Popis: |
The development of facile methods to produce hetero-structural metal oxides with highly controlled nano-scale morphology could have implications for resistive-type gas sensing as well as related fields such as catalysis and energy applications. Nano-scale metal oxides are ideal materials for resistive-type sensors because surface processes can cause large modulation in resistance. For n-type metal oxides such as SnO2, ZnO, and TiO2 at elevated temperatures, ambient oxygen adsorbs onto the sensor surface and abstracts electrons from the material. When the metal oxide has dimensions in the range of the Debye length, the presence of this adsorbed oxygen can cause a significant effect on charge carriers in the sensing material. This surface charge accumulation causes upward band bending resulting in a high junction potential and a surface depletion layer which causes increased resistance. When a reducing gas is introduced, oxygen can react with the adsorbed gas molecules and electrons will be released back into the sensing material thereby decreasing resistance. This change in measured resistance can be used to detect ppm levels of a variety of reducing gasses. SnO2 based sensors in particular have shown good sensitivity and stability to a variety of important gasses. Improvements in sensitivity and selectivity would further increase the implementation of this technology. Decoration of p-type materials such as CuO, Co3O4, and NiO onto n-type metal oxides has shown to improve sensor sensitivity and selectivity. The reduction of free carriers in SnO2, due to the presence of a p-n junction results in band bending and resistance modulation. Increased surface reactivity can facilitate more reactions between adsorbed oxygen and the gas analyte. This may both improve resistance modulation and response time as well as improve gas selectivity. Performance improvement in these hetero-structural resistive-type sensors has often been attributed to:1)Fermi level equilibration that results in charge carrier depletion in the major conducting channels and 2)Increased surface activity due to the catalytic nature of the decorated p-type metal oxides. However, fundamental understanding of the mechanisms dictating gas selectivity in hetero-structural sensors is lacking, necessitating the need for further investigation of selective sensing behavior. Selectivity improvements have been attributed to gas-surface interactions and the electronic structure of the two materials. In this dissertation, concepts relating both to receptive and transduction sensor mechanisms are explained. The effects due to gas surface interactions and electronic equilibration are compared and discussed. Both modeling efforts and experimental literature are presented to explain fundamental mechanisms that control sensor selectivity. In this dissertation, an overview of the mechanisms responsible for selective detection will be covered in Chapter 1 and Chapter 2. Chapter 3 will discuss synthesis and oxide selection. Chapter 4 and Chapter 5 will discuss the development of a database to find trends in resistive-type sensing research. Chapter 6 will show sensing results obtained to examine selectivity changes due to changes in oxide selection and oxide ratio. Chapter 7 will present mechanistic models that may be used to design selective sensing material. Finally, Chapter 8 will discuss future outlook and conclusions of this research. |