22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma modification of MWCNTs: increasing hydrophilization toward stable
dispersion in aqueous solutions
M. Garzia Trulli1,2, E. Sardella2,3, F. Palumbo2,3, G. Palazzo1, S. Musso4 and P. Favia1,2
1
2
Department of Chemistry, University of Bari “Aldo Moro”, Bari, Italy
Research Unit of the Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM),
Department of Chemistry, University of Bari “Aldo Moro”, Bari, Italy
3
CNR - Institute for Inorganic Methodologies and Plasmas (CNR-IMIP), Bari, Italy
4
Schlumberger-Doll Research Laboratory, Cambridge, Massachusetts, United States
Abstract: Low pressure plasma processes were used to modify multi-walled carbon
nanotubes powders, in order to increase their hydrophilic character and facilitate their
dispersion in aqueous media, by grafting polar functional groups. Different experimental
conditions were tested, including process gases, input power, times and pressure values, in
order to study and optimize their influence on process efficiency.
Keywords: multi-walled carbon nanotubes, plasma treatment, aqueous dispersion
1. Introduction
Carbon nanotubes (CNTs) are an interesting molecular
form of carbon in the fullerene family, which could be
described as hollow tubular channels of one or more
layers of graphene, denoted respectively single wall
(SWCNT) or multiwall (MWCNT) [1]. Due to their
unique physical and chemical properties, CNTs are
promising for a variety of potential applications in the
fields of nanoelectronics, sensors, biosensors, catalysis,
nanocomposites and biomedicine, just to mention a few of
them. However, their low solubility in most solvents,
weak affinity with most of polymer matrices, poor
chemical and biological compatibility, and the
hydrophobic and inert nature of the surface of as-prepared
CNTs greatly hinder their practical applications. Besides
many other applications, CNTs have been proposed as
reinforcing nanoparticles in high-performance composite
materials, such as ceramics, polymers and cement
matrices, that should greatly improve the properties of the
composites as compared with traditional reinforcing
materials, such as glass or carbon fibers. CNTs exhibit
great mechanical properties and the high stiffness,
strength and toughness could be achieved simultaneously
because nanotubes will deform prior to breaking [2].
Furthermore, because of their size (from 1 to tens of nm)
and high aspect ratios (length-to-diameter ratio), CNTs
can be distributed in a much finer scale than common
fibers, giving as a result a more efficient crack bridging at
the very preliminary stage of crack propagation within
composites [3]. However, to achieve this result, it is
necessary to create strong bridges between the matrix and
the nanotubes that will transfer the load. This issue is
much more critical for CNTs than for carbon microfibers,
because the interfacial area is much higher in the case of
CNTs and their surface is chemically inert. Poor
dispersion of nanotubes in the matrix is also a problem
because large aggregates of CNTs easily initiate cracks in
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composites rather than reinforce them. Therefore, a
proper durable functionalization of CNTs is a prerequisite
for their successful application in composites. The
chemical functionalization of nanotubes is widely studied,
often achieved by means of the oxidation of their surface
by etching acids [4]. Because of the harsh conditions of
wet chemical methods, the structure of nanotubes can be
damaged, their length may be shortened and their peculiar
properties can be seriously compromised. Alternative
methods, such as plasma functionalization, are proposed
[5,6,7]. The excited molecules and radicals generated
during plasma discharge attack C=C bond, creating open
ends and defect sites as prime sites for functionalization.
Various functional can be grafted at CNTs surface by
exposing it to reactive gases under a glow discharge.
Different types of discharge configurations (e.g. dielectric
barrier discharge and plasma jet) in different conditions
have been utilized for surface modification of
carbonaceous materials. This research is aimed to
improve the hydrophilic character of MWCNTs by means
of low pressure plasma treatments, in order to facilitate
their dispersion in aqueous media and in other polar
matrices, such as those often used for composites.
Different experimental conditions were tested, changing
process gases, input power, time and pressure. N 2 , O 2 and
H 2 O have been selected as gas feed in order to graft
nitrogen or oxygen containing groups (like COOH and
NH 2 ), which should improve the hydrophilic character of
CNTs. A pre-treatment step has also been tested in order
to
activate
the
surface
towards
following
functionalizations. A deep characterization of the
dispersibility of the plasma treated carbon nanotubes has
been accomplished.
2. Materials and methods
Commercially available Multi Walled Carbon
Nanotubes (Cheap Tubes Inc., Cambridge, USA) were
1
used for this study. They are 8-15 nm in diameter with a
length of 10-50 μm. N 2 , O 2 and Ar (99.99%, Air Liquide)
and vapors of H 2 O (DI, 18 Ω) were used alone or in
mixtures as the gas/vapor feed. Plasma surface
modification was performed in a glass tubular reactor, 30
mm in diameter and 118 cm long, described in detail
elsewhere [8]. CNTs samples were placed in vials with a
hole that allows the entry of the feed gas and prevents
leakage of the nanotubes under vacuum conditions. The
plasma discharge was initiated with three external
capacitively-coupled copper ring-electrodes, connected to
a 13.56 MHz Radio Frequency (RF) power source
(RFX600, Advanced Energy) through a home-made
matching unit. The gas pressure in the chamber was
monitored using a Baratron capacitance manometer (MKS
626, 1 Torr). Different experimental conditions (process
feed, input power, process time and pressure) were tested.
