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

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 Coand Co2C-encapsulated CNCs can be produced by varying heating temperature.


Method
We synthesized Co-doped C 60 NWs using the LLIP method [25][26][27].C 60 powders were dissolved in toluene to prepare a C 60 -saturated solution with a solubility of 2.8 g/L.Co nitrate hexahydrate (Co(NO 3 ) 3 •6H 2 O) was dissolved in 2-propanol.The C 60 toluene solution was poured into a glass vial, and the 2-propanol dissolving Co(NO 3 ) 3 •6H 2 O 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.

Results and Discussion
Figure 1 shows a bright field image of an as-precipitated C 60 NW.The diameter of as-precipitated C 60 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 C 60 NWs are surrounded by plane surfaces [25][26][27].Miyazawa and Suga used C 60 derivatives in LLIP methods and showed that the crystal growth of C 60 -derivative NWs is inhibited [29].Therefore, the porous surfaces of the as-precipitated NWs in this study are attributed to the addition of Co(NO 3 ) 3 •6H 2 O.No Co clusters were observed in the as-precipitated NWs, indicating that the doped Co component is mixed with atomic states in the NWs.
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 a = 0.25 nm and c = 0.40 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 (011) 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.
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 Co 2 C crystals with an orthorhombic lattice with lattice constants of a = 0.47 nm, b = 0.44 nm, and c = 0.33 nm, as shown in   4(c) were 0.17 nm and 0.34 nm, respectively, corresponding to (121) of Co 2 C 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 Co 2 C 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.
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.
The synthesis process for encapsulated CNCs is explained as follows.First, Co atoms that were doped by the addition of Co(NO 3 ) 3 •6H 2 O in the NWs aggregated to form Co clusters by heating around 873 K. Subsequently, C 60 molecules in the NWs decomposed at temperatures higher than 873 K.This temperature is lower than that at which pure C 60 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 C 60 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 Co 2 C clusters were formed after cooling from 1273 K.

Conclusion
We successfully synthesized Co-doped C 60 NWs using the liquid-liquid interfacial precipitation method with the additive Co(NO 3 ) 3 •6H 2 O in 2-propanol.Electron microscopy directly showed that Co-and Co 2 C-encapsulated CNCs were produced by heating the NWs.It was found that the formation of Co-and Co 2 C-encapsulated CNCs can be controlled by varying the heating temperature.

Figure 4 (
Figure 4(b).The lattice spacing of the Co 2 C cluster and graphitic layers observed in Figure4(c) were 0.17 nm and 0.34 nm, respectively, corresponding to (121) of Co 2 C 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 Co 2 C 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.Figure5shows 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.The synthesis process for encapsulated CNCs is explained as follows.First, Co atoms that were doped by the addition of Co(NO 3 ) 3 •6H 2 O in the NWs aggregated to form Co clusters

Figure 3 :Figure 4 :
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 5 :
Figure 5: Variation in the diameter of the carbon nanocapsules and the encapsulated clusters and the thickness of graphitic layers in Coencapsulated 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 Co 2 C (the filled triangles in the middle frame) at 1273 K.