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), Mössbauer spectroscopy, and magnetometry. The results unambiguously indicated that the obtained products consisted of CoFe 2 O 4 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, CoFe 2 O 4 nanocrystals with narrow size range were obtained.