Solution–processed Cu2O/ZnO/TiO2/Pt nanowire photocathode for efficient photoelectrochemical water splitting

•Cu2O NWs with ultralow VCu concentration is grown on Cu foil via simple wet chemistry combined with post–thermal treatment.•Photoexcited charge carriers are efficiently rectified by the surface band bending extended deep into Cu2O NWs .•Charge separation is further reinforced by conventional Cu2O/ZnO/TiO2/Pt multi–junction approach prepared via CBD method.•Photoexcited electrons are transferred from Cu2O through ZnO, TiO2 and Pt, and eventually injected into the electrolyte.•The synergistic effect renders the photocurrent density of the Cu2O/ZnO/TiO2/Pt NWs amounted to −8.2 mA cm-2. [Display omitted] State–of–the–art Cu2O photocathodes are mostly fabricated by means of electrodeposition and have been reported to be flooded with the copper vacancies (VCu), into which the photoexcited electrons are rapidly trapped and thereby hardly injected into the electrolyte for photoelectrochemical water splitting. To address this issue, an alternative approach consisting of simple wet chemistry combined with post–thermal treatment is employed in the present contribution to grow the Cu2O nanowires (NWs) on the Cu foil, which provides the Cu ions to fill VCu. An important result of the depletion of VCu is the space charge layer extending deep into the Cu2O NWs, wherein the photoexcited charge carriers are rectified by the surface band bending for efficient separation. The charge separation is further reinforced by using ZnO to first extract the photogenerated electrons from Cu2O, TiO2 to further glean the electrons from ZnO, and the Pt co–catalyst to facilitate their injection into the electrolyte. These materials are deposited sequentially on the Cu2O NWs by means of chemical bath deposition, which is put forward in the present contribution as a cost–effective and readily scalable alternative to the conventional atomic layer deposition technique. The synergistic effect leads to a photocurrent density of the Cu2O/ZnO/TiO2/Pt NWs of −8.2 mA cm−2, well outperforming that of the pristine Cu2O NWs by more than 2.5–fold and, moreover, being comparable to that of the electrodeposited counterparts reported in the literature.