Showing papers on "Ion published in 2010"

SRIM – The stopping and range of ions in matter (2010)

Abstract: SRIM is a software package concerning the S topping and R ange of I ons in M atter. Since its introduction in 1985, major upgrades are made about every six years. Currently, more than 700 scientific citations are made to SRIM every year. For SRIM-2010 , the following major improvements have been made: (1) About 2800 new experimental stopping powers were added to the database, increasing it to over 28,000 stopping values. (2) Improved corrections were made for the stopping of ions in compounds. (3) New heavy ion stopping calculations have led to significant improvements on SRIM stopping accuracy. (4) A self-contained SRIM module has been included to allow SRIM stopping and range values to be controlled and read by other software applications. (5) Individual interatomic potentials have been included for all ion/atom collisions, and these potentials are now included in the SRIM package. A full catalog of stopping power plots can be downloaded at www.SRIM.org . Over 500 plots show the accuracy of the stopping and ranges produced by SRIM along with 27,000 experimental data points. References to the citations which reported the experimental data are included.

Toxicity of silver nanoparticles increases during storage because of slow dissolution under release of silver ions

Abstract: The dissolution of citrate-stabilized and poly(vinylpyrrolidone)-stabilized silver nanoparticles in water was studied by dialysis for up to 125 days at 5, 25, and 37 °C. The particles slowly dissolve into ions on a time scale of several days. However, in all cases, a limiting value of the released silver was observed, i.e., the particles did not completely dissolve. In some cases, the nanoparticles released up to 90% of their weight. Formal kinetic data were computed. Rate and degree of dissolution depended on the functionalization as well as on the storage temperature. The release of silver led to a considerably increased toxicity of silver nanoparticles which had been stored in dispersion for several weeks toward human mesenchymal stem cells due to the increased concentration of silver ions. Consequently, “aged” (i.e., immersed) silver nanoparticles are much more toxic to cells than freshly prepared silver nanoparticles.

Real-time observation of valence electron motion

Abstract: Chemical reactions are triggered by the dynamics of valence electrons in molecular orbitals. These motions typically unfold on a subfemtosecond scale and have eluded real-time access until now. Attosecond spectroscopy (an attosecond is 10−18 seconds), first applied to tracking electronic transitions from one quantum state to another, has now been extended to follow the hyperfast (subfemtosecond) motion of electron wavepackets in the valence shell — the bond-forming electrons — of krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy of this type will ultimately reveal the elementary electron motions in molecules and solid-state materials that determine physical, chemical and biological properties. Attosecond technology (1 as = 10−18 S) promises the tools needed to directly probe electron motion in real time. These authors report attosecond pump–probe measurements that track the movement of valence electrons in krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy will ultimately also reveal the elementary electron motions that underlie the properties of molecules and solid-state materials. The superposition of quantum states drives motion on the atomic and subatomic scales, with the energy spacing of the states dictating the speed of the motion. In the case of electrons residing in the outer (valence) shells of atoms and molecules which are separated by electronvolt energies, this means that valence electron motion occurs on a subfemtosecond to few-femtosecond timescale (1 fs = 10−15 s). In the absence of complete measurements, the motion can be characterized in terms of a complex quantity, the density matrix. Here we report an attosecond pump–probe measurement of the density matrix of valence electrons in atomic krypton ions1. We generate the ions with a controlled few-cycle laser field2 and then probe them through the spectrally resolved absorption of an attosecond extreme-ultraviolet pulse3, which allows us to observe in real time the subfemtosecond motion of valence electrons over a multifemtosecond time span. We are able to completely characterize the quantum mechanical electron motion and determine its degree of coherence in the specimen of the ensemble. Although the present study uses a simple, prototypical open system, attosecond transient absorption spectroscopy should be applicable to molecules and solid-state materials to reveal the elementary electron motions that control physical, chemical and biological properties and processes.

Kitaev-Heisenberg Model on a Honeycomb Lattice: Possible Exotic Phases in Iridium Oxides A 2 IrO 3

Abstract: We derive and study a spin one-half Hamiltonian on a honeycomb lattice describing the exchange interactions between Ir4+ ions in a family of layered iridates A2IrO3 (A=Li,Na). Depending on the microscopic parameters, the Hamiltonian interpolates between the Heisenberg and exactly solvable Kitaev models. Exact diagonalization and a complementary spin-wave analysis reveal the presence of an extended spin-liquid phase near the Kitaev limit and a conventional Neel state close to the Heisenberg limit. The two phases are separated by an unusual stripy antiferromagnetic state, which is the exact ground state of the model at the midpoint between two limits.

