D.M. Kolb

In-situ STM characterisation of the surface morphology of platinum single crystal electrodes as a function of their preparation

Abstract A systematic study of the surface morphology of Pt(111) and Pt(100) electrodes as prepared by flame-annealing and cooling in different atmospheres (air, N 2 , H 2 +N 2 and CO+N 2 ) is presented. The electrodes were characterised by cyclic voltammetry and in-situ STM in 0.1 M H 2 SO 4 . Preliminary voltammetric results for Pt(110) are also shown. In this case, Cu upd served as a structure sensitive probe. It was observed that the presence of oxygen during cooling induces surface defects and leads to rough surfaces. Cooling in pure N 2 preserves the reconstructed Pt(100) and Pt(110) surfaces, while cooling in H 2 +N 2 or CO+N 2 lifts the reconstruction. The use of CO as the cooling gas turned out to be advantageous for the preparation of clean and well-ordered (1×1)-surfaces. The stability of reconstructed Pt surfaces in an electrochemical environment is discussed. For H 2 +N 2 -cooled Pt electrodes, a clear influence of the H 2 -concentration on the surface morphology was observed.

An atomistic view of electrochemistry

Abstract One of the most important tasks of modern, physical electrochemistry is the development of an atomistic picture of the solid/liquid interface in order to provide the basis for a mechanistic understanding of electrochemical processes. Electrochemists seek answers to the same questions as their surface science colleagues (e.g., electronic and structure properties of surfaces and adlayers), but are faced with the fact that in electrochemistry the contact of the solid with a condensed phase, the electrolyte, makes life much more difficult. Nevertheless, electrochemists succeeded in the last 20 years to develop an electrochemical surface science by adopting experimental techniques and theoretical concepts from surface physicists. This article describes the various routes electrochemists have used to obtain a detailed characterization of electrode surfaces in particular, and of the electrochemical interface in general. Success in physical electrochemistry is based on the development of non-traditional in situ methods to complement the classical, current- and voltage-based techniques. The former range from optical spectroscopies, linear and non-linear, to in situ X-ray diffraction and scanning tunneling microscopy. The current status of electrochemical surface science and its most important future goals are briefly addressed.

Initial stages of Pt deposition on Au(111) and Au(100)

Abstract The deposition of Pt onto unreconstructed Au(111) and Au(100) was studied with cyclic voltammetry and in-situ STM. The latter revealed that in [PtCl 4 ] 2− containing electrolytes, both surfaces are covered by an ordered adlayer of the complex. For the adsorbed [PtCl 4 ] 2− a slightly compressed (√7×√7) R19.1°-structure was assumed for Au(111) and a (3×√10) for Au(100). In both cases, a rather high overpotential for Pt deposition was observed, most probably due to the high stability of the [PtCl 4 ] 2− complex. Nucleation of Pt starts mainly at defects like step edges for low deposition rates and three-dimensional clusters are formed. Due to the high overpotential, some nuclei appear also on terraces at random sites. Higher coverages of Pt lead to a cauliflower like appearance. It is not possible to dissolve the platinum clusters at positive potentials without severely roughening the gold surface. The [PtCl 4 ] 2− complex is oxidized to the [PtCl 6 ] 2− complex at about 0.7 V, when metallic Pt is on the surface.

Double layer capacitance of Pt(111) single crystal electrodes

Abstract In order to determine the double layer capacitance of the Pt(111) electrode, impedance and capacitance measurements were carried out in neutral and acidic aqueous perchlorate solutions. Separation of the double layer and adsorption contributions of the interfacial capacitance were based on the adsorption impedance theory. The double layer capacitance versus potential plot exhibits a peak at about 0.12 V versus SCE in the 1.5–7 pH range; from here towards cathodic potentials the capacitance attains a value of about 20 μF/cm 2 . The peak may be related to the potential of zero free charge of the Pt(111) electrode.

