氧化鋅奈米柱與奈米材料應用於光電化學分解水

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國立臺灣大化學研究所博士論文

Department of Chemistry College of Science

National Taiwan University Ph.D. Thesis

氧化鋅奈米柱與奈米材料應用於光電化學分解水 Nanomateriasl modified ZnO nanorod for

Photoelectrochemical Water Splitting

陳致凱 Chih Kai Chen

指導教授：劉如熹博士 Advisor: Ru-Shi Liu, Ph.D.

中華民國 103 年 6 月

June 2014

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謝誌

想當年從中央大學畢業，懵懵懂懂進入台大化學研究所，基於對材料化學具相當興趣而選擇了劉如熹老師的材料化學實驗室。剛進實驗室摸索，從燃料電池觸媒到光電化學分解水，越做越有興趣，因緣際會之下逕讀博士班。

回想過去五年在實驗室所遇到的人事物、學到的實驗與各種經驗，都讓我覺得這個博士班真的很值得。首先，感謝劉如熹老師，實驗室的大家長，從新生進來到現在，一直孜孜不倦地教導，不論實驗安排規劃、時間管理、做事態度、人際關係與禮儀各方面都指導許多，使我各方面有長足進步，越磨越成器。老師亦給予多次出國交流之機會，讓我在這五年中，不論是在實驗室交流與國際會議都有所獲益，足跡遍布大陸、日本、法國、英國、新加坡與美國等國家。在即將畢業之際，老師亦幫學生介紹工作真的很感謝老師的栽培、指導與幫忙。在進入實驗室之後，幫我最多的學長，陳浩銘，除了在實驗給予的指導，在食衣住行育樂方面也都給予了許多實質上的建議，讓我在實驗室的時光能較快的進入狀況，完成許多事情。歷屆以來的學長姐與學弟，

在實驗室不論實驗或事務上也給予許多幫忙。在這邊還要感謝大雄、智文、

景弘、柏源、毓娟、怡君、政屹、宏昌、美伶、巧雯、子晨、良謙、韋廷、

趴趴、孟修、致融、博翔、筱姍、玠瑋、宜庭、柏漢、岱穎、元騤、Mori、

國崗、依蓉等人，不論是在實驗、事務或是生活上給予的幫助。沒有他們的幫忙，博士論文必定不會順利完成。

最後，要感謝我的家人，爸、媽、姊姊與妹妹，在這五年期間給予我無限的支持，給予我很大的依靠，讓我可以無憂無慮的完成學業。

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I

摘要

石化能源為現今主要之能源來源，然其具儲量漸趨耗竭與伴隨環境汙染問題，發展永續且綠色之能源為各國首要目標。氫氣能量密度高，且由燃料電池能產生電力，具潔淨無汙染之優點。利用光電化學分解水產氫，為氫能利用發展最具潛力之方法。本研究以利用奈米材料設計之奈米微結構之光電極，提升光電化學分解水效率為主要目標。本研究以氧化鋅奈米柱陣列電極為基礎，利用一維之奈米結構，有效增加電子之傳導，提升光電極之光電化學分解水效率。

配合不同奈米材料，分成三部分探討奈米結構對光電化學分解水反應之影響。

第一部分為利用量子點敏化之氧化鋅奈米柱電極。氧化鋅為寬能隙之半導體材料，其僅可利用紫外光進行光電化學分解水反應。利用碲化鎘與磷化銦量子點調控粒徑大小改變吸收波長，達到有效吸收太陽光，以增加光電化學分解水效率之目的。除增加光吸收，藉由量子點與氧化鋅奈米柱形成之異質結 (heterojunction)幫助光生電子-電洞對之分離。第二部分為利用量子點搭配氧化鋅奈米柱形成固融體(solid solution)之結構。藉由量子點之奈米尺寸，有效地達到異原子(heteroatom)之遷移，成功地合成固融體結構，延伸光電極吸光範圍與光電轉換效率。第三部分為利用電漿(plasmon)於光電化學分解水。利用具奈米金修飾之氧化鋅奈米柱陣列為光電極為本實驗之平台，藉由一系列實驗所得之結果辨別電漿子效應所造成之影響。並證實此一提升機制源自於金屬奈米粒子產生之電漿誘導效應。此外，我們亦利用電漿效應提升上轉換奈米粒子之放光效率，利用上轉換奈米粒子、量子點與氧化鋅奈米柱形成之奈米複合系統，成功地利用紅外光進行光電化學分解水反應。

本研究成功利用奈米材料，設計不同奈米微結構之光電極，利用不同之策略，改進光電化學分解水效率，並探討不同策略對光電化學分解水反應之影響。

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II

Abstract

The fossil fuels drove the development of global economy, it needs to face the problem that source shortage and price and soars. Hydrogen is abundant, pollution-free, high energy density and high efficiency. The photoelectrochemical water splitting could convert the solar energy to hydrogen. Nanotechnology were employed to improve the conversion efficiency of the photoelectrochemical water splitting. In this study, we use the ZnO nanowire-array as the photoelectrode. The one-dimension nanostructure of the ZnO can be a potential solution for raising the light absorption, increasing the surface area and providing electron transport pathway to improve efficiency. Three main approaches will be employed to improve the performance of the photoelectrochemical water splitting. First, using the CdTe and InP quantum dots to sensitize the ZnO photoanode. The sensitization strategy could increase the absorbance range of the light and thus improve the photoactivity of the photoelectrode. Second, using doping strategies to narrow the bandgap of the ZnO. This solid solution structure can performance a better enhancement effect in photoactivity than traditional quantum dots sensitization, and also reveals the hydrogen generation. Third, using gold and silver nanomaterials to decorate on the ZnO to investigate the plamonic enhanced photoelectrochemical water splitting reaction. Both hot electron transfer and electromagnetic field are available for improving the capture of sunlight and collecting of charge carrier.

We had demonstrated that the nano-strucute controlled photoelectrochemical water splitting strategy could enhance the conversion efficiency of the photoelectrochemical water splitting, supporting a new approach to investigating the photoelectrochemical water splitting.

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Content

謝誌 ... I 摘要 ... I Abstract ... II

Content... i

Figure Caption... vi

Table Caption ... xviii

Chapter 1. Introduction ... 1

1.1 Sunlight Conversion Technologies... 2

1.1.1 Photovoltaic Cell ... 2

1.1.2 Thin-film Photovoltaic Cells ... 3

1.1.3 Wet-chemical Photosynthesis ... 3

1.1.4 Photoelectrolysis... 4

1.2 Photoelectrochemical Water Splitting ... 5

1.2.1 Photoelectrochemical Activity ... 10

1.3 Photoelectrode Materials ... 11

1.4 Strategies to Improve Photoelectrochemical Performance ... 18

1.4.1 Doping... 19

1.4.2 Sensitization ... 22

1.4.2.1. Dye Sensitization ... 23

1.4.2.2. Quantum Dot Sensitization ... 25

1.4.2.3 Upconversion Nanoparticle Sensitization ... 27

1.4.3 Plasmonic Association ... 28

1.5 Research Motivation ... 32

References (Chapter 1) ... 36

Chapter 2. Materials Preparation and Characterization Techniques ... 42

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2.1 Materials ... 43

2.2 Syntheses of Nano-materials ... 46

2.2.1 Synthesis of ZnO Seed on the Fluorine-doped Tin Oxide Coated Glass ... 46

2.2.2 Synthesis of the ZnO Nanorod ... 46

2.2.3 Synthesis of the CdTe Quantum Dots (QDs) ... 47

2.2.4 Synthesis of the InP Quantum Dots (QDs) ... 47

2.2.5 Synthesis of ZnS Quantum Dots (QDs) ... 49

2.2.6 Synthesis of the Gold Nanoparticles... 49

2.2.7 Synthesis of the Gold nanorods ... 50

2.2.8 Synthesis of the NaYF4 Upconversion Nanoparticles (UCNs) and Gold Nanoparticles-coated Upconversion Nanoparticles (Au-UCNs) ... 50

