Formation Mechanism and Shape Control of Monodisperse Magnetic CoFe2O4 Nanocrystals

Abstract

The formation mechanism and shape control of monodisperse magnetic cobalt ferrite (CoFe2O4) nanocrystals produced by thermolysis of a stoichiometric Co2+Fe23+−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 °C, the nucleation and growth dynamics dictate the size and shape evolution of the nanocrystals. Prenucleation of CoFe2O4 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 Co2+Fe23+−oleate complex, the monomer concentration increases rapidly resulting in homogeneous nucleation. Atomic clusters of CoFe2O4 with size <2 nm ar...