Nature of Nitrogen-Doped Anatase TiO₂ and the Origin of Its Visible-Light Activity

Nitrogen doped anatase TiO₂ is studied using hybrid density functional theory (DFT) calculations. It is shown that under typical experimental conditions nitrogen prefers to substitute at a Ti-site and bond to two or three neighboring oxygen ions, forming nitrite or nitrate molecules. This leaves one Ti-vacancy and under-coordinated oxygen ions that produce a large electron density of states at the valence band edge, with localized characters slightly extending into the pristine TiO₂ band gap, due to an altered local electrostatic potential and orbital interactions. We suggest that this large electronic density of states at the valence band edge is responsible for the enhanced subgap tail absorption observed in N-doped anatase TiO₂ and hence the visible-light photocatalytic activity.