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The structure of the water/Pt(111) interface has been a subject of debate over the past decades. Here, we report the results of a room temperature molecular dynamics study based on neural network potentials, which allow us to access long time scale simulations while retaining ab initio accuracy. We find that the water/Pt(111) interface is characterized by a double layer composed of a primary, strongly bound adsorption layer with a coverage of ∼ 0.15 ML, which is coupled to a secondary, weakly bound adsorption layer with a coverage of ∼ 0.58 ML. By studying the order of the primary adsorption layer, we find that there is an effective repulsion between the adsorbed water molecules, which gives rise to a dynamically changing, semi-ordered interfacial structure, where the water molecules in the primary adsorption layer are distributed homogeneously across the interface, forming frequent hydrogen bonds to water molecules in the secondary adsorption layer. We further show that these conclusions are beyond the time scales accessible to ab initio molecular dynamics. [ABSTRACT FROM AUTHOR]

The interactions between liquid water and hydroxyl species on Pt(111) surfaces have been intensely investigated due to their importance to fuel cell electrocatalysis. Here we present a molecular dynamics study of their structure and energetics using an ensemble of neural network potentials, which allow us to obtain unprecedented statistical sampling. We first study the energetics of hydroxyl formation, where we find a near-linear adsorption energy profile, which exhibits a soft and gradual increase in the differential adsorption energy at high hydroxyl coverages. This is strikingly different from the predictions of the conventional bilayer model, which displays a kink at 1/3ML OH coverage indicating a sizeable jump in differential adsorption energy, but within the statistical uncertainty of previously reported ab initio molecular dynamics studies. We then analyze the structure of the interface, where we provide evidence for the water–OH/Pt(111) interface being hydrophobic at high hydroxyl coverages. We furthermore explain the observed adsorption energetics by analyzing the hydrogen bonding in the water–hydroxyl adlayers, where we argue that the increase in differential adsorption energy at high OH coverage can be explained by a reduction in the number of hydrogen bonds from the adsorbed water molecules to the hydroxyls. [ABSTRACT FROM AUTHOR]

The galvanic replacement reaction (GRR) is a convenient method for synthesizing hollow/porous noble metal nanostructures with energy, health, and environmental applications. Understanding the reaction mechanism is important for optimizing the produced nanostructures' physicochemical properties. Using liquid-phase scanning transmission electron microscopy (LPSTEM), we quantitatively analyzed the GRR process in individual silver nanowires (AgNWs) reacting with an aqueous HAuCl4 solution. The experiments and atomic-scale simulations show that GRR is a highly selective process with respect to the exposed surface facets, and we discover that the process progression is influenced by the internal crystal domains. We observe that the etching of AgNWs starts preferentially from facets with high energy sites while not favorable on low energy {111} facets, where even the internal twin facets within the nanostructures are found to be temporarily stable. The LPSTEM-observed etch rates in single or multiple crystal segments in AgNWs are shown to approach diffusion-limited conditions. These results provide intricate and detailed insights into the GRR process, which are difficult to achieve by other methods, and such studies will be beneficial for the understanding of how the surface energy and number of available surface sites influence the initiation probability, which will theoretically guide the synthesis of nanostructures, also supported with the deeper understanding of how the internal structure may influence the process. [ABSTRACT FROM AUTHOR]

We investigate the importance of including quantized initial conditions in Langevin dynamics for adsorbates interacting with a thermal reservoir of electrons. For quadratic potentials the time evolution is exactly described by a classical Langevin equation and it is shown how to rigorously obtain quantum mechanical probabilities from the classical phase space distributions resulting from the dynamics. At short time scales, classical and quasiclassical initial conditions lead to wrong results and only correctly quantized initial conditions give a close agreement with an inherently quantum mechanical master equation approach. With CO on Cu(100) as an example, we demonstrate the effect for a system with ab initio frictional tensor and potential energy surfaces and show that quantizing the initial conditions can have a large impact on both the desorption probability and the distribution of molecular vibrational states. [ABSTRACT FROM AUTHOR]

