Multifunctional structural, optical, magnetic and electrochemical properties of Co-ZnO nanoparticles for supercapacitor applications

Abstract

This manuscript reports the synthesis and multifunctional characterization of cobalt-zinc oxide (Co-ZnO) nanoparticles prepared by room-temperature co-precipitation followed by calcination at 400 °C. X-ray diffraction (XRD) confirmed a dominant hexagonal wurtzite ZnO framework with an average crystallite size of 13.4 ± 0.8 nm and weak cobalt-oxide-related contributions. High-resolution FESEM and TEM revealed aggregated quasi-spherical nanoparticles; re-analysis of 19 clearly resolvable TEM features gave an equivalent projected diameter of 12.2 ± 3.0 nm. Fitted XPS spectra confirmed Zn ²⁺ and Co ²⁺ -containing environments and resolved lattice, defect-/hydroxyl-associated, and surface-adsorbed oxygen components, with an estimated surface Co/Zn atomic ratio of 0.42. UV–Vis analysis using a direct-allowed Tauc representation gave an apparent optical band gap of approximately 2.10 eV. Magnetic measurements indicated dominant paramagnetic behaviour at 300 K with a weak ferromagnetic contribution and a stronger response at 10 K. Electrochemical evaluation showed a CV-derived specific capacitance of 13.36 ± 0.42 F g ⁻¹ at 0.01 V s ⁻¹ and a GCD-derived capacitance of 1.57 ± 0.08 F g ⁻¹ , both exceeding the corresponding pure-ZnO control under identical conditions. Cycling tests showed 90.1 % capacitance retention after 3,000 cycles and an average coulombic efficiency of 98.4 %. These results show that Co incorporation modifies the structural, optical, magnetic, and electrochemical response of ZnO while retaining stable, moderate charge-storage behaviour.