Biogenic synthesis, characterization, antimicrobial and photocatalytic studies of zinc oxide nanoparticles prepared using neem and moringa leaf aqueous extracts

Abstract

The drive towards sustainability and green nanotechnology has sparked increasing interest in the development of environmentally benign processes for synthesis of nanomaterials, especially using plant aqueous extracts as
capping and reducing agents. In this study, two zinc oxide nanoparticle samples were fabricated using leaf aqueous extracts of neem (ZnO NLE) and moringa (ZnO MLE) as
bio reducing and stabilizing agents. Successful synthesis was confirmed using comprehensive characterization techniques including Fourier transform infrared spectroscopy, x ray diffraction, ultraviolet visible spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, and energy dispersive x ray spectroscopy. Fourier transform infrared spectroscopy revealed zinc oxygen vibrations alongside organic residues from leaf phytochemicals, while x ray diffraction confirmed hexagonal wurtzite crystal structures with crystallite sizes of 24.5 nanometres for ZnO NLE and 17.5 nanometres for ZnO MLE, as determined using Scherrer equation. Ultraviolet visible spectroscopy results indicated strong ultraviolet light absorption characteristics and optical band gaps of 2.98 and 3.02 electron volts respectively, estimated using Tauc plot method. Scanning electron microscopy images showed quasi spherical morphology with moderate agglomeration for ZnO NLE, while ZnO MLE sample revealed smaller and more densely clustered nanoparticles. Transmission electron microscopy images confirmed these morphological observations, with ZnO MLE showing broader particle size distribution compared to ZnO NLE, and average particle sizes of 35 nanometres for ZnO NLE and 28 nanometres for ZnO MLE. The biosynthesized samples were systematically tested for antimicrobial and photocatalytic applications. Antimicrobial assay results demonstrated that ZnO NLE possesses stronger antibacterial properties, particularly against Bacillus subtilis and Pseudomonas aeruginosa, with minimum inhibitory concentration values as low as 25 milligrams per millilitre. In contrast, ZnO MLE showed limited antifungal activity and required higher minimum inhibitory concentrations. Photocatalytic degradation studies of organic dyes under natural solar irradiation revealed that ZnO MLE is a more efficient photocatalyst, achieving 34 and 35 percent degradation of methyl orange and Congo red dyes respectively within 120 minutes, with pseudo first order rate constants of 0.0044 and 0.0046 inverse minutes. These comprehensive results demonstrate that ZnO NLE is a more promising antimicrobial agent while ZnO MLE exhibits greater potential as a photocatalyst, thereby emphasizing the critical role of plant phytochemicals in tailoring functional properties of zinc oxide nanoparticles toward specific applications.