Probing the dynamics of CO2 and CH4 within the porous zirconium terephthalate UiO-66(Zr): a synergic combination of neutron scattering measurements and molecular simulations

Quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations were conducted to deeply understand the concentration dependence of the self- and transport diffusivities of CH(4) and CO(2), respectively, in the humidity-resistant metal-organic framework UiO-66(Zr). The QENS measurements show that the self-diffusivity profile for CH(4) exhibits a maximum, while the transport diffusivity for CO(2) increases continuously at the loadings explored in this study. Our MD simulations can reproduce fairly well both the magnitude and the concentration dependence of each measured diffusivity. The flexibility of the framework implemented by deriving a new forcefield for UiO-66(Zr) has a significant impact on the diffusivity of the two species. Methane diffuses faster than CO(2) over a broad range of loading, and this is in contrast to zeolites with narrow windows, for which opposite trends were observed. Further analysis of the MD trajectories indicates that the global microscopic diffusion mechanism involves a combination of intracage motions and jump sequences between tetrahedral and octahedral cages.     

Theoretical study of the stability of lithium atoms in alpha-rhombohedral boron

The stability of lithium atoms in alpha-rhombohedral boron was investigated by first-principles calculations of total energies and molecular dynamics (MD) simulations. In the case of a low concentration (1.03 at. %), Li at the center of the icosahedral B12 site (the I-site) had a negative binding energy, which suggests Li at the I-site is unstable. However, MD simulations at temperatures below 750 K indicated that Li is still confined in the B12 cage under these conditions, which means Li at the I-site is metastable. Over 800 K, Li began to move away from the B12 site and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site also had a negative binding energy, but MD simulations indicated it was metastable up to 1400 K and did not move to other sites. Li at the O-site was energetically the most favorable, having a positive binding energy. In the case of a high concentration (7.69 at. %), the I-site changed to an unstable saddle point. At this concentration, the T-site was metastable and the O-site became the most stable. In MD simulations at 1400 K, Li atoms at the O-site never jumped to other sites regardless of concentration. Considering these facts, the diffusion coefficient of Li in alpha-rhombohedral boron would have to be very small below 1400 K.     

Microwave-solvothermal synthesis of various polymorphs of nanostructured TiO2 in different alcohol media and their lithium ion storage properties

The various polymorphs (anatase, rutile, and brookite) of TiO(2) with different nanomorphologies have been synthesized by a facile microwave-assisted solvothermal process without surfactants, employing TiCl(4) or TiCl(3) as precursors in various alcohol (ethanol, propanol, butanol, and octanol) media. The samples have been characterized by X-ray diffraction (XRD), electron microscopy, and Brunauer-Emmett-Teller (BET) surface area analysis. The Ti/Cl ion concentration, reaction pH, and size of the alcohol molecule are found to control the morphology, crystal structure, and crystallite size of the TiO(2) particles. Among the various TiO(2) polymorphs synthesized, the rutile TiO(2) spheres built up of nanorods that were synthesized with TiCl(4) in octanol have an average pore size and surface area of, respectively, 5 nm and 404 m(2)/g and exhibit the best electrochemical performance with a capacity of >200 mAh/g after 100 cycles and high rate capability. The excellent electrochemical properties originate from the nanorod-building morphology and mesoporosity of TiO(2) spheres that provide good electrical contact, accommodates the strain smoothly, and facilitates facile lithium-ion diffusion.     

Characterization of V5+/TiO2-Rutile Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations with a quantum correction were used to study the 0.5 mol% V5+/TiO2 rutile under conditions of 300 K and 101 kPa. The interatomic potential function in MD simulations is composed of Coulomb, short-range repulsion, van der Waals, and Morse interactions. The topology of rutile was found to undergo a large deformation when Ti4+ is replaced by V5+, which can be related to the difference of valence and ionic radius between V5+ and Ti4+. The graphed distributions of Ti-O and O-O bond lengths, and O-Ti-O angles are all broaden. The simulations showed that when Ti4+ is replaced by V5+, the V5+ moves out of the O6 polyhedron and toward the interstice between TiO6 octahedra, which results in serious distortion of the octahedra near the interstice, but the phase with 0.5 mol% V5+ doping still remains in rutile. The structural features, such as the bond lengths, migration of the dopant, and crystal phase in the MD simulation are consistent with the experimental observations by FTIR, Raman, XRD and ESR.     

