SAW humidity sensor with oleic acid-modified SiO₂ microsphere-supported GO film and acoustic wave driving optimization: Rapid response and low hysteresis

Surface acoustic wave (SAW) humidity sensors have found successful applications in meteorological monitoring, respiratory diagnostics, and industrial process control. However, achieving both rapid response and low hysteresis while maintaining high sensitivity remains a significant challenge. To address this issue, we propose a novel SAW humidity sensor based on a three-dimensional (3D) composite film, where graphene oxide (GO) is supported by oleic acid (OA)-modified SiO₂ microspheres (SiO₂@OA/GO) to enhance water molecule transport while maintaining acoustic compatibility with the substrate. Furthermore, the intrinsic self-excited vibrations of the SAW device are harnessed to actively accelerate the adsorption and desorption of water molecules. Experimental results demonstrate that the SiO₂@OA/GO-based sensor exhibits significantly improved performance, achieving a response/recovery time of 2.6/1.2 s and a hysteresis of 2.7 % relative humidity (RH) under 0 dBm driving power. When driven at the optimal power of 20 dBm, the response/recovery time is further reduced to 1.3/0.7 s, the hysteresis decreases to 1.1 % RH, and the quality factor increases to 2299. The sensor also shows good long-term stability and is successfully applied in real-time respiratory monitoring. These findings highlight the effectiveness of integrating material engineering with SAW excitation, providing a viable route toward efficient humidity sensing.