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(O ⁱPr)₄ (O ⁱPr = isopropoxide) in acetic acid solution or from Ti(O ⁱPr)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(O ⁱPr)₄ varied from 16-36 nm over the hydrothermal temperature range of 180-300 °C, while the average size of those prepared from Ti(O ⁱPr)₃(OAc) varied from 13-23 nm over temperatures of 200-250 °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.