2.2.9 Synthesis of Nitrogen-doped ZnO (ZnO:N) Nanorods... 51

2.2.10 Fabrication of the ZnO@QDs, ZnO@Au Nanoparticles and ZnO@CdTe-Au- UCNs. ... 51

2.2.11 Fabrication of Water Splitting Photoelectrode ... 52

2.2.12 Characterization of Water Splitting Photoelectrode ... 52

2.3 The Instruments for Characterization ... 53

2.3.1 X-ray Diffraction (XRD) ... 53

2.3.2 Transmission Electron Microscopy (TEM)... 57

2.3.3 Scanning Electron Microscopy (SEM) ... 59

2.3.4 Ultraviolet-Vis (UV-Vis) Spectroscopy ... 62

2.3.5 X-ray Photoelectron Spectroscopy (XPS) ... 64

2.3.6 Photoluminescence (PL) ... 66

2.3.7 X-ray Absorption Spectroscopy (XAS) ... 68

2.3.7.1 The Absorption Edge ... 68

2.3.7.2 Pre-edge and X-ray Absorption Near Edge Spectroscopy (XANES) ... 70

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2.3.7.3 Extended X-ray Absorption Fine Structure (EXAFS) ... 71

2.3.8 Photoelectrochemical water splitting measurement ... 75

2.3.8.1 Two-electrode System ... 78

2.3.8.2 Three-electrode system ... 79

2.3.8.3 Solar Conversion Efficiency ... 80

2.3.8.4 Incident-Photon-to-Current-Conversion Efficiency (IPCE) ... 80

2.3.8.5 Gas evolution measurement ... 81

Chapter 3. Quantum Dots Sensitized ZnO Nanowire-array Photoelectrochemical Water Splitting ... 84

3.1 Introduction ... 84

3.2 Results and Discussion ... 90

3.2.1 ZnO@CdTe Photoelectrochemical Cell... 90

3.2.2 ZnO:N@CdTe Photoelectrochemical Cell ... 97

3.2.3 InP@ZnO Photoelectrochemical Cell ...109

3.3 Summary ... 126

Chapter 4. A New Approach to Solar Hydrogen Production: ZnO-ZnS Solid Solution Nanowire Arrays Photoanode ...132

4.2 Experimental Sections ... 134

4.2.1 Fabrication and Characterization of Photochemical Electrode ...134

4.2.2 EXAFS Data Analysis ...135

4.4 Summary ... 152

References (Chapter 4) ... 153 Chapter 5. Plasmon Inducing Effects for Enhanced Photoelectrochemical Water

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Splitting: X-ray Absorption Approach to Electronic Structures ...156

5.2 Experimental Section ... 159

5.2.1 Photoelectrochemical Characterization ...159

5.2.2 Simulation ...159

5.3.1 Plasmon-induced Hot Electron-hole Pairs ...170

5.3.2 Plasmon-induced Electromagnetic Field ...172

5.3.3 Plasmon-induced Heating ...175

5.4 Summary ... 180

Chapter 6. Plasmonic Photoelectrochemical System with Near-infrared Damping for Wild Absorption Solar Water Splitting ...185

6.2.1 Simulation ...191

6.3.1 Plasmonic photocathode characterization ...191

6.3.2 Plasmon damping for hot electrons ...196

6.3.3 Plasmon damping for electromagnetic field ...199

6.4 Conclusions ... 201

Chapter 7. Plasmonic ZnO/Ag Embedded Structures as Collecting Layers for Photogenerating Electrons in Solar Hydrogen Generation Photoelectrode...206

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7.2.1 Fabrication of Photoelectrode ...210

7.2.2 Fabrication and Characterization of Photochemical Electrode ... 211

7.2.3 FEM Simulation ...212

7.4 Summary ... 231

Chapter 8. Plasmon-enhanced Near-infrared-active Material in Photoelectrochemical Water Splitting ...238

8-2 Experimental Section ... 240

8.2.1 Photoelectrode Preparation ...240

8.2.2 Photoelectrochemical Characterization ...241

8.2.3UV-Vis Spectrum Measurement ...241

8.2.4 Upconversion Emission Spectrum Measurement ...242

8.2.5 Upconversion Emission Quantum Yield Measurement ...242

8.2.6 Band Position Measurement ...243

8.4 Summary ... 254

Chapter 9. Conclusions ...257

Scientific Journal Publication List ...261

Conference Publications List ...264

Honor ...266

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Figure Caption

Figure 1-1. Sketch diagram for basic principles of water splitting on photoelectrochemical cells with an n-type semiconductor photoanode, where oxygen is evolved, and a photocathode (Pt sheet), where hydrogen is evolved. ... 6 Figure 1-2. Illustration of the band edge potentials of different semiconductors. ... 8 Figure 1-3. Mechanism of the sacrificial reagent. ... 10 Figure 1-4. Summary of controlling parameters for photoelectrochemical water splitting. ... 19 Figure 1-5. IPCE spectra of TiO1.957N0.043 photoelectrode with cobalt treatment, unmodified TiO2 photoelectrode with cobalt treatment, and TiO1.988N0.012 photoelectrode with cobalt treatment. ... 22 Figure 1-6. (a) Sketch diagram for a photoelectrochemical cell with RuP-sensitized TiO2 photoanode coated with a Nafion® film that has been by penetrated complex 1 and a photocathode (Pt sheet); (b) Corresponding short-circuit current responses to on–off cycles of illumination. ... 25 Figure 1-7. (a) Corresponding photographs and photoluminescence spectra of quantum dots in various particle sizes; (b) qualitative alternatives of quantum dots with increasing particle size. ... 26 Figure 1-8. (a) Corresponding I–V plot for the double-sided CdS and CdSe QD co- sensitized ZnO nanowire array photoanode; (b) IPCE spectra of the double-sided CdS and CdSe QD co-sensitized ZnO nanowire array photoanode. ... 27 Figure 1-9. Sketch diagram of plasmonic effect in the photoelectrochemical cell. (a) hot-electron injection; (b) induced electromagnetic field effect; and (c) scattering and refracting effect. ... 30

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Figure 1-10. (a) Photocurrent of TiO2 with/without gold nanoparticles under irradiation (633 nm); (b) Photocurrent versus wavelength for TiO2 with/without gold

nanoparticles. ... 32

Figure 1-11. Strategies of controlling parameters for this study. ... 35

Figure 2-1. Summarization of instruments for characterization and applications. .... 42

