Clay-modified electrodes as studied by the quartz crystal microbalance: Redox processes of ruthenium and iron complexes

The electrochemical quartz crystal microbalance (EQCM) has been employed to investigate the mass transport processes on a clay-modified electrode. The systems investigated were the redox couples of [Ru(bpy)₃]²?₆ (bpy = 2,2′-bipyridine), [Ru(NH₃)]₆^2+/3+ and [Fe(CN)₆]^{4 ?/3?}. A clay-modified electrode was prepared by depositing synthetic saponite onto a gold coated quartz crystal. An electrode was allowed to swell for more than 50 h in 0.01 M Na₂SO₄ or NaCl or NaClO₄ prior to the adsorption and electrochemical measurements. The EQCM results revealed that the charge balancing during a redox reaction was accomplished by leaching or incorporating mobile ions in the clay film. For the [Ru(bpy)₃]^2+/3 couple, one [Ru(bpy)₃]^3? molecule was eliminated from the clay film when three [Ru(bpy)₃]²⁺ ions were oxidized. For the [Ru(NH₃)₆]^{2+ 3+} couple, one SO₄² ion, which was co-adsorbed with the ruthenium complex, was removed from the clay film when two [Ru(NH₃)₆]³⁺ molecules were reduced. For the [Fe(CN)₆]^4?/3? couple, a part of the excess charge generated by the initial oxidation of [Fe(CN)₄]^4? was canceled out by the elimination of a sodium ion bound by a clay layer. No mass transfer was detected during the oxidation at the later stage. This was probably because sodium ions in the aqueous medium within a clay film carried excess charge. The results are discussed in relation to the reported electrochemical behavior of these metal complexes.     

Electrochemical quartz crystal microbalance study of copper adatoms on Au(111) electrodes in solutions of perchloric and sulfuric acid

The adsorption of anions on highly ordered (111)-type gold electrodes with and without Cu adatoms in perchloric and sulfuric acid electrolyte solutions was investigated on an electrochemical quartz crystal microbalance. A distinct increase in mass due to the specific adsorption of hydrated bisulfate and perchlorate anions was observed in Cu²⁺-free solutions and their amounts were determined as a function of the potential. The composition of the Cu-adlayer (θ_Cu and θ_anion, coverages of Cu adatoms and co-adsorbed anions) during formation or removal in the potential sweep was also determined in both electrolyte solutions. A pair of reversible anodic and cathodic peaks in the voltammogram for the first Cu-upd region in H₂SO₄ was found to correspond well to the reversible changes in θ_Cu and θ_anion. In contrast, asymmetric features of the voltammograms for the second Cu-upd region in H₂SO₄ and for whole processes in HClO₄ corresponded to the large hysteresis in changes of θ_Cu and θ_anion, probably originating from a slow rearrangement of both species.     

Influence of the nature of the supporting electrolyte on the formation of poly[4,4′-bis(butylsulphanyl)-2,2′-bithiophene] films. A role for both counter-ion and co-ion in the polymer growth and p-doping processes

Electrode deposits of a polythiophene with particularly interesting characteristics, i.e., poly[4,4^′-bis(butylsulphanyl)-2,2^′-bithiophene], have been generated in the presence of different supporting electrolytes. The formation of the coatings has been studied by coupling conventional electrochemical techniques with quartz crystal microbalance measurements. The results of simultaneous voltammetric or chronoamperometric and microgravimetric experiments allow us to show the influence exerted by the nature of the supporting electrolyte on the polymerisation and concomitant charging–discharging processes of the conducting polymer.     

Redox and mass transport characteristics of domain-free mixed ferrocenyloctanethiol/alkanethiol monolayers on gold

Mixed monolayers of ferrocenyloctanethiol (FcC8SH) and alkanethiols of various chain lengths were constructed after predetermining the surface composition by the initial coverage of Pb underpotentially deposited on a bare gold electrode during the first step of the preparation. Single waves observed in the reductive desorption voltammograms show no formation of single-component domains. These mixed monolayers showed the following characteristics. First, a considerable amount of the ferrocene moieties were not electroactive. Second, the mass change during the redox reaction of the ferrocene was greater than that expected from the simple association of the ferrocenium cation with the electrolyte anion, and that for the mixed monolayers prepared from mixed solutions of the precursor thiols. This large mass change was interpreted by the incorporation of the anion and water upon oxidation into the pocket surrounded by alkanethiol and FcC8SH at the bottom. These results are consistent with those expected for the FcC8SH molecules in a domain-free environment, or surrounded by more alkanethiol molecules than FcC8SH molecules.     

