Mechanism of enhanced photocatalytic activities on N-doped La_2Ti_2O_7: An insight from density-functional calculations

摘要

Recently, a wide-band-gap layered perovskite compound La_2Ti_2O_7 (LTO) has been reported to show a significant enhancement of both the visible and UV light photocatalytic activities after doping with N atoms. It is known that the doping sites often act as recombination centers of the photo-exited carriers and block the redox reactions. In this work, we investigated the origin of the enhancement in photocatalytic activity of N-doped LTO by using density functional theory (DFT) calculations. More than 40 models were constructed by considering ionic state of dopants, distance between dopants, doping concentrations, and formation of oxygen vacancy. The structure-properties relation of N-doped LTO was established. We found that the model of Sub_(3N-3O)~(dv) (with three dispersed substitutional N atoms at O sites and one oxygen vacancy in 87-atom LTO supercell) well explains both the shift-up of the valence bands and the narrowed band gap observed in experiment. The obtained band gap of 2.46 eV agrees well with the experimental value of 2.51 eV. For the models with interstitial N atoms, the impurity states are mainly localized at the higher-energy region of the band gap, which may trap the photo-excited carriers and decrease the photocatalytic activity. The work provides a potential implication for effective band-gap narrowing of wide-gap photocatalysts.