Effects of non-stoichiometry on electronic structure of CuxSy compounds studied by first-principle calculations

Non-stoichiometric CuxSy (1 <= x/y <= 2) is a potential photocatalytic material. In this article, the non-stoichiometric models are constructed by Cu2S with copper vacancies and CuS with sulfur vacancies. The crystal structure, electronic properties, and optical properties of different stoichiometries of CuxSy were studied by First-principle calculations. Their similarities and differences in electronic structure and optical properties and the changing trends have been systematically compared and analyzed. From Cu2S to CuS, the metallic Cu-Cu bonds gradually disappear; the covalent S-S bonds gradually appear. Most important, the presence of S-S layer is enhancing the covalentity of CuxSy compound, as the x/y ratio is increasing. Comparison to the stoichiometric Cu2S and CuS, both copper vacancy and sulfur vacancy can induce obvious unoccupied energy band on the top of valence band. Namely, the Fermi energy level enters the valence band, resulting in the degenerate semiconductor. The little unoccupied band is the key factor to determine the photo-electronic properties of CuxSy, in which the most important effect is the potential local surface plasmon resonance. All of the above factors lead to changes in optical properties and ultimately affect the photocatalytic properties of non-stoichiometric CuxSy. Therefore, the photocatalytic performance of CuxSy can be adjusted by the stoichiometries. These calculated results can provide reliable data and support for the development of CuxSy and copper sulfide-based photocatalysts.