Figure 2-2. Schematic of interaction between X-ray and material... 54

Figure 2-3. (a) Generation of Cu Kα X-rays. The 1s electron is ionized, follow by 2p electron relaxed into the empty 1s level (o) and the excess energy is released as X- rays. (b) A representative X-ray emission spectra.^[8] ... 56

Figure 2-4. Derivation of Bragg’s law. ... 56

Figure 2-5. Scheme of basic component of TEM instrument. ... 59

Figure 2-6. Photograph of JSM-6700F SEM instrument. ... 61

Figure 2-7. Photograph of UV-Vis spectrometer. ... 63

Figure 2-8. Basic principle of XPS. ... 65

Figure 2-9. Photograph of home-made photoluminescence device. ... 67

Figure 2-10. X-ray energy losing intensity via interactions with material.^[10] ... 69

Figure 2-11. Classification of X-ray absorption spectroscopy. ... 70

Figure 2-12. Scheme for multiple scattering and single scattering.^[11] ... 72

Figure 2-13. Cyclic voltametry diagram.^[12] ... 75

Figure 2-14. Ideal and actual fuel cell voltage vs current characteristic.^[12] ... 76

Figure 2-15. A typical voltammetry curve for photoresponse of an n-type photoelectrode. ... 78

Figure 2-16. A typical voltammetry measurement by two-electrode system. ... 79

Figure 2-17. A typical voltammetry measurement by three-electrode system. ... 80

Figure 2-18. A typical setup of IPCE measurement. ... 81

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Figure 3-1. Sketch showing ZnO nanowires decorated with CdTe QDs and charge- transfer processes. ... 87 Figure 3-2. Sketch showing nitrogen doped ZnO nanowires decorated with CdTe QDs and charge-transfer processes. ... 87 Figure 3-3. Sketch showing different-sized InP QDs sensitized ZnO nanowires array and working strategy... 89 Figure 3-4. (a) SEM image of ZnO nanowires on FTO substrate. (b) Crosssectional image of ZnO nanowires. ... 90 Figure 3-5. (a) TEM image of ZnO nanowires decorated with CdTe QDs. (b) Corresponding electron diffraction pattern of (a). (c) HRTEM image of a ZnO nanowire decorated with CdTe QDs. (d) Elemental mapping of Zn, Cd, and Te. ... 92 Figure 3-6. (a) Linear-sweep voltammograms from pristine ZnO nanowires, ZnO nanowires loaded with CdTe QDs at various deposition times, and dark scan. (b) Photoconversion efficiency of the pristine ZnO nanowires and ZnO nanowires loaded with CdTe QDs for various deposition times. (100 mW/cm² solar simulator in Na2SO4 as electrolyte). ... 93 Figure 3-7. HRTEM images of ZnO@CdTe (120 h). ... 95 Figure 3-8. (a) Amperometric I–t curves of the ZnO nanowires with/without CdTe QDs at 100 mWcm^-2 with on/off cycles in Na2SO4 as supporting electrolyte. (b) Stability of PEC performance after 50 scans at 100 mWcm^-2 in Na2SO4 as supporting electrolyte. (c) HRTEM of the ZnO@CdTe after the stability measurement. ... 95 Figure 3-9. Measured IPCE spectra of pristine ZnO nanowires and ZnO nanowires loaded with CdTe QDs (24 h) in the region of 350–600 nm at a potential of 0 V versus Ag/AgCl in Na2SO4 electrolyte... 97 Figure 3-10. Scanning electron microscopy (SEM) images of top views of ZnO.

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SEM images of ZnO:N at annealing temperatures of (a) 500, (b) 600 and (c) 700°C.97 Figure 3-11. (a) High-resolution transmission electron microscopy (HRTEM) image of the CdTe QDs. (b) The absorption spectrum of the CdTe QDs suspensions in water.

The inset displays photographs of CdTe QDs solution under ambient and UV light (365 nm)... 98 Figure 3-12. X-ray diffraction (XRD) patterns of ZnO, ZnO:N (500), ZnO:N (600) and ZnO:N (700). ... 99 Figure 3-13. (a) X-ray photoelectron (XPS) plot of the ZnO:N nanowire nitrogen concentration versus the annealing temperature. High-resolution XPS of (b) ZnO:N (500), (c) ZnO:N (600) and (d) ZnO:N (700). ...101 Figure 3-14. UV-vis absorption spectra of (a) ZnO, ZnO:N (500), ZnO:N (600) and ZnO:N (700) nanowires. (b) ZnO:N (700), ZnO:N (500)@CdTe, ZnO:N (600)@CdTe and ZnO:N (700)@CdTe nanowires. ...102 Figure 3-15. Photoelectrochemical (PEC) performance measurement in 0.5 M Na2SO4 under 100 mW/cm². Linear-sweep voltammograms of (a) ZnO:N and (b) ZnO:N@CdTe. (c) photoconversion efficiency of the bare ZnO, ZnO:N and ZnO:N@CdTe (d) Measured IPCE spectra of bare ZnO , ZnO:N and ZnO:N@CdTe.

(e) Chronoamperometric measurement of ZnO:N@CdTe. (f) Gas evolution of the ZnO and ZnO:N(700)@CdTe nanowire photoelectrodes (100 mW/cm² solar simulator in Na2SO4 as electrolyte). ...106 Figure 3-16. X-ray absorption near-edge structure measurement of the ZnO:N nanowire O K-edge. ...108 Figure 3-17. EXAFS spectra of Zn K-edge for ZnO, ZnO:N (500), ZnO:N (600) and ZnO:N (700) nanowires. ...109 Figure 3-18. (a) The absorption spectra of the six different-sized InP QDs prepared

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in present study and (b) corresponding photoluminescence of these six different- sized InP QDs. ... 111 Figure 3-19. (a) The absorption spectrum of mixture solution (InP-1, InP-4, and InP- 6) and insert was the photographs of mixture solution under ambient and UV light with a wavelength of 365 nm. (b) TEM micrograph of the mixture InP QDs. ... 112 Figure 3-20. (a) The absorption spectrum of mixture solution (InP-1, InP-4, and InP- 6) and insert was the photographs of mixture solution under ambient and UV light with a wavelength of 365 nm. (b) TEM micrograph of the mixture InP QDs ... 114 Figure 3-21. (a) TEM image and corresponding selected electron diffraction pattern of individual nanowires taken along the [2110] zone axis. (b) High-resolution TEM image of ZnO nanowires decorated with an ensemble of InP QDs. (c) TEM image of InP-ZnO nanowires heterostructure and corresponding EDX elemental mapping of Zn, In, and P. ... 115 Figure 3-22. A polt of j2 measured at incident light wavelength kept at 395 nm and 660 nm versus applied voltage for ZnO rods and InP, respectively. ... 116 Figure 3-23. (a) A set of linear sweep voltammagrams recorded on these nanowires under illumination of 100 mW/cm². (b) Photoconversion efficiency of the bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires. (100 mW/cm² solar simulator in Na2SO4 as electrolyte). ... 118 Figure 3-24. (a) Measured IPCE spectra of bare ZnO nanowires and different-sized InP QDs sensitized ZnO nanowires. (solar simulator in Na2SO4 as electrolyte with 0.5 V bias). (b) Amperometric I-t curves of the different-sized InP QDs sensitized ZnO nanowires at 100 mW/cm² with on/off cycles with 0.5 V bias. ...120 Figure 3-25. (a) Zn K-edge XANES spectra of ZnO, ZnO@InP QD-1, and ZnO@InP QD-mix. (b) Zn K-edge XANES spectra of ZnO under dark/illumonatio. (c) Zn K-

