Oriented Attachment of Cubic or Spherical BaTiO3 Nanocrystals by van der Waals Torque

It has been believed that dipole-dipole interaction is the reason for the oriented attachment (OA) of nanocrystals if they have electric or magnetic dipoles. In this study, the role of van der Waals torque (Casimir torque) originated in dielectric anisotropy of a nanocrystal is numerically studied for the first time on OA of cubic or spherical BaTiO3 nanocrystals. The present numerical simulations of OA have indicated that the alignment of the crystal axes is mainly due to van der Waals torque when the crystal size is smaller than 5 nm both in aqueous and organic solutions with and without electric charge screening by ions, respectively. For larger crystals, the electric dipole-dipole interaction dominates. The mesocrystal formation of 5 nm BaTiO3 nanocrystals in aqueous solution is due to van der Waals torque.