Antoinette Hamelin

Cyclic voltammetry at gold single-crystal surfaces. Part 2. Behaviour of high-index faces

Typical cyclic voltammograms at the three low-index faces of gold are given and discussed comparatively. The influences of various experimental conditions or parameters, e.g. temperature, concentration and nature of the electrolyte, ohmic potential drop, potential limits, etc. are described and related to the atomic surface structures. These findings are recommended as a starting point for further studies on these faces and for obtaining a more satisfactory intercomparison of results from one laboratory to another.

Cyclic voltammetric characterization of oriented monocrystalline gold surfaces in aqueous alkaline solution

Abstract Monocrystalline gold surfaces having five different crystallographic orientations have been characterized by means of cyclic voltammetry in aqueous 0.11 M NaOH in order to examine the electrochemistry of oxide formation and removal in comparison with the well-characterized behavior in acidic media. For the higher-index faces Au(210) and (311), these voltammetric features in 0.11 M NaOH are closely similar to those observed in acidic electrolytes, although the occurrence of substantial OH− adsorption is signaled by a broad reversible current-potential feature at less positive potentials. For the low-index faces Au(100) and (111), however, the voltammograms in 0.11 M NaOH differ substantially from those in acid. These differences are ascribed to the influence of potential-dependent OH− adsorption, which overlaps with oxide formation and especially reduction, and attendant potential-induced surface reconstruction. The behavior of Au(110) is intermediate between that of these other two low-index and the higher-index faces. As for the voltammetric features in acidic electrolytes, such crystal face-dependent voltammetry in 0.11 M NaOH can be employed as a sensitive (albeit empirical) check of the chemical and physical state of the interface prior to other electrochemical measurements.

The temperature dependence of the double layer parameters of the (111) face of silver

Abstract For the (111) face of silver in KPF 6 solutions, the temperature coefficient of the potential of zero charge was found to be positive; the capacitance at zero charge was found to decrease with increasing temperature. Preliminary results for the (111) face in a NaF solution are also given. Differences have been observed with respect to the behaviour of the (110) face investigated previously. The use of the test of the bisecting line of the Parsons-Zobel plot can explain the difficulties existing for the determination of the inner layer capacity at zero charge; however, the sign of its variations with temperature can be derived from a modification of the same plot. The entropies of formation of the interface and of the inner part of the double layer as a function of the charge density are reported.

The temperature dependence of the double-layer properties of gold faces in perchloric acid solutions

The temperature dependence of the differential capacity curves for the (210) gold face in perchloric acid solutions of concentrations ranging from 10 mM to 2 M has been examined. The temperature coefficients of the Galvani potential drops are almost the same as that of mercury in sodium fluoride. The variations of the inner layer capacity, as a function of the charge density, are discussed tentatively. The entropies of formation of the interface and of the inner part of the double layer, as a function of the charge density, are reported.

Oxide formation on gold single crystal stepped surfaces

Abstract Oxide formation and anion adsorption have been studied on gold single crystal stepped surfaces. Most of the measurements were performed in sulphuric and some in perchloric acid solutions by cyclic voltammetry and by double layer capacity measurements. A pronounced structural sensitivity of both oxide formation and anion adsorption was found. The effects of the terrace orientation, step orientation and step density are reflected in voltammetry and the double layer capacity measurements. A systematic dependence of both processes on step density was found and their interrelation is discussed. The results contradict some gas-phase data reporting no critical role of steps in oxygen chemisorption on gold. A correlation between the relative surface energy of gold single crystals and the peak potentials of the initial stage of oxide formation was found.

A double layer study of the (210) and (111) faces of gold in aqueous NaBF4 SOLUTIONS

Abstract Au(111) and Au(21O) faces of gold in aqueous NaBF4 solutions have been investigated using cyclic voltammetry and admittance measurements. On Au(21O) no specific adsorption of B F 4 − has been found. The results were first analysed accepting the Gouy-Chapman-Stern model for the double layer; like Ag(11O) and Ag(100) the inner layer capacity versus charge density curve has a broad maximum. For Au(111) it was not possible to draw clear conclusions and the variation of the inner layer capacity versus charge density seems to correspond to the accepted model only for the more negative densities of charge investigated. Models for the inner layer were discussed for the (210) face.

