A molecular perspective of water at metal interfaces

Modeling Vibrational Sum Frequency Generation Spectra of Interfacial Water on a Gold Surface: The Role of the Fermi Resonance

Studying the hydrogen bonding structure of H2O at the metal-water interface is a highly complex yet fascinating endeavor. The intricate interactions and diverse orientations of water molecules on metal surfaces with varying potentials pose a significant challenge in elucidating the coupling between O-H stretching and H-O-H bending modes. In this study, we employed DFT-MD simulation to explore how the orientation of interfacial water molecules changes with the applied potential on the Au(111) surface. Based on the surface-specific velocity-velocity correlation function (ssVVCF) formula, we calculated vibrational sum frequency generation (VSFG) spectra for the O-H stretches. We found that three assigned peaks (∼3300, ∼3450, and 3650 cm-1) shifted toward lower frequencies as the potential moved toward more negative values. Our results align remarkably well with experimental Raman spectroscopy data. Notably, our VSFG analysis revealed a significant change in the VSFG spectra of the hydrogen-bonded O-H groups (∼3300 cm-1), switching from a negative to a positive sign with decreasing potential. This alteration suggests a substantial change in the orientation of these low-frequency O-H groups owing to their increased interactions with the Au surface. In contrast, the orientations of both the high-frequency O-H groups (∼3450 cm-1) and the dangling O-H groups (∼3650 cm-1) remained unaffected by the applied potentials. Furthermore, our analysis of the decomposed vibrational density of states (VDOS) for the H-O-H bending mode uncovered the coupling between the H-O-H bending and O-H stretching vibrations, known as the Fermi resonance. Our work suggests that the H-O-H bending vibration becomes restricted when water molecules transition from the ″one-H-down″ to the ″two-H-down″ conformation, leading to a redshift in the O-H stretching vibration through the Fermi resonance. By constructing the VSFG and decomposed VDOS spectra, we gained valuable insights into the structural changes that Raman spectra alone cannot fully interpret. Specifically, our analysis revealed the critical role of the Fermi resonance effect in shaping the spectroscopic signature of interfacial water molecules on the Au(111) surface.

Ab initio molecular dynamics investigation of the Pt(111)-water interface structure in an alkaline environment with high surface OH-coverages

In this study, we investigate the structure of the Pt(111)-water interface in an alkaline environment with large OH coverages of 1/3, 2/3 and 1 monolayer using a large well-equilibrated system. We observe that the OH coverage influences both the orientational distribution of the water molecules and their density, with more structure associated with higher coverage. At the same time, there is evidence of a highly dynamic hydrogen bond network on the lower coverage systems with substantial exchange of water between the surface and the solvent. In addition to OH and H2O species, which are preferentially located at the top sites, the 1/3 and 2/3 monolayer surfaces also contain O atoms, which are relatively stable and prefer the hollow sites. In contrast, the 1 monolayer surface shows none of these dynamics, and is unlikely to be active. The dynamic coexistence of O, OH and H2O on Pt(111) electrodes in alkaline conditions necessitates the investigation of several possible reaction paths for processess like ORR and water splitting. Finally, the exchange processes observed between the solvent and the interface underscore the need to explicitly include liquid water in simulations of systems similar to Pt(111).

Benchmarking water adsorption on metal surfaces with ab initio molecular dynamics

Solid-water interfaces are ubiquitous in nature and technology. In particular, technologies evolving in the green transition, such as electrocatalysis, heavily rely on the junction of an electrolyte and an electrode as a central part of the device. For the understanding of atomic-scale processes taking place at the electrolyte-electrode interface, density functional theory (DFT) has become the de facto standard. The validation of DFT's ability to simulate the interfacial solid/water interaction is crucial, and ideal simulation setups need to be identified in order to prevent avoidable systematic errors. Here, we develop a rigorous sampling protocol for benchmarking the adsorption/desorption energetics of water on metallic surfaces against experimental temperature programmed desorption, single crystal adsorption calorimetry, and thermal energy atom scattering. We screened DFT's quality on a series of transition metal surfaces, applying three of the most common exchange-correlation approximations: PBE-D3, RPBE-D3, and BEEF-vdW. We find that all three xc-functionals reflect the pseudo-zeroth order desorption of water rooted in the combination of attractive adsorbate-adsorbate interactions and their saturation at low and intermediate coverages, respectively. However, both RPBE-D3 and BEEF-vdW lead to more accurate water adsorption strengths, while PBE-D3 clearly overbinds near-surface water. We relate the variations in binding strength to specific variations in water-metal and water-water interactions, highlighting the structural consequences inherent in an uninformed choice of simulation parameters. Our study gives atomistic insight into water's complex adsorption equilibrium. Furthermore, it represents a guideline for future DFT-based simulations of solvated solid interfaces by providing an assessment of systematic errors in specific setups.

Orientational dynamics of the water layer adjacent to Au surface accelerated by polarization effect

The orientation and rearrangement of water on a gold electrode significantly influences its physicochemical heterogeneous performance. Despite numerous experimental and theoretical studies aimed at uncovering the structural characteristics of interfacial water, the orientational behavior resulting from electrode-induced rearrangements remains a subject of ongoing debate. Here, we employed molecular dynamics simulations to investigate the adaptive structure and dynamics properties of interfacial water on Au(111) and Au(100) surfaces by considering a polarizable model for Au atoms in comparison with the non-polarizable model. Compared to the nonpolarizable systems, the polarization effect can enhance the interaction between water molecules and the gold surface. Unexpectedly, the rotational dynamics directly associated with the orientational behavior of water adjacent to the gold surface is accelerated, thereby reducing the hydrogen bond lifetime. The underlying mechanism for this anomalous phenomenon originates from the polarization effect, which induces the attraction of the positive hydrogen atoms to the surface by the negative image charge. This leads to a change in orientation that disrupts the hydrogen bonds in the first water layer and subsequently accelerates reorientation dynamics of water molecules adjacent to the gold surface. These results shed light on the intricate interplay between polarization effects and water molecule dynamics on metal surfaces, establishing the foundation for the rational regulation of the orientation of interfacial water.

