First-Principles Investigation of a Bilayer g-C3N4/Bilayer h-BN Van der Waals Type-II Heterostructure for Photocatalytic Water Splitting

Two-dimensional van der Waals heterostructures provide a promising platform for improving charge separation and tuning the optical and electronic properties of metal-free photocatalysts. In this work, first-principles density functional theory (DFT) calculations were employed to investigate the structural, electronic, optical, and photocatalytic properties of bilayer g-C3N4 ((2L) g-C3N4), bilayer h-BN ((2L) h-BN), and their vertically stacked bilayer g-C3N4/bilayer h-BN ((2L) g-C3N4/(2L) h-BN) heterostructure. The optimized heterostructure is found to be energetically and thermally stable, with interlayer interaction characteristic of weak van der Waals coupling. Hybrid-functional calculations reveal a reduced band gap relative to the isolated bilayers and an optical absorption edge shifted toward lower photon energies, extending closer to the visible-light region. Vacuum-level band alignment shows that (2L) g-C3N4 provides a photoresponsive framework but insufficient oxidation power for overall water splitting, whereas (2L) h-BN exhibits favorable redox-band positions despite its weak visible-range optical response. After contact, work-function mismatch drives electron transfer from h-BN to g-C3N4, inducing band bending and a built-in electric field at the interface. Under illumination, the heterostructure follows a type-II charge-separation pathway, in which electrons accumulate on the h-BN side while holes remain on the g-C3N4 side. Gibbs free-energy (ΔG) analysis indicates near-thermoneutral hydrogen adsorption under weakly acidic conditions, whereas the oxygen evolution reaction becomes thermodynamically feasible only under sufficiently high hole chemical potential, highlighting the intrinsic catalytic capability of the heterostructure. These results demonstrate that bilayer interface engineering effectively enhances charge separation and identify the (2L) g-C3N4/(2L) h-BN heterostructure as a promising metal-free platform for photocatalytic water splitting.