Surface Structure Of Au Research Articles

Tracking the surface structure and the influence of cations and anions on the double-layer region of a Au(111) electrode.

We examined the electric double-layer (EDL) of a Au(111) electrode in a dilute perchloric acid solution using a combination of capacitance measurements and in situ scanning tunnelling microscopy under electrochemical conditions (ECSTM). The "camel-shaped" capacitance curve of the EDL is studied with different cations and anions, including their impact on the potential of zero charge (PZC). We show that the ECSTM images of thermally reconstructed and of the potential-induced surface reconstruction of Au(111) in perchloric acid electrolyte resemble previous work in sulphuric acid, displaying a herringbone pattern for a thermally reconstructed surface. Once the reconstruction is lifted, the Au(111) forms islands with an average of 1 atomic step height. When the potential is lowered below that of the PZC, the potential-induced surface reconstruction results in a more disoriented pattern than the thermally reconstructed surface. ECSTM images at different potentials are correlated with the voltammogram to understand the time and potential dependence of the surface. This correlation has led to the development of a potential window technique that can be used to reveal the surface structure of Au(111) based on observing the changes in PZC in the voltammogram. This method provides an indirect approach to understanding the surface structure without always relying on ECSTM. From the voltammogram, we also observed that anions (SO42-, CH3SO3-, ClO4-, F-) interact more strongly with the Au(111) surface than the alkali cations. The cation capacitance peak shape does not depend strongly on the identity of the alkali metal cation (Li+, Na+, K+). However, the anion capacitance peak depends strongly on the anion identity. It suggests that some level of specific adsorption cannot be excluded, even for anions that are traditionally not considered to adsorb specifically (perchlorate, fluoride).

Adsorption of 2-Mercapto-1-methylimidazole on the (√3 × 22) Reconstructed and (1 × 1) Phases of an Ordered Au(111) Electrode under Potential Control

The self-assembled monolayers (SAMs) of organosulfur compounds bearing an imidazole (Im) functional group can form hydrogen bonds with others and tailor specific interfacial structures used for CO2 reduction, biosensing, and corrosion prevention. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to address the adsorption and structure of 2-mercapto-1-methylimidazole (MMI) on the reconstructed and unreconstructed Au(111) electrodes as a function of potential. The adsorbed MMI molecule assumed the unprotonated form, allowing its S- and N-ends to bind with the Au electrode at a positive potential. Molecular resolution STM images showed an ordered Au(111)–(√21 × √21)R10.9°–MMI structure. The Au–N bond could weaken and be broken at a negative potential, leading to a disordered MMI adlayer. This restructuring event was coupled with the protonation of the Im group. These processes resulted in a sharp peak, which shifted negatively by 60 mV with an increase of 1 pH unit. MMI admolecules were desorbed at a more negative potential to unveil a pitted Au surface, resulting from etching of MMI adsorption. This feature supports the S–Au–S motif for MMI adsorbed to the Au(111) electrode, irrespective of the initial Au surface structure. The effect of MMI as an additive to the deposition bath of Ni was also explored.

Cathodic corrosion of Au in aqueous methanolic alkali metal hydroxide electrolytes: Notable role of water

AbstractCathodic corrosion is an electrochemical process that induces restructuring, roughening, and etching of metal surfaces at a highly negative surface charge density, yet, details of the reaction mechanism are not fully resolved. An in‐depth fundamental understanding of the processes and parameters underlying cathodic corrosion is crucial for tailoring the surface structure of the metal electrodes and for synthesizing shape‐ and size‐controlled nanoparticles. Here, we investigate the relevance of water and hydrogen evolution in the cathodic corrosion process. To achieve this aim, Au electrodes were polarized at ‐1.6 V versus RHE in KOH and NaOH electrolytes prepared using different water + methanol mixtures. Structural changes of the Au surfaces were studied by cyclic voltammetry and monitored by scanning electron microscopy (SEM). Most importantly, cathodic corrosion does not take place in the absence of water. There is no detectable bubble formation due to the hydrogen evolution reaction on Au in purely methanolic alkali. Furthermore, the electrochemically active surface area, facet distribution, and surface morphology of Au electrodes are significantly altered upon cathodic polarization as a function of the water concentration. Cathodic corrosion features become more and more pronounced with a further increase in water content. In addition, substantial differences in the surface structure of Au are observed as a function of the nature and concentration of alkali metal cations. Overall, this study provides a more detailed understanding of the role of water and the hydrogen evolution reaction in dominating cathodic corrosion, which might advance the understanding of this phenomenon.

