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are the most common diluting/carrier gases. These inert gases are not
supposed to chemically participate in the decomposition of the carbon
precursor molecules. Interestingly, a recent study by Harutyunyan et al.
40
demonstrated that a particular inert gas can induce restructuring of catalyst
particles that enable a chiral-selective growth of nanotubes.
Whereas the surface chemistry of catalysts has been a major subject
for growth mechanism studies, recent reports unveiled the significant
effects of gas-phase reactions (GPRs) on growth kinetics and nanotube
structure. Several GPR products, such as benzene,
41
acetylene,
42-44
and vinyl
acetylene,
45
were found to be more ecient than the original methane
or ethylene carbon feedstock. These species showed higher reactivity to
accelerate the growth speed with improved growth yields. Meshot et al.
17
proved the striking effects of GPRs in a more obvious way by decoupling gas
heating from substrate heating of their cold-wall reactor.
This new aspect of the CVD growth of CNTs raises an important question:
what is the real (or most effective) carbon precursor? The answer should be
valuable not only for growth mechanism studies but also for the ecient use
of carbon gases and energy resources.
25
In a landmark study, Eres et al.
6
examined various precursor gases to compare their growth eciencies
(Table 3.1). They used a molecular jet growth setup at a relatively low tem-
perature (650 1C) to minimize possible GPRs before the precursor molecule
adsorbs on catalysts. They found that acetylene (C
2
H
2
) was the most ecient
carbon precursor among the selected gases. It is interesting that ethylene
(C
2
H
4
) and methane (CH
4
) gases could not yield CNTs in their growth con-
dition, although these gases have been widely used for nanotube growth but
at rather high temperatures. Indeed, acetylene has been commonly used for
low temperature growth of CNTs.
The most common CVD gases are supplied from gas cylinders and in-
evitably contain traces of gas impurities. In general, ultra-high purity (UHP)
grade gases contain ppm-level impurities such as water vapor, oxygen and
other process-related by-products. Gas impurity effects on the CVD growth of
CNTs had not drawn much attention before a recent report from In et al.
46,47
They found that even minute levels of gas impurities could have striking
effects on CNT growth kinetics; interestingly, the impurities significantly
enhanced growth yields. Especially, by using inline gas purifiers (deoxo
units, Figure 3.5A), they revealed that the growth enhancement was due to
oxygen-containing impurity molecules such as water (H
2
O), oxygen (O
2
), and
oxides of carbon (CO and CO
2
) as shown in Figure 3.5B.
From another point of view, this enhancing effect of gas impurities can
actually be utilized to promote growth. For instance, water is a well-known
growth promoter. A controlled amount (approximately 10-200 ppm) of a
water vapor mixed with other feedstock gases can dramatically improve
catalyst lifetime, growth speed, and graphitic qualities of nanotubes; this is
the essence of the so-called ''supergrowth'' process.
48,49
Many studies show
that there exists an optimal level of water as a growth promoter, and ex-
cessive water is deleterious to growth most
d
n
3
r
4
n
g
|
6
.
likely by oxidizing metal
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