Identifying Copper Vacancies and Their Role in the CuO Based Photocathode for Water Splitting.

Metal oxides are an important family of semiconductors for effective photoelectrodes in solar-to-chemical energy conversion. Defect engineering, such as modification of oxygen vacancy density, has been extensively applied in tailoring the optoelectric properties of photoelectrodes. Very limited attention has been paid to the influence of metal vacancies. Herein, we study metal vacancies in a typical CuO photocathode for photoelectrochemical (PEC) water splitting. The Cu vacancies can improve the charge carrier concentration, and facilitate the charge separation and transfer in the CuO photocathode. By changing the O2 partial pressure, the density of Cu vacancies can be tuned, which 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 vacancies on the PEC is also observed in other Cu based photocathodes, showing the generic role of metal vacancies in efficient photocathodes.