Nanocrystalline TiO2 powders synthesized by in-flight oxidation of TiN in thermal plasma:: Mechanisms of phase selection and particle morphology evolution

Titanium dioxide nanopowders were synthesized by in-flight oxidation of titanium nitride (TiN) in radio-frequency (rf) induction thermal plasma. The powders were characterized by x-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy, Raman spectroscopy, and optical microscopy to reveal the mechanisms of phase selection and particle morphology evolution. The reaction began with surface oxidation of TiN particles, leading to the formation of core-shell composites with oxidized shells and TiN cores, followed by gas-phase condensation of TiO2 nanoparticles. Phase selection of the resultant TiO2 powders was found to largely depend on the oxidation potential of the thermal plasma rather than on the heat transfer itself. Anatase content of the products increased steadily with increasing the O-2 input, and TiO2 nanoparticles (similar to 50 nm) containing similar to 90% of anatase were obtained through O-2/Ar plasma treatment. Phase-pure rutile nanoparticles (similar to 50 nm, on average) were also synthesized in H-2/Ar plasma injected with O-2 as the powder carrier gas.