Green synthesis of ZnO nanoparticles using Cosmos caudatus: Effects of calcination temperature and precursor type on photocatalytic and antimicrobial activities

• Synthesis using Cosmos caudatus leaf extract produced ZnO nanoparticles • Effect of calcination temperature and type of precursor were investigated • The calcination temperature and precursor type greatly altered particle morphology • Precussor Zn(NO 3 ) 2 .4H 2 O at 600 °C degraded methylene blue 81.5% after 75 minutes • The gram-negative bacteria exhibited the most significant inhibition zone. The development of an efficient and eco-friendly production method for zinc oxide nanoparticles (ZnO NPs), which are inorganic compounds used in various industries, is crucial. The biological synthesis of ZnO NPs has shown great application potential. Thus, this study aims to investigate the effect of calcination temperature and precursor type on the physicochemical characteristics, photocatalytic activity, antimicrobial properties, and reusability of ZnO NPs by synthesizing such nanoparticles from Cosmos caudatus leaf extract. The morphology of the obtained ZnO NPs was influenced by the calcination temperature and precursor type, as determined by SEM analysis. In addition, XRD analysis revealed that the Zn(NO 3 ) 2 .4H 2 O precursor and calcination temperature of 600°C produced ZnO NPs with a hexagonal wurtzite crystal structure. The nanoparticles examined had a minor average diameter of 23.9 nm, high methylene blue degradation efficiency of 81.5% after 75 min, and large inhibition zone for Escherichia coli at 21.38 nm. The second recycling decreased the degradation efficiency by 11.7%. The maximum UV–visible absorption was measured at a wavelength of 305 nm with a band gap energy of 3.85 eV.