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 TiO 2 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 TiO 2, 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 TiO 2 . 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.