Correlation and size dependence of the lattice strain, binding energy, elastic modulus, and thermal stability for Au and Ag nanostructures (original) (raw)
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Atomic undercoordination entitles the inert gold even more noble at sites pertained to adatoms, defects, kinks, and skins of bulk and sized crystals with unclear mechanism. Systematic analysis of the electron emission spectra of XPS and STM/S of the undercoordinated gold atoms revealed that the 4f and 5d bands undergo quantum entrapment while the 6s level is subject to localization and polarization because of the undercoordinationinduced local bond contraction and bond strength gainnamed bond order-length-strength correlation and nonbonding electron polarization (BOLS-NEP). Such a bonding and electronic relaxation result in the undercoordination derivacy of properties such as its extraordinary catalytic ability that the bulk gold does never demonstrate. The BOLS-NEP in the skin-electrical-double-layer shell dictates the nanostructure size dependency of the known bulk properties such as the chemical potential, inner potential constant, elasticity, and thermal stability. The exercise not only establishes a powerful means monitoring the Hamiltonian perturbation by atomic undercoordination but also offers information on the atomic-site-resolved local bond length, bond energy, chemical potentials, energy density, and atomic cohesive energy and the associated properties.
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We employ first-principles and empirical computational methods to study the surface energy and surface stress of silver nanoparticles. The structures, cohesive energies, and lattice contractions of spherical Ag nanoclusters in the size range 0.5-5.5 nm are analyzed using two different theoretical approaches: an ab initio density functional pseudopotential technique combined with the generalized gradient approximation and the embedded atom method. The surface energies and stresses obtained via the embedded atom method are found to be in good agreement with those predicted by the gradient-corrected ab initio density functional formalism. We estimate the surface energy of Ag nanoclusters to be in the range of 1.0-2.2 J / m 2 . Our values are close to the bulk surface energy of silver, but are significantly lower than the recently reported value of 7.2 J / m 2 for free Ag nanoparticles derived from the Kelvin equation.
Size-dependent surface energies of Au nanoparticles
Motivated by often contradictory literature reports on size dependence of surface energy of gold nanoparticles, we performed an atomistic study combining molecular dynamics and ab initio calculations. We show that in the case of nanocubes, their surface energy converges to a value for (0 0 1) facets of bulk crystals. A fast convergence to a single valued surface energy is predicted also for nanosheres. Here, however, the value is larger than the surface energy of any low-index surface facet of bulk Au crystal. This is explained by the complex structure of the surface with an extensive number of broken bonds due to edge and corner atoms. A similar trend was obtained also for the case of cuboctahedron-shaped nanoobjects. As the exact surface area of the nanoparticles is an ill-defined quantity, we introduced the surface-induced excess energy and discussed this quantity as a function of (i) number of atoms forming the nanoobject or (ii) its characteristic size. In the former case, a universal power-law behaviour was obtained independent of the nanoparticle shape.
Structural and Energetic Properties of Au@Ag and Ag@Au Core-Shell Nanoparticles
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Under tensile deformation, Au nanowires ͑NWs͒ elongate to form single atom chains via a series of intermediate structural transformations. These intermediate structures are investigated using static densityfunctional theory, with particular attention paid to their behavior under load. The accessibility of these structures and their stability under load are found to be key factors governing the morphological evolution of the NW, while the ground-state energy of the unstrained structures does not correlate well with the observed behavior. Reverse loading conditions are also studied, where a NW is first deformed in tension and then deformed in compression. Again, accessibility and stability under load are the key criteria for predicting the evolution of the NW. Finally, electronic structure studies show abrupt opening and closing of small band gaps during tensile deformation, possibly explaining conductance oscillations observed experimentally. An analysis of the orbital interactions responsible for this unusual band-gap behavior is presented.
ACS Nano, 2021
It is well known that in the case of bulk polycrystalline metals, a reduction in the grain size leads to material hardening, since the grain boundaries represent efficient barriers for slip transfer between the adjacent crystalline grains. Here we show that coating single crystalline Ag nanoparticles with a thin polycrystalline Au layer leads to a weakening of the particles. Moreover, while the single crystalline Ag nanoparticles yield in a single large displacement burst when loaded in compression, their Ag-Au core-shell counterparts demonstrate a more homogeneous deformation with signs of strain hardening. Our molecular dynamics simulations demonstrate that particle weakening at low strains is attributed to the plasticity confinement in the polycrystalline shell, in which the grain boundaries play a dual role of dislocations sources and sinks. At higher strains the plasticity within the Ag core is initiated by the dislocations
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Noble and Precious Metals - Properties, Nanoscale Effects and Applications, 2018
In this chapter, experimental and theoretical studies on surface segregation in Ag-Au systems, including our own thermodynamic studies and molecular dynamics simulations of surface restructuring, on the basis of density functional theory are reviewed. The restructuring processes are triggered by adsorbed atomic O, which is supplied and consumed during catalysis. Experimental evidence points to the essential role of Ag impurities in nanoporous gold for activating O 2. At the same time, increasing Ag concentration may be detrimental for the selectivity of partial oxidation. Understanding the role of silver requires a knowledge on its chemical state and distribution in the material. Recent studies using electron microscopy and photoelectron spectroscopy shed new light on this issue revealing a non-uniform distribution of residual Ag and coexistence of different chemical forms of Ag. We conclude by presenting an outlook on electromechanical coupling at Ag-Au surfaces, which shows a way to systematically tune the catalytic activity of bimetallic surfaces.
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In this investigation, we employed simulations and computational methodologies to examine pure silver and gold nanoparticles (NPs), alongside their core−shell configurations. Our research delved into, assessing the effects and adsorption tendencies of methanethiol molecules on diverse sites within these NPs. In our calculations, the effects of the inclusion of dispersion forces are analyzed. Structural analysis unveiled contractions in larger NPs and notable variations in adsorption energies across silver and gold surfaces. Furthermore, our study scrutinized not only the growth process but also the adsorption behavior of a model molecule within core−shell structures. We found that the arrangement of metal layers within these structures significantly impacted the adsorption energies of methanethiol, closely resembling the behavior observed in the smaller pure gold NPs. Notably, even a single shell layer led to discernible changes in the electronic structure. Overall, our investigation underscored the profound influence of the NP size, composition, and arrangement on adsorption energies. Interestingly, introducing methanethiol molecules to larger-scale NPs exhibited minimal impact on the electronic structure despite the evident changes in adsorption behaviors.
Surface Energy of Au Nanoparticles Depending on Their Size and Shape
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Motivated by often contradictory literature reports on the dependence of the surface energy of gold nanoparticles on the variety of its size and shape, we performed an atomistic study combining molecular mechanics and ab initio calculations. We show that, in the case of Au nanocubes, their surface energy converges to the value for ( 0 0 1 ) facets of bulk crystals. A fast convergence to a single valued surface energy is predicted also for nanospheres. However, the value of the surface energy is larger in this case than that of any low-index surface facet of bulk Au crystal. This fact can be explained by the complex structure of the surface with an extensive number of broken bonds due to edge and corner atoms. A similar trend was obtained also for the case of cuboctahedrons. Since the exact surface area of the nanoparticles is an ill-defined quantity, we have introduced the surface-induced excess energy and discuss this quantity as a function of (i) number of atoms forming the nano-o...