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edge XANES spectra of ZnO@InP QD-1 under dark/illumination. (d) Zn K-edge XANES spectra of ZnO@InP QD-mix under dark/illumination. ...124 Figure 3-26. Band structural evolution of ZnO with InP QDs decoration and solar illumination. ...125 Figure 4-1. (a) Schematic of the fabrication of ZnO@ZnOxS1-x nanowire photoanode. (b) TEM image of ZnO nanowires with deposition of ZnS QDs (c) Elemental profile extract from STEM in direction of nanowires (indicated by arrow in TEM). ...137 Figure 4-2. (a) X-ray diffraction patterns of ZnO, ZnO nanowires sensitized with ZnS QDs, and ZnO@ZnOxS1-x nanowires. (b) Band gap plots of (F(R) ℎ )²for ZnO and ZnO@ZnOxS1-x nanowires. ...139 Figure 4-3. (a) Linear-sweep voltammograms from ZnO nanowires, ZnO nanowires with sensitization of ZnS QDs, and ZnO@ZnOxS1-x nanowires. (b) IPCE spectra of ZnO, ZnO nanowires with sensitization of ZnS QDs, and ZnO@ZnOxS1-x nanowires at a potential of 0.5 V versus Ag/AgCl. (c) H2 production upon illumination of each samples with the surface area photoelectrode (6 cm²). (100 mW/cm² solar simulator in Na2S and 0.5 M Na2SO3 as electrolyte). ...142 Figure 4-4. Magnification linear-sweep voltammograms from ZnO nanowires, ZnO nanowires with sensitization of ZnS QDs and ZnO@ZnOxS1-x nanowires. (100 mW/cm² solar simulator in Na2S and 0.5 M Na2SO3 as electrolyte). ...143 Figure 4-5. UV-Vis spectra of ZnO nanowires, ZnO nanowires with sensitization of ZnS QDs and ZnO@ZnOxS1-x nanowires. ...143 Figure 4-6. (a) EXAFS spectra of Zn K-edge for ZnO and ZnO@ZnOxS1-x

nanowires. (b) Wurtzite structure of ZnO and each scattering shell modes of central Zn atom. (c) XANES spectra of Zn K-edge for ZnO nanowires, ZnO@ ZnS QDs,

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and ZnO@ZnOxS1-x nanowires. ...147 Figure 4-7. EDX of ZnO@ZnOxS1-x nanowires collected from STEM...148 Figure 4-8. Polt of j² measured at incident light wavelength kept at 395 nm versus applied voltage for ZnO rods, ZnO@ZnOxS1-x and ZnO@ZnS QDs...149 Figure 4-9. Band position of samples determined by electrochemical measurement in present study. ...149 Figure 4-10. Sketch of nanostructure of ZnO@ZnOxS1-x nanowires. Bottom part displays pathway of electrons for generating hydrogen and relative energy diagram. ...151 Figure 5-1. SEM images of ZnO@Au photoelectrodes with nanoparticles deposited for various periods: (a) bare ZnO, (b) 1 h deposition, (c) 3 h deposition, (d) 6 h deposition, (e) 12 h deposition and (f) 24 h deposition. ...162 Figure 5-2. Absorption spectra of (a) ZnO@Au photoelectrodes with Au nanoparticles deposited over various periods and (b) pure Au nanoparticles suspension. ...164 Figure 5-3. (a) Linear-sweep voltammograms of ZnO@Au photoelectrodes with nanoparticles deposited for various periods, and a dark scan, performed in a 0.5 M aqueous Na2SO4 with a pH of 6.8 under an AM 1.5 solar simulator. (b) Linear-sweep voltammograms of ZnO@Au photoelectrodes with nanoparticles deposited for various periods, obtained under illumination by visible light (> 420 nm). ...166 Figure 5-4. (a) TEM micrographs and distribution of sizes of as-prepared gold nanoparticles (12 h). (b) TEM micrographs and (c) high-resolution image of ZnO@Au photoelectrodes. (d) Corresponding electron diffraction pattern of ZnO@Au photoelectrode. ...169 Figure 5-5. Absorption spectrum of Au nanospheres, and plots of photocurrent

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versus wavelength, fitted to Fowler’s law, indicating that photocurrent comprises

mainly hot electron flow, with additional contribution from hot electrons that are injected from Au under plasmonic induced irradiation that is amplified by localized

surface plasmon resonance. ...171

Figure 5-6. Amperometric I–t curves of ZnO@Au photoelectrode (12 h) with on/off cycles under a solar simulator with intensity of 100 mW/cm². ...172

Figure 5-7. (a) Density of states for ZnO calculated by density function theory. (b) The relative vacancies for ZnO rods (dark condition), ZnO rods (@ UV illumination), ZnO@Au (dark condition), ZnO@Au (@ 530 nm illumination). ...175

Figure 5-8. (a) Simulated heat generation distribution map of Au nanospheres-ZnO nanorod (12 h). (b) The relative vacancies of ZnO@Au photoelectrode at desired temperature. ...177

Figure 5-9. Time courses of H2 and O2 evolution using ZnO@Au and ZnO photoelectrodes under AM 1.5G solar simulator in 0.5 M Na2SO4 aqueous solution. ...178

Figure 5-10. Schematic illustration of the plasmon-induced effects on ZnO@Au photoelectrode. ...179

Figure 6-1. Schematic structure of a ZnO nanorod@plasmonics // TiO2 nanorod@quantum dot photoelectrochemical cell. ...189

Figure 6-2. SEM and TEM micrograph of TiO2 nanorod decorated with CdTe QDs. ...190

Figure 6-3. Absorption spectrum of the ZnO, CdTe QDs and the Au nanorods. ...190

Figure 6-4. Structural characterization of ZnO@Au rod photoelectrode. ...192

Figure 6-5. TEM image of the ZnO@Au rods with different aspect ratio. ...193 Figure 6-6. Photoelectrochemical properties of plasmon-nanostructural

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photoelectrodes. a. Linear-sweep voltammograms from the ZnO@Au rod (various SPR wavelengths)//TiO2@QDs, ZnO@Au NP// TiO2@QDs, ZnO@Pt NP //

TiO2@QDs, and Pt foil// TiO2@QDs cells. (100 mW/cm² solar simulator in 0.5 M Na2SO4 as electrolyte). b. Amperometric I-t curves of the ZnO@Au rod (various SPR wavelengths) and ZnO@Au NP photoelectrodes under plasmonic inducing illumination with desired wavelength under on/off cycles. (100 mW/cm² solar simulator in 0.5 M Na2SO4 as electrolyte). c. Photocurrent from plasmon inducing hot electrons was plotted as a function of the resonance energy of illumination light on the ZnO@Au rod photoelectrode. d. photocurrent was measured as a function of the incident power of illumination light on the ZnO@Au rod(SPR-722) photoelectrode. ...195 Figure 6-7. TEM images of ZnO nanorod@metal nanoparticles. ...196 Figure 6-8. Plots of photocurrent as function of wavelength and the fit to Fowler’s law. ...199 Figure 6-9. Finite element method simulation of electric field intensity. ...201 Figure 7-1. Steps of the fabrication procedure for ZnO/Ag plasmonic photoelectrode.

