Specific adsorption of anions in the course of underpotential and bulk depositions and alloy formation of cadmium on a smooth gold support in acid medium

 The specific adsorption of ³⁶Cl-labelled Cl⁻ ions and ³⁵S-labelled HSO₄ ⁻ ions was studied in 1 mol dm⁻³ HClO₄ supporting electrolyte in the presence of Cd²⁺ ions at a gold support over a wide potential range corresponding to electrodeposition, alloy formation, underpotential deposition of Cd species and existence of an adatom-free surface. The distinct sections in the potential dependence of the adsorption of anions together with the potential versus time curves obtained under open circuit conditions reflect the changes in the state of the electrode surface, the dissolution of the bulk Cd phase and the slow elimination of Cd species from the Cd/Au alloy. Received: 16 March 1999 / Accepted: 5 May 1999     

Underpotential deposition studies of copper on glassy carbon

Studies on the deposition and dissolution of copper from 0·5 M sulphuric acid solutions onto glassy carbon (GC) using potential sweep techniques indicated that an additional peak occurs at higher positive potentials than the bulk stripping peak. This peak is identified as due to the stripping of underpotential deposited (UPD) copper. Results of investigations on the effect of sweep rate, deposition potential and time of deposition on the peak characteristics of UPD and bulk deposited copper are also reported.     

Peculiarities of copper underpotential deposition and nucleation on polycrystalline platinum in the presence of acetonitrile: Rotating ring-disk electrode

The initial stages of copper electrocrystallization on polycrystalline platinum in 0.5 M H₂SO₄ + 10 mM CuSO₄ + 0–200 mM acetonitrile (AcN) solutions are studied by the methods of cyclic voltammetry and potentiostatic current transients on a ring-disk electrode. Adsorbed AcN molecules accelerate both the underpotential deposition and the bulk deposition of copper due to the local electrostatic effects on the charged interface. With the increase in the additive concentration in solution, the contribution of the production of copper ions Cu⁺ is observed to increase due to the formation of Cu(AcN) _x ⁺ comlexes, particularly, for [AcN] ≥ 4 mM when the concentrations of acetonitrile and copper sulfate become comparable. In the presence of AcN, as well as in the copper sulfate supporting electrolyte, the adatomic layer is formed via the mechanism of the two-dimensional growth of Cu(1 × 1) phase islets.     

Enhancement of the electrochemical reduction of oxygen at platinum by nickel underpotential deposition

Polycrystalline Pt electrode was modified by underpotential deposition (upd) of nickel. The modification was performed by potential cycling in phosphate buffer pH 7. 0 containing NiSO₄, in which hydrogen and nickel upd processes were well separated. The maximum Ni upd coverage was found to be 0.3. Oxygen reduction was studied at bare and nickel upd-modified Pt. It was found that the reaction rate increased with increasing Ni upd coverage. At θ(Ni)=0.3, the current density was a factor of 2 higher compared to bare Pt (at the potential of 0.85 V). The capacitance of the electrode interface was determined in potential-relaxation experiments following interruption of the polarization current. It was found that the pseudocapacitance owing to a coverage by the adsorbed reaction intermediates was higher on the Ni-modified Pt surface than on bare Pt, which resulted in higher reaction rate. The influence of Ni adatoms on the surface coverage by the reaction intermediates was attributed to the inhibition of OH adsorption on Pt by OH ligands attached on neighboring Ni atoms.     

Detection of lead ions in picomolar concentration range using underpotential deposition on silver nanoparticles-deposited glassy carbon electrodes

The efficacy of silver-deposited glassy carbon electrode for the determination of lead ions at the sub-nanomolar concentration ranges is investigated. The silver nanoparticles are electrodeposited on glassy carbon electrode using chronoamperometry and the electrode surface is characterized using SEM. Lead ions are detected in the region of underpotential deposition. The analysis is performed in square wave mode in the stripping voltammetry without the removal of oxygen. The detection limit of 10 pM has been obtained with a constant potential of −0.7 V during the electrodeposition step for a period of 50 s. The interference of surfactants in the detection of lead ions is also studied.     

