PbTe electrodeposition studied by combined electrochemical quartz crystal microbalance and cyclic voltammetry

Electrodeposition of PbTe thin films from alkaline solutions, and the electrochemical behavior of the related precursors TeO₃^2? and PbEDTA^2?, were studied for the first time by means of an electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry. PbTe was found to form by an induced codeposition mechanism via six electron reduction. The reduction mechanism of Te was complicated and sensitive to the substrate surface. On the other hand, the reduction of the PbEDTA^2? complex to Pb⁰ was a simple two electron reaction.     

Self-assembled monolayers of long-chain xanthic acids on gold studied by voltammetry

A new method for the formation of xanthic acids H(CH₂)_nCS₂H (n=8, 10, 12) in a non-polar solvent from the corresponding potassium salts was developed. It was found that the xanthic acids spontaneously adsorb onto gold electrodes, via a mechanism that is proposed to be similar to the adsorption of thiols on gold surfaces. The xanthate modified gold electrodes were studied by cyclic voltammetry. It was found that the xanthate layers efficiently block the current from a ferro/ferri redox couple in aqueous solution. Studies of the electrochemical desorption of the xanthate layer supported the fact that xanthic acids spontaneously form a self-assembled monolayer on the electrode.     

The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V²⁺) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V⁰) in the presence of H⁺ (from HCl), CH₃COOH (pK_a=4.75), ClCH₂CH₂COOH (pK_a=4.00), HCOOH (pK_a=3.75) in aqueous media was examined by cyclic voltammetry according to the EEC_i mechanism. To avoid the electrodeposition of V⁰, we used a 9:1 (v/v%) H₂O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V²⁺, the ratio of the anodic and cathodic peak current (I_pa2/I_pc2) corresponding to V⁰–e^??V⁺ was plotted against log τ, where τ is the time between E_1/2 and the switching potential, at various scan rates of 0.02–3.5 V s^?1. The chemical reaction was found to be a parallel reaction consisting of H⁺-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as k_H⁺=3.5×10³ M^?1 s^?1, k_CH3COOH=5.7 M^?1 s^?1, k_HCOOH=4.6×10¹ M^?1 s^?1, and k_ClCH2CH2COOH=3.2×10¹ M^?1 s^?1 using the Nicholson–Shain method and k_H2O was estimated as <3×10^?6 M^?1 s^?1. The CVs were digitally simulated under the assumption of a two-step reaction of V⁰ following the two-step electrode reactions of V²⁺ to V⁰. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V⁰ is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V⁰ is in pseudo-equilibrium with H⁺ or HA to produce VH⁺ which undergoes a reaction with H₂O.     

Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-d-gluconate complex by cyclic voltammetry

Electron transfer reactions between iron^II/III-complexes are of general interest for corrosion processes, biological processes and technical applications. The cathodic reduction of Fe^III-triethanolamine (TEA) complex to form the Fe^IITEA-form of the complex has been examined by cyclic voltammetry. In the presence of Fe^III-d-heptagluconate (HDGL), complex catalytic currents are observed due to homogenous chemical reaction between Fe^IITEA and Fe^III(HDGL)₂ which regenerates Fe^IIITEA. This method can be used to determine the rate constant of the redox reaction between Fe^IITEA and Fe^III(HDGL)₂ with $k_{\mathrm{f}}^{\prime }=88\pm 12{\mathrm{dm}}^3{\mathrm{mol}}^{-1}{\mathrm{s}}^{-1}$.     

Studies of structural disorder of self-assembled thiol monolayers on gold by cyclic voltammetry and ac impedance

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z=R_s+1/[σ(jω)^α] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the α value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine α. Results of EIS experiments show that α increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, α can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth.     

Study of the DNA hybridization transduction behavior of a quinone-containing electroactive polymer by cyclic voltammetry and electrochemical impedance spectroscopy

We report the characterization of a new bifunctional electroactive polymer, poly(5-hydroxy-1,4-naphthoquinone (juglone)-co-5-hydroxy-3-thioacetic acid-1,4-naphthoquinone), which can be used for direct electrochemical detection of DNA hybridization. The polymer structure was characterized based on molecular orbital calculations and ex situ and in situ FT-IR spectroscopy. The quinone group integrated in the polymer presents very stable electroactivity in neutral aqueous medium and can act as a redox marker for hybridization. The carboxylic function allows the preparation of oligonucleotides (ODN) probe-modified films by covalent binding. The mechanism of hybridization detection was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The “signal-on” upon hybridization could be attributed to a change in conformation from random ODN coils on the probe-modified film to helix double stranded ODNs upon hybridization.     