Lithium Diffusion in Graphitic Carbon

Abstract: Graphiticcarboniscurrentlyconsideredthestate-of-the-artmaterial for the negative electrode in lithium ion cells, mainly due to its high reversibility and low operating potential. However, carbon anodes exhibit mediocre charge/ discharge rate performance, which contributes to severe transport-induced sur- face structural damage upon prolonged cycling and limits the lifetime of the cell. Lithium bulkdiffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized nonisotropic particles. To solvethis problem forgraphite, weuse the Devanathan- Stachurski electrochemical methodology combined with ab initio computations to deconvolute and quantify the mechanism of lithium ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium ion diffusivityinthedirectionparalleltothegrapheneplane(∼10 -7 -10 -6 cm 2 s -1 ),as comparedtosluggishlithiumiontransportalonggrainboundaries(∼10 -11 cm 2 s -1 ), indicating the possibility of rational design of carbonaceous materials and com- posite electrodes with very high rate capability. SECTION Energy Conversion and Storage

Anion recognition by hydrogen bonding: urea-based receptors.

Abstract: Since 1992 a variety of urea-based anion receptors have been synthesised, of varying complexity and sophistication. This critical review will focus on some distinctive aspects of anion recognition by urea derivatives, with a special reference to: (i) design and synthesis, (ii) methodologies for the investigation of the receptor–anion interaction in solution, (iii) the interpretation of the solution behaviour on the basis of the structural interplay between the receptor and the anion. It will be shown that the efficiency of urea as a receptor subunit depends on the presence of two proximate polarised N–H fragments, capable (i) of chelating a spherical anion or (ii) of donating two parallel H-bonds to the oxygen atoms of a carboxylate or of an inorganic oxoanion, a property which is shared with other diamides, e.g. squaramide. The wide use of urea in the design of neutral anion receptors seems to depends on the ease of its synthesis, in particular through the reaction of a primary amine group with an isocyanate, which allows the high-yield preparation of symmetrically and unsymmetrically substituted derivatives (83 references).

Cooperativity in Ion Hydration

Abstract: Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect (i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.

Cation ordering in perovskites

Abstract: Although both A- and B-site cations have the same simple cubic topology in the perovskite structure they typically adopt different patterns of chemical order. As a general rule B-site cations order more readily than A-site cations. When cation ordering does occur, rock salt ordering of B/B′ cations is favored in A2BB′X6 perovskites, whereas layered ordering of A/A′ cations is favored in AA′B2X6 and AA′BB′X6 perovskites. The unexpected tendency for A-site cations to order into layers stems from the bond strains that would result at the anion site if A and A′ cations of different size were to order with a rock salt arrangement. The bonding instabilities that are created by layered ordering are generally offset either by anion vacancies or second order Jahn–Teller distortions of a B-site cation. Novel types of A-site cation ordering can be stabilized by a+a+a+ or a+a+c− tilting of the octahedra.

Cobalt(II) single-molecule magnets.

Abstract: This short tutorial review covers recent progress in the field of polynuclear cobalt(II)-based complexes, which display slow magnetic relaxation at low temperature. Cobalt(II) single-molecule magnets (SMMs) can display much larger magnetic anisotropies and hence, potentially higher blocking temperatures than SMMs based on ions where the zero-field splitting originates from a second order spin–orbit coupling, such as manganese(III).

On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials†

Abstract: To initiate wider discussion about promising research directions, this paper highlights a number of challenges in the development of rechargeable Mg batteries, especially those related to the slow solid-state Mg diffusion in common hosts. With a focus on the intercalation mechanism, we compare for the first time different strategies proposed in the literature for developing Mg battery cathodes, like the use of (i) nanoscale cathode materials; (ii) hybrid intercalation compounds containing bound water or other additional anion groups that can presumably screen the charge of the inserted cations, (iii) cluster-containing compounds with efficient attainment of local electroneutrality. This comparative analysis shows that cathodes whose function is based on a combination of the two first strategies, e.g., V2O5 gels and their hybrids, can exhibit relatively high voltage and capacity upon Mg insertion, but their kinetics is insufficiently fast. A proper intercalation mechanism for such materials is still unknow...

