Application of Undoped and Al<sub>2</sub>O<sub>3</sub>-Doped ZnO Nanomaterials as Solid-State Humidity Sensor and Its Characterization Studies

Abstract

This paper reports characterization and humidity sensing studies of pure zinc oxide (ZnO) andAl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -doped ZnO nanomaterials prepared by solid-state reaction route. Pellet samples of ZnO-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanocrystalline powders with 3, 5, 10, 15, and 20 wt% of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 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 Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 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% Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 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 Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -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 Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -doped ZnO (AZ-15) 114 nm suggesting agglomeration of the crystallites in the sensing element to form larger grains.