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

• Published in: Materials chemistry and physics 2012-08, Vol.135 (2-3), p.587-593
• Main Authors: Kurajica, S., Popović, J., Tkalčec, E., Gržeta, B., Mandić, V.
• Format: Article
• Language: English
• Subjects: Cobalt aluminate; Diffuse reflectance spectroscopy; Rietveld's refinement; Sol–gel; Spinel
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2012.05.030

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. UV–vis DRS spectra of cobalt aluminate powders heat treated at various temperatures. [Display omitted] ► Sol–gel derived spinel-type CoAl2O4 thermal evolution was monitored. ► Below 700 °C part of Co exists in 3+ state (octahedral sites), yielding green color. ► Above 700 °C Co3+ reduction to Co2+ (tetrahedral sites) is promoted, changing to blue. ► Treatment at 500–600 °C resulted in crystal spinel-type phase Co2AlO4. ► At 700–900 °C Co2AlO4 and amorphous alumina react, producing CoAl2O4.