Kinetics of crystallization and crystal growth of nanocrystalline anatase in nanometer-sized amorphous titania

The kinetics of crystallization and crystal growth of nanocrystalline anatase in amorphous titania (2.5-3 nm) samples in the temperature range 300-400 degreesC was studied by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). A kinetic model adopting the Smoluchowski coagulation formulation, combined with our phenomenologically derived model kernels, was used to quantitatively interpret the observed kinetic data. It was revealed that the transformation in the amorphous titania comprises four steps: interface nucleation of anatase on contact areas of amorphous particles, with an activation energy of 147 kJ/mol; crystal growth of anatase by redistribution of atoms from either amorphous particles or smaller anatase crystals onto nanocrystal surfaces, both with an activation energy of 78 kJ/mol; and oriented attachment of adjacent anatase particles that are in appropriate orientations, which is less temperature dependent.