Structure of water adsorbed on the open Cu(110) surface: H-up, H-down, or both?

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

Wetting of a Stepped Platinum (211) Surface

Steps stabilize water adsorption on metal surfaces, providing favorable binding sites for water during wetting or ice nucleation, but there is limited understanding of the local water arrangements formed on such surfaces. Here we describe the structural evolution of water on the stepped Pt(211) surface using thermal desorption, low-energy electron diffraction, and scanning tunneling microscopy to probe the water structure. At low coverage water forms linear structures comprising zigzag chains along the steps that are decorated by H-bonded rings every one or two units along the terrace. Simple 2-coordinate H-bonded chains are not observed, indicating the Pt step binds too weakly to compensate entirely for a low water H-bond coordination number. As the coverage increases, water chains assemble into a disordered (2 × 1) structure, likely made up of the same narrow water chains along the steps with little or no H-bonding between adjacent structures. The chain structure disappears as water adsorption saturates the surface to form an incommensurate, disordered network of water rings of different size. Although the steps on Pt(211) clearly stabilize water adsorption and direct growth, the surface does not support the simple 1D chains previously proposed or an ordered 2D network such as seen on other surfaces. We discuss reasons for this and the factors that determine the behavior of the first water layer on stepped metal surfaces.

Ab Initio Simulations of Water/Metal Interfaces

Structures and processes at water/metal interfaces play an important technological role in electrochemical energy conversion and storage, photoconversion, sensors, and corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid-liquid interfaces, which requires combining concepts from the chemistry and physics of crystalline materials and liquids. Particularly interesting is the fact that the water-water and water-metal interactions are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. Because water is a polar molecule and water and metal surfaces are both polarizable, explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters, and icelike layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water-water and water-metal interactions will be discussed, and electrochemical aspects will be addressed. Finally, more approximate but numerically less demanding approaches to treat water at metal surfaces from first-principles will be briefly discussed.

Simulations of x-ray absorption spectra for CO desorbing from Ru(0001) with transition-potential and time-dependent density functional theory approaches

The desorption of a carbon monoxide molecule from a Ru(0001) surface was studied by means of X-ray Absorption Spectra (XAS) computed with Transition Potential (TP-DFT) and Time Dependent (TD-DFT) DFT methods. By unraveling the evolution of the CO electronic structure upon desorption, we observed that at 2.3 Å from the surface, the CO molecule has already predominantly gas-phase character. While C 1s XAS is quite insensitive to changes in the C-O bond length, the O 1s excitation is very sensitive with the π* coming down in energy upon CO bond stretching, which competes with the increase in orbital energy due to the repulsive interaction with the metallic surface. We show in a systematic way that the TP-DFT method can describe the XAS rather well at the endpoints (chemisorbed and gas phase) but is affected by artificial charge transfer and/or incorrect spin treatment in the transition region in cases like CO, where there are low-lying π* orbitals and large exchange interactions between the core 1s and valence-acceptor π* orbitals. As an alternative, we demonstrate by comparing with experimental data that a linear response approach using TD-DFT employing common exchange-correlation functionals and finite-size clusters can yield a good description of the spectral evolution of the 1s → π* transition with correct spin and gas-to-chemisorbed chemical shifts in good agreement with experiment.

Water Dissociation and Hydroxyl Formation on Ni(110)

