| Chemical sensors that monitor gas concentrations through changes in the electrical resistance of their sensing elements are called resistive type detectors. Gas specificity is a highly desirable property in gas sensing, and is defined as the preferred response to a particular gas in the presence of other interfering compounds. The lack of specificity and limited selectivity of existing chemosensors often results in false alarms, thus reducing the reliability of these devices. This work focuses on specific gas-oxide interactions in transition metal oxides that exhibit structure sensitivity in catalytic processes. The hypothesis is that gas specificity depends on the oxide's polymorph phase used in sensing. MoO3 and WO3 thin films have been chosen for this study as the model gas sensing elements. Ammonia, nitrogen oxides and hydrocarbons are the target gases selected.; Gas sensing films were fabricated using ion beam deposition and sol-gel techniques. The effects of processing parameters, film thickness, stabilization heat treatment, and sensing temperature on the films' microstructures have been studied. The microstructures of the films are characterized using transmission electron microscopy, X-ray diffraction, and scanning electron microscopy. Differential scanning calorimetry experiments are performed to establish the phase stability fields for the various polymorphs of the model systems under study. Gas sensing tests were carried out using the orthorhombic and monoclinic phases of MoO3 and WO3.; It was found that it is possible to control the microstructure and operating temperature of a single semiconducting metal oxide film (viz, MoO3 or WO3) so as to produce polymorphs that are sensitive to a particular gas. A suitable sensor array is proposed consisting of un-doped metal oxide which can detect different target gas species selectively. |
