Ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices

•We demonstrate an ammonia gas sensor based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices.•The ZnO/SiO2 bi-layer nanofilms are prepared by combining a sol–gel process and a spin-coating technique.•The ZnO/SiO2 bi-layer nanofilms inherit the porous structure of SiO2 film.•The gas sensing property of ZnO/SiO2 nanofilm is dependent on its sheet conductivity value.•The sensor exhibits the ability to detect ammonia gas in a concentration as low as 5ppm, and shows an excellent reproducibility and stability.