Diazonium-Mediated Chemisorption and Polymerization of Aryl Ligands on Gold Nanoparticle Surfaces.

Surface-capping molecular ligands are an integral part of virtually all colloidal inorganic nanoparticles that are chemically synthesized. The rational design of nanoparticle-adsorbate hybrid materials with specifically targeted properties and functionalities relies crucially on our understanding of ligand chemistry at the nanoparticle surfaces. Organic ligand molecules can interact covalently with metal surfaces through chemisorption using a variety of surface-binding moieties, most commonly thiol groups, to form self-assembled monolayers of monomeric adsorbates when reaching the saturated surface-coverage. Although monolayer formation of chemisorbed monomeric ligands on metal surfaces has been investigated intensively, diazonium-mediated surface-grafting of polymeric ligands remains underexplored and poorly understood, well-worthy of in-depth investigations. In this work, we use surface-enhanced Raman spectroscopy as a molecule-fingerprinting tool to study how aryl ligands evolve from monomeric adsorbates into surface-grafted branched polymers on nanotextured Au surfaces through diazonium-mediated chemisorption and polymerization. The results of our spectroscopic measurements reveal that the diazonium-mediated molecule-grafting process exhibits several singular characteristics, differing strikingly from the chemisorption of those monolayer-forming monomeric ligands. Through catalytic transfer hydrogenation reactions, -nitrothiophenol monolayers on Au nanoparticle surfaces are chemoselectively converted into -aminothiophenol via a bimolecular pathway with ,'-dimercaptoazobenzene serving as a key intermediate, whereas the poly nitrophenylene multilayer ligands derived from the diazonium-mediated ligand-grafting process appear substantially more reactive upon exposure to hydrogen donors, evolving into surface-adsorbed aminobenzene via a fundamentally different reaction pathway involving rapid polymer decomposition followed by stepwise nitro reduction in monomeric adsorbates. This work provides useful insights that guide us to judiciously leverage diazonium-mediated ligand chemistry for advancing surface-functionalization of nanostructures and kinetic modulation of catalytic reactions toward a higher level of precision and versatility.