Single Crystal Gold Electrodes Research Articles

Electrochemical and spectroscopic studies of the mechanism of monolayer and multilayer adsorption of an insoluble surfactant at the Au(111) | electrolyte interface

The adsorption of an insoluble monolayer of 12-(9-anthroyloxy) stearic acid (12-AS) onto a single crystal gold electrode has been described. The spreading of the insoluble surfactant onto the metal | solution interface of the gold electrode was initially investigated with the help of cyclic voltammetry and differential capacity. Electrochemical studies indicate that the film of 12-AS molecules spreads onto the metal surface at potentials close to the potential of zero charge (pzc) and desorbs from the electrode surface at potentials that are sufficiently negative. The spreading of the film and its subsequent desorption are repeatable. The 12-AS molecule is a surfactant dye which may be used to perform spectroelectrochemical experiments. Electroreflectance spectroscopy, fluorescence spectroscopy and measurements of the light scattered by the desorbed surfactant molecules were employed to determine the mechanism by which molecules of the insoluble surfactant spread onto and desorb from the metal | solution interface of the gold electrode. With the help of the spectroelectrochemical experiments we have demonstrated that the repeatable potential-induced desorption and adsorption (spreading) of the insoluble molecules involves formation of micelles (or similar molecular organizations) in the subsurface region and spreading of the micelles onto the electrode surface.

Electrochemical Oxidation of Glucose on Single Crystal and Polycrystalline Gold Surfaces in Phosphate Buffer

The electrochemical oxidation of glucose has been studied in phosphate buffer (pH = 7.4) on single crystal and polycrystalline gold electrodes using electrochemical techniques, ex situ NMR, in situ FTIR spectroscopy, and isotope labeling. Under these conditions, the results indicate that the rate determining step for the electro‐oxidation of glucose is bond breaking between the hydrogen atom and the carbon atom. Gluconolactone appears to be an intermediate and sodium gluconate is the reaction product. First order kinetics with respect to glucose were also found. Gold would be a useful electrode for sensor applications, if the inhibition of the glucose oxidation by the adsorption of could be avoided.

The influence of OH − chemisorption on the catalytic properties of gold single crystal surfaces for oxygen reduction in alkaline solutions

Oxygen reduction on gold single crystal electrodes of various orientations, Au( hkl), in 0.1 M NaOH was studied by rotating disk electrode and rotating disk ring electrode techniques, with a goal to correlate the physical and chemical properties of the electrode surface with its catalytic activity. The characterization of the state of the electrode surface and the interface was carried out by cyclic voltammetry and differential capacity measurements. The geometrical arrangement and symmetry of surface atoms, and the presence of chemisorbed OH − anions, were found to play a key role in determining the catalytic properties of Au( hkl) surfaces for O 2 reduction. It was proposed that chemisorbed anions act as a precursor for the dissociative adsorption of molecular oxygen or for chemisorption of reaction intermediate, catalyzing the 4e −-reduction of O 2.

The role of the metal in determining electrosorption valency in the case of strong molecular adsorption at the electrode|solution interface

Abstract : The electrosorption valency for the adsorption of parabonic acid has been determined on single crystal gold electrodes and mercury near the point of zero charge. A new technique to perform the measurement using cyclic voltammetry is described. The electrosorption valency was found to be a linear function both of the potential of zero charge and of the work function of the metal. (AN)

Impedance aspects of anion adsorption on gold single crystal electrodes

Impedance aspects of anion adsorption on gold single crystal electrodes

Impedance aspects of anion adsorption on gold single crystal electrodes

In order to explore the origins of the capacitance dispersion observed for single crystal electrodes, voltammetric and impedance measurements on Au(l 11) and Au(100), in the absence and the presence of specifically adsorbed anions, mostly Br −, have been carried out. Interfacial capacitances and rate coefficients characterizing the kinetics of adsorption have been evaluated from the impedance spectra. We present evidence that the double layer behaves as an ideal capacitance in the absence of specific adsorption. The approximation with a constant phase element yields an exponent larger than 0.99. The “capacitance dispersion” observed in the presence of specific adsorption is either assigned to a slow diffusion or adsorption process, or to transformations within the adlayer or the substrate surface.

