ISRN NanotechnologyVolume 2012 (2012), Article ID 871208, 4 pagesdoi:10.5402/2012/871208

Research Article

Synthesis of Cobalt-Encapsulated Carbon Nanocapsules Using Cobalt-Doped Fullerene Nanowhiskers

Daisuke Matsuura,¹ Kun'ichi Miyazawa,² and Tokushi Kizuka¹

¹Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki Tsukuba, 305-8753, Japan²Fullerene Engineering Group, Advanced Materials Processing Unit, National Institute for Materials Science, Namiki, Ibaraki Tsukuba, 305-0044, Japan

Received 24 December 2011; Accepted 11 January 2012

Academic Editors: C. S. Casari and Y. Song

Copyright © 2012 Daisuke Matsuura 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.

Abstract.

We synthesized cobalt- (Co-) doped C₆₀ nanowhiskers (NWs) by applying a liquid-liquid interfacial precipitation method using a C₆₀-saturated toluene solution and 2-propanol with Co nitrate hexahydrate (Co(NO₃)₃·6H₂O). Heating the NWs at 873–1173 K produced carbon nanocapsules (CNCs) that encapsulated Co clusters with a hexagonal-closed-packed structure. After heating at 1273 K, the encapsulated Co clusters in CNCs were transformed into orthorhombic Co₂C clusters. It was found that Co- and Co₂C-encapsulated CNCs can be produced by varying heating temperature.

1. Introduction

Carbon nanocapsules (CNCs), which are hollow multiwalled graphitic nanoparticles, exhibit high chemical and thermal stabilities [1–8]. The encapsulation of metallic and carbide clusters in CNCs leads to the formation of catalysts and drug delivery components [4, 9–17]. Such pristine and encapsulated CNCs have been synthesized by arc discharge, chemical vapor deposition, electron irradiation, and thermal decomposition [1, 3–5, 9, 11, 13–15]. Recently, a new synthesis method for CNCs using single-crystal fullerene nanowhiskers (NWs) has been developed by Asaka et al. and Kizuka et al. [18–22]. Fullerene NWs have been synthesized by liquid-liquid interfacial precipitation (LLIP) methods. Metal elements can be doped in fullerene NWs using C₆₀ derivatives and additives in solutions [23–31]. It is expected that the alloying of CNCs and metals can be performed using such metal-doped fullerene NWs. Some of magnetic materials, for example, cobalt (Co) and iron, form solid solutions with carbon. It is expected that various systems with crystal structures, solid solutions, and intermetallics are combined with CNCs. In this study, we demonstrate the synthesis of Co-doped C₆₀ NWs and CNCs encapsulating Co-based clusters.

2. Method

We synthesized Co-doped C₆₀ NWs using the LLIP method [25–27]. C₆₀ powders were dissolved in toluene to prepare a C₆₀-saturated solution with a solubility of 2.8 g/L. Co nitrate hexahydrate (Co(NO₃)₃·6H₂O) was dissolved in 2-propanol. The C₆₀ toluene solution was poured into a glass vial, and the 2-propanol dissolving Co(NO₃)₃·6H₂O was then added to form a liquid-liquid interface. After the vial was maintained at 278 K for one week, the solution was filtered to extract precipitates. The precipitates were dried and heated in high vacuum at 773–1273 K for 1 h. The rate of temperature increase was 10 K/min. The cooling of the specimens was performed by furnace cool. After cooling, the specimens were dispersed on microgrids and observed by transmission electron microscopy.

3. Results and Discussion

Figure 1 shows a bright field image of an as-precipitated C₆₀ NW. The diameter of as-precipitated C₆₀ NWs ranged from 190 to 940 nm with an average of 440 nm. Electron diffraction showed that the NWs had a tetragonal lattice. The surfaces of the Co-doped NWs are porous, as shown in Figure 1. This feature is similar to Ni-doped NWs [31], whereas pure C₆₀ NWs are surrounded by plane surfaces [25–27]. Miyazawa and Suga used C₆₀ derivatives in LLIP methods and showed that the crystal growth of C₆₀-derivative NWs is inhibited [29]. Therefore, the porous surfaces of the as-precipitated NWs in this study are attributed to the addition of Co(NO₃)₃6H₂O. No Co clusters were observed in the as-precipitated NWs, indicating that the doped Co component is mixed with atomic states in the NWs.

Figure 1: Bright field image of as-precipitated Co-dope C₆₀ nanowhisker. The diameter is 230 nm.

After heating the as-precipitated specimens at 773 K, the shape of the NWs was similar to that of the as-precipitated specimen. After heating at 873 K, the structure of a portion of the NWs was transformed from a tetragonal to an amorphous form. After heating at temperatures higher than 973 K, all NWs were transformed into amorphous forms, and CNCs were observed in the amorphous regions.

