J. Phys. Chem. C 2009, 113, 4047–4052
4047
Synthesis of Monodispersed Co(Fe)/Carbon Nanocomposite Microspheres with Very High Saturation Magnetization Yiwen Chu, Peng Zhang, Jianghua Hu, Wuli Yang, and Changchun Wang* Key Laboratory of Molecular Engineering of Polymers (Minister of Education) and Department of Macromolecular Science, AdVance Materials Laboratory, Fudan UniVersity, Shanghai 200433, China ReceiVed: NoVember 26, 2008; ReVised Manuscript ReceiVed: January 21, 2009
Through the complexation of cyano groups in polyacrylonitrile (PAN) microspheres with cobalt (iron) ions under mild conditions, cobalt (iron)-containing PAN microspheres could be prepared effectively, then after pyrolyzation of the composite microspheres at 800 °C under N2 atmosphere, Co(Fe)/carbon nanocomposite microspheres could be prepared, and the metal content is up to ∼71 wt % in the Co(Fe)/carbon nanocomposite microspheres. Thermogravimetric analysis, transmission electron microscopy, and vibrating sample magnetometry investigation proved that almost all the cobalt nanoparticles were embedded in the interior of the carbon microspheres, which protected the metal nanocrystals (Co or Fe) from oxidation. The microspheres possess high magnetization (Ms is up to ∼161 emu/g) and show near-zero remanence and coercivity (Hc is down to ∼0.012 KOe) at room temperature, suggesting that they are outstanding soft ferromagnets with high magnetic susceptibilities and practically nil hysteresis loss. 1. Introduction The preparation of superparamagnetic microspheres has been intensively pursued not only for the fundamental scientific interest of the magnetic materials but also for their wide technological applications, especially in biomedicine and biochemistry.1-5 Because specifically required for in vitro and in vivo tests,6 the size-controllable superparamagnetic microspheres with high saturation magnetization is of key importance for fast and sensitive magnetic signal control, manipulation, and detection. Recently, ferromagnetic transition metal nanoparticles have attracted much attention owing to their excellent magnetic properties (the saturation magnetization of Fe nanoparticles is twice that of magnetite, which is a very popular magnetic material). Among them, Co and Fe nanoparticles and their alloys are of intensive interest. Sun and co-workers synthesized cobalt nanocrystals from the reduction of cobalt chloride in the presence of oleic acid as stabilizing agents.7 Fe nanocrystals were synthesized by decomposition of Fe(CO)5 in octadecene.8 Even though highly crystalline magnetic nanoparticles can be produced, the experiments reveal that the prepared nanoparticles are extremely reactive and subject to oxidation, resulting in antiferromagnetic oxides, and thereby, they significantly lose their main advantage of high magnetic moment. As a result, these syntheses have difficulties in producing air-stable metal nanoparticles with high saturation magnetization. Then, intensive efforts have been made toward coating and protecting metal crystalline nanoparticles; for example, embedding the particles into polymer matrixes9,10 or passivating them with an oxide shell,11 gold layer12 or carbon.13-16 Due to the facts that the loading of metal nanoparticles in polymer matrices is usually low,17,18 the oxide layers can cause the deterioration of the magnetic properties, and the use of gold does not seem to be cost-efficient, then the choice of carbon coating becomes the most promising option. Wang et al.13 synthesized carboncoated cobalt nanoparticles by a chemical vapor-condensation * Corresponding author. E-mail:
[email protected].
