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28 Mar 2013

Volume 138, Issue 12, Articles (12xxxx)

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J. Chem. Phys. 138, 124701 (2013); http://dx.doi.org/10.1063/1.4794685 (9 pages)

Luying Wang, Randall S. Dumont, and James M. Dickson
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Nonequilibrium molecular dynamics simulation of pressure-driven water transport through modified CNT membranes

Luying Wang, Randall S. Dumont, and James M. Dickson

J. Chem. Phys. 138, 124701 (2013); http://dx.doi.org/10.1063/1.4794685 (9 pages)

Online Publication Date: 22 March 2013

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Nonequilibrium molecular dynamics (NEMD) simulations are presented to investigate the effect of water-membrane interactions on the transport properties of pressure-driven water flow passing through carbon nanotube (CNT) membranes. The CNT membrane is modified with different physical properties to alter the van der Waals interactions or the electrostatic interactions between water molecules and the CNT membranes. The unmodified and modified CNT membranes are models of simplified nanofiltration (NF) membranes at operating conditions consistent with real NF systems. All NEMD simulations are run with constant pressure difference (8.0 MPa) temperature (300 K), constant pore size (0.643 nm radius for CNT (12, 12)), and membrane thickness (6.0 nm). The water flow rate, density, and velocity (in flow direction) distributions are obtained by analyzing the NEMD simulation results to compare transport through the modified and unmodified CNT membranes. The pressure-driven water flow through CNT membranes is from 11 to 21 times faster than predicted by the Navier-Stokes equations. For water passing through the modified membrane with stronger van der Waals or electrostatic interactions, the fast flow is reduced giving lower flow rates and velocities. These investigations show the effect of water-CNT membrane interactions on water transport under NF operating conditions. This work can help provide and improve the understanding of how these membrane characteristics affect membrane performance for real NF processes.
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47.60.Dx Flows in ducts and channels
02.70.Ns Molecular dynamics and particle methods
47.10.ad Navier-Stokes equations
47.11.Mn Molecular dynamics methods
47.56.+r Flows through porous media

Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

Pablo G. Lustemberg and Damián A. Scherlis

J. Chem. Phys. 138, 124702 (2013); http://dx.doi.org/10.1063/1.4796199 (8 pages)

Online Publication Date: 26 March 2013

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The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm−1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm−1 below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti4 + sites. At a higher density of defects, adsorption on Ti3 + sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti4 + sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm−1. These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.
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68.43.Mn Adsorption kinetics
61.66.Bi Elemental solids
61.66.Dk Alloys
71.55.Ht Other nonmetals
61.72.jd Vacancies
78.30.Hv Other nonmetallic inorganics

Structure and chemical reactivity of the polar three-fold surfaces of GaPd: A density-functional study

M. Krajčí and J. Hafner

J. Chem. Phys. 138, 124703 (2013); http://dx.doi.org/10.1063/1.4795435 (20 pages)

Online Publication Date: 26 March 2013

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The polar threefold surfaces of the GaPd compound crystallizing in the B20 (FeSi-type) structure (space group P213) have been investigated using density-functional methods. Because of the lack of inversion symmetry the B20 structure exists in two enantiomorphic forms denoted as A and B. The threefold {111} surfaces have polar character. In both nonequivalent (111) and (mathmathmath) directions several surface terminations differing in structure and chemical composition are possible. The formation of the threefold surfaces has been studied by simulated cleavage experiments and by calculations of the surface energies. Because of the polar character of the threefold surfaces calculations for stoichiometric slabs permit only the determination of the average energy of the surfaces exposed on both sides of the slab. Calculations for nonstoichiometric slabs performed in the grand canonical ensemble yield differences of the surface energies for the possible terminations as a function of the chemical potential in the reactive atmosphere above the surface and predict a transition between Ga- and Pd-terminated surfaces as a function of the chemical potential. The {100} surfaces are stoichiometric and uniquely defined. The calculated surface energies are identical to the average energies of the {100} surfaces of the pure metals. The {210} surfaces are also stoichiometric, with an energy very close to that of the {100} surfaces. Assuming that for the {111} surfaces the energies of different possible terminations are in a proportion equal to that of the concentration-weighted energies of the {111} surfaces of the pure metals, surface energies for all possible {111} terminations may be calculated. The preferable termination perpendicular to the A⟨111⟩ direction consists of a bilayer with three Ga atoms in the upper and three Pd atoms in the lower part. The surface energy of this termination further decreases if the Pd triplet is covered by additional Ga atom. Perpendicular to the A〈mathmathmath〉 direction the lowest energy has been found for a bilayer with three Ga atoms per surface cell in the upper layer and one Ga and one Pd in the lower part. The calculated surface energies are in agreement with a simulated cleavage experiment. However, cleavage does not result in the formation of the lowest-energy surfaces, because all possible {111} cleavage planes expose a low-energy surface on one, and a high-energy surface on the other side. The prediction of Ga-terminated surfaces has been tested against the available experimental information. The calculated surface electronic density of states is in very good agreement with photo-emission spectroscopy. Calculated STM images of the most stable surfaces agree with all details of the available experimental images. The chemical reactivity of the most stable surfaces has been studied by the adsorption of CO molecules. The adsorption energies and maximum coverages calculated for the Ga-terminated surfaces permit a reasonable interpretation of the observed thermal desorption spectra, whereas for the Pd-terminated surfaces the calculated adsorption energies are far too high.
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61.66.Fn Inorganic compounds
68.03.Fg Evaporation and condensation of liquids
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
64.70.dg Crystallization of specific substances
65.40.gp Surface energy
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling

