Prof. Dr. Eckhard Spohr

Theoretical Chemistry
University of Duisburg-Essen

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  • Studying the mechanism of phase separation in aqueous solutions of globular proteins via molecular dynamics computer simulations
    Brudar, S. and Gujt, J. and Spohr, E. and Hribar-Lee, B.
    Physical chemistry chemical physics : PCCP 23 (2021)
    Proteins are the most abundant biomacromolecules in living cells, where they perform vital roles in virtually every biological process. To maintain their function, proteins need to remain in a stable (native) state. Inter- and intramolecular interactions in aqueous protein solutions govern the fate of proteins, as they can provoke their unfolding or association into aggregates. The initial steps of protein aggregation are difficult to capture experimentally, therefore we used molecular dynamics simulations in this study. We investigated the initial phase of aggregation of two different lysozymes, hen egg-white (HEWL) and T4 WT* lysozyme and also human lens γ-D crystallin by using atomistic simulations. We monitored the phase stability of their aqueous solutions by calculating time-dependent density fluctuations. We found that all proteins remained in their compact form despite aggregation. With an extensive analysis of intermolecular residue-residue interactions we discovered that arginine is of paramount importance in the initial stage of aggregation of HEWL and γ-D crystallin, meanwhile lysine was found to be the most involved amino acid in forming initial contacts between T4 WT* molecules.
    view abstract10.1039/d0cp05160h
  • Computer modeling of semiconductor nanotubes for water splitting
    Kenmoe, S. and Spohr, E.
    Current Opinion in Electrochemistry 19 (2020)
    One-dimensional nanostructures such as nanorods and nanotubes (NTs) are regarded as promising materials for photoelectrochemical water splitting. Modeling the electronic properties of oxidic NTs with diameters in the range of 3–30 nm in contact with liquid water is challenging owing to the fact that the systems are too large for direct ab initio molecular dynamics simulations. Here we summarize recent efforts to develop strained two-dimensional model systems for pristine and doped titania NTs. We have studied their structural, optical and photoelectrochemical properties by a number of different techniques attributable to quantum mechanical density functional theory. © 2019 Elsevier B.V.
    view abstract10.1016/j.coelec.2019.10.013
  • Impact of Solvation on the Structure and Reactivity of the Co3O4 (001)/H2O Interface: Insights From Molecular Dynamics Simulations
    Kox, T. and Spohr, E. and Kenmoe, S.
    Frontiers in Energy Research 8 (2020)
    The spinel Co3O4 has many beneficial properties for potential use in catalysis. In operando, water is always present and alters the properties of the catalyst. We have used ab initio molecular dynamics to understand the effect of water and solvation on the structure and reactivity of the Co3O4 (001) A-type and B-type surface terminations. Water adsorbs on both terminations via a partial dissociative mode, and the A-termination is seen to be more reactive. On this surface, a higher degree of dissociation is observed in the topmost layers of the crystal in contact with water. Water dissociates more frequently on the Co2+ sites (about 75%) than on the adjacent Co3+ sites, where the degree of dissociation is about 50%. Increasing water coverage does not change the degree of water dissociation significantly. OH− adsorption on the Co2+ sites leads to a reduction of the amount of reconstruction and relaxation of the surface relative to the clean surface at room temperature. Proton transfer within the water films and between water molecules and surface has localized character. The B-terminated interface is less dynamic, and water forms epitactic layers on top of the Co3+ sites, with a dissociation degree of about 25% in the contact layer. © Copyright © 2020 Kox, Spohr and Kenmoe.
    view abstract10.3389/fenrg.2020.604799
  • Electrochemical CO Reduction: A Property of the Electrochemical Interface
    Bagger, A. and Arnarson, L. and Hansen, M.H. and Spohr, E. and Rossmeisl, J.
    Journal of the American Chemical Society 141 (2019)
    Electrochemical CO reduction holds the promise to be a cornerstone for sustainable production of fuels and chemicals. However, the underlying understanding of the carbon-carbon coupling toward multiple-carbon products is not complete. Here we present thermodynamically realistic structures of the electrochemical interfaces, determined by explicit ab initio simulations. We investigate how key CO reduction reaction intermediates are stabilized in different electrolytes and at different pH values. We find that the catalytic trends previously observed experimentally can be explained by the interplay between the metal surface and the electrolyte. For the Cu(100) facet with a phosphate buffer electrolyte, the energy efficiency is found to be limited by blocking of a phosphate anion, while in alkali hydroxide solutions (MOH, M = Na, K, Cs), OH∗ intermediates may be present, and at high overpotential the H∗ coverage limits the reaction. The results provide insight into the electrochemical interface structure, revealing the limitations for multiple-carbon products, and offer a direct comparison to experiments. © 2019 American Chemical Society.
    view abstract10.1021/jacs.8b08839
  • Electronic and optical properties of pristine, N- and S-doped water-covered TiO 2 nanotube surfaces
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Chemical Physics 150 (2019)
    For rational design and improvement of electronic and optical properties of water-splitting photocatalysts, the ability to control the band edge positions relative to the water redox potentials and the photoresponse as a function of environmental conditions is essential. We combine ab initio molecular dynamics simulations with ab initio many-body theoretical calculations to predict the bandgap and band edge energies, as well as the absorption spectrum of pristine and N- and S-doped TiO 2 nanotubes using the DFT+U and G 0 W 0 approaches. Both levels of theory show similar trends, and N+S-codoping appears to be the optimal system for photocatalytic water splitting both in dry and humid conditions. However, the effect is rather moderate. Compared to DFT+U, the enhanced many-body effects in the G 0 W 0 calculations push the absolute energies of the band edges to higher values and yield increased quasi-particle bandgaps in better agreement with experiment. In dry and humid conditions, the electronic bandgap for all systems is found to be in the range of 6.0-6.2 eV with a redshift from electronic gap to optical gap. The absorption spectra show an optical anisotropy and different absorption thresholds for different light polarizations. © 2019 Author(s).
    view abstract10.1063/1.5050090
  • First-Principles Evaluation of the Morphology of WS2 Nanotubes for Application as Visible-Light-Driven Water-Splitting Photocatalysts
    Piskunov, S. and Lisovski, O. and Zhukovskii, Y.F. and D'Yachkov, P.N. and Evarestov, R.A. and Kenmoe, S. and Spohr, E.
