Simulat ion stud ies on the responses of ZnO-Cu O/CNT nanocomp osite based SAW sensor to various volatile organic chemic als

Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

Contents lists available at ScienceDirect

Journal of Science: Advanced Materials and Devices
journal homepage: www.elsevier.com/locate/jsamd

Original Article

Simulation studies on the responses of ZnO-CuO/CNT nanocomposite
based SAW sensor to various volatile organic chemicals
Nelsa Abraham a, b, *, R. Reshma Krishnakumar a, C. Unni b, Daizy Philip c
a

Department of ECE, Government Engineering College Barton Hill, Thiruvananthapuram, 695035, India
Centre for Development of Imaging Technology, Chitranjali Hills, Thiruvallom, Thiruvananthapuram, 695027, India
c
Department of Physics, Mar Ivanios College, Thiruvananthapuram, 695015, India
b

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 16 October 2018
Received in revised form
19 December 2018
Accepted 19 December 2018
Available online 28 December 2018

Surface acoustic wave (SAW) sensors offer a sensitive platform for monitoring important physical entities
with several advantages. They can operate well in extreme conditions such as high temperature, high
pressure and toxic environment. This work presents a 2D model of SAW sensor with carbon nano tubes
(CNT) as the adsorbent material. A second model was also created by incorporating ZnO and CuO
nanospheres into the sensing layer. The responses of the two sensors towards various gases were analysed at room temperature. The design was modelled and analysed using COMSOL Multiphysics software
which applies the finite element analysis to solve for Eigen frequencies. The shift in the resonant frequencies with and without the presence of gases, which is a measure of sensitivity has been estimated
for all the gases. The second model showed improved response. This novel ZnO-CuO/CNT SAW sensor
combining the sensing properties of metal oxide nanostructures and CNT with improved characteristics
can be used as a promising candidate for sensing important volatile organic chemicals at room
temperature.
© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
ZnO
CuO
Surface acoustic waves
Sensors
Carbon nano tubes

1. Introduction
Gas sensing plays a pivotal role in multidisciplinary areas like industrial, medical, ventilation system design, power plants and environmental pollution monitoring. Nowadays pollution is a vexing
problem in urban areas, hence air quality monitoring is becoming
vital. The current trend in gas sensor development is miniaturization
which provides inexpensive, robust and safe sensors [1,2]. In addition,
it helps to achieve multiplexing of sensor arrays as well [3]. Catalytic,
optical or electrochemical sensing mechanisms are mostly used for
gas sensors [4e7]. High sensitivity, low cost gas sensors working at
room temperature required for a real time detection of toxic gases.
Semiconductor type gas sensors were developed in 1962 on the basic
of the resistive changes in the semiconducting metal oxide. A wide

variety of gas sensors based on several metal oxides like ZnO, SnO2,
TiO2, Fe2O3 and Bi2O3 have already been studied [8e11]. The general
gas sensing principle is the adsorption and the desorption of analyte
molecules on the sensing material. So the sensitivity can be enhanced

* Corresponding author. Department of ECE, Government Engineering College
Barton Hill, Thiruvananthapuram, 695035, India.
E-mail address: (N. Abraham).
Peer review under responsibility of Vietnam National University, Hanoi.

by increasing the contact interfaces which are able to achieve through
the use of nanomaterials. Surface Acoustic Wave (SAW) resonators
are a class of Micro-Electromechanical Systems (MEMS) that can also
be used in gas sensing applications [12]. For chemical sensing applications they are the most prominent candidates as their resonant
frequency ranges from several MHz to GHz, which is much higher
than that of quartz crystal microphones (QCM). This wider frequency
range makes them more sensitive and opens the possibility of operating in wireless mode. They are able to detect analytes at ambient
temperatures and can work efficiently even in inert atmospheric
conditions. Besides this, SAW renators are cheap, possess high thermal stability, easy to fabricate, highly selective and more efficient
compared to conventional gas sensors. These special characteristics
make them highly suitable as smart transducers which can be combined with a variety of sensitive coating layers including metal oxides,
carbon nanotubes (CNTs), graphene layers and functional polymers
[13]. Thanks to the enhanced absorption characteristics, new materials such as metal organic frameworks and porous materials are
increasingly utilized in these fields.
SAW sensors are basically piezoelectric crystals which can sense
and detect the masses of chemical vapours adsorbed on the
chemically sensitive coatings. In this case, inter digitated transducers (IDTs) are placed on the surface of a piezoelectric substrate
to generate and receive acoustic waves. The confinement of the

/>2468-2179/© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license

( />

126

N. Abraham et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

Table 1
Model parameters.

