The Relationship between the Gas Sensing Response and the Phase Transitions occurring in MoO dollar _ 3dollar and WO dollar_3 dollar Thin Film Detectors

摘要

Semiconducting metal oxides such as MoO3 and WO3 have been considered as gas sensing materials. Both of these materials exist as several polymorphs. The polymorphic phase transition temperature depends strongly on the crystallite size of the sensing films. In this study, gas sensing elements consisting of thin films of MoO3 and WO3 were prepared by either ion beam deposition or sol-gel processing. Selective ammonia and NO2 sensors have been developed using these MoO3 and WO3 respectively. Extensive microstructure characterization was performed for each of the films before and after sensing using electron microscopy techniques related the selective gas sensitivity observed to the orthorhombic polymorphs of MoO3 and WO3. The former has an open layered structure while the latter has the cubic ReO3-type configuration. DSC/TGA studies were performed to identify the phase stability fields for the orthorhombic phases of these oxides. XPS measurements were carried out before and after sensing to understand the oxidation state of the films during sensing. A correlation is made between the sensing response and the microstructure of these oxides.