ReaxFF Potential Research Articles

Partially hydrogenated and fluorinated graphene: Structure, roughness, and negative thermal expansion

The structural properties of partially hydrogenated and fluorinated graphene with different percentages of H/F atoms are investigated using molecular dynamics simulations based on reactive force field (ReaxFF) potentials. We found that the roughness of graphene varies with the percentage $(p)$ of H or F and in both cases is maximal around $p=50%$. Similar results were obtained for partially oxidized graphene. The two-dimensional area size of partially fluorinated and hydrogenated graphene exhibits a local minimum around $p=35%$ coverage. The lattice thermal contraction in partially functionalized graphene is found to be one order of magnitude larger than that of fully covered graphene. We also show that the armchair structure for graphene oxide (similar to the structure of fully hydrogenated and fluorinated graphene) is unstable. Our results show that the structure of partially functionalized graphene changes nontrivially with the C : H and C : F ratio as well as with temperature.

Molecular-dynamics simulations of hillocks induced by highly-charged Arq+, Xeq+ ions impact on HOPG surface

The hillocks on highly oriented pyrolytic graphite (HOPG) surface induced by highly charged Arq+, Xeq+ ions are studied by using molecular-dynamics (MD) simulations. And a hybrid potential created by combining the ReaxFF potential with the repulsive ZBL potential is used to describe the interatomic interactions. The effects of incident highly charged ion (HCI)’s kinetic energy and the energy gain due to the interaction of HCI with its own image on the formation of the hillocks are considered in the present simulations. Our results show that both potential and kinetic energy of HCI may affect the hillock size. However, the potential energy of HCI increases dramatically with charge state, which is more important than kinetic energy in the formation of the hillock in extremely high charge states. And it is found that both the height and width of the hillock agree well with experimental data. In addition, the bond breaking and bond formation during the formation of the hillock are also investigated, and the results show that there are many σ bonds breaking and interlayer bonds formation in one layer or between two layers during this process. Furthermore, most of the interlayer bonds in HOPG surface induced by HCI impact are sp2 bond, although some interlayer sp3 bonds are also observed in the present work.

Reactive Force-Field Development for Metal/Ceramic SOFC Anode Modeling

A solid oxide fuel cell (SOFC) system anode material is usually composed of a mixture of Ni and other oxide ceramics, such as doped-CeO2, and Sc stabilized ZrO2, due to their high activity and structural stability. Among them the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet is widely used today as an anode material of SOFC due to its good electrocatalytic property against low price. For SOFC performance, it is essential to improve the stability of the Ni/YSZ cermet anode. Combination of Ni and YSZ has been studied experimentally, but the interaction is not well understood in the boundary between Ni and YSZ. However, there is no evidence about the atomic-scale Ni/YSZ interface dynamics and chemical changes in the structure during the operation of SOFC. Unfortunately, most classical force fields are incapable of describing chemical reactions. To overcome this difficulty, reactive force field (ReaxFF) potential was developed by van Duin and co-workers. Recently, Ni/YSZ/H2 interactions are reported by ReaxFF. But in this interaction Ni/YSZ interface stability is not taken into account. As a first step we developed our own ReaxFF potential to investigate Ni/ZrO2 interface and for determining stable Ni/YSZ interface for modeling of the triple-phase-boundary in SOFCs. Big difference is found between our developed and reported ReaxFF and that is work of separation. Using our ReaxFF potential, work of separation is in excellent agreement with literature. It is found that O around Zr4+cation is reduced in case of bulk and interface regions. Although interface region exhibited less reduction compared to bulk region. More details and Ni/YSZ interface stability through O vacancy may present at conference.

Simulations of tensile bond rupture in single alkane molecules using reactive interatomic potentials

Molecular simulations were performed to study the energetics and geometries of bond rupture in single alkane molecules using three reactive hydrocarbon potentials: (1) modified embedded-atom method (MEAM) for saturated hydrocarbons, (2) ReaxFF, and (3) second-generation REBO. The total energy/force versus strain, strain at fracture, and strain energy release were compared for a homologous series of normal alkanes (ethane to undecane) with generalization to polyethylene. The CC bond distances and CCC bond angles were quantified, and a fragment analysis was performed. Overall, the MEAM and ReaxFF potentials are in reasonable agreement with first-principles data with MEAM matching DFT-calculated lowest energy fragments.

