Laser-assisted production of multi-walled carbon nanotubes from acetylene

Article abstract of DOI:10.1016/S0009-2614(98)00975-0

A production method of carbon nanotubes using the catalytic decomposition of acetylene has been developed. Metal clusters are formed by laser vaporisation of solid nickel in situ in a hot reaction tube containing a mixture of acetylene and argon. The laser generated clusters act as catalysts for the growth of nanotubes. With this method multi-walled tubes were obtained with inner diameters of 3-10 nm and outer diameters of 10-100 nm. Transmission electron microscopy shows that the tubes are highly graphitic. The results obtained are explained by a growth model.

Full text of DOI:10.1016/S0009-2614(98)00975-0

23 October 1998
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Chemical Physics Letters 295 1998 525–530
Laser-assisted production of multi-walled carbon nanotubes from
acetylene
)
Henning Kanzow, Andrea Schmalz, Adalbert Ding
Technische UniÕersitat Berlin, Optisches Institut, Sekr. P 1-1, Str. d. 17. Juni 135, D-10623 Berlin, Germany
¨
Received 18 June 1998; in final form 20 August 1998
Abstract
A production method of carbon nanotubes using the catalytic decomposition of acetylene has been developed. Metal
clusters are formed by laser vaporisation of solid nickel in situ in a hot reaction tube containing a mixture of acetylene and
argon. The laser generated clusters act as catalysts for the growth of nanotubes. With this method multi-walled tubes were
obtained with inner diameters of 3–10 nm and outer diameters of 10–100 nm. Transmission electron microscopy shows that
the tubes are highly graphitic. The results obtained are explained by a growth model. q 1998 Elsevier Science B.V. All
rights reserved.
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1. Introduction
ved recently 16 . Using a laser vaporisation method,
tubes can be generated with a very high relative
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For at least four decades the chemical vapour
yield 17,18 . This technique allows the independent
controlling of most of the experimental parameters,
therefore permitting a good optimisation of the pro-
cess even with the highest laser power available.
However, the total amount of material evaporated is
much less than with the more common arc discharge
method.
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deposition CVD has been known as a technique
suitable for the production of filamentous carbon by
the catalytic decomposition of carbon monoxide or
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hydrocarbons on metal catalysts 1–12 . During the
past few years new methods for the production of
carbon fibrils have been developed which are based
on the co-vaporisation of pure carbon and a metal
We have combined the laser vaporisation tech-
nique with the CVD method. In our apparatus the
laser is used to vaporise the solid catalyst and gener-
ate catalytic metal aggregates in situ in the hot
reaction tube while the carbon is being introduced as
a gaseous compound. In this way it is possible to
achieve higher absolute yields, since the supply of
carbon is not limited by the vaporisation of solid
graphite. The growth of tubes occurred in the gas
phase as well as on the walls of the reaction tube.
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catalyst either in an electric arc discharge 13–16 or
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by high-power laser irradiation 17,18 . The fibrils
usually have an almost perfect graphitic tube struc-
ture. However, the samples produced with the arc
discharge method always contain a significant amount
of amorphous carbon together with the nanotubes
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13–15 , though significant progress has been achie-
)
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Most of the process parameters e.g., laser parame-
Corresponding author. E-mail: ding@physik.tu-berlin.de
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 98 00975-0

