22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Oxygen functionalization of graphene nanoflakes in thermal plasma
recombination zone
U. Legrand, J.-L. Meunier and D. Berk
Plasma Processing Laboratory (PPL), McGill University, Chemical Engineering Montreal, Québec, Canada
Abstract: Graphene nanoflakes (GNFs) are a stack of 5 to 15 layers of graphene sheets
measuring approximatively 100x100 nm, and produced here through homogeneous
nucleation following the plasma decomposition of a carbon source in an ICP thermal plasma
reactor. The GNFs are functionalized downstream of the plasma zone with the addition of
oxygen to the main plasma stream. Oxygen functionalization up to 14.2 at% was reached on
the surface of the particles. While the non-functionalized GNFs were hydrophobic, the
O-functionalized GNFs were shown to be stably dispersed in water and ethanol.
Keywords: graphene nanoflakes, thermal plasma, nanofluids, inductively coupled plasma
1. Introduction
A method to produce highly crystalline carbon powders
in the shape of graphene flakes was described by
Pristavita using the plasma decomposition of methane [1].
Modelling and experimental optimization enabled the
ability to obtain pure, homogeneous, and well crystalized
powders having between 5 - 15 graphitic layers (10 layers
being typical) and in-plane dimensions of roughly
100x100 nm. The high surface area and the crystallinity
of the graphene nano-flakes (GNFs) made them good
candidates to support catalytic sites. The original work on
GNFs focused on adding nitrogen functionality through
the addition of nitrogen in the main plasma stream during
the GNF nucleation phase. This resulted in low nitrogen
contents below 2 at% [2]. In a follow up study, these
N-GNFs were extracted and functionalized with iron by a
wet chemical method for the replacement of the platinum
catalyst typically used in fuel cells. The catalyst obtained
showed both activity and full stability towards the oxygen
reduction reaction (ORR) over 100-hour tests in polymer
membrane fuel cells (PEM-FC) [3].
The iron
functionalization step relied on the dispersion of GNFs in
a mixture of water and ethanol, however the hydrophobic
nature of GNFs made iron incorporation inefficient
because of the partial agglomeration of nanoparticles in
solution. A hydrophilic version of the same GNFs would
eliminate this problem and improve the iron incorporation
method, yielding a better catalyst. In the present work,
fully hydrophilic GNFs are produced by adding oxygen
functionalities to the nanomaterial directly within the
plasma reactor. The new material is characterized and
compared to hydrophobic GNFs to determine differences
in the chemical structure.
2. Methods
GNFs are grown following the Pristavita, et al.
procedure [1], where a carbon source, methane, is
decomposed using an argon plasma. In this first stage of
the process, GNFs nucleate and grow within the high
P-II-8-17
temperature plasma stream downstream of the ICP plasma
torch, and the nanoparticles are deposited on the side
walls and on the downstream bottom plate of the reactor
set in an axisymmetric stagnation point flow geometry
pattern. Immediately after the growth, a second step of
the process is set by changing the gas inlet from methane
and nitrogen to 15 slpm of air (79% nitrogen and 21%
oxygen). This two-stage process is favoured over a onestage process with air injected simultaneously with the
carbon source; it has been observed experimentally, and
evaluated using equilibrium thermodynamic calculations,
that the oxygen from the air consumes the carbon and
produces carbon monoxide and carbon dioxide instead of
GNFs.
Several conditions were tested to see the influence on
the oxygen implementation on the GNFs and on the
dispersion quality. The power delivered to the plasma
and the pressure in the reactor during the functionalization
step was either 20 kW/55.2 kPa (8 psia), which
corresponds to the conditions of GNF growth, or
25 kW / 13.8 kPa (2 psia). It has been shown previously
that a lower pressure, coupled with a higher power of the
plasma, leads to greater nitrogen functionalities added on
the GNFs in the same configuration [4]. The duration of
the oxygen functionalization step was optimized, as it was
observed that a ‘short’ functionalization time produces
low oxygen content on the nanoparticles, while a ‘long’
time would allow the oxygen to consume the deposited
GNFs.
The conditions used throughout the
functionalization step are listed in Table 1.
The powders were collected and weighed to obtain the
yield of both non-functionalized GNFs (NF-GNFs) and
oxygen functionalized GNFs (O-GNFs), knowing that a
batch of NF-GNFs from the initial nucleation step yields
approximatively 200 mg.
Dispersion tests were realized in water and ethanol.
The tests were conducted using 5 mg of powder from the
different tested conditions and 8 mL of solvent. The
stability of the dispersion in water was measured by zeta
1
Table 1. Conditions for the functionalization step.
