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Synthesis of Single-walled Carbon Nanotubes with Rare-earth Metal as Catalyst Component by Arc Discharge and Investigations of Their High Pressure Induced Structural Transition

Author: YaoMingGuang
Tutor: LiuBingBing
School: Jilin University
Course: Condensed Matter Physics
Keywords: Multi - walled carbon nanotubes Single-walled carbon nanotubes Jilin University Carbon nanotubes Doctoral Dissertation Metal catalysts Rare earth metals Catalytic effect Diameter distribution Rare earth elements
CLC: O613.71
Type: PhD thesis
Year: 2007
Downloads: 253
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Abstract


The unique structure of single-walled carbon nanotubes (SWNTs) and their fascinating physical and chemical properites imply a great potential in nanoscience and in technological applications. In this thesis, we systemically study the synthesis and characterization of SWNTs with a series of novel rare-earth/Ni as catalyst by arc discharge and investigate the high pressure induced structural transition of SWNTs.The properties of SWNTs are strongly dependent on their structure, such as helicity and diameter. The applications of SWNTs require large-scale synthesis of SWNTs with desired nanostructures and with high purity. One of the most critical parameters in SWNTs synthesis is the catalyst used. It has been found that the catalyst has strong effect on the nanostructure and the yield and even the macroscopical morphology of as-synthesized SWNTs. Therefore, to explore and study new high efficiency catalyst for large-scale synthesis of SWNTs is always nessary and important. Such studies will provide valuable information to control the synthesis of SWNTs and further understand the growth mechanism of SWNTs.One of the most widely used methods, the DC arc discharge method, can produce SWNTs with a high degree of crystallinity and low defect concentration on a large scale. Here, we synthesize SWNTs in high yield and high purity by this method with a new bimetallic catalyst Ho/Ni for the first time. The morphologies, diameter distribution and the content of SWNTs in the products are characterized by SEM, TEM, Raman spectroscopy, TGA and UV-NIR spectroscopies. Long SWNTs ribbons, consisting of roughly aligned and relatively high purity SWNTs bundles have been synthesized with Ho/Ni as catalyst by using a simple modified arc discharge apparatus. The introduction of Ho/Ni as catalyst and the convection enhanced by the modification of the apparatus play important roles in the formation of these ribbons. Changing the Ho and Ni concentrations in the catalyst hardly affects the diameter distribution but strongly affects the yield of SWNTs. An optimal range of Ho/Ni composition for synthesis of SWNTs with relatively high purity and yield has been obtained. The best yield of SWNTs obtained by Ho/Ni catalyst is as good as the highest yield of SWNTs by Y/Ni catalyst ever reported. The present synthesis process can be easily manipulated and is promising for large-scale production. We also discuss the growth mechanism of SWNTs and suggest that Ho play important roles for enhancing the nucleation ratio of SWNTs in the SWNTs growth process.A systematic experimental study has been carried out on the efficiency of a series of novel bimetallic catalysts based on Ni and the rare-earth elements in the synthesis of SWNTs. Most of rare earth metals studied here have not been reported as catalyst component for the synthesis of SWNTs, except for La, Ce, Tb and Y. We employ SEM, TEM, Raman spectroscopy and UV-NIR spectroscopy to study the effect of novel catalysts on the purity and nanostructure of as-synthesized SWNTs. The results indicate that the elements with a valence of +3 or +4 have an obvious catalytic effect and increase the yield of SWNTs dramatically, including La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu and Y. Among them, Ho/Ni, Er/Ni and Y/Ni give the best yield. However, those rare earth elements having a valence of +2 are not efficient catalysts, including Sm, Eu, Tm and Yb. We also find a rule of the influence of rare-earth elements on the diameter distribution of as-synthesized SWNTs when analyzed by a combination of Raman and UV-NIR spectroscopy. For the metals having an obvious catalytic effeciency, there is a tendency that the fraction of small-diameter tubes decreases while large-diameter tubes increases with decreasing ionic radius of the rare-earth element used. EDX and X-ray analyses indicate that the rare earth metals having an obvious catalytic effect deposit on the cathode deposits and form rare-earth carbides, whereas the inefficient catalyst metals have not been found in cathode deposits, except for a small amount of Tm present in the form of thulium carbide. Further analysis indicates that there is a very strong correlation between the ability to form rare-earth carbides and the catalytic efficiency for the formation of SWNTs.SWNTs can be taken as a representative qusi-one-dimensional model material. The investigation of their structural evolution under high pressure is always an important topic, which can provide information on the stability and phase transitions of SWNTs under compression. In this work, we employ three excitation wavelengths to study the high pressure Raman spectra of SWNTs under hydrostatic and non-hydrostatic conditions. A near IR laser (830nm) is used in this work for the first time. For comparison, other two excitation wavelengths, 514.5 nm and 633 nm are used as well. The IR laser produces a strong Raman signal and gives us the opportunity to observe radial breathing modes even up to 14GPa for the first time. We find that a plateau occurs for the G-band under high pressure. But unlike what is observed in previous studies, the RBM frequencies shift up almost linearly throughout the plateau of the G-band, without an abrupt change. However, near the critical onset pressure where the G-band plateau starts, the intensity of the RBM significantly decreases. This phenomenon occurs regardless of whether a PTM is used or not, and does not depend on the identity of the PTM, on the diameter of tubes, or on the excitation laser. Based on the first principles calculations using the Local Density Approximation (LDA) in density functional theory (DFT), we simulate the structural changes of the chosen SWNT model and calculate the Raman frequencies of the deformed nanotube under compression, yielding the frequencies of the Raman modes as functions of structural evolution. The results indicate that when the structural transition of SWNTs cross-section from an ellipse-like shape to a flattened oval shape is induced, the change of both RBM and G-band in theory is well consistent with that observed in our experiments. The emergence of a pressure plateau for the G-band accompanied by a significant decrease in the intensity of RBM peaks thus can be used as a signature of the structural transition of SWNTs under high pressure.

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CLC: > Mathematical sciences and chemical > Chemistry > Inorganic Chemistry > Non-metallic elements and their compounds > Part Ⅳ family of non-metallic elements (carbon and silicon ) and its compounds > Carbon C
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