Single Crystal Gold Electrodes Research Articles

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.

Characteristic cyclic voltammograms of alkyl viologens at single crystal gold electrodes

Symmetric dialkyl viologens showed characteristic current response in their cyclic voltammograms (CVs) depending on the atomic arrangement of the gold electrode surfaces. Several sharp peaks responsible for the adsorption of the alkyl viologens in the monolayer were observed at Au(111), Au(100) and Au(110) electrodes in a potential region that is more positive than the main reduction waves. The current peaks appeared irregularly independent of the alkyl chain length in contrast to the case of mercury and HOPG electrodes. It was pointed out that the CV features of the alkyl viologens were analogous to those of the metal UPD processes, which may reflect similar adsorption processes. The recrystallization occurred more easily on the electrode surfaces in the order Au(110) < Au(111) < Au(100), which indicated that the states of the bulk accumulated viologen cation radical layer were influenced by the atomic arrangement of the gold electrode surfaces, i.e. the adsorption structure of the first adlayer on them.

Electrochemical nucleation and growth of rhodium on gold substrates

The mechanism of rhodium adlayer formation on the (100) face of a gold single crystal and on a polycrystalline gold electrode has been studied using cyclic voltammetry and chronoamperometry. The voltammogram shows the presence of two peaks associated with the deposition of rhodium from the Na 3RhCl 6 salt in 0.1 M HClO 4. The results obtained on the (100) face of a gold single crystal indicate that the two monolayers of rhodium can be deposited before the nucleation and growth of the three-dimensional (3D) rhodium clusters. The experimental results described in this paper show that the bulk rhodium electrodeposition on a gold electrode includes a nucleation process combined with a diffusion-controlled growth of the stable rhodium crystals. The nucleation kinetics is compared for the 3D electrodeposition on the 〈100〉 face of a single crystal gold electrode and on a polycrystalline gold electrode.

A voltammetric study of 6-mercaptopurine monolayers on polycrystalline gold electrodes

6-Mercaptopurine (6MP) monolayers were formed spontaneously on different gold surfaces by chemisorption from 6MP solutions. The present cyclic voltammetric study reveals that the monolayer formed is stable in a wide potential range up to values where the oxidative desorption occurs. The reductive desorption of 6MP at a polyfaceted gold electrode shows a multi-wave response. This has been analysed by comparing the observed peak potentials with those obtained at low-index single crystal gold electrodes. The film has no apparent effect on the electron transfer rate constant of Ru(NH 3) 6 3+/2+ and Fe(CN) 6 3−/4− at the substrates studied, but modifies the electron transfer rate of the benzoquinone redox couple. This behaviour is typical of thin films where only the slower reactions should display a larger apparent decrease of the rate constant. In contrast, the 6MP-monolayer shows a strong ability to inhibit electroreduction of dioxygen at polyfaceted gold while it remains stable.

The role of charge distribution in the reactant and product in double layer effects for simple heterogeneous redox reactions

The role of charge distribution in transition metal complexes is considered for simple heterogeneous electron transfer reactions. On the basis of quantum chemical calculations for the couples [Co(NH3)6]3+/2+, [Fe(H2O)6]3+/2+, and Fe(CN)63−/4−, and a simple model for the double layer, it is shown that the double layer effect for these reactions depends more on the charge on the ligands than on that on the central metal ion. The results of calculations based on the double layer model are discussed with respect to experimental data obtained at mercury and single crystal gold electrodes.

Adsorption of 4-cyanopyridine on Au(111): ab initio calculations and SFG measurements

The adsorption of 4-cyanopyridine (4-CP) on a gold single crystal electrode in sodium perchlorate supporting electrolyte has been investigated by voltammetry, sum frequency generation (SFG) and ab initio calculations. Combination of in situ vibrational SFG spectra and DFT, MP2 and MP2 + SQM calculations allows us to obtain fundamental information on the behavior of adsorbed 4-CP as a function of the electrode potential. The configuration of the adsorbed 4-CP is dependent on the electrode potential. 4-CP is adsorbed perpendicular to the electrode surface through the pyridine nitrogen at negative potential, and through the nitrile nitrogen at positive potential. In the intermediate potential range, the molecule is adsorbed parallel to the electrode surface, through the π electrons of the aromatic ring.