Two-dimensional ice-like water adlayers on a mica surface with and without a graphene coating under ambient conditions

Water tends to wet all hydrophilic surfaces under ambient conditions, and the first water adlayers on solids are important for a broad range of physicochemical phenomena and technological processes, including corrosion, wetting, lubrication, anti-icing, catalysis, and electrochemistry. Unfortunately, challenges in characterizing the first water adlayer in the laboratory have hampered molecular-level understanding of the contact water structure. Herein, we present the first ab initio molecular dynamics simulation evidence of a previously unreported ice-like adlayer structure (named as Ice-AL-II) on a prototype mica surface under ambient conditions. Calculation showed that the newly identified Ice-AL-II structure is more stable than the widely recognized ice-adlayer structure on mica surfaces (named as Ice-AL-I). Ice-AL-II exhibited a face-centered corner-cut tetragon (or a face-centered irregular pentagon) pattern of a hydrogen-bonded network. The center of the corner-cut tetragon was occupied by either a K+ cation or a water molecule with two H atoms pinned by the mica (100) via double hydrogen bonds. Our simulation also suggested that bilayer Ice-AL-II favors AA stacking rather than AB stacking. Interestingly, when a graphene sheet was coated on top of the ice-like adlayer, the stability of Ice-AL-II was further enhanced. In contrast, due to its strongly puckered structure, the Ice-AL-I structure could be crushed into a near-Ice-AL-II structure by the graphene coating. Ice-AL-II is thus proposed as a promising candidate for the ice-like structure on a mica surface detected by scanning polarization force microscopy and by atomic force microscopy between a graphene coating and a mica surface.

Superdiffusive Rotation of Interfacial Water on Noble Metal Surface

Interfacial water on a metal surface acts as an active layer through the reorientation of water, thereby facilitating the energy transfer and chemical reaction across the metal surface in various physicochemical and industrial processes. However, how this active interfacial water collectively behaves on flat noble metal substrates remains largely unknown due to the experimental limitation in capturing librational vibrational motion of interfacial water and prohibitive computational costs at the first-principles level. Herein, by implementing a machine-learning approach to train neural network potentials, we enable performing advanced molecular dynamics simulations with ab initio accuracy at a nanosecond scale to map the distinct rotational motion of water molecules on a metal surface at room temperature. The vibrational density of states of the interfacial water with two-layer profiles reveals that the rotation and vibration of water within the strong adsorption layer on the metal surface behave as if the water molecules in the bulk ice, wherein the O-H stretching frequency is well consistent with the experimental results. Unexpectedly, the water molecules within the adjacent weak adsorption layer exhibit superdiffusive rotation, contrary to the conventional diffusive rotation of bulk water, while the vibrational motion maintains the characteristic of bulk water. The mechanism underlying this abnormal superdiffusive rotation is attributed to the translation-rotation decoupling of water, in which the translation is restrained by the strong hydrogen bonding within the bilayer interfacial water, whereas the rotation is accelerated freely by the asymmetric water environment. This superdiffusive rotation dynamics may elucidate the experimentally observed large fluctuation of the potential of zero charge on Pt and thereby the conventional Helmholtz layer model revised by including the contribution of interfacial water orientation. The surprising superdiffusive rotation of vicinal water next to noble metals will shed new light on the physicochemical processes and the activity of water molecules near metal electrodes or catalysts.

The structure-giving role of Rb+ ions for water-ice nanoislands supported on Cu(111)

We characterize the effect of rubidium ions on water-ice nanoislands in terms of area, fractal dimension, and apparent height by low-temperature scanning tunneling microscopy. Water nanoislands on the pristine Cu(111) surface are compared to those at similar coverage on a Rb+ pre-covered Cu(111) surface to reveal the structure-giving effect of Rb+. The presence of Rb+ induces changes in the island shape, and hence, the water network, without affecting the nanoisland volume. The broad area distribution shifts to larger values while the height decreases from three bilayers to one or two bilayers. The nanoislands on the Rb+ pre-covered surface are also more compact, reflected in a shift in the fractal dimension distribution. We relate the changes to a weakening of the hydrogen-bond network by Rb+.

Controlling Interfacial Hydrogen Bonding at a Gold Surface: The Effect of Organic Cosolvents

Water often serves as both a reactant and solvent in electrocatalytic reactions. Interfacial water networks can affect the transport and kinetics of these reactions, e.g., hydrogen evolution reaction and CO2 reduction reaction. Adding cosolvents that influence the hydrogen-bonding (H-bonding) environment, such as dimethyl sulfoxide (DMSO), has the potential to tune the reactivity of these important electrocatalytic reactions by regulating the interfacial local environment and water network. We investigate interfacial H-bonding networks in water-DMSO cosolvent mixtures on gold surfaces by using surface-enhanced infrared absorption spectroscopy and molecular dynamics simulations. Experiments and simulations show that the gold surface is enriched with dehydrated DMSO molecules and the mixture phase-separates to form water clusters. Simulations show a "buckled" water conformation at the surface, further constraining interfacial H-bonding. The small size of these water clusters and the energetically unfavorable H-bond conformations might inhibit H-bonding with bulk water, suppressing the proton diffusion required for efficient hydrogen evolution reaction processes.

