Spherical TiO2 nano-to microparticles are rapidly synthesized by spraying clear stock solution as a nano aerosol into an atmospheric-pressure microwave plasma torch at 550-600 degrees C. The stock solution was prepared by an initial H2O/TiCl4 (titanium tetrachloride) volume ratio of 8. At about 0.15 s, anatase TiO2 powders were produced and collected by 14-stage impactors. The size distribution shows that 98.47% of the particles in number were in the < 10-nm impactor, with the normalized particle number percentage (dn'/dlogDp) of 327.1, although the mass distribution of the powders was mainly in the size range of 2.5-6.2 mu m. Scanning electron microscopy and transmission electron microscopy images show that most of the spherical TiO2 particles had a primary particle size of about 10 nm. Moreover, larger secondary particles (several mu m), likely aggregated small particles were also presented. The chemical composition and elemental analyses show that the structure of the powders is mainly composed of TiO2 with traces of TiO2-xNx. Hence, the total reaction pathway is proposed as Ti(OH)(x)Cl4-x + H2O + N-2 + O-2 -> TiO2 nuclei + HCl + other byproducts. Moreover, the ultraviolet-visible absorption spectrum and photocatalytic performance of the synthesized powders during exposure to ultraviolet or visible light are comparable to those of commercial TiO2 powders.
Aerosol And Air Quality Research, v.13 n.4, pp.1313-1320