Near Room Temperature Synthesis of Monodisperse TiO2 Nanoparticles: Growth Mechanism

Hydrolysis of TiCl4 was used to form monodisperse nanoparticles of TiO2 with dean surfaces. The solid fraction and solution composition during synthesis were simulated using equilibrium data, and formation and growth was followed with two complementary techniques, an electrospray-scanning mobility particle sizer (ES-SMPS) and dynamic light scattering (DLS). In ES-SMPS the number density of particles is measured. Droplets formed in the spraying step mainly contain electrolyte, giving rise to residue particles that are detected together with the nanoparticles of interest. Discrimination between the two kinds of particles can be made by changing the flow conditions and applicability of the method for in situ measurements of particle size during growth is demonstrated. In DLS the hydrodynamic mobility is measured, and further insight into the initial growth mechanism was revealed by observation of slow, sustained oscillations in the scattered intensity, indicating a dissolution-precipitation mechanism at the lowest pH values. The size of the particles formed in the dissolution-precipitation step is most likely determined by the surface charge, and larger particles are formed by aggregation.