Controlled Synthesis And Growth Mechanism Of Multi-walled Carbon Nanotubes

To explore the controlled growth of multi-walled carbon nanotubes (MWCNTs) and understand the mechanism of growing carbon nanotubes, two different methods (the floating—based method and magnetron sputtering method) have been used to synthesize carbon nanotubes. Special designed experiments and analyses were carried out and the main results are summarized as follows:High quality and clean carbon nanotubes were synthesized by the new floating—based method by decomposing acetylene at 700°C and using Fecl₃ as the catalyst precursor. This new method combined the merits of the floating catalyst method and the based method. The influence on diameter and defects of carbon nanotubes owing to the different H₂ pre-reduction time of deposited Fe cluster was discussed. It was found that the longer the H₂ pre-reduction time of deposited Fe cluster, the fewer the defects of CNTs and the more uniform of diameter of CNTs. The experiment shows that this floating—based method with Fecl₃ as the catalyst precursor is a promising method for cheap and mass growth of carbon nanotubes.High density, well-aligned MWCNTs were prepared by thermally decomposing acetylene at 700°C with the help of Ni-Cr alloy as catalyst in a thermal chemical vapor deposition system. The density and alignment of MWCNTs were characterized by scanning electron microscope (SEM). It was found that the density of the MWCNTs can be remarkably increased and the alignment can be improved with the decrease of the thickness ratio of Ni:Cr. Also found in our experiment was that the catalyst encapsulated in MWCNT was single crystal Ni, which was confirmed by high-resolution transmission electron microscopy (HRTEM) and electron dispersion X-ray spectrum (EDX). Finally, the growth model of MWCNTs was discussed based on the Ni-Cr alloy catalysts under our experimental conditions: only Ni has catalysis and Cr acts as dispersant. Changing the layer thickness ratio of Ni:Cr can change the density and alignment of MWCNTs dramatically. The results are helpful in providing a better understanding of the role of catalyst and the controlling of the desirable density and alignment of MWCNTs for various applications.Further investigations were carried out upon MWCNTs synthesized under the typical experiment conditions. SEM and transmission electron microscopy (TEM) studies showed that CNTs have been prepared at a temperature range of 650-900℃. Only the MWCNTs synthesizedwithin the temperature range of 700-800 °C were filled with metal nanoparticles or nanowires while no nanoparticles or nanowires were found in the cavity of prepared CNTs if the growing temperature was low down 650°C. While even no CNTs were synthesized if the growing temperature is high up to 900 "C. Also found in our experiment was that the nanoparticles encapsulated in CNT was single crystal Ni, which was confirmed by HRTEM and EDX. From the shape of nanoparticle inside the CNT it is concluded that the metal Ni must be liquid when the CNTs were synthesized. A Vapor-Solid/Liquid-Solid (V-S/L-S) growth mechanism is proposed: in the process of grown CNT. The Ni particles have a solid core and a liquid surface covered the solid Ni particles. The solid Ni core is the active catalyst to grow CNTs meanwhile the surface liquid Ni flow into the cavity of grown CNTs. At suitable temperature, the Ni particle keeps liquid-solid phase and the surface liquid Ni fills in the CNTs. Filling liquid metal Ni and growing CNTs is a dynamic equilibrium. This process will be ended if the solid Ni nucleus become smaller and smaller and gradually disappears. This V-S/L-S model can explain some experimental phenomena very well, which cannot be explained by traditional Vapor-Liquid-Solid (V-L-S) model.