    ACS Omega 4 (2019)
    One-dimensional tungsten disulfide (WS2) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters dNT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV < Δϵgap < 2.6 eV). We have simulated the electronic structure of nanotubes with diameters up to 12.0 nm. The calculated top of the valence band and the bottom of the conduction band (ϵVB and ϵCB, respectively) have been properly aligned relatively to the oxidation (ϵO2/H2O) and reduction (ϵH2/H2O) potentials of water. Very narrow nanotubes (0.5 < dNT < 1.9 nm) are unsuitable for water splitting because the condition ϵVB < ϵO2/H2O < ϵH2/H2O < ϵCB does not hold. For nanotubes with dNT > 1.9 nm, the condition ϵVB < ϵO2/H2O < ϵH2/H2O < ϵCB is fulfilled. The values of ϵVB and ϵCB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree-Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)at(n, n) and (m, 0)at(n, 0) as well as triple-walled (l, l)at(m, m)at(n, n) and (l, 0)at(m, 0)at(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS2 NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap. © 2019 American Chemical Society.
    view abstract10.1021/acsomega.8b03121
  • Photooxidation of Water on Pristine, S- And N-Doped TiO2(001) Nanotube Surfaces: A DFT + U Study
    Kenmoe, S. and Spohr, E.
    Journal of Physical Chemistry C 123 (2019)
    Using density functional theory calculations, we study the structure, energetics, and the photoelectrochemical oxidation of water on pristine, S-, N-, and (N + S)-doped anatase TiO2(001) nanotube (NT) surfaces. We found that water adsorbs molecularly on pristine and S-doped surfaces, while N doping promotes dissociative adsorption (both in the presence and absence of the S codopant) and leads to more favorable adsorbate-substrate interactions. Under photoelectrochemical conditions, OH groups are the most stable species on each surface with decreasing stability in the sequence (N + S) ≈ N → S → pristine. Surface Ti5C are the active sites and the anion impurity sites are not structurally affected during the water oxidation reaction. Nanostructuring TiO2 by forming three monolayer-thick (3 ML) TiO2(001) NT surfaces and subsequent anion doping yield an overpotential drop from 1.31 V on the flat (2D) TiO2(001) surface to 0.90, 0.71, 0.94, and 0.96 V on pristine, S-, N-, and (N + S)-doped nanotube surfaces, respectively. This reduction is a consequence of the strain-induced weakening of hydroxyl adsorption on the NT surfaces; the presence of an N dopant atom does not change the overpotential relative to the pristine nanotube, irrespective of the presence of a codoped S atom, while single S doping produces a slight decrease of the overpotential by 0.2 V. In all cases, the overpotential-determining step is the hydroxyl group dehydrogenation. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b01166
  • Influence of Chain Length and Branching on the Structure of Functionalized Gold Nanoparticles
    Giri, A.K. and Spohr, E.
    Journal of Physical Chemistry C 122 (2018)
    Functionalized gold nanoparticles (GNPs) in aqueous NaCl solutions have been studied using molecular dynamics simulations to assess the suitability of various functionalization chemistries to effectively shield the metallic core. Alkane thiol chains of various chain length (Cl) containing 6, 12, 18, and 24 carbon atoms are grafted onto the surface of the gold core. We compare the properties of GNPs functionalized with nonpolar CH3-terminated and polar COO-- and NH3 +-terminated chains, where the nanoparticle charge is compensated by appropriate numbers of excess Na+ or Cl- counterions. In addition to linear chains, we also investigate branched Y-shaped chains with the branching sites at the 4th, 8th, or 12th carbon atom from the sulfur atom that connects the chain to the gold core. The penetration depth of water and ions into the diffuse hydrocarbon shell region and its dependence on chain length, branching, and terminating group is found to increase with decreasing chain length irrespective of termination. Long linear chains, however, tend to form bundles independent of terminal group and can thus leave fractions of the nanoparticle surface exposed to small molecules, whereas shorter and branched chains do not form bundles and can cover the GNPs more homogeneously. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b08590
  • Validation of a constrained 2D slab model for water adsorption simulation on 1D periodic TiO2 nanotubes
    Lisovski, O. and Kenmoe, S. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Spohr, E.
    Computational Condensed Matter 15 (2018)
    Solar light driven hydrogen evolution is one focus of modern materials research. Among the different emerging technologies, particular interest is devoted towards metal oxide photocatalysts in the form of various 1D nanostructures. Presently, the mismatch between regular structures that can be synthesized and the largest structures that are feasible for computer simulation is still very large. For example, an in-depth study of water adsorption on nanotube (NT) surfaces requires, in addition to DFT calculations, molecular dynamics simulations to take into account the disordered nature of the aqueous phase. To completely immerse even a very thin nanotube into an aqueous system requires very large system sizes, the modeling of which is computationally extremely expensive. On the other hand, the typical surface science approach when modeling planar interfaces is computationally much less demanding, but can be adequate only for NTs of very large diameters possessing low strain energies. Here, we present three simplified slab models derived from local NT geometries as approximations for inner and outer interfaces of thin TiO2 NTs. In order to describe the role of NT strain on the electronic structure of the water/NT interface, these models contain additional constraints on some of the atoms to alleviate some of the shortcomings of the slab approach. We use the energy of water adsorption, equilibrium interatomic distances, calculated densities of states (DOS) and Mulliken charge distribution as criteria for estimating the model quality. Specifically we analyze the suitability of this approach for the very thin TiO2 (001) slab and the thicker TiO2 (001) NT. © 2017 Elsevier B.V.
    view abstract10.1016/j.cocom.2017.11.004
  • Water Adsorption on Clean and Defective Anatase TiO 2 (001) Nanotube Surfaces: A Surface Science Approach
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Physical Chemistry B 122 (2018)
    We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO 2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH - group and an NH2 + cationic site on the surface. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcb.7b11697
  • Water Adsorption on Clean and Defective Anatase TiO2 (001) Nanotube Surfaces: A Surface Science Approach
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Physical Chemistry B 122 (2018)
    We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH- group and an NH2+ cationic site on the surface. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcb.7b11697
  • Cluster formation of NaCl in bulk solutions: Arithmetic vs. geometric combination rules
    Giri, A.K. and Spohr, E.