Phonon thermal transport outside of local equilibrium in nanowires via molecular dynamics

Ya Zhou and Alejandro Strachan

J. Chem. Phys. 138, 124704 (2013); http://dx.doi.org/10.1063/1.4793530 (6 pages)

Online Publication Date: 27 March 2013

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We study thermal transport through Pt nanowires that bridge planar contacts as a function of wire length and vibrational frequency of the contacts. When phonons in the contacts have lower average frequencies than those in the wires thermal transport occurs under conditions away from local equilibrium with low-frequency phonons experiencing a higher thermal gradient than high-frequency ones. This results in a size-dependent increase in the effective thermal conductivity of the wire with decreasing vibrational frequencies of the contacts. The interfacial resistivity when heat flows from the wire to the contact is also size-dependent and has the same physical origin in the lack of full equilibration in short nanowires. We develop a model based on a 1D atomic chain that captures the salient physics of the MD results.
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81.05.Bx Metals, semimetals, and alloys
63.22.Gh Nanotubes and nanowires
81.07.Gf Nanowires
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys

Knudsen layer formation in laser induced thermal desorption

Akihiko Ikeda, Masuaki Matsumoto, Shohei Ogura, Tatsuo Okano, and Katsuyuki Fukutani

J. Chem. Phys. 138, 124705 (2013); http://dx.doi.org/10.1063/1.4795827 (6 pages)

Online Publication Date: 27 March 2013

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Laser induced thermal desorption of Xe atoms into vacuum from a metal surface following the nano-second pulsed laser heating was investigated by the time-of-flight (TOF) measurement. The desorption flow was studied at a wide range of desorption flux by varying the initially prepared Xe coverage Θ (1 ML = 4.5 × 1018 atoms/m2). At Θ = 0.3 ML, the TOF of Xe was well represented by a Maxwell-Boltzmann velocity distribution, which is in good agreement with thermal desorption followed by collision-free flow. At Θ > 0.3 ML, the peak positions of the TOF spectra were shifted towards the smaller values and became constant at large Θ, which were well fitted with a shifted Maxwell-Boltzmann velocity distribution with a temperature TD and a stream velocity u. With TD fixed at 165 K, u was found to increase from 80 to 125 m/s with increasing Θ from 1.2 to 4 ML. At Θ > 4 ML, the value of u becomes constant at 125 m/s. The converging feature of u was found to be consistent with analytical predictions and simulated results based on the Knudsen layer formation theory. We found that the Knudsen layer formation in laser desorption is completed at Knudsen number Kn <0.39.
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68.43.Vx Thermal desorption
68.43.Nr Desorption kinetics
79.20.Ds Laser-beam impact phenomena

Electronic and optical properties of graphene and graphitic ZnO nanocomposite structures

Wei Hu, Zhenyu Li, and Jinlong Yang

J. Chem. Phys. 138, 124706 (2013); http://dx.doi.org/10.1063/1.4796602 (5 pages)

Online Publication Date: 28 March 2013

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Electronic and optical properties of graphene and graphitic ZnO (G/g-ZnO) nanocomposites have been investigated with density functional theory. Graphene interacts overall weakly with g-ZnO monolayer via van der Waals interaction. There is no charge transfer between the graphene and g-ZnO monolayer, while a charge redistribution does happen within the graphene layer itself, forming well-defined electron-hole puddles. When Al or Li is doped in the g-ZnO monolayer, substantial electron (n-type) and hole (p-type) doping can be induced in graphene, leading to well-separated electron-hole pairs at their interfaces. Improved optical properties in graphene/g-ZnO nanocomposite systems are also observed, with potential photocatalytic and photovoltaic applications.
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71.20.Nr Semiconductor compounds
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.72.up Other materials
78.40.Fy Semiconductors