Samples
A
B
C
D
Pressure/Power
conditions
55.2 kPa / 20 kW
55.2 kPa / 20 kW
13.8 kPa / 25 kW
13.8 kPa / 25 kW
Duration of the
functionalization
10 min
20 min
10 min
5 min
potential, while the stability in ethanol was observed
visually.
X-ray photoelectron spectroscopy (XPS) was done on a
VG Scientific ESCALAB MK II, operating at a pressure
of 10-9 Torr, and using an aluminium X-ray source. Zeta
potential was measured on a Zetasizer Nano ZS from
Malvern. Scanning Electron Microscopy (SEM) was
performed on a FEI Inspect F-50 FE-SEM. The Raman
instrument used was an inVia Reflex confocal microRaman (Renishaw) with a laser emitting at a wavelength
of 514.5 nm.
3. Results and discussion
3.1. Oxygen content
XPS was used to determine the atomic composition of
the O-GNFs surface. This non-destructive technique
gives information on the elemental composition of the
first few nanometres of the sample. While the GNFs are
oriented in all the directions, XPS gives a good average
for the atomic composition. The XPS survey for
functionalized in a 13.8 kPa/25 kW argon plasma during
10 min shows the carbon and oxygen peak, with some
traces of nitrogen (Fig. 1).
contents, 13.9 and 14.2 at% respectively, showed a loss in
yield by the end of the oxygen functionalization step; a 74
and 44% loss based on a normal batch of 200 mg of
GNFs.
Table 2. Yields and atomic composition after oxygen
functionalization.
Sample
Yield (mg)
C at%
N at%
O at%
A
196.6
98.1
0.6
1.3
B
52.5
85.3
0.8
13.9
C
111.7
84.9
0.9
14.2
D
205
92.8
0.8
6.4
The oxygen and carbon peaks were studied in more
detail through a deconvolution using Gaussian-Lorentzian
peaks. This analysis extracts the state(s) of the element,
and the functional groups formed with that element can be
retrieved.
The deconvolution of oxygen and carbon peaks is
presented here for sample C, which gave the best result in
terms of oxygen content and yield. In examining the
oxygen peak (Fig. 2), it appears that the main binding
energies involved for the oxygen are 532.1 and 533.3 eV.
These energies are associated with carbonyl oxygen in
ester, anhydrides, oxygen atoms in hydroxyls or ester for
532.1 eV, and ether oxygen atoms in esters and
anhydrides for 533.3 eV [5]. While the GNFs are mainly
graphitic shaped, the hydrophilic groups must be formed
on the edges of the nanoparticles, and on the structural
defects all over the surface.
Fig. 1. XPS survey scan of O-GNFs functionalized in a
13.8 kPa/ 25 kW argon plasma during 10 min.
Fig. 2. High resolution and deconvolution of oxygen
peak and deconvolution for O-GNFs functionalized in a
13.8 kPa / 25 kW argon plasma during 10 min.
The oxygen content was found to be up to 14.2 at.%,
while the usual content for NF-GNFs is approximately
between 1 and 2 at.%, and originates mainly from
contamination. The oxygen contents for all of the
samples, as well as their yields, are summarized in
Table 2.
The samples having the highest oxygen
The deconvolution of the carbon peak (Fig. 3) confirms
the presence of the hydrophilic groups with the peaks at
285.5, 287.7, and 286.7 eV corresponding respectively
with C-O, C=O, and O=C-O groups. A high number of
hydrophilic groups on the O-GNFs would allow them to
be perfectly dispersed in a polar solvent.
2
P-II-8-17
work do not exhibit the same behaviour, and instead show
excellent stability in both water and ethanol for long time
periods. To further confirm this, samples were left
undisturbed for 6 weeks and then observed visually
(Fig. 5).
Fig. 3. High resolution and deconvolution of carbon peak
and deconvolution for O-GNFs functionalized in a
13.8 kPa / 25 kW argon plasma during 10 min.
3.2. Dispersion tests
It has already been shown that in the presence of water,
the NF-GNFs do not mix at all and stay at the surface of
the liquid [1]. However, when functionalized with
oxygen, the O-GNFs with oxygen contents of 14.2 and
13.9 at% mixed perfectly with water. To measure the
stability in water, the zeta potential was measured on all
the prepared samples (Fig. 4). Zeta potential enables a
quantification of the stability of a suspension by
measuring the electrophoretic mobility of particles in
suspension. It is commonly accepted that a zeta potential
of less than -40 mV (or more than + 40 mV) denotes a
good stability. A suspension having a zeta potential
between ±10 and ±30 mV is considered as moderately
stable. However, for samples having a zeta potential in
this range, an agglomeration occurring over time has been
observed.
