22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Diagnostics of arc plasma generated in He between graphite electrodes under
the presence of ethanol vapor
H. Lange, O. Łabędź, M. Bystrzejewski and A. Huczko
Faculty of Chemistry, University of Warsaw, ul. Pasteur1, PL-02093 Warsaw, Poland
Abstract: The emission and absorption spectra originating from the transition between
d 3Π g and a 3Π u (0,0) electronic states were used for determination of temperature and C 2
column density distributions in the DC arc discharge between graphite electrodes in He and
ethanol vapour. The tests aimed at synthesis of graphene-like structures. The products were
characterised by electron microscopy.
Keywords: plasma spectroscopy, emission, absorption, self-absorption, graphene
1. Introduction
The carbon arc route has been used for fullerene [1],
carbon nanotube [2, 3] and carbon-encapsulated magnetic
nanoparticles [4] synthesis for at least for 20 years. For
several years a great attention has been paid to graphemelike structures which can be synthesized, e.g., using
plasma enhanced CVD technique [5] and thermal plasma
jets [6, 7]. In the latter two works ethanol was
decomposed and graphene-like structures were found in
the products. This study has a similar goal, however
carbon arc plasma between graphite electrodes under the
presence of ethanol vapor was investigated. The work is
mostly devoted to the plasma diagnostics which
accompanied on-line the graphene arc synthesis. The
attention was focused on the local mapping of the
temperature and C 2 radical content in the plasma zone.
Despite of a high variety of species from atoms, ions and
simple molecules in the interelectrode gap to the complex
carbon clusters at the arc periphery only few of them can
be easily detected in the optical spectral window by
conventional emission and absorption techniques. Thus,
among them C 2 radical is considered to be one of the
main precursors of carbon nanostructures. Additionally,
this specie possesses strong transition between the ground
a 3Π u and excited d 3Π g electronic states which enables
the application of the emission as well absorption
techniques for plasma diagnostics, OES and OAS,
respectively. Also, preliminary characterization of the
solid products was performed by TEM and SEM.
2. Theory
The diagnostics by OES was performed using the selfabsorption phenomenon, while OAS was associated
with classical measuring of continuum source absorption.
Both ways have already been used before and are
described in details elsewhere [8, 9]. Nevertheless, we
highlight here the most important points.
2.1. Optical emission approach (OES)
First of all, in this method the homogeneous C 2 radical
concentration, N, and temperature distributions along a
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plasma column of length L, is assumed. In consequence,
the following formula was used, which connects the
spectral intensity (I λ ) with the spectral emission (ε λ, ) and
absorption (κ λ ) coefficients:
Iλ =
ελ L
(1 −e −κ λ L )
κλ L
(1)
The spectra C 2 (d 3Π g – a 3Π u , ∆v = 0) are calculated for
the discrete wavelengths separated by 1.6 x 10-4 nm and
assuming Voigt profile with the damping constant equal
to 1. The as-computed spectra are then convoluted with
the apparatus function to be Gaussian with the full line
width equal to 0.023 nm. Then, a large number of spectral
points is reduced to match the bins on the CCD detector.
It was shown that the direct fit of the normalized, against
the band head, experimental spectra affected by the
self-absorption to the computed spectra, can provide the
average rotational temperature and column density (NL)
[9].
2.2. Optical absorption approach (OAS)
As in the case of OES, the emission spectra the
absorption spectra have been computed for different
temperatures and column densities, and also assuming
homogeneous absorber distributions along an isothermal
column
Aλ = − ln Tλ = kλ L
(2)
A λ and T λ are the absorbance and transmittance,
respectively.
Afterwards, the temperature (obtained from Boltzmann
plots) and absorbance at the band head allow to deduce
the column density using the dependence of the computed
absorbance growth curves of C 2 (a-d, 0-0) band head on
column density for different temperatures.
Neither the temperature nor the radical concentration is
constant along any arc plasma column. Therefore, the use
of synthetic spectra calculated for uniform plasma leads to
values of column density and average rotational
temperature of a limited accuracy. The discussion on the
1
Absorbance
3. Experimental
The homogenous graphite electrodes, 6 mm in
diameter, were used for experiments. The experiments
were conducted under He atmosphere at presence of
C 2 H 5 OH vapour and at the constant pressure of 60 kPa.
The liquid ethanol was placed 13 cm below the
horizontally situated electrodes (Fig. 1). The discharge
current was 30, 40 and 60 A and the voltage drop was
between 20 and 27 V. The reactor and experimental
system for spectroscopic studies are described elsewhere
[10, 11].
0.10
0.08
0.06
0.04
0.02
0.00
4.2 512
4.0
3.8
0.2
Intensity, a.u.
possible errors which can appear when applying both
OES and OAS methods is elsewhere [9].
514
516
Ilamp+plasma
Iplasma
0.1
512
514
516
l, nm
Fig. 2.
Example of emission (Iplasma), absorption
lamp+plasma
) and absorbance spectra of C 2 . y = 5 mm,
(I
I = 60 A.
4. Results and discussion
4.1. Plasma diagnostics
The temperature and column density distributions of
C 2 (a 3Π u , v = 0) are shown in Figs. 3, 4 and 5 for arc
currents 30, 50 and 60 A, respectively. In the case of arc
discharge at 30 A the self-absorption was very low and
only temperature distribution could be evaluated.
5500
OES
OAS
The emission and absorption spectra were acquired
using a CCD camera coupled with a 3 m focal length
spectrograph of 1.7 mm/nm in dispersion. The light of a
200 Watt Xe arc lamp was used as the background for
absorption spectra. The transmittance in the spectral
region of C 2 (0-0) Swan band was determined by
acquiring two radiation records.
