Construction And Photocatalytic Properties Of Nano TiO₂Films

Titanium dioxide is considered as a high-efficiency, low poison and cheap photocatalysts,which is widely concerned and has a great development recent years. Titania can be excited toproduce photogenerated electron-hole pairs under the irradiation of high energy UV light.These electron-hole pairs with strong redox properties can react with the H2O, oxygen, andorganic pollutant on the surface of titania via a series of redox reaction, in which processorganic pollutant is finally degradated to be nontoxic micromolecules, such as CO₂, H2O, etc.However, the practical application of titanium oxide has been limited by two problems.Titanium oxide has a large band gap （band gap of anatase is3.2eV）, which means only underirradiation of ultraviolet light can the reaction occur to create electrons （e-） and holes （h+）, butthe proportion of the UV light in sunlight is very low. In addition, the photogeneratedelectrons and holes are extremely unstable and recombination easily, which also affect thephotocatalytic activity of titanium dioxide. Therefore, how to widen the optical responserange of the titanium dioxide, reduce the recombination rate of photo-generated electrons andholes, becomes a key problem to enhance the photocatalytic activities and the applications oftitanium dioxide.In order to solve the above problems, the titanium oxide should be modified. Currently,many methods were found to modify the titanium dioxide. And the methods of ion-surfacedoped, ion-gradient doped and silica modified are used in this article. Ion doping can increasethe light response range of TiO₂, and also can suppress the recombination rate ofphoto-generated electrons and holes, thus improving the photocatalytic activities of TiO₂. Bychanging the concentration distribution of the dopant ions, gradient doping can make thephotogenerated electrons and holes transfer to the surface more easily, but also can preventthe negative impact resulting from excess dopant ions on the surface. Silica modification canincrease the surface area and pore volume of composite materials, and can also improve thethermal stability of the anatase, which are conducive to improve the photocatalytic activity ofTiO₂. In this article, metal ions Ag, Co, Ce, and non-metal ion B are used as dopant ions. Andthe mechanisms of photocatalytic reaction are studied by observing the photocatalytic degradation of dyes using single doped, co-doped and gradient-doped composite films.This paper mainly includes the following three parts,（1） Preparation and property study of composite filmsThe TiO₂colloid and SiO₂colloid are prepared by sol-gel method, the films are pullingon glass substrates by dip-coating method. The best doping concentration and codoped ratioof Ag, Co, Ce, and B ions are determined by the degradation of the dyes using ions dopedTiO₂films, and based on the best doping concentration or codoped ratio, the effects ofphotocatalytic activities could be studied through the degradation of dyes using laminatedfilms or gradient doped films. The experimental results show that, the photocatalytic activitiesof ions codoped composite microlaminated films or gradient doped films have enhancedobviously, and the range of light response is also increased.（2） Characterization of composite filmsThe ultraviolet-visible absorption spectroscopy （UV-Vis）, simultaneous thermal analysis（DTA/TG）, X-ray diffraction （XRD）, field emission scanning electron microscopy （FE-SEM）,photoluminescence （PL） spectroscopy, and N2adsorption-desorption measurements （BET） areused to explore the surface structure and optical absorption behaviors of the composite films,and the reaction mechanisms of codoped films are also discussed. Among various crystals ofTiO₂, only anatases have the photocatalytic avtivity. The experimental results show that, whenonly doped with ions, the quantities of rutile and brookite will be decreased, and anatase willbe more obvious; and when SiO₂added into materials, the rutile and brookite will bedisappeared. In addition, ions doping or SiO₂modifing can inhibit the surface agglomerationof TiO₂films, leading the surface becomes uniformly and smoothly, and can also enhance thespecific surface area of the films.（3） The exploration of photocatalysis mechanismThe photocatalytic activities are improved significantly after ions doped in TiO₂. It canbe speculated that the structures of TiO₂are changed after ions doped, leading thephotocatalytic activities changed. When TiO₂doped with ions, ions can enter into the latticeof TiO₂causing lattice distortion, which can generate more electrons and holes; in addition,the dopant ions can become the electronic adsorption sites on the surface of TiO₂, which canprevent the recombination of electrons and holes, thereby improving the photocatalytic efficiency. For gradient doped films, the gradient distribution of doping ions make electronsand holes move to the surface much easier; at the same time, it can also make full use of thedopant ions, and does not make the excess ions gathered in the surface of the film, turninginto the recombination centers of electrons and holes. The excellent photocatalytic activitiesexhibited by micro-laminated films were primarily because the interface effect between thelayers inhibited the recombination of electrons and holes; in addition, SiO₂can provide alarger specific surface area for the materials, which further improved the photocatalyticactivity of the materials.