STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
PRODUCTION OF CARBON NANOTUBES BY THE CATALYSED VAPOUR
PHASE PYROLYSING PROCESS
Al. Darabont1, K. Kertész1, C. Neamţu2, Zs. Sárközi1, L. Tapasztó1,
L.P. Biró3, Z.E. Horváth3, A.A. Koós3, Z. Osváth3, Z. Vértesy3
Babeş-Bolyai University, Faculty of Physics, str.
Kogălniceanu nr.1, Cluj-Napoca, 3400, Romania
2
National Institute for Research and Development of
Isotopic and Molecular Technologies, 71-103 Donath St.,
P.O. Box 700, Cluj-Napoca, 3400, Romania
3
Hungarian Academy of Sciences, Research Institute for
Technical Physics and Materials Science, P.O. Box 49, H1525, Budapest, Hungary
1
ABSTRACT. This paper confirmes that single-walled carbon nanotubes
(SWCNTs) and multi-walled carbon nanotubes (MWCNTs), as well as
bundles of well aligned MWCNT films can be obtained simultaneously
by injecting a solution of ferrocene in benzene into a reaction furnace in
Ar atmosphere. We have also justified that using a solution of ferrocene
in thiophene, the reaction products contain Y-junction CNTs. There are
data concerning the home made experimental set-up used. The reaction
product was analysed on the basis of TEM, STM, FESEM and XRD
studies. Besides the CNTs, the reaction product contains as byproducts Fe
and Fe3C, which can be eliminated by the described purification process.
Some CNTs are partially filled with Fe-catalyst.
INTRODUCTION
Since the discovery in 1991 [1] of the carbon nanotubes (CNTs), they have
been promising candidates for various applications. Thus, they are effective
emitters for field emission due to he large current density at low threshold voltages
[2]. The small size and high toughness make them suitable for new advanced
scanning probes [3]. As CNTs are generally metallic or semiconducting, depending
on the helicity and diameter [4], they can be used to construct nanoelectronic
devices. In particular the Y and T shaped CNT junctions are considered likely to be
the basic building units for this purpose. The CNTs are up to 100 times stronger
than steel, able to withstand repeated bending, buckling and twisting, which results
in building lightweight metal-matrixes [5, 6] and polymer composites [7]. They
have also potential use as molecular pressure sensors [8] and chemical sensors [9].
There are many methods of producing CNTs, as electric arc discharge [10],
laser evaporation [11], chemical vapour deposition (CVD) [12, 13], plasmaenhanced CVD [14] etc. Amongst the CVD methods, pyrolysis of hydrocarbons in
the presence of a metal catalyst constitutes a simple and efficient process [15].
Generally, two methods are used to introduce the carbon source material into the
AL. DARABONT et al.
pyrolysis furnace: either as a vapour in a gas stream (Ar) [15] or by liquid injection
using an atomizer (sprayer) [16]. We used the latter method to produce CNTs at
laboratory level.
EXPERIMENTAL
The scheme of the home
made
experimental
spraypyrolysis set-up used for the
synthesis of CNTs is represented
in Fig. 1. This set-up uses the
single step synthetic route, which
involves the spray-pyrolysis of
ferrocene-benzene or ferrocenethiophene solutions in an Ar
atmosphere. The essential parts
are: a) an 1 meter long quartz tube
(the reactor) with ~20 mm inner
Fig.1. Scheme of the spray-pyrolysis set-up. 1diameter, which supports well the
gas inlet; 2-container for the solution; 31000ºC temperature, and at the
solution
flow-meter; 4-gas flow-meter; 5-Ar gas
same time plays the role of
cylinder; 6-alumina tube; 7-porcelaine tube; 8support for the catalyst particles
thermal and electrical insulation; 9-heating
resulting from the active solution
element; 10-quartz tube; 11-autotransformer;
and for the final product; b) a
12-Pt-Pt(Rh) thermocouple; 13-teflon tube; 14sprayer (atomizer) of the active
rubber tubing; 15- outlet to the air.
solution, which consists of a glass
nozzle (capillary) with 0.65 mm inner diameter at the end, surrounded by another
glass tube with 2 mm inner diameter also at the end. The outer glass tube of the
nozzle directs the carrier gas (Ar) flow around the nozzle. We consider that the
surface area between the inner- and outer tube has a decisive role in the spraying
process of the solution. In our case this area was 3.14 mm2.
