Au Submonolayer Research Articles

Enhancement of hydrogen response by forming an Au submonolayer on nanogap Pd nanoparticles

When Pd nanoparticles dispersed on a glass substrate with nanometer order gaps are exposed to H2 gas, H atoms are adsorbed on the nanoparticle surface, and the electrical resistance between the nanoparticles increases because of the tunneling current suppression. In contrast, when Au nanoparticles are exposed to H2 gas, the resistance remains unchanged because H atoms are not adsorbed on the Au surface. Considering these behaviors, the change ratio of the electrical resistance is expected to be smaller when the surface of Pd nanoparticles is partially covered with Au. However, the experimental results show the opposite resistance change. Density functional theory simulation indicates that H atoms are adsorbed and absorbed on the pure Pd surface, but H atoms are adsorbed and tend to remain on the partially covered Pd surface. These results indicate that the decrease in the resistance due to the gap narrowing by hydrogen absorption occurs in Pd nanoparticles, but it does not occur in Au/Pd nanoparticles, resulting in a larger resistivity increase compared with the Pd nanoparticles. This result implies that in certain cases, the low reactivity of Au to H2 contributes to the enhancement of the electrical resistance response.

Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO2(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO2, Au9/s-TiO2, and reduced (r) Au9/r-TiO2. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au9 clusters on stoichiometric TiO2 are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor.

Promoting Effects of Au Submonolayer Shells on Structure-Designed Cu-Pd/Ir Nanospheres: Greatly Enhanced Activity and Durability for Alkaline Ethanol Electro-Oxidation.

Rationally engineering the surface physicochemical properties of nanomaterials can improve their activity and durability for various electrocatalytic and energy conversion applications. Cu-Pd/Ir (CPI) nanospheres (NSs) anchored on N-doped porous graphene (NPG) [(CPI NSs/NPG)] have been recently demonstrated as a promising electrocatalyst for the alkaline ethanol oxidation reaction (EOR); to further enhance their electrocatalytic performance, the NPG-supported CPI NSs are coated with Au submonolayer (SML) shells (SMSs), through which their surface physicochemical properties can be tuned. CPI NSs/NPG is prepared by our previously developed method and possesses the special structures of composition-graded Cu1Pd1 and surface-doped Ir0.03. The Au SMSs with designed surface coverages are formed via an electrochemical technology involving incomplete Cu underpotential deposition (UPD) and Au3+ galvanic replacement. A distinctive volcano-type relation between the EOR electrocatalytic activity and the Au-SMS surface coverage for CPI@AuSML NSs/NPG is revealed, and the optimal CPI@Au1/6ML NSs/NPG greatly surpasses commercial Pd/C and CPI NSs/NPG in electrocatalytic activity and noble metal utilization. More importantly, its electrocatalytic durability in 1 h chronoamperometric and 500-cycle potential cycling degradation tests is also significantly improved. According to detailed physicochemical characterizations, electrochemical analyses, and density functional theory calculations, the promoting effects of the Au SMS for enhancing the EOR electrocatalytic activity and durability of CPI NSs/NPG can be mainly attributed to the greatly weakened carbonaceous intermediate bonding and properly increased surface oxidation potential. This work also proposes a versatile and effective strategy to tune the surface physicochemical properties of metal-based nanomaterials via incomplete UPD and metal-cation galvanic replacement for advancing their electrocatalytic and energy conversion performance.

Surface Structure Effects of High-Index Faceted Pd Nanocrystals Decorated by Au Submonolayer in Enhancing the Catalytic Activity for Ethanol Oxidation Reaction

Pd-based nanoparticles are considered as the most promising electrocatalysts for ethanol oxidation reaction (EOR) in alkaline direct ethanol fuel cells (DEFCs). However, the existing catalysts stil...

An exploration of the use of Au submonolayer decorated Pd7Ir nanoparticles as a highly active electrocatalyst for the ethanol oxidation reaction in alkaline media

An activity promotion over 130% from the use of Au submonolayer on carbon supported Pd7Ir nanoparticles as highly active electrocatalyst for EOR.

