Band gap engineering of SnS nanosheets by anion-anion codoping for visible-light photocatalysis.

SnS nanosheets with three atom thickness have previously been synthesized and it has been shown that visible light absorption and hydrogen evolution through photocatalytic water splitting are restricted. In the present study, we have systematically investigated the electronic structures of anionic monodoped (N and P) and codoped (N-N, N-P, and P-P) SnS nanosheets for the design of efficient water redox photocatalysts by adopting first principles calculations with the hybrid HSE06 functional. The results show that the defect formation energies of both the anionic monodoped and all the codoped systems decrease monotonically with the decrease of the chemical potential of S. The P-P codoped SnS nanosheets are not only more favorable than other codoped systems under an S-poor condition, but they also reduce the band gap without introducing unoccupied impurity states above the Fermi level. Interestingly, although the P-P(ii) codoped system gives a band gap reduction, this system is only suitable for oxygen production and not for hydrogen evolution, which indicates that it may serve as a Z-scheme photocatalyst for water splitting. The P-P(i) codoped system may be a potential candidate for photocatalytic water splitting to generate hydrogen because of the appropriate band gap and band edge positions, which overcome the disadvantage that the pure SnS nanosheet is not beneficial for hydrogen production. More importantly, the result of optical absorption spectral analysis shows that the P-P(i) codoped SnS nanosheet absorbs a longer wavelength of the visible light spectrum as compared to the pristine SnS nanosheet. The P-P(I) codoped system with a lower doping concentration also has an absorption shift towards the visible light region.