An Investigation On Synthesis, Purification And Application Of Large-diameter Single-walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) have potential applications in many fields, such as composite materials, nano-electronic devices, and catalyst supports, due to their unique structure and good mechanical, physical and chemical properties. However, there exist some technical hurdles for the applications. For example, the electronic structure of a SWCNT is related to its tube diameter. The currently available techniques for CNT synthesis always produce tubes with a wide range of diameters. As a result, metallic and semiconducting SWCNTs are always coexistent. As a matter of fact, it has been a bottleneck for many fundamental studies and applications how to control the electric properties by increasing the diameter of SWCNTs. The second hurdle is that initial SWCNTs synthesized by arc-discharge, laser vaporization, and chemical vapor deposition (CVD) methods contain a large quantity of impurities (catalytic nano-particles). These impurities need to be purified by strong oxidation methods, but during such purification process, structural destruction of SWCNTs is inevitable. Thus, it is necessary to develop techniques for non-destructive purification. A third hurdle concerns dispersion, Current SWCNTs are bound into entangled ropes or masses with bad dispersion. They have to be separated from each other and have good dispersion for many applications. In this dissertation, the research route follows"controllable synthesis-purification- dispersion-application", and new approaches are proposed to resolve the above-mentioned issues with SWCNTs.A CVD method is used for synthesizing SWCNTs in this study. Ethanol is used as the carbon feedstock, ferrocene as the catalyst, and thiophene as the growth promoter. Under keeping the other experimental condition, the diameter of SWCNTs is increased effectively only by changing the amount of thiophene addition. SWCNT samples with different average diameters from 1 to 5.8 nm can be synthesized continuously, with a production rate of about 1 g h-1. Based on experimental results, a growth model is proposed: S atoms from thiophene decomposition are absorbed on the surface of an Fe particle. The absorption leads to the formation of a liquidus region on the metal surface, and this liquidus region becomes the nucleation core of SWCNTs. With changing thiophene addition, the size of the liquidus region and thus the tube diameter change.The electrical properties of SWCNTs have been studied. Results show that different-diameter SWCNTs have different electrical properties: the electric resistivity of SD-SWCNTs is 0.5m·cm and that for LD-SWCNTs is 0.1m·cm. Additionally, voltammetry curves suggest that the resistance behavior of SD-SWCNTs is non-linear, but that of LD-SWCNTs is of a linear nature. This may be attributed to the possibility that the SD-SWCNT sample contains a high proportion of semiconducting variants, the number of carrier increases for temperature effects with the increasing of the voltage, and thus the electric resistivity decreases.In this dissertation, a high-effective and non-destructive purification method is proposed. Fe catalytic particles are heated to be oxidized at a low temperature in air, and then the sample is heat-treated under the protection of Ar gas at a high temperature to induce the reduction of Fe2O3 by the encapsulating carbon to Fe. Finally, the naked Fe or FeO particles are dissolved easily by HCl. During this purification process, little damage is induced to the graphitic structure of SWCNTs as evidenced by TG, Raman and NIR spectroscopic studies.SWCNTs have high length-diameter ratios, and thus are usually bundled and entangled together with low dispersion in any solutions. The LD-SWCNTs, however, are found to be dispersible uniformly in water or ethanol without any modification after purification. In contrast, the purified SD-SWCNTs could not be dispersed. Instead, they tend to form large aggregates in solutions. Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy results indicate that SD- and LD-SWCNTs have the same types and the same quantities (mol) of functional groups on their surfaces. Thus, it may be concluded that the good dispersion observed for LD-SWCNTs is a result of their large diameters.To improve the dispersion of SD-SWCNTs, a cryogenic freezing and crushing method is proposed in this dissertation. Ethanol and water were used as solvents, and liquid nitrogen as a freezing medium. The frozen mixture of ethanol, water, and SWCNTs was crushed with high-speed blades. After dissolution and filtering, the obtained SD-SWCNTs could be dispersed uniformly in ethanol. The results of nano-laser particle size analyzer indicate that the average particle size of such SWCNTs is 165.1 nm, which are 3 orders of magnitude smaller than the SD-SWCNTs without the treatment.The application of SWCNTs as a catalyst support for fuel cells has been studied. Pt nanoparticles are deposited on LD- and SD-SWCNTs by a glycol reduction method. Electrochemical tests show that the catalytic activity of Pt particles on LD-SWCNTs is 2 times of that for Pt particles on SD-SWCNTs or multi-walled CNTs and also much higher than the commercial catalyst from Johnson Matthey Co. The excellent activity observed for LD-SWCNTs may be attributed to their good electrical conductivity and dispersion property.