NHS-Enabled Molecular Stitching of g-C3N4/Graphitic-Carbon In-Plane Heterojunctions Activates n →π * Transitions for Efficient Visible-Light Photocatalytic Hydrogen Production

Abstract An NHS-assisted molecular self-assembly (“molecular stitching”) route is presented for constructing g-C3N4/graphitic-carbon (g-C3N4/Cg) in-plane heterojunction nanosheets with tunable n→π* transitions. Using N-hydroxysuccinimide and melamine as precursors, covalent coupling to a conductive sp2-carbon network perturbs heptazine symmetry, activates n→π* absorption, and forms electronically coupled 2D/2D interfaces. By adjusting the NHS:melamine feed ratio, the electronic structure is modulated, and the optical response extends from 400 to 660 nm, enhancing visible-light harvesting. The sheet-like, mesoporous architecture affords abundant accessible sites, while the in-plane junction and favorable band alignment promote charge separation and transport. Under visible irradiation with TEOA as sacrificial agent, the optimized composite (g-C3N4/Cg-1.5) achieves a hydrogen evolution rate of 10.35 mmol·g−1·h−1, approximately 47 times that of pristine g-C3N4, attributed to the synergy between high surface area/mesoporosity and n→π* activation across the covalently integrated interface. This NHS-enabled “molecular stitching” strategy provides a general platform for fabricating in-plane carbon nitride/carbon heterostructures with tunable optical transitions and charge–transport properties, offering a viable path to high-efficiency, visible-light photocatalytic hydrogen production from water.