Localized Excitation of Ti³⁺ Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO₂

In reduced TiO₂, electronic transitions originating from the Ti³⁺-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material's blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO₂(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti³⁺ species is identified at 2.5 ± 0.2 eV above the Fermi level (E_F) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below E_F, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti³⁺ ions in reduced TiO₂. Localized excitation of Ti³⁺ ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO₂ photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO₂ through Ti³⁺ self-doping. Our work reveals the physical origin of the Ti³⁺ related photoabsorption and visible light photocatalytic activity in prototypical TiO₂ and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides. (Figure Presented).