Dual Z-Scheme g-C3N4/GQDs/ZnIn2S4 Composite 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 to mineralizing flotation reagents in wastewater. In this work, we construct a ternary g-C3N4/graphene quantum dots/ZnIn2S4 (CN/50GQDs/ZIS) heterojunction via a two-step pyrolysis–hydrothermal route, and elucidate its reactivity and mechanism for visible-light-driven photodegradation of xanthate. The optimized ternary composite achieves 98.67% removal of 100 mg L-1 xanthate in 60 min, surpassing CN/ZIS binary composite and both individual components. The GQDs boost light harvesting and function as an electron mediator, accelerating interfacial charge migration and suppressing the recombination of photogenerated carriers. Radical quenching and band analyses identify O2·⁻ as the predominant oxidant. In situ irradiated XPS, band/redox-potential analysis, and DFT calculations corroborate a dual Z-scheme heterojunction in which GQDs bridge CN and ZIS, triggering interfacial charge redistribution and establishing built-in electric fields that drive directional carrier separation. The adsorption-energy analyses indicate that, on the catalyst surface, H2O and O2 are preferentially adsorbed and activated to generate reactive radicals, whereas xanthate exhibits comparatively weaker adsorption, supporting a radical-dominated degradation pathway. The catalyst retains 92% of its initial activity after five cycles, and post-cycling XPS confirms good surface chemical stability. This work demonstrates that the CN/GQDs/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.