22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Investigation of carbon microparticle synthesis using high-flux plasma exposure
D.U.B. Aussems1, K. Bystrov1, M. Rasinski2, L. Marot3, I. Dogan4, M.A. Gleeson1 and M.C.M. van de Sanden1
1
Dutch Institute for Fundamental Energy Research (DIFFER), Association EURATOM-FOM, Trilateral Euregio
Cluster, De Zaale 20, 5612 AJ Eindhoven, the Netherlands
2
Forschungszentrum Jülich, Wilhelm-Johnen-Strasse, 52428 Jülich, Germany
3
Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
4
Eindhoven University of Technology, Den Dolech 2, 5612 AZ Eindhoven, the Netherlands
Abstract: This study investigates the feasibility of using high-flux plasma exposure of
highly oriented pyrolytic graphite for synthesis of carbon microstructures for possible
energy applications. To build a solid foundation for this work, experiments were
performed to investigate the morphology of the microparticles as function of the ion
energy, plasma flux and surface temperature. The samples are analyzed post-mortem by
SEM and XPS. Preliminary results show a variety of microparticle shapes as function of
predominantly the surface temperature.
Keywords: carbon nanostructures, PECVD, high-flux plasma, CNW, plasma dust
1. Introduction
Many important energy applications require nanoengineered materials [1]. Up to day relevant examples of
these applications are energy storage systems,
watersplitting and CO 2 capture. Materials with a high
surface area and porosity are often desired [2]. One
example of a material with these properties is carbon
microparticles [3][4]. Previous work on plasma enhanced
chemical vapor deposition (PECVD) of graphite by highflux hydrogen plasma
has shown that carbon
microparticles can be grown without the use of precursor
gas injection and surface pretreatment [5]. These
microparticles can be candidates for aforementioned
applications. This requires however the precise
engineering of specific structural/chemical properties. As
a solid foundation for this research, we need to understand
how these structures grow. In this work experiments are
performed to investigate the dependence of the structural
properties on several key parameters, such as the ion
energy, plasma flux and surface temperature.
2. Experimental setup
The experimental setup of Pilot-PSI linear plasma
generator is depicted in Fig. 1 and is described in more
detailed in [6]. In our experiments on Pilot-PSI, hydrogen
plasma is generated by a cascaded arc source and
exhausts into the vessel along a magnetic field of 0.4-0.8
T, generated by electromagnetic field coils. The radially
resolved plasma parameters are measured by a Thomson
scattering system[7] and show a typically high peak
density (>1020 m-3) and an electron temperature of 1–2
eV. The plasma flows towards a water cooled, electrically
biased target. In this way a typical ion flux of 5 × 1023 m2 -1
s - 4 × 1024 m-2s-1 is achieved while the ions have energy
in the range of 10-60 eV, depending on the bias voltage It
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has to be noted that such ion fluxes are up to four orders
of magnitude larger than in conventional plasmaenhanced chemical vapor deposition processing. The
target surface temperature is measured by both an fast
infrared camera (FLIR, SC7500-MB) and a multiwavelength pyrometer (FAR Associates, FMPI
SpectroPyrometer). Because the emissivity of the surface
changes rapidly during the exposure, the infrared camera
data is only used to give qualitative information.
Thomson scattering
Fig. 1. Linear Plasma Generator Pilot-PSI.
In this research, the samples are exposed to a hydrogen
plasma reaching a fluence of the order 1025 - 1026 m-2.
3. Results and discussion
In previous research it was shown that the exposure of
fine-grain graphite to chemically reactive plasma leads to
the formation of several types of microparticles and other
nanostructures such as multi-wall nanotubes, nanowalls
1
and nanotips. The initial surface roughness of fine-grain
graphite is quite large as indicated in Fig. 2. This may
have a strong effect on the onset of structures growth as it
influences the plasma sheath - which is very thin (~10
μm) in our typical plasma - and hence the redeposition of
sputtered carbon. In order to investigate this effect in our
experiments we used highly oriented pyrolytic graphite
(HOPG, NT-MDT Co., 0.8-1.2 degrees mosaic spread, 1
mm thickness), which has a much lower surface
roughness than fine-grain graphite.
a)
5 μm
b)
5 μm
Fig 2. Scanning electron microscopy (SEM) images of a)
highly-oriented pyrolytic graphite (HOPG) and b)
polished fine-grain graphite. The SEM images show that
the HOPG sample has a much lower surface roughness.
•
•
this abstract). In the temperature range around
~1250 oC the microparticles show large sizes and
a high surface coverage. At T surf > 1600 oC
microparticle do not appear.
Biasing the sample leads to cavity formation (not
in this abstract).
No correlation could be found with the ion flux
in the investigated parameter range.
Growth of microparticles
For the selected samples, the structure of the
microparticles is analyzed in more detail. Sample S10
shows large microparticle agglomerations on the surface
which can even be seen by the naked eye. The photograph
of the surface and the SEM image of the particles are
given in Fig. 4.
A cross section of a microparticle from the sample S6 is
depicted in Fig. 5. The microparticle is cut by a focused
ion beam (FIB). The SEM image shows a spherical core
and two elongated rings. This sample was exposed to
three subsequent plasma shots. This may explain the three
separate growth regions. Backscattering Electron (BSE)
imaging (not in this abstract) show that the microparticle
consists purely of carbon.
