Ionic Surface Research Articles

A tight-binding model of a carbon nanotube interacting with TiO2 rutile (110) surface

The electronic and the spatial structure of an armchair carbon nanotube (CNT) deposited on a (110) surface of titanium dioxide (rutile) is studied with a help of the density functional theory (DFT) and using a tight binding model. It is shown that an electrostatic potential due to an ionic surface of the rutile crystal may induce an energy gap at the Fermi level in the electronic structure of the nominally metallic CNT. The gap may reach a magnitude 0.02 eV depending on a symmetry of an arrangement of the CNT at the surface. The electron site occupations obtained from the DFT computations are used to deduce an analytic form of the external potential applied in the model. It is shown that the potential induces a substantial lattice distortion which contributes to an enhancement of the energy gap. The electronic structure obtained from the calculation based on the model is in a good quantitative agreement with the DFT results.

Energy transfers in a weakly coupled gas-surface system: The scattering of CO from MgO(001)

This work reports on quasi-classical dynamics simulations of the non-reactive scattering of CO from a prototypical cubic ionic surface. We shed light on the role of the incidence energy, the initial ro-vibrational state of impinging CO, and the effect of surface atom motion on the scattering behaviour of the molecules. Our results rely on a novel high-dimensional potential energy surface, which is fitted using a modified reactive bond order force field to reproduce reference energy data obtained from periodic embedding density functional theory. The scattering process is found to be strongly influenced by the anisotropy of the potential for CO impinging either from the C-end or O-end of the molecule. The distinct scattering features are significant at low incidence energies when surface atom motion is included in the simulations. This suggests that modelling energy transfer to phonons is very important to describe the dynamics of such weakly interacting systems.

High-performance Li-Ion capacitor constructed from biomass-derived porous carbon and high-rate Li4Ti5O12

In this study we prepared a biomass-derived porous carbon and a high-rate Li4Ti5O12 (LTO) as cathode and anode materials, respectively, for Li-ion capacitors (LICs). We synthesized the bio-derived carbon through pyrolysis of rubberwood as the carbon source. The as-synthesized carbon featured a hierarchical micro/mesoporous architecture with a surface area of 1365 m2 g−1 and excellent electrochemical properties. A symmetric supercapacitor (SC) based on the bio-derived carbon material exhibited excellent capacitance characteristics and remarkable cycling stability. Furthermore, the rate performance of the spray-dried LTO modified through ionic doping and surface coating was much better than that of the unmodified LTO, due to enhanced conductivity and ionic diffusivity. Because of the outstanding rate capability of the modified LTO, the kinetic mismatch between the cathode and anode—a general problem for LICs—was overcome. After coupling the bio-derived porous carbon with the high-rate LTO, the as-fabricated LIC displayed a high energy/power density of 142 Wh kg−1/253 W kg−1, and even provided a value of 52.9 Wh kg−1 at 4556 W kg−1. In addition, this LIC retained 85.7% of its original capacity after 10,000 cycles.

Effect of Molecule–Substrate Interactions on the Adsorption of meso-Dibenzoporphycene Tautomers Studied by Scanning Probe Microscopy and First-Principles Calculations

meso-Dibenzoporphycene molecules adsorbed on the Ag(111) surface and on 2-monolayer-thick NaCl films were studied using submolecular resolution atomic force microscopy (AFM), scanning tunneling microscopy (STM), and first-principles calculations to clarify their stability and tautomerization behavior. We have found that the bonding of the molecules with the surface is determined by the interplay of different contributions, including the interaction of the π-aromatic orbitals of the benzene rings and the metal-coordination bond of the lone-pair electrons of the imine nitrogen atoms with the metal atoms (Ag, Na) on each substrate. The strength of the latter ultimately governs the molecular adsorption configuration and determines the nature and energy barriers for tautomerization. On Ag(111), the interaction of the imine nitrogen atoms with the Ag atoms deforms the macrocycle of porphycene, leading to a distinct AFM contrast that allows a clear identification of the molecule in its cis tautomeric form. In contrast, on NaCl films, the weaker interaction with the Na atoms leads to a flatter geometry and very similar adsorption configurations for the cis- and trans-forms, which cannot be distinguished in AFM experiments. Although weak, the dominant role of this local N–Na interaction, compared to the essentially nondirectional dispersive interactions, results in a new type of tautomerization process. In this case, the transfer of hydrogen atoms within the porphycene cavity is accompanied by a significant displacement of the whole molecule to a new site to reach a new minimum energy adsorption configuration. Our theoretical calculations indicate that this lateral translation, rather than the intramolecular H transfer, dominates the activation energy on NaCl. This novel tautomerization behavior, which we have identified on a rather inert ionic surface, might also be present on other weakly interacting substrates.

