Low-Temperature Preparation of Superparamagnetic CoFe2O4 Microspheres with High Saturation Magnetization
© The Author(s) 2010
Received: 23 May 2010
Accepted: 26 July 2010
Published: 11 August 2010
Based on a low-temperature route, monodispersed CoFe2O4 microspheres (MSs) were fabricated through aggregation of primary nanoparticles. The microstructural and magnetic characteristics of the as-prepared MSs were characterized by X-ray diffraction/photoelectron spectroscopy, scanning/transmitting electron microscopy, and vibrating sample magnetometer. The results indicate that the diameters of CoFe2O4 MSs with narrow size distribution can be tuned from over 200 to ~330 nm. Magnetic measurements reveal these MSs exhibit superparamagnetic behavior at room temperature with high saturation magnetization. Furthermore, the mechanism of formation of the monodispersed CoFe2O4 MSs was discussed on the basis of time-dependent experiments, in which hydrophilic PVP plays a crucial role.
Over the past decades, superparamagnetic (SP) nanostructures of spinel ferrites (MFe2O4: M = Fe, Co, Cu, Zn, etc.) have drawn intense scientific and technological interests because they possess a wide range of applications in magnetic fluid [1–3], magnetic resonance imaging (MRI) [4, 5], and drug delivery technology [6–8]. Recently, spinel cobalt ferrite (CoFe2O4) nanostructure materials have been extensively studied because they form a magnetic system which is an ideal candidate toward understanding and controlling magnetic properties at the atomic level through chemical manipulation . Based on different techniques, SP CoFe2O4 nano-scale particles have been synthesized by several research groups [10–13]. Unfortunately, these small nanoparticles, especially those with sizes below 10 nm, have poor magnetic response abilities . The low magnetization properties caused by small size limit their usage in a number of practical applications since they cannot be effectively manipulated by using moderate magnetic fields [15, 16]. In order to obtain a high saturation magnetization (Ms), simply making particles larger cannot be an option at all, resulting in a strong aggregation due to the non-SP ferromagnetic attraction .
Recently, much effort has been focused on the preparation of large-size SP particles with high Ms using simple composites [18, 19]. For example, Lee et al.  reported the fabrication of highly uniform SP mesoporous spheres with sub-micrometer scale, composed of silica and CoFe2O4 achieved a high magnetization value. Similar efforts were done by Bao et al.  in which controlled growth of SP nanostructures of spherical and rod-like CoFe2O4 nanocrystals. However, their high Ms required a high reaction temperature and expensive high-boiling point toxic solvents, which are disadvantages for their biological applicability. Moreover, such composite nanostructures are incompatible with low expense of preparing procedure. Herein, we report a single-step process for high-performance monodisperse SP CoFe2O4 MSs at low temperature (at only 180°C). As a consequence of the low-temperature synthesis, the as-obtained samples possess a high Ms (over 55 emu/g) and biocompatibility.
Cobalt acetate [Co(CH3COO)2 · 4H2O], anhydrous ethanol(CH3CH2OH), iron(III) nitrate nonahydrate [Fe(NO3)3 · 9H2O], and poly(vinyl pyrrolidone) (PVP, K30) are of analytic grade reagents and purchased without further treatment. In a typical reaction, 2 mmol Co(CH3COO)2 · 4H2O and 4 mmol Fe(NO3)3 · 9H2O were dissolved in 35 mL CH3CH2OH forming a reddish brown solution, after which 0.2 g PVP was added. The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined stainless steel autoclave (50 mL capacity). The autoclave was heated to and maintained at 180°C for 12–24 h and then was allowed to cool to room temperature. The dark products were subjected to magnetic decantation, followed by repeated washing with distilled water, ethanol, and acetone. The final products were dried in a vacuum oven at 60°C for 8 h.
Phase and composition analyses of the products were performed by X-ray diffraction/photoelectron spectroscopy [XRD/XPS (Philips X Pert Prodiffractometer with Cu Kα (λ = 1.54056 Å) radiation)/(AXIS ULTRA XPS, Al Kα)]. The size and morphology of the as-synthesized MSs were investigated by using scanning/transmission electron microscopy (SEM/TEM, JSM5600LV/JEOL100CX-II). The magnetic measurements were carried out by a vibrating sample magnetometer (VSM).
Results and Discussion
In summary, SP CoFe2O4 MSs have been synthesized through a simple surfactants-assisted solvothermal method at a relatively low temperature. All of the reactants are common reagents and inexpensive, as well as environment benign. The as-prepared MSs, composed of about 8-nm CoFe2O4 nanoparticles, have uniform sizes (diameters up to several hundred nanometers), high Ms and well water-dispersible make them good candidates for not only in advanced magnetic materials and ferrofluid technology, but also in biomedical fields such as biomolecular separations, targeted drug delivery, as well as magnetic resonance imaging.
This work was partially supported by the Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2008 HASTIT002), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 094100510015), and by the Natural Science Foundation of China under Grant No. 20971036.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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