Voltammetric behaviour at gold electrodes immersed in the BCR sequential extraction scheme media: Application of underpotential deposition-stripping voltammetry to determination of copper in soil extracts

The development of mercury-free electroanalytical systems for in-field analysis of pollutants requires a foundation on the electrochemical behaviour of the chosen electrode material in the target sample matrices. In this work, the behaviour of gold working electrodes in the media employed in the BCR sequential extraction protocol, for the fractionation of metals in solid environmental matrices, is reported. All three of the BCR sequential extraction media are redox active, on the basis of acidity and oxygen content as well as the inherent reducing or oxidising nature of some of the reagents employed: 0.11 M acetic acid, 0.1 M hydroxylammonium chloride (adjusted to pH 2) and 1 M ammonium acetate (adjusted to pH 2) with added trace hydrogen peroxide. The available potential ranges together with the demonstrated detection of target metals in these media are presented. Stripping voltammetry of copper or lead in the BCR extract media solutions reveal a multi-peak behaviour due to the stripping of both bulk metal and underpotential metal deposits. A procedure based on underpotential deposition-stripping voltammetry (UPD-SV) was evaluated for application to determination of copper in 0.11 M acetic acid soil extracts. A preliminary screening step in which different deposition times are applied to the sample enables a deposition time commensurate with UPD-SV to be selected so that no bulk deposition or stripping occurs thus simplifying the shape and features of the resulting voltammograms. Choice of the suitable deposition time is then followed by standards addition calibration. The method was validated by the analysis of a number of BCR 0.11 M acetic acid soil extracts. Good agreement was obtained been the UPD-SV method and atomic spectroscopic results.     

Cu + Au alloy particles formed in the underpotential deposition region of copper in acid solutions

Cu + Au alloy particles electrodeposited on an amorphous carbon electrode at the underpotential region of Cu in both perchloric acid and sulfuric acid solutions were investigated by means of transmission electron microscopy. The fraction of Cu in the Cu + Au alloy particles grown in both acid solutions with a concentration of 1 mM Au ion increased while the underpotential deposition (UPD) potential was decreased. However, it was independent of the concentration of Cu ion in solution. It is inferred that the composition of the Cu + Au alloy particles is dependent on the UPD potential. The fraction of Cu in the Cu + Au alloy particles grown at around the reversible Nernst potential of Cu in 0.1 mM HAuCl₄ + 50 mM Cu(ClO₄)₂ containing perchloric acid solution was 505. This result suggests a layer-by-layer formation of the Cu + Au alloy particles. The fraction of Cu in the Cu + Au alloy particles formed in the presence of sulfate was lower than that in the perchloric acid solution as the UPD potential and the concentration of Cu ion were the same. This is attributed to an influence of coadsorbed sulfate ions.     

Changes of the Cu electrode real surface area during the process of electroless copper plating

The surface area (nanoscale roughness) of copper coatings deposited from electroless plating solutions containing Quadrol, L(+)- and DL(∓)-tartrate as Cu(II) ion ligands was measured using underpotential deposition thallium monolayer formation. Surface roughness of Cu coatings depends on the plating solution pH and the Cu(II) ligand, and varies over a wide range. In L(+)-tartrate and Quadrol solutions (pH 12.5–13.3) the roughness factor R _f is low and is equal to 2–3 and 4–6, respectively (substrate: electrodeposited Cu; R _f=2.2). Cu coatings of higher surface area are obtained in DL(∓)-tartrate (pH 12.3–12.7) and Quadrol (pH 12.0) solutions: R _f reaches 20–30. The correlation between R _f and Cu deposition rate was found in L(+)-tartrate solution. The Cu surface area changes are discussed in terms of partial electrochemical reactions of the autocatalytic Cu deposition process, and the decisive role of cathodic Cu(II) reduction from adsorbed Cu(II) complex species. Received: 2 November 1999 / Accepted: 22 February 2000     

Underpotential deposition of tin(II) on a gold disc electrode and determination of tin in a tin plate sample

