Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment.

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Cation Modifies Interfacial Water Structures on Platinum during Alkaline Hydrogen Electrocatalysis.

The molecular details of an electrocatalytic interface play an essential role in the production of sustainable fuels and value-added chemicals. Many electrochemical reactions exhibit strong cation-dependent activities, but how cations affect reaction kinetics is still elusive. We report the effect of cations (K+, Li+, and Ba2+) on the interfacial water structure using second-harmonic generation (SHG) and classical molecular dynamics (MD) simulation. The second- (χH2O(2)) and third-order (χH2O(3)) optical susceptibilities of water on Pt are smaller in the presence of Ba2+ compared to those of K+, suggesting that cations can affect the interfacial water orientation. MD simulation reproduces experimental SHG observations and further shows that the competition between cation hydration and interfacial water alignment governs the net water orientation. The impact of cations on interfacial water supports a cation hydration-mediated mechanism for hydrogen electrocatalysis; i.e., the reaction occurs via water dissociation followed by cation-assisted hydroxide/water exchange on Pt. Our study highlights the role of interfacial water in electrocatalysis and how innocent additives (such as cations) can affect the local electrochemical environment.

Steric Effects on Single-Molecule Conductance in Flat-Lying Phenanthrene.

A previous combined experimental and theoretical study found that the position of anchoring groups on a phenanthrene (PHE) backbone played a large role in determining the single-molecule conductance of the PHE derivative. However, a consistent 0.1 G0 feature was found across all PHE derivatives. To understand this, the previously investigated PHE derivatives were placed flat on a simulated Au substrate with a scanning tunneling microscope (STM) tip over PHE and conductance was calculated using the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT). The location of the tip was varied to find the most conductive and most energetically favorable arrangements, which did not coincide. Furthermore, the variation in conductance found in erect junctions was not present when PHE derivatives were lying flat, with all derivatives calculated to have conductance values around 0.1 G0.

Reversible solvent interactions with UiO-67 metal-organic frameworks.

The utility of UiO-67 Metal-Organic Frameworks (MOFs) for practical applications requires a comprehensive understanding of intermolecular host-guest MOF-analyte interactions. To investigate intermolecular interactions between UiO-67 MOFs and complex molecules, it is useful to evaluate the interactions with simple polar and non-polar analytes. This problem is approached by investigating the interactions of polar (acetone and isopropanol) and non-polar (n-heptane) molecules with functionalized UiO-67 MOFs via temperature programmed desorption mass spectrometry and temperature programmed Fourier transform infrared spectroscopy. We find that isopropanol, acetone, and n-heptane bind reversibly and non-destructively to UiO-67 MOFs, where MOF and analyte functionality influence relative binding strengths (n-heptane ≈ isopropanol > acetone). During heating, all three analytes diffuse into the internal pore environment and directly interact with the µ3-OH groups located within the tetrahedral pores, evidenced by the IR response of ν(µ3-OH). We observe nonlinear changes in the infrared cross sections of the ν(CH) modes of acetone, isopropanol, and n-heptane following diffusion into UiO-67. Similarly, acetone's ν(C=O) infrared cross section increases dramatically when diffused into UiO-67. Ultimately, this in situ investigation provides insights into how individual molecular functional groups interact with UiO MOFs and enables a foundation where MOF interactions with complex molecular systems can be evaluated.

Probing Heterogeneous Charge Distributions at the α-Al2O3(0001)/H2O Interface.

Unlike metal or semiconductor electrodes, the surface charge resulting from the protonation or deprotonation of insulating mineral oxides is highly localized and heterogeneous in nature. In this work the Stark active C≡N stretch of potassium thiocyanate is used as a molecular probe of the heterogeneity of the interfacial electrostatic potential at the α-Al2O3(0001)/H2O interface. Vibrational sum frequency generation (vSFG) measurements performed in the OH stretching region suggest that thiocyanate species organize interfacial water similarly to halide ions. Changes in the electrostatic potential are then tracked via Stark shifts of the vibrational frequency of the thiocyanate stretch. Our vSFG measurements show that we can simultaneously measure the vSFG response of SCN- ions experiencing charged and neutral surface sites. We assign local potentials of +308 and -154 mV to positively and negatively charged aluminol groups that are present at pH = 4 and pH = 10, respectively. Thiocyanate anions at positively charged surface sites and negatively charged surface sites and those participating in contact ion pairing adopt similar orientations and are oppositely oriented relative to thiocyanate ions near neutral surface sites. All four species followed Langmuir adsorption isotherms. Density functional theory-molecular dynamics (DFT-MD) simulations of SCN- near the neutral α-Al2O3(0001)/H2O interface show that the vSFG response in the C≡N stretch region originates from a SCN-H-O-Al complex, suggesting the surface site specificity of these experiments. To our knowledge this is the first spectroscopic measurement of local potentials associated with a heterogeneously charged surface. The ability to probe the evolution of local charges in situ could provide vital insight into many industrial, electrochemical, and geochemically relevant interfaces.

