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

초록

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.

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