Optimal Electrical and Optical Heterostructure of Copper Oxide for Photoelectrochemical Water Splitting

Climate change is a serious global challenge arising from exponentially increased carbon emissions. With the extensive increase in atmospheric greenhouse gas concentration and its chain effect on the environment, the development of eco-friendly and sustainable sunlight harvesting technologies has been highly demanded. The continuing efforts for sustainable energy have triggered the renaissance of a plenty of solar-to-fuel strategies such as CO₂ reduction, oxygen reduction, and water splitting. In particular, photoelectrochemical water splitting stands out due to its critical role in converting abundant water into hydrogen energy. Hydrogen functions as an indispensable resource in many fields such as public transport, business, and green energy.<br/><br/>In current developments, the most paramount feature in a photocatalyst is the selection of materials from among the various available semiconductors. Owing to sufficient band alignment near the reduction potential, several p-type semiconductors are appropriate to convert water to hydrogen energy. Especially, cuprous oxide (Cu₂O) and cupric oxide (CuO) are spotlighted as promising materials for photocathode due to high abundance on Earth, ease of processing, and cost-friendly fabrication process. Moreover, Cu₂O has spacious inclusion of reduction potential and CuO can highly absorb overall visible light. Accordingly, the strategic design of an innovative heterostructure of copper oxides is vital for evolving the water splitting field.<br/><br/>Here, we propose a novel heterostructure that consists of Cu₂O nanowires on a CuO/Cu₂O mixed-phase film. This design operates as the highly efficient photocathode because CuO components in the mixed-phase film contribute to high absorption from light incidence while the nanostructure of Cu₂O results in an enlarged reactive area and outstanding electrical properties. To design the optimal heterostructure, we introduce a Cr intermediate layer and conditional adaptation of the anodization time, which induces various dimensions of slanted nanowires. The Cr-based layer lying between Cu₂O nanowires and the mixed-phase film has functional roles such as adhesion, protection for the beneath film, and the aid of optical improvement. Cr converts into chromium oxide during the annealing process, which does not obstruct light propagation to underlying film. Chromium oxides can assist efficient water splitting by blocking the backward reaction and maintaining stable operation. Additionally, the anodization time determines elemental change and structural parameters such as the diameter, length, and fill factor of the nanowire array. The straightforward fabrication process of electrodeposition, sputtering, and thermal treatments support the successful formation of Cu₂O nanowires on the mixed-phase film. Finally, the atomic layer deposition of TiO₂ improves the durability of the photocathode.<br/><br/>Last, we computationally and experimentally demonstrate the remarkable photoelectrochemical performances of an ideal configuration of copper oxides. We numerically calculated optical efficiencies of the photocathode. Light penetration is observed from Cu₂O nanowires at short wavelengths to the mixed-phase film at longer wavelengths. Also, linear sweep voltammetry and electrochemical impedance spectroscopy indicate the electrical characteristics, which consequently affect the incident photon-to-electron conversion efficiency. Compared with single films and the over-anodized sample (<i>i.e.,</i> CuO nanowires), Cu₂O nanowires on the mixed-phase film verify the utmost photoelectrochemical capabilities due to the high absorptance (~90%) in the visible region and lowered the impedance component. With the superior ability to generate hydrogen, this advanced design could be applied to CO₂ reduction, leveraging the extensive reduction capacities of nanostructured Cu₂O.