    Journal of Molecular Liquids 228 (2017)
    We have investigated the usability of three common ionic force fields, the AMBER-99, the OPLS-AA and the CHARMM-27 parameter sets for simulation of intermediate concentration NaCl solutions. We have found that the Amber and Opls force fields produce NaCl crystallites at concentrations between 1 and 2 m, when used with arithmetic combination rules to derive the Lennard-Jones σij,i≠j parameters. When switching to a geometric rule to derive these parameters, the NaCl solubility improves somewhat, but crystallisation still occurs at higher electrolyte concentrations. On the other hand, when using the Charmm force field, we observe no signs of crystallisation up to 2.0 m already for the arithmetic combination rules. In addition to the simulations with these ‘well-tempered’ parameter sets we have also performed simulations, in which the combination rules were applied individually for cation–anion, cation–(water) oxygen and anion–oxygen pairs. The altogether eight different parameter sets that can be obtained from combining three interactions with two combination rules have revealed that the Cl −–oxygen interactions are the most sensitive quantity. When switching from arithmetic to geometric combination rules, the value of the size parameter σij,i≠j is always smaller than the corresponding one for arithmetic averaging, which gives rise to larger Coulomb and thus larger total interaction energies. As a consequence, applying geometric combination rules to the Cl −–oxygen interactions improves solubility, applying it to the Cl −–Na + interactions reduces solubility and increases crystallisation; because the sodium cations are usually quite strongly solvated, the effect of combination rules is small for the cation–oxygen interactions. © 2016 Elsevier B.V.
    view abstract10.1016/j.molliq.2016.09.089
  • Developing adaptive QM/MM computer simulations for electrochemistry
    Dohm, S. and Spohr, E. and Korth, M.
    Journal of Computational Chemistry 38 (2017)
    We report the development of adaptive QM/MM computer simulations for electrochemistry, providing public access to all sources via the free and open source software development model. We present a modular workflow-based MD simulation code as a platform for algorithms for partitioning space into different regions, which can be treated at different levels of theory on a per-timestep basis. Currently implemented algorithms focus on targeting molecules and their solvation layers relevant to electrochemistry. Instead of using built-in forcefields and quantum mechanical methods, the code features a universal interface, which allows for extension to a range of external forcefield programs and programs for quantum mechanical calculations, thus enabling the user to readily implement interfaces to those programs. The purpose of this article is to describe our codes and illustrate its usage. © 2016 Wiley Periodicals, Inc. © 2016 Wiley Periodicals, Inc.
    view abstract10.1002/jcc.24513
  • First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production
    Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E. and D'yachkov, P.N.
    Vacuum 146 (2017)
    We have estimated theoretically the photocatalytic suitability of thinnest single-wall fluorite-structured titania (4,4) nanotube (NT) possessing three layers each (O-Ti-O) and doped by Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms substituted for host Ti atoms. For this goal, we have performed large-scale ab initio calculations on TiO2 NTs with three-layer morphology doped by 3d transition metals, using (i) the method of linear combination of atom-centered Gaussian-type orbitals (LCAO) based on the hybrid density functional theory (DFT) incorporating the Hartree-Fock (HF) exchange contribution (DFT+HF) and (ii) the method of linearized augmented cylindrical waves (LACW) with the muffin-tin approximation based on the local density functional approach (LDA). We have compared the ground state electronic structure, particularly the one-electron densities of states (DOSs) from the LCAO and LACW calculations for periodic arrangements of the 3d-metal dopant atoms. The results show clear evidence for a potential photocatalytic application for water splitting in the case of the Sc-doped titania nanotubes only. These NTs show both a reduced band gap of 2.0 eV relative to the pristine NT and an absence of defect-induced levels between the redox potentials of hydrogen and oxygen, so that electron-hole recombination becomes unlikely. Other 3d dopants with higher atomic number, although their band gap also covers the favorable green to orange region of the solar spectrum, are unsuitable because their defect-induced levels are positioned between the redox potential of oxygen and hydrogen, which can be expected to lead to rapid electron-hole recombination. © 2017 Elsevier Ltd
    view abstract10.1016/j.vacuum.2017.05.002
  • Merging Empirical Valence Bond Theory with Quantum Chemistry to Model Proton Transfer Processes in Water
    Dohm, S. and Spohr, E. and Korth, M.
    Electrocatalysis 8 (2017)
    Proton transfer processes in water are of fundamental importance for, among others, electrochemical proton discharge. Empirical valence bond (EVB) approaches were shown in the past to be a versatile tool for modeling complex phenomena such as proton discharge at metal electrodes. By replacing empirical fitting procedures with on-the-fly quantum chemistry (QC) calculations, we arrive at a transferable and systematically tunable description of proton transfer in water with EVB. [Figure not available: see fulltext.]. © 2017, Springer Science+Business Media, LLC.
    view abstract10.1007/s12678-017-0396-4
  • Molecular dynamics study of stability and disintegration of long rod-like micelles: Dodecyltrimethylammonium chloride in solutions of hydroxybenzoates
    Gujt, J. and Bešter-Rogač, M. and Spohr, E.
