Au Facets Research Articles

Au-induced nanofaceting and the stoichiometry of the Si(7 7 5)–Au surface

The Au-induced changes in the surface morphology of a Si(1 1 1) sample miscut 8° towards [ 1 1 2 ¯ ] have been measured using room temperature scanning tunneling microscopy and low energy electron diffraction. Au coverages of less than 0.06 ML up to 0.43 ML have been investigated. In all cases Au adsorption produces dramatic changes in surface morphology. The Au-induced surface exhibits nanofacets with orientations that depend critically on the amount of Au deposited. Below 0.32 ML, the restructured surface always includes (7 7 5)-Au nanofacets suggesting that the (7 7 5)-Au facet is energetically preferred on this surface. The (7 7 5)-Au facet is oriented 8.5° from [1 1 1] towards [ 1 1 2 ¯ ] and is characterized by 1-d chains spaced 21.3 Å apart that run along the [ 1 1 ¯ 0 ] direction. By maximizing the surface area of the (7 7 5) facets and optimizing the associated diffraction pattern we determine that the (7 7 5)-Au reconstruction is optimized at 0.24 ML and corresponds to a stoichiometry of 1.5 Au atoms per 1 × 1 unit cell. We believe that the local Au coverage on the (7 7 5) facet is 0.24 ML, and that the deficit/extra of Au at any particular coverage is accommodated by non-(7 7 5) facets. For example at 0.06 ML the regions of step bunching observed on the clean surface are eliminated and Au-induced (7 7 5) and Au-free (1 1 1)7 × 7 facets are already visible. Up to 0.18 ML the non-(7 7 5) facet is Au free. Beyond 0.32 ML, the (7 7 5)-Au reconstruction is no longer stable and the extra Au is accommodated by the formation of higher angle facets with smaller chain spacings.

Oxygen adsorption on gold nanofacets and model clusters

We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra.

Crystal Structures and Growth Mechanisms of Au@Ag Core−Shell Nanoparticles Prepared by the Microwave−Polyol Method

Au@Ag core−shell nanocrystals have been synthesized by using a microwave−polyol method, and their growth mechanisms have been studied. When HAuCl4·4H2O was reduced in ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP) as a polymer surfactant, a mixture of triangular twin platelike, octahedral, and multiple-twinned decahedral Au nanocrystals, having only {111} facets, was prepared. When Ag+ was reduced by using these Au nanocrystals as seeds, triangular-bipyramidal, cubic, and rod/wire Ag shells, having {100} facets, were overgrown, respectively. It was concluded that morphology changes between Au cores and Ag shells arise from changes in the adsorption selectivity of PVPs from {111} facets of Au to {100} facets of Ag. Total space volumes of Ag shells overgrown on Au cores were on the order of cubes > triangular-bipyramidal crystals > rods and wires. These findings provide general information on the growth mechanism of face-centered cubic (FCC) types of single crystals in the presence of a pol...

Single-Crystalline Gold Microplates: Synthesis, Characterization, and Thermal Stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.

Anomalous Enhancement of Light Emission by Au Adsorption on a Si(001) Vicinal Surface

Anomalous enhancement of light emission from a Si(001) vicinal surface induced by Au adsorption was observed using a light detecting system combined with a transmission electron microscope. The Si(001) vicinal surface was separated into Si(001)5×3.2–Au terraces and Si(119)“8×2”–Au facets by Au adsorption. It was found that the electron-beam-induced light emission is highly polarized due to anisotropy of the surface structure. The emission intensity from the Si(001)–Au terrace is enhanced for the s-polarized direction parallel to the surface steps in the wavelength range of 270–450 nm, and reduced for the p-polarized direction. This result indicates that the Au adsorption on the Si(001) vicinal surface modifies the optical properties of the materials (in the present case Si), suggesting the formation of a new material phase on the surface.

