An insight in the structural, morphological, electrical and optical properties of spray pyrolysed Co3O4 thin films

Abstract

The spray pyrolysis deposition of stable and single phase cobalt oxide (Co3O4) thin films at three different deposition temperatures viz. 350 °C, 420 °C and 470 °C was undertaken in the present work. It was found that substrate temperature played a pivotal role in obtaining pure thin layers. X-ray diffraction analysis confirmed the cubic spinel type structure (a = 8.094 Å) of polycrystalline Co3O4 with predominant orientation along (311) plane. The average crystallite size was found to be around 49 nm. Fourier transform IR spectra revealed the presence of an OB3 vibration at 540 cm−1 and an ABO3 vibration at 640 cm−1 where, B denotes Co3+ ions in an octahedral hole and A denotes Co2+ ions in a tetrahedral hole. The Raman spectra recorded in the wave number range 200–800 cm−1 indicated five Raman peaks and the strong characteristic peak at 694 cm−1 was assigned to the A1g mode which corresponds to Co–O breathing vibration of Co2+ ions in tetrahedral coordination. Surface analysis was carried out with the aid of SEM and AFM. The formation of Co3O4 was confirmed by analyzing the binding energies and intensities of Co 3s, Co 2p and O 1s obtained from XPS. Temperature dependent resistivity was realized from the two probe electrical measurements validating the semiconducting behaviour of the prepared films. Co3O4 films had low transmittance values in the order of 23%–43% in the UV–Vis-NIR region. Estimation through Tauc plots revealed direct optical band gap energies around 2–2.24 eV due to charge transfer transition between p states of O2− and t2 states of Co2+. Related optical parameters like extinction coefficient (k) and refractive index (μ) were calculated using Swanepoel method.