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The electrochemical behaviour and molecular structure of a layer of water-soluble 5,10,15,20-Tetrakis-(N-methyl-4-pyridyl)-porphyrin-Fe(III) pentatosylate, abbreviated as FeTMPyP, on a chloride modified Cu(100) electrode surface were investigated by means of cyclic voltammetry (CV) and in-situ electrochemical scanning tunneling microscopy. Voltammetric results of HOPG in an electrolyte containing FeTMPyP molecules indicate three distinguishable redox steps involving both the central iron metal and the π-conjugated ring system. However, only the first two reduction steps are observable within the narrow potential window of CVs of Cu(100) measured in the same electrolyte. In the potential range below the first reduction peak, at which the [Fe(III)TMPyP](5+) molecules are reduced to the corresponding [Fe(II)TMPyP](4+) species, in-situ scanning tunneling microscopy (STM) images revealed, for the first time, a highly ordered adlayer of this reduced porphyrin species on the chloride terminated Cu(100) surface. The ordered adlayer exhibits a (quasi)square unit cell with the lattice vectors |a→2|=|b→2|=1.53±0.1 nm and an angle of 93° ± 2° between them. A model is proposed based on the STM observation illustrating the arrangement of the [Fe(II)TMPyP](4+) molecules at the electrolyte/copper interface.

The redox behaviour and potential-dependent adsorption structure of heptyl viologen (1,1'-diheptyl-4,4'-bipyridinium dichloride, DHV(2+)) on a Cu(100) electrode was investigated in a chloride-containing electrolyte solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). The dicationic DHV molecules generate a few pairs of current waves in CV measurements which are ascribed to two typical one-electron transfer steps. STM images obtained in a KCl-containing electrolyte solution disclose a well-ordered c(2x2) chloride adlayer on a Cu(100) electrode surface. After injecting DHV(2+) molecules into the KCl electrolyte solution, a highly ordered 2D "dot-array" structure in STM images emerges on the c(2x2)-Cl modified Cu(100) electrode surface. DHV(2+) molecules spontaneously arrange themselves with their molecular planes facing the electrode surface and their long molecular axis parallel to the step edge. Such adsorption structure can be described by mirror domains and rotational domains which stably exist between 200 mV and -100 mV. One-electron reduction of the dications DHV(2+) around -150 mV causes a phase transition from a 'dot-array' assembly to a stripe pattern formed by DHV(*+) radical monocations in STM images which has a bilayer structure. With a further decrease of the applied electrode potential, the structure of the DHV(*+) adlayer undergoes a change from a loose stripe phase to a more compact stripe phase, a subsequent decay of the compact structure, and finally the formation of a new dimer phase. A further electron transfer reaction at -400 mV causes the formation of an amorphous phase on the chloride free electrode surface. In a reverse anodic sweep, the reproduction of the ordered DHV(*+) stacking phase occurs again on top of the chloride lattice.

The structure and reactivity of a Cu(100) single crystal electrode surface covered with free base meso-tetra (N-methyl-4-pyridinium) porphyrin (abbreviated as H(2)TMPyP) as a function of electrode potential have been investigated with cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (ECSTM), and UV-Vis and Raman spectroscopy. The well-ordered self-assembled layer of the porphyrin is consistent with the adsorption of the reduced porphyrin species after the first two-electron reduction step. The copper dissolution reaction in the presence of the stable self-assembled porphyrin layer starts at step edges on both upper and lower terraces and coincides with the preferential oxidation of reduced porphyrin species at step sites. The dissolved copper cations are incorporated into the free base porphyrin molecules leading to the formation of CuTMPyP. As a consequence this new species accumulates in the solution with time and a copper redeposition in the cathodic potential scan is lacking.

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.

The adsorption of sulfate anions ( S O 4 2 − ) and copper cations on Au(1 1 1) from CuSO 4 containing H 2 SO 4 solution was investigated by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). The underpotential deposition from solution results first in the formation of 2/3 monolayer of copper with the well known ( 3 × 3 ) structure of sulfate followed by the completion of the first copper monolayer at lower potentials. Just after formation of 1ML of Cu STM reveals a disordered surface. At more negative potential values, already in the overpotential deposition region, the STM images reflect a ( 3 × 7 ) sulfate structure on terraces of the first pseudomorphic Cu layer, a ( 3 × 3 ) -like structure on terraces of the second uncompleted Cu layer with sulfate anions residing in atomic vacancies, and the known Moiré structure on terraces of the multi-layer Cu deposit. The formation of the ( 3 × 7 ) sulfate structure on pseudomorphic monolayer Cu terraces on Au(1 1 1) is discussed in the context of results obtained for sulfate on bare Au(1 1 1) and Cu(1 1 1) electrodes in H 2 SO 4 solution. [ABSTRACT FROM AUTHOR]