Nickel is an active catalyst for hydrogenation and re-forming reactions, with the reactions showing a strong dependence on the surface exposed. Here, we describe the mixed hydroxyl-water phases formed during water dissociation on Ni(110) using scanning tunneling microscopy and low-current low-energy electron diffraction. Water dissociation starts between 150 and 180 K as the H-bond structure evolves from linear one-dimensional (1D) chains of intact water into a two-dimensional (2D) network containing short rows of face-sharing hexagonal rings. As further water desorbs, the hexagonal rows adopt a local (2 × 3) arrangement, forming small, disordered domains separated by strain relief features. Decomposition of this phase occurs near 220 K to form linear 1D structures consisting of flat, zigzag water chains, with each water stabilized by donating one H to hydroxyl to form a branched chain structure. The OH-H₂O chains repel each other, with the saturation layer ordering into a (2 0, 1 4) structure that decomposes to OH near 245 K as further water desorbs. The structure of the mixed OH/H₂O phases is discussed and contrasted with those found on the related Cu(110) surface, with the differences attributed to strain in the 2D H-bond network caused by the short Ni lattice spacing and strong bond to OH/H₂O.

Hydration of a 2D Supramolecular Assembly: Bitartrate on Cu(110)

Hydration layers play a key role in many technical and biological systems, but our understanding of these structures remains very limited. Here, we investigate the molecular processes driving hydration of a chiral metal–organic surface, bitartrate on Cu(110), which consists of hydrogen-bonded bitartrate rows separated by exposed Cu. Initially water decorates the metal channels, hydrogen bonding to the exposed O ligands that bind bitartrate to Cu, but does not wet the bitartrate rows. At higher temperature, water inserts into the structure, breaks the existing intermolecular hydrogen bonds, and changes the adsorption site and footprint. Calculations show this process is driven by the creation of stable adsorption sites between the carboxylate ligands, to allow hydration of O–Cu ligands within the interior of the structure. This work suggests that hydration of polar metal–adsorbate ligands will be a dominant factor in many systems during surface hydration or self-assembly from solution.

Understanding Cement Hydration of Cemented Paste Backfill: DFT Study of Water Adsorption on Tricalcium Silicate (111) Surface

Understanding cement hydration is of crucial importance for the application of cementitious materials, including cemented paste backfill. In this work, the adsorption of a single water molecule on an M3-C3S (111) surface is investigated using density functional theory (DFT) calculations. The adsorption energies for 14 starting geometries are calculated and the electronic properties of the reaction are analysed. Two adsorption mechanisms, molecular adsorption and dissociative adsorption, are observed and six adsorption configurations are found. The results indicate that spontaneous dissociative adsorption is energetically favored over molecular adsorption. Electrons are transferred from the surface to the water molecule during adsorption. The density of states (DOS) reveals the bonding mechanisms between water and the surface. This study provides an insight into the adsorption mechanism at an atomic level, and can significantly promote the understanding of cement hydration within such systems.

Theoretical Insights into the Solvent Polarity Effect on the Quality of Self-Assembled N-Octadecanethiol Monolayers on Cu (111) Surfaces

The effect of solvent polarity on the quality of self-assembled n-octadecanethiol (C₁₈SH) on Cu surfaces was systematically analyzed using first-principles calculations. The results indicate that the adsorption energy for C₁₈SH on a Cu surface is −3.37 eV, which is higher than the adsorption energies of the solvent molecules. The higher adsorption energy of dissociated C₁₈SH makes the monolayer self-assembly easier on a Cu (111) surface through competitive adsorption. Furthermore, the adsorption energy per unit area for C₁₈SH decreases from −3.24 eV·Å⁻² to −3.37 eV·Å⁻² in solvents with an increased dielectric constant of 1 to 78.54. Detailed energy analysis reveals that the electrostatic energy gradually increases, while the kinetic energy decreases with increasing dielectric constant. The increased electrostatic energies are mainly attributable to the disappearance of electrostatic interactions on the sulfur end of C₁₈SH. The decreased kinetic energy is mainly due to the generated push force in the polar solvent, which limits the mobility of C₁₈SH. A molecular dynamics simulation also confirms that the -CH₃ site has a great interaction with CH₃(CH₂)₄CH₃ molecules and a weak interaction with CH₃CH₂OH molecules. The different types of interactions help to explain why the surface coverage of C₁₈SH on Cu in a high-polarity ethanol solution is significantly larger than that in a low-polarity n-hexane solution at the stabilized stage.