Microgravimetric studies of silver electrocrystallization on polycrystalline gold surfaces

Cyclic voltammetry and chronoamperometry were performed in conjunction with electrochemical quartz crystal microbalance studies with the aim of making a new contribution to electrocrystallization studies. The nucleation and growth of silver on polycrystalline gold surfaces was selected as the probe system due to its simplicity and the availability of previous publications. To obtain thin films of silver on gold, the electrochemical experiments were conducted in an ammonium hydroxide solution (pH 11.8). The simultaneous current and mass variation transients obtained allowed for the proposal of a mathematical model, based on the well-established model of Scharifker and Hills (Electrochim. Acta 28 (1983) 879) model, in which parameters such as the number of nuclei and the nucleation rate were determined without contributions from non-faradic currents.     

Electrochemical quartz crystal microbalance investigation of surface fouling due to phenol oxidation

An electrochemical quartz crystal microbalance is employed for investigating the formation of inhibitory polyphenol layers on the surface of gold electrodes. The frequency–time profiles are indicative of the formation of a thin insulating polymeric layer, that meets the rigid film requirement, and are in good agreement with the voltammetric response. Once access to the monomer is blocked, no further frequency change is observed. The passivation process is explored as a function of the specific phenolic compound, its concentration, and operational conditions. Comparison to the charge passed during the film deposition indicates that the efficiency of the polymerization process is dependent upon the phenol concentration. Valuable insights are also gained for the entrapment of glucose oxidase within the electrochemically grown polyphenol.     

Reduction of peroxodisulfate on gold(111) covered by surface oxides: inhibition and coupling between two oxide reduction processes

Cyclic voltammetry and potentiostatic current transient measurements with the rotating disc electrode are used to study the reduction of peroxodisulfate (PDS) on Au(111) covered by surface oxides. A strong inhibition is observed, but the PDS reduction rate does not correlate with the oxide surface coverage through a simple geometric blocking factor. Evidence is provided that the inhibition is related to the coverage of Au(111) by the more stable oxide, which blocks the PDS reaction sites located probably on the surface step edges. The difference between the kinetics of the reduction of the two main oxide species on Au(111) is ascribed to a coupling between the two processes. A model of the coupling based on the diffusion controlled proton catalysis of the oxide reduction is proposed and verified by numerical simulation.     

Analyses of passivation films on lithium and lithium alloys by electrochemical quartz crystal microbalance

Electrochemical quartz crystal microbalance (EQCM) measurements were carried out during electrochemical deposition/dissolution of Li on the Ti and Pt substrate electrodes of AT-cut quartz crystals in a 1 M LiPF₆ in propylene carbonate (PC) and dimethoxyethane (DME) (1:1 in volume) and a 1 M LiPF₆ in ethylene carbonate (EC) and DME (1:1 in volume). The mass change during Li deposition on the Pt substrate was about half that on the Ti substrate because the alloy formation of Li–Pt suppressed passivation reactions of deposited Li. Furthermore, it was also suggested that the amount of organic compounds in passivation films formed in the PC-based electrolyte was larger than that in the EC-based electrolyte due to PC reacting more easily with Li than EC. The EQCM measurement of the passivation film formations during alloying Li with Pb electrodeposited on the Ti substrate was also conducted successfully. The Li–Pb alloy formation occurred under 0.5 V, in which the potential profile was divided into two regions associated with the phase diagram of Li–Pb alloy. The mass change due to passivation film formations increased in accordance with the changes of Li–Pb alloy compositions into Li rich phases, which were thought either to enhance the reactiveness of Li or decrease the diffusiveness of Li in Li–Pb alloy.     