Combination of Electrochemical Surface Plasmon Resonance and Molecular Dynamics Simulation to Evaluate the Quasi-Static Capacitance at the Ionic Liquid/Electrode Interface

1. Introduction Composed of ions solely, ionic liquids (ILs) are low-melting point salts exhibiting excellent properties for electrolytes such as ultra-low volatility, non-flammability, thermal/electrochemical stability. The structure and dynamics of the electric double layer (EDL) are critical in the process of charge/mass transfer. However, different from the conventional electrolytes, ILs exhibit strong Coulomb correlation with high concentration and density, hence Gouy-Champman-Stern model is not applicable to describe the EDL of ILs. For the experimental measurement of the differential capacitance in the EDL, electrochemical impedance spectroscopy (EIS) is the most popular method. But the fast ac potential oscillation in EIS measurements can’t avoid the influence of the slow dynamics of the EDL such as ultraslow relaxation[1] and hysteresis.[2] In the present study, electrochemical surface plasmon resonance (ESPR) is proposed as an effective measurement method of static differential capacitance at the IL/electrode interface. The influences of the slow relaxation are demonstrated at different scan rates of potential. In order to achieve quantitative measurement, molecular dynamics (MD) simulations of the IL between two single-layer graphene electrodes are performed. Moreover, through the data analysis of MD, the variations of ionic layer structure and ionic orientation are revealed and described in detail. 2. Methods 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][TFSA−]) was chosen as the IL material for research. In the ESPR experiments, a laser beam was incident through a prism and totally reflected on a 50-nm golden film deposited on a SF15 glass. The golden film was the working electrode, a Pt wire was set as the counter electrode, and an Ag/AgCl wire was inserted into the IL as the quasi-reference electrode. The experiments were performed at different scan rates to determine the influence of slow dynamics.[3] The all-atom MD simulations were performed by using DL_POLY classic. The results in all the charged conditions were simulated from the same initial configuration, which was prepared from IL bulk simulations, vacuum-liquid interface simulations, and uncharged graphene-IL interface simulations.[4] There were 720 ion pairs between two single-layer graphene electrodes within 125 Å. In each charged condition, the charge was allocated evenly to every C atom on the electrodes. The temperature was controlled (NVT ensemble) by using the Berendsen thermostat (0.5 ns as the quick equilibration process) and the Nose-Hoover thermostat (2 ns). Thus, the trajectories in the last 1 ns were chosen as the statistical data for analysis. 3. Results and Discussion By ESPR experiments, the change in SPR angle is recorded simultaneously with cyclic voltammetry, exhibiting a sigmoidal shape with the maximum slope at around −0.6 V (PZC). We find that the variation ranges by the negative scans are larger than those by the positive scans, and the variation range by positive scan increases as the scan rate decreases. These phenomena indicate that the influence of slow dynamics is insignificant for negative scans as was previously observed for a different IL.[1] To study the cause of the sigmoidal change in SPR angle, we propose a mathematical model,[3] by which a linear relationship between SPR angle and the surface charge density at the IL/electrode interface can be established. Consequently, the relative value of the quasi-static differential capacitance is shown in Fig. 1. Comparing with the previous study,[2] we determine the potential of zero charge in this system as −0.61 V, noted as the grey dashed line. This is the first time to obtain the static differential capacitance at the IL/solid electrode, in spite of the relative value. The differential capacitance exhibits a camel-shaped potential dependence, agreeing with the mean-field lattice gas model.[5] For quantitative measurement, MD simulations are performed to produce ionic distributions and surface charge density with respect to potential. By the data fitting of surface charge density and ionic concentration, we estimate the X − 1 as 3.1 mdeg∙cm2∙μC−1. Furthermore, by the observation of the ionic distribution, we confirm the butyl group of C4mim+ in the first ionic layer would provide extra room for the cations in the second layer, resulting in that the state of “ionic crowding” is hardly to occur and the interval of ionic layers varies with the potential on the negatively charged electrode.

