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

Abstract

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.