22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Atmospheric pressure plasma functionalization of carbon nanotubes
N. Hordy, F.P. Sainct and S. Coulombe
Plasma Processing Laboratory - PPL, Department of Chemical Engineering, McGill University, Montréal, Québec,
Canada
Abstract: Carbon nanotubes (CNTs) were surface treated using a helium glow discharge
stamp (APGD-s) operating in open air in order to add oxygen functionalities. X-ray
photoelectron spectroscopy (XPS) analysis confirmed that the oxygen addition occurs
quickly (5.5 at.% after 1 min) and up to distances as far away as 8 cm. When suspended in
water, the APGD-s functionalized CNTs demonstrated a stability almost comparable to the
ones obtained with a low-pressure glow discharge under more favorable functionalization
conditions.
hese preliminary results demonstrate the potential to transition a
technologically important functionalization process outside the vacuum chamber.
Keywords: atmospheric pressure glow discharge, carbon nanotubes, functionalization
1. Introduction
Due to either their unique or often non-equaled
individual properties, carbon nanotubes (CNTs) have
been hyped for almost 25 years as a “super-material” for
use in a wide range of possible applications.
Unfortunately it has proven quite difficult to transfer these
unique properties (e.g., mechanical strength, electrical and
thermal conductivity) from the nanoscale to the mesoscale
for real-world commercial use [1]. One of the main
difficulties to overcome is the CNTs’ propensity to
agglomerate together due to strong van der Waals
interactions. Surface modification of the CNTs, often
through oxygen functionalization, has been shown to be
the best way to inhibit this process [2]. Traditionally wet
chemistry techniques, such as boiling in harsh acids, have
been used to graft oxygen moieties to the surface of the
CNTs. However these processes tend to be time
consuming, complex and difficult to scale up. Plasma
treatment in comparison is solvent free, dry, time efficient
and versatile.
Our recent work has demonstrated the efficient and
rapid functionalization of CNTs using a classical lowpressure capacitively-coupled RF glow discharge
configuration [3]. Given that our CNT growth process is
accomplished by atmospheric pressure thermal chemical
vapor deposition (t-CVD), there is a strong incentive to
move the functionalization step outside the vacuum
chamber to reduce capital and operation costs.
Atmospheric pressure plasma functionalization of CNTs
has been attempted through the use of various types of
configurations, with the three most often employed being
arc-like discharge [4], dielectric barrier discharge [5-7],
and atmospheric pressure glow discharge (APGD) [8-12].
APGDs bear the most promise for this particular
application given their ability to functionalize large areas.
In this work we demonstrate a novel radio frequency
(RF) APGD device (referred to as an APGD “stamp”;
APGD-s), which generates a glow discharge over a
surface area of approx. 1.5 cm2 (Fig. 1). Using this
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APGD-s we then functionalize CNTs in an open
atmosphere (air) and compare these results to those
obtained using a low-pressure RF glow discharge
(LPGD). Nanofluids, which are defined as a suspension
of nanoparticles (CNTs) in a base fluid (water), were also
produced as a measure of the effectiveness of the
functionalization process.
Fig 1. Image (left) and schematic (right) of the
APGD-s treating a CNT-covered SS mesh. Sourceto-sample distance: 1 cm.
2. Method
The CNTs used in this study were grown directly from
stainless steel (SS) 316 mesh using a t-CVD process [13].
The surface functionalization of the CNTs was achieved
using our custom designed APGD-s. The flowing glow
discharge was sustained by applying 20 W of continuous
RF (13.56 MHz) power to a He flow of 20 L min-1.
Further details on the specifications of the APGD-s, along
with optical emission spectroscopy CFD modeling and
optimization results can be found in the accompanying
proceeding authored by Sainct et al. [14]. Results
obtained using the APGD-s were compared to previouslyreported results obtained using a low-pressure
capacitively-coupled RF glow discharge [3].