Table 3
Frequency shift of various gases for the two different sensors.

Variable

Expression

Description

P
T
C0
C_gas_air
M_gas
K
rho_gas_CNT

1 atm
25  C
100
1eÀ6*C0*P/(R- Constant*T)

M
K
K*M*C_gas_air

Rho_CNT
E_CNT
nu_CNT
eps_CNT
vR
Width
f0
t_CNT

1.49 g/cm3
350 GPa
0.269
À8
3488 m/s
4 mm
vR/width
0.5 mm

Air pressure
Air temperature
Gas concentration ppm
Gas concentration in air
Molar mass of gas
CNT/air partition constant
Mass concentration
of gas in CNT

Density of CNT
Young's modulus of CNT
Poisson's ratio of CNT
Relative permittivity f CNT
Rayleigh wave velocity
Width of unit cell
Estimated SAW frequency
CNT thickness

Gases

acoustic energy into the near surface region can increase its
sensitivity manifold. The area between the IDTs (delay line) is made
sensitive by coating with chemically active species which reacts
with the target molecules. Coating SAW sensors with various
nanostructured materials allow gas detection by varying in the
resonant frequency. Acoustic resonators are considered as universal
transducers since they detect the mass, which is a primary feature
of any target [14]. On combining with suitable molecular layers
they can be well designed for remote sensing applications. Metal
oxide semiconductor based gas sensors mostly operate at very high
temperatures which require large power and special packaging.
This urges the need for developing new metal oxide sensing layers
that can work safely at room temperature.
CNT and its composites owing to their unique physical and
chemical properties have received considerable attention in recent
years [15,16]. The properties of CNTs and their composites change
upon exposure to various gases which can be detected by various
methods. The electronic properties of CNTs change on interaction
with gas molecules which can be attributed to the charge transfer

between gas molecules and nanotubes. This can make them more
sensitive to gases with large binding energies.
Many studies have already been conducted on hybrid metal
oxide-CNT sensors namely SnO2, WO3, TiO2 eCNT for room temperature operation [17,18]. Herein we report the simulation studies of
two different CNT based SAW gas sensors for the detection of
different volatile organic chemicals (VOCs). We have incorporated a
metal oxide compositae in to the CNT layer to enhance the efficiency.
Tasaltin et al. [19] studied the 433 MHz Rayleigh wave based SAW
sensor coated with ZnO for these VOCs. To the best of our knowledge
no studies have been reported on this particular ternary nanocomposite based SAW gas sensor. Even though metal oxide semiconductor (MOX) gas sensors have several advantages, their biggest
downsides are problems related to drift and significant energy

Trichloromethane
Dichloromethane
n-Hexane
n-Pentane
Diethylether
Acetone
Acetonitrile
2-Propanol
Ethanol
Methanol

Sensing layer
CNT (Df in kHz)

ZnO-CuO/CNT (Df in kHz)

47.82
22.45

18.95
6.61
76.19
93.23
7.33
183.88
92.7
13.42

59.8
33.42
30.76
10.89
89.46
107.23
10.64
200.26
100.69
20.99

consumption. In addition, a large amount of energy is needed to
activate the interaction of the sensing layer with the gas molecules.
So the sensing layer of MOX needs to be heated up for long periods.
Such high temperatures can even alter the structure and properties
of the sensing layer. At elevated temperatures, the mobility of oxygen
vacancies will become appreciable leading to the mixed ionicelectronic conduction mechanism. This type of (oxygen vacancies)
diffusion could produce long term drift in MOX sensors [20]. Now the
prime motive of the researchers is to reduce the power consumption
which could prolong the battery life [21]. CNTs have been deeply
explored by the research world because of their ability to detect

gases at ambient temperatures. Thus researchers were able to bring
down the power consumption to a few mW [22]. CNT based sensors
also face few challenges like long response and recovery times which
impede them from directly replacing metal oxides in MOX based
sensors [23]. So it is expected that doping metal oxide with CNT
could surely enhance the sensitivity, lower preheating of the work
body and also could lower the response/recovery time [24]. The aim
of this paper is to model a SAW based sensor with MOX-CNT nanocomposite as sensing layer to combine the advanced features of MOX
and CNT and also to study the effect of adding ZnO-CuO nanocomposites in to the CNT layer in sensitivity enhancement.
2. Model design
COMSOL Multiphysics is a platform where models can be developed and analysed using Finite Element Analysis (FEA). Modern
computer-aided design techniques realized in commercial softwares
like ANSYS and COMSOL Multiphysics offer powerful and robust
simulation tools for designs which can accurately predict the system
performance avoiding the physical prototype fabrication [25].
By metal patterning SAW sensors can be configured as one
port or two port resonators and delay lines. Appropriate methods

