Formation Mechanism and Shape Control of Monodisperse Magnetic CoFe₂O₄ Nanocrystals

The formation mechanism and shape control of monodisperse magnetic cobalt ferrite (CoFe 2 O 4 ) nanocrystals produced by thermolysis of a stoichiometric Co 2+ Fe 2 3+ −oleate complex in organic solution has been investigated. Synthesis of the pure ternary CoFe 2 O 4 inverse spinel phase, without formation of any intermediate binary cobalt and iron oxides, is favored by the close thermal decomposition temperature of the Co 2+ −oleate and Fe 3+ −oleate precursors. For reaction temperatures between 250 and 320 °C, the nucleation and growth dynamics dictate the size and shape evolution of the nanocrystals. Prenucleation of CoFe 2 O 4 occurs at 250−300 °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 °C, which is above the thermolysis temperature of the mixed Co 2+ Fe 2 3+ −oleate complex, the monomer concentration increases rapidly resulting in homogeneous nucleation. Atomic clusters of CoFe 2 O 4 with size <2 nm are initially formed at 314 °C that then grow rapidly when the temperature is raised to 320 °C in less than a minute. The shape of the CoFe 2 O 4 nanocrystals can be reproducibly controlled by prolonging the aging time at 320 °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 CoFe 2 O 4 nanocrystals with high yield.