The critical size mechanism for the anatase to rutile transformation in TiO2 and doped-TiO2

A series of titania and doped titania materials have been prepared from sol–gel methods using a titanium isopropoxide precursor. Powder X-ray diffraction (PXRD) and secondary electron microscopy (SEM) have been used to follow the anatase to rutile transformation (ART) phase change in this system. PXRD was used to estimate the relative amounts of each phase and the average particle size at a series of temperatures. Importantly, very careful choice of reaction precursors were made so that a wide range of similar samples could be compared, thus removing effects due to preparation. It was found that doping with Si, Zr, Al and tertiary mixtures of these produced an elevated ART temperature whilst Co, Mn and V had the opposite effect. The most likely explanation for the elevation in the ART temperature is the presence of dopant strain fields, which limit mass transport in the system. Lowering of the ART temperature is probably related to creation defect sites, which provide low energy mass transport routes. It was also found that in the majority of samples, the mechanism for phase change was related to attaining a critical particle size. This was measured at around 450 Å independent of the dopant used. Results are discussed in terms of previous work.