Seeing is believing: what is on the surface of silver nanocrystals suspended in their original reaction solution

Colloidal synthesis of inorganic nanocrystals always involves a multitude of ionic and molecular species. How the chemical species affect the evolution of nanocrystals remains a black box. As an essential ingredient in the polyol synthesis of Ag nanocubes, Cl^- has been proposed to co-adsorb on the surface with poly(vinyl pyrrolidone) (PVP) to facilitate shape evolution. However, there is still no direct evidence to confirm the presence of Cl^- on the surface of Ag nanocubes while they are suspended in the original reaction solution. By leveraging the high sensitivity of surface-enhanced Raman scattering, here we offer direct evidence, for the first time, by resolving the Ag-Cl vibrational peak at 240 cm^-1. This characteristic peak disappears if the synthesis is conducted in the absence of Cl^-. Instead, three peaks associated with CF₃COO^- (from the precursor to Ag) are observed. When the sample is diluted with ethylene glycol, all the peaks associated with CF₃COO^- decrease proportionally in intensity, implying the involvement of chemisorption and negligible desorption during dilution. The chemisorbed CF₃COO^- is readily replaced by Cl^- due to their major difference in binding strength. The co-adsorbed Cl^- forces the carbonyl group of PVP binding to the Ag surface to take a more perpendicular configuration, enhancing its peak intensity. Altogether, these findings shed new light on the roles played by various chemical species in a successful synthesis of Ag nanocubes.