Enriching Structural Diversity of Alkynyl-Protected Gold Nanoclusters with Chlorides.

The synthesis and isolation of alkynyl/chloride-protected gold nanoclusters is described. Silica gel column chromatography is effective in isolating gold nanoclusters from the as-synthesized cluster mixture to give the clusters Na[Au25 L18 ] (Au25 ), [HNEt3 ]3 [Au67 L32 Cl4 ] (Au67 ), [HNEt3 ]4 [Au106 L40 Cl12 ] (Au106 ), L=3,5-bis(trifluoromethyl)-phenylacetylide. Au67 and Au106 are new clusters; the structures were determined by X-ray single-crystal diffraction. Au67 contains a distorted Au18 Marks decahedron shelled by an irregular Au32 and further protected with two V-shaped Au2 L3 , 13 linear AuL2 staples and 4 chlorides. Au67 is the first structurally determined 34e superatomic gold nanocluster. Au106 is composed of 106 Au atoms co-protected by alkynyls and chlorides. It has a Au79 kernel, like in Au102 (p-MBA)44 . The surface structure of Au106 includes 20 linear Au-alkynyl staples, 5 Cl-Au-Cl and 2 Cl-Au motifs. These three gold nanoclusters show size-dependent electrochemical properties.

Effect of surface structure of Au(1 0 0) electrode on phase transitions within adsorbed adlayer of coumarin in halide electrolytes in the presence of tetrabutylammonium cations

Effect of surface structure of Au(1 0 0) electrode on the phase transitions within adsorbed adlayer of coumarin (CUM) in halide electrolytes in the presence of tetrabutylammonium cations (TBAc) was studied by means of cyclic voltammetry and differential capacity measurements. At the nearly unreconstructed surface of Au(1 0 0) electrode in chloride and bromide solutions, TBAc induced this transition, while at the reconstructed surface these cations caused a shift of it to more negative potentials. In KI as the supporting electrolyte, such phase transition was not detected at the reconstructed surface, but it was induced at the unreconstructed one. Additionally, new phase transitions in CUM adlayer have been found at the unreconstructed surface in bromide electrolyte, which are, however, completely inhibited by TBAc.

RoDSE Synthesized Fine Tailored Au Nanoparticles from Au(X)4– (X = Cl–, Br–, and OH–) on Unsupported Vulcan XC-72R for Ethanol Oxidation Reaction in Alkaline Media

An electrosynthesis method to obtain Au nanoparticles dispersed on carbon Vulcan XC-72R support material was done using AuX4– (X = Cl–, Br–, and OH–) as molecular precursors and different electrolyte media. The Au surface structure was significantly enhanced using KOH as an electrolyte as opposed to KBr and H2SO4. Cyclic voltammetry was used as a surface sensitive technique to illustrate the Au/Vulcan XC-72R catalytic activity for the ethanol oxidation reaction (EOR). The Au electroactive surface areas obtained were 1.88, 5.83, and 13.96 m2 g–1 for Au/C–H2SO4, Au/C–KBr, and Au/C–KOH, respectively. The latter compares to chemically reduced Au/C–spheres that had an electroactive surface area of 15.0 m2 g–1. The electrochemical Au electrodeposition, in alkaline media (Au/C–KOH), exhibited the highest catalytic activity for the EOR with a 50% increase in peak current density when compared with Au nanoparticles prepared by the chemical reduction route. Raman and X-ray photoelectron spectroscopies analyses of t...

Electrochemical detection of cytosine and 5-methylcytosine on Au(111) surfaces

In this communication we report a voltammetric study of the adsorption–desorption of cytosine (C) and methylcytosine (mC) on well-defined gold (Au) electrodes. The voltammetric measurements clearly indicate that these processes are extremely sensitive to the Au surface structure and in particular to the presence of (111) surface domains. Interestingly, on Au(111) surfaces, a linear correlation between the C and mC concentrations (logarithm scale) and the peak potential of the main voltammetric feature is found. In addition, in the simultaneous presence of both molecules, mC governs the electrochemical response, which has allowed its accurate quantification in C-mC mixtures. In situ FTIR spectroscopic measurements have been carried out to deepen on this mC electrochemical sensitivity. This research may contribute to the future development of an electrochemical sensor for the determination of the degree of methylation in DNA.