Water at Interfaces

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Tribological Behavior of Aqueous Copolymer Lubricant in Mixed Lubrication Regime

Although a number of experiments have been attempted to investigate the lubrication of aqueous copolymer lubricant, which is applied widely in metalworking operations, a comprehensive theoretical investigation at atomistic level is still lacking. This study addresses the influence of loading pressure and copolymer concentration on the structural properties and tribological performance of aqueous copolymer solution of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) (PPO-PEO-PPO) at mixed lubrication using a molecular dynamic (MD) simulation. An effective interfacial potential, which has been derived from density functional theory (DFT) calculations, was employed for the interactions between the fluid's molecules and iron surface. The simulation results have indicated that the triblock copolymer is physisorption on iron surface. Under confinement by iron surfaces, the copolymer molecules form lamellar structure in aqueous solution and behave differently from its bulk state. The lubrication performance of aqueous copolymer lubricant increases with concentration, but the friction reduction is insignificant at high loading pressure. Additionally, the plastic deformation of asperity is dependent on both copolymer concentration and loading pressure, and the wear behavior shows a linear dependence of friction force on the number of transferred atoms between contacting asperities.

Comparison of x-ray absorption spectra between water and ice: new ice data with low pre-edge absorption cross-section

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.

A different view of structure-making and structure-breaking in alkali halide aqueous solutions through x-ray absorption spectroscopy

X-ray absorption spectroscopy measured in transmission mode was used to study the effect of alkali and halide ions on the hydrogen-bonding (H-bonding) network of water. Cl(-) and Br(-) are shown to have insignificant effect on the structure of water while I(-) locally weakens the H-bonding, as indicated by a sharp increase of the main-edge feature in the x-ray absorption spectra. All alkali cations act as structure-breakers in water, weakening the H-bonding network. The spectral changes are similar to spectra of high density ices where the 2nd shell has collapsed due to a break-down of the tetrahedral structures, although here, around the ions, the breakdown of the local tetrahedrality is rather due to non-directional H-bonding to the larger anions. In addition, results from temperature-dependent x-ray Raman scattering measurements of NaCl solution confirm the H-bond breaking effect of Na(+) and the effect on the liquid as similar to an increase in temperature.

Co-adsorption of water and glycine on Cu{110}

In this study, we use density functional theory (DFT) to investigate the surface co-adsorption of glycine with water on Cu{110}. Our results show that, under UHV conditions and for a wide range of temperatures, a pure glycine monolayer is more stable than either mixed gly-water phases or pure water (ice) monolayers, but for a high water pressure half-dissociated water layers can appear on the surface at low and medium temperatures.

A unified study for water adsorption on metals: meaningful models from structural motifs

We present a comprehensive structural model that allows the rapid assessment of the first layer of water adsorption on metals for different motifs.

A seven-crystal Johann-type hard x-ray spectrometer at the Stanford Synchrotron Radiation Lightsource

We present a multicrystal Johann-type hard x-ray spectrometer (~5-18 keV) recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The instrument is set at the wiggler beamline 6-2 equipped with two liquid nitrogen cooled monochromators--Si(111) and Si(311)--as well as collimating and focusing optics. The spectrometer consists of seven spherically bent crystal analyzers placed on intersecting vertical Rowland circles of 1 m of diameter. The spectrometer is scanned vertically capturing an extended backscattering Bragg angular range (88°-74°) while maintaining all crystals on the Rowland circle trace. The instrument operates in atmospheric pressure by means of a helium bag and when all the seven crystals are used (100 mm of projected diameter each), has a solid angle of about 0.45% of 4π sr. The typical resolving power is in the order of E/ΔE ~ 10,000. The spectrometer's high detection efficiency combined with the beamline 6-2 characteristics permits routine studies of x-ray emission, high energy resolution fluorescence detected x-ray absorption and resonant inelastic x-ray scattering of very diluted samples as well as implementation of demanding in situ environments.

