Publications by authors named "Adri C T van Duin"

180 Publications

Atomistic Insights Into the Degradation of Inorganic Halide Perovskite CsPbI: A Reactive Force Field Molecular Dynamics Study.

J Phys Chem Lett 2021 Jun 7:5519-5525. Epub 2021 Jun 7.

Materials Simulation & Modelling, Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

Halide perovskites make efficient solar cells but suffer from several stability issues. The characterization of these degradation processes is challenging because of the limited spatiotemporal resolution in experiments and the absence of efficient computational methods to study these reactive processes. Here, we present the first reactive force field for molecular dynamics simulations of the phase instability and the defect-induced degradation in CsPbI. We find that the phase transitions are driven by the anharmonic fluctuations of the atoms in the perovskite lattice. At low temperatures, the Cs cations tend to move away from their preferential positions, resulting in worse contacts with the surrounding metal halide framework which initiates the conversion to a nonperovskite phase. Moreover, our simulations of defective structures reveal that, although both iodine vacancies and interstitials are mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, leading to the decomposition of perovskites to PbI.
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http://dx.doi.org/10.1021/acs.jpclett.1c01192DOI Listing
June 2021

Recent Advances for Improving the Accuracy, Transferability, and Efficiency of Reactive Force Fields.

J Chem Theory Comput 2021 Jun 10;17(6):3237-3251. Epub 2021 May 10.

Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, United States.

Reactive force fields provide an affordable model for simulating chemical reactions at a fraction of the cost of quantum mechanical approaches. However, classically accounting for chemical reactivity often comes at the expense of accuracy and transferability, while computational cost is still large relative to nonreactive force fields. In this Perspective, we summarize recent efforts for improving the performance of reactive force fields in these three areas with a focus on the ReaxFF theoretical model. To improve accuracy, we describe recent reformulations of charge equilibration schemes to overcome unphysical long-range charge transfer, new ReaxFF models that account for explicit electrons, and corrections for energy conservation issues of the ReaxFF model. To enhance transferability we also highlight new advances to include explicit treatment of electrons in the ReaxFF and hybrid nonreactive/reactive simulations that make it possible to model charge transfer, redox chemistry, and large systems such as reverse micelles within the framework of a reactive force field. To address the computational cost, we review recent work in extended Lagrangian schemes and matrix preconditioners for accelerating the charge equilibration method component of ReaxFF and improvements in its software performance in LAMMPS.
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http://dx.doi.org/10.1021/acs.jctc.1c00118DOI Listing
June 2021

ReaxFF reactive molecular dynamics simulations to study the interfacial dynamics between defective h-BN nanosheets and water nanodroplets.

Phys Chem Chem Phys 2021 May;23(18):10822-10834

Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA-16802, USA.

In this work, the authors have developed a reactive force field (ReaxFF) to investigate the effect of water molecules on the interfacial interactions with vacancy defective hexagonal boron nitride (h-BN) nanosheets by introducing parameters suitable for the B/N/O/H chemistry. Initially, molecular dynamics simulations were performed to validate the structural stability and hydrophobic nature of h-BN nanosheets. The water molecule dissociation mechanism in the vicinity of vacancy defective h-BN nanosheets was investigated, and it was shown that the terminal nitrogen and boron atoms bond with a hydrogen atom and hydroxyl group, respectively. Furthermore, it is predicted that the water molecules arrange themselves in layers when compressed in between two h-BN nanosheets, and the h-BN nanosheet fracture nucleates from the vacancy defect site. Simulations at elevated temperatures were carried out to explore the water molecule trajectory near the functionalized h-BN pores, and it was observed that the intermolecular hydrogen bonds lead to agglomeration of water molecules near these pores when the temperature was lowered to room temperature. The study was extended to observe the effect of pore sizes and temperatures on the contact angle made by a water nanodroplet on h-BN nanosheets, and it was concluded that the contact angle would be less at higher temperatures and larger pore sizes. This study provides important information for the use of h-BN nanosheets in nanodevices for water desalination and underwater applications, as these h-BN nanosheets possess the desired adsorption capability and structural stability.
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http://dx.doi.org/10.1039/d1cp00546dDOI Listing
May 2021

Dynamics of the Chemically Driven Densification of Barium Titanate Using Molten Hydroxides.

Nano Lett 2021 Apr 14;21(8):3451-3457. Epub 2021 Apr 14.

Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Molten hydroxides, often used for crystal growth and nanoparticle synthesis, have recently been applied for the single step densification of several inorganic materials under moderate uniaxial pressures and 1000 °C below their usual sintering temperatures. The latter approach, termed cold sintering process (CSP), is a mechanochemically driven process that enables the densification of inorganic materials through a dissolution-precipitation creep mechanism. In this study, we report the main densification mechanisms of BaTiO in a NaOH-KOH eutectic mixture. A chemical insight at the atomistic level, investigated by ReaxFF molecular dynamics simulations, offers plausible ionic complex formation scenarios and reactions at the BaTiO/molten hydroxide interface, enabling the dissolution-precipitation reactions and the subsequent cold sintering of BaTiO.
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http://dx.doi.org/10.1021/acs.nanolett.1c00069DOI Listing
April 2021

Atomistic-scale insight into the polyethylene electrical breakdown: An eReaxFF molecular dynamics study.

