Growth window and possible mechanism of millimeter-thick single-walled
  carbon nanotube forests
Microsoft Word - NodaSuper2pdf.doc
Growth window and possible mechanism 
of millimeter-thick single-walled carbon nanotube forests 
Kei Hasegawa1, Suguru Noda1,* , Hisashi Sugime1, Kazunori Kakehi1, 
Shigeo Maruyama2 and Yukio Yamaguchi1 
1 Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, 
Bunkyo-ku, Tokyo 113-8656, Japan 
2 Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, 
Bunkyo-ku, Tokyo 113-8656, Japan 
Corresponding author. E-mail address: noda@chemsys.t.u-tokyo.ac.jp 
Our group recently reproduced the water-assisted growth method, so-called "super growth", of millimeter-thick 
single-walled carbon nanotube (SWNT) forests by using C2H4/ H2/ H2O/ Ar reactant gas and Fe/ Al2O3 catalyst. 
In this current work, a parametric study was carried out on both reaction and catalyst conditions. Results 
revealed that a thin Fe catalyst layer (about 0.5 nm) yielded rapid growth of SWNTs only when supported on 
Al2O3, and that Al2O3 support enhanced the activity of Fe, Co, and Ni catalysts. The growth window for the 
rapid SWNT growth was narrow, however. Optimum amount of added H2O increased the SWNT growth rate 
but further addition of H2O degraded both the SWNT growth rate and quality. Addition of H2 was also essential 
for rapid SWNT growth, but again, further addition decreased both the SWNT growth rate and quality. Because 
Al2O3 catalyzes hydrocarbon reforming, Al2O3 support possibly enhances the SWNT growth rate by supplying 
the carbon source to the catalyst nanoparticles. The origin of the narrow window for rapid SWNT growth will 
also be discussed.  
Keywords: Single-Walled Carbon Nanotubes, Vertically Aligned Nanotubes, Combinatorial Method, 
Growth Mechanism, Growth Window 
1. INTRODUCTION  
 Single-walled carbon nanotubes (SWNTs) 
have unique mechanical and electrical properties, 
and many applications for them have been proposed 
and researched. To realize practical applications, 
mass production of SWNTs is essential, and various 
catalytic chemical vapor deposition (CCVD) 
methods have been developed to achieve this mass 
production. There are two types of CCVD; one 
involving nanoparticle catalysts suspended in the gas 
phase and the other involving nanoparticle catalysts 
supported on substrates. A gas-phase production 
process, the so-called "HiPco process", is the first 
process to be used in the mass production of 
SWNTs.1 Recently, remarkable progress has been 
made in CCVD processes using supported catalysts. 
Submicrometer-thick films of randomly-oriented 
SWNTs have been the typical product when 
supported catalysts are used. In 2003, vertically 
aligned single-walled carbon nanotubes 
(VA-SWNTs) were realized2 by using alcohol 
catalytic CCVD (ACCVD).3 VA-SWNTs have now 
been achieved using several CVD methods and 
conditions.4-7 As a result, CCVD from substrates 
now has potential as a process in the mass 
production of SWNTs. 
 Among those growth methods, the 
water-assisted method, so-called "super growth",4 
realized an outstanding growth rate of a few 
micrometers per second, leading to millimeter-thick 
VA-SWNTs forests. However, many research groups 
have failed in reproducing "super growth", and the 
underlying mechanism of the growth rate 
enhancement by water remained unclear. By using 
our combinatorial method for catalyst 
optimization,8,9 we recently reproduced the "super 
growth" method and showed the important role of 
catalyst supports.10 In this current work, by doing a 
parametric study, we report in detail the effect of the 
catalyst and reaction conditions determined, and 
discuss the novel mechanism essential for rapid 
growth VA-SWNTs.   
2. EXPERIMENTAL 
 Catalysts were prepared on SiO2 substrates 
by sputter-deposition. An Al2O3 layer was formed by 
depositing 15-nm-thick Al on a substrate and then 
exposing the layer to ambient air. Fe was deposited 
on Al2O3 layers or directly on SiO2 substrates. For a 
separate experiment, gradient-thickness profiles of 
Fe were formed by using combinatorial masked 
deposition (CMD) method previously described.9 
The catalysts were set in a tubular CVD reactor (22 
mm in diameter and 300 mm in length), heated to a 
target temperature (typically 1093 K), and kept at 
that temperature for 10 min while being exposed to 
27 kPa H2/75 kPa Ar at a flow rate of 500 sccm, to 
which H2O vapor was added at the same partial 
pressure as for the CVD condition (i.e., 0 to 0.03 
kPa). During this heat treatment, Fe forms 
nanoparticles of a certain diameter and areal density 
depending on the initial Fe thickness.8 After the heat 
treatment, CVD was carried out by switching the gas 
to C2H4/ H2/ H2O / Ar. The standard condition was 
8.0 kPa C2H4/ 27 kPa H2/ 0.01 kPa H2O/ 67 kPa Ar 
at 500 sccm at 1093 K for 10 min. The samples after 
CCVD were analyzed by using transmission electron 
microscopy (TEM) (JEOL JEM-2000EX), field 
emission scanning electron microscopy (FE-SEM) 
(Hitachi S-900), and micro-Raman scattering 
spectroscopy (Seki Technotron, STR-250) with an 
excitation wavelength at 488 nm. 
