J. Phys. Chem. C 2007, 111, 6303-6307
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Solid-Phase Synthesis of Carbon-Encapsulated Magnetic Nanoparticles Cheng Fa Wang, Jian Nong Wang,* and Zhao Min Sheng School of Materials Science and Engineering, Shanghai Jiao Tong UniVersoty, 1954 Huashan Rooad, Shanghai 200030, P. R. China ReceiVed: January 28, 2007; In Final Form: February 24, 2007
Carbon-encapsulated magnetic nanoparticles (CEMNPs) have wide applications. However, synthesis of such particles with superparamagnetic properties is still a great challenge. The present study reports a new method for the production of CEMNPs in the solid state. An Fe/C solid solution was first prepared by pyrolysis of Fe(CO)5 and C2H2. Heat treatment of this material at low temperatures (e600 °C) generated fine nanoparticles (∼7 nm) with superparamagnetic properties, but treatment at high temperatures (>600 °C) produced larger nanoparticles (g30 nm) exhibiting permanent magnetic behavior. The advantages of the present solid-phase approach over previous gas-phase methods include good controllability of particle size and thus magnetic properties and suitability for large-scale production.
Introduction Carbon-encapsulated magnetic nanoparticles are of intense research interest because of their many important applications in areas such as magnetic data storage, xerography, and magnetic resonance imaging.1,2 The role of the carbon layer is to isolate the particles magnetically from each other, thus avoiding problems caused by interactions among closely compacted magnetic bits, and to provide oxidation resistance to the bare metal nanoparticles. Moreover, carbon encapsulation can endow these magnetic particles with biocompatibility and stability in many organic and inorganic media. Of special interest are those materials that exhibit a superparamagnetic behavior at room temperature. This is because carbon-encapsulated superparamagnetic nanoparticles (CESNs) are needed for applications in separation,3,4 in the magnetically guided transport of anticancer drugs,5 and so on. Superparamagnetic properties have been extensively studied using the nanoparticles of pure metals such as Fe, Co, and Ni particles.6 Because iron or iron-based alloy nanoparticles have an advantage of high saturation magnetization, various techniques have been adopted to obtain such particles with superparamagnetic properties. Most CESNs have been synthesized by the arc discharge method in which metal precursors are usually packed inside a cave drilled into a graphite electrode and then subjected to arc vaporization.7 Magnetic metal carbides can be encapsulated in carbon using this method. Dravid and co-workers7,8 modified the arc discharge method and successfully produced nanophase Ni encapsulated in graphitic shells. In this case, the product usually consisted of mixtures of different forms of carbon including carbon nanotubes, carbonencapsulated metal particles, and graphitic flakes. In addition, the metal particles had a wide size distribution. Recently, catalytically assisted chemical vapor deposition (CCVD)9 is becoming increasingly important because of its potential for scalable production. However, this potential cannot be practically realized until some obstacles have been overcome,10,11 such as the relatively low productivity, the existence of complex phases, and the difficulty in separating CESNs from impurities. * Corresponding author. Tel.: +86 21 62932015. Fax: +86 21 62932587. E-mail:
[email protected].
To overcome the inherent shortcomings of gas-phase synthesis approaches such as arc discharge and CVD, this study reports the solid-phase synthesis of CESNs with high controllability of chemical composition and particle size. This approach is based on the initial preparation of an Fe/C nanocomposite and subsequent heat treatment at low temperature to give rise to fine magnetic particles uniformly distributed in a carbon matrix. Experimental Methods The starting material of an Fe/C nanocomposite powder was synthesized by catalytic chemical vapor deposition. A gas mixture of C2H2 and volatile iron pentacarbonyl [Fe(CO)5] was introduced into a vertical quartz tube furnace set at 700 °C with N2 as the carrier gas. The diameter of the quartz tube was 3 cm, and the heating length was 70 cm. To add Fe(CO)5 reactant, N2 and C2H2 flowed through the liquid iron pentacarbonyl, which was held at 0 °C. The experimental flow fluxes were 60 L/h and 20 mL/min for N2 and C2H2, respectively. The reactants flowed into the quartz tube at the upper end, and black powder was collected in a glass bottle connected to the lower end of the quartz tube. Little powder deposited on the wall of the quartz tube. Heat-treatment experiments were conducted as follows: The obtained black powder was first placed in a quartz tube with both ends confined, and then the tube was evacuated and filled with argon to atmospheric pressure. The tube was subsequently heated to a given temperature (500, 600, 700, 800, and 1000 °C) in an electric furnace. After heat treatment for 1 h, the tube was cooled to room temperature. For the purpose of description, the starting material is denoted as SM, and the materials obtained after heat treatment are denoted as HT-x, where x represents the temperature used for heat treatment. X-ray diffraction (XRD) experiments were conducted on SM and HT-x. The X-ray diffractometer (Bruker D8 Advance, Bruker AXS, Germany) was operated at 40 kV and 40 mA. Nickel-filtered Cu KR radiation was used in the incident beam. A high-resolution electron transmission microscope (HRTEM) (JEOL 2010) with a cold field emission gun was used to study the microstructures and morphologies of the samples. The chemical compositions of the nanoparticles in the samples were
10.1021/jp0707283 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007
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Figure 2. Analysis of HT-600: (a) XRD pattern showing the presence of a ferrous phase and iron oxide. (b) TEM image exhibiting fine nanoparticles less than 10 nm in size embedded in a carbon matrix. (c) HRTEM image corresponding to b. (d) Enlarged image of a ferrous particle with an interlayer spacing of ∼0.2 nm. Similar XRD and TEM results were obtained for HT-500.
