Synthesis and Purification of Carbon Nanotubes

Abstract

Over the last one and a half decades, a great amount of research on the synthesis of carbon nanotubes has been carried out [1–5] since the first pioneering report of the discovery of multiwalled carbon nanotubes (MWCNTs) by Iijima [6]. Carbon nanotubes (CNTs) are generally produced by three main techniques: arc discharge, chemical vapor deposition (CVD), and laser ablation, although scientists are researching more economic ways to produce these structures. In arc discharge, a vapor is created by an arc discharge between two carbon electrodes with or without catalyst. Nanotubes self-assemble from the resulting carbon vapor. CVD synthesis is achieved by putting a carbon source in the gas phase and using an energy source, such as plasma or a resistively heated coil, to transfer energy to a reactive gaseous carbon molecule. This reactive carbon will deposit and grow on the catalyst (usually a first row transition metal such as Ni, Fe, or Co). The CVD method can result in MWCNTs or single-walled carbon nanotubes (SWCNTs). The CVD method is easy to scale up, which favors commercial production. For the laser ablation method, a mixture of carbon and transition metals such as Co and Ni is vaporized directly by a laser impinging on a metal–graphite composite target. In contrast to the arc discharge method, direct vaporization allows a far greater control over growth conditions, permitting continuous operation and preparation of nanotubes in higher quality. The laser ablation method tends to produce a small amount of clean nanotubes, whereas the arc discharge method generally produces larger quantities of nanotubes but also much more impurity.