Molecular dynamics simulations of nanoporous organosilicate glasses using Reactive Force Field (ReaxFF)

The structure and properties of nanoporous organosilicate glass (OSG) structure models with 30–70% porosities and different pore morphologies were simulated using molecular dynamics simulations with Reactive Force Field (ReaxFF) potential. The OSG structures were created from nanoporous silica structures generated using classical molecular dynamics (MD) simulations with partial charge pairwise potentials and a subsequent step of adding hydroxide and methyl groups to the dangling bonds and coordination defects. The nanoporous OSG systems were then fully relaxed using molecular dynamics simulations with ReaxFF and detailed structure analysis performed and properties calculated. Analysis of the OSG systems indicated that the structural features (bond distances and angles) as well as the Qn distribution of the nanoporous silica backbone structure are consistent with the features of dense silica and that the geometry of the added methyl groups is experimentally accurate, even after ReaxFF relaxation. The elastic modulus of the nanoporous silica was calculated and found to be 24–31GPa for system with 30% porosity and 0.5–2.5GPa for those with 70% porosity, consistent with previously reported experimental results.

Molecular dynamics simulation of amorphous SiO2thin films

Amorphous SiO 2 thin films have been studied via molecular dynamics (MD) simulations. Thin film models (with two free surfaces) have been obtained by cooling from the melt with reactive force field (ReaxFF) potential. Structural and dynamic properties of the thin films are analyzed via radial pair distribution functions (RPDFs), coordination number distributions, ring statistics and bond-angle distributions. Fraction and role of structural defects have been analyzed and discussed. We also show temperature dependence of various thermodynamic quantities of the system. We found that structure of interior of thin films is close to that of the bulk while surface region contains a large amount of structural defects including dangling bonds, undercoordinated sites, small membered rings. Small and large membered rings concentrate mainly in the surface region and their fraction has a tendency to decrease with decreasing temperature.

On the interaction between charged ions and graphene on a substrate

The interaction between graphene on a substrate and charged particles moving with different rates (from thermal to Bohr’s) is studied by the molecular dynamics method. An approach is used, in which chemical-bond formation and breakage are taken into account by the ReaxFF potential. The characteristic dependences of the interaction forces of moving charged particles on the graphene-substrate distance are considered. Data on the polarization energy of the system as a function of the graphene-substrate distance are obtained.

Graphdiyne as a promising substrate for stabilizing Pt nanoparticle catalyst

At present, Pt nanoparticle catalysts in fuel cells suffer from aggregation and loss of chemical activity. In this work, graphdiyne, which has natural porous structure, was proposed as substrate with high adsorption ability to stabilize Pt nanoparticles. Using multiscale calculations by ab initio method and the ReaxFF potential, geometry optimizations, molecular dynamics simulations, Metropolis Monte Carlo simulations and minimum energy paths calculations were performed to investigate the adsorption energy and the rates of desorption and migration of Pt nanoparticles on graphdiyne and graphene. According to the comparison between graphdiyne and graphene, it was found that the high adsorption ability of graphdiyne can avoid Pt nanoparticle migration and aggregation on substrate. Then, simulations indicated the potential catalytic ability of graphdiyne-Pt-nanoparticle system to the oxygen reduction reaction in fuel cells. In summary, graphdiyne should be an excellent material to replace graphite or amorphous carbon matrix for stabilizing Pt nanoparticle catalysts.

Atomistic simulation and virtual diffraction characterization of homophase and heterophase alumina interfaces

