Efficient charge separation and visible-light response in bilayer HfS₂-based van der Waals heterostructures

Two-dimensional (2D) hafnium disulfide (HfS₂) has been synthesized and is expected to be a promising candidate for photovoltaic applications, and at the same time the hexagonal BN sheet (h-BN) and graphene-like C₃N₄ sheet (g-C₃N₄) have also been fabricated and are expected to be applied in photocatalysis. In this work, we employ hybrid density functional theory to investigate HfS₂-based van der Waals (vdW) heterojunctions for highly efficient photovoltaic and photocatalytic applications. HfS₂/h-BN and HfS₂/g-C₃N₄ heterostructures with direct bandgaps and efficient charge separation are both typical type-II semiconductors and have potential as photovoltaic structures for solar power. Moreover, compared with h-BN and g-C₃N₄ single-layers, HfS₂/h-BN heterostructures with 6% tensile strain and HfS₂/g-C₃N₄ heterostructures with 9% tensile strain have moderate bandgaps, whose optical absorption is obviously enhanced in the ultraviolet-visible (UV-VIS) light range and whose bandedges are suitable for photocatalytic water splitting. HfS₂/h-BN heterostructures with 6% applied strain, being different from HfS₂/g-C₃N₄ heterostructures with 9% strain, possess a direct bandgap and show complete separation of the photoinduced electron-hole pairs. Thus the HfS₂/h-BN heterojunction with 6% strain has bright prospects for use in visible light photocatalytic water splitting to produce hydrogen.