Influence Of Oxygen On The Synthesis Of Carbon Nanotubes And Related Materials

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 pretreatment progress of catalysts, reacting gas species, deposition t pressure and time on the CNT growth, is systematically investigated. The role of oxygen, which was used during the catalyst etching stage and the CNT growth process, was investigated for the high yield synthesis and purity of vertical aligned CNTs.CNTs with the high quality have been prepared via optimizing the experimental parameters, which may provide valuable choices for application. Moreover, we report a synthesis method for obtaining CNT/graphite shell encapsulated Co nanoparticles composite, in which CNTs are highly aligned, and try to reveal the growth mechanism and growth model of the CNTs/graphite shell encapsulated Co nanoparticle.The resuts and conclusions are shown as follows:1. The optimization of growth parameters on CNTsThe main growth parameters of Carbon Nanotubes included catalyst category, the pretreatment progress of catalysts, reacting gas species, gas pressure and growth time. These conditions are important influence on the preparation carbon nanotubes.(1) We prepared Fe-Ti- Ni,Ni,Fe-Ti,Fe-Ni,Co and Co-Nb catalysts film with 14nm thickness by the physical vapor deposition (PVD) method in Si(100) substrate as catalysts and prepared CNTs. After characterization to CNTs we thought, that Co 14 nm is the most suitable as a catalyst for the growth of CNTs. In the same growth conditions, using Co 14nm catalyst prepared CNTs are high-density, diameter of the same size, well-aligned, fast growing, the high yield synthesis and fewer impurities.(2) Co thin films with a thickness of about 14 nm were first sputtered on Si (1 0 0) substrates by dc magnetron sputtering. Before CNT growth, the catalyst films were etched in H2 or O2 atmospheres, referred to as H2-etched or O2-etched catalysts, respectively. The Co thin film etched by hydrogen, the components of film haven't been changed , but the Co thin film etched by oxygen ,cobalt oxide has been formed. For the case of H₂-etching Co film,H2 can not suppress the agglomeration of Co particles,but, as the Co film is O2-etched, O2 can suppress the agglomeration of Co particles.When the reacting gases are been introduce into the chamber,the CoO nanoparticles will be deoxidized. This results in the formation of Co nanoparticles as a catalyst, promoting the CNTs growth. The carbon nanotubes prepared are high-density, even smaller diameter, well-aligned, and fewer impurities.(3) We control growth time of CNTs were respectively: 1min, 3min, 30 min and 60 min to study effect of growth time on preparation CNTs. Growth times were 1 min and 3min, 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, and exist a lot of impurities in CNTs. 30 min is right time for preparation CNTs, CNTs growth full, well-aligned, less impurities.(4) On the Si (1 0 0)substrate of O₂-etching ,we control CH₄ and H₂ flux ratios were respectively: 20:80,50:50,80:20 sccm for preparation CNTs. With the hydrogen decrease and methane increase, density increase, aligned orientation become better, impurity reduction of growth CNTs. When CH4: H2 = 80:20 sccm, the growth CNTs are tidy, uniform distribution, high-density, and well-aligned.(5) We adding a small amount of oxygen at the basis of CH4 and H2 to study the influence of oxygen on the growth of carbon nanotubes. Some differences in the CNTs growth with and without oxygen addition in the growth stage could be observed. Firstly, hardly any particles can be seen on the CNTs surface grown with oxygen addition, which suggests that the oxygen addition during the CNT growth improves the catalytic activity. Secondly, the addition of oxygen can enhance the CNT yielding rate. Previous studies on the effect of oxygen on CNT growth also suggested that the oxygen can remove the amorphous carbon through oxidation. By the SEM,we can see high purity CNTs can be synthesized with a small quantity of oxygen addition compare to without oxygen.(6) We further study the influence of oxygen flow rate on the growth of carbon nanotubes. At other conditions being equal, we introduce 1,2,4,6 sccm of O₂ addition into the reaction gas, respectively. We have found that 1,2 sccm oxygen flow rate prepare the carbon nanotubes which have a large density , and when a further increase in the amount of O₂, a reduction the density of carbon nanotubes. When the O₂ flow rate is 6sccm the obtained amount of the carbon nanotubes are only a few.(7) We prepared CNTs respectively in 700 Pa, 1000 Pa, 1300 and 1500 Pa conditions, we found that 1300 Pa is befitting gas pressure, growth CNTs are high-density,well-aligned, fewer impurities. In 700Pa and 1000pa,the growth of CNTs are few. In 1500 Pa, the growth of CNTs have a lot of impurities and are poor-aligned.In summary, we have educed the best conditions of preparation high-quality CNTs: Co14 nm as a catalyst, 800 oC, flux ratios CH₄ : H₂: O₂= 20:80 sccm, gas pressure 1300 Pa, growth time 30 min.2.The study of CNT/graphite shell encapsulated cobalt nanoparticles compositeIn this work, 14 nm thickness of Co thin film was used as the catalyst,at 800oC,1300 Pa , H2:CH4=20:80 sccm conditions, we prepared CNT/graphite shell encapsulated cobalt nanoparticles composite by PECVD. We characterized these patterns CNTs by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction pattern (SAED) and high-resolution transmission electron microscopy (HRTEM). By these characterizationg we found CNTs are highly aligned, and a high density of nanoparticles (with the size of about 70¹00 nm) clung to CNT walls can be observed. We demonstrate that the nanoparticle clung to CNT walls is a graphite shell/encapsulated Co particle.Finally,we try to reveal the growth mechanism of the CNTs/graphite shell encapsulated Co nanoparticle.