Metal-water Interface Research Articles

First-principles Calculations under an External Electric Field: Electrochemical Reactions and Control of Electronic Structures

Electronic structure study under an external electric field is one of the important subjects in the electronic device development and sustainable energy technologies. We developed a calculation method, called effective screening medium (ESM) method, to include the external electric field or bias potential within the density functional theory. In this article, the details of the ESM method are explained and two recent researches are reviewed. First, a simulation of the electrochemical reaction on a metal-water interface is reviewed. We have elucidated an electronic state modulation of metal surface during the reaction. Second, a study about the electronic structure of an ABC-stacked graphite thin film with or without an external electric field is reviewed. We have found that ferrimagnetic spin polarization occurs on the both surface sides of the thin film. The polarization is associated with the peculiar surface localized state that possesses the same characteristics as the edge-localized states of graphene nanoribbons. The spin polarization is, therefore, controllable by applying an external electric field.

A gentler approach to RNEMD: Nonisotropic velocity scaling for computing thermal conductivity and shear viscosity

We present a new method for introducing stable nonequilibrium velocity and temperature gradients in molecular dynamics simulations of heterogeneous systems. This method extends earlier reverse nonequilibrium molecular dynamics (RNEMD) methods which use momentum exchange swapping moves. The standard swapping moves can create nonthermal velocity distributions and are difficult to use for interfacial calculations. By using nonisotropic velocity scaling (NIVS) on the molecules in specific regions of a system, it is possible to impose momentum or thermal flux between regions of a simulation while conserving the linear momentum and total energy of the system. To test the method, we have computed the thermal conductivity of model liquid and solid systems as well as the interfacial thermal conductivity of a metal-water interface. We find that the NIVS-RNEMD improves the problematic velocity distributions that develop in other RNEMD methods.

Scannable Plasmonic Trapping Using a Gold Stripe

Using counterpropagating surface plasmon polaritons (SPPs) on a gold stripe, we demonstrate a scannable integrated optical tweezer. We demonstrate the trapping of individual fluorescent beads on the stripe, which supports a single quasi-transverse magnetic (TM) mode at the metal-water interface. The beads are localized to the stripe center, with a standard deviation of 51 nm transverse to the stripe, corresponding to a trap stiffness of 1.7 pN/microm. The localization along the stripe is achieved by balancing the scattering forces from the two counterpropagating SPPs excited by prism coupling. The particle position along the stripe can be controlled by varying the relative intensity of the two input beams. This work adds an important new capability to plasmonic optical tweezers, that of scanning. We anticipate that this will broaden the range of applications of plasmonic optical manipulation.

Broadband optoacoustic measurements of ultrasound attenuation in severely plastically deformed nickel

Laser optoacoustics and immersion techniques allowed a broadband ultrasound spectroscopy which was used for measuring the attenuation of severely plastically deformed nickel. A disk shaped specimen of nickel of about 33 mm diameter and 2.5 mm thickness was prepared by the high pressure torsion method. The produced equivalent shear strain linearly increased from a minimum at the center up to 1000% at the edge, gradually refining the grain size distribution down to 200 nm. The metal water interface was illuminated by 5 ns laser pulses, generating longitudinal ultrasound pulses with a pronounced compression phase and a smooth spectrum covering the range from 0.1 up to 150 MHz. The laser beam spot diameter was 6 mm, yielding a maximum power density below 15 MW/cm2. The ultrasound passed through the sample thickness and a 2 mm layer of coupling water. The pulse was detected by a 25 μm thick piezoelectric foil transducer with a diameter of the sensitive area of 2 mm. The transient signals were locally measured at different radii of the specimen. The attenuation almost linearly increases with frequency while its absolute value decreases from the center to the edge of the specimen.

Properties of metal–water interfaces studied from first principles

Properties of the metal–water interface have been addressed by periodic density functional theory calculations, in particular with respect to the electronic and geometric structures of water bilayers on several transition metal surfaces. It will be demonstrated that the presence of the metal substrate leads to a significant polarization of the water bilayer. This causes a substantial water-induced reduction of the work function in spite of the weak water–metal interaction, but it is not associated with a significant change of the electronic structure of the metal substrates. The structure and the vibrational spectra of water bilayers at room temperatures have been studied performing ab initio molecular dynamics simulations. The simulations suggest that the water bilayer structure on noble metals is not stable at room temperature, whereas on more strongly interacting metal surfaces some ordering of the water layer persists. In addition, metal–water interfaces under electrochemical conditions, i.e. for charged metal substrates, are addressed. Our simulations show that the charging of the surface leads to characteristic changes in the wall–oxygen distribution and the vibrational spectra.

P-Benzosemiquinone radical anion on silver nanoparticles in water.

This communication reports the SERS observation of p-benzosemiquinone radical anion, produced on reduction of p-benzoquinone by Ag nanoparticles at the metal-water interface. The species is positively identified by comparison of the SERS spectrum with the resonance Raman spectra of the radical anion in aqueous solution. This is a rare SERS observation of a radical intermediate formed by surface reaction on nanosize silver particles in solution.

