Synthesis and Mechanistic Study of Carbon Nanotubes and Tungsten Oxide Nanowires by Chemical Vapor Deposition Methods
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Chemical vapor deposition (CVD) is a versatile method for material preparation. Materials from films to nanostructures can be obtained by tuning the parameters of the CVD method. In the field of one-dimensional nanostructures, CVD is the most extensively employed method. This dissertation is focused on the synthesis of carbon nanotubes (CNTs) and tungsten oxide nanowires by the CVD method as well as the growth mechanism of these two kinds of one-dimensional materials. The first part of this dissertation focuses on CNTs. After an introduction to CNTs, the synthesis of bulk CNTs with high purity is explored. In chapter 2, the synthesis of few-walled carbon nanotubes (FWNTs) with ethanol as carbon source is explored. It is proved that addition of methanol to ethanol increases the purity of raw FWNT materials. Methanol acts as "carbonaceous impurities remover" to remove impurities deposited on the MgO support and also hinders the formation of such impurities during FWNTs growth. In Chapter 3, the synthesis of double-walled carbon nanotubes (DWNTs) with high purity and small size distribution using FeSi2 as catalyst is described. In FeSi2, iron is dispersed at atomic scale, and thus it is possible to provide uniform iron particles for CNT growth. In chapter 4, a two-stage model for the growth of SWNTs is proposed, which may explain the low yield of SWNT growth in some early results. In this model, growth of SWNTs is divided into two stages, nucleation and growth. These two stages need different carbon feeding rates. To obtain a high yield of SWNTs, different carbon feeding gases should be applied to different stages. In chapter 5, a small amount of Zn is reported to increase the yield of SWNTs grown on SiO2/Si wafers. This phenomenon is abnormal and can not be explained with general accepted CNT growth mechanism. More effort is needed to elucidate the mechanism of high SWNT yield obtained under assist of Zn.The second part of this dissertation is focused on tungsten oxide nanowires. Potassium doped tungsten oxide and a tungsten bronze nanowires were synthesized with a simple CVD system on tungsten plate. A vapor-liquid-solid (VLS) mechanism is employed to explain the growth of these two kinds of nanowires.
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