Polycrystalline Gold Surface Research Articles

Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes.

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Noble Metal Nanoparticles Networks Stabilized by Rod‐Like Organometallic Bifunctional Thiols

AbstractRod‐like organometallic dithiol containing square‐planar Pt(II) centers, i. e., trans,trans‐[(H3COCS)Pt(PBu3)2(C≡C−C6H4−C6H4−C≡C)(PBu3)2Pt(SCOCH3)] was used as bifunctional stabilizing agent for the synthesis of Pd‐, Au‐, and AgNPs (MNPs). All the MNPs showed diameters of about 4 nm, which can be controlled by carefully modulating the synthesis parameters. Covalent MNPs stabilization occurred through a single S bridge between Pt(II) and the noble metal nanocluster surfaces, leading to a network of regularly spaced NPs with the formation of dyads, as supported by SR‐XPS data and by TEM imaging analysis. The chemical nature of NPs systems was also confirmed by EDS and NMR. Comparison between SR‐XPS data of MNPs and self‐assembled monolayers and multilayers of pristine rod‐like dithiols deposited onto polycrystalline gold surfaces revealed an electronic interaction between Pt(II) centers and biphenyl moieties of adjacent ligands, stabilizing the organic structure of the network. The possibility to obtain networks of regularly spaced MNPs opens outstanding perspectives in optoelectronics.

Observing Real-Time Formation of Self-Assembled Monolayers on Polycrystalline Gold Surfaces with Scanning Electrochemical Cell Microscopy.

Self-assembled monolayers (SAMs) of alkanethiols on gold have become a central focus of controllable surface chemistry because they can be easily formed from the solution phase and characterized using various techniques. Understanding the formation processes occurring at a nanoscale level is crucial to form defect-free SAMs for tailored applications in bio- and nanotechnology. Although many reports study and characterize SAMs after they are formed on gold surfaces, typical methods have not extensively studied the SAM formation process at the nanoscale. This paper focuses on the formation of defect-free SAMs and elucidates the formation mechanism occurring at the nanoscale level during the formation process. Exploiting the strength of scanning electrochemical cell microscopy, we monitored SAM formation via a soluble redox reporter on a polycrystalline gold foil using voltammetric and amperometric techniques. We formed SAMs by varying the concentration of 3-mercapto-1-propanol [HS(CH2)3OH], 6-mercapto-1-hexanol [HS(CH2)6OH], and 9-mercapto-1-nonanol [HS(CH2)9OH] to determine the effects of the thiol chain length, concentration, and location on the substrate (grain boundaries) on monolayer formation. We observed real-time changes in the quasisteady-state current of our redox reporter during the self-assembly process. Importantly, we formed defect-free SAMs at the nanoscale level using different concentrations of HS(CH2)6OH and HS(CH2)9OH and found that SAM formation at the nanoscale is concentration-dependent and varies when at a boundary between two crystal grains.

Self-Assembled Monolayer of Monomercaptoundecahydro-closo-dodecaborate on a Polycrystalline Gold Surface.

In this work, we present an electrochemical study of the boron cage monomercaptoundecahydro-closo-dodecaborate [B12H11SH]2− in solution and in a self-assembled monolayer over a polycrystalline gold electrode. Cyclic voltammetry of the anion [B12H11SH]2− in solution showed a shift in the peak potentials related to the redox processes of gold hydroxides, which evidences the interaction between the boron cage and the gold surface. For an Au electrode modified with the anion [B12H11SH]2−, cyclic voltammetry response of the probe Fe(CN)63−/Fe(CN)64− showed a ΔEp value typical for a surface modification. Electrochemical impedance spectroscopy presented Rtc and Cdl values related to the formation of a self-assembled monolayer (SAM). A comparison of electrochemical responses of a modified electrode with thioglycolic acid (TGA) reveals that the boron cage [B12H11SH]2− diminishes the actives sites over the Au surface due to the steric effects. Differential capacitance measurements for bare gold electrode and those modified with [B12H11SH]2− and (TGA), indicate that bulky thiols enhance charge accumulation at the electrode–solution interface. In addition to electrochemical experiments, DFT calculations and surface plasmon resonance measurements (SPR) were carried out to obtain quantum chemical descriptors and to evaluate the molecular length and the dielectric constant of the Boron cage. From SPR experiments, the adsorption kinetics of [B12H11SH]2− were studied. The data fit for a Langmuir kinetic equation, typical for the formation of a monolayer.