The objective of this work is to elucidate the structure and energy of 12 homophase and heterophase alumina interfaces using atomistic simulations with the ReaxFF potential. First, the computational methods are validated by exploring a set of five α-Al2O3 symmetric tilt twin interfaces. The interface structures and energies for most homophase α-Al2O3 twins are in good agreement with prior atomistic studies; however, small deviations occur for select α-Al2O3 interfaces due to the larger, more appropriate interface areas explored in this work. Next, select experimentally observed κ-Al2O3, γ-Al2O3, and θ-Al2O3 homophase interfaces as well as heterophase α-Al2O3//γ-Al2O3 and θ-Al2O3//γ-Al2O3 interfaces are investigated for the first time using atomistic simulations to elucidate their atomic structure, including terminating plane(s) and relaxations, and to compute interface energies. ReaxFF predicts that the γ-Al2O3 {111} twin and the θ-Al2O3 {200} twin interfaces have energies of the same order as the lowest-energy α-Al2O3 prismatic twin boundary and that the heterophase α-Al2O3 (0001)//γ-Al2O3 (111) interface has the lowest energy of all interfaces studied. Lastly, virtual selected-area electron diffraction patterns of select interfaces are used to experimentally validate the predicted interface structures. Because a consistent computational method is implemented throughout this work, the computed interface energies can be incorporated in future predictive mesoscale simulations of polymorphic alumina.

How reliable is the ReaxFF potential for describing the structure of alkanethiols on gold? A molecular dynamics study

The structures of self-assembled monolayers (SAMs) of short (methyl) and long (hexyl) chain alkyl thiols on the clean gold(111) surface were modeled using for the Au – S interactions either the reactive ReaxFF potential or the well-known nonreactive Morse potential, while for the Au – Au interactions either the ReaxFF potential or an embedded-atom method (EAM). Analysis of the molecular dynamics (MD) trajectories of possible SAM structures suggests that disordering of interfacial Au atoms is definitely driven by the gold–sulphur interactions. Our MD results reveal a novel structure where two methanethiol molecules are bound to a gold adatom that has been lifted from the surface at 300 K, and the same kind of RS– Au –SR motif was also observed for hexanethiol at 600 K but not at 300 K. What is more, the above motif is only observed for the reactive ReaxFF potential. Moreover, these results are in clear agreement with recent experiments and more costly first principles-based MD simulations. These findings strongly support the use of reactive potentials such as ReaxFF for gathering an accurate description of Au – S interactions in inexpensive classical MD simulations.

Atomistic simulation and virtual diffraction characterization of stable and metastable alumina surfaces

The structures of select alumina surfaces are studied using molecular statics and molecular dynamics simulations and are characterized using virtual diffraction methods. First, bulk alumina simulations are performed to validate the transferability of the ReaxFF potential to model different alumina phases. Bulk alumina simulations accurately predict α-Al2O3 as the lowest energy crystalline phase; however, they unexpectedly predict an even lower-energy amorphous phase. At 0K, virtual X-ray diffraction patterns of the bulk crystalline phases and select alumina surfaces are validated by experimental studies. Molecular statics simulations of select alumina surfaces are consistent with prior first-principles studies. However, molecular dynamics simulations show that many surfaces experience significant reconstructions at temperatures below what is expected from experiments. It is believed that premature surface reconstructions are biased by the predicted lower-energy amorphous phase and occur due to the extra degrees of freedom allowed by the free surfaces as well as the available thermal energy during dynamics. Discrete peaks appearing in virtual selected-area electron diffraction patterns indicate that the reconstructions are not fully amorphous due to lattice constraints imposed by the internal bulk structure. Bulk and surface energies are tabulated for each simulation to be used in future predictive mesoscale models of polymorphic alumina.

Atomistic Mechanisms of Phase Boundary Evolution during Initial Lithiation of Crystalline Silicon

In lithium-ion batteries, the electrochemical reaction between Li and Si causes structural changes in the negative electrode. The dynamics of lithiation of Si can be further complicated by the crystalline-to-amorphous phase transition. In situ TEM experiments show that a sharp interface, known as phase boundary, is formed in between c-Si and a-LixSi during initial lithiation. Despite intensive study of the mixing mechanism during lithiation of Si negative electrode, the atomistic investigation of the formation and propagation of phase boundary for different orientation of Si remains unclear. We, therefore, performed molecular dynamics simulations to characterize the structural evolution of the phase boundary with a newly developed reactive force field (ReaxFF) potential for Li–Si. Our results confirm the phase boundary formation in between c-Si and a-LixSi. Structure and dynamics of the phase boundary depend on the crystalline phase of the Si. In particular, the location of the (111) plane plays a key role in crystal-to-amorphous phase transformation. A relatively thick phase boundary is developed at the (100) surface, while an atomically sharp interface of negligible thickness is formed at the (111) surface. An amorphous phase of lithiated Si is developed beyond the phase boundary, in which the ratio of lithium to silicon atoms is steady at 0.8. Partial RDF studies revealed that the structures of the phase boundary and the lithiated Si region are c-LiSi and a-Li15Si4, respectively.

