Localized and delocalized electron states in TiO₂ revealed using resonant inelastic soft X-ray scattering

Excess carriers in defective TiO₂ play a key role in determining the functionality of numerous applications including photocatalysts, memristors, etc. The electronic structures of the excess electron states derived from inherent oxygen deficiencies in a TiO₂ crystal and a TiO₂ thin film are investigated by using resonant inelastic soft X-ray scattering. The results show that there exist two separable states associated with these excess electrons, that is, either local (bound) or delocalized (band-like) d electrons, and that their abundance differs significantly depending on the sample type. In the case of thin film, delocalized electron states (d-fluorescence) dominate over the localized defect states (dd excitation) showing a strong resonance effect at Ti³⁺’s excitation energy, whereas in the case of crystal, both localized and delocalized electron states weakly contribute to the defect states. Most plausibly, this discrepancy originates mainly from the distinct crystal structure of a rutile (crystal) and an anatase (thin film), implying that understanding the crystal structure dependence of intrinsic defects in TiO₂ polymorphs is crucial for engineering TiO₂'s electronic transport properties.