Nanoscale one-dimensional close packing of interfacial alkali ions driven by water-mediated attraction

The permeability and selectivity of biological and artificial ion channels correlate with the specific hydration structure of single ions. However, fundamental understanding of the effect of ion-ion interaction remains elusive. Here, via non-contact atomic force microscopy measurements, we demonstrate that hydrated alkali metal cations (Na+ and K+) at charged surfaces could come into close contact with each other through partial dehydration and water rearrangement processes, forming one-dimensional chain structures. We prove that the interplay at the nanoscale between the water-ion and water-water interaction can lead to an effective ion-ion attraction overcoming the ionic Coulomb repulsion. The tendency for different ions to become closely packed follows the sequence K+ > Na+ > Li+, which is attributed to their different dehydration energies and charge densities. This work highlights the key role of water molecules in prompting close packing and concerted movement of ions at charged surfaces, which may provide new insights into the mechanism of ion transport under atomic confinement.

On the nature of Con±/0 clusters reacting with water and oxygen

Bulk cobalt does not react with water at room temperature, but cobalt nanometals could yield corrosion at ambient conditions. Insights into the cobalt cluster reactions with water and oxygen enable us to better understand the interface reactivity of such nanometals. Here we report a comprehensive study on the gas-phase reactions of Con±/0 clusters with water and oxygen. All these Con±/0 clusters were found to react with oxygen, but only anionic cobalt clusters give rise to water dissociation whereas the cationic and neutral ones are limited to water adsorption. We elucidate the influences of charge states, bonding modes and dehydrogenation mechanism of water on typical cobalt clusters. It is unveiled that the additional electron of anionic Con- clusters is not beneficial to H2O adsorption, but allows for thermodynamics- and kinetics-favourable H atom transfer and dehydrogenation reactions. Apart from the charge effect, size effect and spin effect play a subtle role in the reaction process. The synergy of multiple metal sites in Con- clusters reduces the energy barrier of the rate-limiting step enabling hydrogen release. This finding of water dissociation on cobalt clusters put forward new connotations on the activity series of metals, providing new insights into the corrosion mechanism of cobalt nanometals.

Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.

Cation-Induced Interfacial Hydrophobic Microenvironment Promotes the C-C Coupling in Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) toward C2 products is a promising way for the clean energy economy. Modulating the structure of the electric double layer (EDL), especially the interfacial water and cation type, is a useful strategy to promote C-C coupling, but atomic understanding lags far behind the experimental observations. Herein, we investigate the combined effect of interfacial water and alkali metal cations on the C-C coupling at the Cu(100) electrode/electrolyte interface using ab initio molecular dynamics (AIMD) simulations with a constrained MD and slow-growth approach. We observe a linear correlation between the water-adsorbate stabilization effect, which manifests as hydrogen bonds, and the corresponding alleviation in the C-C coupling free energy. The role of a larger cation, compared to a smaller cation (e.g., K+ vs Li+), lies in its ability to approach the interface through desolvation and coordinates with the *CO+*CO moiety, partially substituting the hydrogen-bonding stabilizing effect of interfacial water. Although this only results in a marginal reduction of the energy barrier for C-C coupling, it creates a local hydrophobic environment with a scarcity of hydrogen bonds owing to its great ionic radius, impeding the hydrogen of surrounding interfacial water to approach the oxygen of the adsorbed *CO. This skillfully circumvents the further hydrogenation of *CO toward the C1 pathway, serving as the predominant factor through which a larger cation facilitates C-C coupling. This study unveils a comprehensive atomic mechanism of the cation-water-adsorbate interactions that can facilitate the further optimization of the electrolyte and EDL for efficient C-C coupling in CO2RR.

Configuration of ammonia on Cu{311}: Infrared spectroscopy and first-principles theory

We describe Reflection Absorption Infrared Spectroscopy (RAIRS) and first-principles Density Functional Theory (DFT) studies of ammonia adsorption on the Cu{311} surface. Our experimental results indicate an upright chemisorbed species at low coverages, with at least one additional species accompanying this at higher coverages. Our high-coverage RAIRS data cannot be fully explained by DFT models containing only ammonia or its dissociation products, even allowing for molecular tilt and/or the formation of a bilayer. We therefore also consider urea and formamide as possible products of surface reaction with residual carbon monoxide, but these species are again not fully compatible with our observed spectra. The overlayer composition at high coverages remains mysterious.

Structure Discovery in Atomic Force Microscopy Imaging of Ice

The interaction of water with surfaces is crucially important in a wide range of natural and technological settings. In particular, at low temperatures, unveiling the atomistic structure of adsorbed water clusters would provide valuable data for understanding the ice nucleation process. Using high-resolution atomic force microscopy (AFM) and scanning tunneling microscopy, several studies have demonstrated the presence of water pentamers, hexamers, and heptamers (and of their combinations) on a variety of metallic surfaces, as well as the initial stages of 2D ice growth on an insulating surface. However, in all of these cases, the observed structures were completely flat, providing a relatively straightforward path to interpretation. Here, we present high-resolution AFM measurements of several water clusters on Au(111) and Cu(111), whose understanding presents significant challenges due to both their highly 3D configuration and their large size. For each of them, we use a combination of machine learning, atomistic modeling with neural network potentials, and statistical sampling to propose an underlying atomic structure, finally comparing its AFM simulated images to the experimental ones. These results provide insights into the early phases of ice formation, which is a ubiquitous phenomenon ranging from biology to astrophysics.

