st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Synthesis of single-walled carbon nanotubes by induction thermal plasma using
different catalysts
J.-F. Carrier1, A. Hekmat-Ardakan1, G. Soucy1, B. Simard2
1
Dept of Chemical Engineering and Biotechnology Engineering, Université de Sherbrooke, J1K 2R1
2
National Research Council Canada, 100 promenade Sussex, Ottawa, K1A 0R6, Canada
Abstract: The use of cobalt as a catalyst has been identified as a potential health issue in the
synthesis of single-walled carbon nanotubes using ITP (induction thermal plasma) reactor.
Four alternative recipes of catalysts were compared to a ternary reference containing Co, Ni
and Y2O3. Thermodynamic model was initially used to screen the properties of the five
recipes. The results were coherent with TEM, TGA and Raman spectroscopy analysis. The
best alternative for the reference recipe was concluded to be a binary mixture of Ni and Y2O3.
The three other recipes gave lower yield.
Keywords: Single-walled carbon nanotubes (C-SWNT), thermal induction plasma, cobalt
1. Introduction
Since the discovery of carbon nanotubes, in 1991,
much research has been published and patented about
their synthesis methods. Among them, induction thermal
plasma (ITP) process is one of the interesting methods,
which is capable of producing a gram per minute of
product.
In the past years, the growing concerns about the
health and the safety issues of C-SWNT, has influenced
the approach of experimental research. These issues
comprise how to 1) reach similar production yield while
improving occupational safety and, 2) reduce the health
hazards associated with the material handling.
This could be partly done by reviewing the content of
the catalytic mixture used in the process. Although a
common catalyst is used in many synthesis methods, the
use of cobalt has been identified as a health concern [1].
minute of carbon and catalyst (feedstock). The plasma
and transport gases used are presented at Table 1.
Table 1. Operating conditions of the plasma and transport
gases.
Flow (slpm)
Species
Central gas
27
Ar
Sheath gas
180
He
Feeding (carrier)
5
Ar
gas
The reactor zone next to the plasma torch is thermally
insulated with a cylindrical graphite insert that helps to
maintain the temperature high enough for a longer section
in the system. This also augments the residence time of
the reactive intermediates, which occur in a specific
temperature range. A photo of the reactor is presented in
Fig. 1.
In this study, four different recipes of catalysts
without any Co content were compared to a reference
recipe containing (C + Ni + Co + Y2O3) [a]. The first
alternative was the same as original recipe without Co (C
+ Ni + Mo + Y2O3) [b]. The three others were made by
replacing Co with the following catalyst: zirconia oxide
(C + Ni + ZrO2 + Y2O3) [c], manganese oxide (C + Ni +
MnO2 + Y2O3) [d] and molybdenum (C + Ni + Mo +
Y2O3) [e].
2. Experimental Procedure
The plasma system used is a Tekna PL-50 induction
thermal plasma torch with a five-pass solenoid at 3 MHz
and 44 kW of power. The reactor is at a pressure of
66 kPa and operates at a feeding rate of about 2 grams per
Fig. 1 Picture of the induction thermal plasma reactor
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21 International Symposium on Plasma Chemistry (ISPC 21)
For all recipes, the mass fraction of the catalyst in
mixtures is maintained at 5.2 % followed by the addition
of carbon black (Monarch280) and yttrium oxide in a
mass percentage of 87.3 %, and 7.5 %, respectively. The
recipes are presented in more detail in Table 2.
Three analysis methods have been used in order to
investigate the quality of C-SWNT produced. Firstly,
transmission electronic microscopy (TEM) has been used
in order to compare the morphology of the specimens
produced by ITP. Secondly, thermo gravimetric analysis
(TGA) was performed to compare the relative content of
the different carbon allotropes and the mass distribution
of the products. Eventually, Raman spectroscopy has been
operated at three different wavelengths (514, 633 and
785 nm). It is used for identifying the relative content of
the different allotropes of carbon and the various type of
C-SWNT.
