Surface Gold Atoms Research Articles

Identification of surface species that inhibit and passivate thiosulfate leaching of gold

The anodic dissolution of gold in thiosulfate solution is accelerated by the presence of ammonia and significantly enhanced by the addition of copper. It is inhibited by the presence of adsorbed surface species and passivation is observed under certain conditions. Sulfur, polythionate and copper sulfide species have been identified on gold surfaces in thiosulfate solutions using surface enhanced Raman scattering spectroscopy (SERS) under potential control and open-circuit leaching conditions. Correlations are made between the gold leaching rate and the extent and composition of surface layers, determined quantitatively by anodic stripping voltammetry. In thiosulfate alone, the inhibiting surface layer on gold is sulfur with the coverage of one sulfur atom per three surface gold atoms ( 1 3 ML ) in the potential region of leaching, increasing to close to 1 ML at higher potentials. When ammonia is added, the sulfur coverage under the same conditions remains at 1 3 ML , but co-adsorption of a polythionate occurs at higher potentials and causes passivation. Ammonia copper polysulfide is formed on the gold surface when copper is present as well as ammonia, with the surface layer under conventional leaching conditions developing to a composition of 2 3 ML S and 1 3 ML Cu.

Oxidation of the polycrystalline gold foil surface and XPS study of oxygen states in oxide layers

Gold oxide films obtained on the surface of polycrystalline gold foil upon oxidation by oxygen acti- vated by a high-frequency discharge have been studied by X-ray photoelectron spectroscopy. High-frequency O 2 activation affords oxide films more than 3-5 nm thick. As follows from Au4 f spectra, the surface gold atoms are oxidized to the oxidation state +3. The O1 s spectra have a composite shape and are decomposed into four components that characterize nonequivalent states of oxygen in the resulting oxide films. It is assumed that the two major oxygen states ( E b (O1 s ) = 529.0 and 530.0 eV) correspond to the oxygen atoms in two- and three- dimensional gold oxide Au 2 O 3 , respectively. The oxygen states characterized by the higher binding energies ( E b (O1 s ) = 531.8 and 535.2 eV) likely correspond to molecular oxygen in peroxide and superoxide groups, respectively. DOI: 10.3103/S0027131407060090

Estimation of Surface Oxide on Surfactant-Free Gold Nanoparticles Laser-Ablated in Water

Surfactant-free gold nanoparticles were prepared by laser ablation of a gold metal plate in water. The nanoparticles were characterized by absorption spectroscopy, ζ-potential measurements, and XPS spectroscopy. The nanoparticles are negatively charged because the surface atoms are partially oxidized to Au−O-, according to the literature by Sacher and co-workers (J. Phys. Chem. B, 2004, 108, 16864). We further examined electrostatic interactions between nanoparticle and cationic surfactants. It was found that the surfactant cations attach to the particle surface, neutralizing the particle charge. Taking advantage of the electrostatic interactions, we estimated that 3.3−6.6% of the surface gold atoms was oxidized in water.

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation

The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/CeO2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2 through both the decomposition of the formate species and the formation of the [Au(CO)2]+ species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) chan...

Allylic isomerization of allylbenzene on nanosized gold particles

Nanosized gold particles immobilized on γ-Al2O3 exhibited catalytic activity in the allylic isomerization reaction of allylbenzene. As the size of gold nanoparticles was decreased from 40 to 2 nm, their specific activity per surface gold atom nonmonotonically increased from 0.5 to 110 (mol products) (mol Ausurface)−1 h−1. The particles greater than 40 nm were practically inactive.

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3) R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450–525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S–Au(1 1 1) interaction.

Carboranethiol-Modified Gold Surfaces. A Study and Comparison of Modified Cluster and Flat Surfaces

Four different carboranethiol derivatives were used to modify the surfaces of gold nanoparticles and flat gold films. The novel materials engendered from these modifications are extraordinarily stable species with surfaces that support self-assembled monolayers of 1-(HS)-1,2-C2B10H11, 1,2-(HS)2-1,2-C2B10H10, 1,12-(HS)2-1,12-C2B10H10, and 9,12-(HS)2-1,2-C2B10H10, respectively. Surprisingly, characterization of these materials revealed that a number of molecules of the carboranethiol derivatives are incorporated inside the nanoparticles. This structural feature was studied using a number of techniques, including X-ray photoelectron spectroscopy (XPS), UV-vis, and IR spectroscopies. Thermal desorption experiments show that carborane molecules detach and leave the nanoparticle surface mostly as 1,2-C2B10H10 isotopic clusters, leaving sulfur atoms bound to the gold surface. The surfaces of both the gold nanoparticles and the flat gold films are densely packed with carboranethiolate units. One carborane cluster molecule occupies an area of six to seven surface gold atoms of the nanoparticle and eight surface gold atoms of the flat film. XPS data showed that molecules of 1,12-(HS)2-1,12-C2B10H10 bind to the flat gold surface with only half of the thiol groups due to the steric demands of the icosahedral carborane skeleton. Electrochemical measurements indicate complete coverage of the modified gold surfaces with the carboranethiol molecules.

