22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
VOC sensing properties of plasma coated QCM based sensor related to
monomer flow rate
M. Boutamine1, O.C. Lezzar1, A. Bellel1*, S. Sahli2, Y. Segui3 and P. Raynaud3
1
Laboratoire des Etudes des Matériaux Electronique pour Applications Médicales (LEMEAmed), Faculté des Sciences
de la Technologie, Université des Frères Mentouri Constantine, Algeria
2
Laboratoire de Microsystèmes et Instrumentation (LMI), Faculté des Science de la Technologie, Université des Frères
Mentouri Constantine, Algeria
3
Laboratoire Plasma et Conversion de l’Energie (LAPLACE), Université Paul Sabatier, 118 Route de Narbonne,
FR-31062 Toulouse Cedex, France
Abstract: Hexamethyldisiloxane (HMDSO) thin films coated quartz crystal microbalance
(QCM) electrodes have been characterized for the detection of volatile organic compounds
(VOCs). The sensitive coatings were plasma polymerized in pure vapor of HMDSO with
different monomer flow rate. The sensor sensitivity was evaluated by monitoring the
frequency shift (∆f) of the coated QCM electrode exposed to different concentrations of
benzene, chloroform and their mixture. Fourier Transform Infrared Spectroscopy (FTIR)
and Atomic Force Microscopy (AFM) were used to study the chemical composition and
surface morphology of the plasma polymerized sensitive layers. The results show that
surface roughness combined with chemical structure significantly affects the sensitivity of
the QCM-based sensor.
Keywords: monomers, PECVD, Volatile organic compounds, AFM, FTIR
1. Introduction
Volatile organic compounds (VOCs) are found in
ambient air due to human activities and natural sources.
The identification and monitoring of VOCs have become
serious tasks in many countries of the world and are
important for the early control of environmental pollution
[1]. The analysis of gases represents one of the main
objectives of current research in the sensor field [2, 3].
Due to their low price, fast response and ready integration
into electronic circuits, one widely investigated device is
the Quartz Crystal Microbalance (QCM) [4]. In order to
be used as chemical sensor, the QCM electrode is coated
with sensitive layer capable of interacting with the
chemical species of interest. At present, a great variety of
sensitive polymers materials have been successfully
employed as coating of QCM sensors especially for
monitoring environmental pollutants [5, 6]. There is an
interest in using plasma polymerized films as sensitive
layers in chemical sensors because they can be deposited
on any substrate and feature excellent mechanical and
thermal stability and have low fabrication cost [7, 8].
In this study, plasma polymer materials were deposited
in low frequency plasma reactor from pure vapor of
HMDSO on QCM gold electrode substrates. Among the
numerous parameters, which have an influence on the
synthesized plasma film structure, the monomer flow rate
is fundamental. In fact, this parameter could be related to
the monomer fragmentation degree and radicals
concentration in the plasma reactor.
During
polymerization the partial pressure of HMDSO has been
varied in order to elaborate VOC sensor with different
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sensing properties. The sensor response in terms of QCM
frequency shift was evaluated towards different
concentration of chloroform, benzene and their mixture.
The structural analysis of the elaborated sensitive layers
was carried out by Fourier transform infrared (FTIR)
spectroscopy and the morphological properties were
investigated by Atomic Force Microscopy (AFM).
2. Experimental
Thin sensitive layers were elaborated using plasma
Enhanced chemical vapor deposition (PECVD) technique
[9]. The films were deposited in low frequency plasma
reactor from pure vapor of HMDSO at different monomer
flow rate.
The power during polymerization was
controlled by a 19 kHz generator. The system consisted
of parallel symmetrical electrodes, vacuum system
(composed of Alcatel primary pump) and a monomer inlet
system. The pressure in the reactor was monitored by a
pressure measurement system (Pirani). Samples were
placed in the grounded lower electrode and the reactor
chamber was pumped down to 10 Pa. A partial monomer
pressure was adjusted to 15, 30, 40 and 50 Pa to
investigate their effects on the sensing properties of the
elaborated sensor. The evaluation of sensors responses
were carried out by monitoring the frequency shifts of the
quartz exposed to two various VOCs vapors and their
mixture. The experimental setup is illustrated in Fig.1. In
the step of individual detection, a liquid of known volume
and density was introduced in the testing cell and heated
to evaporate freely. After evaporation and diffusion
towards the electrode surface, the injected vapor was
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subsequently adsorbed onto the functionalized QCM
electrode surface, which induced a frequency shifts.
Fig. 1. Sensor response characterization of individual and
binary gas mixture.
The sensitivity of the elaborated QCM sensors was
evaluated towards different concentrations varied from
about 40 to 200 ppm. In the step of gas mixture detection,
the individual VOCs (benzene and chloroform) were
introduced separately at the same time in the testing cell
and heat together to evaporate. The values of ∆f were
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3. Results and discussion
Fig. 2 shows the relation between concentration and
shift frequency of QCM coated from HMDSO at different
pressure. It is clearly seen that ∆f versus analyte
concentrations exhibited satisfactory linear relationship.