Grain boundary melting in ice

E. S. Thomson, Hendrik Hansen-Goos, J. S. Wettlaufer, and L. A. Wilen

J. Chem. Phys. 138, 124707 (2013); http://dx.doi.org/10.1063/1.4797468 (5 pages) | Cited 1 time

Online Publication Date: 28 March 2013

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We describe an optical scattering study of grain boundary premelting in water ice. Ubiquitous long ranged attractive polarization forces act to suppress grain boundary melting whereas repulsive forces originating in screened Coulomb interactions and classical colligative effects enhance it. The liquid enhancing effects can be manipulated by adding dopant ions to the system. For all measured grain boundaries this leads to increasing premelted film thickness with increasing electrolyte concentration. Although we understand that the interfacial surface charge densities qs and solute concentrations can potentially dominate the film thickness, we cannot directly measure them within a given grain boundary. Therefore, as a framework for interpreting the data we consider two appropriate qs dependent limits; one is dominated by the colligative effect and other is dominated by electrostatic interactions.
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64.70.dj Melting of specific substances
61.72.Mm Grain and twin boundaries

Deposition of model chains on surfaces: Anomalous relation between flux and stability

Pritam Kumar Jana and Andreas Heuer

J. Chem. Phys. 138, 124708 (2013); http://dx.doi.org/10.1063/1.4795316 (7 pages)

Online Publication Date: 29 March 2013

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Model chains are studied via Monte Carlo simulations which are deposited with a fixed flux on a substrate. They may represent, e.g., stiff lipophilic chains with an head group and tail groups mimicking the alkyl chain. After some subsequent fixed simulation time we determine the final energy as a function of flux and temperature. Surprisingly, in some range of temperature and flux the final energy increases with decreasing flux. The physical origin of this counterintuitive observation is elucidated. In contrast, when performing equivalent cooling experiments no such anomaly is observed. Furthermore, it is elaborated whether flux experiments give rise to configurations with lower energies as compared to cooling experiments. These results are related to recent experiments by the Ediger group where very stable configurations of glass-forming systems have been generated via flux experiments.
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68.43.Mn Adsorption kinetics
64.70.P- Glass transitions of specific systems
65.40.gp Surface energy

Interaction of magnetic transition metal dimers with spin-polarized hydrogenated graphene

S. W. Ong, J. Wu, A. Z. H. Thong, E. S. Tok, and H. C. Kang

J. Chem. Phys. 138, 124709 (2013); http://dx.doi.org/10.1063/1.4795500 (12 pages)

Online Publication Date: 29 March 2013

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The coadsorption of hydrogen and transition metal dimers Fe2, Co2, Ni2, and FeCo on graphene is investigated using density functional theory calculations. Our work is motivated by observations that the magnetic moments of these transition metal dimers are large and that hydrogen adsorption partitions the graphene lattice into magnetic subdomains. Thus, we expect the magnetic dimers to interact strongly with the lattice. Our results show that the majority-spin direction of the lattice electronic states depends upon the dimer identity, the lattice spin polarization being in the same direction as the dimer spin polarization for Fe2 and FeCo, but opposite for Co2 and Ni2. We can understand this by examining the electronic density of states of the dimer and the lattice. We also show that coadsorption significantly increases the adsorption energies of both dimer and hydrogen leading to a more strongly-adsorbed dimer, while the bond length and magnetic moment of the upper dimer atom, the latter important for potential magnetic storage applications, are negligibly changed. Our work shows that the coadsorbed hydrogen and metal dimer interact over a long-range, this interaction being mediated by the hydrogen-induced spin-polarization of the graphene lattice. We obtain general insight into how the elemental identity of these magnetic dimers determines the spin-polarized states on the hydrogenated graphene lattice. These results could be important for potential applications of magnetic properties of decorated graphene lattices.
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68.43.Mn Adsorption kinetics
72.25.-b Spin polarized transport
73.20.Hb Impurity and defect levels; energy states of adsorbed species
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ch Domain walls and domain structure
75.76.+j Spin transport effects
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