This work describes a study of the underpotential deposition (UPD) of Sn²⁺ on a polycrystalline gold disc electrode using cyclic voltammetry (CV) and chronocoulometry (CC). Sn²⁺ ions showed well-defined peaks from UPD and UPD stripping (UPD-S) in 1 mol/L HCl solutions, while bulk deposition (BD) and BD stripping (BD-S) of the ions were also observed. The measured UPD shifts, E_UPD, between the UPD-S and the BD-S peaks were more than 200 mV. The UPD charge and the surface coverage of tin were measured by CC. A new method for determining Sn²⁺ was therefore developed, based on the excellent electrochemical properties of the Au/Sn UPD system. A plot of the UPD-DPASV (differential pulse anodic stripping voltammetry) signal versus the Sn(II) concentration was obtained for [Sn(II)] of 1.98×10^–7 to 3.64×10^–5 M. The method developed here has been applied to determine the tin in a tin plate sample.     

Simultaneous oscillations of surface energy, superficial mass and electrode potential in the course of galvanostatic oxidation of formic acid

Simultaneous oscillations of electrode potential, surface mass and specific surface energy have been detected in the course of galvanostatic oxidation of formic acid on platinum by using a Koesters laser interferometer combined with an electrochemical quartz crystal microbalance. Changes of surface energy data measured with the electrochemical Koesters laser interferometer and with the electrochemical bending beam technique are shown to be equivalent. Problems related to the interpretation of the measured data are discussed.Dedicated to Professor György Horányi on the occasion of his 70th birthday     

Characteristics of subtractive anodic stripping voltammetry of Pb and Cd at silver and gold electrodes

Silver and gold electrodes are useful for the quantitative determination of lead and cadmium with subtractive anodic stripping voltammetry (SASV). The use of SASV is essential for achieving good separation between the two peaks, to eliminate the interference of nitrates when cadmium is present and to allow analysis at very low concentrations without the removal of oxygen. The deposition and dissolution of Pb²⁺ and Cd²⁺ proceed at underpotential (UPD) on both electrodes. The UPD properties of the deposits are the main factor determining the analytical characteristics of the ASV method and are strongly affected by the type and concentration of the electrolyte. The effects of anions (Cl⁻, Br⁻, SO₄²⁻, NO₃⁻) and acids (HNO₃, HClO₄, H₂SO₄, HCl) are shown. The two electrodes complement each other and, in addition, enable the qualitative identification of Pb²⁺ and Cd²⁺, since the peaks appear in opposite order on the two electrodes. Analysis of mixtures of the two analytes is restricted on gold but not on silver. At gold the two peaks overlap: (i) at concentrations of cadmium higher than 250 nM at deposition times greater than 30 s, (ii) in the presence of copper at concentrations higher than 1 μM, and (iii) in the presence of Triton X-100 at concentrations above 10 mg/l. The repeatability at 10 nM analyte is better than 2.5%. The detection limits for Pb²⁺ and Cd²⁺ at 120 s deposition time and 3500 rpm rotation rate are: dl_Pb/Ag=0.04 nM; dl_Cd/Ag=0.7 nM; dl_Pb/Au=0.1 nM; dl_Cd/Au=0.3 nM. The analysis of lead and cadmium in natural waters has been performed.     

Catalytic effect of Pd nanoparticles on electroless copper deposition

This study elucidates the application of Pd nanoparticles as catalysts of electroless copper deposition and their catalytic effect on the deposition kinetics and microstructure in an electroless copper bath. Quartz crystal microgravimetry (QCM) and high-resolution field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that the kinetic changes associated with electroless copper deposition (ECD) comprised two stages—the incubation period and the acceleration period. In the incubation period, small copper particles were deposited. In the acceleration period, the ECD rate increased rapidly and continuously conducting films with large grains were formed. Leaner sweep voltammetry (LSV) and mixed potential theory (MPT) were applied to examine the catalytic powers of the prepared Pd nanoparticles and the related electrochemical kinetics in the ECD bath.     