Ions Tune Interfacial Water Structure and Modulate Hydrophobic Interactions at Silica Surfaces.

The structure and ultrafast dynamics of the electric double layer (EDL) are central to chemical reactivity and physical properties at solid/aqueous interfaces. While the Gouy-Chapman-Stern model is widely used to describe EDLs, it is solely based on the macroscopic electrostatic attraction of electrolytes for the charged surfaces. Structure and dynamics in the Stern layer are, however, more complex because of competing effects due to the localized surface charge distribution, surface-solvent-ion correlations, and the interfacial hydrogen bonding environment. Here, we report combined time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD) to get direct access to the molecular-level understanding of how ions change the structure and dynamics of the EDL. We show that innersphere adsorbed ions tune the hydrophobicity of the silica-aqueous interface by shifting the structural makeup in the Stern layer from dominant water-surface interactions to water-water interactions. This drives an initially inhomogeneous interfacial water coordination landscape observed at the neat interface toward a homogeneous, highly interconnected in-plane 2D hydrogen bonding (2D-HB) network at the ionic interface, reminiscent of the canonical, hydrophobic air-water interface. This ion-induced transformation results in a characteristic decrease of the vibrational lifetime (T1) of excited interfacial O-H stretching modes from T1 ∼ 600 fs to T1 ∼ 250 fs. Hence, we propose that the T1 determined by TR-vSFG in combination with DFT-MD simulations can be widely used for a quantitative spectroscopic probe of the ion kosmotropic/chaotropic effect at aqueous interfaces as well as of the ion-induced surface hydrophobicity.

Potential-Induced High-Conductance Transport Pathways through Single-Molecule Junctions.

Employing single molecules as electronic circuit building blocks is one promising approach to electronic device miniaturization. We report single-molecule junction formation where the orientation of molecules can be controlled externally by the working electrode potential. The scanning tunneling microscopy break junction (STM-BJ) method is used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic acid (TPA) molecules between the Au(111) electrode and the STM tip to measure the single-molecule conductance through the junction. When the Au(111) electrode is at negative potentials (with respect to the zero-charge potential), a highly ordered and flat-oriented superstructure forms, allowing for direct contact between the π system of the benzene ring of the molecules and the Au(111) electrode, leading to junction formation with no anchoring group involvement. Our first-principles nonequilibrium Green's function (NEGF) computation shows a flat configuration yields a conductance that is 3 orders of magnitude larger than for a molecule vertically connected to the electrodes via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ± 0.02 G0 are experimentally measured with the flat configurations of TFTPA and TPA, respectively. These values are at least 2 orders of magnitude higher than the experimental values previously reported for the conductance of TPA bridged through carboxylic acid anchoring groups (3.8 × 10-4-3.2 × 10-3 G0). In contrast, a positively charged surface triggers an order-disorder transition eliminating the high-conductance states, most likely because the formation of the flat-oriented junction is prevented. The dependence of TFTPA conductance on the electrode potential (electrode Fermi level) suggests a LUMO mediated transport mechanism. Calculation confirms the lack of an effect of the addition of an electron-withdrawing group are investigated.

Bond-Dependent Thole Model for Polarizability and Spectroscopy.

We present a new model for the calculation of molecular polarizabilities from effective atomic polarizabilities. This model is based on the Thole modified dipole interaction model for molecular polarizabilities, where the total polarizability is computed as a sum of effective atomic polarizabilities modified by dipole-dipole interactions. We extend this model by making the atomic polarizabilities explicit functions of the interatomic distances, scaling them by the radius of the volume an atom occupies in a molecule. We use the SCAN functional to show that this model, denoted TholeL, yields accurate molecular polarizabilities with little dependence on the training set. We also demonstrate that the TholeL model yields accurate polarizabilities for configurations far from the ground state structure for a wide range of molecules. Finally, we show that the TholeL model can be used to generate accurate Raman spectra for water, crystalline urea, and urea in water from ab initio molecular dynamics simulations.