3. RESULTS AND DISCUSSION 
3.1. Standard condition of "super growth" 
Figure 1a shows a photograph of CNT 
samples grown by a combinatorial catalyst library 
under the standard condition. Millimeter-thick 
vertically aligned CNTs (VA-CNTs) were grown at 
regions where the Fe thickness was 0.4 nm or more. 
The maximum thickness of VA-CNTs was 1.2 mm at 
a Fe thickness of 0.5 nm. The thickness of VA-CNTs 
decreased when Fe thickness exceeded 0.5 nm. 
Figure 1b shows TEM images of CNTs 
grown under the same condition as Fig. 1a on 
substrates with uniform Fe thicknesses of 0.5 and 1.0 
nm. SWNTs with a diameter around 4 nm mainly 
grew for 0.5-nm-thick Fe catalyst, whereas thicker 
CNTs grew for 1.0-nm-thick Fe catalyst. This 
difference in CNTs is because a thicker initial Fe 
layer yields larger Fe nanoparticles,8 indicating a 
narrow VA-SWNTs growth window for the initial Fe 
thickness. 
 Figure 1c shows Raman spectra of the 
same CNT sample as Fig. 1a taken at Fe thicknesses 
of 0.5, 0.8, and 1.0 nm. Sharp, branched G-bands 
with a small D-band and peaks of radial breathing 
mode (RBM) were detected, indicating the existence 
of SWNTs. The G/D peak area ratios were smaller 
for thicker Fe layers (≥ 1 nm), because multi-walled 
CNTs (MWNTs) became the main product when a 
thicker Fe layer was used as catalyst. 
 These figures show that "super growth" 
was achieved in this work. The growth temperature 
of the standard condition of this work is higher than 
that of the original "super growth" 4 because both the 
CNT thickness and the G/D ratio increased with 
increasing growth temperature. 
3.2. Effects of catalyst metals and supports 
 Effects of catalyst metals and their 
supports were investigated next. Figure 2a shows 
top-view photographs of CNT samples grown on the 
Fe/Al2O3, Co/Al2O3, and Ni/Al2O3 combinatorial 
catalyst libraries under the standard condition. The 
0.5 0.8 1.0
Fe thickness / nm
2 mm(a)
0.5 0.8 1.0
Fe thickness / nm
2 mm(a)
RBM ×10
Fe 0.5 nm
Fe 0.8 nm
Fe 1.0 nm
1200 1400 1600
Raman shift / cm-1
100 200 300
RBM ×10
Fe 0.5 nm
Fe 0.8 nm
Fe 1.0 nm
1200 1400 1600
Raman shift / cm-1
100 200 300
20 nm
20 nm20 nm
Fig. 1 CNTs grown on the Fe/Al2O3 catalyst 
library under the standard condition (8.0 kPa 
C2H4/ 27 kPa H2/ 0.010 kPa H2O/ 67 kPa Ar at 
1093 K for 10 min) 
. (a) Side view photograph of CNTs grown on 
the combinatorial catalyst library. (b) Raman 
spectra of the same sample at Fe thickness of 
0.5, 0.8 and 1.0 nm. Intensity at the low 
wavenumber region (< 300 cm-1) was 10x 
magnified. (c) TEM images of CNTs grown on 
substrates with uniform Fe thickness of 0.5 nm 
(left) and 1.0 (right) nm. 
surfaces of both Fe/Al2O3 and Co/Al2O3 libraries 
became black at regions where the Fe thickness was 
0.4 nm or more. On the other hand, the surface of 
Ni/Al2O3 was somewhat darkened only at the 
relatively thin Fe region around 0.5 nm. Figure 2b 
shows cross-sectional SEM images of these samples. 
About 200-μm-thick VA-CNTs grew for 
0.5-nm-thick Co and about 0.4-μm-thick VA-CNTs 
grew for 0.5-nm-thick Ni.. 
