Physical & Electrochemical Properties of Green Synthesized Bunsenite NiO Nanoparticles via Callistemon Viminalis’ Extracts

P-type Bunsenite NiO powders with an average crystallite size of 21 nm (as shown by x-ray diffraction analysis) were produced via biosynthesis and heat treatment using aqueous extracts from red flowers of the plant, Callistemon viminalis. SEM showed that the NiO powders consisted of particles with sizes in the 20-35 nm range while XPS confirmed the formation of highly pure NiO. From Raman spectroscopy, strong 1 phonon vibrations at 507.4 cm-1 and the existence of a broadened 2-phonon band of reduced intensity at 1096 cm-1confirmed that biosynthesized NiO powders were not only defect-rich/ rich in surface effects but were also nanosized with dimensions less than 100 nm. Through UV-Vis-NIR spectroscopy, the optical band gap for an annealed spin-coated thin film of NiO obtained using the green coloured Ni2+-containing extract of Callistemon viminalis, was calculated to be 3.35 eV. A cyclic voltammetric study of the NiO nanopowders on Ni showed the redox processes to be quasi-reversible with the films showing potential for pseudocapacitance and the specific capacitance of the NiO thin films on Ni being estimated at 101 F/g. Electrochemical impedance spectroscopy showed the associated redox processes to be primarily diffusion controlled at room temperature. Through these findings the use of natural plant extracts is hereby shown to be a cost-effective and environmentally friendly alternative to preparing Nickel oxide nanosized powders that can be of use in a variety of energy storage applications.