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H. Kanzow et al.rChemical Physics Letters 295 1998 525–530
ters, pressure, gas composition, flow, temperature,
from the target holder, suspended in acetone, toluene
or chloroform, and subsequently treated in a super-
sonic bath. The samples were examined with scan-
.
reaction time can be controlled independently.
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ning electron microscopy SEM, Hitachi S-2700 ,
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transmission electron microscopy TEM, Philips CM
200 FEG and energy-dispersive X-ray spectroscopy
2. Experimental
.
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EDX .
The experiments have been performed in a reac-
tion chamber originally designed for the laser vapori-
sation of graphite and modified according to Fig. 1.
3. Results
Two types of Nd:YAG laser arrangements were
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A nickel target 2 cm diameter, 99.2 % Ni was
attached to a water-cooled rod in order to suppress
unwanted surface reactions. It was placed in the
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centre of the quartz reaction tube 3.5 cm inner
used to generate nickel aggregates. The laser parame-
ters were not optimised with respect to the particle
size. From TEM pictures of our samples the size of
the nickel aggregates were estimated to be between
;1 and 100 nm. The reaction product removed from
the chamber usually contains ;30% multi-walled
nanotubes. The remainder consists of encapsulated
large nickel particles and amorphous carbon. Some
parts consist almost entirely of multi-walled nan-
otubes as shown in Fig. 2.
These tubes mainly had outer diameters of 10–40
nm, and in some cases 50–100 nm. Inner diameters
were between 3 and 10 nm. High-resolution TEM
clearly showed an almost perfect crystalline graphitic
structure of the inner walls, while the outer shells
.
diameter . The reaction chamber was pumped down
to 10y4 mbar for at least 15 min. After closing the
valve to the vacuum pump, the quartz tube was filled
with a mixture of argon and acetylene at a pressure
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of 300 mbar P Ar s260 mbar, P C2 H2 s40
mbar . The focussed laser beam ;1 mm focus
diameter of the high-power laser was scanned over
.
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the target in a circle using a rotating mirror. Experi-
mental results were obtained using two different
pulsed Nd:YAG lasers with ls1064 nm, ns20
Hz, pulse length 5 or 30 ns and pulse energy 700 or
1000 mJ. The laser was operated for 2–4 min gener-
ating highly excited nickel aggregates.
The reaction was stopped by switching off the
laser and pumping the reaction chamber down to
10y4 mbar. While cooling down, the reaction cham-
ber was kept under vacuum. The reaction products
were removed from the walls of the chamber and
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were partially amorphous Fig. 3 . Some of the tubes
had a nickel aggregate at the end of the tube. Others
had open ends, most likely due to the loss of the
catalyst particle. The size of the metal aggregates
Fig. 1. Experimental arrangement: water-cooled nickel target, pulsed Nd:YAG laser with ls1064 nm, ns20 Hz, pulse energy 0.7 or 1 J
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in 5 or 30 ns, respectively, focal lengths750 mm, oven temperature 7508C, acetylene–argon mixture 1:6.5 , pressure 300 mbar.

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H. Kanzow et al.rChemical Physics Letters 295 1998 525–530
527
Fig. 2. TEM image of the reaction product containing multi-walled carbon nanotubes, encapsulated nickel particles and some amorphous
carbon.
found at the end of the tubes were 15–25 nm for the
smallest dimension. They were elongated rather than
spherical. Sometimes the tubes were partially filled
with nickel. The particles incorporated in the tubes
varied between 5 and 10 nm in diameter. Small
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nickel clusters -5 nm were found to be embedded
Fig. 3. High-resolution TEM image showing the highly graphitic nature of the multi-walled carbon nanotube.

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H. Kanzow et al.rChemical Physics Letters 295 1998 525–530
in the amorphous carbon between the tubes. Large
mental results. The growth model describes the for-
mation of carbon nanotubes in four stages:
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particles )25 nm were encapsulated by a multi-
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layer graphitic skin.
1 Hydrocarbon molecules are adsorbed on dif-
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The large number experimental parameters oven
ferent parts of the surface of small metal aggregates.
temperature, laser parameters, pressure, metal, reac-
The different surfaces have different catalytic activ-
.
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tion time would require many experiments to opti-
ity due to their crystal orientation 7,20 Fig. 4a .
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mise the system. In the following we give some
preliminary quantitative results of this procedure:
lowering the temperature below 7008C resulted in a
very low product yield with a much higher relative
content of amorphous carbon. Higher temperatures
have not yet been studied. The laser focusing is less
critical. Changing the focus diameter of the laser
beam on the target surface in the range of 0.7–1.5
mm did not significantly influence the quantity and
quality of our product. However, if the nickel target
was placed precisely in the distance of the focal
length of the lens, the product yield decreased due to
the reduced amount of nickel evaporated under these
2 The active metal surfaces crack the carbon–
hydrogen bonds. The carbon diffuses into the bulk
material. For unsaturated hydrocarbons this process
is highly exothermic, increasing the surface tempera-
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ture of the catalytic active sites 5 Fig. 4b .
Ž .
3 When the saturation limit for carbon on the
cooler parts of the surface of the particle is reached,
the carbon is precipitated off the metal surface: this
is an endothermic process 5 . The resulting tempera-
ture gradient between the active, carbon dissolving
surface and the opposite carbon emitting side leads
to further carbon diffusion through the particle. The
relatively slow diffusion determines the overall reac-
tion rate. The only way to avoid the energetically
unfavourable dangling bonds of the carbon deposit is
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conditions. Lasers Nd:YAG, Eximer with the same
pulse energy but much longer pulse time e.g., 1 ms
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generated relatively large nickel droplets. Hardly any
carbon was deposited on their surfaces. Only two
alternative experiments to study the effect of the
acetylene pressure were done. An overall pressure of
300 mbar with a content of 200 mbar acetylene led
to polymerisation of the acetylene. When only a
to form a carbon tube with a closed cap Fig. 4c .
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4
If excess carbon on the surface does not
diffuse fast enough to build up the filament, the
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small amount of acetylene 20 mbar was used, the
product yield was low.
4. Discussion
The growth model for CVD grown carbon nano-
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tubes was adapted from Baker et al. 5 who found a
remarkable correlation between the activation ener-
gies directly measured for the growth of the carbon
fibrils and those for the diffusion of carbon through
the corresponding metals. His proposed
temperature-driven diffusion mechanism is strongly
supported by the experimental results of Yang and
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Yang 19 . They let different hydrocarbon gases
stream past one side of a thin nickel film and found
that the side of the carbon deposit formations was
dependent on the heat of decomposition of the hy-
drocarbon gases tested. Contrarily to our results, the
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nanofibres produced by Baker 5 were filled with
amorphous carbon. We made only small changes to
his model in order to describe most of our experi-
Fig. 4. Proposed mechanism of carbon nanotube formation.