On the electrooxidation of glucose on gold: Towards an electrochemical glucaric acid production as value-added chemical

The electrocatalytic oxidation of glucose to value-added chemicals, such as glucaric acid, has gathered increased interest in recent years. Glucose oxidation is a promising process which has the potential to contribute to establishing renewable resources as alternatives to fossil carbon sources. Herein, we present rotating disk electrode (RDE) studies on polycrystalline gold surfaces and subsequent Koutecký-Levich analysis as a benchmark to expand the understanding of reaction kinetics and competition between glucose, reaction intermediates and OH− at the catalyst surface. Based on the obtained results it follows that the glucose oxidation reaction (GOR) is predominately mass-transport controlled. Combining electrochemical studies and Raman spectroscopy, it is shown that increasing glucose concentrations lead to a delayed oxidation of the gold catalyst surface, presumably by increased consumption rates of Au-hydroxide species.

RETRACTED: Covalent functionalization mediated anchoring of gold nanoparticles on graphene oxide surface: A unique approach for electrode preparation

Abstract Now a days, the electrochemical approach is employed in various technological applications ranging from sensing to electrochemical energy storage/conversion. However, the efficiency of the process strictly depends on the type of electrode material and process of electrode fabrication. Here, graphene oxide surface is functionalized using 3-(mercaptopropyl)-trimethoxy silane (MPTS) and attached to polycrystalline gold surface (AuE) at room temperature. Thereafter, a hybrid electrode (AuE/FGO-AuNP) has been fabricated by anchoring the gold nanoparticles (Au NPs) onto the functionalized surface by simple dipping into the colloidal Au NPs solution under stirring condition. After the systematic characterization, the fabrication of the hybrid electrode has been validated by collecting the cyclic voltammetry and Nyquist impedance in a redox system (ferrocyanide/ferricyanide solution). Interestingly, the AuE/FGO-AuNP shows an efficient sensing platform for the oxidative detection of Nicotinamide adenine dinucleotide (NADH), an important co-enzyme. From this preliminary observation, it has been presumed that this process of electrode fabrication will be a potential approach for future electrochemical applications.

Large-Area, Two-Dimensional MoS2 Exfoliated on Gold: Direct Experimental Access to the Metal-Semiconductor Interface.

Two-dimensional semiconductors such as MoS2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal–semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal–semiconductor interface by large-area exfoliation of single-layer MoS2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS2–gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS2–gold interface, we can explain the anomalous Raman signature of MoS2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A1g mode with a reduced Raman shift (397 cm–1) due to the weakening of the Mo–S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS2–gold interface [Nano Lett.14, 2014, 1714]. As a consequence, the strength of the MoS2–gold contact can be determined from the intensity ratio between the reduced A1greduced mode and the unperturbed A1g mode.

Formation of HCN+ in collisions of N+ and N2+ with a self-assembled propanethiol surface on gold.

Collisions of N+ and N2+ with C3 hydrocarbons, represented by a self assembled monolayer of propanethiol on a polcrystalline gold surface, were investigated by experiments over the incident energy range between 5 eV and 100 eV. For N+, formation of HCN+ is observed at incident energies of projectile ions as low as 20 eV. In the case of N2+ projectile ions, the yield of HCN+ increased above zero only at incident energies of about 50 eV. This collision energy in the laboratory frame corresponds to an activation energy of about 3 eV to 3.5 eV. In the case of N+ projectile ions, the yield of HCN+ was large for most of the incident energy range, but decreased to zero at incident energies below 20 eV. This may indicate a very small energy threshold for the surface reaction between N+ and C3 hydrocarbons of a few tenths of an eV. Such a threshold for the formation of HCN+ may exist also for collisions of N+ with an adsorbed mixture of hydrocarbon molecules.

Unoccupied Electron States of Ultrathin Films of Thiophene–Phenylene Cooligomers on the Surface of Polycrystalline Gold

Unoccupied electronic states in the energy range from 5 to 20 eV above the Fermi level have been studied in ultrathin films of dimethyl-substituted thiophene–phenylene cooligomers CH3-phenylene–thiophene–thiophene–phenylene–CH3 (CH3–PTTP–CH3) on polycrystalline gold surfaces of two types: the ex situ Au layer thermally deposited in a special chamber and the in situ Au surface prepared inside an analytical chamber. The film structure is studied by the X-ray diffraction (XRD) method. The formation of a superposition of the amorphous phase and the crystalline phase with period 3.8 nm is discussed. The energy positions of the maxima of the unoccupied electronic states and the character of formation of the boundary potential barrier have been studied by the total current spectroscopy (TCS). The structures of the FSTCS maxima of the 5–7-nm-thick CH3–PTTP–CH3 films are not different when using various types of Au substrates and the ZnO semiconductor surface prepared by atomic layer deposition (ALD). As a CH3–PTTP–CH3 layer is deposited on the ex situ Au and in situ Au surfaces, the electron work function increases insignificantly (by ~0.1 eV) as the coating thickness increases to 5–7 nm. At such thicknesses of the CH3–PTTP–CH3 films, the electron work function is 4.7 ± 0.1 eV in the case of the ex situ Au substrate and 4.9 ± 0.1 eV in the case of the in situ Au substrate. A possible influence of the processes of physicochemical interaction at the boundary between the film and the substrate on the formation of the boundary potential barrier in the structures under study is discussed.

