We report on the interaction of light with a particular class of media--wet gels, which in contrast to sols of nanoparticles possess a macroscopic bulk structure, and which differ from conventional solids by the existence of the internal liquid-solid interface. We show, taking an absorption cross section of trapped electrons from Safrany, Gao, and Rabani [J. Phys. Chem. B 104, 5848 (2000)], that a separation of charges with quantum efficiency as high as 46% appears under the band-gap irradiation of titanium oxide gels: electrons are stored in the gel network as Ti3+ centers, whereas holes are stored in the liquid phase. Under a prolonged UV-laser irradiation, more than 14% of Ti4+ centers can be converted into Ti3+ . Their lifetime can be extremely long and exceeds months at room temperature. The trapped electrons are responsible for a "dark" absorption continuum covering the spectral range from 350 nm to 2.5 mum .

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).

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