Numerical determination of extended semi integrals and semi differentials by using spline cubic functions. Applications to an EE reversible mechanism in cyclic voltammetry

In this work, we propose a new general method based on the use of cubic spline functions, suitable for the numerical determination of extended semi integrals and semi differentials. The procedure combines these interpolation functions with different numerical integration and differentiation standard methods, including commercial software. The method has been applied to the electrochemical reduction of the meso-tetra(4-N-methylpyridyl)porphyrin in acidic aqueous solution, where this compound exhibits a two-electron wave in cyclic voltammetry. In this work, we determine the semi differential of the current and demonstrate that the signals obtained correspond to an EE process, where both electrochemical steps are reversible and their formal potentials are very close.     

Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.     

Investigations of a novel ferrocene-containing lipid by cyclic voltammetry and cryogenic transmission electron microscopy

A novel chiral redox-active ferrocene compound (FcVI) with amphiphilic properties has been synthesized. Cryogenic Transmission Electron Microscopy (cryo-TEM) has been used to estimate the shape and size of the FcVI aggregates in solution. Uni- and multi-lamellar vesicles (between 40 and 300 nm in diameter) were observed in water. Large particles (of more than 1 μm in diameter) with a hexagonal fine structure were found in 50 mM aqueous Na₂SO₄ solution. Sonication transformed the latter into ‘rosette’-like structures. Cyclic voltammetry has been employed to investigate the electrochemical properties of FcVI. The amphiphile adsorbed on graphite electrodes and a reversible electrochemical behaviour, characteristic of ferrocene, was observed with redox potentials between 330 and 350 mV.     

Kinetics of CH3OH oxidation on PtRu/C studied by impedance and CO stripping voltammetry

The kinetics of the CH₃OH oxidation reaction at 60 °C on well-alloyed platinum–ruthenium supported on carbon (Pt₅₀Ru₅₀/C) was studied by electrochemical impedance spectroscopy and compared with carbon-supported platinum (Pt/C). The reaction rate of the overall CH₃OH oxidation increased with increasing electrode potential for both Pt/C and PtRu/C. In the case of Pt/C, when the electrode potential was E ? 450 mV vs. RHE only a capacitive behavior was observed. Resistive and pseudo-inductive types of behavior were evident above 500 and 600 mV vs. RHE. In the case of PtRu/C, a similar change in behavior was observed, except that the two types of behavior were observed at 200 mV lower electrode potentials than for Pt/C. Correlation of the impedance data with pre-adsorbed carbon monoxide (CO_ad) stripping voltammetry allowed the understanding of the methanol oxidation reaction. The change in the reaction rate of the oxidation of CH₃OH to CO_ad as a function of the electrode potential as well as the promotional effect of Ru was evident from a change in the frequency where the frequency deviated from the ～90° phase angle. The change in the reaction rate of the oxidation of CO_ad to CO₂ as a function of the electrode potential as well as the alloying with Ru was evident from a change in the frequency where the phase angle approached zero.     

Catalysis by immobilized redox enzymes. Diagnosis of inactivation and reactivation effects through odd cyclic voltammetric responses

Odd cyclic voltammetric responses, with an inverted peak appearing on the reverse scan, have been recently reported for the catalysis of immobilized enzymes involved in a direct electron transfer at the electrode surface and implicated in a chemical inactivation/redox reactivation mechanism. In this work, it is shown that this twisted reverse trace behavior can be related rigorously and quantitatively to such a reaction scheme by means of a minimal number of dimensionless parameters. As a prelude, the requirements for ‘pure catalytic’ conditions to be achieved are established quantitatively. It is also shown that simple irreversible or reversible inactivation does not entail the appearance of twisted reverse traces. The quantitative analysis of the inactivation/reactivation mechanism does not lead to a closed form expression of the current responses, but rather requires the numerical resolution of the pertinent differential equations. This approach may be readily extended to virtually any kind of mechanism, including more complex reactions schemes, distance-dependent electron transfer kinetics, the use of immobilized or free-moving redox cosubstrates, consideration of substrate mass transport limitations, etc.     

Conducting films obtained by electro-oxidation of p-aminodiphenylamine (ADPA) in the presence of aniline in buffer aqueous solution at pH 5

The electrochemical oxidation of aniline and p-aminodiphenylamine (ADPA) was carried out in a phosphate buffer solution at pH 5. Cyclic voltammetry and “in situ” FTIR spectroscopy were combined to study the redox behavior of the oxidation products. When ADPA is oxidized in the presence of aniline, two redox processes are observed. One of these redox processes is related to a soluble species, while the other pertains to a species attached to the electrode surface. Only those films synthesized from ADPA oxidation in the presence of aniline are electroactive in this buffered medium. No evidence pointing to the occurrence of autocatalytic polymerization exists under the experimental conditions employed.     

