Effects of Catalyst Promoters on the Growth of Single-Layer Carbon Nanotubes
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CHING-HWA KIANGab, WILLIAM A. GODDARD IIlb, ROBERT BEYERS', JESSE R. SALEM', AND DONALD S. BETHUNE' 'IBM Research Division, Almaden Research Center, San Jose, CA 95120 bMaterials and Molecular Simulation Center, Beckman Institute, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125
ABSTRACT The discovery of a catalytic route to the growth of single-layer carbon nanotubes suggests that it may be possible to produce these materials with better selectivity and in higher yield12 . Increasing the production efficiency is essential for characterization and application of these materials. We have discovered several catalyst promoters, in particular S, Bi, and Pb, that greatly enhance the single-layer carbon nanotube yield, and extend the distribution of nanotube diameters to much larger sizes (> 3 nm). Co crystallites encapsulated in graphitic polyhedra also form abundantly when S, Bi, or W is present. Understanding these catalytic process is of substantial scientific and technological importance. CARBON NANOTUBES The term "carbon nanotube" generally refers to carbon with a cylindrical graphene structure. Multilayer carbon nanotubes are generally produced on the cathode tip deposit in an electric arc3 . These nanotubes are typically composed of 2 to 50 layers of cylindrical shells, with tube diameters ranging from several to several tens of nm. Single-layer carbon nanotubes can be produced with or without a catalyst. Carbon can self-assemble into tubes without a catalyst by either hydrocarbon decomposition 4 or carbon vapor condensation 5 on a substrate surface. The yield is low and the nanotubes produced usually have more than one layer. On the other hand, catalytic synthesis of carbon nanotubes produces nanotubes exclusively with a single layer and in higher yields. Fe, Co, and Ni catalyze the growth of single-layer carbon nanotubes when co-vaporized with carbon in an arc"''2'. The nanotubes produced have a very high aspect ratio (length to diameter), and have diameters from subnanometer to several nanometers and lengths from hundreds of nm to pm. Metals such as Y and Gd have also been shown to catalyze nanotube growth 8'9. These tubes, however, are rooted on a metal carbide (or metal oxide) crystal, and have diameters ranging from 1 to 2 nm and lengths from 10 to 200 nm. This article focuses on the single-layer carbon nanotubes produced using Co catalyst, and the effects of certain catalyst promoters on the nanotube growth.
69 Mat. Res. Soc. Symp. Proc. Vol. 359 @1995 Materials Research Society
CATALYTIC SYNTHESIS OF SINGLE-LAYER CARBON NANOTUBES We produce nanotubes with an electric arc, using a dc current of 95 A with a supply voltage of 25 V, under 300 to 500 Ton" helium. The cathode is a 6 mm graphite rod, and the anode is a graphite rod cored at the center and filled with a mixture of C, Co, and additives such as S, Bi, Pb, and W. The metal concentration is 4 atomic % Co and an equal amount (in atomic %) of the additive is used. We sonicate the soot in ethanol
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