In situ x-ray-diffraction and -reflectivity studies of the Au(111)/electrolyte interface: Reconstruction and anion adsorption

X-ray diffraction and STM study of reactive surfaces under electrochemical control: Cl and I on Cu(100)

The surface structure of Cu(100) modified by chloride and iodide has been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface X-ray diffraction with a particular focus on adsorbate and potential dependent surface relaxation phenomena. For positive potentials close to the on-set of the copper dissolution reaction, the X-ray data disclose an extraordinarily large Cu-Cl bond length of 2.61 A for the c(2 x 2)-Cl phase. This finding points to a largely ionic character of the Cu-Cl interaction at the Cu(100) surface, with chloride particles likely to retain their full charge upon adsorption. Together with the positive surface charging at these high potentials, this ionic Cu-Cl bond drives the observed 2.2% outward relaxation between the first two copper layers. These results indicate that the bond between the first and the second copper layer is significantly weakened which appears as the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 A the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.

X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering. The electron-density profile across the n-hexane/silica sol interface (solutions with 5, 7, and 12 nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na(+), a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at the interface reflects the difference in the potentials of "image forces" between the cationic Na(+) and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field (approximately 10(9)-10(10) Vm) of the transition region and the layering of silica in the diffuse layer is discussed.

Water Density in the Electric Double Layer at the Insulator/Electrolyte Solution Interface

I studied the spatial structure of the thick transition region between n-hexane and a colloidal solution of 7-nm silica particles by X-ray reflectivity and grazing incidence small-angle scattering. The interfacial structure is discussed in terms of a semiquantitative interface model wherein the potential gradient at the n-hexane/sol interface reflects the difference in the potentials of "image forces" between the cationic Na(+) and anions (nanoparticles) and the specific adsorption of surface charge at the interface between the adsorbed layer and the solution, as well as at the interface between the adsorbed layer and n-hexane. The X-ray scattering data revealed that the average density of water in the field approximately 10(9)-10(10) V/m of the electrical double layer at the hexane/silica sol interface is the same as, or only few percent higher (1-7%) than, its density under normal conditions.

Structural Kinetics Studies on Phase Transitions of the Bi UPD Layer between the (2 × 2) and (p × √3) Structures Using Surface X-ray Diffraction

The kinetics of the phase transition between the (2 x 2) and (p x square root[3])-Bi structures on Au(111) was investigated using electrochemical methods and time-resolved surface X-ray diffraction. The temporal changes in the current value and the diffracted X-ray intensity that originated from the (2 x 2)-Bi overlayer were monitored during the phase transitions at various over-potentials. The phase transition models and kinetics parameters were deduced from each of the current and X-ray intensity transient curves. We also carried out comparative studies of the phase transition from the structural and electrochemical points of view. For the (p x square root[3]) --> (2 x 2) phase transition, the phase transition models determined by the X-ray and electrochemical measurements were a surface-diffusion controlled instantaneous nucleation-growth process and a Langmuir process, respectively. For the reverse transition, the phase transition models determined by X-ray and electrochemical measurements were a Langmuir adsorption process and a surface diffusion controlled nucleation-growth process, respectively. Our results revealed that the current transient curve does not always reflect the phase transition model in both cases and suggest that a structural analysis is fundamental in the phase transition studies. The disagreements between the phase transition models and their mechanisms are discussed.

Electrocompression of the Au(111) surface layer during Au electrodeposition

In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a significantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress.

Potential-induced strain relaxation in au mono- and bilayer films on Pt(111) electrode surfaces

In situ scanning tunneling microscopy observations of 1-2 monolayer thick Au films on Pt(111) electrodes are presented, which show a complex, potential-dependent structural phase behavior of the Au film. Starting from a pseudomorphic structure at <0.35 V(Ag/AgCl), a sequence of transitions into dislocation network structures occurs with increasing potential: (1x1)--> "striped phase" --> "hex phase" in the Au bilayer, (1x1)--> striped phase in the Au monolayer. This is explained by a reduction of the in-plane stress in the Au surface layer(s) due to anion adsorption and a strain energy minimization as described by the Frenkel-Kontorova model.

Effect of local environment on nanoscale dynamics at electrochemical interfaces: anisotropic growth and dissolution in the presence of a step providing evidence for a schwoebel-ehrlich barrier at solid/liquid interfaces

The dynamics of nanoscale islands in the vicinity of a monoatomic step on a metal electrode surface under potential control have been investigated by potential pulse perturbation time-resolved scanning tunneling microscopy (P3 TR-STM). As the metastable islands decay, a clear anisotropy develops in the spatial distribution of islands, relative to the monoatomic step. Islands appear to be stabilized above the step, while a denuded region develops below the step over a distance of about 100 nm. Vacancy islands are observed to be more stable than adatom islands. These results provide evidence for an electrochemical Schwoebel-Ehrlich barrier to diffusion of atoms across the step.