(354d) Graphene Quantum Dots Based ZnO Nanomaterials for Degradation of Pollutant and Microbial Deactivation in Wastewater Treatment Plant.

Advanced oxidation processes (AOPs) offer enhanced contaminants destruction and low by-product and secondary pollution problems compared to conventional treatments. Photocatalysis is the mostly used form of AOPs. The photocatalyst can inactivate microorganisms and degrade organic materials by producing reactive oxygen species (ROS) upon photon irradiation. ZnO is an effective photocatalyst. It is characterized by a large bandgap energy, cost-effectiveness, biocompatibility and a high electron mobility. However, the efficiency of ZnO is reduced due to electron-hole (e^−/h⁺) pairs rate recombination and inability to harvest a large part of the sunlight. In this work, different shapes of ZnO nanomaterials nanoflakes (ZnO-NFs) and zinc oxide nanorods (ZnO-NRs) were covalently linked to graphene oxide quantum dots (GQDs). They were linked to reduce the bandgap and decrease the e^−/h⁺ pairs rate of recombination of the photocatalysts. The photocatalytic and antimicrobial behaviour of the graphene quantum dots based ZnO nanomaterials (ZnO-NFs@GQDs and ZnO-NRs@GQDs) were investigated. Several microscopic, spectroscopic and thermal analytic techniques were employed to confirm the successful preparation of the nanomaterials. Optical analysis showed that the calculated bandgap decreased from 2.98 eV to 2.61 eV and from 3.00 eV to 2.79 eV for the linked ZnO-NFs@GQDs and ZnO-NRs@GQDs, respectively. Data obtained from photolumiscence (PL) showed the same trend with a larger decrease in PL obtained for ZnO-NFs@GQDs indicating a large decrease in the e^−/h⁺ pairs rate of recombination, thus a better photocatalyst. The photocatalytic experimental results revealed that the dyes were absorbed at 573 nm. A noticeable red-shifting of the dyes peak was evident with increase in time. The maximum absorption peak of the dyes at 573 nm decreased significantly with time for ZnONFs@GQDs than for ZnO-NRs@GQDs. The reduction in absorbance in the visible region indicated the decolourization through deprotonation (cleavage of the N−N bond) and desulfonation (ring opening). However, ZnO-NRs@GQDs showed greater efficiency in inhibiting bacterial growth for E. coli and S. aureus strains under the experimental conditions. The antimicrobial efficiency of the nanocomposites was in the order of ZnO-NFs < ZnO-NFs@GQDs < ZnO-NRs < ZnO-NRs@GQDs with greater efficiency being against S. aureus. The findings in this study imply that graphene quantum dots based ZnO nanomaterials can be used for the removal of dye pollutants and bacterial deactivation in wastewater treatment plant.