Ion transport in nanofluidic channels

Abstract: In this tutorial review, recent developments in modeling and experimental studies on nanofludics were reported. Nanofluidic studies were categorized into two groups depending on the characteristic length scale. When the size of the nanochannels and pores is 5–100 nm, electrostatic interactions are dominant, and ion and fluid flow can be analyzed by continuum dynamics. Various nanofluidic devices were proposed to manipulate aqueous solutions and biomolecules at the nanoscale. The successful development of such systems has major implications for technologies focusing on water purification and processing of complex biological solutions. When the size is less than 5 nm, steric interactions and hydration affect ion and fluid flow, which is analyzed by stochastic and/or molecular dynamics.

Hydrated metal ions in aqueous solution: How regular are their structures?

Abstract: The hydration reaction is defined as the transfer of an ion or neutral chemical species from the gaseous phase into water, M n+ (g) → M n+ (aq). In this process, water mole- cules bind to metal ions through ion-dipole bonds of mainly electrostatic character. The hy- dration reaction is always strongly exothermic with increa sing heat of hydration with in- creasing charge density of the ion. The structures of the hydrated metal ions in aqueous solution display a variety of configurations depending on the size and electronic properties of the metal ion. The basic configurations of hydrated metal ions in aqueous solution are tetrahedral, octahedral, square antiprismatic, and tricapped trigonal prismatic. This paper gives an overview of the structures of hydrated metal ions in aqueous solution with special emphasis on those with a non-regular coordination figure. Metal ions without d-electrons in the valance shell form regular aqua complexes with a coordination figure, allowing a maxi- mum number of water molecules to be clustered around the metal ion. This number is de- pendent on the ratio of the metal ion radius to the atomic radius of oxygen in a coordinated water molecule (1.34 A). The lighter lanthanoid(III) ions have a regular tricapped trigonal prismatic configuration with the M-O distance to the capping water molecules somewhat longer than to the prismatic ones. However, with increasing atomic number of the lan- thanoid(III) ions, an increasing distortion of the capping water molecules is observed, result- ing in a partial loss of water molecules in the capping positions for the heaviest lanthanoids. Metal ions with d 4 and d 9 valance shell electron configuration, as chromium(II) and cop- per(II), respectively, have Jahn-Teller distorted aqua complexes. Metal ions with low charge and ability to form strong covalent bonds, as silver(I), mercury(II), palladium(II), and plat- inum(II), often display distorted coordination figures due to the second-order Jahn-Teller ef- fect. Metal ions with d 10 s 2 valence shell electron configuration may have a stereochemically active lone electron pair (hemi-directed complexes) or an inactive one (holo-directed). The hydrated tin(II), lead(II), and thallium(I) ions are hemi-directed in aqueous solution, while the hydrated bismuth(III) ion is holo-directed. The structures of the hydrated cationic oxo- metal ions are reported as well.

Structural mechanism of C-type inactivation in K + channels

Abstract: Interconversion between conductive and non-conductive forms of the K(+) channel selectivity filter underlies a variety of gating events, from flicker transitions (at the microsecond timescale) to C-type inactivation (millisecond to second timescale). Here we report the crystal structure of the Streptomyces lividans K(+) channel KcsA in its open-inactivated conformation and investigate the mechanism of C-type inactivation gating at the selectivity filter from channels 'trapped' in a series of partially open conformations. Five conformer classes were identified with openings ranging from 12 A in closed KcsA (Calpha-Calpha distances at Thr 112) to 32 A when fully open. They revealed a remarkable correlation between the degree of gate opening and the conformation and ion occupancy of the selectivity filter. We show that a gradual filter backbone reorientation leads first to a loss of the S2 ion binding site and a subsequent loss of the S3 binding site, presumably abrogating ion conduction. These structures indicate a molecular basis for C-type inactivation in K(+) channels.

