Preparation And Characterization Of Carbon Nanotubes By Plasma-enhanced Chemical Vapor Deposition

Since carbon nanotubes (CNTs) have been discovered in 1991, they have become one of the most active research subjects because of their unique structures and excellent physical and chemical properties as well as the enormous potential applications. CNTs can be regarded as seamless concentric rolled up graphitic carbon atom layers. According to the number of carbon atom layer, CNTs can be categorized into single-walled CNTs (SWCNT) and multi-walled CNTs (MWCNT). CNTs possess outstanding properties in mechanics, electromagnetism, optics, field emission, hydrogen storage areas, and so on. These properties of CNTs are closely related to their structure. Investigation on the preparation methods and growth mechanism of CNTs is essential in terms of further application purposes. CNTs have been successfully synthesized by many techniques, such as arc discharge, chemical vapor deposition, laser vaporization, and etc. Here CNTs are synthesized by PECVD technique by which CNTs with high quality can be synthesized at a lower temperature with good repeatability. The effect of many experimental conditions, including catalyst category, the thickness of catalyst film, reacting gas species, deposition temperature, pressure and time on the CNT growth, is systematically investigated. CNTs with the high quality have been prepared via optimizing the experimental parameters, which may provide valuable choices for application. Moreover, amorphous carbon nanotubes (ACNTs) with crystalline branches have been successfully synthesized from catalytic Ni film in hydrogen air and methane atmosphere by PECVD for the first time. The branched ACNTs have been carefully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The growth mechanism of the branched ACNTs has also been explored.The resuts and conclusions are shown as follows:1. The optimization of growth parameters on CNTs Carbon Nanotubes main growth parameters included the kinds of catalysts, thickness of catalyst film, growth temperature, components of reacting gas, gas pressure and growth time. These conditions are important influence on the preparation carbon nanotubes.(1) We prepared Fe, Ni, FeNi, Co, CoNb and CoN catalysts film with 5nm thickness by the physical vapor deposition (PVD) method in SiO2 substrate as catalysts and prepared CNTs. After characterization to CNTs we thought, that Ni 5 nm is the most suitable as a catalyst for the growth of CNTs. In the same growth conditions, using Ni 5nm catalyst prepared CNTs are high-density, diameter of the same size, well-aligned, fast growing, and fewer impurities.(2) We prepared respectively 0.5 nm, 5 nm, 10 nm and 30 nm four different Ni catalyst film thickness as catalysts and prepared CNTs. After characterization of CNTs we thought, that Ni 5 nm thickness film formed uniform nanoparticles when heat that is conducive to catalyze growth of CNTs, and preparation CNTs are well-aligned, fewer impurities,but 10 nm, 30 nm thick catalyst films formed large catalyst particles, growth CNTs have a lot of impurities and are poor-aligned. A 0.5 nm thin catalyst film formed uneven size particles, growth CNTs are small density.(3) We prepared CNTs respectively at 500 oC, 600 oC, 700 oC, 800 oC in the other conditions being same. The growth CNTs are best quality in 700 oC, CNTs are uniform distributing, high-density, well-aligned, fewer impurities, 700 oC is the most suitable temperature in our experiment. In 500 oC, the catalyst film did not become particles, so do not growth carbon nanotubes. In 600 oC, the catalytics are low activity, growth CNTs are small-density, poor-aligned, many impurities. In 800 oC, decomposition of CH4 gained many activated carbon atoms, the catalyst activity decreased, so growth CNTs have a lot of impurities.(4) We control H2 and CH4 flux ratios were respectively: 0:100, 20:80,40:60,50:50,60:40 80:20sccm for preparation CNTs, to study the effect of gas composition on growth CNTs. Only methane without hydrogen as reactive gas could not growth CNTs. When H2: CH4 = 20:80 sccm, the growth CNTs are tidy, uniform distribution, high-density, and well-aligned. With the hydrogen increase and methane reduce, density reduction, aligned orientation break, impurity increase of growth CNTs.(5) We prepared CNTs respectively in 500 Pa, Pa 1000, 1500 and 2000 Pa conditions, we found that 1000 Pa is befitting gas pressure, growth CNTs are high-density, well-aligned, fewer impurities. In 500 Pa, growth carbon nanotubes are few. In 1500 Pa, growth CNTs have a lot of impurities and are poor-aligned. In 2000 Pa, the gas ionization becomes difficult, growth CNTs are few.(6) We control growth time of CNTs were respectively: 5 min, 10 min, 30 min and 60 min to study effect of growth time on preparation CNTs. Growth times were 5 min and 10min, preparation CNTs were fewer impurities and defects, but CNTs do not full growth , they are so short. When 60min, the catalyst has been lost the activity, exist a lot of impurities in CNTs. 30 min is right time for preparation CNTs, CNTs growth full, well-aligned, less impurities.In summary, we have educed the best conditions of preparation high-quality CNTs: Ni5 nm as a catalyst, 700 oC, flux ratios H2 :CH4 = 20:80 sccm, gas pressure 1000 Pa, growth time 30 min.2. Research of amorphous CNTs with crystallization branch. 25 nm thickness of Ni thin film was used as the catalyst,at 800 oC,1300 Pa,H220 sccm, CH480 sccm, air5 sccm conditions, we prepared amorphous CNTs with crystallization branch by PECVD. This is the first time prepared such special ACNTs. We characterized these patterns CNTs by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (Raman) and high-resolution transmission electron microscopy (HRTEM), showing that the sidewall of the ACNT is composed of curved and randomly distributed carbon clusters and the size of the clusters is less than 5 nm. Crystallization branches are growing from the CNTs wall and are composed of the ordered carbon atoms layer. Size is very greatly of the main tube body of CNTs and branches, Ni catalyst particles are on the top of branches. We indicate the growth mechanism ofaCNTs with crystallization branch, branches follows a self-seeded branching mode, hydrogen atmosphere, excessive methane gases and morphology and size of Ni catalyst are crucially impact for growth amorphous CNTs with crystallization branch.