J Chem Phys 2021 Jan;154(2):024904

Department of Mechanical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA.

Cross-linked polyethylene (XLPE) has been recognized as an outstanding insulator for high-voltage power cables due to its favorable structural integrity at high temperature, low moisture sensitivity, chemical resistance, and low rates of failure due to aging. However, the roles of by-products and amorphous regions generated during the XLPE production are not clearly known at the atomistic scale. In this study, we present an eReaxFF-based molecular dynamics simulation framework with an explicit electron description verified against density functional theory data to investigate the roles of XLPE by-products and processing variables such as density and voids on the time to dielectric breakdown (TDDB) of polyethylene (PE). Our simulation results indicate that an increase in density of PE increases the TDDB; however, adding a by-product with positive electron affinity such as acetophenone can reduce the TDDB. Furthermore, during the electrical breakdown in PE, electrons tend to migrate through voids when transferring from the anode to cathode. In comparison with neutral acetophenone, we find that the acetophenone radical anion can significantly reduce the energy barrier and the reaction energy of secondary chemical reactions.
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http://dx.doi.org/10.1063/5.0033645DOI Listing
January 2021

Wafer-Scale Lateral Self-Assembly of Mosaic TiCT MXene Monolayer Films.

ACS Nano 2021 Jan 6;15(1):625-636. Epub 2021 Jan 6.

Department of Physics, Northeastern University, Boston, Massachusetts 02115, United States.

Bottom-up assembly of two-dimensional (2D) materials into macroscale morphologies with emergent properties requires control of the material surroundings, so that energetically favorable conditions direct the assembly process. MXenes, a class of recently developed 2D materials, have found new applications in areas such as electrochemical energy storage, nanoscale electronics, sensors, and biosensors. In this paper, we present a lateral self-assembly method for wafer-scale deposition of a mosaic-type 2D MXene flake monolayer that spontaneously orders at the interface between two immiscible solvents. ReaxFF molecular dynamics simulations elucidate the interactions of a MXene flake with the solvents and its stability at the liquid/liquid interface, the prerequisite for MXene flakes self-assembly at the interface. Moreover, facile transfer of this monolayer onto a flat substrate (Si, glass) results in high-coverage monolayer films with uniform thickness and homogeneous optical properties. Multiscale characterization of the resulting films reveals the mosaic structure and sheds light on the electronic properties of the films, which exhibit good electrical conductivity over cm-scale areas.
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http://dx.doi.org/10.1021/acsnano.0c06393DOI Listing
January 2021

Structure and Dynamics of Aqueous Electrolytes Confined in 2D-TiO/TiCT MXene Heterostructures.

ACS Appl Mater Interfaces 2020 Dec 18;12(52):58378-58389. Epub 2020 Dec 18.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania 16802, United States.

The synthesis of heterostructures of different two-dimensional (2D) materials offers an approach to combine advantages of different materials constituting the heterostructure and ultimately enhance their performance for applications such as electrochemical energy storage, achieving high energy, and high-power densities. Understanding the behavior of ions and solvents in confinement between these dissimilar layers is critical to understand their performance and control. Considering aqueous electrolytes, we explore the heterostructure of 2D lepidocrocite-type TiO (2D-TiO) and hydroxylated or O-terminated TiC MXene using ReaxFF molecular dynamics simulations and elastic/quasielastic neutron scattering techniques. Simulating a bilayer water intercalation, we find that the extent of interlayer hydration is impacted most by the surface terminations on the MXene and is marginally affected by 2D-TiO. However, the introduction of 2D-TiO decreases the water self-diffusion due to the notch sites (i.e., surface oxygen ridges) entrapping water molecules. Intercalating alkali cations into the heterostructures, we find that Li is predominantly adsorbed at the 2D-TiO surface instead of the MXenes with the preferential occupation of the notch sites. In contrast, Na forms a planar solvation with water, while K is adsorbed both at the O-terminated MXene and 2D-TiO. This behavior is altered when OH-terminated MXene is involved-the repulsion from the protons on the MXene surface forces the K ions to be adsorbed exclusively to 2D-TiO, while Na retains some of its solvation in the water layer due to its smaller size. In OH-terminated MXenes, we see a consistent transfer of protons from the MXene surface toward 2D-TiO, implying a greater capacity to store protons in the heterostructures. Of the three cations simulated, Na hinders the proton migration the least and Li the most because of its position near the 2D-TiO surface. Therefore, 2D-TiO/MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na intercalation.
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http://dx.doi.org/10.1021/acsami.0c17536DOI Listing
December 2020

Atomistic Mechanisms of Thermal Transformation in a Zr-Metal Organic Framework, MIL-140C.

