Elucidation of Rh-Induced In-Gap States of Rh:SrTiO₃ Visible-Light-Driven Photocatalyst by Soft X‑ray Spectroscopy and First-Principles Calculations

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO₃ photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh⁴⁺ acceptor state was found 1.5 eV below the SrTiO₃ conduction band minimum. Both Rh⁴⁺ and Rh³⁺ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO₃. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO₃ is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO₃ an attractive material for use as a visible-light-driven H₂-evolving photocatalyst in a solar water splitting reaction