Synergistic Niobium Doped Two-Dimensional Zirconium Diselenide: An Efficient Electrocatalyst for O2 Reduction Reaction

The development of high-activity and low-price cathodic catalysts to facilitate the electrochemically sluggish O2 reduction reaction (ORR) is very important to achieve the commercial application of fuel cells. Here, we have investigated the electrocatalytic activity of the two-dimensional single-layer Nb-doped zirconium diselenide (2D Nb-ZrSe2) toward ORR by employing the dispersion corrected density functional theory (DFT-D) method. Through our study, we computed structural properties, electronic properties, and energetics of the 2D Nb-ZrSe2 and ORR intermediates to analyze the electrocatalytic performance of 2D Nb-ZrSe2. The electronic property calculations depict that the 2D monolayer ZrSe2 has a large band gap of 1.48 eV, which is not favorable for the ORR mechanism. After the doping of Nb, the electronic band gap vanishes, and 2D Nb-ZrSe2 acts as a conductor. We studied both the dissociative and the associative pathways through which the ORR can proceed to reduce the oxygen molecule (O2). Our results show that the more favorable path for O2 reduction on the surface of the 2D Nb-ZrSe2 is the 4e- associative path. The detailed ORR mechanisms (both associated and dissociative) have been explored by computing the changes in Gibbs free energy (ΔG). All of the ORR reaction intermediate steps are thermodynamically stable and energetically favorable. The free energy profile for the associative path shows the downhill behavior of the free energy vs the reaction steps, suggesting that all ORR intermediate structures are catalytically active for the 4e- associative path and a high 4e- reduction pathway selectivity. Therefore, 2D Nb-ZrSe2 is a promising catalyst for the ORR, which can be used as an alternative ORR catalyst compared to expensive platinum (Pt).

How Water Binds to Microcline Feldspar (001)

Microcline feldspar (KAlSi3O8) is a common mineral with important roles in Earth's ecological balance. It participates in carbon, potassium, and water cycles, contributing to CO2 sequestration, soil formation, and atmospheric ice nucleation. To understand the fundamentals of these processes, it is essential to establish microcline's surface atomic structure and its interaction with the omnipresent water molecules. This work presents atomic-scale results on microcline's lowest-energy surface and its interaction with water, combining ultrahigh vacuum investigations by noncontact atomic force microscopy and X-ray photoelectron spectroscopy with density functional theory calculations. An ordered array of hydroxyls bonded to silicon or aluminum readily forms on the cleaved surface at room temperature. The distinct proton affinities of these hydroxyls influence the arrangement and orientation of the first water molecules binding to the surface, holding potential implications for the subsequent condensation of water.

DFT study of water on graphene: Synergistic effect of multilayer p-doping

Recent experiments related to a study concerning the adsorption of water on graphene have demonstrated the p-doping of graphene, although most of the ab initio calculations predict nearly zero doping. To shed more light on this problem, we have carried out van der Waals density functional theory calculations of water on graphene for both individual water molecules and continuous water layers with coverage ranging from one to eight monolayers. Furthermore, we have paid attention to the influence of the water molecule orientation toward graphene on its doping properties. In this article, we present the results of the band structure and the Bader charge analysis, showing the p-doping of graphene can be synergistically enhanced by putting 4-8 layers of an ice-like water structure on graphene having the water molecules oriented with oxygen atoms toward graphene.

Illuminating the Role of Mo Defective 2D Monolayer MoTe2 toward Highly Efficient Electrocatalytic O2 Reduction Reaction

The fuel cell is one of the solutions to current energy problems as it comes under green and renewable energy technology. The primary limitation of a fuel cell lies in the relatively slow rate of oxygen reduction reactions (ORR) that take place on the cathode, and this is an all-important reaction. An efficient electrocatalyst provides the advancement of green energy-based fuel cell technology, and it can speed up the ORR process. The present work provides the study of non-noble metal-based electrocatalyst for ORR. We have computationally designed a 3 × 3 supercell model of metal defective (Mo-defective) MoTe2 transition metal dichalcogenide (TMD) material to study its electrocatalytic activity toward ORR. This work provides a comprehensive analysis of all reaction intermediates that play a role in ORR on the surfaces of metal-deficient MoTe2. The first-principles-based dispersion-corrected density functional theory (in short DFT-D) method was implemented to analyze the reaction-free energies (ΔG) for each ORR reaction step. The present study indicates that the ORR on the surface of metal-defective MoTe2 follows the 4e- transfer mechanism. This study suggests that the 2D Mo-defective MoTe2 TMD has the potential to be an effective ORR electrocatalyst in fuel cells.

Realistic Modeling of the Electrocatalytic Process at Complex Solid-Liquid Interface

The rational design of electrocatalysis has emerged as one of the most thriving means for mitigating energy and environmental crises. The key to this effort is the understanding of the complex electrochemical interface, wherein the electrode potential as well as various internal factors such as H-bond network, adsorbate coverage, and dynamic behavior of the interface collectively contribute to the electrocatalytic activity and selectivity. In this context, the authors have reviewed recent theoretical advances, and especially, the contributions to modeling the realistic electrocatalytic processes at complex electrochemical interfaces,  and illustrated the challenges and fundamental problems in this field. Specifically, the significance of the inclusion of explicit solvation and electrode potential as well as the strategies toward the design of highly efficient electrocatalysts are discussed. The structure-activity relationships and their dynamic responses to the environment and catalytic functionality under working conditions are illustrated to be crucial factors for understanding the complexed interface and the electrocatalytic activities. It is hoped that this review can help spark new research passion and ultimately bring a step closer to a realistic and systematic modeling method for electrocatalysis.

