Layered WO3/ZnO/36° LiTaO3 SAW gas sensor sensitive towards ethanol vapour and humidity

Ethanol sensing in dry and humid air by layered surface acoustic wave (SAW) devices is presented. The transducing platform is based on 36° YX LiTaO3 layered SAW device, utilising a 1.2 μm zinc oxide (ZnO) intermediate layer and 150 nm tungsten trioxide (WO3) sensing layer. Sheet conductivity calculations show that maximum sensitivity is achieved for ZnO layer thickness between 1 and 1.5 μm. Sensor performance was analyzed in terms of response magnitude as a function of operational temperature and different relative humidity (RH). Frequency shifts of 119, 90 and 86 kHz towards 500 ppm of ethanol in synthetic air were observed for 0, 25 and 50% RH, respectively. All RH levels were measured at 20 °C. At an operating temperature of 300 °C, the largest response towards 500 ppm of ethanol was observed. Response magnitude was found to decrease with increasing RH and decreasing operating temperature. Furthermore, the effect of elevated temperatures on the sensors surface morphology is characterised by AFM and SEM techniques. It is suggested that the morphological modifications, due to elevated temperatures play an important role in the sensing behaviour of the WO3 films.