The sensitive layer was found to be reasonably selective
and more sensitive to chloroform vapor than benzene
vapors. The sensor coated from HMDSO at 40 Pa
presents a great sensitivity to VOC than other monomer
layers.
Benzene (a)
Chloroform
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Shoft frequency (Hz)
Shoft frequency (Hz)
20
transferred to a computer via RS232 interface.
Chemical structure and composition of the deposited
films were characterized by FTIR spectroscopy. All
spectra were acquired in absorbance mode in the
400 - 4000 cm-1 range using a Nicolet Avatar 360 FTIR
spectrometer.
Surface morphology of the QCM coating was studied
using the Agilent Technologies 5420 Scanning Probe
Microscope. The surface roughness was examined on a
nanometer scale and AFM images were recorded in
contact mode with scan area of 5 µm × 5 µm.
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240
220
200
180
160
140
120
100
80
60
40
20
40
60
80
100 120 140 160
Concentration (ppm)
180
200
40
200
360
340
320
300
280
260
240
220
200
180
160
140
120
100
40
Benzene (c)
Chloroform
Shoft frequency (Hz)
Shoft frequency (Hz)
6
40
60
80
100 120 140 160
Concentration (ppm)
180
30
28
26
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20
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10
Benzene (b)
Chloroform
60
80
100 120 140 160
Concentration (ppm)
180
200
Benzene (d)
Chloroform
60
80
100 120 140 160
Concentration (ppm)
180
200
Fig. 2. Variation in shift frequency of QCM coated by HMDSO at: (a) 5 Pa, (b) 20 Pa, (c) 30 Pa and (d) 40 Pa as a
function of the concentration of chloroform and benzene.
Fig.3 showed the differential frequency shifts of the
four QCM sensors exposed to four different
concentrations of binary mixture of chloroform and
benzene. It can be seen from the bar chart (Fig. 3) that
there is significant statistical variation between the
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frequency response curves of the sensors.
FTIR spectra of the HMDSO films deposited at various
pressures are shown in Fig. 4.
When plasma
polymerization is performed in pure vapour of HMDSO,
IR spectra exhibit a main absorbance in the
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220
200
180
160
140
120
100
80
60
40
20
0
5 Pa
20 Pa
30 Pa
40 Pa
coated at high pressure of HMDSO.
Si-O-Si
Si-(CH3)3
Absorbance [arb.unit.]
Shift frequency (Hz)
1= 47 ppm chloroform+ 47 ppm benzene
2= 95 ppm chloroform+ 95 ppm benzene
260 3= 142 ppm chloroform+ 95 ppm benzene
240 4= 189 ppm chlorofom+ 47 ppm benzene
(d)
Si-OH CHx
Si-CH2
Si-H
(c)
(b)
(a)
1
2
3
Binary mixture
4
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber [cm-1]
Fig. 3. Frequency response of QCM for a binary mixture
of chloroform and benzene at different concentration.
2960 - 2850 cm-1 range related to CH x groups. It clearly
seen that increasing the HMDSO flow rate, there is an
increase of hydrocarbons (CH x ), methyl groups Si–
(CH 3 ) 3 and Si-H with the decrease of hydroxyl groups.
The increase of methyl groups makes the film more
hydrophobic. It would be expected that QCM sensor
coated with hydrophobic film can detect volatile organic
compounds with decreasing water effect from ambient
conditions because of hydrophobicity. These behaviors
justify the high sensitivity observed for QCM sensor
Fig. 4. FTIR spectra of plasma-polymerized coating of
HMDSO at: (a) 5 Pa, (b) 20 Pa, (c) 30 Pa and (d) 40 Pa.
Fig. 5 depicts a typical high resolution AFM images.
The elaborated sensitive coatings are continuous and
pinhole-free with no cracks.
Fig. 5. AFM characterization of the QCM sensor surface coated at: (a) 5 Pa, (b) 20Pa, (c) 30 Pa and (d) 40 Pa.
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Nanoparticles with varying sizes and quantities were
seen to be randomly distributed throughout the surface of
the deposited film. The aggregates dimensions were in
the range of about 50 - 300 nm. The surface roughness
increases from 6.97 nm to about 20 nm with increasing
HMDSO partial pressure. The increase in surface
roughness leads to the increase in the specific surface area
due to large surface to volume ratio. The enhanced sensor
surface area can then accommodate more adsorption sites,
which justifies the enhanced adsorptive properties of
QCM senor coated from HMDSO film deposited at
monomer flow rate of about 40 Pa.
4. Conclusion
As demonstrated by FTIR and AFM analyses, the
chemical structure and surface morphology of the coated
QCM sensors are clearly correlated with their sensing
properties. The QCM sensor coated with hydrophobic
film would be sensitive to VOCs molecules due to the
interaction with methyl groups on the sensitive coating
surface. AFM images revealed the increase of the surface
roughness, leading to the increase of the sensor
sensitivity. Surface morphology combined with the
chemical composition strongly affects the sensitivity of
the coated QCM sensor.
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