(50c) Role of Chloride in the Shape-Selective Growth of Ag Nanocrystals

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 the precursor AgNO₃ salt, the 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, but elusive.

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 octahedral 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 high Cl surface coverages, at which Cl continues strengthening the binding of PVP to Ag(100). 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 see 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. Our results also indicate that Cl and PVP could work synergistically to produce five-fold twinned Ag nanowires under kinetic control.