Structure sensitivity of irreversibly adsorbed tin on gold single-crystal electrodes in acid media

Tin adlayers formed on well defined gold electrodes upon irreversible adsorption from SnII solutions have been characterized by means of cyclic voltammetry in sulfuric and perchloric acid solutions. The surface redox reactions undergone by the tin adatom have been found to be structure sensitive. From the analysis of the voltammetric charges involved in these processes it can be suggested that adsorbed tin is oxidized to oxygenated SnII species, which are stable at the electrode surface as long as the electrode potential is kept below 0.25 V. At higher potentials, soluble SnIV species are formed and the adlayer is removed from the electrode surface. The stability of the adlayer is strongly dependent on the solution pH. On Au(111), adsorbed tin is directly oxidized to SnIV species.

Electrochemical Desorption and Adsorption of Nonyl Mercaptan at Gold Single Crystal Electrode Surfaces

The electrochemical behavior of a nonanethiol layer adsorbed on Au(111), Au(110), and on a Au polycrystal has been examined using cyclic voltammetry. The reductive desorption and the oxidative redeposition of the nonanethiol molecules at the Au(111) surface have been shown to depend strongly on the pH of the electrolyte solutions. While the amount of material reductively desorbed from the surface on the first cathodic cycle is independent of the pH, the amount of material that is oxidatively redeposited increases significantly as the pH is lowered. This behavior is ascribed to a reduction in solubility of the product of desorption (thiolate or thiol) as pH decreases. At high pH the redeposition of the layer seems to occur in one step, but at pH values that are lower than the pKa of the nonanethiol the redeposition seems to occur in two steps. In an alkaline solution, the reductive desorption of the nonanethiols from a Au(110) surface is similar to the same process at the Au(111) surface. The double layer ...

Adsorption of insoluble surfactants at the Au(111)/solution interface

The adsorption of surfactants, which form insoluble monolayers on an aqueous substrate, onto a single crystal gold electrode have been described. Adsorption of this class of surfactants have been characterized using a combination of electrochemistry and Langmuir-Blodgett techniques. We have developed a technique to simultaneously measure the film pressure at the gas-solution (GS) interface and the film pressure of the surfactants that spread to the metal-solution (MS) interface. We have shown that surfactants such as octadecanol and stearic acid, which interact weakly with the metal surface, adsorb at an uncharged MS interface (at the potential of zero charge) and progressively desorb when the electrode surface is charged negatively. The electrode potential (charge density at the metal surface) influences the transfer of the surfactant from the GS interface to the MS interface. The transfer ratio is 1:1 at an uncharged MS interface, and is progressively reduced to zero when the MS interface is charged. We have employed 12-(9-anthroloxy) stearic acid, a surfactant dye molecule, to study the mechanism of potential induced desorption and adsorption of the film of insoluble molecules. With the help of electroreflectance spectroscopy and light scattering measurements, we have shown that if desorbed, the surfactant molecules form micelles (flakes or vesicles) that are trapped under the electrode surface. The micelles spontaneously spread back onto the electrode surface when the charge density at the metal approaches zero. The repeatable desorption and readsorption involve micellisation of the film at negative potentials and spontaneous spreading of the micelles to reform the monolayer at potentials close to pzc.

Structural effects on ethine adsorption at gold single-crystal electrodes

The adsorption of ethine at +0.60 V versus RHE in 0.1M HClO 4 was studied on different gold crystal faces ((111), (110), (100) and (210)) as well as on polycrystalline gold. For all studied surfaces 90% of the adsorbate formed was reduced in the first negative-going potential scan at about +0.25 V versus RHE, except for Au(100) where 50% of the adsorbed species remained on the surface after the first reduction cycle. With respect to oxidation of the adsorbate the crystal faces under investigation displayed distinctive differences. Whereas Au(210) and Au(poly) exhibited only a single voltammetric peak at comparable potentials (approximately 1.30 V), on the other orientations several peaks at potentials differing by about 0.30 V were observed. On Au(111) the lowest oxidation peak potential (1.13 V) as well as the highest onset potential for the electro-oxidation (0.97 V) were found.

