Abstract
Metal-encapsulating onion-like fullerenes (M@OLFs) were synthesized by CVD at relatively low temperature (420) using Fe and Co nanoparticles impregnated into NaCl as catalyst. The morphology and structure of the products were characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The results show that Fe@OLFs and Co@OLFs with stacked graphitic fragments were prepared using Fe/NaCl or Co/NaCl as catalysts; after Co@OLFs were immersed in concentrated HCl for 48 hours, Co nanoparticles encapsulated by carbon shells were not removed, meaning that carbon shells can protect the encapsulated Co cores against environmental degradation. The coercivity value (750.23 Oe) of Co@OLFs showed an obvious magnetic property.
1. Introduction
Since the report by Ugarte in 1992 [1], onion-like fullerenes (OLFs) have been expected to have good prospects in some aspects of energy materials, high-performance/high temperature wear-resistance materials, superconductive materials, and biomaterials [2]. Metal-encapsulating OLFs (M@OLFs) have potential application in many fields such as magnetic data storage, xerography, and magnetic resonance imaging [3]. At present, M@OLFs have been prepared by various techniques, such as electron irradiation [4], arc discharge [5], plasma [6], explosive decomposition of organometal [7], and chemical vapor deposition (CVD) [8, 9]. Among these methods, CVD appears promising because of its relatively low cost and potentially high yield. The catalyst, which can be categorized into floating catalyst and supported catalyst, plays essential role in CVD. Compared with the floating precursors, the supported catalyst plays the role of dispersing active components and adjusting catalyst properties via the chemical or physical interaction of support with metal nanoparticles. Various kinds of catalyst supports such as Al2O3, MgO, CaCO3, and zeolite were used to synthesize nanocarbon materials [10–13]. However, it is difficult to separate these supports from the final products. On the other hand, water-soluble materials as catalyst supports can be easily separated from the product. Soluble silicate, carbonate, and chloride were employed as catalyst supports to synthesize carbon nanofibers or carbon nanotubes (CNTs) [14–16]. However, little attention was paid to the synthesis of M@OLFs using water-soluble materials as catalyst supports. Liu et al. [17] employed cobalt supported on NaCl prepared by mechanical milling as catalyst to synthesize carbon-encapsulated cobalt nanoparticles, but the method to fabricate catalyst was somewhat complicated.
In the present study, M@OLFs were synthesized by CVD at relatively low temperature (C) using Fe and Co nanoparticles impregnated into NaCl support as catalysts, and the magnetic property of Co@OLFs was also studied. This work is of interest for the low cost production of OLFs.
2. Experimental
2.1. Synthesis of Fe/NaCl and Co/NaCl Catalysts
Fe/NaCl and Co/NaCl with about 2 wt% Fe or Co were prepared by means of impregnation. First, 1.76 g of Fe(NO3)3·9H2O or 1.49 g of Co(NO3)2·6H2O was dissolved in an appropriate amount of distilled water. Then 14.85 g of NaCl was added into Fe(NO3)3 or Co(NO3)2 solution. To get homogeneous mixture, the mixture was stirred for 1 h and then dried at C. The obtained catalysts were ground into fine particles.
2.2. Synthesis of M@OLFs
The catalytic decomposition of C2H2 was carried out at C in a horizontal furnace, using Fe/NaCl and Co/NaCl as catalysts separately. An 18 mm-long quartz boat with about 2.50 g of catalyst was placed at the isothermal zone in a horizontal quartz tube reactor. Initially, the tube was heated up to C in 100 ml·min−1 of steady Ar flow. The catalysts were reduced at C in a hydrogen atmosphere for 1 h. Then synthesis reactions were carried out at C by introducing a mixture of C2H2-Ar (C2H2: 30 ml·min−1, Ar: 300 ml·min−1) into the reactor. After 1 h, the reactor was cooled to room temperature in Ar atmosphere (80 ml·min−1) and the black powders were collected.