Differential reactivity of alkanethiols with Si(111)–Au 2D surface alloys

We study the differential reactivity of alkanethiols with submonolayers of Au on Si(111), where the surface is a mixture of the Au-rich 3×3 structure and the Au-poor 5×2 structure. We expose these binary 2D alloy samples to alkanethiols, while monitoring the induced spatial and structural transformations in real time using a low energy electron microscope (LEEM). Energy-dependent diffraction measurements show that, surprisingly, there is no alkanethiol growth on the Au submonolayers. Instead, S–Si interactions induce a disproportionation of the 5×2 structure into the unexpectedly inert 3×3 phase and a Au-depleted phase. We show how this process can be used to reduce the dimensionality of a 2D electron system to 1D.

Enhanced Oxygen Reduction Activity of Platinum Monolayer with a Gold Interlayer on Palladium

The activity of Pt monolayer supported on conventional (cuboctahedral) Pd nanoparticles for the oxygen reduction reaction was improved by 2-fold by introducing an Au submonolayer between the Pd core and Pt shell. By controlling the shape of the Pd cores (cubic and octahedral), we are able to distinguish the role of Au on the activity improvement at different facets. The Au interlayer can enhance the Pt monolayer activity toward oxygen reduction on both (100) and (111) surfaces, with the enhancement on (100) much more pronounced. The larger enhancement degree at the (100) sites may be due to the larger decrease of oxygen binding energy caused by Au interlayer (0.275 eV) than that at (111) sites (0.075 eV).

Titration of submonolayer Au growth on Si(111)

We study and analyze the growth of submonolayers of Au on Si(111) by a complementary set of surface techniques. Specifically, we focus on the $5\ifmmode\times\else\texttimes\fi{}2$ and the $\ensuremath{\alpha}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structures. We determine the gold coverage of these structures as a function of temperature by means of low energy electron diffraction (LEED) and low energy electron microscopy (LEEM). These results are independently calibrated by ex-situ ion scattering experiments. This allows us to present a phase diagram for this system. Remarkably, for all temperatures considered (820--1040 K), we find a coverage for the $5\ifmmode\times\else\texttimes\fi{}2$ phase that is significantly ($\ensuremath{\approx}10%$) higher than the value of 0.6 monolayers which is assumed in the latest structural models. Therefore, a further refinement of the present picture of the quasi-one-dimensional $5\ifmmode\times\else\texttimes\fi{}2$ structure is required.

Molecular and Atomic Hydrogen Interactions with Au−Ir Near-Surface Alloys

Using a combination of density functional theory and experimental electrochemical methods, we have investigated Au−Ir near-surface alloys and their ability to take up hydrogen (H). Despite the relative instability of H in the subsurface of pure Au(111) and Ir(111), H in the subsurface of a near-surface alloy made of a monolayer of Au over Ir(111) (Au*/Ir) is stabilized compared to surface H. While dissociative adsorption of H2 is activated on this alloy surface, the presence of subsurface H stabilizes the transition state for further H2 dissociation. This is explained by the upshift in the d-band center of the surface Au atoms induced by the presence of subsurface H in Au*/Ir. Submonolayers of Au on Ir(111) stabilize H at the Au−Ir interface while allowing for nonactivated H2 dissociation at the exposed Ir atoms. Synthesis of submonolayer alloys of Au on Ir(111) shows that Au does not block hydrogen adsorption on the Ir surface, despite the inability of pure Au to take up hydrogen under these conditions. The possible existence of subsurface hydrogen in these alloys may explain this unexpected behavior.

Electrical Conduction on Various Au/Si(111) Surface Superstructures

Submonolayers of Au on Si(111) exhibit various surface superstructures, 5×2, α-√3×√3, 2√21×2√21, 6×6, and β-√3×√3, depending on the Au coverage and heat treatments. We have succeeded in detecting the clear difference in temperature dependence of electrical conductivity among these Au/Si(111) surface superstructures by using the micro-four-point probe method. The conductivity of 6×6 is higher than that of the β-√3×√3 in the whole temperature range (130-300 K), though both surfaces have the same Au coverage, but are different in long-range order of atomic arrangement. The surface conductivity of both surfaces exhibit semiconducting temperature dependence of conductivity, while that of the α-√3×√3 show a metallic character, which is consistent with surface-state bands revealed by previous photoemission spectroscopy. [DOI: 10.1380/ejssnt.2005.497]