    Journal of Molecular Liquids 228 (2017)
    Recently it was found out that different positions of the hydroxylic group on hydroxybenzoate anions (HB) crucially affect the thermodynamics of the self-organization of the cationic surfactant dodecyltrimethylammonium chloride (DTAC) and also the structure of resulting aggregates. In our previous work, the properties of stable long cylindrical DTAC micelles in the presence of NaHB at 1:1 DTAC/NaHB molar ratio in aqueous solutions were investigated by atomistic molecular dynamics simulations. In the present work, we first study the decay of cylindrical DTAC micelles in water without added salt and then extend our research to systems with low NaHB concentrations (DTAC/NaHB molar ratios of 4:1 and 2:1) in order to approach the real experimental conditions more closely. The geometry and structural properties of DTAC micelles in water are investigated, and also the decomposition of long cylindrical micelles and the solvent accessible surface area of micelles is discussed. We observe that the initial DTAC micelle without NaHB quickly disintegrates into smaller stable spherical micelles. At the 2:1 DTAC/NaHB molar ratio we find all initial DTAC micelles to remain stable; however, their geometry deviates significantly from initial cylindrical one. Furthermore, it is observed that o-HB induces a more ordered internal structure of the micelle, and is more strongly oriented than the other two isomers, which agrees well with the experiments and observations reported in our previous work. When the NaHB concentration is decreased to 4:1 DTAC/NaHB molar ratio, an initial DTAC micelle disintegrates forming smaller aggregates of spherical or elongated shapes regardless of the nature of the HB isomer present. The microscopic structure of the resultant micelles is very similar to the structure observed at higher NaHB concentration, however, the effect of HB ions is smaller. It was also observed that the micelle remains stable longer in the presence of o-HB than in the presence of the other two isomers. © 2016 Elsevier B.V.
    view abstract10.1016/j.molliq.2016.09.067
  • Water Structure and Mechanisms of Proton Discharge on Platinum Electrodes: Empirical Valence Bond Molecular Dynamics Trajectory Studies
    Wiebe, J. and Spohr, E.
    Electrocatalysis 8 (2017)
    We present a comparative analysis of molecular dynamics trajectory studies of the influence of surface charge, ion strength, and ion adsorption on the interfacial water structure and the possible pathways of proton transport and discharge on negatively charged platinum(111) electrodes. The model used is a reactive force field based on a nine-state empirical valence bond model. It incorporates both proton transfer between water molecules and simultaneous electron and proton transfer to the metal (discharge). The interfacial water polarization is the result of the competition between the electrical field influence of the smooth surface charge and the point-like local charges of adsorbed positive or negative ions, which leads to variations of the prevalent proton discharge pathways depending on system composition. [Figure not available: see fulltext.]. © 2017, Springer Science+Business Media, LLC.
    view abstract10.1007/s12678-017-0398-2
  • Ab initio calculations of doped TiO2 anatase (101) nanotubes for photocatalytical water splitting applications
    Lisovski, O. and Chesnokov, A. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Materials Science in Semiconductor Processing 42 (2016)
    TiO2 (titania) is one of the promising materials for photocatalytic applications. In this paper we report on recently obtained theoretical results for N and S doped, as well as N+S co-doped 6-layer (101) anatase nanotube (NT). First principles calculations in our study have been performed using a modified B3LYP hybrid exchange-correlation functional within density functional theory (DFT). Here we discuss the energy of defect formation mechanism and electronic band structure for nanotubes under study. We also report on influence of dopant concentration on the NT's band structure and discuss the defect-defect interactions. © 2015 Elsevier Ltd.
    view abstract10.1016/j.mssp.2015.09.003
  • Quantum chemical simulations of doped ZnO nanowires for photocatalytic hydrogen generation
    Zhukovskii, Y.F. and Piskunov, S. and Lisovski, O. and Spohr, E. and Evarestov, R.A.
    Physica Status Solidi (B) Basic Research 253 (2016)
    Zinc oxide (ZnO) is considered in general as a promising material for solar water splitting. Its wurtzite-structured bulk samples, however, can be considered as active for photocatalytic applications only under UV irradiation, where they possess ∼1% efficiency of sunlight energy conversion due to their wide band gap (3.4 eV). Although pristine ZnO nanowires (NWs) possess noticeably narrower band gaps than the bulk, the tendency of band gap reduction with increasing NW diameter is insufficient, and further modification is required. We have contributed to filling this gap by performing a series of ab initio calculations on ZnO NWs of different diameters (dNW), which are mono-doped by metal (Ag) and non-metal atoms (C, N) or contain oxygen vacancies with varied concentration (∼3 vs. ∼6%). To reproduce qualitatively the energies of one-electron states of nanowires in our calculations, the hybrid DFT + HF Hamiltonian has been used, based on the PBE0 exchange-correlation functional. We have analyzed changes in the electronic structure induced in a few defect composition scenarios, showing that, for specific concentrations and locations of the dopants, the optical absorption peak of doped ZnO can be shifted to the visible light range with promising efficiency. In agreement with experimental observation, the most significant results have been achieved for carbon-doped ZnO nanowires. They possess the highest photocatalytic suitability, since the band gap is reduced in this case down to 2.1–2.2 eV (for nanowire diameters of 2.9–3.5 nm), which corresponds to optimal 15–17% efficiency of solar energy conversion. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201600452
  • Structure and Stability of Long Rod-like Dodecyltrimethylammonium Chloride Micelles in Solutions of Hydroxybenzoates: A Molecular Dynamics Simulation Study
    Gujt, J. and Bešter-Rogač, M. and Spohr, E.
    Langmuir 32 (2016)
    The relative position of the hydroxylic and carboxylic groups in the isomeric hydroxybenzoate (HB) anions is experimentally known to have a large impact on the thermodynamics of micellization of cationic surfactants, such as dodecyltrimethylammonium chloride (DTAC), and on the structure of the resulting micelles. To understand the effect of the different isomers on the molecular level, we employed atomistic molecular dynamics simulations to study systems containing infinitely long cylindrical DTAC micelles in aqueous solutions of the sodium salts of all three isomers of HB at a temperature and a pressure of 298.15 K and 1 atm. In all studied systems, the number of DTAC unimers is identical to the number of HB anions. At this concentration, the initially cylindrical micelles remain stable, irrespective of the nature of the isomer, whereas micelles rapidly disintegrated in the absence of HB anions. The HB isomers decrease the line density of unimers along the micellar axis and its concomitant thickness in the order o-HB > m-HB > p-HB. It is further observed that o-HB anions penetrate more deeply into the micellar core, induce a more ordered internal structure of the micelle, and are oriented more strongly than the other two isomers. In addition, the ortho isomer shows two different preferential orientations with respect to the radial direction of the cylindrical micelle; it can either be incorporated almost completely into the micelle or it can be attached through hydrogen bonding to one of those o-HB anions that are already incorporated into the micelle, and thus stick out of the micellar surface. © 2016 American Chemical Society.
