High and stable photoelectrochemical activity of ZnO/ZnSe/CdSe/CuxS core–shell nanowire arrays: nanoporous surface with CuxS as a hole mediator
Advanced materials for electrocatalytic and photoelectrochemical water splitting are key for taking advantage of renewable energy. In this study, ZnO/ZnSe/CdSe/CuxS core–shell nanowire arrays with a nanoporous surface were fabricated via ion exchange and successive ionic layer adsorption and reaction (SILAR) processes. The ZnO/ZnSe/CdSe/CuxS sample displays a high photocurrent density of 12.0 mA cm−2 under AM 1.5G illumination, achieves the highest IPCE value of 89.5% at 500 nm at a bias potential of 0.2 V versus Ag/AgCl, and exhibits greatly improved photostability. The functions of the ZnSe, CdSe, and CuxS layers in the ZnO/ZnSe/CdSe/CuxS heterostructure were clarified. ZnSe is used as a passivation layer to reduce the trapping and recombination of charge carriers at the interfaces of the semiconductors. CdSe functions as a highly efficient visible light absorber and builds heterojunctions with the other components to improve the separation and transportation of the photoinduced electrons and holes. CuxS serves as a passivation layer and an effective p-type hole mediator, which passivates the defects and surface states of the semiconductors and forms p–n junctions with CdSe to promote the hole transportation at the semiconductor–electrolyte interface. The nanoporous surface of the ZnO/ZnSe/CdSe/CuxS core–shell nanowire arrays, together with the tunnel transportation of the charge carriers in the thin films of ZnSe and CdSe, also facilitates the kinetics of photoelectrochemical reactions and improves the optical absorption as well.