Synthesis, electrical and gas sensing properties of polycrystalline ZnO films

The motivation for this thesis work is to identify, assess, and address the challenges associated with the use of polycrystalline ZnO as a gas sensing material in variable humidity environments at room temperature. In this work, synthesis--structure--property relations link the chemistry and physics of the materials involved.;The effects of humidity on gas sensing are investigated by looking at three specific parameters: baseline drift, sensitivity, and response time. Ethanol, a regulated volatile organic compound, is the test gas.;In identical humidity conditions, solid state synthesized ZnO films work better as capacitive ethanol sensors than as resistive sensors, due to faster recovery times (tens of seconds), no baseline drift, and comparable sensitivities. Further capacitive sensing measurements show that increasing the humidity content of either the target gas or the sensor's environment enhances sensitivity without impacting the recovery times. In comparison, the recovery times for a test gas of water vapor alone are longer than for any ethanol--water vapor mixture. These findings suggest that water vapor may interact with the target gas, in addition to the sensing material. At very high environmental humidities (90% RH), the sensing responses for ethanol mixed with relatively dry air show surface dehydration and subsequent re-hydration in the recovery stage due to the difference in the moisture content between the target gas and the sensor's environment.;Comparisons between the electrical properties of ZnO prepared by two different synthesis methods (solid state metathesis ZnO and Zn metal oxidation) in different humidity conditions revealed that synthesis tuning mitigates the influence of humidity on these properties. Solid state synthesized ZnO shows non-linear asymmetric current-voltage (I-V) responses at moderate (40% RH) and high (60% RH) environmental humidities, with three orders of magnitude increase in sensing current relative to dry (15% RH) conditions. In contrast, films obtained by oxidation showed less than an order of magnitude increase in sensing current under the aforementioned humidity conditions. The asymmetry in the I-V responses of solid state synthesized films at moderate and high humidity is due to water electrolysis.