Janus Aluminum Oxysulfide Al₂OS: A promising 2D direct semiconductor photocatalyst with strong visible light harvesting

Hydrogen production via solar light-driven water dissociation has been regarded as an artificial and effective process to overcome the environmental problem as well as solving the current energy crisis. In this regard, numerous works have mainly been devoted to developing the appropriate photocatalyst which satisfies the conditions for water splitting and understanding the photocatalysis process. In this study, we propose for the first time the potential application of the two-dimensional Janus aluminum oxysulfide Al₂OS as an efficient photocatalyst material for hydrogen-production H₂ through the first-principles calculations. Janus Al₂OS monolayer has been designed from the parental binary aluminum sulfide AlS by substituting one sub-layer of sulfide atoms (S) to oxygen atoms (O). The electronic properties of the pristine AlS and the derived Janus Al₂OS were computed using GGA-PBE and HSE06 functionals. According to the band structure, AlS monolayer shows a semiconductor behavior with an indirect bandgap of 2.14 eV whereas, the Janus Al₂OS exhibits a direct bandgap of 1.579 eV. Motivated by the desirable bandgap of the Janus Al₂OS, the absorption-coefficient of Janus Al₂OS shows strong visible light harvesting compared to the parental AlS. Furthermore, the photocatalytic performance of Al₂OS has been investigated. Our calculations demonstrate that the band edge position of Al₂OS is suitable for the hydrogen evolution reaction (HER). More importantly, based on the reaction coordinate, it was found that the Gibbs free-energy ΔG_H^∗ of Al₂OS is 0.97 eV which is smaller than of the two-dimensional Janus Ga₂XY (X, Y = S, Se, Te with X ≠ Y) reported recently. Moreover, this value decreases from 0.97 eV to 0.69 eV under 0.5 V/Å of an external electrical field. Our results indicate that Janus Al₂OS fulfills the fundamental requirements for efficient photo-catalyst under visible light and provides new guidance for hydrogen-production via water splitting.