Synthesis of Zinc Oxide-based Nanomaterials for Catalytic and Antibacterial Applications

Abstract

Nanomaterial-based catalytic conversion of hazardous organic pollutants into benign substances is an attractive method for wastewater treatment. In this study, metal oxide-based nanomaterials (CuO, ZnO, CuO-ZnO, and rGO-ZnO/CuO) were prepared by a microwave assisted method using plant extracts and low toxicity level reagent for wastewater remediation and antibacterial activities. In this regard, CuO nanoparticles were synthesized by a microwave-assisted method using Cordia africana Lam. leaf extracts. Also, CuO-ZnO nanocomposites (NCs) were synthesized by using an extract of Verbascum sinaiticum Benth plant. Similarly, rGO-ZnO/CuO nanocomposites (NCs) were synthesized via a simple, microwave (MW)-assisted method. X-ray diffraction analysis (XRD), spectroscopic (UV-visible, Fourier Transform-infrared spectroscopy, and photoluminescence), microscope (scanning electron microscope, transmission electron microscope), and electrochemical methods were used to explore the crystallinity, optical characteristics, morphology, composition, and electrochemical properties of the synthesized nanomaterials, respectively. The X-ray diffraction analysis and scanning electron microscope/energy dispersive spectroscopy analyses indicated the formation of nanocrystals of monoclinic CuO phase having a cluster of spherical-shaped morphology using Cordia africana Lam. extract, plate-like CuO-ZnO NCs using the extract of Verbascum sinaiticum Benth, rod-shaped rGO-ZnO/CuO and flower-like rGO-ZnO/CuO using PEG-200. Compared to pristine ZnO, all nanocomposites (i.e., plate-like CuO-ZnO, rod-shaped rGO-ZnO/CuO, and flower-like rGO-ZnO/CuO) have showed enhanced visible light harvesting capability and suppressed rate of electron-hole recombination. The performance of the as synthesized nanomaterials was tested against catalytic reduction of 4-nitrophenol, photocatalytic degradation of methylene blue, and disinfection of E. coli, S. aureus, P. aeruginosa, and S. pyogenes bacterial strains. According to the result, the reduction of 4- nitrophenol to 4-aminophenol was achieved with ratio constants of 75.8 min-1 g -1 (CuO), 5.9 min-1 g -1 (plate-like CuO-ZnO), and 11.7 min-1 g -1 (rod-shaped rGO-ZnO/CuO). Similarly, 82%, 90%, and 86% photocatalytic degradation of methylene blue initial concentration were achieved within 105 min using plate-like CuO-ZnO, rod-shaped rGO-ZnO/CuO, and flower-like rGO-ZnO/CuO nanocomposites, respectively. Among the prepared nanomaterials, CuO (using Cordia africana Lam) and CuO-ZnO (using PEG-200) showed antibacterial activities against all four bacterial strains (E. coli, S. aureus, P. aeruginosa, S. pyogenes) with an approximate inhibition zone (mm) of (20.5, 10.0, 22.0, 24.8, respectively) for CuO and (13.8, 11.0, 18.5, 24.0, respectively) for CuO-ZnO at 37.5 μg/mL concentration. Modification of ZnO with CuO and rGO showed enhanced photocatalytic activities due to the improved visible light absorption and suppressed electron-hole (e- /h+ ) recombination. Therefore, nanomaterials prepared using low toxicity level reagents including CuO nanoparticles (Cordia africana Lam.), CuO-ZnO NCs (Verbascum sinaiticum Benth), and rGO-ZnO/CuO NCs (PEG-200) could be a desirable catalyst for the elimination of organic pollutants for the remediation of wastewater.