Figure 1. Analysis of starting material (SM): (a) HRTEM image of SM showing a uniform Fe/C composite and SAED image (inset) indicating poor crystallization. (b) Energy-dispersive X-ray spectrum from the Fe/C composite circled in a. The copper peak is due to the copper grid used to support the specimen. (c) XRD pattern of SM with no strong peaks.
analyzed by the equipped energy-dispersive X-ray spectrometer (EDX). For TEM studies, the samples were dispersed in ethanol by means of a sonicator and scooped up with a holy amorphous carbon film. The magnetic properties of the samples were recorded on a vibrating sample magnetometer (MLVSM9 MagLab 9 T, Oxford Instruments). Acrylic capsules (diameter, 6 mm; thickness, 8 mm) were used to pack the samples for VSM measurements. The magnetization measurements (M) were made at varying applied fields (H) from -20000 to +20000 Oe at room temperature, with 100 Oe spacings between -1000 and 1000 Oe. The saturation magnetization (MS), remanence magnetization (MR), and coercivity field (HC) values were obtained from the hysteresis loops registered up to a field of 5 T. A dc magnetic field of 0.8 MA m21 was applied to record hysteresis loops, and a field of 1.6 MA m21 was applied to determine saturation magnetization values. Results and Discussion Figure 1a shows a high-resolution TEM image and the corresponding selected area diffraction pattern for SM. It can be seen that ferrous nanoclusters are uniformly embedded in an amorphous carbon matrix, forming particles with an ironcarbon solid solution and a size of about 60 nm. As the inset in Figure 1a shows, the broad and diffuse Debye rings in the selected-area electron diffraction image of the sample demon-
strate that the SM is amorphous. The elemental compositions of the iron-carbon solid solution were analyzed by EDX and are shown in Figure 1b. The peaks around 8.00 and 9.00 keV correspond to Cu KR and Cu Kβ, which result from the copper of the copper grid used to support the nanoparticles. The EDX results show that the solid solution consisted of iron and carbon. The XRD pattern confirms the HRTEM observations (Figure 1c), i.e., an amorphous mixture of iron and carbon, as no strong peaks are present. Such an iron-carbon solid solution can be heat-treated at different temperatures. Heat treatment can improve the crystallization and change the particle size and magnetic properties.12 Thus, ferrous particles can be prepared to have different sizes and magnetic properties in the solid phase. After heat treatment at 600 °C (HT-600), the crystallization of the nanoparticles is apparent by XRD (Figure 2a). The main peaks appearing at 2θ ) 42.98°, 44.76°, and 49.14° can be ascribed to the diffraction of the (121), (103), and (122) planes, respectively, of Fe3C (JCPDS No. 85-0871), suggesting that the present particles are mainly Fe3C. According to JCPDS No. 19629 for magnitite (Fe3O4) and No. 39-1346 for maghemite (γFe2O3), the two peaks at 2θ ) 35.5° and 62.7° can be attributed to the (311) and (440) planes, respectively, of the oxides. It is difficult to differentiate between Fe3O4 and γ-Fe2O3 only on the basis of XRD. Further identifications can be done using Raman13 and Mossbauer14 spectroscopies. The presence of iron oxide might be due to air oxidation during sample handling. The mean particle sizes of iron carbide and iron oxide calculated by Scherrer equation are 8.1 and 4.3 nm, respectively. Figure 2b shows a typical TEM image of HT-600. It can be seen that quasi-spherical nanoparticles are uniformly dispersed in the carbon matrix and have diameters of less than 10 nm. This is consistent with the mean particle size of 8.1 nm estimated from XRD. The HRTEM image of HT-600 reveals that crystalline nanoparticles smaller than 10 nm are separated by the amorphous carbon matrix (Figure 2c). As the arrow indicates, incomplete graphitic layers formed during the heat treatment. For the particle shown in Figure 2d, the interlayer spacing is about 0.20 nm, which is in accordance with the value of d103 for Fe3C.