AGNES at vibrated gold microwire electrode for the direct quantification of free copper concentrations

The free metal ion concentration and the dynamic features of the metal species are recognized as key to predict metal bioavailability and toxicity to aquatic organisms. Quantification of the former is, however, still challenging. In this paper, it is shown for the first time that the concentration of free copper (Cu²⁺) can be quantified by applying AGNES (Absence of Gradients and Nernstian equilibrium stripping) at a solid gold electrode. It was found that: i) the amount of deposited Cu follows a Nernstian relationship with the applied deposition potential, and ii) the stripping signal is linearly related with the free metal ion concentration. The performance of AGNES at the vibrating gold microwire electrode (VGME) was assessed for two labile systems: Cu-malonic acid and Cu-iminodiacetic acid at ionic strength 0.01 M and a range of pH values from 4.0 to 6.0. The free Cu concentrations and conditional stability constants obtained by AGNES were in good agreement with stripping scanned voltammetry and thermodynamic theoretical predictions obtained by Visual MinteQ. This work highlights the suitability of gold electrodes for the quantification of free metal ion concentrations by AGNES. It also strongly suggests that other solid electrodes may be well appropriate for such task. This new application of AGNES is a first step towards a range of applications for a number of metals in speciation, toxicological and environmental studies for the direct determination of the key parameter that is the free metal ion concentration.     

Adsorption of pyridine on a polycrystalline gold electrode surface studied by infrared reflection absorption spectroscopy

The adsorption behavior of pyridine on a smooth polycrystalline gold electrode surface was investigated over a wide wavenumber region (2000–500 cm⁻¹) by in situ infrared reflection absorption spectroscopy (IRAS). The reversible adsorption/desorption of pyridine was observed upon the change in applied electrode potential, and the adsorption state at positive potentials was found to depend strongly on the kind of halide ion used as a supporting electrolyte. Symmetry analysis of absorption bands observed revealed that pyridine molecules adsorb with the molecular axis (C₂ axis) perpendicular to the electrode surface (vertical configuration) at positive potentials in 0.5 M KF, KCl and KBr solutions. A band due to the out-of-plane bending mode of the adsorbed pyridine molecule was observed at potentials more negative than ca. 0 V for 0.5 M KF solution containing 100 mM pyridine. We concluded that even in the 100 mM pyridine solution, adsorbed pyridine forms a monolayer and that the molecules reorient from a flat (parallel) to the vertical configuration as the potential becomes less negative. No bands due to adsorbed pyridine were detected for 0.5 M KI solution. The amount of adsorbed pyridine was found to depend strongly on the strength of specific adsorption of halide ions.     

Measurements of trace concentrations of mercury in sea water by stripping chronopotentiometry with gold disk electrode: influence of copper

A stripping chronopotentiometric method, using a rotating gold disk electrode for mercury measurements in sea water is described. Compared with a same method using a stationary gold film electrode, this method has a eight times higher sensitivity and a detection limit of 5 ng l⁻¹ after 10 min deposition time. Moreover, the time needed for gold plating is eliminated. Compared with other electrochemical methods capable of measuring mercury at low concentrations, the present method is more simplified with no degassing step and no need to use a medium-exchange procedure before the stripping step. These characteristics render the method easily practicable on board oceanographic vessels for ‘in situ’ measurements.     

Mechanism for nucleation and growth of electrochemical palladium deposition on an Au(111) electrode

The initial kinetics of electrochemical deposition of palladium on an Au(111) electrode in 0.1 M H₂SO₄ solution containing K₂PdCl₄ have been investigated quantitatively by cyclic voltammetry (CV) and potentiostatic current transient measurements. A cathodic current maximum was observed in the initial stage of Pd deposition process in 0.1 mM K₂PdCl₄ solution. The analysis shows that the deposition of palladium in this solution proceeds by an instantaneous nucleation and two-dimensional (2D) growth mode. When the concentration of K₂PdCl₄ became higher, a potential-dependent cathodic current ‘plateau’ after the current maximum was observed. Simple nucleation-growth models cannot fit the transients in the region of the ‘current plateau’. The present current transient results were discussed in comparison with the results obtained by in situ scanning tunneling microscopy (STM).     

