, S. S., Semiconductor-based Photocatalytic Hydrogen

Generation. Chemical Reviews 2010, 110 (11), 6503-6570.

[28]. Silva, V. F., Bouquet, V., Deputier, S., Boursicot, S., Ollivier, S., Weber, I. T., Silva, V. L., Santos, I. M.

G., Guilloux-Viry, M. and Perrin, A., Substrate-controlled allotropic phases and growth orientation of TiO2

epitaxial thin films. Journal of Applied Crystallography 2010, 43 (6), 1502-1512.

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Chapter 5

Summary and Future Perspectives

5.1 Summary

Energy is at the heart of most critical economic, environmental and

developmental issues facing the world today. Clean, efficient, affordable and reliable

energy services are indispensable for global prosperity. This thesis describes our

efforts to develop a plasmon-assisted water splitting system under visible light

irradiation using gold nanostructured strontium titanate single crystals so as to address

the energy issues. This work defines a potential route for constructing efficient

composite photoelectrodes comprising a semiconductor substrate, a nanostructured

photo-sensitizer and a noble metal co-catalyst functioned for H2 evolution.

A novel water splitting system using two sides of the same SrTiO3 single crystal

substrate was designed and fabricated. The experimental apparatus has two reaction

chambers, separating the evolution of the H2 and O2. Numerical simulations indicated

that a plasmonic near field enhancement was formed at the interface between the

Au-NPs and the SrTiO3 single crystal substrates. Under the influence of this near field

enhancement, the optical absorption of the substrate was extended from UV light to

visible light region with a peak at 600 nm. In this water splitting system, unlike the

conventional ones, the reaction proceeded without any external electric bias. A

chemical bias formed by aqueous solutions with different pH values between the front

and back side of the composite photoelectrode was involved as a substitution. By

analyzing the action spectrum, the water splitting yields were found to be in good

consistent with the segmented integral area of the extinction spectrum of the Au-NPs.

H2 evolution from the water splitting is thus verified to be caused by the absorption of

the visible light. The pH value dependence of the water splitting activity was

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systematically investigated. It has been discovered that the efficient water oxidation

due to the plasmon effect reduced the minimum chemical bias that capable of driving

the visible-light response water splitting reaction to 0.23 V, which is the smallest

value ever reported so far. In addition, the stability of this water splitting system was

verified to be longer than 48 hours.

Based on the rational design of this reaction apparatus, the appropriate H2

evolution co-catalyst was explored in order to further increase the water splitting

activity. The schottky barrier was eliminated by using In/Ga alloy between the SrTiO3

substrate and the Pt board. Various noble metal and metal oxide thin film were

deposited on the Pt board as a H2 evolution co-catalyst, among which the metallic Rh

exhibited highest water splitting efficiency enhancement as 3 fold.

Besides the chemical bias regulation, semiconductor energy band engineering

was also taken into account in this thesis. Specifically, TiO2 thin films were deposited

on the SrTiO3 substrates as anatase and rutile crystal type by calcining in different

temperatures. The composite substrate with a rutile TiO2 under layer exhibited higher

H2 and O2 evolution rates because of its conduction band position difference with that

of the SrTiO3. The electron transfer through the substrate was thus facilitated and the

recombination between electrons and holes was hindered. Moreover, the size and

shape of the Pt, which was used as a H2 evolution co-catalyst, had a strong impact on

the activity of the water splitting reaction. The optimal thickness of the Pt thin film

was determined to be 1 nm.

In conclusion, it has been demonstrated in this thesis that a facile artificial

photosynthesis reaction system utilizing solar irradiation was successfully developed.

It is anticipated that the study presented here introduces new possibilities for efficient

and stable water splitting catalytic systems for clean fuel generation leading towards a

greener energy technology and sustainable future.

