Influence of grain size on photoelectrocatalytic performance of CuBi2O4 photocathodes

CuBi₂O₄ is an ideal photocathode material owing to its suitable optical bandgap (～1.8 eV), positive onset potential (～1 V vs. RHE), and high theoretical photocurrent density (19.7–29 mA/cm²). However, its relatively poor efficiency in transporting carriers hinders it from achieving high photoelectrocatalytic performance. In this study, we propose the preparation of phase-pure large-grain CuBi₂O₄ thin film photocathodes through solvent pre-annealing and two-step annealing, with the aim of improving the carrier transport efficiency. The maximum grain size of CuBi₂O₄ reached an astonishing 1 μm at the optimal ethanol vapor concentration. Through time-resolved photoluminescence, we discovered that after treating CuBi₂O₄ with the proposed technique, the carrier lifetime improved by more than one order of magnitude. This improvement was achieved because the large grain size reduced the inhibition of carrier transport through grain boundaries. Therefore, the photocurrent density of large-grained CuBi₂O₄ reached 0.27 mA/cm², which is 27 times that of a direct annealing treatment. Finally, we used atomic layer deposition to load a ZnO protective layer and Pt catalyst onto the surface of CuBi₂O₄ photocathodes, and the photocurrent density of CuBi₂O₄/ZnO/Pt was further increased to 0.46 mA/cm² without using an electron scavenger (0.4 V vs. RHE).