A Capacitance and Infrared Study of the Electrical Double Layer Structure at Single Crystal Gold Electrodes in Acetonitrile

AbstractThe electrical double layer at Au(100) and Au(111) electrodes in acetonitrile containing tetraethylammoniumper‐chlorate (TEAP) was studied by means of differential capacitance and in‐situ FTIR spectroscopy measurements. For both surfaces, a preponderant capacitance maximum was observed at a positive charge of about 8 μC cm−2. Two types of acetonitrile molecules were identified in the interfacial infrared spectra. One of these concerns adsorbed acetonitrile with a strongly blue shifted vCN mode at 2337 cm−1. The potential dependence of the spectra provides a clear evidence for dielectric saturation of the solvent and specific adsorption of the ions at large electrode charges. The spectra also indicate that the maximum ability of the solvent to reorientate in the interfacial field is in an adsorption geometry with the molecular axis inclined with respect to the surface. The data are discussed in context of the current models of the double layer, and compared with the related literature data obtained in an aqueous electrolyte.

Adsorption and Film Formation of Thymine on a Gold (100) Single Crystal Electrode

AbstractThe film formation of thymine adsorbed at a Au (100) surface in aqueous solution has been analysed by means of cyclic voltammetrie and capacity‐potential measurements. On the freshly flame annealed and therefore reconstructed (100) surface the formation of a 2‐dimensional condensed adlayer of thymine has been observed. On the unreconstructed (100) surface exists no condensed adsorbate state.A certain amount of charge transfer from adsorbed thymine molecules is detected at potentials near to the potential of zero charge. Variation of the pH of the solution reveals a transfer of nominally 0.81 electrons per molecule. From this the surface concentration of chemisorbed thymine at positive potentials is estimated. The surface concentration correlates with an adsorption state in which the molecular plane of thymine is parallel oriented to the surface.

Electrosorption of unsaturated alcohols on a gold(100) electrode

The electrosorption of 2-propen-1-ol, 2-propin-1-ol, 2-butene-1, 4-diol, and 2-butyne-l,4-diol from an aqueous solution of NaClO4 at a gold(100) single crystal electrode has been studied by tensammetry. The results are discussed with respect to the surface properties of the single crystal electrode, the different structures of the adsorbed molecules, and possible contributions from the multiple bonds and functional groups therein.

Molecular adsorption at metal electrodes

This paper gives a brief review of our efforts to describe adsorption of organic molecules at gold and silver electrodes. We are using, as an example, pyridine to show the differences between adsorption of molecules at group IB metals and their adsorption at mercury ( sp metal) or platinum ( d metal) electrodes. We discuss the relationship between the character of adsorption and the electronic structure of the metal. Later, we describe the effect of surface crystallography on pyridine adsorption at gold single crystal electrodes. Finally, we assess the effect of molecular adsorption on the reconstruction phenomena using second harmonic generation (SHG) and electroreflectance (ER) spectroscopies.

A study of the adsorption of aromatic sulphonate anions at the gold (210) single-crystal electrode

The adsorption behaviour of benzenesulphonate (BS) and p-toluenesulphonate (PTS) anions at an Au(210) single-crystal electrode was investigated employing cyclic voltammetry and capacity measurements at constant ionic strength. It was found that the charge σ i due to specifically adsorbed anions is significantly greater (in the region of saturation) for the BS anion than for the PTS anion. It was postulated that BS anions can assume two different orientations at the electrode surface. A more detailed study of the adsorption process based on the Frumkin isotherm was performed for PTS anions. The inner-layer capacity was analysed into its components. The analysis indicated that the molecular structure of a given anion determines its behaviour during the adsorption process.

Adsorption equilibria in two-dimensional systems

This paper describes first equilibrium studies of partitioning an insoluble surfactant (octadecanoic acid) between the gas—solution and solid—solution interfaces. A novel experimental approach is described in which a Wilhelmy plate is used to measure the film pressure of the insoluble organic monolayer spread at the gas—solution interface while simultaneously the film pressure of surfactants that spread from the gas—solution interface to the metal—solution interface of the electrode is monitored in situ by electrochemical means. Using as an example gold single-crystal electrodes, it is demonstrated that the equilibrium between the two-dimensional phases is strongly dependent not only on the electrode potential but also on the morphology of the solid electrode surface. The significance of these experiments for the choice of optimum conditions for transfer of insoluble monolayers from the gas solution onto the solid surface using the Langmuir—Blodgett technique is discussed.

