Thiolate-Capped Silver Nanoparticles: Discerning Direct Grafting from Sulfidation at the Metal-Ligand Interface by Interrogating the Sulfur Atom

Grafting thiol-bearing molecules at the surface of silver nanoparticles (AgNPs) is a successful strategy to tune their optical and antibacterial properties. The capping layer generated from self-assembly of the ligands at the nanoparticle surface determines the range of possible applications of the resulting material. In particular, direct grafting of the thiol heads to surface Ag(I) can occur, with various hybridizations of the S atom, sp versus sp(3). Alternatively, a passivating Ag2S layer can form. We make use of S K-edge X-ray absorption near edge structure (XANES) and synchrotron-based X-ray photoelectron spectroscopy (XPS) to probe the metal-ligand interface in different thiol-capped AgNPs. The use of cryogenic conditions for XAS analyses reveals a peculiar spectral signature for thiolates chemisorbed on the AgNPs surface, unambiguously distinguished from that of Ag2S. Ab initio simulations of XANES spectra and XPS analyses are used to predict the grafting mode, suggesting that different ligand architectures promote slightly different proportions of sp/sp(3) sites, and a dramatic variability in the stability of the nanomaterial that can evolve toward either self-assembly or dissolution of the AgNPs.