Magnetic control of synthesis and characterization of carbon
nanostructures produced in an anodic arc plasma
Jian Li, Alexey Shashurin and Michael Keidar
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC
20052
Abstract: Anodic arc discharge supported by the erosion of the anode material is
one of the most practical and efficient methods to synthesize various high-quality
carbon nanostructures including nanotubes, graphene due to its relatively higher
growth temperature and eco-friendly procedures. In order to increase the arc
controllability of the as-synthesized products, we introduced a non-uniform
magnetic field which has component normal to the current in the arc, so that the
Lorentz force along the J×B direction can generate the plasma jet and make
effective delivery of carbon ion particles and heat flux to samples. As a result,
large-scale graphene flakes and high-purity single-walled carbon nanotubes were
simultaneously generated by the magnetically-enhanced anodic arc method. Arc
imaging, SEM, TEM and Raman spectroscopy analysis were employed. These
findings indicate a wide spectrum of opportunities to manipulate with the
properties of nanostructures produced in magnetically-enhanced arc discharge by
means of control of arc conditions.
Keywords: arc discharge, carbon nanostructures, magnetic control, plasma jet
1. Introduction
carbon nanotubes, [9] and provide conditions for the
synthesis of large-scale and high-quality graphene
flakes. [10] In this paper, the controllability of arc
plasma jet by means of external magnetic field in
conditions of atmospheric anodic arc and associate
SWCNT and graphene synthesis with directing of
arc jet in J×B direction was demonstrated. Arc
images were recorded simultaneously through the
front and left viewports of reaction chamber, while
the properties of carbon nanotubes and graphene are
analyzed by scanning electron microscope (SEM),
transmission electron microscope (TEM) and Raman
spectroscopy. We also demonstrate the effect of
perturbation of arc plasma column due to Lorentz
force applied to current channel, and furthermore
establish the correlation between arc conditions and
properties of synthesized carbon nanostructures.
Carbon nanostructures such as single-walled carbon
nanotubes (SWCNT) and graphene flakes attract a
deluge of interest of scholars and boosted the
energy-related research nowadays. Their promising
application led to many breakthroughs in nanotransistors, infrared sensors, photodiodes, hydrogen
storage, molecular sensors, field effect transistor,
super thin electronic devices and many other
applications recently. [1-6] Arc discharge is one of
the most practical and efficient methods, which can
provide non-equilibrium process and a high influx of
carbon material to the developing structures at
relatively higher temperature. Therefore the resulting
carbon nanostructures have few structural defects
and better crystallinity comparing to the other
methods, such as laser ablation and chemical vapor
deposition. [7]
2. Experimental methods
In order to improve the controllability of synthesis
of carbon nanostructures in arc discharge, nonuniform magnetic fields was applied. [8] It was
demonstrated that the magnetically-enhanced arc
discharge can increase the length of single-walled
The arc discharge system for synthesis of SWCNT
has the following set up. A cylindrical chamber
made from stainless steel with 254-mm length and
152-mm diameter was the reaction chamber where
arc discharge occurs. Initially the chamber was
pumped down to the pressure less than 10-1 Torr
vacuum and then filled in by helium with purity of
99.995%. The anode was attached to a linear drive
system controlled by LabVIEW software, which
keeps the predetermined gap distance according to
the desired arc voltage after the discharge is initiated
via contact ignition. All experiments were done with
arc current of about 60-90 A, discharge voltage of 30
V and the helium pressure of 500 Torr. Cathode was
made of graphite with the diameter of 13 mm, while
anode contains a hollow graphite rod with the outer
and inner diameter of 5 and 3.2 mm, respectively.
Carbon powder, catalyst powder of nickel (300 mesh)
and yttrium (40 mesh) were mixed and filled into the
graphite rod with the total molar radio of 56:4:1. A
cuboid permanent magnet (Alnico, Grade 8) with
dimensions of 25×25×100 mm was placed inside the
chamber at about 30-mm distance from the
interelectrode axis and creating magnetic field B in
the range from 0.02 to 0.1 Tesla (depending on
magnet positions) in the gap. Figure 1(a) shows the
case when the interelectrode gap was placed at the
distance of about h=75 mm from the bottom of
permanent magnet and presents the magnetic field
distribution simulated by FEMM 4.2 software.
Samples from different locations on the surface of
permanent magnet and the surface of cathode were
collected after 30-second run of the arc discharge
and then observed under Zeiss LEO 1430VP SEM
with the high voltage of 30 KV and JEOL 1200 EX
TEM with the voltage of 100 KV, respectively. The
morphology of as-synthesized black soot adhered by
carbon tape to aluminum substrate will be observed
directly under SEM. Regarding the sample
preparation for TEM analysis, the thin films of
SWCNT were obtained by drop casting a suspension
of methanol-dispersed SWCNT solution after
sonicating for 60 minutes using Fisher Scientific
ultrasonic dismembrator (Model 150T) with 50%
sonicating amplitude.
