Microstructure control of thermally stable TiO2 obtained by hydrothermal process for gas sensors

Nanocrystalline titanium dioxide with enhanced thermal stability was prepared by subjecting the alkoxide-derived TiO2 gel to a hydrothermal treatment at 150 degreesC for 3 h in dilute HNO3 solution (pH 3 or 2). Structural modifications of TiO2 were analyzed by XRD and the morphology of the powders observed by FE-SEM. The hydrothermal. treatment stabilized TiO2 in two ways, i.e., suppressing thermal growth of TiO2 crystallites and shifting up the temperature of anatase-to-rutile phase transformation, although the degree of stabilization differed considerably depending on the pH of the HNO3 solution used. The TiO2 hydrothermally treated at pH 3 consisted of small anatase nanospheres of 13 and 34 nm in average diameter after calcination at 600 and 800 degreesC, respectively, whereas the untreated TiO2 was predominated by rutile phase at 700 degreesC. Suppression of crystallite growth was even more conspicuous with the treatment at pH 2; the average crystallite size of anatase was 11 and 26 nm after calcined at 600 and 800 degreesC, respectively. In this case, however, the transformation was less hindered, with rutile phase occupying 9, 22 and 67% of TiO2 after calcinations at 600, 700 and 800 degreesC, respectively. This indicates that the phase transformation is not always related uniquely with crystallite size of anatase phase. The thick films fabricated with these TiO2 powders did not show much differences in sensor response (ratio of resistance in air to that in gas) to dilute CO in air at 400-550 degreesC, though the powder hydrothermally treated at pH 3 tended to give the highest response. However, the hydrothermal treatment was found to improve much the sensor response transients, suggesting that it was effective in developing mesopores inside the films. (C) 2004 Elsevier B.V. All rights reserved.