Showing papers in "NANO in 2004"

Supermolecular Self-Assembly of Graphene Sheets: Formation of Tube-in-Tube Nanostructures

Abstract: Graphitic impurity nanoparticles were reorganized outside and inside of carbon nanotubes to produce novel tube-in-tube nanostructures. The graphitic nanoparticles were disintegrated into small graphene sheets by an intercalation-exfoliation process with nitric acid, during which the graphene sheets were simultaneously modified with carboxyl and hydroxyl groups at their edges. The modified graphene sheets were self-organized outside and inside of pristine carbon nanotubes in an acid-catalyzed esterification process, leading to an assembly of wellconstructed tube-in-tube nanostructures. Carbon nanotubes are basically constituted by sp 2 C-C covalent bonds as in graphite planes. Their syntheses have been highly successful following various routes, such as laser evaporation or arc-discharge of graphite, catalytic chemical vapor deposition, and decomposition of organic explosives. 1-4 These methods are based on a common key process: the assembly of small carbon species (Cn) generated at high temperatures. The studies on the structures of carbon nanotubes have shown that the practically obtained nanotubes are highly defective and have a local structure similar to that of turbostratic graphite. 5,6 The presence of discontinued defects in the tube structures means that an individual tube could be actually viewed as an assembly of small graphene sheets and that they could be directly synthesized from the graphene sheets under mild conditions if proper organization technology is available. Because of the anisotropic lamellar structure of graphite, single or thinly stacked graphene sheets can be easily obtained by an intercalation-exfoliation process against bulk graphite with inorganic acids such as nitric, suphuric, and perchloric acids. 7-10 This process has been developed industrially over 15 years to produce flexible graphite for the application of sealing gaskets. In the synthesized carbon nanotube samples, graphitic impurity nanoparticles are always present. They seriously hamper the accurate characterization of the bulk properties of nanotubes and affect their practical applications. To remove these impurities, various purification methods have been developed. 11-17 Although the graphitic nanoparticles intrinsically contain richer sub-stable nonhexagonal rings and thus are more reactive than carbon nanotubes, 11 the presence of defects in the tube structures renders the purification difficult. Furthermore, carbonaceous impurities are also frequently present in the inner voids of tubes. 5 These internal