Decomposition of Hydrocarbons-Pyrolytic Methods

MultiwaUed carbon nanotubes may be prepared from organic precursor molecules. These serve exclusively as carbon source here. The decomposition takes place on a catalyst at high temperatures, and even some methods capable of doing without a catalyst have been developed. 

The conversion of acetylene on an iron catalyst on Si02-support is a typical example. In this process, acetylene is thermally decomposed by leading it over a bed of catalyst within a quartz tube heated at about 700 °C (500-1000 °C, generally). Apart from the desired MWNT, there are also larger, fibrous structures and layers of amorphous graphene observed. These tend to coat the catalyst particles. The bamboo-like nanotubes (Section 3.3.4) usually obtained from this method are often covered with amorphous carbon too and, in parts, they are considerably curved. In addition to these bent species, there is also a spiral or helical structure 

The length of the tubes can range up to many micrometers and depends on the reaction time. The diameter of the MWNT can be controlled as well by coating suitably sized catalyst particles onto the substrate. Generally, the samples obtained are soiled with other structures, mainly with graphite-coated catalyst particles and so-called carbon nanoparticles (refer to Chapter 4). 

The chemical dehalogenation of perfluorinated hydrocarbons Uke perfluorocy-clopentene, perfluoronaphthalene, or perfluorodecahn leads to carbon materials, too. At first, unstable carbyne phases are generated, which may be transformed then into nanotubes. The same is true for the decomposition of 1,3,5-hexatriyne that forms nanotubes as well. The yields, however, are rather low, with only 1-2% of the resulting material actually being MWNT. In addition, fullerenes and onionlike carbon particles are obtained. 

A method to prepare nanotubes in a fashion similar to carbon fiber production consists in the pyrolysis of carbon-rich polymers. For example, the thermal 

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