Materials Chemistry and Physics 239 (2020) 122026 Available online 24 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved. Highly sensitive surface acoustic wave HCl gas sensors based on hydroxyl-rich sol-gel AlO x OH y flms Yongliang Tang a, * , Dengji Li b , Dongyi Ao c , Yuanjun Guo c , M. Bilal Faheem b , Bilawal Khan c , Xiaotao Zu b, ** , Sean Li d a School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, 610031, PR China b Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu, 610054, PR China c School of Physics, University of Electronic Science and Technology of China, Chengdu, 610054, PR China d School of Materials Science & Engineering, UNSW SYDNEY, NSW, 2052, Australia HIGHLIGHTS G R A P H I C A L ABSTRACT � The sensor can detect HCl gas with a concentration as low as 0.05 ppm. � The hydroxyl groups and H 2 O on the sol-gel AlO x (OH) y flms assist the response. � The frequency shift of sensor is derived from the change of weight of the flms. A R T I C L E INFO Keywords: Surface acoustic wave HCl gas sensor Sol-gel AlO x OH y flm Hydroxyl group H 2 O ABSTRACT Gas sensors detecting low HCl gas concentrations used for early detection and continuous monitoring of leakage is of great concern because HCl gas is extremely hazardous. In order to tackle the potential hazardous threats, surface acoustic wave HCl gas sensors based on hydroxyl-rich sol-gel AlO x OH y flms are reported. The fabricated SAW sensors show negative frequency shifts with exposure to HCl gas. The negative frequency shift is found to be caused by the increasing weight of sensitive flms induced by the adsorption of HCl assisted by the hydroxyl groups and H 2 O captured on the flms. The sensing performance of sensors is highly dependent on the heat- treatment temperature and thickness of flms, as well as the ambient humidity because these parameters are highly infuential on the amount of hydroxyl groups, H 2 O on flms and the pore volume of flms. For a sensor with a 198-nm-thick flm annealed at 300 � C, it shows a response of ~ -1.4 KHz to HCl at a low concentration level (0.05 ppm) in an environment with a temperature of 25 � C and a RH value of 40%. * Corresponding author. ** Corresponding author. E-mail addresses: tyl@swjtu.edu.cn (Y. Tang), xtzu@uestc.edu.cn (X. Zu). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys https://doi.org/10.1016/j.matchemphys.2019.122026 Received 29 March 2019; Received in revised form 9 August 2019; Accepted 14 August 2019