The aim of this study was to detennine the effect of different complexing agents and of the annealing tem perature on the structural, morphological and optical properties of the synthesized precursors. Thus, cobalt aluminate nanoparticles were prepared by the sol-gel method using polyacrylic acid, glycine or citric acid as complexing agents. The synthesis performed at 500 °C for 5 hours led to dark green powders composed of solid solutions denoted Co 2+ [ Agci-x) Co�;i]O4 and of amorphous alumina (39.8wt%). Calcination at temperatures above 900 °C caused the powder colour to change from dark green to blue. A direct spinel structure Co 2+ [ Al� + ] 04 is all the more achieved as the annealing temperature is high. The powder obtained using polyacrylic acid as the complexing agent at 900 °C for 5 hours revealed the best morphology; it consisted of agglomerates of primary particles with a quasi-spherical shape and a size in the range 20-40 nm.
Nanocrystals of zinc-cobalt molybdate (Zn1-xCc>xMo04) were prepared by the glycine-nitrate process (GNP) route with a glycine/nitrate ratio of 2:3 and compared to the solid state synthesis. The obtained powders were characterized by thermal analysis (TGA-TDA), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The morphology was examined by scanning electron microscopy (SEM) and Brunauer-Ernrnett-Teller (BED. The particle size was determined by transmission electron micros copies (TEM). UV-Visible spectroscopy and CIE L*a*b* colorimetric parameters have been used for the colour characterisation and measurement. The compounds obtained present two single-phased domains regardless of the synthesis method. The first domain of blue colour corresponding to a level of cobalt x such as: 0 = x S 0.3, isostructural with a-ZnMo04 of triclinic symmetry and the second domain of green colour, for cobalt x levels such as: 0.45 = x S 1 isostructural with cr-CoMo04 of monoclinic structure. The Zno,7Coo.3Mo04 powders obtained at 700 °c by GNP route and synthesized in acidic environment were fonned of particles of quasi-spherical morphology, with average size of crystallites estimated between 80-140 nm.
Magnesium-cobalt molybdate composites (Mg 1-x Co x MoO 4 : x = 0, 0.3, 0.4, 0.6, 0.8 and 1) were successfully synthesized through a facile sol-gel synthesis at low temperature, and characterized by thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), UV-Vis spectroscopy and colorimetric measurements using the CIE L*a*b* colour system. The surface specific area was calculated using the Brunauer-Emmett-Teller analysis in the adsorption/desorption isotherm. The examination of the X-ray diffractograms of the unground milled solid solutions of Mg 1-x Co x MoO 4 (0 ≤ x ≤ 1) presented a single continuous system related to monoclinic β-MgMoO 4 . The absorbance spectra of the Mg 1-x Co x MoO 4 pigments confirmed the insertion of cobalt in the β-MgMoO 4 matrix. The CIE-L*a*b* colour coordinates indicated that the intense purple colour was obtained for x = 0.6.
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