Cross-over mechanism of the melting transition in monolayers of alkanes adsorbed on graphite and the universality of energy scaling

http://arxiv.org/ftp/arxiv/papers/0902/0902.4422.pdfThe interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In molecular simulations often ad-hoc values for the scaling factor (SF) are adopted without adequate justification. In this letter we demonstrate for the first time that the precise value of the SF has direct consequences on the critical properties and mechanisms of systems undergoing a phase transition. By analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not a universal feature, that it monotonically decreases with the molecular length, and that it drives a cross-over between two distinct mechanisms for melting in such systems.Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5) for the support of this research. This material is based upon work supported in part by the Department of Energy under award number DE-FG02-07ER46411. Computational resources were provided by the University of Missouri Bioinformatics Consortium

Structural and energetic factors in designing a perfect nano-porous sorbent for hydrogen storage [abstract]

Only abstract of poster available.Track IV: Materials for Energy ApplicationsCarbons are one of several promising groups of materials for hydrogen storage by adsorption. However, the heat of hydrogen physisorption in such materials is low, in the range of about 4-8 kJ/mol which limits the total amount of hydrogen adsorbed at P = 100 bar to ~2 wt% at room temperature and about ~10 wt% at 77 K. To get better storage capacity, the adsorbing surfaces must be modified, either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. Here we analyze the variation of interaction energy between a molecule of hydrogen and graphene-based sorbents prepared as hypothetical modifications of the graphene layer. In particular, we show that partial substitution of carbons (for example, by boron) modifies both the symmetry of the energy landscape and strength of hydrogen physisorption. The effect of substituent extends over several sites of graphene lattice making the surface more heterogeneous. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411."This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411.

Melting of hexane monolayers adsorbed on graphite: the role of domains and defect formation

http://arxiv.org/ftp/arxiv/papers/0903/0903.1065.pdfWe present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduces all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) fully atomistic representation of hexane; (ii) explicit site-by-site interaction with carbon atoms in graphite; (iii) CHARMM force field with carefully chosen adjustable parameters of non-bonded interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation time of ca. 10 CPU-years. This has allowed us to determine correctly the mechanism of the transition: molecular reorientation within lamellae without perturbation of the overall adsorbed film structure. We observe that the melted phase has a dynamically reorienting domain-type structure whose orientations reflect that of graphite.This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411. Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5). Computational support was provided by the University of Missouri Bioinformatics Consortium

Quantum energy levels of hydrogen adsorbed on nanoporous carbons: an intrinsic probe for pore structure, and improving Monte Carlo simulations of adsorption [abstract]

Only abstract of poster available.Track IV: Materials for Energy ApplicationsHydrogen is the lightest molecule in nature, making both rotational and translational degrees of freedom eminently quantum mechanical (especially at low temperatures). For isolated molecules the first excited (degenerate) rotational states are at about 175 K above the (non-degenerate) ground state. When the hydrogen molecule is adsorbed, however, interaction with the substrate partially eliminates this degeneracy due to the different adsorption strengths of the different rotational states of the molecule. In this talk, we consider the adsorption of hydrogen in nanometer-size pores in carbon. We show that the rotation-vibration energy levels are strongly dependent on the pore structure (geometry and size). This dependence may be probed by inelastic neutron scattering as a local, non-destructive, probe intrinsic to the system, to characterize nanopores (in fact, using H2 as the probe makes sure that the pore structure probed is relevant for H2 adsorption). The rotation-vibration energy levels were also used as input for grand canonical Monte Carlo simulations of H2 adsorption, improving the accuracy of the simulations. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411."This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411.

