We have investigated the electronic structure of anionic mono- (S, N, and C) and co-doping (N-N, C-N, S-C, and S-N) on BiNbO₄ for the visible-light photocatalysis. The maximum band gap reduction of pure BiNbO₄ is possible with the (C-S) co-doping and minimum with N mono-doping. The calculated binding energies show that the co-doped systems are more stable than their mono-doped counterparts. Our optical absorption curves indicate that the mono- (C) and co-anionic doped (N-N and C-S) BiNbO₄ systems are promising materials for visible light photocatalysis.

Interaction between water and oxide surfaces plays an important role in many applications. In the present study, we use a density functional theory + U method to study the initial adsorption behavior of water on various α-Fe₂O₃ (0001) terminations. In particular, adsorption and dissociation of water at 1 monolayer and 0.5 monolayer coverage on the Fe-terminated, O-terminated, and ferryl-terminated (0001) surfaces are studied. The results show that the dissociated state is preferred, which is in agreement with theoretical and experimental observations. However, experiments show the co-existence of molecular water in the initial adsorption on hematite, which, so far, has no theoretical support. To explain this, we investigate OH-covered Fe-terminated surfaces and find that on R–Fe–(OH)_x terminations, molecular water is actually energetically favorable. Thus, we propose that the existence of R–Fe–(OH)_x terminations is a requirement for molecular water in the initial adsorption process. This proposal is further supported by comparing the calculated core level shift of R–Fe–(OH)_x structures with experimental data.