Formation Mechanism and Shape Control of Monodisperse Magnetic CoFe2O4 Nanocrystals

The formation mechanism and shape control of monodisperse magnetic cobalt ferrite (CoFe2O4) nanocrystals produced by thermolysis of a stoichiometric Co2+ Fe-2(3+)-oleate complex in organic solution has been investigated. Synthesis of the pure ternary CoFe2O4 inverse spinel phase, without formation of any intermediate binary cobalt and iron oxides, is favored by the close thermal decomposition temperature of the Co2+-oleate and Fe3+-oleate precursors. For reaction temperatures between 250 and 320 degrees C, the nucleation and growth dynamics dictate the size and shape evolution of the nanocrystals. Prenucleation of CoFe2O4 occurs at 250-300 degrees C but without any growth of nanocrystals, because the monomer concentration is lower than the critical nucleation concentration. For temperatures in the range of 300-320 degrees C, which is above the thermolysis temperature of the mixed Co2+ Fe-2(3+)-oleate complex, the monomer concentration increases rapidly resulting in homogeneous nucleation. Atomic clusters of CoFe2O4 With size <2 nm are initially formed at 314 degrees C that then grow rapidly when the temperature is raised to 320 degrees C in less than a minute. The shape of the CoFe2O4 nanocrystals can be reproducibly controlled by prolonging the aging time at 320 degrees C, evolving from initial spherical, to spherical-to-cubic, cubic, corner-grown cubic, or starlike shapes. Thus, with careful choice of reaction parameters, such as the precursor concentration and the heating rate, it is possible to achieve large-scale synthesis of shape-controlled monodisperse CoFe2O4 nanocrystals with high yield.