Nanotube Growth Methods

Arc-discharge and laser ablation methods for the growth of nanotubes have been actively pursued in the past ten years. Both methods involve the condensation of carbon atoms generated from evaporation of solid carbon sources. The temperatures involved in these methods are close to the melting temperature of graphite, 3000-4000°C.

1.1 Arc-Discharge and Laser Ablation

In arc-discharge, carbon atoms are evaporated by plasma of helium gas ignited by high currents passed through opposing carbon anode and cathode

(Fig. 1a). Arc-discharge has been developed into an excellent method for producing both high quality multi-walled nanotubes and single-walled nano-tubes. MWNTs can be obtained by controlling the growth conditions such as the pressure of inert gas in the discharge chamber and the arcing current. In 1992, a breakthrough in MWNT growth by arc-discharge was first made by Ebbesen and Ajayan who achieved growth and purification of high quality MWNTs at the gram level [5]. The synthesized MWNTs have lengths on the order of ten microns and diameters in the range of 5-30 nm. The nanotubes are typically bound together by strong van der Waals interactions and form tight bundles. MWNTs produced by arc-discharge are very straight, indicative of their high crystallinity. For as grown materials, there are few defects such as pentagons or heptagons existing on the sidewalls of the nanotubes. The by-product of the arc-discharge growth process are multi-layered graphitic particles in polyhedron shapes. Purification of MWNTs can be achieved by heating the as grown material in an oxygen environment to oxidize away the graphitic particles [5]. The polyhedron graphitic particles exhibit higher oxidation rate than MWNTs; nevertheless, the oxidation purification process also removes an appreciable amount of nanotubes.

For the growth of single-walled tubes, a metal catalyst is needed in the arc-discharge system. The first success in producing substantial amounts of SWNTs by arc-discharge was achieved by Bethune and coworkers in 1993 [6]. They used a carbon anode containing a small percentage of cobalt catalyst in the discharge experiment, and found abundant SWNTs generated in the soot material. The growth of high quality SWNTs at the 1-10 g scale was achieved by Smalley and coworkers using a laser ablation (laser oven) method (Fig. 1b) [7]. The method utilized intense laser pulses to ablate a carbon target containing 0.5 atomic percent of nickel and cobalt. The target was placed in a tube-furnace heated to 1200°C. During laser ablation, a flow of inert gas was passed through the growth chamber to carry the grown nanotubes downstream to be collected on a cold finger. The produced SWNTs are mostly in the form of ropes consisting of tens of individual nanotubes close-packed into hexagonal crystals via van der Waals interactions (Fig. 2). The optimization of SWNT growth by arc-discharge was achieved by Journet and coworkers using a carbon anode containing 1.0 atomic percentage of yttrium and 4.2 at. % of nickel as catalyst [8].

In SWNT growth by arc-discharge and laser ablation, typical by-products include fullerenes, graphitic polyhedrons with enclosed metal particles, and amorphous carbon in the form of particles or overcoating on the sidewalls of nanotubes. A purification process for SWNT materials has been developed by Smalley and coworkers [9] and is now widely used by many researchers. The method involves refluxing the as-grown SWNTs in a nitric acid solution for an extended period of time, oxidizing away amorphous carbon species and removing some of the metal catalyst species.

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