Nanosize titania synthesis: Kinetic and colloidal stability aspects

We report on the formation of nanosize titania (TiO₂) particles by hydrolysis and condensation of alkoxides under a large excess of water. This process is characterized by a rapid precipitation of large aggregates, followed by a slow peptization (precipitate breakup). The mechanisms of peptization are revealed to consist of two competing processes: deaggregation of larger aggregates into primary particles and possibly small aggregates driven by the action of nitric acid, and aggregation of these primary particles and small aggregates driven by their insufficient colloidal stability. In order to provide the quantitative description of this process, we formulate a model for simultaneous aggregation and deaggregation. The model predicts that the steady-state size is approached exponentially, that the final size scales with the mass concentration of particles, and that the steady-state size depends on the ratio of the aggregation and deaggregation reaction rate constants. The predictions are tested against experiments on acid peptization of titania nanoparticles produced from titanium-isopropoxide. We find that the model provides a quantitative description of the experimental data and allows the calculation of the aggregation and deaggregation rate constants from measurements of size as a function of time.; The kinetic model has been applied to understand the effects of various experimental parameters, in particular temperature and alcohol. The increase in temperature and alcohol molecular weight causes the increase in the aggregation reaction rate constants. This observation occurs mostly because of the decrease in dielectric constants of liquid medium with temperature or alcohol molecular weight and hence, the decrease in the magnitude of repulsive interactions between particles as proven by the colloidal stability theory. In contrast, the deaggregation reaction is affected entirely by the change in temperature whereas the presence of alcohol showed no effect on deaggregation. The rate constants of the deaggregation reaction increase rapidly as the reaction temperature increases. It has been determined by experiment that the deaggregation exhibits a high activation energy which indicates that this process is a chemical reaction involving the breakdown of Ti-O-Ti bonds between titania particles via the action of nitric acid.