Motivated by some incomprehensible aspects of the electro-deposition of copper on a Au(111) surface from copper sulfate containing sulfuric acid solution published in the literature, we have revisited this system using cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Specifically, we have examined the different stages of Cu deposition associated with 2/3 ML, 1 ML, supposedly 5/3 ML and multilayer coverages, and present a consistent structure model for the coadsorbed sulfate overlayers on the copper deposits of varying thickness. At a Cu coverage of 2/3 ML the coadsorbed sulfate anions exhibit the well known ( 3 × 3 ) R30° structure. In the regime where the full pseudomorphic Cu monolayer is supposed to exist first the formation of a disordered sulfate layer is found, which only at more negative potentials transforms into a ( 3 × 7 ) R19.1° sulfate structure (on the first completed Cu monolayer). In view of the ( 3 × 7 ) R19.1° structures found on both the Au(111) and the bulk Cu(111) surface a ( 3 × 7 ) R19.1° structure on a pseudomorphic Cu monolayer on Au(111) appears much more likely than a ( 3 × 3 ) R30° structure as claimed in the literature. Moreover, this ( 3 × 7 ) R19.1° structure on 1ML Cu/Au(111) could only be found at rather negative potentials and, in particular, low Cu concentrations in the solution. Further deposition of Cu then leads again to the formation of a ( 3 × 3 ) R30° structure on a 2/3 filled and slightly contracted second Cu layer, i.e. at a local copper coverage of 5/3 ML. Consistent with this contraction the STM images indicate a weak Moiré superstructure of the second layer. The formation of this ( 3 × 3 ) R30° structure is again rationalized by the adsorption of sulfate anions in atomic vacancies within the second Cu layer, similar to what was previously established for a copper coverage of 2/3 ML on Au(111). Cu islands, which exhibit multilayer thickness show the characteristic sulfate induced Moiré structure similar, but not identical, to that known from bulk Cu(111). [ABSTRACT FROM AUTHOR]

The co-adsorption of submono-, mono- and multi-layers of Cu with sulfate anions on a Au(111) electrode surface was investigated in electrochemical environment (0.1 mM CuSO 4 + 0.1 M H 2 SO 4 ) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). Correlated with the STM investigations the CV measurements indicate co-adsorption/-desorption processes of Cu of submono-, mono- and multi-layer coverages with sulfuric acid species on Au(111). The formation of a quasi-hexagonal Moiré superstructure on terraces of copper multilayers was observed in situ by STM. In detail the observed Moiré-structure is similar but not identical to the one observed on the (111) surface of bulk copper. High resolution STM images show the formation of a 3 × 7 -like sulfate structure on all multilayer copper terraces. [ABSTRACT FROM AUTHOR]

Exposing a Cu(100) electrode surface to an acidic and iodide containing electrolyte (5 m M H2SO4/1 m M KI) leads to the formation of an electro-compressible/electro-decompressible c(p × 2)-I adsorbate layer at potentials close to the onset of the copper dissolution reaction. An increase of mobile CuI monomers on-top of the iodide modified electrode surface causes the local CuI solubility product to be exceeded thereby giving rise to the nucleation and growth of a laterally well ordered 2D-CuI film at potentials below 3D-CuIbulk phase formation. Step edges serve as sources for the consumption of copper material upon compound formation leading to accelerated copper dissolution at the step edges. The 2D-CuI film exhibits symmetry properties and nearest neighbor spacings that are closely related to the (111) lattice of the crystalline CuIbulk phase. Intriguingly, the 2D-CuI film on Cu(100) does not act as an efficient passive layer. Copper dissolution proceeds at slightly higher potentials even in the presence of this binary 2D-compound via an inverse step flow mechanism. Further dissolution causes the nucleation and growth of 3D-CuI clusters on-top of the 2D-CuI film. This several nanometer thick 3D-CuIbulk phase passivates the electrode against further dissolution. Characteristically, the formation/dissolution of the 3D-CuIbulk phase reveals a significantly larger potential hysteresis of about Δ E = 320 mV while the appearance/disappearance of the 2D-CuI film is reversible with a potential hysteresis of only Δ E = 20 mV. [ABSTRACT FROM AUTHOR]