Decomposition of Carbon Tetrachloride on Gold Surfaces

Banned for already 20 years, carbon tetrachloride is still used industrially. The ingestion of the halogen compound (inhalation, dermal contact, drinking) is known to induce serious damage to the liver, for which gold particles might represent one of the most efficient treatment. Here, using the temperature-programmed desorption technique, we show that carbon tetrachloride is decomposed by a polycrystalline gold surface producing Cl2 at a minimum rate of 25% of the deposited molecules. The adsorption and decomposition explored via density functional theory (DFT) periodic calculations show that a cooperative decomposition of a pairwise CCl4 is energetically favorable on the (110) surface. Our results may contribute to develop better strategies for optimizing the design of gold nanosurfaces and improve the efficiency of the treatment of the hepatotoxicity induced by carbon tetrachloride.

Enhanced Oxidation of Uric Acid at Thiourea-Modified Gold Electrode in Alkaline Media

The surface of polycrystalline gold (Au (poly)) electrode was ex situ modified with self-assembled monolayer (SAM) of thiourea (TU) with a view of applying the electrode for the electrochemical oxidation of uric acid (UA) in alkaline media. UA undergoes an enhanced electrochemical oxidation at the monolayer of TU-modified Au (poly) (TU|Au (poly)) electrode at a remarkably lower potential with a higher current density as compared to the bare Au (poly) electrode. The enhanced activity of the electrode was achieved due to the effective blocking of harmful adsorption of UA that takes place strongly at the bare Au (poly) electrode surface. The TU|Au (poly) electrode was also found to be very effective towards simultaneous oxidations of UA, ascorbic acid (AA) and dopamine (DA) in their mixture with well separation of the peak current of the individual substance. This technique may be advantageously utilized for developing sensor for the purpose of detection of UA.

An electrochemical peptide-based biosensor for the Alzheimer biomarker amyloid-β(1-42) using a microporous gold nanostructure.

Alzheimer's disease (AD) is connected to aggregation of amyloid-β (Aβ) peptide and formation of insoluble plaques in the brain. Aβ level can be monitored as an AD early diagnosis route. In this study, an irregular shaped microporous gold nanostructure with a typical size of 150 × 250nm was electrodeposited on a polycrystalline gold surface at 0mV (vs. AgCl) using sodium alendronate. The nanostructure was then characterized by field-emission scanning electron microscopy. An electrochemical peptide-based biosensor was fabricated by immobilizing an Aβ(1-42)-binding peptide on the gold nanostructure. Binding of Aβ(1-42) by the peptide was followed electrochemically using ferro/ferricyanide as a redox probe. Differential pulse voltammograms in a potential range of 0-500mV (vs. AgCl) with typical peak potentials at 224mV are linear in the 3-7000pgmL-1 Aβ(1-42) concentration range, with a 0.2pgmL-1 detection limit. The biosensor is free of interferences and was applied to the quantitation of Aβ(1-42) in artificial cerebrospinal fluid and spiked serum samples. Graphical abstractSchematic presentation of an immobilized amyloid-β(1-42)-specific peptide on the surface of a microporous gold nanostructure to fabricate an electrochemical biosensor for early diagnosis of Alzheimer's disease. Aβ(1-42) capturing by the peptide led to repulsion of ferrocyanide/ferricyanide redox couple.

The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission

Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources.

Atomic Composition and Morphology of Thin Films of Resveratrol Deposited on Oxidized Silicon and Polycrystalline Gold Surfaces

The atomic composition of films of a polyphenol antioxidant, namely, resveratrol (RVL), with a thickness of up to 50 nm thermally deposited on an oxidized silicon surface is studied by the method of X-ray photoelectron spectroscopy (XPS). It is found that the surface area of pores in the RVL film is about 15% of the total surface area. The results of studying the stability of the RVL films when their surface is treated with Ar+ ions of 3 keV under the electric current of 1 μA passing through the sample for 30 s are given. The treatment gives rise to an increase in the area of pores to 30–40%, while the ratio of the concentration of C atoms to the concentration of O atoms in the RVL film both before and after the treatment of the surface with ions does not correspond to the chemical formula of RVL molecules. Using the method of atomic force microscopy (AFM) in contact mode with a scanning area size of about 10 × 10 μm, RVL coatings deposited on the oxidized silicon and polycrystalline Au surfaces are studied. It is found that the RVL films produce grainy and porous coatings on the substrate surfaces. The typical size of grains in the sample surface plane is 150–300 nm, and the characteristic elevation reaches 30 nm.

