Stability and Growth of Titanium-oxo-alkoxy Ti x O y (OiPr) z Clusters

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).