Abstract: We present a detailed analysis of the electronic and geometric bonding properties of the model alkene ethene on different mono- and bimetallic surfaces to establish the difference between adsorption energy and interaction energy and to elucidate the chemical character of a single platinum atom in different chemical environments. The adsorption of ethene on Pt(111) at 100K leads to two adsorption states, which are commonly described as being of di-σ-type (bidentate, μ2η2) and π-type (monodentate, μ1η2). While the later is the minority species on Pt(111) it is of larger abundance on the platinum alloys. We have chosen π-bonded ethene for our study since it can be found on Pt(111), the Pt3Sn and Pt2Sn surface alloys, and Cu3Pt(111). Density functional theory calculations of the adsorption structures, site and decomposed densities of states, as well as partial charge densities in conjunction with vibrational spectroscopy show that the bonding, i.e. the interaction energy, of the π ethene is only weakly influenced by alloying. Even in a copper matrix – as in the case of Cu3Pt(111) – the bonding platinum atom essentially keeps its chemical identity and the interaction energy is reduced by only 14% compared to Pt(111). This observation suggests that bonding on surfaces is a strongly localized phenomenon. However, the adsorption energy decreases significantly due to alloying, which is attributed to the varying local relaxation of the different metal surfaces. [Copyright &y& Elsevier]

Abstract: The electrooxidation of formic acid, potentially important for future fuel cells, was investigated by means of cyclic voltammetry (CV) and in situ infrared reflection absorption spectroscopy (IRRAS) both on a pure and on a copper-modified Pd(111)-electrode in sulphuric acid solutions. In situ IR spectra recorded under open-circuit conditions exhibit several vibrational bands in the carbonyl region characteristic of “free” and adsorbed formic acid as well as of decomposition intermediates. A detailed analysis of the intensity of the bands as a function of time leads to a reaction mechanism for the mere “catalytic” oxidation of formic acid at the Pd(111)/electrolyte interface. The “electro-catalytic” oxidation of formic acid under potential control starting above 0.35V is investigated by following the evolution of carbon dioxide as a function of electrode potential. IR measurements at different but constant potentials point to an electronic structure of the solid/liquid interface in the open-circuit HCOOH/Pd(111)–H2SO4 system which resembles that at an electrode potential of about 0.4V. Modification of the Pd(111) surface is achieved by copper deposition from a Cu2+/HCOOH/H2SO4 solution. Simultaneously, the influence of the foreign metal on the electro-catalytic oxidation of formic acid is studied in dependence on the potential scan direction. In the positive-going scan copper deposited on Pd(111) at cathodic potential inhibits the electro-catalytic oxidation of formic acid below 0.45V by blocking catalytically active Pd sites. Conversely, during the negative-going scan the Cu2+/HCOOH/Pd(111)–H2SO4 system shows an increased oxidation activity between 0.6V and 0.2V due to a direct redox reaction between copper ions and formic acid. The amount of copper deposited on the surface was determined by ex situ Auger Electron Spectroscopy after a contamination free sample transfer into UHV. [Copyright &y& Elsevier]

Abstract: The self-assembly of meso-tetra (N-methyl-4-pyridyl) porphyrine tetratosylat (H2TMPyP) on an iodide modified Cu(111) surface has been studied by means of in situ electrochemical-scanning tunneling microscopy (EC-STM). The I/Cu(111) surface was found to be a good substrate for the self-assembly of highly ordered layers of porphyrin cations. Furthermore, a reversible structural phase transition within the highly ordered porphyrin layer was observed during variation of the electrode potential, as a consequence of changing electronic properties of the substrate. [Copyright &y& Elsevier]

Abstract: The reversible formation of a 2D-CuI film on Cu(100) is studied by means of cyclic voltammetry in combination with electrochemical scanning tunneling microscopy. Exposing the Cu(100) electrode surface to an acidic and iodide containing electrolyte (5mM H2SO4/1mM KI) leads to the formation of a well ordered c(p ×2)-I adsorbate layer at potentials close to the onset of the anodic copper dissolution reaction. Copper dissolution starts at slightly more positive potentials preferentially at step edges in the presence of the iodide adlayer via the removal of copper material from kink sites at step edges. This increase of mobile Cu+ ions causes the local exceeding of the CuI solubility product (pKL=11.3), thereby giving rise to the nucleation and growth of a laterally well ordered 2D-CuI film. Key structural motifs of the growing CuI film are closely related to the (111) plane of bulk CuI. Quite intriguing, the 2D-CuI film does not act as a passive layer. Copper dissolution proceeds even in the presence of this binary compound via an inverse step flow mechanism. [Copyright &y& Elsevier]

