The effect of magnetic fields on the electrodeposition of CoFe alloys

The effect of a uniform magnetic field with a flux density up to 1 T and different configurations relative to the electrode surface, on the electrodeposition of Co, Fe and CoFe alloys from acidic sulphate electrolytes has been investigated. It was found that a magnetic field applied parallel to the electrode surface increases the limiting current density and deposition rate due to the magnetohydrodynamic (MHD)-effect caused by the Lorentz force. No significant changes of the limiting current density and deposition rate with a magnetic field applied in the perpendicular to electrode configuration were observed. But in this configuration desorption of hydrogen was found to be supported by a magnetic field. The chemical composition of the deposited alloys was found to be unaffected by the magnetic field. The deposition of CoFe at the highest applied potential (−1600 mV_MSE) was found to be anomalous. At this potential the increase of the interface pH value is sufficient for hydroxyl species formation.     

Electrodeposition of CdTe thin films onto n-Si(1 0 0): nucleation and growth mechanisms

The mechanisms related to the initial stages of the nucleation and growth of cadmium telluride (CdTe) thin films on the rough face side of a (1 0 0) monocrystalline n-type silicon have been studied as a function of different potential steps that varied from an initial value of −0.200 V to values comprised between −0.515 and −0.600 V versus saturated calomel electrode (SCE). The analysis of the corresponding potentiostatic j/t transients suggests that the main phenomena involved at short times is the formation of a Te-Cd bi-layer (BL). For potentials below −0.540 V, the formation of this bi-layer can be considered independent of potential. At greater times, the mechanisms is controlled by two process: (i) progressive nucleation three dimensional charge transfer controlled growth (PN-3D)_ct and (ii) progressive nucleation three dimensional diffusion controlled growth (PN-3D)_diff, both giving account for the formation of conical and hemispherical nuclei, respectively. Ex situ AFM images of the surface seem to support these assumptions.     

Electrocrystallization of Au nanoparticles on glassy carbon from HClO4 solution containing [AuCl4]

The mechanism and kinetics of electrocrystallization of Au nanoparticles on glassy carbon (GC) were investigated in the system GC/1 mM KAuCl₄ + 0.1 M HClO₄. Experimental results show that the gold electrodeposition follows the so-called Volmer-Weber growth mechanism involving formation and growth of 3D Au nanoparticles on an unmodified GC substrate. The analysis of current transients shows that at relatively positive electrode potentials (E ≥ 0.84 V) the deposition kinetics corresponds to the theoretical model for progressive nucleation and diffusion-controlled 3D growth of Au nanoparticles. The potential dependence of the nucleation rate extracted from the current transients is in agreement with the atomistic theory of nucleation. At sufficiently negative electrode potentials (E ≤ 0.64 V) the nucleation frequency becomes very high and the nucleation occurs instantaneously. Based on this behaviour is applied a potentiostatic double-pulse routine, which allows controlled electrodeposition of Au nanoparticles with a relatively narrow size distribution.     

Electrodeposition of Te onto monocrystalline n- and p-Si(1 0 0) wafers

Electrochemical deposition of Te onto n- and p-Si(1 0 0) wafers from 0.1 M HNO₃ + 1 mM TeO₂ solution was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM and XRD. Electrodeposition of Te onto n-Si takes place both in the dark and under illumination. Electrodeposition of Te onto p-Si proceeds only under illumination, when the photoelectrons are generated in silicon substrate and reduce Te(IV) species in solution. Electrochemical reduction of Te(IV) on n- and p-Si occurs with large cathodic overvoltage (0.22-0.62 V). Nucleation of Te on n- and p-Si proceeds via 3D island growth, it is characterised correspondingly by progressive and instantaneous nucleation mechanisms followed by diffusion limited growth. Cathodic deposition of Te onto n- and p-Si is irreversible. Anodic stripping of Te electrodeposited onto p-Si occurs both in the dark and under illumination and anodic stripping of Te from n-Si proceeds only under illumination.     

