22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Deposition of amine functions onto ZnO nanoparticles
S. Mathioudaki and S. Lucas
Research Centre for the Physics of Matter and Radiation (PMR) University of Namur (FUNDP), 61 rue de Bruxelles,
BE-5000 Namur, Belgium
Abstract: Low-pressure plasma treatment of cyclopropylamine (CPA) can achieve the
grafting of stable amine-rich thin films onto ZnO nanoparticles, reduce the agglomeration
and favor dispersion. In this work, first results of surface functionalization of ZnO
nanoparticles by plasma polymerization are presented.
Keywords: ZnO nanoparticles, cyclopropylamine, plasma polymerization
1. Introduction
Low pressure plasma polymerization process is gaining
increasing interest due to the ability of grafting functional
groups and produce thin polymeric films of organic
compounds on most substrates, by using a relatively
simple one-step coating procedure while additionally is a
low cost process and environmental friendly. The
modification of surfaces can be done by using different
precursors, which can generate functional groups as
amine –NH 2 , carboxyl –COOH or hydroxyl –OH [1].
The deposition of thin films rich in amine by plasma
polymerization can be carried out by the use of saturated
organic monomers as butylamine [2] and propylamine
[3] or unsaturated organic monomers as allylamine [4] or
propargylamine [3] which can lead to a highly branched
and cross-linked polymeric film onto the substrates.
In this study we investigated the amine-containing
plasma polymers utilizing cyclopropylamine as precursor
monomer. To optimize the conditions of deposition
(power, flow rate of monomer), initially silicon wafers
were used and in sequel the plasma polymerization
occurred onto the ZnO nanoparticles. We used a
homemade, cylindrical hollow cathode involving a fixed
magnetron setup (Fig 1). The drum was able to rotate
(25rpm), mix the nanoparticles and expose their surface to
the discharge in order to graft amine functions from
cyclopropylamine (CPA).
2. Chemicals
Cyclopropylamine (98% purity) and was purchased
from Sigma-Aldrich and utilized as received without any
other purification. Commercial Silicon wafer substrates
prepared and cleaned by immersing in acetone in an
ultrasonic bath for 15 minutes, followed by immersing in
ethanol for 15 minutes and finally dried with flowing
nitrogen. ZnO nanoparticles were purchased from Nano4
[5] and used as received.
3. Plasma Polymerization
To decide on the best conditions, first plasma polymers
of CPA were deposited on flat silicon wafers (1×1cm2),
and in a second set of experiments onto the ZnO
nanoparticles.
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Fig.1. Sectional scheme of the reactor, A: rotating
cylindrical hollow cathode, B: fixed magnetron set-up, C:
gas shower, D: Plasma ring and E: Nanoparticles load.
For the plasma polymerization of CPA the homemade
set-up utilized and the whole set-up was placed in a
vacuum chamber. The monomer was introduced in a glass
vacuum flask and connected to the vacuum chamber. A
flow control valve was placed between the chamber and
the flask to obtain a stable pressure. The polymerization
occurred in the pulsed mode, in which is feasible to
control the chemistry of polymeric films. Moreover,
experiments occurred with CPA in addition with Argon in
several flow rates. The flow of Argon was regulated with
a mass flow controller in each case. The pressure before
starting plasma was 10−6 Torr and was obtained with a
turbomolecular pump. The working pressure in all
experiments regulated at 10−1 Torr. Table 1 presents the
samples and the plasma polymerization conditions.
Table 1. Description of samples. Experimental conditions
employed.
Sample Name
Si(CPA)-28
Si(CPA)-16
Si(CPA+Ar30)-28
Si(CPA+Ar30)-16
Si(CPA+Ar60)-28
Precursor(s)
CPA
CPA
CPA+Ar 30sccm
CPA+Ar 30sccm
CPA+Ar 60sccm
Duty Cycle %
28
16
28
16
28
P (W)
14
9.6
16.8
9.6
11.2
1
28
11.2
5. Results
The chemical composition of amine thin layers of the
Si-samples is presented in Table 2 as measured by XPS.
The coating was mainly composed of carbon, nitrogen
and oxygen. It is well known that the chemical
characteristics of the plasma polymeric films depend on
the plasma conditions and it can be seen that the nitrogen
atomic concentration can vary between 10-11.7 atomic%.
The highest concentration of N appears in the case when
precursor was the mixture of CPA and Argon (60sccm),
and that means that the addition of Argon ions can
dissociate easier the monomer and keep the plasma stable.
Table 2. Elemental composition of CPA layers.
Sample Name
Si(CPA)-28
Si(CPA)-16
Si(CPA+Ar30)-28
Si(CPA+Ar30)-16
Si(CPA+Ar60)-28
Precursor(s)
CPA
CPA
CPA+Ar 30sccm
CPA+Ar 30sccm
CPA+Ar 60sccm
C[at%]
84.4
81.2
81.8
83.8
83.3
N[at%]
10.4
10.1
11.1
10.6
11.7
O[at%]
5.2
8.7
7.1
5.6
5
Plasma polymerization of CPA in addition with Ar
(60sccm) was chosen for the treatment of ZnO
nanoparticles. The polymerization lasted two minutes and
a thin amine layer was deposited. The suitable rotating
reactor has permitted the surface modification of the
nanoparticles after their exposure to plasma. In figure 2
are presented the survey spectras of ZnO nanoparticles
before and after plasma polymerization. In the spectra of
untreated ZnO nps two different peaks are observed that
can be attributed to Zn and O elements in energies
~1022eV and ~531eV respectively, while in the spectra of
treated nps four peaks exist, which correspond to Zn
(~1022eV), O (~532eV), N (~399eV) and C (~285eV) . It
is interesting to note the incorporation of nitrogen and
carbon with only two minutes of plasma treatment. The
2
chemical composition for the treated ZnO nanoparticles is
Zn~39at.%, O~41at.%, C~18at.% and N~2at.%.
untreated ZnO nps
treated ZnO nps
200000
2.0x106
Intensity (a.u.)
