Phyto-mediated synthesis of ZnO nanoparticles and their sunlight-driven photocatalytic degradation of cationic and anionic dyes

Abstract

In this study, zinc oxide-based nanocatalysts were biosynthesized using Ocimum basilicum (OB) and Olea africana (OA) leaf aqueous extracts, termed OB-ZnO and OA-ZnO, as a simple, affordable, and environmentally friendly approach. Their characteristics and efficacy in photodegrading cationic dyes (crystal violet and methylene blue) and anionic dyes (methyl orange and naphthol blue black) were investigated. The catalyst’s properties were analyzed using various techniques, including Fourier transform infrared, X-ray diffraction, photoluminescence, thermogravimetric analysis, UV-Vis, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer–Emmett–Teller. Analysis revealed pure products having a hexagonal wurtzite structure, crystallite sizes of 15.04 and 21.46 nm, surface areas of 23.65 and 7.97 m2/g, particle sizes of 35 and 170 nm with spherical (uniform) and oval-like (non-uniform) shapes, and optical bandgaps of 3.15 and 3.05 eV, respectively. Photocatalytic applications under sunlight indicated excellent activity of both catalysts against targeted cationic and anionic dyes. Most notably, even though OA-ZnO has a lower surface area than OB-ZnO, it demonstrated greater efficiency. The variation in effectiveness is explained by the lower bandgap value of OA-ZnO and its ability to reduce electron–hole recombination due to its larger crystal size, which accelerates the degradation process. Additionally, both catalysts exhibited high stability after being used four times.

Keywords

• biosynthesis
• ocimum basilicum extract
• olea africana extract
• organic dyes
• photodegradation
• zinc oxide