Integrating heteromixtured Cu2O/CuO photocathode interface through a hydrogen treatment for photoelectrochemical hydrogen evolution reaction

Mono and divalent copper oxide (Cu2O and CuO) has been considered as a potential semiconductor photocathode with capability for hydrogen (H2) production. The use of one-dimensional (1D) nanostructures as photocathode is desirable for the photoelectrochemical water-splitting device due to the large surface areas, lateral carrier extraction property, and excellent light-harvesting potentiality. Accordingly, the numerous attempts to develop copper oxide heterojunctions via a bottom-up approach has been achieved, but, the resulting composites have suffered from the poor interfacial configuration. Here, we demonstrate the intimate modulation through the different ambient annealing of photocathodes could alter the nanostructure, light-harvesting, and optical bandgap. Accordingly, the Cu2O/CuO heterostructure was synthesized by in situ growth via simple electrochemical anodization of Cu foil followed by annealing in air, and then the surfaces of the heterostructure were sequentially modified by the formation of Cu(OH)2 via the post-annealing process under 4% hydrogen-argon mixed gas at 300 °C. As a result, a mild hydrogen treatment (H2@1min) was found to be effective in enhancing the PEC properties, due to the favorable formation of Cu2O/CuO/Cu(OH)2 layer, promoting the photogenerated charge collection yield; the generation of sufficient cation vacancies was closely related to the electronic conductivity and the charge-transfer yield. The TEM and XPS results confirmed that the hydrogen treatment found a slightly thin Cu(OH)2 layer was produced as an efficient co-catalyst from the inherent chemical reduction of prominent CuO phase to develop the junction of Cu2O/CuO/Cu(OH)2. Overall, the Cu2O/CuO/Cu(OH)2(H2@1min) film exhibited an obvious improvement in photocurrent density of −2.3 mA/cm2 at 0 V vs. reversible hydrogen electrode (RHE) (abbreviated as 0 VRHE) than those nanowires having −1.72 and −1.44 mA/cm2 of both Cu2O/CuO/Cu(H2@3min) and Cu2O/CuO(Air), respectively.