Hetero-junction and Nanomaterial Systems for Metal Oxide Semiconductor based Gas Sensing

Investigations into a number of hetero-junction and nanoceramic materials systems for metal oxide semiconductor (MOS) gas sensing for potential environmental and bio-sensing applications are presented. The hetero-junction study encompasses investigations into various composite n-n hetero-contact systems such as WO3-ZnO and SnO2-ZnO and a p-n hetero-contact system, specifically CTO (Chromium Titanium Oxide) - ZnO. The facile fabrication of various arrays of hetero-junction MOS gas sensor devices has been demonstrated. A simple change in the compositional contribution of an individual metal oxide within a composite, exhibits the ability to tune the composite’s responsivity and selectivity. The hetero-junction systems were characterized by various techniques including Scanning Electron Microscopy (SEM), Raman spectroscopy, X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) and the influence of the physical and chemical properties of these materials towards the associated gas sensing properties, deduced. Further, the influence of fundamental properties of junctions such as contact potential and packing structure, towards the sensing properties, are also discussed. The nanomaterials study encompasses investigation into ZnO semiconducting oxides fabricated by various emerging fabrication technologies including Continuous Hydrothermal Flow Synthesis (CHFS) and other relatively high temperature routes. The chemical and physical properties of the nanoceramics have been investigated by various techniques including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Brunauer Emmett Teller (BET) surface area measurements. The investigation demonstrates emerging techniques for the production of nanomaterials, which can be successfully used in MOS gas sensing for the desired applications. Further, the study shows that the behaviour of the nanomaterials is complex and material surface area is not the only deterministic factor of enhanced responsivities, but microstructural factors such as morphology and particle size, as well as heat-treatment conditions are all influential over the overall sensing properties. This thesis presents an overview of emerging material systems for MOS gas sensing applications