...213 Figure 7-2. (a)-(c) SEM images of AgO thin film after femtoseconf laser treatment on ITO substrate. ...215 Figure 7-3. (a) Cross-section image of interface between ZnO nanorods and ITO substrate. (b) Magnification image of square region in Figure (a). (c) HRTEM image of Ag nanoparticle embedded structure in ZnO/ITO interface. (d) High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image of Ag nanoparticle embedded structure in ZnO/ITO interface and corresponding EDX elemental mapping of Ag, Zn, O, and In. ...216

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Figure 7-4. (a) Linear sweep voltammagrams from ZnO/Ag plasmonic nanostructures photoelectrodes with various laser power treatment, ZnO/Ag nanoparticles photoelectrode (via chemical bath deposition: CBD), and ZnO nanorods photoelectrode under AM 1.5G solar simulator illumination in 0.5 M Na2SO4 as supporting electrolyte. (b) Photocurrent obtained at +0.5V from various photoelectrodes and corresponding SEM images of Ag nanostructures. ...218 Figure 7-5. SEM images of femtosecond laser induced Ag nanostructure with various power density treatment: (a) 60W, (b) 80W, (c) 100W, and (d) 150W. ...219 Figure 7-6. (a) Absorption spectrum of plasmonic ZnO/Ag nanostructural photoelectrode (80W) and corresponding photocurrent response as function of wavelength under chopped illumination. (b) Measured IPCE spectra of plasmonic ZnO/Ag nanostructure photoelectrode (80W) and ZnO nanorods photoelectrode at an applied bias of +0.5 V. ...224 Figure 7-7. (a) Density of states for ZnO calculated by density function theory. (b) The relative vacancies for ZnO rods (dark condition), ZnO/Ag (@ 380 nm illumination-30 mW/cm²), ZnO/Ag (@ 410 nm illumination-15 mW/cm²), ZnO/Ag (@ 410 nm illumination-30 mW/cm²)...226 Figure 7-8. (a) SEM image of laser treated Ag plasmonic nanostructures on ITO substrate. (b) Corresponding electric field distribution maps from FEM simulations.

(c) 3D hotspots distribution maps of Figure (b). ...228 Figure 7-9. Measured hydrogen production of the ZnO/Ag plasmonic nanostructural photoelectrode (80W) and ZnO photoelectrode under at +0.5 V vs Ag/AgCl as a function of time under AM 1.5G solar simulator illumination (100 mW/cm²) in 0.5 M Na2SO4 as suppoting electrolyte. ...231 Figure 8-1. Sketch of the (a) Au-UCNs–CdTe-ZnO photoelectrode and (b)

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mechanism of energy conversion from NIR to chemical fuel. ...240 Figure 8-2. TEM image. (a) CdTe-ZnO and (b) HRTEM image of the CdTe-ZnO;

TEM images of the UCNs modified with (c) 0.2 wt.%; (d) 0.4 wt.%. (e) 1.0 wt.%. (f) 2.0 wt.% Au nanoparticles; (g) TEM image of the ZnO@ Au 1.0 wt UCNs-CdTe. ..244 Figure 8-3. X-ray diffraction (XRD) patterns of a) ZnO nanorod and b) Er³⁺/Yb³⁺

co-doped NaYF4. ...245 Figure 8-4. TEM (left) and HRTEM (right) images of the as-synthesized Er³⁺/Yb³⁺

co-doped NaYF4 nanoparticles. ...246 Figure 8-5. UV-Vis spectra. (a) Au-x-UCNs nanoparticles suspension in water (x = 0, 0.2, 0.4, 1.0 and 2.0 wt%); (b) CdTe QDs and Au NPs suspension in water, and (c) upconversion emission spectra of the Au-x-UCNs nanoparticles (x = 0, 0.2, 0.4, 1.0 and 2.0 wt%) suspending in water with controlled concentration in 10 mg/mL of UCNs in the suspension. ...248 Figure 8-6. (a) TEM image of the Au 1.0% wt.-UCNs nanoparticles. (b) TEM image of the Au 1.0% wt.-UCNs nanoparticles after coated with ZnS. ...250 Figure 8-7. (a) TEM image of the ZnO@Au-CdTe. (b) HRTEM image of the ZnO@Au-CdTe. ...250 Figure 8-8. (a) TEM image of the ZnO@Au 1.0%-UCN–CdTe. (b) HRTEM image of the ZnO@Au 1.0%-UCN–CdTe. ...251 Figure 8-9. (a) Linear-sweep voltammograms and (b) Gas evolution of the ZnO@Au x-UCN-CdTe (x = 0, 0.2, 0.4, 1.0 and 2.0 wt%) and ZnO as photoanode in 0.5 M Na2SO4 solution. (980 nm laser, 100 nm/cm² electrolyte : 0.5 M Na2SO4, pH = 6.8) ...253 Figure 8-10. (a) Linear-sweep voltammograms of the ZnO@Au-CdTe, ZnO@Au 1.0%-UCN and ZnO@UCN photoelectrode. (b) The chronoamperometry

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measurement of the ZnO@Au 1.0%-UCN-CdTe. (electrolyte : 0.5 M Na2SO4, pH = 6.8) ...253 Figure 8-11. (Left) A polt of j² measured at incident light wavelength kept at 395 nm versus applied voltage for ZnO rods and CdTe. (Right) Band position of ZnO rods and CdTe. (electrolyte : 0.5 M Na2SO4, pH = 6.8)...254

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Table Caption

Table 1-1. Semiconductor materials of photoelectrode for photoelectrochemical water splitting. ... 17 Table 3-1. Zn K-edge EXAFS structural parameters of ZnO, ZnO:N (500), ZnO:N (600) and ZnO:N (700) nanowires. ...109 Table 3-2. Absorption properties, emission properties, and corresponding size of each InP QDs solution. ... 112 Table 4-1. Zn K-edge EXAFS structural parameters of ZnO and ZnO@ZnOxS1-x

nanowires. ...145 Table 5-1. Elemental analysis of gold from inductively coupled plasma atomic emission spectrometer. ...164 Table 8-1. Upconversion emission quantum yields of the Er³⁺/Yb³⁺ co-doped NaYF4

suspending in the cyclohexane and water soluble colloid suspension of the Au-x- UCNs (x = 0, 0.2, 0.4, 1.0 and 2.0 wt.%). ...249 Table 9-1. Summarized photocurrent and photoconversion efficiency in different strategies. ...260

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Chapter 1. Introduction

The annual worldwide energy consumption in a person’s lifetime is estimated to be approximately 15 TW. In the past decade, fossil fuel has become the primary energy consumed. Recent reports indicate that the consumption rate of fossil fuel will be greater than its generation rate in the next 20 years.^[1] Issues involving fossil fuel include soaring prices, carbon release, and resource shortage. The release of carbon dioxide causes the greenhouse effect, which has increased the temperature of the Earth over the past 100 years and has caused rising sea levels, air pollution, and global climate change. The increasing demand for energy has motivated scientists to seek sustainable energy sources that are clean, renewable, abundant, and safe. Solar, wind, hydropower, biomass, and hydrogen energy are currently the foci for research on future energy sources. Solar energy has gradually been taken seriously because it currently produces approximately 120,000 TW per year.^[2] It is considered an inexhaustible, clean, cost effective, highly practical, versatile in application, and pollution-free energy resource. Using this vast energy source well will simultaneously solve the energy shortage, various environmental problems, and other issues. Solar energy is convertible into variable energy through electric energy, chemical energy, and harnessed heat from sunlight, methods that have been investigated since ancient times. The daily and seasonal variations of sunlight are incompatible with the continuous energy demand of daily life. To facilitate future application, solar energy is converted into an ordered energy form, such as electric or chemical energy, to overcome variations and obtain a higher potential for application. The manipulation of photovoltaic and artificial photosynthesis has resulted in four kinds of major technologies currently

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available for converting sunlight into useful forms, such us electricity and hydrogen.

