Diversity of Behavior after Collisions of Sn and Si Nanoparticles Found Using a New Density Functional Tight-Binding Method.

We present a new approach to studying nanoparticle collisions using density functional based tight binding (DFTB). A novel DFTB parametrization has been developed to study the collision process of Sn and Si clusters (NPs) using molecular dynamics (MD). While bulk structures were used as training sets, we show that our model is able to accurately reproduce the cohesive energy of the nanoparticles using density functional theory (DFT) as a reference. A surprising variety of phenomena are revealed for the Si/Sn nanoparticle collisions, depending on the size and velocity of the collision: from core-shell structure formation to bounce-off phenomena.

Density Functional Tight-Binding Model for Lithium-Silicon Alloys.

The predictive power of molecular dynamic simulations is mainly restricted by the time scale and model accuracy. Many systems of current relevance are of such complexity that they require addressing both issues simultaneously. This is the case of silicon electrodes in Li-ion batteries, where different LixSi alloys are formed during charge/discharge cycles. While first-principles treatments for this system are seriously limited by the computational cost of exploring its large conformational space, classical force fields are not transferable enough to represent it accurately. Density Functional Tight Binding (DFTB) is an intermediate complexity approach capable of capturing the electronic nature of different environments with a relatively low computational cost. In this work, we present a new set of DFTB parameters suited to model amorphous LixSi alloys. LixSi is the usual finding upon cycling the Si electrodes in the presence of Li ions. The model parameters are constructed with a particular emphasis on their transferability for the entire LixSi composition range. This is achieved by introducing a new optimization procedure that weights stoichiometries differently to improve the prediction of their formation energies. The resulting model is shown to be robust for predicting crystal and amorphous structures for the different compositions, giving excellent agreement with DFT calculations and outperforming state-of-the-art ReaxFF potentials.

Galvanostatic Fast Charging of Alkali-Ion Battery Materials at the Single-Particle Level: A Map-Driven Diagnosis.

In this work, we develop a new tool to provide a diagnostic map for alkali-ion intercalation materials under galvanostatic conditions. These representations, stated in the form of capacity level diagrams, are built from hundreds of numerical simulations representing different experimental conditions, summarized in two dimensionless parameters: a kinetic parameter denominated Ξ and a finite diffusion parameter l. To lay the theoretical and methodological foundations, a general model is used here. This model can be adapted to the thermodynamic and kinetic framework of specific systems. We provide two representative examples.

Sodiation Of Hard Carbon: How Separating Enthalpy And Entropy Contributions Can Find Transitions Hidden In The Voltage Profile.

Sodium-ion batteries (NIBs) utilize cheaper materials than lithium-ion batteries (LIBs) and can thus be used in larger scale applications. The preferred anode material is hard carbon, because sodium cannot be inserted into graphite. We apply experimental entropy profiling (EP), where the cell temperature is changed under open circuit conditions. EP has been used to characterize LIBs; here, we demonstrate the first application of EP to any NIB material. The voltage versus sodiation fraction curves (voltage profiles) of hard carbon lack clear features, consisting only of a slope and a plateau, making it difficult to clarify the structural features of hard carbon that could optimize cell performance. We find additional features through EP that are masked in the voltage profiles. We fit lattice gas models of hard carbon sodiation to experimental EP and system enthalpy, obtaining: 1. a theoretical maximum capacity, 2. interlayer versus pore filled sodium with state of charge.

Voltammetric Behaviour of LMO at the Nanoscale: A Map of Reversibility and Diffusional Limitations.

Understanding and optimizing single particle rate behaviour is normally challenging in composite commercial lithium-ion electrode materials. In this regard, recent experimental research has addressed the electrochemical Li-ion intercalation in individual nanosized particles. Here, we present a thorough theoretical analysis of the Li+ intercalation voltammetric behaviour in single nano/micro-scale LiMn2 O4 (LMO) particles, incorporating realistic interactions between inserted ions. A transparent 2-dimensional zone diagram representation of kinetic-diffusional behaviour is provided that allows rapid diagnosis of the reversibility and diffusion length of the system depending on the particle geometry. We provide an Excel file where the boundary lines of the zone diagram can be rapidly recalculated by setting input values of the rate constant, k0 and diffusion coefficient, D . The model framework elucidates the heterogeneous behaviour of nanosized particles with similar sizes but different shapes. Hence, we present here an outlook for realistic multiscale modelling of real materials.

