First-principles calculation of chalcogen-doped Sr2M2O7 (M=Nb and Ta) for visible light photocatalysis

Doped Sr2M2O7 (M = Nb and Ta) crystals with various chalcogen X (X = S, Se, and Te) concentrations have been studied by using first-principles calculations combined with hybrid functional (HSE06). Our results indicate that the more stable doping site of X generally induces the less distortion of the lattice. The calculated formation energies reveal that the doping becomes more difficult in the order S < Se < Te. Some localized X np states are located above the VBM of pure semiconductors, which results in the narrowing of band gaps. For Sr2Nb2O7, an increase of the X concentration has negative effect on the narrowing of band gap since the X np states moves towards the VBM of pristine semiconductor. As to Sr2Ta2O7, the higher doping level of X elements leads to the further narrowing of energy gap compared with the lower doping level. The formation of Sr2M2X7 (X = S and Se) by replacing all the O atoms in crystal structures by chalcogens extends the absorption edges to near-infrared region and loses the abilities to oxidize water. Our findings demonstrate that S-doped Sr2Nb2O7, Se-doped Sr2Nb2O7 and Se-doped Sr2Ta2O7 at a higher doping level, and Te-doped Sr2Ta2O7 at a lower doping level can catalyze overall splitting water reaction under visible light irradiation. Furthermore, they possess stronger capabilities of photo-oxidation and photo-reduction than pristine semiconductors. Therefore, these systems mentioned above are potential visible-light-driven photocatalytic materials, which are worthy of further experimental investigations.

Automated summary

In this study, the effects of doping chalcogen elements on the properties of Sr2M2O7 crystals were analyzed using theoretical calculations. The results indicate that the more stable doping sites of chalcogens result in less distortion of the lattice. The formation energies of the dopants follow the order S < Se < Te, indicating increasing difficulty in doping. Doping chalcogens leads to a narrowing of the band gap and potential for photocatalytic reactions.