Morphology oriented surfactant dependent CoO and reaction time dependent Co3O4 nanocrystals from single synthesis method and their optical and magnetic properties

Abstract

A simple solvothermal strategy has been developed to synthesize highly stable, pure phased CoO and Co₃O₄ nanoparticles from Co(CH₃COO)₂·4H₂O precursor in ethanol solvent at 220 °C. The various phases of cobalt oxide (cubic CoO and Co₃O₄) with controlled size and morphologies viz, nanocubes, hexagonal platelets and nanospheres could be obtained via varying only one parameter (either surfactant or reaction time) in this single synthesis method. XRD and HRTEM analyses revealed pure cubic phase CoO and Co₃O₄ nanoparticles. The formation and shapes of CoO nanoparticles are surfactant dependent, whereas the formation and morphologies of Co₃O₄ are time dependent. The optical absorbance studies showed two distinct broad absorption peaks and hence two optical band gaps for both CoO and Co₃O₄ nanoparticles. CoO nanoparticles showed a wide band gap difference of 2.36 eV, while Co₃O₄ nanoparticles were found to be semiconducting in nature with a 0.6 eV–1 eV band gap difference, which showed characteristic features of CoO and Co₃O₄ nanoparticles, respectively. CoO nanoparticles showed interesting magnetic behaviour due to intrinsic antiferromagnetic structure and uncompensated surface spins, which was confirmed by VSM and EPR studies.