Energetics and optical properties of nitrogen impurities in SrTiO3 from hybrid density-functional calculations

Semiconductor photocatalysts that can produce hydrogen from water splitting are of great interest. Among the various possibilities, nitrogen (N)-doped $\mathrm{SrTi}{\mathrm{O}}₃$ is a promising candidate for hydrogen evolution under visible-light irradiation. In this study, hybrid density-functional calculations are employed to investigate the stability and impact of nitrogen impurities on the electronic and optical properties in $\mathrm{SrTi}{\mathrm{O}}₃$. We find that the substitutional N on O site (${\mathrm{N}}_{\mathrm{O}}$) is a deep acceptor in $\mathrm{SrTi}{\mathrm{O}}₃$. Moreover, ${\mathrm{N}}_{\mathrm{O}}$ predominates over other N-related defects under equilibrium growth conditions in $n$-type $\mathrm{SrTi}{\mathrm{O}}₃$. Our results reveal that ${\mathrm{N}}_{\mathrm{O}}$ gives rise to visible-light absorption in agreement with experimental observations. In addition, we find that hydrogen can bind to ${\mathrm{N}}_{\mathrm{O}}$, forming ${\mathrm{N}}_{\mathrm{O}}\ensuremath{-}{\mathrm{H}}_i$ complex which leads to a blueshift in the optical absorption. Other N configurations can also contribute to optical absorption in the visible range. The vibration frequencies of different N configurations are provided to support identification using vibrational spectroscopy techniques.