Electron Population and Water Splitting Activity Controlled by Strontium Cations Doped in KTaO₃ Photocatalysts

KTaO 3 was doped with Sr cations, and its photocatalytic activity for the overall water splitting was investigated. By doping with Sr, a Sr-rich shell was formed over a Sr-poor core. Extended X-ray absorption fine structure spectroscopy revealed the simultaneous occupation of the K sites and Ta sites by Sr cations. A concentration gradient of Sr cations occupying the Ta sites was suggested to be produced after doping with Sr. The concentration gradient of Sr cations simultaneously induced an energy gradient of the conduction band edge bottom. Hence, photoexcited electrons are separated, which are driven away to the bulk by the energy gradient, from the complementary holes present on the surface. The higher the Sr cation concentration, the higher the energy gradient produced. Thus, the electron-hole separation is improved and the electron population accordingly increases with the increase in the Sr cation concentration. Nevertheless, the increase in the Sr cation concentration led to the decrease in the water splitting activity. The highest water splitting activity was observed for bare KTaO 3 . The decreased water splitting activity was proposed to be primarily due to the blockage of the electron transfer to the cocatalysts by the Sr-rich shell and the poor O 2 evolution ability of the Sr-rich shell.