Synthesis and Magnetic Properties of Nearly Monodisperse CoFe2O4Nanoparticles Through a Simple Hydrothermal Condition
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- Li, XH., Xu, CL., Han, XH. et al. Nanoscale Res Lett (2010) 5: 1039. doi:10.1007/s11671-010-9599-9
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Nearly monodisperse cobalt ferrite (CoFe2O4) nanoparticles without any size-selection process have been prepared through an alluring method in an oleylamine/ethanol/water system. Well-defined nanospheres with an average size of 5.5 nm have been synthesized using metal chloride as the law materials and oleic amine as the capping agent, through a general liquid–solid-solution (LSS) process. Magnetic measurement indicates that the particles exhibit a very high coercivity at 10 K and perform superparamagnetism at room temperature which is further illuminated by ZFC/FC curves. These superparamagnetic cobalt ferrite nanomaterials are considered to have potential application in the fields of biomedicine. The synthesis method is possible to be a general approach for the preparation of other pure binary and ternary compounds.
CoFe2O4, as a type of magnetic materials, has long been of intensive importance in the fundamental sciences and technological applications in various fields of electronics , photomagnetism , catalysis , ferrofluids , hyperthermia , cancer therapy , and molecular imaging agents in magnetic resonance imaging (MRI) . The applications of CoFe2O4 are strongly influenced by its magnetic properties. For biomedical applications, CoFe2O4 nanoparticles are required to have a narrow size distribution, high magnetization values, a uniform spherical shape, and superparamagnetic behavior at room temperature. So far, various synthetic routes have been explored for the preparation of CoFe2O4 nanoparticles, such as hydrothermal , coprecipitation [9, 10], microemulsion , forced hydrolysis , reduction–oxidation route . However, the main difficulty of these traditional methods is that the as-prepared nanoparticles are severely agglomerated with poor control of size and shape in most cases, which greatly restrict their applications . In order to solve the above problems, thermal decomposition of organometallic precursors in high-boiling organic solution has been explored [15, 16] for the preparation of size- and shape-controlled monodisperse CoFe2O4 nanoparticles [14, 17–19]. However, the major disadvantages of this method are the need of toxic and expensive reagents, high reaction temperature, and complex operations. To address these concerns, Li et al. adopted a general liquid–solid-solution (LSS) phase transfer and separation method . This strategy is based on a general phase transfer mechanism occurring at the interfaces of the liquid, solid, and solution phases present during the synthesis. Through this general method, Li et al. successfully synthesized Fe3O4 doped with Co, which has a coercivity about 250 Oe at room temperature . However, the synthesis of CoFe2O4 nanoparticles with a superparamagnetic behavior at room temperature has not been reported. In this letter, we report a significant improvement of the method of Li et al.  to synthesize nearly monodispersed CoFe2O4 nanoparticles and systematically investigate the magnetic properties of the as-prepared nanomaterials. At room temperature, these as-prepared nanoparticles were found to have high saturation magnetization values of 50 emu/g and superparamagnetic behavior with negligible coercivity, which is expected to have potential application in biomedicine.
Synthesis of CoFe2O4Spherical Nanoparticles
The process for synthesizing nearly monodisperse CoFe2O4 with superparamagnetic behavior at room temperature was carried out as follows: In a typical synthesis, 1.6 g (6 mmol) of FeCl3·6H2O and 0.7 g of (3 mmol) CoCl2·6H2O were dissolved in the solvent composed of 80 ml of water and 40 ml of ethanol. After that, 7.3 g (24 mmol) of sodium oleate and 7 ml of oleic amine were added into the above solution with strongly stirring at room temperature for 2 h. Then, the reaction precursor was transferred into a Teflon-lined stainless autoclave with a capacity of 150 ml. In order to crystallize the particles, the reaction temperature of the autoclave was increased and maintained at 180°C for 12 h. Then, the system was cooled down to room temperature naturally. The products were separated from the final reaction solution by the addition of hexane. The red supernatant liquor containing CoFe2O4 nanoparticles was separated by a separating funnel. The as-prepared cobalt ferrite could be deposited by adding ethanol and obtained by centrifugating at a high speed (10,000 rpm) without any size-selecting process. The as-prepared samples could be well redispersed in a hexane solvent and stored for several months without delamination.
Properties of the as-synthesized samples were charactered through several techniques. The phase contents and crystal structures of the samples were analyzed by X-ray diffraction (XRD) with Cu Kα radiation on a Philips X’pert diffractometer. Elemental analysis for metal iron was measured by an IRIS ER/S inductively coupled plasma emission spectrometer (ICP-ES). High-resolution TEM (HRTEM) analysis was carried out on a JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV. One droplet of hexane dispersion of CoFe2O4 nanoparticles was dropped on a carbon-coated copper grid and then dried naturally before recording the micrographs. FTIR spectra of the samples capped with oleic amine were performed on a 170SX spectrometer in the range of 500–4,000 cm−1. Magnetic properties of the products were characterized at room temperature with a Lake Shore 7,304 vibrating sample magnetometer (VSM). Temperature and field dependences of the samples were recorded on a Quantum Design MPMS-XL superconducting quantum interference device (SQUID). ZFC/FC measurements were carried out in the temperature range of 10–330 K with an applied field of 100 Oe.
Results and Discussion
The chemical composition of the as-synthesized products is further analyzed by the inductively coupled plasma atomic emission spectroscopy (ICP-AES). The result reveals that the molar ratio of Co and Fe is 1:2.05, which is nearly consistent with the expected stoichiometry of CoFe2O4.
Comparison of magnetic properties of the as-synthesized cobalt ferrites and the reported CoFe2O4 measured at room temperature
The magnetic properties of the as-synthesized CoFe2O4 measured at different temperature
where MFC (FC magnetization) involves the total magnetization from the contribution of all particles, MZFC (ZFC magnetization) is determined by the magnetization from only the contribution of the nanoparticles whose energy barriers are overcomed by the thermal energy (kBT) at the measuring temperature, and f(T) reflects a quantitative characterization for superparamagnetism of the magnetic nanoparticles.
In conclusion, nearly monodispersed CoFe2O4 nanoparticles were prepared under a simple hydrothermal condition. The as-synthesized samples are considered to have potential applications in biomedicine for its narrow particle size distribution, high saturation magnetizations, and superparamagnetization at room temperature. The simple synthesis route used in this work is expected to be a general approach for the preparation of binary and ternary metal oxide, especially spinel ferrite.
An erratum to this article can be found athttp://dx.doi.org/10.1007/s11671-010-9777-9
This work is supported by China Postdoctoral Science Foundation Funded Project and the National Natural Science Foundation of China under Grant Nos. 50602020.
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