Development of a new preparation method of thermally stable mesoporous titania and its application to gas sensors

TiO/sub 2/ powder was prepared by employing a modified sol-gel method with Ti(NO/sub 3/)/sub 4/ and polyethylene glycol (average molecular weight: 6000, PEG6000). The as-prepared powder was in anatase phase with a crystallite size of 4.4 nm, and showed a high specific surface area of 214 m/sup 2/ g/sup -1/ involving a small fraction of ordered mesopores with a d-spacing of ca. 4.1 nm. Calcination of the powder at 500/spl deg/C for 1 h led to fracture of the ordered mesoporous structure along with a partial phase transition to rutile and a decrease in specific surface area to 25.5 m/sup 2/ g/sup -1/, so that the ordered mesoporous structure was not thermally stable enough. Phosphoric acid (PA) treatment was found to be effective for improving the thermal stability of the ordered mesoporous structure. In addition, no phase transition and little change in crystallite size were observed, and a high specific surface area of 217 m/sup 2/ g/sup -1/ was maintained even after the calcination. Sensitivities of the sensor fabricated with the PA-treated powder to H/sub 2/ and CO was not so high as those of the sensor fabricated with the untreated powder, irrespective of its high specific surface area, small crystallite size and ordered mesoporous structure involved. Thus, the PA treatment was effective for restricting the crystallite growth and then for maintaining the ordered mesoporous structure, but it may result in inactivation of surface sites for gas detection.