Table 2
Air partition constant and molar mass for different gases.
Gas

CNT/air partition constant

Molar mass

Trichloromethane
Dichloromethane
n-hexane
n-pentane

Diethylether
Acetone
Acetonitrile
2-Propanol
Ethanol
Methanol

10^4.36
10^4.18
10^4.10
10^3.72
10^4.70
10^4.87
10^4.01
10^5.24
10^5.06
10^4.38

119.38
84.93
86.18
72.15
106.12
58.68
41.05
60.1
46.06
32.06

Fig. 1. SAW gas sensor, showing the IDT electrodes (in black), the thin PIB film (light

grey), and the LiNbO3 substrate (dark grey). A slice of geometry is removed to reveal
the modelled unit cell.


N. Abraham et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

127

Fig. 4. Mode shape plot showing deformation for CNT based sensor.

Fig. 2. Geometry of SAW gas sensor with ZnO-CuO nanospheres in CNT adsorbent film.

have been developed by Tsai et al. [26] to design SAW sensors
based on mass loading principle using FEM. To develop a 2D
model of the sensor, the model geometry is reduced to a periodic
unit cell as shown in the Fig. 1. When a voltage is applied to the
input electrodes, by converse piezoelectric effect mechanical
perturbations are generated on the surface. These acoustic waves
will propagate along the surface and on reaching the output
electrodes, by direct piezoelectric effect voltage gets developed
across them. The wave velocity depends on many factors like
conductivity, sensing layer thickness as well as the spacer thickness. The different model parameters are given in Table 1.
In a piezoelectric material the propagation of the wave is governed by the equation

T ¼ CE S À e T E

(1)

here T represents the stress matrix, C the elasticity matrix, eT the
piezoelectric matrix and E represents the electric field intensity.

This formula serves as the basis for building the geometry. Even
though there are different variants of acoustic waves (SH-SAW,
love, Lamb and leaky wave), Rayleigh mode is most popular as they
are extremely sensitive to a number of quantities. If a gas-phase
analyte of certain concentration is coming in contact with its

surface, the sensing layer will adsorb these molecules until thermodynamic equilibrium is attained. Due to the increased adsorption, the layer becomes denser and heavier. As a result, the
propagation velocity of the surface wave decreases which leads to
frequency downshift (Df ¼ f e f0) [25]. On the assumption that the
analyte forms a non-piezoelectric, isotropic and non-conducting
layer with a thickness of t, partition constant K and vapour concentration Cv, the change in frequency can be written in simplified
form as

Df ¼ ðk1 þ k2 Þf 0 2 t K Cv

(2)

where k1 and k2 are coupling coefficients depending on the different
displacement components of the wave in the substrate and f0, the
operating frequency in the absence of sensing layer [25]. Air partition
constants and molar masses of different gases are given in Table 2.
This equation implies that the shift in frequency is proportional to
the mass loaded on the surface. The sensitivity of a gas sensing device
(S) is given as the ratio of device response (R) to gas concentration
(n). The device response for an uncoated substrate is given as

R ¼ f 0 2 ðk1 þ k2 ÞDm=As

(3)

where Dm/As represents the mass loaded per unit surface area [27].
The piezoelectric material used in this study is YZ cut Lithium
Niobate (LiNbO3) crystal. It has a higher wave velocity than quartz,

Fig. 3. Mesh mapped onto the geometry.


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N. Abraham et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

3. Results and discussion

Fig. 5. Mode shape plot showing deformation for ZnO-CuO/CNT based sensor.

Frequency shift (kHz)

another commonly used piezoelectric material. The designed
sensor works in the MHz frequency range. LiNbO3, CNT and
aluminium electrodes were modelled in rectangular shapes of
4 mm, 500 nm and 250 nm respectively (Fig. 2). Aluminium electrodes are predominantly used since they have low attenuation. In
the first model the adsorbing layer is only CNT while in the second
model ZnO-CuO nanocomposites were modelled as nano spheres of
25 nm diameter in the CNT adsorbent layer.
After specifying the material properties, the appropriate
boundary conditions were applied to the model. The ground potential and floating potential were applied to the left and right
electrodes respectively. This is equivalent to the open circuit condition, which is ideal for gas sensing. After specifying the boundary
conditions, a mesh has been created in each domain. The mesh
consists of 29 boundary elements. The complete mesh consists of
4438 domain elements and 505 boundary elements (Fig. 3). After

completing the mesh mapping, the solution of each individual
mesh has been calculated and integrated over the entire surface.

