A novel route for the preparation of CoCr₂O₄/SiO₂ nanocomposite starting from Co(II)–Cr(III) carboxylate complex combinations

This study reports the preparation and characterization of the spinel CoCr₂O₄. In order to obtain 20% CoCr₂O₄/80% SiO₂ and 50% CoCr₂O₄/50% SiO₂ (mol%) nanocomposites, we have used a versatile pathway based on the thermal decomposition of some particular precursors, Co(II) and Co(III) carboxylate-type complex combinations, inside the SiO₂ matrix. The ligands of these coordination compounds result in the redox reaction between Co(II) and Cr(III) nitrates and 1,3-propylene glycol by heating at 150 °C of the gels (tetraethylorthosilicate–metal nitrates–1,3-propylene glycol). The as-obtained precursors, embedded in silica gels, were characterized by FT-IR spectrometry and thermal analysis. Both precursors decompose up to 350 °C, leading to the corresponding metal oxides inside the silica matrix. X-ray diffraction of the powders annealed at different temperatures has evidenced the formation of CoCr₂O₄ starting with 400 °C for 20% CoCr₂O₄/SiO₂ and 300 °C for 50% CoCr₂O₄/SiO₂. This behaviour can be explained by the fact that, by thermal decomposition of the chromium carboxylates, a nonstoichiometric chromium oxide Cr₂O_3+x is formed. At ~ 400 °C, Cr₂O_3+x turns to α-Cr₂O₃, which interacts with CoO leading to cobalt chromite nuclei inside the pores of the silica matrix. CoCr₂O₄ has been obtained as nanocrystallites homogenously dispersed within the silica matrix as resulted from XRD, TEM and EDX mapping, with mean particle size in the range 5–20 nm.