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 sp³. 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/sp³ sites, and a dramatic variability in the stability of the nanomaterial that can evolve toward either self-assembly or dissolution of the AgNPs.