Formation of TiO2 sols, gels and nanopowders from hydrolysis of Ti(OiPr)4 in AOT reverse micelles

Titania sols, gels and nanopowders have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation have been investigated by photon correlation spectroscopy, the crystal phases by FT-Raman spectroscopy, and the crystallite dimensions of the precipitates by transmission electron microscopy. Nanoparticles could be produced at relatively high Ti(IV) concentrations (up to 0.05 mol dm(-3)). These nanoparticles aggregated into sols, with colloid sizes of 20-300 nm, eventually forming gelatinous precipitates. The kinetics of particle formation and aggregation were controlled by varying the primary process parameters [TPT], [H2O]/[AOT] (w(0)), and [H2O]/[Ti(IV)] (R), yielding a range of products including stable, transparent sols, precipitates and monolithic gels. The aggregation kinetics and physical properties of the sols depended strongly on w(0). Different titania phases were produced, depending on w(0); w(0) less than or equal to 6 yielded amorphous particles, while w(0) greater than or equal to 10 produced anatase. The dimensions of the crystallites were comparable to those of the parent reverse micelles. A model was developed to interpret the effect of the primary process parameters on colloidal stability: (1) nucleation to form primary crystallites occurs by rapid hydrolysis and condensation reactions within the reverse micelle and (2) subsequent colloidal growth by aggregation occurs by reverse micellar exchange, where the rate of growth is governed by electrostatic and steric stability factors which increase as [AOT]/[TPT] (S) and residual [H2O]/[AOT] (w(r)) increase.