Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 40

the atomic form in a nanotube. Therefore, we start by assuming that the
catalyst nanoparticle is in its ''ready for growth'' state.

3.2.2.1 CVD Growth Process
Regardless of the exact types of process gases and catalysts, nanotube growth
by CVD shares several common mechanistic steps (Figure 3.6). A controlled
amount of carbon feedstock enters the process vessel and undergoes gas-
phase reactions depending on the thermal environment of the reactor. Then
the produced active carbon species are delivered to the gas-catalyst interface
by gas-phase diffusion as well as by convective flow of the gases. The arriving
molecules adsorb to the catalyst surface and subsequently dissociate into
carbon adsorbates. The hydrogen products desorb back into the gas stream.
Now the carbon adatoms diffuse through the catalyst to the open end of the
nanotube, or nucleate into a graphitic cage to initiate growth if there is no
pre-existing nanotube.

d n 3 r 4 n g | 6

3.2.2.2 Gas-phase Reactions
Modern CVD literature contains ample proof that gas-phase reactions (GPRs)
have significant effects on deposition kinetics and film morphology. 53 GPRs
(or pyrolysis) of light hydrocarbons are not negligible at CNT growth tem-
peratures, 54,55 and their profound effects on the growth of CNTs are critical
for understanding the growth mechanism. As mentioned before, recent lit-
erature has revealed the existence of ecient pyrolysis products for CNT
growth. However, pyrolysis of hydrocarbon species involves many elem-
entary steps comprising initiation, radical propagation, production of
intermediate species, and reaction termination with final products. Un-
fortunately, the complicated self-pyrolysis chemistry is also coupled with
fluid dynamics of the gas flow and mass transport of the products to the

.

Figure 3.6 Main stages of the CNT growth process.