Green synthesis of ZnO nanoparticles from a <i>Cassia siamea</i> leaf extract: Characterization and its application for the photocatalytic degradation of methylene blue (MB) dye and antibacterial activity

Zinc oxide (ZnO) nanoparticles were successfully synthesized via a green route using Cassia siamea leaf extract as a natural reducing and capping agent. Zinc sulfate served as the precursor, and nanoparticles were prepared through a precipitation method followed by calcination at 600 °C. The nanoparticles were characterized using UV-Vis, FTIR, FESEM, EDX, XRD, TEM and TGA analyses. UV-Vis spectroscopy exhibited a sharp absorption peak at 369 nm, corresponding to a band gap of 3.36 eV, while FTIR confirmed Zn–O bond formation. EDX verified the elemental composition, and XRD revealed a highly crystalline hexagonal wurtzite structure. FESEM and TEM showed hierarchical flower-like morphologies composed of interconnected nanosheets. TGA analysis showed ∼5.5% moisture loss, followed by ∼18.8% decomposition of plant-derived residues, confirming the involvement of organic components during synthesis. The stable final mass verified the formation of thermally robust ZnO nanoparticles after calcinations. Photocatalytic studies demonstrated 94% degradation of methylene blue within 60 min. Furthermore, antibacterial assays indicated strong activity against Staphylococcus aureus and Escherichia coli. The study highlights an eco-friendly approach for producing multifunctional ZnO nanoparticles with potential applications in environmental remediation and biomedical fields.