(Invited) Theoretical Analysis of Microscopic Interface Ionics at Heterogeneous Solid-Solid Interfaces in Batteries

All-solid-state battery (ASSB) has attracted considerable attention for decades, and the developments of many new materials appropriate for ASSB have been made. In contrast, less is known about the interfacial processes, especially at the solid electrolyte (SE)-electrode interfaces. The microscopic origins of interfacial resistance and degradation are still crucial issues for the efficient ASSBs, which are closely related to the interface chemistry and electrochemistry on the ionic and electronic scales. Therefore, understanding of the microscopic interface ionics and electronics is indispensable for the cutting-edge ASSB development.We have addressed establishing a universal calculation framework based on density functional theory (DFT) for heterogeneous solid-solid interfaces. Typical DFT geometry optimization or molecular dynamics does not work well for the atomistic structure search due to the coexistence of collective and local geometry relaxations inherent to the solid-solid interfaces. The methods with empirical force field are also insufficient because their parameters are usually fitted to bulk properties, not interfacial ones.To overcome these issues, we have developed a novel computational technique, “Heterogeneous Interface (HI)-CALYPSO method” [1], for systematic sampling of heterogeneous solid-solid interface structures. The breakthrough was achieved by applying CALYPSO structure prediction method for all types of interfacial geometry relaxations, allowing to explore a variety of interfacial disordered structures [1]. This approach is compatible to massive parallel calculations on the supercomputers like Fugaku.Then, the method was applied to a variety of solid-solid interfaces in ASSBs. As a representative model, we investigated the system with LiCoO2 (LCO), β-Li3PS4 (LPS), and LiNbO3 (LNO) acting as a cathode, a sulfide electrolyte, and a buffer layer, respectively [1-3]. For the LCO/LPS interfaces, we sampled over 20000 configurations and found several stable disordered structures involving mixing cations and anions, leading to the formation of a reaction layer. On the other hand, Li-ion sites that can be preferentially depleted upon charging always exist around the cathode-SE interfaces irrespective of the interfacial order/disorder. Therefore, we conclude that the dynamic Li-ion depletion is related to suppression of successive Li-ion transport, leading to the interfacial resistance. Through the electronic states analysis, we also deduced a probable origin for the interfacial Li-ion depletion and a mean to suppress this problematic behavior. Finally, we provided ionic potential surface across the interfaces, governing ion dynamics across the interfaces. In consequences, sampling probable interface structures enabled to analyze statistically probable characteristics of the ionics and electronics at the solid-solid interfaces and provide a possibility of space-charge layer as well as electric double layer.We have also applied the method to the interface between Li metal anode and Li7La3Zr2O12 electrolyte [4]. Again, we succeeded in sampling of the probable interface structures, which have many interfacial Li-O bonds and unsaturated (i.e. coordination number < 6) Zr sites. These undercoordinated Zr sites are reduced once the LLZO surface is in contact with the Li metal, leading to chemical instability of the LLZO/Li interface.Recently, the interfaces between the Li1.17Al0.17Ti1.83(PO4)3 SE and LCO cathode as well as Li metal anode have been investigated as well. Though it’s not related to ASSB, we examined the heterogeneous interfaces between the graphite anode and Li2CO3 model SEI layer [5]. All of these studies suggest a unified insight into the interfacial ionic transport in ASSBs. In this talk, I’ll introduce our recent methodological and application studies on the ASSB interfaces and discuss the unified insight obtained.Acknowledgements: These works were done in collaboration with Dr. Bo Gao, Dr. Hong-Kang Tian, and Dr. Randy Jalem in NIMS. The works were partly supported by MEXT as “Program for Promoting Researches on the Supercomputer Fugaku” (Fugaku Battery & Fuel Cell Project) and JSPS KAKENHI “Interface IONICS” Grant Number JP19H05815.

A solvent-based surface cleaning and passivation technique for suppressing ionic defects in high-mobility perovskite field-effect transistors

Organometal halide perovskite semiconductors could potentially be used to create field-effect transistors (FETs) with high carrier mobilities. However, the performance of these transistors is currently limited by the migration of ionic surface defects. Here, we show that a surface cleaning and passivation technique, which is based on a sequence of three solution-based steps, can reduce the concentration of ionic surface defects in halide-based perovskites without perturbing the crystal lattice. The approach consists of an initial cleaning step using a polar/nonpolar solvent, a healing step to remove surface organic halide vacancies and a second cleaning step. The surface treatment is shown to restore clean, near hysteresis-free transistor operation, even if the perovskite films are formed under non-optimized conditions, and can improve room-temperature FET mobility by two to three orders of magnitude compared to untreated films. Our methylammonium lead iodide (MAPbI3) FETs exhibit high n- and p-type mobilities of 3.0 cm2 V−1 s−1 and 1.8 cm2 V−1 s−1, respectively, at 300 K, and higher values (9.2 cm2 V−1 s−1; n-type) at 80 K. We also show that the approach can be used to transform PbI2 single crystals into high-quality, two-dimensional perovskite single crystals. A three-stage solution-based cleaning technique can increase the room-temperature mobility and reduce the hysteresis of organometal halide perovskite transistors by decreasing the surface defects in the perovskite films.