    view abstract10.1021/acs.langmuir.6b02076
  • Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications
    Chesnokov, A. and Lisovski, O. and Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Physica Scripta 90 (2015)
    In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO<inf>2</inf> nanotubes, undoped, doped, and codoped with N<inf>O</inf> and S<inf>O</inf> atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree-Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO<inf>2</inf> nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen 2p character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O<inf>2</inf>/H<inf>2</inf>O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level. © 2015 The Royal Swedish Academy of Sciences.
    view abstract10.1088/0031-8949/90/9/094013
  • C-, N-, S-, and Fe-Doped TiO2 and SrTiO3 Nanotubes for Visible-Light-Driven Photocatalytic Water Splitting: Prediction from First Principles
    Piskunov, S. and Lisovski, O. and Begens, J. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Journal of Physical Chemistry C 119 (2015)
    The ground state electronic structure and the formation energies of both TiO<inf>2</inf> and SrTiO<inf>3</inf> nanotubes (NTs) containing C<inf>O</inf>, N<inf>O</inf>, S<inf>O</inf>, and Fe<inf>Ti</inf> substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO<inf>2</inf> NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H<inf>2</inf>O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N<inf>O</inf>+S<inf>O</inf> codoped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that ε<inf>HOIL</inf> < ε<inf>O<inf>2</inf>/H<inf>2</inf>O</inf> < ε<inf>H+/H<inf>2</inf></inf> < ε<inf>LUIL</inf>), thus decreasing the photon energy required for splitting of H<inf>2</inf>O molecule. For C- and Fe-doped TiO<inf>2</inf> NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO<inf>3</inf> NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b03691
  • Conformational Equilibria of Organic Adsorbates on Nanostructures in Aqueous Solution: MD Simulations
    Giri, A.K. and Spohr, E.
    Journal of Physical Chemistry C 119 (2015)
    We have performed atomistic molecular dynamics (MD) simulations of gold nanoparticles (GNPs) in aqueous NaCl solution. Alkanethiol chain-covered GNPs at grafting densities between approximately one-third and full coverage were studied with nonpolar CH3 and charged COO- and NH3 terminations. Special attention was given to the penetration depth of water and ions into the diffuse shell of the functionalized alkanethiol chains and its dependence on grafting density and functionalization. Solutions with polar terminations were neutralized by an excess of Na+ and Cl- ions. The penetration of water and ions into the hydration shell increases with decreasing grafting density irrespective of termination. High grafting densities lead to more extended hydrocarbon chains. Charged functionalized GNPs produce nonmonotonous counter charge distributions with reduced ion mobility. Partial replacement of first shell solvation water by the charged groups leads to a drastic increase in torsional relaxation times of the chain termini. Due to the large curvature of the GNPs with a diameter of 2 nm, gold cores remain accessible to both ions and water even at the highest studied grafting densities of about 5 chains/nm2. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b06249
  • Electrolyte effects in a model of proton discharge on charged electrodes
    Wiebe, J. and Kravchenko, K. and Spohr, E.
    Surface Science 631 (2015)
    We report results on the influence of NaCl electrolyte dissolved in water on proton discharge reactions from aqueous solution to charged platinum electrodes. We have extended a recently developed combined proton transfer/proton discharge model on the basis of empirical valence bond theory to include NaCl solutions with several different concentrations of cations and anions, both stoichiometric (1:1) compositions and non-stoichiometric ones with an excess of cations. The latter solutions partially screen the electrostatic potential from the surface charge of the negatively charged electrode. 500-1000 trajectories of a discharging proton were integrated by molecular dynamics simulations until discharge occurred, or for at most 1.5 ns. The results show a strong dependence on ionic strength, but only a weak dependence on the screening behavior, when comparing stoichiometric and non-stoichiometric solutions. Overall, the Na+ cations exert a more dominant effect on the discharge reaction, which we argue is likely due to the very rigid arrangements of the cations on the negatively polarized electrode surface. Thus, our model predicts, for the given and very high negative surface charge densities, the fastest discharge reaction for pure water, but obviously cannot take into account the fact that such high charge densities are even more out of reach experimentally than for higher electrolyte concentrations. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.susc.2014.06.016
  • Double layer effects in a model of proton discharge on charged electrodes
    Wiebe, J. and Spohr, E.
    Beilstein Journal of Nanotechnology 5 (2014)
    We report first results on double layer effects on proton discharge reactions from aqueous solutions to charged platinum electrodes. We have extended a recently developed combined proton transfer/proton discharge model on the basis of empirical valence bond theory to include specifically adsorbed sodium cations and chloride anions. For each of four studied systems 800-1000 trajectories of a discharging proton were integrated by molecular dynamics simulations until discharge occurred. The results show significant influences of ion presence on the average behavior of protons prior to the discharge event. Rationalization of the observed behavior cannot be based solely on the electrochemical potential (or surface charge) but needs to resort to the molecular details of the double layer structure. © 2014 Wiebe and Spohr; licensee Beilstein-Institut.
    view abstract10.3762/bjnano.5.111
  • Ion mobility and clustering of sodium hydroxybenzoates in aqueous solutions: A molecular dynamics simulation study
    Gujt, J. and Podlipnik, Č. and Bešter-Rogač, M. and Spohr, E.
    Physical Chemistry Chemical Physics 16 (2014)
    The relative position of the hydroxylic and the carboxylic group in the isomeric hydroxybenzoate (HB) anions is known to have a large impact on transport properties of this species. It also influences crucially the self-organisation of cationic surfactants. In this article a systematic investigation of aqueous solutions of the ortho, meta, and para isomers of the HB anion is presented. Molecular dynamics simulations of all three HB isomers were conducted for two different concentrations at 298.15 K and using two separate water models. From the resulting trajectories we calculated the self-diffusion coefficient of each isomer. According to the calculated self-diffusion coefficients, isomers were ranked in the order o-HB > m-HB > p-HB at both concentrations for both the used SPC and SPC/E water models, which agrees very well with the experiment. The structural analysis revealed that at lower concentration, where the tendency for dimerisation or cluster formation is low, hydrogen bonding with water determines the mobility of the HB anion. o-HB forms the least hydrogen bonds and is therefore the most mobile, and p-HB, which forms the most hydrogen bonds with water, is the least mobile isomer. At higher concentration the formation of clusters also needs to be considered. The ortho isomer predominantly forms dimers with 2 hydrogen bonds per dimer between one OH and one carboxylate group of each anion. m-HB mostly forms clusters of sizes around 5 and p-HB forms clusters of sizes even larger than 10, which can be either rings or chains. © 2014 The Owner Societies.
    view abstract10.1039/c4cp02425g
  • Molecular dynamics studies of poly(N-isopropylacrylamide) endgrafted on the surfaces of model slab pores
    Lorbeer, L. and Alaghemandi, M. and Spohr, E.