2.3. Purification of M@OLFs
To separate NaCl, the products were dissolved in an appropriate amount of distilled water and filtered, which are denoted as H2O-washed samples. In addition, to remove the residual metal catalyst, the as-synthesized samples were immersed in concentrated HCl solution at room temperature for 48 h, and then washed and filtrated with distilled water, which are denoted as HCl-washed OLFs.
2.4. Characterization of M@OLFs
The morphology and structure of the samples were characterized using field emission scanning electron microscope (FESEM, JSM-6700F, operated at 10 kV), high resolution transmission electron microscope (HRTEM, JEM-2010, working at an accelerating voltage of 200 kV), X-ray powder diffractometer (XRD, D/Max-3C, Cu-Kα radiation, Å), and thermogravimetric analyzer (TGA, Netzsch TG 209 F3, between room temperature and in air atmosphere at heating rate of /min). The magnetic property measurement was conducted with a vibrating sample magnetometer (VSM, Lake Shore 7307).
3. Results and Discussion
FESEM was used to investigate the morphologies of the products (Figure 1). It can be obviously observed that there were large quantities of nanoparticles in the products without accompanying CNTs or nanofibers (Figure 1(a)). To analyze the elemental composition for the products, the EDS spectra were shown in the insert. From the insert in Figure 1(a), C, Fe, and O signals were observed. And from the insert in Figure 1(b), C, Co, and O signals were clearly observed. No Na and Cl signals were exhibited, indicating the complete removal of NaCl.
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The products were further characterized by HRTEM to study the particle size and structure. Figure 2(a) shows the TEM image of H2O-washed OLFs synthesized using Fe/NaCl as catalyst. Most metal nanoparticles were wrapped by carbon layers, and the sizes were in the range of 10–50 nm. The diversity in the shapes of the carbon nanocages encapsulating metallic particle reflects the shapes of the encapsulated metallic particles. Figure 2(b) shows the TEM image of H2O-washed OLFs synthesized using Co/NaCl as catalyst. A mass of metal-encapsulating carbon nanoparticles can be seen ranging in diameter from 10 to 60 nm besides little hollow carbon indicated by an arrow. Figures 2(c) and 2(d) show the TEM images of HCl-washed OLFs synthesized using Fe/NaCl and Co/NaCl as catalysts, respectively. The metal nanoparticles encapsulated by carbon shells were not removed after immersing in concentrated HCl for 48 h. It means that carbon shells can protect the encapsulated metal cores against environmental degradation. The HRTEM images of HCl-washed OLFs synthesized using Fe/NaCl and Co/NaCl as catalysts (Figures 2(e) and 2(f)) indicate that the metallic cores were encapsulated by graphitic sheets. This implied that both Fe/NaCl and Co/NaCl can be used as catalysts to synthesize M@OLFs by CVD using acetylene as carbon resource at , and carbon shells can protect the encapsulated metallic cores.
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Based on our experimental results and previous investigations, a vapor solid (VS) growth model of M@OLFs at low temperature was suggested [18]. Firstly, C2H2 was absorbed onto metal nanoparticle surface and decomposed into carbon atoms; secondly, assembled carbon atom clusters began to diffuse in the crystal lattice of metal particles until carbon species got supersaturated; thirdly, carbon species precipitated and nucleated on catalyst nanoparticle surface, resulting in the formation of small graphitic fragments with a lot of defects. The small graphitic fragments combined with each other by their dangling bonds in order to reach a more stable state, and at the same time, the defects on the surface of the graphitic fragments might act as nucleation sites for the deposition of decomposed carbon species followed by the nucleation of pentagonal or hexagonal rings. Thus OLFs grew in isotropic way continuously until no carbon source was supplied. Because the reaction temperature was too low to supply enough energy to induce the rearrangement of carbon atoms in the graphitic fragments, the formed OLFs had a structure of stacked graphitic fragments.
