Studying electron transfer reaction at the Au/n-decanethiol/aqueous solution of NaNO3 interface by electrochemical impedance spectroscopy

The electrochemical behavior of the gold/electrolyte interface in aqueous 1 M NaNO₃ solutions in the presence of an organic monolayer of n-decanethiol (CH₃(CH₂)₉S) is studied by electrochemical impedance spectroscopy in the frequency range of 10–10⁵ Hz and also by cyclic voltammetry. It is experimentally shown that in the potential interval from 0 to ?0.5 V (vs. SCE), the dense monolayer film decreases the measured current density approximately 40-fold. The measured capacitance falls down to 1–2 μF/cm². Based on the analysis of impedance characteristics acquired with the use of empirical equivalent circuits comprising ideal and nonideal analogues of electric circuits, the tentative estimates of the thickness of organic monolayers formed on Au electrodes with various roughness factors are obtained. Using the complex nonlinear regression (CNLS) method and a model of microarray electrode, the porous structure of adsorbed monolayers is revealed and the transition frequency of interfaces under study is determined. The degree of inhibition of the electron transfer across the Au/n-decanethiol/solution interface is determined by comparing the rate constants for the Ru[(NH₃)₆]^3+/2+ redox process on clean and modified electrodes. The acquired results are compared with available literature data.     

The impedance of liquid (Ga-In)-electrode of eutectic composition in aqueous electrolyte solutions in the absence and in the presence of surface-active ions

Adsorption behavior of anions at liquid (Ga-In)-electrode at a temperature of 305 K is studied by electrochemical impedance spectroscopy and cyclic voltammetry. The above-listed methods allowed evaluating the adsorbability of different ions. Equivalent circuit describing the experimental data in the presence and in the absence of ions Br⁻ and Cl⁻ is a contour comprising a resistance connected in series to a capacitance whose value remains constant over the frequency range from ∼300 Hz to 10 kHz. Analysis of the experimental data obtained by the mixed electrolyte method with excess of surface-inactive ion Cl and constant ionic strength 0.1 M in electrolyte solutions acidified down to pH 3 gave the charge of specifically adsorbed ions Br⁻ and Cl⁻ (σ₁) at the liquid (Ga-In)-electrode surface as 5.24 and 1.67 μC/cm², respectively, at the adsorbate maximal concentration and zero-charge potential. These values are characteristic of very weak specific adsorption. The σ₁ values found for the (Ga-In)-electrode were used in the calculations of different isotherms, aiming at the determination of adsorption parameters. The results of the study were compared with literature data obtained by different researchers for different metals in the presence of specifically adsorbing bromide and chloride ions.     

Electrochemical behavior of silver single-crystal electrodes in 1 M NaNO<Subscript>3</Subscript> solution in the absence and in the presence of decanethiol films

The surface properties of silver single-crystal faces (111) and (210) in 1 M NaNO₃ aqueous solution in the absence and in the presence of monolayer n-decanethiol (DT) films are studied by the methods of cyclic voltammetry and electrochemical impedance using the meniscus contact technique. It is experimentally shown that, in the potential range from 0 to −0.5 V, the faradaic processes are absent and low currents are recorded on the unmodified silver surfaces. It is shown that the DT molecules, which are adsorbed on the nonoxidized surfaces of silver faces, spontaneously form (during 72 h) compact structurized films. The films are stable in the air and in the electrolyte solutions and totally block the surfaces of both faces. The impedance spectra, which were measured for each interface in a wide range of frequencies, are analyzed and interpreted. The measurements were performed by the use of the corresponding empirical equivalent circuits containing perfect and imperfect analogs of electrical circuits; complex nonlinear least squares (CNLS) regression method was applied for the calculations. The capacitance, ohmic resistance, and adsorbed monolayer thickness were estimated.     

Effect of Surface Structure on Behavior of RuO2 Electrodes in Sulfuric Acid Aqueous Solution

The electrochemical properties of RuO₂ electrodes are studied by means of cyclic voltammetry, potential step, and impedance measurements in aqueous 0.5 mol/dm³ H₂SO₄ as a supporting electrolyte solution and applying the hanging electrolyte meniscus technique. Two types of the electrodes are used: bulk as-grown single-crystal having (101) exposed interface and 500 nm film produced by reactive RF magnetron sputtering at 450°C. The surface structure of the RuO₂ electrodes prepared by different techniques is defined from X-ray LAUE backscattering. Experimental data show that, for both RuO₂ electrodes, the charging components include a slow diffusion-controlled contribution, due to proton injection–ejection. The diffusion characteristics and the diffusion coefficients for the same electrodes are estimated and reported.     