GaAs deposition on the (100) and (110) planes of gold by electrochemical atomic layer epitaxy: A low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy study

Preliminary studies on the deposition of GaAs by electrochemical atomic layer epitaxy (ECALE) were performed. ECALE is based on the alternated underpotential deposition (UPD) of atomic layers of different elements to form a compound. Oxidative UPD of As and reductive UPD of Ga on the low-index planes of gold were studied using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and coulometry. AES and LEED were performed in an ultrahigh vacuum (UHV) surface analysis instrument interfaced to an electrochemical cell in an antechamber. This instrument configuration allowed the electrochemical treatment of the samples and their subsequent analysis in UHV without the need to transfer the samples through air. STM was performed under nitrogen at atmospheric pressure. AES and coulometry were used for surface composition analysis while LEED and STM provided structural information. The substrate was a gold single-crystal electrode with three oriented faces, each to a different low-index plane. Oxidative UPD of arsenic was observed only on the (100) and (110) faces. The resulting structures were a Au(100)(2×2)-As at 1/4 coverage and an Au(110)c(2×2)-As at 1/2 coverage. Reductive UPD of Ga was observed on all three faces, although it resulted in disordered layers of Ga oxide upon removal of the substrate from solution, due to partial oxidation of the Ga in contact with water in the absence of potential control. Stoichiometric coverages of Ga and As were obtained on the (100) and (110) surfaces when Ga was underpotentially deposited on the As covered surfaces [Au(100)(2×2)-As at 1/4 coverage and Au(110)c(2×2)-As at 1/2 coverage]. Structures displaying (2×2) and c(2×2) LEED patterns were observed on the (100) and (110) faces, respectively.

A study of p-toluenesulphonate anion adsorption at the gold (311) single crystal electrode

The adsorption of p-toluenesulphonate anions at the gold (311) single crystal electrode has been investigated by cyclic voltammetry and differential capacity measurements. The results have been fitted with the Frumkin isotherm. The adsorption parameters such as the surface coverage, the standard Gibbs energy of adsorption and the electrosorption valencies have been determined. The present data are compared with those obtained at polycrystalline gold.

The effects of adsorbed anions on the oxidation of D-glucose on gold single crystal electrodes

The effects of various anions on the oxidation of glucose on single crystal gold electrodes were studied in HClO 4, CF 3SO 3H, HNO 3, H 2SO 4, H 3PO 4 and HCl solutions. Contrary to common belief, glucose can be oxidized on gold in acid solutions, but only in the absence of strongly adsorbed anions such as chlorides, sulfates and phosphates. The reaction is partly or completely inhibited in solutions containing specifically adsorbed anions. The data show some indication of an (OH) ads layer on gold at potential cathodic to +1.2 V vs rhe in HClO 4 and CF 3SO 3H solutions. Based on the rates of oxidation of glucose, the following sequence has been found for the inhibition of the glucose oxidation by adsorbed anions: ClO − 4 ≈ CF 3SO − 3 ⪡ NO − 3 ⪡ HSO − 4 (SO 2− 4) < H 2PO − 4 (HPO 2− 4 < Cl −.

Adsorption of benzonitrile at the aqueous solution/gold (111) interface

The adsorption of benzonitrile (BN) at a gold (111) single crystal electrode has been investigated quantitatively using chronocoulometry. The procedure described in this paper differs slightly from previous work in that experiments were performed not only in neutral pH solutions but also at pH 2.4 in order to shift the potential corresponding to the oxidation of the metal surface toward more positive values and hence extend the double layer region effectively. The following adsorption parameters: film pressure, Gibbs surface excess, Gibbs energy of adsorption and electrosorption valency, were determined as functions of the electrode potential and the charge density for concentrations of BN up to 0.03 M (close to saturation). The values of the relative Gibbs excess indicate that adsorbed BN molecules are oriented parallel to the electrode surface at negative and moderately positive potentials and low bulk concentrations. The Gibbs energy at maximum adsorption is equal to −27 kJ mol −1 which suggests that BN is chemisorbed weakly on Au(111). The results presented here are compared with those obtained from other studies of BN adsorption on gold and mercury.