N-doped TiO₂: Theory and experiment

Nitrogen doped titanium dioxide is attracting a continuously increasing attention because of its potential as material for environmental photocatalysis. In this paper we review experimental and theoretical work done on this system in our groups in recent years. The analysis is largely based on electron paramagnetic resonance (EPR) spectra and on their interpretation based on high-level ab initio calculations. N-doped anatase TiO₂ contains thermally stable single N-atom impurities either as charged diamagnetic N_b^- centers or as neutral paramagnetic N_b^{{radical dot}} centers (b stays for bulk). The N-atoms can occupy both interstitial or substitutional positions in the solid, with some evidence for a preference for interstitial sites. All types of N_b centers give rise to localized states in the band-gap of the oxide, thus accounting for the related reduction of absorption band edge. The relative abundance of these species depends on the oxidation state of the solid. In fact, upon reduction, oxygen vacancies form and transfer electrons from Ti³⁺ ions to the N_b^{{radical dot}} with formation of Ti⁴⁺ and N_b^-. EPR spectra measured under irradiation show that the N_b centers are responsible for visible light absorption with promotion of electrons from the localized N-impurity states to the conduction band or to electron scavengers like O₂ adsorbed on the surface. These results provide an unambiguous characterization of the electronic states associated with N-impurities in TiO₂ and a realistic picture of the processes occurring in the solid under irradiation with visible light.