In this paper, we explore the notion that a negative alloying energy may act as a descriptor for long term stability of Pt-alloys as cathode catalysts in low temperature fuel cells. Using density functional theory calculations, we show that there is a correlation between the alloying energy of an alloy, and the diffusion barriers of the minority component. Alloys with a negative alloying energy may show improved long term stability, despite the fact that there is typically a greater thermodynamic driving force towards dissolution of the solute metal over alloying. In addition to Pt, we find that this trend also appears to hold for alloys based on Al and Pd. [ABSTRACT FROM AUTHOR]

A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67 μs at 135 K. Similar final configurations were obtained starting from different initial structures: (i) by bringing the two grains into contact without any intermediate layer, and (ii) by inserting an amorphous region between the grains. The results obtained were analyzed with a radial distribution function and a common neighbor analysis. Annealing events leading to lowering of the energy typically involved concerted displacement of several atoms—even as many as 10 atoms displaced by more than half an Ångström. Increased local icosahedral ordering is observed in the boundary layer, but local HCP coordination was also observed. In the final low-energy configurations, the thickness of the region separating the crystalline grains corresponds to just one atomic layer, in good agreement with reported experimental observations. The simulated system consists of 1307 atoms and atomic interactions were described using effective medium theory. [ABSTRACT FROM AUTHOR]

In the present study we have used in situ EXAFS to provide experimental evidence for a reversible change in the morphology of the metallic particles in a high surface area, porous catalyst system, Cu/ZnO, containing small metallic copper particles. By changing the oxidation potential in the synthesis gas mixture, it is found that the apparent Cu-Cu coordination number changes in an essentially reversible manner suggesting that the small metallic Cu particles dynamically change morphology. This indicates that a wetting/non-wetting phenomenon takes place in the Cu/ZnO system with changing partial pressures of oxygen. Under similar conditions, these effects are not observed when copper is supported on, for example, SiO. A model based on surface and interface free energies provides a simple explanation for the observed results. Since the wetting/non-wetting processes are accompanied by a change in the active surface area, the observed behavior has important general implications and such effects must be incorporated into microkinetic models in order to provide a proper description of the catalyst performance. [ABSTRACT FROM AUTHOR]

Presents research which conducted computer simulations of the deformation of nanocrystalline copper to show a softening with grain size for the smallest sizes. Properties of nanocrystalline solids; Producing nanocrystalline metals; What the Hall-Petch effect is; Reason for most of the plastic deformation; Limit on strength of nanocrystalline metals.

Nanoparticles have an immense importance in various fields, such as medicine, catalysis, and various technological applications. Nanoparticles exhibit a significant depression in melting point as their size goes below ≈10 nm. However, nanoparticles are frequently used in high temperature applications such as catalysis where temperatures often exceed several 100 degrees which makes it interesting to study not only the melting temperature depression, but also how the melting progresses through the particle. Using high‐resolution transmission electron microscopy, the melting process of gold nanoparticles in the size range of 2–20 nm Au nanoparticles combined with molecular dynamics studies is investigated. A linear dependence of the melting temperature on the inverse particle size is confirmed; electron microscopy imaging reveals that the particles start melting at the surface and the liquid shell formed then rapidly expands to the particle core. The depression of the melting point of nanoparticles compared to the bulk value is studied using transmission electron microscopy in combination with molecular dynamics simulations. The melting point is a crucial parameter for the optimized use of this category of materials. Understanding this fundamental property helps tailoring, e.g., catalytic materials with superior performance. [ABSTRACT FROM AUTHOR]

Through evaporation of dense colloids of ferromagnetic ~13 nm ε-Co particles onto carbon substrates, anisotropic magnetic dipolar interactions can support formation of elongated particle structures with aggregate thicknesses of 100-400 nm and lengths of up to some hundred microns. Lorenz microscopy and electron holography reveal collective magnetic ordering in these structures. However, in contrast to continuous ferromagnetic thin films of comparable dimensions, domain walls appear preferentially as longitudinal, i.e., oriented parallel to the long axis of the nanoparticle assemblies. We explain this unusual domain structure as the result of dipolar interactions and shape anisotropy, in the absence of inter-particle exchange coupling. [ABSTRACT FROM AUTHOR]