Bright blue pigment CoAl₂O₄ nanocrystals prepared by modified sol–gel method

Abstract

Bright blue ceramic CoAl₂O₄ nanocrystals was prepared by sol–gel technique using aluminum nitrate, cobalt nitrates, diethylene glycol monoethyl ether, and citric acid were used as precursor materials. This method starts from of the precursor complex, and involves formation of homogeneous solid intermediates, reducing atomic diffusion processes during thermal treatment. The formation of pure crystallized CoAl₂O₄ nanocrystals occurred when the precursor was heat-treated at 350 °C in air for 2 h. The stages of the formation of CoAl₂O₄, as well as the characterization of the resulting compounds were done using thermo-gravimetric analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The structure, coloration, particle size, and temperature of formation of the resulting CoAl₂O₄ phases were found to depend on the precursors and methods used for preparation and the calcination temperature. The UV–vis spectra presented three bands at 538,575 and 617 nm attributed to the Co²⁺ spin transitions in tetrahedral sites. UV–vis spectroscopy of the samples allows to compare the band shift with the size variation. These results improve that the bright blue nanopigment was obtained at the presence of citric acid as capping agent. The lowest temperature for preparation of the blue cobalt aluminate is about 400 °C. This temperature is much lower than that needed for preparation of the compound through traditional solid-state reactions (above 1,000 °C).