The first record
(Ilamp+plasma) includes the light from Xe lamp after passing
through the arc discharge added to the plasma emission,
and the second one (Iplasma ) is the plasma emission when
Xe lamp is off. Therefore the spectral transmittance (T λ )
is deduced from
lamp + plasma
plasma
I
−I
l
T = l
l
Io
(3)
l
where I λo is the value of the numerator in the continuum
adjacent to the absorption band head. The example of the
emission and absorption spectra recorded at position
y = 5 mm below the arc discharge and the resulted
absorbance spectrum are shown in Fig. 2.
2
4500
4000
3500
3000
2500
C2(a, v=0) column density, x1015 cm-2
Fig. 1. Pictorial representation of electrode configuration
and arrangement for OES and OAS measurement.
Temperature, K
5000
0.20 0
2
4
6
0.15
0.10
0.05
0.00
0
1
2
3
4
5
Plasma coordinate y, mm
6
Fig. 3. Average rotational temperature (upper panel) and
column density (lower panel) distributions. I = 30 A and
U = 22 V.
The error bars shown in the figures result from
averaging of the data obtained from 20 consecutive
spectra acquired during 3 minutes.
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5000
OES
OAS
Temperature, K
4500
4000
3500
C2(a, v=0) column density, x1015 cm-2
3000
0.40
0
1
2
3
4
5
4
5
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
1
2
3
Plasma coordinate y, mm
Fig. 4. Average rotational temperature (upper panel) and
column density (lower panel) distributions. I = 50 A and
U = 25 V.
5000
close to the discharge centre. Obviously, the rotational
temperature derived from the absorption spectra concerns
the ground electronic state, a 3Π u , and can be interpreted
as the average gas temperature, whilst the temperature
values obtained from OES characterize the population
distributions of rotational levels in the excited d 3Π g
state. It points to some non-equilibrium excitation of C 2
radical. It is possible therefore, that a part of excited C 2
radicals is formed through the association of carbon atoms
C(3P) with an excess of the rotational energy. Since the
C 2 molecule is homonuclear with the zero nuclear spin,
the collision relaxation is not very effective for the
removal of the excess of rotational energy prior to the
radiation transition to the ground a 3Π u state [12]. For
this reason, it was assumed, in further consideration, that
the results obtained from absorption spectra are
undoubtedly more realistic taking into account both the
column density and average temperature distributions.
The C 2 column densities (lower panels in Figs. 3, 4 and 5)
evaluated on the basis of emission and absorption
measurements are also different, the latter ones are lower,
especially at the highest current discharge.
For the sake of comparison the diagnostics of carbon
arc discharge under the presence of helium only was also
performed. The results for the arc current of 60 A are
shown in Fig. 6.
OES
OAS
5000
4000
3500
3000
4000
3500
3000
2500
2.5 0
1
2
3
4
5
2000
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
6
C2(a,v=0) column density, x1015 cm-2
C2(a3Πu, v''=0) column density, x1015 cm-2
OES
OAS
4500
Temperature, K
Temperature, K
4500
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
6
Plasma coordinate y, mm
0
1
1
2
3
4
2
3
4
Plasma coordinate y, mm
5
6
5
6
Fig. 5. Average rotational temperature (upper panel) and
column density (lower panel) distributions. I = 60 A and
U = 27 V.
Fig. 6. Average rotational temperature (upper panel) and
column density (lower panel) distributions. I = 60 A.
Please notice that in all discharge conditions the
rotational temperature values obtained from the emission
spectra significantly exceed those ones determined from
the absorption spectra. The difference is about 1000 K
It should be noted that in these conditions the reactor
transparency rapidly decreases.
Therefore it was
impossible to acquire many times the radiation from the
Xe lamp. By this reason the temperature and column
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3
density values could not be evaluated by OAS at the
plasma coordinate between 0.8 and 3.5 mm. In these sites
the plasma radiation exceeded the absorption effect.
Nevertheless one can notice that the gas temperature is
much lower than in the case of the discharge under the
presence of ethanol vapour, whilst the rotational
temperature of excited C 2 remains at the same level.
Probably, the lower temperature results from high thermal
conductivity of the buffer gas (helium).
There is also a significant difference between the
respective column densities (Figs. 5 and 6) evaluated by
OAS. It is very likely that the observed overestimation of
the column densities determined using OES approach is a
consequence of the assumption of the same rotational
temperature in both, ground and excited electronic states
of C 2 when the influence of the self-absorption on the
emission spectra was considered. To proof this a new
approach in the spectra computing should be applied.
4.2. Products morphology
The product morphology was studied by transmission
and scanning electron microscopy. The representative
images for the products obtained with and without ethanol
vapour are shown in Fig 7a and 7b, and Fig. 7c and 7d,
respectively.
like particles can also be spotted in the SEM image (D).
The TEM image, not shown here, also resembled the
morphological features as in Fig 7a. Probably by
changing the distance between the liquid ethanol and the
arc plasma and also the arc gap one can minimize the
amount of soot in the solid products.
5. Acknowledgement
The work supported by the National Research Center
under Grant UMO-2012/05/B/ST5/00709.
6. References
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Fig. 7. TEM (left panel) and SEM (right panel) images of
products. A, C: arc discharge in He; and B, D: in
He-C 2 H 5 OH. I = 60 A.
There is a profound difference in the morphology of the
products obtained when only He was used and that under
the presence of ethanol vapour. The first image (A)
shows structures for typical soot while the second (B)
resembles a few-layered graphene [12].
Soot-
4
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