The development of titanium dioxide (TiO 2 ) as a gas sensor for combustion and exhaust air pollutants monitoring is strongly dependent on its properties such as thermal stability, grain size and surface area. In this study, nanostructure TiO 2 with its thermal stability enhanced by niobium dopant (Nb-TiO 2 ) was synthesized using the water-in-oil (w/o) microemulsion system of n-heptane/water/sodium bis (2-ethylhexyl) sulfosuccinate (AOT) surfactant and was compared with undoped TiO 2 . It was found that the synthesized powder was of uniform size (14 nm) and high surface area (80 m 2 /g). Nb-doped TiO 2 at a level of 3-5 mole% clearly hinders the anatase to rutile phase transformation and inhibits the grain growth in comparison with pure TiO 2 . The nanostructure of anatase was maintained even after the powder was fired at 850 8C. The result indicates that sensitivity of CO is significantly increased with an increase of thermal stability of Nb-doped TiO 2 in comparison with those of undoped TiO 2 and thus is useful for CO sensing studies at high temperatures. q

Titanium dioxide (TiO2) was synthesized in the water-in-oil (w/o) microemulsion system of n-heptane/water/sodium bis (2-ethylhexyl) sulfosuccinate (AOT) surfactant. This study reports the effect of microstructure of the synthesized TiO2 nanoparticles on the photocatalysis degradation of phenol in an aqueous solution, as compared to commercial TiO2 (P25) and TiO2 synthesized by bulk precipitation. The results indicated that the rate of phenol decomposition catalyzed by the synthesized TiO2 from microemulsion techniques is faster than those of TiO2 synthesized by bulk precipitation and commercial P25. This is due to the small crystal size of TiO2 prepared by the microemulsion technique. The exposed titanium sites on the surface controlled by the morphology of synthesized TiO2 are critical for photocatalytic activity.

Nanostructured powders of Nb-doped TiO2 (TN) and SnO2 mixed with Nb-doped TiO2 in two different atomic ratios-10 to 1 (TSN 101) and 1 to 1 (TSN 11)-were synthesized using the reverse micelle microemulsion of a nonionic surfactant (brine solution/1-hexanol/Triton X-100/cyclohexane). The powders were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Thick films were fabricated for gas sensors and characterized by XRD analysis and field emission scanning electron microscopy (FE-SEM). The effects of the film morphology and firing temperature in the range 650-850 °C on CO sensitivity were studied. The best gas response, expressed as the ratio between the resistance in air and the resistance under gas exposure (R air/R gas), was measured for TSN 11 at 11 for 1,000 ppm CO exposure. All types of sensors showed good thermal stability. The electrochemical impedance spectroscopy (EIS) measurements were performed in different gas atmospheres (air, O2, CO and NO 2) to better understand the electrical properties of the nanostructured mixed metal oxides. © 2007 Springer Science+Business Media, LLC

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