Tungstate ion (WO42-) confined in hydrophilic/hydrophobic nanomaterials functionalized brönsted acidic ionic liquid as highly active catalyst in the selective aerobic oxidation of alcohols in water

A Brönsted acidic Ionic Liquid containing tungstate anion functionalized polysiloxane network (PMO-IL-WO42-) was synthesized by simple self-condensation of tungstic acid and zwitterionic organosilane precursor possessing both imidazolium and sulfonate groups. Characterization by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), thermal gravimetric analysis TGA, nitrogen porosimetry, solid-state NMR spectroscopy and elemental analysis confirmed that both imidazolium cation and tungstate anion of zwitterion are successfully incorporated inside the organosilica framework. The catalytic activity of resulting hybrid PMO-IL- WO42- material was studied in the selective aerobic oxidation of primary and secondary alcohols using an atmospheric pressure of air in pure water. Due to the ionic liquid-based charged surface containing hydrophilic sulfonic acid and tungstate group, the synergistic hydrophilic/hydrophobic and redox effect of PMO-IL-WO42- as water-friendly catalyst facilitates and enhances the activity and selectivity toward the target oxidative products in water and proved to have a particularly broad substrate scope for reliable aerobic oxidation reaction. Furthermore, the catalyst showed outstanding stability and could be easily separated and reused at least ten reactions run under the same conditions as fresh catalyst without any loss of catalytic activity and product selectivity.

Microphase separation and aggregate self-assembly in brushes of oppositely charged polyelectrolytes triggered by ion pairing.

This work applies a molecular theory to study the formation of lateral self-assembled aggregates in mixed brushes composed of polyanion and polycation chains. In order to overcome the well-known limitations of mean-field electrostatics to capture polyelectrolyte complexation, the formation of ion pairs between anionic and cationic groups in the polyelectrolytes is explicitly modeled in our theory as an association reaction. This feature is essential to capture the microphase separation of the mixed brush and the formation of lateral aggregates triggered by polyelectrolyte complexation. The effects of solution pH and ionic strength, surface coverage, and chain length on the morphology of the mixed brush are systematically explored. It is shown that increasing salt concentration leads to the rupture of polyelectrolyte complexes and the stabilization of the homogeneous, non-aggregated brush, providing that the formation of ion pairs between the polyelectrolytes and the salt ions in solution is explicitly accounted for by the theory. The inclusion of ion-pairing association reactions between oppositely charged polyelectrolytes within a mean-field description of electrostatics emerges from this work as a useful and simple theoretical approach to capture the formation of polyelectrolyte complexes and their responsiveness to solution ionic strength and pH.

Simultaneous doping of higher ionic state metal and surface plasmon resonance-inducing element with ZnO: an effective approach to improve photocatalytic dye degradation

This study aims to improve the photocatalytic activity of zinc oxide thin films by simultaneously doping with a higher oxidation state transition element and another element with localized plasmon resonance. To achieve this hypothesis, tungsten (W) and copper (Cu) were added to ZnO and the effects of W + Cu co-doping on the photocatalytic activity have been investigated. In addition, the influence of W + Cu co-doping on the structural, optical and surface morphological properties of ZnO has been studied and the obtained results have been correlated with the enhancement in the photocatalytic activity. The concentration of W was kept as 3 wt% and that of Cu was varied as 1, 3 and 5 wt%. The films were deposited on stainless steel mesh substrates. Antibacterial activity test was carried out for all the prepared film samples. The co-doped film with W + Cu doping concentrations 3 + 3 wt% exhibits superior photocatalytic and antibacterial activities when compared with other samples. The reasons and the mechanism behind this enhancement in the photocatalytic and antibacterial activities have been addressed in this paper.

