Oxygen-vacant TiO2 (TiO2-x) nanoparticles and TiO2-x/activated carbon fiber (ACF) composites were developed via a N-2/Ar/He thermal plasma system. The TiO2-x nanoparticles and TiO2-x/ACF composites were characterized with TEM, XRD, UV-Vis, ESEM and N-2 adsorption isotherms. The removal effectiveness of TiO2-x/ACF for gaseous Hg-0 at ppb concentration level and various conditions was subsequently evaluated. The experimental results indicated that the formed TiO2-x nanoparticles had a size within 10-40 nm and a mixture of anatase and rutile phases. The TiO2-x formed at 7% N-2 concentration had an evident red-shift in wavelength absorption. The ESEM and N-2 adsorption results suggested that the synthesized TiO2-x nanoparticles unevenly deposited on the ACF surface causing a decrease in total and micropore surface areas/volumes. Hg breakthrough tests revealed that TiO2-x/ACF composites had a greater Hg removal under UV or visible-light irradiation than those obtained in the dark condition. The presence of O-2 up to 12% greatly enhanced the Hg removal, implying the positive effects of catalytic oxidation. However, moisture reduced Hg removal performance, especially when visible-light irradiation was applied. These results revealed the competitive adsorption between Hg species and H2O and the physisorption nature of Hg species on the light-induced hydrophilic TiO2-x/ACF surface. (C) 2012 Elsevier B.V. All rights reserved.