Abstract: Combined cyclovoltammetry (CV) and electrochemical scanning tunneling microscopy (ECSTM) studies on the electrochemical behavior of a Cu(111) single crystal electrode in a mixed solution of equal volumes of 0.5mM H2SO4 and 1mM Na2S are presented. After an overview over all structural surface phases observed at different electrode potentials E, i.e. a chain-structure (E =+200 to −150mV versus RHE), a Moiré-structure (E =−50 to −300mV versus RHE) as well as a (√7×√7)R19.1°-S-(E =−100 to −450mV versus RHE) and a (2√7×2√7)R19.1°-S-structure (E =−300 to −650mV versus RHE), this paper focuses on a detailed description of the latter two. All phases are regular adsorbate structures on the surface and reversibly transform into each other. Except step alignment no reconstruction of the copper surface is observed. The unit cell of the √7-structure is rotated by ±19.1° with respect to that of the substrate lattice and contains, in contrast to a publication by Wang et al. (D. Wang, Q.M. Xu, L.J. Wan, C. Wang, C.L. Bai, Surf. Sci. 499 (2002) L159.), only one extra S-atom and therefore corresponds to a coverage of 0.286 (2/7)ML. Detailed atomistic models are proposed for the structure of the rotational domain boundaries in perfect agreement with the experimental observation. With decreasing electrode potential defects develop within the √7-structure due to partial sulfide desorption until a perfect 2√7-structure is formed. At given potential the creation and annihilation of defects occurs by an exchange process with the solution and not by intralayer hopping of adsorbed sulfur atoms. [Copyright &y& Elsevier]

The potential-dependent phase behavior of specifically adsorbed iodide on Cu(1 1 1) in 10 mM HClO4 containing either 0.1 or 0.05 mM KI has been studied using cyclic voltammetry in combination with in situ scanning tunneling microscopy. The adsorption of iodide on Cu(1 1 1) at negative electrode potentials close to the onset of the hydrogen evolution reaction leads to the formation of a (√3×√3)R30°-I structure. However, at more positive electrode potentials a transition to a uniaxially incommensurate (UIC) iodide adlayer takes place caused by a unidirectional compression of the (√3×√3)R30°-I phase. This compressed adlayer shows an additional one-dimensional long-range height modulation superimposed on the atomic corrugation. The process of compression can be described rather by the insertion of super-heavy domain-walls (shdw) into the (√3×√3)R30°-I phase than by a uniform and continuous compression of the adsorbate adlayer. Within the UIC phase regular (√3×√3)R30°-I regions are separated by broad and ‘fuzzy’ shdw instead of sharp dislocation lines as expected for ideal shdw. This observation points to a partial relaxation of these walls. Within these domain-walls the smallest nearest neighbor distance (NND) of 3.7 A˚ is significantly smaller than the van-der-Waals diameter of the uncharged iodine particle (3.9 A˚) while the NND within adjacent commensurate (√3×√3)R30°-regions exceeds the ‘Pauling’ diameter of the single charged iodide (I−) by 1.85%. Both the NND at domain-walls and the wave-length of the striped superstructure change with the applied potential. [Copyright &y& Elsevier]

Abstract: The behavior of specifically adsorbed bromide on Cu(110) in 10 mM HBr has been studied using cyclic voltammetry in combination with high resolution in situ electrochemical scanning tunneling microscopy (EC-STM). At cathodic potentials near the onset of the hydrogen evolution reaction the structure of the bare copper surface was observed by EC-STM. A variation of the electrode potential in positive direction causes a specific anion adsorption leading to the formation of a highly ordered superstructure with a quasi-hexagonal symmetry. The two-dimensional lattice of this adlayer can be described by a c(3×2) unit cell. The bromide anions are arranged parallel to the close packed copper rows and are located in three different types of adsorption sites. These inequivalently bound bromide species are imaged with different brightness in the STM-pictures. As a result the bromide adlayer exhibits a long-ranged wavy superstructure superimposed on the atomic corrugation. Tip induced copper corrosion is used to obtain highly ordered nanostructures due to a locally confined electrochemical annealing process proceeding along the [001]-direction of the substrate. This annealing process results in the formation of Cu(100)-facets. [Copyright &y& Elsevier]

