Growth, electronics, processing and toxicology of carbon nanotubes

Abstract

The growth of carbon nanotubes by chemical vapour deposition has been investigated using sodium chloride for facile post-CVD removal of the catalyst support. Saturated methanol vapour and acetylene gas have been used to grow tubes from bimetallic Co/Mo salts and cobalt-based nanoparticles at temperatures below 900 °C. Multi-walled nanotubes and carbon fibres have been produced. Absence of single-walled tubes in the carbon product is shown to be characteristic of catalysts prepared using sodium chloride and other inorganicsalt supports, and the factors contributing to this outcome are explored. It is shown that sodium chloride actively inhibits carbon deposition and the formation of single-walled tubes in specific catalytic systems. The effect of other inorganic metal salts on tube growth has been investigated. The mechanism for inhibition is discussed in terms of both the surface and bulk diffusion mechanisms of nanotube growth.Samples consisting of types and forms of nanotubes typically found in research labs were prepared and comprehensive physico-chemical characterisation of the samples was carried out. Co-workers at the Queen's Medical Institute, Edinburgh, have been performing in vivo and in vitro studies on these samples. In conjunction with the physico-chemical data, their results should provide better understanding of the pulmonary toxicology of carbon nanotubes. L(c)-phosphatidylcholine, a model for pulmonary surfactant, has been shown to disperse carbon nanotubes to a degree commensurate with commonly employed nanotube surfactants (e.g., Triton X-lOO and SDS). The results obtained using this model surfactant have been used in understanding the potential interaction between nanotubes and the lung environment, and the relevance to issues surrounding the toxicology of carbon nanotubes is discussed.Novel techniques for purifying and processing carbon nanotube samples have been investigated. Dispersion of nanotubes in surfactants prior to microwave heating has shown substantial increase in the purity of the tubes compared with standard non-dispersive microwave heating. Thorough investigation has unearthed no evidence for the selective destruction of specific types of nanotubes; therefore, microwave-based purification is deemed a useful method to remove residual metal originating from the catalyst. A timedependant study on the oxidative shortening of carbon nanotubes has been carried out and compared with a basic theoretical model for nanotube cutting. The solubility of carbon nanotubes in novel aqueous monomeric and polymeric surfactants has also been demonstrated: improving the toolkit for dispersing and processing nanotubes.In collaboration with co-workers at the Scottish Microelectronics Centre, single-walled carbon nanotubes have been exposed to CF₄ and SF₅ plasmas to control their degree of functionalisation before substitution with 1 ,2-diaminoethane. The degree of amino functionalisation has been shown to depend on the degree of initial fluorination rather than oxygen or carbon defects and thereby presents a replicable route to n-type doping. The different types of nanotube-fluorine bonding produced by the plasma processes (e.g., covalent, semi-ionic) have been investigated as well as the effect of different plasmas on the doping process. Electrical characterisation has shown p-type semiconducting behaviour for CF₄ functionalised tubes and n-type semiconducting behaviour for amino functionalised tubes. The degree of n-type behaviour increases with the amount of nitrogen attached.