High Quality Photocatalytic TiO2 Films Deposited by Reactive Sputtering in Mid-Frequency Mode with Dual Cathodes

Titanium dioxide (TiO2) films were deposited on unheated non-alkali glass substrates by reactive mid-frequency (mf) magnetron sputtering using dual cathodes 1,2) with two Ti metal targets. In order to keep the very high deposition rate, the experiments were carried out in the “transition region” between the metallic and the reactive (oxide) sputter mode. For stabilizing the discharge in the transition region, we used the plasma control units (PCU) which is a closed loop control for the reactive gas inlet to keep the optical emission intensity of Ti line (500nm) at the specified constant value. The dual cathodes sputtering system was made up of two magnetron sources with planar targets sized 400 × 130 mm2. They were connected with a bipolar mf pulse generator so that the targets alternately acted as cathode and anode of the dc discharge. The impressed voltage had the approximate shape of a square-wave pulse with pulse frequency of 50 kHz, which can decrease the abnormal arc discharge drastically under the high sputtering power impression. The films were deposited at total gas pressure of 1Pa and 3Pa. For the all depositions the discharge power was adjusted to 2kW per each target. Stable deposition was carried out in the whole transition region at the both total pressure of 1Pa and 3Pa, using the PCU. XRD patterns of all the films deposited in the transition region showed amorphous structure. These amorphous films didn’t show any photocatalytic activities. On the other hand, XRD patterns of the films deposited in reactive region showed anatase polycrystalline structure, which performed clear photocatalytic activities, ie. photo-induced hydrophilicity and decomposition of the acetaldehyde by UV light irradiation (Black Light, 1.0mW/cm2). However the deposition rate in the reactive region was very small of about 2nm/min because the oxidized Ti surface had a very small sputtering yield. The films deposited in the transition region, which had amorphous structure, were annealed in air at 200? for an hour. After the annealing, XRD patterns of these films showed anatase polycrystalline structure. They performed high Photocatalytic ability, the both decomposition of the acetaldehyde and hydrophilicity after the UV irradiation. Additionally, the films deposited at the pressure of 3Pa had higher photocatalytic activities than those deposited at the pressure of 1Pa. The highest deposition rate in this study for the photocatalytic TiO2 films deposited in transition region was over 40 nm/min, which was about twenty times higher than that for the films deposited in the reactive region