Initial surface structure of flame-annealed Au(100) in aqueous solution: Consistency between in-situ scanning tunneling microscopy and electrochemical results

Abstract : The aim of this note is to clarify the initial superficial structure of gold electrodes following the commonly utilized flame-annealing pretreatment procedure, by using atomic - resolution scanning tunneling microscopy (STM), and to show the consistency of electrochemical and STM results. Initially in 1974 it was reported that the Au(100) electrode on aqueous electrolyte exhibited some hysteresis in the differential capacity-electrode potential curves and the cyclic voltammograms within the double-layer region. This hysteresis depends on the negative potential limit and the potential-time history. In electrolyte exhibiting only slight ionic adsorption at the potential of zero change such as perchloric acid, the potential of the capacity minimum arising from the diffuse layer was shown to be shifted in the positive direction following an excursion to more negative potentials.

Dependence of the kinetics of proton reduction at gold electrodes on the surface cyrstallographic orientation

Rate constant-potential data are reported for aqueous proton electroreduction at seven single-crystal gold faces with the objective of ascertaining the origin, as well as extent, of the dependence of the proton discharge kinetics upon the surface crystallographic structure. In addition to the low-index faces (111), (100), and (110), the high-index planes (311), (332), (210), and (531) were examined to investigate the effect of introducing various surface atomic steps and kinks. Lithium and sodium perchlorate electrolytes of 0.1 and 1 M ionic strength containing 0.01 M HClO4 were employed in addition to 0.1 M HClO4 in order to examine the possible influence of the supporting electrolyte cation. In each case the observed rate constants, kob, at a given electrode potential, increase in the sequence (111) < (100) < (332) < (110) < (311) < (210) < (531). These rate variations span factors of ca. 3.5-fold in the NaClO 4 - and ca. 2.5-fold in the LiClO 4 - and HClO 4 -based electrolytes. An identically prepared polycrystalline gold sample yielded kinetic behavior very similar to the (332) plane. This reactivity sequence largely mirrors the corresponding variations in the potential of zero charge, Epzc, for these faces, which lie in the sequence (111) ⪢ (100) ⪢ (311) ∼ (332) ⪢ (110) ⪢ (531) ⪢ (210). Plots of the overpotential at constant kob versus Epzc are roughly linear with slopes of around −0.16 to −0.18. This dependence of the proton-discharge kinetics upon Epzc for different single-crystal faces of gold in qualitatively different from, and markedly smaller than, the corresponding dependence upon Epzc for different polycrystalline metals. The former is most likely due to an increase in the stability of the adsorbed hydrogen intermediate as the surface atomic step density is increased, associated with stronger hydrogen adsorption at gold atoms of lower surface coordination number.

The temperature coefficient of the potential of zero charge of the gold single-crystal electrode/aqueous solution interface: Possible relevance to gold-water interactions

Abstract The minimum in the differential capacity of (111), (100), (110), (210) and (311) single-crystal gold faces in contact with dilute solutions of HClO 4 as a function of the temperature was used to estimate the temperature coefficient of the potential of zero charge of these interfaces. The results are discussed in terms of the interaction of water with the metallic surfaces and the role played by the surface atomic arrangement. The strength of Au ( hkl )-water interactions are proposed to be in the order Au (111) > Au (311) > Au (100) > Au (110) > Au (210) ≈ Hg.

The surface state and the potential of zero charge of gold (100): a further assessment

Abstract Electrochemical results obtained in neutral solutions, sulphuric acid and perchloric acid for the gold (100) face are described with the primary objective of providing an assessment of apparently differing interpretations published by other workers. Values of the potential of zero charge in these different media are given for mostly unreconstructed surfaces.