(318h) Shape-Selective Formation of Ag Nanocrystals: Insights From Atomic-Scale Simulations

The ability of plasmonic Ag and Au nanostructures to concentrate UV-vis radiation in small volumes makes it attractive to use these building blocks in the design of composite photocatalysts for the production of solar fuels, as nanometallic plasmonic structures to enhance the efficiency of photovoltaics, or as flexible electrode materials.  A popular method of synthesizing Au and Ag nanocrystals is the solution-phase polyol process, in which a structure-directing agent (commonly PVP) directs the growth of seed crystals to form shape-selective nanostructures.  The exact role of PVP in this synthesis is unclear.  It is likely that PVP protects growing Ag nanocrystals from uncontrolled aggregation.  PVP also promotes the formation of {100}-faceted Ag nanostructures in solution, while the {111} facet has the minimum energy in vacuum.  The reasons for this facet selection are unclear and raises the question:  Does PVP affect the fluxes of solution-phase Ag atoms to promote growth of nonequilibrium structures with {100} facets?  Or does PVP simply alter the solution-phase interfacial energies of the two facets, so that the {100} facet is thermodynamically preferred in solution?  To answer these questions, we used molecular dynamics simulations based on force fields fitted to first-principles density-functional theory calculations to calculate the interfacial energies of PVP-covered Ag surfaces in ethylene glycol solvent.  In this way, we mimic the solution-phase environment that occurs for Ag nanostructure synthesis.  We investigate the interfacial energies of PVP-covered Ag(100) and Ag(111).  Additionally, we calculate the flux of Ag atoms to Ag surfaces that are both bare (in solvent) and covered with PVP.  We find that the flux of Ag atoms to PVP-covered Ag(111) is larger than to Ag(100), which would promote the growth of Ag(100) facets in a crystal.  Interestingly, the flux of Ag atoms to PVP-covered surfaces is larger than that to bare surfaces in solvent.  This suggests a non-equilibrium growth scenario, in which PVP channels atoms selectively to {111} facets to promote the growth of {100}-faceted nanostructures.