1.1 Sunlight Conversion Technologies

1.1.1 Photovoltaic Cell

Photovoltaic cells can convert sunlight directly into electricity. It has been recognized as one of the most sustainable clean energy technologies for generating electrical power on a large scale and has attracted worldwide attention. Few processes use photovoltaic cells to generate electricity through solar energy. First, sunlight is collected and induced into a charge-separated state to generate electron–hole pairs. The photon absorption process occurs in the silicon semiconductor film that causes the migration of excitons to a p-n junction in which charge separation occurs, producing an electromotive force. This p-n junction is connected to an external electrically circuit. A photo-current flows through the circuit, generating electrical power and completing the photo-to-electricity conversion process.^[3] The highest energy conversion efficiency of commercially available crystal silicon photovoltaics is approximately 18%. Apart from improving the efficiency of photo-electricity conversion, increased attention must be paid to the storage and distribution of solar energy for practical applications.

The most reasonable technology involves converting electrical energy into a form of chemical fuel, such as hydrogen or low-carbon organics, which can be easily stored and transported.

Although solar energy is clean and rich, the major challenge of current technology in making photovoltaic cells that can be competitive with fossil fuels is their high cost, which is two to five times higher than that of conventional energy

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technologies.^[4] The most used materials in photovoltaics are crystalline silicon wafers with thicknesses of around 180 μm to 300 μm. The production of crystalline silicon-based photovoltaic cell technology also requires high temperature, high pressure, and complex processes. Both the materials and their associated processing are expensive. To overcome the challenge of high material cost and to produce thin-film photovoltaic cells that can reduce the usage quantity of silicon- based materials, various type of technologies that increase the absorbance of visible light have been explored, including dye-sensitized solar cells and organic photovoltaics.^[5,6] These systems use relatively cheap materials and have higher potential for future utilization than crystalline silicon photovoltaic cells.

1.1.2 Thin-film Photovoltaic Cells

To reduce the cost of photovoltaic cells, thin-film photovoltaic cells should be explored because they use less expensive crystalline silicon-based photo- materials. Some thin-film solar cells with 1 μm- to 2 μm-thick photo-material film fabricated on cheap substrates, such as conductive glass, stainless steel, or plastic, have been investigated and validated. In terms of improving the efficiency of photo-electricity conversion, semiconductors used to harvest solar energy in thin- film photovoltaic cells are typically made of GaAs, CdTe, CuInSe2, and amorphous/polycrystalline silicon to substitute for expensive, conventional crystalline silicon.^[4]

1.1.3 Wet-chemical Photosynthesis

New sunlight conversion technology has been explored by learning from nature. Wet-chemical photosynthesis technology has been examined to achieve

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efficient and stable energy conversion, which operates under mild condition and uses only earth-abundant materials. In the long-term, photosynthesis technology should be able to generate solar fuels by simulating chemical reactions that occur naturally in biological systems. Photosynthesis is a wet-chemical process that transforms carbon dioxide and water into organic compounds, particularly sugars (chemical fuels), by using energy from sunlight. Harvesting solar energy, and then converting and storing it via chemical methods, similar to what occurs naturally in photosynthesis in green plants, is a possible strategy for meeting the challenges of solar energy applications.

1.1.4 Photoelectrolysis

Photoelectrolysis, which combines the concepts of photovoltaic cells and wet-chemical photosynthesis, uses photovoltaic cells to split water into hydrogen (chemical fuel), a process called solar water splitting. Photoelectrolysis can be regarded as an artificial form of photosynthesis. This process is assisted electrochemically: a solid photoelectrode is adopted to convert sunlight energy into splitting water and generating chemical energy. Hydrogen plays a critical role in the development of green energy because it is the ultimate clean energy and can be used in fuel cells. However, hydrogen is primarily formed by steam reforming, in which fossil fuels are consumed and carbon dioxide is generated.^[7] The chemical reaction of the cracking process may be represented as

(1-1) Photoelectrolysis can be utilized to accomplish water splitting and produce hydrogen and oxygen, a sustainable energy source without any byproducts that contain carbon. Nevertheless, the conversion efficiency of photoelectrolysis

4 2 2 2

CH H OheatH CO

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remains lower than that of photovoltaic cells, which is limited mainly by the low performance of photoelectrodes.

In 1972, Honda and Fujishima accomplished the earliest work on photoelectrolysis by using a photoelectrochemical cell that contains a photoanode and a photocathode; in this cell, the anodic/cathodic reactions in the splitting of water was conducted using TiO2 as the photoelectrode.^[8] They discovered that both electrons and holes were generated on the semiconductor photoanode (TiO2) when the UV illumination experienced radiation. Moreover, the excited photo- electrons went through the out circuit to reduce water and form hydrogen on a Pt counter-electrode as the holes oxidized water to form oxygen on the surface of the TiO2-based electrode, which was maintained at a certain electrode potential. The present investigation offers the possibility for a renewable, carbon-free source of high-quality hydrogen that can undergo processing, storage, and later usage through photoelectrochemical water splitting.

1.2 Photoelectrochemical Water Splitting

Under standard conditions, the change in free energy (ΔG) associated with the conversion of one molecule of H2O into H2 and ½ O2 is 237.2 kJ/mol, which corresponds to an electrolysis cell voltage (ΔE°) of 1.23 V per electron transferred.

As shown in Figure 1-1, photoelectrochemical water splitting involves two electrodes: the anode and the cathode. The anode is an electrode based on a photoactive material or a semiconducting material. A light beam irradiates the anode during water splitting. The cathode is a counter-electrode (also called a photocathode), although it is not irradiated by light. To drive this reaction, the layer of the photoactive material on the photoanode must absorb radiant light to make

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its electrode potential higher than 1.23 V. Thus, the water molecule that can be oxidized to form O2 and proton (H⁺) can be simultaneously reduced to form H2 at the cathode. If the photoanode is irradiated by light that has an energy greater than the bandgap (EBG) of the photoactive material, then the electrons of the valence band will be excited into the conduction band (CB) while the holes remain in the valence band. These photogenerated electrons will then pass through the external load and reach the surface of the cathode to react with protons, generating H2. In addition, the holes at the photoanode will diffuse to the surface of the photoanode and oxidize the H2O to produce O2.

