Understanding the Forces that Regulate Crystallization by Particle Attachment

Nanoparticles in solution can grow, aggregate, assemble, attach, or remain dispersed due to the interplay of energetics and dynamics. These phenomena have been exploited to create an impressive array of nanomaterials such as colloidal crystals, mesocrystals, highly branched nanowires, and adaptive materials that respond reversibly to external stimuli. When the nanoparticles fuse to create larger single crystals, they often become crystallographically coaligned before the point of contact in a process referred to as oriented attachment (OA). Traditional colloidal theories such as DLVO provides a traditional framework for evaluating the forces that underly the interactions and dynamics of nanoparticles but fail to capture key particle interactions due to the atomistic details of both the crystal structure and the interfacial solution structure that are part and parcel of the OA process. Moreover, despite the fact that attachment on any two identical facets would create an energetically favorable product, OA is well known to occur preferentially on specific facets, implying that forces which oppose particle-particle approach may be as important as those the drive it. In particular, solvation barriers, which are intimately related to the structure of the underlying crystalline lattice, provide an obvious source of barriers to particle-particle contact but are not captured by DLVO theory. Here we explore the non-DLVO forces, both attractive and repulsive, that regulate the dynamics and outcomes of particle aggregation, coalignment and attachment. Using ZnO as a model system, we investigate the effect of dipole-dipole interactions on the long-range forces and torques that drive particle approach and alignment by combining by <i>in situ</i> TEM observations of ZnO nanoparticle OA events with Langevin dynamics simulations for solvents whose dielectric constants differ by an order of magnitude. We compare the magnitude of these forces to the electrostatic and van der Waals forces calculated using DLVO theory and show that they non-DLVO forces both dominate and provide a rationale for the discrepancies observed in the different solvents. We also investigate the short-range repulsive forces arising from the structuring of the solvent near the surface using 3D AFM. We find that the solvation force is stronger in water compared to ethanol and methanol, due to the stronger hydrogen bonding and denser packing of water molecules at the interface. To further understand the nature of the solvation force, we used 3D AFM to measure the repulsive hydration force between the AFM probe and mica for a series of monovalent and divalent electrolytes and low and high concentrations. The results exhibit a direct scaling between the work required to bring the tip to the surface and well-known proxies for the strength of the ion-water and ion-ion interactions. Our results highlight the importance of non-DLVO forces in a general framework for understanding and predicting particle aggregation and attachment and suggest a scheme for controlling the outcomes of particle aggregation and attachment by varying the solution conditions to tune the solvation forces.