Fundamental Study of the Interaction of Carbon Nanotube Electrodes
with Glow Discharges: A comparative study of the role of plasma
composition
L. Vandsburger1, S. Coulombe1, J.-L. Meunier1
1
Plasma Processing Laboratory, Department of Chemical Engineering
McGill University, Montréal, Canada
Abstract: Studies in which MWNTs have been exposed to RF plasma afterglows in N2, O2,
and Ar have revealed the respective effects of gas composition and electron bombardment on
MWNT plasma degradation. A new means of providing specifically tailored surface groups
has been developed, where plasma polymers such as pyrrole, carbazole and acridine are
deposited onto the surfaces of MWNTs, producing MWNT-polymer composite materials.
Introduction
Multi-walled carbon nanotubes (MWNTs) are perfectly
suited for use in advanced materials, because their surfaces
can be modified through simple organic chemistry. This is
done by reactive addition through the formation of new C-C
or C=O bonds. Although this disrupts the graphitic lattice
of the outermost surfaces, MWNTs have numerous
concentric shells and can maintain axial electronic and
thermal conductivity after such modifications. The
importance of these techniques is clear, but their limitation
lies in method and control. Wet chemical modification is the
principal approach, which does not control agglomeration or
settling in the early stages of the process, and
functionalization or modification cannot be localized on the
surface. The former limits the effectiveness of chemical
treatments, while the latter prevents the development of
important new applications that rely on unimpeded
electrical conductivity in the outer shells of MWNTs.
Methods
A fundamental study has been undertaken to address these
issues, taking advantage of a unique procedure that
produces MWNT forests grown directly from stainless-steel
mesh samples [1]. Whereas normal chemical modification
occurs in aqueous suspension, a new approach has been
developed to recruit functional species directly to the tips of
MWNTs. Desired functional groups and charged particles
are produced in a glow discharge, and are accelerated to the
tips of MWNTs by an electric field emanating not from the
bulk surface, but from the MWNTs themselves [2].
A system, shown in Fig. 1, was designed using a
capacitively-coupled radio-frequency (RF) afterglow to
supply free-electrons and ions for functionalization. By
applying a positive bias, MWNTs were degraded by
electron bombardment from the plasma. The products of the
plasma-decomposition were identified using FTIR, Optical
Absorption Spectroscopy (OAS) and Optical Emission
Spectroscopy (OES), SEM imaging, and
Gas
Figure 1: RF Plasma Setup. (1) DC feedthrough and
MWNT sample holder, (2) Pressure gauge, (3)
Vacuum valve (4) MS sample collection, (5) OAS
path and MWNT sample, (6) Plasma viewport and
RF electrode, (7) Gas inlet, (8) RF feedthrough
Chromatograph-Mass Spectrometry (GC-MS) analysis of
vapor phase products that had been condensed onto
activated carbon pellets downstream of the MWNT
samples.
Plasma power was set at 30 W for all gases. Chamber
pressure was set at 0.75 Torr, with a gas flowrate of 5 sccm
throughout experiments. Free electrons produced in the
plasma were attracted to the MWNTs by a DC sample bias,
set at +200 VDC. The voltage was controlled to maintain an
electron bombardment current of 5 mA on the sample
surface for a total duration of 60 minutes.
Confirmation of MWNT degradation, and of deposition of
surface plasma polymer layers was obtained by SEM
microanalysis. The spatial development of degradation
during an experiment was determined by imaging the
sample at regularly spaced intervals across its length. This is
considered to be analogous to the temporal development of
degradation, considering that the intensity of electron
bombardment is inversely proportional to the distance from
the RF glow discharge.
The arrangement for in situ spectroscopy measurements is
given in Fig. 2. It shows the techniques and typical types of
spectra produced during experiments. The figure also shows
the position of the MWNT sample relative to the RF
plasma, as well as the positions of the OAS beam-path and
the optical probe for each type of measurement. OAS
measurements were done using an incandescent tungsten
light source, to provide a continuous spectrum for detection
of broad-band absorption patterns. The beam path was
located outside the RF glow, to prevent changes in plasma
composition
during
experiments
from
affecting
measurements. The OAS setup is labeled A in Fig. 2. OES
measurements, labeled B, were collected directly from the
RF glow. Typical spectra produced by the incandescent
source and the RF glow discharge are shown in the graphs
labeled C and D, respectively.