    Journal of Molecular Liquids 189 (2014)
    We report first results of a systematic study of the properties of thermo-responsive polymer chains of poly(N-isopropylacrylamide) (PNIPAM), which are endgrafted onto the inner surfaces of a slab pore of approximately 9 nm width. We have systematically varied the strength of the PNIPAM-surface interaction energy to estimate the variation of the extent of the thermo-responsive effect on different surfaces. For weak to intermediate PNIPAM-surface interactions, the MD simulations show thermo-responsive behavior as characteristic changes of the radius of gyration and other measures of the polymer structure and polymer-water interactions, when comparing simulations below (at 280 K) and above (at 320 K) the lower critical solution temperature of PNIPAM, which is at 305 K. When the PNIPAM-surface interactions become stronger, the polymer loses its thermo-responsive behavior and is adsorbed flatly on the pore walls at both investigated temperatures. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.molliq.2013.05.022
  • A molecular dynamics study of poly(N-isopropylacrylamide) endgrafted on a model cylindrical pore surface
    Alaghemandi, M. and Spohr, E.
    RSC Advances 3 (2013)
    Structure and dynamic behavior of the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) endgrafted onto the inner surface of a simple cylindrical pore model that resembles a carbon nanotube (CNT) with a diameter of 8.4 nm is studied as a function of temperature, of surface-polymer interaction strength, and of pore water content. A free PNIPAM chain in water shows thermo-responsive behavior with a lower critical solution temperature (LCST) of about 305 K. We have investigated two different strengths of PNIPAM-pore interactions. In the strong interaction case, which corresponds to force-field parameters taken without change from the AMBER force field, the endgrafted PNIPAM chain collapses onto the surface at all temperatures studied and hence does not adopt a brush structure. In the weak interaction case the PNIPAM-pore interaction strengths were scaled by a factor of 10, and the temperature-responsive behavior of the PNIPAM chain re-emerges. End-to-end distances, radii of gyration, density profiles, number of hydrogen bonds, and radial distribution functions demonstrate the temperature-dependent structural changes of endgrafted PNIPAM in the pore. Analysis of the translational motion of water molecules in the pore shows that the ratio of the water self diffusion coefficient in a pore with a free pore surface relative to the self diffusion coefficient in a pore containing an end-grafted PNIPAM molecule is less strongly reduced above the LCST than below the LCST, where the chain is in a more extended state. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3ra22266g
  • A new class of nanoengines based on thermoresponsive polymers: Conceptual design and behavior study
    Alaghemandi, M. and Spohr, E.
    Chemical Physics Letters 581 (2013)
    A model nanoengine based on endgrafted Poly(N-isopropylacrylamide) (PNIPAM) on graphene-like sheets is proposed. The nanoengine consists of a water-filled slab and four PNIPAM chains, which are at one end grafted to one of the slab walls and on the other end to a mobile square graphene 'piston'. The basis of the reciprocating motion of the piston is the reversible coil-to-globule transition of polymer chains when changing the temperature of the aqueous environment. Molecular dynamics simulations have been used to investigate the behavior of the proposed system at the full atomistic level. At temperatures below the lower critical solution temperature (LCST) PNIPAM chains are swollen and the nanopiston is in an expanded open state. Above the LCST, the PNIPAM chains are shrunken and the piston is retracted. The studied nanopiston exhibits an amplitude of approximately 10 Å when the temperature is reduced from 310 to 300 K or increased from 300 to 310 K with a frequency of about 10 9 rotations per minute; however the efficiency is very low. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cplett.2013.07.017
  • Molecular simulations of hydrated proton exchange membranes: The structure
    Marchand, G. and Bopp, P.A. and Spohr, E.
    Zeitschrift fur Naturforschung - Section C Journal of Biosciences 68 A (2013)
    The structure of two hydrated proton exchange membranes for fuel cells (PEMFC), Nafion® (Dupont) and Hyflon® (Solvay), is studied by all-atom molecular dynamics (MD) computer simulations. Since the characteristic times of these systems are long compared to the times for which they can be simulated, several different, but equivalent, initial configurations with a large degree of randomness are generated for different water contents and then equilibrated and simulated in parallel. A more constrained structure, analog to the newest model proposed in the literature based on scattering experiments, is investigated in the same way. One might speculate that a limited degree of entanglement of the polymer chains is a key feature of the structures showing the best agreement with experiment. Nevertheless, the overall conclusion remains that the scattering experiments cannot distinguish between the several, in our view equally plausible, structural models. We thus find that the characteristic features of experimental scattering curves are, after equilibration, fairly well reproduced by all systems prepared with our method. We thus study in more detail some structural details. We attempt to characterize the spatial and size distribution of the water rich domains, which is where the proton diffusion mostly takes place, using several clustering algorithms. © 2013 Verlag der Zeitschrift für Naturforschung, Tübingen.
    view abstract10.5560/ZNA.2012-0089
  • Probing the structures of hydrated nafion in different morphologies using temperature-accelerated molecular dynamics simulations
    Lucid, J. and Meloni, S. and MacKernan, D. and Spohr, E. and Ciccotti, G.