Interstitialcy diffusion of oxygen in tetragonal La2CoO(4+δ)

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La(2)CoO(4+δ). The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La(2)CoO(4+δ). Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principles-based calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling-untilting of the CoO(6) octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800-1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT- and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31-0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden-Popper structures.     

Structure and Dynamics of Adsorbed Dopamine on Solvated Carbon Nanotubes and in a CNT Groove

Advanced carbon microelectrodes, including many carbon-nanotube (CNT)-based electrodes, are being developed for the in vivo detection of neurotransmitters such as dopamine (DA). Our prior simulations of DA and dopamine-o-quinone (DOQ) on pristine, flat graphene showed rapid surface diffusion for all adsorbed species, but it is not known how CNT surfaces affect dopamine adsorption and surface diffusivity. In this work, we use molecular dynamics simulations to investigate the adsorbed structures and surface diffusion dynamics of DA and DOQ on CNTs of varying curvature and helicity. In addition, we study DA dynamics in a groove between two aligned CNTs to model the spatial constraints at the junctions within CNT assemblies. We find that the adsorbate diffusion on a solvated CNT surface depends upon curvature. However, this effect cannot be attributed to changes in the surface energy roughness because the lateral distributions of the molecular adsorbates are similar across curvatures, diffusivities on zigzag and armchair CNTs are indistinguishable, and the curvature dependence disappears in the absence of solvent. Instead, adsorbate diffusivities correlate with the vertical placement of the adsorbate’s moieties, its tilt angle, its orientation along the CNT axis, and the number of waters in its first hydration shell, all of which will influence its effective hydrodynamic radius. Finally, DA diffuses into and remains in the groove between a pair of aligned and solvated CNTs, enhancing diffusivity along the CNT axis. These first studies of surface diffusion on a CNT electrode surface are important for understanding the changes in diffusion dynamics of dopamine on nanostructured carbon electrode surfaces.     

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study.

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.     

First-principles study of lattice dynamics and thermodynamics of TiO2 polymorphs

The structural, phonon, and thermodynamic properties of six TiO(2) polymorphs, i.e., rutile, anatase, columbite, baddeleyite, orthorhombic I, and cotunnite, have been systematically investigated by density functional theory. The predicted volumes, bulk modulus, and Debye temperature are in good agreement with experiments. The phonon dispersions of the TiO(2) polymorphs were studied by the supercell approach, whereas the long-range dipole-dipole interactions were calculated by linear response theory to reproduce the LO-TO splitting, making accurate prediction of phonon frequencies for the polar material TiO(2). The calculated phonon dispersions show that all TiO(2) polymorphs are dynamically stable at ambient pressure, indicating the high-pressure phases might be quenched to ambient conditions as ultrahard materials. Furthermore, the finite temperature thermodynamic properties of TiO(2) polymorphs were predicted accurately from the obtained phonon density of states, which is critical in the future study of the pressure-temperature phase diagram of TiO(2). The calculated Gibbs energies reveal that rutile is more stable than anatase at ambient pressure. We derived the Gibbs energy and heat capacity functions for all TiO(2) polymorphs for use in thermodynamic modeling of phase equilibria.     

O2 interaction and reactivity on a model hydroxylated rutile(110) surface

Recently several theoretical studies have examined oxygen adsorption on the clean, reduced TiO2(110) surface. However the photocatalytic behavior of TiO2 and the scavenging ability of oxygen are known to be influenced by the presence of surface hydroxyls. In this paper the chemistry of O2 on the hydroxylated TiO2 surface is investigated by means of first-principles total energy calculations and molecular dynamics (MD) simulations. The MD trajectories show a direct, spontaneous reaction between O2 and the surface hydroxyls, thus supporting the experimental hypothesis that the reaction does not necessarily pass through a chemisorbed O2 state. Following this reaction, the most stable chemisorbed intermediates are found to be peroxide species HO2 and H2O2. Although these intermediates are very stable on the short time scale of MD simulations, the energetics suggests that their further transformation is connected to a new 300 K feature observed in the experimental water temperature programmed desorption (TPD) spectrum. The participation of two less stable intermediate states, involving terminal hydroxyls and/or chemisorbed water plus oxygen adatoms, to the desorption process, is not supported by the total energy calculations. Analysis of the projected density of states, however, suggests the possibility that these intermediates have a role in completing the surface oxidation immediately before desorption.     

