Emerging van der Waals Heterostructures for Enhanced Photocatalytic Water Splitting through Type-II Band Alignment

The structural, electronic, optical, and photocatalytic properties of individual two-dimensional (2D) ZnS, CdS, and NiI₂ monolayers and corresponding van der Waals (vdW) heterostructures are thoroughly investigated from the perspective of band-edge alignment and its repercussion on photocatalytic water splitting. We performed first-principles electronic structure calculations within the framework of the density functional theory to envisage the possibility of such heterostructures to be used efficiently for hydrogen evolution reaction (HER) activity. Both vdW heterostructures of NiI₂ with ZnS and CdS have been found to be energetically stable, followed by the most favored AB stacking. Individual monolayers are predicted to be semiconducting in nature, and corresponding heterostructures are staggered in the type-II band alignment. The potential drop in between monolayers creates an electric field for the separation of photogenerated electron and hole pairs for enhancing the photocatalytic performance. Our study reveals that the NiI₂ surface side in the CdS/NiI₂ interface is the most active for HER. Moreover, the CdS/NiI₂ heterostructure is found to be an efficient candidate for adsorbing a wider range of solar energy, and it could emerge as a promising vdW candidate for photocatalytic water splitting through type-II band-edge alignment.