The synthesis and filling of single-walled carbon nanotubes

This thesis is concerned with the synthesis, properties and application of single-walled carbon nanotubes (SWNTs). The two main objectives of the work were the development of a continuous-flow synthesis of SWNTs, using chemical vapour deposition (CVD) techniques, and the application of the hollow SWNTs as moulds for the study of the crystallisation behaviour of inorganic materials in the confined space of their inner cavity. The latter study was mainly performed by interpreting high-resolution transmission electron microscopy (HRTEM) images of the filled SWNTs. A so-called focal series restoration approach, which enhances the resolution of the images and thereby increases the information content, was employed where possible. Chapter I reviews the previous work in the field of SWNTs and introduces their basic structure, symmetry, physical and mechanical properties and the common methods of SWNT synthesis. The chapter ends with an overview of the techniques used in the present work for the characterisation of carbon nanotube samples by giving a description of the high-resolution transmission electron microscopy (HRTEM) techniques and the digital image processing method. Other physical measurement techniques used, such as Raman spectroscopy and thermogravimetric analysis (TGA), are discussed with reference to their application for the characterisation of carbon nanotubes. Chapter II describes the development of an improved synthesis strategy for SWNTs. A continuous-flow chemical vapour deposition (CVD) method was explored using carbon monoxide or mixtures of methane and hydrogen as the carbon feedstock gas and introducing various volatile organometallic compounds to catalyse the formation of SWNTs. In this study, a special water-cooled copper nozzle was designed and built so as to prevent the premature decompositiont (disproportionation) of the reactants (the carbon monoxide gas) and to allow their direct introduction into the centre of the hot reaction zone. A thorough investigation of all synthesis parameters was conducted in order to evaluate their effect on the formation of SWNTs. The screened parameters were the gas flow rate, the reaction temperature, the temperature gradient experienced by the reactants, the reaction time and the residue time of the reactants in the hot reaction zone, the molecular ratio of carbon to metallic catalyst precursor and the composition of the carrier gas mixture. Furthermore, a large number of different volatile organometallic compounds was tested for their ability to catalyse the formation of SWNTs. The highest yield of SWNTs was achieved using carbon monoxide as the carbon feedstock gas, enriched with less than 100 ppm Fe(CO)5 as the organometallic precursor. The product obtained under these optimised conditions was characterised by HRTEM, Raman spectroscopy and TGA. The chapter concludes with the evaluation of the CVD synthesis by comparing it to other SWNT synthesis routes. The outlook for a possible scale-up towards the industrial production of SWNTs is subsequently discussed. Comparison is made with respect to a similar method, published and patented by the group of Professor R. E. Smalley at Rice University, during the course of this work. Chapter III is concerned with the characterisation of the crystallisation behaviour of nine inorganic materials, encapsulated within SWNTs. The characterisation was carried out by the interpretation of conventional HRTEM images for the systems Sb2O3-SWNT, Mnl2-SWNT, ln2l6-SWNT, SnI4-SWNT and lodine-SWNT. For the encapsulation composites Sb2O3-SWNT, Rbl-SWNT, PbI2-SWNT and LaI2-SWNT, focal series of HRTEM images were acquired and restored to retrieve the phase of the respective system, providing improved resolution and more information about the imaged composite. Significant lattice distortions were observed in nearly all the encapsulated crystals with respect to their bulk structure. In some cases, the crystal fragments revealed atom arrangements unknown to their bulk structure. For example a helical arrangement of a MnI2 fragment inside a SWNT and the crystallisation of bivalent LaI2 in the 'reduced' lattice of the trivalent Lal3. In the case of the focal series restoration of the Sb2O3-SWNT and Lal2-SWNT inclusion composites, it was found possible to characterise the chirality of the surrounding SWNT and to assign a probable symmetry for the graphitic cylinder. The chapter concludes with a comparison and interpretation of the observed crystallisation behaviour. Chapter IV provides details of the experimental procedures and physical measurements. (author)