At the beginning of each experiment the quartz tube is flushed with Ar, to
eliminate the oxygen from the reaction chamber. Then the tube is gradually
preheated to temperatures between 750–975ºC and the Ar flow-rate is set at the
desired value. The ferrocene-benzene or ferrocene-thiophene solution is introduced
into the sprayer and pulverized into the reaction chamber by the Ar gas flowing
around the nozzle. The flow-rate of the solutions is adjusted. The final product (the
carbonaceous material deposited on the wall of the reactor chamber) is analysed by
means of Transmission Electron Microscopy (TEM), Scanning Tunnelling
Microscopy (STM), Field Emission Scanning Electron Microscopy (FESEM) and
X Ray Diffraction (XRD).
We have performed growth experiments at different values of furnace
temperature, solution concentration, and solution flow-rate. The values of the
investigated parameters are resumed in Table 1. In addition, we investigated as
carbon source material also thiophene (sample S14), because in the literature this
PRODUCTION OF CNTS BY THE CATALYSED VAPOUR PHASE PYROLYSING PROCESS
material is known as promoting Y-junction growth of CNTs [17]. The parameters
of this growth process were: 1 ml/min solution flow-rate, 3 g ferrocene/ 50 ml
thiophene solution concentration, 875ºC.
RESULTS
Table 1
AND DISCUSSION
Investigated
parameters.
Systematic analysis of TEM images of
the samples makes
possible to determine
the most suitable values
of the investigated
parameters. These are:
875–925C temperature
range, ~1 ml/min ferrocene-benzene solu-tion flow-rate, ~3 g ferrocene in 50 ml
benzene catalyst concentration, while
in all experiments a constant Ar flowrate of 500l/h was maintained. TEM
and FESEM images of the most
characteristic samples are given in Fig.
1–3. The TEM images also indicate
that the samples, beside the CNTs,
contain impurities as: Fe, Fe3C, and a
small amount of amorphous carbon (as
byproducts). A considerable part of
a)
b)
byproducts may be removed by heating
Fig.1. Characteristic TEM images of
the samples first in dilute nitric acid for
sample S8 a) before and b) after the
several hours, than in distilled water,
purification treatment.
finally the samples are washed with
distilled water. The purification process
do not affect the catalyst particles
encapsulated in the CNTs (compare
Fig. 1 a) with b)). Fig. 2 (FESEM
image of sample S9) indicates that the
sample contains large areas of aligned
CNT films. The diameters of MWCNTs
decrease with increasing growth
temperature. It is probable, that the
SWCNTs visibles in STM appears as
the result of an overgrow of fine
Fig.2. Characteristic FESEM image of
particles and fibrous materials which
S9.
appears on the top of the aligned
nanotube films, and this overgrown material contains the SWCNTs [18]. Fig. 3
gives a characteristic TEM image of sample S14, obtained using thiophene as
AL. DARABONT et al.
carbon source material and ferrocene as catalyst. Thus, we have obtained Y-type
CNTs, which confirm that the presence of sulphur atoms promote the formation of
ramified CNTs [17].
Fig. 3. Characteristic TEM
image of sample S14.
CONCLUSIONS
The used spray-pyrolysis method yields
both MWCNTs and SWCNTs. The prepared
samples contain also bundles of well aligned CNTs.
Probable the SWCNTs are included in the
overgrown material which appears on the top of the
aligned CNT films. Pyrolysis of thiophene with
ferrocene yields Y-junction CNTs.
The purification of the samples in hot, dilute
nitric acid and in distilled water eliminates the
byproducts as Fe, Fe3C deposited between the tubes,
but the impurities encapsulated in the tubes are not
affected. The spray-pyrolysis method is a promising
technique for production of large quantities of
CNTs.
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