Dissertation
Titanium dioxide-based carbon monoxide gas sensors: effects of crystallinity and chemistry on sensitivity
Washington State University
Doctor of Philosophy (PhD), Washington State University
12/2009
DOI:
https://doi.org/10.7273/000005979
Abstract
Among metal-oxide gas sensors which change electrical resistive properties upon exposure to target gasses, titanium dioxide (TiO2) has received attention for its sensitivity and stability during high temperature (>500 C) operation. However, due to the sensing mechanism sensitivity, selectivity, and stability remain as critical deficiencies to be resolved before these sensors reach commercial use. In this study, TiO2 thick films of approximately 30 m and thin films of approximately 1 m thick were fabricated to asses the influence of their material properties on gas sensing mechanism. Increased calcination temperature of TiO2 thick films led to grain growth, reduction in specific surface area, and particle-particle necking. These properties are known to degrade sensitivity; however the measured carbon monoxide (CO) gas response improved with increasing calcination temperature up to 800 C. It was concluded that the sensing improvement was due to increased crystallinity within the films. Sensing properties of TiO2 thin films of were also dependent on crystallization, however; due to the smaller volume of material, they reached optimized crystallization at lower temperatures of 650 C, compared to 800 C for thick films. Incorporation of tungsten (W) and nickel (Ni) ions into the films created donor and acceptor defect sites, respectively, within the electronic band gap of TiO2. The additional n-type defects in W-doped TiO2 improved n-type CO response, while p-type defects in Ni-doped TiO2 converted the gas response to p-type. Chemistry of thin films had a more significant impact on the electrical properties and gas response than did microstructure or crystallinity. Doped films could be calcined at higher temperatures and yet remain highly sensitive to CO. Thin films with p-n bi-layer structure were fabricated to determine the influence of a p-n junction on gas sensing properties. No effect of the junction was observed and the sensing response neared the average of the layers; however, electrical and gas response studies revealed that the majority of the conductivity and gas-surface reactions took place on the outer layer of the film. Further research is necessary to understand the influence of p-n junctions on the gas sensing behavior.
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Details
- Title
- Titanium dioxide-based carbon monoxide gas sensors
- Creators
- Zachary Mark Seeley
- Contributors
- Susmita Bose (Chair)Amit Bandyopadhyay (Committee Member) - Washington State University, School of Mechanical and Materials EngineeringJeanne Louise McHale (Committee Member) - Washington State University, Department of Chemistry
- Awarding Institution
- Washington State University
- Academic Unit
- Materials Science and Engineering Program
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 133
- Identifiers
- 99901055136001842
- Language
- English
- Resource Type
- Dissertation