Electrodeposition of copper into functionalized polypyrrole films

Copper microparticles have been dispersed in anion-exchange polymer films coated on carbon electrodes by oxidative electropolymerization of pyrrole–alkylammonium monomers. Incorporation of copper in polymeric films was effected by impregnation of copper-oxalate anionic complexes followed by an electroreductive precipitation to copper metal, or by electroreduction of polymer coated electrodes immersed in copper–oxalate aqueous solutions. The deposition process was examined by voltammetry. Scanning electron microscopy showed a dispersion of metal particles from 50–100 up to 300 nm, according to the lipophilic character of the polymer, located in a layer near the carbon ∣ polymer interface. A study of the electrocatalytic activity in the reduction of nitrobenzene to aniline at these materials was performed. The cathodes appeared much more active than bulk copper electrodes.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

Simultaneous surface plasmon optical and electrochemical investigation of the electropolymerization of bithiophene at elevated pressures

A novel high-pressure set-up is presented that allows for the simultaneous characterization of interfacial reactions and thin films by surface plasmon optical techniques, as well as by electrochemical methods. The current configuration uses a grating coupler for the resonant excitation of surface plasmons at the (gold) metal ∣ dielectric interface with the metallic substrate serving at the same time as the working electrode in a three electrode electrochemical configuration. Using the same electrolyte as the pressure medium experiments up to about 50 MPa are reported. Firstly, some performance characteristics of the cell we discussed on the basis of CV measurements with hexacyanoferrate as a quasi-reversible redox system. Then the electropolymerization of bithiophene in acetonitrile solution at different cell pressures is presented.     

Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.     

Electrochemical impedance spectroscopy of thin films with two mobile charge carriers: effects of the interfacial charging

In the electrochemical systems containing an excess of the background electrolyte, the faradaic process and the interfacial (‘double-layer’) charging are coupled to the fluxes of different charge carriers, the former being related to the diffusional transport of electroactive entities while the latter being realized mostly by ions of the supporting electrolyte. As a result, the interfacial capacitance C^dl may simply be added in parallel to the faradaic impedance specific for each particular system (Randles & Ershler). This simple treatment is not justified in the absence of an indifferent electrolyte, if the same charged species take part in both the electrode reaction and the double layer charging. This is the case where at least one of the ions of a binary electrolyte participates in the electron transfer process or cross the interface. A similar situation arises in the case of an electrochemically active polymer film in that mixed electronic–ionic conductivity prevails, i.e. the charging of the polymer (electron transport) is accompanied by the motion of the charge-compensating counterions (or co-ions). In such systems both interfacial processes are coupled with the same flux of the ‘electroactive’ component. Moreover, the distributions of both charged species inside the film are interrelated due to the electroneutrality condition and the self-consistent electric field so that their transport cannot be considered as pure diffusion. This paper presents an analysis of the alternating current passage across a thin film containing mobile charge carriers of two types, which for the sake of specificity are referred to as ‘electrons’ and ‘counterions’ (although the results are also valid for other systems with two charge carriers). The contributions at the interfaces related to the faradaic processes (or the ion exchange) and to the double-layer charging have been taken into consideration ‘ab initio’. Analytical expressions for the complex impedance have been obtained for three principal arrangements of the system, m ∣ f ∣ m′ (film between two electronic conductors), s′ ∣ f ∣ s (‘membrane’, film between two solutions) and m ∣ f ∣ s (modified electrode). Besides the parameters characterizing the transport processes of the charged species in the bulk film, these formulae contain four characteristics of each interface α (α=m ∣ f or f ∣ s), R^α (charge transfer resistance), C^α (interfacial capacitance), t_e^α=1?t_i^α (‘double-layer numbers’) and C_μ^α (‘asymmetry factor’). The predicted complex-impedance plots demonstrate a greater variety of shapes compared to those predicted by traditional approaches.     

Mechanistic studies of electrodeposition for bioceramic coatings of calcium phosphates by an in situ pH-microsensor technique

A pH-microsensor technique was developed to measure in-situ the pH value at the titanium alloy ∣ solution interface for the first time during the electrodeposition process of calcium phosphate coatings. Electrochemically prepared iridium oxide was used as a pH sensitive material, which is of a good response behavior in the measured solutions. The pH response slope was ?70.2 mV/pH at 60°C. The results indicated that the interfacial pH increased with the applied current density, first jumped to a maximum, then slowly decreased at the controlled current density during the electrodeposition process. Incorporating the results of composition and structure of electrodeposition coatings characterized by X-ray diffraction (XRD) patterns and Raman spectra, the measurement of the interfacial pH was informative for further understanding of the electrodeposition mechanism of calcium phosphates. The interfacial pH may determine the precipitation reaction and the electrodeposition products at the titanium surface.     