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H. Kanzow et al.rChemical Physics Letters 295 1998 525–530
529
particle becomes completely encapsulated, stopping
tubes after their formation and carbonises further.
This explains the amorphous coating of our graphitic
multiple tubes. By changing the reaction gas from
acetylene to carbon monoxide it may be possible to
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further growth Fig. 4d .
Several studies have suggested that the inner di-
ameter of the nanofibres produced by the CVD
method is determined by the size of the catalyst
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suppress the polymerisation 12 , but even in this
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particles 9,12 . However, from our study, it seems
that this is only very roughly the case. Although
cluster sizes vary between 1 and 100 nm, the tubes
are generated in most cases with inner diameters of
3–10 nm. Our findings suggest rather that only
case we expect some amorphous product which is
inevitable if one has a plasma flame inside the
.
reactor . Further optimisation of laser and reaction
parameters should also lead to a higher quality of the
samples.
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particles of a certain narrow size range from 15 to
.
25 nm are active for the formation of nanotubes.
5. Conclusions
The longish form of the inclusions we find indicates
that the nickel particles are at least partially molten
during the growth of the tube took place. From our
experiment we cannot decide whether the fibre for-
mation starts from a molten metal droplet, or if the
exothermic nature of the reaction combined with the
freezing-point depression due to the solvating of
carbon in the metal causes the melting. Bearing in
mind that similar multi-walled carbon nanotube for-
mation can take place at temperatures as low as
We have reported a new laser method for the
production of carbon nanotubes by the catalytic de-
composition of acetylene. With this method, multi-
walled tubes were obtained with inner diameters of
3–10 nm and outer diameters of 10–100 nm. TEM
analysis shows a high degree of graphitisation of
these tubes. The results obtained are in good agree-
ment with a growth model developed by Baker et al.
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5 for the formation of carbon nonofibres.
3908C 3 , we did not address the melting by the
growth model we used.
The model restricts the range of catalyst particle
diameter where effective growth is possible. For
large particles the reaction is too slow due to the
long diffusion lengths. This explains why there is
almost no contribution of aggregates larger than 25
nm. These large particles become easily encapsulated
with excess carbon and completely inactive before
any fibre growth can start. The smaller the particles
are, the faster the growth rate should be. However,
Acknowledgements
This work has been funded by the German Min-
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istry of Science and Technology BMBF under con-
tract No. 13N6705. The SEM pictures were taken by
J. Nissen, Central Institute for Electron Microscopy
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ZELMI , Technical University Berlin. TEM was
performed by T. Link, Technical University Berlin,
and N. Pfander, Fritz-Haber-Institut Berlin. The lasers
¨
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very small metal particles -3 nm are also not
useful for the growth of carbon nanotubes. To avoid
dangling bonds during the growth process the whole
tube cross-section must be in contact with the metal
surface. However, tubes of very small diameters are
energetically unfavourable because of their inner
strain. This implies that there is a minimum particle
were provided by the Laser and Medicine Technol-
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ogy Berlin LMTB .
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