Parameter optimization
Since a large effective surface area is desired, we aim to
balance between the surface coverage and size of
Fig 3. SEM images show microparticle formation for specific experimental conditions (indicated in the table). For each
sample SEM images were taken at four radial positions from the center to the edge (i.e. 0 to 6 mm). The table colors
green, yellow, orange and red indicate the microstructure-formation performance from optimal to poor, respectively.
For the sample S1, S7 and S8 the temperature of the pyrometer was below the detection limit. These temperatures were
estimated by the IR camera data with an assumed surface emissivity of 0.2. The temperature of sample S2 has a high
standard deviation due to a high pulse-to-pulse variability.
microparticles. The microparticle formation was
investigated as function of ion energy, ion flux and
surface temperature, see Fig 3.
• In the temperature range around ~1050 oC the
surface morphology is dominated by relative
small (~1 μm) microparticles/structures (not in
2
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1 mm
50 μm
Fig 6. The XPS spectrum showing various impurity
species. The binding energy (BE) is uncorrected, but
typically only 5 eV off.
The presence of Mo line for the as-received sample is
likely caused by the Mo clamping ring in the XPS system.
On the other hand, the as-received sample does have some
contamination in the form of metal particles on the
surface, see Fig. 7 (the light spots). The cause of the
surface contamination is unclear and will be investigated
in further work. The bulk HOPG is not expected to be
contaminated, as the purity according to the manufacturer
is 10 ppm.
10 μm
Fig 4. a) Sample after exposure to hydrogen plasma b)
SEM image of close up on microparticles
ring 2
5 μm
ring 1
core
Fig 5. SEM image of a cross section of a microparticle.
The two (white) guidelines separate the three different
growth regions.
Fig. 7 Backscattering electron imaging of a reference
sample.
A possible reason for the increase in the molybdenum
content for the biased samples is sputtering of the
molybdenum clamping-ring surrounding the sample likely
due to energetic C, CH or C x H y atoms/molecules [8].
Despite the contamination of metals, carbon
microstructures grow. In one case, by using BSE
imaging, we even observe a metal particle at the base of a
carbon microparticle, see Fig. 8.
2 μm
Detection of impurities
The samples were analyzed by X-ray photoelectron
spectroscopy (XPS) to detect possible traces of impurities
see Fig. 6. The spectra show an increase of the
Molybdenum content for the exposed samples compared
to the as-received ones and an even further increase for
the samples biased to -60V.
Fig 8. SEM image of a cross section of a carbon
microparticle with a metal particle at the base.
4. Discussion
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The results show that the carbon microparticles growth
highly depends on the surface temperature which is in
agreement with [5]. Biasing the samples leads to cavities
formation in the microstructures. No clear dependence on
the ion flux is found yet. The scenario with the highest
surface coverage and size of microparticles in this
campaign is Γ i =2.3×1024 m-2s1, V bias =-30 V, T surf
=1240 oC.
The carbon microstructures grown on HOPG show a
similar morphology to those grown on fine-grain graphite
previously [5]. In the case of HOPG, the microstructures
appear to be more isolated. Possibly the erosion of HOPG
is lower than that of graphite, so there are less building
blocks available for structural growth.
The question still remains whether the microparticles
are created in plasma or on the surface. In this work we
observe microparticles containing a spherical carbon
microparticle in the core as well as a metal contamination
at the base. This suggests two possible (concurrent)
growth mechanisms. Initial or plasma exposure induced
surface roughness leads to distortions of the plasma
sheath and may effectively direct carbon towards specific
locations on the surface. Alternatively, the particles may
be created in the plasma by atomic-molecular accretion of
eroded carbon from the surface as suggested by [9]. This
is also supported by the spherical shape of some of the
microparticles, found in previous work [5]. Additionally,
if particles are indeed formed in the plasma and deposited
at the surface, they may agglomerate further by radical
deposition.
5. Conclusions
This study shows the formation of microparticles on
highly oriented pyrolytic graphite with a similar
morphology as on fine-grain graphite [5]. A variety of
4
microparticle morphologies were identified as function of
the surface temperature. Lastly, microparticles with and
without metal contamination are observed suggesting two
growth mechanisms.
6. Outlook
In further work measurements will be performed on the
microparticle’s catalytic performance. A CO 2 capture
experiment will be conducted with a Thermal Desorption
Spectroscopy setup. The surface area and porosity of the
microparticles will be measured by a Brunauer–Emmett–
Teller setup. Moreover, a pure-graphite clamping ring is
designed to mitigate the contamination of the sample by
metal impurities from the clamping ring.
7. Acknowledgements
The authors wish to thank DIFFER technical staff for
their professional skill and dedicated support. This work
was supported by FOM Program 148.
8. References
[1] K. Ostrikov, et al., Advances in Physics 62, 113
(2013)
[2] S. Park, et al., Chem. Mater., 27, 457 (2015)
[3] An-Hui Lu, et al., Porous materials for carbon dioxide
capture, Springer (2014)
[4] C. Liang, et al., J. Am. Chem. Soc. 128, 5317 (2006 )
[5] K. Bystrov, et al., Carbon, 68, 695 (2014).
[6] W. Vijvers, et al., Phys. Plasmas 15,13 (2008)
[7] H. J. Van der Meiden, et al., Rev. Sci. Instrum. 79,
013505 (2008)
[8] W. Eckstein, Calculated sputtering, reflection and
range values, IPP-Report (2002)
[9] C. Arnas, et al., J. Nucl. Mater. 337, 69 (2005)
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