Identifying Copper Vacancies and their Role in CuO based Photocathodes for Water Splitting

Metal oxides represent an important family of semiconductors for fabricating effective photoelectrodes in solar-to-chemical energy conversion application. Defect engineering such as modification of oxygen vacancy density has been widely studied in tailoring the optoelectronic properties of photelectrodes, but very limited attention has been paid to the influence of metal vacancies. In this work, we focus on the understanding of metal vacancies in a typical CuO photocathode for photoelectrochemical (PEC) water splitting. We find that the presence of Cu vacancies can improve the charge carrier concentration, and facilitate the charge separation and transfer in the CuO photocathode. By changing O2 partial pressure during heat treatment of thin film samples at 450oC, the density of Cu vacancies can be effectively tuned, which subsequently leads to improved PEC performance. The CuO photocathode prepared in pure O2 exhibits a 100 % photocurrent increase compared to that prepared in air. The promotion effect of Cu vacancy on the PEC is also observed in other Cu based photocathodes (e.g., CuBi2O4 and CuFe2O4), highlighting the generic role of metal vacancies in achieving highly efficient photocathodes.