Controlled Synthesis of Near-Stoichiometric Cobalt Ferrite Nanoparticles

Large batches of more than 18 g of cobalt ferrite nanoparticles (Co x Fe3 - x O4, x being close to 1) have been prepared by the “chemie douce approach” using aqueous solutions of metal salts at 90 °C mixed with solutions of hydroxide ions under air atmosphere. By suitable choice of the metal ion to hydroxide ion ratio, it was possible to prepare nanoparticles with the stoichiometric composition (CoFe2O4). The composition and the density of the nanoparticles could be controlled by varying the metal ion to hydroxide ion molar ratio in the reactor. Adjusting the initial concentration ratios of the reactants prior to the mixing allowed the variation of the average size of the nanoparticles. The repeatability of the average particle diameter of the synthesis was typically 5 nm and average particle sizes could be controlled between 50 and 80 nm determined by nitrogen adsorption measurements (consistent with the number size average 35−60 nm obtained by transmission electron microscopy studies). Aging of the suspensions resulted in a narrowing of the initial broad unimodal distribution. The narrowing of the size distribution was associated with the phase transformation of δ-FeOOH platelets to spinel phase. The spinel nanoparticles had different morphologies: cubic, spherical, and occasionally irregular. Nanoparticles with the stoichiometric composition were a mixture of cubical and spherical shapes. Nanoparticles with less than the stoichiometric cobalt content had an irregular morphology, whereas nanoparticles with greater than the stoichiometric concentration of cobalt were predominantly spherical.