No abstract

A tunnel junction device was made by immersing mercury electrodes in an aqueous nitrate solution. The junction conductance was measured at zero bias as the two mercury surfaces were brought together in the solution. Changes in separation between the mercury surfaces were calculated from changes in the junction conductance using a simple model of elastic electron tunneling, due to Simmons. An absolute distance scale was established using the estimated hard-sphere diameter of water as an internal standard. Discrete changes in junction conductance were observed when the metal surfaces were separated by less than about 1 nm. We interpret this behavior to be due to the presence of quasi-equilibrium junction geometries which are themselves due to time-averaged structuring of liquid water near the metal surfaces. The longitudinal structuring in the water was found to decay normal to the metal surface with a characteristic length on the order of the molecular diameter. The time-averaged structures of the liquid water domains appear to be similar to the structure of hexagonal ice Ih and do not resemble hard-sphere packing. At zero bias, there appears to be no strong preference for one type of ordered water structure over another, suggesting that hydrogen bonding is the dominant factor determining structure in the liquid water near the metal surface and not metal-water bonding in this case. Our experimental data are in substantial agreement with recent molecular dynamics and Monte Carlo simulations and with analytic theory. There are significant differences between our results for ordering of liquid water at metal surfaces and the results reported previously for local ordering of liquid water in the mica/water/mica surface force-balance apparatus.

The anodie behavior of copper in lithium hydroxide solution was investigated at room temperature under controlled potential conditions. The electrodes were either saturated copper amalgams or small polycrystalline wires. Particular attention was focused on the copper (I) region. Two layerplanes of Cu~O were formed at anodic overpotentials following the dissolution of copper (I) hydroxy complexes on the COl~per amalgam electrodes. Some novel data are reported concerning the nucleation of holes in these layers during the reverse reaction. Behavior analogous to that of the amalgam electrodes was observed on the polycrystalline electrodes, except that a thick layer of Cu20 was also formed On top of the previously formed layer-planes. A model is proposed for the growth of this thick layer, and it is demonstrated that this model also describes the galvanostatic data acquired by earlier workers.* Electrochemical Society Active Member. ** Electrochemical Society Student Member.

The hydrolysis of maleimide has been investigated in the pH range 8.5–14. Polarographic limiting currents were well-defined, so that the bulk concentration of reactant during reduction could be clearly followed as a function of time. Logarithmic analysis of derived data indicated an arrest in the rate of reaction at circa pH 12, and a reaction scheme is proposed to explain this. In this scheme, the neutral maleimide molecule exists in equilibrium with its anion, and both of these species may undergo hydrolysis at the appropriate pH. From derived rate equations, the pKa of maleimide was found to be 10.0, whilst estimates of the rates of hydrolysis were also calculated.

Solid-water films thinner than 0.8 nrn were confined between the mercury surfaces of a squeezable tunnel junction at 265~3 K. Uniaxial compression of the films was performed electrostatically by changing the junction bias. Mean compressive displacements were calculated from the nonlinear current

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Using standard multilayer and effective medium models, we determine microstructures that optimize the near-IR-visible normal-incidence optical transmittance of electrically conducting metal films intended for use as semitransparent contacts for semiconductor devices such as photodetectors or photoelectrochemical converters. Various conditions are considered, including unpolarized and linearly polarized light and electrical conduction both parallel and perpendicular to the surface. For linearly polarized light. the optimum microstructure consists of parane! metal lines of nominally square cross section oriented perpendicular to the polarization vector of the incident light, regardless of the direction of electrical conduction. The line separation and cross-sectional dimensions must both be small compared to the wavelength A. For unpolarized radiation. the optimum microstructure depends on the direction of electrical conduction. For conduction parallel to the surface, the optimum microstructure again consists of parallel lines with the lines oriented perpendicular to the residuallinear polarization, if any, of the incident flux. For conduction perpendicular to the surface, the optimum microstructure consists of cylindrical metal posts of dimension small compared to A. Expressions are derived that allow the thicknesses and refractive indices of protective antireflection coatings to be calculated to first order in the thicknesses of the metal. films. The more general case of antireflection coatings for anisotropic structures is briefly discussed.

Results are presented for the dark electrochemistry and photoelectrochemistry of CdS and TiO2 colloidal particles studied with the optical rotating disk electrode (ORDE). The electron-transfer reactions between the particles and the electrode are found to be irreversible. However, in agreement with theoretical predictions, the kinetics obey both the Tafel and Levich relations. Several hundred electrons may be transferred on each encounter between a particle and the ORDE. Added halide ions are shown to be good scavengers for photogenerated holes. Results for the quantum efficiencies for the photogeneration of majority carriers are reported.