| TiO2thin film has drawn much research attention due to its versatile applications in photocatalysis, photovoltaics, biocompatibility and sanitary disinfection. Low temperature film-fabrication processes are essential for all thermally sensitive substrate materials such as plastic and textiles. Cold plasma chemical vapor deposition (CVD) is highly efficient for low temperature fabrication. However, it has mainly been low pressure incorporated with sophisticated discharge and vacuum systems that have been adopted. Dielectric barrier discharge (DBD) is a simple and easily operated approach for generating atmospheric-pressure cold plasma. In this paper, atmospheric-pressure cold plasma was employed to prepare TiO2photocatalytic films from TiCl4and O2with one-step procedure. The results are as follows:1. Using surface DBD, atmospheric-pressure cold plasma was employed to prepare TiO2photocatalytic films with one-step procedure.At a lower surface power density (0.6W·cm-2), the TiO2films fabricated were amorphous as evidenced by Raman analysis, and showed a very poor photocatalytic response. After calcination at350or450℃, characteristic peaks corresponding to anatase TiO2were observed, and the TiO2films were notably photocatalytically active for oxidation of HCHO. The crystallinity and photocatalytic activity of the TiO2films calcinated at450℃were higher than those calcinated at350℃.Increasing the surface power density to1.2W·cm-2, the as-deposited TiO2films were notably active for the photocatalytic oxidation reaction of HCHO without extra heat treatment. In addition, TiO2photocatalytic films were also successfully fabricated on plastic substrates with one-step procedure at this surface power density.2. Using volume DBD, atmospheric-pressure cold plasma was employed to prepare TiO2photocatalytic films with one-step procedure.To deposit TiO2films through plasma CVD, the partial pressure ratio of O2to TiCl4should be greater than stoichiometric ratio (pO2/pTicl4>1). However, this may result in the formation of powder instead of film on the substrate using volume DBD at atmospheric pressure. In this study, by adding N2into the working gas Ar, TiO2photocatalytic films were successfully fabricated in the presence of excess O2(pO2/pTiCl4=2.6) using atmospheric-pressure volume DBD with wire-to-plate electrodes. The tuning effect of N2on the as-deposited TiO2film was studied at different N2contents with the same input power or discharge power. The results showed that by increasing N2content, deposition rate of the as-deposited TiO2film was reduced. SEM images of all six samples showed evenly distributed granular surface morphologies, but the particle size was reduced significantly by increasing N2content. The UV-Vis spectra demonstrated the high visible light transparency and strong UV absorption of the as-deposited TiO2film. The visible light transmittance was enhanced by increasing N2content. According to the XPS results, there was little difference in the chemical binding state and composition among six samples of TiO2film deposited at different N2content with the same input power or discharge power, and the O/Ti atomic ratio of these samples was in the range of1.9-2.0. No N-doped TiO2was formed during the deposition process. The photocatalytic activity increased significantly by increasing N2content.Analysis of the OES spectra and the gas temperature during deposition of the TiO2films showed that, with the increase of N2content, the vibrational temperature of N2increased, the excitation temperature of Ar atom decreased, and there was no obvious change in gas temperature. Thus, the tuning mechanism of N2can be explained as follows:on the one hand, the increase in N2content led to the reduction in excitation temperature of Ar atom, which may lead to the reduction in reaction rate, and therefore the reduction in deposition rate and particle size of TiO2film; on the other hand, the increase in N2content led to the increase in vibrational temperature, which may facilitate the formation of photocatalytic active phase, and therefore led to the enhancement in photocatalytic activity of as-deposited TiO2film.3. The uniformity of the deposited TiO2films using wire-to-plate volume DBD was investigated on surface and cross section by UV-Vis absorption spectra. The film thickness increased linearly with deposition time, and the UV absorbance increased linearly with the thickness of TiO2films. This proved the uniformity of TiO2films on cross section. UV-Vis absorption spectra of5testing points on surface of the TiO2film were in good agreement, which showed the uniformity of TiO2films on surface.The kinetics of photocatalytic oxidation for TiO2films in HCHO removal from simulated air was investigated in a continuous flow reactor under UV irradiation of a254nm lamp. Distance between the quartz window and the film was kept at1mm and TiO2film of783nm thickness (deposition time=15min) was used. The influences of residence time, light intensity, HCHO initial concentration and humidity on phtocatalytic oxidation rate of HCHO were investigated. At short residence time (τ<0.7s, linear velocity>4cm·s-1), the overall rate was controlled by surface reaction, and the apparent rate constant of photocatalytic oxidation of HCHO k was0.7cm2·mJ-1.In the range of the incident light intensities investigated (0.9-1.84mW·cm-2), the experimental data of HCHO conversion to CO2and the value calculated by k=0.7cm2·mJ-1were in good agreement, and the photocatalytic oxidation reaction rate of HCHO increased linearly with the increase in incident light intensity.In the range of the initial concentrations of HCHO investigated (24-58ppm), the experimental data of CO2formation and the value calculated by k=0.7cm2·mJ-1were in good agreement, and the photocatalytic oxidation of HCHO can be assumed to be pseudo-first-order reaction.In the range of the humidity from0.1vol%to0.5vol%, photocatalytic conversion of HCHO to CO2kept almost constant, while in the range of the humidity from0.5vol%to1.7vol%, photocatalytic conversion of HCHO to CO2decreased with increasing the humidity. According to the results, the reaction rate equation for photocatalytic oxidation of HCHO including the influence of humidity was preliminarily given.
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