Characterization of amorphous LixSi structures from ReaxFF via accelerated exploration of local minima.

Motivated by the abundant experimental work in the area of Li-ion batteries, in the present work we characterize via computer simulations the structure of Si-Li amorphous alloys in a wide range of compositions. Using a reactive force field we propose a novel accelerated exploration of local minima to obtain amorphous structures close to equilibrium. The features of this system analyzed for different alloy compositions are the partial radial distribution functions g(r), the first and second nearest neighbour coordination numbers and the short-order structure. The complex structure of the second peak of the Si-Li g(r) is elucidated using a cluster-connection analysis.

Role of the solvent in the activation of Li2S as cathode material: a DFT study.

Lithium-sulfur batteries are considered one of the possible next-generation energy-storage solutions, but to be commercially available many drawbacks have yet to be solved. One solution with great potentiality is the use of lithium sulfide as cathode material since it can be coupled to Li-free anodes, such as graphite, Si or Sn. Nevertheless, Li2S, like sulfur, is electronically and ionically insulating, with a high activation potential for its initial oxidation step. To overcome this issue, different strategies have been explored, one of them being the use of catalytic surfaces. In the present article, we study using first principles calculations the effect of the dielectric constant of the solvent on the activation energy of the cleavage reaction of Li2S on different catalytic surfaces. To the best of our knowledge, this is the first time that such a study is undertaken. We find that the effect of the solvent should be twofold: on one side, it should decrease the interaction between the Li2S molecule and the surface. On the other side, since the species arising in the dissociation reaction are charged, the solvent should decrease the activation barrier for the dissociation of the Li2S molecule, when compared with the reaction in vacuum. These theoretical findings are discussed in connection with experimental results from the literature, where the behaviour of the Li-S cathode is studied in different solvents.

Design-Considerations regarding Silicon/Graphite and Tin/Graphite Composite Electrodes for Lithium-Ion Batteries.

An analytical model is proposed to investigate properties of composite electrodes that utilize more than one active material. We demonstrate how the equations can be applied to aid in the design of electrodes by comparing silicon-graphite and tin-graphite composite negative electrodes as examples with practical relevance. Based on simple assumptions, the results show how volume expansion tolerance and initial porosity are important factors for the achievable gravimetric and volumetric capacities as well as volumetric energy density. A Si-alloy/graphite composite electrode is used as an experimental system to corroborate the formulated analysis. Kinetic limitations are also addressed based on a novel heuristic approach.

Coalescence of Nanoclusters Analyzed by Well-Tempered Metadynamics. Comparison with Straightforward Molecular Dynamics.

The coalescence process of two nanoparticles to yield a core-shell structure is analyzed by a well-tempered metadynamics procedure. This methodology has been shown to be useful in understanding the present phenomenon in terms of two collective variables: the distance between the center of mass of the coalescing particles and the gyration radius of the resulting core element. The free-energy contour plots clearly show that the coalescence process involves the deformation of the core material, which is manifested in the residence of the system in regions with a larger gyration radius. Results from molecular dynamics for the same system were found helpful to reach the definition of this second collective variable. The advantages and limitations of the latter approach are discussed.

The Role of Tris(2-carboxyethyl)phosphine Reducing Agent in the Controlled Formation of α,ω-Alkanedithiols Monolayers on Au(111) with Monocoordinated and Bicoordinated Configurations.

