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URN etd-0218105-093810 Statistics This thesis had been viewed 3042 times. Download 1413 times. Author Tsung-Yeh Yang Author's Email Address firstname.lastname@example.org Department Materials Engineering Year 2004 Semester 1 Degree Ph.D. Type of Document Doctoral Dissertation Language zh-TW.Big5 Chinese Page Count 128 Title The Gas Sensing Properties of Nanocrystalline TiO2 based Systems Keyword XRD TEM Nanocrystalline TiO2 Gas sensor AFM AFM Gas sensor Nanocrystalline TiO2 TEM XRD Abstract The gas sensing properties of nanocrystalline TiO2 are enhanced by submitting the sensor system to UV light irradiation. Titanium dioxide nanoparticles are produced by gas condensation technique under 5-mbar of helium and oxygen atmosphere, and then quenched to the liquid nitrogen temperature. The as prepared titanium dioxides of nanoparticles are porous free with uniform size. The powder X-ray diffraction (XRD) spectrum confirms that the as-prepared and after sinter treatments samples. The crystal structure and ceramic microstructure of the powders are determined by transmission electron microscope (TEM). The size of granules and surface morphology are analyzed by atomic force microscopy (AFM). Structural and morphological studies as well as UV irradiation are carried out in order to investigate their influence on the sensing properties of TiO2 sensor.
Preliminary X-ray diffraction results indicate that the structure of TiO2, that is transformed to the rutile phase up 1000℃. There are two phases exist at 800℃, one is anatase phase and another one is rutile phase. An anatase phase exist at 600℃. In order to understand the better distinguish the structural properties of the titanium dioxide Nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or titanium can be determined. According to the XAS result, the shape of the Ti K-edge undergoes no distinct changes. This infers that structure transformation of titanium dioxide nanoparticle may be caused by the migration of oxygen after sintering. The other reason is when increases of calcine temperature caused the oxidation more obvious. After calcine process the position of the oxygen doesn't shift. These results indicate that heat treatment to approximately from 600℃ to 1000℃ produced stable structures of nanocrystalline of TiO2. It is important information to the sensing material.
It is easily to adsorb the water vapor on the surface of porous nano-TiO2. The resistance decreases while the surface of TiO2 capacitance increases with highly humidity. The gas sensing properties can be obtained from the changes of the resistance as a function of time during different CO concentrations range. The gas-sensing properties of nano-TiO2 have been demonstrated to be enhanced by UV irradiation. The sensitivity to gaseous CO increases by a factor of 30 with UV light enhancement. The model of photocatalysis can be used to interpret the enhancement of the sensing properties by UV stimulation. The UV enhancement of the gas sensing property can be explained by the decrease of the activity energy of surface reactions.
The results of vacuum environment experiment indicate the effect of UV radiation on the sensing mechanism of TiO2 gas sensor can be carried out based on theoretical assumptions. The UV radiation can significantly enhance the conductivity and sensitivity of a TiO2 gas sensor at high temperature. The sensitivity increases with the increasing UV radiation flux density. Also, the current of the nano TiO2 thin films due to the UV irradiation at lower temperature is comparable with that of the nano TiO2 at high temperature without UV irradiation. A new model has been created in this study to explain the mechanism of gas/soild interaction between surface of TiO2 and CO molecular.
The light activates the TiO2 gas sensor increasing both the changes in conductance also increasing the rate of reaction. The sensor exhibited good sensitivity to CO (200-800 ppm) with UV irradiated. It shows that the response times increased slightly and the recovery times increased with increasing CO concentration. Surface FTIR spectrometry proved to be a powerful technique for studying in situ detection the gas/soild surface interactions and electric response occurring in nano TiO2 gas sensor. This technique has proved that surface reactivity of gas sensor depends on the activation conditions of UV source. In the other words, to ensure better response, reproducibility, higher sensitivity and selectivity of the gas sensor, the material of gas sensor must be carefully prepared and the working conditions shall well control.
Advisor Committee Hong-Ming Lin - advisor
C. K. Lin - co-chair
T. T. Chung - co-chair
W. C. Chen - co-chair
Yi Hu - co-chair
Yung-Jen Lin - co-chair
Files Date of Defense 2005-01-31 Date of Submission 2005-02-18