Synthesis of Carbon-Encapsulated Magnetic Nanoparticles
Figure 3. Analysis of sample HT-800: (a) XRD pattern with sharp diffraction peaks of Fe3C (overlapping with the R-Fe peak) and the graphite (002) peak. (b) TEM image showing spheroidal nanoparticles of around 20-30 nm. (c) HRTEM image revealing a typical nanoparticle wrapped by graphitic shells. Similar XRD and TEM results were obtained for HT-700.
When the heat-treatment temperature was increased to 800 °C, sharp peaks of Fe3C were observed in the XRD pattern (Figure 3a). This can be regarded as a standard Fe3C diffraction pattern without considering the graphite and R-Fe peaks. The peaks at 44.9°, 64.9°, and 82.3° can be identified as the (110), (200), and (211) planes, respectively, of R-Fe (JCPDS No. 6-0696). The diffraction peak at about 26.2° can be assigned to the (002) planes of a hexagonal graphite structure with an interlayer spacing of 0.34 nm. The fact that the peak is broader and lower than that of well-crystallized graphite suggests a relatively low degree of crystallization. The amorphous carbon matrix in the sample contributes to the broad diffraction peak compared to the graphite peak. The mean particle size of iron carbide can be calculated to be around 30 nm by the Scherrer equation. This is in good accordance with the TEM observations (Figure 3b) of ferrous nanoparticles uniformly dispersed in a carbon matrix with diameters ranging from 20 to 40 nm. There was an increase not only in grain size but also in crystallization when the heat-treatment temperature was elevated to 800 °C. HRTEM analysis revealed that all of the spheroidal particles had a core-shell structure with a ferrous core and carbon shells. The carbon shells tightly encapsulated the core nanoparticles (Figure 3c), and no obvious voids could be observed between the cores and the shells. The shells were uniform in thickness and usually consisted of 6-20 layers. The spacing of the lattice fringes was about 0.33 nm (Figure 3c), which is close to that of graphite (002) planes. The phase encapsulated inside the graphite shell is likely to be immune to environmental attack or degradation owing to the presence of the protective carbon shell.15 The dependence of nanoparticle size on heat-treatment temperature is shown schematically in Figure 4. First, the starting material consisted of an Fe/C solid solution. After heat treatment at 600 °C or lower, iron clusters grew to form ferrous nanoparticles of around 8 nm embedded in a carbon matrix. When the heat-treatment temperature was higher than 600 °C, ferrous nanoparticles larger than 30 nm were encapsulated by
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Figure 4. Schematic illustration of the present solid-phase approach for the synthesis of ferrous nanoparticles with different sizes and structures. (a) Starting material: Fe/C solid solution. (b) Ultrafine particles (30 nm) encapsulated by graphitic shells by heat treatment at high temperature.