Nitrite oxidation on RuO<Subscript>2</Subscript> electrodes

An electrochemical kinetic investigation of nitrite oxidation to nitrate on RuO₂ is discussed. The process is studied by cyclic voltammetry, steady-state measurements and potential step measurements. The overall oxidation reaction is a two-electron process where the first step involves a reversible charge transfer: NO₂⁻ ⇔ NO₂ + e⁻ The one-electron oxidation of nitrite yields adsorbed NO₂ which is further oxidized to adsorbed (NO₂)⁺ and subsequently desorbed via a chemical reaction. In the general case, fit of experimental data is obtained with adsorption described by a Temkin isotherm unless the electrode is pre-treated at a cathodic potential where the (NO₂)_ads is removed. This treatment lowers the degree of coverage by intermediates but not the nature of the slow step. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 1, pp. 142–149. The text was submitted by the authors in English.     

Electron transfer through single-crystal silver electrode/<Emphasis Type="Italic">n</Emphasis>-decanthiol monolayer/NaNO<Subscript>3</Subscript> solution interfaces

The kinetics of Ru[(NH₃)₆]³⁺ reduction in 1 M NaNO₃ solution at Ag(210) and Ag(111) singlecrystal electrodes modified by n-decanthiol monolayer is studied by electrochemical impedance spectroscopy and cyclic voltammetry. By using these two methods, standard rate constants of the redox reaction involving Ru[(NH₃)₆]^3+/2+ redox couple in the absence and in the presence of the n-decanthiol film were estimated. The equivalent circuit describing the experimental data in the presence of the self-assembled organic monolayer and in the absence of redox reaction is an electrical circuit comprising a large resistance (∼10⁶ Ω) connected in parallel with a capacitance (∼10⁻⁸ F). Analysis of kinetic data and extrapolation of Tafel lines resulted in the determination of the rate constant at unmodified Ag-electrode, which is characteristic of very fast heterogeneous electron transfer. The calculated rate constants for n-decanthiol-modified silver singlecrystal faces (210) and (111) in 1 M NaNO₃ solution (pH 6.3) equal 4.63 × 10⁻⁵ and 3.05 × 10⁻⁵ cm/s, respectively. The results are compared with the data at hand reported by different authors for gold electrodes in indifferent electrolyte solution in the absence and in the presence of self-assembled monolayer.     

Uniqueness and partial identification in a geometric inverse problem for the Boussinesq system

We analyze the inverse problem of the identification of a rigid body immersed in a fluid governed by the stationary Boussinesq system. First, we establish a uniqueness result. Then, we present a new method for the partial identification of the body. The proofs use local Carleman estimates, differentiation with respect to domains, data assimilation techniques and controllability results for PDEs. To cite this article: A. Doubova et al., C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

Acetonitrile as Adsorbate or Solvent to Probe the Crystal Face Specificity of Metal–Water Interaction at Silver Electrode/Solution Interfaces

The crystal face specificity of metal–water interaction at Ag electrode/solution interfaces is investigated by using acetonitrile (ACN) as a probe molecule of the water interfacial structure or as a solvent in which water is a solute. Capacitance and voltammetric curves suggest that ACN is weakly adsorbed from aqueous solution on Ag in the order (111) > (100) > (110). Apparent inconsistencies of adsorption parameters are explained by the occurrence of two ACN adsorption modes: (i) directly on the metal surface and (ii) on the water layer adsorbed on the metal surface. Ag surface oxidation in ACN in the presence of variable amounts of water suggests that water has an inhibiting effect on Ag oxidation, the diminution of the water content in ACN leading to free anodic dissolution of the metal surface.     

Application of global Carleman estimates with rotated weights to an inverse problem for the wave equation

We establish geometrical conditions for the inverse problem of determining a stationary potential in the wave equation with Dirichlet data from a Neumann measurement on a suitable part of the boundary. We present the stability results when we measure on a part of the boundary satisfying a rotated exit condition. The proofs rely on global Carleman estimates with angle type dependence in the weight functions. To cite this article: A. Doubova, A. Osses, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Specific adsorption of iodide-ion at liquid (Ga-In)-electrode of eutectic composition from aqueous solutions with constant ionic strength

The adsorption behavior of iodide-ion at liquid (Ga-In)-electrode in aqueous electrolyte solutions at 305 K is studied by the electrochemical impedance spectroscopy, differential capacitance, and cyclic voltammetry. The equivalent circuit describing the experimental data in the presence and in the absence of the I⁻ ion is a series connection of a resistance and a capacitance that is frequency-independent over a ∼500 Hz to 100 kHz range. The experimental data were obtained by the mixed electrolyte method in electrolyte solutions acidified with HClO₄ down to pH 3, with excess of surface-inactive ion ClO₄⁻ and constant ionic strength (0.1 M). The analysis resulted in the determination of the charge σ₁ of the I⁻ ion specifically adsorbed at the liquid (Ga-In)-electrode at the adsorbate maximal concentration: σ_I = 7.73 μC/cm² in the case of analysis at σ = const (at the zero charge potential) or σ_I = 7.50 μC/cm² in the case of analysis at E = const. These values are characteristic of rather strong specific adsorption. The values of σ_I of the studied anion were used in the calculations of different isotherms with the purpose of the adsorption parameters determination. The obtained results were compared with literature data determined on other metals in the presence of specifically adsorbable I⁻ ion.