Au Step Edges Research Articles

Gold oxide formation on Au(111) under CO oxidation conditions at room temperature.

Although gold-based catalysts are promising candidates for selective low-temperature CO oxidation, the reaction mechanism is not fully understood. On a Au(111) model catalyst, we observe the formation of gold oxide islands under exposure to atmospheric pressures of oxygen or CO oxidation reaction conditions in an in situ scanning tunneling microscope. The gold oxide formation is interpreted in line with the water-enabled dissociation of O2 on the step edges of Au(111). Contaminants on the gold surface can strongly promote the gold oxide formation even on the terraces. On the other hand, TiO2 nanoparticles on the Au(111) do not show any influence on the formation of the gold oxide and are thus not providing a significant amount of atomic oxygen to the gold at room temperature. Overall, the presence of gold oxide is likely under industrial conditions.

Facile room-temperature self-assembly of extended cation-free guanine-quartet network on Mo-doped Au(111) surface.

Guanine-quadruplex, consisting of several stacked guanine-quartets (GQs), has emerged as an important category of novel molecular targets with applications from nanoelectronic devices to anticancer drugs. Incorporation of metal cations into a GQ structure is utilized to form stable G-quadruplexes, while formation of a cation-free GQ network has been challenging. Here we report the room temperature (RT) molecular self-assembly of extended pristine GQ networks on an Au(111) surface. An implanted molybdenum atom within the Au(111) surface is used to nucleate and stabilize the cation-free GQ network. Additionally, decoration of the Au(111) surface with 7-armchair graphene nanoribbons (7-AGNRs) enhances the GQ domain size by suppressing the influence of the disordered phase nucleated from Au step edges. Scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations confirm the formation of GQ networks and unravel the nucleation and growth mechanism. Our work, utilizing a hetero-atom doped substrate, provides a facile approach to enhance the stability and domain size of the GQ self-assembly, which would be applicable for other molecular structures.

Methanol oxidation on the Au(3 1 0) surface: A theoretical study

This work establishes a comprehensive analysis of the role of steps and kinks of the Au(3 1 0) surface in the generation of active oxygen species and elucidates the reaction pathway for methanol oxidation to formaldehyde. We have considered two possibilities for the OO bond splitting in molecular O2: via a dissociative O2 adsorption or a direct reaction of O2 with methanol. Both reaction pathways more favorably take place on the step edges of Au(3 1 0) than on flat Au(1 1 1). Depending on the adsorption positions of O2, different dissociation pathways are possible, where the most favored path shows a substantially lower activation barrier compared to the flat gold surface. Atomic oxygen generated on the surface, prefers to build OAuO fragments with stronger binding to the surface than individually adsorbed O. The reaction pathways for methanol reaction with O and O2 have been analyzed and compared. The direct reaction of methanol with molecular oxygen (an associative mechanism) proceeds via a formation of the OOH intermediate and its further dissociation to O and OH. This reaction sequence has low activation barriers of 0.3–0.4 eV and therefore should be preferred over O2 dissociation to atomic O and the following reaction of O with methanol. By analogy, O2 can react with water generating OOH with similarly low activation barriers. We have supported the proposed associative mechanism by microkinetic modeling.

Concave Cubes as Experimental Models of Catalytic Active Sites for the Oxygen-Assisted Coupling of Alcohols by Dilute (Ag)Au Alloys

A major challenge in understanding structure–function relationships in heterogeneous catalysis is bridging the materials complexity gap between the well-ordered surfaces used in fundamental experimental and computational studies and the more complex and dynamic materials that exist under catalytic operating conditions. In this work, we utilized (Ag)Au concave cube nanoparticles as experimental models to test a prediction made by theory regarding a potential bimetallic active site for the dissociation of molecular oxygen, which is a key initiating step in the selective oxygen-assisted coupling of alcohols. As a consequence of their method of synthesis, the concave cubes have surfaces that are rich in Ag-stabilized Au step edges, which is the active site proposed by theory, and thus we predicted that they would have high activity for the methanol coupling reaction. Indeed, in addition to 99% selectivity toward the desired coupling product, methyl formate, the concave cubes show a major increase in activity compared to ozone-activated nanoporous gold, a comparable dilute (Ag)Au alloy catalyst for the same reaction, even without an activating ozone pretreament. Further, the well-defined surfaces of these concave cubes open up opportunities for in-situ microscopy and spectroscopy experiments that can provide a better understanding of (Ag)Au active sites. More broadly, this work highlights how nanoparticles with controlled shapes and well-defined surfaces can be rationally tailored to experimentally validate predictions from theory.

