Temperature Dependence of Structural, Morphological and Porosity Features in Co1+xFe2-xO4 (x = 0; 1) Spinel Oxides and Electrical Studies

Abstract

Spinel-type oxides containing cobalt and iron have been intensively investigated in recent years due to their remarkable electrical, magnetic and electrocatalytic properties and wide practical applications. Since the theoretical interpretation of the chemical and physical properties strongly depends on the site assigned to the cations, determination of the cation distribution between tetrahedral and octahedral sites is of major importance. We have prepared samples of Co1+xFe2-xO4 (x = 0; 1) at 353 K, using a coprecipitation method, dried at 473 K in a sand bath and afterwards characterized. Annealing temperatures considered for thermal treatments were 573, 773, 1073 and 1173 K. Samples were quenched in air till room temperature. In order to evaluate phase stability, slow cooled (3 K min-1) samples of FeCo2O4, from 1173 K, were also obtained. Powder XRD technique was used to control structural behaviour. Spinel-type phases were obtained for all samples; nevertheless, below 1073 K, it was detected the existence of chemical heterogeneities. Samples prepared at 473, 573 and 1173 K were chosen for a more detailed characterization. Rietveld refinement of XRD patterns and 57Fe Mössbauer spectroscopy allowed a proposal for a cationic distribution for the high-temperature samples.The morphology, grain size and porosity were evaluated by SEM studies and nitrogen adsorption measurements. Surface characterization by XPS confirmed the homogeneity observed for the bulk in samples prepared at 1173 K and FAB results were in agreement with the major binding energy for Co-O suggested by FT-IR studies. The TGA analysis strengthened the fact that FeCo2O4 spinel phase, once prepared at 1173 K, presents a high structural stability even when it is annealed at temperatures that were supposed to produce sample decomposition. Electrical conductivity studies agreed with conduction by hopping mechanism of carriers, depending on cation distribution.