Physical properties and antioxidant activity of ZnO nanoparticles synthesized by Moringa oleifera leaf extract at different pH values

We have demonstrated green synthesis of ZnO nanoparticles using Moringa oleifera leaves (Mor-ZnO NPs) extract by co-precipitation method under 450^oC for 2 hours with different pH-5, pH-7, and pH-8. The physical properties (i.e. structural, morphology, and absorbance) have been analyzed using X-ray diffraction (XRD) measurement, scanning electron microscopy (SEM), and ultraviolet-visible (UV-Vis) spectrophotometer. Furthermore, to investigate the functional group and antioxidant activity we performed Fourier transform infra-red (FTIR) analysis and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Structurally, all samples possess polycrystalline hexagonal wurtzite structure. Morphologically, the pH values affect the morphology of Mor-ZnO NPs. Optically, the Mor-ZnO NPs with pH-8 have strong absorption in the range of short ultraviolet (200-280 nm). We predict that the basic pH value on green synthesize of Mor-ZnO NPs tune the near band edge absorption (NBA), which relates to the recombination of the free excitons. Whereas, based on the FTIR spectra analysis, the Zn-O stretching can be identified at 442.67 cm^-1, 458.67 cm^-1, and 442.67 cm^-1 for Mor-ZnO NPs with the pH-5, pH-7, and pH-8, respectively. According to DPPH assay, the Mor-ZnO NPs with pH-8 at 200 mg/ml has higher inhibition rate (67.7%). Hence, we believed that the Mor-ZnO NPs are potential antioxidants.