Structural Stability and Phase Transitions of Octanethiol Self-Assembled Monolayers on Au(111) in Ultrahigh Vacuum.

To understand the structural stability of as-prepared octanethiol (OT) self-assembled monolayers (SAMs) with a fully covered c(4 x 2) phase on Au(111) in ultrahigh vacuum (UHV) conditions of 3 x 10(-7) Pa at room temperature, we examined OT SAM samples obtained as a function of storage period using scanning tunneling microscopy (STM). STM imaging revealed that phase transition of OT SAMs after storage in UHV for 3 days occurs from the c(4 x 2) phase to the mixed phase containing ordered c(4 x 2) and disordered phases. It was also observed that the disordered phase was mainly located at around vacancy islands and near step edges of Au(111) terraces, implying that desorption of OT molecules chemisorbed on Au(111) in UHV occurs more quickly in these regions compared with in the closely packed and ordered domains. After a longer storage in UHV for 6 days, OT SAMs with the c(4 x 2) phase were changed to the disordered phase containing a partially ordered domain with a row structure. From this study, we clearly demonstrated that OT molecules in SAMs on Au(111) are desorbed spontaneously in UHV at room temperature, resulting in the formation of disordered and row phases.

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface.

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm2) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e., parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO2(111)-type O–Ce–O single and double trilayer structures for submonolayer coverages.

Ni electrochemical epitaxy on unreconstructed Au(111): An in-situ STM study

Abstract Gold (111) is a widely used substrate in vacuum and in electrolytes for studying the initial stages of metal growth. Its surface reconstruction is unavoidable using common preparation procedures in vacuum and this affects the growth mechanism of metal overlayers. In the electrochemical environment, the same surface reconstruction is obtained at potentials negative of the Au(111) potential of zero charge, whereas above this potential the surface structure is (1 × 1). In this paper we present a preparation procedure which allows blocking the Au step edges by Pd islands, inhibiting the formation of the Au(111) reconstruction and yielding the unreconstructed Au(111) surface (1 × 1) at any electrochemical potential. Using in-situ scanning tunnelling microscopy, we compare the Ni/Au(111) electrochemical epitaxy on reconstructed and on unreconstructed Au(111). Observations show that performing Ni growth on the (1 × 1) surface drastically improves the long-range order of the (8 × 8) moire pattern created by the lattice mismatch between the Au substrate and Ni adlayer. Detailed characterizations of the moire structure and of its electrochemical dependence provide new insights into the relative energy landscape of the multiple adsorption sites of the Ni atoms within the (8 × 8) unit cell. The possibility of stabilizing a Au(111)-1 × 1 surface opens up new perspectives for studying metal growth and molecular organisation.

Electrochemical Cu Growth on MPS-Modified Au(111) Electrodes

Au(111) electrodes have been modified with self-assembled monolayers (SAM) of 3-mercapto-1-propanesulfonic acid (MPS) and used as a substrate for Cu electrodeposition. Aqueous plating solutions contained 0.1 M H2SO4, low Cu concentrations (≤80 μM), and, optionally, 1.4 mM Cl ions. The deposition process was characterized by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM) as a function of the electrode potential. At potentials positive of Cu growth (≥0.7 VRHE), freshly modified electrodes are covered by an ordered (5√3 × √21) MPS adlayer (α) both in Cl-free and Cl-containing electrolytes. The α adlayer becomes disordered at more negative potentials prior to the onset of Cu deposition (≤0.65 VRHE). In the potential regime of Cu underpotential deposition (UPD) (≈0.2–0.65 VRHE), the surface morphology strongly depends on the presence of Cl. In the absence of Cl, a transient, ordered Cu/MPS adlayer phase (δ) forms via 2D growth and covers the entire Au(111) surface. Subsequently, the δ phase transforms into a disordered Cu/MPS phase (σCu) with small, embedded Cu islands. In Cl-containing electrolyte, a disordered Cu/MPS/Cl phase (γ) nucleates at Au step edges or surface defects and spreads laterally. Cu islands form simultaneously within the γ phase. Two-dimensional growth of these islands results in a pure Cu-UPD layer. Overpotential deposition (OPD) proceeds via layer-by-layer mode with second layer nucleations at surprisingly small critical coverages (θC ≪ 0.5). Our observations differ significantly from those in previous studies, demonstrating that the Cu growth behavior critically depends on the concentrations of MPS, Cu, and Cl at the interface.

