Photocatalytic Activity Of Nanocrystalline TiO₂Composite Films

Titanium dioxide is considered as a high efficient, clean, cheap and easily availablecatalysts, which are used as environmental materials increasingly. It’s a kind of N-typesemiconductors, which could produce electron-hole pairs in the surface of TiO₂under theexcitation of UV light. The electron-hole pairs can degrade some organics that produced somefree radicals. These free radicals will undergo a chemical reactions which product H2O, CO₂and other small molecules. These reactions may degrade organics and avoid pollution.However, TiO₂has a band gap over a wide range at Eg=3.2eV, it’s hard to be excited of sunlight, because ultraviolet light in natural light is lower than5%, so TiO₂materials cannot usethe visible light that accounted for43%of the nature light. As a result of photoinducedelectron-hole pairs compound, their lifetime reduced. Therefore, how to improve utilizationrate of TiO₂nanoparticles on sun light and reduce the electron-hole pairs’ recombination rates,are the key topics on wider applications of TiO₂.Surface modification of TiO₂is one solution of the problems above. Now, ion-doping isone of the most common methods. Doping of metal cations into TiO₂can increase its opticalresponse and enhanced photocatalytic ability. This paper mainly uses co-doping of vanadiumand other metals or non-metallic ions to modify the films. With the observation ofphotocatalytic property of modified films on degradation of organic dyes, we can study thephotocatalytic performance of modification and mechanisms.This paper mainly includes the following three parts:（1） Property study and preparation of TiO₂filmsThe TiO₂films were prepared by sol-gel method, the films were pulling on glasssubstrates and then calcined. Through the results of dyes degradation about V and W, Ce, Fe,Zn, F co-doped films, we can find the optimal concentration, as well as the photocatalyticefficiency changes. Experimental results indicate that: the composited TiO₂films withvanadium and other metal ions showed an improvement of photocatalytic properties, and to acertain extent, they improved visible light response.（2） Characterization of TiO₂composite films The thesis uses characterization methods like ultraviolet-visible absorption spectroscopy（UV-VIS）, X-ray diffraction （XRD）, field emission scanning electron microscopy （FE-SEM）,photoluminescence （PL） spectroscopy, simultaneous thermal analysis （DTA/TG） and nitrogenadsorption-desorption measurements （BET） to explore the surface structure and opticalabsorption behaviors of the composite films and reaction mechanism of the photocatalyst andthe reaction mechanism of the co-doping. Titanium dioxide with high catalytic performanceshould have mainly anatase crystal type, containing a small amount of rutile and brookite. Theexperimental results show that, when doped with metal or non-metal ions, the quantities ofrutile and brookite will decrease, and anatase will be more obvious; while the surfaceagglomeration of TiO₂will be relieved after ions doping, their that their morphologies are flat,the pore size decreases, and the specific surface area of particles increase.（3） Analysis of the Mechanism of PhotocatalysisThe doping of vanadium and other ions can remarkably improve the optical catalyticactivity. We can infer that TiO₂band width was reduced after co-doping. As V5+and Ti4+have similar radius and arrangement structures, vanadium ions can easier access to the TiO₂lattice and create many defects, which lead more electron-hole pairs occur and reduce therecombination, so the photocatalytic efficiency can be raised.