Figure 1-1. Sketch diagram for basic principles of water splitting on photoelectrochemical cells with an n-type semiconductor photoanode, where oxygen is evolved, and a photocathode (Pt sheet), where hydrogen is evolved.^[8]

Semiconductor electrodes used in photoelectrochemical cells must satisfy several criteria. First, as a result of the existence of kinetic sluggishness for driving

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the reactions of water splitting, energy is lost during the transfer of electrons at the photoanode/electrolyte interface. The energy required for photoelectrolysis is frequently given as at least 1.7 eV to provide the potential necessary for electrolysis and to overcome other energy losses in the system. Based on electrochemical principles, the water-splitting reaction can only be driven when the irradiation energy exceeds 1.23 eV (~1,000 nm), indicating that the energy of the light must be larger than the bandgap to separate the electrons and holes. In practical operation, the minimum energy requirements plus the thermodynamic/overpotential loss must be at least 1.7 eV to 1.9 eV for photoelectrochemical water splitting, which corresponds to an onset of light absorption at a wavelength of 730 nm.^[9,10]

In addition to the bandgap requirement, a second factor that commonly affects the use of water splitting is the energy band edge of the semiconductor. In water splitting, the bottom level of the CB must be located at a more negative potential than the reduction potential of H⁺/H2. The top of the valence band must also be more positive than the oxidation potential of H2O/O2. With respect to the fundamental requirements of conduction and valence bands, the band edge positions must straddle the hydrogen and oxygen redox potentials. Figure 1-2 illustrates the different semiconductors and their corresponding potential band edges.^[5] Some semiconductor materials can reduce but not oxidize water. However, several metal-oxide materials, such as Fe2O3, can oxidize but not reduce protons.

For the majority of such materials, an external bias is essential to reduce protons and assist in the measurement of photocurrents in the photoelectrochemical cell;

thus, the onset potential of the photoresponse from the I–V curve always shifts to a higher potential region.^[11]

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Figure 1-2. Illustration of the band edge potentials of different semiconductors.^[5]

Another critical factor is electrochemical stability or resistance to photocorrosion, which may limit the usefulness of several photocatalytic materials.

Most metal-oxide semiconductor materials are thermodynamically unstable. Thus, the photogenerated holes may oxidize themselves rather than water (photocorrosion and/or anodic photodecomposition). These undesired photodecompositions in various photoactive materials commonly depend on the pH value of electrolytes and frequently limit their utilization in certain conditions.

TiO2 and SnO2 are highly stable over a wide range of pH values in aqueous environments upon illumination. The stability of hematite strongly depends on the presence of dopants, pH values, and oxygen stoichiometry. Many non-oxide semiconductor materials may either dissolve or form a thin oxide film on their surface, preventing the transfer of electrons through the interface between semiconductor/electrolyte interfaces. Photocorrosion or anodic decomposition is

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expected to be markedly inhibited if the transfer of carriers for water oxidization through the interface is faster than a competing reaction. Thus, the development of semiconductor materials with excellent stability against photocorrosion/anodic decomposition becomes a critical issue for future applications. To prevent serious problems, scientists add redox couples into the photoelectrochemical cell to scavenge the photoexcited electron/hole pairs. This technique improves the charge separation and controls the desired product. The redox couples are also called sacrificial reagents.^[7] The mechanism of the sacrificial reagent is shown in Figure 1-3. The sacrificial reagents are used in the following conditions: (1) The CB of the semiconductor is more positive than the hydrogen redox potential, such as for Fe2O3 and WO3. The oxidizing reagents (electron acceptor), such as S^2- and SO32-, are added to the water-splitting system to scavenge the photoexcited electrons, which facilitate the diffusion of the photoexcited hole to the surface of the semiconductor and, thus, the generation of oxygen gas. (2) The valence band of the semiconductor is more negative than the oxygen redox potential, such as for silicon and InP. Reducing reagents (electron acceptor), such as Ag⁺ and Fe³⁺, are added to the water-splitting system to scavenge the photoexcited holes, which facilitate the migration of the photoexcited electrons to the surface of the semiconductor and, thus, the generation of hydrogen gas. (3) To increase the charge separation efficiency of the semiconductor materials. (4) To investigate the activity or reaction condition of the specific reaction, such as hydrogen generation.

The sacrificial reagent facilitates the electron/hole transfer. Thus, the generated photocurrent does not represent the activity of the materials, and the photocurrent cannot be used when calculating efficiency. Moreover, the reaction of the sacrificial reagent consumes the reducing/oxiding reagent. This consumption is not reversible, and the reagent cannot be recycled. As a result,

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sacrificial reagents cannot be used in practical applications.

Figure 1-3. Mechanism of the sacrificial reagent.

1.2.1 Photoelectrochemical Activity

The bandgap energy requirements of the semiconductors for photoelectrochemical water splitting must be at least 1.7 eV to 1.9 eV, which corresponds to an onset of light absorption at a wavelength of 730 nm. The intensity of the solar spectrum also dramatically falls below 350 nm, resulting in an upper limit on the bandgap of approximately 3.5 eV. For this reason, the desired optimum value of the semiconductor bandgap should be between 1.9 and 3.5 eV, which is within the viable range of the solar spectrum. In practical cases, the flux of solar photons in the wavelength range of 680 nm to 280 nm (1.8 eV–4.4 eV) represents 27.5% of the total solar photon flux, which is the maximum efficiency predicted in various investigations based on the bandgap of the semiconductor material, the solar spectrum, and various losses.^[12] However, the efficiency in

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energy conversion using a single bandgap material is too low to satisfy the requirements of actual application, even if a perfect photocatalytic material can be developed. As a result, a more efficient configuration has to be developed.

Some important factors other than bandgap energy must be considered to improve water-splitting reaction, including charge separation, charge mobility, and the lifetime of photogenerated electron–hole pairs. These factors also critically affect the photoactivity of semiconductor materials. The structural and electronic properties of co-catalysts on the surface of photoelectrodes also strongly affect the generation and separation of electron–hole pairs. In the general case, highly crystalline materials with a low density of defects are beneficial for water-splitting reaction because defects may serve as recombination centers for photogenerated electrons/holes.

1.3 Photoelectrode Materials

Photoelectrode materials are crucial to water splitting. Thus, chemists and scientists continually seek the best photo-active materials that exhibit the following: a small semiconductor bandgap that covers a large fraction of the sunlight spectrum, a conduction and valence band energy that straddles water oxidization and reduction potentials, a stable property in an aqueous environment, high conversion efficiency for water splitting, and low cost. Unfortunately, no single material has yet been found that satisfies all of these requirements, though combinatorial methods have been used to quickly search for and optimize ideal materials.^[5,11] Transition metal oxides, including TiO2 and ZnO, exhibit good stability but only absorb a small fraction of incident sunlight because they have a large bandgap. Given that most sunlight is visible light, the use of visible light to

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irradiate a photoelectrode and generate hydrogen (oxygen) seems more practical than the use of UV light. Hence, research on effective visible-light photoelectrodes for water splitting using new materials for both anodic/cathodic processes should focus on the goal of realizing an efficient photoelectrochemical cell that can simultaneously drive both hydrogen generation and water oxidation reaction under visible-light radiation.

The combination of either a photoanode and a photocathode or a photoelectrode and a photovoltaic device has also been investigated to construct a tandem cell that facilitates overall water splitting associated with multi-photon absorption events.^[13] A solar energy conversion efficiency of over 12% has been achieved using III–V semiconductor materials, but the cost and stability of such materials remain major disadvantages.^[14] Although the efficiency of photoelectrochemical cells for solar energy conversion is still low, research on water splitting remains popular in solving problems associated with energy and the environment.

