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)/VO_<i>x</i> photoanode, simultaneously introducing V⁵⁺ into the BiVO₄ lattice during the VO_<i>x</i> 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)/VO_<i>x</i>/FeNiO_<i>x</i> 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.