Anisotropic Growth of Silver Nanocubes: The Role of Bromide Adsorption and Hydrophilic Polymers

The application of capping agents to modulate the colloidal synthesis of metal nanocrystals offers an effective avenue for shape control, but the roles of such agents are not yet completely understood. This study uses seed-mediated growth, single-crystal electrochemistry, and surface-enhanced Raman spectroscopy (SERS) to illuminate the roles of polyvinylpyrrolidone (PVP) and bromide (Br – ) in the anisotropic growth of Ag nanocubes. Synthetic results show that Ag nanocubes only form in the presence of both PVP and sufficiently high concentrations of Br – . Truncated octahedra form in the presence of PVP, and truncated cubes form in the presence of Br – alone. Electrochemical measurements indicate that elevated concentrations of Br – consistently passivate Ag(100) facets more than Ag(111) facets regardless of the presence of PVP. The critical condition for the growth of cubes, however, materializes only when both PVP and Br – are present, which sufficiently suppresses atomic deposition to Ag(100) relative to Ag(111). SERS measurements show that high concentrations of Br – displace PVP from Ag(100) and Ag(111), but electrochemical measurements suggest PVP acts as a strong passivator under such conditions. We propose that the chemisorption of Br – beneath a physisorbed layer of PVP creates a unique condition for the growth of Ag nanocubes. Furthermore, our investigation points to a generalizable adsorption structure for the synthesis of {100}-faceted Ag nanocrystals, where Br – adsorption under other hydrophilic polymers can similarly guide the formation of Ag nanocubes. This study enhances our understanding of the synergistic roles of Br – and hydrophilic polymers in the controlled morphogenesis of Ag nanocrystals.