OAS spectra were processed using an initial spectrum as a
reference, according to the Beer-Lambert law for gases.
This approach was considered appropriate because the
concentration of absorbing species was low, so scattering
could be neglected. The governing equation is summarized
in Eq. 1, where A’ is absorbance, Io is the initial absorbance
spectrum and I is the sample spectrum, at time t.
(1)
Grazing-Angle FTIR spectroscopy examined the changes in
MWNT surface chemistry resulting from exposure to RF
afterglows. Samples were measured using a transmission
FTIR microscope. Bare stainless steel meshes were used as
a background reference to compensate for the signal from
the metal substrate as well as the atmosphere around the
sample. FTIR spectra were collected in a range from 750 –
4000 cm-1 and were corrected to adjust for smoothness and
were adjusted to fit a flat baseline.
GC-MS samples were collected during the course of the
experiments by condensing volatile organic vapors in a cold
trap. This trap was constructed by placing mesh-screened orings on either side of a short Kwik-Flange - NW25 section
in the exhaust line of the vacuum chamber. Activated
carbon pellets were placed in the screened section prior to
the experiment and left under vacuum to degas. Prior to
experimental runs and while under vacuum, the cold trap
was cooled externally by dry ice to -80 OC. After the end of
each experiment the carbon pellets were stored in a freezer
to prevent re-volatilization prior to GC-MS analysis.
Figure 2: Diagram detailing in situ spectroscopy
measurements. A: OAS setup, B: OES setup, C:
Spectrum produced by W incandescent source, D: OES
spectrum produced by Ar glow discharge.
For analysis, the activated carbon pellets were heated and
sampled using a headspace analyzer connected to a GC-MS.
This procedure allowed for direct sampling of the
condensable components of the plasma gas, and avoided
cross contamination with non-volatile carbonaceous species
that could come from the activated carbon. Such species
would likely be detected by liquid chromatography.
Results and Discussion
SEM microanalysis provides clear evidence of tip-localized
degradation of MWNTs, resulting from electric fieldconcentrated electron bombardment. A series of three
images, in Fig. 3, show the effect of electron bombardment
strength, and in this way also show the temporal progression
of MWNT degradation and the morphological changes
associated with electron bombardment-induced MWNT
degradation. The figure shows that degradation proceeds
from the tips of the MWNTs to the point that the stainless
steel surface beneath is exposed. In addition, the
morphology of the MWNTs that make up the forest is
unaffected by the degradation that occurs at their outermost
edges. What is seen is a uniform reduction in average
MWNT length, without the development of MWNT lattice
defects that typically occurs during RF plasma
functionalization.
Figure 3: SEM images showing tip-localized effect of
degradation of MWNTs by electron bombardment.
OAS spectra taken during electron bombardment
experiments in N2, Ar, and O2 are presented in Fig. 4. The
spectra each show the characteristic absorption pattern of
poly-pyrrole (PPy), with a broad absorption peak at
approximately 450 nm, and a near-IR tail that was sensitive
to the oxygen content of the plasma gas. In oxygen-rich
discharges the spectra contained absorbance bands at longer
visible wavelengths (600-900 nm) and increasing toward
the IR range; such absorbance is absent from samples
produced in N2 and Ar RF afterglows, and has been
reported in literature to occur as a result of oxidation of PPy
[3]. Spectra from Ar and N2 plasma tests also contained an
absorption band centered at approx. 360 nm, which was not
observed in O2 experiments. Based on searches of literature
sources, this peak has been attributed to acridine, a threering polycyclic aromatic hydrocarbon [4].
The location of the OAS pathway at the same position as
the MWNT sample gives a good indication that PPy and
other species are produced concurrently with MWNT
degradation by electron bombardment. The sensitivity of
OAS spectra to plasma composition confirms that plasma
reactions play a role in the synthesis of PPy during
experiments.
