Gas Discrimination Using Screen-printed Piezoelectric Cantilevers Coated with Carbon Nanotubes Academic research paper on "Materials engineering"

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Procedía Engineering 120 (2015) 987 - 992

Procedía Engineering

www.elsevier.com/locate/procedia

EUROSENSORS 2015

Gas discrimination using screen-printed piezoelectric cantilevers

coated with carbon nanotubes

P. Clementa' b, Eduard Llobeta, C. Lucatb, H. Debedab*

aMinos-EMas, Rovira i Virgili University, Tarragona, Spain bIMS, Bordeaux University, Talence, France

Abstract

In this paper, a screen-printed piezoelectric cantilever coated with carbon nanotubes (CNTs) as sensitive layer is used as a resonant type gas sensor. In parallel, the resistance of the CNT layer is measured in static mode thanks to a modification of the cantilever top electrode. Positive and negative shifts of the resonance frequency are observed at low and high gas concentrations, respectively. These are attributed to stress or to mass effects becoming dominant at low or high gas concentration levels. Monitoring the resistance of the p-type CNT film helps discriminating gases/ vapours according to their oxidizing or reducing character. The responses towards benzene, CO and NO2 concentrations are discussed.

© 2015TheAuthors. PublishedbyElsevierLtd.Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015 Keywords: Cantilever, carbon nanotubes, double transduction, mass, resistive, screen-printed

1. Introduction

The high sensitivity of resonant microcantilevers make them interesting for many applications including gas sensing [1-2]. To perform detection of gas species, the microcantilevers are coated with a chemically sensitive layer that aims to selectively sorb the target analyte. The sorbed species modifies the mechanical properties of the sensitive coating and therefore the mass, the rigidity and the surface stress of the microstructure. Usually, the resonating cantilevers used for chemical sensing are vibrating in their first transverse flexural mode (out-of-plane vibrations), also called transverse bending mode, which is the resonant mode having the smallest resonant frequency. Due to the

* Corresponding author. Tel.: +33-540008336; fax: +33- 556371545 E-mail address: helene.debeda@ims-bordeaux.fr, IMS, University of Bordeaux, UMR5218, 33405 Talence Cédex, France

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015 doi: 10.1016/j.proeng.2015.08.638

fact that the sensor sensitivity increases with the resonant frequency, alternative operational modes are attractive and use of an in-plane mode also called longitudinal is proposed here [3]. To actuate and have an integrated read-out of this particular vibration mode, symmetric screen-printed piezoelectric cantilevers are used. These cantilevers have already been studied in a previous work covered with PEUT or CNTs to detect respectively toluene or benzene [4]. More recently, we have also shown the possibility of coupling a double transduction with the CNTs-coated piezoelectric printed cantilever [5]. For this purpose, the top electrode, which normally covers the full surface of the cantilever was replaced by interdigitated electrodes. Therefore, the two transduction mechanisms i.e. changes in the resonance frequency of the cantilever and in the equilibrium conductivity of CNTs (translated into resistance shifts) helped us to obtain a better identification/quantification of vapors. In this paper, the piezoelectric cantilever platform is tested under gases/vapors employing this double transduction mechanism. A better comprehension of the phenomena occurring in presence of oxidizing and reducing species (benzene, CO and NO2) is proposed.

2. Sensor fabrication

2.1. Transducer

The transducer is composed of a piezoelectric PZT cantilever (2x8x0.1 mm3) with top and bottom Au electrodes. The whole Au/PZT/Au component is fabricated on an alumina substrate by associating the screen-printing technique with the sacrificial layer process [6]. Each screen-printed layer is dried at 120°C during 30 min. Then, to improve sample's densification before firing, the dried layers are isostatically pressed at 40 MPa and 60°C during 4 min. The sample is afterwards fired 2 h at 900°C. Finally, dissolution of the SrCO3 based sacrificial layer is performed in an acidic solution. For simultaneous resonance frequency shift and resistance variation measurements of the CNTs sensitive coating, the initial top rectangular electrode is replaced by two interdigitated electrodes as shown in Fig. 1. Finally, to exhibit piezoelectric properties, the cantilever is progressively polarized between the bottom electrode e1 and the top electrodes, e3 and e2 (the latter are kept under short circuit). An electric field is applied under nitrogen atmosphere at 280°C, just below the Curie temperature of the PZT. After polarization, the piezoelectric electromechanical properties of the cantilever are measured with an Agilent E5061B impedance meter and show a resonant frequency fr ^ 70 kHz with a high quality factor Q ^ 1500 for the first in plane 31-longitudinal mode.

Fig.1. Picture showing the double transduction method allowing resistance and resonance frequency measurement: the bottom electrode is ei and the interdigitated electrodes are e2 and e3.