Overpotential deposition of copper on an iodine-modified Au(1 1 1) electrode

Cyclic voltammetry, chronoamperometry and in situ electrochemical scanning tunneling microscopy were used to study the kinetics of nucleation and crystal growth during the initial stages of copper overpotential deposition (OPD) on a previously iodine-modified Au(1 1 1) electrode, from an aqueous solution 10⁻³ M CuSO₄ in 0.05 M H₂SO₄. The starting potential during step experiments was chosen in the region where the gold electrode was completely free of the copper deposit. The recorded current transients for copper deposition onto the iodine-modified Au(1 1 1) electrode surface appear to be very complex, with the unusual presence of two or more current maxima. A new method was used for quantitative evaluation of current transients that involves the transition UPD-OPD, developed by our group [M. Palomar-Pardavé, I. González, N. Batina, J. Phys. Chem. B 104 (2000) 3545], was used for the quantitative interpretation. Our results show that, within a single current transient, copper adsorption and two types of nucleation process: two-dimensional (2D) and three-dimensional (3D) limited by lattice incorporation of copper adatoms and diffusion of Cu(II) ion, respectively, take place simultaneously. STM images revealed the enhanced growth of 3D copper on edge of I-Au(1 1 1) during the early stages of deposition. Moreover, our results strongly suggest that the iodine adlayer is constantly present, even after the striping Cu that was overpotential deposited.     

Copper electrodeposition on pyrolytic graphite electrodes: Effect of the copper salt on the electrodeposition process

The electrodeposition of copper on pyrolytic graphite from CuSO₄ or Cu(NO₃)₂ in a 1.8 M H₂SO₄ aqueous solution was investigated. The Cu deposits were formed potentiostatically and characterized by electrochemical methods, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy. It was found that the deposition of copper in the presence of CuSO₄ induced the codeposition of sulfate anions. In addition, electrochemical quartz crystal microbalance revealed that the increase of the Cu mass was higher than expected from Faraday's law with the CuSO₄/H₂SO₄ solution. These results confirmed the specific adsorption of anions during the Cu deposition. On the other hand, the use of Cu(NO₃)₂ resulted in a non-contaminated surface with different surface morphologies. The Cu nuclei size, the population density and the surface coverage were monitored as a function of the deposition potential. From the analysis of the chronoamperometric curves, the nucleation kinetics was studied by using various theoretical models. Independently of the Cu source, the nucleation mechanism follows a three-dimensional (3D) process. Copper nucleates according to an instantaneous mode when the deposition potential is more negative than −300 mV versus Ag/AgCl, while the nucleation was interpreted in terms of a progressive mode at −150 mV. The nuclei population densities were also determined by using two common fitting models for 3D nucleation and growth (Scharifker-Mostany and Mirkin-Nilov-Heerman-Tarallo). Their values are reported here as a function of the deposition potential.     

Zinc electrodeposition in the presence of polyethylene glycol 20000

The influence of polyethylene glycol 20000 (PEG₂₀₀₀₀) on the mechanism of zinc deposition and nucleation was studied by voltammetry, chronoamperometry, and atomic force microscopy (AFM). The electrodeposition of zinc in an electrolytic bath containing PEG₂₀₀₀₀ occurs via two reduction processes with different energies that involve the same species, ZnCl₄²⁻: the first reduction process occurs at E_PI′c = −1.25 V, SCE, whereas the second process, which corresponds to the bulk deposition of Zn, occurs at E_PII′c = −1.6 V, SCE without significant interference from the hydrogen evolution reaction. Analysis of chronoamperograms obtained in the absence and presence of PEG₂₀₀₀₀ indicates that distinct nucleation mechanisms are involved during the initial stages of Zn deposition. In the absence of PEG₂₀₀₀₀, the transients are consistent with the model of 3D diffusion-controlled nucleation. In the presence of PEG₂₀₀₀₀, however, the transients exhibit a more complex form involving two simultaneous nucleation and growth processes: 2D instantaneous nucleation limited by the incorporation of adatoms (2D_i-li) and a diffusion-controlled 3D nucleation mechanism (3D_-dc). Characterization of the surface morphologies of the zinc deposits by AFM imaging confirmed our conclusions drawn from the electrochemical studies. SEM analysis showed that the zinc coatings obtained in the presence of PEG₂₀₀₀₀ at −1.6 V, SCE are smoother and more compact.     

Electrochemical behavior of fission palladium in 1-butyl-3-methylimidazolium chloride

Electrochemical behavior of palladium (II) chloride in 1-butyl-3-methylimidazolium chloride has been investigated by various electrochemical transient techniques using glassy carbon working electrode at different temperatures (343-373 K). Cyclic voltammogram consisted of a prominent reduction wave at −0.61 V (vs. Pd) due to the reduction of Pd(II) to Pd, and two oxidation waves at −0.26 and 0.31 V. A nucleation loop is observed at −0.53 V. The diffusion coefficient of palladium (II) in bmimCl (∼10⁻⁷ cm²/s) was determined and the energy of activation (63 kJ/mol) was deduced from the cyclic voltammograms at various temperatures. Nucleation and growth of palladium on glassy carbon working electrode has been investigated by chronoamperometry and chronopotentiometry. The growth and decay of chronocurrents measured for palladium deposition has been found to follow the instantaneous nucleation model with three-dimensional growth of nuclei. The surface morphology of the deposit obtained at various applied potentials revealed the formation of dendrites immediately after nucleation and spread in all the directions with time.     

