Near-Ideal Copper Oxide Heterostructure Design for Photoelectrochemical Water Splitting.

In recent years, photoelectrochemical (PEC) hydrogen generation through water splitting has gained significant attention as a carbon-free solar-to-energy conversion strategy. Among various materials, copper oxides, specifically cupric oxide (CuO) and cuprous oxide (CuO), have been extensively investigated for their suitable band positions and prominent performance, particularly in heterostructures. However, previously reported heterostructures, such as CuO layers on CuO, are not ideal configurations in terms of photoelectrical properties. In this study, we introduce the fabrication approach for an ideal heterostructure consisting of CuO nanowires on a CuO/CuO mixed-phase film, fabricated by a straightforward electrochemical/thermal method. The CuO nanowire with Cr layer (CNwC) shows potential for solar energy harvesting due to its suitable band positions and narrow bandgap, enabling enhanced photoabsorption across the entire visible spectrum. A thin chromium (Cr) layer underlying the nanostructure contributes to the formation of the ideal copper oxide heterostructure, acting as an adhesive and protective layer. The Cr layer is oxidized during the fabrication process of the CNwC and supports the hydrogen evolution reaction for water splitting. Moreover, the anodization time critically influences the phase composition, size, and density of the nanowires. Under optimal conditions, collective and slanted CuO nanowires can absorb incident light, maximizing both photon absorption and photon-to-energy conversion efficiency.