Kinetics of Hydrothermally Driven Anatase to Rutile Phase Transformation

Abstract

Titanium dioxide is a unique inorganic material that contributes to a wide variety of applications such as catalysis, ceramics, cosmetics or dye production. It can also exist in many different phases with specific chemical or physical properties. The most common and stable phases are brookite, rutile, and anatase. Phase transformations of these polymorphs are size dependent. In order to achieve higher control over phase composition of a desired material, it is important to understand mechanisms behind size dependent phase transformations. This project investigated anatase to rutile transformation kinetics using pure anatase samples with different particle sizes. With the assumption that very high acidic conditions phase transformation occur via dissolutionprecipitation mechanism, a kinetic model was used to identify rate constants, activation energies and frequency factors. Due to the fact that there is little understanding for transformation kinetics under hydrothermal conditions, this project focused on anatase nanocrystals aged in an aqueous medium with sizes ranging from 3.1nm to 6.0nm and specific aging conditions at 200-250 degrees celcius with pH=1. Extensive analysis of nanoparticles showed hydrothermal processing of titanium dioxide makes growth of larger crystals and transforms them to a more stable phase. It is clear that smaller particles will transform faster than the larger nanoparticles that may have less solubility causing the slower growth. Lastly the dissolution-precipitation mechanism showed that the transformation rate constant decreases slightly with increasing size of nanoparticles and activation energies and frequency factors were slightly different for nanoparticles with different sizes. Overall this project proved to gain more insight about the mechanism of size dependent phase transformations.