Origin of the enhanced visible-light absorption in N-doped bulk anatase TiO₂ from first-principles calculations

Extension of the absorption properties of TiO₂ 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 TiO₂ material using spin-polarized density functional theory (DFT) and the range-separated hybrid HSE06 functional. The thermodynamic stability of competitive N-doped TiO₂ structural configurations is studied as a function of the oxygen chemical potential and of various chemical doping agents: N₂, (N₂ + H₂), NH₃, N₂H₄. We show that the diamagnetic TiO _(2-3x)N_2x 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 NH₃, N₂H₄, and (N₂ + H₂) 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_(2-3x)N_2x system exhibits the enhanced optical absorption in N-doped TiO₂ 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.