Hydrogen Chemisorption on Al2O3-Supported Gold Catalysts

Hydrogen is dissociatively adsorbed on the gold particles in Au/Al(2)O(3) catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experiments. This chemisorption of hydrogen induces changes in the Au L(3) and L(2) X-ray absorption near-edge structures. The gold atoms on corner and edge positions dissociate the hydrogen, which does not spill over to the face sites. Therefore, the average number of adsorbed hydrogen atoms per surface gold atom increases with decreasing particle size. With temperature, the hydrogen uptake by supported gold increases or remains constant, whereas it decreases for platinum. Furthermore, in H/D exchange experiments, the activity of Au/Al(2)O(3) increases strongly with temperature. Thus, the dissociation and adsorption of hydrogen on gold is activated.

Preparation and photophysical properties of a hyperbranched conjugated polymer-bound gold nanoassembly

A novel kind of redissolvable gold nanoparticles was prepared by using a poly(phenylene vinylene)-based hyperbranched conjugated polymer as stabilizing agent. The complexation between the surface gold atoms and the bipyridyl groups at the periphery of the hyperbranched polymer prevents the nanoparticles from aggregation. The surface plasmon band of the nanoparticles is insensitive to surrounding media due to the capping with organic layer. The fluorescence of the surface-attached conjugated macromolecule is significantly quenched by the gold nanocore. Solvent polarity effect suggests the occurrence of a photo-induced electron transfer process from the excited bound fluorophore to the nanocore.

Catalytic Gold Nano‐Particles

Although gold is the most inert of all metallic elements, it has interesting properties as a heterogeneous catalyst. There are a number of curious aspects to catalysis by gold that are currently attracting academic investigation, while the observation that gold-based catalysts are active at room temperature and below is driving considerable industrial interest. However, much is still not understood about these catalysts and, for example, apparently similar preparation techniques result in activities of hugely varying magnitude. In the present paper we assess the known phenomenology of heterogeneous catalysis by gold, with particular reference to the material properties of the individual nano-particles of catalyst and the many disagreements in the literature. Even the structure of the nano-particles is uncertain, with claims being made for truncated octahedra, cub-octahedra, icosahedra, various kinds of decahedra, and amorphous structures. As far as uncertainty concerning the mechanism of catalysis is concerned, we show that the situation has not yet been resolved, with evidence that catalysis can proceed even in the absence of either a discrete particulate morphology or an oxide support. One possibility is that more than one mechanism applies. Alternatively, the explanation may be that the activity of gold as a catalyst is determined only by the availability of surface gold atoms with low coordination numbers and an associated electron density suitable for whatever reaction is being catalysed. In this case, the role of the oxide support and of gold particle size and structure is indirect, and they would serve mainly to modulate the specific surface area of the gold, and the electronic configuration of its surface atoms.

Scope for new applications for gold arising from the electrocatalytic behaviour of its metastable surface states

The electrocatalytic behaviour of gold in aqueous media is surprisingly complex and a novel view of gold electrode surfaces is proposed to explain the observed electrochemical responses. The metal surface is considered as a chemically (or redox) modified electrode, with equilibrated, low energy, surface gold atoms functioning as a relatively inert support and low coverage, high energy, protruding gold atoms (or minute clusters of same) functioning as electrocatalytic redox mediators. The protruding atoms are viewed as surface active sites and the complex electrocatalytic behaviour is attributed to the basic instability of the mediators, the diversity (and poor control) of same, and the acid/base (or hydrolysis) behaviour of the oxidized states of the mediators. It is assumed that major advances in the application of gold in the heterogeneous catalysis of gas phase reactions will be accompanied by similar advances in the use of gold in the electrocatalysis area; however, this will require considerable work of both a practical and theoretical nature.

