Oxygen-vacant titanium dioxide (TiO2-x) nanoparticles possessing a narrower band gap were synthesized with a single-step process using N-2/Ar/He thermal plasma as a heating source. Sample characterization showed that the formed TiO2-x nanoparticles had a size within 5-40 nm and were in a mixing form of anatase and rutile. An evident absorption red shift to 470 nm wavelength was observed. The deconvolution results of XPS spectra supported the formation of TiO2-x due to the presence of Ti(3+)2p peaks. The Hg removal efficiency of TiO2-x nanoparticles increased with increasing O-2 concentration and in the presence of light irradiation, suggesting an enhancement in Hg-0 oxidation. H2O molecules, however, greatly reduced the Hg removal of TiO2-x under visible-light (VL) irradiation. The photoinduced hydrophilicity of TiO2-x under VL irradiation was suspected to amplify the competitive adsorption of H2O that decreased Hg capture. Hg removal performances at 50 degrees C were better than those at 25 and 100 degrees C, suggesting that both adsorption and catalytic oxidation limit the Hg removal. These observations also implicate that the bonding between TiO2-x and adsorbed Hg was weak and adsorption reaction may be reversible. (c) 2012 Elsevier B.V. All rights reserved.