Electrodeposition of palladium in a hydrophobic 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquid

The electrochemical reduction of palladium halide complexes such as PdBr₄²⁻ and PdCl₄²⁻ was investigated in a hydrophobic room-temperature ionic liquid (RTIL), 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI). The irreversible electrode reaction between Pd(II) and Pd(0) was observed in BMPTFSI containing PdBr₄²⁻ or PdCl₄²⁻ by cyclic voltammetry. The diffusion coefficient of PdBr₄²⁻ was estimated to be (1-2) × 10⁻⁷ cm² s⁻¹ by choronopotentiometry and chronoamperometry. The deposition of crystalline Pd metal was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. It was suggested by the chronoamperometric measurements on PdBr₄²⁻ in BMPTFSI that the initial stage of the electrodeposition of Pd on the polycrystalline Pt electrode surface involves three-dimensional progressive nucleation under diffusion control. The reduction potential of PdCl₄²⁻ was more negative than that of PdBr₄²⁻, reflecting the difference in the donor property between chloride and bromide.     

Electrodeposition of copper on Ru(0 0 0 1) in sulfuric acid solution: Growth kinetics and nucleation behavior

The electrodeposition of Cu on Ru(0 0 0 1) from 0.1 M CuSO₄/0.5 M H₂SO₄ solution has been studied by cyclic voltammetry, current-time transient measurements, and by in situ electrochemical atomic force microscopy (EC-AFM). Cyclic voltammetry measurements show that the as-prepared Ru(0 0 0 1) electrode exhibits a UPD peak, while EC-AFM data indicate a broadly terraced surface with step heights of atomic dimensions. Kinetic data show that the electrodeposition/nucleation process is not well described by 3D or 2D nucleation models. The EC-AFM data show that at potentials near the OPD/UPD threshold, Cu crystallites exhibit pronounced growth anisotropy, with lateral dimensions greatly exceeding vertical dimensions. AFM data also show that deposition at more cathodic potentials result in smaller crystallites.     

Electrochemical evaluation of the surface of chalcopyrite during dissolution in sulfuric acid solution

The dissolution of a massive chalcopyrite electrode (98.1% chalcopyrite, 1.9% siderite) was studied in 0.5 M sulfuric acid solution. Different anodic potentials were applied and the behavior of the electrode was observed by means of EIS, potentiodynamic, and Mott-Schottky techniques. Electrochemical impedance spectroscopy studies at open circuit potential (around −235 mV vs. MSE) proved the existence of a thin surface layer on the electrode. This layer was stable up to 100 mV vs. MSE and was assumed to be Cu_1−xFe_1−yS₂ (y?x) based on reports from previous studies. By increasing the potential to the range of 100-300 mV vs. MSE, the previously formed layer partially dissolved and a second layer (Cu_1−x−zS₂) formed on the surface. Both of the layers showed the characteristics of passive layers at low potentiodynamic scan rate (0.05 mV s⁻¹) while at high scan rates they acted like pseudo-passive layers. However, in the potential range of 300-420 mV vs. MSE, both of these surface layers dissolved and active dissolution of the electrode started. Further increase in potential caused the formation of a CuS layer which hindered the dissolution rate of the electrode. The formation of CuS is concomitant with Fe₂(SO₄)₃ formation and the latter may act as a nucleation precursor for jarosite at higher potentials (around 750 mV vs. MSE). Jarosite precipitation on the electrode surface hindered the dissolution of chalcopyrite at higher potentials. Different equivalent electrochemical circuits were modeled for each potential range and the model regression results compared with the experimental results of EIS to determine the proposed sequence of chalcopyrite dissolution.     

Electrochemistry of terbium in the eutectic LiCl-KCl

The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl-KCl molten was investigated in the temperature range 673-823 K, by means of cyclic voltammetry, chronopotentiometry and chronoamperometry. It was found that during cathodic polarization, deposition of metallic Tb from the chloride mixture onto the tungsten surface proceeds in a single step, and electrocrystallization plays an important role in the whole process. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. From chronopotentiometric measurements, the diffusion coefficient of the Tb(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 33.4 ± 1.5 kJ mol⁻¹.The standard apparent potential value of the Tb(III)/Tb(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Tb(III) in the molten chloride media based on a pure supercool reference state has also been calculated.     

