Structure And Phase Transformation Of Nanocrystalline Titanium Dioxide Induced By Mechanical Activation

Nanocrystalline anatase TiO2 powder was prepared by the method of precipitation-solution-gelation, using metatitanic acid, hydrogen peroxide and ammomia as reactants. Photoacitivity of nanocrystalline TiO2 with the smaller crystallite size is better than that with the larger crystallite size. The optimum TiO2 loading exists to effectively degrade the dye solution. The degradation efficiency of the three dyes is as follows: Reactive red > Reactive red-violet>Reactive blue. Ag-doping can improve the photocatalytic acitivity of nanocrystalline TiO2, especially moderate the photoacitivity deterioration of nanocrystalline TiO2 calcinated at high temperature. In comparison with pure TiO2, photoacitivity of Ag-doped TiO2 is better although it contains less anatase and larger crystallite.The two nanocrystalline TiO2 powders with different crystallite size were studied from 25 to 1200 C by a high temperature Raman spectroscopy with a function of in-situ heating. During experiments, it was first observed that the temperature effect existed in the high Raman spectra of nanocrystalline TiO2, that is, broadening of Raman bands, the blue-shifting of the 144 and 196 cm-1 bands, the red-shifting of the 395, 514 and 640 cm-1 bands with an increase in temperature. In the two nanocrystalline TiO2 powders, the frequency shifting of the Raman bands depending on the temperature is different although the changing trend of the frequency is the same. As the temperature increases, the broadening speed of Raman bands in the nanocrystalline TiO2 with the smaller size declines while the linewidth of Raman bands in the nanocrystalline TiO2 with the larger crystallite size linearly largens. And the linewidths of Raman bands in the two nanocrystalline TiO2 approach at high temperature. Raman bands intensify first, and then weaken with the temperature increasing. Only a few weak and broad bands are exhibited in Raman spectra measured at high temperature.Phase transformations in nanocrystalline TiO2 powder induced by high energy ball milling were conducted in a planetary mill. The mechanism of the milling-induce transformations was studied according to the collision model of the balls, the theory of the local temperature rise and the theory of defects in crystals, which not only benefits to the study on phase transformations of nanocrystalline TiO2 and the theory of the milling-induce solid transformation, but also develop a new and practicable technique of preparing nanocrystalline rutile TiO2 powder. During ball milling, mechanical activation induces the transformations of nanocrystalline TiO2 from anatase to srilankite and rutile at room temperature and ambient pressure, which should primarily be attributed to the rise of local temperature and pressure at the collision sites of the powder and the balls. In addition, the additional energy caused by defects, lattice distortion and the refinement of the crystallite is responsible for the transformations. The increase in free energy by the interfacial energy and the surface stress is relatively large in comparison with that by lattice vacancies and dislocations. The two pathways exist in the milling-induce transformations of nanocrystalline TiO2: one is the anatase-srilankite-rutile transformation; the other is the anatase-rutile transformation. As milling time increases, the anatase content reduces and the amounts of both srilankite and rutile increase. And the transformation from srilankite to rutile phase takes place by further milling. In milled anatase, the crystallite size decreases and the lattice strain rises with milling time, and the anisotropic lattice expansion occurs.Many milling parameters, such as the rotation velocity, the filling of the vial, the properties ofthe balls, have a significant influence on the milling-induce transformations of nanocrystalline TiO2, due to the differences in impact pressure, the local temperature rise, the additional energy, the effective impact frequency, and the power transmitted by the balls to the powders in a collision event. The transfor...