Comparative study on the preparation procedures of cobalt ferrites by aqueous processing at ambient temperatures

Cobalt ferrite particles were synthesized by aerial oxidation of ferrous and cobalt ions at alkaline condition and room temperature. It was found that mixing procedures of ferrous and cobalt ions and alkaline in initial stage of reaction were critical in mechanisms of the formation of cobalt ferrite and sizes of particles. Smaller particles were precipitated in the procedure in which alkaline solution was dropped into the mixture solution of ferrous and cobalt ions (procedure 1, denoted as normal pH regulation procedure), and with the increase of content of cobalt in the initial solution, the content of Fe(III) in the product decreased which suggested that cobalt ions occupied the location of Fe(III) in the spinel structure, and it is concluded from Mössbauer measurement at room temperature that the transformation from sextet to doublet recorded in the spectrum is typical superparamagnetic (SPM) transition, which depend on increasing of Fe(III) in the Co(II) gelatin with the Co concentration in initial solution. In procedure 2, referred to as reverse pH regulation procedure, where ferrous and cobalt ions were dropped into alkaline solution, with the increase of the ratio of Fe3+/Fe2+(from 0 to 1) of initial solution, the ratio of Co2+/Fe(total) in the end product decreased, and the products exhibited strong magnetization that would be in a tight correlation with the ferrite structure, evidenced by XRD analyses. But, the precipitate with the ratio of Fe3+/Fe2+ = 1 of initial solution produced by procedure 1 exhibited no magnetization and brown-red color. The possible mechanisms of the formation of cobalt ferrite in procedure 1 and 2 were given based on analysis of XRD and IR patterns along the reaction time.The cobalt ferrite were produced through green-rust intermediate phase in procedure 1, while were precipitate directly in procedure 2.