Studies of Double-Layer Effects at Single Crystal Gold Electrodes II. The Reduction Kinetics of Hexaaquairon(III) Ion in Aqueous Solutions

The reduction kinetics of hexaaquairon(III) was studied at different concentrations of perchloric acid at four single-crystal gold electrodes. The electron-transfer rate constants obtained at constant electrode potential increase in the following order: Au(210) < Au(110) < Au(100) < Au(111). The experimental transfer coefficients increase in the same order. An analysis of double-layer effects within the framework of the classical Frumkin approach was consistent with the transport of a reactant of charge 2+ through the double-layer region and gave a value of the apparent transfer coefficient αa equal to 0.3 for all single-crystal electrodes studied. The αa value was rationalized by the distribution of charges within the reactant and product. The differences between double-layer effects on the reduction kinetics of hexaaquairon(III) and hexaamminecobalt(III) ions are discussed.

Surface structural effects on specific adsorption of oxoanions on gold single crystal electrodes

Abstract Cyclic voltammetry and capacitance measurements were performed using gold single crystal electrodes with 13 different crystallographic orientations. The experiments were performed in solutions of four different oxoanions: perchlorate, nitrate, sulphate and phosphate. Evidence of surface structure sensitivity was deduced for both the anion adsorption and surface oxidation phenomena. The current density and differential capacity profiles exhibit features that seem characteristic of the orientation of the terraces but are also sensitive to the density and orientation of the steps. For all of the gold faces under study, the anion adsorbability increases in the following order: ClO 4 − 3 − 4 2 2 PO 4 − . A parallel between this order of adsorbability and the degree of stereochemical ‘matching’ between the anions and the surface atomic arrangement is suggested

Adsorption of pyridine on gold, studied by difference frequency generation (DFG) using the CLIO-FEL

The geometry of the adsorption of pyridine on gold single-crystal electrode has been investigated by in situ difference frequency generation (DFG) in aromatic ring spectral range, by using the CLIO free electron laser. We found that pyridine adsorbs in an end-on configuration, via the nitrogen lone pair, the molecular plane being tilted. The potential dependence of the vibrational modes showed evidences of a reorientation of the molecule around the potential of zero charge.

The preparation of platinum and gold single crystal surfaces and voltammetric behaviours of these electrodes

The surface crystal structures of polycrystalline electrodes that are not stable and the difficulties in obtaining reproducible results lead to several difficulties in working with such electrodes. For this reason especially in the recent years intensive search is being carried out. For this goal noble metal platinum is especially selected because of its high electrocatalytic effect. But obtaining single crystal surfaces and preparing electrode from these surfaces turn out to be difficult. During the thermal procedure, surface changes and due to the pretreatment that are adapted different current-potential curves are obtained. In this review by giving the preparation methods of single crystal platinum and gold electrodes voltammetric studies have been explained with these electrodes.

SNIFTIRS studies of the electrical double layer at gold electrodes

Studies of double layer structure at gold electrodes using subtractively normalized interfacial FTIR spectroscopy are presented. It is shown that, when a hemispherical window is used to introduce the infrared radiation into the electrochemical cell, a significant increase in sensitivity results. Data are presented illustrating the reorientation of solvent dipoles in the double layer. In addition, results obtained for the accumulation of perchlorate and nitrate ions at single crystal gold electrodes are discussed. In the former case, analysis of the integrated intensity of the perchlorate band at 1109 cm −1 is used to show the potential region over which the surface excess follows the Gouy–Chapman theory and the potential region in which perchlorate ion is specifically adsorbed.

