Au Adatoms Research Articles

Self-assembly of methanethiol on the reconstructed Au(111) surface

We present a combined experimental and theoretical study of molecular methanethiol $({\text{CH}}_{3}\text{SH})$ adsorption on the reconstructed Au(111) surface in the temperature range between 90 and 300 K in UHV. We find that the simplest thiol molecules form two stable self-assembled monolayer (SAM) structures that are created by distinct processes. Below 120 K, a solid rectangular phase, preserving the herringbone reconstruction, emerges from individual chains of spontaneously formed dimers. At higher adsorption temperatures below 170 K, a close-packed phase forms via dissociative ${\text{CH}}_{3}\text{SH}$ adsorption and the formation of Au adatoms that are not incorporated into the SAM. We show that the combination of a strong substrate-mediated interaction with nondissociative dimerization and temperature activated removal of the Au(111) reconstruction drives the large-scale assembly of molecular ${\text{CH}}_{3}\text{SH}$ into two distinct phases.

Tailoring the Interaction Strength between Gold Particles and Silica Thin Films via Work Function Control

The possibility to modify the adsorption properties of a porous silica/Mo(112) film by controlling its work function has been studied by a combined STM and density-functional theory approach. While the original film is inert towards metal adsorption, Au atoms and clusters can be stabilized on the surface after Li doping. The Li atoms penetrate the topmost silica layer and bind as Li+ cations at the metal-oxide interface, thereby reducing the oxide work function. This induces a charge transfer into Au adatoms, which in turn enables strong Au-silica interaction mediated by a polaronic distortion of the oxide lattice.

Reaction Mechanisms for the CO Oxidation on Au/CeO2 Catalysts: Activity of Substitutional Au3+/Au+ Cations and Deactivation of Supported Au+ Adatoms

Density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U) reveal the mechanisms for the oxidation of CO catalyzed by isolated Au atoms as well as small clusters in Au/CeO(2) catalysts. Ceria (111) surfaces containing positively charged Au ions, either as supported Au(+) adatoms or as substitutional Au(3+) ions, are shown to activate molecular CO and to catalyze its oxidation to CO(2). In the case of supported single Au(+) adatoms, the limiting rate for the CO oxidation is determined by the adsorbate spillover from the adatom to the oxide support. The reaction then proceeds with the CO oxidation via lattice oxygen and O vacancy formation. These vacancies are shown to readily attract the supported Au(+) adatoms and to turn them into negatively charged Au(delta-) adspecies that deactivate the catalyst, preventing further CO adsorption. Au(3+) ions dispersed into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle consisting of three individual steps maintaining their activity along the reaction process: Au cations in Au(x)Ce(1-x)O(2) systems promote multiple oxidations of CO without any activation energy via formation of surface O vacancies. Molecular oxygen adsorbs at these vacancies and forms O adspecies that then catalyze the oxidation of molecular CO, closing the catalytic cycle and recovering the stoichiometric Au(x)Ce(1-x)O(2) system. The interplay between the reversible Ce(4+)/Ce(3+) and Au(3+)/Au(+) reductions underpins the high catalytic activity of dispersed Au atoms into the ceria substrate. It is shown that the positive oxidation state of the substitutional Au ions is retained along the catalytic cycle, thus preventing the deactivation of Au(x)Ce(1-x)O(2) catalysts in operation conditions. Finally, although a single Au(+) adatom bound to an O vacancy is shown to deactivate during CO oxidation, the calculations predict that the reactivity of gold nanoparticles nucleated at O vacancies can be recovered for cluster sizes as small as Au(2).

Au adatom-linked CH 3S–Au–SCH 3 complexes on Au(1 1 1)

A novel chain structure made of gold adatoms and gold–thiolate complexes on Au(1 1 1) is predicted here by first principles density functional theory. In this structure, the CH 3S–Au–SCH 3 complexes are parallel to each other and linked together by interactions between Au adatoms and Au atoms in the complexes as well as by bonding between the Au adatoms and the CH 3S– groups in the CH 3S–Au–SCH 3 complexes. This new structure is compared with previously proposed gold–thiolate polymers on Au(1 1 1) and found to be energetically competitive with previous models at both low and high coverages.

A benign route to fabricate nanoporous gold through electrochemical dealloying of Al–Au alloys in a neutral solution

Nanoporous gold (NPG) ribbons have been fabricated through electrochemical dealloying of melt-spun Al–Au alloys with 20–50 at.% Au in a 10 wt.% NaCl aqueous solution under potential control at room temperature. The microstructures of NPG were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDX) analysis. The microstructures of the NPG ribbons strongly depend upon the phase constitutions of the starting Al–Au alloys. The single-phase Al2Au or AlAu intermetallic compound can be fully dealloyed, resulting in the formation of NPG with a homogeneous porous structure. The separate dealloying of Al2Au and AlAu in the two-phase Al–45 Au alloy leads to the formation of NPG composites (NPGCs). In addition, the dealloying of the Al–20 Au alloy comprising α-Al and Al2Au leads to the formation of NPG with bimodal channel size distributions. According to the ligament size, the surface diffusivity of Au adatoms along the alloy/electrolyte interface has been evaluated and increases with increasing applied potential. The dealloying mechanism in the neutral NaCl solution has been explained based upon pourbaix diagram and chloride ion effect.

