Application of Undoped and Al2O3-Doped ZnO Nanomaterials as Solid-State Humidity Sensor and Its Characterization Studies

This paper reports characterization and humidity sensing studies of pure zinc oxide (ZnO) andAl₂O₃-doped ZnO nanomaterials prepared by solid-state reaction route. Pellet samples of ZnO-Al₂O₃ nanocrystalline powders with 3, 5, 10, 15, and 20 wt% of Al₂O₃ in ZnO were prepared. Pellet samples of pure ZnO were also prepared. Pellets were annealed at temperatures of 400 °C to 700 °C. Prepared powders were given pellet shape by applying pressure of 260 MPa. When samples were exposed to humidity it was found that as relative humidity (RH) increased, resistance of pellets decreased for the range of humidity from 10% to 90% RH range. Sample with 15 wt% of Al₂O₃ in ZnO and annealed at 700 °C showed the best results with sensitivity of 14.98 MΩ/%RH. For this sensing element repeatability over different cyclic operations was within ±2% of the measured values of sensitivity after six months. Response and recovery time of sensing element of 15% Al₂O₃ in ZnO were found to be 85 and 286 s, respectively. This sensing element manifested lower hysteresis, less effect of ageing and high reproducibility for annealing temperature 700 °C. X-ray diffraction pattern of this sensing element showed peaks of cubic gahnite and hexagonal corundum. Scherrer´s formula gave crystallite size for the sensing elements of pure ZnO and Al₂O₃-doped ZnO in 18-96- and 23-98-nm range, respectively. Average grain size measured from scanning electron micrograph for pure ZnO was 107 nm and for Al₂O₃-doped ZnO (AZ-15) 114 nm suggesting agglomeration of the crystallites in the sensing element to form larger grains.