Sensitized Surface Acoustic Wave (SAW) Sensors for Room-Temperature Gas Detection
Authors: Liu Huan , Zhou Licheng , Zhai Bohui , Hu Zhixiang , Li Hua-Yao , Zhang Guangzu , Jiang Shenglin 
Published: May 2020

Introduction
Identifying and quantifying gas components in mixtures is an important direction for the development of gas sensors.
So far, many types of gas sensors have been developed, which can be mainly classified into semiconductor, catalytic combustion, and electrochemical, solid electrolyte, infrared optical, quartz crystal microbalance type (QCM), surface acoustic wave type (SAW) [1-4], according to the detection mechanism of the sensor.
The semiconductor sensor relies on the change of the resistance of the sensitive film, and has nothing to do with the mass of the film. Combining sensor arrays and pattern recognition algorithms, identification of mixed atmospheres has been achieved. Conversely, the QCM sensor depends on the change of the film mass, and has nothing to do with the resistance. Recently, Wang et al. achieved NH3 and H2S sensing based on QCM sensor, and each gas concentration can be quantified individually.[5] The QCM sensor is based on Janus mesoporous carbon/silica films with asymmetric mesostructures and disparate active sites (–NH2 and –COOH groups). However, SAW based gas sensors have mass loading effects, acoustoelectric effects and elastic loading effects.[6]
Here, we investigate the NO2 and NH3 detection and mechanisms at room temperature of surface acoustic wave sensors based on Bi2S3 nanobelts. When exposed to NO2, the acoustic and electrical effects dominate. When exposed to NH3, the mass loading effects dominate. Bi2S3-based semiconductor and QCM sensors were used to demonstrate this effect.
Experiment
Bi2S3 nanobelts were prepared using a hydrothermal method. Briefly, triphenyl bismuth (C18H15Bi, 98%, Aladdin, 0.6 mM) and dibenzyl disulfide(C6H5CH2SSCH2C6H5, 98%, Aladdin, 0.6 mM) were dissolved in 4 ml oleylamine (OLA), and then 0.4 g polyvinyl pyrrolidone (PVP,Aladdin, MW ≈ 24,000-28,000) were dissolved into 26 ml absolute ethanol. After that, these solutions were thoroughly mixed together and transferred into a 50 ml autoclave to react at 180oC for 8 hours. Finally, the product was washed with hexane and dissolved in ethanol at a concentration of 30 mg/ml
for use.
In this study, lithium niobate is chosen as the piezoelectric substrate for making the SAW delay-line device. For the sensitive layer deposition, 100 µL of Bi2S3 nanobelts solution was spin-coated (at a speed of 2000 rpm) onto the sensitive area of the SAW resonator as a gas sensing film. The central frequency of SAW delay-line device was measured using a network analyzer.
Results and Conclusions
Bi2S3/SAW gas sensor showed a response of 5 kHz toward 20 ppm of NO2, and 12 kHz upon 20 ppm NH3 at room temperature, with fast response and recovery speed, respectively. Bi2S3 nanobelts can be used as a sensitive material for high performance NH3 and NO2 gas sensors. The semiconductor type and QCM type gas sensor were used to explain the sensitive mechanism. This provides a reference for us to use different mechanisms to make different types of sensors, and recognize and quantify gas components in mixtures.
References
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[2] Hu, J et., Light-assisted recovery for a highly-sensitive NO2 sensor based on RGO-CeO2 hybrids, Sensors and Actuators B: Chemical. 270 (2018), 119-129. doi: 10.1016/j.snb.2018.05.027.
[3] Hodgkinson, J et., Optical gas sensing: a review, Measurement Science and Technology. 24, 012004. dio: 10.1088/0957-0233/24/1/012004
[4] Zhu, Y et.,. Synthesis of mesoporous SnO2–SiO2 composites and thei application as quartz crystal microbalance humidity sensor, Sensors and Actuators B: Chemical. 193(2014), 320-325. doi: 10.1016/j.snb.2013.11.091
[5] Wang, R et., Janus Mesoporous Sensor Devices for Simultaneous Multivariable Gases Detection, Matter. 1(2019), 1274-1284. doi: org/10.1016/j.matt.2019.07.004
[6] Länge et., Surface acoustic wave biosensors: a review, Anal Bioanal Chem. 391(2008), 1509-1519. dio: 10.1007/s00216-008-1911-5
Figure. 1 NO2 and NH3 gas detection and mechanisms of semiconductor, QCM and SAW sensors based on Bi2S3 Nanobelts.

Figure 1