Electrochemical Preparation of a Au Crystal with Peculiar Morphology and Unique Growth Orientation and Its Catalysis for Oxygen Reduction

Gold nanocrystals with a peculiar pin-like morphology have been obtained on glassy carbon (GC) electrodes by electrochemical deposition from 0.5 M solution containing 1.0 mM in the presence of cysteine as an additive. The amount of such electrodeposited Au particles as well as the particle size increased with the increase of deposition time . Crystalline facets of Au(100) and Au(110) were significantly enriched in the presence of cysteine during the electrodeposition, and the Au crystals grew along a unique crystalline orientation of Au(111). Finally, Au crystals (with the average diameter of ca. 100 nm and length of ca. 1660 nm at ) with a peculiar pin-like morphology were obtained, in contrast to the plump Au particles prepared in the absence of cysteine. The cathodic peak current density for the reduction in acidic electrolyte (0.5 M ) at the Au crystals-electrodeposited GC electrode prepared in the presence of cysteine was larger than that at the electrode prepared in its absence. This behavior could be reasonably explained in view of the higher specific surface area of the Au crystals prepared in the presence of cysteine than those prepared in the absence of cysteine.

Size and Crystallographic Orientation Controls of Gold Nanoparticles Electrodeposited on GC Electrodes

Gold nanoparticles were electrodeposited onto glassy carbon-electrodes (nano-Au/GC) in the presence of two different additives, namely, cysteine and iodide ions. The electrochemical characterization of the electrodeposited nano-Au/GC electrodes was performed via the measurements of the reductive desorption patterns of a thiol (e.g., cysteine) self-assembled monolayer as well as the cyclic voltammetric response toward the oxygen reduction reaction in alkaline medium. The structural characterization of the electrodeposited Au nanoparticles was performed by the scanning electron microscope. The nano-Au/GC electrodes prepared in the presence of 100 μM cysteine were surprisingly found to be enriched in the Au(100) and Au(110) facets and are characterized by a relatively big particle size up to 300 nm as well as low particle density (number of particles per unit area). The Au nanoparticles prepared in the presence of ions were found to be much enriched in the Au(111) facets and are characterized by a relatively narrow particle size distribution range (10-40 nm) as well as a high particle density. Analysis of the X-ray diffraction data revealed a significantly decreased ratio of Au(111) domains of the Au nanoparticles electrochemically deposited in the presence of cysteine. These preliminary results suggest a simple way to control the size as well as the preferential crystallographic orientations of gold nanoparticles. © 2004 The Electrochemical Society. All rights reserved.

Surface morphology of Au-adsorbed Si(001) vicinal surfaces studied by reflection electron microscopy

Surface morphology after Au-adsorption-induced faceting on a 4° off Si(001) vicinal surface was studied by reflection electron microscopy. The surface morphology changes as a function of temperature (780–940°C) and of deposition rate of Au. Steeper facets are formed at a lower temperature and a higher deposition rate. The kinetics and energetics in the system govern the surface morphology (facet orientation) and later become dominant at high temperature. The transition between the step bands and facets was also observed and it was found that the (119)-Au facet is stable at 920°C.

Observation of a new Au ( 111 ) reconstruction at the interface of an adsorbed C 60 overlayer

Scanning tunneling microscope studies of monolayer films of C 60 sublimed onto Au ( 111 ) facets reveal a new Au reconstruction at the C 60-Au ( 111 ) interface after annealing to a temperature of 700 K. C 60 forms two well-ordered hexagonal phases upon which is superimposed a striped texturing with a long-range height modulation of ≈ 1 Å. and a periodicity of 90–160 Å. This texturing is shown to arise from a modification of the reconstruction of clean Au ( 111 ) present at the C 60-Au interface. The observations indicate that the C 60-Au interaction reduces energetic factors favoring compression of the first Au layer resulting in a substantial increase in the unit cell of the Au reconstruction along 〈110〉 directions. This decompression of the first Au layer also favors the formation of a commensurate C 60 layer.

Detailed underpotential deposition of copper on gold(III) in aqueous solutions

In situ electrochemical scanning tunneling microscopy (ESTM) is applied to a gold(111) surface with atomic resolution in aqueous sulfuric add solutions. Real-space images of close-packed Au atoms are stably resolved on an Au(111) facet in a “pure” H 2SO 4 solution. It is concluded that (3 1 2 × 3 1 2 )R30° is the structure for the first underpotential deposition of copper on Au(111). The (3 1 2 × 3 1 2 )R30° is the only observable structure for a long period of time in the entire potential range between 0.18 V and 0.1 V vs. a saturated calomel electrode. Two distinctly different phase boundaries are found between nearly perfect (3 1 2 × 3 1 2 )R30° domains. The initial stages of the UPD of Cu have been investigated carefully.