Dual Z-scheme g-C3N4/GQD/ZnIn2S4 composites with enhanced photocatalytic degradation of xanthate under visible light irradiation

Abstract

Rational interface design that combines efficient charge separation with strong redox capacity is crucial for mineralizing flotation reagents in wastewater. In this work, we constructed a ternary g-C₃N₄/graphene quantum dot/ZnIn₂S₄ (CN/50GQD/ZIS) heterojunction via a two-step pyrolysis–hydrothermal route and elucidated its reactivity and mechanism for visible-light-driven photodegradation of xanthate. The optimized ternary composite achieved 98.67% removal of 100 mg L⁻¹ xanthate in 60 min, surpassing a CN/ZIS binary composite and both individual components. The GQDs boosted light harvesting and function as an electron mediator, accelerating interfacial charge migration and suppressing the recombination of photogenerated carriers. Radical quenching and band analyses identified O₂˙⁻ as the predominant oxidant. In situ irradiated XPS, band/redox-potential analysis, and DFT calculations corroborated a dual Z-scheme heterojunction, in which GQDs bridged CN and ZIS, triggering interfacial charge redistribution and establishing built-in electric fields that drive directional carrier separation. The adsorption-energy analyses indicated that, on the catalyst surface, H₂O and O₂ were preferentially adsorbed and activated to generate reactive radicals, whereas xanthate exhibited comparatively weaker adsorption, supporting a radical-dominated degradation pathway. The catalyst retained 92% of its initial activity after five cycles, and post-cycling XPS confirmed good surface chemical stability. This work demonstrates that the CN/GQD/ZIS heterojunction is an effective photocatalytic platform for xanthate removal and supports a GQD-mediated dual Z-scheme charge-transfer pathway in this specific system.