(28h) Shape-Selective Synthesis of Au Nanoparticles: The Role of PVP

Structure-directing agents, such as polyvinylpyrrolidone (PVP), play an important role in determining the shapes of metal nanostructures in solution-phase syntheses. It is usually hypothesized that structure-directing molecules bind more strongly to certain crystal facets, which grow at the expense of facets on which they are less strongly bound. In this study, we use dispersion-corrected density functional theory to resolve the role of PVP in the shape-selective synthesis of {111}-faceted Au nanostructures in the solution-phase polyol process.   Interestingly, Ag nanostructures grown via the polyol process are {100}- faceted and we discern the differences between the two metals.  We calculate binding energies for various conformers of the 2-pyrrolidone ring of PVP on Au(111) and (5x1) Au(100)-hex – the energetically favored reconstruction of the (100) surface.  We find a wide variety of binding configurations on the two surfaces, with binding energies that vary from 0.92 to 1.14 eV. On Au(111), there is high and uniform density of PVP binding sites, all with binding energies that fall in the middle of the binding-energy range. On (5 x 1) Au(100)-hex, the binding sites form an interesting striped pattern, with alternating stripes of high, medium, and low binding energies.  We discuss the origins of these binding-energy stripes.  The stripes of high binding energies are the thinnest and they are widely spaced, making it impossible that a single polymer could have many segments contacting these sites.  We also analyze the surface energies of the two surfaces with adsorbed PVP and find that the surface energy of Au(111) is lower than (5x1) Au(100)-hex. Therefore, PVP is more inclined to bind to Au(111) rather than (5x1) Au(100)-hex, causing the formation of nanostructures with (111) facets that is seen experimentally.  Differences between the experimentally observed performances of PVP for forming Au and Ag nanostructures stem from the propensity of Au(100) to reconstruct to lower its surface energy, while Ag(100) retains the bulk truncation.