Adsorption mechanism of water molecules on the surface of ZnO-SAW sensors

In this work, the ZnO-SAW sensor was successfully prepared for the detection of relative humidity by employing the magnetron sputtering technology. The sensitivity of the ZnO-SAW sensor to different RH levels was investigated. The adsorbing mechanisms of water molecules on the ZnO surface were also investigated by using the density functional theory. [Display omitted] In the present study, the ZnO thin film was deposited on the surface acoustic wave (SAW) chip for sensitive humidity sensors by using radio frequency magnetron sputtering method. The microstructure of the ZnO thin film was characterized by using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The ZnO film exhibited rough morphology with the average thickness of about 0.3 μm. The humidity-sensing properties of the sensors were examined by changing the relative humidity (RH) within the range of 20%–60% RH at ambient temperature. The resonant frequency shifts increased gradually with the rising of RH levels. It was found that the ZnO-SAW sensors showed high sensitivity, mainly owing to the formation of the liquid water layer and superposition effect of water molecules (WMs). Moreover, the density functional theory (DFT) calculation revealed that the adsorption and desorption of WMs on the ZnO film were physical processes.