Volume 221, Issue 9-10

By employing in-situ STM, we have studied precursor adsorption and initial stage of Sb UPD on Au(111) and Au(100) in a SbBr 3 -containing BMIBF 4 ionic liquid. It has been observed that the adsorption of SbBr 3 selectively takes place on Au(100), in contrast to that of SbCl 3 , with formation of chains with structure as well as squares with structure. The chains are composed of pairs of SbBr 3 molecules, which are bridged with the adjacent SbBr 3 molecules in the chain to form the anisotropy along either of the two characteristic directions of Au(100). The square configured structure is composed of periodically arranged monomeric and dimeric SbBr 3 , which will gradually replace the chain structure. Regardless of the precursor adsorption, however, the initial stage of Sb UPD is characterized by the formation of atomic strips, like in the SbCl 3 -containing system, which reveals the consistency of discharging behavior of Sb(III) from the SbCl 3 and SbBr 3 molecules. Together with the findings from the SbCl 3 -containing system, the results are discussed in view of matching of structures between the pyramidal antimony tri-halides and the crystalline surfaces. The role of the ionic liquid as well as properties of the electrified interface will also be mentioned.

The electroreduction of nitrate ions is investigated in acid and neutral aqueous solutions (HClO 4 and NaClO 4 as electrolytes) at polycrystalline copper electrodes, copper single crystals and at copper particles deposited in polyaniline (PANI) layers. In the presence of low nitrate concentrations (5 mM), the reduction of nitrate is not significantly different on various copper atomic surface structures but is greatly dependent on the local pH at the electrode. In contrast to strong acidic solutions, two separate waves are detected when nitrate ions are present in neutral solutions irrespective of the use of copper polycrystalline or single crystal electrodes. The first of the two waves leads to the formation of nitrite ions. When copper particles are dispersed in polyaniline layers it is demonstrated that the electrocatalytic activity is strongly dependent on the way of depositing copper in the polymer layer. A clear difference is observed in the current response in absence and presence of nitrate ions for copper deposited in the reduced state of the PANI layer, whereas copper deposited in the oxidized state of the PANI layer remains still electrocatalytically rather inactive. Copper crystalline species act effectively for the investigated reaction only if copper conducting paths are available through the polymer matrix up to the underlying metal surface.

When adsorption and desorption are the only electrode processes on an electrode, the voltammetry curves are symmetric to the potential axis and the charge vs. potential curve exhibits no hysteresis, provided that the sweeprate and adsorbate concentration are sufficiently small and large, respectively. If the latter conditions do not hold, the adsorption process becomes diffusion-controlled, and the voltammograms become asymmetric. Assuming Langmuirian adsorption, a procedure employing a combination of integration and semiintegration of the measured current is suggested by which the effect of diffusion can be removed from the measured voltammograms. The procedure is supported by numerical simulations.

A way is suggested for determining the effective three phase boundary width of electrodes on solid electrolytes from the difference between the electrolyte resistance of the primary and the secondary current distribution at a circular microelectrode. It is shown by means of numerical finite element calculations how the potential distribution in the solid electrolyte depends on the frequency and how the corresponding variation of the electrolyte resistance can lead to an additional semicircle in the complex impedance plane. An equivalent circuit is introduced for analyzing such a situation and thus for determining the additional ohmic resistance due to current constriction close to the three phase boundary region of the electrode. This additional resistance is quantitatively related to the effective three phase boundary width and thus can serve as a tool to estimate the size of the electrochemically active region of electrode reactions such as oxygen reduction or hydrogen oxidation on oxide ion conducting solid electrolytes.

Rigid carbon nanotube (CNT) polymer composites were characterized as bare electrochemical transducers and as support for (bio)functionalization. In order to accomplish that, cyclic voltammetry and impedance spectroscopy in presence of benchmark redox systems were performed. The response and sensitivity can be enhanced if the density of edges is increased in the CNT systems. Bamboo-like CNTs seem to be the best choice for composite electrodes. Additionally, the CNT electrodes result in a very robust support for chemical functionalization. Direct electron transfer of myoglobin has been verified without the need of electrochemical mediators and without compromising the biomolecule activity. Finally, some other exciting applications of CNT electrochemistry at the nanoscale regime are also highlighted.

