(167e) Preferential Binding of Polyvinylpyrrolidone (PVP) Is Not Responsible for Shape Control in Ag Nanoparticle Synthesis

The role of polyvinylpyrrolidone (PVP) in the shape control synthesis of Ag nanoparticles (NPs) has been attributed to its preferential binding to Ag (100) facets compared to Ag (111) facets. Theoretical studies have validated this hypothesis, however, only at the limit of molecular or small oligomeric analogs of PVP in the absence of solvent and modifiers (i.e., Cl^-) present during synthesis. We experimentally measured the surface density of PVP on different shaped Ag NPs in order to build equilibrium adsorption isotherms. Based on a Langmuir isotherm, a weak (5 M^-1) equilibrium adsorption constant for PVP adsorbed to Ag nanocubes was determined. For PVP molecular weights > 10K, constant PVP monomer surface densities were determined, implying a similar equilibrium adsorption constant. The equilibrium adsorption constant for Ag octahedron decreased to half the value of the Ag cubes, confirming the existing hypothesis that PVP selectively adsorbs on the cubic (100) surface over the (111) surface. The difference in apparent affinity is significantly less than the difference determined by first-principle simulations. The final Ag nanocube product contains a surface layer of adsorbed chlorine due to the inclusion of hydrochloric acid in a typical synthesis. However, the exact role of Cl^- has on shape control has not been fully elucidated. We combined various ex-situ characterization techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS) to probe the growth mechanisms of Ag nanocubes. The initially formed nanostructures during the early stages of synthesis were AgCl nanocubes which were subsequently etched by incoming Ag salt flux to form metallic Ag nanocubes containing a nanometer thick chloride layer. Variation of Cl^- concentration alone at constant PVP concentration led to modification of the nanoparticle shape. As the concentration of Cl^- decreased, truncated cubes, cuboctahedron and truncated octahedron formed, which confirms the importance of Cl^- for Ag NPs shape control. PVP serves only to stabilize Ag nanocubes in solution.