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 O-2 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. (C) 2010 Elsevier B.V. All rights reserved.