Low-temperature STM/STS investigation of nanostructures created by an STM tip on Au(111) surfaces

We have investigated the electronic structure of atomic size particles on clean Au(111) surfaces by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) at low temperatures. Au adatoms were created on Au(111) with high reproducibility by bringing the STM tip in (near) contact with the surface, providing a rather easy means of creating larger nanostructures out of atomic size particles. Scattering of Au(111) surface state electrons at the adatom site gives rise to the formation of complex standing wave patterns. The electronic structure of the adatoms appears to be dominated by a pronounced resonance below the onset of the Au(111) surface state. The observed resonance is interpreted as a bound state that is split off from the 2D surface state of the Au(111) surface.

Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

We present a scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 500 °C for 15 min. I (V) curves taken via STS measurements show that large gold nanoparticles (∼8 nm) exhibit a metallic electronic structure and, thus, are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the Fe3O4(111) surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size-sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged. When this Au/Fe3O4(111) system is dosed with CO at 260 K, adsorption of CO molecules normal to...

Porphyrin-Based Two-Dimensional Coordination Kagome Lattice Self-Assembled on a Au(111) Surface

A two-dimensional network of metal-coordinated Kagome lattice has been self-assembled by free-base tetrapyridyl porphyrin molecules on a Au(111) surface. Au ad-atoms offered by the surface coordinate the pyridyl functions of the neighboring porphyrins in a 2-fold linear coordination. Comparison of the self-assemblies of the same molecules on various surfaces (Au, Ag, and Cu) underlines the fact that both structural and chemical characteristics of surfaces play important roles in determining the supramolecular adlayer structures.

Formation of large ordered domains in benzenethiol self-assembled monolayers on Au(1 1 1) observed by scanning tunneling microscopy

Benzenethiol (BT) self-assembled monolayers (SAMs) on Au(1 1 1) were prepared as a function of solution temperature after immersion in a 1 mM ethanol solution for 1 day. The surface structures of BT SAMs were examined by means of scanning tunneling microscopy (STM). Although BT molecules usually form disordered SAMs containing the Au adatom islands at room temperature, we found that they formed two-dimensional ordered SAMs containing a large size domain at a high solution temperature of 50 or 75 °C. High-resolution STM imaging revealed that BT SAMs on Au(1 1 1) formed at 50 °C have a (2×3√2)R23° packing structure. From our STM study, we revealed that two-dimensional ordered BT SAMs on Au(1 1 1) can be obtained by changing the solution temperature.

The structure of the Au(111)/methylthiolate interface: New insights from near-edge x-ray absorption spectroscopy and x-ray standing waves

The local structure of the Au(111)(square root(3) x square root(3))R30 degrees-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular sigma(*) S-C resonance but also into a sigma(*) S-Au orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second sigma(*) S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the sigma(*) S-C resonance. The new NIXSW data show the S-C bond to be tilted by 61 degrees relative to the surface normal, with a preferred azimuthal orientation in <211>, corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data.

Local Methylthiolate Adsorption Geometry on Au(111) from Photoemission Core-Level Shifts

The local adsorption structure of methylthiolate in the ordered Au(111)-(sqrt[3]xsqrt[3])R30 degrees phase has been investigated using core-level-shift measurements of the surface and bulk components of the Au 4f(7/2) photoelectron binding energy. The amplitude ratio of the core-level-shift components associated with surface Au atoms that are, and are not, bonded to the thiolate is found to be compatible only with the previously proposed Au-adatom-monothiolate moiety in which the thiolate is bonded atop Au adatoms in hollow sites, and not on an unreconstructed surface, or in Au-adatom-dithiolate species.

Mass Thickness Analysis of Gold Thin Films Using Room Temperature Gas-Phase Chlorination

Chemical interaction of ultrathin gold films with gaseous chlorine at room temperature results in quantitative metal chlorination and formation of Au(III) chloride. Combination of gas-phase chlorination of Au nanostructures with dissolution of the chloride product and spectrophotometric determination of the chloroaurate concentration presents a simple procedure for quantitative determination of the mass thickness of Au nanostructures. The method is demonstrated by mass thickness analysis of the condensation coefficient of Au adatoms on solid substrates, the extinction coefficient of Au nanoparticles, as well as study of the integrity of Au island films upon interaction with solvents.

