Defect Engineering of a CuBi₂O₄ Photocathode for Bias-Free Photoelectrochemical Water Splitting

The performance of metal oxide semiconducting electrodes in photoelectrochemical water splitting is often limited by complex defects that obscure the role of individual defect species and hinder rational optimization. Taking CuBi₂O₄ photocathodes as an example, the overlapping effects of Cu⁺ ions (Cu⁺) and oxygen vacancies (O_V) complicate the understanding of charge-carrier recombination. Here, we propose a methodology for the precise modulation of defect concentrations by meticulously controlling the oxygen content during the postannealing process of vacuum-evaporated Cu/Bi precursors. This approach reveals that Cu⁺ species act as the dominant recombination centers, whereas the presence of O_V plays a secondary role. Consequently, reducing the Cu⁺ content in the CuBi₂O₄ photocathode yields a 6.39-fold increase in charge-separation efficiency and a positive shift in the onset potential (0.96 V vs RHE). Coupling the optimized CuBi₂O₄ photocathode with a BiVO₄-based photoanode results in an efficient bias-free solar water-splitting device, which delivers a photocurrent density of 0.22 mA cm^-2 under simulated one-sun illumination. This performance places the device at the forefront of unbiased solar water-splitting systems based on metal oxide photoelectrodes without interfacial modification.