(Invited) Synergy between Halides and Capping Molecules in the Shape-Selective, Solution-Phase Growth of Metal Nanocrystals: A Theoretical Perspective

Abstract

Ag nanocrystals have promising applications in many cutting-edge technologies, which are highly dependent on their shapes. These nanomaterials are typically synthesized using the solution-phase polyol method, which involves AgNO3 salt, capping agent polyvinylpyrrolidone (PVP), and ethylene glycol as the solvent and reducing agent. Chloride and bromide are often added to achieve robust shape control. In the interest of achieving shape-selective syntheses, understanding the mechanistic origins of shape selectivity is important.Previous theoretical/experimental studies indicate that the preferential binding of PVP to Ag(100) can produce kinetic nanocubes of sufficiently large sizes. Small nanocubes can be synthesized experimentally when chloride is present, while theory predicts that truncated nanocubes are thermodynamically preferred in the presence of chloride and truncated octahedra are thermodynamically preferred without chloride. Here, we use dispersion-corrected density-functional theory to investigate the possibility that there is a synergy in the binding of PVP and chlorine to Ag surfaces, through which nanocubes emerge as the preferred thermodynamic shape.We study the co-adsorption of PVP dimers and Cl to Ag(100) and Ag(111). We find that the adsorption of Cl influences the binding of PVP in a facet-selective way, such that Cl weakens the binding of PVP to Ag(111) at moderate to high Cl surface coverages, while Cl strengthens the binding of PVP to Ag(100) at low to moderate coverages, before weakening it at high coverages. We construct a contour plot of Ag-PVP-Cl surface energies as a function of the solution-phase Cl and PVP chemical potentials and find the associated Wulff shapes of the Ag nanocrystals. We find that truncated octahedra are thermodynamically preferred at low Clconcentrations / chemical potentials under full protection of PVP and Ag nanocubes are obtained at high Cl concentrations. We also identify a regime at which PVP binds to Ag(100) facets with adsorbed Cl, but is absent from Ag(111) facets. In this regime, kinetic {100}-faceted nanoshapes are likely to be preferred. Time permitting, we will also discuss recent results with bromide. This synergy between PVP and halide indicate a rich ground for kinetic control of Ag nanocrystal structures.