Unraveling and Compensating Surface Vanadium Vacancies in BiVO ₄ Photoanodes via a Hydrothermal Diffusion Strategy for Enhanced Photoelectrochemical Water Oxidation

Although cocatalysts have been employed to enhance the photoelectrochemical (PEC) performance of BiVO₄, it remains unclear whether the improvements primarily arise from suppressed charge recombination or accelerated surface oxygen evolution reaction (OER) kinetics, as the role of surface defects in BiVO₄ is still poorly understood. Herein, we report a hydrothermal diffusion strategy to construct a hybrid BVO(V)/VOx photoanode, simultaneously introducing V⁵⁺ into the BiVO₄ lattice during the VOx deposition process, effectively compensating for surface vanadium vacancies. Comprehensive analyses reveal that the PEC enhancement primarily originates from suppressed surface recombination rather than merely improved OER kinetics. The optimized dual-overlayer BVO(V)/VOx/FeNiOx photoanode demonstrates a photocurrent density of 5.82 mA cm⁻² at 1.23 V vs RHE, which is 5.18 times higher than that of pristine BiVO₄, achieving near-unity charge separation efficiency and excellent long-term durability. These findings suggest that surface vanadium vacancies are the primary cause of interfacial charge recombination in BiVO₄, and hydrothermal V-source diffusion offers a viable strategy for defect compensation and performance enhancement. This work reveals the underlying nature of surface defect states in BiVO₄ and offers a robust surface engineering strategy for photoanodes in solar fuel applications.