Cobalt oxide-based films: sol-gel synthesis and characterization

Recently, much attention has been focused on cobalt oxide-based materials. In this study, Co3O4, CoO and CoO–SiO2 coatings were synthesized via sol–gel using cobalt acetate [Co(CH3COO)2·4H2O] and tetraethoxysilane [Si(OC2H5)4] (TEOS) as starting compounds. The precursor choice was made taking into account that acetate decomposes under thermal treatment without leaving residual contaminants inside the coatings. The films were prepared by a dip-coating procedure from alcoholic solutions of the proper precursors and subsequently annealed under different conditions. Co3O4 coatings were obtained after heating in air or nitrogen at 300 °C. Treatments in reducing atmosphere (H2/Ar) yielded CoO layers up to 500 °C, and films of metallic cobalt for firing at higher temperatures. CoO–SiO2 coatings were heated in air between 300 and 900 °C. CoO cluster formation in the silica glass layer is not yet well clarified. The chemical composition and the microstructural evolution of the coatings were studied by X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), UV–Vis spectroscopy and X-ray diffraction (XRD).