Theoretical study on oxidation reaction mechanism on Au catalyst in direct alkaline fuel cell

Abstract We theoretically analyzed the surface structure of Au in alkaline solution and the glucose oxidation reaction to understand the electrooxidation process in direct alkaline glucose fuel cell. We find the oxidation reaction mechanism of glucose on Au catalyst in alkaline solution theoretically as follows. i) Glucose adsorbs on OH of Au surface. ii) OH− in alkaline solution interacts with CHO group of glucose. iii) Water is formed by proton transfer from CHO group of glucose to OH− in solution. iv) Gluconic acid is formed by OH transfer from Au surface.

Surface structures of Au–Pt bimetallic nanoclusters on thin film Al2O3/NiAl(100) probed with CO

With infrared reflection absorption spectra and temperature-programmed desorption of CO as a probe molecule, we investigated the surface structure of Au–Pt bimetallic nanoclusters on thin-film Al2O3/NiAl(100) under ultrahigh vacuum conditions. Vapor Au and Pt (0.0–2.0ML) were sequentially deposited onto Al2O3/NiAl(100) at 300K to form bimetallic clusters; under the kinetic constraint, the order of the metal deposition made a variation in the surface structure of the grown clusters. For the deposition of Au onto Pt clusters (first Pt and then Au), the deposited Au decorated preferentially and then aggregated the edge sites of Pt clusters, instead of decorating the rest of Pt surface. For the metal deposition in the reverse order, the deposited Pt covered uniformly the surface of existing Au clusters and no preferential adsorption site was indicated. The infrared absorption for CO on Au sites was significantly enhanced when alloying occurred; this enhancement is primarily a local field effect depending on the coordination of CO-binding Au, and hence provides additional structural characterization of the grown clusters.

Potential-induced phase transition of low-index Au single crystal surfaces in propylene carbonate solution

In situ scanning tunneling microscopy (STM) was employed to examine the surface structures of Au(111), Au(100), and Au(110) single crystals in propylene carbonate (PC) containing tetrabutylammonium perchlorate (TBAP). All three electrodes exhibited potential-induced phase transition between the reconstructed and unreconstructed (1 × 1) structures at negative and positive potentials, respectively. The potential-induced phase transition of the Au electrode surfaces is attributed to the interaction of the TBA cation and the perchlorate anion at the electrode surface, which is similar to that which takes place in aqueous solutions. In addition to static atomic structures, dynamic processes of both the reconstruction and the lifting of the reconstruction were investigated by means of in situ STM. The lifting of reconstructed Au(111)-(√3 × 22) on Au(111) to the (1 × 1) structure is completed within 1 min at a positive potential. The diffusion of Au atoms on the Au(100) plane in the PC solution proceeds more rapidly than that in the aqueous solution, suggesting that the PC solvent plays an important role in accelerating the diffusion of Au atoms.

Direct observation of Au(111) electrode surface structure in bis(trifluoromethylsulfonyl)amide-based ionic liquids using surface X-ray scattering

The surface structure of Au(111) electrodes in hydrophobic ionic liquids, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]TFSA) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM]TFSA) was investigated using surface X-ray scattering. In both [BMP]TFSA and [BMIM]TFSA, the reconstruction of the surface structure was observed and the phase transition between the (1 × 1) and the reconstructed structure was found to be reversible. On the other hand, the area of the reconstructed structure in [BMP]TFSA was larger than that in [BMIM]TFSA. These differences suggest that the [BMIM] +–Au interaction is stronger than the [BMP] +–Au interaction.

An Analysis of the Si(111)-5*2 (s)-Au Surface Structure -The Ordered Structure of the Intermediate State of One and Two Dimensional Structures-

