Crystal and electronic structure of the sub-stoichiometric photocatalyst Cu₄Se₆: DFT study

Copper selenides being earth-abundant advanced functional materials have paramount significance due to their substantial absorption characteristics and structural diversity. In this study, the crystal and electronic structure of hypothetical visible-light photocatalyst Cu₄Se₆ have been systematically investigated by density functional theory (DFT) calculations. Given a primitive unit cell of the parent pristine CuSe (P6₃/mmc) compound at most five symmetry-unique copper-deficient Cu₄Se₆ polymorphs can be constructed. DFT calculations with PBE (Perdew-Burke-Ernzerhof) density functional identify two of them (P6₃/mc and P6₃/mmc) as potential photocatalysts. It was found that accurate description of their electronic structures requires inclusion of spin-orbit coupling (SOC). Temperature effect on the electronic structures was modeled via uniform crystal lattice expansion/contraction (<5%). The computed PBE + SOC bandgap maxima of 1.57 eV (P6₃/mc) and 1.41 eV (P6₃/mmc) correspond to the lattice expansion of 4% and 3%, respectively. Since the P6₃/mc Cu₄Se₆ polymorph is lower in energy than its P6₃/mmc counterpart by 2.56 eV we concluded that the former would be preferentially formed in actual experiment. This work might prove to be instrumental in the design of new photocatalysts from the Cu_2-xSe family.