Versatile Co3O4@metal oxide nanocomposites: from photocatalytic pollutant degradation to high-efficiency energy supercapacitor storage.

Co3O4-based heterojunction nanocomposites (Co3O4@M-O x ; where M = Zn, Ti or Cu) were synthesized using a simple, cost-effective, eco-friendly hydrothermal method. These nanocomposites can be used in a variety of applications. Characterisation using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy, high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET) analysis and complex impedance spectroscopy (EIS) confirmed the presence of pure nanoscale phases measuring 76-91 nm and mesoporous type IV structures. The formation of successful p-n or p-p heterojunctions with spherical morphology was also confirmed. Band gap tuning revealed that Co3O4@CuO narrows the gap for visible-light photocatalysis, whereas Co3O4@ZnO and Co3O4@TiO2 widen it for high-energy redox applications. The composites achieved 90% degradation of Rhodamine B (RhB) and Methylene blue (MB) within 135 minutes (pseudo-first-order kinetics), which can be attributed to Fermi-level-driven band bending and the formation of internal electric fields that separate electrons and holes, thereby suppressing recombination and enhancing the generation of reactive oxygen species. Complex impedance spectroscopy (EIS) was used to characterise the electrical properties. Electrical conduction is thermally activated (100-200 °C) with higher activation energies (0.178-0.245 eV) reflecting the depletion-layer barriers at the grain boundaries. Thanks to interfacial synergy, these materials demonstrate improved conductivity and fast ion/electron transport, showing great promise for use in supercapacitors, water-splitting electrodes, zinc-air batteries, flexible solid-state storage and photocatalytic pollutant degradation.