Raman spectroscopy was performed on a microRaman system based on a 200 mW Lexel 3000 Ar
ion laser (tunable single line output), with
holographic optics, a 0.5 m spectrometer and a liquid
nitrogen cooled CCD detector; wavelength 514 nm
which corresponds to the energy of 2.33 eV. Raman
measurements covered the range of 1200 cm-1 to
3200 cm-1, and were carried out on bulk samples of
arc produced carbon soot.
Figure 1. Distribution of magnetic field (a), photograph of arc
plasmas jet from the right porthole (b), schematic diagram of
electrodes position and direction magnetic field in the gap for
the case when the interelectrode gap is positioned about 75 mm
above the bottom of permanent magnet (c), and photograph of
arc plasmas jet from the front porthole (d).
3. Results and discussions
The video snapshots obtained simultaneously from
side and front viewports of the chamber are shown
in Figure 1(b, d) for h=75 mm. These images
illustrate significant perturbation of arc plasma
column in presence of external magnetic field in
comparison with axially symmetric arc column
observed in the case without a magnetic field. [9] It
should be noted that change of magnet position (we
tested magnet shift along z-axis and turning the
magnet over) results in deviation of arc jet flow in xdirection corresponding to direction of J×B force. It
was also observed that geometry of arc plasma
column did not change at removing of nickel catalyst
from the anode meaning that influence of magnetic
field on nickel catalyst particles motion does not
affect overall geometry of plasma column. We can
control distribution of magnetic field by changing
the position of permanent magnet. In the case of
h=95 mm, the plasma jets also obey the direction of
J×B force and become more downward comparing
the position of h=75 mm.
Figure 2 displays the typical morphology of
SWCNT and catalyst particles collected on the
surface of cathode with the magnetic field of B=0.06
Tesla under TEM. It can be seen that SWCNT are
close-packed into bundles with diameters ranging
from 2 to 20 nm due to the van der Waals interaction
between individual SWCNT. In contrast to the
SWCNT without magnetic field, the magneticallysynthesized SWCNT has relatively larger length,
smaller diameter and narrow diameter distribution.
[8, 9] Also the analysis of the diameter distribution
of catalyst particles shows the same trend of catalyst
particle diameter decrease with a magnetic field. [11]
Figure 3. Typical SEM images of graphene flakes
Figure 2. TEM images of as-synthesized SWCNT bundles
In addition to SWCNT, graphene flakes can be
obtained from the surface of permanent magnet in
the same process. Figure 3 and 4 show the graphene
flakes as well as few-layer graphene obtained from
the sample taken at the surface of the magnet in the
location corresponded to arc plasmas jets. The inset
of Figure 4 shows the electrons diffraction pattern
associated with the graphene. The hexagonal dots
pattern of electron diffraction presents the evidence
of few-layer graphene. The arc plasma jets by
Lorentz force play an important role during the
graphene synthesis process in that it can make
effective delivery of carbon ion particles and heat
flux along the J×B direction. Hence, it provides an
easy method to control the sample deposition.
Figure 4. TEM images of graphene. Inset of (a) is the electron
diffraction pattern showing the crystalline structure of graphene.
Raman spectrum is a powerful tool for
characterization of graphene flakes. The typical
peaks observed in graphene are the G and 2D peaks
at ~1600 cm-1 and ~2700 cm-1 respectively, using the
excitation wavelength of 514 nm. The G peak stems
from in plane vibrations which can be observed in
all sp2 carbon materials. The 2D peak is a second
order of the D peak but is seen even in non
disordered systems, due to the fourth order phonon
momentum exchange double resonance process. It
plays a crucial role in the characterization of
graphene. The intensity of I(2D)/I(G) is
approximately 4 for monolayer graphene and
decreases with the addition of subsequent layers,
thus making it possible to estimate the thickness of
graphene layers. [12] Figure 5 indicates that the
value of I(2D)/I(G) is around 0.45, which can be the
evidence of few-layer graphene.
Acknowledgments
The authors thank the kind help about Raman
spectroscopy from Dr. I. Calizo, National Institute of
Standards and Technology, Gaithersburg, MD. This
work was supported by NSF/DOE Partnership in
Plasma Science and Technology (NSF Grant No.
CBET-0853777 and DOE Grant No. DESC0001169).
References
Figure 5. Raman spectrum of large-scale graphene flakes. The
number of graphene flakes layers can be determined by the ratio
of intensity of G and 2D peaks
In conclusion, we demonstrated the simultaneous
method to synthesize SWCNT and graphene by
magnetic-controlled arc. The experiments were
carried out for various electrode gaps, magnitudes
and geometries of external magnetic field created by
the different positioning of permanent magnet. The
properties of carbon nanotubes and graphene are
analyzed by SEM, TEM and Raman spectroscopy.
These findings indicate a wide spectrum of
opportunities to manipulate the properties of
nanostructures produced in magnetically-enhanced
arc discharge.
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