In situ femtosecond spectroelectrochemistry of Au(1 1 1) in an aqueous chloride solution

In this paper we report the first in situ femtosecond spectroelectrochemistry experiment employing a broadband probe, allowing the measurement of ultrafast transient visible spectra with a fixed pump wavelength. We investigated in situ femtosecond transient reflectivity of a Au(1 1 1) electrode in contact with an aqueous KCl solution. The pump wavelength was set at 780 nm and a supercontinuum probe was employed, yielding ultrafast spectral information on the electron thermalisation dynamics in the range 450–650 nm. Electrochemical control allowed to investigate the dynamic response to different Cl^? adsorption conditions.     

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.     

The oxidation of trivalent chromium at Au(1 1 1) electrode

Electro-oxidation of Cr(III)–Cr(VI) species was investigated on the (1 1 1) plane of a gold single crystal in a highly alkaline solution. This process was found to be structure sensitive due to the involvement of the OH⁻ anion in the rate-determining step. It was noticed additionally, that in the ionic system the lifting of the reconstruction could be due to the coadsorption of the anions from the supporting electrolyte and the anions supplied from the electro-oxidation process.     

Gap-mode SERS studies of azobenzene-containing self-assembled monolayers on Au(1 1 1)

Self-assembled monolayers of azobenzene-containing thiols on smooth Au(1 1 1) surfaces were studied by gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS). By adsorption of colloidal Au nanoparticles on top of the organic adlayer highly reproducible spectra with strongly enhanced intensities are obtained. The observed bands indicate a trans conformation of the azobenzene moieties and are in agreement with structural data for the molecular layer. A characteristic dependency on the terminal and the spacer groups of the molecules is found. Samples prepared during illumination with UV light show pronounced spectral differences that can be attributed to azobenzene in cis conformation.     

Modelling of benzene-1,4-dithiol on a Au(1 1 1) surface

The bonding of benzene-1,4-dithiol to a Au(1 1 1) surface is studied using a Au₂₅–SC₆H₄SH model. We find that the most favorable structure has the S directly above an Au atom, i.e., an on-top site. If the molecule is constrained to be perpendicular to the surface, the on-top site is the least favorable site and the threefold hollow is the most favorable one.     

Surface structure for Au on (0 0 1) SrTiO3

First-principle calculations are performed to study the crystal structure, formation energies, and electronic structures of (0 0 1) SrTiO₃ surfaces with/without Au covered. The initiative Au additive layer is crystallized in a fcc structure with (0 0 1) face on SrO-terminated surface. The bimodal growth trend of Au on TiO₂-terminated surface is qualitatively consistent with the experimental observations. The defect structure of Au occupying the oxygen (O) vacancies of TiO₂-terminated surface is energetically favorable under oxygen-poor conditions, and a feature corresponding to gap states appear and the occupied Ti 3d states disappear.     

Electronic structure of the Au–Si(1 1 1) interface as a function of Au coverage

We have investigated the Au–Si(1 1 1) interface as a function of the Au coverage by the core-level photoemission spectroscopy. With increasing the Au coverage, the spectral features in the Si 2p core-level changed remarkably and some fine structures in both Si 2p and Au 4f spectra were observed. Based on the curve fitting analysis, the Si 2p and Au 4f spectra at more than 20 Å Au coverage were decomposed into three chemically different components, respectively. The assignments of their components were performed. In addition, we have compared these results for the Au–Si(1 1 1) interface with our previous study for the Au–Si(1 0 0) interface. It was found that the electronic structures for the Au–Si(1 1 1) interface is essentially identical to those of the Au–Si(1 0 0) interface except at the initial Au deposition.     

Modeling the environment influence on the anodic metal dissolution

The influence of the electrolyte composition on the anodic indium dissolution is studied by the RRDE method. A model is designed for the active dissolution of indium according to the hydroxide and anion mechanisms. The model is used for calculating polarization curves and the limiting current of the oxidation of intermediate ions of univalent indium on the ring electrode at different solution compositions and electrode rotation rates. The calculated curves agree well with the experimental ones, which confirms validity of the model. The paper illustrates the way one can analyze the active metal dissolution kinetics in the case of the hydroxide and anion mechanisms. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

EC-STM observation on electrochemical response of fluidic phospholipid monolayer on Au(1 1 1) modified with 1-octanethiol

Electrochemical scanning tunneling microscopy (EC-STM) was applied to observe phospholipid layers over thiol-modified gold substrates as a model biological cell membrane. On a monolayer of 1-octanethiol on Au (1 1 1), a synthetic lipid, 1,2-dihexanoyl-sn-glycero-3-phosphocholine, was introduced in a neutral 0.05 M NH₄ClO₄ buffer solution. The lipid molecules formed a fluidic layer at 0.0 V vs. RHE of the substrate electrode potential. By cycling the electrode potential between +0.2 V and −0.2 V, the lipid layer reversibly changed over between the fluidic phase and a striped/grainy structure. This structural change might involve partial decomposition and oligomerization of phospholipids. This method will contribute for molecular biology by revealing the nanometer-scale structure of cell membrane.     

NEXAFS study of a Pt-containing rod-like organometallic polymer (Pt-DEBP): molecular orientation onto HOPG, Au/Si(1 1 1), Cr/Si(1 1 1) and Si(1 1 1) surfaces

The influence of different substrates on the molecular orientation of organometallic polymer Pt-DEBP, [Pt(PBu₃)₂CCC₁₂H₈CC]_n, has been investigated by NEXAFS spectroscopy. Thin films were deposited on HOPG, Au/Si(1 1 1), Cr/Si(1 1 1), Si(1 1 1) and stainless steel. The assignment of the spectral features has been carried out on the basis of previous STEX calculations performed on phenylacetylene model molecule in gas phase and adsorbed on Pt(1 1 1) and Cu(1 0 0). Angular dependent analysis of the π* resonance occurring at 285.50 eV photon energy deriving by the benzene carbon orbitals showed a polarisation effect for all substrates. A preferential molecular orientation at nearly 40° to the surface normal was observed. This result might be explained by the strong interaction between sp and sp² carbons of the organic diethynylbiphenyl DEBP moiety contained in close chains, leading to polymer self-assembling.     

