Rational Design of Star-Shaped Donor−Acceptor Organic Small-Molecule/GCN Heterostructures for Superior Photocatalytic Hydrogen Evolution and Photoelectrocatalytic Water Splitting Applications

Abstract Graphitic carbon nitride (GCN) is a preferred metal-free, sustainable photocatalyst for solar hydrogen production; however, its efficiency is severely limited by rapid charge carrier recombination and poor electronic transport properties. To overcome these limitations, this study describes the design and synthesis of two donor−acceptor (D−A)-conjugated small molecules (TRZH and TRZBR) that have been heterojunctioned with GCN. The heterojunction was fabricated by integrating these organic small molecules (OSMs) with GCN (g-C3N4) through a facile sonication-assisted method, facilitating the formation of OSM/GCN heterostructures via non-covalent π−π interactions. These organic small molecules (OSMs) have a triazine central core with extended π-conjugation to carbazole via olefin double bonds and distinct hexyl and 2-ethylhexyl chains on the carbazole N-position, resulting in enhanced solubility and π−π stacking interactions. In contrast to the linear hexyl chains in TRZH, the branched 2-ethylhexyl chains in TRZBR significantly improve interfacial contact and decrease self-aggregation, resulting in more efficient interaction with the GCN surface and accessible active sites. Under visible-light irradiation, the GCN/TRZBR heterojunction exhibited a superior hydrogen evolution rate of 780 μmol g−1 h−1, while GCN/TRZH achieved 570 μmol g−1 h−1, both significantly outperforming pristine GCN (250 μmol g−1 h−1). This enhanced performance is attributed to the synergistic effect of broadened visible light absorption and accelerated interfacial charge transfer across the D−A heterojunction that effectively suppresses exciton recombination and boosts photocatalytic activity.