Search

Herein, we report nanosecond, single-pulse laser post-processing (PLPP) in a liquid flat jet with precise control of the applied laser intensity to tune structure, defect sites, and the oxygen evolution reaction (OER) activity of mesostructured Co3O4. High-resolution X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) are consistent with the formation of cobalt vacancies at tetrahedral sites and an increase in the lattice parameter of Co3O4after the laser treatment. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) further reveal increased disorder in the structure and a slight decrease in the average oxidation state of the cobalt oxide. Molecular dynamics simulation confirms the surface restructuring upon laser post-treatment on Co3O4. Importantly, the defect-induced PLPP was shown to lower the charge transfer resistance and boost the oxygen evolution activity of Co3O4. For the optimized sample, a 2-fold increment of current density at 1.7 V vs RHE is obtained and the overpotential at 10 mA/cm2decreases remarkably from 405 to 357 mV compared to pristine Co3O4. Post-mortem characterization reveals that the material retains its activity, morphology, and phase structure after a prolonged stability test.

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.

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.

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 CH3and 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.

The ground state electronic structure and the formation energies of both TiO2and SrTiO3nanotubes (NTs) containing CO, NO, SO, and FeTisubstitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO2NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H2O 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 NO+SOcodoped 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 εHOIL< ϵO2/H2O< ϵH+/H2< ϵLUIL), thus decreasing the photon energy required for splitting of H2O molecule. For C- and Fe-doped TiO2NTs, 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 SrTiO3NTs 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.

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.

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.

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.

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.

An empirical valence-bond (EVB) model is developed to describe the transfer of a proton from aqueous solution bulk to a metal electrode surface. The results of density functional calculations are used for parametrizing the model for a Pt(111) surface, which was chosen as a model system. Thereby, the metal surface is addressed in terms of the cluster model. The developed EVB model makes it possible to perform large-scale molecular dynamics (MD) simulations for a metal/electrolyte solution interface. Using MD we explore the proton transfer to a Pt(111) electrode surface with different negative surface charge densities. Preliminary conclusions are reported to elucidate some aspects of the proton transfer mechanism.

We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pores in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydration of the pore. We have performed five simulations of systems between 19 and 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two layers of water molecules are affected strongly by the interactions with the glass surface. With decreasing degree of hydration an increasing volume in the center of the pore is devoid of water molecules. At 96 % hydration the center is a continuous and homogeneous region that has, however, a lower density than bulk water at ambient conditions. A well-pronounced mobility profile exists, where molecules in the center of the pores have substantially higher self diffusion coefficients than molecules on the pore surface. The spectral densities of center of mass and hydrogen atom motion show the signature of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk water. The molecular dynamics results are in good agreement with recent experiments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative contribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages.

Molecular dynamics (MD) simulation studies of the mercury-water interface are presented. A slab of water molecules, about 25Åwide, confined by mercury phases, is investigated. A rigid water model and a flexible water model are used to describe the water-water interactions. Recently, we parameterized ab initio calculations on mercury-water clusters to describe the mercury-water potential. The liquid mercury phase is approximated as a rhombohedral lattice. The mercury-mercury interaction is modelled by a harmonic potential. Our results show a significant influence of the mercury phase on the structural and dynamic properties of the water phase. The structure of the interface is analysed in terms of density functions, radial distribution functions, orientation of the water molecules, potential drop, and hydrogen bonding characteristics.

We present recent calculations of pair correlation functions and partial structure factors derived from molecular dynamics simulations of large bulk water systems (up to 12 167 water molecules). We observe small amplitude oscillations of the pair correlation functions in the range between 10 and 20 Ä.

Cover:Responsive macromolecules are materials which undergo a physical change in a controllable, reproducible, and reversible manner in response to an external stimulus, for example, change of solvent, pH, or temperature. The temperature‐responsive polymer poly(N‐isopropylacrylamide), PNIPAM, is investigated in aqueous solution using molecular dynamics (MD) simulations in order to determine its lower critical solution temperature (LCST) and to study the physical properties below and above this temperature. A critical number of PNIPAM monomer units must be present before MD simulations are capable of detecting conformational changes. Further details can be found in the article by M. Alaghemandi and E. Spohr* on page 106.

For Abstract see ChemInform Abstract in Full Text.

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.)