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

Two-dimensional (2D) hafnium disulfide (HfS) 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 CN sheet (g-CN) have also been fabricated and are expected to be applied in photocatalysis. In this work, we employ hybrid density functional theory to investigate HfS-based van der Waals (vdW) heterojunctions for highly efficient photovoltaic and photocatalytic applications. HfS/h-BN and HfS/g-CN 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-CN single-layers, HfS/h-BN heterostructures with 6% tensile strain and HfS/g-CN 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. HfS/h-BN heterostructures with 6% applied strain, being different from HfS/g-CN heterostructures with 9% strain, possess a direct bandgap and show complete separation of the photoinduced electron-hole pairs. Thus the HfS/h-BN heterojunction with 6% strain has bright prospects for use in visible light photocatalytic water splitting to produce hydrogen.