Temperature driven high-performance pseudocapacitor of carbon nano-onions supported urchin like structures of α-MnO2 nanorods

The promising pseudocapacitive charge storage insists the strategic design of three-dimensional architecture of nanostructures with extra-active redox sites on and/or near surface with high capacity of ionic insertion into the porous network of electrodes. Herein, we report the urchin like structure of α-MnO2 nanorods grown on carbon nano-onion (CNO) support via simple, cost-effective wet chemical method. The highly crystalline α-MnO2 nanorods with larger spinel sizes of 0.46 nm and 0.18 nm are characterized by SEM, TEM, XRD and Raman spectra. These spinel structures of α-MnO2 nanorods offers large cavity for ions insertion, which further improved by increasing temperature due to fast ionic mobility. The temperature driven increment of specific capacity from 202 Cg-1 to 363 Cg-1 at current density 0.3 Ag-1 is accomplished owing to the improved ionic conductivity, surface area and fast charge transfer. Further the 78% capacitance retention after 1000 cycles at 70 °C is realized, which is just 17% lower that the cyclic stability at 4 °C. It emphasizes an importance of three-dimensional design/architecture of electrode materials for the high-temperature pseudocapacitor applications.

Synergistic effect of Li2MgTi3O8 coating layer with dual ionic surface doping to improve electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials

Surface modification plays a vital role in improving the rate performance and cycling stability of layered Ni-rich LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials for Li-ion batteries. In this study, Li2MgTi3O8 (LMTO)-coated NCM622 was successfully synthesized by wet coating combined with sintering process. The results of morphological analysis showed a uniform LMTO layer coated on the surface of NCM622 materials. The dual ions of Ti4+ and Mg2+ on surface structure of NCM622 were identified by X-ray diffraction and X-ray photoelectron spectroscopy. The surface coating layer prevented the direct contact between the active cathode material and the electrolyte which significantly reduced the side reactions, and the doping of Ti4+ and Mg2+ improved the structural stability of the NCM622 materials. The electrochemical performance indicates that NCM622 cathode with a coating layer of 0.5 wt% LMTO exhibited excellent cycle stability and maintained a capacity retention up to 76% after 200 cycles at 25 °C at 1 C-rate, which was higher than the value of 52% for the NCM622.

Three-dimensional solvation structure of ethanol on carbonate minerals.

Calcite and magnesite are important mineral constituents of the earth’s crust. In aqueous environments, these carbonates typically expose their most stable cleavage plane, the (10.4) surface. It is known that these surfaces interact with a large variety of organic molecules, which can result in surface restructuring. This process is decisive for the formation of biominerals. With the development of 3D atomic force microscopy (AFM) it is now possible to image solid–liquid interfaces with unprecedented molecular resolution. However, the majority of 3D AFM studies have been focused on the arrangement of water at carbonate surfaces. Here, we present an analysis of the assembly of ethanol – an organic molecule with a single hydroxy group – at the calcite and magnesite (10.4) surfaces by using high-resolution 3D AFM and molecular dynamics (MD) simulations. Within a single AFM data set we are able to resolve both the first laterally ordered solvation layer of ethanol on the calcite surface as well as the following solvation layers that show no lateral order. Our experimental results are in excellent agreement with MD simulations. The qualitative difference in the lateral order can be understood by the differing chemical environment: While the first layer adopts specific binding positions on the ionic carbonate surface, the second layer resides on top of the organic ethyl layer. A comparison of calcite and magnesite reveals a qualitatively similar ethanol arrangement on both carbonates, indicating the general nature of this finding.

Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization.

The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

Condensation of Satellite Droplets on Lubricant-Cloaked Droplets.

Condensation on lubricant-infused micro- or nanotextured superhydrophobic surfaces exhibits remarkable heat transfer performance owing to the high condensation nucleation density and efficient condensate droplet removal. When a low surface tension lubricant is used, it can spread on the condensed droplet and “cloak” it. Here, we describe a previously unobserved condensation phenomenon of satellite droplet formation on lubricant-cloaked water droplets using environmental scanning electron microscopy. The presence of satellite droplets confirms the cloaking behavior of common lubricants on water such as Krytox oils. More interestingly, we have observed satellite droplets on BMIm ionic liquid-infused surfaces, which is unexpected because BMIm was used in previous reports as a lubricant to eliminate cloaking during water condensation. Our studies reveal that the cloaking of BMIm on water droplets is theoretically favorable due to the fast timescale spreading during initial condensation when compared to the long timescale required for dissolution of the lubricant in water. We utilize a novel characterization approach based on Raman spectroscopy to confirm the existence of cloaking lubricant films on water droplets residing on lubricant-infused surfaces. The selected lubricants include Krytox oil, ionic liquid, and dodecane, which have drastically different surface tensions and polarities. In addition, spreading dynamics of cloaking and noncloaking lubricants on water droplets show that ionic liquid has the capability to mobilize water droplets spontaneously owing to cloaking and its relatively high surface tension. Our studies not only elucidate the physics governing cloaking and satellite droplet condensation phenomena at micro- and macroscales but also reveal a subset of previously unobserved lubricant–water interfacial interactions for a large variety of applications.

