Biomedical Engineering Reference
In-Depth Information
9.3.3
Chemical Vapor Deposition
This synthesis technique utilizes small metal catalyst nanoclusters
in the gas phase or on substrate surfaces to decompose a carbon-
containing feedstock gas, such as methane (CH
), acetylene (C
H
),
4
2
2
ethylene (C
), carbon monoxide (CO), or their combination in a
proper gas mixture. The resulting carbon atoms dissolve in, or are
adsorbed on, the catalyst particles and are released in the form of a
nanotube starting with a buckyball-type cap when the concentration
exceeds the maximum solubility. The nanotube continues to grow
as long as carbon continues to be delivered at the right rate and the
form of catalyst does not change. The growth temperature depends
on the type of nanotube to be grown, the catalyst composition and
size, and lies in the range 400-1100°C, lower than temperatures in
the arc discharge or laser ablation processes. For this reasons, it is
believed that CVD nanotubes have a higher density of defects.
The advantage of CVD nanotube production is the possibility of
structuring the catalyst particles, and, hence, selectively growing
the nanotubes where they are required. Moreover, under the right
experimental conditions, only nanotubes are fabricated and minor
unwanted graphitic material.
The general nanotube growth mechanism in a CVD process
involves the dissociation of hydrocarbon molecules catalyzed by the
3d transition metal (Co, Fe, Ni, etc.), and dissolution and saturation
of carbon atoms in the metal nanoparticles. The precipitation of the
carbon atoms from the saturated metal particle leads to the formation
of the tubular carbon solids in sp
H
2
4
2
structure. The tube formation is
favored over other forms of the carbon such as graphitic sheets with
open edges. This is because a tube contains no dangling bonds and
therefore is in a low-energy form. For MWCNT growth, most of the
CVD methods employ ethylene or acetylene as the carbon feedstock
and the growth temperature is typically in the range of 550-750°C.
Iron, nickel, or cobalt nanoparticles are often used as catalyst. At
high temperatures, carbon has finite solubility in these transition
metals, which leads to the formation of metal-carbon solutions and
therefore the previously mentioned growth mechanisms. Extensive
literature on the CNTs growth mechanisms including experimental
and theoretical aspects has been reviewed in various reports
[98, 123, 124].
