Preparation Of TiO₂-based Composite Films And Their Photocatalysis For Organic Pollutants

TiO2 as an n-type semiconductor, it is highly photocatalytic, nontoxic, inexpensive and stable. However, TiO2 has two shortcomings: one is that the width of forbidden band is larger and the band gap energy is 3.2eV, that is to say, only when TiO2 is illuminated under UV light which wavelength is smaller than 387.5nm can the photogenerated electrons and holes be produced. But the ratio of UV light in the sun is less than 5%, which leads the lower utilization of sunlight by TiO2. Two is that photogenerated electrons and holes are so unstable that they can easily recombine, which reduces the photocatalytic activity of TiO2. Therefore, to narrow band gap and to prohibit the recombination of photogenerated electrons and holes have become two important problems, in order to broaden the application area of TiO2 and improve its photocatalytic activity.The modification research of TiO2 is necessary to realize the above problems. So far, the researchers have found many methods applied in the modification of TiO2. Among all kinds of methods, ions doping and semiconductor composition are adopted in our research. The modification can not only broaden the light response range of TiO2 to visible light area, but also greatly reduce the recombination rate of electronic- hole pairs.The research of the paper mainly contains the following aspects:(1)The preparation and performance tests of TiO2 and TiO2-ZnO films.We selected Ti(C4H9O)4 and Zn(CH3COO)2.2H2 O as precursors and used sol-gel method to prepare TiO2 colloid and ZnO colloid in experiment, respectively. TiO2-ZnO composite colloid can be made by mixing TiO2 colloid and ZnO colloid in a volume ratio of 2/1. The pure TiO2 and TiO2-ZnO composite films were prepared by dip-coating process on clean and dry glass substrates. The doping ions Ce, F, Ca, Fe and La were doped on the surface of films to prepare the ion doped films. After the ion aqueous solution on surface were dry, the films were put in the furnace to burn. The best ion doping concentration was determined by the performance test of films in degradation of formaldehyde. The experiment results of degradation of organic pollutants showed that the the photocatalytic activity of TiO2-based composite films was obviously improved.(2)The characterization of TiO2 and TiO2-ZnO catalystsThe optical properties and structure of prepared catalysts were characterized by photoluminescence(PL) spectra, UV-Vis diffraction reflectance absorption spectra(DRS), X-ray diffraction(XRD), differential thermal analysis-thermogravimetry(DTA-TG), field emission scanning electron microscopy(FE-SEM), and N2 adsorption/desorption isotherms. The DRS and PL spectra results showed that multi-modification not only induced strong visible light absorption of TiO2 but also reduced the recombination rate of electron-hole pairs in the composite film. The DTA-TG and XRD results indicated that the crystal type of the TiO2 composite film is mostly stabilized in anatase. The FE-SEM and BET surface area results revealed that the small nanoparticles constituted modified TiO2 composite film with the large specific surface area.(3)The description of photocatalytic mechanismsFirstly, the presence of doping energy level in coupled materials is relevant to this behavior: the electrons on the conduction of ZnO can be easily inspired to the conduction of TiO2 under irradiation, on the contrary, the generated holes on the valence band of TiO2 transfer to the valence band of ZnO, as a result, the recombination rate of the pairs is decreased and the availability of the pairs in redox process is increased. Secondly, the doping level can be formed after doping of the ions which have smaller radius than Ti; doping lanthanide ions which have larger radius than Ti can contribute to the formation of titanium dioxide lattice distortion, which could accept more photoexcited holes and produce more surface free radicals to oxidate adsorptive molecules. In addition, the doping ions can capture electrons or holes to prevent their recombination(4)Development in the field of applicationDifferent from the before researches, the ion doped Ti O2-ZnO composite films were used in the degradation of formaldehyde which is a emerging contaminant, in this way, the actual application value of TiO2 is increased.