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: N-2, (N-2 + H-2), NH3. N2H4. We show that the diamagnetic TiO(2-3x)N2x 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 (N-2 + H-2) chemical doping agents presenting a dual nitrating-reducing character. The simulated UV-vis absorption spectra using the perturbation theory (DEPT) approach demonstrates unambiguously that the diamagnetic TiO(2-3)N2x 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 TiO2. A fruitful comparison with experimental data is furnished.