Effects of Catalyst Characteristics on the Growth of Aligned Carbon Nanotubes

Autor: Kun-Hou Liao, 廖坤厚
Rok vydání: 2006
Druh dokumentu: 學位論文 ; thesis
Popis: 94
Microwave plasma-enhanced chemical vapor deposition (MPCVD) system was employed to grow one-dimensional carbon nanomaterials at low temperature of 370℃ under a methane and hydrogen gas mixture in this research. Five different catalysts were used, including transition metals Fe, Co, Ni and novel catalyst Co52Fe20Zr8B20, Fe-Si alloys. A low-temperature, nonlinear rapid growth of aligned carbon nanotubes with high area-density had been obtained using Fe-Si thin film as a catalyst. Carbon nanowires (CNWs) were grown on Fe thin film catalyst. The growth rate and area-density of CNWs is 0.3 μm/min and 108/cm2, respectively. When the Co thin films were used as the catalyst, the diameter of carbon nanotubes (CNTs) with broadening distribution range is 57±39 nm. The CNTs exhibited a twist and winding growth morphology. The growth rate and area-density of CNTs is 0.12 μm/min and lower 108/cm2, respectively. The use of amorphous Ni as the catalyst led to the formation of crystalline CNW but not amorphous CNW. In general, a catalyst can be seen at the tip of each CNW. The carbon nanowires were also found to exhibit an interesting microstructure where the basal planes were perpendicular to the wire axial direction and parallel to the closest packing plane of the catalyst. The growth rate and area-density of CNTs is 2.3 μm/min and lower 108/cm2, respectively. When the Co52Fe20Zr8B20 thin films were used as catalysts, the average diameters of CNTs increase with the film thickness. However, the average length of the CNTs was found to decreases with the film thickness. The growth rate and area-density of CNTs are 0.9 μm/min and 108/cm2, respectively. Depending on the diameter of CNT, the diffusion of carbon in the catalyst particle are either a bulk diffusion (larger diameter) or surface diffusion (smaller diameter). A low-temperature (370℃), nonlinear rapid (18 μm/min) growth of aligned carbon nanotubes with high area-density (1010/cm2) had been obtained using Fe-Si thin film as the catalyst. All the CNTs with the base growth mode have the same average diameter near 12±5 nm. The Raman signatures of the CNTs consist of two main peaks near 1326 cm-1 (D-band) and 1580cm-1 (G-band) with a shoulder peak D’-line (1610 cm-1). As usual, the ID/IG ratio decreases with increasing methane/hydrogen, and the G-band broadens with increasing methane/hydrogen ratio. The Raman analyses indicated that the sp2 graphite cluster La of carbon products surrounding with carbon or graphite layer decreased. In other words, the growth carbon materials on Fe-Si catalyst are near amorphous carbon under a higher methane concentration. When the CNTs growth, the etched Fe-Si films were divided into two layer, base layer and aggregate layer. The base layer provided the aligned CNTs growth sites and the aggregate layer formed the impurity on the tip of CNTs. The growth of well-aligned carbon nanotubes (CNTs) having increased growth rate at reduced growth temperature as a result of using Fe-Si as the catalyst. Fe-Si catalyst leads to not only a low growth temperature of 370℃ but also an unprecedented, rapid growth kinetics. The lengthening of CNTs was found to be proportional to t15 where t is the growth time. The use of Fe-Si as the catalyst enhances the carbon diffusivity in the catalyst. Due to the enhanced carbon diffusivity and the nonlinear growth kinetics, a reaction-controlled growth model is suggested.
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