Synthesis of titanium dioxide in O2/Ar/SO2/TiCl4 microwave torch plasma and its band gap narrowing

Titanium dioxide (TiO2) nanoparticles were synthesized directly by injecting gas-phase titanium tetrachloride (TiCl4) bubbled with a mixture of Ar and SO2 into a 2.45 GHz microwave torch plasma operated in O2 gas at atmospheric pressure. The absorption edge of the as-produced TiO2 powders in the UV–Visible spectrum shifted from 457 nm to 483 nm as the SO2 gas content in the mixture gas carrying the TiCl4 was increased. According to X-ray photoelectron spectroscopy (XPS) and UV–Visible spectroscopy, the band gap narrowing was due to the incorporation of sulphur (S) species from the Ar and SO2 mixture in the oxygen microwave torch plasma into the O site of TiO2.