Characterizing The Mechanism Of Nanocrystalline Anatase To Rutile Phase Transformation

Abstract

Phase transformations are important processes by which materials evolve in both natural and synthetic materials. Understanding the nature, mechanisms, and kinetics of phase transformations, as well as the micro structural changes that accompany them, require comprehensive characterization so as to gain a deeper understanding of atomic scale mechanisms and better control materials properties. A combination of experimental and theoretical techniques has led to improved understanding of how phase transformations are initiated at interfaces and then propagate by growth of the more stable phase at the expense of meta-stable phase. In this work, nanocrystalline TiO₂ is used as a model system to systematically explore the atomic level mechanism of particle mediated phase transformation. First, a number of important research studies are highlighted to improve our understanding of this aggregation based phase transformation. Second, the dependence of anatase to rutile phase transformation and crystal growth kinetics on crystallite size and aggregation state of the particles in aqueous suspension is explored by varying reaction conditions. Rates of anatase growth and its transformation to rutile increase with decreasing initial grain size under hydrothermal conditions. Overall, rates are slower at the higher pHs employed. Furthermore, densely aggregated particles show higher transformation and growth rates, compared to loosely aggregated ones. Third, macroscopic modeling was used to characterize the kinetics of anatase to rutile phase transformation. Kinetic data under acidic, hydrothermal conditions are consistent with a two-step phase transformation mechanism: interface-nucleation followed by dissolution-precipitation. Fourth, a kinetic model that enables quantitative assessment of the contribution to the rate of phase transformation by dissolution-precipitation and by interface-nucleation has been developed. Generally speaking, interface-nucleation plays a critical role during the early stages of the transformation, regardless of pH, whereas dissolution-precipitation dominates the later stages of the transformation. Finally, anatase to rutile phase transformation kinetics was exploited to produce nanoporous rutile nanocrystals by controlling the solubility of TiO₂ nanocrystals. All in all, the observations and results obtained in this work might enable new insights into the mechanism of particle-mediated phase transformation and better control over the mechanism to produce materials with desired properties.