Boosting Oxygen Evolution Reaction Performance on BiVO4 Photoanode via Gradient Oxygen Vacancies

BiVO₄ is extensively studied in photoelectrochemical water splitting systems. Its relatively large bandgap and low charge carrier mobility severely limit the solar-driven oxygen evolution reaction on BiVO₄. A gradient distribution of oxygen vacancies can modulate the structural and electrical properties of BiVO₄. However, the formation of oxygen vacancies remains a daunting challenge. Here, we propose a simple hydrothermal post-treatment method to deposit a thin amorphous BiO_x layer (∼8 nm) with gradient oxygen vacancies on the surface of BiVO₄ and enable directional hole migration. The resultant BiVO₄/BiO_x/NiFeO_x photoanode achieves a high photocurrent density of 6.34 mA cm^–2 at 1.23 V (vs RHE) in 1.0 M potassium borate solution. A solar-to-hydrogen conversion efficiency of 0.89% is obtained in a device using a BiVO₄/BiO_x/NiFeO_x photoanode and a CuBi₂O₄ photocathode at zero bias. This work highlights a novel strategy for modulating the oxygen vacancy gradient and provides insights into the design of unbiased solar water splitting systems.