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.
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