J Phys Chem Lett 2021 Jan 15;12(1):177-184. Epub 2020 Dec 15.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

To understand the mechanisms responsible for thermal decomposition of a Zr-MOF (MIL-140C), we perform atomistic-scale molecular dynamics (MD) simulations and discuss the simulation data in comparison with the TEM images obtained for the decomposed Zr-MOF. First, we introduce the ReaxFF parameters suitable for the Zr/C/H/O chemistry and then apply them to investigate the thermal stability and morphological changes in the MIL-140C during heating. Based on the performed simulations we propose an atomic mechanism for the collapse of the MIL-140C and the molecular pathways for carbon monoxide formation, the main product of the MIL-140C thermal degradation. We also determine that the oxidation state of the ZrO clusters, evolved due to the thermal degradation, approximates the tetragonal phase of ZrO. Both simulations and experiments show a distribution of very small ZrO clusters embedded in the disrupted organic sheet that could contribute to the unusual high catalytic activity of the decomposed MIL-140C.
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http://dx.doi.org/10.1021/acs.jpclett.0c02930DOI Listing
January 2021

Stable metal anodes enabled by a labile organic molecule bonded to a reduced graphene oxide aerogel.

Proc Natl Acad Sci U S A 2020 Dec 16;117(48):30135-30141. Epub 2020 Nov 16.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802;

Metallic anodes (lithium, sodium, and zinc) are attractive for rechargeable battery technologies but are plagued by an unfavorable metal-electrolyte interface that leads to nonuniform metal deposition and an unstable solid-electrolyte interphase (SEI). Here we report the use of electrochemically labile molecules to regulate the electrochemical interface and guide even lithium deposition and a stable SEI. The molecule, benzenesulfonyl fluoride, was bonded to the surface of a reduced graphene oxide aerogel. During metal deposition, this labile molecule not only generates a metal-coordinating benzenesulfonate anion that guides homogeneous metal deposition but also contributes lithium fluoride to the SEI to improve Li surface passivation. Consequently, high-efficiency lithium deposition with a low nucleation overpotential was achieved at a high current density of 6.0 mA cm A Li|LiCoO cell had a capacity retention of 85.3% after 400 cycles, and the cell also tolerated low-temperature (-10 °C) operation without additional capacity fading. This strategy was applied to sodium and zinc anodes as well.
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http://dx.doi.org/10.1073/pnas.2001837117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7720105PMC
December 2020

ReaxFF/AMBER-A Framework for Hybrid Reactive/Nonreactive Force Field Molecular Dynamics Simulations.

J Chem Theory Comput 2020 Dec 3;16(12):7645-7654. Epub 2020 Nov 3.

Department of Computer Science and Engineering, Michigan State University, 428 S. Shaw Lane, East Lansing, Michigan 48824-1322, United States.

Combined quantum mechanical/molecular mechanical (QM/MM) models using semiempirical and methods have been extensively reported on over the past few decades. These methods have been shown to be capable of providing unique insights into a range of problems, but they are still limited to relatively short time scales, especially QM/MM models using methods. An intermediate approach between a QM based model and classical mechanics could help fill this time-scale gap and facilitate the study of a range of interesting problems. Reactive force fields represent the intermediate approach explored in this paper. A widely used reactive model is ReaxFF, which has largely been applied to materials science problems and is generally used as a stand-alone (i.e., the full system is modeled using ReaxFF). We report a hybrid ReaxFF/AMBER molecular dynamics (MD) tool, which introduces ReaxFF capabilities to capture bond breaking and formation within the AMBER MD software package. This tool enables us to study local reactive events in large systems at a fraction of the computational costs of QM/MM models. We describe the implementation of ReaxFF/AMBER, validate this implementation using a benzene molecule solvated in water, and compare its performance against a range of similar approaches. To illustrate the predictive capabilities of ReaxFF/AMBER, we carried out a Claisen rearrangement study in aqueous solution. In a first for ReaxFF, we were able to use AMBER's potential of mean force (PMF) capabilities to perform a PMF study on this organic reaction. The ability to capture local reaction events in large systems using combined ReaxFF/AMBER opens up a range of problems that can be tackled using this model to address both chemical and biological processes.
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http://dx.doi.org/10.1021/acs.jctc.0c00874DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8145783PMC
December 2020

Development and initial applications of an e-ReaxFF description of Ag nanoclusters.

J Chem Phys 2020 Sep;153(10):104106

Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA.

Metal nanocrystals are of considerable scientific interest because of their uses in electronics, catalysis, and spectroscopy, but the mechanisms by which nanocrystals nucleate and grow to achieve selective shapes are poorly understood. Ab initio calculations and experiments have consistently shown that the lowest energy isomers for small silver nanoparticles exhibit two-dimensional (2D) configurations and that a transition into three-dimensional (3D) configurations occurs with the addition of only a few atoms. We parameterized an e-ReaxFF potential for Ag nanoclusters (N ≤ 20 atoms) that accurately reproduces the 2D-3D transition observed between the Ag and Ag clusters. This potential includes a four-body dihedral term that imposes an energetic penalty to 3D structures that is significant for small clusters but is overpowered by the bond energy from out-of-plane Ag-Ag bonds in larger 3D clusters. The potential was fit to data taken from density-functional theory and coupled-cluster calculations and compared to an embedded atom method potential to gauge its quality. We also demonstrate the potential of e-ReaxFF to model redox reactions in silver halides and plasmon motion using molecular dynamics simulations. This is the first case in which e-ReaxFF is used to describe metals. Furthermore, the inclusion of a bond-order dependent dihedral angle in this force field is a unique solution to modeling the 2D-3D transition seen in small metal nanoclusters.
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http://dx.doi.org/10.1063/5.0018971DOI Listing
September 2020

Optimization of the Reax force field for the lithium-oxygen system using a high fidelity charge model.