Interpretation of the ultra-fast electropolymerization of pyrrole in aqueous media on zinc in a one-step process: The specific role of the salicylate salt investigated by X-ray photoelectron spectroscopy (XPS) and by electrochemical quartz crystal microbalance (EQCM)

The use of sodium salicylate as an electrolyte makes it possible to deposit polypyrrole (PPy) films on oxidizable metals such as zinc by the electrochemical oxidation of pyrrole. In spite of the very large difference between the pyrrole and zinc oxidation potentials, which thermodynamically should lead to metal dissolution and not polymer formation, PPy films are formed as easily as on a platinum electrode. X-ray photoelectron spectroscopy (XPS) and in situ electrochemical quartz crystal microbalance (EQCM) experiments reveal that a very thin composite passivating zinc salicylate layer is formed prior to pyrrole electropolymerization and prevents zinc dissolution without inhibiting polymer formation. Ex situ XPS analysis of the surface at different potentials (?1 to 0.7 V) at a grazing and normal incidence of the photoelectrons shows that pyrrole is adsorbed on zinc at an early stage of the polarization and remains on the metal surface up to the beginning of electropolymerization. In situ EQCM measurements indicate that this passivation layer is very thin (about 5 nm), and does not desorb at the potential where pyrrole is oxidized. The rapid oxidation of pyrrole through this passivating layer is explained by the presence of conductive pyrrolic paths inside the salicylate layer which make this composite layer as active as a noble metal.     

Nucleation at liquid ∣ liquid interfaces: electrodeposition without electrodes

A model has been developed for diffusion controlled electrodeposition of metallic particles at the interface between two immiscible electrolyte solutions. A rate law was derived for the case where no preferential nucleation sites are present. Palladium particles were deposited at the water ∣ 1,2-dichloroethane interface by reduction of aqueous ammonium palladate using butylferrocene in the organic phase as electron donor. Experimental results were in good agreement with the theoretical model derived. The potential dependence of the nucleation rate was found to follow a classical exponential law.     

Finite element simulation of ion transfer reactions at a single micro-liquid ∣ liquid interface supported on a thin polymer film

The electrochemical response of an ion transfer reaction at a polarised micro liquid ∣ liquid interface has been studied by a finite element method. A new penalty method developed for this application has been first validated by comparison with experimental and analytical results obtained for a symmetric micro interface. Then, asymmetric geometries have been studied and the influence on the voltammetric response of parameters such as micro hole depth, interface position within the hole and diffusion coefficient ratio is investigated.     

Linear correlation between surface stress and surface charge in anion adsorption on Au(111)

Voltammetry and quantitative surface stress measurements of anion adsorption on Au(111) have been performed simultaneously at the metal ∣ electrolyte interface. A linear correlation between surface stress and interfacial charge was observed. It is shown that this representation is to be preferred to the usual voltstressogram, since the structure of the latter is dominated by the differential capacity.     

In-situ FTIR studies on the electrochemical oxidation of histidine and tyrosine

The electrochemical oxidation of cyclic amino acids such as histidine and tyrosine has been studied in a strongly basic solution by means of an in-situ FTIR spectroscopic method. The terminal COO^– group of fully unprotonated anions was adsorbed on a platinum electrode from +0.4 V versus Ag ∣ AgCl ∣ sat KCl, and the concentration of the adsorbent increased with increasing potential. However, the adsorption strength of the terminal carboxyl was weakened at +1.0 or +1.1 V for histidine or tyrosine, respectively. In the oxidation of tyrosine, the benzene ring started to be oriented in parallel to the electrode surface at +0.3 V and the flat orientation reached completion at +0.6 V. Histidine and tyrosine were both oxidized from +1.1 V in a NaOH solution of pH 13.     

