Influence of the Silica Support on the Structure and the Morphology of Silver Nanoparticles: A Molecular Simulation Study
Résumé
We investigate the effect of the support on the structure and morphology of deposited metallic nanoparticles via molecular simulations at zero temperature. We focus on silver clusters in contact with a substrate which exhibits weak interactions, as amorphous silica, frequently used in atomic deposition experiments. Three levels of approximation are used to describe the substrate: (i) the smooth wall approximation where the support is modeled by a square-well whose depth is related to the adhesion energy of the nanoparticle, (ii) an atomistic model of flat amorphous silica obtained from Monte Carlo simulations with effective pair potentials, and (iii) the same silica substrate partially melted in order to roughen the surface at the nanometric scale. The metal metal interactions are modeled using tight-binding semiempirical potentials, and the metal silica interactions are modeled with Lennard-Jones potentials fitted to ab initio or experimental data. The icosahedron/decahedron/ truncated octahedron sequence found for free Ag nanoparticles is shown to be affected by the substrate (square-well), which favors truncated octahedrons deposited on their (111) facets. The aspect ratio is shown to follow the Wulff-Kaishew theorem for large clusters. The introduction of an atomistic description of the flat SiO2 surface does not change the results significantly, while the presence of a nanometric roughness induces drastic changes. Vertex orientations of nanoparticles are possible in the surface hollows, and decahedron structures are slightly favored for sizes smaller than 1200 atoms.
Domaines
Chimie
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