UV Raman Spectroscopic Study on TiO2. II. Effect of Nanoparticle Size on the Outer/Inner Phase Transformations

The mechanism of the size-dependent outer/inner phase transformation in ultrafine TiO2 particles with narrowed size distribution was intensively investigated using UV Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Particle size is found to be the critical parameter determining the onset transition temperature and nucleation performance. The transformation temperature was decreased with the decrease of initial particle size. Rutile nucleates at interfaces of the contacting anatase grains (<60 nm). The free surface, interface, and bulk are all likely to work as rutile nucleation sites for large particles(>= 60 nm). Thermal stability of nanoscale titania is significantly enhanced by the presence of La2O3 even with surface coverage less than 5%, which blocks the outer/inner phase transformation of nano anatase. A model was devised to describe the phase transformation as a function of particle size and to predict the phase stability.