Electrochemical defect control of bulky crystalline CuBi2O4 film and the band edge alignment for photoelectrochemical water reduction

Hetero-metal oxides have been used in photoelectrochemical (PEC) water splitting systems, but synthetic-originating defects and charge recombination process degrades PEC cell performance. Herein, we studied intrinsic physical properties of pure-phase copper bismuth oxide (CuBi2O4, CBO) photocathode through controlling defects and band edge alignment. Preparation of pure-phase CBO film with large grain sizes (average similar to 290 nm) enabled to investigate the correlation between CBO's crystal structure and charge carrier transport efficiency. The Cu1+-V-o point defects were regulated through electrochemical oxidation or thermal oxygenation under argon, air, and O-2 atmosphere. The thermal treatment in an O-2-saturated environment significantly reduced Cu1+-V-o defects, increasing charge carrier density, thereby reducing band gap, which eventually facilitated the charge transport. Moreover, electrochemical oxidation produced similar band structure to thermal oxygenation under O-2, demonstrating a high level of Cu1+ defect control could be achieved through electrochemical oxidation as well as thermal oxygenation, showing systematic adjustment of the CBO band edge. Additionally, the hole-transfer heterojunction at the CBO film's back side was engineered using copper oxide (CuO) thin film for interfacial band alignment. As a result, band edge-aligned FTO|CuO|CBO heterojunction exhibited a remarkably increased photocurrent density up to 2.63 mA/ cm(2) at 0.4 V vs. RHE in alkaline electrolyte.