Structural characterization and optical properties of Co3O4 and CoO films

Thin-film coatings based on cobalt oxides are obtained. For depositing cubic spinel Co 3 O 4 and CoO films the sol-gel dip-coating technique was used. The as-deposited films were subjected to different thermal treatments in air and in reducing atmosphere (H 2 /N 2 ), respectively. Optical absorption spectroscopy, X-ray diffraction and atomic force microscopy (AFM) were used to characterize the samples. The Swanepoel method was employed for the study of the films optical constants as a function of wavelength. By XRD studies it was established that the annealed films become polycrystalline. The films exposed in air consist in Co 3 O 4 phase, while the films annealed in reducing atmosphere (H 2 /N 2 ) exhibit the CoO phase. AFM studies showed uniform films with smooth surface. By exposing the film to H 2 /N 2 , the average rugosity increases ten times. After treatment, the cobalt oxide films transmittance values decrease. The decrease is more meaningful when the films are exposed in reducing atmosphere. The UV-VIS spectra of the films exposed in air present an absorption peak at λ= 730 nm, which corresponds to a charge transfer ligand-metal: O 2- →Co 3+ The peak which corresponds to λ= 690 nm is due to Co 2+ . These transitions confirm the existence of Co 3 O 4 . Concerning the films annealed in reducing atmosphere (H 2 /N 2 ), the band at 594 nm which appears in the spectra is typical for Co 2+ in octahedral coordination and ascribed to the 4 A 2 → 4 T 1 (P) transition. The refraction index presents a normal dispersion, the films exposed in forming gas having higher values than the films exposed in air. The films annealed in a reducing atmosphere feature only direct transitions, representing an internal oxido-reduction process Co 3+ →Co 2+ . The films exposed in air present both direct and indirect transitions. Co 3 O 4 films present an allowed direct interband transition of 1.4-1.5 eV and 2.18-2.23 eV, respectively, while CoO films have an optical band gap energy of 2.2-2.8 eV.