Zn3In2S6/pyridine-bridged g-C3N4 S-scheme heterojunctions for high-efficiency photocatalytic H2 production and tetracycline degradation

The photocatalytic performance of graphitic carbon nitride (CN) is limited by electron-localized sp³-hybridized nitrogen bridges and restricted visible-light absorption. Here, we construct an S-scheme Zn₃In₂S₆/pyridine-modified CN heterojunction through a combined strategy of high-temperature calcination and in situ hydrothermal growth. The dual modification strategy—the simultaneous engineering of π-electron delocalization via pyridine bridging and the construction of S-scheme heterojunctions—results in enhanced charge separation and broadened visible-light absorption. The optimized composite photocatalyst achieves 91 % tetracycline (TC) degradation within 120 min with concurrent H₂ production at a rate of 30.3 mmol·g^–1 over 6 h, showing 3.71-fold and 33.70-fold enhancements over pristine g-C₃N₄, respectively. Notably, the photocatalyst also exhibits exceptional stability, maintaining 97.8 % of its degradation efficiency and 86.5 % of its H₂ evolution activity after four cycles. The mechanisms of charge transfer and the dynamics of charge migration were investigated through a combination of in situ XPS, ESR spectroscopy, DFT modeling, and fs-TAS. Molecular transformation pathways were mapped using HRMS, and the potential toxicity of intermediates along these pathways was estimated with T.E.S.T. software. This research offers a strategic framework for developing multifunctional photocatalytic systems but also provides deep insights into the structure-property-activity correlations by synergistically tuning π-conjugation and engineering heterojunction interfaces.