Note on the oxidation of methanol at Pt(1 1 1) and Pt(3 3 2) electrodes in alkaline solutions

The cyclic voltammetric behavior of different concentrations of CH₃OH has been studied in alkaline solutions on Pt(1 1 1) and Pt(3 3 2). The oxidation of CH₃OH gives well-defined current density–potential curves at around the potential where the adsorption of OH occurs on Pt.The current–potential behavior of the hydrogen adsorption–desorption shows that CH₃OH or related species adsorbs on the step sites of (3 3 2) surface, but less on the (1 1 1) surface.The analysis of the voltammograms with less concentrated methanol solutions is suggested in order to understand the mechanism of CH₃OH oxidation reaction in alkaline media.     

The effect of acetate concentration,solution pH and conductivity on the anodic stripping voltammetry of lead and cadmium ions at in situ bismuth-plated carbon microelectrodes

The use of carbon microdisc electrode substrates allowed Pb(II) and Cd(II) anodic stripping voltammetry at in situ plated bismuth films to be studied in an extended acetate buffer/electrolyte concentration range that includes very low or zero acetate levels. The change of the Pb and Cd Square Wave Anodic Stripping Voltammetry (SWASV) peak height with acetate concentration, pH and conductivity has been studied systematically. It was found that the stripping peak signal is considerably enhanced in the absence of added acetate or in acidic solutions. This behaviour has been attributed to the extent of metal ion complexation at different levels of free acetate ions that are present at different buffer concentrations or pH values. Recording both the forward and the reverse voltammograms of each SWASV curve, the signal variation could be attributed to changes in the metal ion deposition rate (as its complexation state is varied) both during the preconcentration step and the reverse pulse of the SWASV potential sequence.     

Hydrogen bond complexation of dimethyl-[1-butyl-2,4-dioxo(1H,3H)pyrimido]tetrathiafulvalene with aminopyridine derivatives probed by cyclic voltammetry

The cyclic voltammetric response of dimethyl-[1-butyl-2,4-dioxo(1H,3H)pyrimido]tetrathiafulvalene 1 is studied in the presence of aminopyridine derivatives. The first oxidation potential of 1 increases with an increase in 2,6-di(N-acetylamino)pyridine concentration, with the maximum shift being 30 mV. This is assigned to the formation of a multihydrogen-bond complex in the neutral state with the binding constant estimated to be ca. 1000 M^?1. An irreversible following chemical reaction is suggested for the explanation of the complicated behaviour of the second redox wave of 1 in the presence of 2,6-di(N-acetylamino)pyridine.     

The electrode reactions of Fe(CN)63− and Fe(CN)64− ions studied at temperatures below the melting point of stoichiometric electrolytes

The electrode reactions of Fe(CN)₆^3? and Fe(CN)₆^4? ions have been studied at temperatures below the melting point of stoichiometric electrolytes. Tetramethylammonium cation hydrates: (CH₃)₄NOH·nH₂O(n = 5, 7.5, 10) and (CH₃)₄NF·4H₂O were selected as electrolytes. The redox active ion with higher electric charge, Fe(CN)₆^4?, is more stable at temperatures below as well as above the melting point of these electrolytes. However, the temperature dependence of the redox potential of the Fe(CN)₆^3?/Fe(CN)₆^4? couple is more pronounced in these conditions. In the limited temperature range below the electrolyte melting point the kinetics of the reaction are controlled by the rate of reactant transport towards the electrode surface. The apparent diffusion coefficient of redox active ions does not change substantially at temperatures around the electrolyte melting point. The activation energy of reactant transport is twice as large in frozen than in liquid electrolyte. It has been concluded that the motion of the redox active ions is restricted to a limited volume—the intergrain space of the electrolyte. This conclusion is supported by results of experiments performed in a cell filled with chemically inert beads and liquid electrolyte.     

