Synthesis Of Carbon Nanotubes By Catalytic Chemical Vapor Deposition Method

Since the discovery of carbon nanotubes （CNTs） was reported on Nature in 1991, the investigation on CNTs has become a hot topic in the world. CNTs has excellent properties and wide potential application. However, the limitation of large-scale production has restrained the development of the research on their application. Comparing to arc discharge and laser ablation, catalytic chemical vapor deposition （CCVD） is the most possible method with large-scale, low-cost and high-yield advantage to fabricate CNTs. Therefore, we used CCVD to synthesize CNTs and to increase the yield of CNTs mainly from two aspects: one aspect was that, developing new catalysts for synthesis of CNTs; the other aspect was that, improving the method of CCVD. The main researches in this paper are as follows:（1） Water was introduced into the carbon nanotubes growth reaction by the gas in the catalytic chemical vapor deposition （CCVD） to study the effect of water on the multi-walled carbon nanotubes （MWNTs）. The obtained MWNTs was characterized by transmission electron microscopy （TEM）. The results show that, the MWNTs obtained with suitable amount of water is easy to be purified and water with suitable amount could not affect the obtained MWNTs structure and patterns. The water has its double actions: too much or too little water both affected the MWNTs yield. But with adding suitable amount water, MWNTs yield can have a great increase.（2） We used Fe/MgO, Fe/La₂O₃ as catalysts to synthesize CNTs by CCVD. The effects of MgO and La₂O₃ on synthesizing CNTs were investigated. The obtained CNTs morphology and the catalyst structure and component were characterized by the transmission electron microscopy and X-ray diffraction, respectively. The results show that, on the different support of MgO and La₂O₃, the phase structures of Fe were a and y respectively. Due to the diffusion coefficient of carbon atom in a-Fe is higher than in y-Fe, the yield of CNTs synthesized onα-Fe/MgO was higher than on onγ-Fe /La₂O₃. The yields are about 2897%（gcNTs/gFe） onα-Fe/MgO and 1021 %（gCNTs/gFe） onγ-Fe/La₂O₃.（3） Carbon nanotubes （CNTs） were prepared by CCVD using the Fe-Ni dual active metal catalyst made by citric acid complexation. The obtained CNTs morphology and the catalyst structure and component were characterized by the transmission electron microscopy and X-ray diffraction, respectively. The research results show that the obtained CNTs yield from the Fe-Ni dual active metal catalyst is clearly higher （about six or four times） than that from the single Fe or Ni as the catalyst. Especially, the CNTs yield has its maximum （2000% gCNTs / gCatayst precursor·h） when the Fe atomic percentage is 75% in Fe and Ni atoms. The reason may be that the Fe and Ni in dual active metal catalyst at high temperature come into being solid solution of （γ-Fe, Ni） , which has the higher diffusion coefficient of carbon atom than Fe or Ni does.（4） The small tube diameter CNTs was prepared by CCVD using Co-Mo/MgO and CH₄ as the catalyst and carbon source, respectively. The catalyst was prepared by citric acid complexation. In order to find the suitable reaction condition to prepare small tube diameter CNTs, the reaction gas atmosphere, reaction temperature and the mol ratio of Co/Mo were researched in detail. The transmission electron microscopy and high-resolution transmission electron microscopy were used to character the obtained CNTs pattern and structure. The results show that the excellent reaction condition is that the reaction temperature is 950℃, and the gas flow rate of the H₂, N₂ and CH₄ are 200, 200 and 50 mL/min, respectively. Under this reaction condition, using the most preferable catalyst of Co:Mo:Mg: citric acid=0.0375 : 0.0375 : 0.925 : 1, we can obtain good structure small tube diameter CNTs with about 2 nm inner diameter and 8nm outer diameter. Moreover, the small tube CNTs yield is higher than 110%.