Functionalization of larger surface area CNT-covered
1
SS meshes was achieved by slowing passing the sample
under the APGD-s. For these experiments a 4 x 1 cm2
strip of CNTs-covered mesh was placed on a motorized
translation stage and moved under the APGD-s at a speed
of 2 cm min-1. Both sides of the sample were treated at a
source-to-sample distance of 1 cm.
Surface analysis of the CNTs was accomplished using
an X-ray photoelectron spectrometer (XPS, Thermo
Scientific K-alpha) with a monochromatized Al Kα
photon source (hν = 1486.6 eV). Data was collected and
analyzed using the Thermo Scientific Avantage XPS
software package, with the Shirley/Smart method for
background removal. Nanofluids produced by dispersing
the functionalized CNTs, removed from the SS mesh
substrate by ultrasonication in deionized water, were
characterized for stability by UV-visible absorption
spectroscopy.
3. Results
3.1. Effect of exposure time and source-to-sample
distance
The surface oxygen concentration, as determined by
XPS analysis, was used as in indication of the degree of
functionalization. Little-to-no nitrogen addition was
found to occur for any of the samples. As can be seen in
Fig. 2, a significant amount of oxygen is grafted to the
surface of the CNTs in the first 60 sec of treatment, after
which the oxygen concentration essentially plateaus. The
same trend can be seen for samples positioned 0.15 and
1 cm away from the source.
Fig. 2. XPS-derived surface atomic oxygen
concentration as a function of treatment time. Error
bars represent ± 1 standard deviation.
A similar maximum at approximately 5.7 at.% oxygen
occurs when measuring oxygen content as a function of
source-to-sample distance (Fig. 3).
No significant
difference can be seen when the sample is located 2 cm
2
away from the plasma source (possibly even up to a
distance of 4 cm). The level of functionalization appears
to decrease between 4 and 8 cm, after which no change is
observed. For comparison, the level of oxygen addition
obtained using the LPGD the same type of CNTs is
9.6 ± 0.8 at.% for a power of 20 W and in a gas mixture
of Ar/O 2 /C 2 H 2 at flow rates of 250/5/1 sccm [3].
Fig. 3. XPS-derived surface atomic oxygen
concentration as a function of source-to-sample
distance. The initial oxygen concentration of the
non-functionalized (NF)-CNTs is indicated as a
reference. Error bars represent ± 1 standard
deviation.
3.2. Characterization of oxygen functional groups
High resolution scans of both the C1s and O1s lines
before and after the APGD-s treatment were compared to
CNTs treated with LPGD, both for a 2 min treatment time
(Fig. 4). These spectra can be used to provide an
indication of which functional groups are present.
However, due to the high number of potential oxygen
moieties, combined with the asymmetric nature of the C1s
peak for CNTs, we will not attempt any quantification.
When examining the C1s peak (Fig. 4a), one can see that
a small increase occurs in the range in which
carbon-oxygen bonds are located (286 - 290 eV) [3]. This
increase is still significantly less than for the samples
treated with the LPGD, especially for the range in which
carboxylic-type bonds (COO-) are found (~289 eV).
When comparing the O1s peak (Fig. 4b) for the low- and
high-pressure treatments, a small shift to lower binding
energy can be seen; possibly indicating a slightly larger
proportion of carbonyl-type bonds (C=O) for the APGD-s
treated samples.
3.3. Nanofluid synthesis and stability
XPS mapping was conducted on a sample that was
slowly scanned under the APGD-s to test the uniformity
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of this process. As Fig. 5 shows, the scanning process
worked extremely well in uniformly functionalizing the
CNTs. The only areas without significant oxygen
addition were the edges of the sample that were covered
during the treatment (see Fig. 1). Nanofluids produced in
this study were compared to the ones produced using the
LPGD functionalization technique (Fig. 6). It was found
that the nanofluid made with the APGD-s treated CNTs,
although extremely stable in comparison to conventional
CNT suspensions, was slightly less stable that the
nanofluid made up of LPGD-functionalized CNTs.
functionalized using the APGD-s for nanofluid
production. Spatial resolution of the map: 1 mm2.
