It has been experimentally observed that during the crystallization of aqueous nanocrystalline titanium dioxide (TiO2), two mechanisms of crystal growth exist: traditional coarsening via Ostwald ripening and oriented attachment [1,2]. Oriented attachment is the non-random aggregation of two nanocrystals along a preferred crystallographic direction [3,4]. The ability to direct crystallization processes through mechanisms such as oriented attachment could allow for the creation of new nanostructures with well-defined sizes and shapes. However, the origins and mechanisms underlying oriented attachment are not completely understood.

In an effort to understand the mechanisms of oriented attachment, we simulated the sintering of two TiO2 (anatase) nanocrystals in vacuum using molecular dynamics (MD) in the microcanonical ensemble, as implemented in the DL-POLY simulation package. Interatomic interactions are modeled using the Matsui-Akaogi force field. We consider nanoparticles in the 5-6 nm size range, with shapes dictated by the Wulff construction. By considering nanoparticles in vacuum, we remove effects of solvent and retain the role of intrinsic nanoparticle-nanoparticle forces. Moreover, the sintering of titania nanocrystals in vacuum is of interest in industrial aerosol syntheses. Finally, previous MD studies of sintering have focused on amorphous nanoparticles and our study highlights differences between these and nanocrystals.

Our studies reveal that the nanoparticles do exhibit oriented attachment: We observe a preference for the nanocrystals to initially contact one another along distinct crystallographic edges, with the edge between the (001) and (101) surfaces of one particle in contact with the edge between the (101) and (101) surfaces of another particle. Subsequently, one of the particles flips so that the (001) surface of one particle contacts the (101) surface of the other particle and a long-time restructuring process ensues. We determine that oriented attachment in this system arises from the unscreened electrostatic interaction between under-coordinated Ti and O atoms along the two different edges.

[1] R.L. Penn, J.F. Banfield, American Mineralogist 83, 1077 (1998).

[2] R.L. Penn, J.F. Banfield, Geochim. Cosmochim. Acta 63, 1549 (1999).

[3] R.L. Penn, J.F. Banfield, Science 281, 969 (1998).

[4] M. Niederberger, H. Colfen, Phys. Chem. Chem. Phys. 8, 3271 (2006).