Characteristics of TiO2 thin films surfaces treated by O2 plasma in dielectric barrier discharge with the assistance of external heating

TiO2 thin films were treated with plasma with the assistance of external heating. This plasma was produced with an O2 dielectric barrier discharge (DBD) at atmospheric and near-atmospheric gas pressures. For comparison, the samples were annealed in O2 gas and also treated with O2 DBD plasma alone. The effects of these treatments on photocatalytic activity were compared by evaluating the photodecomposition of methylene blue dye in aqueous solution. The photocatalytic activity of the heat-assisted plasma-treated sample was the most enhanced among the treatment processes. The net photodecomposition of the heat-assisted plasma-treated sample was slightly higher than or comparable to that of TiO2 anatase-phase nanoparticle powder. The photocatalytic activity enhancement can be attributed to an increase in the effective concentration of photogenerated carriers on the surface. This increase is driven by the synergistic effect between an increase in the depletion layer width and an increase in the optical absorption due to the crystalline transition from the bronze to the anatase phases. This crystalline transition occurred at a lower temperature than the annealing transformation temperature.