Synthesis of uniform anatase TiO_2 nanoparticles by gel-sol method 3. Formation process and size control

摘要

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 deg C for 72h, while the yield of the anatase TiO_2, 100% below pH 9.6, started to decreased from pH 10, to 67% at pH11.5 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 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 realtively 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.