The IDTs generate harmonic frequencies in addition to the
fundamental mode. The presence of the aluminium IDT electrodes
and the piezo electric material causes the lowest SAW mode to split
up into two Eigen solutions. The lowest one represents series
resonance, where propagating waves interfere constructively and
the other one represents parallel (“anti-”) resonance, where they
interfere destructively. The sensor has been tested with 100 ppm of
methanol, ethanol, trichloromethane, dichloromethane, acetonitrile, diethyl ether, acetone, n-pentane, n-hexane, and 2-propanol.
The mode shape plots in Figs. 4 and 5 show the decay of the
surface displacement with the depth since the mode of propagation
is Rayleigh mode. The resonant frequency is found to be 915 MHz.
The shift in resonant frequency in the presence of gases lies in kHz
range. Both the models showed maximum response to the 2propanol (with a shift in resonant frequency of 183.88 and
200.26 kHz for CNT and ZnO-CuO/CNT respectively). The 2-propanol
is a highly inflammable gas, the detection of which is very important
in many fields. The enhancement in sensitivity has been observed for
the second model.
Most SAW devices operate in the 100e600 MHz resonant
frequency range. Dickert et al. [28] experimentally proved that the
variation in the resonant frequency could increase the sensor
response in a parabolic fashion. In order to study the resonant
frequencyefrequency down shift relationship, we have simulated
the sensor response for the different gases (100 ppm) in the second
model. The obtained results is shown in Fig. 6. It can be seen that the
frequency shift increases with the resonant frequency in agreement
with equation (2). Venema et al. [27] done similar studies and found
that for a given sensing layer thickness, highest sensitivity has been

obtained for the sensor with high operating frequency. Optimizing
sensor response can surely reduce the sensing layer thickness. This
can further enhance the adsorption process which in turn reduces
the response time. The ability to decreasing sensing layer thickness,
however, depends on the smallest electrode distance feasible for the
process of photolithography [28].
The increase in sensitivity of the second model can be primarily
attributed to the increase in surface area of the adsorbing layer. The
specific surface area of CNT is very large and it possesses a hollow
structure, which can expose a large number of reaction sites. In
addition, they have much higher electrical conductivity than metal

Resonant Frequency

Fig. 6. Frequency shift of different VOCs with varying resonant frequency.


129

Frequency shift (kHz)

N. Abraham et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

Fig. 7. Frequency downshift for the two sensor models to different VOCs.

oxides. The strong sp2 bonding in CNTs makes them chemically
inactive. Functionalization with MOX nanocomposites can surely
improve the chemical reactivity which could further enhance the
oxygen adsorption on the compositae surface. On combining these
two, there will be a reduction in the resistance of the sensing layer.

The surface resistivity increases with the amount of the adsorbed
oxygen which could be removed by the reduction with the gas
molecules [29]. Previous studies [30,31] on CuOeZnO nanocomposites showed their potential use as an efficient photoanode
material in dye sensitized solar cells. These studies also revealed
that, the tendency to form significant agglomerates in the case of
ZnO has been reduced due to the addition of CuO, which can
significantly increase the surface area. The reduction in the resistance of these hybrid nanocomposites could also increase the
overall sensitivity. Studies on selective CO sensing with CuOeZnO
heterocontact [32] show that surface resistance of CuO will be
increased by the oxidative reaction of reducing gases. However it
will be opposite in the case of ZnO. Since the resistance of ZnO is ten
times that of CuO, the total resistance of the CuOeZnO heterocontact will be governed by the surface resistance of ZnO. The

Fig. 8. Frequency shift with sensor layer thickness for 2-propanol.

