Mitigating Humidity Interference of Surface Acoustic Wave Ultraviolet Sensor With Hexamethyldisiloxane-Silica Nanoparticle Modified ZnO Nanowires

Owing to its wide direct bandgap and high exciton binding energy, zinc oxide (ZnO) is commonly used as a sensitive layer for surface acoustic wave (SAW) based ultraviolet (UV) sensors. However, the signal of such sensors is often adversely affected by environmental conditions such as humidity, owing to the strong affinity between hydrophilic hydroxyl groups of ZnO and water molecules. Herein, an effective mitigation method for humidity interferences of SAW UV sensors was proposed based on a hydrophobic sensing layer using ZnO nanowires (NWs) coated with hexamethyldisiloxane (HMDS)-modified silica nanoparticles. With the effectively hydrophobic modification process and high concentration of −Si(CH $_{\mathbf {{3}}}$ ) $_{\mathbf {{3}}}$ bonds formed on the hydrophobic sensing layer, a hydrophilic-to-hydrophobic transition was achieved for the coated ZnO NWs, whose water contact angle (WCA) was increased from 5° to 155°. The ZnO NWs-coated SAW devices effectively prevented water molecules from attaching onto the ZnO NWs, and thus achieved a superior antihumidity interference, even at 90% relative humidity. The developed UV sensor showed a UV sensitivity of 6.82 ppm (mW/cm $^{{2}}) ^{\mathbf {-{1}}}$ , a high linearity with an ${R} ^{{2}}$ value of 0.99277, a small hysteresis of less 2.9%, a fast response of 9 s, and reliable repeatability. Our approach effectively solves long-term issues of humidity sensitivity for ZnO NWs-based UV sensors.