ABSTRACT
Titanium dioxide (TiO2) is found primarily in three main natural crystalline forms that are, from the most common (and stable) to the least: rutile, anatase, and brookite crystals (see Fig. 5.6 for crystalline structures). However, TiO2 industrial production is based on ilmenite-deposits (FeTiO3) treatment in order to manufacture particles that eventually form a chemically and thermally stable white powder. To date, there are two main methods for nanoscale-TiO2 particle production: the vapor phase and the liquid phase process. The vapor phase process is the hydrogen-oxygen-ϐlame-hydrolysis deposition using titanium tetrachloride as a raw material, which is the most competitive method because of its economy, environmental protection, and ϐlexible process technology. The liquid-phase
process is the chemical deposition that uses titanium tetrachloride and titanium oxysulfate as raw materials to process the dissolution, sublimation, hydrolysis, deposition, and heat decomposition. Nanoparticles (NPs or ultraϐine particles) are by deϐinition particles whose size ranges from 1 to 100 nanometers along at least one axis. It is clear that the size limit distinction between ultraϐine and ϐine particles is purely theoretical. Hence it is not related to any physico-chemical criterion for which a sudden change in one or several intrinsic properties would occur when crossing this size limit. However, the evolution of the surface area as a function of the particle size exhibits different regime below and above ~100 nm (see Eq. 5.1 and Fig. 5.4). In some cases, nanomaterials might present some bulk properties differing from their larger counterparts, which can be illustrated, for instance, that TiO2 powder becomes transparent for an average particle diameter inferior to 60 nm. Figure 5.1 shows such engineered ϐine and ultraϐine TiO2 particles observed by electron microscopy.