An in situ scanning tunnelling microscopy study of bulk copper deposition and the influence of an organic additive

Abstract In situ scanning tunnelling microscopy (STM) was used to study the electrodeposition of copper onto Au(111) and Au(100) electrodes from sulphuric and perchloric acid electrolytes. We also followed copper deposition in the presence of crystal violet, which has properties typical of additives used in metal plating baths. For both crystallographic orientations, and also with or without the additive, the very initial stages of copper electrocrystallization occurred predominantly at surface defects, such as step edges of the substrate surface. In the absence of additive the copper crystallites grew in three dimensions and the number of growth centres remained relatively constant, implying an instantaneous nucleation mechanism. However, the presence of small quantities of crystal violet in the electrolyte solution markedly influenced the growth of the copper crystallites, on both Au(111) and Au(100) substrates. Instead of growing in three dimensions they spread essentially parallel to the surface, to produce copper films which were much smoother on the microscopic scale.

Structural transitions in uracil adlayers on gold single crystal electrodes

The formation of uracil adlayers on Au(111), Au(100) and Au(110) in aqueous solutions has been studied by current-potential, capacitance-potential and current-time measurements. Besides adsorption of uracil on gold at low coverages and negative potentials, the formation of two distinctly different organized uracil layers was found. The first one at medium coverages has been observed only for the densely-packed surfaces of Au(111) and reconstructed Au(100) and it corresponds to a two-dimensional condensed physisorbed film. The second type of adlayer is formed at very positive potentials on all gold surfaces, irrespective of their crystallographic orientation. This layer, which has been imaged by in-situ STM, has a hcp structure with a next-neighbour distance of 0.4 nm and is assigned to chemisorbed uracil. The kinetics of the structural transitions within the uracil adlayer has been studied by current transients and analyzed by standard nucleation-and-growth models which were modified to account for the influence of surface defects.

Comments on the thermodynamics of solid electrodes

The thermodynamics of solid electrodes are discussed in light of the recent measurements of surface stress. Interfacial tension and surface stress are not even approximately equal, and they generally exhibit a different dependence on the electrode potential. The variation of the interfacial strain with potential is small so that the Lippmann equation for a solid is practically the same as for a liquid electrode. Changes in the interfacial tension can be obtained by integrating the charge density over the potential.

The pzc of Au(111) and Pt(111) in a perchloric acid solution: an ex situ approach to the immersion technique

Clean and well-ordered Au(111) and Pt(111) electrodes were prepared in a UHV chamber, transferred to an electrochemical cell by a closed-transfer system and immersed in 0.1 M HClO 4 at various potentials. From the charge flowing during contact with the electrolyte under potential control, the potential of zero charge (pzc) was derived. Au(111) was used as a test system and very good agreement was obtained between the pzc values determined by in-situ capacity measurements in dilute solutions and by the immersion technique. For Pt(111) the pzc was found to be very positive, within the oxide region, and hence not directly accessible by experiment. However, from charge measurements in the double layer region, the pzc of Pt(111) could be estimated to lie around 0.9 V vs. Ag AgCl.

Electrochemical fabrication of large arrays of metal nanoclusters

Abstract Tip-induced metal deposition was employed to fabricate large, micrometer-size arrays of Cu nanoclusters on Au(111) electrodes. The fully-automated process allowed to produce clusters of uniform size at a rate of 50xa0Hz. An array of 10 4 Cu clusters placed on Au(111) at intervals of 11xa0nm was generated within 17xa0min. It is demonstrated that the cluster fabrication procedure allows us to surmount several monoatomically high steps in the substrate, indicating that this type of nanostructuring does not require perfectly flat surfaces. No sign of a depletion of the deposited metal has been noted even after production of 10 4 Cu clusters. The externally controlled tip approach employed for the cluster generation, was also used to determine the tunnel barrier in an electrochemical environment. With this method a tunnel barrier of 1.5±0.1xa0eV was found.

On the valence state of bismuth adsorbed on a Pt(111) electrode: an electrochemistry, LEED and XPS study

The valence state of Bi on Pt(111) which is irreversibly deposited at underpotentials from perchloric acid solutions, has been determined ex-situ by XPS for various emersion potentials. From chemical shift measurements it is inferred that the adsorbed Bi remains in its zero-valent state over the entire potential range under study, despite a substantial charge flow during cycling. This charge is attributed to OH-adsorption on the Bi-covered Pt(111) surface rather than to a change in valence state of the adsorbed Bi, as was previously suggested. Superstructures were observed in the LEED patterns and are assigned to ordered Bi adlayers.