Coverage-dependent Orientations of Dy@C82 Molecules on Au(111) Surface

The adsorption and molecular orientation of Dy@C82 isomer I on Au(111) has been investigated using ultrahigh-vacuum scanning tunneling microscopy at 80 K. At low coverages, the Dy@C82 molecules tend to grow along the step edges of Au(111), forming small clusters and molecular chains. Adsorption of Dy@C82 on the edges is dominated by the fullerene-substrate interaction and presents various molecular orientations. At higher coverages, the Dy@C82 is found to form ordered islands consisting of small domains of equally oriented molecules. The Dy@C82 molecules in the islands prefer the adsorption configurations with the major C2 axis being approximately parallel to the surface of the substrate. Three preferable orientations of the Dy@C82 molecules are found in a two-dimensional hexagonal close packed overlayer. These observations are attributed to the interplay of the fullerene-substrate interaction and dipole-dipole interaction between the metallofullerenes.

Site- and Configuration-Selective Anchoring of Iron–Phthalocyanine on the Step Edges of Au(111) Surface

Adsorption behavior of iron–phthalocyanine (FePc) at low submonolayer coverage on a reconstructed Au(111) single crystalline surface was investigated by a combination of low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. A site- and orientation-selective adsorption was found at different temperatures and molecular coverages by means of STM. Further DFT calculations demonstrate that the energy difference between different adsorption configurations leads to the selectivity, and thus the formation of one-dimensional molecular chains on the monatomic step edges in the fcc surface reconstruction domains. The exact adsorption site and configuration of the FePc molecule as well as the simulated STM images are obtained on the basis of DFT calculations, which is in good agreement with experimental observations.

Molecular chains and carpets of sexithiophenes onAu(111)

The two organic molecular materials $\ensuremath{\alpha}$-sexithiophene (6T) and $\ensuremath{\alpha},\ensuremath{\omega}$-dihexylsexithiophene (DH6T) adsorbed on $\mathrm{Au}(111)$ in the (sub)monolayer range were investigated by scanning tunneling microscopy (STM) in order to explore the effect of alkyl substitution on the self-assembly at surfaces. Metal substrate step edges are identified as preferred nucleation sites for 6T, while stable nucleus formation for DH6T occurs at kinks of the $\mathrm{Au}(111)$ herringbone reconstruction. At low coverage, 6T forms continuous chains of single-molecular width along Au step edges, involving molecular conformation changes by rotations around C-C bonds of neighboring thiophene units. In contrast, DH6T exhibits no ordered structures in the submonolayer range. At monolayer coverage, substantially different structures were observed for the two molecules. 6T forms rows of molecules with parallel long molecular axes, whereas DH6T forms lines along these axes, where the conjugated cores are embedded in a matrix of hexyl chains. Because of different preferred nucleation sites, 6T forms a continuous molecular carpet on extended $\mathrm{Au}(111)$ terraces, whereas DH6T resembles a patchworklike carpet as domain boundaries are induced by the $\mathrm{Au}(111)$ herringbone surface structure, leading to reduced domain sizes. Alkylation of 6T thus drastically changes the adsorption behavior and the resulting layer structure on the Au surface. These results should be valuable for developing new directed self-assembly schemes on prepatterned surfaces.