CPA+Ar 60sccm
Intensity (a.u.)
ZnO
4. Characterization Methods
X-ray photoelectron spectroscopy was carried out using
a SSX 100 Spectrometer system equipped with a
hemispherical electron analyzer, while the photon source
was the monochromatized Al Kα line (hv=1486.6eV).
The XPS characterization occurred after depositing the
polymeric layer on the Si samples, while for the ZnO
nanoparticles occurred both before and after the grafting
of amine groups. The survey and the high resolution scan
measurements were performed at pass energies of 200eV
and 30eV respectively.
Transmission Electron Microscopy was carried out
using a Philips-Tecnai 10 microscope at 80kV, for the
determination of the particle size distribution before the
deposition of plasma polymers and an estimation of the
agglomeration. In addition, particle number distributions
were measured with a disk centrifuge DC 24000 System,
which based on the centrifugal liquid sedimentation
method (CLS), in order to evaluate the effect of the thin
film polymers.
6
1.5x10
100000
C1s
N1s
0
420
350
280
Binding Energy (eV)
1.0x106
5.0x105
0.0
1000
800
600
400
200
0
Binding Energy (eV)
Fig.2. Survey spectras of untreated and plasma
polymerization treated nanoparticles. Inset shows the
peaks of N1s and C1s of treated nps.
In order to characterize the morphology of
nanoparticles produced by CPA plasma polymerization,
Transmission Electron Microscopy (TEM) was used to
determine their size distribution. For ZnO nanoparticles
the size distribution gives 22±5nm before the plasma
polymerization, while after the treatment, an increase in
their diameter observed.
The significant feature of nanoparticles, the high
surface energy, can lead to agglomeration. Figure 3
graphs the relationship between the treated and untreated
ZnO nanoparticles. Regardless the fact that the treatment
was not homogeneous, the amine thin layer of ~15.5±3nm
can be clearly seen. Grafting amines onto the ZnO
nanoparticles will probably lead to enhanced dispersion
and might solve the problem of agglomeration.
Fig.3. Untreated
nanoparticles.
(left)
and
treated
(right)
ZnO
ZnO nanoparticles were further characterized by a
different analytical technique, the centrifugal liquid
sedimentation (CLS) in order to give complementary size
information about the suspension. The CLS method is
measuring the velocity of particles moving in a
suspending density gradient medium and under the action
of an increasing gravitational field due to the rotation of
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the disc. Depending on their size and density, particles
will sediment at different velocities. Particles of a given
density but with decreasing sizes will sediment with
decreasing velocities.
CLS occurred before and after the plasma treatment of
ZnO nanoparticles by CPA. The results showed that the
diameter of the nanoparticles increased. Figure 4 presents
the particle number distribution as a function of size of
the ZnO nanoparticles dispersion before and after plasma
polymerization. The average diameter for the untreated
nanoparticles is 14nm with range from 10-18nm at
FWHM and also a population of bigger particles or
agglomerations is detected at ~100nm. For the treated
nanoparticles, the particle number distribution shows that
the average diameter is 42nm with range from 35-60nm at
FWHM, due to the deposition of the amine polymeric
film. Nanoparticles increased their hydrodynamic
diameter almost three times with two minutes treatment.
Relative number (%)
100
untreated ZnO nps
treated ZnO nps
50
0
10
100
Hydrodynamic diameter (nm)
1000
6. Conclusions
In this work we have demonstrated that the grafting of
amine functions onto nanoparticles is feasible. The
characterization of nanoparticles showed that two minutes
of plasma treatment by CPA can lead to the incorporation
of amine functions, increase their hydrodynamic diameter
and might lead to enhanced dispersion.
In the next steps, plasma polymerization of
cyclopropylamine will be used for the modification of
ZnO nanoparticles in different parameters and more
characterization will be employed in order to compare our
results. Also, the stability of the thin polymer will be
tested as well as the homogeneity, the effect of aging and
the reproducibility.
7. Acknowledgments
This research was supported by the Flycoat project
financed by the Marshall Plan 2.vert from the Walloon
Region.
8. References
[1] K.S. Siow, L. Britcher, S. Kumar, H.J. Griesser,
Plasma Processes and Polymers 392, 3 (2006).
[2] G.Pozniak, I. Gancarz, M. Bryjak, W. Tylus,
Desalination 293, 146 (2002).
[3] D. Mangindaan, W. Kuo, C.C. Chang, S.L. Wang,
H.C. Liu, M.J. Wang, Surface and Coatings
Technology 1299, 206 (2011).
[4] C. Rigaux, F. Tichelaar, P.Louette, J.L. Colaux, S.
Lucas, Surface and Coatings Technology S601, 205
(2011).
[5] http://www.nano4-materials.com/content.aspx?id=1
Fig. 4. CLS particle size distribution: relative number
distributions for untreated and plasma treated ZnO
nanoparticles by CPA.
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