Preparation of thermally stable mesoporous TiO2 powder and its gas sensor application

TiO2 powder was prepared by employing a modified sol-gel method with Ti(NO3)(4) and polyethylene glycol having an average molecular weight of 6000. 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(2) g(-1) involving a small fraction of ordered mesopores with a d-spacing of ca. 4.1 nm. Calcination of the powder at 500degreesC for 1 h led to fracture of the ordered mesoporous strucutre along with a partial phase transition to rutile and a decrease in specific surface area to 25.5 m(2) g(-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(2) g-1 was maintained even after calcination at 500degreesC for 1 h. Sintering of the calcined powder at 550degreesC for 3 h resulted in a decrease in specific surface area to 157 m(2) g(-1) in the case of the PA-treated powder, but the usefulness of the PA treatment to restrict the growth of TiO2 crystallites was again confirmed. Sensitivities of the sensor fabricated with the PA-treated powder to H-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 maintainig the ordered mesoporous structure, but it may result in inactivation of surface sites for gas detection.