INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 16 (2005) 532–535
doi:10.1088/0957-4484/16/4/033
Effect of temperature, pressure, and gas
ratio of methane to hydrogen on the
synthesis of double-walled carbon
nanotubes by chemical vapour deposition
G Y Xiong1 , Y Suda2 , D Z Wang1 , J Y Huang1 and Z F Ren1,3
1
Department of Physics, Boston College, Chestnut Hill, MA 02467, USA
Division of Electronics and Information Engineering, Graduate School of Engineering,
Hokkaido University, Sapporo 060-8628, Japan
2
E-mail: [email protected]
Received 5 November 2004, in final form 23 December 2004
Published 21 February 2005
Online at stacks.iop.org/Nano/16/532
Abstract
Double-walled carbon nanotubes (DWCNTs) were synthesized by chemical
vapour deposition using Fe–Mo catalyst supported on magnesium oxide
particles. A mixture of gases of methane and hydrogen was used for the
growth. The effect of gas pressure, ratio of methane to hydrogen, and
growth temperature on the structure, purity, and yield of DWCNTs has been
studied in detail. Transmission electron microscope studies revealed that the
growth temperature is the crucial factor determining the size of the catalyst
particles. Scanning electron microscope studies were also carried out to
provide information on the purity and diameter of DWCNTs synthesized
under various conditions.
1. Introduction
Ever since the first observation of double-walled carbon
nanotubes (DWCNTs) [1], they have received great attention
because of their unique structure [2–4] and promising
applications in microelectronic devices such as field
emitters [5]. In this report, the chemical vapour deposition
(CVD) method has been employed to synthesize DWCNTs
on Fe–Mo catalyst [6] supported on magnesium oxide (MgO)
particles.
Many growth conditions affect the CVD growth of carbon
nanotubes (CNTs). Among them, the growth temperature, gas
pressure, and ratio of the carbon source gas to the carrying gas
are the most important factors in determining the structure,
purity, and yield of multi-walled carbon nanotubes [7, 8]. In
the present report, we have studied how these three factors
affect the growth of DWCNTs.
2. Experimental details
The experiments were carried out in a horizontal tube furnace
with a quartz tube of 2 inch diameter. The catalyst used
3 Author to whom any correspondence should be addressed.
0957-4484/05/040532+04$30.00 © 2005 IOP Publishing Ltd
in this study is Fe–Mo nanoparticles supported on MgO
particles. MgO powders are used as catalyst support because
they have a relatively large surface area and are porous [9].
To load the Fe–Mo catalyst onto the MgO powder, first, a
certain amount of iron sulfate heptahydrate (FeSO4 ·7H2 O) was
dissolved in 100 ml distilled water by magnetic stirring for
about 30 min, then a certain amount of ammonium molybdate
((NH4 )6 Mo7 O24 ·7H2 O) was added, and the mixture was
stirred for 2 h. Second, MgO powder was immersed in the
solution by sonicating for 30 min and then stirred for 3 h to form
a uniform suspension. Third, the suspension was continuously
stirred and dried at 80 ◦ C on a heated plate until the water was
completely vaporized, followed by baking at 130 ◦ C for 15 h
to eliminate any remaining moisture. Finally, the dried bulk
catalyst was ground into fine powder in a mortar. The final
catalyst has a planned atomic ratio of Fe/Co/MgO = 5:1:60.
For DWCNT growth, a two-step procedure was used.
After the catalyst (100 mg) was spread onto a Mo boat, it
was first reduced at 820–900 ◦ C for 10 min in flowing gas of
H2 (100 sccm) at a pressure of 100 Torr to generate Fe–Mo
nanoparticles on the supporting MgO particles, immediately
followed by growth at 820–900 ◦ C for 1 h in flowing gases of
Printed in the UK
532
Effect of temperature, pressure, and gas ratio of methane to hydrogen on the synthesis of DWCNTs by CVD
Scanning electron microscopes (SEM, JEOL JSM-6340F and
JEOL JSM-7400F) and a transmission electron microscope
(TEM, JEOL 2010F) were used to characterize the catalyst
and CNTs.
3. Results and discussion
Figure 1. High-magnification backscattering SEM image of Fe–Mo
catalyst supported on MgO (reduced at 860 ◦ C for 10 min with
flowing 100 sccm H2 gas at 100 Torr) to show the distribution of
Fe–Mo nanoparticles. The bright spots represent nanosized Fe–Mo
particles, whereas the background is porous MgO particles.
H2 and CH4 with different ratios at various pressures of 60–
120 Torr. The amount of product (defined as the net weight
gain divided by the sum of the remaining catalyst and MgO
support) was carefully monitored with the growth conditions.
Figure 1 shows the backscattering SEM image of the Fe–
Mo catalyst reduced at 860 ◦ C for 10 min with flowing
100 sccm H2 gas at 100 Torr. The background is porous
MgO particles. The bright spots represent nanosized Fe–Mo
particles in the range of 2–10 nm measured under TEM in
an independent experiment. It is clearly shown that after the
reduction procedure lots of nanoparticles (bright spots) were
generated and distributed fairly uniformly on the supporting
MgO particles.
