The presence of Ti(II) centers in doped nanoscale TiO2 and TiO2−xNx

Unusual trends are observed in the Ti (2s, 2p) XPS spectra of Fe(II) doped TiO2 and TiO2−xNx. The binding energy of Ti (2s, 2p) initially decreases with increasing Fe(II) concentration, as expected, but increases at higher Fe(II) doping levels. Density functional theory is used to analyze the results. The observed VB-XPS and core level XPS spectra are consistent with the facile charge transfer sequence Ti(IV) + Fe(II) → Ti(III) + Fe(III) followed by Ti(III) + Fe(II) → Ti(II) + Fe(III). The formed Ti(II) sites may be relevant to nanoparticle catalysis on TiO2 surfaces.