Rhodium-Doped Barium Titanate Perovskite as a Stable p-Type Photocathode in Solar Water Splitting

Solar water splitting from a p-n-conjugated photoelectrochemical (PEC) system is a promising way to produce hydrogen sustainably. At present, finding a compatible p-type photocathode material for the p-n system remains a great challenge in consideration of the photocurrent and stability. This paper highlighted a promising candidate, Rh/BaTiO₃, by switching BaTiO₃ from an n-type photoanode to a p-type photocathode upon Rh doping. The dopant activated visible light absorption up to 550 nm and an onset potential as high as 1.0 V (vs RHE). Using surface photovoltage spectroscopy as a powerful characterization tool, the n- to p-type transition of the semiconductor was studied and explained microscopically by which we quantitatively isolated the cathodic contribution caused by the Rh dopant. Unbiased overall solar water splitting was accomplished by serially connecting the Pt/Rh/BaTiO₃ photocathode to a CoO_x/Mo/BiVO₄ photoanode, which produced a solar to hydrogen conversion efficiency of 0.1% and an excellent stability over 100 h of operation at ambient pressure. This work revealed the key role that the Rh dopant played in the n- to p-type adjustment of titanate semiconductors and demonstrated its great potential for application in PEC water splitting.