Salvia officinalis leaf extract-stabilized NiO NPs, ZnO NPs, and NiO@ZnO nanocomposite: Green hydrothermal synthesis, characterization and supercapacitor application

In this study, NiO nanoparticles (NiO NPs) and NiO@ZnO nanocomposite were synthesized for the first time using a Salvia officinalis (S. officinalis) extract-assisted hydrothermal process. The S. officinalis leaf extract served as a natural reducing and capping agent. The synthesized NiO NPs, ZnO NPs, and NiO@ZnO nanocomposite were thoroughly characterized using various techniques, including Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), energy-dispersive spectrometry (EDS) mapping, vibrating sample magnetometer (VSM), and cyclic voltammetry (CV) analysis. The direct and indirect band gap energies of NiO NPs, ZnO NPs, and NiO@ZnO were found to be 3.00, 2.28, and 2.71 eV, and 2.63, 1.91, and 2.23 eV, respectively. The crystallite sizes were analyzed using PXRD spectra through Scherrer and Williamson-Hall (W-H) methods. TEM analysis revealed that the average particle sizes of NiO NPs, ZnO NPs, and NiO@ZnO were 16.0, 207.5, and 31.0 nm, respectively. The magnetic properties of all nanomaterials were assessed via the VSM technique. Specific capacitance (Cs) values, determined from CV voltammograms, were 196.8, 632.4, and 785 Fg-1 at a scan rate of 25 mVs-1 for NiO NPs, ZnO NPs, and NiO@ZnO, respectively. These findings suggest that the green-synthesized NiO@ZnO nanocomposite holds significant potential as a high-performance electrode material for supercapacitor applications.