carbon shells. Therefore, starting from an Fe/C solid solution, the nanoparticle size and structure can be controlled. The VSM magnetic properties of SM, HT-500, HT-600, HT800, and HT-1000 measured at room temperature are reported in Table 1 and Figure 5. The results show a ferromagnetic response for SM, HT-800, and HT-1000 but a superparamagnetic response for HT-500 and HT-600 as suggested by the data on HC, MR, and S (MR/MS). Particularly, the M-H curve for HT-600 shows a rapid increase of magnetization with increasing applied magnetic field without saturation and exhibits the highest initial susceptibility χ among the three samples, because of the superparamagnetic relaxation in the smaller particle assemblies.16 Superparamagnetic behavior was also obtained for HT500, which was heat-treated at a lower temperature of 500 °C. Nevertheless, the M-H curve (not shown in Figure 5) has distorted and shifted from the origin around M ) 0, indicating that more energy would be consumed per circulation.17 It is well-known that magnetic behavior is size-dependent and should be understood in conjunction with thermal energy and surface anisotropy aspects.18 Coercivity is a direct measure of the strength of the anisotropy. However, as the particle size decreases, the magnetocrystalline anisotropy barrier is reduced, and therefore, the coercive field decreases with decreasing particle size. In particular, single-domain particles below a certain critical diameter are greatly affected by temperature fluctuations and usually exhibit superparamagnetic properties. The single magnetic domain size in a magnetic material is proportional to the factor, A1/2/MS, where A and Ms are the exchange energy constant and the theoretical maximum magnetization, respectively.19 The exchange energy of metallic pure iron is quite small, leading to an ultrafine single domain of less than 10 nm.20 Preparation of superparamagnetic iron particles is, therefore, very difficult. Carbide nanoparticles with an average size of about 8 nm in the HT-600 sample can be considered to have a single magnetic domain. Moreover, ultrafine iron carbide magnetocrystallines were isolated by nonmagnetic amorphous carbon and weakly crystallized graphite layers. Therefore, the interaction between carbide particles was greatly suppressed. The single-domain particle-size effect
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Wang et al. larger than the single-domain size (∼10 nm), and superparamagnetic behavior was not observed. The Fe or Fe-rich particles in SM (Figure 1) are presumably smaller than those in HT-600 (Figure 2). However, SM did not show a superparamagnetic behavior (Table 1). This observation suggests that the mechanisms of superparamagnetism for amorphous nanoparticles in SM and crystallized nanoparticles in HT-600 might be different. The imposition of heat treatment must have changed the phase constitution and structure. Therefore, the increase in particle size and the change in structure of the Fe/C composite during heat treatment, in particular, the transition of the particles from the amorphous to the crystalline state, would have modified the coercivity, as this parameter is structurally sensitive.21 The present study demonstrates a solid-phase approach to the synthesis of carbon-encapsulated magnetic nanoparticles. The unique feature of the present approach is that such particles can be prepared to have different sizes and thus different magnetic properties just by annealing an Fe-C solid solution at different temperatures. Solid-phase approaches were also used in some previous studies. Examples are those based on high-temperature annealing of materials such as Fe2O3 plus C powders,22 elementary Fe plus C powders,23 and Co nanoparticles plus copolymers.24 However, the size and thus the magnetic properties of the final particles could hardly be controlled, and superparamagnetic particles could not be obtained as the starting particle size was usually much larger than 10 nm. The additional feature of the present approach is that the starting Fe-C solid solution can be easily prepared and annealed at different temperatures using ordinary furnaces. The whole procedure is relatively simple, as it does not involve complicated preparations or special conditions. A number of experiments have shown that the present results, in terms of both the morphologies and magnetic properties of the nanoparticles, can be well reproduced within experimental error. Thus, the present approach is likely scalable for mass production. Conclusions
Figure 5. M versus H isotherms at 300 K: (a) SM indicating ferromagnetic response, (b) HT-600 showing a superparamagnetic response, and (c) HT-800 with an obvious ferromagnetic response.