The addition of the reducing agent tris(2-carboxyethyl) phosphine (TCEP) during the formation of α,ω-alkanedithiols monolayers on Au(111) using the immersion method produces the assembly of monolayers with bicoordinated molecules (both S-terminal groups bound to the surface) that have a reductive desorption potential that is more positive than for monolayers with monocoordinated molecules in a standing up configuration. We show that the use of TCEP either during formation of the monolayer or as a post treatment procedure allows the controlled formation of monolayers with bicoordinated or monocoordinated configurations. Density functional theory (DFT) calculations were performed to elucidate the role of TCEP in the formation of the bicoordinated configuration. We investigated the TCEP-dithiol interaction in ethanol solvent as well as the coadsorption of trimethylphosphine with 1,2-ethanedithiol on Au(111). The Brønsted base character of the phosphine facilitates the H exchange from the -SH groups of the dithiol to the phosphorus atom of TCEP with very low activation energy barriers, thus allowing the thiolate groups to bind to the Au(111) surface, thus yielding the bicoordinated configuration. Dithiol lifting mechanisms such as H exchange between S atoms and the formation of intra/inter layer disulfide bonds have much higher energy barriers.

Time Recovery for a Complex Process Using Accelerated Dynamics.

The hyperdynamics method (HD) developed by Voter (J. Chem. Phys. 1996, 106, 4665) sets the theoretical basis to construct an accelerated simulation scheme that holds the time scale information. Since HD is based on transition state theory, pseudoequilibrium conditions (PEC) must be satisfied before any system in a trapped state may be accelerated. As the system evolves, many trapped states may appear, and the PEC must be assumed in each one to accelerate the escape. However, since the system evolution is a priori unknown, the PEC cannot be permanently assumed to be true. Furthermore, the different parameters of the bias function used may need drastic recalibration during this evolution. To overcome these problems, we present a general scheme to switch between HD and conventional molecular dynamics (MD) in an automatic fashion during the simulation. To decide when HD should start and finish, criteria based on the energetic properties of the system are introduced. On the other hand, a very simple bias function is proposed, leading to a straightforward on-the-fly set up of the required parameters. A way to measure the quality of the simulation is suggested. The efficiency of the present hybrid HD-MD method is tested for a two-dimensional model potential and for the coalescence process of two nanoparticles. In spite of the important complexity of the latter system (165 degrees of freedoms), some relevant mechanistic properties were recovered within the present method.

Monte Carlo simulation of elongating metallic nanowires in the presence of surfactants.

Nanowires of different metals undergoing elongation were studied by means of canonical Monte Carlo simulations and the embedded atom method representing the interatomic potentials. The presence of a surfactant medium was emulated by the introduction of an additional stabilization energy, represented by a parameter Q. Several values of the parameter Q and temperatures were analyzed. In general, it was observed for all studied metals that, as Q increases, there is a greater elongation before the nanowire breaks. In the case of silver, linear monatomic chains several atoms long formed at intermediate values of Q and low temperatures. Similar observations were made for the case of silver-gold alloys when the medium interacted selectively with Ag.

Non-covalent interactions at electrochemical interfaces: one model fits all?

The shift with increasing concentration of alkali-metal cations of the potentials of both the spike and the hump observed in the cyclic voltammograms of Pt(111) electrodes in sulfuric acid solutions is shown to obey the simple model recently developed by us to explain the effect of non-covalent interactions at the electrical double layer. The results suggest that the model, originally developed to describe the effect of alkali-metal cations on the cyclic voltammogram of cyanide-modified Pt(111) electrodes, is of general applicability and can explain quantitatively the effect of cations on the properties of the electrical double layer.

Stretching single atom contacts at multiple subatomic step-length.

This work describes jump-to-contact STM-break junction experiments leading to novel statistical distribution of last-step length associated with conductance of a single atom contact. Last-step length histograms are observed with up to five for Fe and three for Cu peaks at integral multiples close to 0.075 nm, a subatomic distance. A model is proposed in terms of gliding from a fcc hollow-site to a hcp hollow-site of adjacent atomic planes at 1/3 regular layer spacing along with tip stretching to account for the multiple subatomic step-length behavior.

Anchoring sites to the STM tip can explain multiple peaks in single molecule conductance histograms.