STM tip-induced nanostructuring of Zn in an ionic liquid on Au(1 1 1) electrode surfaces

In this communication, the “jump-to-contact” based STM tip-induced nanostructuring is extended to BMIBF₄ ionic liquid for the first time. It is demonstrated successfully that Zn, as an example of less noble metal and being hard to deposit from aqueous solutions, can be nanostructured on Au(1 1 1) surfaces in the ionic liquid. Due to the large effective tunnel barrier in the ionic liquid, the Z-pulse required to create Zn nanoclusters in ionic liquid is about twice as large as for Cu nanoclusters of similar size in aqueous solutions. Patterns as well as large-scale arrays consisting of 100 × 100 Zn nanoclusters have been produced. The present work demonstrates the feasibility for surface nanostructuring a new category of systems that have not been possible in aqueous solutions, which could open up new opportunities for studies of nanoscopic effects from various aspects.     

In situ and real-time surface differential diffraction (SDD) study of Cd underpotential deposition on Au(1 1 1)

In situ and real-time surface differential diffraction (SDD) has been used to study the underpotential deposition (UPD) of Cd on Au(1 1 1) in sulfuric acid media. Comparison of SDD results in sulfate electrolytes with and without the presence of Cd²⁺ ions reveals that the surface reconstruction associated with the sulfate adsorption and desorption dominates the structural effect. It is also found that the reconstructed gold surface is stable upon Cd UPD process. In the initial stages of UPD, Cd atoms bind to the surface in bridge sites. This is followed by an adlayer structure with Cd adsorption in threefold hollow sites before Au/Cd intermixing takes place.     

Hot band spectroscopy of CCO in the C–O stretching region: The (ν1 + 2ν3) − 2ν3 and (2ν1 + ν3) − (ν1 + ν3) bands

Two C–O stretching hot bands, (ν₁ + 2ν₃) − 2ν₃ and (2ν₁ + ν₃) − (ν₁ + ν₃), of the CCO radical in the ground electronic state were measured. These hot bands are red shifted by approximately 70 cm⁻¹ compared to the C–O stretching fundamental. CCO was produced in a discharge through a flowing mixture of carbon suboxide and helium. The spectra were recorded using a diode laser spectrometer. The band origins were determined to be 1904.32512(62) and 1902.69130(56) cm⁻¹ for (ν₁ + 2ν₃) − 2ν₃ and (2ν₁ + ν₃) − (ν₁ + ν₃), respectively. The measurements in this band together with previously reported frequencies in the C–C and C–O stretching regions were analysed to determine harmonic frequencies and anharmonicity constants.     

The interfacial chemistry and energy level structure of a liquid crystalline perylene derivative on Au(1 1 1) and graphite surfaces

We compare the energy level structure of ordered monolayers of N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) on Au(1 1 1) and highly oriented pyrolytic graphite (HOPG). Both ultraviolet photoemission spectroscopy and scanning tunneling spectroscopy measurements show the formation of an occupied interface state for PTCDI-C13 on Au(1 1 1), whereas no such state is observed for PTCDI-C13 on HOPG. Comparative density functional theory calculations for various isolated perylene derivatives in the neutral and anionic states are unable to explain differences in observed interface state formation, which suggests that the interface state for PTCDI-C13/Au(1 1 1) is a consequence of a weak chemical interaction of this molecule and the Au substrate.     

Stability of underpotentially deposited Ag layers on a Au(1 1 1) surface studied by surface X-ray scattering

Stability of underpotentially deposited (upd) Ag layers on Au(1 1 1) surface was investigated by surface X-ray scattering (SXS). While the complete pseudomorphic Ag bilayer on Au(1 1 1) surface obtained by upd at 10 mV (vs. Ag/Ag⁺) was maintained its structure even after the circuit was disconnected and the surface was exposed to ambient atmosphere, the pseudomorphic Ag monolayer obtained by upd at 50 mV was converted to a partial bilayer with the coverage of 0.66 and 0.46 ML for the 1st and 2nd layer, respectively. These results show that Ag bilayer is structurally more stable than Ag monolayer on Au(1 1 1) and Ag atoms of the upd monolayer move around on the Au(1 1 1) surface without potential control.     

Spontaneous deposition of Sn on Au(1 1 1). An in situ STM study

The tin adlayer formed by spontaneous deposition on Au(1 1 1) was characterized by cyclic voltammetry and in situ scanning tunneling microscopy (STM) in sulphuric acid solution. Cyclic voltammetry measurements showed oxidation peaks in the potential range −0.60 ⩽ E/V vs SSE ⩽ 0, which can be ascribed to the dissolution of the Sn adsorbed layer. STM images of the Au(1 1 1)/Sn modified surface showed that tin nucleated both on step edges and on the flat terraces forming two-dimensional islands. The anodic polarization of this modified surface produced the gradual dissolution of the Sn adlayer which was evidenced by the formation of some holes and the reduction of the initial terraces to many small islands. STM images with atomic resolution obtained on these islands displayed an hexagonal expanded atomic structure. After the anodic stripping of this Sn adsorbed layer the images exhibited the typical Au(1 1 1) terraces with a (1 × 1) atomic structure. However, at more anodic potentials another dissolution process was observed producing noticeable changes on the surface morphology which could be ascribed to the dissolution of a Au–Sn surface alloy.