Purification and Characterization of Single-Walled Carbon Nanotubes
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much attention because of their unique electrical properties and their potential for a variety of application [2, 3]. For a wide range of proposed application, especially for the construction of electrical nanodevices [4,5], purity remains a crucial problem. The electric arc method is a wellknown way of production of SWNTs [6], but this as-grown materials contain a substantial fraction of nanoscale impurities such as bucky onions, spheroidal fullerenes, amorphous carbons, and metal catalysts. Also they are made up of the tubes having a range of lengths and diameters. The SWNTs are only about 60% in weight, so must be purified. Various gas-phase purification methods based on oxidation have been proposed for multiwalled carbon nanotubes (MWNTs), but SWNTs cannot be purified by such methods because they are so reactive to oxygen that they are consumed before the extraneous material [7]. So efforts have been directed at modifying other techniques originally developed for MWNTs [810]. In this paper, we describe a liquid-phase purification technique of SWNT using typical oxidants such as HNO 3, H2 80 4, and their mixture. Recent studies indicate that the tube tips have electronic properties that differ from the cylindrical parts of the nanotubes and they determine furthermore to a great extent the field emission properties. It is therefore of great importance to obtain purified tubes with intact ends and non-damaged side wall. The purifed SWNTs were analyzed by SEM,TEM and EDS. It was also observed by Raman scattering using Ar excitation ( X=514.5nm) whether the structural change of SWNT was generated after purification or not . 123 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
Moreover, Field Flow-Fractionation (FFF) method was employed to determine the size distribution of the purified SWNTs [11-12]. EXPERIMENT The SWNTs were obtained by the conventional graphite electric arc method in a helium atmosphere. As a catalyst, metal mixture of nickel and cobalt (1:1 weight %) was used, which leads to an additional contamination with metal clusters, partly incorpoated in carbon. One gram of raw material containing 60% (weight %) of SWNTs was stirred in 200mL concentrated nitric/sulfuric acid solution of various volume ratio for a few minutes. The SWNT-acid solution was refluxed under magnetic stirring at 100-120TC for a few hours. Dense yellow vapors of nitrogen dioxide evolved during the first a few hours, indicating a high rate of oxidation of hydrocarbons into alcohols, carboxylic acids, ketons, aldehydes and of metallic particles into their corresponding ions. Acid solutions were made by mixing concentrated nitric / sulfuric acid in the volume ratio of 1:1 and using only concentrated nitric acid. The reflux time is also varied from I h to 6h. The suspension was then filtered on poly tetra-fluoro ethylene (PTFE) filter paper. The filtered wet powder was washed with distilled water several times and dried at room temperature. For characterization of the purified SWNTs, SEM , TEM, Raman scattering,
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