X-ray absorption from large molecules at metal surfaces: theoretical and experimental results for Co-OEP on Ni(100)

Metal octaethylporphyrins (M-OEP), M-N(4)C(20)H(4)(C(2)H(5))(8), adsorbed at a metallic substrate are promising candidates to provide spin dependent electric transport. Despite these systems having been studied extensively by experiment, details of the adsorbate geometry and surface binding are still unclear. We have carried out density functional theory calculations for cobalt octaethyl porphyrin (Co-OEP) adsorbate at clean and oxygen-covered Ni(100) surfaces as well as for the free Co-OEP molecule where equilibrium structures were obtained by corresponding energy optimizations. These geometries were then used in calculations of Co-OEP carbon and nitrogen 1s core excitations yielding theoretical excitation spectra to be compared with corresponding K-edge x-ray absorption fine structure (NEXAFS) measurements. The experimental NEXAFS spectra near the carbon K-edge of Co-OEP bulk material show large intensity close to the ionization threshold and a triple-peak structure at lower energies, which can be reproduced by the calculations on free Co-OEP. The experimental nitrogen K-edge spectra of adsorbed Co-OEP layers exhibit always a double-peak structure below ionization threshold, independent of the layer thickness. The peaks are shifted slightly and their separation varies with adsorbate-substrate distance. This can be explained by hybridization of N 2p with corresponding 3d contributions of the Ni substrate in the excited final state orbitals as a result of adsorbate-substrate binding via N-Ni bond formation.

Nano-ice models for the water aggregates observed on the h-BN/Rh(111) nanomesh

When a large amount of water is deposited onto a bare h-BN/Rh(111) nanomesh, the formation of ordered and stable nano-ice crystals in the pores has been experimentally observed. The present work proposes different possible models for the structure of the observed clusters, based on density functional theory calculations of two-dimensional water lattices adsorbed on free-standing hexagonal BN. Through the investigation of the electronic properties, the interaction with BN, and the distribution of the molecular dipoles, the most probable two-dimensional arrangement has been identified. Finally, a model is proposed for the nano-ice cluster trapped in the pore of the nanomesh, which constitutes 38 molecules distributed according to the most probable two-dimensional arrangement on free-standing BN. Structural and electronic properties of the optimized nano-ice cluster are also reported, and it is shown that the model is consistent with the experimental observation.

Strain relief and disorder in commensurate water layers formed on Pd(111)

Water adsorbs and desorbs intact on Pd(111), forming a hydrogen-bonded wetting layer whose structure we examine by low energy electron diffraction (LEED) and He atom scattering (HAS). LEED shows that water forms commensurate (√3 × √3)R30° clusters that aggregate into a partially ordered, approximately (7 × 7) superstructure as the layer completes. HAS indicates that the water layer remains disordered on a local (approximately 10 Å) scale. Based on workfunction measurements and density functional theory simulations we propose that water forms small, flat domains of a commensurate (√3 × √3)R30° water network, separated by disordered domain boundaries containing largely H-down water. This arrangement allows the water layer to adapt its density and relieve the lateral strain associated with adsorbing water in the optimum flat atop adsorption site. We discuss different possibilities for the structure of these domain walls and compare this strain relief mechanism to the highly ordered, large unit cell structures formed on surfaces such as Pt(111).

