Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Park, Jinsoo, Nanostructured semiconducting metal oxides for use in gas sensors, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3441
A gas sensor is a functional device to warn us of dangerous gases in our environment. Its reliable and effective functions protect our property or lives from unexpected accidents or harm. So, it can be said that human safety is closely related to features of the gas sensors, that is, sensitivity and selectivity. To improve the properties of gas sensors, this thesis has attempted to apply nanotechnology to their fabrication. Commercially available metal oxides or rarely investigated types of metal oxides were chosen to investigate possible enhancements from the size-related and structural effects.
Tin(IV) oxide was investigated, although it is a conventional material in the gas sensor field. It was synthesized in the form of nanotubular structures using the template filling method. The results confirmed that the nanotube is a good nanostructure to enhance the sensing properties and was superior to nanoparticles.
One-dimensional nanostructured Indium oxide (In2O3) and Gallium oxide (Ga2O3) were prepared by carbothermal chemical vapor deposition (CVD). They were successfully synthesized according to the Vapor-Liquid (VL) or Vapor-Liquid-Solid (VLS) theories. Their perfect crystallinity enabled an investigation into the crystallographic effects from exposed planes. So, comprehensive analysis using high-resolution transmission electron microscopy (HRTEM) was carried out. However, due to having very few active sites and thick nanowires diameters, In2O3 only exhibited moderate sensing properties, and Ga2O3 did not respond to any target gases. For the study of crystallographic effects, it is necessary to scale down the lateral thickness to less than twice the thickness of the depletion layer.
Hematite (α-Fe2O3) was studied in various porous nanostructures for gas sensors: (1) porous nanorods and branched nanostructures, (2) porous nanowires, and (3) porous nanospheres. These nanostructures were synthesized via the hydrothermal growth route by changing experimental parameters such as source materials, temperature, reaction time, etc. This investigation was an attempt to engineer gas sensors by changing architectures in consideration of the geometric aspect. Sensitivities obtained at 1000 ppm of any chosen gas are similar, but for the range down to 200 ppm, the morphology has an influence on the gas sensing properties. The insight from the results is that there may be potential for improvement of gas sensing properties such as sensitivity and selectivity by various synthetic techniques.
Nanoribbons of Cupric oxide (CuO), a p-type semiconducting metal oxide, were investigated for gas sensors. CuO has barely been studied for gas sensors due to its lower sensitivity compared to n-type semiconducting metal oxides. However, CuO nanoribbons 2 – 8 nm in thickness showed remarkably high sensitivities compared to their counterparts. Moreover, functionalizing them by loading with Au or Pt proved its effectiveness in enhancment of the gas sensing performance.
Cobalt oxide (Co3O4, p-type semiconductor) hollow nanospheres were synthesized by a surfactant-assisted solvothermal method. Co3O4 is widely used as a functional material due to its high catalytic reactivity, which may be effective in gas sensing performance, especially towards organic solvents and fuels. Co3O4 hollow nanospheres with a diameter of 200-300 nm showed that they can achieve good sensing performance in detecting toluene, acetone, etc.