The products were further characterized by XRD (Figure 3). Figure 3(a) shows the XRD patterns of Fe@OLFs synthesized using Fe/NaCl. The peak attributed to the diffraction of carbon at 2θ= indicates that OLFs had a structure of stacked graphitic fragments, which was between amorphous carbon and concentric graphitic layers. The peaks at 2θ= and can be ascribed to the diffraction of Fe3C, indicating that the metallic cores inside the OLFs were Fe3C. And the peak attributed to the diffraction of Fe2O3 at 2θ=was also observed. After immersing the products in concentrated HCl for 48 h, the peak attributed to the diffraction of Fe2O3 disappeared. It means that some catalyst nanoparticles were not encapsulated completely and converted to Fe2O3 when exposed to air. Figure 3(b) shows the XRD patterns of H2O-washed OLFs (A) and HCl-washed OLFs (B) synthesized using Co/NaCl as catalyst. A broad diffraction peak at about was assigned to the (002) planes of hexagonal graphite structure. The peak at 2θ= was identified to the (111) planes of Co with a face-centered cubic (fcc) structure, indicating that the metallic cores inside the OLFs were Co. After immersing the products in concentrated HCl for 48 h, the peaks attributed to the diffraction of Co became weaker, indicating the removal of bare Co nanoparticles.
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Co@OLFs synthesized using Co/NaCl as catalysts were further investigated by TG. The content of Co in H2O-washed and HCl-washed OLFs was calculated as 25.6 wt% and 14.2 wt%, respectively, in accordance with the XRD measurement.
The magnetic property of the HCl-washed Co@OLFs at room temperature is shown by the magnetization hysteresis loop (Figure 5). For magnetic nanoparticles, the magnetic properties, especially the saturation magnetization Ms and coercive force Hc, are dependent upon chemical composition and particle size [19]. The curve is symmetric around , with a saturation magnetization (Ms) of 17.70 emu/g and a coercivity (Hc) of 750.23 Oe. Here, the Ms of Co@OLFs is much lower than that of bulk Co (Ms = 162.5 emu/g) [19]. The decrease of the saturation magnetization may be attributed to the inclusive phases of carbon (e.g., carbon’s diamagnetic contribution), and the surface coating effects [19], in view of the fact that Co nanoparticles were entirely encapsulated by carbon. These effects are expected to become more prominent for smaller particles owing to their larger surface-to-volume ratio. Moreover, the data for the ratio of remanence to saturation magnetization () indicates the good ferromagnetism of Co@OLFs at room temperature.
The magnetic property of the Co@OLFs can also be tested qualitatively, as shown in Figure 6. The Co@OLFs were dispersed homogeneously in ethanol solution in a colorimetric tube by ultrasonic vibration (Figure 6(a)). After a magnet was placed on the outer wall of colorimetric tube for 10 min, the black products were aggregated on the inner wall of tube (Figure 6(b)), suggesting the ferromagnetic property of Co@OLFs, which may be of potential application in electronic devices, high-density magnetic memories, sensors, and electrochemical energy storage.
4. Conclusions
Fe/NaCl or Co/NaCl were used as catalysts to fabricate Fe@OLFs with diameters of 10–50 nm or Co@OLFs with diameters of 10–60 nm by CVD using acetylene as carbon resource at . NaCl was easily separable from the product just by a washing process. The metallic cores inside the OLFs were Fe3C when Fe/NaCl was used as catalyst and Co nanoparticles when Co/NaCl was used as catalyst. After immersing the as-synthesized products in concentrated HCl for 48 h, bare metal nanoparticles were removed while the metal nanoparticles encapsulated by carbon shells were unaffected. It means that carbon shells can protect the encapsulated metallic cores against environmental degradation. The coercivity value (750.23 Oe) of Co@OLFs showed an obvious magnetic property.
Acknowledgments
The authors acknowledge financial support from Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), National Natural Science Foundation of China (20971094), International S&T Co-operation Program (2007DFA50940), International S&T Cooperation Program of Shanxi Province (2009081018, 2010081017), Natural Science Foundation of Shanxi Province (2009011012-4), and Shanxi Research Fund for Returned Scholars (2008-31).