Aqueous synthesis and computational modelling of metal oxide nanostructures for gas sensing applications
Creators: name: Breedon, Michael
Publisher: name: RMIT University
Year: 2024

description: In this thesis, the author pursued a holistic, multi-faceted approach to the development of ZnO, WO3 and WO3•nH2O nanostructures grown from aqueous chemical techniques, and their application to nitrogen dioxide (NO2) and hydrogen (H2) gas sensing; and understanding the adsorption processes of nitrogen oxides (NOx) on ZnO nanomaterials with Density Functional Theory (DFT). <br><br>Several new deposition techniques were developed for the fabrication of corrugated ZnO nanorods and interconnected ZnO nanowires. The choice of seed/nucleation layer used during ZnO deposition was investigated by the author, and was identified to be a major variable in the resultant nanostructured morphology. <br><br>The use of hemispherically terminated seed/nucleation layers resulted in arrays of nanorods emanating perpendicularly from the plane of the substrate. Interestingly, rougher polygonal seed/nucleation layers were found to facilitate the growth of an interconnected nanostructured morphology. A search of current literature revealed that many nanostructured WO3 deposition techniques required the use of expensive, dangerous and/or exotic reagents. Considering this, the author pursued the development of an aqueous deposition method capable of forming a nanostructured thin film via inexpensive and relatively innocuous chemical precursors. This led to the development of two new methods: the first being a successive ionic layer adsorption reaction; and the second, a sol-gel method. For the sol-gel method, it was found that the humidity of the atmosphere in which the samples were dried had a significant impact on both the nanostructured morphology and the crystallography of the resultant nanostructures. <br><br>Using the developed deposition techniques, conductometric gas sensors based on nanostructured ZnO and WO3 were fabricated, and evaluated towards H2 and NO2. ZnO nanorod arrays demonstrated the highest sensitivity of the developed sensors towards NO2. Reducing the dimensions of the ZnO nanomaterial further, to an interconnected nanowire array, made it possible to reduce the operational temperature of the sensor by almost 60%, with only a marginal decrease in sensitivity. <br><br>Optical gasochromic H2 sensors based on Pt/WO3 thin films exhibited excellent baseline stability, and the low operational temperature of 100 °C, showing promise for remote sensing applications. All concentration ranges investigated were below the lower explosive threshold of H2, and below NO2 concentrations that cause negative physiological effects in healthy adults. <br><br>The author used DFT to investigate gas interactions of NOx molecules and its dissociation products with the ZnO(2ĪĪ0) surface, at the atomic scale. Charge transfer between the surface and adsorbate was found to occur, regardless of the strength of adsorption. The direction of the charge transfer was in good agreement with experimentally observed sensor resistance changes during exposure to NO2. The calculated binding energies of atomic N and O onto the ZnO(2ĪĪ0) surface were indicative of chemisorption. It was found that NO and NO2 preferentially physisorbed onto ZnO(2ĪĪ0), over one or more surface zinc atoms, with minimal changes to the surface geometry. On oxygen defect containing ZnO(2ĪĪ0) surfaces, NO2 typically chemisorbed onto two or more defect site zinc atoms, leading to significant surface reconstruction. Comparisons between experimental and theoretical observations were made whenever possible. <br>