 Figure 2c shows top-view photographs of 
combinatorial catalyst libraries of Fe/SiO2, Co/SiO2, 
and Ni/SiO2 after CVD under the standard condition. 
The results were completely different from those 
libraries with Al2O3 support layer (i.e., Fe/Al2O3, 
Co/Al2O3, and Ni/Al2O3); VA-CNTs did not form on 
any library and only part of the surface of Fe/SiO2 
for the thickness range from 0.4 to 0.6 nm darkened 
slightly compared with the library before CVD. As 
for Co/SiO2 and Ni/SiO2, negligible change appeared 
in color. Figure 2d shows the Raman spectra for the 
Fe/ SiO2 sample in Fig. 2b at Fe thickness of 0.5 nm, 
indicating growth of SWNTs. Fe catalyst supported 
on SiO2 can actually grow CNTs, including SWNTs, 
although the CNT yield is much smaller 
(submicrometer thickness or less) than that grown 
using catalysts supported on Al2O3 (up to millimeter 
thickness), regardless of the thickness of the catalyst.  
 Next, we examined if an Al2O3 catalyst 
support layer actually acts as Al2O3 rather than 
metallic Al. Another catalyst library was prepared by 
first depositing 15-nm-thick Al on SiO2, then 
exposing the layer to ambient air, then oxidizing the 
layer under air at 973 K for 5 min, and finally 
depositing a gradient thickness profile of Fe on the 
layer at ambient temperature. Figure 3a shows 
photographs of the Fe/ Al2O3 catalyst libraries before 
CVD. In the thick Fe region (≥ 2 nm), both libraries 
were metallic silver in color. In the thinner region (< 
2 nm), the Al layer only exposed to air was gray 
metallic in color, indicating the existence of metallic 
Al, whereas the Al layer oxidized at 973 K was 
completely transparent, indicating the complete 
oxidation of the Al layer. Then, CVD was carried out 
on these libraries. Figure 3b shows the photograph 
after CVD of the library with the Al layer oxidized at 
high temperature. The growth of the VA-CNT films 
was similar to that without high-temperature 
oxidation of the Al layer (Fig. 1a), indicating that the 
surface of the Al layer was oxidized by just being 
exposed to air, and that the layer acted as an Al2O3 
support.  
 Fe nanoparticles can grow SWNTs on 
either SiO2 or Al2O3 supports, but they need to be 
0.5 0.8 1.0
Metal thickness / nm
0.5 0.8 1.0
Metal thickness / nm
0.5 0.8 1.0
Metal thickness / nm
0.5 0.8 1.0
Metal thickness / nm
RBM ×10
1200 1400 1600
Raman shift / cm-1
100 200 300
RBM ×10
1200 1400 1600
Raman shift / cm-1
100 200 300
Fig. 2 Effects of catalyst metals and supports on 
CNT growth under the standard condition. (a,c) 
Top-view photographs of CNTs samples grown 
by combinatorial catalyst libraries prepared on 
Al2O3 (a) and SiO2 (c). All of the catalyst metals 
(i.e. Fe, Co, Ni) had the same thickness profiles 
between 0.2 nm (left) and 3 nm (right). (b) 
Cross-sectional FE-SEM images of the CNTs 
samples grown by 0.5-nm-thick Co and Ni 
catalysts prepared on Al2O3. (d) Raman 
spectrum of the CNTs samples grown by 
0.5-nm-thick Fe catalyst prepared on SiO2 
supports. Intensity at the low wavenumber 
region (<300 cm-1) was 10x magnified.  
supported on Al2O3 to achieve rapid growth of 
VA-CNTs at a rate of micrometers per second. In 
addition, Al2O3 support enhances the CNT growth by 
other catalysts (i.e., Co and Ni). Al2O3 support layer 
should have an essential role in growing CNTs. 