Selective Desorption of Ethylene after Dimethyl Sulfide Reaction on Cold Gold Surface

Ethylene is mostly generated from nonrenewable petroleum feedstocks. To reduce the environmental footprint, sustainable sources and less energy demanding methodologies are highly encouraged. Here we show that ethylene is produced from the reaction of dimethyl sulfide on the polycrystalline gold surface, maintained at 90 K. The ion detection ratio is measured to (8.5 ± 1.1)%, leading to a conversion rate of 52% in (450 ± 60)s. The reaction is investigated theoretically for three specific surfaces, Au(111), Au(110)(1 × 1) and Au(110)(1 × 3). The flat surface is found to be ineffective for conversion, while cooperative pairwise molecular mechanism is operative at the Au(110) surfaces. Pure ethylene is finally selectively desorbed from the surface by a thermal heating at 105 K. The laboratory findings can be applicable to nanoscale chemistry but may guide the optimization of larger scale methodologies.

The aggregation behaviour of 2H-imidazole-2-thione derivatives in solution, the solid state and over polycrystalline gold surface

The structure-forming properties of three 2H-imidazole 2-thione derivatives were investigated in solution, the solid state as well as over polycrystalline gold surface. The molecules were synthesized in one of our labs, and their interactions were investigated by a range of instrumental methods (FT-IR spectroscopy, FT-IR and FT-Raman microscopies, scanning electron and atomic force microscopies) as well as molecular modelling. Short-range (in solution; NH⋯S hydrogen bonding) as well as long-range ordering (in the crystalline form; SH⋯N hydrogen bonding) of the molecules were observed, and pillared-like structures were seen over the polycrystalline surface.

Study of the Wetting of Paraffin Films on the Metal Surface in the Dynamic Dip-Coating Process Using Compressive-Sensing Sum-Frequency Generation Microscopy

A compressive-sensing sum-frequency generation microscope (CS-SFGM) is used to investigate the adsorption of the interfacial dotriacontane (n-C32H66) molecules on polycrystalline gold surfaces in t...

Influence of argon ion beam etching and thermal treatment on polycrystalline and single crystal gold electrodes Au(100) and Au(111)

The effect of argon ion beam etching and subsequent annealing on the surface morphology and electrochemical response of poly- and monocrystalline gold electrodes was studied with the help of scanning electron microscopy, cyclic voltammetry and measurements of under-potential deposition of lead. While the samples were cleaned and restructured by the argon ion beam the following thermal treatment produced stable and very smooth highly ordered surfaces. The specific behaviour of polycrystalline and monocrystalline gold surfaces is discussed.

Nonlinear optical response of a gold surface in the visible range: A study by two-color sum-frequency generation spectroscopy. III. Simulations of the experimental SFG intensities.

We model the amplitude line shape and absolute phase of the infrared-visible sum-frequency signals produced by a thiolated polycrystalline gold surface as a function of the visible wavelength. We follow two hypotheses: in the interband scenario, the resonant features are attributed to interband transitions, whereas in the effective surface state scenario, they stem mostly from the excitation of surface transitions. We find that both scenarios lead to a satisfactory account of the experimental data and that only free electrons may spill out of the gold bulk, as expected. For the interband scenario, the balance between free and bound electron contributions to sum-frequency generation has to be adjusted to fit the data. The surface transitions are shown to take their origin inside gold and we investigate the surface states involved in such transitions, with a comparison to the silver surfaces. We finally provide a work program dedicated to discriminate between the two scenarios.

Assignment of NEXAFS Resonances in Alkanethiols and Their Implication on the Determination of Molecular Orientation of Aliphatic SAMs

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has become a widely used technique to analyze the molecular orientation of functional adlayers by analyzing the dichroism of NEXAFS resonances. In contrast to π-conjugated systems, which exhibit distinct π* resonances whose transition dipole moments are oriented perpendicular to the aromatic planes, the analysis of purely aliphatic systems is not as straightforward and requires a precise identification of the underlying resonances and their polarization. Here, we analyze the carbon K-edge NEXAFS signature and its dichroism of a self-assembling monolayer (SAM) formed by octadecanethiol (ODT) on gold, representing the widely studied class of alkanethiol SAMs. Employing density functional theory calculations and using the transition potential method, the polarizations of all NEXAFS resonances at excitation energies around and below the ionization threshold are precisely determined. This information is then used to simulate expected dichroism curves for various adlayer arrangements that are compared with our experimental findings for ODT SAMs on single-crystalline Au(111) and polycrystalline gold surfaces. This analysis demonstrates a novel strategy to precisely deduce the molecular orientation from the experimental NEXAFS data of aliphatic SAMs.