Synthesis of uniform anatase TiO 2 nanoparticles by gel–sol method : 3. Formation process and size control

Abstract

Uniform anatase-type TiO 2 nanoparticles were prepared by the gel–sol process from a condensed Ti(OH) 4 gel preformed by the hydrolysis of a Ti–triethanolamine (TEOA) complex. The particle size of the anatase TiO 2 was increased from ca. 5 to 30 nm with pH increasing from 0.6 to 12 by aging the Ti(OH) 4 gel at 140 °C for 72 h, while the yield of the anatase TiO 2, 100% below pH 9.6, started to decrease from pH 10, to 67% at pH 11.5 and only 9% at pH 12.2. These results reveal a significant reduction of the nucleation rate of the anatase TiO 2 with increasing pH, as is explained by the reduction of the concentration of a precursor complex, Ti(OH) 3 +, and the adsorption of hydroxide ion onto the embryos of TiO 2. Triethanolamine appeared to enhance the pH effect on the nucleation rate of anatase TiO 2 particles by adsorption onto their embryos, leading to the wide range of the size control. Triethanolamine was also found to act as a shape controller of the anatase TiO 2 particles for yielding ellipsoidal particles from Ti(OH) 4 gel at a relatively high pH above 11. The particle size was also controlled by seeding of anatase TiO 2. Moreover, the seeding experiment suggested that the rate-determining step of the gel–sol process was not the dissolution of the hydroxide gel, but the deposition of the monomeric precursor from the solution phase.