3.3. Effects of H2O and H2  
 Figure 4a shows the thickness profiles of 
CNTs grown on Fe/Al2O3 catalyst libraries under the 
standard condition, except that the amount of added 
H2O (i.e. partial pressure) was varied. Without H2O, 
CNTs grew only at the relatively thin Fe region (0.3- 
1.0 nm) and the maximum thickness of VA-CNTs 
was 0.7 mm at the 0.5 nm-thick Fe region. Addition 
of 0.010 kPa H2O enhanced the growth, especially at 
the thick Fe region (> 0.7 nm), and the maximum 
VA-CNT thickness increased to 1.0 mm at the 
0.5-nm-thick Fe region. Further H2O addition (0.030 
kPa) inhibited CNT growth at the thin Fe region 
(0.3- 0.6 nm) where SWNTs grew at the lower H2O 
partial pressures (≤ 0.010 kPa). Figure 4b shows the 
G/D ratios for each CVD condition at Fe thickness of 
0.5, 0.8, and 1.0 nm. Slight addition of H2O (0.010 
kPa) did not affect the G/D ratio at the thin Fe region 
(0.5 nm), but decreased the G/D ratio at the thicker 
regions (0.8 and 1.0 nm). Further addition of H2O 
(0.030 kPa) significantly decreased the G/D ratio at 
all regions. These results show that proper amount of 
H2O enhances the growth rate of CNTs, but 
excessive H2O decreases the growth rate of CNTs 
from small nanoparticles (i.e. small Fe thickness) 
and degrades the quality of CNTs possibly by 
oxidation. In conclusion, excess H2O totally inhibits 
"super growth" of SWNTs.  
 Figure 5a shows thickness profiles of 
CNTs grown under the standard condition except 
that the amount of added H2 (i.e. partial pressure) 
was varied. When a lower amount of H2 was added 
(2.7 kPa), VA-CNTs became much thinner at Fe 
thickness of 0.7 nm or less. When a large amount of 
H2 was added (54 kPa), CNTs grew at any Fe 
thickness at a reduced VA-CNT thickness (around 
0.4 mm). Figure 5b shows the G/D ratios for each 
CVD condition at Fe thickness of 0.5, 0.8, and 1.0 
0.5 0.8 1.0
Metal thickness / nm
0.5 0.8 1.0
Metal thickness / nm  
0.5 0.8 1.0
Fe thickness / nm
2 mm(b)
0.5 0.8 1.0
Fe thickness / nm
2 mm(b)
Fig. 3 Effect of the preparation process of Al2O3 
layer. (a) Photographs of Fe/Al2O3 catalyst 
libraries before CVD. Al2O3 layer was formed by 
oxidizing 15-nm-thick Al layer only by exposure 
to ambient air (top) and then by oxidizing in air at 
973 K (bottom). (b) Photograph of CNT grown 
on the Fe/Al2O3 library with Al2O3 layer oxidized 
at 973 K (same library as Fig. 3a, bottom).   
0.2 0.4 0.6 0.8 1T
Fe thickness / nm  
0.4 0.6 0.8 1.0
Fe thickness / nm
0 kPa
0.010 kPa
0.030 kPa
0.4 0.6 0.8 1.0
Fe thickness / nm
0 kPa
0.010 kPa
0.030 kPa
Fig. 4 Effect of H2O on CNT growth. CVD was 
carried out by using Fe/Al2O3 combinatorial 
catalyst libraries under the standard condition 
except the amount of added H2O (partial 
pressure) was varied. (a) Relationship between 
thickness of VA-CNTs and Fe thickness at 
different H2O pressures. (b) Relationship 
between G/D ratio of Raman spectra and Fe 
thickness at different H2O pressures.  
nm. The G/D ratios decreased in either case of lower 
(2.7 kPa) and higher (54 kPa) H2 was added. These 
results indicate that an optimum amount of H2 is 
needed for rapid growth of VA-SWNTs of relatively 
good quality. 
3.4. Possible mechanism of rapid growth of 
VA-SWNTs 
 Based on the results discussed above, we 
propose three necessary conditions for "super 
growth" of SWNTs. The first condition is that the Fe 
catalyst needs to be thin enough, about 0.5 nm, so 
that the catalyst nanoparticles are small enough to 
grow SWNTs with small diameters. The second 
condition is that the catalyst nanoparticles need to be 
supported on Al2O3. The third is that the partial 
pressures of both H2O and H2 need to be carefully 
adjusted; these gases are essential but excessive 
amounts degrade the growth rate and/or the quality 
of SWNTs. 
 Al2O3 and its related materials have long 
been used as catalysts for decomposition and 
dehydrogenation of hydrocarbons.11,12 Figure 6 
shows a schematic of our proposed mechanism 
explaining the enhancement effect of Al2O3 on CNT 
growth. In this mechanism, C2H4 or its derivatives 
adsorb on Al2O3, diffuse over Al2O3 surface to 
catalyst nanoparticles, and then are incorporated in 
Fe nanoparticles. In contrast, when Fe is deposited 
directly on SiO2, C2H4 can be decomposed only on 
Fe nanoparticles whose surface is largely covered 
with growing CNTs. As a result, the growth rate of 
CNTs on Al2O3 is much faster than that on SiO2. In 
addition, the catalytic activity of Al2O3 strongly 
depends on its crystalline structure.11 α-Al2O3, the 
most stable phase, has low activity,11 and thus, rapid 
CNT growth does not occur when catalysts are 
supported on sapphire (i.e. monocrystalline 
α-Al2O3). 
