TiO2 (anatase) nanocrystals have numerous applications in science and technology. In many applications (e.g., in dye-sensitized solar cells), these nanocrystals typically exist as aggregates. The aggregation of anatase nanocrystals has been studied experimentally, where it was observed that they tend to aggregate along certain preferred directions to form single or twinned crystals . This phenomenon, known as oriented attachment, can be a powerful mechanism for synthesizing new nanostructures with controlled sizes and shapes. Unfortunately, no thorough insight into the origins of this phenomenon has yet been achieved. To gain insight into the mechanisms of oriented attachment, we use molecular dynamics (MD) to study the aggregation of anatase nanocrystals in the 2-6 nm size range, with shapes dictated by the Wulff construction. In vacuum, the nanocrystals aggregate almost exclusively along preferred crystallographic directions due to multipole interactions . However, the structures of the experimental (aqueous) aggregates are different than those in vacuum, possibly indicating that water plays a role in directing aggregation.
There is no published force field to describe the interaction of water, hydrogen and hydroxyl groups with anatase surfaces. In this work, we adapt a potential developed for the rutile-water interaction for anatase. Using MD simulations, we obtain the coverage-dependent geometries of adsorbed / dissociated water on various surfaces of anatase. The MD results are in a good agreement with first-principles DFT and experiment. Using this force field, we simulate aqueous anatase nanocrystals and characterize the ordering of water around the nanocrystal surfaces. We consider the approach of two nanocrystals and discuss the ramifications of our findings for the oriented attachment of anatase nanocrystals observed experimentally.
1. R.L. Penn, J.F. Banfield, Geochim. Cosmochim. Acta 63, 1549 (1999).
2. M. Alimohammadi, K.A. Fichthorn, Nano. Lett. 9, 4198 (2009).