Enhanced hydrogen adsorption in boron substituted carbon nanospaces

doi:10.1063/1.3251788Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4-8 kJ/mol, which limits the total amount of hydrogen adsorbed at P = 100 bar to ∼ 2 wt % at room temperature and ∼ 8 wt % at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%-10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity ( ∼ 5 wt % at 298 K, 100 bar) with a 97% delivery rateThis material is based on work supported in part by the Department of Energy under Award Nos. DE-FG02-07ER46411 and DE-FC36-08GO18142 (L.F., B.K., P.F., and C.W.). We acknowledge the Wroclaw and Poznan Supercomputing and Networking Centers and University of Missouri Bioinformatics Consortium for the use of their computational facilities

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite. Explicit Hydrogen Molecular Dynamics study

http://arxiv.org/ftp/arxiv/papers/0805/0805.1435.pdfWe discuss Molecular Dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche defect formation, a weak smectic mesophase (observed experimentally but never reproduced in United Atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as it is in UA force field), the melting temperature increases by ~70 K with respect to the experimental value. When the non-bonded 1-4 interaction is not scaled, the melting temperature decreases by ~90 K. If the scaling factor is set to 0.5, the melting occurs at T = 350 K, in very good agreement with experimental data.Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277 - B5), and the University of Missouri Research Board, for the support of this research. This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411

Monte Carlo simulations of krypton adsorption in nanopores: Influence of pore wall heterogeneity on the adsorption mechanism

We present molecular simulation results of the adsorption of krypton in a model of MCM-41 mesoporous material. The adsorption isotherm and adsorption enthalpies have been studied at 77 K. The comparison of experimental and simulation data allows us to analyze how the available interaction models (Kr–Kr and Kr–walls) are able to reproduce the experimental situation. The role of the heterogeneous interactions versus homogenous model is studied and compared with the previous simulation results of nitrogen adsorption in MCM-41. The results show that a model of ideal cylindrical pores gives qualitatively and quantitatively different results. A distribution of the adsorption sites must exist to explain the loading at low pressure (below capillary condensation). Such distribution in MCM-41 is a consequence of non-homogenous walls that contain a wide variety of attractive sites ranging from weakly attractive silica-type to highly attractive regions. In our simulations, the MCM-41 structure is modeled as an amorphous array of oxygen and silicon atoms, each one interacting with an adsorptive atom via the atom-atom potential. The distribution of the adsorption sites is merely a consequence of local atomic structure. Such a model of the wall reproduces the smooth increase in loading seen experimentally

How dense is the gas confined in nanopores?

International audienceIt is well know that the properties of nano-objects differ from those of their macroscopic analogs. Any system of nanometric size shows characteristics that strongly depend on its size and geometric form. It is mainly because the major part of atoms (or molecules) of nano-volume are located at the object surface and their cohesive energy is smaller than for the atoms in the bulk. As a consequence, the density of the nanoobjects is not homogeneous, and may decrease close to the object boundary. Here we show that when a fluid is confined in nano-volume, delimited by non-interacting pore walls, its density is on average smaller than the bulk density. The heterogeneous distribution of fluid density results from the nano-confinement, and progressively weakens when the pore size increases: it disappears for pores larger than 5 nm. On the other side, the fluid density approaches the ideal gas values in the limit of very small pores. This effect should be distinguished from the well know heterogeneity of density of fluids adsorbed in nanopores, driven by the difference between the strength of fluid-fluid and fluid-pore wall interactions. The reported observation has non-trivial influence on evaluation of excess/total adsorption in nanopores, as these two quantities are calculated assuming the known – and homogeneous – bulk density of gas in the pore. Additionally, the gas density in the pores depends on the definition of the pore volume which is neither straightforward nor unique. We analyze this phenomenon on an example of five gases: H2, CH4, the two intensively studied energy vectors, and N2, Ar, and Kr, commonly used for characterization of porous structures. Two model pore geometries with not adsorbing soft walls are analyzed (slit-shaped and cylindrical). For H2, the distributions of densities of gas confined in adsorbing and not adsorbing pores are compared and commented

