Preparation And Properties Of CoO And Co₃O₄ Thin Films Deposited By PLD

Among the transition metal oxides, cobalt oxide is one of the most studied oxides due to its importance for various scientific technologies. There are two known stable oxides of cobalt; CoO and Co₃O₄. both being cubic in structure, the latter having a wider range of stability. Of these, Co₃O₄ belongs to the normal spinel crystal structure of the type AB₂O₄. The monoxide CoO crystallizes in the rock salt structure. Cobalt oxide has many industrial applications, such as solar selective absorber, catalyst in the hydrocracking process of crude fuels, pigment for glasses and ceramics and catalyst for oxygen evolution and oxygen reduction reaction. It is also widely used as perpendicular magnetic recording medium, magneto-optical recording medium, sensors electrochemical anodes and newly invented application in supercapacitors. Cobalt oxides also show electrochromic properties and are potential materials for non-emitting displays, smart windows, and thermal control for space vehicles. They are attractive in solar cells as selective absorbers and corrosion protective coatings. In this paper, we report the details of acomparative study of cobalt oxide structural analysis, phase formation, surface morphology, and optical properties of thin films deposited by PLD using Co target in oxygen reactive atmosphere.CoO and Co₃O₄ thin films have been deposited on glass substrates at different temperature and with various oxygen flow rate by reactive pulsed laser deposition techniques (PLD). Co (99.9%) was used as target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. High purity metallic Co disk was used as the target material, and high purity oxygen was used as the reactive gas. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible spectrometry were applied to characterize the crystalline phases of the deposited films. Results show that when the oxygen flow rate is less than 15sccm, the deposited films is (111)-oriented rocksalt CoO, while films deposited at oxygen flow rate greater than 15sccm, the deposited films are spinel Co₃O₄. The indirect band structures of both the CoO and Co₃O₄ phases are proved from UV-Vis absorbance spectra, with bandgaps of 0.82eV for CoO and 1.21eV for Co₃O₄, respectively.