Formation Mechanism of Amorphous TiO2 Spheres in Organic Solvents 3. Effects of Water, Temperature, and Solvent Composition

Abstract

In a sol−gel system for the formation of spherical hydrous titania particles by hydrolysis of titanium butoxide (TBO) in a mixed solvent of n-butanol and acetonitrile with ammonia at 25 °C, effects of water content, temperature, and solvent composition have been studied. Water and ammonia were admixed with TBO in ca. 1 s, and the hydrolysis of TBO and about 80% of the subsequent precipitation of the hydrolysis product were completed earlier than 2.5 s in a standard homogeneous system with a butanol/acetonitrile (vol. ratio 1:1) solvent, as found in the preceding study of this series. In the standard system, dramatic size reduction was observed with increase in the content of water. The marked size reduction was explained in terms of enhanced nucleation of the hydrolysis product by the increase in supersaturation ratio due to the drop of the solubility of the hydrated hydrolysis product with the excess water. But, even though the temperature was raised up to 80 °C from 25 °C, the final mean particle size in the standard system (ca. 0.45 μm) was virtually left unchanged. From this striking result, it was concluded that the nucleation of the hydrolysis product started after the complete hydrolysis of TBO in the homogeneous system. The homogeneous system turned to a heterogeneous TBO emulsion system under agitation, when the temperature was lowered to 5 °C, the volume fraction of acetonitrile in the mixed solvent was raised to 0.6 or higher, or butanol was replaced by a higher alcohol such as hexanol or octanol. In the emulsion systems, titania particles were found to be formed in each solvent phase by dissolution of the TBO droplets, but their size distribution was broad due to constant nucleation near the surfaces of the TBO droplets. Also, dramatic reduction of the final particle size with the increasing fraction of acetonitrile was observed as well, as explained in terms of the constant nucleation near the surfaces of TBO droplets in addition to the lowered growth rate of the generated nuclei with the reduction of the solubility of the hydrolysis product due to the low affinity of acetonitrile to the butanol-solvated hydroxide monomer. A possibility of direct conversion of the alkoxide droplets to titania particles was also discussed.