In order to exactly analyze a crystal structure it is necessary to consider from two viewpoints of long range order (LRO) and short range order (SRO). The information on the reciprocal lattice which is obtained by electron diffraction or X-ray diffraction concerns mainly on the long range order. On the other hand, the images observed by STM (tunnel scanning microscopy) concern mainly with the short range order. For the analysis of the STM images, we must construct the structure which does not contradict to the diffraction patterns. We observe sometimes the streak reflections in LEED (low energy electron diffraction) and RHEED (reflection high energy electron diffraction) patterns taken from the surface structures. When such streak patterns appear, we are apt to deal with it as the case that the crystallinity is not good. There are actually such cases, but some cases suggest a deep meaning on the structure. In this paper, from the standpoint of diffraction theory, we consider the streak reflection patterns which were observed often in 5×2 (s)-Au surface structure formed on the Si(111) surface when Au atoms are deposited on the Si(111) surface. Since by coexisting with the streak reflections and the sharp diffraction points of 5 times structure in the pattern of this surface structure, it is the case of that we cannot simply neglect it by saying that the crystallinity is not good. When this diffraction pattern is examined in detail, an important hint of the structure is obtained which shows an intermediate state of one dimensional and two dimensional lattices. A suggested structure is that for one dimensional lattice of Au atoms (Au chain), the position of the neighboring chain is decided with a connection probability α. The calculation method of diffracted intensity of the streaks in the 5×2 (s)-Au structure is a typical example of diffraction theory and worthwhile to consider in details and instructive also to understand many other diffraction phenomena.

In situ diffraction studies of electrode surface structure during gold electrodeposition

Surface X-ray scattering (SXS) in transmission geometry provides a valuable tool for in situ structural studies of electrochemical interfaces under reaction conditions, as illustrated here for homoepitaxial electrodeposition on Au(1 0 0) and Au(1 1 1) electrodes. Employing diffusion-limited deposition conditions to separate the effects of potential and deposition rate, a mutual interaction between the interface structure and the growth behavior is found. Time-dependent SXS measurements during Au(1 0 0) homoepitaxy show with decreasing potential transitions from step flow to layer-by-layer growth, then to multilayer growth, and finally back to layer-by-layer growth. This complex growth behavior can be explained within the framework of kinetic growth theory by the effect of potential, Cl adsorbates and the Au surface structure, specifically the presence of the surface reconstruction, on the Au surface mobility. Conversely, the electrodeposition process influences the structure of the reconstructed Au surface, as illustrated for Au(1 1 1), where a significant deposition-induced compression of the Au surface layer as compared to Au(1 1 1) surfaces under ultrahigh vacuum conditions or in Au-free electrolyte is found. This compression increases towards more negative potentials, which may be explained by a release of potential-induced surface stress.

Structure of Au(111) and Au(100) Single-Crystal Electrode Surfaces at Various Potentials in Sulfuric Acid Solution Determined by In Situ Surface X-ray Scattering

Potential-dependent surface structures of Au(111) and Au(100) single-crystal electrodes in a 50 mM H2SO4 solution were investigated at an atomic level using in situ surface X-ray scattering (SXS) techniques. It was confirmed that both the Au(111) and Au(100) surfaces were reconstructed with an attached submonolayer of an oxygen species, most probably water, at 0 V (vs Ag/AgCl). Results at +0.95 V supported a previously suggested model for both the Au(111) and the Au(100) electrodes that, based on infrared and scanning tunneling microscopy measurements, the surfaces were a (1 × 1) structure with the coadsorbed sulfate anion and hydronium cation (H3O+). At +1.05 V, where a small amount of an anodic current flowed, adsorption of a monolayer of oxygen species was observed on both surfaces. When the single-crystal gold electrodes were electrochemically oxidized at +1.40 V, the expansion of the gold surface by about one monolayer of Au atoms was observed, suggesting the penetration of oxygen into the surface gold layers (i.e., the formation of two layers of surface oxide). When the surface oxide was reduced at +0.65 V, the surface structure returned back to the structure observed at +0.95 V before the oxide formation (i.e., a (1 × 1) structure with coadsorbed sulfate anion and H3O+). When the potential was reduced to 0 V, the surfaces were reconstructed again but with slightly more random structures than those before the potential cycle.

Electrochemical STM investigation of 1,8-octanedithiol monolayers on Au(1 1 1). : Experimental and theoretical study

The surface structure of Au(1 1 1) electrodes covered by 1,8-octanedithiol self-assembled monolayers (SAMs) was studied with in situ scanning tunnelling microscopy (STM) as a function of the electrode potential in acidic and alkaline electrolytes. We investigated the correlation between the dynamics of the SAM and the underlying gold atoms during the reductive desorption and oxidative readsorption processes. The reductive desorption of 1,8-octanedithiol is characterized by a transition from a compact monolayer with an upright molecular configuration to a striped phase with flat lying dithiol molecules, whereas during the oxidative readsorption process the surface first becomes increasingly covered by the striped phase until the transition to the compact phase occurs. We also considered under equivalent conditions 1-octanethiol/Au(1 1 1) SAMs which were used as a reference to evaluate the influence of the –SH terminal group on the structure and stability of dithiol SAMs. The desorption and readsorption of both the dithiol and the monothiol have a considerable influence on the substrate morphology which is manifested in the dynamics of vacancy islands, gold islands and indentation of step edges. Quantum mechanical calculations in the framework of density functional theory (DFT) show that adsorbed thiols greatly facilitate the detachment of gold atoms from step edges. The high mobility of gold atoms observed experimentally is compatible with the presence of a defected layer of gold atoms. The DFT results suggest that the formation of a monolayer may involve the diffusion and self assembly of thiolate–Au moieties rather than the diffusion of the bare thiolates across the surface. This mechanism explains the formation of a defected layer of gold atoms.

