Au LSPR-boosted S-scheme carbon nitride heterostructures for H₂O₂ photocatalytic production

Photocatalytic oxygen reduction offers a sustainable, solar-driven route for the green synthesis of hydrogen peroxide (H₂O₂), however it suffers from limited efficiency due to rapid charge recombination and poor selectivity toward the two-electron reduction pathway. Herein, crystalline poly(heptazine) imide (PHI) and poly(triazine) imide (PTI) were synthesized via an ionothermal method and assembled into a PHI/PTI heterostructure, leading to a high H₂O₂ productivity of 1.27 ± 0.03 mmol L^-1. The performance of the composite was benchmarked against the individual pristine components, graphitic carbon nitride (GCN) and carbon-doped GCN, with the heterostructure exhibiting superior activity attributed to an S-scheme charge-transfer mechanism promoting efficient charge separation. Au nanoparticles were subsequently deposited onto the heterostructure surface to yield a 1Au/PHI/PTI catalyst, which further enhanced the performance by ca. 74% through the S-scheme heterojunction, the Schottky barrier formation and the localised surface plasmon excitation. Under optimised continuous irradiation conditions, this catalyst achieved an apparent quantum yield of 18.8% and produced 8.32 ± 0.13 mmol L^-1 H₂O₂ over 5 h, outperforming the recovering-and-reusing route. This work highlights the synergistic effect of Au LSPR with the PHI/PTI S-scheme heterostructure as a promising strategy for enhancing photocatalytic O₂ reduction to H₂O₂, while also underscoring the importance of strong metal-support interactions for long-term catalyst stability.