A kinetic study of the intercalation of lithium salts into Al(OH)3

The intercalation of five lithium salts into the gibbsite and bayerite polymorphs of Al(OH)3 has been studied using in situ energy-dispersive X-ray diffraction. The kinetics and mechanisms of the reactions have been modeled using the Avrami-Erofe'ev model. The kinetic data suggest that the reaction mechanisms are predominantly nucleation controlled, although the intercalation of LiNO3 into bayerite and of Li2SO4 into gibbsite proceed via two-stage mechanisms, one part of which is diffusion controlled. All the reactions proceed directly from the host to the product, except for the intercalation of Li2SO4 into gibbsite where a more hydrated intermediate form of [LiAl2(OH)6]2SO4 x yH2O is generated prior to the final product.     

Li-KCl熔融界面上离子扩散的分子动力学研究

采用分子动力学(MD)方法研究了熔融Li(电极)-KCl(电解质)界面上离子的扩散行为。熔融界面上离子的扩散动力学通过离子质心的均方位移(MSD)和速度自相关函数(VACF)进行研究,扩散系数由MSD(t)函数线性区间的斜率和VACF(t)函数积分得到。模拟结果表明,在熔融的Li-KCl界面上,Li+离子在浓度梯度的驱动下穿过界面发生定向迁移,导致双电层的形成和外电路上电流的输出。Li+离子的扩散系数比K+和Cl-离子的大7～8倍,说明在界面上Li+离子是主要的载荷子,热电池的电荷传输机制主要与Li+离子的扩散运动有关。由Nernst-Einstein公式对电导率进行估算,由扩散到KCl层中的Li+离子产生的电导率约为0.4 S.cm-2,对应的电流密度估算值为3.27×105A.cm-2。     

Transport coefficients, Raman spectroscopy, and computer simulation of lithium salt solutions in an ionic liquid

Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.     

LiTFSI structure and transport in ethylene carbonate from molecular dynamics simulations

Molecular dynamics (MD) simulations using a many-body polarizable force field were performed on ethylene carbonate (EC) doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperature and salt concentration. At 313 K Li+ was coordinated by 2.7-3.2 EC carbonyl oxygen atoms and 0.67-1.05 TFSI- oxygen atoms at EC:Li = 10 and 20 salt concentrations. In completely dissociated electrolytes, however, Li+ was solvated by approximately 3.8 carbonyl oxygen atoms from EC on average. The probability of ions to participate ion aggregates decreased exponentially with an increase in the size of the aggregate. Ion and solvent self-diffusion coefficients and conductivity predicted by MD simulations were in good agreement with experiments. Approximately half of the charge was transported by charged ion aggregates with the other half carried by free (uncomplexed by counterion) ions. Investigation of the Li+ transport mechanism revealed that contribution from the Li+ diffusion together with its coordination shell to the total Li+ transport is similar to the contribution arising from Li+ exchanging solvent molecules in its first coordination shell with solvents from the outer shells.     