strong interaction between CNT and carboxyl groups present in the
CuOeZnO nanocomposite would also help in the sensitivity
enhancement at room temperature.
Most of the available sensors operate at higher temperatures
except sensors based on polymers. Zheng et al. [33] studied CNT/
CuO based chemical sensor and they found that these hybrid
composites can enhance the sensitivity at room temperature. According to them the enhancement in sensitivity is mainly due to the
strong interaction between CNTs and carboxyl groups present in
the CuO. Hieu et al. [34] studied enhanced sensing properties of tin
oxide doped with CNTs and metal oxide semiconductors to sense
the liquid petroleum gas and ethanol. Functionalization of CNT is a
viable and easy method to improve the sensitivity. Filling with
metal oxides and noble metals is an easy method to functionalize
and broaden their applications. Zhang and his team [35] binded
ZnO quantum dots on to CNTs by covalent coupling. Comparing

with single wall carbon nanotubes (SWCNT) the gas adsorption
mechanism in multiwall CNT (MWNT) is more complicated. However they exhibit high sensitivity to certain gases.
The three key parameters which play major role in the
sensing mechanism of SAW based sensors are mass, conductivity and elasticity of the sensing material. So in these sensors
the surface wave can interact with the sensing layer to cause the
velocity variation in three different ways a) variation in the
mass of the layer b) acoustoelectric effect c) viscoelasticity. The
elastic loading has been neglected in majority of SAW sensors.
In the case of metal oxides, the conductivity cannot play a
dominant role because when this parameter changes against
various VOCs, the sensor has to be heated up to 200e300  C. So
in such cases the mass effect will become the prominent factor
which decides the sensor response. Former studies [36,37] on
metal/semiconductor (SC) layered structures reveal that
improved sensitivity of these structures (compared to metal or
SC layers) can be attributed to the highly active conductivity
regime leading to better acoustoelectric coupling between the
layers and the surface wave. Earlier studies on SnO2/CNT
nanocomposites show that better sensitivity is due to the existence of two different depletion layers and associated potential
barriers [38]. Electron density studies of CNT based NO2 gas
sensors reveal that charge transfer takes place from the NO2 gas


130

N. Abraham et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 125e131

molecules leading to the p-type doping of nanotubes [39]. The
adsorbed gas molecules on the CNT produce charge transfer
which will be enhanced by the CuOeZnO nanostructures. CNTs

also have special ability to sense the corresponding changes
when gas molecules get attached or detached from their surface.
Chemisorbed molecules can also act as interfacial states through
which electrons and holes are captured and emitted [40].
Similar studies [41] on SnO2-CNT hetero-structure based gas
sensors reveal that the enhanced performance of these hybrid
structures is due to the nanochannels formed on the MOX
semiconductor surface which can augment the diffusion of gas
molecules in to the metal oxide surface as well as increase in
the local electric field at the interface. Two different types of
depletion layers co-exist in these types of metal oxide/CNT
hybrid structures, one at the surface of the metal oxide and
other at the interface between CNT and metal oxide [42]. The
adsorption of various gases can modify these depletion layers
which has a significant effect on the sensor response [43]. The
two port SAW sensors will monitor these changes and
produce corresponding changes in the resonant frequencies. The
frequency shift has been measured for the different gases with
same concentration and is shown in Fig. 7 and corresponding
values are given in Table 3.
The sensing layer thickness is a crucial factor which affects the
sensitivity of SAW sensors. The frequency variation with sensing
layer thickness has been investigated for 2-Proponol and the obtained results are shown in Fig. 8. The highest frequency shift is
observed at the thickness of 236 nm. After this critical thickness
the frequency shift shows a downshift. This downshift is due to
the change in the interaction mechanism from mass or acoustoelectric effect to elastic effect. Similar results were observed for
SnO2 and ZnO based SAW gas sensors [44].
4. Conclusion
The SAW based sensor technology is very promising for
sensing any analyte through the optimized integration with

sensing layers. SAW gas sensors based on CNT and ZnO-CuO/CNT
nanocomposites were modelled in COMSOL Multiphysics. Their
response has been tested for different VOCs at room temperature.
Compared to the CNT based sensor, the ZnO-CuO/CNT sensor
showed an improved performance. However, in both the cases the
response maximum is found for 2-Propanol, a highly inflammable
gas. The enhancement in sensitivity can be attributed to the
increased surface area, the reduction in the resistance of the
sensing layer as well as the strong interaction between CNT and
the carboxyl groups present in the hybrid nanocomposite. In
these types of CNT-MOX hybrid structures, the nanochannels
formed on the MOX surface can also augment the diffusion of
analyte molecules. The incorporation of polymer layers in between could bring further sensitivity enhancement, which is a
future scope of the present work.
Acknowledgements
The authors are pleased to acknowledge, Department of mechanical Engineering, Govt. Engineering College Barton Hill, Thiruvananthapuram, Kerala, India, for providing the lab facilities.
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