Different steps in the electrosynthesis of poly(3,4-ethylenedioxythiophene) on platinum

The initial stages of poly(3,4-ethylenedioxythiophene) (PEDOTh) film growth on platinum electrodes from TBAClO₄/acetonitrile solution are investigated by means of current-time transient measurements and tapping mode atomic force microscopy. It is shown that, for growth potentials in the range 1.17-1.29 V vs. SCE, the deposition process involves the formation of oligomers in the solution, progressive nucleation of centres of the new phase and three-dimensional growth of a first compact layer followed by a non-uniform distribution of granular-like clusters, whose number and size increase with the synthesis potential and charge. The obtained results reveal that PEDOTh films prepared at distinct potentials but with the same growth charge (Q_g) display similar electroactivities. They also depict that the electrochemical behaviour of the films is a function of the charge used for the synthesis, namely the reduction of tick PEDOTh layers (Q_g > 20 mC cm⁻²) includes more that one step, as a consequence of the formation of a two-layered polymer film.     

In situ radiotracer and voltammetric study of the formation of surface adlayers in the course of Cr(VI) reduction on polycrystalline and (1 1 1) oriented platinum

This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO₄⁻/SO₄²⁻ adsorption at Pt(poly) and Pt(1 1 1) surfaces in 0.1 mol dm⁻³ HClO₄ solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO₄⁻/SO₄²⁻ ions on bare Pt(poly) and Pt(1 1 1) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(1 1 1) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (1 1 1) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO₄⁻/SO₄²⁻ ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E > 0.20 V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode.     

Electrochemistry of Sn(II)/Sn in a hydrophobic room-temperature ionic liquid

The electrochemical reaction of Sn(II)/Sn was investigated in a room-temperature ionic liquid, 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) at 25 °C. The anodic dissolution of Sn metal proceeded by a two-electron transfer reaction with a current efficiency of nearly 100%. Electrodeposition of Sn on a Cu substrate is possible in BMPTFSI containing Sn(II). The formal potential of the Sn(II)/Sn is −0.57 V vs. Ag/Ag(I). The diffusion coefficient of Sn(II) was estimated to be ∼1 × 10⁻⁷ cm² s⁻¹ from chronoamperometric and chronopotentiometric techniques. The initial stage of nucleation of Sn on a polycrystalline Pt substrate was found to be classified into a three-dimensional progressive nucleation under diffusion control by chronoamperometry, suggesting the rate of nucleation is faster than that of crystal growth.     

On the performances of lead dioxide and boron-doped diamond electrodes in the anodic oxidation of simulated wastewater containing the Reactive Orange 16 dye

The performances of the Ti-Pt/β-PbO₂ and boron-doped diamond (BDD) electrodes in the electrooxidation of simulated wastewaters containing 85 mg L⁻¹ of the Reactive Orange 16 dye were investigated using a filter-press reactor. The electrolyses were carried out at the flow rate of 7 L min⁻¹, at different current densities (10-70 mA cm⁻²), and in the absence or presence of chloride ions (10-70 mM NaCl). In the absence of NaCl, total decolourisation of the simulated dye wastewater was attained independently of the electrode used. However, the performance of the BDD electrode was better than that of the Ti-Pt/β-PbO₂ electrode; the total decolourisations were achieved by applying only 1.0 A h L⁻¹ and 2.0 A h L⁻¹, respectively. In the presence of NaCl, with the electrogeneration of active chlorine, the times needed for total colour removal were markedly decreased; the addition of 50 mM Cl⁻ or 35 mM Cl⁻ (for Ti-Pt/β-PbO₂ or BDD, respectively) to the supporting electrolyte led to a 90% decrease of these times (at 50 mA cm⁻²). On the other hand, total mineralization of the dye in the presence of NaCl was attained only when using the BDD electrode (for 1.0 A h L⁻¹); for the Ti-Pt/β-PbO₂ electrode, a maximum mineralization of 85% was attained (for 2.0 A h L⁻¹). For total decolourisation of the simulated dye wastewater, the energy consumption per unit mass of dye oxidized was only 4.4 kWh kg⁻¹ or 1.9 kWh kg⁻¹ using the Ti-Pt/β-PbO₂ or BDD electrode, respectively. Clearly the BDD electrode proved to be the best anode for the electrooxidative degradation of the dye, either in the presence or absence of chloride ions.     

