Effects of Dipole Moment and Temperature on the Interaction Dynamics of Titania Nanoparticles during Agglomeration

The interactions between nanoparticles in high-temperature vapor-synthesis environments have important implications on their self-assembly into specific structures. We apply classical molecular dynamics (MD) simulations, with the Matsui-Akaogi interatomic potential, to study the interaction forces and the resulting dynamics between pairs of cooriented and counteroriented charge-neutral TiO2 anatase nanoparticles in vacuum. The distributions of Ti and O ions at the nanocrystal surface are asymmetric due to limited surface sites preventing perfect sphericity, resulting in permanent dipoles that are approximately proportional to the surface area in magnitude. For two approaching cooriented nanoparticles, with parallel dipole moments, the nanoparticles translate toward each other with minimal rotation, where the attractive Coulomb dipolar force is much larger than the van der Waals force at long-range distances. For two approaching counteroriented nanoparticles, with antiparallel dipole moments and initially repulsive interaction, the nanoparticles experience mutual rotations, and the Coulomb force changes from repulsive to attractive during translation. Both the Hamaker approximation and the dipole-dipole approximation are compared with the MD simulation results to assess their validity as a function of particle separation distance. As the temperature increases from 273 to 1673 K, the fluctuation of the dipole vector (in both magnitude and direction) increases, resulting in a dramatic decrease of the time-averaged dipole moment from 60 to 2.1 D, as well as a reduction of the Coulomb dipolar force between the two nanoparticles.