Comparative theoretical study of the structure and bonding of propyne on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of propyne with the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied by means of the generalised gradient approach of density functional theory using periodic slab models. For both surfaces, the most stable adsorption mode of propyne is di-σ/π mode where the hydrocarbon is σ-bonded to two metal atoms with some additional π bonding to a third adjacent surface atom. The adsorption geometry is a highly distorted propyne with the C₁ and C₂ in a nearly sp² hybridisation. Two equivalent surface structures have been found on Pt and Pd. These correspond to the adsorption on the fcc or hcp hollow sites. The adsorption energies on Pt(1 1 1) and Pd(1 1 1) are predicted to be ∼−197 and −161 kJ mol⁻¹, respectively. The electronic factors that control the chemisorption have been analysed by means of the projected density of states.     

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.     

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.     

Electrochemical deposition of PbTe onto n-Si(1 0 0) wafers

Electrochemical deposition of PbTe from 50 mM Pb(NO₃)₂ + 1 mM TeO₂ + 0.1 M HNO₃ solution onto n-Si(1 0 0) wafers was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM, XRD and EDX. Electrochemical behavior of n-Si(1 0 0) electrode in electrolytes 50 mM Pb(NO₃)₂ + 0.1 M HNO₃ and 1 mM TeO₂ + 0.1 M HNO₃ was also studied. No underpotential deposition (UPD) of Pb and Te onto n-Si was observed in the investigated systems indicating weak Pb–Si and Te–Si interactions. Deposition of Pb and Te on n-Si occurred with overvoltage via 3D island growth. Electrosynthesis of PbTe (NaCl-like structure, a = 0.650 nm) takes place due to codeposition of Pb and Te at potentials E > E_{Pb²⁺/Pb⁰} (lead UPD onto tellurium). Cathodic deposition of PbTe onto n-Si(1 0 0) is irreversible – there is no anodic current in the CV curve. Oxidation of PbTe on n-Si is observed only under illumination, when photoelectrons and photoholes are generated in silicon substrate.     

Adsorption of alkali metals on Ge(0 0 1)(2 × 1) surface

Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(0 0 1)(2 × 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge–Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.     

The influence of chloride on the initial anodic dissolution of Cu3Au(1 1 1)

We report a detailed in situ X-ray diffraction study of the influence of chloride on the atomic structure evolution at the solid-electrolyte interface during the selective dissolution of Cu from a Cu₃Au(1 1 1) surface immersed in 0.1 M H₂SO₄. We disclose that the formation of the initial ultrathin Au-rich (1 1 1) with an inverted stacking sequence, as recently observed at Cu₃Au(1 1 1) in contact with pure 0.1 M H₂SO₄, is strongly influenced by adding 5 mM HCl. The main finding is a negative shift of about 150 mV of the critical potential at which the ultra-thin Au-rich layer transforms into thicker Au islands. The presented results support the view that it is not a thermodynamic driving force, but rather the rate of surface diffusion that dominates the formation of the structures of the metallic layer.     

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.     

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.     

An ab initio study of CO adsorption on ceria(1 1 0)

Hartree–Fock and DFT calculations are reported for the CO/CeO₂(1 1 0) surface system. The electron density, electrostatic potential, atomic charges and projected electronic density of states have been calculated from an ECP-and-point-charge-embedded cluster model and is compared with periodic calculations. The agreement between the two surface models is reasonably good. A number of weakly bonding CO adsorption sites were found, with E_ads (BSSE-corrected) ranging from 0.01 to 0.22 eV per adsorbed molecule. The two most favourable sites are found in the vicinity of surface cerium ions, with the CO molecule oriented in a tilted fashion, C-end down. The surface-induced CO stretching vibrational frequency shifts on these sites are a redshift of ≈−30 cm⁻¹ and a blueshift of ≈25 cm⁻¹, respectively.     

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.     

Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR

Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO₄. We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H_ad). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H_ad is in fact a spectator in the HOR.     

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.     

A possible approach towards spin-polarized transport through single molecule magnets: Mn12 on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0)

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn₁₂ single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn₁₂ monolayers was studied. The results demonstrate that Mn₁₂ can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.     