Frequency dependent electrolyte electroreflectance at semiconductor ∣ electrolyte interfaces

Electrolyte electroreflectance (EER) measurements at different modulation frequencies are carried out for the characterization of semiconductor ∣ electrolyte (SC ∣ EL) junctions. It is shown that the frequency dependence of the intensity and lineshape of EER signals is governed by the electrical impedance of the junction. Examples are given for n-RuS₂ and n-GaAs electrodes in contact with aqueous electrolyte. At the n-RuS₂ ∣ H₂O interface, electroreflectance spectra reveal the presence of an interfacial layer, most probably of RuO_x composition, which determines the behavior of this oxygen evolving electrode. Modulation of the electric field in this layer, and thus observation of its electroreflectance signal, requires the trapping of photogenerated holes at the RuS₂ ∣ RuO_x interface followed by recombination with conduction band electrons. At the n-GaAs ∣ H₂O interface, the oscillatory reduction of H₂O₂ is followed through EER measurements. Oscillation in baseline, lineshape and peak-to-peak intensity of the signals is observed. Baseline oscillation can only be observed at low modulation frequency and is attributed to a surface layer, probably As⁰. The lineshape reflects an oscillating inhomogeneity in the scl electric-field attributed to H⁺ intercalation in the near-surface of the GaAs electrode.     

Electrochemistry of Langmuir–Blodgett and self-organized monolayers of an azocrown ether,both pure and mixed with a phospholipid

The electrochemical behavior of monolayers of an azocrown ether (L16), both pure and mixed with dioleoylphosphatidylcholine (DOPC) was investigated using a mercury electrode. The monolayers formed at the air ∣ water interface were transferred onto the electrode using the Langmuir–Blodgett approach. The reversibility of the electrode processes depends on the surface pressure during the transfer of the monolayer. The reduction mechanism of the azo to the hydrazo group was studied in acidic medium by cyclic voltammetry and potential-step chronocoulometry. The dependence of the faradaic charge due to azo group reduction at constant pH upon scan rate (for voltammetry) or upon electrolysis time (for chronocoulometry) was examined on the basis of a general kinetic approach. The same approach was used to interpret the dependence of the faradaic charge upon pH at constant scan rate or electrolysis time. The reduction of the azo to the hydrazo group takes place via the reversible uptake of one electron, followed by the rate determining protonation of the resulting radical anion. When L16 is in the form of a pure monolayer its electroreduction is accompanied by a 2D phase transition involving the passage from a liquid-like to a solid-like structure. No such phase transition is observed in mixed L16–DOPC monolayers. At intermediate compositions of this mixture, strong attractive interactions between L16 and DOPC molecules decrease the mean area per molecule with respect to the ideal behavior at the air ∣ water interface, and prevent complete electroreduction of the L16 molecules in the monolayer.     

Electroless deposition of Pt at a Pd electrode by reaction with sorbed H in Pd/H

In the course of cycling a Pt electrode into the platinum surface oxide formation and reduction region in an aqueous Cl^? ion solution, using a Pd ∣ H reference electrode, it was found that the Pd surface becomes coated with Pt. It is shown that this phenomenon arises from electroless deposition of Pt from Pt anodically dissolved from the Pt working electrode; the adsorbed H at Pd ∣ H is the reducing agent. These observations provide a warning that care must be exercised in the use of Pd ∣ H as a reference electrode.     

Polarization phenomena at the water ∣ o-nitrophenyl octyl ether interface: Part III. Kinetics of tetraethylammonium ion transfer across the polymer supported interfaces

Kinetics of the tetraethylammonium (TEA⁺) ion transfer across the interface between an aqueous electrolyte solution (W) and the o-nitrophenyl octyl ether (o-NPOE) plasticized polyvinylchloride or the polyvinylidene difluoride supported membrane is examined by impedance spectroscopy at the equilibrium cell potential difference. Impedance data are analyzed by using the Randles-type equivalent circuit. As compared with the unsupported W ∣ o-NPOE interface, the polymer supported interfaces do not exhibit the enhanced differential capacity in the presence of the TEA⁺ ion, while they exhibit a lower charge transfer resistance (i.e. a higher rate constant of ion transfer). It is shown that these differences can be due to mechano–electric oscillations, which are related to capillary waves at the liquid ∣ liquid interface. An equivalent electrical circuit involving these oscillations comprises an internal ac voltage source in series with the interfacial capacity having the same frequency but the opposite sign to the applied voltage. At a polymer supported interface, the mechano–electric oscillations are likely to be suppressed, and the Randles-type equivalent circuit becomes a realistic representation. A revision of the previous impedance analysis of the TEA⁺ ion transfer across the unsupported W ∣ o-NPOE interface yields kinetic data which are consistent with the present results. The ion transfer rate constant still shows a correlation with the viscosity of the organic phase.     