Ion pair receptors

Abstract: Compared with simple ion receptors, which are able to bind either a cation or an anion, ion pair receptors bearing both a cation and an anion recognition site offer the promise of binding ion pairs or pairs of ions strongly as the result of direct or indirect cooperative interactions between co-bound ions. This critical review focuses on the recent progress in the design of ion pair receptors and summarizes the various binding modes that have been used to accommodate ion pairs (110 references).

Fracture of electrodes in lithium-ion batteries caused by fast charging

Abstract: During charging or discharging of a lithium-ion battery, lithium is extracted from one electrode and inserted into the other. This extraction-insertion reaction causes the electrodes to deform. An electrode is often composed of small active particles in a matrix. If the battery is charged at a rate faster than lithium can homogenize in an active particle by diffusion, the inhomogeneous distribution of lithium results in stresses that may cause the particle to fracture. The distributions of lithium and stress in a LiCoO2 particle are calculated. The energy release rates are then calculated for the particle containing preexisting cracks. These calculations predict the critical rate of charging and size of the particle, below which fracture is averted.

Effects of packing fraction and bond valence on microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics

Abstract: Microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence. For A2+B6+O4 specimens sintered at 800–1100 °C for 3 h, a single phase with a tetragonal scheelite structure was detected, and the theoretical density was higher than 93% throughout the composition. Although the ionic polarizability of Ba2+ ion was larger than that of Ca2+ ion, the dielectric constant (K) of BaB6+O4 showed a smaller value than that of CaB6+O4. These results could be attributed to changes of the packing fraction due to the effective ionic size. The Q·f value was largely dependent on the packing fraction, as well as the percentages of theoretical density. The temperature coefficients of the resonant frequencies (TCFs) of the specimens were affected by the bond valence of oxygen. The specimens of CaMoO4 sintered at 1000 °C for 3 h showed the K of 10.8, Q·f of 76,990 GHz and TCF of −22.8 ppm/°C, respectively.

Crystal Habit-Tuned Nanoplate Material of Li[Li1/3-2x/3NixMn2/3-x/3]O-2 for High-Rate Performance Lithium-Ion Batteries

Abstract: Major State Basic Research Development Program of China [2009CB220102]; National Science Foundation of China [20833005, 20773102, 20931160426]

Charge Efficiency: A Functional Tool to Probe the Double-Layer Structure Inside of Porous Electrodes and Application in the Modeling of Capacitive Deionization

Abstract: Porous electrodes are important in many physical−chemical processes including capacitive deionization (CDI), a desalination technology where ions are adsorbed from solution into the electrostatic double layers formed at the electrode/solution interface inside of two juxtaposed porous electrodes. A key property of the porous electrode is the charge efficiency of the double layer, Λ, defined as the ratio of equilibrium salt adsorption over electrode charge. We present experimental data for Λ as a function of voltage and salt concentration and use this data set to characterize the double-layer structure inside of the electrode and determine the effective area for ion adsorption. Accurate experimental assessment of these two crucial properties of the electrode/solution interface enables more structured optimization of electrode materials for desalination purposes. In addition, detailed knowledge of the double-layer structure and effective area gives way to the development of more accurate dynamic process mode...

Anomalous ion transport in 2-nm hydrophilic nanochannels

Abstract: Transmembrane proteins often contain nanoscale channels through which ions and molecules can pass either passively (by diffusion) or actively (by means of forced transport). These proteins play important roles in selective mass transport and electrical signalling in many biological processes. Fluidic nanochannels that are 1-2 nm in diameter act as functional mimics of protein channels, and have been used to explore the transport of ions and molecules in confined liquids. Here we report ion transport in 2-nm-deep nanochannels fabricated by standard semiconductor manufacturing processes. Ion transport in these nanochannels is dominated by surface charge until the ion concentration exceeds 100 mM. At low concentrations, proton mobility increases by a factor of four over the bulk value, possibly due to overlapping of the hydrogen-bonding network of the two hydration layers adjacent to the hydrophilic surfaces. The mobility of K+/Na+ ions also increases as the bulk concentration decreases, although the reasons for this are not completely understood.

