Atmospheric-pressure Rf Plasma Chemical Vapor Deposition Of TiO₂Photocatalytic Films

Anatase titanium dioxide (TiO2) films have been studied extensively as semiconductor photocatalyst for solar energy conversion, purification of air and water, self-cleaning and antibacterial disinfection. Atmospheric-pressure dielectric barrier discharge (DBD) has a potential competitive advantage because of its low deposition temperature, simple device and easy operation for preparation of TiO2photocatalytic films. Atmospheric-pressure radio frequency (RF) plasma was employed to prepare the anatase TiO2photocatalytic films with one-step procedure on the substrate without additional heating, and deposition mechanism and kinetics study of photocatalytic reaction for TiO2films were investigated in this paper. The results are as follows:1. Using atmospheric-pressure RF plasma, the anatase TiO2photocatalytic films were prepared using titanium tetraisopropoxide (TTIP) and O2as the precursors with one-step procedure at input power of50W. The results demonstrated that the as-deposited TiO2film was anatase structure with high crystallinity, and showed high photocatalytic activity in complete oxidation of formaldehyde (HCHO) to CO2and degradation of stearic acid, respectively. Subsequently, the effect of flow rate of carrier gas for TTIP and input power were investigated on the crystallization of TiO2filmThe gas temperature in the atmospheric-pressure RF plasma was estimated at about500K by the rotational temperature via optical emission spectra (OES). No photocatalytic activity was observed when the amorphous TiO2film was treated either by Ar+O2plasma or by Ar+O2+isopropyl alcohol (IPA) plasma at the same conditions of TiO2film deposition. Thereby, the thermal effect of the atmospheric-pressure RF plasma on crystallization of TiO2film was excluded, which confirmed that there does exist non-thermal effect of the atmospheric-pressure RF plasma on the high photocatalytic activity of the as-deposited TiO2film.2. Using diagnosis of optical emission spectra, the dependences of relative intensity of atomic oxygen line, Ar excitation temperature, OH rotational and vibrational temperatures were investigated on total flow rate, O2and TTIP partial pressure and input power in the system of Ar+O2and TTIP+Ar+O2, respectively. Subsequently, RF discharge was compared with kHz discharge in the respects of relative intensity of atomic oxygen line, OH rotational and vibrational temperatures and variation of spectral lines of CH and Ti with O2partial pressure.TiO2powders were prepared at different O2partial pressures and input powers, and the XRD results of TiO2powders were compared with results of OES and mass spectra (MS). It showed that relative intensity of atomic oxygen line, Ar excitation temperature, OH rotational and vibrational temperatures were not all immediate cause for change of crystallinity of TiO2powders. The energy for crystallinity of TiO2powders originated from the de-excitation of metastable argon atoms (Ar*). Thereby, anatase TiO2powders can be prepared successfully by atmospheric-pressure RF plasma, which can be explained as follows:on the one hand, RF discharge generated many reactive species such as O atoms which led to the oxidation of organic group from dissociation of TTIP and promoted generation of TiO2molecules; on the other hand, RF discharge generated many metastable argon atoms which provided energy in the process of nucleation and growth of TiO2and may facilitate the improvement of crystallinity.3. Using a continuous flow reactor, kinetics study of photocatalytic removal of formaldehyde from air over anatase TiO2films of230nm thickness was investigated under irradiation of UV lamps. In the experiment, effect of residence time on photocatalytic oxidation of HCHO was investigated over three TiO2films with different area, and determined the linear velocity of the reaction control, which was greater than5.1cm·s-1(r0.79s). And then the effects of UV light sources, incident light intensity and humidity on photocatalytic removal of HCHO were analyzed quantitatively in the reaction control region to obtain the rate equation of photocatalytic reaction containing two key influencing factors: incident light intensity and humidity. Subsequently, in the range of the initial concentrations of HCHO investigated (28-62ppm), the rate equation of photocatalytic reaction was verified by the experimental results of CO2formation rate. Finally, the effect of incident light intensity, humidity and HCHO initial concentration on formal quantum efficiency (FQE) were examined, and the results of FQE were explained by the rate equation of photocatalytic oxidation.