Engineering plasmonic–semiconductor interfaces in Au/g-C ₃ N ₅ for mechanistic insights into piezo–photocatalytic H ₂

Engineering plasmonic–semiconductor interfaces offers a powerful means of steering charge dynamics in polymeric carbon nitrides for artificial photosynthesis. Herein, a series of [Formula: see text]% Au/g-C ₃ N ₅ ([Formula: see text], 1, 1.5, 2) Schottky-junction composites was constructed by the in situ photo-deposition of Au nanoparticles onto azo-linked g-C ₃ N ₅ synthesized via a melem–hydrazine route and evaluated for sacrificial-agent-free H ₂ O ₂ production under photocatalytic, piezocatalytic, and piezo–photocatalytic conditions. Au decoration preserves the layered g-C ₃ N ₅ framework while leaving the optical bandgap essentially unchanged (2.01 versus 1.95[Formula: see text]eV) and introducing an intense localized surface plasmon resonance absorption at 631[Formula: see text]nm; the pronounced photoluminescence quenching evidences efficient interfacial electron transfer through the Schottky contact. The optimized 1.5% Au/ g-C ₃ N ₅ delivers a photocatalytic H ₂ O ₂ production rate of 216.70[Formula: see text][Formula: see text]mol g[Formula: see text] h[Formula: see text] under simulated solar light in pure water (solar-to-chemical conversion efficiency of 0.0036% and apparent quantum yield of 0.21% at 420[Formula: see text]nm), nearly twice that of pristine g-C ₃ N ₅ . Under coupled ultrasonic and visible-light excitation, the rate reaches 449.00[Formula: see text][Formula: see text]mol g[Formula: see text] h[Formula: see text], and under coupled ultrasonic and simulated solar-light excitation it reaches 1455.43 and 1768.68[Formula: see text][Formula: see text]mol g[Formula: see text] h[Formula: see text] in air-equilibrated and O ₂ -saturated water, respectively, far exceeding the sum of the individual photocatalytic and piezocatalytic contributions under the same lamp. Reactive-species trapping identifies photogenerated electrons, •O[Formula: see text], and ¹ O ₂ as the dominant species, supporting a sequential two-electron oxygen reduction pathway mediated by an O ₂ /•O[Formula: see text]/ ¹ O ₂ redox shuttle, while water oxidation sustains the supply of O ₂ and protons. The catalyst retains 91.2% of its initial activity over six consecutive cycles. This work demonstrates that coupling work-function-guided Schottky junctions with plasmonic hot-electron injection under combined photo–mechanical excitation is an effective strategy for high-rate, selective, and durable green H ₂ O ₂ synthesis.