Mechanistic insights into the role of empty mid-gap states in Al- and Rh-doped SrTiO3 for photocatalytic water splitting

SrTiO₃, a widely studied photocatalyst, exhibits enhanced efficiency in water splitting by heteroatom doping. Al and Rh are key dopants that enable SrTiO₃ to achieve a solar-to-hydrogen conversion efficiency of ∼ 1 %. However, Al-doped SrTiO₃ can catalyze overall water splitting, while Rh-doped SrTiO₃ can only catalyze hydrogen evolution. The mechanism behind this difference remains unclear. Both Al and Rh dopants introduce empty mid-gap states into SrTiO₃ which can facilitate the reaction. Our first-principles calculations reveal that the empty mid-gap states in Al-doped SrTiO₃ are fully localized on the surface oxygen atoms, where they directly participate in the oxygen evolution reaction (OER). In contrast, for Rh doping, half of the empty mid-gap states are localized on Rh atoms. For OER to proceed, a high energy barrier must be overcome to facilitate the migration of holes from the Rh atoms to oxygen. Taking the SrTiO₃ (001) surface as an example, OER is an endothermic reaction with a heat absorption of 1.3 eV on the perfect surface, but it becomes exothermic by 1.2 eV upon Al doping. After Rh doping, although the heat absorption decreases to 0.4 eV, the energy barrier of the rate-determining step increases by 0.1 eV compared to the perfect surface.