Identification of a common ice nucleus on hydrophilic and hydrophobic close-packed metal surfaces

Establishing a general model of heterogeneous ice nucleation has long been challenging because of the surface water structures found on different substrates. Identifying common water clusters, regardless of the underlying substrate, is one of the key steps toward solving this problem. Here, we demonstrate the presence of a common water cluster found on both hydrophilic Pt(111) and hydrophobic Cu(111) surfaces using scanning tunneling microscopy and non-contact atomic force microscopy. Water molecules self-assemble into a structure with a central flat-lying hexagon and three fused pentagonal rings, forming a cluster consisting of 15 individual water molecules. This cluster serves as a critical nucleus during ice nucleation on both surfaces: ice growth beyond this cluster bifurcates to form two-dimensional (three-dimensional) layers on hydrophilic (hydrophobic) surfaces. Our results reveal the inherent similarity and distinction at the initial stage of ice growth on hydrophilic and hydrophobic close-packed metal surfaces; thus, these observations provide initial evidence toward a general model for water-substrate interaction.

Insights into the Rearrangement of the Molecular Assembly Structure of 6-Aminonicotinic Acid in Its Hydrated Environment

Water, as a ubiquitous and essential component of life, is known to have a significant impact on the structure and function of organic molecules. In this study, we investigate the role of water in the phase transition of organic molecular assembly structures by scanning tunneling microscopy at room temperature. The results show that the -O-H···O hydrogen induced by water molecules can lead to a significant structural transition in the molecular assembly, specifically through selective weakening of -C-H···O between 6-aminonicotinic acid and the formation of new -O-H···O bonds between 6-aminonicotinic acid and water molecules. Subsequent thermal treatment of these molecular assembly structures reveals that the formation of -N-H···O hydrogen bonds induced by water molecules has created a different pathway for the phase transition of the molecular assembly structure. This knowledge has important implications for the design of organic molecules with specific nanostructures that can be controlled through hydration.

Influence of Wettability and Geometry on Contact Electrification between Nonionic Insulators

Contact electrification is an interfacial process in which two surfaces exchange electrical charges when they are in contact with one another. Consequently, the surfaces may gain opposite polarity, inducing an electrostatic attraction. Therefore, this principle can be exploited to generate electricity, which has been precisely done in triboelectric nanogenerators (TENGs) over the last decades. The details of the underlying mechanisms are still ill-understood, especially the influence of relative humidity (RH). Using the colloidal probe technique, we convincingly show that water plays an important role in the charge exchange process when two distinct insulators with different wettability are contacted and separated in <1 s at ambient conditions. The charging process is faster, and more charge is acquired with increasing relative humidity, also beyond RH = 40% (at which TENGs have their maximum power generation), due to the geometrical asymmetry (curved colloid surface vs planar substrate) introduced in the system. In addition, the charging time constant is determined, which is found to decrease with increasing relative humidity. Altogether, the current study adds to our understanding of how humidity levels affect the charging process between two solid surfaces, which is even enhanced up to RH = 90% as long as the curved surface is hydrophilic, paving the way for designing novel and more efficient TENGs, eco-energy harvesting devices which utilize water and solid charge interaction mechanism, self-powered sensors, and tribotronics.

Orientational Water Bonding on Pt(111): Beyond the Frontier Orbital Principle

For decades, our understanding of water-metal bonding has been dominated by the frontier orbital principle in which globally stable water-metal interactions are ruled by HOMO interacting with metal surfaces. Using density functional theory calculations, herein, we have revealed that the frontier orbital principle cannot be applied to metastable water bonding on Pt(111), where the decisive role of HOMO is replaced by HOMO-1 in terms of the greatest orbital shifts and depopulations as the two different bonding indicators. Unlike the stable water configuration in which both HOMO-1 and HOMO prefer to overlap with metal states through σ-like orbital interactions, metastable configurations exhibit delicate competition or balance between σ-like and π-like orbital interactions exerted by HOMO-1 and HOMO, respectively. These findings have significantly deepened our understanding of orbital roles in water-metal bonding interactions and bridged the gap between theoretical understanding of electrified waters at electrochemical interfaces and water science on metal surfaces.

Transient Adsorption Behavior of Single Fluorophores on an Electrode-Supported Nanobubble

Here we report the use of a Langmuir isotherm model to analyze and better understand the dynamic adsorption and desorption behavior of single fluorophore molecules at the surface of a hydrogen nanobubble supported on an indium tin oxide (ITO) electrode. Three rhodamine dyes, rhodamine 110 (R110, positively charged), rhodamine 6G (R6G, positively charged), and sulforhodamine G (SRG, negatively charged) were chosen for this study. The use of the Langmuir isotherm model allows us to determine the equilibrium constant and the rate constants for the adsorption and desorption processes. Of the three fluorophores used in this study, SRG was found to have the greatest equilibrium constant. No significant potential dependence was observed on the adsorption characteristics, which suggests the nanobubble size, geometry, and surface properties are relatively constant within the range of potentials used in this study. Our results suggest that the use of the Langmuir isotherm model is a valid and useful means for probing and better understanding the unique adsorption behavior of fluorophores at surface-supported nanobubbles.