An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films

An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films in aqueous media is presented. It has been shown that these films exhibit significant mass changes upon electrochemical cycling. These mass changes have been shown to be controlled by the ingress and egress of solvated anions during oxidation and reduction of the film, respectively. The mass changes within the film are shown to tend to steady state after a prolonged period of cycling. Chrono-amperometric studies have additionally shown that the movement of cations also has a role in the electrochemistry and mass transport within these films, but generally these events occur on a shorter timescale than those involving the solvated anions, and are hidden during most potential cycling studies.     

Symmetric current oscillations at tip and substrate electrodes of scanning electrochemical microscope during silver deposition/stripping

Symmetric current oscillations at adjacent tip and substrate electrodes of the scanning electrochemical microscope during silver deposition and dissolution were described. Effects of the tip–substrate distance, the applied potential, the radius of the glass around the tip, diameters of the tip and substrate electrodes as well as the solution ionic strength (I) on the silver growth were examined and discussed. Electrochemical quartz crystal microbalance (EQCM) experiments were also conducted to examine the solution density–viscosity properties and the diffusion coefficients of Ag⁺ (D) in solutions of different ionic strength, and the obtained results are discussed according to the theoretical D?I relationship with the diffusion coefficient of Ag⁺ at infinite dilution and solvent factor. It is concluded that the lateral transport of silver ions via diffusion from the bulk solution to the thin-layer space enclosed by the substrate electrode and the tip remarkably affect the oscillations. An equivalent circuit is proposed to evaluate the resistance for the localized nanoscale silver contact between tip and substrate electrodes. When the tip was located far away from the substrate, instead, a current–time waveform at the tip similar to that at the dropping mercury electrode and periodic current pulses at the substrate electrode were observed.     

Voltammetric and EQCM investigation of conductive films formed by electroreduction of Ru(L)(CO)2Cl2 compounds

Electroreduction of trans(Cl) and cis(Cl)Ru(L)(CO)₂Cl₂ (L=bpy=2,2′-bipyridine, dmbpy=4,4′-dimethyl-2,2′-bipyridine) in CH₃CN and DMSO resulted in electrode-adhering films and the reactions were studied by voltammetry and the electrochemical quartz crystal microbalance. These methods were complemented by hydrodynamic rotating ring-disc and quartz crystal impedance methods. The deposition was strongly influenced by the type of solvent, polarisation mode and electrolyte stirring, but for all compounds, rapid charge transfer was found. For monomers with trans(Cl)Ru(bpy)(CO)₂Cl₂, the two-electron reduction and Ru⁰Ru⁰ polymerisation process suggested in the literature was confirmed. Some adsorption of CH₃CN was also indicated and for potentiostatic deposition in static solution the impedance measurements indicated increased film viscoelasticity. Depositions with cis(Cl)Ru(bpy)(CO)₂Cl₂ were more complex and the rate declined gradually in static solution. In stirred solution a straightforward polymer formation took place. For trans(Cl)Ru(dmbpy)(CO)₂Cl₂ the literature mechanism could not be confirmed and large deviations were observed during the early stages of deposition. The trans(Cl) dimer, [Ru(bpy)(CO)₂Cl]₂ displayed a lower current efficiency during film formation than the monomer and no simple deposition mechanism could be suggested. For the trans(Cl)[Ru(dmbpy)(CO)₂Cl]₂ dimer a straightforward deposition was found. The films could be removed by electrooxidation and mass/charge data show that the films break up in large fragments. The reduction and deposition process for trans(Cl) monomers (L=bpy, dmbpy) was light sensitive and the mass of films deposited during cyclic voltammetry (CV) was doubled when the electrode was irradiated.     

Electrochemical study of methylalkylviologens using an electrochemical quartz crystal microbalance