process with cobalt carbonyl as a precursor and carbon monoxide as the carrier gas. The obtained cobalt nanoparticles were coated completely by amorphous carbon, which resisted oxidation and dilute acid. The saturation magnetization went up with increasing decomposition temperature due to the increase in the size of the magnetic particles, but Co2C and Co3C were inevitably formed during the process, which reduced the magnetization of the nanocomposites. The cobalt/carbon nanocomposite particles could be functionalized by use of diazonium chemistry to yield chloro-, nitro-, and amino-functionalized magnetic nanoparticles, providing a potential way to functionalize metal/carbon nanocomposite particles for biotechnological application.19 Pyrolyzing organometallic polymers provided a new way to fabricate nanostructured materials. It was found that hyperbranched organometallic polymers are better precursors with higher iron contents than their linear counterparts. Cobalt-containing hyperbranched polyynes were used as precursors and pyrolyzed to nanostructured cobalt ceramics, with a saturation magnetization of 118 emu/g.20 However, all the reported methods could not well resolve the problems of uncontrollable shapes and uncertain composition, which obstruct their potential and practical application. Herein, we report for the first time a novel method to prepare air-stable Co(Fe)/carbon nanocomposite microspheres with very high metal loading and a controllable size. This process is based on entrapping of Co2+ (Fe3+) into polyacrylonitrile (PAN) template microspheres, followed by pyrolysis of the template microspheres containing these metal complexes under a nitrogen atmosphere. The maximum saturation magnetization of the pyrolyzed nanocomposites reached to 161.5 emu/g. PAN template microspheres, as a well-known precursor of carbon, could maintain their shapes well after pyrolysis. 2. Experimental Section 2.1. Materials. Acrylonitrile was purchased from Fluka. Divinyl benzene (DVB) was obtained from Aldrich. Sodium chloride, potassium persulphate, sodium nitrate, N,N-dimethyl formamide (DMF), cobalt(II), and iron(III) chloride were
10.1021/jp810395j CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
4048 J. Phys. Chem. C, Vol. 113, No. 10, 2009
Chu et al.
TABLE 1: The Influence of Cross-Linked Degree of PAN Microspheres on the Cobalt-Loading Amount in Co(II)/PAN Microspheres at 60°Ca
a
sample
Co/PAN-5
Co/PAN -10
Co/PAN -15
Co/PAN -20
cross-linked degree of PAN microspheres Co(II) wt % in Co(II)/PAN microspheres b
5% 26.1
10% 25.3
15% 7.8
20% 5.7
In preparation, the molar feed ratio of Co2+ to CN is 2:1. b Calculated by TGA (see Supporting Information for the details).
analytical grade and commercially available products. All chemicals were used as received. 2.2. Synthesis of Monodispersed PAN-Template Microspheres. Monodispersed PAN-template microspheres were prepared via soap-free emulsion polymerization. A typical synthesized procedure is described as follows: A solution of NaCl (0.004 g), acrylonitrile (7.00 g), divinyl benzene (1.05 g), and 133 mL of deionized water was charged into a flask, which was equipped with a mechanical stirring bar, a thermometer, and a condenser. The solution was purged with nitrogen to remove oxygen for 30 min and then heated to 70 °C. The initiator solution of potassium persulphate was charged into the flask and this reaction system continued heating for 8 h. The obtained microspheres were washed with deionized water by centrifugation for several times and then dispersed in DMF with a solid content of 1 wt % for further use. 2.3. Preparation of Co(II)/PAN Microspheres. A suitable amount of CoCl2 (according to the molar ratio of Co2+/CN) was added into the above dispersion of PAN-template microspheres. The mixture dispersion was kept at different temperatures with mechanical stirring for 24 h. Then excess NaNO2 was added into the solution and stirring continued for another 24 h. The products were precipitated by deionized water and filtered, then washed several times and dried off. 2.4. Pyrolysis of the Co(II)/PAN Microspheres. Pyrolysis of the Co(II)/PAN microspheres was performed in a tube furnace under nitrogen atmosphere at 800 °C for 1 h, then Co/Carbon (Co/C) microspheres were obtained. Due to the fact that Co nanoparticles are encapsulated by carbon, the nanocomposites are very stable in air. 2.5. Characterization. Transmission electron microscopy (TEM) images were obtained using a Hitachi H-600 transmission electron microscope operating at 250 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope operating at 200 kV. Thermogravimetric analysis (TGA) measurements were carried out on a Perkin-Elmer Pyris-1 series thermal analysis system under a flowing nitrogen or air atmosphere at a scan rate of 10 °C/min from 100 to 800 °C. Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAXIIA diffractometer using Cu KR radiation (λ ) 0.154 nm) operated at 40 mA and 40 kV. The XPS spectra were recorded under vacuum (
