Effective approach to improve photocatalytic dye degradation ability of ZnO based nanopowder: simultaneous cationic and anionic doping, and enzyme coupling

In this study, vermiwash derived-enzyme enriched ZnO NPs co-doped with copper and nitrogen were synthesized using soft chemical method for cost-effective and eco-friendly photocatalytic dye degradation. Undoped and copper (cationic), nitrogen (anionic) and copper + nitrogen (cationic + anionic) doped ZnO NPs were synthesized separately and compared with their vermiwash derived-enzyme enriched counterparts. Vermiwash-an aqueous liquid obtained from vermi reactor of Eudrilus eugineae earthworm-was used as a reducing agent along with NaOH for the synthesis process. The presence of four enzymes viz. phosphatase, urease, protease and amylase was confirmed through enzymes activity studies. The enzymes quantitative study clearly showed that the enzymes prevalent in the vermiwash are inherited by the vermiwash activated nanoparticles as well as by the photocatalytically treated water. The photocatalytic dye degradation efficiency of the ZnO:Cu:N:vermi nanopowder against the methylene blue dye was found to be 99% which is the best among all the other synthesized counterparts. The photocatalytic dye degradation efficiencies of the enzyme enriched samples were in the order: ZnO:vermi < ZnO:N:vermi < ZnO:Cu:vermi < ZnO:Cu:N:vermi. The same order is observed in the bare samples also: ZnO < ZnO:N < ZnO:Cu < ZnO:Cu:N. The mechanism lying behind the enhanced photocatalytic ability of the vermiwash activated ZnO:Cu:N NPs is illustrated in the paper. The results obtained from the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) and enzyme quantitative studies were correlated with the proposed dye degradation mechanism of the vermiwash activated ZnO:Cu:N nanomaterial. The synthesized nanomaterials have good reproducibility and stability. The results showed that the synthesized vermiwash activated ZnO:Cu:N NPs have great potential for the effective degradation of various toxic organic dyes.