The Synthesis of Carbon Nanotubes

Abstract

The synthesis and study of carbon nanotubes is a relatively young field of research but one that is stimulating great interest, with groups around the world working on increasing the understanding of the physical and chemical nature of these materials. The synthesis of carbon nanotubes in a controllable and tunable manner is of great importance to both research and the development of applications hut it has not as yet been achieved, particularly with regards to the synthesis of particular types of single-walled carbon nanotubes. The aim of the work reported here was to study the synthesis of carbon nanotubes using metal-containing clusters of a known size as catalysts. It was hoped that variation of the size of the cluster would result in a correlated change in the diameter of the nanotubes produced. Two main synthetic techniques were used here, and two variations or each technique were studied. The first of these involved the introduction of die catalyst to a reaction furnace in the form of an aerosol. Controlling the concentration of the catalyst in droplets of a known size would allow control of the catalytic particle diameter. Two different aerosol generation techniques were used, electrohydrodynamic atomisation and hydraulic atomisation. A variety of catalysts were tested in this experimental system, including iron pentacarbonyl and triiron dodecacarbonyl, larger nickel and nickel-cobalt carbonyl clusters, and compounds based on giant polyoxomolybdates, termed Müller clusters. The products resulting from these experiments were studied by a number of techniques and shown to contain a variety of carbonaceous materials. These products ranged from amorphous carbon, to nanospheres, to single- and multi-walled carbon nanotubes. Analysis of these results showed that the use of catalyst precursors with high volatilities counteracts the purpose of the exercise as they volatilise out of the droplets and undergo an uncontrollable agglomeration process in the reaction chamber. This process tends to result in relatively consistently sized nanotubes but allows no control over the diameter through concentration variation. By comparison the use of less volatile catalyst precursors shows some promise for the control of the catalytic particle size. This has been shown in the case of the Müller clusters when concomitantly atomised with a refractory precursor such as a metal alkoxide. The second main technique used was chemical vapour deposition over supported catalysts, with the variation being the use of either high surface area or monolithic supports. The catalysts used under this scheme were the Müller clusters mentioned above, with the original purpose being to determine if they could catalyse the synthesis orcarbon nanotubes prior to testing them under the aerosol methodology. A number of different support materials were used in this work, including refractory materials such as silica, alumina, magnesia and titania. Also tested were two techniques for tethering the catalyst to the monolithic surface. In the cases of both the high surface area and the monolithic systems, the strength of the interactions between the catalyst and the support was found to play a vital role in the successful synthesis of single-walled carbon nanotubes. Stronger interactions reduce the mobility of the catalysts and hence the likelihood of agglomeration, increasing the selectivity of the synthesis process.