Electrochemical and topological characterization of gold(111) ∣ oligo(cyclohexylidene) ∣ gold nanocrystal interfaces

Self-assembled monolayers (SAMs) of functionalized oligo(cyclohexylidene) molecules on gold(111) surfaces have been studied by measurement of the interfacial capacitance in concentrated electrolyte solutions and by cyclic voltammetry with Fe(CN)₆^3?/4? as the redox system. The morphology of the layers has been studied by scanning tunneling microscope (STM) and AFM. We found that sulfide-terminated oligo(cyclohexylidenes) form a well-ordered SAM on gold(111) and that the oxime end-functionality inhibits the formation of ordered SAMs. Charge-stabilized gold nanocrystals attach to sulfide-terminated SAMs on gold(111) to form robust gold(111) ∣ oligo(cyclohexylidene) ∣ gold nanocrystal molecular junctions. We show that the nanocrystal ∣ water interface can be charged by electron transport through the molecular layer.     

Fabrication of agar-gel microelectrodes and their application in the study of ion transfer across the agar–water ∣ 1,2-dichloroethane interface

In this paper, we describe a simple procedure to make agar-gel microelectrodes by filling micropipettes. These microelectrodes were used to study K⁺ transfer across the agar–water ∣ 1,2-dichloroethane interface facilitated by dibenzo-18-crown-6 (DB18C6), and the transfer of tetraethylammonium (TEA⁺). The results observed were similar to those obtained at micro-liquid ∣ liquid interfaces. The effect of various amounts of agar in the aqueous phase was optimized and 3% agar was chosen based on the potential window and solidification time. The different shapes of micro-agar-gel electrodes were prepared in a similar way. The fabricated agar-gel microelectrodes obey the classical micro-disk steady-state current equation, which is different from the behavior of a normal micropipette filled with aqueous solution without silanization.     

Square-wave voltammetry of an aqueous solution of indigo

Indigo is dissolved in water in the concentration of 5×10^?5 M and measured by highly sensitive adsorptive stripping square-wave voltammetry in order to determine the parameters of its electrochemical reduction. The formal potential of the indigo/leucoindigo redox couple dissolved in the aqueous 0.7 M KNO₃, pH 12 (NaOH) is ?0.566±0.003 V versus Ag ∣ AgCl ∣ 3 M KCl. Both indigo and leucoindigo are adsorbed on the surface of the mercury electrode, and the standard rate constant of this redox reaction is 920±130 s^?1. The ratio of adsorption constants of indigo and leucoindigo is 0.75.     

In situ infrared spectroscopy of the semiconductor ∣ electrolyte interface

Infrared spectroscopy has been developed as a powerful tool for the in situ study of the electrochemical interface. The possibility of using a multiple-internal-reflection geometry makes it especially suitable for the study of the semiconductor ∣ electrolyte interface. For high sensitivity to surface species, it is implemented as a differential or a modulation technique. Absorption due to vibrational transitions provides straightforward characterisation of the changes in the chemical state of the surface (interfacial films, adsorbed species, structure of the double layer); it can also be used to probe the composition of the electrolyte in the diffusion layer (redox species, reaction intermediates, pH) or even of the bulk of the electrode, in the case of intercalation reactions. Electronic absorption provides direct information on the semiconductor space-charge layer and on electronic states at the interface. Inherent possibilities of the infrared technique include the use of the polarisation of the infrared light, which gives information on the orientation of interface species, and the analysis of the time response of the infrared absorption, which gives direct information on the interfacial kinetics. Careful analysis of the changes in electrolyte absorption and spectrum baseline may provide valuable indirect information, for instance about infrared non-active species, surface roughening, or the formation of a porous layer at the interface. In situ infrared spectroscopy clearly offers a wealth of information about the semiconductor ∣ electrolyte interface, and it is sufficiently convenient and versatile that it could be used to a much wider extent than has been the case so far.     

Langmuir–Blodgett films of azocrown ethers on electrodes — voltammetric recognition of isomers

Monolayers of amphiphilic derivatives of crown ethers bearing electroactive azo groups in the macrocycle were transferred from the air ∣ water interface onto indium–tin oxide (ITO) or thin mercury film electrodes (TMFE) using the Langmuir–Blodgett technique. Differences in the electrochemical reversibility of the systems observed on these two electrodes were explained by different orientation of the azocrown molecules on the hydrophobic (TMFE) and hydrophilic (ITO) electrode surfaces. The electrochemical studies of the monolayer modified electrodes proved that the organization of the monolayer and the redox properties of the azocrown molecules depend on the geometry around the NN moiety. Voltammetry allowed Z- and E-isomers of the azocrowns to be recognized and their isomerization processes in monolayers to be followed. The effects of pH and of the alkali metal cations on the stability of the isomers were studied. Isomerization to the more stable E-isomer was hindered in alkaline solutions and under these conditions well separated reduction peaks of the E- and Z-isomers were obtained.