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

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.