Multivalent Ions as Reactive Crosslinkers for Biopolymers-A Review.

Many biopolymers exhibit a strong complexing ability for multivalent ions. Often such ions form ionic bridges between the polymer chains. This leads to the formation of ionic cross linked networks and supermolecular structures, thus promoting the modification of the behavior of solid and gel polymer networks. Sorption of biopolymers on fiber surfaces and interfaces increases substantially in the case of multivalent ions, e.g., calcium being available for ionic crosslinking. Through controlled adsorption and ionic crosslinking surface modification of textile fibers with biopolymers can be achieved, thus altering the characteristics at the interface between fiber and surrounding matrices. A brief introduction on the differences deriving from the biopolymers, as their interaction with other compounds, is given. Functional models are presented and specified by several examples from previous and recent studies. The relevance of ionic crosslinks in biopolymers is discussed by means of selected examples of wider use.

Growth and aggregation of Cu nanocrystals on ionic liquid surfaces**Project supported by the National Natural Science Foundation of China (Grant No. 11374082).

We report a catalyst-free growth of Cu nanocrystals on ionic liquid surfaces by thermal evaporation method at room temperature. After deposition of Cu on ionic liquid surfaces, ramified Cu aggregates form. It is found that the aggregates are composed of both granules and nanocrystals with triangular or hexagonal appearances. The sizes of the nanocrystals are in the range of tens to hundreds of nanometers and increase with the nominal deposition thickness. The growth mechanism of the Cu aggregates and nanocrystals is presented.

Dynamical Quantum Filtering via Enhanced Scattering of para-H2 on the Orientationally Anisotropic Potential of SrTiO3(001)

Quantum dynamics calculation, performed on top of density functional theory (DFT)-based total energy calculations, show dynamical quantum filtering via enhanced scattering of para-H2 on SrTiO3(001). We attribute this to the strongly orientation-dependent (electrostatic) interaction potential between the H2 (induced) quadrupole moment and the surface electric field gradient of ionic SrTiO3(001). These results suggest that ionic surfaces could function as a scattering/filtering media to realize rotationally state-resolved H2. This could find significant applications not only in H2 storage and transport, but also in realizing materials with pre-determined characteristic properties.

Nanomolar Surface-Active Charged Impurities Account for the Zeta Potential of Hydrophobic Surfaces.

The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ potential of hydrophobic surfaces for varying pH and salt concentration by solving the Poisson-Boltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the experimentally measured surface tension of the air-electrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H3O+, OH-, HCO3-, and CO32- cannot account for the ζ potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impurities' acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pKa = 5-7) and weak bases (pKb = 12) at a concentration of the order of 10-7 M.

Pore-scale study of water salinity effect on thin-film stability for a moving oil droplet

HypothesesThe interfacial dynamics in natural porous media are affected not only by the interplay between viscous and capillary forces but also the solid surface wettability. It has been hypothesized that the wettability alteration induced by changes in the water salinity is primarily caused by electric double-layer force expansion, which strongly affects the multiphase flow dynamics. SimulationsWe investigate the effect of water ionic composition and surface roughness on pore-scale wettability alteration. Multiphase hydrodynamics is numerically captured by a lubrication approximation describing the evolution of thin-films coupled with a multiscale level-set approach. An oil blob mobilized by water within a single pore is considered as a case study. The effect of brine ionic composition is accounted for by an electric double-layer through the water ionic strength and zeta-potential parameters. FindingsWe demonstrate that high-salinity water thin-films collapse to an adsorbed nanometer layer, leading to a large pressure drop during mobilization of the blob induced by the attractive surface forces. However, low-salinity water thin-films are stable due to the repulsive electric double-layer forces, leading to less pressure drop during mobilization of the blob. The novelty of this work lies in efficiently capturing the nanoscale effects of the electric double-layer in pore-scale multiphase flow displacements. Our quantitative investigations provide fundamental insights into the efficiency of low-salinity waterflooding.