Fig. 6. Relative CNT concentration in CNT-DI water
nanofluid as determined by absorption spectroscopy,
comparing the APDG-s and LPGD methods of
functionalization. Trend lines added for visual
clarity.
Fig. 4. High-resolution XPS C1s (A) and O1s (B)
spectra comparing APGD-s and LPGD
functionalization techniques (2 min treatment time).
NF-CNTs are provided as a reference.
Fig. 5. XPS mapping of the C1s and O1s peaks of a
4x1 cm2 CNT-covered SS mesh sample,
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4. Discussion
Given that only pure helium was injected in the APGDs, all of the reactive oxygen species, which cause the
functionalization of the CNTs, can be attributed to the
intrusion of oxygen and water vapor from the surrounding
atmosphere. During the plasma treatment, recirculation
of ambient air (relative humidity ~ 32%) around the
APGD-s leads to the formation of various ions and
radicals (emission bands from OH, N 2 , N 2 + and emission
lines from atomic O are detected by OES) [14]. However
as the lifetimes of the charged species and He metastable
produced in the glow discharge are on the order of
microseconds [15], these species cannot be responsible
for the functionalization of the CNTs. Seeing as up to
5.1 at.% oxygen addition was found to occur at distances
as far as 4 cm from the plasma source, lifetimes on the
order of milliseconds are required (the max centre-line
velocity is ~10 m sec-1).
We hypothesize that it is primarily singlet molecular
oxygen, O 2 (1D), that binds with the CNTs to form the
oxygenated functional groups. Many recent studies on
APGD-jets have shown that this long-lived (up to 75 min
at 1 atm [16]) reactive oxygen metastable has the highest
density of all the species outside of the plasma core [15].
In addition, singlet molecular oxygen is known to
covalently bond relatively easily to CNTs, especially at
defect sites such as Stone-Thrower-Wales defects [17].
Previous work on the specific CNTs used in this work
found them to be heavily defective, with bond rotations
(e.g., Stone-Thrower-Wales, 5-7-7-5 defects) as likely the
3
primary type. It is possible that atomic oxygen could also
cause functionalization, although as it decays much faster
than O 2 (1D) it is unlikely to be the primary contributor.
If singlet molecular oxygen is the cause of the
functionalization, it would also explain why essentially no
carboxylic (COO-) addition was seen for the APGD-s
functionalized samples (as opposed to the LPGD treated
ones). Addition of carboxylic functions is not expected
for the APGD-s, as there no external carbon source (in the
LPGD, ethane is injected along with oxygen). The lack of
carboxylic functions could explain the slightly lower
stability of the nanofluids in comparison to the LPGD
functionalized-CNTs ones, as carboxylic groups can
easily deprotonate in polar protonic solvents, leading to a
negative surface charge on the CNTs, thus limiting
agglomeration over time. Without this negative charge
present, the produced nanofluids may not be as stable,
especially under harsh conditions such as intense heating.
However, given that some applications only require the
CNTs to be hydrophilic (electrode applications for
example [18]), this simple open-air treatment may be all
that is required.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
5. Conclusions
This study demonstrates that a significant amount of
oxygen can be grafted to the surface of CNTs through the
use of a pure helium APGD-s operating in ambient air.
Although the level of oxygen addition, as determined by
XPS (~5.5 at.%), was less than that achieved through
LPGD process (9.6 at.%), this atmospheric pressure
method of treatment is successful in changing the
hydrophobicity of the CNT surface.
As a result,
nanofluids produced with CNTs functionalized by
scanning the CNT-covered SS meshes under the APGD-s,
remained over 80% suspended after 3 weeks. The next
phase of this work will transition the APGD-s into a
controlled atmosphere, in which the reactive species
(oxygen and ethane) will be injected directly into the
carrier gas.
[12]
[13]
[14]
[15]
[16]
[17]
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