On the mechanism of carbon nanotube formation by means of catalytic chemical vapour deposition

Abstract

Despite enormous strides in the synthesis of carbon nanotubes (CNTs), the mechanism for growth is still a highly debated issue. It is generally accepted that the model for carbon filament growth [1, 2], derived from concepts of vapour-liquid-solid theory, also applies to CNT growth. However, this model fails to account for the growth of CNTs from noble metal [3{7], ceramic [8, 9] and semiconducting nanoparticles [10{13], all of which are regarded as unable to catalyse the dissociation of hydrocarbons. In addition, in their bulk form, these materials do not have a catalytic function to produce graphite. This work examines non-traditional catalyst assisted chemical vapour deposition of CNTs with a view to determine the essential role of the catalyst in nanotube growth. CNT synthesis based upon noble metal and two approaches using germanium nanoparticles are presented. Extensive characterisation has been undertaken of each step of the growth process, and the synthesized carbon nanotubes are analysed by atomic force microscopy, electron microscopy and Raman spectroscopy. The results indicate that good densities of high quality single-walled carbon nanotubes are produced by these techniques. Additionally, the effects of different catalyst support interactions were explored by testing combinations of metal catalysts and support media. This study showed that the support has a strong effect on the chemical activity and morphology of the catalyst. The results presented show that the commonly utilised model of carbon filament growth is inadequate to describe CNT growth from non-traditional catalysts. A model for CNT growth consistent with the experimental results is proposed, in which the structural reorganisation of carbon to form CNTs is paramount

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