Laser ablation at the solid-liquid interface: transient absorption of continuous spectral emission by ablated aluminium atoms

A light emission from aluminium atoms produced by a pulsed laser irradiation to the aluminium metal-water interface was studied by emission spectra and analysed on the basis of a rate model.The time dependence of the emission spectra was obtained from the 20-80 ns time range. Early in this time range the Al (2P°-2S) transition appears as an absorption line in a continuous spectrum, suggesting that the optical thickness of the light-emitting region is considerably high and that aluminium atoms present in the light-emitting region are cold enough to absorb the continuous spectral emission.Later the emission surmounts the absorption, i.e. a positive emission peak over the continuous spectrum is observed.The time of the transition from the absorption regime to the emission regime depends on the irradiation laser fluence.These behaviours can be well described by the model calculation based on the kinetics of the two energy levels.By comparison between the model calculation and the experimental results, the time behaviour of the species in the light-emitting region is discussed.

Effects of Natural Organic Matter, Anthropogenic Surfactants, and Model Quinones on the Reduction of Contaminants by Zero-Valent Iron

Recent studies of contaminant reduction by zero-valent iron metal (Fe 0) have highlighted the role of iron oxides at the metal–water interface and the effect that sorption has at the oxide–water interface on contaminant reduction kinetics. The results suggest that a variety of organic surface-active substances might enhance or inhibit contaminant degradation, depending on the degree to which they promote solubilization, sorption, and/or reaction. Of particular interest is the effect of natural organic matter (NOM), because of its ubiquitous presence in natural waters and amphiphilic properties; anthropogenic surfactants, because of their use in groundwater remediation; and certain quinones, which represent the redox-active functional groups associated with NOM. In this study, no well-defined effects of these substances were found on the reduction of nitro benzene by Fe 0, but the reduction of carbon tetrachloride and trichloroethylene (TCE) was inhibited by NOM. Results with carbon tetrachloride showed that the inhibitory effect of humic acids was greater than fulvic acids, but that several quinonoid NOM model compounds (juglone, lawsone, and anthraquinone disulfonate) increased the rate of reduction by Fe 0. Isotherms for adsorption of TCE and NOM onto Fe 0 showed evidence of competition for surface sites.

Self-consistent description of a metal–water interface by the Kohn–Sham density functional theory and the three-dimensional reference interaction site model

We have developed a self-consistent description of an interface between a metal and a molecular liquid by combination of the density functional theory in the Kohn–Sham formulation (KS DFT) for the electronic structure, and the three-dimensional generalization of the reference interaction site model (3D RISM) for the classical site distribution profiles of liquid. The electron and classical subsystems are coupled in the mean field approximation. The procedure takes account of many-body effects of dense fluid on the metal–liquid interactions by averaging the pseudopotentials of liquid molecules over the classical distributions of the liquid. The proposed approach is substantially less time-consuming as compared to a Car–Parrinello-type simulation since it replaces molecular dynamics with the integral equation theory of molecular liquids. The calculation has been performed for pure water at normal conditions in contact with the (100) face cubic centered (fcc) surface of a metal roughly modeled after copper. The results are in good agreement with the Car–Parrinello simulation for the same metal model. The shift of the Fermi level due to the presence of water conforms with experiment. The electron distribution near an adsorbed water molecule is affected by dense water, and so the metal–water attraction follows the shapes of the metal effective electrostatic potential. For the metal model employed, it is strongest at the hollow site adsorption positions, and water molecules are adsorbed mainly at the hollow and bridge site positions rather than over metal atoms. Layering of water molecules near the metal surface is found. In the first hydration layer, adsorbed water molecules are oriented in parallel to the surface or tilted with hydrogens mainly outwards the metal. This orientation at the potential of zero charge agrees with experiment.

Molecular dynamics, density functional theory of the metal–electrolyte interface

Quantitative, predictive theories for metal–electrolyte interfaces require an atomic-scale representation of the interface, which must include an accurate statistical description of a polar fluid in contact with a solid surface; and also a description of the electronic density and structure of a metal surface in contact with a fluid. Such a complex system presents a difficult computational problem, and has been dealt with in the past essentially by parts; either by molecular dynamics calculations of the fluid structure, or density functional calculations of the metal–surface electronic structure. A complete and self-consistent determination of the surface structure would, however, involve a simultaneous calculation of both the atomic and electronic structure of the interface. This suggests a combination of these two calculational techniques, and it is just this sort of molecular dynamics and density functional combination which comprises the Car–Parrinello, and related, methods. We have developed a Car–Parrinello type combination of molecular dynamics and density functional methods, suitable for application to the metal–electrolyte interface. We briefly describe this calculation and discuss our initial results for a fairly simple metal–water interface.

Molecular dynamics of adiabatic and nonadiabatic electron transfer at the metal–water interface

A molecular model for an electron transfer reaction at a solution–electrode interface is developed. The solvent diabatic free energy curves for the reaction Fe+3+e−→Fe+2 are calculated using an umbrella sampling procedure. These are used to calculate the rate of electron transfer as a function of the electrode–solution potential difference in the electronically nonadiabatic region. A model Hamiltonian for the adiabatic case is also developed and used to calculate the adiabatic free energy surface. Reactive flux correlation functions are used to determine the solvent dynamical corrections to the rate. A comparison between the molecular dynamics and the Kramers and Grote–Hynes theories is made.

Wettability of clean metal surfaces

Experimental and theoretical investigations bearing on the question of the wettability, by water, of clean oxygen-free metal surfaces are reviewed. Results on gold, silver, and copper are discussed in terms of surface cleanliness, surface structure, and extent of dispersion (London) force interaction. It is concluded that clean solid metal surfaces are hydrophilic. They will yield a zero degree contact angle when prepared in the amorphous state and possibly in the perfect crystalline state as well. These results do not necessarily preclude the possibility that physical interaction at the metal-water interface consists solely of dispersion forces.