BTEX Detection Using Piezoelectric Cantilever with Mesocellular Foam Silica Functionalized with Hexamethyldisilazane Sensitive Coating

Abstract

Introduction Gas detection with gravimetric sensors such as SAW, MEMS, FBAR or QCM is based on a frequency shift when the target specie is (ad)sorbed on a sensitive layer covering the sensor [1]. Sensor’s sensitivity and reliability depend on transducer type and sensitive coating, while selectivity and reversibility are correlated with the sensitive coating’s nature. Research studies are still active on the development of transducers to reduce their cost and power consumption but sensitive coating optimization remains a hot topic for sensitive and selective sensor achievements. Among attractive sensitive coating for gravimetric sensors, high specific area coatings such as CNTs, graphene oxide, microporous/mesoporous materials or MOFs are nowadays majorly studied one [2-3]. Their performances in gas detection can be modified by tuning of pores or surface functionalization. In this work we are interested in applying mesoporous foam silica (MCF), a type of mesoporous silica, as a base for hydrophobic hexamethy-disilazane (HDMS) functionalization, and tested for BTEX detection. Also, low cost printed piezoelectric MEMS cantilevers are used as gravimetric transducers. Piezoelectric cantilever sensor fabrication and tests One type of inorganic mesoporous silica (MPS), mesocellular foam (MCF), is synthesized by using P123 as a template and 1,3,5-trimethylbenzene (TMB) as a pore expander as described in [4]. After the template removal by ethanol extraction, the template removed MCFs are then functionalized by hexamethyldisilazane (HDMS) following protocol described in [5]. For the piezoelectric PZT based transducer fabrication, the process consists of screen-printing of all the layers before a co-sintering at 900°C allowing releasing of the PZT cantilever with Au electrodes. Details on the process can be found in [6]. Photographs of the fired cantilever (~ 3x2x0.08 mm3) are shown Fig. 1. The HDMS functionalized MCF (F_MCF) powder is dispersed in deionized water with the ratio of 100 mg/1 mL before depositing by drop-coating with 1µl of this suspension, followed by a drying . The cantilever is self-actuated and self-readout thanks to piezoelectric effect. Admittance circle B(G) (Fig.3a and Fig. 3b)) shows the piezoelectric effect for the 31 in-plane longitudinal mode. Conductance (G(f)) curve before and after F_MCF deposit (weight ~0.05mg) shows the damping effect due to the additional mass (Fig.3c). Based on the measured dimensions of the cantilever, its density and its resonance, frequency f0 , a theoretical sensitivity S=-f0/2m of ~23Hz/µg is calculated. Finally, the cantilever is tested under gas. A PULL110 vapor generator controls N2 flow (100ml/min) and toluene concentration sent in a home-made PTFE cell. Temperature is controlled during the measurements, and acquisition data (f0 as a function of time) are extracted from G measurement using a polynomial fit described in [6]. Results and Conclusions After washing and drying of the MCF, interconnected spherical pore structure are observed by TEM analysis even after functionalized by HMDS (Fig.2). The analysis results of the N2 adsorption-desorption isotherms of the MCFs are shown in Table 1. Pore and window sizes are not affected by functionalization whereas BET surface areas and BJH pore volumes decrease with the increasing HMDS ratio. Also, after functionalization, as expected, reduction of water adsorbed on the surface of the MCF has been observed by TGA and FTIR analysis, and the optimum HDMS:SiO2 for VOCs detection has been shown to be 0.15 (for benzene detection in fact) [5]. This composition is selected for further experiment under gas. Tests under toluene with F_MCF show a sensitivity of 241 mHz/ppm at f0 ~ 256kHz (Figs. 4a and 3b). The blank cantilever also demonstrates slight sensitivity to toluene (~6 mHz/ppm) (Fig. 4b), attributed to density/viscosity changes of the surrounding fluid and cantilevers’s stiffness. In previous work using polymer PEUT coating, silicon cantilever (0.15x0.15x0.095mm3 ) showed sensitivity of 125mHz/ppm at 350 kHz (out of plane mode), whereas similar printed PZT cantilever of size 8x2x0.08mm3 gave sensitivity of 15mHz/ppm at 70kHz (in plane vibration mode) [7]. Size reduction leading to higher sensitivity because of the increased f0 and lower m, combined with the F_MCF coating clearly improves the sensor performances. Good reversibility is also observed. The noisy curve is mainly due to temperature fluctuation which is not precisely controlled in the cell (±0.2°C). The sensor is indeed sensitive to temperature as shown Fig. 3c. Tests under humidity and benzene are ongoing and temperature compensation with another dummy cantilever fabricated in the same conditions could be implemented.