Dynamics of l-Phenylalanine Sputtering by Argon Cluster Bombardment

We simulate the impact of an Ar1000 cluster (energy 10 keV, impact angle 55°) into an amorphous l-phenylalanine target. By use of a ReaxFF potential, it is possible to model not only the emission dynamics of intact Phe molecules but also the fragmentation and reaction pathways taken. The simulated sputter yield is in close agreement with experiment. The simulated emission mass spectrum features both emission of large Phen clusters and entrainment of reaction products in the ejected flow, again in agreement with experimental observation. While H abstraction is a common fragmentation channel, the H radicals quickly combine with Phe in the amino group; no isolated H atom is ejected.

How reliable is the ReaxFF Potential for Describing the Structure of Alkanethiols on Gold? A Molecular Dynamics Study

The structures of self-assembled monolayers (SAMs) of short (methyl) and long (hexyl) chain alkyl thiols on the clean gold (111) surface were modelled using for the Au-S interactions either the reactive ReaxFF potential or the well known non-reactive Morse potential, while for the Au-Au interactions either the ReaxFF potential or an embedded-atom method (EAM). Analysis of the MD trajectories of possible SAM structures suggests that disordering of interfacial Au atoms is definitely driven by the gold-sulphur interactions. Our MD results reveal a novel structure where two methanethiol molecules are bound to a gold adatom that has been lifted from the surface at 300 K, and the same kind of RS-Au-SR motif was also observed for hexanethiol at 600 K but not at 300 K. What is more, the above motif is only observed for the reactive ReaxFF potential. Moreover, these results are in clear agreement with recent experiments and more costly first principles-based MD simulations. These findings strongly support the use of reactive potentials such as ReaxFF for gathering an accurate description of Au-S interactions in inexpensive classical MD simulations.

Interactions of plasma species on nickel catalysts: A reactive molecular dynamics study on the influence of temperature and surface structure

Methane reforming by plasma catalysis is a complex process that is far from understood. It requires a multidisciplinary approach which ideally takes into account all effects from the plasma on the catalyst, and vice versa. In this contribution, we focus on the interactions of CHx (x={1,2,3}) radicals that are created in the plasma with several nickel catalyst surfaces. To this end, we perform reactive molecular dynamics simulations, based on the ReaxFF potential, in a wide temperature range of 400–1600K. First, we focus on the H2 formation as a function of temperature and surface structure. We observe that substantial H2 formation is obtained at 1400K and above, while the role of the surface structure seems limited. Indeed, in the initial stage, the type of nickel surface influences the C–H bond breaking efficiency of adsorbed radicals; however, the continuous carbon diffusion into the surface gradually diminishes the surface crystallinity and therefore reduces the effect of surface structure on the H2 formation probability. Furthermore, we have also investigated to what extent the species adsorbed on the catalyst surface can participate in surface reactions more in general, for the various surface structures and as a function of temperature. These results are part of the ongoing research on the methane reforming by plasma catalysis, a highly interesting yet complex alternative to conventional reforming processes.

Development of a ReaxFF potential for Pt-O systems describing the energetics and dynamics of Pt-oxide formation.

ReaxFF force field parameters describing Pt-Pt and Pt-O interactions have been developed and tested. The Pt-Pt parameters are shown to accurately account for the chemical nature, atomic structures and other materials properties of bulk platinum phases, low and high-index platinum surfaces and nanoclusters. The Pt-O parameters reliably describe bulk platinum oxides, as well as oxygen adsorption and oxide formation on Pt(111) terraces and the {111} and {100} steps connecting them. Good agreement between the force field and both density functional theory (DFT) calculations and experimental observations is demonstrated in the relative surface free energies of high symmetry Pt-O surface phases as a function of the oxygen chemical potential, making ReaxFF an ideal tool for more detailed investigations of more complex Pt-O surface structures. Validation for its application to studies of the kinetics and dynamics of surface oxide formation in the context of either molecular dynamics (MD) or Monte Carlo simulations are provided in part by a two-part investigation of oxygen diffusion on Pt(111), in which nudged elastic band (NEB) calculations and MD simulations are used to characterize diffusion processes and to determine the relevant diffusion coefficients and barriers. Finally, the power of the ReaxFF reactive force field approach in addressing surface structures well beyond the reach of routine DFT calculations is exhibited in a brief proof-of-concept study of oxygen adsorbate displacement within ordered overlayers.