TiB2/均相硼掺杂TiO2核/壳结构提升光催化氧化水产氧活性

光催化可实现污染物降解、分解水制氢和CO2还原等多种氧化还原反应, 因而受到了广泛关注. 光催化材料中光生电荷的数目与氧化还原能力直接影响光催化反应效率, 在许多光催化反应中, 光生空穴氧化反应被认为速控步骤. 以光催化分解水为例, 质子的还原是单电子过程, 水氧化产生氧气则涉及四个电子. 空穴的高能量不仅可赋予其高的氧化能力,还能提高其迁出表面的能力, 因此具有重要研究价值.我们组的前期工作表明, 以TiB2作为前驱体, 采用水热合成和焙烧两步法可制备出间隙硼掺杂的金红石相或锐钛矿相TiO2, 间隙硼掺杂可显著降低价带顶, 提升光催化氧化水产氧性能. 然而, 在已有的结果中, 间隙硼掺杂浓度在TiO2中均呈现从内向外逐渐增加的梯度分布, 这意味着硼掺杂浓度有限, 且表层更低的价带顶不利于体相光生空穴向表面迁移, 因此亟需实现TiO2中均相的间隙硼掺杂.本文以湿化的氩气为水解环境, 将水解过程限域在TiB2的表面以减少硼原子流失; 同时提高水解温度, 使残留的硼原子形成间隙掺杂, 避免其在二次焙烧时扩散, 从而在TiB2核的表面所形成的TiO2壳层中实现均相间隙硼掺杂, 显著提高了光催化氧化水产氧活性. 多种表征结果表明, 直径约为6-10 μm的TiB2核表面形成了厚约400 nm的TiO2壳层, 在TiO2/TiB2中TiO2壳层重量比约为30%, TiO2壳层中锐钛矿相TiO2占比为65 wt%, 金红石相TiO2占比为35 wt%. TiO2壳层中间隙硼为均相分布, 硼掺杂显著降低了价带顶位置, 提高了光生空穴的氧化能力, 从而使得TiB2/TiO2展现出比未掺杂的金红石、锐钛矿相及两者混合相的TiO2均具有更高的光催化氧化水产氧的能力.     

Internal concentration gradients of guest molecules in nanoporous host materials: measurement and microscopic analysis

Evolution of internal concentration profiles of methanol in 2-D pore structure of ferrierite crystal was measured in the pressure range of 0 to 80 mbar with the help of the recently developed interference microscopy technique. The measured profiles showed that both a surface barrier and internal diffusion controlled the kinetics of adsorption/desorption. Furthermore, they indicated that in the main part of the crystal, the z-directional 10-ring channels were not accessible to methanol and that the transport of methanol mainly occurred via 8-ring y-directional channels. The roof-like part of the crystal was almost instantaneously filled/emptied during adsorption/desorption, indicating accessible 10-ring channels in this section. The measured profiles were analyzed microscopically with the direct application of Fick's second law, and the transport diffusivity of methanol in ferrierite was determined as a function of adsorbed phase concentration. The transport diffusivity varied by more than 2 orders of magnitude over the investigated pressure range. Transport diffusivities, calculated from measured profiles from small and large pressure step changes, were all found to be consistent. Simulated concentration profiles obtained from the solution of Fick's second law with the calculated functional dependence of diffusivities on concentration compared very well with the measured concentration profiles, indicating validity and consistency of the measured data and the calculated diffusivities. The results indicate the importance of measuring the evolution of concentration profiles as this information is vital in determining (1) the direction of internal transport, (2) the presence of internal structural defects, and (3) surface/internal transport barriers. Such detailed information is available neither from common macroscopic methods since, they measure changes in macroscopic properties and use model assumptions to predict the concentration profiles inside, nor from microscopic methods, since they only provide information on average displacement of diffusing molecules.     

Enhancement of intercalation properties of V2O5 film by TiO2 addition

Although it is well-known that TiO2 incorporation can greatly improve the cyclic stability of V2O5, the influences of TiO2 addition on the Li+ intercalation properties of V2O5 remain an issue of debate in literature. In this paper, we report on a systematic investigation of the preparation and intercalation properties of V2O5-TiO2 mixture films. The present work demonstrates that high Li+ intercalation rates and capacity in V2O5 films are achievable with TiO2 addition. For example, the addition of 20 mol % Ti into V2O5 polycrystalline demonstrated an approximated 100% improvement in Li+ intercalation performance as compared to single V2O5 electrodes. Such enhancement in intercalation properties of V2O5 films with TiO2 addition was attributed to changes in microstructure, crystallinity, and also a possible lattice structure and interaction force between adjacent layers in V2O5.     

