Effect of Annealing Temperature on the Optical Properties of Cobalt Oxide Thin Films

Abstract

A spectrometric technique, UV-Vis spectroscopy was used to measure the optical properties of cobalt oxide (Co₃O₄) thin films synthesized via the sol-gel spin coating technique. Thin films were annealed at temperatures ranging from 250 °C to 400 °C and characterized by spectrophotometer for their optical absorption, transmittance, reflectance, absorption coefficient, extinction coefficient, optical band gap, refractive index, and optical dielectric constant. The results reveal that increasing the annealing temperature enhances the optical absorption while reducing both transmittance and reflectance, indicative of improved crystallinity and surface morphology. The optical band gap decreased from 2.75 eV at 250 °C to 1.80 eV at 400 °C, corresponding to a red shift in the absorption edge. A progressive increase in the refractive index and optical dielectric constant was also observed, suggesting a denser and more compact film structure at higher temperatures. These results demonstrate that thermal treatment plays a crucial role in tailoring the optical performance of cobalt oxide thin films. With increasing annealing temperature, absorption intensity and absorption coefficient increased, while transmittance and reflectance decreased, indicating enhanced structural uniformity and reduced surface scattering. These findings highlight the potential of thermally treated cobalt oxide thin films for use in optoelectronic, photovoltaic, sensor, and solar energy conversion.