3. Results and Discussion
3.1 Thermodynamic computation
In order to evaluate the state properties and
equilibrium chemical composition of different reactants,
as they pass through the reactor, thermodynamic
computation software was used through Fact-Sage 6.4.
The two coupled databases chosen were FACTPS and
FSSTEL. FACTPS covers all the gas species present in
this study, across the temperature gradient, going to 6 000
K. while FSStel has a higher level of precision for the
content of the metallic catalysts in the both of liquid and
solid state.
The calculation illustrates the chemical composition of
the reference system containing 87.3 % C + 2.6 % Ni +
2.6 % Co + 7.5 % Y2O3 (wt. %) [a] throughout the
temperature gradient of the reaction. The calculation
shown in Fig. 2 reveals the important and sudden change
of the gas phases, at around 3 790°C, and the appearance
of solid carbon.
Temperature (°C)
Fig. 2 Thermodynamic computation calculated for the
reference recipe.
The next revealing feature is the transition of the
liquid phase composition, until the final cooling of all the
species. All four different mixtures have a somewhat
similar general composition profile. As it will be
discussed later, different recipes (with different metallic
catalysts) vary mainly in their liquid phase.
In order to get closer to the results, the profile
composition of different liquid phase is presented at
Fig. 3. It is observed that the liquid fraction and their
temperature formation differ noticeably for each recipe.
Some become liquid at higher temperature, Ni + ZrO 2 [c]
and Ni + Mo [e], while another stay liquid at a lower
temperature (Ni + MnO2 [d]). One interesting feature,
however, is the similarity between the reference mixture
with Ni + Co [a] and the one with only Ni [b].
log10 (mole)
Table 2. Content of the different feedstock mixtures.
Catalyst 1
Catalyst 2
Mass
Mass
Spec
fraction
Species fraction
ies
(%)
(%)
[a]
Ni
2.6
Co
2.6
[b]
Ni
5.2
[c]
Ni
2.6
ZrO2
2.6
[d]
Ni
2.6
MnO2
2.6
[e]
Ni
2.6
Mo
2.6
log10 (mole)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Temperature (°C)
Fig. 3
Thermodynamic computation of the liquid
phase of the five mixtures
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
log10 (mole)
The TEM results, presented in Fig. 5, show similarity
in the structure of the SWCNTs bundle. Single-walled
carbon nanotubes can also be observed alone. Some small
catalyst can be seen at the growing point of the nanotubes,
which is consistent with the V-L-S model.
Temperature (°C)
[a]
[b]
[d]
[e]
[c]
Fig. 4 Thermodynamic computation of the carbon
content in liquid phase of the five mixtures
Fig. 4 shows the difference in the carbon content in
liquid solution of each recipe. One will notice how the
graphic is similar to Fig. 3. This is because the carbon
content is expressed in absolute term and is therefore
correlated to the total amount of the liquid phase.
However in terms of C-SWNT synthesis, these two
figures indicate that some recipes have a heterogeneous
liquid phase. When compared to Ni + Co [a] and Ni [b],
all three others seem to have an uneven composition
across the phase’s temperature gradient. These are in fact
indications that the liquid phase and its dissolved carbon
are reacting into another compound (like carbide) than the
formation of SWCNT. It is put forward that these
reactions could enter in competition with the main
synthesis; suggesting a poorer yield of C-SWNT.
3.2 C-SWNT Synthesis
Three different characterization methods that exist for
the carbon nanotubes have been chosen to determine the
relative quality of C-SWNT: TEM for the morphology, a
thermo gravimetric analyser (TGA) for the mass
distribution of the products and Raman spectroscopy for
quantifying the relative content of carbon allotropes.
Fig. 5 TEM images of the five different mixtures
TGA 2050 Seteram was used for TGA test. The
operating conditions were a heating ramp of 10°C/min
with gas content of 20 % O2 and 80 % Ar at a flow of 40
Std cm3/min.