Molecular Insights into the Potential and Temperature Dependences of the Differential Capacitance of a Room-Temperature Ionic Liquid at Graphite Electrodes

Abstract: Molecular dynamics simulation studies of the structure and the differential capacitance (DC) for the ionic liquid (IL) N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonyl imide ([pyr13][TFSI]) near a graphite electrode have been performed as a function temperature and electrode potential. The IL exhibits a multilayer structure that extends 20−30 A from the electrode surface. The composition and ion orientation in the innermost layer were found to be strongly dependent on the electrode potential. While at potentials near the potential of zero charge (PZC), both cations and anions adjacent to the surface are oriented primarily perpendicular to the surface, the counterions in first layer orient increasingly parallel to the surface with increasing electrode potential. A minimum in DC observed around −1 VRPZC (potential relative to the PZC) corresponds to the point of highest density of perpendicularly aligned TFSI near the electrode. Maxima in the DC observed around +1.5 and −2.5 VRPZC are associated...

Power generation from concentration gradient by reverse electrodialysis in ion-selective nanochannels

Abstract: In an aqueous solution, the surface of inorganic nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions. In the presence of a concentration gradient, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of one type of ions. The Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using inorganic ion-selective nanochannels. Silica nanochannels with heights of 4, 26, and 80 nm were used in this study. We experimentally investigated the power generation from these nanochannels placed between two potassium chloride solutions with various combinations of concentrations. The power generation per unit channel volume increases when the concentration gradient increases, and also increases as channel height decreases. The highest power density measured was 7.7 W/m2. Our data also indicate that the energy conversion efficiency and the ion selectivity increase with a decrease of concentrations and channel height. The best efficiency obtained was 31%. Power generation from concentration gradients in inorganic ion-selective nanochannels could be used in a variety of applications, including micro batteries and micro power generators.

Average local ionization energy: A review.

Abstract: The average local ionization energy I(r) is the energy necessary to remove an electron from the point r in the space of a system. Its lowest values reveal the locations of the least tightly-held electrons, and thus the favored sites for reaction with electrophiles or radicals. In this paper, we review the definition of I(r) and some of its key properties. Apart from its relevance to reactive behavior, I(r) has an important role in several fundamental areas, including atomic shell structure, electronegativity and local polarizability and hardness. All of these aspects of I(r) are discussed.

Crystal Structure and Electrochemical Properties of A2MPO4F Fluorophosphates (A = Na, Li; M = Fe, Mn, Co, Ni)†

Abstract: We report new solid state and hydrothermal synthetic routes to (Li,Na)2FePO4F that incorporate carbon-containing additives and result in good electrochemical properties of this Li (or Na) ion electrode material. Single crystal X-ray diffraction analysis of Na2FePO4F prepared by flux growth confirms the unusual structural features of this compound that include pairs of face-sharing metal octahedra and [6 + 1] coordination of the sodium ions. Facile Na−Li ion-exchange occurs upon reflux with lithium salts, upon electrochemical cycling in a cell (vs. Li), and also in a cell simply equilibrated at OCV. The material does not exhibit typical two-phase behavior on electrochemical cycling. A combination of a redox process which occurs with little structural strain, and ion scrambling give rise to a solid solution-like sloping voltage profile on charge−discharge, although localization of the Fe2+/3+ in the mixed valence single phase intermediate, Na1.5FePO4F drives a very small structural distortion. Temperature-d...

Ca9Lu(PO4)7:Eu2+,Mn2+: A Potential Single-Phased White-Light-Emitting Phosphor Suitable for White-Light-Emitting Diodes

Abstract: A novel white-light-emitting phosphor Ca9Lu(PO4)7:Eu2+,Mn2+ has been prepared by solid-state reaction. The photoluminescence properties indicate that there is an efficient energy transfer from the Eu2+ to Mn2+ ions via a dipole−quadrupole reaction. The obtained phosphor exhibits a strong excitation band between 250 and 430 nm, matching well with the dominant emission band of a UV light-emitting-diode (LED) chip. Upon excitation of UV light, white light is realized by combining a broad blue-green emission band at 480 nm and a red emission band at 645 nm attributed to the Eu2+ and Mn2+ ions. The energy-transfer efficiency and critical distance were also calculated. Furthermore, the phosphors can generate lights from blue-green through white and eventually to red by properly tuning the relative ratio of the Eu2+ to Mn2+ ions through the principle of energy transfer. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for a UV white-light LED.

Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces

Abstract: We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than ∼2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1–2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.