Impact of annealing temperature on the structure, magnetic properties, and organic dyes adsorption capacity of Fe0.5Co2.5O4 nanoparticles obtained by combustion

Fe-doped cobalt oxide nanoparticles, FexCo3-xO4, attract considerable attention due to their unique properties, high application potential, and the ability to vary the properties over a wide range by changing the technological conditions. The annealing temperature is one of the decisive parameters critically affecting the properties of nanoparticles. In this work, we studied the Fe0.5Co2.5O4 (Co3O4 type) nanoparticles synthesized by the combustion method: the impact of annealing temperature (400, 500, 600, 700 and 800 degrees C) on their structural, morphological, magnetic characteristics and on their ability to absorb organic dyes. X-ray diffraction analysis revealed that the nanoparticles under study consisted of a crystalline phase of Fe0.5Co2.5O4, with an admixture of the iron oxide phase in trace concentrations. A giant increase in the crystallite size due to annealing was observed. Overall, the average crystallite size increased from similar to 5 nm in the as-prepared sample to similar to 110 nm in the sample annealed at 800 degrees C. These changes in size are explained by the efficiency of the Ostwald ripening process. They are accompanied by an increase in the magnetization of nanoparticles and an appearance of magnetic hysteresis. The study of the adsorption properties of the nanoparticles with respect to the most important water pollutants - organic dyes, i.e. cationic methylene blue (MB) and anionic Congo red (CR) revealed their high selective adsorption capacity to CR. The as-prepared nanoparticles had the highest capacity: the CR dye concentration in water decreased by 90 percent within 5 minutes of exposure to the nanoparticles in concentration of 1 g/L. When the annealing temperature increased from 400 to 800 degrees C, the adsorption capacity decreased by approximately half, but remained high enough to consider these nanoparticles as a promising material for CR selective removal from water.