| Summary: | Hybrid density functional theory has been employed to study the influence of interfacial oxygen (O), sulfur (S) and zinc (Zn) vacancies on the optoelectronic properties of ZnO/ZnS heterostructure. The results show that the O, S, and Zn vacancies can decrease cell volume of the ZnO/ZnS heterostructure, leading to slight deformation from the perfect heterostructure. The quasi-band gap of ZnO/ZnS heterostructure is remarkably reduced compared to the ZnO surface. Hence, the visible light response is enhanced in ZnO/ZnS heterostructure, which can be further improved by creating an interfacial S or O vacancy. Moreover, the removal of S or O atoms can generate lone electrons in the system, which can enhance <i>n</i>-type conductivity of the heterostructure. The O and S vacancies improve the contribution of the atomic orbitals of Zn<sub>ZnO</sub> (Zn atom in ZnO), Zn<sub>ZnS</sub> (Zn atom in ZnS), S and O to the valence band maximum (VB) of the heterostructure; while the Zn-vacancy remarkably improves the contribution of S states to the conduction band minimum (CB). The resultant type-II band alignment and large difference between the migration speed of electrons and holes can efficiently separate the photogenerated electron-hole pairs. The CB edge positions are more negative than the redox potentials of CO<sub>2</sub>/CO and H<sub>2</sub>O/H<sub>2</sub>, and the VB edge positions are more positive than the redox potential of O<sub>2</sub>/H<sub>2</sub>O. Hence, all the systems under investigation can be potentially used as efficient photocatalysts for various applications like CO<sub>2</sub> reduction and water splitting.
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