Ortho-para interconversion of nuclear states of H2O through replica transition state: prospect of quantum entanglement at homodromic Bjerrum defect site

CONTEXT

From a nuclear spin prospective, water exists as para and ortho nuclear spin isomers (isotopomers). Spin interconversions in isolated molecules of water are forbidden, but many recent reports have shown them to happen in bulk, through dynamic proton exchanges happening between interconnected networks of a large array of water molecules. In this contribution, a possible explanation for an unexpected slow or delayed interconversion of ortho-para water in ice observed in an earlier reported experiment is provided. Using the results of quantum mechanical investigations, we have discussed the roles played by Bjerrum defects in the dynamic proton exchanges and ortho-para spin state interconversions. We guess that at the sites of the Bjerrum defects, there are possibilities of quantum entanglements of states, through pairwise interactions. Based on the perfectly correlated exchange happening via a replica transition state, we speculate that it can have significant influences on ortho-para interconversions of water. We also conjecture that the overall ortho-para interconversion is not a continuous process, rather can be imagined to be happening serendipitously, but within the boundary of the rules of quantum mechanics.

METHODS

All computations were performed with Gaussian 09 program. B3LYP/6-31++G(d,p) methodology was used to compute all the stationary points. Further energy corrections were computed using CCSD(T)/aug-cc-pVTZ methodology. Intrinsic reaction coordinate (IRC) path computations were carried out for the transition states.

Structure prediction in low dimensions: concepts, issues and examples

Structure prediction of stable and metastable polymorphs of chemical systems in low dimensions has become an important field, since materials that are patterned on the nano-scale are of increasing importance in modern technological applications. While many techniques for the prediction of crystalline structures in three dimensions or of small clusters of atoms have been developed over the past three decades, dealing with low-dimensional systems-ideal one-dimensional and two-dimensional systems, quasi-one-dimensional and quasi-two-dimensional systems, as well as low-dimensional composite systems-poses its own challenges that need to be addressed when developing a systematic methodology for the determination of low-dimensional polymorphs that are suitable for practical applications. Quite generally, the search algorithms that had been developed for three-dimensional systems need to be adjusted when being applied to low-dimensional systems with their own specific constraints; in particular, the embedding of the (quasi-)one-dimensional/two-dimensional system in three dimensions and the influence of stabilizing substrates need to be taken into account, both on a technical and a conceptual level. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Evaluating Adsorbate-Solvent Interactions: Are Dispersion Corrections Necessary?

Incorporating solvent-adsorbate interactions is paramount in models of aqueous (electro)catalytic reactions. Although a number of techniques exist, they are either highly demanding in computational terms or inaccurate. Microsolvation offers a trade-off between accuracy and computational expenses. Here, we dissect a method to swiftly outline the first solvation shell of species adsorbed on transition-metal surfaces and assess their corresponding solvation energy. Interestingly, dispersion corrections are generally not needed in the model, but caution is to be exercised when water-water and water-adsorbate interactions are of similar magnitude.

Promotion Effect of Well-Defined Deposited Water Layer on Carbon Monoxide Oxidation Catalyzed by Single-Atom Alloys

Single-atom alloys (SAAs) exhibit excellent catalytic performance and unique electronic structures, emerging as promising catalysts for potential industrial reactions. While most of them have been widely employed under reducing conditions, few are applied in oxidation reactions. Herein, using density functional theory calculations and microkinetic simulations, we demonstrate that a well-defined one water layer can improve CO oxidation on model SAAs, with reaction rates increased by orders of magnitude. It is found that the formation of hydrogen bonds and the transfer of charges effectively enhance the adsorption and activation of oxygen molecules at the H₂O/SAA interfaces, which not only increases the surface coverage of O₂ species but also reduces the energy barrier of CO oxidation. The proposed strategy in this work would extend the application range of SAA catalysts to oxidation reactions.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Hydrogen Exchange through Hydrogen Bonding between Methanol and Water in the Adsorbed State on Cu(111)

The interaction between submonolayers of methanol and water on Cu(111) is studied at 95-160 K temperature range with surface-sensitive infrared spectroscopy using isotopically labeled molecules. The initial interaction of methanol with the preadsorbed amorphous solid water at 95 K is through hydrogen-bonding with the dangling hydroxyl groups of water. Upon increasing the temperature up to 140 K, methanol and deuterated water form H-bonded structures which allow hydrogen-deuterium exchange between the hydroxyl group of methanol and the deuterated water. The evolution of the O-D and O-H stretching bands indicate that the hydrogen transfer is dominant at around 120-130 K, slightly below the desorption temperature of methanol. Above 140 K, methanol desorbs and a mixture of hydrogen-related water isotopologues remains on the surface. The isotopic composition of this mixture versus the initial D₂O:CH₃OH ratio supports a potential exchange mechanism via hydrogen hopping between alternating methanol and water molecules in a hydrogen-bonded network.

Machine Learning Assisted Simulations of Electrochemical Interfaces: Recent Progress and Challenges

The electrochemical interface, where the adsorption of reactants and electrocatalytic reactions take place, has long been a focus of attention. Some of the important processes on it tend to possess relatively slow kinetic characteristics, which are usually beyond the scope of ab initio molecular dynamics. The newly emerging technique, machine learning methods, provides an alternative approach to achieve thousands of atoms and nanosecond time scale while ensuring precision and efficiency. In this Perspective, we summarize in detail the recent progress and achievements made by the introduction of machine learning to simulate electrochemical interfaces, and focus on the limitations of current machine learning models, such as accurate descriptions of long-range electrostatic interactions and the kinetics of the electrochemical reactions occurring at the interface. Finally, we further point out the future directions for machine learning to expand in the field of electrochemical interfaces.