Numerical modeling of melting of nano-clusters. Microscopic mechanism

International audienceThe fundamental physical properties of nanocrystals, such as their electronic band structure and optical activity, can be drastically different from those of the corresponding bulk materials, mainly due to their large surface-to-volume ratio. Among variety of properties, variation (lowering) of melting temperature of nanoparticles is a very important issue as it decreases the functional range of the solid phase. In this talk we discuss mechanism of melting of nanoclusters from a perspective of numerical modeling and relate the computational results to experimental observations. Conventionally, when studying melting of nanoparticles, the particles and the host medium are both in direct contact with a temperature source. In such conditions, melting of the nanoparticles starts at their surfaces, at reduced temperatures [1,2]. The typical dependence of the melting temperature on the size of nanoparticles is presented in the Figure 1. It shows that generally for particles built from smaller number of atoms melting is observed at lower temperature. However, the microscopic mechanism of melting may be affected by many factors. In particular, the irradiation with laser light can selectively and homogeneously excite the metallic nanoparticles without direct heating of the particle environment. The time of the laser pulse can be very short (fs) and conse-quently the melting may happen in non-equilibrium conditions. Also, the initial structural changes may lead to intermediate non-homogenous structures. Therefore, the microscopic mechanism of melting may be complex, and depending not only on the size of particles but also on theirs initial structures, rate of heating and the local temperature (kinetic energy of particles) distribution. We will show how the above factors affect the mechanism of melting of nanoparticles. The numerical simulation results will be confronted with the experimental observations which show that, although the as-prepared nanorods are defect-free, point and planar defects are present in gold nanorods after laser irradiation. The defects are mainly (multiple) twins and stacking faults (planar defects). They are the precursors that drive the convertion of nanorods (110) facets into the more stable (100) and (111) facets and hence minimize their surface energy. These observations suggest that short-laser pulsed photothermal melting begins with the creation of defects inside the nanorods followed by surface reconstruction and diffusion [3]

Nanoengineering of molecular machines. A review

International audienceA molecular machine, or nanomachine, refers to any nano-sized system that produces quasi-mechanical movements (output) in response to a specific stimuli (input). Nano-engineering is today a rapidly growing, innovative field of science and technology. The 2016 Nobel Prize in Chemistry recompensing the chemical research effort in this area confirmed that engineering of the nano-machines is not any more an object of science-fiction stories. We are at the advent of new engineering methodology, in which the nano-systems can be numerically designed and chemically synthesized. In order to construct a complex machine, several challenging problems must be solved. First, a number of pre-assembled building blocks have to be prepared, as the function of the device is intended to be a consequence of their assembly. Furthermore, to obtain a predetermined function of the machine, this assembly should provide a possibility to precisely controlled translational and rotational movements of the machine components, stimulated by an inflow of external energy. The machine also needs an interface with its environment. Ultimately, it should be able to overcome thermal fluctuation (Brownian motion) that influences its mechanical action, and to operate out of equilibrium in dissipative systems, when controlled and driven by an external fueling, by light or other energy source.Starting from Richard Feynman prediction from 1984 that it would be possible to build machines with nanometric dimensions we show how the notion of the nano-machine has evolved during the last century. In 1983 chemists succeeded to produce molecular chains, catenanes, in which ring-shaped molecules were linked together by mechanical bonds, but the interlocked molecules did not form any intermolecular, strong covalent bonds. The catenanes were not only a new class of macromolecules: they also represented the first step towards creating a molecular machine. Then, in 1994 a rotaxane, a ring-shaped molecule that is mechanically attached to an axle has been created. The first molecular motor was proposed in 1999. Since then the rotaxane has been used to construct numerous molecular machines, including a molecular elevator, which can raise itself 0.7 nanometers above a surface, and an artificial muscle (2000), that is able to bend a very thin gold lamina. Many new ideas have been published and many new systems have been already synthesized. This presentation will be focused on the nanomachines in which the light is used as a photonic factor actuating and allowing control of the nano-machines movements