Size and shape control of nanocrystallites in mesoporous TiO2 films

Abstract

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(OⁱPr)₄ (OⁱPr = isopropoxide) in acetic acid solution or from Ti(OⁱPr)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(OⁱPr)₄ varied from 16–36 nm over the hydrothermal temperature range of 180–300 °C, while the average size of those prepared from Ti(OⁱPr)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.