Phase separation of binary self-assembled thiol monolayers composed of 1-hexadecanethiol and 3-mercaptopropionic acid on Au(111) studied by scanning tunneling microscopy and cyclic voltammetry

The mixing characteristics of binary self-assembled monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) on Au(111) have been studied by cyclic voltammetry and scanning tunneling microscopy. Two distinctive peaks, ～0.45V apart, are observed over the entire range of surface composition on cyclic voltammograms for the reductive desorption of the adsorbed thiol molecules, which reflects the presence of two different types of phase-separated domains greater than several tens of nm² which can be imaged by scanning tunneling microscopy. The peak potential of 1-hexadecanethiol is nearly independent of the surface composition whereas a slight shift of the peak potential is observed in the case of MPA, suggesting that the HDT is slightly soluble in the MPA domains, while MPA is insoluble in the HDT domains. The minimum number of the adsorbed thiol molecules required for exhibiting two distinctive peaks (i.e. two-dimensional bulk properties) is estimated to be ca. 50 by comparing the cyclic voltammograms with the distribution of the domain size observed by scanning tunneling microscopy.     

An improved algorithm for the numerical simulation of cyclic voltammetric curves affected by the ohmic potential drops and its application to the kinetics of bis(biphenyl)chromium(I) electroreduction

Previously published algorithms for the modeling of cyclic voltammetric curves affected by ohmic potential drops, in terms of the classical explicit finite differences method, are discussed briefly. A fast and efficient numerical procedure suitable for such simulations is described. This approach exhibits high numerical stability for both high scan rates and large uncompensated ohmic resistances. The procedure is based on the calculation of the faradaic current as a root of the non-linear equation, with a simultaneous calculation of the capacitive current. Comparison of the cyclic voltammograms obtained using this method with those calculated using alternative published procedures proves its validity. For comparison with the experimental data, the cyclic voltammetric response for the reduction of bis(biphenyl)chromium(I) tetraphenylborate in N,N-dimethylformamide is shown and the kinetic parameters of this process are fitted. Compared to earlier modelings, they show better concordance with the results of studies at microelectrodes. In conjunction with earlier successful applications of the analogous numerical procedure to the realistic modeling of electrochemical oscillations and multistability at a constant external voltage, the algorithm presented appears to be one of the most applicable methods of calculation of the electrochemical responses affected by the ohmic potential drops.     

Phenomenology of oscillatory electro-oxidation of formic acid at Pd: role of surface oxide films studied by voltammetry,impedance spectroscopy and nanogravimetry

Concepts of electrocatalysis are applied to examination of the oscillatory kinetics that are observed in the oxidation of formic acid in aqueous HClO₄ and H₂SO₄ at Pd. The phenomenology and interpretation of such behavior at Pd are compared and contrasted with related phenomena observed at Pt electrodes reported in earlier work by the present authors and also in more recent literature. Special attention is given to the role of formation and reactivity of the surface oxide anodically formed at Pd, up to various potentials, studied by voltammetry and nanogravimetry, in generating oscillatory kinetics, particularly in oscillation transients that extend to ca. 1.6 V(RHE), unlike the behavior at Pt where they extend only to ca. 0.83 V. Remarkably reproducible oscillations are recorded up to 11.1 h of constant polarization, with periods of up to 1000 s, i.e. frequencies of 10^?3 s^?1, dependent on polarization current-density. Major effects of solution-stirring are shown to arise in the oscillatory behavior of formic acid oxidation at Pd. Also the effects of increase of temperature from 273.1 K, upwards, on the form of the oscillations are examined. The oscillatory behavior is attributable to a combination of the following factors, also recognized in our earlier works and other literature: (a) negative resistance over the parts of the anodic polarization curve associated with (b) S-shapes of such curves exhibiting hysteresis between positive-, and negative-directions of polarization and (c), importantly, the participation of autocatalysis in chemical/or electrochemical steps, e.g. reactions of an intermediate or H.COOH with oxide-film species at the Pd electrode. The appearance of both potential and current oscillations at Pd in the presence of H.COOH under conditions of stirring is a new and significant observation, not observed in previous studies.     

Mechanistic study of the oxidation of 2-amino-3-cyano-4-naphtylthiophene by cyclic voltammetry and UV–Vis–NIR absorption spectroscopy

The anodic oxidation of 2-amino-3-cyano-4-naphtylthiophene (ACNT) was studied for the first time by cyclic voltammetry, digital simulation and in situ spectroelectrochemical techniques. The kinetic data were estimated using the results of fitting the digitally simulated voltammograms to the experimental data. All electrochemical evidence indicates that the oligomerization mechanism is ECE and the main reaction path of the oligomer formation involves the coupling of the dimeric cation radical of RC–PM with neutral ACNT molecules or other radical species. The in situ UV–Vis–NIR spectra of the dimers and oligomeric mixture indicated that new species with a low energy absorption (365, 537, 828, and 1055 nm) formed during the electrooxidaton of ACNT.