The origin of facet selectivity and alignment in anatase TiO2 nanoparticles in electrolyte solutions: implications for oriented attachment in metal oxides.

Oriented attachment (OA) is an important nonclassical pathway for crystal growth from solution, occurring by the self-assembly of nanoparticles and often leading to highly organized three-dimensional crystal morphologies. The forces that drive nanocrystal reorientation for face-selective attachment and exclude improperly aligned particles have remained unknown. Here we report evidence at the microscopic level that ion correlation forces arising from dynamically interacting electrical double layers are responsible for face-selective attraction and particle rotation into lattice co-alignment as particles interact at long range. Atomic-to-mesoscale simulations developed and performed for the archetype OA system of anatase TiO2 nanoparticles in aqueous HCl solutions show that face-selective attraction from ion correlation forces outcompetes electrostatic repulsion at several nanometers apart, drawing particle face pairs into a metastable solvent-separated captured state. The analysis of the facet and pH dependence of interparticle interactions is in quantitative agreement with the observed decreasing frequency of attachment between the (112), (001), and (101) face pairs, revealing an adhesion barrier that is largely due to steric hydration forces from structured intervening solvents. This finding helps open new avenues for controlling crystal growth pathways leading to highly ordered three-dimensional nanomaterials.