Conformal Cu₂S-coated Cu₂O nanostructures grown by ion exchange reaction and their photoelectrochemical properties

Cuprous oxide Cu₂O is a promising p-type semiconductor for photoelectrochemical (PEC) solar hydrogen generation because it has a suitable bandgap (E_g = 2.0-2.2 eV) and a band alignment adapted to water reduction. In addition, metallic Cu is earth-abundant thus making Cu₂O a low-cost material. However, the reduction potential of Cu₂O into metallic Cu (0.47 V versus RHE) is lower than that of water which induces a severe instability under irradiation in a PEC cell. Therefore, our recent efforts focused on the growth of a protective overlayer on top of Cu₂O in order to stabilize Cu₂O when used as a photocathode in an aqueous electrolyte. Among potential protective materials cuprous sulphide Cu₂S is another p-type semiconductor with a 1.2 eV bandgap and an appropriate energy level alignment with Cu₂O that would allow electrons flowing to the interface. We present here an original and simple method aimed at protecting a compact layer (CL) or nanowires (NWs) of Cu₂O with a Cu₂S coating. Our method is based on the ions exchange reaction (IER) of O^2- into S^2- at the surface of Cu₂O itself in a solution-containing Na₂S as the sulphur source. The local surface IER implies the formation of a conformal and uniform coating independently on the starting Cu₂O morphology, CLs or NWs. As expected, coating Cu₂O photocathodes by a conformal Cu₂S layer improves their stability and PEC performances.