Gold-induced surface reconstruction on GaAs(111)B surface

We have investigated the atomic structure of the Au/GaAs(111)B- surface and have performed scanning tunnelling microscopy (STM) simulations of the surface by using first-principles calculations. The atomic geometry determined with Au adatoms located at every third threefold hollow hcp site of the Ga lattice, agrees well with results reported from STM and low-energy electron diffraction experiments. The simulated STM images support experimental observations of bright centers with triangular spots. The calculated value of the diffusion barrier height of the Au adatom quantitatively explains the experimentally observed reconstruction of the wetting layer of Au on the Au/GaAs(111)B- surface.

The role of gold adatoms in self‐assembled monolayers of thiol on Au(111)

AbstractThe role of Au adatoms in high‐density alkanethiol self‐assembled monolayers (SAMs) has been investigated. Surface reconstructions in ethylthiol (ET) SAMs due to gold adatoms have been studied using DFT calculations. Relative energies are compared based on binding and surface energies calculated for different lattices using the same unit cell. We have found a structure with two ET‐Au‐ET adatom moieties binding on top of surface gold atoms as the most stable structure. Our model is consistent with the structural parameters obtained from experiments. In particular, the simulated STM image displays the characteristic zig‐zag pattern. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Electronic charge transfer between ceria surfaces and gold adatoms: a GGA+U investigation

We use density functional theory calculations with Hubbard corrections (DFT+U) to investigate electronic aspects of the interaction between ceria surfaces and gold atoms. Our results show that Au adatoms at the (111) surface of ceria can adopt Au(0), Au(+) or Au(-) electronic configurations depending on the adsorption site. The strongest adsorption sites are on top of the surface oxygen and in a bridge position between two surface oxygen atoms, and in both cases charge transfer from the gold atom to one of the Ce cations at the surface is involved. Adsorption at other sites, including the hollow sites of the surface, and an O-Ce bridging site, is weaker and does not involve charge transfer. Adsorption at an oxygen vacancy site is very strong and involves the formation of an Au(-) anion. We argue that the ability of gold atoms to stabilise oxygen vacancies at the ceria surface by moving into the vacancy site and attracting the excess electrons of the defect could be responsible for the enhanced reducibility of ceria surfaces in the presence of gold. Finally, we rationalise the differences in charge transfer behaviour from site to site in terms of the electrostatic potential at the surface and the coordination of the species.

First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces

In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.

Effect of Ag Adatoms on High-Coverage Alkanethiolate Adsorption on Au(111)

Alkanethiol adsorption on Ag-adatom-modified Au(111) surfaces is studied by means of electrochemical techniques combined with Auger electron spectroscopy and periodic density functional (DF) calculations. The bimetallic surfaces are prepared by Ag underpotential deposition, and only the alkanethiolate high-coverage regime is considered. Alkanethiolate electrodesorption from Ag-modified Au(111) surfaces requires potentials shifted 0.3 eV in the negative direction with respect to the value corresponding to desorption from Au(111) surfaces. In agreement with this experimental result, DF calculations show that alkanethiolates prefer to be laterally adsorbed to Ag adatoms, the interaction energy being 0.3 eV larger than that corresponding to the same thiolate on Au(111). Therefore, it is concluded that alkanethiolates adsorbed on Au are likely to be observed only when the Ag adatom surface density is small enough or all Ag sites are occupied. The preference for alkanethiolate adsorption on adatoms is also found for a model surface containing Au adatoms on Au(111). This indicates that no significant differences in electrodesorption potentials should be expected between Ag-adatom- and Au-adatom-rich surfaces. These findings and the analysis of experimental results strongly suggest that, under experimental conditions, the Au(111) surface does not exhibit a significant amount of Au adatoms, thus casting reasonable doubts on the adatom reconstruction models proposed for thiolates on Au(111).

The c(4 × 2) Structure of Short- and Intermediate-Chain Length Alkanethiolate Monolayers on Au(111): A DFT Study

We report on first principles density functional theory calculations for a new adatom + vacancy model (ad+va) of the c(4 × 2) structure of short- and intermediate-length alkanethiolate self-assembled monolayers (SAMs) on Au(111). This model includes the same structural motif, two thiolates bound to an Au adatom and a surface vacancy, that has been recently proposed for the low-coverage and (√3 × √3)R30° phases of short-chain alkanethiolate SAMs on Au(111). We show that the ad+va model is more stable than all available (√3 × √3)R30° one-chain models and can also satisfactorily describe the experimental scanning tunneling microscopy images of the c(4 × 2) phase.

Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects

We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO2(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of Oa was examined using temperature programmed desorption (TPD). The Oa is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this Oa to make CO2, probably via adsorbed CO. The reactivity of Oa with CO increases with increasing particle size, as expected based on Bronsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO2(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.

CO adsorption on a Au/Ni(111) surface alloy—a DFT study

The adsorption of CO on a Au/Ni(111) surface alloy has been investigated using ab initiodensity-functional calculations. We show that, in contrast to a Au adatom on a Ni(111)surface, a Au impurity binds CO only very weakly. In addition, the impurity induces areduction of the adsorption energies which is strictly localized to its immediateneighbourhood. The consequences for possible reaction scenarios are discussed.