Purification of single-walled carbon nanotubes synthesized by the hydrogen arc-discharge method
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A simple procedure for the purification of the single-walled carbon nanotube (SWNT) product synthesized by the hydrogen arc-discharge method was proposed and discussed. The procedure involves ultrasonication in alcohol, oxidation in fixed air, and soaking in hydrochloric acid. Most of the amorphous carbon and carbon nanoparticles as well as metal particles in the product was successfully removed, according to the results obtained from transmission electron microscopy, thermogravimetric analysis, and resonant laser Raman measurements. With this procedure, a 41 wt% yield of the SWNTs with a purity of about 96% was achieved after purification.
I. INTRODUCTION 1–3
Recently, several methods for large-scale synthesis of single-walled carbon nanotubes (SWNTs) have opened the door to study their physical and chemical properties, which were expected to be unique.4,5 However, the as-prepared SWNTs synthesized by any method usually contain a large amount of impurities, such as metal particles, amorphous carbon, and carbon nanoparticles. These impurities are closely entangled with SWNTs or stick on the outer surface of the SWNTs. The impurities present in the as-prepared products are deleterious to the precise measurement for the properties of SWNTs. Various oxidation methods6–13 have been explored for the purification of SWNTs. One simple purification method is the gas-phase oxidation of the SWNT soot at elevated temperatures.6 However, the SWNTs are so fragile during high-temperature oxidation that the yield is less than 1 wt%. Tohji10 et al. used an improved gas oxidation, in which they incorporated a hydrothermal treatment for the easy removal of the impurities of both metal particles and carbonaceous materials; the yield they obtained increased to some 20 wt%. In this paper, we report a simple method that appears to be quite suitable for the purification of SWNTs synthesized by the hydrogen arc-discharge method. The purification procedure includes ultrasonication, oxidation in fixed air, and acid washing. With this procedure, the impurities of metal particles, amorphous carbon, and
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 16 Sep 2014
carbon nanoparticles could be effectively removed; a 41 wt% yield of the SWNTs with a purity of about 96% was achieved.
II. EXPERIMENTAL A. Synthesis of SWNTs
The SWNTs were synthesized by the hydrogen arcdischarge method, which has been described elsewhere.14 The electric arc was typically operated in a direct current (dc) mode using 150 A under an atmosphere of 200 torr H2, with a distance of approximately 2 mm maintained between the two electrodes. Transition metals, including iron, nickel, and cobalt, were selected as co-catalysts. A mixture of 2.6 at.% Ni, 0.7 at.% Fe, 0.7 at.% Co, 0.75 at.% FeS, and graphite powders gave a yield of more than 1 g SWNTs within 3 min of synthesis. The morphology of the raw sample is usually in large slices of thin films and weblike s
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