Cinnamon-assisted synthesis of defective ZnO nanoparticles for enhanced antibacterial and photocatalytic applications

This study reports a plant-mediated synthesis of zinc oxide nanoparticles (G-ZnO-NPs) using cinnamon bark extract during the zinc oxalate precipitation step. The extract acts as both an aqueous precipitation medium and a capping agent with cinnamaldehyde identified as the predominant phytochemical and coumarin as a comparatively minor constituent. These phytochemicals modulate particle growth and defect formation, thereby tailoring the physicochemical properties of the resulting nanoparticles. The properties of G-ZnO-NPs were systematically compared with those of commercial ZnO (C-ZnO) and ZnO nanoparticles synthesized via zinc oxalate precipitation in water (W-ZnO-NPs). Relative to both reference materials, G-ZnO-NPs exhibit reduced particle agglomeration, a remarkable particle size reduction (15.9 nm), an increased specific surface area, a lower band gap energy, and more efficient separation of photogenerated electron-hole pairs partly attributable to defect-mediated charge trapping, thereby resulting in a higher photocurrent density and a lower charge-transfer resistance. Collectively, these features contribute to enhanced photocatalytic and antibacterial performance. Under xenon-lamp irradiation, G-ZnO-NPs achieve 98.7% degradation of methyl orange (MO) within 150 min, compared with 83.6% and 88.9% for W-ZnO-NPs and C-ZnO, respectively, with corresponding first-order kinetic constants of 0.0289, 0.0118 and 0.0146 min -1 . Moreover, G-ZnO-NPs exhibit good structural stability and satisfactory recyclability after five consecutive photodegradation cycles, with hydroxyl radicals (OH . ) and photogenerated holes (h + ) identified as the predominant reactive species involved in MO degradation. In addition, G-ZnO-NPs demonstrate superior antibacterial activity than both reference ZnO samples particularly against Escherichia coli and Bacillus subtilis . These results highlight the potential of the cinnamon-based synthesis method as an eco-friendly strategy for producing ZnO-NPs with tailored properties offering a dual benefit for addressing water pollution and antibacterial resistance.