    Journal of Physical Chemistry C 117 (2013)
    We perform combined temperature-accelerated and standard molecular dynamics (MD) simulations to elucidate the atomistic structure of hydrated Nafion (hydration level λ = 6.5) in the slab and cylinder morphologies. Our samples are initially made of elongated Nafion strands with a relatively small fraction of gauche defects. Our simulations show that even very long (>50 ns) "brute force" MD simulations are insufficient to reach equilibrated structures. In fact, ∼30-40 ns long temperature-accelerated molecular dynamics (TAMD) simulations started from the same initial conditions explore more stable (lower potential energy) stationary structures. The effect of TAMD is to allow a rearrangement of the backbone consisting of an increase in gauche defects, which cannot be obtained by "brute force" MD because the trans-gauche transition is a rare event at room temperature. Associated with the backbone rearrangement, we observe a change in the structure of the water layers/tubes as measured by the size and number of bulk (four-fold coordinated water molecules) and surface-like water clusters. At equilibrium, the mean size of bulk-like water clusters is small, typically between 10 and 20 molecules, depending on the morphology. Larger clusters are also present in our samples, the largest being made of ∼350 molecules, but even the latter is too small for percolation. This suggests that the proton transport through each morphology might be a two-step process: Grotthuss-like within bulk-like water clusters and of a different type (e.g., diffusive or even transport across fluctuatively opening necks) between clusters. © 2012 American Chemical Society.
    view abstract10.1021/jp309038n
  • Probing the temperature dependence of proton transfer to charged platinum electrodes by reactive molecular dynamics trajectory studies
    Schmickler, W. and Wilhelm, F. and Spohr, E.
    Electrochimica Acta 101 (2013)
    We have performed reactive trajectory calculations of proton discharge on charged platinum surfaces as a function of temperature and charge. A recently developed 9-state empirical valence bond model has been employed. The temperature dependence follows an Arrhenius law with activation energies in the range of 0.1 eV. The activation energy for the discharge reaction decreases significantly with increasing driving force as modeled by an increasingly negative surface charge on the electrode. The analysis shows that the average orientation of molecules in the adsorbed water layer reacts to the approaching proton. Within increasing temperature, configurations become more prevalent which facilitate fast proton transfer by Grotthuss style proton hops from the second to the first layer. This effect becomes more pronounced near more negatively charged surfaces and leads to the computed reduction of the activation energy. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2013.01.146
  • Why silver deposition is so fast: Solving the enigma of metal deposition
    Pinto, L.M.C. and Spohr, E. and Quaino, P. and Santos, E. and Schmickler, W.
    Angewandte Chemie - International Edition 52 (2013)
    A perfect match: Silver deposition is one of the fastest electrochemical reactions, even though the Ag+ ion loses more than 5 eV solvation energy in the process. This phenomenon, an example of the enigma of metal deposition, was investigated by a combination of MD simulations, DFT, and specially developed theory. At the surface, the Ag+ ion experiences a strong interaction with the sp band of silver, which catalyzes the reaction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201301998
  • Molecular dynamics investigation of the thermo-responsive polymer poly(N-isopropylacrylamide)
    Alaghemandi, M. and Spohr, E.
    Macromolecular Theory and Simulations 21 (2012)
    Using molecular dynamics simulations with an OPLS force field, the lower critical solution temperature (LCST) of single- and multiple-chain PNIPAM solutions in water is investigated. The sample containing ten polymer chains shows a sudden drop in size and volume at 305 K. Such an effect is absent in the single-chain system. Large fluctuations of the physical properties of a short single-chain prevent any clear detection of the LCST for the chosen model system, at least on the time scale of 200 ns. The results provide evidence that a critical number of PNIPAM monomer units must be present in the simulated system before MD simulations are capable to detect conformational changes unambiguously. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mats.201100071
  • Sampling excited state dynamics: Influence of HOOP mode excitations in a retinal model
    Klaffki, N. and Weingart, O. and Garavelli, M. and Spohr, E.
    Physical Chemistry Chemical Physics 14 (2012)
    Zero point energy and classical thermal sampling techniques are compared in semi-classical photodynamics of the pentadienyliminium cation, a minimal retinal model. Using both methods, the effects of vibrational hydrogen-out-of-plane (HOOP) excitations on the photo-reactivity are probed at the ab initio CASSCF level. With 2376 individual trajectories the calculations reveal a clear picture of the relation between the excited state reaction coordinate, surface crossing and product generation. The productivity is strongly coupled with hydrogen torsion and the number of hopping attempts before the molecule finally decays. © 2012 the Owner Societies.
    view abstract10.1039/c2cp41994g
  • Thermal conductivity of polyamide-6,6 in the vicinity of charged and uncharged graphene layers: A molecular dynamics analysis
    Alaghemandi, M. and Gharib-Zahedi, M.R. and Spohr, E. and Böhm, M.C.
    Journal of Physical Chemistry C 116 (2012)
    The thermal conductivity (λ) of nanoconfined polyamide-6,6 (PA) oligomers in polymer-graphene nanocomposites has been investigated by reverse nonequilibrium molecular dynamics (RNEMD) simulations. The preferential alignment of the PA chains parallel to the graphene plane as well as their elongation implies that λ of the polymer in nanocomposites is larger than that in the neat polymer system. The ordering of the polymer phase is enhanced in an arrangement of charged graphene surfaces made of one layer with a charge deficit and one with a charge excess. The consequence of the enhanced polymer ordering as well as the denser packing is an increase in λ in the polymer network. Differences in the thermal conductivity for an armchair and zigzag arrangement of the graphene sheets in the direction of the heat transfer are almost negligible. In contrast with this insensitivity, the present RNEMD simulations predict the largest value of λ for composites with the smallest number of PA chains between adjacent graphene sheets. The modifications in the polymer thermal conductivity are rationalized via several structural parameters such as PA bond orientation relative to the graphene sheets, end-to-end distance of polymer chains, and density profiles. © 2012 American Chemical Society.
    view abstract10.1021/jp301452z
  • Ab initio molecular dynamics of proton networks in narrow polymer electrolyte pores
    Ilhan, M.A. and Spohr, E.
