ZnO-graphene oxide nanocomposites for efficient photocatalytic degradation of organic dyes: investigation of natural solar light driven photocatalysis, recyclability and synergetic mechanism

Producing sustainable nanomaterials for real practical applications is the need of the day. In this regard, functional nanocomposite photocatalysts are promising materials with fascinating structural and optoelectronic properties for variety of energy and environmental applications. The current work investigates the synthesis and characterization of zinc oxide-graphene oxide (ZnO–GO) functional nanocomposite which shows direct sunlight driven photocatalysis attributed to the synergetic photocatalytic effect of ZnO and GO resulting from enhanced structural and optoelectronic properties. The ZnO–GO nanocomposite (with 5 wt% GO) was synthesized via simple ultrasonication technique and characterized with various spectroscopic and microscopic techniques that confirm the successful formation of nanocomposite as well as strong interfacial interactions between ZnO and GO nanostructures. Interestingly, ZnO–GO nanocomposite demonstrates superior performance achieving more than 98% photodegradation efficiency for organic water pollutant (dye) under direct sunlight for 180 min which is markedly higher than that of ZnO, showing only 66.6% degradation under similar conditions. This excellent photocatalytic performance is attributed to the improved visible light absorption, reduced electron–hole recombination, increased surface area for dye adsorption, and efficient charge separation through Z-scheme mechanism. The kinetic analysis investigations reveal that it is a pseudo-first-order photodegradation with rate constants of 0.01383 min⁻¹ for ZnO–GO nanocomposite which is more than 2 times of the rate constant of ZnO of approximately 0.00581 min⁻¹. The synergetic photodegradation mechanism involves formation of superoxide anions (·O₂⁻), hydroxyl radicals (·OH), electrons, and holes as confirmed by radical scavenging experiments. Furthermore, the natural sunlight driven recyclability investigation exhibits excellent reusability of ZnO–GO nanocomposite photocatalyst over five consecutive cycles with minimal activity loss. This work demonstrates the potential of ZnO–GO nanocomposites as cost-effective, sustainable, and highly efficient recyclable photocatalysts for solar-driven wastewater treatment applications.