Ar, N 2 and O 2 flow were controlled through a MKS
mass-flow controller (20 sccm) and H 2 O (g) flow was
controlled via a needle valve (LV10K Leak Valve, BOC
Edwards). In these conditions, up to 50 mg of CNTs can
be treated homogeneously. Plasma treated CNTs (PTCNTs) were dispersed in water and their behavior was
observed over time, compared with untreated samples (UCNTs). Supernatant solutions were then characterized by
means of UV-VIS absorption measurements with a
Agilent 8453 diode array spectrophotometer. X-ray
photoelectron spectroscopy (XPS) was performed to
chemically characterize CNTs samples. The analysis was
carried out on powders supported on a conductive
adhesive aluminum tape suitable for ultra high vacuum,
with a Theta Probe Thermo VG instrument
(monochromatic Al Kα X-ray source, 1486.6 eV; take-off
angle 54.5°; 300 µm spot size). Binding energies were
charge corrected by setting the C1s aliphatic carbon signal
at 285.0 eV. FT-IR spectra were obtained in transmission
mode with a Vertex 70V Bruker spectrometer from CNTs
dispersed in KBr tablets. Scanning electron microscopy
(SEM) characterization of U- and PT-CNTs was carried
out with a SEM-FEG (ZEISS SUPRA 40). Dynamic light
scattering (DLS) characterizations have been used for zeta
potential determination of suspended CNTs, using a
Nanosizer ZS (Malvern instruments).
3. Results and discussion
To improve the efficiency of the treatments toward the
dispersibility of CNTs in water, an optimization study of
the plasma processes was performed. Initially different
process gases (N 2 , H 2 O, O 2 , and N 2 /H 2 O blends) were
tested and the samples obtained were dispersed in water,
in order to evaluate the efficiency of the dispersion. It was
observed that, after vigorously shaking, the U-CNTs
deposit at the bottom of vials in less than one hour, while
PT-CNTs disperse in water, attesting for their
functionalization. The best dispersion improvement was
obtained when CNTs were treated with O 2 , as they
remain dispersed for more than 1 week, demonstrating a
large increase in wettability. O 2 treatments performed in
2
our conditions resulted in negligible morphological
changes. Figure 1 shows, in fact, the SEM image of UCNTs and O 2 PT-CNTs, where no differences are
observed.
Fig. 1. SEM image of U-CNTs (a) and O 2 PT-CNTs (b)
Keeping the O 2 flow rate fixed at 15 sccm, the power
was changed from 20 to 100W, treatment time from 5 to
30 min and pressure from 150 to 300 mTorr. After plasma
treatment, CNTs were dispersed in water. As shown in
figure 2, part of PT-CNTs precipitate, while part remain
dispersed in water for more than a week, showing an
improvement of the dispersion character compared to the
native sample. Only O 2 PT-CNTs treated at 100W
presents, instead, a dispersion behaviour similar to the
native one. The best results in terms of CNTs dispersion
have been found at higher pressure (600 mTorr) and for
longer time (30 min). Instead, an increase of power seems
to worsen the efficiency of the treatment. These results
were confirmed by UV-VIS absorption measurements of
the supernatant and are in agreement with XPS analyses.
Samples treated for 30 minutes and at 600 mTorr present
the highest O/C ratio, about 0,14 ± 0,01. Clearly, in these
conditions the grafting efficiency of the oxygen radicals
produced in the plasma is highest, both because of the
longer duration of the process and because their density in
the reactor is higher as a consequence of the higher
pressure. Conversely, the 100W PT-CNTs shows a low
O/C ratio of 0,06 ± 0,02, comparable to that one of native
CNTs (0,06 ± 0,01). A competition between etching and
grafting was hypothesized to explain such results, that
practically leaves PT-CNTs as untreated. The XPS C1s
signal of PT-CNTs obtained at different RF power values
is shown in figure 3, along with that of U-CNTs, reported
as reference. FT-IR analyses confirm the presence of
oxygen polar groups (e.g. OH groups) as a result of O 2
plasma treatments.
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potentials more positive than +30 mV or more negative
than -30 mV are normally considered stable, since they
avoid coagulation by electrostatic repulsion, overcoming
the van der Waals forces between tubes.
Fig. 3. XPS C1s spectra of U-CNTs and PT-CNTs
(15 sccm O 2 , 15 min, 150 mTorr).
4. Conclusion
The results presented in this paper demonstrate that
plasma processing of commercial CNTs allows enhancing
their dispersion in water by modifying their surface
properties. The resulting PT-CNTs dispersions in water
remained stable in time for several months. A detailed
characterization has been carried out to understand the
effect of plasma treatments on CNTs in terms of better
dispersibility, which is an extremely important property
for their application in nanocomposite materials. In
particular, this set of experiments in O 2 discharges has
highlighted that the best results can be obtained at long
process time and at high pressure.
Fig. 2. Dispersion of plasma treated CNTs in the different
experimental conditions.
DLS measurements show that the zeta potentials of UCNTs dispersed in water is about -22 ± 1 mV, instead an
increase of zeta potentials is observed on PT-CNTs,
which is in the range from -31 to -50 mV, depending on
the experimental conditions used. The higher zeta
potential values found on PT-CNTs can certainly be
attributed to the presence of polar ionizable moieties, such
as carboxylic functional groups, grafted at the surface of
CNTs. Considering that the general dividing line between
stable and unstable suspensions is generally taken at
either +30 or -30 mV [9], these values attest for the high
stability in water of PT-CNTs. In fact, CNTs with zeta
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5. Acknowledgements
Mr. S. Cosmai (CNR-IMIP) and Mr. D. Benedetti
(Univ. Bari) are gratefully acknowledged for their
technical contribution. Schlumberger-Doll, INSTM and
the projects LIPP (Rete di Laboratorio 51, Regione
Puglia) and SISTEMA (PON MIUR) are gratefully
acknowledged for funding and supporting this research.
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