Size and shape control of nanocrystallites in mesoporous TiO2 films

We investigated the influences of the titania precursor chemistry and hydrothermal temperature in the sol - gel procedure on the growth of TiO2 nanocrystallites. The nanocrystallites were prepared either from Ti((OPr)-Pr-i)(4) ((OPr)-Pr-i = isopropoxide) in acetic acid solution or from Ti((OPr)-Pr-i)(3)(OAc) ( OAc = acetate) in water followed by nitric acid acidification. Nanocrystallites formed from the first method were 20 - 40% larger than those produced from the second method. The average size of calcined TiO2 nanocrystallites formed from Ti((OPr)-Pr-i)(4) varied from 16-36 nm over the hydrothermal temperature range of 180-300 degrees C, while the average size of those prepared from Ti((OPr)-Pr-i)(3)(OAc) varied from 13-23 nm over temperatures of 200-250 degrees C. Significantly, the size of the crystallites displayed a linear dependence on the hydrothermal temperature. Crystal growth in acetic acid was enhanced in the [001] direction with increased autoclave temperature, whereas crystallites formed in the presence of nitric acid grew less rapidly in the [001] direction. The mechanisms for the preferential growth direction of the nanocrystals are discussed.