J Chem Phys 2020 Aug;153(8):084107

Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, USA.

Studies using molecular dynamics (MD) have long struggled to simulate the failure modes of materials, predicting unrealistically high ductility and failing to capture brittle fracture. The primary cause of this shortcoming is an inadequate description of bond breaking. While reactive force fields such as ReaxFF show improvements compared to traditional force fields, the charge models used yield unphysical partial charges, especially during dissociation of ionic bonds. This flaw may be remedied by using the atom-condensed Kohn-Sham density functional theory (DFT) approximated to a second order (ACKS2) charge model for determining partial charges. In this work, we present a new ACKS2-enabled Reax force field for fracture simulations of lithium oxide systems, which was obtained by training against an extensive set of DFT, multireference configuration interaction (MRCI), and MRCI+Q reference data using genetic optimization techniques. This new force field significantly improves the bond breaking behavior, but still cannot fully capture the brittle fracture in MD simulations, suggesting more research is needed to improve simulation of brittle fracture.
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http://dx.doi.org/10.1063/5.0014406DOI Listing
August 2020

Reactive molecular dynamics simulation for isotope-exchange reactions in H/D systems: ReaxFF development.

J Chem Phys 2020 Jun;152(22):224111

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.

To investigate the chemical isotope-exchange reactions within a system composed of a mixture of hydrogen and deuterium (H/D) in the plasma media, the ReaxFF potential was parameterized against an appropriate quantum mechanics (QM)-based training set. These QM data involve structures and energies related to bond dissociation, angle distortion, and an exchange reaction of the tri-atomic molecular ions, H, D, HD, and DH, produced in the hydrogen plasma. Using the ReaxFF potential, a range of reactive molecular dynamics simulations were performed on different mixtures of H/D systems. Analysis of the reactions involved in the production of these tri-atomic molecular ions was carried out over 1 ns simulations. The results show that the ReaxFF potential can properly model isotope-exchange reactions of tri-atomic molecular ions and that it also has a perfect transferability to reactions taking place in these systems. In our simulations, we observed some intermediate molecules (H, D, and HD) that undergo secondary reactions to form the tri-atomic molecular ions as the most likely products in the hydrogen plasma. Moreover, there remains a preference for D in the produced molecular ions, which is related to the lower zero-point energy of the D-enriched species, showing the isotope effects at the heart of the ReaxFF potential.
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http://dx.doi.org/10.1063/5.0008386DOI Listing
June 2020

Graphene reinforced carbon fibers.

Sci Adv 2020 Apr 24;6(17):eaaz4191. Epub 2020 Apr 24.

Department of Mechanical and Aerospace Engineering, University of Virginia, 122 Engineer's Way, Charlottesville, VA 22904, USA.

The superlative strength-to-weight ratio of carbon fibers (CFs) can substantially reduce vehicle weight and improve energy efficiency. However, most CFs are derived from costly polyacrylonitrile (PAN), which limits their widespread adoption in the automotive industry. Extensive efforts to produce CFs from low cost, alternative precursor materials have failed to yield a commercially viable product. Here, we revisit PAN to study its conversion chemistry and microstructure evolution, which might provide clues for the design of low-cost CFs. We demonstrate that a small amount of graphene can minimize porosity/defects and reinforce PAN-based CFs. Our experimental results show that 0.075 weight % graphene-reinforced PAN/graphene composite CFs exhibits 225% increase in strength and 184% enhancement in Young's modulus compared to PAN CFs. Atomistic ReaxFF and large-scale molecular dynamics simulations jointly elucidate the ability of graphene to modify the microstructure by promoting favorable edge chemistry and polymer chain alignment.
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http://dx.doi.org/10.1126/sciadv.aaz4191DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7182419PMC
April 2020

Full-Scale Ab Initio Simulation of Magic-Angle-Spinning Dynamic Nuclear Polarization.

J Phys Chem Lett 2020 Jul 2;11(14):5655-5660. Epub 2020 Jul 2.

U.S. DOE, Ames Laboratory, Ames, Iowa 50011, United States.

Theoretical models aimed at describing magic-angle-spinning (MAS) dynamic nuclear polarization (DNP) NMR have great potential in facilitating the in silico design of DNP polarizing agents and formulations. These models must typically face a trade-off between the accuracy of a strict quantum mechanical description and the need for using realistically large spin systems, for instance, using phenomenological models. Here, we show that the use of aggressive state-space restrictions and an optimization strategy allows full-scale ab initio MAS-DNP simulations of spin systems containing thousands of nuclei. Our simulations are shown to reproduce experimental DNP enhancements quantitatively, including their MAS rate dependence, for both frozen solutions and solid materials. They also reveal the importance of a previously unrecognized structural feature found in some polarizing agents that helps minimize the sensitivity losses imposed by the spin diffusion barrier.
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http://dx.doi.org/10.1021/acs.jpclett.0c00955DOI Listing
July 2020

Lithium-electrolyte solvation and reaction in the electrolyte of a lithium ion battery: A ReaxFF reactive force field study.

J Chem Phys 2020 May;152(18):184301

Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.

