Microstructural and thermal analyses of crystallization in ultrafine amorphous titania particles

Ultrafine amorphous titania particles, with a mean particle size of 145 nm, were prepared using a hydrolysis technique in an aerosol reactor. The crystallization behavior of these particles has been studied using transmission electron microscopy, differential scanning calorimetry, x-ray diffraction, and scanning electron microscopy. The amorphous powder crystallized into anatase with a crystallization enthalpy of 27 kJ/mol and an apparent average activation energy of 2.0 eV. Microscopy results suggest that the contact regions between neighboring particles are the likely nucleation sites for the anatase phase. This proposed crystallization mode is interpreted as a consequence of the presence of appreciable local pressure (stress) which was predicted by model calculations and observed under TEM. An analysis suggests that such a pressure and its gradient reduces the relative stability of the amorphous phase by enhancing the thermodynamic driving force for nucleation and atomic movements to accelerate the crystallization kinetics. The nucleation and growth behavior has important implications when these amorphous particles are used as precursors to form nanocrystalline TiO₂. The results are also discussed in comparison with the crystallization behavior reported previously for other ultrafine-structured amorphous oxides.