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Solution Synthesis and Texture Engineering of BiVO4 Photoelectrodes for Enhanced Photoelectrochemical Activity

  • 황성원 (아주대학교 일반대학원)

초록/요약

Bismuth vanadate (BiVO4, BVO) has emerged as a promising photoanode material in photoelectrochemical water splitting (PEC), attracting significant research interest in recent years. This material has a narrow band gap (2.4 eV), allowing it to absorb a broader range of visible light than other photoanodes (e.g., WO3 and TiO2). Additionally, the valence and conduction band positions of BVO are well-aligned, positioned around +2.8 and 0.4 V versus the Standard Hydrogen Electrode (SHE), respectively. This alignment allows efficient charge transfer for PEC water oxidation. However, despite its theoretical potential, pristine BVO photoanodes exhibit a significant performance gap. While theoretical calculations predict a maximum photocurrent density of 7.5 mA/cm² under AM 1.5 G illumination (100 mW/cm²), experimentally observed values typically fall short. In order to enhance the PEC activity of BVO photoanodes, various strategies are carried out, including morphology control, heterojunction formation, defect, and texture/facet engineering. In this thesis, three different strategies (morphology control, triple layered heterojunction design, and texture engineering) are studied to improve the PEC activity of BVO photoanode. First, we introduce an electron-beam evaporation (EB) method to deposit phase-pure and large-grained BVO photoanode: varying substrate temperature and emission current control BVO film phase purity and grain size. Optimally prepared EB-BVO exhibits large grains (~400 nm) with oxygen vacancies, enhancing photoelectrochemical (PEC) performance. Finally, a photocurrent density of ~1.0 mA/cm2 at 1.23 V versus a reversible hydrogen electrode, 50% higher than the conventional sol-gel derived BVO. CoOx oxygen evolution electrocatalyst (OEC) deposition further increases photocurrent density up to 2.4 mA/cm2, significantly improving stability. Second, we designed a triple-layered TiO2/BiVO4/SnO2 (T/B/S) photoanode fabricated via sol-gel spin-coating, yielding improved PEC water-oxidation performance and high visible transmittance (>510 nm). The T/B/S structure features a bottom SnO2 layer that increases BiVO4 grain size (~600 nm) and forms a type-II heterojunction, enhancing charge separation and electron transport. A top TiO2 layer protects against photocorrosion. The resulting photoanode, devoid of electrocatalysts, achieves photocurrent densities of ~2.3 mA/cm2 and ~3.7 mA/cm2 at 1.23 V versus reversible hydrogen electrode for water oxidation and H2O2 oxidation, respectively, with higher stability compared to other configurations. Next, we describe a one-pot solution synthesis of (00l)-textured and surface- reconstructed BiVO4 photoanode (namely, ts-BVO), enhancing bulk and surface charge transport efficiencies through a stepwise dual reaction (SDR) mechanism. Ethylene glycol (EG) addition facilitates texture development and surface reconstruction. Optimal ts-BVO achieves significantly improved bulk charge transport (70%) and surface charge transfer (85%) efficiencies compared to non- textured BVO. Deposition of CoBi oxygen evolution electrocatalyst results in stable photocurrent density of 4.3 mA/cm2 at 1.23 V versus reversible hydrogen electrode and high faradaic efficiency of 98% under one sun illumination. The texture and surface reconstruction engineering effectively improve intrinsic material properties for PEC water splitting. Hence, our studies provide the novel synthesis methods and texture engineering approaches for developing efficient BiVO4 photoanodes for PEC water splitting and hydrogen production. This work paves the way for further advancements in photoanode design, potentially leading to even higher efficiencies in future research. The optimized approach for texture growth control and surface reconstruction also holds promise for broader applications beyond PEC water splitting. It could apply to various metal oxide preparation methods in diverse energy fields, including photocatalysis, Li-ion batteries, supercapacitors, and upcycling of wastewater and biomass, etc. KEYWORDS: Bismuth vanadate, e-beam evaporation deposition, large grain, oxygen vacancy, triple-layer, heterojunction, sol-gel method, one-pot hydrothermal synthesis, texture engineering, surface reconstruction, photoelectrochemical water splitting, electrochemical properties, and hydrogen production.

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목차

Chapter 1. Introduction 1
1.1 Global energy consumption, CO2 emission and climate crisis 1
1.2 Renewable and carbon free energy source 6
Chapter 2. Research background 7
2.1 Hydrogen production method 7
2.2 Solar-driven photoelectrochemical (PEC) water splitting 9
2.2.1 PEC cell 12
2.2.2 Working principle of PEC device 14
2.3 Bismuth vanadate (BiVO4, BVO) as photoanode material 16
2.4 Electron beam evaporation method 23
2.5 Sol-gel method 25
2.6 Hydrothermal method 27
2.7 Texture engineering 28
Chapter 3. Synthesis of BiVO4 Photoanode using Electron-beam Evaporation of a Single Precursor Source for Enhanced PEC Activity 30
3.1 Introduction 30
3.2 Experimental 32
3.2.1 Preparation of BiVO4 source powder and sintered pellet 32
3.2.2 Electron-beam evaporation deposition 33
3.2.3 Characterization of Materials 33
3.2.4 Electrochemical (EC) and photoelectrochemical (PEC) analysis 34
3.3 Results and Discussion 36
3.3.1 Electron-beam evaporation synthesis of BiVO4 fil 36
3.3.2 Optical properties and electrochemical characterization 47
3.3.3 PEC performance 55
3.4 Conclusion 64
Chapter 4. Solution-processed TiO2/BiVO4/SnO2 Triple-layer Photoanode with Enhanced Photoelectrochemical Activity 65
4.1 Introduction 66
4.2 Experimental 68
4.2.1 Sol-gel synthesis of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode 68
4.2.2 Material characterization 70
4.2.3 PEC measurements 70
4.3 Results and Discussion 72
4.3.1 Characterization of TiO2/BiVO4/SnO2 (T/B/S) triple-layer photoanode 72
4.3.2 Optical properties of three photoanodes 80
4.3.3 PEC performance 84
4.4 Conclusion 97
Chapter 5. Stepwise Dual Reaction Synthesis of Textured and Surface-reconstructed BiVO4 with Enhanced PEC Water-splitting Activity 98
5.1 Introduction 98
5.2 Experimental 101
5.2.1 Preparation of BVO seed-layer 101
5.2.2 Hydrothermal growth of textured and surface-reconstructed BVO (ts-BVO) photoanode 101
5.2.3 Photo-electrodeposition of CoBi electrocatalyst 102
5.2.4 Material characterizations 102
5.2.5 Photoelectrochemical measurements 103
5.2.6 Gas Chromatography (GC) measurements 104
5.3 Result and discussion 105
5.3.1 One-pot hydrothermal synthesis of (00l)-textured and surface reconstructed BiVO4 (ts-BVO) 105
5.3.2 Growth and texture development of ts-BVO 113
5.3.3 Surface reconstruction of BVO photoanode 123
5.3.4 Photoelectrochemical (PEC) performance 132
5.4 Conclusion 150
Chapter 6. Conclusion 151
References 154

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