Co2+-laddered heterojunction a next-generation solar-photocatalyst: Unusually improved activity for the decomposition of pharmaceuticals, dyes, and microplastics

Abstract

Novel and versatile photocatalysts that can work under direct sunlight are in high demand especially for mitigating water contamination. Some of the burgeoning pollutants in water are textile dyes, organic molecules, pharmaceutical products etc. In view of their extensive use, polymer wastes such as microplastics in water bodies are a new cause of concern. Using direct sunlight for the degradation of such pollutants needs the development of solar photocatalysts. We report on a novel next-generation solar-photocatalyst, consisting laddered-heterojunction formed between Co2+- substituted zinc-ferrite core & zinc-oxide shell, to harvest full-solar-spectrum in scavenger-free photodegradation of dyes, pharmaceuticals and microplastics. Using in-house developed protocols and Microwave-assisted-solvothermal-technique (MAST), nanospheres of ∼20–40 nm were synthesized first followed by ZnO shell growth in a controlled manner (∼80–180 nm) to obtain the core-shell photocatalyst nanospheres using another microwave approach. The absorption of the photocatalyst could be extended upto 852 nm by judiciously doping with Co2+- enabling the utilization of UV–Vis-NIR region of sunlight. As evident from the valence band spectra, the Co2+ substitution introduced free electrons in the conduction band of ZnFe2O4 that resulted in the formation of laddered type-1 heterojunction. With the optimized Co2+content and ZnO-shell thickness, solar-photocatalytic degradation of Methyl-Orange enhanced 6-&12-times respectively. Complete degradation of antibiotics like Ciprofloxacin (CF), Norfloxacin (NF), and Ofloxacin (OF) under direct sunlight was achieved within an hour. This unusual enhanced activity was attributed to the inclusion of Co2+, conducive band positions leading to higher absorption and reduced recombination. We also showed the degradation of polypropylene microfibers used in face masks to combat the COVID-19 outbreak could also be degraded., indicating their potential to combat microplastic pollution. Our novel photocatalyst holds promise for sunlight-assisted degradation of a wide range of hazardous pollutants.