CONICET | Buscador de Institutos y Recursos Humanos

Interest in small metal clusters, nanoparticles (NPs) has progressively grown in the past decade. These nanomaterials have a large area-to-volume ratio. It implies that a large part of the material is in direct contact with the environment. The general chemical synthesis principle of metal NPs is the reduction of metal salts using appropriate reducing agents, in the presence of ligand molecules, which form self-assembled monolayers (SAMs) on the metal surface and thus stabilize the NPs and prevent sintering. Besides the choice of the ligand and the reaction conditions, the question is still how the atomic structure of the metal NPs is modified or affected by the ligand adsorption and consequently most of the mechanical, optical and electronic properties of the synthesized NPs. Thiolates are strong passivating agents, and due to this tight binding, notable surface deformation is observed in thiolate- protected gold NPs, as was recently shown by STEM-HAADF imaging and theoretical calculations.1,2 Although studies of SAM formation of other ligands are scarce, it is already known that amines act as softer passivating agents on palladium and gold surfaces. The main advantage of this effect is that morphology and therefore structural and optical properties could be conserved after passivation.3 In this work we present an atomistic simulation study analyzing the effect of ligand molecules on the morphology and crystalline structure of monolayer protected gold nanoparticles (NPs). In particular, we focused on Au NPs covered with alkyl thiolates (?SR), which form a strong covalent bond with the Au surface, and alkyl amines (?NH2R), which physisorb onto gold. The atomic interactions between gold and the head groups of ligand molecules were represented by means of a recently developed bond-order potential [Olmos-Asar et al., Phys. Chem. Chem. Phys., 2011, 13, 6500]. We found in the case of strong passivants (i.e. ?SR) significant surface damage and/or amorphous-like structures, whereas soft passivants (?NH2R) do not produce almost any distortion in the crystalline structure of the metallic NPs. The enriched coverage degree related to flat surfaces is also discussed. We have also demonstrated that the new semi-empirical potential can reproduce low-coordinated adsorption sites, unlike usual pairwise classical potentials. In general, our simulations provide a direct observation of the structure of ligand protected gold nanoparticles. Langevin dynamics has been used to describe the motion of all atoms. The semi-empirical potential cited has been used, but now re-parameterized to represent amine?gold interactions.4