This study aimed to investigate the photoelectrochemical properties of GaN for solar hydrogen gas applications. The thin film case is initially considered. In order to understand the effect of the polar crystallographic facets of GaN, photoelectrochemical measurements of free-standing GaN film was investigated. In 1 M HCl and 100 mW/cm2 Xe lamp illumination, the Ga-polar demonstrates a more negative onset potential compared to the N-polar surface. At more positive applied voltages, however, the N-polar shows higher photocurrent. Investigations indicate that these PEC performance profile may be explained by difference in the positions of the conduction and valence band edges of the polar surfaces.

Using a simple and inexpensive photoelectrochemical and crystallographic etching process of a GaN thin film, the hydrogen generation efficiency was increased by 100%. Prior to etching, the thin film’s efficiency at the applied bias of 0.5 V versus counter electrode in 1.0 M HCl solution is 0.37%. After etching, the efficiency doubled to 0.75%. After five hours of continuous gas collection, the unetched GaN thin film yielded a stable photocurrent of 0.41 mA cm-2 which produced 0.10 mL of H2 gas. The etched sample, on the other hand, resulted to an improved stable photocurrent of 0.83 mA cm-2 and yielded a greater volume of 0.70 mL of H2 gas, with the presence of H2 confirmed through gas chromatography.

The oxidation catalyst cobalt phosphate was also deposited as a thin film on GaN. The cobalt phosphate improved on the oxidation properties of GaN. Furthermore, the deposition of cobalt phosphate improved the photostability of the GaN surface.

Finally, GaN nanowires were tested for their photoelectrochemical performance. Nanowires were successfully synthesized through Au-catalyzed thermal reconstruction method on GaN thin film. Photoelectrochemical measurements in 1 M HCl solution under 100 mW/cm2 of Xe light illumination of the nanowire system has shown markedly improved on the performance of the GaN nanowires compared to GaN nanowires synthesized by other methods (thermal chemical vapor deposition and metallo-organic chemical vapor deposition methods). High density nanowires fabricated from the three methods shows those that were synthesized through thermal reconstruction demonstrated photocurrent that was about one order higher than those synthesized through other techniques. The improved photoelectrochemical performance is attributed to the good interfacial contact between the nanowires and the GaN thin film substrate that allows good charge transfer during the photoelectrochemical process, and some improvements in the photoelectrochemical areas. One aspect of the nanowires growth that needed to be addressed, however, is the oxygen content of the sample. Further tests indicate the presence of oxides in the system, which may be core-shell structure with the GaN, or mixed GaN-GaOx system, a N-doped Ga2O3, or an O-doped GaN system.

Table of Contents
Abstract ii
Acknowledgements vi
Table of Contents ix
List of Figures xv

Chapter 1 Introduction to Solar Hydrogen Gas Generation 1
1.1 Importance of Solar Hydrogen Gas Generation 1
1.2 Modes for Water Solar Hydrogen Gas Generation 3
1.3 Requirements for Solar Hydrogen Gas Generation 7
1.4 The Photoelectrochemical Voltammetry Curve 8
1.5 GaN for Solar Hydrogen Gas Generation 10
1.6 Objective and Approaches to the Study 13
1.7 References 15

Chapter 2 Experimental Details 17
2.1 Introduction. 17
2.2. Characterization of the GaN System 18
2.3 Electrode Preparations 18
2.4 Determination of Mott-Schottky Plots 18
2.5 Synthesis of Nanowires 19
2.6. References 21

Chapter 3 Photoelectrochemical Hydrogen Production Properties of Ga-polar and N-polar GaN
Films 23
3.1 Introduction 23
3.2. Experimental 24
3.2.1 Characterization of the Free Standing GaN 24
3.2.2 Distinguishing the Polarity of the Free Standing GaN Sample 24
3.2.3 Electrode Preparations 25
3.2.4 Determination of the Mott-Schottky Plots 26
3.3 Results and Discussion 26
3.3.1 Quality of the Free Standing GaN 26
3.3.2 Photoelectrochemical Properties of the Free Standing GaN 28
3.4 Conclusion and Summary 38
3.5 References 39

