(627g) Facet-Selective Roles of Chloride and Polyvinylpyrrolidone (PVP) in Facilitating the Growth of Ag Nanowires and Nanocubes

Ag nanocrystals are used in various applications in which the performance relies on their morphology. Though Ag nanocrystals with different shapes and sizes have been synthesized, control over the yield and selectivity is still difficult to achieve due to a lack of understanding of the growth mechanisms. Chloride and the capping agent PVP are usually hypothesized to affect Ag nanocrystal growth, but their exact roles remain controversial.

In this work, we applied density-functional theory (DFT) calculations to investigate the co-adsorption of Cl^- and PVP on Ag(100) and Ag(111) at different Cl^- coverages, with the aim to understand their synergistic roles in facilitating the growth of Ag nanocrystals. A weak facet-selectivity of PVP binding is found at low (or zero) Cl^- coverages and Ag truncated octahedra are thus predicted. A selective desorption of the PVP adlayer on Ag(111) [but not on Ag(100)] at 0.50 monolayer coverage of Cl^- is observed, which may facilitate anisotropic growth kinetically. No facet-selectivity presents at higher Cl^- coverages, as PVP leaves both surfaces due to stronger repulsion when more chloride is introduced.

Using ab initio thermodynamics calculations, a phase diagram, in which regions are classified based on various coverages of Cl^- and PVP, was constructed as a function of the chemical potentials of Cl^- and PVP. We see growth under thermodynamic and kinetic controls at different conditions. One pathway with increasing Cl^- concentration is predicted to thermodynamically produce truncated octahedra, cuboctahedra, and cubes, while it also indicates the switch to kinetic control to grow nanowires with decahedral seeds, which agrees well with experimental observations.