In the electrode/electrolyte interface of a typical lithium-ion battery, a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from the anode to the electrolyte reduces the Li-ion and makes it more reactive, resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange, and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended the existing ReaxFF reactive force field parameter sets to organic electrolyte species, such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate, and LiPF salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to the existing ReaxFF training data, and subsequently, we trained the ReaxFF parameters with the aim of finding the optimal reproduction of the DFT data. In order to discern the characteristics of the Li neutral and cation, we have introduced a second Li parameter set to describe the Li-ion. ReaxFF is trained for Li-neutral and Li-cation to have similar solvation energies, but unlike the neutral Li, Li will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li-ions are reduced to Li-atoms, which can be simulated using a Monte Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of ethylene carbonate molecules solvating the Li-atom.
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http://dx.doi.org/10.1063/5.0003333DOI Listing
May 2020

Reactive Molecular Dynamics Simulations and Quantum Chemistry Calculations To Investigate Soot-Relevant Reaction Pathways for Hexylamine Isomers.

J Phys Chem A 2020 May 18;124(21):4290-4304. Epub 2020 May 18.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Sooting tendencies of a series of nitrogen-containing hydrocarbons (NHCs) have been recently characterized experimentally using the yield sooting index (YSI) methodology. This work aims to identify soot-relevant reaction pathways for three selected CHN amines, namely, dipropylamine (DPA), diisopropylamine (DIPA), and 3,3-dimethylbutylamine (DMBA) using ReaxFF molecular dynamics (MD) simulations and quantum mechanical (QM) calculations and to interpret the experimentally observed trends. ReaxFF MD simulations are performed to determine the important intermediate species and radicals involved in the fuel decomposition and soot formation processes. QM calculations are employed to extensively search for chemical reactions involving these species and radicals based on the ReaxFF MD results and also to quantitatively characterize the potential energy surfaces. Specifically, ReaxFF simulations are carried out in the ensemble at 1400, 1600, and 1800 K, where soot has been identified to form in the YSI experiment. These simulations account for the interactions among test fuel molecules and pre-existing radicals and intermediate species generated from rich methane combustion, using a recently proposed simulation framework. ReaxFF simulations predict that the reactivity of the amines decrease in the order DIPA > DPA > DMBA, independent of temperature. Both QM calculations and ReaxFF simulations predict that CH, CH, and CH are the main nonaromatic soot precursors formed during the decomposition of DPA, DIPA, and DMBA, respectively, and the associated reaction pathways are identified for each amine. Both theoretical methods predict that sooting tendency increases in the order DPA, DIPA, and DMBA, consistent with the experimentally measured trend in YSI. This work demonstrates that sooting tendencies and soot-relevant reaction pathways of fuels with unknown chemical kinetics can be identified efficiently through combined ReaxFF and QM simulations. Overall, predictions from ReaxFF simulations and QM calculations are consistent, in terms of fuel reactivity, major intermediates, and major nonaromatic soot precursors.
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http://dx.doi.org/10.1021/acs.jpca.0c03355DOI Listing
May 2020

Atomically thin half-van der Waals metals enabled by confinement heteroepitaxy.

Nat Mater 2020 Jun 10;19(6):637-643. Epub 2020 Mar 10.

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA.

Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.
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http://dx.doi.org/10.1038/s41563-020-0631-xDOI Listing
June 2020

Timescale prediction of complex multi-barrier pathways using flux sampling molecular dynamics and 1D kinetic integration: Application to cellulose dehydration.

J Chem Phys 2020 Jan;152(2):024123

CNRS/MIT/Aix Marseille University Joint Lab "MultiScale Materials Science for Energy and Environment", Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.

Reactive molecular dynamics (MD) simulations, especially those employing acceleration techniques, can provide useful insights on the mechanism underlying the transformation of buried organic matter, yet, so far, it remains extremely difficult to predict the time scales associated with these processes at moderate temperatures (i.e., when such time scales are considerably larger than those accessible to MD). We propose here an accelerated method based on flux sampling and kinetic integration along a 1D order parameter that can considerably extend the accessible time scales. We demonstrate the utility of this technique in an application to the dehydration of crystalline cellulose at temperatures ranging from 1900 K to 1500 K. The full decomposition is obtained at all temperatures apart from T = 1500 K, showing the same distribution of the main volatiles (HO, CO, and CO) as recently obtained using replica exchange molecular dynamics. The kinetics of the process is well fitted with an Arrhenius law with E = 93 kcal/mol and k = 9 × 10 s, which are somehow larger than experimental reports. Unexpectedly, the process seems to considerably slow down at lower temperatures, severely departing from the Arrhenius regime, probably because of an inadequate choice of the order parameter. Nevertheless, we show that the proposed method allows considerable time sampling at low temperatures compared to conventional MD.
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http://dx.doi.org/10.1063/1.5126391DOI Listing
January 2020

Unveiling Carbon Ring Structure Formation Mechanisms in Polyacrylonitrile-Derived Carbon Fibers.

ACS Appl Mater Interfaces 2019 Nov 1;11(45):42288-42297. Epub 2019 Nov 1.

Department of Mechanical and Aerospace Engineering , University of Virginia , 122 Engineer's Way , Charlottesville , Virginia 22904 , United States.

