Temperature-induced variations in the structural, morphological and optical features of cobalt oxide nanoparticles synthesized via hydrothermal method

Cobalt oxide is a promising material that attracts the researchers to find its application in various fields like catalysts, dielectric applications, gas sensors and electronics devices, etc. In this work, we investigated the effect of temperature in the structural, elemental, morphological and optical properties of Co3O4 nanoparticles synthesized by hydrothermal method. The as-prepared and annealed samples were observed for their structural and optical properties. The structural analysis and micro-strain effects of Co3O4 nanoparticles with cubic spinel structure were carried out through X-ray diffraction (XRD) studies. The crystallite size of as-prepared and annealed samples were around 27 nm and 29 nm, respectively. Raman spectrum showed the lattice vibrations of both oxidation states of cobalt oxide nanoparticles. Elemental analysis were observed through X-ray photoelectron spectroscopy (XPS). The morphology and chemical composition of the samples were determined from the field emission scanning electron microscope (FESEM) image and energy dispersive X-ray spectroscopy (EDAX). Moreover, the internal structure of both samples were compared through transmission electron microscopic (TEM) studies. The optical studies were done using the absorption spectrum and photoluminescence spectroscopy. Two direct band gaps (1.8 and 3.2 eV) were observed in the as-prepared, which got red-shifted on annealing (1.7 and 2.5 eV) due to size effects. The charge transfer process in two oxidation states of cobalt oxide could be identified from these bandgaps. Emission peaks in the photoluminescence spectrum of samples are due to the recombination of free-excitons and point defects in the crystal lattice.