X-ray Raman scattering provides evidence for interfacial acetonitrile-water dipole interactions in aqueous solutions

Aqueous solutions of acetonitrile (MeCN) have been studied with oxygen K-edge x-ray Raman scattering (XRS) which is found to be sensitive to the interaction between water and MeCN. The changes in the XRS spectra can be attributed to water directly interacting with MeCN and are reproduced by density functional theory calculations on small clusters of water and MeCN. The dominant structural arrangement features dipole interaction instead of H-bonds between the two species as revealed by the XRS spectra combined with spectrum calculations. Small-angle x-ray scattering shows the largest heterogeneity for a MeCN to water ratio of 0.4 in agreement with earlier small-angle neutron scattering data.

c(2×2) water-hydroxyl layer on Cu(110): a wetting layer stabilized by Bjerrum defects

Understanding the composition and stability of mixed water-hydroxyl layers is a key step in describing wetting and how surfaces respond to redox processes. Here we show that, instead of forming a complete hydrogen bonding network, structures containing an excess of water over hydroxyl are stabilized on Cu(110) by forming a distorted hexagonal network of water-hydroxyl trimers containing Bjerrum defects. This arrangement maximizes the number of strong bonds formed by water donation to OH and provides uncoordinated OH groups able to hydrogen bond multilayer water and nucleate growth.

A one-dimensional ice structure built from pentagons

Heterogeneous ice nucleation has a key role in fields as diverse as atmospheric chemistry and biology. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular scale on a well-defined, planar interface. A common feature of structural models for such films is that they are built from hexagonal arrangements of molecules. Here we show, through a combination of scanning tunnelling microscopy, infrared spectroscopy and density-functional theory, that about 1-nm-wide ice chains that nucleate on Cu(110) are not built from hexagons, but instead are built from a face-sharing arrangement of water pentagons. The pentagon structure is favoured over others because it maximizes the water-metal bonding while maintaining a strong hydrogen-bonding network. It reveals an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favour non-hexagonal structural units.

Simulating ice nucleation, one molecule at a time, with the 'DFT microscope'

Few physical processes are as ubiquitous as the nucleation of water into ice. However, ice nucleation and, in particular, heterogeneously catalyzed nucleation remains poorly understood at the atomic level. Here, we report an initial series of density functional theory (DFT) calculations aimed at putting our understanding of ice nucleation and water clustering at metallic surfaces on a firmer footing. Taking a prototype hydrophobic metal surface, Cu(111), for which scanning tunneling microscopy measurements of water clustering have recently been performed, possible structures of adsorbed clusters comprised of 2-6 H2O molecules have been computed. How the water clusters in this size regime differ from those in the gas phase is discussed, as is the nature of their interaction with the substrate.

Influence of aggregation, defects, and contaminant oxygen on water dissociation at Cu(110) surface: A theoretical study

The DFT-PW91 slab model approach is employed to investigate the influence of aggregation, surface defects, and contaminant oxygen on water dissociation on Cu(110) at low temperatures. The dissociation barriers of water in various aggregate states are calculated in the range of 60-75 kJ/mol on the clean surfaces, in nice agreement with the experimentally determined values. It is revealed that the aggregation of water shows no propensity to reduce the activation barrier for the O-H bond breaking on Cu(110), at variance with the water chemistry on Ru(0001). The calculated activation energy on Cu(211) which is the most active stepped surface investigated is equal to the value on the (110) surface, indicating that the hydroxyl groups observed on Cu(110) at low temperatures may not stem from surface defects. The coadsorbed oxygen, whether as a "spectator" or a "participant," facilitates the water dissociation both kinetically and thermodynamically.

Ice nanoclusters at hydrophobic metal surfaces

Studies of the structure of supported water clusters provide a means for obtaining a rigorous molecular-scale description of the initial stages of heterogeneous ice nucleation: a process of importance to fields as diverse as atmospheric chemistry, astrophysics and biology. Here, we report the observation and characterization of metal-supported water hexamers and a family of hydrated nanoclusters--heptamers, octamers and nonamers--through a combination of low-temperature scanning tunnelling microscopy experiments and first-principles electronic-structure calculations. Aside from achieving unprecedented resolution of the cyclic water hexamer--the so-called smallest piece of ice--we identify and explain a hitherto unknown competition between the ability of water molecules to simultaneously bond to a substrate and to accept hydrogen bonds. This competition also rationalizes previous structure predictions for water clusters on other substrates.