As the demand for electric vehicles (EVs) and autonomous vehicles (AVs) rapidly grows, lower-cost, lighter, and stronger carbon fibers (CFs) are urgently needed to respond to consumers' call for greater EV traveling range and stronger safety structures for AVs. Converting polymeric precursors to CFs requires a complex set of thermochemical processes; a systematic understanding of each parameter in fiber conversion is still, to a large extent, lacking. Here, we demonstrate the effect of carbonization temperature on carbon ring structure formation by combining atomistic/microscale simulations and experimental validation. Experimental testing, as predicted by simulations, exhibited that the strength and ductility of PAN CFs decreased, whereas the Young's modulus increased with increasing carbonization temperature. Our simulations unveiled that high carbonization temperature accelerated the kinetics of graphitic phase nucleation and growth, leading to the decrease in strength and ductility but increase in modulus. The methodology presented herein using combined atomistic/microscale simulations and experimental validation lays a firm foundation for further innovation in CF manufacturing and low-cost alternative precursor development.
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http://dx.doi.org/10.1021/acsami.9b15833DOI Listing
November 2019

ReaxFF Parameter Optimization with Monte-Carlo and Evolutionary Algorithms: Guidelines and Insights.

J Chem Theory Comput 2019 Dec 12;15(12):6799-6812. Epub 2019 Nov 12.

Center for Molecular Modeling (CMM) , Ghent University , Technologiepark-Zwijnaarde 46 , B-9052 Ghent , East Flanders , Belgium.

ReaxFF is a computationally efficient force field to simulate complex reactive dynamics in extended molecular models with diverse chemistries, if reliable force-field parameters are available for the chemistry of interest. If not, they must be optimized by minimizing the error ReaxFF makes on a relevant training set. Because this optimization is far from trivial, many methods, in particular, genetic algorithms (GAs), have been developed to search for the global optimum in parameter space. Recently, two alternative parameter calibration techniques were proposed, that is, Monte-Carlo force field optimizer (MCFF) and covariance matrix adaptation evolutionary strategy (CMA-ES). In this work, CMA-ES, MCFF, and a GA method (OGOLEM) are systematically compared using three training sets from the literature. By repeating optimizations with different random seeds and initial parameter guesses, it is shown that a single optimization run with any of these methods should not be trusted blindly: nonreproducible, poor or premature convergence is a common deficiency. GA shows the smallest risk of getting trapped into a local minimum, whereas CMA-ES is capable of reaching the lowest errors for two-third of the cases, although not systematically. For each method, we provide reasonable default settings, and our analysis offers useful guidelines for their usage in future work. An important side effect impairing parameter optimization is numerical noise. A detailed analysis reveals that it can be reduced, for example, by using exclusively unambiguous geometry optimization in the training set. Even without this noise, many distinct near-optimal parameter vectors can be found, which opens new avenues for improving the training set and detecting overfitting artifacts.
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http://dx.doi.org/10.1021/acs.jctc.9b00769DOI Listing
December 2019

How to characterize interfacial load transfer in spiral carbon-based nanostructure-reinforced nanocomposites: is this a geometry-dependent process?

Phys Chem Chem Phys 2019 Nov;21(43):23880-23892

School of Mechanical Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran 1417466191, Iran.

There is a great deal of attention given to spiral carbon-based nanostructures (SCBNs) because of their unique mechanical, thermal and electrical properties along with fascinating morphology. Dispersing SCBNs inside a polymer matrix leads to extraordinary properties of nanocomposites in diverse fields. However, the role of the interfacial mechanical properties of these nanocomposites remains unknown. Here, using molecular dynamics simulations, the characteristics of interfacial load transfer of SCBN-polyethylene nanocomposites are explored. Considering the geometric characteristics of SCBNs, new insight into the separation behavior of nanoparticles in normal and sliding modes is addressed. Interestingly, the results show that the maximum force and the separation energy of the SCBNs are much larger than those of graphene because of interlocking of the coils and polymer. The heavy influence of changes in the geometric characteristics of SCBNs on the separation behavior is observed. Pullout tests reveal that the influence of parameters such as the length and number of polyethylene chains, temperature, and functionalization of the SCBNs on the interfacial mechanical properties is also significant. This study sheds new light in understanding the crucial effect of the interaction of SCBNs with polymer chains on the interfacial mechanical properties, which can lead to better performance of nanocomposites.
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http://dx.doi.org/10.1039/c9cp04276hDOI Listing
November 2019

A ReaxFF molecular dynamics study of molecular-level interactions during binder jetting 3D-printing.

Phys Chem Chem Phys 2019 Oct 19;21(38):21517-21529. Epub 2019 Sep 19.

Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802, USA.

