(650i) Molecular Dynamics Simulation of Aggregation Phenomena of TiO2 Nanoparticles in Aqueous and Vacuum Environments

The aggregation of nanosize clusters is an important phenomenon that occurs in materials synthesis through both colloidal and vapor-phase routes. Particularly interesting is the experimental observation that nanocrystals can also approach one another and merge along specific crystallographic directions, in the oriented attachment mechanism. The ability to direct crystallization through oriented attachment is an exciting prospect that could allow for the creation of new nanostructures with well-defined sizes and shapes. To this end, insight into its origins, which are not currently clear in all cases, would be beneficial.

In this work, we employ molecular dynamics (MD) simulations to understand the origins of oriented attachment in colloidal titanium dioxide (anatase). We use the DL-POLY simulation package to model nanoparticles in the 2-3nm size range, with shapes dictated by the Wulff construction. Our initial studies probe the aggregation (sintering) of two anatase particles in vacuum. We find that oriented attachment occurs in vacuum and is due to electrostatic interactions between under-coordinated O atoms along [101] facet edges and under-coordinated Ti atoms on [001] facets. Oriented attachment is observed experimentally for colloidal nanocrystalline anatase. However, the crystallographic direction is different than what we observe in vacuum, indicating that water plays a role in directing aggregation. In initial efforts to understand the role of water in the aggregation process, we simulate an anatase nanocrystal in water. We characterize the orientation of water around the facets and edges of the nanocrystal, as well as its density distribution. We add surface hydroxyls and hydrogens to probe the role of water dissociation in the hydration of the nanoparticles. Our results indicate that nanoparticle forces in water can significantly differ from those in vacuum.