Humidity-sensing properties of ZnO QDs coated QCM: Optimization, modeling and kinetic investigations

Uncapped 8 nm average diameter ZnO QDs were successfully prepared by the sol-gel method and studied for moisture detection application by QCM technique. The developed humidity sensors were built by a quartz crystal microbalance (QCM) as the transducer and ZnO thin layer as a humidity sensing element. The synthesized ZnO QDs were characterized by powder X-ray diffraction (PXRD) and UV-visible. The experimental investigation indicates that the sensitivity as well as the response and recovery times of the sensors were greatly influenced by the thickness of the deposited layer. The optimized thickness of 370 nm shows a good sensitivity, a good long-term stability and reproducibility, a short response and recovery time and a small hysteresis. To elucidate the effect of the thickness on the response of the sensors, the thermodynamic and the kinetics adsorption-desorption processes were investigated in real time by the multi layer Guggenheim-Anderson-deBoer model (GAB model) and bimodal exponential kinetic model, respectively. The constructed adsorption isotherms coincide and show a sigmoidal shape (S-shaped, type II) for all sensors. The estimated characteristic times, deduced from the time-dependent frequency shift, were correlated to different kinetic steps involved during sorption processes. (C) 2014 Elsevier Ltd. All rights reserved.