Unexpectedly Spontaneous Water Dissociation on Graphene Oxide Supported by Copper Substrate

Water dissociation is of fundamental importance in scientific fields and has drawn considerable interest in diverse technological applications. However, the high activation barrier of breaking the OH bond within the water molecule has been identified as the bottleneck, even for the water adsorbed on the graphene oxide (GO). Herein, using the density functional theory calculations, we demonstrate that the water molecule can be spontaneously dissociated on GO supported by the (111) surface of the copper substrate (Copper-GO). This process involves a proton transferring from water to the interfacial oxygen group, and a hydroxide covalently bonding to GO. Compared to that on GO, the water dissociation barrier on Copper-GO is significantly decreased to be less than or comparable to thermal fluctuations. This is ascribed to the orbital-hybridizing interaction between copper substrate and GO, which enhances the reaction activity of interfacial oxygen groups along the basal plane of GO for water dissociation. Our work provides a novel strategy to access water dissociation via the substrate-enhanced reaction activity of interfacial oxygen groups on GO and indicates that the substrate can serve as an essential key to tuning the catalytic performance of various two-dimensional material devices.

A General Framework of Scanning Tunneling Microscopy Based on Bardeen's Approximation for Isolated Molecules

Scanning tunneling microscopy (STM) is one of the most popular techniques for precise characterization. Yet, its current theoretical implementation is often based on the periodic boundary condition with the Tersoff-Hamann approximation, which is inefficient to explore the tip states other than the s-wave and to treat properly the charged molecules that are ubiquitous in chemistry. In this work, we establish a general theoretical framework for STM image simulations, which is based on the Bardeen's approximation and utilizes the boundary condition of the cluster model. We develop an analytic algorithm for the precise evaluation of the transfer Hamiltonian matrix, addressing correctly the asymptotic behaviors of the tip states. Numerical results demonstrate that the molecular images under different STM tip states and mapping modes can be quantitatively simulated in the present framework, which paves the avenue for the conclusive investigation of the ground state electronic structures for either neutral or charged molecules.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Cluster Formation Effect of Water on Pristine and Defective MoS2 Monolayers

The structure and electronic properties of the molybdenum disulfide (MoS₂) monolayer upon water cluster adsorption are studied using density functional theory and the optical properties are further analyzed with the Bethe-Salpeter equation (BSE). Our results reveal that the water clusters are electron acceptors, and the acceptor tendency tends to increase with the size of the water cluster. The electronic band gap of both pristine and defective MoS₂ is rather insensitive to water cluster adsorbates, as all the clusters are weakly bound to the MoS₂ surface. However, our calculations on the BSE level show that the adsorption of the water cluster can dramatically redshift the optical absorption for both pristine and defective MoS₂ monolayers. The binding energy of the excitons of MoS₂ is greatly enhanced with the increasing size of the water cluster and finally converges to a value of approximately 1.16 eV and 1.09 eV for the pristine and defective MoS₂ monolayers, respectively. This illustrates that the presence of the water cluster could localize the excitons of MoS₂, thereby greatly enhance the excitonic binding energy.

Water Structures Reveal Local Hydrophobicity on the In2O3(111) Surface

Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In₂O₃(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, nc-AFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets.

Direct Measurement of Solid-Liquid Interfacial Energy Using a Meniscus

Solid-liquid interactions are central to diverse processes. The interaction strength can be described by the solid-liquid interfacial free energy (γ_{SL}), a quantity that is difficult to measure. Here, we present the direct experimental measurement of γ_{SL} for a variety of solid materials, from nonpolar polymers to highly wetting metals. By attaching a thin solid film on top of a liquid meniscus, we create a solid-liquid interface. The interface determines the curvature of the meniscus, analysis of which yields γ_{SL} with an uncertainty of less than 10%. Measurement of classically challenging metal-water interfaces reveals γ_{SL}∼30-60  mJ/m^{2}, demonstrating quantitatively that water-metal adhesion is 80% stronger than the cohesion energy of bulk water, and experimentally verifying previous quantum chemical calculations.

Water adsorption and dynamics on graphene and other 2D materials: Computational and experimental advances

The interaction of water and surfaces, at molecular level, is of critical importance for understanding processes such as corrosion, friction, catalysis and mass transport. The significant literature on interactions with single crystal metal surfaces should not obscure unknowns in the unique behaviour of ice and the complex relationships between adsorption, diffusion and long-range inter-molecular interactions. Even less is known about the atomic-scale behaviour of water on novel, non-metallic interfaces, in particular on graphene and other 2D materials. In this manuscript, we review recent progress in the characterisation of water adsorption on 2D materials, with a focus on the nano-material graphene and graphitic nanostructures; materials which are of paramount importance for separation technologies, electrochemistry and catalysis, to name a few. The adsorption of water on graphene has also become one of the benchmark systems for modern computational methods, in particular dispersion-corrected density functional theory (DFT). We then review recent experimental and theoretical advances in studying the single-molecular motion of water at surfaces, with a special emphasis on scattering approaches as they allow an unparalleled window of observation to water surface motion, including diffusion, vibration and self-assembly.

Unraveling the liquid gliding on vibrating solid liquid interfaces with dynamic nanoslip enactment

Slip length describes the classical no-slip boundary condition violation of Newtonian fluid mechanics, where fluids glide on the solid surfaces. Here, we propose a new analytical model validated by experiments for characterization of the liquid slip using vibrating solid surfaces. Essentially, we use a microfluidic system integrated with quartz crystal microbalance (QCM) to investigate the relationship between the slip and the mechanical response of a vibrating solid for a moving fluid. We discover a liquid slip that emerges especially at high flow rates, which is independent of the surface wetting condition, having significant contributions to the changes in resonant frequency of the vibrating solid and energy dissipation on its surface. Overall, our work will lead to consideration of ‘missing slip’ in the vibrating solid-liquid systems such as the QCM-based biosensing where traditionally frequency changes are interpreted exclusively with mass change on the sensor surface, irrespective of the flow conditions.