In situ observation on Au(1 0 0) surface in molten EMImBF 4 by electrochemical atomic force microscopy (EC-AFM)

The Au(1 0 0) surface structure in contact with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4) has been observed using electrochemical atomic force microscopy (EC-AFM) under an electrochemically controlled potential. The AFM images, taken in EMImBF 4 in the potential range from −0.6 to 0.2 V vs. Ag/Ag(I), shows a fourfold symmetry with the distance between protrusions of ≈0.30–0.32 nm. This structure agrees well with the ideal surface structure of Au(1 0 0)-(1 × 1) and it is very similar to that previously obtained in a sulfuric acid aqueous solution.

Wave field restoration using three-dimensional Fourier filtering method

A wave field restoration method in transmission electron microscopy (TEM) was mathematically derived based on a three-dimensional (3D) image formation theory. Wave field restoration using this method together with spherical aberration correction was experimentally confirmed in through-focus images of amorphous tungsten thin film, and the resolution of the reconstructed phase image was successfully improved from the Scherzer resolution limit to the information limit. In an application of this method to a crystalline sample, the surface structure of Au(1 1 0) was observed in a profile-imaging mode. The processed phase image showed quantitatively the atomic relaxation of the topmost layer.

Direct methods determination of the Si(111)-(6×6)Au surface structure

The atomic structure of the Au 6×6 on Si(111) phase has been determined using direct methods and surface X-ray diffraction data. This surface structure is very complicated, with 14 independent gold atoms, relaxations in 24 independent silicon sites and three partially occupied gold sites. In one sense the structure can be described as microdomains of the parent 3 × 3 Au on Si(111) structure. A better description is in terms of a tiling of incomplete pentagonal and trimer units, essentially a pseudo-pentagonal glass. In terms of these structural units it is possible to explain all the gold structures in the coverage range 0.8–1.5 monolayers as pseudo-glasses with strong short-range order but varying degrees of long-range order.

The surface structure of Au(111) in the presence of organic adlayers: a combined electrochemical and surface X-ray scattering study

In situ X-ray scattering studies of the Au(111) electrode surface have been carried out in KClO 4 solutions containing uracil, pyridine or 2,2′-bipyridine using grazing-incident-angle diffraction. At sufficiently negative potentials, the ( p × √3) surface reconstruction forms with a stripe separation p = 23. The formation of a two-dimensional condensed uracil film or a complete monolayer of planar oriented pyridine or 2,2′-bipyridine hinders the lifting of the reconstruction. Whereas the stripe separation increases continuously over a 0.6 V range in the base electrolyte, the presence of the organic compounds blocks this process considerably. Only a small, step-like increase in p is observed, where the adlayer is disordered. There exists a 0.7 V hysteresis between the lifting and the subsequent formation of the reconstruction. The ( p × √3) phase reappears at potentials where a ‘gas-like’ adlayer is formed once again. Cyclic diffractograms and potential step measurements have been employed to study the time dependence of these substrate transitions for the system uracil | Au(111).

The surface structure of flame-annealed Au(100) in aqueous solution: An STM study

The initial structure of flame-annealed Au(100) surfaces has been studied in air and in 0.1 M H 2SO 4 by scanning tunnelling microscopy (STM). It is shown that before, during and after contact with the electrolyte, at potentials sufficiently negative to prevent specific adsorption of anions, the flame-annealed Au(100) surface is reconstructed into exactly the same “hex” form as a surface which has been prepared by annealing in ultrahigh vacuum (UHV). However, the quality of the reconstructed surface depends sensitively on the sample preparation and on the experimental conditions of the flame-annealing procedure. The influence of the cooling procedure after flame annealing on the initial surface structure of the Au(100) electrode is demonstrated and briefly discussed in the light of results published previously.