Determining in situ phases of a nanoparticle catalyst via grand canonical Monte Carlo simulations with the ReaxFF potential

Catalyst design requires a detailed understanding of the structure of the catalyst surface as a function of varying reaction conditions. Here we demonstrate the capability of a grand canonical Monte Carlo/molecular dynamics (GC-MC/MD) method utilizing the ReaxFF potential to predict nanoparticle structure and phase stability as a function of temperature and pressure. This is demonstrated for Pd nanoparticles, which readily form oxide, hydride, and carbide phases under reaction environments, impacting catalytic behavior. The approach presented here can be extended to other catalytic systems, providing a new tool for exploring the effects of reaction conditions on catalyst activity, selectivity, and stability.

The effect of empirical potential functions on modeling of amorphous carbon using molecular dynamics method

Empirical potentials have a strong effect on the hybridization and structure of amorphous carbon and are of great importance in molecular dynamics (MD) simulations. In this work, amorphous carbon at densities ranging from 2.0 to 3.2g/cm3 was modeled by a liquid quenching method using Tersoff, 2nd REBO, and ReaxFF empirical potentials. The hybridization, structure and radial distribution function G(r) of carbon atoms were analyzed as a function of the three potentials mentioned above. The ReaxFF potential is capable to model the change of the structure of amorphous carbon and MD results are in a good agreement with experimental results and density function theory (DFT) at low density of 2.6g/cm3 and below. The 2nd REBO potential can be used when amorphous carbon has a very low density of 2.4g/cm3 and below. Considering the computational efficiency, the Tersoff potential is recommended to model amorphous carbon at a high density of 2.6g/cm3 and above. In addition, the influence of the quenching time on the hybridization content obtained with the three potentials is discussed.

Development of a ReaxFF potential for Pd/O and application to palladium oxide formation

Oxide formation on palladium surfaces impacts the activity and selectivity of Pd-based catalysts, which are widely employed under oxygen rich operating conditions. To investigate oxidation processes over Pd catalysts at time and length scales inaccessible to quantum based computational methods, we have developed a Pd∕O interaction potential for the ReaxFF reactive force field. The parameters of the ReaxFF potential were fit against an extensive set of quantum data for both bulk and surface properties. Using the resulting potential, we conducted molecular dynamics simulations of oxide formation on Pd(111), Pd(110), and Pd(100) surfaces. The results demonstrate good agreement with previous experimental observations; oxygen diffusion from the surface to the subsurface occurs faster on the Pd(110) surface than on the Pd(111) and Pd(100) surfaces under comparable conditions at high temperatures and pressures. Additionally, we developed a ReaxFF-based hybrid grand canonical Monte Carlo∕molecular dynamics (GC-MC∕MD) approach to assess the thermodynamic stability of oxide formations. This method is used to derive a theoretical phase diagram for the oxidation of Pd935 clusters in temperatures ranging from 300 K to 1300 K and oxygen pressures ranging from 10(-14) atm to 1 atm. We observe good agreement between experiment and ReaxFF, which validates the Pd∕O interaction potential and demonstrates the feasibility of the hybrid GC-MC∕MD method for deriving theoretical phase diagrams. This GC-MC∕MD method is novel to ReaxFF, and is well suited to studies of supported-metal-oxide catalysts, where the extent of oxidation in metal clusters can significantly influence catalytic activity, selectivity, and stability.

Reactive Potentials for Advanced Atomistic Simulations

This article reviews recent advances in the development of reactive empirical force fields or potentials. In particular, we compare two widely used reactive potentials with variable-charge schemes that are desirable for multicomponent or multifunctional systems: the ReaxFF (reactive force field) and charge-optimized many-body (COMB) potentials. Several applications of these approaches in atomistic simulations that involve metal-based heterogeneous systems are also discussed.