The electrochemical behavior of various 1-methyl-1′-alkylviologens (C₁C_nV²⁺: n=1, 7, 8, 9, 10, 12, 14, 16 and 18) was studied using an electrochemical quartz crystal microbalance (EQCM). Three different types of the frequency change-potential (Δf–E) curves were observed depending on the length of alkyl substituent during the cyclic voltammetry (CV) of the first redox step: for n=1 and 7, Δf was less than 30 Hz (reversible); for n=9, 10, 12 and 14, Δf=200 Hz indicating electrodeposition of C₁C_nV⁺; for n=16 and 18, Δf=5000 Hz with two anodic peaks implying reorientation of the electrodeposited C₁C_nV⁺ occurring on the electrode surface. These results are consistent with our previous report where apparent dimerization of C₁C_nV⁺ was observed. The effect of cyclodextrins (CDs) on the frequency change-potential (Δf–E) curves was also investigated. Δf for 1 mM C₁C₁₆V²⁺ solution was 6000 Hz, while Δf was below 100 Hz in the presence of 13 mM of α-CD. However, Δf was 5000 Hz in the presence of 13 mM of β-CD. This implies that the complexation ability between C₁C₁₆V⁺/C₁C₁₆V²⁺ and α-CD is larger than that of β-CD through the tighter binding as per the report by Diaz et al. (J. Phys. Chem., 92 (1988) 3537). Comproportionation between C₁C₇V²⁺ and C₁C₇V⁰ in the presence of α- and β-CD was also carried out. A negligible effect was observed in the CV or Δf–E curve by α-CD. With β-CD, the anodic stripping wave (C₁C₇V⁰?e^?→C₁C₇V⁺) disappeared, however, Δf was 9500 Hz (1100 Hz without CDs). It is clear that the complexation ability between C₁C₇V⁰ and β-CD is larger than that of α-CD. This might be due to the limited solubility of β-CD upon complexation with C₁C₇V⁰. A similar result was obtained in a 1 mM C₁C₄V²⁺ with 30 mM γ-CD experiment. The mechanism of the cyclodextrin-induced comproportionation reaction as well as electrodeposition pathways are discussed in this paper based on EQCM and spectroelectrochemical experimental results.     

A piezoelectric gene-sensor using actinomycin D-functionalized nano-microspheres as amplifying probes

A gene-sensing system has been developed using actinomycin D-functionalized magnetic nano-microspheres, which can interact with double-stranded DNAs (dsDNAs) anchored on the gold film electrode of an electrochemical quartz crystal microbalance (EQCM). Actinomycin D acts as a guide that leads heavy microspheres onto the dsDNAs at the EQCM film. A magnetic separation shelf could separate unreacted microspheres conveniently. The modification and DNA hybridization at EQCM electrodes were examined by microgravimetric and electrochemical methods. In this way, an outstanding change in frequency decrease has been monitored owing to the mass increase on the EQCM electrodes. The limit for the determination of target DNA could be improved from 6.2×10^?8 to 2.0×10^?12 mol l^?1 by the amplifying technique.     

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.     

Electrocatalytic effects in the electrochemical reduction of pyrazine on single crystal IB metal electrodes

Due to the presence of the second nitrogen atom, pyrazine has an increased affinity for electrons and lower energies of the empty orbitals than pyridine. For this reason the electrochemical reduction of pyrazine takes place at considerably less negative potentials than that of pyridine. The reduction of pyrazine on Cu(111), Ag(111) and Au(111) electrodes in neutral aqueous media has been found to take place in one step as previously reported for Hg. The potential region and the current densities associated with this process were, however, observed to depend significantly on the electrode nature. Pyrazine was also found to be involved in a chemical interaction with the hydrogen atoms resulting in the water electro-reduction process occurring on Au(111) and Cu(111). The specific interaction of water with the single crystal electrodes turned out to be an important factor for developing the reduction of the organic molecule, both chemically and electrochemically.     

The hanging meniscus contact: geometry induced diffusional overpotential. The reduction of oxygen in dimethylsulphoxide at Au(111)

Mass transport in the hanging meniscus configuration has been studied. The effects of this geometry on an electrochemical reaction of a gas phase species with a high solubility in the electrolyte phase have been investigated, both numerically and experimentally. The steady-state process is modelled in cylindrical polar coordinates using a fully implicit finite-difference numerical method and solved with a preconditioned Krylov subspace method. The numerical method incorporates an expanding mesh technique to optimise electrode flux error-cancelling. The nature of the meniscus boundary such as to permit the ingress of electroactive material mentioned, but not the egress of electrode reaction products is shown to result in a diffusional overpotential in the steady-state voltammogram. This observation has been examined experimentally by using the reduction of O₂ in dimethylsulphoxide solution on a Au(111) single crystal electrode.     

