Studies On Synthesis And Filling Of Double-Walled Carbon Nanotubes

Being a transition from single-walled carbon nanotubes (SWNTs) to multi-walled carbon nanotubes (MWNTs), double-walled carbon nanotubes (DWNTs) with unique structures and extraordinary properties are ideal and unique thinnest MWNTs, possessing the advantages of both SWNTs and MWNTs, and have been attracting considerable interests and broad attention of scientists all over the world. DWNTs have become one of the most prominent materials in nanoscience and are of great potential in nanoelectronics, nanobiology and other related fields. Filling foreign materials into the hollow cavities of carbon nanotubes may significantly modify their electronic and mechanical properties, as well as alter the properties of the filled materials. This thesis is mainly focused on the synthesis and filling of DWNTs, and the properties of substance filled in the nanospace, i.e. the central cavity of the DWNTs.1. Mass preparation of double-walled carbon nanotubes with perfect structural integrity has been achieved in high yiled by a modified DC arc discharge method. By adding trace amount of halide typically KC1 into the iron sulfide catalyst, the yield of DWNTs could be increased dramatically from about 10 wt% in related references to over 50 wt%. The quality of DWNTs can be greatly improved by using a newly developed purification method; DWNTs with a purity of over 90wt% have been obtained. It is believed that chlorine in halide plays an important role in the formation and growth of DWNTs. The successful synthesis of high quality DWNTs in large quantity made it possible for further studying the properties of DWNTs and for exploring their potential applications.2. Some novel properties of DWNTs are revealed for the first time. Raw soot consisting of thread-like DWNTs exhibits very high mechanical strength, and the DWNTs in random orientation would gradually become to be arranged in order under the effect of foreign force. Intramolecular nanotube junctions are found in the DWNTs bundles, which can be regarded as a kind of heterojunctions with unique electron transportation property, and may be of potential in the next generation electronic devices based on carbon nanotubes.3. Fullerene-filled SWNTs and DWNTs (so-called nanopeapods) with a purity of over 80% have been synthesized in high yield, in which the C60 molecules are introduced into the hollow cavities of nanotubes by a vapor phase diffusion method. HRTEM examination has revealed that C60 molecules are aligned with a distance of about 1.0 nm in the central cavity of SWNTs and DWNTs with a small diameter, the stacking phases of C60 inside DWNTs with a larger diameter are totally different. Depending on the inner diameter of DWNTs, the stacking phase of C60 varies, which can be the zigzag phase, double-helix phase, and phase of two molecule layers. This new observation provides direct solid experimental evidence to the previous theoretical prediction that the stacking phases would be sensitively dependent on the inner-tube diameters of the nanotubes.4. Raman measurements on DWNTs peapods have revealed for the first time that there are obvious differences between the spectrum of DWNTs peapods and SWNTs peapods in terms of the shifts, and the strengthened or weakened intensity of certain characteristic peaks. The difference is believed to be due to the special double-layer structure of DWNTs. C60 fullerenes would polymerize via a dipolymerization scheme under the laser irradiation, and C60 fullerenes would interact with the inner walls of DWNTs via weak force of van der Waals, during which charge transfers may take place between C60 molecules and carbon nanotubes.5. Ferrocene, a small metallorganic molecule with very good redox activity, has been successfully filled into the central cavity of DWNTs, resulting in a novel nanohybrid material Fc@DWNTs that is of potential as air-stable n-type transistors, building blocks and biosensors for various devices based on the redox activities of ferrocene and electronic properties of nanotubes. FT-IR study reveals that there exists a strong interaction between the ferrocene molecules inside the cavity of DWNTs and the nanotubes, and n-doped DWNTs are formed because of the electrons transfer from ferrocene molecules. The electrochemical properties of as-synthesized Fc@DWNTs have been evaluated using the cyclic voltammetry, showing the Fc@DWNTs are different from the Fc@SWNTs in terms of the electrochemical property. For the Fc@DWNTs materials, surface-confined thin-layer electrochemical behavior is observed at low scan rate, while a diffusion-confined electrochemical behavior would dominate at high scanning rate.6. The thermochemical behaviors of ferrocene molecules inside the confined nano-space of the DWNTs have been studied for the first time using the thermogravimetric analysis-mass spectrum (TG-MS) technique. The results show that the decomposion temperature of ferrocene molecules in the confined nano-space is 540℃, which is 40℃higher than the decomposion temperature of 500℃under normal conditions. The variation in thermochemical properties of ferrocene molecules is believed to be due to the confinement effect and protection of carbon layers. 7. Triple-walled carbon nanotubes (TWNTs) have been prepared for the first time by annealing of Fc@DWNTs nanohybrid materials at high temperature, in which the filled-ferrocene molecules decompose, resulting in a new carbon nanotube inside the central cavity of DWNTs that function as template at nanoscale. The detailed HRTEM examination confirms that the formation of the new nanotubes inside DWNTs follows a root-growth mechanism that has been predicted in previous theoretical studies but lacs solid evidence before the present work. This opens a new approach to the controllable synthesis of TWNTs, and may give a new impetus to the study of TWNTs.