Nonstoichiometry, amorphicity and microstructural evolution during phase transformations of photocatalytic titania powders

The performance of photocatalytic titania powders is regulated, in part, by nonstoichiometry and the proportions of the crystalline and amorphous components. These variables can be quantitatively established by Rietveld analysis of diffraction data when internal standards are used to fix absolutely the crystallochemical parameters during quantitative phase analysis and to correct for mass absorption. Here, fixed-wavelength neutron and multiple-wavelength X-ray powder diffraction are used to assess phase development in alkoxide-derived titania gel as a function of temperature. In this manner, it is shown that the amorphous gel is progressively replaced by anatase for temperatures <= 773 K, and that during the reconstructive transition to rutile (773-873 K) aperiodicity increases as anatase is broken down to clusters of TiO6 octahedra, with a fraction (similar to 10 wt%) of this short-range order persisting to 1273 K. Microabsorption correlates with X-ray energy, leading to systematic aberrations in the Rietveld scale factors connected to microstructural evolution which accompanies phase development during heat treatment. These changes are consistent with encapsulation of anatase and rutile by ubiquitous non-diffracting materials. The appearance of significant quantities of an intervening disordered phase during the dimorph transformation is supportive of recent kinetic models; however, its impact on catalytic activity remains to be determined.