Atomic Rearrangements during the Electrochemical Treatments of Au(111) Covered with Irreversibly Adsorbed Sb

A morphological variation of Au(111) covered with irreversibly adsorbed Sb was investigated with cyclic voltammetry and EC-STM. At open circuit potential (approximately 0.0 V vs a Ag/AgCl reference electrode), the oxygenated Sb layers were formed as an island on the wide terraces and a terrace at the step edges of Au(111). The ultimate morphology at the open circuit potential was a network adlayer with a (radical3 x radical3)R30 degrees atomic arrangement. When the oxygenated layer was reduced, the adsorption features, such as the island, shrunk or disappeared depending on their sizes. This modification was interpreted in terms of an alloy formation of Sb and Au. All of the Sb atoms, however, were not involved in the alloy formation, although the alloyed and unalloyed domains showed (radical3 x radical3)R30 degrees atomic structures with different brightness in EC-STM images. During oxidation of the reduced Sb layers, the alloyed and unalloyed domains of Sb behaved in a different way: the alloyed Sb was stripped to a soluble species to leave pits, while the unalloyed Sb became an oxygenated adspecies, which desorbed very slowly. A long oxidation led to a Au(111) covered with pits and islands of (1 x 1) without any adsorbed Sb.

Electrochemical in situ STM study of Al and Ti–Al alloy electrodeposition on Au(111) from a room temperature molten salt electrolyte

The electrodeposition of Al and Ti–Al alloys on Au(111) from an acidic aluminium chloride-1-methyl-3-butylimidazolium chloride (AlCl3–MBImC) room-temperature molten salt electrolyte containing TiCl4 was studied by cyclic voltammetry and chronoamperometry and in particular, nucleation and growth processes were monitored in situ by means of electrochemical scanning tunneling microscopy (EC-STM) for the first time. Two underpotential deposition (UPD) states for Al were observed in the cyclic voltammogram at 0.51 and +0.22 V vs. Al/Al3+. At corresponding UPD conditions STM images show different nucleation behavior for Al: first a growth of clusters all over the Au surface, and later preferentially along the step edges leading to a step edge decoration. Potential step experiments imply progressive nucleation and growth and are evidenced by STM images obtained at different time intervals showing the growth of Al clusters of up to 8 nm diameter in size. After anodic dissolution of the deposited Al film, the appearance of circular holes of a size comparable with the Al clusters notably along Au step edges indicates Al–Au surface alloy formation. Tunneling current vs. bias voltage (I–U) curves of the Al clusters exhibit a reduced tunneling barrier height (Φ) of 0.23 eV. A distinct two-dimensional nucleation of Ti–Al clusters begins to occur at 0.5 V vs. Al/Al3+ exclusively on the Au terraces. They grow in the form of chains reaching a size of about 2–3 nm diameter in size, smaller in comparison with Al clusters. X-Ray photoelectron spectra of the potentiostatically deposited film under overpotential deposition (OPD) conditions confirm the formation of a Ti–Al alloy phase.

Electrons in image states near roughened metal surfaces

Electrons near roughened Ag and Au surfaces with chemisorbed dielectric overlayers of alkanethiol or alkaneselenol self-assembled monolayers are shown to move within the sulfur or selenium head-group layer on the metal terraces. The electrons exist in image states with respect to Ag or Au step edges. There is no substantial image force between the electrons and the terraces.

Surface structure of a fluorinated thiol on Au(111) by scanning force microscopy

Scanning force microscopy (SFM) was used to study the surface structure of CF 3CF 2(CH 2) 6SH (7,7,8,8,8-pentafiuorooctanethiol, PFOT) monolayers self-assembled from a 1 mM trichloroethene (TCE) solution on Au(111). We observed many depressions with several to 20 nm diameters and 0.24 nm depth. In addition, especially around the step edges of Au, mounds 0.3–0.8 nm in height and with lower friction and higher stiffness than those of other regions were observed. These properties would result from the difference in the molecular orientation between on the mounds and on the fiat parts. The molecular arrangement with the ( √3 × √3) R30° structure was observed on the terraces of Au(111). By comparing the film thickness and the molecular length, the tilt angle of the molecular chain was found to be 56–76° with respect to the surface normal. This large tilt angle allows the PFOT SAMs with the relatively large CF 3CF 2- end group to pack closely on the ( √3 × √3) R30° structure.

STM Observations of the Underpotential Deposition and Stripping of Pb on Au(111) under Potential Sweep Conditions

We report the observation of electrochemical deposition and stripping of a monolayer of Pb on the surface of Au(111) using an in situ scanning tunneling microscope. Sequential STM scans taken during slow (1 mV/s) potential sweeps show that the Pb layer forms by initial growth at Au step edges followed by Pb island nucleation on Au terraces. When the potential sweep is reversed to remove the Pb monolayer, the surface is restored to a state similar to the original, but the details of the Au topography are modified.