Electrocatalytic Reduction of Oxygen on a Pd Ad-Layer Modified Au(111) Electrode in Alkaline Solution

Oxygen reduction was studied on palladium, cadmium and zinc ad-atom modified single crystal Au(111) electrodes. The electrodes were modified by underpotential deposition process and their activity towards oxygen reduction was studied in alkaline media by voltammetry. The reduction peaks obtained were compared with those of bare Au(111), Pd disc and bulk deposited Cd electrodes. Enhanced catalytic activity of the Au(111) electrode in the presence of Pd, Cd and Zn ad-layer can be attributed to a change in surface charge and energy by ad-layer formation. In oxygen saturated medium a well defined sharp reduction peak was observed at ?0.12 V for 1/5 ML Pd ad atom modified Au(111) electrode while it was positioned at ?0.18 V on a Pd disk electrode. The best shift in reduction peak potential was obtained with 2/5 ML Pd ad atom modified Au(111) electrode with similar current density of Pd disc electrode.     

Adsorption characteristics of OH-terminated alkanethiol and arenethiol on Au(111) surfaces

Adsorption behaviors of hydroxyl-terminated alkanethiol, 3-mercapto-1-propanol (MPO), and arenethiol, 4-mercaptophenol (MPH), on Au(111) electrodes were studied by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM). The effects of the molecular structure and the chain length on the characteristics of the self-assembled monolayers (SAMs) were investigated by comparison with the results of 11-mercapto-1-undecanol (MUO) and 6-mercapto-1-hexanol (MHO) reported in the literature. All the alkanethiol SAMs have the same coverage ratio (0.33 ML). For MUO which has a longer chain length, a hexagonal lattice, the (√3 ×√3) structure was observed. However, for shorter thiols (MHO and MPO), the adsorbed molecules exhibit different contrasts under the imaging of STM, ascribed to the different conformations of adsorbed molecules. The CV results indicated that a longer chain length triggers a SAM with higher adhesion and higher resistance to the charge transfer across a SAM. For the SAM of arenethiol, MPH, the π-π stacking interaction of the phenyl ring leads to a lower surface mobility of thiol/Au complexes, lower coverage ratio, and less uniform structure of the adlayer. Furthermore, the vacancy islands commonly observed on alkanethiol-modified Au(111) electrodes do not appear on the MPH-modified surface. Instead, 2-dimensional patch islands formed on the terrace due to the aggregation of moveable MPH/Au complexes.     

硫醇分子在Au(111)表面吸附结构的DFT研究

采用分子模拟方法对硫醇分子在Au(111)表面上的吸附行为进行了研究。对于化学吸附过程,采用了密度泛函理论(DFT)的算法对甲基硫醇和2-巯基乙醇的吸附结构和能量进行了分析。优化结果显示在头基硫原子的多种吸附位置中bridge-fcc位是最稳定的,S-C键相对于金表面法线形成53~54°的倾斜角。对于全覆盖率下的自组装有序结构,采用了分子动力学方法比较了3种不同排布形式的稳定性,结果证明含有3种链取向的C(4×2)结构最稳定。另外,采用DFT方法对电子结构的分析显示,在bridge-fcc吸附位上,硫原子的p电子和邻近的金原子的d电子通过杂化形成了新的成键轨道和反键轨道,这一过程通过对硫和金原子反应前后的局部态密度的分析,进行了详细的论证。     

Initial stages of Pt deposition on Au(111) and Au(100)

The deposition of Pt onto unreconstructed Au(111) and Au(100) was studied with cyclic voltammetry and in-situ STM. The latter revealed that in [PtCl₄]²⁻ containing electrolytes, both surfaces are covered by an ordered adlayer of the complex. For the adsorbed [PtCl₄]²⁻ a slightly compressed (√7×√7) R19.1°-structure was assumed for Au(111) and a (3×√10) for Au(100). In both cases, a rather high overpotential for Pt deposition was observed, most probably due to the high stability of the [PtCl₄]²⁻ complex. Nucleation of Pt starts mainly at defects like step edges for low deposition rates and three-dimensional clusters are formed. Due to the high overpotential, some nuclei appear also on terraces at random sites. Higher coverages of Pt lead to a cauliflower like appearance. It is not possible to dissolve the platinum clusters at positive potentials without severely roughening the gold surface. The [PtCl₄]²⁻ complex is oxidized to the [PtCl₆]²⁻ complex at about 0.7 V, when metallic Pt is on the surface.     

Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunneling Microscopic Study

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold’s catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO₂ and O₂ dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO₂ decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.     

Light Emission from M-Type Enantiomer of 2,13-bis(hydroxymethyl)[7]-thiaheterohelicene Molecules Adsorbed on Au(111) and C60/Au(111) Surfaces Investigated by STM-LE

Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C₆₀-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C₆₀ layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C₆₀ buffer layers due to the small interaction of the molecules and C₆₀ islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C₆₀ layers grown on the Au(111) surface, leading to the fact that the helicene/C₆₀ multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.     

Creation of Low-Coordination Gold Sites on Au(111) Surface by 1,4-phenylene Diisocyanide Adsorption

The adsorption of CO on a saturated overlayer of 1,4-phenylene diisocyanide (PDI) adsorbed on a Au(111) surface at 300 K is studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations and reflection absorption infrared spectroscopy (RAIRS). The PDI forms closed-packed rows of gold-PDI chains by extracting gold atoms from the Au(111) substrate. They are imaged by STM and the structure calculated by DFT. The adsorption of CO is studied on the low-coordination gold sites formed on the PDI-covered surface where it adsorbs exhibiting a CO stretching frequency of 2004 cm^?1, consistent with adsorption on an atop site. It is found that CO is stable on heating the sample to ~150 K and is only removed from the surface by heating to ~180 K. Since low-coordination gold atoms are suggested to be the active catalytic sites on supported gold nanoclusters, ??embossing?? the surface to form similar low-coordination sites using PDI might offer a strategy for tailoring the catalytic activity of gold.     

In/Au(111)和Ir/Au(111)面上巴豆醛选择加氢的机理研究及比较

基于巴豆醛在M/Au（111）合金表面（M=In,Ir）垂直吸附的最稳定吸附结构,采用密度泛函理论对其不完全加氢的反应机理进行探究。从不同加氢机理下各基元反应的活化能、反应热计算以及构型变化分析中可知,巴豆醛在M/Au（111）面上均优先对距离合金表面较近的C＝O进行加氢,且以C为活性中心优先进行加氢为最优机理,其中第1步加氢反应的活化能较高,是该机理的控速步骤。反应物巴豆醛的O原子与合金的掺杂原子M形成较强的化学吸附,提高了M/Au（111）面对C＝O加氢的选择性。巴豆醛按最优机理加氢的基元反应中在In/Au（111）面上最高反应能垒为0.969 eV,比在Ir/Au（111）面的最高反应能垒1.332 eV低,因此认为In/Au合金对其不完全加氢有更好的催化活性。     

Decomposition of Furan on Pd(111)

Periodic density functional theory calculations (GGA-PBE) have been performed to investigate the mechanism for the decomposition of furan up to CO formation on the Pd(111) surface. At 1/9 ML coverage, furan adsorbs with its molecular plane parallel to the surface in several states with nearly identical adsorption energies of ?1.0?eV. The decomposition of furan begins with the opening of the ring at the C?CO position with an activation barrier of E _a ?=?0.82?eV, which yields a C₄H₄O aldehyde species that rapidly loses the ?? H to form C₄H₃O (E _a ?=?0.40?eV). C₄H₃O further dehydrogenates at the ?? position to form C₄H₂O (E _a ?=?0.83?eV), before the ???C?? C?CC bond dissociates (E _a ?=?1.08?eV) to form CO. Each step is the lowest-barrier dissociation step in the respective species. A simple kinetic analysis suggests that furan decomposition begins at 240?C270?K and is mostly complete by 320?K, in close agreement with previous experiments. It is suggested that the C₄H₂O intermediate delays the decarbonylation step up to 350?K.     

Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.     

Graphene growth on Pt(111) and Au(111) using a MBE carbon solid-source

In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene (G) on different metal surfaces: from a well studied substrate as platinum, to a substrate where it can only be formed using innovative methods, as is the case of gold. For the characterization of the graphene layers we have used in situ surface science techniques as low energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM).One of the main advantages of our methodology is that low surface temperatures are required to form graphene. Thus, by annealing Pt(111) and Au(111) substrates up to 650 °C and 550 °C respectively during carbon evaporation, we have obtained the characteristic LEED diagrams commonly attributed to graphene on these surfaces. STM results further prove the formation of graphene. For the case of G on Pt(111), STM images show a long range ordering associated with moiré patterns that correspond to a monolayer of graphene on (111) platinum surface. On the other hand, G/Au(111) STM results reveal the formation of dendritic islands pinned to atomic step edges. This method opens up new possibilities for the formation of graphene on many different substrates with potential technological applications.     

