Textural evolution and phase transformation in titania membranes: Part 2. Supported membranes

Nanostructural evolution and phase transformation in supported and unsupported titania membranes have been studied using Raman spectroscopy, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Densification of unsupported membranes started at ca. 450 °C and reached more than 97% density at 600 °C, whereas the supported membranes had a density of only ca. 70–75% even at 700 °C when calcined for 8 h. At 700 °C the average crystallite size of supported and unsupported membranes was ca. 20 and 70 nm, respectively. This behaviour is primarily attributed to the decrease in the driving force for sintering due to the stress developed during the constrained sintering of a film attached to a rigid support and to the inhibition of the reorganization process within the film, resulting in lower coordination numbers in supported membranes. Supported membranes showed a higher transformation temperature (slower rate of transformation) than did the unsupported. Supported and unsupported membranes, calcined for 8 h, transformed to ca. 90% rutile (calculated from Raman spectrum) after calcination at 850 and 650 °C, respectively. This difference in phase transformation behaviour is attributed primarily to the large stress which is developed in a constrained environment owing to the negative volume change during the anatase–rutile transformation.