Simultaneously Controllable Doping Sites and the Activity of a W-N Codoped TiO2 Photocatalyst

Tungsten nitrogen (W-N) codoping has been known to enhance the photocatalytic activity of anatase TiO2 nanoparticles by utilizing visible light. The doping effects are, however, largely dependent on calcination or annealing conditions, and thus, the massive production of quality-controlled photocatalysts still remains a challenge. Using density functional theory (DFT) thermodynamics and time-dependent DFT computations (TDDFT), we investigate the atomic structures of N doping and W N codoping in anatase TiO2, as well as the effect of the thermal processing conditions. We find that W and N dopants predominantly constitute two complex structures: an N interstitial site near a Ti vacancy in the triple charge state ((V-Ti-N-i)(3-)) and the simultaneous substitutions of Ti by W and the nearest O by N ((W-Ti-N-O)(+)). The latter case induces highly localized shallow in-gap levels near the conduction band minimum (CBM) and the valence band maximum (VBM), whereas the (V-Ti-N-i)(3-) defect complex yielded deep levels (1.9 eV above the VBM). Electronic structures suggest that (W-Ti-N-O)(+) improves the photocatalytic activity of anatase by band gap narrowing, while (V-Ti-N-i)(3-) degrades the activity by an in-gap state-assisted electron hole recombination, which explains the experimentally observed deep-level-related photon absorption. Through the real-time propagation of TDDFT, we demonstrate that the presence of (V-Ti-N-i)(3-) attracts excited electrons from the conduction band to a localized in-gap state within a much shorter time than the flat band lifetime of TiO2. On the basis of these results, we suggest that calcination under N-rich and O-poor conditions is desirable to eliminate the deep-level states caused by (V-Ti-N-i)(3-) and to improve photocatalysis.