In the present work, we study one of the major additive manufacturing processes, i.e., the binder jetting printing (BJP) process, at the molecular level through atomistic-scale level representations of powders and binder solutions with chromium-oxide (Cr-oxide) nanoparticles and water-based diethylene glycol solutions, respectively. The results show that both diethylene glycol and water contribute to the bonding of Cr-oxide particles during the print and curing stages by forming a hydrogen bond network. Heating the system to the burn-out temperature results in the oxidation of diethylene glycol and the decomposition of the hydrogen bond network. Subsequently, Cr-oxide particles are partially sintered by forming Cr-O bonds. The final sintering facilitates further Cr-O bond formation. Additionally, the influence of the chemical composition of the binder solution is investigated by performing ReaxFF molecular dynamics simulations on two sets of systems, which control the number of water and diethylene glycol molecules, respectively. Our results demonstrate that adding both diethylene glycol and water to the binder solution can raise the number of "useful" hydrogen bonds, resulting in a higher breaking strength at the print and curing stages. During the burn-out and sintering stages, the influence of water on the breaking strength is not obvious. In contrast, an optimal quantity of binder species exists for the breaking strength after sintering. A comparison of the ReaxFF molecular dynamics simulations using 2-ethoxyethanol, diethylene glycol and 1-(2,2,2-trihydroxyethoxy)ethane-2,2,2-triol as the binder phase indicates that an increasing number of hydroxyl groups leads to higher breaking strength at the print and curing stages. The findings from this study can be extended to identify the optimal binder chemistry, curing and sintering conditions for different material systems.
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http://dx.doi.org/10.1039/c9cp03585kDOI Listing
October 2019

Understanding the influence of defects and surface chemistry on ferroelectric switching: a ReaxFF investigation of BaTiO.

Phys Chem Chem Phys 2019 Aug;21(33):18240-18249

Department of Mechanical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA.

Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- and temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) the existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and a reduction in the polarization and the Curie temperature due to the OVs; (3) domain wall interaction with the surface chemistry to influence the ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.
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http://dx.doi.org/10.1039/c9cp02955aDOI Listing
August 2019

Water-Mediated Surface Diffusion Mechanism Enables the Cold Sintering Process: A Combined Computational and Experimental Study.

Angew Chem Int Ed Engl 2019 Sep 30;58(36):12420-12424. Epub 2019 Jul 30.

Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, USA.

The cold sintering process (CSP) densifies ceramics at much lower temperatures than conventional sintering processes. Several ceramics and composite systems have been successfully densified under cold sintering. For the grain growth kinetics of zinc oxide, reduced activation energies are shown, and yet the mechanism behind this growth is unknown. Herein, we investigate these mechanisms in more detail with experiments and ReaxFF molecular dynamics simulations. We investigated the recrystallization of zinc cations under various acidic conditions and found that their adsorption to the surface can be a rate-limiting factor for cold sintering. Our studies show that surface hydroxylation in CSP does not inhibit crystallization; in contrast, by creating a surface complex, it creates an orders of magnitude acceleration in surface diffusion, and in turn, accelerates recrystallization.
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http://dx.doi.org/10.1002/anie.201904738DOI Listing
September 2019

Topological Control of Water Reactivity on Glass Surfaces: Evidence of a Chemically Stable Intermediate Phase.

J Phys Chem Lett 2019 Jul 2;10(14):3955-3960. Epub 2019 Jul 2.

Department of Materials Science and Engineering , The Pennsylvania State University , University Park , Pennsylvania 16802 , United States.

Glass surfaces are of considerable interest due to their disproportionately large influence on the performance of glass articles in many applications. However, the behavior of glass surfaces has proven difficult to model and predict due to their complex structure and interactions with the environment. Here, the effects of glass network topology on the surface reactivity of glasses have been investigated using reactive and nonreactive force field-based molecular dynamics simulations as well as density functional theory. A topological constraint-based description for surface reactivity is developed, allowing for improved understanding of the physical and chemical origins of surface reactivity. Results show evidence for the existence of a chemically stable intermediate phase on the surface of the glass where the glass network is mechanically isostatic.
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http://dx.doi.org/10.1021/acs.jpclett.9b01275DOI Listing
July 2019

Atomistic Scale Analysis of the Carbonization Process for C/H/O/N-Based Polymers with the ReaxFF Reactive Force Field.

J Phys Chem B 2019 Jun 13;123(25):5357-5367. Epub 2019 Jun 13.

During the carbonization process of raw polymer precursors, graphitic structures can evolve. The presence of these graphitic structures affects mechanical properties of the carbonized carbon fibers. To gain a better understanding of the chemistry behind the evolution of these structures, we performed atomistic-scale simulations using the ReaxFF reactive force field. Three different polymers were considered as a precursor: idealized ladder PAN (polyacrylonitrile), a proposed oxidized PAN, and poly( p-phenylene-2,6-benzobisoxazole). We determined the underlying molecular details of polymer conversion into a carbon fiber structure. Because these are C/H/O/N-based polymers, we first developed an improved force field for C/H/O/N chemistry based on the density functional theory data with a particular focus on N formation kinetics and its interactions with polymer-associated radicals formed during the carbonization process. Then, using this improved force field, we performed atomistic-scale simulations of the initial stage of the carbonization process for the considered polymers. On the basis of our simulation data, the molecular pathways for the formation of low-molecular-weight gas species and all-carbon ring formation were determined. We also examined the possible alignment of the developed all-carbon 6-membered ring clusters, which is crucial for the further graphitic structure evolution.
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http://dx.doi.org/10.1021/acs.jpcb.9b04298DOI Listing
June 2019

Evaluation of the effect of nickel clusters on the formation of incipient soot particles from polycyclic aromatic hydrocarbons via ReaxFF molecular dynamics simulations.

