Interfacial Excitons and Band Edge Engineering in SnS₂/h-BN Heterostructures for Efficient Solar Water Splitting

Excitons play a decisive role in governing light absorption, charge separation, and carrier utilization in low-dimensional photocatalysts. In this work, we present a comprehensive first-principles investigation of excitonic effects and their impact on photocatalytic water splitting in a SnS₂/h-BN van der Waals (vdW) heterostructure. Density functional theory (DFT), combined with many-body perturbation theory (MBPT) within the GW approximation and the Bethe-Salpeter equation (BSE), is employed to determine the quasiparticle band edge alignment, exciton binding energies (EBEs), optical absorption, carrier effective masses, and solar-to-hydrogen (STH) conversion efficiency. The SnS₂/h-BN heterostructure exhibits a staggered type-II band alignment with quasiparticle band edges straddling the redox potentials, ensuring thermodynamic feasibility for overall water splitting. Beyond band alignment, the heterostructure supports multiple optically active bright interlayer excitons with spatially separated electrons and holes at the interface. These interlayer excitons display reduced electron-hole (e-h) wave function overlap and favorable effective masses, particularly a highly dispersive SnS₂-derived conduction band that enables efficient electron transport toward hydrogen evolution reaction sites. Despite their sizable binding energies, efficient exciton dissociation is promoted by strong interfacial electric fields and large conduction band offsets, leading to effective charge separation. Consequently, photogenerated carriers are selectively funneled to distinct catalytic surfaces, enabling spatially separated hydrogen and oxygen evolution. The synergistic enhancement of light absorption, carrier lifetime, and charge transport results in a markedly higher STH efficiency (11.04%) compared to that of pristine SnS₂. This work underscores the necessity of explicit excitonic treatment and establishes exciton engineering in vdW heterostructures as a key strategy for the design of efficient photocatalysts for solar water splitting.