Synthesis of Carbon Nanocapsules and Nanotubes Using Fe-Doped Fullerene Nanowhiskers

Hindawi Publishing Corporation Journal of Nanotechnology Volume 2012, Article ID 613746, 6 pages doi:10.1155/2012/613746 Research Article Synthesis of Carbon Nanocapsules and Nanotubes Using Fe-Doped Fullerene Nanowhiskers Tokushi Kizuka,1 Kun’ichi Miyazawa,2 and Daisuke Matsuura1 1 Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8753, Japan 2 Fullerene Engineering Group, Advanced Materials Processing Unit, National Institute for Materials Science, Namiki, Tsukuba, Ibaraki 305-0044, Japan Correspondence should be addressed to Tokushi Kizuka, Received 15 July 2011; Revised 14 October 2011; Accepted 16 October 2011 Academic Editor: Zheng Hu Copyright © 2012 Tokushi Kizuka et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We synthesized iron-(Fe-)doped C60 nanowhiskers (NWs) by applying the liquid-liquid interfacial precipitation method that employs a C60 -saturated toluene solution and a solution of 2-propanol containing ferric nitrate nonahydrate (Fe(NO3 )3 ·9H2 O). Fe particles of 3–7 nm in diameter were precipitated in the NWs. By heating at 1173 K, the NWs were transformed into hollow and Fe3 C-encapsulated carbon nanocapsules and carbon nanotubes. 1. Introduction 2. Method Fullerene nanocages, such as carbon nanocapsules (CNCs) and carbon nanotubes (CNTs), can be used to encapsulate functional nanomaterials, leading to potential applications in catalysis and drug delivery [1–13]. Encapsulation of functional nanomaterials has been performed by simultaneous evaporation of metals and diamond by arc discharge [4, 5, 8– 10]. Chemical vapor deposition, electron irradiation, and thermal decomposition have also been applied to produce CNCs and CNTs [1, 3, 12, 13]. An efficient synthesis method for hollow CNCs using single-crystal fullerene nanowhiskers (NWs) was found by Asaka et al. [14–18]. Fullerene NWs can be synthesized by a simple method, that is, the liquid-liquid interfacial precipitation (LLIP) method [19–23]. In addition, in the LLIP method, fullerene NWs can be doped with metallic particles using C60 derivatives and additives such as metal nitrate nonahydrates in solution [24–26]. Such metallic particles act as catalysts in the syntheses of CNCs and CNTs. In this study, we demonstrate the synthesis of iron-(Fe-) doped C60 NWs using the LLIP method and their application to produce CNCs and CNTs. C60 powders were dissolved in toluene to prepare a C60 saturated solution with a solubility of 2.8 g/L. In addition, ferric nitrate nonahydrate (Fe(NO3 )3 ·9H2 O) was dissolved in 2-propanol to give a concentration of 0.1 M. Next, the C60 toluene solution was transferred to a glass vial, and the solution of 2-propanol containing Fe(NO3 )3 ·9H2 O was added to form a liquid-liquid interface. The vial was maintained at 278 K for one week, and the mixed solution was then filtered to extract precipitates. The precipitates were dried and heated under high vacuum at 1173 K for 1 h. The as-precipitated and heat-treated specimens were dispersed on microgrids and observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 3. Results Figure 1 shows an SEM image of as-precipitated C60 NWs. Figures 2(a) and 2(b) show a bright-field image and a highresolution image of an as-precipitated C60 NW, respectively. The diameters of the as-precipitated NWs ranged from 0.5 to 7.5 μm, as shown in Figure 3. Lattice fringes with a spacing of 2 Journal of Nanotechnology 5 µm 500 nm Vacuum (a) Figure 1: Scanning electron microscopy secondary-electron image of as-precipitated Fe-doped C60 nanowhiskers. The nanowhiskers are supported on microgrid. 004 222 0.52 nm were observed in the NWs, as shown in Figure 2(b). Figure 2(c) shows a selected-area electron diffraction pattern of the NW depicted in Figure 2(a). The high-resolution images and diffraction patterns reveal that the NWs have a tetragonal lattice with lattice constants of a = 0.99 nm and c = 2.1 nm. The lattice fringes with a spacing of 0.52 nm depicted in Figure 2(b) correspond to the (004) plane. The long axis of the NW is parallel to the (110) direction. Figure 2(d) shows a high-resolution image of an NW surface, where Fe particles with diameters in the range 3–7 nm were observed. Thus, the LLIP method using a solution of Fe(NO3 )3 ·9H2 O in 2-propanol resulted in the precipitation of Fe particles in the C60 NWs. Owing to the precipitation of Fe particles, the crystal growth of the NW was inhibited; as a result, the surfaces of the NWs had a rough topography. Therefore, the Fe-doped NWs presented here differ from pure C60 NWs, which are surrounded by plane surfaces [19– 23]. Figure 4 shows a bright-field image of the heat-treated specimen. Hollow and encapsulating CNCs and CNTs were observed in the specimen, as were chains of CNCs. Figure 5 shows a bright-field image and a selected-area diffraction pattern of a CNC encapsulating a particle. The 220, 230, and 050 spots of Fe3 C (cementite) are observed; the particle was identified to be Fe3 C. Figures 6(a) and 6(b) show high-resolution images of an Fe3 C-encapsulated CNC. The diameters of the CNCs and the Fe3 C particles ranged 25–175 nm and 5–100 nm, respectively, as shown in Figure 7. The Fe3 C particle does not completely fill the empty space at the core of the CNC. Figure 6(c) shows a high-resolution image of graphene layers in an Fe3 C-encapsulated CNC. The spacing of the graphene layers around the surface is 0.34 nm, whereas the spacing decreases to 0.31 nm around the graphene/Fe3 C interface. Figure 8(a) shows a high-resolution image of a CNT encapsulating Fe3 C particles (Figures 8(b) and 8(c)), similar to the case of the CNCs. The Fe3 C particles encapsulated by the CNTs showed rod shapes, as shown in Figure 8(a). This is different from the spherical Fe3 C particles observed in CNCs. The diameters of the CNTs and the Fe3 C particles ranged 10– 70 nm and 5–50 nm, respectively, as shown in Figure 9. 110 222 0.52 nm (b) (c) Vacuum 0.2 nm Fe (d) Figure 2: (a) Bright-field image, (b) high-resolution image, and (c) selected-area electron diffraction pattern of as-precipitated Fedoped C60 nanowhisker. The diameter of the nanowhisker is 1.2 μm. (d) High-resolution image of Fe particles in the nanowhisker. The lattice fringes of (110)Fe with a spacing of 0.20 nm are observed. The formation of CNCs and CNTs was not confirmed when the heating temperature was changed to 873 K, 973 K, 1073 K, and 1123 K. When the heating time was shortened to 0.5 h at 1173 K, the size distribution of CNCs and CNTs was similar. 4. Discussion 4.1. Formation of Fe3 C Particles. In the as-precipitated NWs, Fe particles 3–7 nm in diameter were observed. On the other hand, after heating at 1173 K, the diameter of the Fe3 C par (...truncated)