Chemistry Reference
In-Depth Information
that catalysts enhance growth yields remarkably. The best way to understand
the role of the catalyst is to consider the energetics of carbon-carbon or
carbon-hydrogen bonding. When hydrocarbon molecules are used as a
carbon feedstock, the overall chemical reaction of nanotube growth can be
described by the following expression:
d
n
3
r
4
n
g
|
6
C
x
H
y
!
xC
NT
þ
y
2
H
2
ð
g
Þ
(3
:
2)
where the subscript NT stands for a nanotube. When methane (CH
4
) gas is
used as a carbon source, four C-H bonds need to break. However, the energy
of each bond is too high (439 kJ mol
1
) to be overcome by the pure thermal
energy of CVD. At general CVD temperatures, the feedstock dissociation and
carbon polymerization in a gaseous state are very limited; researchers found
only ppm levels of polyaromatic hydrocarbon (PAH) products from such
pyrolysis.
25
With a catalyst, however, adsorbed precursor molecules undergo
multiple dissociation steps. Not only did the energetic modification of
catalysis help the dissociation of the adsorbed precursor molecules, but also
the electron-abundant surface of a metal catalyst facilitates carbon poly-
merization by holding the carbon radicals for an extended residence time.
Catalyst nanoparticles also play a crucial role in determining the
morphologies of the resulting nanotubes. The curvature of a nanoparticle
surface provides favorable nucleation sites to overcome the strain energy
necessary for hemispherical nanotube cap formation. Indeed, CVD techni-
ques for graphene growth use a flat metal foil of copper or nickel as catalytic
substrates.
26,27
The confined domain size of a catalyst nanoparticle has
significant correlation with the diameter of the nanotube grown from that
catalyst. Yamada et al.
28
conducted a systematic study to explore correlation
of particle size (or catalyst film thickness) with diameter, and number of
walls. The data clearly revealed a straightforward relation: larger catalysts
produce nanotubes with larger diameters and more walls. Lin et al.
29
also
observed by in situ TEM that the diameter ratio of a nanotube to its catalyst
particle falls within a finite range (0.5-1).
When it comes to selection of a catalyst composition, each element shows
unique characteristics depending on the detail of the chemical reaction,
operation temperatures and pressures, support materials, etc. In general,
carbide-forming transition metals such as Fe, Co and Ni make the most
ecient catalytic materials for CNT growth. Binary alloys of these metals
also catalyze nanotube growth. A co-catalyst is not an active catalyst by itself,
but it can contribute to enhancing growth activity when combined with
active (Fe, Co, Ni) catalysts. For instance, molybdenum (Mo) is widely used
as a co-catalyst for SWNT growth. While the definitive role of an auxiliary Mo
catalyst remains elusive, several publications proposed that Mo helps the
main catalysts by promoting carbon delivery to active catalyst particles,
30,31
preventing silicate formation on a silicon oxide support,
32
or improving
dispersion of catalyst particles.
10,33
.
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