Atomistic insights into the conversion reaction in iron fluoride: a dynamically adaptive force field approach

Nanoscale metal fluorides are promising candidates for high capacity lithium ion batteries, in which a conversion reaction upon exposure to Li ions enables access to the multiple valence states of the metal cation. However, little is known about the molecular mechanisms and the reaction pathways in conversion that relate to the need for nanoscale starting materials. To address this reaction and the controversial role of intercalation in a promising conversion material, FeF(2), a dynamically adaptive force field that allows for a change in ion charge during reactions is applied in molecular dynamics simulations. Results provide the atomistic view of this conversion reaction that forms nanocrystals of LiF and Fe(0) and addresses the important controversy regarding intercalation. Simulations of Li(+) exposure on the low energy FeF(2) (001) and (110) surfaces show that the reaction initiates at the surface and iron clusters as well as crystalline LiF are formed, sometimes via an amorphous Li-F. Li intercalation is also observed as a function of surface orientation and rate of exposure to the Li, with different behavior on (001) and (110) surfaces. Intercalation along [001] rapid transport channels is accompanied by a slight reduction of charge density on multiple nearby Fe ions per Li ion until enough Li saturates a region and causes the nearby Fe to lose sufficient charge to become destabilized and form the nanocluster Fe(0). The resultant nanostructures are fully consistent with postconversion TEM observations, and the simulations provide the solution to the controversy regarding intercalation versus conversion and the atomistic rationale for the need for nanoscale metal fluoride starting particles in conversion cathodes.     

Diffusion-controlled luminescence quenching in metal-organic frameworks

Diffusion-controlled luminescence quenching of a phosphorescent metal-organic framework built from the Ru(bpy)(3)(2+)-derived bridging ligand (MOF-1) was studied using a series of amines of different sizes as quenchers. The dynamics of amine diffusion into solvent-filled MOF-1 channels was probed by modeling time-dependent luminescence quenching data, which provide quantitative diffusion coefficients for the amine quenchers. Triethylamine, tripropylamine, and tributylamine were found to follow Fickian diffusion with a diffusivity of (1.1 ± 0.2) × 10(-13), (4.8 ± 1.2) × 10(-14), and (4.0 ± 0.4) × 10(-14) m(2)/s, respectively. Diisopropylethylamine (DIPEA), on the other hand, was found to be too large to enter the MOF channels. Despite its size, 4-MeOPhNPh(2) can enter the MOF channels via a slow, complicated framework/guest intercalation process to result in extensive framework distortion as revealed by powder X-ray diffraction. This work represents the first quantitative study of the dynamics of molecular diffusion into solvent-filled MOF channels. Such quantitative information on molecular diffusion in MOFs is of fundamental importance to many of their potential applications (e.g., heterogeneous catalysis).     

Influence of isotherm inflection on the loading dependence of the diffusivities of n-hexane and n-heptane in MFI zeolite. Quasi-elastic neutron scattering experiments supplemented by molecular simulations

Quasi-Elastic Neutron Scattering (QENS) experiments were carried out to determine (a) Fick diffusivity, D (b) self-diffusivity, Dself, and (c) 1/Gamma, the inverse of the thermodynamic correction factor, for n-hexane (nC6) and n-heptane (nC7) in MFI zeolite (all silica silicalite-1) at 300 K for a variety of loadings. These experimental results are compared with configurational-bias Monte Carlo (CBMC) and molecular dynamics (MD) simulations of, respectively, the adsorption isotherms and diffusivities. For n-hexane, the CBMC simulated isotherm shows a slight inflection at a loading=4 molecules per unit cell; this inflection manifests, also, in the loading dependence of 1/Gamma, obtained from QENS. The trend in the loading dependence of the Fick D and Dself of nC6 obtained from QENS matches the MD simulation results. For nC7 the CBMC simulated isotherm shows a strong inflection at a loading=4 molecules per unit cell. At this loading=4, 1/Gamma tends to zero and there is a very good match between QENS and molecular simulations for the loading dependence of 1/Gamma. Both MD simulations and QENS data on the Fick diffusivity shows a sharp maximum at a loading in the region of=4. For both nC6 and nC7 the simulated values of diffusivity are about an order of magnitude higher than those determined from QENS.