Formation Mechanism of Amorphous TiO2 Spheres in Organic Solvents 2. Kinetics of Precipitation

This paper (Part 2) is a report on the kinetics of precipitation in a sol-gel system developed in Part 1 of this series for the formation of monodispersed amorphous TiO2 spheres through hydrolysis of titanium butoxide (TBO) in its homogeneous solution of a mixed solvent of butanol/acetonitrile (vol. ratio 1:1) with ammonia. The precipitation was performed by adding 5 cm(3) of a composite solution of 1.0 M H2O and 0.20 M NH3 to 5 cm(3) of 0.10 M TBO in 1 s and the following aging, under constant agitation at 25 degrees C. From Nielsen's chronornal analysis on the change of the electric conductivity of the system, 83% of the precipitation of the hydrolysis product was found to be completed by 2.5 s from the start of the admixing. The preceding hydrolysis of TBO was likely to have finished earlier than the start of the nucleation of the product particles, as revealed from the kinetic analysis on the hydrolysis and precipitation processes. The particle growth proceeded through a two-step process consisting of a very fast initial step finished at latest by 2.5 s and the following much slower one. The cause of the two-step process was explained in terms of the high initial reactivity of the particle surfaces prior to the adsorption of ammonia, as well as the possibility of precipitation of higher clusters under the extremely high supersaturation, and the following deactivation of the particle surfaces by retarded adsorption of ammonia. The second step was found to proceed in a third-order reaction-controlled kinetics supposedly by direct deposition of the trimer of hydrolysis product in equilibrium with the monomer and dimer. Finally, mechanisms of particle formation in relevant sol-gel systems of TiO2 and ZrO2 are discussed in the light of the present results.