46928-AC9
Atomic Scale Simulation of Many Body Forces in Colloidal Nanoparticle Suspensions
Work in our group has focused on quantifying nanoparticles forces for two different types of materials: silver and titanium dioxide. For both systems, there is experimental evidence that crystal growth is at least partially mediated by the aggregation of nanoparticles and that nanoparticles aggregation occurs along particular crystallographic directions in the oriented attachment mechanism [1-5]. 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 titanium dioxide (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 [6]. We consider nanoparticles in the 3-6 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. Previous MD studies of sintering have focused on amorphous nanoparticles and our study highlights differences between these and nanocrystals.
Our studies reveal that titanium dioxide nanoparticles in vacuum 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. Ms. Mozhgan Alimohamadi Zanjani, a Ph.D. student, is currently in the process of preparing a publication on this work. Mozhgan will give a presentation (the first presentation of her Ph.D.) on this project at the 2008 Annual AIChE meeting. In future work, we will consider the forces between two titanium dioxide particles in water.
In a second effort, Dr. Leonidas Gergidis is working on understanding the oriented attachment of Ag nanoparticles in polar and nonpolar solvent. At this point, he has learned how to do these simulations using the DL-POLY package. He learned how to simulated two Ag nanoparticles in vacuum and he is in the process of studying their interaction in hexane solvent.
References
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. Giersig,
5. M. Niederberger, H. Colfen, Phys. Chem. Chem. Phys. 8, 3271 (2006).
6. M. Matsui and M. Akaogi, Molecular Simulation 6, 239 (1991).