Long-time scale molecular dynamics study of Co diffusion on the Au([formula omitted]) surface

A long-time scale molecular dynamics simulation of a Co adatom on an unreconstructed Au(111) surface is performed from 140 to 870 K. Co and Au are modelized by many-body potentials determined within the second-moment approximation of the tight-binding theory. From 260 to 750 K, the diffusion exhibits an Arrhenius behavior with a migration energy of Em=0.160 eV and a pre-exponential factor of D0=5.8×10−4 cm2/s. From 810 K, an exchange process of the Co adatom with a surface gold atom of surface is observed.

Characterization of a 1,4-dithiane gold self-assembled monolayer: an electrochemical sensor for the cyt-c redox process

Electrochemical desorption and spectroscopic investigations of the gold electrode surface modified with 1,4-dithiane (1,4-dt) organothiol species were performed. The wave observed at −0.87 V versus Ag ∣ AgCl in the LSV (linear sweep voltammetry) reductive curve of the 1,4-dt compared to that for a similar 4-mercaptopyridine (pyS) system (−0.56 V) is indicative of a most effectively chemisorbed monolayer. The evaluation of the capability of the 1,4-dt self-assembled monolayer (SAM) in assessing the direct electron transfer (ET) of cytochrome c (cyt c) metalloprotein was investigated by cyclic voltammetry. The electrochemical response of the cyt c ( E 1/2 ≈0.0 V vs. Ag ∣ AgCl, Δ E p ≈50 mV) showed the characteristics of a reversible redox process. The cyt c voltammetric parameters acquired with the 24-h air exposure modified electrode, and after 100 cycles suggest a considerable improvement of the 1,4-dt electrode performance. The surface enhanced Raman spectroscopy (SERS) spectra revealed that 1,4-dt species is in a mixed gauche and trans orientation on the gold surface. The shift for higher wavenumbers observed for the C–S stretching modes in the SERS spectra, comparatively to the normal Raman spectrum, is assigned to the 1,4-dt coordination to surface gold atoms via a π interaction with the sulfur p-orbitals. The data collected suggest that this π interaction plays an important role on the stability of the 1,4-dt adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.

Coverage-Dependent Adsorption of CH3S and (CH3S)2 on Au(111): a Density Functional Theory Study

The origin of the superlattice present in the commensurate hexagonal structure of self-assembled monolayers of n-alkanethiols on gold and the question of whether the thiols are adsorbed onto the surface as dimers (disulfides) or monomers (thiolates) have been under debate for many years. Looking for a better understanding of the structural properties of these systems, we have performed a theoretical study of the molecular and dissociative adsorption of dimethyl disulfide on Au(111) as a function of coverage (0.25 ≤ ϑ ≤ 1), using gradient-corrected density functional (DFT) calculations with a slab geometry. For the dissociated state, our results indicate that the hcp hollow site is much less favorable than the fcc site. For the latter site, we find that, because of surface gold atom relaxation, the adsorption energy depends strongly on ϑ, changing from ∼18 kcal/mol at ϑ = 0.25 to ∼3 kcal/mol at ϑ = 1. For the bridge site, instead, the adsorption energy is a weak function of ϑ, and for all investigated coverages, this site is by far the most stable. According to our DFT approach, the adsorption of dimethyl disulfide is dissociative with a thermodynamic gain, at ϑ = 1, of ∼13 kcal/mol with respect to the adsorbed molecular state. We also find, however, that the energy of c(4 × 2) structures containing at least two inequivalent CH3S groups per unit cell (with a minimum S−S distance of ∼3.7 Å) is, within the accuracy of our approach, indistinguishable from the pure ( × ) hexagonal structure. Our results suggest that the full solution of this thorny problem will require, also for the shortest chains, an estimate of the energetic contribution of dispersion forces that are not included in the DFT calculations.

Amphoterization of Colloidal Gold Particles by Capping with Valine Molecules and Their Phase Transfer from Water to Toluene by Electrostatic Coordination with Fatty Amine Molecules

The surface modification of colloidal gold particles with the amino acid valine is demonstrated. Self-assembly of valine on the gold particles is accomplished in the aqueous phase, linkage with the gold particles possibly occurring through covalent interaction of the amine group with the surface gold atoms. Derivatization with the amino acid in this manner imparts amphotericity to the gold particles, the particles being negatively charged at pH values greater than 6 (the isoelectric point of valine) and positively charged below this pH. The charge reversal on the gold particles is demonstrated by electrostatic self-assembly of the colloidal gold particles on glass (which is negatively charged at pH > 3) by immersion of the substrate alternately into the valine-capped gold solutions maintained at pH = 3.5 and 8.5. The phase transfer of the aqueous valine-capped gold colloidal particles by electrostatic linkage with fatty amine molecules present in toluene is also described. The two-phase method described h...

