(151d) On-the-Fly Length Classification of Carbon Nanotube Aerosols and the Kinetics of Growth

We describe an on-the-fly kinetic study of gas-phase growth of carbon nanotubes. The methodology employs electrical mobility classification of the CNT, which enable a direct measure of CNT length distribution in an aerosol reactor. The specific experiment employs two mobility classification steps. In the first step we mobility classify the catalyst particle, in this case Ni, created by pulsed laser ablation, to generate a stream of monodisperse particles. This then determined the diameter of the CNT, when a hydrocarbon/ H2 mix is added in a heated aerosol reactor. A second electrical mobility classification step allows us to determine the length distribution of the CNT's. We found that CNT growth from ethylene required the addition of small amounts of water vapor, while growth from acetylene did not. We have speculated that this result is consistent with the fact that acetylene always has small amounts of acetone present when purchased, which can provide the oxygen source to prevent catalyst coking. By varying the temperature of the growth we were able to extract Arrhenius growth parameters. We found an activation energy for growth ~ 80 kJ mol-1 from both acetylene and ethylene, which is considerably lower than previous works for substrate grown CNT's (Ea=110~150 kJ mol-1). Furthermore we observed that our aerosol CNT growth rates were about two orders of magnitude higher than substrate grown CNT's. The dominant growth mechanism of CNT was previously proposed based upon bulk diffusion of carbon through nickel particles. However, on the basis of lower activation energy found in this approach, we proposed that the possible mechanism of gas-phase growth of CNT is correlated with both surface (Ea=29 kJ mol-1) and bulk diffusion (Ea=145 kJ mol-1) of carbon on nickel aerosol particles.