The effect of annealing temperature on the structure and optical properties of sol–gel derived nanocrystalline cobalt aluminate spinel

The thermal evolution of a sol–gel derived cobalt aluminate (CoAl2O4) of a spinel-type structure was studied. The samples were prepared by the sol–gel technique using aluminum-sec-butoxide [Al(OsBu)3] and cobalt nitrate hexahydrate Co(NO3)2 × 6H2O as starting materials and ethyl-acetoacetate (C6H10O3) as a chelating agent. The powder precursors were annealed at various temperatures ranging from 200 to 900 °C. The properties of products were characterized using differential thermal analysis and thermo-gravimetric analysis (DTA/TGA), UV–vis diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD). The UV–vis DRS spectra revealed that in samples annealed at temperatures below 700 °C a part of Co existed in a 3 + oxidation state. The accommodation of Co3+ in octahedral cation sites yields with a green color. Thermal treatment at higher temperatures promote the reduction of Co3+ ions to Co2+ and a change of color from green to blue, later being characteristic of Co2+ ions in tetrahedral coordination. The XRD revealed that annealing of the precursor powder at a lower temperature (500 and 600 °C) resulted in a crystal spinel-type phase Co2AlO4. Annealing at higher temperatures (700–900 °C) induces a reaction of Co2AlO4 with amorphous alumina producing a phase CoAl2O4 having a partially inverse spinel structure, with δ in the range of 0.198–0.260. The spinel powders obtained at all annealing temperatures were in the nanosize range.