Origin of photoactivity of nitrogen-doped titanium dioxide under visible light

Nitrogen-doped titanium dioxide (N-TiO₂), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains single-atom nitrogen impurities that form either diamagnetic (N_b^-) or paramagnetic (N_b^.) bulk centers. Both types of N _b centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti³⁺ ions to N _b^. results in the formation of Ti⁴⁺ and N _b^-. EPR spectra measured under irradiation show that N_b centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO₂ and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light.