Electronic structure and photoelectrochemical properties of an Ir-doped SrTiO₃ photocatalyst

The effect of iridium valence in Ir:SrTiO₃ on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO₃. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir⁴⁺ or Ir³⁺ dopants in SrTiO₃. The measurements show that Ir³⁺:SrTiO₃ forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir⁴⁺:SrTiO₃ under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir⁴⁺:SrTiO₃ contains an in-gap state close to the top of the SrTiO₃ valence band. The performance of Ir⁴⁺:SrTiO₃ in electrochemical reactions is compared with cathodic Rh³⁺:SrTiO₃, clearly illustrating the importance of strong dopant d-electron hybridization with the oxygen 2p valence band of SrTiO₃ for improving the energy conversion efficiency in SrTiO₃-based photocatalysts.