The results, as indicated in Fig. 6, show that the three
following recipes have a common TGA profile: [a], [b]
and [c]. The two others, [d] and [e], seem to have a
greater amounts of unreacted graphitic carbon due to the
higher mass percent from 560 to 670°C. The slope prior
to that initiated from 300 up to 670°C, is attributed to
amorphous carbon and C-SWNT, in that combustion
temperature orders [5].
Mass percent (%)
The role of these calculations is to help anticipating
the effect of different catalysts on the synthesis of singlewalled carbon nanotubes. According to the V-L-S model,
it is suspected that C-SWNT grows during the
solidification of the active catalysts [4]. Based on this
fact, the computation of their thermodynamic properties
can be used to estimate the quality of C-SWNT
synthesized by each recipe.
Temperature (°C)
Hitachi H-7500 set at 60 kV was used for the images
using transmission electron microscopy.
Fig. 6 TGA profile of the fixe mixtures
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
In order to quantify the relative content of the
different carbon allotropes, such as amorphous carbon
and C-SWNT, a final analysis instrument was used:
Raman spectroscopy. This method relies on the variation
of the inelastic photonic interaction with different carbon
allotropes.
The results of Raman spectroscopy at 633 nm
wavelength are presented at Fig. 7. One can immediately
observe the high G-line and G’-line values of [a] and [b],
which is associated to the E2g mode of graphite; an
indicator of the order the structure, hence the quality of CSWNT produced.[2].
G-line
Intensity
G’-line
RBM
D-line
Temperature (°C)
Fig. 7 Raman spectroscopy profile of Ni + Co [a]
The D-band (Disorder induced), around 1 340 cm -1, is
indicative of defects in the graphitic (C-SWNT)
structures. The RBM section, the Radial Breathing Mode,
is associated to the diameter of the nanotubes.
A frequent method to evaluate the final quality of the
produced C-SWNT containing samples is to compare the
intensity of the G-band versus the D-band. This is used as
an indicator of the crystallographic quality of the
products, coming from to preponderance of single-walled
carbon nanotubes [3] .
The results from Table 3 indicate that the mixture
with Ni [b], has the best quality of single-walled carbon
nanotubes followed by [a], the reference mixture with the
undesirable Co and Ni. In addition, as it was observed in
the all three analysis methods presented in this article, [a]
and [b] have similar results. It is noted that this similarity
was predicted during the thermodynamic computation
with FactSage where [a] and [b] had a somewhat similar
liquid phase profile. However the products with the best
quality seem to come from [b], with G/D ratios higher
than [a].The recipe [c], however, seems to stand at the
midpoint of the G/D results. After that, the quality rapidly
decreases at [d], with MnO2-Ni and [e], with Mo-Ni.
Table 3. G/D Ratio of the final products at 514, 633 and
785 nm
514 nm
633 nm
785 nm
[a]
13,3
3,5
4,7
[b]
17,7
3,9
6,6
[c]
8,2
2,7
4,5
[d]
5,2
2,7
5,0
[e]
4,7
2,2
2,6
4 Conclusion
The search for an alternative to toxic cobalt used as a
one of the ternary metallic catalysts mixture in the
synthesis of single-walled carbon nanotubes by induction
thermal plasma suggest that a binary mixture of Ni and
Y2O3 is suitable as an alternative to the ternary reference
mixture with Co, Ni and Y2O3.
The use of FactSage, as a thermodynamic computation
tool, enables to screen the most suitable alternative recipe.
The key element seems to be the similarity of the liquid
phase profile. A parameter that was not changed during
this study is the operating conditions of the reactor. It
would have been possible to operate at higher or lower
temperatures.
The next step would be to study the effect of different
operating conditions with some of the same catalysts that
were used in this article. The results given from the
thermodynamic computation suggest this could
significantly change the physical properties of the
reactants. Controlling the operating conditions, according
to the catalyst, could help increase the options for the
synthesis of C-SWNT.
Acknowledgements
This research was made possible by the help of
Raymor Nanotech inc. and the National Research Council
Canada (NRC).
References
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