Enhanced moisture sensing properties of a nanostructured ZnO coated capacitive sensor

This work reports the enhancement in sensitivity of a simple and low-cost capacitive moisture sensor using a thin film of zinc oxide (ZnO) nanoparticles on electrodes. The ZnO nanoparticles are systematically characterized using X-ray diffraction, atomic force microscopy, transmission electron microscopy, BET surface area analysis, Fourier transform infrared spectroscopy, and UV-visible and photoluminescence (PL) spectroscopy. The average crystallite size of the ZnO nanoparticles is ∼16 nm with a surface roughness of ∼3 nm. Blue emission in the PL spectrum confirms the presence of oxygen vacancy dipoles, which are responsible for enhancing the dielectric properties of the ZnO nanoparticles. The effect of the ZnO nanoparticles on the sensitivity of a moisture sensor cell has been studied using wheat grains with a moisture content from 7% to 25%. An enhancement in sensitivity of 36.4% at 1 MHz and 97.4% at 500 Hz has been observed. A detailed sensing mechanism is proposed and the enhancement in sensing has been explained based on the interaction of ZnO with water vapor and the dielectric behavior of the nanostructured ZnO. The present results establish ZnO as a sensing material for improving the utility of moisture sensors.