Near Surface Solution Structure Controls the Oriented Attachment of Metal Nanoparticles—Reactive Molecular Dynamics

Oriented attachment (OA) of nanoparticles has been recognized as a common mechanism for crystal growth, affecting the micro- and macroscale morphologies for a large variety of materials. However, the role of environmental constraints such as pH, temperature, and ionic concentration in controlling the pathways and kinetics of OA remains unaddressed, which makes it difficult to control and exploit OA syntheses to create unique materials. Experimental studies with <i>in situ</i> liquid-cell and <i>ex situ</i> transmission electron microscopy indicated that the solution pH affects the OA of Pt nanoparticles. With pH varying from ~3 -1.5, the attachment facet of Pt nanospheres switches from {001} to {111}. The initial {001} attachment of ~2-5 nm nanoparticles leads to the formation of nanocubes (~50-200 nm), and the subsequent {111} attachment results in the extrusion of Pt nanorods from the surface of the nanocubes. Using molecular dynamics simulations based on a ReaxFF reactive force field fit to results from quantum density-functional theory, we simulated the approach of two Pt nanocrystals in HCl and C_l2 solution under conditions of relatively low and high pH. Our studies confirm the experimental findings and reveal the key role of solution environment in controlling OA.