    Journal of Physics Condensed Matter 23 (2011)
    It is well established that proton conductivity in fuel cell membrane materials such as Nafion decreases strongly with decreasing water content. Proton transport in almost dry membranes is thought to proceed through narrow channels. In the present work we investigate proton structure and dynamics in two narrow cylindrical pores, which differ by their radius and the spacing of SO3H groups inside the channel. Pores are modelled through eight CF3CF3 and four CF3SO3H entities in a helical arrangement. The water content λ (the ratio between the number of water molecules and the number of sulfonic acid groups) in the pores varies between 2.5 and 4.5. We observe a transition from the undissociated acid at very low λ through more or less localized H3O+ entities to more delocalized H5O2 + entities for the investigated range of λ. In the narrower pore, where S-S distances vary in a more favourable range (between 6 and 8.5) than in the wider pore, we find that the molecular mobility is significantly higher, even at a rather high density of water molecules inside the pore. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/23/23/234104
  • Hydrogen bonding in narrow protonated polymer electrolyte pores
    Ilhan, M.A. and Spohr, E.
    Journal of Electroanalytical Chemistry 660 (2011)
    Proton conductivity in fuel cell membrane materials such as Nafion® decreases dramatically with decreasing water content. At very low water content proton transport is thought to occur through narrow necks, which can be either static or fluctuatively formed temporarily. In the present work we investigate the properties of hydrogen bonding and protons in a one-dimensional narrow model pore by using ab initio Car-Parrinello molecular dynamics. The pore consists of eight suitably arranged CF3-CF3 and four CF 3-SO3H entities and is filled with water at varying water content λ (the ratio between the number of water molecules and the number of sulfonic acid groups) between 2.5 and 4.5. Proton mobilization in this pore occurs in two steps. First, around λ = 3 sulfonic acid groups dissociate to form sulfonate groups and hydronium ions which form mostly contact ion pairs. Second, increasing the water content to λ = 4.5 leads to an increase of the population of Zundel-like H5O2+ configurations with more or less symmetrically shared protons. Simultaneously, the number of hydrogen bonds increases and the hydrogen bond network becomes more liquid-like. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jelechem.2011.03.035
  • Modeling proton transfer to charged silver electrodes
    Wilhelm, F. and Schmickler, W. and Nazmutdinov, R. and Spohr, E.
    Electrochimica Acta 56 (2011)
    Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study proton transfer from an aqueous solution to an Ag(1 1 1) surface. DFT is applied to study Ag-water and Ag-hydronium interactions as well as proton transfer for small systems based on the cluster model. The data gained are then used to adjust an empirical Ag-water interaction potential and to reparametrize an empirical valence-bond (EVB) model, which has been successfully applied for the study of proton transfer to a Pt(1 1 1) surface before. Employing these force fields in MD simulations enables dynamic modeling of the electrolyte-metal interface on a scale large enough to give realistic results. Results from a MD trajectory study on Ag(1 1 1) are reported and compared to the analogous study for platinum. Low discharge rates on Ag(1 1 1) are observed, and the potential range for hydrogen evolution can be estimated. The different behavior relative to Pt(1 1 1) can be traced to features of the respective potential energy surfaces and to the different structural properties of the aqueous/metallic interfaces. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2011.04.036
  • Oxygen adsorption at La 1-xSr xMnO 3(001) surfaces: Predictions from first principles
    Piskunov, S. and Jacob, T. and Spohr, E.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The interaction of atomic and molecular oxygen with MnO 2- and La(Sr)O-terminated (001) surfaces of Sr-doped LaMnO 3 has been studied using the B3LYP hybrid exchange-correlation functional within the framework of density functional theory (DFT). Using the obtained binding energies in conjunction with the ab initio thermodynamics we predict that adsorbed O atoms stabilize the surface of an LSM cathode at typical SOFC working conditions (T=1100 K, pO 2=0.2 p 0). The most favorable oxygen adsorption sites have been found to be atop surface Mn atoms on the MnO 2-terminated surface and on the hollow positions of the La(Sr)O-terminated LSM(001) surface. An O 2 molecule adsorbed at a hollow position of the La(Sr)O-terminated surface is readily dissociated, which adds a substantial contribution to the ionic transport at the LSM surface. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.073402
  • SrTiO3 nanotubes with negative strain energy predicted from first principles
    Piskunov, S. and Spohr, E.
    Journal of Physical Chemistry Letters 2 (2011)
    On the basis of hybrid density functional theory calculations, we predict that the most energetically favorable single-walled SrTiO3 nanotubes with negative strain energy can be folded from SrTiO3 (110) nanosheets of rectangular morphology. Further formation of multiwalled tubular nanostructure with interwall distance of ∼0.46 nm yields an additional gain in energy of 0.013 eV per formula unit. (The formation energy of the most stable nanotube is 1.36 eV/SrTiO3.) Because of increase in the Ti-O bond covalency in the outer shells, SrTiO3 nanotubes can demonstrate an enhancement of their adsorption properties. Quantum confinement leads to a widening of the energy band gap of single-walled SrTiO3 nanotubes (∼6.1 eV) relative to the bulk (∼3.6 eV), which makes them attractive for further band gap engineering. © 2011 American Chemical Society.
    view abstract10.1021/jz201050e
  • Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study
    Wilhelm, F. and Schmickler, W. and Spohr, E.
    Journal of Physics Condensed Matter 22 (2010)
    A recently developed empirical valence bond(EVB) model for proton transfer on Pt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has been applied in molecular dynamics(MD) simulations of a water film in contact with a charged Pt surface. A total of seven negative surface charge densities σ between - 7.5 and - 18.9νCcm-2 were investigated. For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab were sampled, and the trajectories were integrated until discharge of a proton occurred on the charged surfaces. We have calculated the mean rates for discharge and for adsorption of solvated protons within the adsorbed water layer in contact with the metal electrode as a function of surface charge density. For the less negative values of σ we observe a Tafel-like exponential increase of discharge rate with decreasing σ. At the more negative values this exponential increase levels off and the discharge process is apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwise proton transfer: first, a proton is transferred from the bulk into the contact water layer, which is followed by transfer of a proton to the charged surface and concomitant discharge. At the more negative surface charge densities the proton transfer into the contact water layer and the transfer of another proton to the surface and its discharge occur almost simultaneously. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/22/17/175001
  • density functional theory

  • electronic structure

  • moddeling and simulation

  • molecular dynamics

  • nanostructures

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