Dieter M. Kolb

Advances in electrochemical science and engineering

Microelectrodes in solid-state ionics, Jurgen Fleig nonlinear dynamics in electrochemical systems, Katharina Krischer the electrochemistry of diamond, Yuri V. Pleskov passivity of metals, Hans-Henning Strehblow.

Underpotential deposition of metals and work function differences

Summary The electrolytic deposition of metal atoms onto foreign metal substrates at underpotentials has been studied in aqueous and non-aqueous solutions. It is shown, that the potential difference between monolayer and bulk deposition is closely related to the difference in the work functions of substrate and deposit, causing a partial charge of the adatoms. The ionic contribution to the chemical bond, arising from this partial electron transfer, can account for the favourable deposition of the first monolayer. A formula is given which allows the prediction of the amount of underpotential shift from differences in work function. This is explained in analogy to Paulings treatment of the chemical bond between atoms of different electronegativity.

A hydrogen adsorption and absorption study with ultrathin Pd overlayers on Au(111) and Au(100)

Pd overlayers with (111) and (100) morphology and with average thicknesses between one and ten monolayers were deposited onto Au(111) and Au(100), respectively. In the case of Pd on Au(111), the film growth was characterized by in situ STM. Due to a significant overpotential for hydrogen absorption in the Pd thin films, adsorption of hydrogen can be studied without being masked by the absorption reaction. H adsorption on Pd(111) films with an average thickness of about five monolayers occurs in two steps up to a total amout of about one monolayer, whereas for thicknesses of 10 monolayers and above as well as for all Pd“(100)” films only a single adsorption peak is observed. The H adsorption process is strongly influenced by the anion of the supporting electrolyte with respect to its thermodynamics as well as kinetics. The maximum amount of adsorbed hydrogen increases with the average film thickness and the H/Pd-ratio reaches a limiting value of around 0.6 in acid electrolytes for thicknesses well above 10 monolayers. The influence of organic molecules and of upd Tl on hydrogen adsorption, absorption and evolution is shown and briefly discussed.

Electrochemical Surface Science

The last 30 years have seen remarkable changes in interfacial electrochemistry, particularly in the kind of questions that were addressed in electrochemical studies. Ever since classical surface science, traditionally performed under ultrahigh vacuum conditions, has succeeded in describing surfaces and surface reactions on a molecular level, electrochemists longed for a microscopic understanding of the solid/electrolyte interface and, at the same time, searched widely for new experimental ways to reach that goal. Herein, studies are described concerning the structure and the dynamics of bare and adsorbate-covered electrode surfaces and of metal deposition as a simple, yet important, electrochemical process. In all these cases, the scanning tunneling microscope plays a pivotal role emphasizing the surface-science approach to the problems.

Impedance aspects of anion adsorption on gold single crystal electrodes

Abstract In order to explore the origins of the capacitance dispersion observed for single crystal electrodes, voltammetric and impedance measurements on Au(l 11) and Au(100), in the absence and the presence of specifically adsorbed anions, mostly Br − , have been carried out. Interfacial capacitances and rate coefficients characterizing the kinetics of adsorption have been evaluated from the impedance spectra. We present evidence that the double layer behaves as an ideal capacitance in the absence of specific adsorption. The approximation with a constant phase element yields an exponent larger than 0.99. The “capacitance dispersion” observed in the presence of specific adsorption is either assigned to a slow diffusion or adsorption process, or to transformations within the adlayer or the substrate surface.

Initial stages of Pd deposition on Au(hkl) Part I: Pd on Au(111)

Abstract The electrochemical deposition of palladium onto Au(111) from chloride-containing solutions was investigated by in-situ STM and cyclic voltammetry. These methods are complementary by yielding structural and thermodynamic information. For [PdCl 4 ] 2− adsorbed on Au(111), a distorted hexagonal structure was imaged, and a phase transition within this adsorbed layer was observed. This palladate anion was found to play a crucial role in Pd deposition and dissolution. Pd deposition starts by forming a pseudomorphic layer in the underpotential region, followed by the formation of a second Pd monolayer at overpotentials. By using stepped Au(111) surfaces, an assignment of voltammetric peaks to nucleation of Pd at steps and on terraces was achieved. It is generally observed that Pd nucleates at monoatomic high steps and grows two-dimensionally. The morphology of the overlayers changes with increasing Pd coverage from flat and well-ordered for the first two monolayers to increasingly rough and defect-rich, but retaining Pd(111) characteristics in cyclic voltammetry up to at least 10 monolayers.

The kinetics of structural changes in Cu adlayers on Au(111)

Abstract The kinetics of structural changes in the adlayer formed during underpotential deposition (and dissolution) of copper on Au(111) in sulphuric acid solution has been investigated by potential-step experiments. The measured current transients can be described by a two-dimensional nucleation and growth mechanism which is accompanied by an adsorption (desorption) process; the two reactions occur at different sites on the electrode. The potential dependences of adsorption, desorption and growth rate obey the Butler-Volmer relation.

The influence of steps on the deposition of Cu onto Au(111)

Abstract The deposition of Cu onto stepped Au(111) surfaces from sulfuric acid solutions has been studied in the underpotential and overpotential range in order to elucidate the role of surface defects in deposition kinetics. Au(111) surfaces of different step densities have been prepared by cutting four single crystals between 0.7 and 4.5 degrees off their [111] axis. Au(111) surfaces roughened by oxidationreduction cycles were also included in the study for comparison. Current transients revealed a marked influence of the step density on the nucleation rates, both for structural transitions within the Cu adlayer at underpotentials and for bulk deposition. Variation of the step density allowed for an assignment of voltammetric features to the underpotential deposition of Cu at steps or terraces. For bulk Cu deposition the critical free energy of nucleation and the size of the critical Cu nucleus were determined.

Potential-induced surface reconstruction of Au(100)

Abstract We have found that at the metal-electrolyte interface an Au(100)−(1×1) surface can reconstruct already at room temperature into its hexagonal (5×20) form, when a negative potential is applied. The activation barrier for reconstruction is obviously lowered significantly by charging the electrode negatively, which can be rationalized within the model of Heine and Marks by the accumulation of excess electrons at the surface. The transition (1×1)→(5×20) depends strongly on the electrode potential and, to some extent, on the electrolyte composition. In 5 mM H2SO4 at −0.4 V versus SCE, the Au(100)−(1×1) surface can reconstruct almost completely into (5×20) within a few minutes. The kinetics of this potential-induced surface reconstruction has been investigated and its mechanism will be briefly discussed.

UHV Techniques in the Study of Electrode Surfaces

The emersion of electrodes and their subsequent transfer into a vacuum chamber for detailed surface-analytic and structural studies is now widely used in interfacial electrochemistry. This method rests on the premise that the electrode can be removed from the electrochemical cell with its double layer intact and without any noticeable amount of bulk electrolyte. We specifically address the question of the emersion process and present some recent results from the study of bare and adsorbate-covered electrode surfaces under ultrahigh-vacuum conditions.