2.2. Sensitive coating

CNTs are good candidates as sensitive layer, since they have high specific area (up to 1800 m2/g for single wall CNTs) and the possibility to be chemically functionalized and metal decorated [7]. The used CNTs are multiwall-type obtained by chemical vapor deposition and functionalized by oxygen-argon plasma to improve their surface reactivity (O-MWCNTs). They are deposited on the top interdigitated electrodes by air brushing (Fig. 2a). During this deposition, the monitoring of the CNTs resistance is made for achieving a resistance of a few hundreds of ohms. After CNTs coating, a negative resonance frequency shift of 181 Hz is observed without affecting the quality factor (Fig. 2b). This observed negative shift after the CTNs deposition is linked to mass effect as previously observed

with sensitive coating deposits on the piezoelectric cantilever [8]. The fact that quality factor remains unchanged is explained by the small amount of O-MWCNTs deposited [9].

070200

Af= 181Hz

Uncoated cantilever CNTs coated cantilever

71000 71200

70600 70800 f (Hz)

Fig.2. Optical microscope view of the piezoelectric cantilever with CNTs coated on the interdigitated electrodes (left), Conductance of the PZT cantilever at the resonance frequency before and after CNTs deposition (right).

3. Gas tests and discussions

Before gas tests, the sensor is placed in a gas chamber (350 mm2) and is heated at 150°C in order to desorb contaminants from the cantilever and the CNTs. Then, resonance frequency is followed thanks to a dynamic working mode. Alternatively, the resistance of the CNTs film is measured at 1 kHz in static mode (without cantilever vibration), since they show a resistive behavior at this frequency. All those parameters are controlled with a Labview environment. The gas flow and dilution are controlled with an Environics mass flow system employing calibrated gas cylinders and dry air as carrier.

First detections under low concentrations of CO, benzene and NO2 show a positive shift of the resonance frequency with low noise (Fig.3). By neglecting the viscosity and density effects and assuming that the gas and the cantilever are at the same temperature, the resonance frequency shift of the 31-longitudinal vibration mode related to gas adsorption is:

4f ,31 -

where the frequency variation (Afr,31), is proportional to the resonance frequency f 31) and linked to the mass m and the stiffness k of the cantilever.

These positive resonance shifts are thus attributed to a stress effect under low concentrations of CO, benzene and NO2. Moreover, the resistance shifts of CNTs are positive under benzene and CO and negative under NO2 as expected for a p-type semiconductor. We also observe that baseline recovery is reached for benzene and CO but not for NO2 detection. This is due to the very low dynamic desorption of nitrogen dioxide since these species strongly interacts with O-MWCNTs. This double transduction not only allows for identifying oxidizing or reducing species but also lowers the detection limit. Indeed, CO is detected below 40 ppm employing the resonance frequency shift but not using the resistance change of CNTs.

Higher concentrations of benzene have been also investigated and the measurements show that stress and mass effects nearly compensate at around 2000 ppm of benzene (Fig. 4). The mass effect becomes then the dominant phenomenon at higher benzene concentrations and results in negative shifts of the resonant frequency Also, during

this test, the resistance of the O-MWCNTs films monotonically increases with benzene concentration. The sensor responses towards CO, NO2 and benzene are reported in figure 5.

70491.570491.0 70490.5 70490.0 -

10000 t(s)

-N 70480 4

70480 2

70480 0 -j\Svf E 70479 8

Vi Air

605.5 605.0 604.5 ;

7000 t(s)

70530.3 -,

70530.3-

70530A -

70530.2 -

-- 70530.0E

70529.3-70sas.6

70529.4 -70529.270529.0

( ¡Vyj

319.4 319 2 319 0 516.8 3 616 6 E 616 4 616.2 316 0

Fig.3. Response at room temperature under NO2 (top-lef), under CO (top-right) and under benzene (bottom) of a CNT coated PZT cantilever.

Fig.4: Response under high concentrations of benzene at RT.

Fig.5.

--■--no2 --•--co

-- Benzene

0.1 1 10 100 1000 10000 Concentration (ppm)

1.2 1,0 0.8 0.6 0.4

"> 0 2

- 0.0 X

" -0.2H -0.4

- —-no2

• CO -- Benzene

10 100 1000 Concentration (ppm)

Sensor response in a) frequency and b) resistance toward different gases with SR=Ax/xair (where x is the resonance frequency or resistance

and Ax its difference under air and contaminants).

4. Conclusion

The benefit of the double transduction has been demonstrated for discriminating reducing from oxidizing gases thanks to the sign of the O-MWCNTs resistance shifts. Moreover, positive and negative shifts of the resonance frequency have been observed at low and high gas concentrations, respectively attributed to stress or mass effects. The lower measured concentrations are 2 ppm, 5 ppm and 50 ppb for benzene, CO and NO2, respectively. The limit of detection for CO is found to be lower in case of the resonance shift measurement with a significant signal at 5 ppm. Nevertheless, such a system appears to be sensitive to humidity with negative shifts for both resistance and resonance [5]. In this case, the lowest tested humidity level (25%) was inducing a mass effect (negative resonance frequency shift), as seen for high benzene concentrations. Further tests are in progress to study the influence of the CNTs thickness, and of the piezoelectric cantilever porosity on the gas responses.

Acknowledgements

Région Aquitaine (France) and Generalitat de Catalunya (Spain) have supported this research.

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