 Next, we discuss why the window of 
"super growth" for SWNTs is narrow. One reason 
might be due to the catalyst deactivation mechanism, 
i.e. coking of either Al2O3 surface or Fe 
nanoparticles. In the dehydrogenation process of 
hydrocarbons,11 Al2O3 easily loses its catalytic 
activity due to carbon deposition, and thus, H2O 
vapor has long been used to remove carbon 
byproducts. H2 also keeps the Al2O3 surface reactive 
by controlling the balance between dehydrogenation 
and hydrogenation of carbon surface species, as is 
known in the hydrocracking process.13 In contrast, 
excessive H2 inhibits the growth of SWNTs by 
hydrogenating C2H4-derivatives adsorbed on Al2O3. 
Concerning Fe nanoparticles, when the incoming 
flux of carbon into Fe nanoparticles increases, 
carbon in Fe nanoparticles will be highly 
supersaturated, resulting not only in increased 
SWNT growth rate but also in graphite formation on 
the surface of the nanoparticles. The fewer walls and 
larger free energy of SWNTs than MWNTs may 
0.2 0.4 0.6 0.8 1
Fe thickness / nm
27 kPa
2.7 kPa
54 kPa
0.2 0.4 0.6 0.8 1
Fe thickness / nm
27 kPa
2.7 kPa
54 kPa
27 kPa
2.7 kPa
54 kPa
0.4 0.6 0.8 1
Fe Thickness / nm
27 kPa
2.7 kPa
54 kPa
0.4 0.6 0.8 1
Fe Thickness / nm
Fig. 5 Effect of H2 on CNT growth. CVD was 
carried out by using Fe/Al2O3 combinatorial 
catalyst libraries under the standard condition 
except the amount of added H2 (partial pressure) 
was varied. (a) Relationship between thickness 
of VA-CNTs and Fe thickness at different H2 
pressures. (b) Relationship between G/D ratio of 
Raman spectra and Fe thickness at different H2 
pressures. 
SiO2 Al2O3
C2H4 C2H4
SiO2 Al2O3
C2H4 C2H4
Fig. 6 Schematic of enhancement mechanism of 
SWNT catalytic growth by Al2O3 support. 
make the degree of supersaturation larger for 
nanoparticles growing SWNTs than those growing 
MWNTs. This may be the reason why the "super 
growth" window for SWNTs is such narrow 
compared with MWNTs. (Note that MWNTs grow 
rapidly under a wide window of reaction conditions 
if catalyst nanoparticles are supported on Al2O3). In 
conclusive, two conditions are needed to sustain the 
rapid growth of SWNTs: first, the partial pressures of 
C2H4, H2, and H2O need to be balanced to suppress 
coking of Al2O3, and second, the incoming flux of 
carbon into Fe nanoparticles must not be too large, 
that means nanotubes must not grow too rapid, to 
prevent carbonization of Fe nanoparticles.  
4. CONCLUSION 
 Rapid growth of VA-SWNTs, namely, a 
few micrometers per second, or so-called "super 
growth", was reproduced in this study, and the 
growth window was clarified. The standard 
condition of this work was 8.0 kPa C2H4/ 27 kPa H2/ 
0.01 kPa H2O/ 67 kPa Ar at 500 sccm at 1093 K for 
10 min using a tubular CVD reactor (22 mm in 
diameter and 300 mm in length). Results showed that 
for the rapid growth of SWNTs, small nanoparticles 
formed from a thin Fe layer (about 0.5 nm) need to 
be supported on Al2O3 and that only the optimal 
amounts of H2O and H2 should be used. We 
proposed a novel mechanism by which Al2O3 
enhances the growth rate of SWNTs, and offered a 
simple explanation of the effect of H2O and H2 on 
the growth rate. Namely, Al2O3 supports supply a 
carbon source to Fe nanoparticles, and H2O and H2 
prevent catalyst deactivation by keeping the Al2O3 
surface reactive and by balancing the carbon fluxes 
among the gas-phase, Al2O3 support, Fe 
nanoparticles, and growing SWNTs.  
Acknowledgements:  
The authors thank Z. Zhang for her help in the 
Raman measurements. This work is financially 
supported in part by the Grant-in-Aid for Young 
Scientists (A), 18686062, 2006, from the Ministry of 
Education, Culture, Sports, Science and Technology 
(MEXT), Japan.  
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