Cyanate adlayers were investigated with in situ and ex situ infrared reflection absorption spectroscopy (IRRAS) on a Cu(1 1 1)-electrode. The cyanate layers were adsorbed in 5 mM KOCN + 0.5 mM H2SO4 solution and measured in cyanate-free as well as in cyanate-containing sulphuric acid. The in situ IR spectra exhibit an absorption band near 2270 cm-1 of N-bound cyanate that partly appears bipolar and also shows a potential dependent frequency shift between 30 and 80 cm-1 V-1 depending on the surrounding electrolyte and the immersion potential. Further analysis of the frequency and intensity of the band as a function of the electrode potential during the IR measurements points to cyanate desorption below 0.1 V and a rearrangement of the remaining molecules from on top positions at high surface coverages to bridge or hollow adsorption sites at lower coverages. Simultaneously parts of the remaining adsorbate lie down flat on the surface as the potential and therewith the coverage decreases.Two additional absorption bands at 2169 and 2220 cm-1 are detected which are assigned to solved cyanate and a copper complex in the thin layer solution.After transfer into UHV, the system was studied by means of ex situ IRRAS and Auger electron spectroscopy (AES). The ex situ IR spectra display a similar band at around 2250 cm-1 as the in situ spectra with a clear dependence on the emersion potential. This indicates the successful transfer of the electrochemical double layer including the potential dependent occupation of different adsorption sites. AES data show that K+ ions are replaced by H+ when rinsing the cyanate-covered surface with dilute sulphuric acid in the electrochemical cell. [Copyright &y& Elsevier]

The phase behavior of specifically adsorbed iodide on Cu(1 0 0) has been studied by means of in situ scanning tunneling microscopy in particular focusing on low coverage iodide phases.Using an aqueous solution containing 10 mM KClO4 and 1 mM KI an iodide coverage of Θ=0.4 ML is formed on Cu(1 0 0) at E=−200 mV vs. Ag/AgI. The resulting adsorbate structure can be understood in terms of a uniaxially expanded c(2 × 2) phase that can be described by a c(p × 2) unit-mesh with p=2.5. Characteristically a wavy long-range superstructure is superimposed on the atomic corrugation due to the uniaxial incommensurability of this adlayer.However, injecting only a small volume of the KClO4/KI containing solution into an iodide free electrolyte (10 mM HCl) significantly lower surface concentrations of iodide can be realized on Cu(1 0 0) at the same potential of E=−200 mV. For the first time a low coverage p(2 × 2) halide phase could be observed under electrochemical conditions. It is assumed that the interplay between a strong corrugation of the copper–iodide interaction on the one hand and long-range electrostatic interactions between neighboring adsorbate particles on the other hand allows a stabilization of this low coverage p(2 × 2)-I phase. [Copyright &y& Elsevier]

The structure and composition of a disordered Pt25Cu75(1 1 1) sample was studied by low energy electron diffraction. The r-factor, D1=0.096, illustrates a very good agreement between experimental and theoretical spectra. The Pt concentration versus depth oscillates around the bulk value (C1,C2,C3,C4)=(28,8,48,8 at.% Pt). The most striking result is the quite large and unusual Pt enrichment in layer 3 (C3=48%) followed by the still important Pt depletion in layer 4 as opposed to the damped oscillations observed for similar Pt–Ni(1 1 1) and Pt–Co(1 1 1) alloys. This result, which is in contradiction with a previous LEIS study, is interpreted within the Tight Binding Ising Model and is related to a termination of the L11 phase, which exists only in the Cu–Pt system. [Copyright &y& Elsevier]

The potential dependent phase behavior of specifically adsorbed iodide on Cu(1 0 0) in 10 mM HClO4/1 mM KI has been studied using cyclic voltammetry in combination with in situ STM. At positive electrode potentials near the onset of the copper dissolution reaction the electrode surface is covered by a uniaxially incommensurate iodide adlayer showing an additional one-dimensional long-range height modulation superimposed on the atomic corrugation. The atomic distances and the wave length of the striped superstructure change with the applied electrode potential indicative for an electrocompression process. The iodide layer close to the saturation coverage can be understood in terms of a uniaxially expanded c(2×2)-I structure. The resulting unit cell can be described as c(p×2)-I with p>2aCu.At saturation coverage a nearest neighbor distance of 3.7 A˚ is found which is significantly smaller than the iodine van-der-Waals diameter.In contrast to a uniformly expanded iodide layer at positive potentials, regular domain wall structures are observed at more negative potentials.At a certain potential close to the onset of the hydrogen evolution reaction a first order phase transition leads to the formation of a c(6×2)-I phase which remains stable even under massive hydrogen evolution. Dynamic STM measurements exhibit a preferential direction for this phase transition along the substrate 〈0 1 1〉 directions. The role of defects such as step edges for this phase transition is elucidated by high resolution STM images. [Copyright &y& Elsevier]