2, Procter & Gamble, Cincinnati, Ohio, United States
The importance of stabilizing ligands cannot be understated for the colloidal synthesis of nanoparticles as well as their dispersion and assembly into macro-structures such as electrodes for alternative energy applications. We have recently demonstrated a novel all-inorganic ligand approach to pure Pt and Pt-Bi nanoparticle synthesis where surface-adsorbed Sn serves as both reducing agent and stabilizing ligand, producing remarkably monodispersed nanoparticles. By eliminating the need to remove organic surfactants prior to nanoparticle incorporation into electrodes, we were able to use electrostatic assembly to achieve well-defined nanoparticle dispersions in aqueous media in contrast to the aggregated structures and flammable and toxic organic solvents common in the field. The approach has allowed us to elucidate the nature of structure sensitivity for electrocatalytic oxygen reduction reaction in acidic and alkaline media. In this paper, we provide a detailed investigation of the evolution of surface-adsorbed Sn-Pt and Sn-Bi ligand interaction during nanoparticle growth. We show that surface adsorbed SnCl3– exists as pyramidally coordinated to the Pt and Bi surface through Sn-Pt and Sn-Bi bond formation, resulting in a distorted tetrahedrally coordinated Sn-moiety.. Furthermore, we show the shift in the bond charge distribution as the Pt and Bi are reduced from salt to their metallic states. Direct evidence for the distorted tetrahedral coordination of M-SnCl3- and the shift in Sn-M bond charge emerges from the 119Sn Mossbauer quadrupole splitting (QS) and Isomer-shift (IS) respectively. The evolution of the bond energy corresponds to the growth of the nanoparticle (as measured using using small- and wide-angle x-ray absorption spectroscopy) and its transition from the atomic cluster to single crystal state. Evolution of the structure and chemistry of this surface complex has never before been demonstrated and our work is the first to identify the strength of the Sn-Minteraction in these systems. Such an understanding of nanoparticle surface chemistry will permit extension of this technique to other metals and macro-structures.