Figure 2(a) shows a high-resolution image of a CNC in the specimen heated at 1073 K. The outer graphitic layers of the CNC were three-dimensionally closed, and a cluster was encapsulated. As shown in Figure 2(b), selected-area electron diffraction showed that the encapsulated clusters were Co crystals with a hexagonal-closed-packed (hcp) structure having lattice constants of  nm and  nm. The diameter of the CNCs ranged from 14 to 76 nm with an average of 35 nm. The diameter of the encapsulated Co clusters ranged from 10 to 44 nm with an average of 20 nm. Figure 2(c) shows the enlarged interface between the encapsulated Co cluster and the graphitic layers in the Co-encapsulated CNC depicted in Figure 2(a). The Co cluster directly bonded with the graphitic layers. The lattice spacing of the Co cluster and the graphitc layers depicted in Figure 2(b) were 0.20 nm and 0.34 nm, respectively, corresponding to () of hcp-Co and (002) of graphitic layers, respectively. In the specimens heated at 873–1173 K, Co-encapsulated CNCs were observed. Figure 3 shows the thickness of the graphitic layers in Co-encapsulated CNCs produced by heating at 1073 K against the diameter of the Co clusters. The diameter of the Co clusters ranges from 10 nm to 45 nm. The maximum thickness of the graphitic layers is 20 nm. In Figure 3, although no clear relationship is observed, the graphitic layers seem to increase with the diameter of the Co clusters. If the graphitic layers were formed by deposition of decomposed carbon atoms on the surfaces of the Co clusters, the thickness of the graphitic layers becomes constant with the diameter of the Co clusters at a certain temperature. On the other hand, if the graphitic layers were formed by simple precipitation from the Co clusters, the thickness of the graphitic layers is proportional to the volume of the Co clusters, that is, the cube of the diameter. Thus, the distribution in Figure 3 suggests that the precipitation of carbon atoms from the Co clusters contributes to the formation of the graphitic layers.

(a)

(b)

(c)

Figure 2: (a) High-resolution image, (b) selected-area electron diffraction pattern of Co-encapsulated carbon nanocapsule in specimen heated at 1073 K. (c) high-resolution image of interface between encapsulated Co cluster and graphitic layers in Co-encapsulated carbon nanocapsule depicted in (a).

Figure 3: Variation in the thickness of graphitic layers with the diameter of Co clusters in Co-encapsulated carbon nanocapsules prepared by heating at 1073 K.

Figure 4(a) shows a high-resolution image of CNCs in the specimen heated at 1273 K. As was the case with the specimens heated at 1073 K, the graphitic layers of the CNCs were closed. Selected-area electron diffraction revealed that the encapsulated clusters were Co₂C crystals with an orthorhombic lattice with lattice constants of  nm,  nm, and  nm, as shown in Figure 4(b). The lattice spacing of the Co₂C cluster and graphitic layers observed in Figure 4(c) were 0.17 nm and 0.34 nm, respectively, corresponding to () of Co₂C and (002) of graphitic layers, respectively. In the specimens heated at 1273 K, the diameter of the CNCs ranged from 44 to 66 nm with an average of 54 nm. In addition, the diameter of the Co₂C clusters ranged from 29 to 34 nm with an average of 32 nm. These diameters are larger than those of the specimens heated at 1073 K.

(a)

(b)

(c)

Figure 4: (a) High-resolution image, (b) selected-area electron diffraction pattern of Co₂C-encapsulated carbon nanocapsule in the specimen heated at 1273 K, (c) High-resolution image of the interface between the encapsulated Co₂C cluster and the graphitic layers in Co₂C-encapsulated carbon nanocapsule depicted in (a).

Figure 5 shows the changes in the diameters of both CNCs and encapsulated clusters and in the thickness of the graphitic layers in encapsulated CNCs against the heating temperature. Both the diameters and thickness increase with heating temperature.

(a)

(b)

(c)

Figure 5: Variation in the diameter of the carbon nanocapsules and the encapsulated clusters and the thickness of graphitic layers in Co-encapsulated carbon nanocapsules with heating temperature. The encapsulated clusters are hexagonal-closed-packed Co (the open triangles in the middle frame) at 873–1173 K and orthorhombic Co₂C (the filled triangles in the middle frame) at 1273 K.

The synthesis process for encapsulated CNCs is explained as follows. First, Co atoms that were doped by the addition of Co(NO₃)₃·6H₂O in the NWs aggregated to form Co clusters by heating around 873 K. Subsequently, C₆₀ molecules in the NWs decomposed at temperatures higher than 873 K. This temperature is lower than that at which pure C₆₀ molecules decompose (1073–1173 K) [32]. Hence, in the present NWs, it is inferred that the Co clusters acted as catalysts that decrease the decomposition temperature of C₆₀ molecules. Such decomposed carbon atoms dissolved in the Co clusters during heating. Then, during cooling, carbon atoms precipitated on the surfaces of the Co clusters, followed by the formation of graphitic layers, that is, Co-encapsulated CNCs. At higher heating temperatures (1273 K), more C atoms dissolved in the clusters because of increase in the solubility limit. As a result, the Co₂C clusters were formed after cooling from 1273 K.

4. Conclusion

We successfully synthesized Co-doped C₆₀ NWs using the liquid-liquid interfacial precipitation method with the additive Co(NO₃)₃·6H₂O in 2-propanol. Electron microscopy directly showed that Co- and Co₂C-encapsulated CNCs were produced by heating the NWs. It was found that the formation of Co- and Co₂C-encapsulated CNCs can be controlled by varying the heating temperature.

Acknowledgment

This study was partly supported by Grants-in-Aid from the Ministry of Education, Culture, Sport, Science, and Technology, Japan (nos. 22310065 and 23651127).

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