Oxygen network formation for efficient charge transport and durable BiVO₄ photoanodes with ultra-thin TiO₂ layer in solar water splitting

Bismuth vanadate (BiVO₄) has emerged as a prominent oxide semiconductor in solar water splitting investigations owing to facile synthesis and favorable band-alignment with the water oxidation level. Oxygen and oxygen vacancies (V_O) within the BiVO₄ exhibit multifaceted roles across bulk, surface, and interface. This study presents a method for selectively regulating the surface and bulk V_O in BiVO₄ through facile chemical redox reactions. A large amount of V_O on the BiVO₄ surface enables the formation of a robust networking interface with oxide-based protection overlayers, whereas V_O in bulk region must be effectively suppressed. A significant amount of V_O can be specifically formed on the BiVO₄ surface via a controlled surface chemical reduction reaction (SCR) at the atomic level, precisely controlling the presence of O^2- and OH^- ions. Here, we quantitatively and qualitatively analyze V_O changes and the impact of V_O on photoelectrochemical operation. As a result, The SCR process allows for the strategic control of the BiVO₄ surface into a V_O-rich surface. The controlled surface enhances the charge kinetic by promoting the conformal coating of a n-TiO₂ protective layer and decreasing the charge loss in the interface junction with ensuring stability. Furthermore, the SCR-BiVO₄/TiO₂/CoPi photoanodes exhibit a highly stable photocurrent density of 3.9 mA cm⁻² at 1.23 V_RHE. Surface modification and understanding of the oxygen-end network can be broadly applied to photoelectrodes that require oxide-based overlays.