Porous CuBi₂O₄ photocathodes with rationally engineered morphology and composition towards high-efficiency photoelectrochemical performance

CuBi₂O₄ with a suitable optical bandgap and positive onset potential is a promising photocathode material for solar hydrogen evolution, while fine control of the morphology and chemical composition (Cu/Bi ratio) is currently still highly desirable for pursuing its high photoelectrochemical performance. Here, we demonstrate the facile synthesis of high-quality nanoporous CuBi₂O₄ films through spin coating with a vividly controlled morphology and Cu/Bi composition. Through systematic analysis of energy bands, we found that the electronic structure of CuBi₂O₄ was much sensitive to the variation of the Cu/Bi ratio, which could result in different photoelectrochemical performances. The nanoporous CuBi₂O₄ films with enhanced light absorption and a shortened carrier diffusion pathway exhibited a high photocurrent density of up to 2.45 mA cm^-2 at 0.6 V vs. the reversible hydrogen electrode with H₂O₂ as a sacrificial agent. By further optimizing the chemical compositions, the photocurrent density of the film with a Cu/Bi ratio of 0.55 could increase to 2.66 mA cm^-2, which is among the top recorded values of CuBi₂O₄ based photocathodes. This work reveals the significance of rational control of the morphology and chemical composition for the development of highly efficient CuBi₂O₄ photoelectrodes.