Development and characterization of high temperature, selective titania-based gas sensors

Selective titania-based sensors for high temperature detection of carbon monoxide were developed and characterized. Two approaches to making the sensors selective were taken; the addition of dopants to the anatase phase of TiO 2 and mixing p-type and n-type TiO2 to make composite sensors.;Sensors made of TiO2 (anatase) and TiO2 doped with La2O3 show a resistance change to CO that is larger than the response to CH4. When CuO is added, the sensitivity to methane is minimal, but the sensor maintains sensitivity to CO. The samples were studied with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to determine a sensing mechanism. La2O3 formed a LaOx surface phase on TiO2, strongly adsorbing CO 2 and carbonate-type species. The adsorptive properties of the LaO x allows for the observation of reaction intermediates on the surface of TiO2 and TiO2/CuO at high temperatures. CO oxidation on the anatase/La2O3/CuO sample resulted in the formation of large amounts of carbonate species on the sample surface, while the reaction of methane produced negligible species. This suggests that CH4 may be rapidly oxidizing on CuO without interaction with titania or La 2O3, thus producing minimal resistance change and few infrared observable species. CO oxidation also occurred on the CuO surface but significant reaction also occurred on the anatase surface and produced a resistance change.;The anatase phase of TiO2 sensor shows an n-type response to CO and CH4 while the rutile phase shows a p-type response. A 75% rutile composite mixture of anatase and rutile selectively responds to CO over CH4. SEM micrographs of the sample show that both anatase and rutile particles are percolating in this sample, resulting in an electrical response that is a combination of both material's responses.;Sol-gel titania was investigated as sensor material. Characterization of sol-gel derived powders by XRD and electron microscopy showed that under the high temperature required for the sensor preparation, the TiO2 is rutile and approximately 100 nm in size. The electrical behavior of the sol-gel titania was n-type. Both thick film and thin film sensors made from the sol-gel titania were more sensitive to CO than sensors made from commercial titania.