Thermal Chemical Vapor Deposition Synthesis And Properties Of New One-dimensional Carbon Nanomaterials

Many potential applications have been proposed for carbon nanotubes (CNTs), including nanofabrication, electronical materials and devices, biomedicine, chemistry, physics, composites, energy storage, electron resource and so on, based on their unique physical and chemical properties. Among tens of synthesis methods for CNTs, the thermal chemical vapor deposition (CVD) method attracts more attentions because of its low cost and high efficiency. The catalyst plays a key role in the synthesis procedure. Although the CVD synthesis of CNTs has been developed in recent years, there are many barriers to overcome. The conventional catalyst is not yet sufficiently effective and limited to transition metals such as Fe, Co, Ni and their alloys. Particularly, it is interesting to see the possibility of exploring some new catalysts to synthesize unconventional carbon nanomaterials.In the thesis, we successfully prepared three kinds of catalysts, i.e., transition metals (Fe, Co and Ni) doped MgMoO₄, alkali metal (Li, Na and K) oxide doped Cu/MgO catalyst and nanosized SnO₂ catalyst. Using these catalysts, a series of novel carbon nanomaterials, mainly CNTs, can be synthesized via CVD method. Some properties and applications were studied, based on their special structures.Single phase MgMoO₄ catalyst was prepared by a sol-gel combustion method. We found that the multi-walled carbon nanotube (MWNT) bundles can be produced using this MgMoO₄ catalyst for the first time. The experiments revealed that the transition metals such as Fe, Co and Ni doped MgMoO₄ catalyst have much higer activity and efficiency than the pure MgMoO₄. After reaction for 1 h, the maximum yield of synthesized MWNTs bundles is over 50 times of the pristine doped MgMoO₄ catalyst. With a simple enlarged process, a single furnace can produce over 40 g CNTs from 1 g doped MgMoO₄ catalyst in 30 min. The efficiency of these transition metals doped MgMoO₄ is higher than the conventional catalyst. The transmission electron microscopy (TEM) results proved that most of the MWNTs grow in the base-growth mechanism. It was also found that the reactive activity of the catalyst depends on the reaction time, carbon source and carrier gas. The nitrogen doped MWNTs can also be produced on the transition metals doped MgMoO₄ catalyst using CH₄, H₂ and NH₃ as carbon source, carrier gas and nitrogen source, respectively. After reacted for 1 h, the maximum yield ofsynthesized nitrogen doped MWNTs is over 30 times of the pristine Co doped MgMoO₄ catalyst. By controlling the flow-rate of NH₃, the nitrogen concentration of 0.6 at.% to 3.2 at.% was obtained. X-ray photoelectron spectroscopy (XPS) revealed that three different structures of the nitrogen atoms were involved in the nitrogen-doped MWCNTs, among which graphite-like structure was dominant in these MWCNTs. More defects and disorders are introduced into MWCNTs due to the nitrogen doping.It was found that several categories of novel carbon nanomaterials can be synthesized using alkali metal (Li, Na and K) oxide doped Cu/MgO catalyst via thermal CVD method using C₂H₂ and NH₃ as carbon source and nitrogen source, respectively. By controlling the reaction temperature and the content of alkali metals, various products such as Cu nanocone filled single crystalline carbon nanohorns, carbon nanotubes filled with Cu nanocones at the tips (Cu-CNTs), Cu-CNTs with carbon nanofiber, multi-branched carbon nanotubes and multi-brandched carbon nanofibers with porous structure can be produced. It is proposed that the alkali metal oxide doping changes the electronic structure of the copper catalyst. The carbon nanofiber shows fascinating potential in the electrochemical double-layed capacitors due to its porous structure. Spot welding using Cu-CNTs was investigated experimentally using nanorobotic manipulation inside a TEM. Controlled melting and flowing of copper inside nanotube shells were realized by changing the bias voltage. The self welding of the CNTs was realized based on the melting and flowing property of Cu.Carbon nanotubes filled with single crystalline β-Sn nanowires (β-Sn/CNTs) have been synthesized on the nanosized SnO catalyst by thermal CVD method using C₂H₂ as carbon source. It was found that the SnO₂ is the active component for the deposition of carbon. The products are affected by the growth temperature. The prepared β-Sn/CNTs were used as the anode of lithium ion battery. Upon the electrochemical testing, the β-Sn/CNTs electrodes show better electrochemical prperties than the pure Sn. Upon TEM observasions, it was also found that the Sn can be molten under the irradiation of electron beam and elongation/contraction alone the CNTs with the density change of electron beam. The mechanism of melting and elongation/contraction of the Sn nanowires were discussed.