Direct observation of reaction-induced lateral redistribution of sub-monolayers of Au deposited on a Rh(110) surface

Abstract X-ray photoemission electron microscopy (XPEEM), combined with micro-low energy electron diffraction (LEED) and low energy electron microscopy (LEEM) were employed to study the dynamic changes occurring during the H 2 + O 2 reaction on a Rh(1 1 0) surface covered with a submonolayer of Au. It was found that the propagating reduction/oxidation fronts destroy the lateral homogeneity of the Au adlayer resulting in the development of a “patterned” interface consisting of Au-depleted and Au-enriched areas. The evolution of this reaction-induced morphologically complex interface was examined by varying the reaction conditions, in order to verify the mechanism of the observed Au mass transfer processes. The Au-patterned surface is preserved in oxidation ambient but is rapidly destroyed during reduction.

Three‐layer substrates for surface‐enhanced Raman scattering: preparation and preliminary evaluation

AbstractIn surface‐enhanced Raman scattering (SERS), the magnitude of enhancement is highly dependent on the nanometer‐scale morphological and optical properties of the substrate employed. This paper reports the development of an improved SERS‐active substrate that is based on a combination of nanoparticle assembly and evaporated thin films. Highly enhancing, three‐layer SERS‐active substrates were constructed by evaporation of an Ag film over an Ag‐clad colloidal Au submonolayer; the distinguishing feature of the three‐layer substrate is the formation of evaporated Ag islands between the surface‐bound colloidal particles. UV–visible absorption spectroscopy, atomic force microscopy and field emission scanning electron microscopy were used to characterize the substrate morphology. The thickness of the evaporated layer was varied in order to determine the optimal substrate architecture. The estimated enhancement factor for the optimized substrate architecture was 7 × 104. Furthermore, better than 15% reproducibility in the Raman spectra for adsorbed analytes was observed from spot to spot on a given substrate, and also from substrate to substrate. Finally, a sub‐picogram detection limit was demonstrated with the environmental pollutant 1,3,5‐triazine. Copyright © 2003 John Wiley & Sons, Ltd.

Alumina vapour condensation on Mo( [formula omitted]) surface and adsorption of copper and gold atoms on the formed oxide layer

Aluminium oxide vapour condensation on Mo(1 1 0) surface and subsequent adsorption of Cu and Au submonolayer films on the formed ultrathin oxide layer has been studied by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection–absorption infrared spectroscopy. AES and XPS data suggest that the oxide films formed in this way exhibit the properties characteristic for the bulk alumina, although the intramolecular bond weakening (up to 30%) compared to bulk oxide occurs at the submonolayer coverage. Both Cu and Au form 3D clusters on the surface of the oxide film regardless of its thickness (2–5 ML). At a very low coverage (<0.4 ML) Cu deposited on the oxide strongly interacts with the surface oxygen forming the species resembling the copper oxide(s). At a higher coverage the Cu deposits have metallic character. Unlike Cu, the latter, however, is the case for gold particles regardless of the coverage: No evidence of forming of the Au–O bonds was found at room temperature. Annealing of Au and Cu deposits on Al 2O 3 film results in metal/oxide intermixing and chemical reaction. The properties of the metals deposited on 2 and 5 ML thick oxide films are essentially the same at all coverages studied. The only observed difference is that the gold deposited on a thinner oxide film is characterised by a smaller average sizes of islands compared to Au deposited on a thicker oxide film. We tend to attribute this difference to higher defect density in thinner oxide film compared to thicker film, rather than to the influence of the underlying Mo(1 1 0) support.

Formation of Au fractal nanoclusters during pulsed laser deposition on highly oriented pyrolitic graphite

The topography of the Au submonolayer condensate formed by pulsed laser deposition (PLD) on pyrolitic graphite surface has been studied by scanning-tunneling microscopy (STM). PLD technique is characterized by an extremely high instantaneous deposition rate (up to ${10}^{22} {\mathrm{atoms}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}).$ The formation of Au fractal nanoclusters with $D=1.33\ifmmode\pm\else\textpm\fi{}0.08$ was observed with STM. The mechanism driving the formation of fractal nanoclusters on the surface at high deposition rate is suggested to be the result of the interacting-adatom initial-states evolution in the system far from thermodynamic equilibrium. The geometrical structure of the forming nanoclusters depends critically on the rate at which atoms arrive at the surface as well as on its lattice symmetry.