In a search for electrocatalysts based on other metals with comparable attributes to very scarce and expensive Pt, we studied the kinetics of the oxygen reduction reaction (ORR) on carbon-supported Pd 3 Fe alloy nanoparticles in HClO 4 and NaOH solutions. In acid solution, the electrocatalyst's activity for the ORR is slightly higher than that of commercial Pt/C. The reaction kinetics involves predominantly a four-electron reduction with the first charge-transfer step being the rate-determining one. The synthesis of a Pd overlayer on the Pd 3 Fe alloy's surface at elevated temperatures is due to strong segregation effects. The major cause of this electrocatalyst's high activity appears to be the downshifting of the d -band center of Pd overlayer, resulting in a weaker interaction between the oxygen-containing species and the Pd surface. We demonstrated a further enhancement of the ORR kinetics with a Pt monolayer-covered Pd 3 Fe/C electrocatalyst. In HClO 4 solution, the mass-specific activity of Pt/Pd 3 Fe/C was about five times higher than that of commercial Pt/C, surpassing the Pt/Pd/C. In situ XANES data indicate that this elevated ORR activity may be due to the decreased formation of PtOH, and weaker oxygen adsorption on Pt/Pd 3 Fe/C.

The kinetics of the electrocrystallization of nanostructured gold is investigated on both rotating disc and stationary glassy carbon electrodes. A possible mechanism of gold deposition is presented. A general expression of the current-time transient for the superposition of adsorption, three-dimensional nucleation and mixed charge transfer/diffusion controlled growth based on existing theories is reported for the kinetics on stationary electrodes. The experimentally recorded current-time transients are analyzed in terms of this model. Kinetic parameters are determined and their consequence is discussed.

A simple, first principles-based model of the oxygen reduction reaction (ORR) is used to determine ORR kinetics on the (111), (100), and (211) facets of eleven transition metals (Au, Ag, Pt, Pd, Ir, Cu, Rh, Ni, Ru, Co, Fe). For most metals, the unreconstructed (100) facets are found to have an activity comparable to, or slightly higher than, the (111) facets. In contrast, (211) steps are found to be significantly less active than the terraces, with the exception of the most noble metals. These results are combined with simple models of the geometries of catalytic nanoparticles to estimate the average ORR activity of Pt and Au nanoparticles of various sizes. On Pt, a modest decrease in the activity with decreasing particle size is predicted, while for Au, the opposite trend is found.

A new mechanism of metal deposition on conducting surfaces is presented. But in contrast to former procedures where metal ions were deposited by electrolysis on a conducting cathodically polarised surface, now the deposition occurs without any additional electrochemical energy even if the substrate metal is more noble than the deposited metal. This phenomenon can be explained by the formation of a microgalvanic cell as a consequence of the tip reaction. The process is similar to the effects of a local element well known in corrosion science. The latter one is an undesirable effect due to surface impurities whereas the former one can be targeted on generating microstructures. In this paper, we will show that the mechanism of deposition is congenerous to that one of the positive feedback, since both may occur on a nonpolarised conducting surface. Both are “tip-induced” electrochemical cells with a positive electromotive force.

The covalent attachment of a sol–gel thin film onto a glassy carbon electrode has been accomplished through the electrochemical reduction of 4-hydroxyphenyldiazonium. The latter forms a very thin layer on the glassy carbon with hydroxyl groups, through which the sol–gel is attached. We find that this “molecular glue” improves the adhesion of thin layers of sol–gel to a carbonaceous surface.

The electrodissolution of metal electrodes exhibits often sustained oscillations of the current density. We study the spatio-temporal dynamics of a metal disk electrode embedded in an insulator in a cylindrical electrochemical cell with a ring-shaped CE in the oscillatory region. The chosen cell geometry introduces a strong radial parameter dependence into the system. For low conductivity the dynamics is spatio-temporally chaotic. A small aspect ratio of the cell supports 2-dimensional space-time chaos on the entire disk electrode. For larger aspect ratios and thus larger parameter gradients, however, we observe a ‘self-confinement’ of the turbulent dynamics to the central part of the disk electrode, the outer ring of the electrode exhibiting angularly uniform oscillations. It can be expected that the state of confined chaos is not restricted to electrochemical systems but can be established also in other dynamical systems by introducing an appropriate parameter gradient.

