[en] An atmospheric-pressure dielectric barrier discharge (DBD) gas-liquid cold plasma was employed to synthesize Cu-doped TiO₂ nanoparticles in an aqueous solution with the assistance of [C₂MIM]BF₄ ionic liquid (IL) and using air as the working gas. The influences of the discharge voltage, IL and the amount of copper nitrite were investigated. X-ray diffraction, N₂ adsorption-desorption measurements and UV-Vis spectroscopy were adopted to characterize the samples. The results showed that the specific surface area of TiO₂ was promoted with Cu-doping (from 57.6 m²·g⁻¹ to 106.2 m²·g⁻¹ with 3% Cu-doping), and the content of anatase was increased. Besides, the band gap energy of TiO₂ with Cu-doping decreased according to the UV-Vis spectroscopy test. The 3%Cu-IL-TiO₂ samples showed the highest efficiency in degrading methylene blue (MB) dye solutions under simulated sunlight with an apparent rate constant of 0.0223 min⁻¹, which was 1.2 times higher than that of non-doped samples. According to the characterization results, the reasons for the high photocatalytic activity were discussed. (paper)