Near-Ideal Copper Oxide Heterostructure Design for\nPhotoelectrochemical Water Splitting

In\nrecent years, photoelectrochemical (PEC) hydrogen generation\nthrough water splitting has gained significant attention as a carbon-free\nsolar-to-energy conversion strategy. Among various materials, copper\noxides, specifically cupric oxide (CuO) and cuprous oxide (Cu<sub>2</sub>O), have been extensively investigated for their suitable\nband positions and prominent performance, particularly in heterostructures.\nHowever, previously reported heterostructures, such as CuO layers\non Cu<sub>2</sub>O, are not ideal configurations in terms of photoelectrical\nproperties. In this study, we introduce the fabrication approach for\nan ideal heterostructure consisting of Cu<sub>2</sub>O nanowires on\na CuO/Cu<sub>2</sub>O mixed-phase film, fabricated by a straightforward\nelectrochemical/thermal method. The Cu<sub>2</sub>O nanowire with\nCr layer (CNwC) shows potential for solar energy harvesting due to\nits suitable band positions and narrow bandgap, enabling enhanced\nphotoabsorption across the entire visible spectrum. A thin chromium\n(Cr) layer underlying the nanostructure contributes to the formation\nof the ideal copper oxide heterostructure, acting as an adhesive and\nprotective layer. The Cr layer is oxidized during the fabrication\nprocess of the CNwC and supports the hydrogen evolution reaction for\nwater splitting. Moreover, the anodization time critically influences\nthe phase composition, size, and density of the nanowires. Under optimal\nconditions, collective and slanted Cu<sub>2</sub>O nanowires can absorb\nincident light, maximizing both photon absorption and photon-to-energy\nconversion efficiency.