First-principles calculations of the photocatalytic performance of ZnO-MX (M = Mo, W; X = S, Se) heterojunctions.

In this work, we systematically investigate the stability, electronic structure, optical properties, and photocatalytic performance of four ZnO-MX (M = Mo, W; X = S, Se) heterojunctions. The results indicate that all four heterojunctions exhibit excellent structural stability. In each system, an internal electric field is formed from the ZnO layer to the MX layer, facilitating the effective transfer of electrons. Moreover, the effective mass of holes in these systems is greater than that of electrons, suggesting efficient separation of electron-hole pairs, which enhances photocatalytic efficiency. Compared with monolayer ZnO, the band gap of the heterojunctions is significantly reduced, and all heterojunctions display direct band gap characteristics. Simultaneously, the static dielectric constant of these systems increases, and redshift is observed in their absorption spectra. Both ZnO-MoSe and ZnO-WSe exhibit type I band alignment, making them unsuitable for photocatalytic applications but ideal candidates for solar cells. On the other hand, ZnO-MoS and ZnO-WS exhibit a II-type band alignment. In comparison to ZnO-MSe, they demonstrate a higher static dielectric constant and light absorption coefficient, as well as a larger value (the ratio of the effective mass of electrons to holes), which suggests their superior photocatalytic efficiency. Notably, while ZnO-MoS only possesses hydrogen evolution reaction (HER) capability, ZnO-WS demonstrates both HER and oxygen evolution reaction (OER) capabilities.