The electronic attributes of an electrochemically deposited copper monolayer on Au(111) were compared to its bulk equivalent by XPS experiments. Due to the modified interaction between the Cu-Cu as well as Cu-Au atoms at the surface a core level shift to lower binding energies occurs, which exceeds the values found for surface core level shifts of Cu atoms on Cu(100) surfaces. It could be shown that the thymine orientation on a copper layer depends on the externally applied potential in analogy to the adsorption behaviour on Au(111) and Ag(111) single crystals. The transition between a chemisorbed and a physisorbed phase occurs around the potential of zero charge. Independently of the adsorption state of thymine the electron density around the surface copper atoms is reduced by the interaction between thymine molecules and the copper surface. At electrochemically deposited silver monolayers, bilayers as well as at bulk silver on Au(111) different orientations and adsorption states of thymine could be evaluated. A comparison between XPS and electrochemical data reveal an alteration of the thymine–Ag interaction within the chemisorbed thymine layer deposited on 1 ML Ag/Au(111). It is notable that the transformation between the chemisorbed and physisorbed thymine phase does not seem to be dependent on the thickness of the silver adlayers, while the potential of zero charge differs by about 200 mV. In contrast, the onset of the hydrogen evolution shifts to lower potentials with increasing thickness of the Ag layers.

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.

Kinetic Monte Carlo (KMC) simulations were carried out to simulate kinetic-limited electrodeposition of a metal (M) onto an array of pre-existing metal clusters on a substrate (S) of a second conducting material. Electrochemical reaction and surface diffusion were accounted for in a KMC code which tracked deposit growth with a (2+1)-dimensional approach. Beginning with various arrangements of ten-atom metal seed clusters on a substrate platform of 300 × 300 fcc lattice sites, KMC simulations were carried out to investigate the evolution of the surface morphology. The influence of the number (spacing) of pre-existing seed clusters, the applied potential, and the metal–substrate surface diffusion energy barrier were investigated. It was found that when 16 or fewer seed clusters were present on the surface prior to electrodeposition, the resulting nucleation distribution was dominated by secondary nuclei formed during deposition. For substrates with a metal–substrate surface diffusion energy barrier greater than 3.5 × 10 -20 J, it was more difficult to control the uniform growth of the seed clusters owing to the nucleation of secondary clusters. At lower applied potentials it was found that larger nuclei could be grown with a more controlled size distribution because fewer secondary nuclei were formed. Furthermore, it was found that larger clusters with a more controlled size distribution can be grown when more clusters are seeded onto the surface because the deposited atoms were more likely to attach to existing clusters, than to form secondary nuclei.

The intensity of monochromatic light (λ = 633 nm) reflected off the surface of a polycrystalline Pt electrode, R , has been measured in situ as a function of potential, E , during continuous voltammetric cycles in the range -0.25 < E < 1.05 V vs. SCE, both in 0.5 M HClO 4 and 0.5 M H 2 SO 4 aqueous solutions. Plots of the normalized differential reflectance, Δ R / R = [ R ( E sam ) - R ( E ref )]/ R ( E ref ) vs. Q n , where E sam and E ref refer to the sampling and reference potentials, respectively, and Q n is the coulometrically determined relative oxide charge (in terms of e/Pt surface atom), were found to deviate from linearity in the PtOH formation region. Improved proportionality between Δ R / R and Q n was achieved up to Q n = 1, upon correcting Δ R / R for double layer effects using the method originally reported by Conway et al . [3]. In the case of 0.5 M H 2 SO 4 , however, remaining discrepancies could only be resolved by accounting for the charge due to bisulfate desorption induced by PtOH formation on the surface. On the basis of these results, it has been concluded that for both types of electrolytes, Δ R / R is proportional to the PtOH coverage, θ, and independent of the applied potential for Q n ≤ 1, opening new prospects for monitoring θ quantitatively during the course of faradaic reactions .

On the way to prepare stable platinum electrodes for the characterization of the H upd reaction by electrochemical impedance measurements, a novel, feather-like platinum surface modification was found and characterized. In addition, a precursor morphology was identified. Electrochemical impedance measurements at selected potentials in the H upd potential region yield information on double layer capacity, adsorption capacity, and adsorption resistance of the H upd reaction for these two electrode morphologies.

