Crystallo-Chemical Approach to Gas Selectivity of Metal Oxides

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NON-TECHNICAL DESCRIPTION: Metal oxide-based gas sensing today goes beyond pollution control and environmental monitoring and reaches out to health monitoring applications and non-invasive diagnostics. Therefore, a broader scope of understanding is needed to synthesize and use metal oxides for detecting a specific chemical at trace concentrations with no interference from other compounds in a complex mixture. Within this project, the structure (rather than the composition) of an oxide is being correlated to its ability to detect a specific vapor of interest, whether it is a disease marker in breath or a harmful pollutant in the environment. The experiments being undertaken include novel synthesis of ceramic phases at the nanoscale, and atomic level characterization of gas-oxide interactions. Fundamental knowledge about how to produce tailored ceramic nanostructures with high specificity to the chemical vapors of interest is sought. This project is training scientists to use advanced materials processing and characterization techniques, and is educating the public at large about the benefits of ceramics research to human health and welfare. TECHNICAL DETAILS: While semiconducting metal oxides have been used as resistive gas sensors commercially since 1968, the nature of gas-oxide interactions still remains unknown. These inherently polymorphic ceramics exist in various distinct crystallographic configurations, each behaving as a different material with respect to it?s physical and chemical properties, even though they all have the exact same composition. Nanoscale processing of ceramics has made available a "toolbox" of "metastable phases" at room temperature in high quantities. Sensing elements based on nanostructured binary metal oxides of controlled stoichiometry and phase distribution need to be studied in order to elucidate how gas selectivity is achieved. The underlying hypothesis is that phase distribution, rather than oxide composition, determines the gas sensing properties. Therefore, this study is synthesizing such controlled nanostructures, single crystal nanowires and nanopowders of both stable and metastable phases for common oxides used in sensing (MoO3, WO3, TiO2) by means of blend electrospinning, soft chemistry routes, and a rapid solidification process; and then carrying out detailed characterization studies on gas-oxide interactions, including in situ gas sensing experiments in an electron microscope. The physico-chemical changes occurring on the oxide surfaces in contact with a given gas (e.g., oxidation, reduction, ferroelectric poling) will be assessed for a selected group of oxide crystals (rutile, perovskite, etc.) and a respective set of classes of chemicals (such as amines and alkanes). The expected outcome from this project is a gas-oxide polymorph selection library for building the next generation of gas-sensing systems with inherent selectivity, to be used in health monitoring as non-invasive diagnostics.