Abstract
The kinetics of multi-walled carbon nanotubes (MWCNTs) growth reported so far were mostly based on simulation or deriving the activation energy of one kind of catalyst assuming steady growth. Whether the growth mechanism of MWCNT growth on different catalyst metals can fall into the same mechanism, and whether the nucleation and growth kinetics share the same kinetic controlling mechanism need to be clarified. In this study, we synthesized MWCNTs over different catalysts (iron, cobalt and nickel) deposited by e-gun evaporation and measured the length of MWCNTs varying with time. It was found that there consisted three stages of MWCNTs formation: (1) Nucleation stage by observing the shape transformation of catalyst particles in the initial stage and also by extrapolating the change of length with time to derive the nucleation period. (2) Steady growth stage from which the activation energy of the controlling step could be derived by Arrhenius plot. (3) Growth saturation period, which was caused by amorphous carbon formation. After intensive observations of the relation between the sizes and shapes of the catalyst nanoparticles and the growth rates of MWCNTs at different temperatures over different catalyst metals, it was concluded that the catalyst shape transformation at nucleation stage was important for the growth on nickel and cobalt, but not on iron. As to the steady growth rate, it was found that MWCNTs growth followed the same growth mechanism in all temperature range for iron and cobalt, whereas for nickel there observed a transition of growth mechanism at temperature of ~840oC. There were also observed different growth saturation behaviors on different catalyst metals, which will be discussed in this paper as well.