Anisotropic Conductivity at the Single-Molecule Scale.

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation-dependent conductance is expected. Here, we fabricated single-molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room-temperature single-molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either "ON" or "OFF" conductance states.

Ultrabroadband mid-infrared noncollinear difference frequency generation in a silver thiogallate crystal.

We report the generation of ultrabroadband mid-infrared (mid-IR) pulses by noncollinear difference frequency mixing. The signal and the idler output beams of an optical parametric amplifier are combined in a silver thiogallate crystal (AgGaS2) to generate mid-infrared radiation. We show that a noncollinear geometry facilitates broadband phase matching. Spectral bandwidths up to 1750 cm-1 were obtained at an external noncollinear angle of 4.2 deg, which is more than three times broader than in a collinear geometry. The broadband spectrum is tunable in the range of 1500-4500 cm-1. Pulse energies up to 1 µJ were achieved. The broadband pulses were used in sum frequency generation in ZnSe and in vibrational absorption spectroscopy experiments of liquid samples.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction.

We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS. The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a current density of 1 mA/cm2. Kinetic parameters (η and Tafel slope) associated with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.

Amine-Directed Hydrogen-Bonded Two-Dimensional Supramolecular Structures.

Utilizing pure amine hydrogen bonding is a novel approach for constructing two-dimensional (2D) networks. Further, such systems are capable of undergoing structural modifications due to changes in pH. In this study, we designed a 2D network of triaminobenzene (TAB) molecules that by varying the pH from neutral to acidic, form either ordered or disordered structures on Au(111) surface as revealed in scanning tunneling microscopy images. In near-neutral solution (pH ≈5.5), protonation of TAB generates charged species capable of forming H-bonds between amine groups of neighboring molecules resulting in the formation of a 2D supramolecular structure on the electrified surface. At lower pH, due to the protonation of the amine groups, intermolecular hydrogen bonding is no longer possible and no ordered structure is observed on the surface. This opens the possibility to employ pH as a chemical trigger to induce a phase transition in the 2D molecular network of triaminobenzene molecules.

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation.

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm(-2) . This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst ß-Ni(OH)2 (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

Copper-Intercalated Birnessite as a Water Oxidation Catalyst.

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.

Hapticity-dependent charge transport through carbodithioate-terminated [5,15-bis(phenylethynyl)porphinato]zinc(II) complexes in metal-molecule-metal junctions.

Single molecule break junction experiments and nonequilibrium Green's function calculations using density functional theory (NEGF-DFT) of carbodithioate- and thiol-terminated [5,15-bis(phenylethynyl)-10,20-diarylporphinato]zinc(II) complexes reveal the impact of the electrode-linker coordination mode on charge transport at the single-molecule level. Replacement of thiolate (-S(-)) by the carbodithioate (-CS2(-)) anchoring motif leads to an order of magnitude increase of single molecule conductance. In contrast to thiolate-terminated structures, metal-molecule-metal junctions that exploit the carbodithioate linker manifest three distinct conductance values. We hypothesize that the magnitudes of these conductances depend upon carbodithoate linker hapticity with measured conductances across Au-[5,15-bis(4'-(dithiocarboxylate)phenylethynyl)-10,20-diarylporphinato]zinc(II)-Au junctions the greatest when both anchoring groups attach to the metal surface in a bidentate fashion. We support this hypothesis with NEGF-DFT calculations, which consider the electron transport properties for specific binding geometries. These results provide new insights into the origin of molecule-to-molecule conductance heterogeneity in molecular charge transport measurements and the factors that optimize electrode-molecule-electrode electronic coupling and maximize the conductance for charge transport.

Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling.

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Structure of water at charged interfaces: a molecular dynamics study.

The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (~1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.

Orientation-controlled single-molecule junctions.

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature by a combination of molecular assembly, scanning tunneling microscopy (STM) imaging, and STM break junction (STM-BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat-oriented mesitylene molecules on Au(111) to enable direct tip/π contacts, a result that is not possible by conventional methods. The measured conductance of Au/π/Au junction is about 0.1 G(o) , two orders of magnitude higher than the conductance of phenyl rings connected to the electrodes by standard anchoring groups. Our experiments suggest that long-range ordered structures, which hold the aromatic ring in place and parallel to the surface, are essential to increase probability of the formation of orientation-controlled molecular junctions.