Photocatalytic Properties Of Nonmetal Co-doped Nano-TiO₂ Films

Titanium dioxide TiO₂ is considered a promising photo-catalyst due to its strong oxidizing power, photo-stability, non-toxicity, chemical and biological inertness, as well as its low cost etc. Under the excitation of UV light, TiO₂ forms photoinduced-electron and hole and effectively degrade some organics into H2O, CO₂ and other small molecules so as to eliminate secondary contamination. However, there are some disadvantages in its practical application. Widespread technological use of TiO₂ has been limited by its wide band gap (3.2eV), because only under irradiation of light wavelength<387nm can TiO₂ realize its photo-catalytic activity, while natural solar spectrum contains less than 5%. Photo-catalytic activity of the TiO₂ is due to the production of photo-generate electrons and corresponding positive holes. In fact, the photo-catalytic efficiency depends on the competition between the surface charge carrier transfer rate and the electron-hole recombination rate. If the recombination rate is so fast, there is no photo-catalytic activity. Nonmetal ions doping is to reduce the recombination of the species.In order to avoid the drawbacks of TiO₂ ,we adopt improved sol-gel process and co-doped titanium dioxide films.It is believed that the surface microstructure of the films and the co-doping methods are responsible for improving the photo-catalytic activity.This paper mainly includes the following three parts:(1) Preparation and properties of TiO₂ filmsThe nano-crystalline B³⁺ and F- co-doped titanium dioxide films were successfully prepared by the improved sol-gel process. Based on single doped ions, the thesis studies the degradation of the bi-doped of F-B and tri-doped F-B-S to organic dyes. The experimental results indicate that no matter under the UV-light or visible light, the co-doped nonmetallic ions have better effects of photocatalysis than any single ions. In co-doped ions system, photocatalysis of tri-doping is obviously better than that of bi-doping .The dynamics study shows that the photocatalytic degradation reactions of both under UV light and Visible Light are correspond with the Langmuir-hinshelwood first-order equation(2) Characterization of TiO₂ composite films The as-prepared specimens were characterized using X-ray diffraction (XRD),high-resolution field emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, and UV-vis diffuse reflectance spectroscopy. The results indicated that the TiO₂ films were composed of nano-particles. Nonmental co-doping could obviously not only suppress the formation of brookite phase but also inhibit the transformation of anatase to rutile at high temperature. Diffuse reflectance measurements showed that co-doping could clearly extend the absorbance spectra of TiO₂ into visible region. Compared with pure TiO₂ , B³⁺ doped or F- doped TiO₂ film, the B³⁺,F- and S^2- co-doped TiO₂ film exhibited excellent photo-catalytic activity and can effectively decrease the aggregation of the TiO₂ nanometre particles, making the surface of film uniform and smooth, and therefore keeping their nano-characteristics.(3) Analysis of the Photocatal MechanismThe doping of nonmetallic ions can seriously improve the optical catalytic activity and the co-doping has better effects than single doping. The possible mechanism is as follows: After doped with nonmetallic ions, production of photo-generate electrons and corresponding positive holes and nonmetal ion doping reduces the recombination of the species.the synergistic effects will be more obvious. their impurity states are near the valance band edge and their roles as recombination centers might be minimized, as compared to that of metalcation doping. Therefore, the photo-catalytic efficiency depends on the competition between the surface charge carrier transfer rate and the electron–hole recombination rate.