Interaction of Water with Titania:  Implications for High-Temperature Gas Sensing

Abstract

High-temperature gas sensors based on semiconducting metal oxides show potential for optimization of combustion processes, resulting in efficient energy use and minimization of emissions. Such metal oxides can function as gas sensors because of the reaction of the sensing gas (e.g., CO) with ionosorbed oxygen species on the oxide surface with the resulting increase in conductivity. A limitation of metal oxide sensors is their difficulty of distinguishing between different gases. Designing selectivity into sensors necessitates a better understanding of the chemistry of gas-solid interactions at high temperatures. In this paper, we have used in situ infrared spectroscopy to monitor the dehydration of a hydrated anatase surface up to 600 degrees C and also to examine the hydration/dehydration of anatase held at 400 degrees C. When the O-H stretching region (3000-3800 cm(-1)) was primarily focused on, it was found that water loss from the titania surface proceeded at lower temperatures (<200 degrees C) through desorption, whereas at higher temperatures, water dissociation to terminal (approximately 3710 cm(-1)) and bridged (approximately 3660 cm(-1)) hydroxyl groups was noted. With a further increase in temperature to 600 degrees C, the bridged hydroxyl groups disappeared faster than the terminal ones. The electrical resistance of anatase at 600 degrees C was measured in the presence of moist gas streams and resulted in an increase in conductivity in the presence of water. In situ vibrational spectroscopy indicated a temporal correlation between the appearance of the bridging hydroxyl group and the change in electrical resistance. Several possible mechanisms are discussed. The chemical reaction of water with anatase at high temperatures necessitates that water be removed from the gas stream to avoid interference. A strategy involving the use of a hydrophobic microporous filter that can reject water and let gases such as CO pass unimpeded is examined. Successful use of such a concept has been demonstrated with a silicalite filter using moist CO gas streams.

UV-assisted room-temperature chemiresistive NO2 sensor based on TiO2 thin film

“…2b shows high resolution, deconvoluted XPS scans of the Ti 2p and O 1s regions. A typical Ti 2p doublet peaks centered at 458.3 eV and 464.1 eV verifies the presence of TiO 2 (Ti 4+ ) while the main O 1s peak at 529.6 eV is assigned to lattice oxygen from TiO 2 , and the side peak at higher binding energy is attributed to surface hydroxylation [20, 21]. …”

Photoinduced electron transport in dye-containing titania gel films

“…The spectrum for the untreated sample is broader than the others due to a large amount of hydroxyl groups exhibiting the peak at around 531.0-531.5 eV [28,30,32], as observed in its FTIR spectrum in Fig. 12.…”

“…The spectrum for the steam-treated sample also has a shoulder at around 532 eV assigned to the adsorbed water molecules [28,30].…”

Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO

“…Thus the electron extracted by oxygen will come back to the bulk, and the resistance of the film will change to generate a sensor signal. But for the interaction of water with MOS, equilibrium instead of red-ox reactions, exists on the interface of water vapor and TiO 2 or SnO 2 [24][25][26]: Fig. 9.…”