Theoretical investigation of the metal-doped SrTiO ₃ photocatalysts for water splitting

The effects of a series metal dopants on the photocatalytic activity of SrTiO ₃-based photocatalyst are investigated using first principle DFT calculations. The SrTiO ₃:Rh(1%) loaded with Pt has been found to give the best efficiency in water splitting. However, the same host doped with Ru leads to very low H ₂ evolution rate even it has a better visible light response. The analysis of the density of states and the calculated absorption spectra were used to illustrate the mechanisms that influence the photocatalytic efficiency. Our calculation results suggested that the two competing factors, the free electron generation (via light harvesting) and the charge recombination (due to the presence of recombination centers) process resulted in the existence of the optimal dopant concentration for the transition-metal-doped SrTiO ₃ lattice. The energy states introduced by dopant Rh in the bandgap of SrTiO ₃ were found to be very close to the valence band maximum. These new states thus reduce the bandgap of catalyst and enhance its light absorption capability. Furthermore, the proximity of these states to the valence band allows for efficient electron replenishment and thus reduces the probability of trapping electrons from the conduction band. In contrast, the energy states introduced by dopant Ru are significantly higher than the valence band making them an isolated recombination center. These Ru associated states also reduces the driving force for oxidation reaction. As a result, the Rh-doped SrTiO ₃ catalysts are found to provide a high H ₂ evolution rate.