The stability and electronic and photocatalytic properties of the ZnWO4 (010) surface determined from first-principles and thermodynamic calculations

We present the results of the generalized-gradient approximation of Perdew, Burke and Ernzerhof (GGA-PBE) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional calculations of the atomic and the electronic structures of ZnWO4 (010) surfaces. The total energies obtained from these calculations are used to analyze the thermodynamic stability of the surfaces. The surface phase diagrams are constructed by surface Gibbs free energies obtained as a function of temperature and oxygen partial pressure. Our results suggested that the stable area of the surface terminations of ZnWO4 (010) has little correlation with the functional selected. The stability phase diagram shows that O-Zn, DL-W, and DL-Zn terminations of ZnWO4 (010) can be stabilized under certain thermodynamic equilibrium conditions. Based on the HSE06 hybrid functional, we calculate the electronic structures for three possible stability surface terminations. It is found that there is a fat band of the surface states in DL-W termination, which shows a delocalized feature. This fat band acts as an electron transition bridge between the valence band (VB) and conduction band (CB). It contributes to visible-light absorption by two-step optical transition with the first transition from the VB to the fat band and the second from the fat band to the CB. Significantly, the conduction band minimum (CBM) band edge position of DL-W termination is favourable for H-2 evolution as the CBM edge is located above the water reduction level (H+/H-2). Simultaneously, DL-W termination's valence band maximum (VBM) potential shows a strong potential for O-2 generation from water oxidation because of the higher VBM edge with respect to the water oxidation level (H2O/O-2). These results may help explore ZnWO4 (010) surfaces' intrinsic properties, providing a helpful strategy for experimental studies of ZnWO4-based photocatalysts in the future.