Effect of ammonia on the growth of carbon nanotubes

Carbon nanotubes were grown on hydrogen-treated Fe catalyst at 700 degrees C using a thermal chemical vapor deposition method. During the growth, acetylene was used as the carbon source, which is balanced by hydrogen and/or ammonia. Raman analysis shows that the introduction of ammonia to the gaseous carbon source can lead to defect structures in the carbon nanotubes due to the incorporation of nitrogen atoms into the carbon nanotubes. Furthermore, the growth rate of carbon nanotubes was also affected by the introduction of ammonia into the gaseous carbon source. We show that the dependence of the growth rate on the gaseous source composition is better described in terms of the ratio of ammonia to acetylene than the overall ammonia concentration. It is proposed that there is a competition between the ammonia and the acetylene during the growth of CNTs. At low ammonia/acetylene ratios the growth increases with ammonia concentration; while at high ammonia/acetylene ratios the growth decreases with the ammonia concentration. A critical ammonia/acetylene ratio of 4.4, at which the growth peaks, was found and discussed.