Facile Synthesis and Characterization of Cobalt Ferrite Nanocrystals via a Simple Reduction-Oxidation Route

In this paper, a facile synthetic route of cobalt ferrite nanocrystals with narrow size distribution was reported. The key feature of this method involved a very rapid mixing of reducible metal cations with sodium borohydride and simultaneous reduction in a colloid mill, which is followed by a slow oxidation in a separate hydrothermal treatment. The microstructural and magnetic characteristics of the materials were characterized by powder X-ray diffraction (XRD), chemical analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), transmission electron microscopy (TEM), Mossbauer spectroscopy, and magnetometry. The results unambiguously indicated that the obtained products consisted of CoFe2O4 nanocrystals with good monodispersity and high stoichiometry and that, especially, a 9 nm sample exhibited room-temperature superparamagnetism. The formation mechanism of nanocrystals was proposed. It is believed that the extreme forces, to which the nucleation mixture of metallic cobalt and iron obtained with a very short time was subjected in the colloid mill, prevented aggregation of the newly formed metal nuclei. Consequently, when the resulting metal nuclei were oxidized slowly in a separate hydrothermal treatment, CoFe2O4 narrocrystals with narrow size range were obtained.