An atmospheric-pressure dielectric barrier discharge (DBD) gas-liquid cold plasma was employed to synthesize TiO2 nanoparticles in an aqueous solution, with the assistance of nine imidazolium-based ionic liquids and using air as working gas. X-ray diffraction, N-2 adsorption-desorption measurements, and transmission electron microscopy were used to characterize the samples. The results showed that the samples prepared by the DBD gas-liquid cold plasma were anatase-rutile mixed TiO2 with mesoporous structures. The effects of the cations, anions, and the amount of the ionic liquids on the structure of the TiO2 samples were investigated, and the corresponding mechanism of actions were analyzed. The photocatalytic activity of the TiO2 photocatalysts was evaluated by photodegradation of methylene blue. The TiO2 prepared with the assistance of 0.2 ml 1-ethyl-3-methylimidazolium tetrafluoroborate in Ti precursor solution exhibited the highest photocatalytic activity. Atmospheric-pressure DBD cold plasma was found to be a fast, simple, and environmentally friendly method for fabricating mixed-phase TiO2. (C) 2015 Elsevier B.V. All rights reserved.