Origin of the Enhanced Visible-Light Absorption in N-Doped Bulk Anatase TiO 2 from First-Principles Calculations

Extension of the absorption properties of TiO 2 photocatalytic materials to the visible part of the solar spectrum is of major importance for energy and cleaning up applications. We carry out a systematic study of the N-doped anatase TiO 2 material using spin-polarized density functional theory (DFT) and the range-separated hybrid HSE06 functional. The thermodynamic stability of competitive N-doped TiO 2 structural configurations is studied as a function of the oxygen chemical potential and of various chemical doping agents: N 2, (N 2 + H 2 ), NH 3, N 2 H 4 . We show that the diamagnetic TiO (2–3 x ) N 2 x system corresponding to a separated substitutional N species (with 2–4% N impurities) and formation of one-half concentration of O vacancies (1–2 atom %) is an optimal configuration thermodynamically favored by NH 3, N 2 H 4, and (N 2 + H 2 ) chemical doping agents presenting a dual nitrating–reducing character. The simulated UV–vis absorption spectra using the perturbation theory (DFPT) approach demonstrates unambiguously that the diamagnetic TiO (2–3 x ) N 2 x system exhibits the enhanced optical absorption in N-doped TiO 2 under visible-light irradiation. Electronic analysis further reveals a band gap narrowing of 0.6 eV induced by delocalized impurity states located at the top of the valence band of TiO 2 . A fruitful comparison with experimental data is furnished.