Accelerated molecular dynamics and quantum conductance calculations are employed to shed light onto the electrochemical properties of the Au|1,8-octanedithiol|Au junction. Widely different contact geometries with varying degrees of roughness are examined. Strikingly, the two extreme situations considered in this work, tip-tip and tip-perfect surface junctions, give almost indistinguishable conductances. This result contrasts the usual notion that different S-Au bonding geometries combined with molecular torsions provide the explanation for the experimentally observed sets (low, medium, high) of conductance peaks. In this work, we provide an alternative explanation for the occurrence of these sets in terms of the specific anchoring sites of the molecule to the tip, which in turn determines the interaction of a portion of the carbon chain with the tip.

A Straightforward Approach for the Determination of the Maximum Time Step for the Simulation of Nanometric Metallic Systems.

In the present work, we report on a systematic analysis to determine the maximum time step allowed in molecular dynamics simulations applied to study metal systems of current interest in nanoscience. Using the velocity Verlet integration scheme, we have found that it is possible to use a 20 fs time step for the simulation of gold nanosystems. This is roughly an order of magnitude greater than the usually employed integration step (2 to 5 fs). We also propose a general criterion to select this maximum time step for other metallic nanosystems, even in the case of bimetallic nanosystems.

Configurational Behavior and Conductance of Alkanedithiol Molecular Wires from Accelerated Dynamics Simulations.

An accelerated dynamics scheme is employed to sample the configurational space of a system consisting of an alkanedithiol molecule confined to the gap between a metal tip and a perfect metal surface. With this information and by means of nonequilibrium green functions techniques (NEGF), conductance calculations are performed. The present results show that even for this system, which is one of the most simple conceivable because of the perfectness of the surface, a complex behavior appears due to the occurrence of an unexpected tip-molecule-surface arrangement, where the insertion of one of the molecular ends into the tip-surface gap generates configurations with strongly enhanced conductance. Estimates are also made for the time required to generate the molecular junction, indicating that it should depend on the tip-surface distance, thus opening the way to new experiments in this direction.

Reversible precipitation of casein micelles with a cationic hydroxyethylcellulose.

The cationic hydroxyethylcellulose Polyquaternium 10 (PQ10) was found to produce a dose-dependent destabilization of casein micelles from whole or skim milk without affecting the stability of most of the whey proteins. The anionic phosphate residues on caseins were not determinant in the observed interaction since the destabilization was also observed with dephosphorylated caseins to the same extent. However, the precipitation process was completely inhibited by rising NaCl concentration, indicating an important role of electrostatic interactions. Furthermore, the addition of 150 mM NaCl solubilized preformed PQ10-casein complexes, rendering a stable casein suspension without a disruption of the internal micellar structure as determined by dynamic light scattering. This casein preparation was found to contain most of the Ca2+ and only 10% of the lactose originally present in milk and remained as a stable suspension for at least 4 months at 4 degrees C. The final concentration of PQ10 determined both the size of the casein-polymer aggregates and the amount of milkfat that coprecipitates. The presence of PQ10 in the aggregates did not inhibit the activity of rennet or gastrointestinal proteases and lipases, nor did it affect the growth of several fermentative bacteria. The cationic cellulose PQ10 may cause a reversible electrostatic precipitation of casein micelles without disrupting their internal structure. The reversibility of the interaction described opens the possibility of using this cationic polysaccharide to concentrate and resuspend casein micelles from whole or skim milk in the production of new fiber-enriched lactose-reduced calcium-caseinate dairy products.

Collision as a way of forming bimetallic nanoclusters of various structures and chemical compositions.

In the present work, a new way to obtain bimetallic nanoclusters of different structures and chemical compositions is proposed, which is based on computer simulations. Collision processes between two metal clusters of different natures are simulated through molecular-dynamics simulations using many-body potentials. Diverse diffusion mechanisms and structures can be observed, depending on the metals combined and the initial kinetic energies. The nanostructures we have found are core-shell (Pt-Au), alloyed (Pd-Au), and three-shell onionlike (Cu-Ag).