Precipitation Of Preparation Of Doping Tio < Sub > 2 < / Sub > Light Catalytic Mechanism Research

The photocatalytic technology of semiconductor is a newly emerged interdiscipline with many fields. It has exhibited the great potential applications in new energy sources exploitation, pollutants degradation and other fields due to the advantages of wide application, excellent economy and without secondary pollution. Among the all semiconductor photocatalysts, Titanium dioxide is the most widely researched photocatalytic material for its nontoxicity, cheapness, mild reaction condition, excellent stability and versatility. However, TiO2may show photocatalytic activity only by irradiation of ultraviolet (UV) light for its wide band gap. It exhibits the low efficiency for utilizing solar light. In addition, the high recombination rate of photo-generated carriers results in a low photo-quantum yield. These disadvantages limit the application of TiO2photocatalyst in the industry and life.Metal ions doping is considered to be a simple and effective way to improve TiO2photocatalytic activity. The idea that TiO2photocatalyst is doped with La3+/Cr3+is presented to extend the photo-excited wavelength. On the basis of the mechanism of the thermodynamics of materials, the ions reaction equilibriums of La3+and Cr3+in Men+(La3+/Cr3+)-NH3·H2O system were discussed, when TiO2photocatalyst precursor was prepared by precipitation process and using NH3·H2O as the precipitants. Then the total concentrations of La3+and Cr3+at different pH value and reaction temperature were calculated. The relation curves between logCMe and pH value were also obtained. The optimal parameters of precipitation process which was used to prepare La3+/Cr3+doped TiO2powder photocatalyst were obtained by using the orthogonal experiment design method. The prepared TiO2powders were analyzed by XRD, SEM, XPS and UV-Vis absorption spectra. The photocatalytic mechanism of La3+/Cr3+doped TiO2photocatalyst was preliminarily discussed. The results show that:(1) The total concentrations of La3+and Cr3+in each system decrease with the rising of the pH value at a certain reaction temperature. Meanwhile, the precipitation degree of La3+and Cr3+increases along with the reaction temperature rising. Therefore, the metal ions of La3+and Cr3+can be completely precipitated.(2)The analysis of the orthogonal experiments shows that the optimal parameter of the La3+doped TiO2powder photocatalyst prepared by precipitation process is that the calcinated temperature is800℃, the amount of La3+is0.6mol%, the solution temperature is90℃, and the pH value is8.0. While the optimal parameter of the Cr3+ doped TiO2powder photocatalyst prepared by precipitation process is that the amount of Cr3+is1.0mol%, the calcinated temperature is500℃, the solution temperature is70℃, and the pH value is10.0.(3)The results of XRD, SEM and XPS indicate that the La3+is well-proportioned dispersed on the surface of the TiO2particles as the chemical forms of lanthanum oxide with fine size, and the main crystal structure of the TiO2is not changed. While the Cr3+partially substitutes the Ti4+of TiO2, it also effectively promotes the phase transformation from anatase to rutile. A certain amounts of hydroxyls are also adsorbed on the surface of TiO2due to the La3+and Cr3+doped.(4) The photocatalytic mechanism of La3+/Cr3+doped TiO2photocatalyst is as follows:a certain amounts of defects are caused because of the presence of La3+and Cr3+ion in TiO2. Then the oxygen vacancies and holes can exist to keep the electric neutrality of the doped TiO2system, and the space charge layer is also formed on the surface of TiO2. Therefore, a certain amounts of hydroxyls are adsorbed on the surface of TiO2, and the electron-hole pairs which are photo-induced are also effectively separated as the voltage of the space charge layer rises. The life time of photo-generated is also lengthened. A series of sub-energy levels can be formed under the conduction band of TiO2due to the unique electronic structures of the rare earth La, and the band gap of La3+doped TiO2is narrowed. The impurity energy level can existed above the valence band of TiO2because of the Cr3+doped, and the spectral response range of Cr3+doped TiO2is extended for the impurity energy level.