Carbon nanotubes: synthesis and functionalization
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This thesis focuses on two of the major challenges of carbon nanotube (CNT) research: understanding the growth mechanism of nanotubes by chemical vapour deposition (CVD) and the positioning of nanotubes on surfaces. The mechanism of growth of single–walled nanotubes (SWNTs) has been studied in two ways. Firstly, a novel iron nanoparticle catalyst for the production of single–walled nanotubes was developed. CVD conditions were established that produced high quality tubes. These optimised CVD conditions were then used as the basis of several comparative CVD experiments showing that the quality of nanotubes and the yield of carbon depended on the availability of carbon to react. The availability could be controlled by the varying concentration of methane in the gas phase or the residence time of the methane over the catalyst. Evidence is presented that the diameters of the tubes produced were affected by the availability of methane. A second mechanistic investigation was carried out to study the validity of the previously proposed ring addition mechanism for the growth of carbon nanotubes from camphor. In this mechanism, the formation of tubes is thought to occur through the addition of preformed carbon rings: so it would be expected that there would be a relationship between the molecular structure of the precursor and the resulting SWNTs. To explore this relationship, comparative CVDs were carried out to produce SWNTs with several different cyclic and acyclic compounds similar in structure to camphor. The vapour pressure and the chemical stability of the precursor were found to be important to the formation of nanotubes, while the compound’s structure was not related to the quality of tubes produced. The lack of a relationship between the structure of the precursor and the production of SWNTs suggests that preformed rings are not vital to the production of SWNTs. Differences in the growth of SWNT from benzene and methane were related to the stability of each compound. In particular, differences in the distributions of the diameters of the tubes formed from methane and benzene have been observed. These differences have been explained in terms of the relative kinetic stabilities of these molecules, and in terms of a competition between end–cap and sidewall growth. Positioning of nanotubes on surfaces has been explored using two approaches. In the first approach, commercially obtained SWNTs were functionalized by a sulfur plasma so that the products would form bonds with gold surfaces. The nanotubes were found to selectively deposit themselves onto gold surfaces from ethanolic dispersions of the functionalized samples. This selective deposition of the nanotubes allowed the production of prototype carbon nanotube field–effect transistors with higher device yields than were obtained with unfunctionalized tubes. In a second approach to positioning of carbon nanotubes, the growth of tubes on surfaces by CVD was explored. Iron nitrate and different magnesium compounds were dip–coated onto SiO2 surfaces so that MgO supported–Fe catalysts would be formed by calcination. SWNTs were grown on the surfaces by CVD. Surface area measurements of the equivalent powdered catalysts showed that a high surface area was vital to produce dense growth of high quality SWNTs. The morphology of the surface was also found to play a key role in the growth of the tubes. Patterned growth of carbon nanotubes was accomplished using soft lithography techniques to control the localization of catalyst deposition onto a surface. A long calcination step (10 h, 950 °C) before CVD, was found to improve the quality of nanotubes grown. Catalysts that had been calcined for 10 hours were also found to produce smaller diameter nanotubes than uncalcined samples. The formation of smaller diameter tubes was explained in terms of the formation of MgFe2O4 alloys, consistent with results reported previously in the literature. In addition, Raman spectroscopy of the calcined catalysts with 3% w/w loadings of Fe was used to confirm directly the presence of MgFe2O4.