Quaternary Holey Carbon Nanohorns/SnO2/ZnO/PVP Nano-Hybrid as Sensing Element for Resistive-Type Humidity Sensor

In this study, a resistive humidity sensor for moisture detection at room temperature is presented. The thin film proposed as a critical sensing element is based on a quaternary hybrid nanocomposite CNHox//SnO2/ZnO/PVP (oxidated carbon nanohorns–tin oxide–zinc oxide–polyvinylpyrrolidone) at the w/w/w/w ratios of 1.5/1/1/1 and 3/1/1/1. The sensing structure consists of a Si/SiO2 dielectric substrate and interdigitated transducers (IDT) electrodes, while the sensing film layer is deposited through the drop-casting method. Morphology and composition of the sensing layers were investigated through scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction, and Raman spectroscopy. Each quaternary hybrid nanocomposite-based thin film’s relative humidity (RH) sensing capability was analyzed by applying a direct current with known intensity between two electrodes and measuring the voltage difference when varying the RH from 0% to 100% in a humid nitrogen atmosphere. While the sensor with CNHox/SnO2/ZnO/PVP at 1.5/1/1/1 as the sensing layer has the better performance in terms of sensitivity, the structure employing CNHox//SnO2/ ZnO/PVP at 3/1/1/1 (mass ratio) as the sensing layer has a better performance in terms of linearity. The contribution of each component of the quaternary hybrid nanocomposites to the sensing performance is discussed in relation to their physical and chemical properties. Several alternative sensing mechanisms were taken into consideration and discussed. Based on the measured sensing results, we presume that the impact of the p-type semiconductor behavior of CNHox, in conjunction with the swelling of the hydrophilic polymer, is dominant and leads to the overall increasing resistance of the sensing film.