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°C for 3h 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 34nm in average diameter after calcination at 600 and 800°C, respectively, whereas the untreated TiO₂ was predominated by rutile phase at 700°C. Suppression of crystallite growth was even more conspicuous with the treatment at pH 2; the average crystallite size of anatase was 11 and 26nm after calcined at 600 and 800°C, 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°C, 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°C, 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.