A fluid flowing in solid confinement will glide, rather than stick to, the solid’s surfaces. This is usually described by introducing a concept known as slip length. The liquid slip concept is now extended for the situation of a vibrating solid–liquid interface.

The interaction mechanism of cesium with water on the SrTiO3(100) surface at room temperature

The interaction of water with cesium on the strontium titanate surface SrTiO3(100), was studied, mainly by means of work function measurements and thermal desorption spectroscopy. The catalytic role of cesium with respect to the dissociation of water on surface was investigated, by applying two different adsorption processes at room temperature (RT): (1) The adsorption of water on the cesium covered surface (sequential adsorption), and (2) the co-adsorption process (simultaneous adsorption) on surface. Based on the results and by adopting the Lewis acid–base model, we conclude that during the sequential adsorption the water molecules are mostly adsorbs non-dissociatively on surface, without oxidizing the alkaline overlayer. This seems to be due, first to the strong interaction between the alkaline adatoms and the substrate, and secondly to the limited maximum pre-deposited amount of cesium (≤ 0.45 ML). Instead, water dissociation appears to merely occur on defective sites of the substrate in accordance with previous studies. For a full cesium layer covered surface, the adsorbed water retracts the metallicity of cesium due to electrostatic interactions. In contrast to the sequential adsorption, during the co-adsorption process the oxidation of cesium takes place above a critical coverage of cesium (≥ 0.45 ML). It appears that the co-adsorbed cesium with water modifies the surface potential providing an effective template for cesium oxide, Cs2O development. Based on that, we suggest a catalytic reaction of water dissociation according to the Langmuir–Hinshelwood mechanism. Finally, we propose atomistic adsorption models for both processes of cesium with water adsorption.

A Happy Get-Together - Probing Electrochemical Interfaces by Non-Linear Vibrational Spectroscopy

Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid-electrolyte interfaces are crucial for an in-depth mechanistic understanding of their function. Vibrational sum-frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this review discusses the happy get-together between VSFG spectroscopy and electrochemical interfaces. Methodological approaches for answering core issues associated with the behavior of adsorbates on electrodes, the structure of solvent adlayers, the transient formation of reaction intermediates, and the emergence of solid electrolyte interphase in battery research are assessed to provide a critical inventory of highly promising avenues to bring optical spectroscopy to use in modern material research in energy conversion and storage.

Accurate prediction of ice nucleation from room temperature water

Crystal nucleation is one of the most fundamental processes in the physical sciences and almost always occurs heterogeneously with the aid of a nucleating substrate. No example of nucleation is more ubiquitous and impactful than the formation of ice, vital to fields as diverse as geology, biology, aeronautics, and climate science. However, despite considerable effort, we still cannot predict a priori the efficacy of a nucleating agent. Here we utilize deep learning methods to accurately predict nucleation ability from images of room temperature liquid water-generated from molecular dynamics simulations-on a broad range of substrates. The resulting model, named IcePic, can rapidly and accurately infer nucleation ability, eliminating the requirement for either notoriously expensive simulations or direct experimental measurement. In an online poll, IcePic was found to significantly outperform humans in predicting the ice nucleating efficacy of materials. By analyzing the typical errors made by humans, as well as the application of reverse interpretation methods, physical insights into the role the water contact layer plays in ice nucleation have been obtained. Moving forward, we suggest that IcePic can be used as an easy, cheap, and rapid way to discern the nucleation ability of substrates, also with potential for learning other properties related to interfacial water.

Quantitative Characterization of Oxygen-Containing Groups on the Surface of Carbon Materials: XPS and NEXAFS Study

The results of the comparative quantitative study of oxygen-containing groups adsorbed on the surface of carbonized sponge scaffold (CSS), highly oriented pyrolytic graphite (HOPG), fullerite C60 and multi-walled carbon nanotubes (MWCNTs) introduced into a high vacuum from the atmosphere without any pre-treatment of the surface are discussed. The studied materials are first tested by XRD and Raman spectroscopy, and then quantitatively characterized by XPS and NEXAFS. The research results showed the presence of carbon oxides and water-dissociation products on the surfaces of materials. It was shown that main source of oxygen content (~2%) on the surface of HOPG, MWCNTs, and C60 powder is water condensed from the atmosphere in the form of an adsorbed water molecule and hydroxyl group. On the CSS surface, oxygen atoms are present in the forms of carbon oxides (4–5%) and adsorbed water molecules and hydroxyl groups (5–6%). The high content of adsorbed water on the CSS surface is due to the strong roughness and high porosity of the surface.

The pressure induced phase diagram of double-layer ice under confinement: a first-principles study

Here, we present double-layer ice confined within various carbon nanotubes (CNTs) using state-of-the-art pressure induced (-5 GPa to 5 GPa) dispersion corrected density functional theory (DFT) calculations. We find that the double-layer ice exhibits remarkably rich and diverse phase behaviors as a function of pressure with varying CNT diameters. The lattice cohesive energies for various pure double layer ice phases follow the order of hexagonal > pentagonal > square tube > hexagonal-close-packed (HCP) > square > buckled-rhombic (b-RH). The confinement width was found to play a crucial role in the square and square tube phases in the intermediate pressure range of about 0-1 GPa. Unlike the phase transition in pure bilayer ice structures, the relative enthalpies demonstrate that the pentagonal phase, rather than the hexagonal structure, is the most stable ice polymorph at ambient pressure as well as in a deep negative pressure region, whereas the b-RH phase dominates under high pressure. The relatively short O⋯O distance of b-RH demonstrates the presence of a strong hydrogen bonding network, which is responsible for stabilizing the system.