Gold Cluster Crystals

Abstract Gold clusters are gold compounds with a core of gold atoms and organic groups covalently bound to the surface gold atoms. An example is undecagold, Au11(P(C6H5)3)7, whose structure was solved by x-ray crystallography using 3-dimensional crystals. These differ from colloidal gold, which are suspensions of metal particles, usually formed by metal ion reduction; although the particles may be approximately the same size, they vary due to the statistical process of formation. Gold clusters are compounds with a definite formula, and should all be perfectly identical. However, it is known that there is a family of stable gold cluster compounds, such as Au6, Au11, Au13, AU55, Au67, etc. In a given preparation of gold clusters, there is usually some mixture of these, thus leading to some size variation. Methods such as gel filtration column chromatography and ultrafiltration can be used to separate most of these species, so that relatively pure preparations may be achieved.

Reflection FTIR Studies of the Conformation of 2,2‘-Bipyridine Adsorbed at the Au(111) Electrode/Electrolyte Interface

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the conformation of 2,2‘-bipyridine (22BPY) adsorbed at the Au(111) electrode surface. In addition, ab initio Hartree−Fock calculations were carried out to calculate the IR spectra for the trans-planar, cis-planar, and twisted conformations both for an isolated 22BPY molecule and for the molecule attached to a gold atom or ion. The spectra for the trans- and the cis-conformations were predicted to be distinctly different. The differences between the predicted spectra and the differences for most structural parameters between the cis-planar conformer and the torsional conformations with a twist of 0° to 40° between the two rings were small. SNIFTIRS spectra with a good signal-to-noise ratio were measured only for those potentials where the 22BPY molecules associate into stacks. Efficient packing into stacks requires a planar or very nearly planar conformation for the 22BPY molecules. For these potentials, the SNIFTIRS spectra provided direct evidence that 22BPY molecules coordinated to the surface gold atoms assume a cis- or nearly cis-conformation.

Adsorption of cyanide on gold single crystal investigated by in situ visible–infrared difference frequency generation

The adsorption of CN − on gold single crystal has been monitored by visible–infrared difference frequency generation (DFG). We show a significant effect of the surface orientation on the adsorption strength of CN −, which decreases from (110) to (100) and (111). Both the linewidth and the potential tuning rate of the DFG resonances do not depend on the electrode surface orientation, indicating that CN − is tightly and linearly bound to its nearest surface gold atom.

Cyclic voltammetric studies on the reduction of a gold oxide surface layer

Cathodic reduction of a stationary surface layer of gold compounds formed anodically on a gold electrode in 0.5 M H 2SO 4 solution at E=1.7 V, i.e. prior to oxygen evolution, has been investigated. With the initial amount of gold oxide on the electrode surface being constant, linear dependences of j p on v, log j c on E and log j p on E p were established. It has been found that the parameters d E/d log j c, d E p/d log j p and d E p/d log v at 1.2 V (SHE)> E> E p are close to the theoretical value of 2.3 R T/ F. Such Tafel behaviour is expected for the cathodic process in the case of a reversible metal|metal oxide electrode. It has been proposed that the reversibility of the cathodic process may be determined by the equilibrium of the following surface reaction: Au · H 2O+2H 2O⇔(Au(OH) 3) s+3H ++3e − where Au ·H 2O represents the gold surface atoms with electrostatically adsorbed H 2O, and (Au(OH) 3) s is Au(OH) 3 in the form of the surface layer. This equilibrium may exist at the interface only when the Au(OH) 3 layer is present on the electrode surface. The equilibrium potential of this reaction was evaluated as 1.28±0.01 V.

Electrochemistry in nano-hole formation on gold surface with a scanning tunnelling microscope

By increasing the interaction strength between the tip and the sample in a controllable and reproducible manner, scanning tunnelling microscopy has been used to produce nanometre scale structures. Two types of modifications could be realised: holes or mounds. In this paper we focus on the understanding of the mechanism responsible for nano-hole formation. The smallest stable nano-hole formed is 3 nm in diameter and 0.24 nm in depth, which represents the loss of ca 100 gold atoms. The writing process has an unitary rate in nitrogen gas with the presence of water vapour. The chemical composition of the tip-sample interface is important in nano-hole formation. The etching mechanism seems to be electrochemical in origin, where water or more precisely oxygen atoms could have a predominant play. A place-exchange mechanism between gold surface atoms and oxygen atoms adsorbed on surface seems to be the crucial point of this reaction.