To study the effect of growth temperature on catalyst
particle formation, we preformed several growths without
supplying carbon-containing gas. Figures 2(a)–(c) show the
TEM images of the catalysts reduced at temperatures of
820, 860, and 900 ◦ C for 10 min with flowing 100 sccm H2
gas at 100 Torr, respectively. Clearly, at 820 ◦ C the Fe–
Mo particles were just starting to form (figure 2(a)). At
860 ◦ C, lots of nanosized (2–5 nm) catalyst particles were
formed (figure 2(b)). At 900 ◦ C, nanosized catalyst Fe–Mo
particles have agglomerated into relatively large (8–10 nm)
Figure 2. TEM images of Fe–Mo catalyst on MgO after being reduced at 820 ◦ C (a), 860 ◦ C (b), and 900 ◦ C (c) for 10 min with flowing
100 sccm H2 gas at 100 Torr.
533
G Y Xiong et al
Figure 3. The amount of product as a function of growth
temperature with different gas pressures of 60 Torr (), 80 Torr ( ),
100 Torr (), and 120 Torr ( ).
•
particles (figure 2(c)). It is well known that the size of catalyst
particles decides the diameter of CNTs [1, 10–12]. Using
TEM we demonstrated that the same diameter controlling
mechanism works: growth temperature affects the diameter
of the grown CNTs. It was found that the optimal growth
temperature for the present catalyst is about 860 ◦ C based on
the CNT diameter and amount of product.
Figure 3 shows the amount of product at different growth
temperatures and gas pressures with a fixed flowing gas of
CH4 (300 sccm)/H2 (6 sccm). It is clear that higher growth
temperature results in higher amount of product. The same
is true for the gas pressure. The parameters including growth
temperature, gas pressure, and ratio of methane to hydrogen
not only have great effects on the amount of product, but
also on the structure and purity of DWCNTs. There were
few DWCNTs when the temperature was lower than 800 ◦ C,
where the weight gain (figure 3) is mostly amorphous carbon.
When the temperature was higher than 900 ◦ C, there were few
DWCNTs too. At high temperature, the main part of the weight
increment is amorphous and other forms of crystalline carbon.
In general, during the growth, methane molecules
decompose at the catalyst particles’ surface, with carbon
dissolving into the catalyst particles; the dissolved carbon
atoms diffuse either on the surface or through the bulk of the
Fe–Mo particles upon supersaturation. The DWCNTs are then
formed by precipitation of carbon atoms on the other side of the
catalyst particle [13–16]. When the concentration of methane
is too high, meaning more carbon source is provided than is
needed, the decomposition rate of the carbon source is higher
than the precipitating rate of carbon, which results in oversupply of carbon in the form of amorphous and/or crystalline
carbon. In our experiment, the gas pressure inside the chamber
determines the concentration of methane. Our result proves
that higher gas pressure leads to abundant amorphous carbon,
which is illustrated in the weight increment as shown in
figure 3.
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Figure 4. SEM images of DWCNTs grown at 860 ◦ C and 100 Torr
with various ratios of hydrogen to methane. The ratio is (a) 0 sccm
H2 and 300 sccm CH4 , (b) 6 sccm H2 and 300 sccm CH4 , and
(c) 12 sccm H2 and 300 sccm CH4 .
With TEM studies, we found a rapid increase of
amorphous carbon when the gas pressure exceeds 100 Torr.
With the temperature fixed at 860 ◦ C and pressure at 100 Torr,
different combinations of methane to hydrogen (300/0, 300/6,
and 300/12) were studied to see how this affects the growth
of DWCNTs. Figure 4 shows the SEM images of DWCNTs
grown at different ratios of methane to hydrogen. It is clearly
shown that the concentration of hydrogen obviously affects
DWCNT growth. There were few DWCNTs when 300 sccm
methane without any hydrogen was used (figure 4(a)), but lots
of DWCNTs when 300 sccm methane and 6 sccm hydrogen
were supplied (figure 4(b)). When the hydrogen flow was
increased to 12 sccm, the DWCNT growth (figure 4(c)) turns
out to be less than that in figure 4(b). After growth we checked
the samples using high resolution TEM. Figure 5(a) shows a
bundle of CNTs grown at the optimal conditions of 860 ◦ C,
100 Torr, and 300/6 sccm (CH4 /H2 ). Figures 5(b) and (c) are
the enlarged areas indicated in figure 5(a). It is clearly shown
that most of the CNTs are DWCNTs.
Effect of temperature, pressure, and gas ratio of methane to hydrogen on the synthesis of DWCNTs by CVD
carbon nanotubes, mainly three- and/or four-walled CNTs.
Our explanation here is that hydrogen acts as ‘the promoter
of the catalyst’. Hydrogen helps to keep the activity of the Fe–
Mo catalyst during the growth. When there is no hydrogen, the
catalyst loses activity rapidly and few DWCNTs grow. With
hydrogen being supplied, the catalyst keeps active so that the
decomposition, diffusion, and precipitation of carbon atoms
work smoothly.
4. Conclusions
In summary, we have demonstrated how the growth parameters
affect the synthesis of DWCNTs using the CVD method. The
catalyst reducing temperature determines the average size of
the final CNTs. The gas pressure and growth temperature
dictate the purity and amount of product. The hydrogen
concentration determines the final structure of the product.
Acknowledgments
The work is supported by NSF NIRT 0304506 and by DOE
DE-FG02-00ER45805.
References
Figure 5. (a) TEM images of an as-synthesized DWCNT bundle,
(b) HRTEM image of a bundle of two DWCNTs, and (c) HRTEM
image of a bundle of a few DWCNTs.
Under TEM and high magnification SEM, we observed
that the diameter of DWCNTs increased when the
concentration of hydrogen increased. As hydrogen further
increased above 10 sccm, we started to observe multi-wall
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