A long-standing issue in protein film voltammetry (PFV), particularly electrocatalytic voltammetry of redox enzyme monolayers, is the variability of protein adsorption modes, reflected in distributions of catalytic activity of the adsorbed protein/enzyme molecules. Use of well-defined, atomically planar electrode surfaces is a step towards the resolution of this central issue. We report here the voltammetry of copper nitrite reductase (CNiR, Achromobacter xylosoxidans ) on Au(111)-electrode surfaces modified by monolayers of a broad variety of thiol-based linker molecules. These represent positively charged and electrostatically neutral, hydrophobic and hydrophilic, aliphatic and aromatic, and variable-length micro-environments, as well as their combinations. Optimal conditions for enzyme function seems to be a combination of hydrophobic and hydrophilic surface linker properties, which can lead to close to complete non-catalytic monolayer interfacial electron transfer function and electrocatalysis with activity approaching enzyme activity in homogeneous solution. Thiophenol (combined hydrophobic stacking and interdispersed water molecules), 4-methyl-thiophenol (hydrophobic and water molecules), and 3- and 4-aminothiophenol (hydrophilic, hydrophobic) offer the overall most efficient micro-environments. Subtle differences with even small structural linker differences, however, lead to widely different electrocatalytic properties, strikingly illuminated by the ω-mercaptoamines. CuNiR thus shows highly efficient, close to ideal reversible electrocatalytic voltammetry on cysteamine-covered Au(111)-electrode surfaces, most likely due to two cysteamine orientations previously disclosed by in situ scanning tunnelling microscopy. Such a dual orientation exposes both a hydrophobic and a positively charged, hydrophilic surface feature. In contrast, the higher cysteamine homologues expose only the hydrophilic component with no electrocatalytic activity on these surfaces. These results offer a basis for rational surface design in forthcoming biological electrocatalysis useful both fundamentally and in novel biosensor technology.

The rotating ring disk method (RRDE) is applied to investigate the oxygen reduction reaction (ORR) on Cu(111), Ag(111) and Au(111) single crystal surfaces in alkaline and acidic media. Independently of the pH of solution, we found that at low overpotentials the order of activity increases from Cu(111) to Ag(111) to Au(111). At high overpotentials, however, Cu(111) and Ag(111) are much more active than Au(111). An interpretation of the variation in activity is presented based on the premise that the rate of the ORR is controlled by two factors: a chemical-dependent part, Δ G θ * = Δ G 0 * + γ Δ G Θ 0 Θ ad , and the pre-exponential part which is determined by the adsorption of spectator species (Θ ad ) and the availablitiy of bare metal sites, i.e. , the (1-Θ ad ) term.

Density Functional Theory (DFT) studies were performed on the adsorption of sulfate on Au(111). Focusing on the well-known (√3 × √7)R19.1° structure reported by various surface sensitive techniques, the coadsorption of H 3 O + and/or H 2 O has been considered in different combinations. The calculated binding energies show that the coadsorption of a single H 3 O + per sulfate is the most stable configuration, which is in agreement with experimental observations. Further, we find that in the case of coadsorption of both H 3 O + and H 2 O along with sulfate, one of the protons of hydronium moves to sulfate, finally leading to bisulfate with two coadsorbed water molecules. Besides the morphology and energetics of the different configurations, we also discuss the nature of the surface bonds by analyzing the charge density distribution.

In this paper devoted to Professor Dieter Kolb's 65 th birthday the prospects and challenges of ionic liquids for fundamental investigations at the interface electrode/electrolyte are discussed. Ionic liquids consist solely of mainly organic cations and anions and they have wide electrochemical windows of up to 6 V (approximately ± 3 V vs. NHE) combined with wide thermal windows of up to 300ºC and extremely low vapour pressures between 10 -11 and 10 -10 mbar around room temperature. Thus, thermodynamically they give access to many elements and compounds at variable temperature which due to their reactivity cannot be electrodeposited in aqueous solutions. Apart from a discussion of electrochemical windows examples to be covered in this paper are the local probe deposition of silicon, aluminium and tantalum on Au(111). There is an incredibly high number of possible liquids (between 10 12 and 10 18 liquids, binary and ternary mixtures have been predicted) but also one major challenge for fundamental physicochemical studies, especially with the in situ STM: purity. It is tough to purify ionic liquids as hitherto they can neither be distilled with considerable rates without decomposition nor recrystallized nor sublimed. It will be shortly discussed that even apparently ultrapure ionic liquids can contain low amounts of inorganic impurities leading to inexpected behaviour on the single crystalline surface of Au(111). Due to their importance this paper focuses soleley on the third generation of ionic liquids, i.e. air and water stable ones.