Phys Chem Chem Phys 2019 May;21(19):9865-9875

Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802, USA.

In the present study, the ReaxFF reactive molecular dynamics simulation method was applied to investigate the effect of a small nickel cluster (Ni13) on the formation of nascent soot from polycyclic aromatic hydrocarbon (PAH) precursors. A series of NVT simulations was performed for systems of a Ni13 cluster and various PAH monomers, namely, naphthalene, anthracene, pyrene, coronene, ovalene, and circumcoronene, at temperatures from 400 to 2500 K. At low temperatures, the PAHs form soot particles via binding and stacking around nickel clusters. Larger soot particles are formed due to the early initiation of clustering provided by nickel compared to those observed in homogenous PAH systems. At 1200-1600 K, the PAH monomers show a chemisorption tendency onto the nickel surface, which results in incipient soot particles. Chemical nucleation was observed at 2000 K where nickel-assisted dehydrogenation and chemisorption of PAH led to the growth of stable soot particles, which did not occur in the absence of Ni-clusters. At a high temperature (2500 K), nickel significantly accelerates the ring-opening and graphitization of PAH molecules and increases the size of the fullerene-type soot as compared to that of homogenous systems.
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http://dx.doi.org/10.1039/c9cp00354aDOI Listing
May 2019

Development of the ReaxFF Reactive Force Field for Inherent Point Defects in the Si/Silica System.

J Phys Chem A 2019 May 6;123(19):4303-4313. Epub 2019 May 6.

Department of Physics , Middle East Technical University , Ankara 06800 , Turkey.

We redeveloped the ReaxFF force field parameters for Si/O/H interactions that enable molecular dynamics (MD) simulations of Si/SiO interfaces and O diffusion in bulk Si at high temperatures, in particular with respect to point defect stability and migration. Our calculations show that the new force field framework (ReaxFF), which was guided by the extensive quantum mechanical-based training set, describes correctly the underlying mechanism of the O-migration in Si network, namely, the diffusion of O in bulk Si occurs by jumping between the neighboring bond-centered sites along a path in the (110) plane, and during the jumping, O goes through the asymmetric transition state at a saddle point. Additionally, the ReaxFF predicts the diffusion barrier of O-interstitial in the bulk Si of 64.8 kcal/mol, showing a good agreement with the experimental and density functional theory values in the literature. The new force field description was further applied to MD simulations addressing O diffusion in bulk Si at different target temperatures ranging between 800 and 2400 K. According to our results, O diffusion initiates at the temperatures over 1400 K, and the atom diffuses only between the bond-centered sites even at high temperatures. In addition, the diffusion coefficient of O in Si matrix as a function of temperature is in overall good agreement with experimental results. As a further step of the force field validation, we also prepared amorphous SiO (a-SiO) with a mass density of 2.21 gr/cm, which excellently agrees with the experimental value of 2.20 gr/cm, to model a-SiO/Si system. After annealing the a-SiO/Si system at high temperatures until below the computed melting point of bulk Si, the results show that ReaxFF successfully reproduces the experimentally and theoretically defined diffusion mechanism in the system and succeeded in overcoming the diffusion problem observed with ReaxFF, which results in O diffusion in the Si substrate even at the low temperature such as 300 K.
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http://dx.doi.org/10.1021/acs.jpca.9b01481DOI Listing
May 2019

Multiscale Modeling of Structure, Transport and Reactivity in Alkaline Fuel Cell Membranes: Combined Coarse-Grained, Atomistic and Reactive Molecular Dynamics Simulations.

Polymers (Basel) 2018 Nov 20;10(11). Epub 2018 Nov 20.

Department of Materials Science and Engineering, University of Utah, 122 South Central Campus Drive, Room 304, Salt Lake City, UT 84112, USA.

In this study, molecular dynamics (MD) simulations of hydrated anion-exchange membranes (AEMs), comprised of poly(-phenylene oxide) (PPO) polymers functionalized with quaternary ammonium cationic groups, were conducted using multiscale coupling between three different models: a high-resolution coarse-grained (CG) model; Atomistic Polarizable Potential for Liquids, Electrolytes and Polymers (APPLE&P); and ReaxFF. The advantages and disadvantages of each model are summarized and compared. The proposed multiscale coupling utilizes the strength of each model and allows sampling of a broad spectrum of properties, which is not possible to sample using any of the single modeling techniques. Within the proposed combined approach, the equilibrium morphology of hydrated AEM was prepared using the CG model. Then, the morphology was mapped to the APPLE&P model from equilibrated CG configuration of the AEM. Simulations using atomistic non-reactive force field allowed sampling of local hydration structure of ionic groups, vehicular transport mechanism of anion and water, and structure equilibration of water channels in the membrane. Subsequently, atomistic AEM configuration was mapped to ReaxFF reactive model to investigate the Grotthuss mechanism in the hydroxide transport, as well as the AEM chemical stability and degradation mechanisms. The proposed multiscale and multiphysics modeling approach provides valuable input for the materials-by-design of novel polymeric structures for AEMs.
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http://dx.doi.org/10.3390/polym10111289DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6401961PMC
November 2018