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5.2 Future Perspectives

On the basis of the results obtained in present study, the plasmon-enhanced water

splitting has been successfully demonstrated using solar light irradiation by Au-NPs

loaded strontium titanate single crystal substrates. The innovative design of the water

splitting system with only one substrate separating the H2 and O2 evolution sites

features some unique properties. However, the performances of these composite

photoelectrodes are still poor, and the active area for the solar light harvesting is

restricted to a very small range in the current stage. The present reaction system is

thus far away for large-scale production of water splitting. For the purpose of

large-scale application, several aspects should be considered.

The future outlook of this thesis study can be extended to:

(1) The water splitting reaction mechanism and interaction of the loaded co-catalyst

with the base semiconductor material is still not completely understood. A better

knowledge of this system is of course very useful in designing active photocatalysis

systems. Some attempts to investigate the mechanistic aspect on water splitting and

the co-catalyst/semiconductor interface by means of kinetic analysis, spectroscopic

and photoelectrochemical measurements, and computational approach have been

reported so far. Specifically, understanding the interfacial property between a

photocatalyst and the loaded cocatalyst would be the major challenge in this research

field.

(2) The extinction properties of the Au-NPs have already found to be highly

dependent on their size and shape. In the present study, visible irradiation with a

wavelength at ca. 600 nm was employed as a light source according to the extinction

spectrum. By varying the experimental conditions during the gold sputtering and

annealing process or using electron beam lithography technique to fabricate gold

nanostructure, an extended absorption to the longer wavelength (even in the infrared

region) could be expected. Solar energy conversion efficiency will be greatly

enhanced if we can make use of major part of the solar spectrum.

(3) In despite of the fact that the SrTiO3 substrate adopted in this study was single

125

crystal type, defects are inevitable with such a thickness. Recombination between

electrons and holes are likely to occur at these defects, the water splitting efficiency

would be suffered as a result. Therefore, thinner substrates are considered to be better

for avoiding losses due to electron trapping in the defect level. Furthermore, as a

substitution of the single crystal substrate, semiconductor thin films are recommended

to fabricate by atomic layer deposition or pulse laser deposition etc. so as to facilitate

the electron transfer.

(4) A practical evaluation method of the capability of co-catalyst has been presented

in this study. However, at the current status of the research, there are just a couple of

noble metal and metal oxides examined. Having a lot more co-catalyst materials

evaluated and finally constructing a co-catalyst library is of great guiding significance

for the artificial photosynthesis system design.

(5) While making hydrogen from water is important, it is not the only future target.

With the gold nanostructure as a light harvesting antenna, the present reaction system

is capable of charge separation and electron transportation driven by visible

irradiation. Multiple artificial photosynthesis reactions can be realized in the back side

chamber of the system. For instance, CO2 reduction into useful products and low

carbon fuels like methanol and formic acid, ammonia production by nitrogen gas

fixation, degradation of organic pollutants.

If these proposed goals are achieved, the present system could potentially be

used in constructing efficient and robust artificial photosynthesis systems that respond,

with lower chemical biases, to visible and near-infrared light.

The author believes that the present results are expected to give important

information for this research fields. Finally, the author would like to expect this

research field to be progressed more in the future.

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List of Publications

1. Y. Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H.

Misawa

“Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3

Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy”

Angew. Chem. Int. Ed., 2014, 53, 10350-10354.

2. Y. Zhong, K. Ueno, Y. Mori, T. Oshikiri, H. Misawa

“Co-catalyst Effects on Hydrogen Evolution in a Plasmon-Induced Water

Splitting System”

J. Phys. Chem. C, 2015, 119, 8889-8897.

3. Y. Zhong, K. Ueno, Y. Mori, T. Oshikiri, H. Misawa

“Plasmon-Enhanced Water Splitting Utilizing the Heterojunction Synergistic

Effect between SrTiO3 and Rutile-TiO2”

Chem. Lett., 2015, 44, 618-620.

4. K. Ueno, T. Oshikiri, X. Shi, Y. Zhong, H. Misawa

“Plasmon-Induced Artificial Photosynthesis”

Interface Focus, 2015, 5, DOI: 10.1098/rsfs.2014.0082.