Quantitative studies of benzonitrile adsorption at the low-index gold single crystal electrodes

The adsorption of benzonitrile (BN) on the Au(111), Au(100), and Au(110) electrodes in aqueous solutions was investigated. The chronocoulometric technique was used to measure the charge density at the metal surface as a function of the electrode potential for BN concentrations ranging from 0 to 0.030 M. The adsorption parameters such as the film pressure, the surface concentration of the organic adsorbate, the standard Gibbs energy of adsorption (Δ G ads 0), and the charge flowing to the interface per one adsorbed molecule, were determined from the thermodynamic analysis of the charge density data. The analysis was carried out at constant electrode potential and at constant charge density. In addition, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was employed to determine the orientation of BN molecules at the Au(111) surfaces. The thermodynamic and spectroscopic data indicate that BN molecules assume a flat (π-bonded) state at the negatively charged surface and progressively reorient from the flat to the tilted (N-bonded) state as the charge at the metal surface becomes more positive. At very positive potentials the character of BN adsorption becomes reactive and the adsorbed molecules partially hydrolyze to form benzamide (BA).

On the dissolution kinetics of two-dimensional uridine layers adsorbed on gold single-crystal electrodes

The kinetics of dissolution of a two-dimensional chemisorbed layer made of uridine molecules adsorbed onto gold single-crystal electrodes have been studied by the potential step technique. Current transients have been recorded after single potential steps aimed at triggering the dissolution of the uridine chemisorbed monolayer formed on Au(111), Au(13,13,12) and Au(554). Irrespective of the electrode surface all the transients have satisfactorily been described by a model that combines a desorption step with a simultaneous hole nucleation and growth process. On the basis of a model considering an exponential law of nucleation and a surface diffusion-controlled growth, it is found that an increase of the step density of the electrode surface by affecting the ordering of the organic layer enhances markedly the rate of formation of holes in the film but slows down the expansion of the holes. The influence of the temperature and of the surfactant concentration on the nucleation and growth parameters is also reported.

In situ vibrational spectroscopy analysis of adsorbed phosphate species on gold single crystal electrodes

The adsorption of phosphate species on Au(100) electrode at different pHs is studied by using in situ Fourier Transform Infrared Spectroscopy. The results are compared with those published previously for Au(111) and also for polycrystalline gold. H 2PO 4 − ions present two spectral features at 1000 and 1120 cm −1, consistent with a C 2v symmetry, as expected. At pH 12.6 for the adsorption of PO 4 3− on Au(100) an abrupt change in the band intensity is observed which is consistent with the lifting of the surface reconstruction. This change in the intensity coincides with a current peak observed in the cyclic voltammogram and has been related to the surface lifting of the reconstruction. The PO 4 3− and H 2PO 4 − ions when adsorbed on Au(100) show the same characteristic bands compared to Au(111). The bands of the adsorbed species are located at ca. 1120 cm − and 1050 and 1085 cm −1 for HPO 4 2− and PO 4 3−, respectively. At Au(100) surfaces the three-fold coordination for both PO 4 3− and HPO 4 2− is very unlikely due to the mismatch between the oxygen-oxygen distances and the atomic distances at the (100) surface. For PO 4 3−, where the bands for the solution and adsorbate phosphate species do not overlap, it is possible to identify a consistent difference in the band half-width, when compared with PO 4 3− adsorbed on Au(111). This difference in band width is tentatively assigned to a different surface morphology of Au(100) after the lifting of the reconstruction.

Effect of the crystallographic orientation of gold single-crystal electrodes on the occurrence of 2D phase transitions in adsorbed organic monolayers

The influence of the surface structure of a gold electrode on the occurrence of 2D phase transitions in adlayers of uracil and uridine has been studied by cyclic voltammetry. Au(111) surfaces with different step densities have been prepared by cutting the single crystals between 2 and 15.8° of their (111) axis. Results confirm that flat surfaces are a prerequisite for the formation of a 2D condensed physisorbed film.