Electrochemical properties of ruthenised electrodes in the oxide layer region

The impact of increasing the amount of electrodeposited ruthenium and of potential/time treatment of ruthenium substrates on the electrochemical properties of anodically formed ruthenium oxide films, are investigated by use of cyclic voltammetry, electrochemical quartz crystal microbalance, anodic and cathodic current transients, and electrochemical impedance spectroscopy. In addition to the specific interpretation of results obtained by each of the techniques used, general discussion in terms of the essential charge storage characteristics of ruthenium oxide films is presented. It is shown that an increase in the ruthenium loading acts beneficially on the stability of oxide films over a range of potentials, but detrimentally on the kinetics of the overall charging/discharging reaction, which becomes proton mass transfer limited. On the other hand, potential/time treatment of ruthenium substrates acts detrimentally on the stability of related oxide films, but makes the kinetics of the oxide film charging/discharging reaction more reversible. Total capacitance values of oxide films presented per gram of electrodeposited ruthenium are found to be almost independent of ruthenium loading, and slightly increased by the potential/time treatment performed. This all points to the fact that an increase in the ruthenium loading and potential/time treatment of ruthenium substrates results primarily in beneficial changes in the morphology of related oxide films. The magnitudes of the total capacitances per gram of electroplated ruthenium, however, remain strongly determined by the method of preparation of ruthenium substrates, and cannot be significantly changed by either an increase in the ruthenium loading or by the potential/time treatment of the ruthenium substrates.     

Influence of a static magnetic field on nickel electrodeposition studied using an electrochemical quartz crystal microbalance,atomic force microscopy and vibrating sample magnetometry

We investigated the influence of static magnetic fields up to 740 mT on the electrochemical nickel deposition from a sulphamate electrolyte. The magnetic field was applied parallel to the surface of the working electrode and thus gave rise to magnetohydrodynamic convection by Lorentz forces. An electrochemical quartz crystal microbalance was used to characterize the amount of hydrogen produced on the cathode during the deposition and to calculate the efficiency of the deposition process. The amount of hydrogen produced depends on the magnetic flux density, and this dependence can be explained by the complex interplay of the electrode kinetics and hydrodynamic effects, correlated with the Lorentz force. Atomic force microscopy showed an increase of the surface roughness of the nickel layers deposited in the magnetic field. The absolute values of the magnetic properties of the layers prepared in the magnetic field differed slightly from those prepared in the absence of a magnetic field. This can be explained by the structural changes. No anisotropy of the magnetic properties was observed.     

Electrochemical response of α-H4SiW12O40 on Ag and Au electrodes

Electrochemical measurements are presented for acidic solutions containing α-H₄SiW₁₂O₄₀ adsorbed on Ag(111) and Au(111) electrodes. These measurements show that this molecule passivates the Ag surface towards solution redox events. This passivation is unique to Ag, as it is not observed on Au or carbon electrodes. Depassivation can be accomplished by moving the potential of the Ag electrode into the hydrogen evolution region. These results are discussed in terms of formation of a reduced AgSiW₁₂O₄₀ complex that is stable only at negative potentials.     

Comments on ‘On the calculation of surface concentration from the measurement by the radiotracer ‘electrode lowering’ technique’ by D. Poskus

Quantitative aspects of surface concentration measurements of radiolabeled adsorbates by the thin-gap measuring technique developed by our group have been discussed recently by Poskus (see D. Poskus, J. Electroanal. Chem. 442 (1998) 5). It was argued that surface concentrations obtained by us were underdetermined since the backscattering factor of β-radiation used in our work was too high versus a similar, but more recently obtained value by the quoted author. In response, we have reanalyzed our measuring conditions with a special emphasis on the backscattering phenomena involved in the experiment. Without contradicting the author’s conclusions, the reported results indicate that the backscattering factor used in our formula linking radiation data and surface concentrations was correct. Apparently, in the scintillation-counter mode of the measurements, such a backscattering factor involves a simultaneous detection of reflected light from scintillation events and backscattered β-radiation emitted by the radioactive preparate. Conversely, the Poskus measurements conducted by the use of a gaseous counter determine the second component only. Using a new, experimentally determined backscattering factor and a Au(111) electrode, we present a comparison between sulfate adsorption data obtained by the use of the thin-gap technique and Auger electron spectroscopy and prove, once again, the accuracy of our data.