No abstract

Nucleation and growth of TiO2 clusters and nanoparticles have been studied in the sol−gel process at the hydrolysis ratio, H, between 1.0 and 2.6. A quasi-monodispersed size distribution of the condensed species has been obtained in each experimental series due to an efficient turbulent micromixing of two reactive fluids containing titanium tetraisopropoxide and water in 2-propanol. This approach enables identification of four different domains of the cluster/nanoparticle stability and growth kinetics:  H < 1.45 (I), 1.45 ≤ H ≤ 1.75 (II), 1.75 < H ≤ 2.0 (III), and H > 2.0 (IV). Small stable clusters of radius R = 0.95 ± 0.1 nm appearing in domain I may be assigned to the Keggin-type structure observed earlier for Ti17-oxo-alkoxy clusters by Steunou et al. (J. Chem. Soc., Dalton Trans. 1999, 21, 3653). The next-stable cluster with R = 1.60 ± 0.05 nm appears in domain II as a result of the assembling of five smaller clusters. Domain III is characterized by cluster instability:  they agglomerate in short chains, whose limit size depends on H. After the chain size reaches 2.6 nm (H > 2.0), nuclei are formed and subject to steady irreversible growth until the powder precipitates at the induction time (domain IV).

A spectroscopic study of the initial stage of the Ti(OPr i )4 sol−gel process has been carried out. We have observed that the primary hydrolysis/condensation reaction is complete at low water concentration. At times shorter that the induction time, the reaction results in creation of nanosize nuclei. The most important finding is that the total nuclei mass, which appears almost instantaneously during the stage of mixing, is conserved throughout the process: the particles grow by nucleus agglomeration. The hydrolysis ratio H ≤ 1 is sufficient for nucleus creation; H > 1 is needed for particle growth. The initial reagent mixing seems to be the key stage of the process.