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. This characteristic peak disappears if the synthesis is conducted in the absence of Cl. Instead, three peaks associated with CFCOO (from the precursor to Ag) are observed. When the sample is diluted with ethylene glycol, all the peaks associated with CFCOO decrease proportionally in intensity, implying the involvement of chemisorption and negligible desorption during dilution. The chemisorbed CFCOO 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.