A topographic view of the Pt(1 1 1) surface at the electrochemical interface in the presence of carbon monoxide

In this communication we present topographic images of the Pt(1 1 1) surface in CO saturated 0.1 M HClO₄, obtained by scanning tunneling microscopy.The topography presents two different structures, depending on the CO adsorption potential (E_ad = 0.15 V or E_ad = 0.5 V vs RHE). For adsorption at 0.15 V the system presents a heterogeneous appearance, which totally covers the surface and impedes the observation of steps on the substrate surface. When CO is adsorbed at 0.5 V large clusters forming chains along the steps are observed. These aggregates can be, tentatively, correlated with the H-bonded water structure suggested earlier on the basis of FTIR spectroscopy. The clusters have inhibitory effects on CO oxidation.     

Physicochemical properties of {1-methyl piperazine (1) + water (2)} system at T = (298.15 to 343.15) K and atmospheric pressure

Densities (ρ) and viscosities (η) of aqueous 1-methylpiperazine (1-MPZ) solutions are reported at T = (298.15 to 343.15) K. Refractive indices (n_D) are reported at T = (293.15 to 333.15) K, and surface tensions (γ) are reported at T = (298.15 to 333.15) K. Derived excess properties, except excess viscosities (Δη), are found to be negative over the entire composition range. The addition of 1-MPZ reduces drastically the surface tension of water. The temperature dependence of surface tensions is explained in terms of surface entropy (S^S) and enthalpy (H^S). The measured and derived properties are used to probe the microscopic liquid structure of the bulk and surface of the aqueous amine solutions.     

Simultaneous removal of copper(II), lead(II), zinc(II) and cadmium(II) from aqueous solutions by multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were used successfully for the removal of heavy metals from aqueous solution. Characterization techniques showed the carbon as nanotubes with an average diameter between 40 and 60 nm and a specific surface area of 61.5 m² g^?1. The effect of carbon nanotubes mass, contact time, metal ions concentration, solution pH, and ionic strength on the adsorption of Cu(II), Pb(II), Cd(II) and Zn(II) by MWCNTs were studied and optimized. The adsorption of the heavy metals from aqueous solution by MWCNTs was studied kinetically using different kinetic models. A pseudo-second order model and the Elovich model were found to be in good agreement with the experimental data. The mechanism of adsorption was studied by the intra-particle diffusion model, and the results showed that intra-particle diffusion was not the slowest of the rate processes that determined the overall order. This model also revealed that the interaction of the metal ions with the MWCNTs surface might have been the most significant rate process. There was a competition among the metal ions for binding of the active sites present on the MWCNTs surface with affinity in the following order: Cu(II) > Zn(II) > Pb(II) > Cd(II).     

Highly ordered adlayers of three calix[4]arene derivatives on Au(1 1 1) surface in HClO4 solution: in situ STM study

Highly ordered adlayers of three calix[4]arene derivatives have been prepared on Au(1 1 1) surface in HClO₄ solution. The adlayer structures are characterized by in situ scanning tunneling microscopy (STM). High-resolution STM images reveal the molecular orientation and packing arrangement in the ordered adlayers. All the molecules are upright on Au(1 1 1) surface in pinched cone conformation. The structural difference of these molecules is proposed to be from the different functional groups at the upper and lower positions. The structural models are tentatively proposed for three ordered adlayers.     

In situ study of nitrobenzene grafting on Si(1 1 1)-H surfaces by infrared spectroscopic ellipsometry

The binding of nitrobenzene (NB) molecules from a solution of 4-nitrobenzene-diazonium-tetrafluoroborate on a Si(1 1 1)-H surface was investigated during the electrochemical processing in diluted sulphuric acid by means of infrared spectroscopic ellipsometry (IR-SE). The grafting was monitored by an increase in specific IR absorption bands due to symmetric and anti-symmetric NO₂ stretching vibrations in the 1400–1700 cm^?1 regime. The p- and s-polarized reflectances were recorded within 20 s for each spectrum only. NB molecules were detected when bonded to the Si(1 1 1) surface but not in the 2 mM solution itself. Oxide formation on the NB grafted Si surface was observed after drying in inert atmosphere and not during the grafting process in the aqueous solution.