Chapter 4 Improving the Solar Hydrogen Gas Generation Property of c-Plane GaN Thin Film
through Crystallographic Etching 41

4.1. Introduction 41
4.2. Experimental 42
4.2.1 Preparation of Pristine Sample 42
4.2.2 Preparation of the Etched GaN Electrode 43
4.2.3 Determination of Mott-Schottky Plots 43
4.2.4 Determination of Photoelectrochemical Performance 44
4.2.5 H2 Gas Generation and Collection 45
4.2.6 Probing the Reasons for Efficiency Increase 46
4.3. Results and Discussion 47
4.3.1 Characterization of GaN thin film 47
4.3.2 Effects of Crystallographic Wet Etching on c-plane GaN 48
4.3.3 Obtaining Greater Degree of Crystallographic Etching 51
4.3.4 Two-Electrode Testing and Calculation of Efficiency 55
4.3.4 Probing the Reasons for Efficiency Increase 59
4.3.5.1 Estimation of the Increase in Surface Area 59
4.3.5.2 Decrease in Surface Carrier Concentration 61
4.3.5.3 Appearance of Crystallographic Planes and Stepped Edges 65
4.4 Conclusions 67
4.5 Reference 69

Chapter 5 Thin Film Deposition of Co-Based Oxidation Co-Catalysts GaN Surface for
Photostabilization. 71
5.1. Introduction. 71
5.2 Experimental Details 73
5.2.1 Preparation of the GaN Electrodes 73
5.2.2 Deposition of the Cobalt Catalyst Thin Film 74
5.2.3 Photoelectrochemical Performance 75
5.2.4 Extended Test 76
5.3. Results and Discussion 76
5.4. Conclusions 83
5.5 References 84

Chapter 6 Achieving Good Electrical Contacts Between GaN Nanowires and Thin Film through
Thermal Reconstruction of GaN Thin Film for Water Splitting Applications 87
6.1 Introduction 87
6.2 Experimental 90
6.2.1 Synthesis of GaN NWs 90
6.2.2 Preparation of the Electrodes 91
6.2.3 Determination of Photoelectrochemical Performance 91
6.2.4 Determination of Mott-Schottky Plots. 92
6.3. Results and Discussion 92
6.3.1 Characterization of GaN Nanowires Synthesized Through Thermal
Reconstruction 92
6.3.2 Photoresponse of the Different Nanowire Systems 97
6.3.3 Advantage of the Thermally-Reconstructed Nanowire System 100
6.3.3.1 Changes in Carrier Concentration 100
6.3.3.2 Changes in Electroactive Surface Area 103
6.3.3.3 Effective in Charge Separation and Effect of Porous Layer 104
6.4 On th Formation of Oxides in Thermally Reconstructed Nanowires 110
6.5 Conclusions 113
6.5 References 114

Chapter 7. Summary and Future Perspectives 117

Appendix Electrochemical Characterization and Photoresponse of InN Thin Films for Biosensing
Applications 123
A.1 Introduction 123
A.2 Experimental 125
A.2.1 Synthesis of InN thin film by Gas-Source Molecular Beam Epitaxy 125
A.2.2 Preparation of InN electrode 126
A.2.3 Determination of Electrochemical Properties 126 A.2.3.1 Determination of Potential Window 127
A.2.3.2 Determination of ΔEp 127
A.2.3.3 Electrochemical Response to Different Chemicals. 127
A.2.4 Demonstration of InN Biosensing Properties 127
A.2.5 Photoresponse of InN thin film Electrode 128
A.3 Results and Discussion 128
A.3.1 Quality of InN thin film 128
A.3.2 Potential Window 130
A.3.3 Electron Transfer Reversibility 133
A.3.4 Responses to Various Redox Chemicals 135
A.3.5 Demonstration of InN Thin Film for Biosensing Properties 137
A.3.6 Photoresponse 140
A.4 Conclusions144
A.5 References144

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