Electrodeposition of PbSe onto n-Si(1 0 0) wafers

PbSe was electrodeposited onto monocrystalline n-Si(1 0 0) wafers from 50 mM Pb(NO₃)₂ + 2 mM SeO₂ + 0.1 M HNO₃ solution. The mechanism of PbSe electrocrystallization on n-Si was studied. At initial stage, 3D Pb and 3D Se nuclei are simultaneously codeposited onto Si at potentials more negative than Si flat band potential and chemically interact resulting in PbSe formation. When n-Si/PbSe heterostructure is formed, the overvoltage of bulk lead deposition increases, as a result of redistribution of electrode potential. Further growth of PbSe is realized due to underpotential deposition (UPD) of Pb and overpotential deposition (OPD) of Se onto formed PbSe nuclei. With Pb UPD shift increase, amorphous Se inclusion is registrated in the deposit. When 2D Pb nucleation mechanism is changed to 3D mode, metal Pb cubic phase is codeposited with PbSe. Electrodeposition of PbSe onto n-Si is irreversible. PbSe anodic stripping does not take place in the dark due to the barrier on solid interface. Oxidation of PbSe on n-Si is observed only under illumination, when photoholes are generated in silicon substrate.     

Electrocrystallisation of zinc from acidic sulphate baths; A nucleation and crystal growth process

The electrochemical nucleation and growth of zinc on low-carbon steel from acidic (pH 2.0-4.5) baths containing ZnSO₄, NaCl, and H₃BO₃, was studied by means of chronoamperometry at various cathodic potentials under a charge-transfer controlled regime. It is shown that at overpotentials in the range 0.30-0.55 V (negative to the Zn²⁺/Zn redox value) the electrodeposition proceeds by instantaneous three-dimensional nucleation, which turns to progressive at higher overpotentials and/or very acidic baths. At low cathodic overpotentials (<0.30 V), a two-dimensional contribution limited by the incorporation of Zn ad-atoms in the developing lattice becomes significant at the early stages of deposition, and is more progressive in type the more acidic is the bath pH. Nucleation rate constants were calculated and correlated analytically with the respective potentials, using the classical theory of heterogeneous nucleation, which though fails to lead to reasonable values for the critical nucleus size.     

An electroanalytical application of 2-aminothiazole-modified silica gel after adsorption and separation of Hg(II) from heavy metals in aqueous solution

2-Aminothiazole covalently attached to a silica gel surface was prepared in order to obtain an adsorbent for Hg(II) ions having the following characteristics: good sorption capacity, chemical stability under conditions of use, and, especially, high selectivity. The accumulation voltammetry of mercury(II) was investigated at a carbon paste electrode chemically modified with silica gel functionalized with 2-aminothiazole (SIAMT-CPE). The repetitive cyclic voltammogram of mercury(II) solution in the potential range −0.2 to +0.6 V versus Ag/AgCl (0.02 mol L⁻¹ KNO₃; v=20 mV s−1) show two peaks one at about 0.1 V and other at 0.205 V. The anodic wave peak at 0.205 V is well defined and does not change during the cycles and it was therefore further investigated for analytical purposes using differential pulse anodic stripping voltammetry in differents supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, “cleaning” solution, possible interferences and other variables. The precision for six determinations (n = 6) of 0.02 and 0.20 mg L⁻¹ Hg(II) was 4.1 and 3.5% (relative standard deviation), respectively. The detection limit was estimated as 0.10 μg L⁻¹ mercury(II) by means of 3:1 current-to-noise ratio in connection with the optimization of the various parameters involved and using the highest-possible analyser sensitivity.     

Lead electrodeposition from very alkaline media

The electrodeposition of lead from very alkaline media has been studied by cyclic voltammetry, chronoamperometry under stationary and convective conditions. Experimental parameters like lead concentration and temperature have been varied. From NaOH 6 M the metal deposition takes place at about −0.90 V versus SCE far from the hydrogen evolution reaction (HER) which is seen at −1.30 V, but both processes are favoured by the lead content increase and the NaOH concentration decrease. The analyses of the chronoamperometric responses support the view of a 3D growth and suggest a substantial influence of lead concentration on the type of nucleation. Progressive nucleation is observed for the deposition from solutions with low content in lead but as this concentration increases a tendency towards instantaneous nucleation is revealed. The voltammetry with the rotating platinum disc electrode has confirmed that the lead electrodeposition is a mass transfer controlled process, and also allowed the estimation of diffusion coefficients.