In situ FTIR study of 2-chloropyridine adsorbed on Au(111) and Au(110) electrodes

2-chloropyridine (2Clpy) adsorbed on Au(111) and Au(110) electrodes in 0.1 M KClO₄ containing 2 mM 2Clpy has been investigated using in-situ infrared spectroscopy and differential capacity measurements. For both the electrodes, an in-plane ring vibration due to N-bonded 2Clpy was observed, which was accompanied by a loss in the in-plane ring vibrations due to dissolved 2Clpy. However, the integrated band intensity ratios between the adsorbed 2Clpy and the loss of dissolved 2Clpy for both electrodes are quite different in all potentials measured. The relative band intensity of the N-bonded 2Clpy for Au(110) is much stronger than that for the Au(111). This indicates that for the Au(111) the adsorbed 2Clpy is mainly flat type or the tilt angle of the N-bonded 2Clpy is large. It can be explained by the configuration of adsorbed 2Clpy on the electrode surface. For Au(111) terrace, N-bonded configuration is obstructed by the presence of Cl atom. There is no obstruction for the Au(110) step if the N-bonded 2Clpy is adsorbed on the overtop layer.     

Effect of Au thickness on the evolution of self-assembled Au droplets on GaAs (111)A and (100)

In this paper, we report the effect of Au thickness on the self-assembled Au droplets on GaAs (111)A and (100). The evolution of Au droplets on GaAs (111)A and (100) with the increased Au thickness progress in the Volmer-Weber growth mode results in distinctive 3-D islands. Under an identical growth condition, depending on the thickness of Au deposition, the self-assembled Au droplets show different size and density distributions, while the average height is increased by approximately 420% and the diameter is increased by approximately 830%, indicating a preferential lateral expansion. Au droplets show an opposite evolution trend: the increased size along with the decreased density as a function of the Au thickness. Also, the density shifts on the orders of over two magnitude between 4.23 × 10¹⁰ and 1.16 × 10⁸ cm⁻² over the thickness range tested. At relatively thinner thicknesses below 4 nm, the self-assembled Au droplets sensitively respond to the thickness variation, evidenced by the sharper slopes of dimensions and density plots. The results are systematically analyzed and discussed in terms of atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), cross-sectional surface line profiles, and Fourier filter transform (FFT) power spectra.     

From the nucleation of wiggling Au nanostructures to the dome-shaped Au droplets on GaAs (111)A, (110), (100), and (111)B

In this paper, the systematic evolution process of self-assembled Au droplets is successfully demonstrated on GaAs (111)A, (110), (100), and (111)B. On various GaAs substrates, self-assembled Au clusters begin to nucleate at around 300°C, and then, they develop into wiggly Au nanostructures at 350°C. Between 400°C and 550°C, the self-assembled dome-shaped Au droplets with fine uniformity are fabricated with various sizes and densities based on the Volmer-Weber growth mode. Depending on the annealing temperature, the size including the average height and lateral diameter and the density of Au droplets show the opposite trend of increased size with correspondingly decreased density as a function of the annealing temperature due to the difference in the diffusion length of adatoms at varied activation energy. Under an identical growth condition, depending on the surface index, the size and density of Au droplets show a clear distinction, observed throughout the temperature range. The results are systematically analyzed and discussed in terms of atomic force microscopy (AFM) images, cross-sectional line profiles, and Fourier filter transform (FFT) power spectra as well as the summary plots of the size and density.     

Synergistic Effect in the Dehydrogenation of Cyclohexene on C/W(111) Surfaces Modified by Submonolayer Coverage Pt

The dehydrogenation and decomposition of cyclohexene on the Pt-modified C/W(111) surfaces have been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). The objective of the current study is to investigate how the surface reactivity of tungsten carbide is modified by the presence of submonolayer Pt. Similar to that observed on Pt(111), Pt(100) and C/W(111) surfaces, the characteristic reaction pathway on Pt/C/W(111) is the selective dehydrogenation of cyclohexene to benzene. At a Pt coverage of 0.52 monolayer, the selectivity to the gas-phase benzene product is 86±7%, which is slightly higher than that on Pt(111) (75%) and on C/W(111) (67±7%). More importantly, the desorption of benzene on Pt/C/W(111) is a reaction-limited process that occurs at 290 K, which is much lower than the benzene desorption temperature of 400 K from Pt(111).