Estudos de Síntese e Caracterização de Óxidos Mistos do Tipo Espinela

Abstract

A detailed study of the preparation and the characterization of polycrystalline FeCo2O4 and CoFe2O4 samples with the spinel structure, belonging to the Co1+xFe2-xO4 (x = 0; 1)system has been undertaken, in order to evaluate the ionic competition by site occupation and the relation between structure and properties, regarding to their potential use as electrode materials. Three distinct preparation methods were tested – the direct ceramic method, a wet chemistry route and a sol-gel type process – with different parameters: starting materials, stages in characterization corresponding to distinct annealing treatments and cooling process – quenched (q) and slowly cooled samples at 3 K min-1 (sc). After the optimization of these conditions, stoichiometric single-phase samples with spinel structure were obtained. All compounds were characterized by powder X-ray diffraction and scanning electron microscopy combined with X-ray microanalysis, energy dispersive spectroscopy for structural and morphological information. The total Fe and Co contents were estimated by inductively coupled plasma emission spectroscopy and atomic absorption spectroscopy respectively. Associated with the EDS results, these techniques supplied a useful tool for the evaluation of the chemical homogeneity and stoichiometry of the samples. Crystal data and site occupation factors were estimated from phase analysis and structure refinement by Rietveld analysis of powder diffractograms. The non-linear enthalpy of disordering model of O’Neill and Navrotsky was applied to FeCo2O4 and CoFe2O4 spinel phases and the degree of inversion, λ, could be successfully estimated, and afterwards it was compared with the one derived from the experimental techniques. The wet chemistry route has provided excellent reproducibility and a straightforward and rapid way for powder preparation. Moreover, the samples have shown good stability and chemical and morphological homogeneity. For this reason, these samples were chosen to be the subject of a more fundamental characterization and evaluation of their properties. 57Fe Mössbauer spectroscopy revealed that iron was present as Fe3+ and it allowed an estimation of the relative fractions of Fe3+ in tetrahedral and octahedral sites, which combined with powder XRD, providing a more accurate cation distribution. The surface of the materials was analysed by X-ray photoelectron spectroscopy, which showed cobalt segregation in samples prepared at low temperature for both FeCo2O4 and CoFe2O4 stoichiometries. The powders obtained at 1173 K presented that the overall Co/Fe ratios were in accordance with the theoretical values. The fast atom bombardment technique revealed that Fe+ ion was the preferential species being ejected (sputtered) from the sample, suggesting that Fe-O binding energy was smaller than the Co-O. The result was in agreement with the infrared spectra analyses, which showed a deviation of the absorption bands to lower frequency values when Co3+ ions were replaced by Fe3+ ions, with an increase of the Fe3+ fraction in tetrahedral coordination. Nitrogen adsorption studies at 77 K were carried out and revealed high specific surface areas in the samples prepared at 573 K (>100 m2 g-1). Thermogravimetric analysis supplied information about the samples behaviour in the 293 – 1273 K range and the possibility of Co2+ oxidation at ≈560K. Electronic properties were analysed by electric conductivity studies that pointed out a hopping semiconductor behaviour of carriers between localized states, and magnetization studies with temperature and applied field and hysteresis curves, showing a strong dependence of composition, cation distribution and morphology. The samples presented a ferrimagnetic order and the Néel collinear model successfully described the cation arrangement. As a conclusion, it was possible to synthesize the CoFe2O4 and FeCo2O4 samples using the defined preparing conditions. Furthermore, their characterization with the available techniques showed that these materials present potential interest for application, namely, as electrode materials.