GC-MS data provides the important information about the
molecular weight of PPy produced during plasma tests.
Spectra of short hydrocarbons also help to shed light on the
synthesis mechanism of the PPy, which appears to proceed
from the radical cyclization of 1,3 butadiyne. The typical
MS spectrum from experiments in O2 is shown in Fig. 5.
PPy, with monomer weight of 65 and pyrrole (m/z = 67) are
seen, with polymer lengths of up to five monomer units.
Figure 4: OAS spectra from RF plasma afterglow
experiments in Ar, N2, and O2. The absorption band
seen between 400 and 600, with a peak around 460 nm
indicates the presence of PPy, while the longer
wavelength tail that shows sensitivity to gas
composition indicates the effect of oxidation. The band
centered at 360 nm is attributed to acridine (MM 179), a
polycyclic aromatic hydrocarbon.
This distribution is consistent with plasma polymerization,
and indicates a gas phase synthesis mechanism. The inset
shows the ions detected below m/z = 60. These were found
in much greater quantity than PPy, indicating that they are
the products of MWNT degradation, and that the polymer
detected in OAS is produced by plasma polymerization in
the gas phase. Literature reports have found that plasma
polymerization can produce polycyclic hydrocarbons, which
helps support this finding [5,6].
FTIR measurements confirm that the PPy is deposited as a
film onto the surface of the MWNT sample. Spectra from
MWNT samples taken before and after plasma degradation
treatment in N2 and O2 are presented in Fig. 6. The PPy
coating is revealed by comparing the spectrum from the
treated surfaces to the untreated surface, labeled CNTs. The
difference is emphasized in the development of a peak at
3300 cm−1. This peak is absent in the untreated sample, and
has been shown in literature to be a strong peak in PPy
spectra [7].
Fig. 7. shows the so-called fingerprint region, located
between 850 and 1800 cm−1. For both N2 and O2 tests, these
match the specific peak signature of PPy [8]. This confirms
that the plasma polymer is indeed PPy, but it also reveals
the effect of plasma gas composition on the chemical
100
67
100
Normalized Percent Intensity
40
90
80
80
51
60
70
40
29
60
20
50
0
40
0
10
20
30
40
50
60
129
268
30
328
20
197
10
0
50
100
150
200
250
300
350
M/z [AMU]
Figure 5: GC-MS spectra taken during O2 RF
afterglow test. Spectrum shows repeating monomer
units of 65 amu, corresponding to pyrrole and low
molecular weight PPy. Inset: Spectrum of short
unsaturated hydrocarbons attributed to the products of
CNT degradation.
Figure 6: FTIR-GIR spectra taken of MWNT mesh
samples before and after RF afterglow treatment with
electron bombardment. Development of peak profiles
is consistent with PPy signature.
structure of the deposited film. The spectrum taken from a
sample treated in an O2 plasma afterglow contains
absorbance peaks specific to overoxidized PPy. The peak at
1075 cm−1 is found in literature sources to result from PPy
formed in oxidizing conditions [8]. The ability to control the
plasma polymer film chemistry is important, since it allows
for tailoring the film functionality towards intended
applications, such as photo-luminescent materials.
Conclusions
Plasma decomposition of MWNTs has been examined for
systems where electrons are attracted to the tips of MWNTs
by applied electric fields. The resulting degradation has
been found to produce short hydrocarbons such as propyne
and butadiyne. Concurrent with MWNT degradation,
plasma polymerization reactions produce polycyclic
aromatic plasma polymers that redeposit onto the MWNTs
as plasma polymer coatings composed mostly of PPy that
exhibits chemical sensitivity to gas composition.
Acknowledgments
The researchers would like to acknowledge funding
provided by the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the Fonds de recherche du
Québec: Nature et technologies (FRQ:NT), and technical
assistance provided by Mr. Ranjan Roy.
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Figure 7: FTIR-GIR spectra taken of MWNT mesh
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