STUDY OF THE ANATASE-RUTILE TRANSFORMATION IN TIO2 POWDERS OBTAINED BY LASER-INDUCED SYNTHESIS

Powder samples of pure anatase were produced using laser-induced pyrol ysis of titanium alkoxides, and the catalysts were prepared using conv entional wet impregnation methods. The diffraction patterns were inter preted in microstructural terms by Fourier analysis of their peak prof iles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500-degrees and 550-degrees-C. For t he reflections of the anatase phase, a decrease of their Bragg (2theta ) positions was observed up to 550-degrees-C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the hi ghest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The co rresponding microstructural parameters indicate that, in all cases, th e transformation is accompanied by an increase of the crystallites and /or of the lattice perfection. The evolution of these parameters is in fluenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfec ting by thermal treatments. Only the transition to rutile is capable o f overcoming that restraint by allowing crystallite growth at the expe nse of the smaller and distorted anatase crystallites.