EFFECT OF LOCAL DIFFUSION COEFFICIENT VARIATION ON SUBMONOLAYER GROWTH

In this article Au was deposited on PVF(Polyviny formal) film which was kept at 253 K to grow Au submonolayer film. It is found that there are special areas whose morphology differs from other's experiment in which metal was deposited on single crystal. In these areas especially big clusters were found and they were surrounded by empty bands. Our deduction that this is caused by variation of local coefficient is proved by a modified DDA(Deposition-Diffusion-Aggregation) model which includes edge diffusion. We also argue that the relationship between cluster diffusion coefficient DS, and particles numbers S which the cluster contains should be exponential for Au cluster on PVF film instead of power-dependent in original DDA model.

Super strong effect of surface impurities on the resistance and Hall effect of quench condensed Cs films

Thin films of Cs, quench condensed onto a He cold quartz plate, possess electronic mean free paths which considerably exceed the film thicknesses. When theses films are covered in situ with sub monolayers of Au, Ag, In, Rh and Pb in the range between 0.01 and 1 monolayers the film resistance and the Hall constant increase dramatically. This corresponds to a scattering cross section of the surface impurities which is larger than 1 in units of Various models are explored to explain these unusual behavior. None yields a satisfactory explanation for this extremely strong effect of surface impurities on the resistance and the Hall effect.

Spatio-temporal pattern formation during catalytic CO oxidation on a Pt(100) surface modified with submonolayers of Au

The formation and develpment of spatio-temporal concentration patterns associated with the catalytic oxidation of carbon monoxide on a Pt(100) surface modified by deposition of sub monolayers of Au was investigated by means of photoemission electron microscopy (PEEM). The presence of Au atoms essentially reduces the adsorption probability for oxygen and the diffusion coefficient for chemisorbed CO, leading to alterations of the velocities for propagation of O- and CO-waves. At the boundaries between domains of differing Au content, phenomena of refraction and reflection occurred. The refraction obeys Snell's law, while total reflection as found in optics does not occur for chemical waves at less than the critical angle. The observed effects as well as characteristic differences to the previously studied Pt(110) surfaces were explained qualitatively on the basis of the elementary steps involved.

Photoemission study of Au overlayers on Pd(111) and the formation of a Pd-Au(111) alloy surface

The techniques of photoemission spectroscopy (PES), Auger spectroscopy, and low-energy electron diffraction were used to examine the interaction of Au and Pd for the following: (1) overlayers of Au on Pd(111), ranging from 0.1 to 10 monolayers, (2) Au diffused into the Pd matrix, and (3) a stabilized Pd-Au alloy surface, formed by diffusing a thick Au layer into the Pd until the composition stabilized. An ordered Au(111) layer formed on the Pd. For the Au overlayers, three effects were seen by PES: (1) structures appeared which merged smoothly into the metallic Au structure, (2) structures at the energies of the clean Pd peaks were sharpened when submonolayers of Au were evaporated, and (3) a new structure appeared which could not be definitely attributed to Pd or Au. Au diffused into the Pd evidenced a more atomiclike structure than the overlayers. The stabilized alloy, which was determined to have 30 at.% Au in the surface region, evidenced PES structure strikingly similar to the 0.3 monolayer overlayer. Direct transition effects were seen for the alloy. The Au-derived structure was found to narrow and shift to slightly lower binding energy for the alloy as compared to the 0.3 monolayer overlayer. Comparison is made between the thin layers of Au on Pd and thin layers of Cu on Zn. Comparison is also made between the stabilized Pd-Au alloy and the Cu-Ni alloy system. Theoretical support for direct transitions in alloys is cited.

Influence of Spin-Orbit Coupling on Weak Localization

The spin-orbit coupling modifies the weak localization of the conduction electrons in thin films and reverses the magnetoconductance. This is quantitatively demonstrated in Mg films covered with submonolayers of Au. The effect provides a new and direct method for measuring the spin-orbit coupling.