together with the suppressed interaction effect might have caused the samples that were heat-treated at low temperature (HT-500 and HT-600) to exhibit superparamagnetic behavior. When heat treatment was conducted at temperatures higher than 600 °C (700-1000 °C), the particle sizes in these samples were much
The present study shows that magnetic nanoparticles can be synthesized by a solid-phase approach. To this end, a composite material consisting of an Fe/C solid solution is prepared first by concurrent pyrolysis of Fe(CO)5 and C2H2. By heat treatment of this Fe/C solid solution at different temperatures, the size of the ferrous particles and thus the magnetic properties can be well controlled. After heat treatment at a low temperature such as 600 °C, the majority of the nanoparticles have a size of about 8 nm and are embedded in an amorphous carbon matrix. The material having such fine ferrous nanoparticles shows superparamagnetic behavior at room temperature. However, after heat treatment at a high temperature such as 800 °C, most of the nanoparticles have a size of about 30 nm and are encapsulated in graphitic carbon shells. The material having such large ferrous nanoparticles shows permanent magnetic behavior at room
TABLE 1: Magnetic Properties of the Present Samples mean particle size (nm) sample
XRD
TEM
MS (emu g-1)
MR (emu g-1)
MR/MS
HC (Oe)
HT-500 HT-600 HT-700 HT-800 HT-1000 SM
8.1 30.5 -
5(3 7(3 20 ( 5 29 ( 10 >100 60
30.03 37.72 60.18 22.43 67.21 22.52
1.02 0.95 9.68 5.49 5.22 1.19
0.034 0.025 0.161 0.245 0.078 0.053
0 51 406 465 302 176
Synthesis of Carbon-Encapsulated Magnetic Nanoparticles temperature. The advantages of the present solid-phase approach include good controllability of particle size to be less than 10 nm and thus good controllability of superparamagnetism, generation of few impurities, and suitability for large-scale production. Acknowledgment. J.N.W. acknowledges the Outstanding Youth Fund from the National Natural Foundation of China. References and Notes (1) Scott, J. H.; Majetich, S. A. Phys. ReV. B 1995, 52, 12564. (2) Seraphin, S.; Zhou, D.; Jiao, J. J. Appl. Phys. 1996, 80, 2097. (3) Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. (4) Safarik, I.; Safarikova, M. Biomagn. Reson. Technol. 2004, 2, 7. (5) Kuznetsov, A. A.; Filippov, V. I.; Kuznetsov, O. A.; Gerlivanov, V. G.; Dobrinsky, E. K.; Malashin, S. I. J. Magn. Magn. Mater. 1999, 194, 22. (6) Chen, K. Z.; Zhang, Z. K.; Cui, Z. L.; Zuo, D. H.; Yang, D. Z. Nanostruct. Mater. 1997, 8, 205. (7) Dravid, V. P.; Host, J. J.; Teng, M. H.; Eillott, B.; Hwang, J. H.; Johnson, D. L. Nature 1995, 374, 602. (8) Teng, M. H.; Host, J. J.; Hwang, J. H.; Elliott, B. R.; Weertman, J. R.; Mason, T. O. J. Mater. Res. 1995, 10, 233. (9) Flahaut, E.; Agnoli, F.; Sloan, J.; O’Connor, C.; Green, M. L. H. Chem. Mater. 2002, 14, 2553.
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6307 (10) Wang, Z. H.; Choi, C. J.; Kim, B. K.; Kim. J. C.; Zhang, Z. D. Carbon 2003, 41, 1751. (11) Liu, B. H.; Ding, J.; Zhong, Z. Y.; Dong, Z. L.; White, T.; Lin, J. Y. Chem. Phys. Lett. 2002, 358, 96. (12) Lee, Y. H.; Han, T. C. Jpn. J. Appl. Phys. 2004, 43, 7477. (13) Faria, D. L. A. D.; Silva, S. V.; Oliveira, M. T. D. J. Raman Spectrosc. 1997, 28, 873. (14) Cao, H. Q.; Zhu, M. F.; Li, Y. G. J. Solid State Chem. 2006, 179, 1208. (15) Thorsten, E.; Markus, W.; Branko, S.; Keir, F.; Claudia, F.; Horst, H. J. Appl. Phys. 2006, 99, 044306. (16) Zhang, L.; Ziolo, R. F.; Ying, J. Y. Nanostruct. Mater. 1997, 9, 185. (17) Berkowitz, A. E.; Takano, K. J. Magn. Magn. Mater. 1999, 200, 552. (18) Mørup, S.; Bødker, F.; Hendriksen, P. V.; Linderoth, S. Phys. ReV. B 1995, 52, 287. (19) Kodama, R. H. J. Magn. Magn. Mater. 1999, 200, 359. (20) Lee, D. W.; Yu, J. H.; Jang, T. S.; Kim, B. K. Mater. Lett. 2005, 59, 2124. (21) Yelsukov, E. P.; Ul’yanov, A. I.; Zagainov, A. V.; Arsent’yeva, N. B. J. Magn. Magn. Mater. 2003, 513, 258. (22) Tokoro, H.; Fujiia, S.; Oku, T. J. Mater. Chem. 2004, 14, 253. (23) Yelsukov, E. P.; Ul’yanov, A. I.; Zagainov, A. V.; Arsent’yeva, N. B. J. Magn. Magn. Mater. 2003, 258-259, 513. (24) Zalich, M. A.; Baranauskas, V. V.; Riffle, J. S.; Saunders, M.; Pierre, T. G. St. Chem. Mater. 2006, 18, 2648.
