Abstract
The chemical reactivity of transition metal alkoxides must be controlled in order to obtain sols and gels. In TiO 2-based systems, this control can be achieved through acid catalysts. H + ions catalyze the hydrolysis of Ti(OR) 4 into TiOH through the protonation of the OR groups; however the protonation of TiOH inhibits the nucleophilic attack of Ti by TiOH, hence condensation reactions are hindered. A continuous range of final products including stable sols, transparent gels, turbid gels and precipitates can be synthesized by varying the ratio H +/Ti used for the hydrolysis reaction. The structures of these materials have been investigated through small angle X-ray scattering. The polymers grow as fractal objects; remarkably, the fractal dimension, d f, varies continuously according to the ratio H +/Ti. Sols have d f ≈ 1.5–2; gels range between d f ≈ 2 (transparent) and d f ≈ 2.9 (turbid); precipitates correspond to d f ≈ 3.
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