1. Chemical vapor deposition (CVD): The deposition of nanotubes of carbon employing this technique was first reported ]n 1959. This process takes place in vacuum or with process gases and in the presence of some catalyst particles such as, Ni, Co, Fe or combination of these catalysts. The diameters of the nanotubes that are to be grown are related to the size of the catalyst metal particles. To initiate the growth of nanotubes, two gases are fed into the reactor: a process gas such as ammonia, nitrogen or hydrogen and a carbon-containing gas such as acetylene, ethylene, ethanol or methane. The substrate is heated to 700oC. The carbon-containing gas is broken at the surface of the catalyst particles, and the carbon is transported to the edges of the particles to form the nanotubes. A problem associated with this synthesis is the removal of the catalyst support via an acid treatment, which sometimes can destroy the original structure of the carbon nanotubes. CVD is the most commonly used method for the commercial production of carbon nanotubes on an industrial scale. It has the capability of growing nanotubes directly on a desired substrate, whereas the nanotubes need to be collected in the other growth techniques.
2. Arc discharge method: Carbon nanotubes were first observed in 1991 in the carbon soot of graphite electrodes during an arc discharge when a current of 100 A was used for producing fullerenes. This method has since been the most widely used for nanotube synthesis. During this process, the carbon negative electrode sublimes at high discharge temperatures to yield both single-walled and multiwalled nanotubes with lengths of up to 50 pm in up to 30% yield by weight.
3. Laser ablation: In this technique, pulsed laser vaporizes graphite in a high-temperature reactor while an inert gas is fed into the reactor. Carbon nanotubes grow on the cooler surfaces of the reactor as the vaporised carbon condenses there. The laser ablation method produces mostly SWCNT in about 70% yield, with the operating temperature controlling the diameter of the tubes.
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