Surface Structure of Organosulfur Stabilized Silver Nanoparticles Studied with X-ray Absorption Spectroscopy

With the recent determination of the unexpected surface structure for thiolate-protected gold nanoparticles (-SR-Au-SR-staple-like motif for Au-102), it is of great interest to determine whether or not similar systems such as silver exhibit this special surface structure. A detailed study of the structure and composition of a series of organosulfurstabilized silver nanoparticles (AgNPs) was carried out using X-ray absorption near-edge (XANES) and extended X-ray absorption fine structure (EXAFS) from a multielement (Ag, S) and multicore (Ag K- and L-edge) perspective. It was determined that AgNPs of varied sizes prepared with dodecanethiol did not exhibit either a staple-like surface structure or the traditional metal thiolate structure (e.g., thiolate on 3-fold hollow site of metal surface), and instead adopted a layer of silver sulfide on the surface of metallic silver cores. The amount of the sulfide formed was found to be dependent on the AgNP size. Moreover, a comparison of the surface structure of thiolate-AgNPs with those coated with didodecyl sulfide indicated that the formation of a sulfide layer was inhibited when didodecyl sulfide was used achieving a surface structure more akin to the traditional thiolate bonding. These results show that AgNPs can be tailored to have different surface structure and bonding depending on the silver/sulfur molar ratio of the starting materials and type of organosulfur ligand used and, importantly, that the resulting bonding between silver and sulfur is very different from that of gold and sulfur.