A Study of the Underpotential Deposition of Lead on Gold by UV-Visible Differential Reflectance Spectroscopy

The underpotential deposition of lead on gold has been studied by cyclic voltammetry and UV-visible differential reflectance spectroscopy. Comparative experiments were carried out with a polycrystalline gold electrode and two single-crystal gold electrodes with the orientations (111) and (210). For the single crystal surfaces, the reflectance results confirm the large effect of the crystallographic orientation on the deposition and redissolution of lead on gold surfaces. The existence of different types of structures for adsorbed lead, depending not only on the atomic distribution of the substrate but above all on the electrode potential, is demonstrated. An attempt is made to explain the behavior of polycrystalline gold on the basis of the results obtained for the two investigated gold single crystal faces.

Electrochemical immittance spectroscopy applied to the study of the single crystal gold/aqueous perchloric acid interface

The technique of electrochemical immittance spectroscopy has been used for characterisation of the interface at gold single crystal electrodes in aqueous solutions of perchloric acid in the absence of Faradaic processes. The results obtained for low index crystallographic orientations, namely, (100), (110) and (111) are compared to those obtained earlier for the (210) orientation. The experimental results are analysed by fitting them to a simple equivalent circuit consisting of a resistor in series with a constant phase element representing respectively, the solution resistance, Rs, and the distributed impedance of the double layer, ZCPE. The dependence of the parameters constituting the constant phase element, namely the pre-exponential factor, Y0, and the fractional exponent, nf, on the orientation of the electrode surface, on the electrode potential and on the solution concentration are discussed. The relationship of these parameters to the surface energy of the single crystals is discussed.

Capacitance dispersion on solid electrodes: anion adsorption studies on gold single crystal electrodes

In order to explore the origin of the capacitance dispersion observed on solid electrodes, voltammetric and impedance measurements on single crystal gold electrodes have been carried out in the absence and presence of specifically adsorbed anions, mostly Br −. Evidence is presented showing that (i) in the absence of specific adsorption, the double layer on these electrodes behaves as an ideal capacitance (were the impedance approximated by a constant phase element, the exponent would be larger than 0.99); (ii) in the presence of specific adsorption, capacitance dispersion can be observed due to the sluggishness and/or diffusion control of the adsorption step, to the surface reconstruction/deconstruction and to adlayer structural transformations.

The role of the metal and the solvent in simple heterogeneous electron transfer reactions

Solvent effects in simple electron transfer reactions are reviewed. It is shown that estimation of the outer sphere Gibbs activation energy is often difficult. Data for the electroreduction of Fe(H 2O) 6 3+ at single crystal gold electrodes are also discussed. This system shows a very large double layer effect.

SNIFTIRS studies of the double layer at the metal/solution interface. Part 1. Single crystal gold electrodes in aqueous perchloric acid

In situ infrared reflection spectroscopy has been used to probe the concentration change of the perchlorate anion in the electrical double layer at low-index single crystal gold electrodes. Cyclic voltammetry was used to estimate the amount of anionic charge in the diffuse layer, assuming the absence of specific adsorption near the potential of zero charge (pzc). A correlation between the integrated intensity of the perchlorate band at 1109 cm−1 and the surface excess of the anion in the diffuse layer estimated by the Gouy-Chapman theory was found for σm < ¦10 μCcm−2¦. At higher positive charge densities on the metal, systematic deviations were observed. A strategy to determine the pzc at the higher concentration of perchloric acid (1 M), which is not available from electrochemical techniques, is presented. This involves the previous determination of the effective extinction coefficient of the perchlorate anion in the double layer. This parameter was determined from the analysis of integrated IR bands at a lower concentration (0.1 M), for which the cyclic voltammogram provides the value of the pzc. Based on these results it is shown that SNIFTIRS data can be used to confirm the location of the pzc in both dilute and concentrated solutions under circumstances where more traditional techniques cannot be used.