Green-synthesized Co₃O₄ nanoparticles: Structural, vibrational, magnetic, and electrochemical insights for high-performance energy storage applications

Co₃O₄ nanostructures were synthesized using an aqueous pomegranate (Punica granatum) peel extract as a plant-derived processing aid and subsequently evaluated through an integrated structural, vibrational, morphological, surface-chemical, magnetic, optical, and electrochemical characterization strategy. X-ray diffraction (XRD) confirmed the formation of cubic spinel Co₃O₄ (Fd-3m) with an average crystallite size of approximately 28 nm; Williamson–Hall analysis, where applicable, gave a comparable crystallite size of approximately 29 nm and a low microstrain of approximately 1.5 × 10⁻³ . Raman and FTIR spectra showed the characteristic vibrational signatures of spinel Co₃O₄. FESEM revealed quasi-spherical particles with an image-derived mean particle diameter of approximately 185 ± 35 nm, larger than the XRD crystallite size, indicating that the observed particles are aggregates of multiple crystallites. Nitrogen adsorption–desorption measurements indicated a mesoporous texture with a BET surface area of approximately 52.8 m² g⁻¹ , a total pore volume of approximately 0.26 cm³ g⁻¹ and an average BJH pore diameter of approximately 19.7 nm. XPS confirmed Co and O as the principal surface elements and the mixed Co²⁺/Co³ ⁺ chemical environment expected for spinel Co₃O₄. At 5 K, the M–H response showed a small hysteretic magnetic contribution, which is interpreted cautiously as a nanoscale surface/finite-size effect superimposed on the predominantly antiferromagnetic character of Co₃O₄. UV–Visible analysis yielded an optical gap of approximately 1.83 eV, interpreted here as an indirect optical transition rather than as evidence of a 'narrow-band-gap' semiconductor. Electrochemical measurements in 6 M KOH showed pronounced Faradaic redox activity, with specific capacitances of 304 F/g at a current density of 1 A/g and a capacitance retention of 95% after 5000 at 2 A/g current density. The integrated results indicate that crystallinity, mesoporous architecture, mixed cobalt valence, and nanoscale surface chemistry collectively govern the observed charge-storage response. The study therefore provides a structure–property framework for plant-assisted preparation of Co₃O₄ rather than treating the green synthesis route as the sole source of novelty.