Porous Cellulose Acetate Film-Coated Surface Acoustic Wave Sensor for Highly Sensitive Humidity Sensing

Abstract

The humidity sensing performance and mechanism of surface acoustic wave (SAW) sensors with cellulose acetate (CA) films were studied. The porous microstructure and strong interaction with water molecules of CA films are favorable for its attraction to water molecules, thus greatly improving the humidity sensing performance of SAW sensors, which have been verified by scanning electron microscopy (SEM), N2 adsorption, and Raman characterizations. The SAW sensors coated with 4 wt% CA films show a sensitivity of 9.46 kHz/%RH, linear response, and good stability in the humidity range of 20%–90%, along with a short response time (21 s) and recovery time (14 s). In contrast, extremely low responses were observed in the sensing tests of the same sensors to other gases (CO, H2S, NH3, SO2, CH4, and NO). Both variations of mass and conductivity of sensing films can cause the frequency shift. The conductivity change of the CA film provides only a small part of the frequency response. Conversely, the adsorption of water molecules increases the mass of the CA film, which results in the dramatic and fast frequency response of the sensor to humidity variation.