SOL-GEL PROCESSING OF TITANIA USING REVERSE MICELLAR SYSTEMS AS REACTION MEDIA

Titanium-isopropoxide-derived gels were obtained in reverse micellar media prepared from a decane/TX35/H2O-H2O2 mixture. Polymerization of titanium isopropoxide, which is solubilized in the organic medium, occurs through hydrolysis and condensation reactions, the rates of which are controlled by the frequency of intermicellar exchanges, and by the amount of hydrogen peroxide (or water) contained in the reverse micelles. Viscosity and light scattering measurements have shown that micellar aggregates of small size solubilize eight hydrogen peroxide molecules per surfactant molecule (TX35). The gelation kinetics of titanium dioxide depend on three main parameters: the number of H2O2 and/or H2O molecules per amphiphile (n), the alkoxide hydrolysis rate (h), and the alkoxide molality (m). At the same n, h and m values, the rate of gel formation is greater in the presence of hydrogen peroxide than in the presence of water. Furthermore, gels formed with hydrogen peroxide have a better mechanical strength, as confirmed by various techniques such as thermoanalysis, X-ray and electron diffraction, adsorption isotherms, electron microscopy, thermoporometry and small-angle X-ray scattering. Mesoporous solids were obtained by means of thermal treatment; they possess a structure formed by interconnected rods of titania. Primary aggregates exhibit fractal behaviour in the gel phase, which indicates that their formation occurs by a cluster-cluster aggregation, reaction-limited process. On heat treatment of the gels, pores are formed which appear to have a bottle-like shape with a diameter of approximately 4 nm.