Fig. 4. Zeta potential measurements for all prepared
samples in a 1 mM NaHCO 3 solution.
From past work, it is known that when NF-GNFs are
mixed with ethanol, there is very little stability and the
material settles to the bottom after a couple of minutes.
The O-GNFs with high oxygen content produced in this
P-II-8-17
Fig. 5. Observed stability after 6 weeks for a) GNFs with
14.2 at% oxygen in ethanol, b) GNFs with 14.2 at%
oxygen in water, c) GNFs with 6.4 at% oxygen in water,
and d) NF-GNFs in water.
Dispersion tests and visual observation showed that
only the samples with the highest oxygen content can be
perfectly mixed with water and ethanol. Their stability
can be attributed to their high oxygen content, which is
directly related to the number of hydrophilic groups
surrounding the nanoparticles.
If the number of
hydrophilic groups is insufficient, the stability of the
nanoparticles in water is eventually lost.
3.3. Homogeneity and crystallinity
The samples functionalized with the different
conditions were analysed by SEM and compared to
NF-GNFs (Fig. 6). The O-GNF samples showed the
same appearance as the NF-GNFs. It is important to note
the absence of any amorphous carbon or polymer in the
samples, thus demonstrating the purity of the O-GNFs.
While SEM allows preliminary analysis on the overall
sample, Raman spectra (Fig. 7) gives concrete parameters
to evaluate the graphitisation of GNFs. When analysing
carbon based samples, three peaks need to be examined.
The D peak, found in the 1250 to 1450 cm-1 range,
provides information on non-graphitized carbon, while
the G peak, in the 1500 to 1700 cm-1 range, is associated
to the graphitic content of the sample. Finally, the G’
peak in the 2550 and 2850 cm-1 range becomes sharper
and intensifies as the number of graphene layers
increases. The purity, the crystallite size (L a ), and the
average length of graphene planes (L eq ) are all relations
defined by Larouche, et al. [6] and can be deduced from
the intensity and the underlying area from the three peaks
(Table 3). The Raman spectra obtained for NF-GNFs and
sample C are also presented (Fig. 7).
3
Fig. 7. Raman spectrum for O-GNFs (sample 14.2 at% of
oxygen) and NF-GNFs.
Table 3. Graphitic indices relations.
Indices
Purity
L a (nm)
L eq (nm)
Formula
I G /I D
4.44xA G /A D
8.8xA G’ /A D
NF-GNFs
2.83 ± 0.56
8.76 ± 0.93
28.16 ± 5.34
Sample C
1.92 ± 0.17
6.24 ± 0.29
14.50 ± 1.65
It can be inferred from the graphitic indices that
GNFs still have a graphitic structure. However,
oxygen functionalization causes partial damage to
structure of the O-GNFs, by decreasing the purity,
length of the crystallite and the average length of
graphene planes, and so increasing the defects.
Othe
the
the
the
4. Conclusion
By adding a functionalization step with an argon-air
plasma, hydrophobic GNFs became hydrophilic, and
stable in a suspension, with a solvent such as water, or
ethanol. This study was made to facilitate the use of wet
chemical methods on GNFs for catalytic applications;
however a large series of other potential applications can
be considered for this type of nanofluid.
5. Acknowledgments
We acknowledge Dr. J. Lefebvre (Ecole Polytechnique
de Montreal) for her help on the XPS analysis, M. Basnet
(McGill Univ.) for zeta potential measurements, and
P. Pascone for the Raman measurements.
We
acknowledge the funding contributions from the McGill
Engineering Doctoral Award (MEDA), the Fonds de
Recherche Nature et Technologie du Québec (FRNTQ)
and from Natural Science and Engineering Research
Council (NSERC) of Canada.
Fig. 6. SEM picture of a) NF-GNFs, and b) O-GNFs for a
functionalization step occurring at 55.2 kPa/ 20 kW for
10 min, for a length scale of 2 µm. No structural
difference following the O-functionalization step.
4
6. References
[1] R. Pristavita, et al. Plasma Chem. Plasma Process.,
30, 267-279 (2010)
[2] R. Pristavita, et al. Plasma Chem. Plasma Process.,
31, 393-403 (2011)
[3] P.-A. Pascone, et al. Catalysis Today, 211, 162-167
(2013)
[4] D. Binny, et al. in: 12th Int. Conf. Nanotechnology
(IEEE
Nano).
(Birmigham,
UK),
doi: 10.1109/NANO.2012.6322208 (2012)
[5] P. Lakshminarayanan, et al. Carbon, 42, 2433-2442
(2004)
[6] N. Larouche, et al. Carbon, 48, 620-629 (2010)
P-II-8-17