A wide range of semiconductor materials has been developed for photoelectrodes used for water-splitting reaction. Table 1-1 presents several representative photo-active materials. The medium of photoelectrochemical measurement has a significant effect on the stability of materials in terms of photocorrosion and photodecomposition.^[15] For example, the photoelectrode made of TiO2 can be operated under neutral or basic conditions, and ZnO can be measured under neutral conditions. The properties of the medium strongly affect the performance of the photoelectrode. For instance, the pH value of the electrolyte clearly affects the redox potential of the water and the position of the CB/valence band of the semiconductor materials.

TiO2 was the first reported photoelectrode for water splitting under UV

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irradiation.^[8] It was adopted to produce hydrogen and/or oxygen from an aqueous solution. Since then, various studies have focused on reducing the probability of recombination, increasing the surface area, and enhancing the carrier mobility of TiO2 to improve the photoconversion efficiency of the photoelectrochemical cell.^[16,17] In this regard, nanoarchitecture has been regarded as a candidate for fulfilling these demands. A mesoporous structure has been adopted to reduce the probability of charge-carrier recombination because such a structure is composed of smaller particle sizes. One-dimensional (1D) nanostructures (such as nanowires and nanotubes) can provide a high surface area and rapid diffusion in a single direction, yielding a low recombination of electron–hole pairs. Mor et al. ^[18]

formed TiO2 nanotube arrays by anodizing a Ti sheet and measured an efficiency of 4% under UV irradiation (λ = 320 nm–400 nm). Subsequently, Paulose et al. ^[19]

prepared TiO2 nanotubes with a length of 45 μm using an anodizing method and achieved a photocurrent of 26 mA/cm² and a conversion efficiency of 16.25%

under UV irradiation (λ = 320 nm–400 nm). Hartmann et al. ^[20] reported that mesoporous TiO2 films prepared using the sol-gel method were approximately 10 times as efficient in water-splitting reaction than their counterparts obtained from crystalline TiO2 nanoparticles because of their thick, continuous pore walls and lower recombination rate.

ZnO may become an alternative to TiO2 for fabricating photoelectrodes because of its various advantages, including higher carrier mobility, greater chemical and thermal stability, and lack of toxicity. The CB of ZnO is also at a more negative position than that of TiO2, such that the photogenerated electrons are more able to reduce protons, an ability that corresponds to greater solar energy conversion efficiency. In addition, the morphology of ZnO can be easily controlled by adjusting synthesis conditions. Variously shaped ZnO nanostructures have been

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demonstrated.^[21] Ahn et al. ^[22] produced a ZnO nanocoral and measured a photocurrent of 0.25 mA/cm² under a tungsten halogen lamp (λ = 350 nm–750 nm). Qiu et al. ^[23] prepared ZnO nanotetrapods by using a multi-step growth process, and measured a photocurrent of 0.12 mA/cm² and a conversion efficiency of 0.045% under irradiation from a solar simulator (λ = 350 nm–800 nm). ZnO was easy to produce into many nanostructure, for example nanorods, nanocorals or nanoflowers. The one-dimesional ZnO nanorod has many advantage for using in photoelectrode material as list in follow: 1. The one-dimensional nanorods nanostructures exhibit the high surface area which could increase the the contact interface between ZnO and electrolytes. 2. The one-dimensional nanorods provide an electron transportation route for the major carrier (electron in ZnO) and could shorten the diffusion length of the minor carrier simultaneously which inhibit the recombination possibility of the photoexcited electron hole pairs. 3. The one- dimesional nanorods also improved the light absorption owing to the increase of the light diffucsion length. 4. The one-dimensional ZnO nanorods could lower the overpotential due to increase the reaction sites.

In general, the ZnO exhibit higher carrier mobility, greater chemical and thermal stability and lack of toxicity than TiO2. The higher conduction band level could generate more energytic electron than TiO2. In this thesis, we chosen the ZnO nanorods as our major photoelectrode material.

Although both TiO2 and ZnO have high performance in the UV region, their wide bandgap nature cannot capture solar light in the visible region, in which photons of sunlight are most abundant and thus result in low energy utilization.

Therefore, materials with a narrower bandgap are required.

With respect to narrow-bandgap materials, BiVO4 [24,25], WO3 [26,27], and Fe2O3 [27-30] are very attractive narrow-bandgap materials.

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BiVO4 thin films comprised of ordered arrays of pyramidal-shaped nanowires have been successfully fabricated through seed-mediated growth in an aqueous BiVO4 suspension, and a photocurrent of 0.4 mA/cm² was measured under AM 1.5 irradiation (λ = 400 nm–700 nm; 100 mW/cm²).^[31] Tungsten trioxide (WO3) and hematite (α-Fe2O3) have bandgaps of 2.2 and 2.6 eV respectively, which make them effective semiconductor oxides that serve as oxygen-evolving anodes to absorb photons from the blue regions of the solar spectrum. Cristino et al. ^[32]

prepared mesoporous WO3 films using an anodization method and measured a photocurrent of 9 mA/cm² under AM 1.5 irradiation (λ = 300 nm–700 nm; 370 mW/cm²). Kim et al. ^[33] prepared a WO3 photoelectrode with a mesoporous nanostructure by using polyethyleneglycol as a surfactant, and the photoelectrochemical properties of the photoelectrode revealed a photocurrent of 3.7 mA/cm² under AM 1.5 irradiation (λ = 300 nm–700 nm). Su et al. ^[34]

successfully prepared vertically aligned WO3 nanoflaskers by using a solvothermal technique and was able to fabricate nanowires and nanoflakes by adjusting the composition of the solution. A photocurrent of 1.43 mA/cm² and an incident-photon-to-current-conversion efficiency (IPCE) as high as 60% at λ = 400 nm were measured.

α-Fe2O3 , which has a small bandgap, low cost, and high chemical stability, is one of the most promising metal oxide semiconductor materials for the splitting of water using solar energy. Mohapatra et al. ^[35] reported an anodization method for growing Fe2O3 nanotubes of an ultrathin wall with a thickness of 5 nm to 7 nm and Fe foil with a length of 3 μm to 4 μm. A photocurrent density of 1.41 mA/cm² was obtained using hematite nanotube arrays, and a maximum solar-to-hydrogen conversion efficiency of 0.84% was reached under AM 1.5 conditions. Brillet et al. ^[36] elucidated a solution-based strategy and fabricated a porous electrode by

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encapsulation with an SiO2 confinement scaffold, yielding a high water oxidation photocurrent of 2.34 mA/cm² under AM 1.5 illumination. This high photocurrent density was attributed to the activation of nanostructured hematite photoanodes with control over the a wide range of particle sizes in the porous film. Generally, hematite has a relatively poor photoelectrochemical performance, which is attributable to its short charge carrier diffusion length and the slow kinetics of oxidation of water by the valence band holes.^[37]

A photoelectrode made of single-component photo-active materials cannot deliver the required performance in terms of either photocurrent or conversion efficiency to meet the demands of daily life. The limitations on their performance include low absorbance in the visible region, poor charge-carrier transportation, poor collection of photogenerated electrons, and limited chemical stability in an electrolyte under illumination.