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.
Journal of Science: Advanced Materials and Devices (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 (http://creativecommons.org/licenses/by/4.0/) Keywords: ZnO CuO Surface acoustic waves Sensors Carbon nano tubes 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: nelsaarun2016@gmail.com (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 https://doi.org/10.1016/j.jsamd.2018.12.006 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 (http://creativecommons.org/licenses/by/4.0/) 126 N Abraham et al / Journal of Science: Advanced Materials and Devices (2019) 125e131 Table Model parameters Table 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 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 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 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 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 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 (2019) 125e131 127 Fig Mode shape plot showing deformation for CNT based sensor Fig 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 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 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 simplied form as Df ẳ k1 ỵ k2 Þf 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 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 ẳ f k1 ỵ k2 ịDm=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 Mesh mapped onto the geometry 128 N Abraham et al / Journal of Science: Advanced Materials and Devices (2019) 125e131 Results and discussion Fig 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 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 and 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 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 Frequency shift of different VOCs with varying resonant frequency 129 Frequency shift (kHz) N Abraham et al / Journal of Science: Advanced Materials and Devices (2019) 125e131 Fig 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 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 (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 and corresponding values are given in Table 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 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] 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 References [1] K Khun, Z.H Ibupoto, X Liu, N.A Mansor, A.P.F Turner, V Beni, M Willander, An electrochemical dopamine 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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