The renovated silver ring electrode in determination of lead traces by differential pulse anodic stripping voltammetry

The study of a new type of working electrode - the renovated silver ring electrode (RSRE) - for lead ions detection via differential pulse anodic stripping voltammetry (DP ASV) without removal of oxygen is reported. The only four constituents of the RSRE: a specially constructed silver ring electrode, a silver sheet used as silver counter/quasi-reference electrode and a silicon O-ring, are fastened together in a polypropylene body. The renovation of this electrode is carried out through mechanical removal of solid contaminants and electrochemical activation in the electrolyte which fills the RSRE body. Excellent repeatability and reproducibility - also in organic samples solutions - were reached in a period of a few weeks through the renovation of the electrode surface before each measurement. The reduction and stripping of lead on silver electrode under the DP ASV conditions are underpotential deposition/dissolution phenomena. The RSRE is used for the determination of Pb ions in concentrations ranging from 1 × 10⁻⁹ to 1 × 10⁻⁷ M. The repeatability of DP ASV runs in synthetic solutions covering the entire concentration range is better than 2%. Obtained calibration curves are represented by a correlation coefficient of at least 0.999. The detection limit (LOD) for the time of electrodeposition equal to 60 s is 0.2 × 10⁻⁹ M. LOD for Pb²⁺ detection at the RSRE is similar to this reported for a rotating silver electrode in subtractive anodic stripping voltammetry (E. Kirowa-Eisner, et al., Anal. Chim. Acta, 385 (1999) 325). The analysis of Pb²⁺ in synthetic solutions with and without surfactants, certified reference materials and natural water samples have been performed.     

A strategy for improving the sensitivity of molecularly imprinted electrochemical sensors based on catalytic copper deposition

A novel method to improve the sensitivity of molecularly imprinted polymer sensors was developed. Oxytetracycline (OTC), which was selected as the template molecule, was first rebound to the imprinted cavities. Gold nanoparticles were then labeled with the amino groups of OTC molecules via electrostatic adsorption and non-covalent interactions. Copper ions were catalytically reduced by the gold nanoparticles, and copper was deposited onto the electrode. The deposited copper was electrochemically dissolved, and its oxidative currents were recorded by differential pulse voltammetry (DPV). OTC could be determined indirectly within the concentration range of 3.0 × 10⁻¹⁰ to 1.5 × 10⁻⁷ mol L⁻¹ with a detection limit of 6.8 × 10⁻¹¹ mol L⁻¹.     

Influence of the adsorption on the oxidation of oxalic acid on a gold electrode in acid media

Summary The influence of an adsorption process on the oxidation of oxalic acid on a gold electrode has been analysed. To disclose this influence, different supporting electrolytes have been used with different adsorption of the anion (SO ₄ ^2– , NO ₃ ^– and ClO ₄ ^– ). Moreover, it has been studied the modifications produced in the oxidation process by the presence of some species in solution with a strong adsorption (halides), but at low concentration levels.

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Der Einfluß der Adsorption auf die Oxidation von Oxalsäure an der Goldelektrode in saurem Medium

Zusammenfassung Um den Einfluß der Adsorption auf die Oxidation von Oxalsäure an der Goldelektrode in saurem Medium zu untersuchen, wurden Support-Elektrolyten mit verschiedener Anionenadsorption verwendet (SO ₄ ^2– , NO ₃ ^– und ClO ₄ ^– ). Außerdem wurden Modifikationen des Oxidationsprozesses in der Gegenwart von stark adsorbierten Spezies, diese allerdings in geringen Konzentrationen, untersucht (Halogenide).

     

Anion adsorption in the course of dichromate reduction on a smooth gold electrode in aqueous acid solutions

The specific adsorption of anions (HSO₄ ⁻, Cl⁻) present in low concentration (c < 10⁻³ mol dm⁻³) was studied by radiotracer techniques in the course of the reduction of dichromate (chromate) species in 1 mol dm⁻³ HClO₄ supporting electrolyte. In accordance with the results of preliminary studies reported earlier, enhancement of the anion adsorption was found, induced by some adsorbed intermediates of the reduction process. Potential dependence of the induced adsorption and its correlation with the reduction rate was investigated. The role of adsorption competition between various anions is discussed. It is concluded that study of the induced anion adsorption could be a tool for the investigation of the sorption of intermediates formed in the course of the reduction. Received: 3 May 1999 / Accepted: 10 June 1999