Surface X-ray Scattering SXS

Besides crystal planes parallel to the surface, further planes exist that might reflect impinging X-rays. The scattered X-rays are not in the plane of reflection and are called off-specular reflections. Their intensity is also high at the Bragg angles. As with the specularly reflected X-rays, the weak signal between these peak intensities contains surface-relevant information. A combined analysis of these reflections often leads to a complete picture of the surface structure. 

In reciprocal space, these reflections form streaks of scattered X-ray intensity with a narrow but elongated cross section in the direction of the surface normal. With a flat surface parallel to a crystallographic axis, these streaks connect the Bragg 

Instrumentation. Investigations require an X-ray source of sufficient intensity that provides monochromatic radiation, usually a synchrotron. The schematic setup is shown in Fig. 6.6. Typically, X-rays in the energy range 6 to 17 kV (X = 70 to 200 pm) are used. 

Two basically different cell geometries can be used. In Fig. 6.7 these designs are shown together with their analogous arrangements for single crystal studies. 

In the reflection geometry (or in analogy, the Bragg geometry), the cell window is parallel to the electrolyte surface. The current distribution and iR-dvop is 

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Surface Enhanced Raman Spectroscopy (SERS), 176, 184, 194 Surface X-Ray Scattering (SXS), 247-248... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

Bromide adsorption on Au(lll) has also been studied, applying in situ surface X-ray scattering (SXS) and STM [56]. The potential-dependent adlayer density agreed well with the earlier pubKshed bromide surface excess densities, obtained in electrochemical measurements. At very positive potentials, a bromide-induced step-flow etching of Au occurred. 

STM is one of only a few techniques that can be used to obtain detailed structural information at the solid/liquid interface. Surface x-ray scattering (SXS) can be used and probes the local order of the surface with higher resolution than STM, but the information is averaged over an extended area of the surface. STM has the advantage that it can image lighter atoms that do not scatter x-rays well, but it is fairly insensitive for distinguishing between atomic or molecular species. Thus, as usual, a combination of the two techniques can provide a more detailed description of the solid-liquid interface. 

Since the early days of modern surface science, the main goal in the electrochemical community has been to find correlations between the microscopic structures formed by surface atoms and adsorbates and the macroscopic kinetic rates of a particular electrochemical reaction. The establishment of such relationships, previously only developed for catalysts under ultrahigh vacuum (UHV) conditions, has been broadened to embrace electrochemical interfaces. In early work, determination of the surface structures in an electrochemical environment was derived from ex-situ UHV analysis of emersed surfaces. Although such ex-situ tactics remain important, the relationship between the structure of the interface in the electrolyte and that observed in UHV was always problematic and had to be carefully examined on a case-by-case basis. The application of in-situ surface-sensitive probes, most notably synchrotron-based surface X-ray scattering (SXS) [1-6] and scanning tunneling microscopy (STM) [7, 8], has overcome this emersion gap and provided information on potential-dependent surface structures at a level of sophistication that is on a par with (or even in advance of) that obtained for surfaces in UHV. 

Surface X-ray scattering (SXS) The first SXS study of an underpotentially deposited metal monolayer was reported more than ten years ago. In a recent review [132] it is demonstrated that this method is well suited for the study of the structure of metals, halides, and metal-halide adlayers on single-crystal electrodes. As another example, the study of the distribution of water at Ag(lll) surface can be mentioned [133]. 

One typically finds that the order parameter of a continuous phase transition varies in the critical region as E - Ec) [53] or (T — Tc) [1, 33]. The numerical value of the critical exponent f depends only on a few physical properties, such as the dimension of the local variable (order parameter) in the Hamiltonian, the symmetry of the coupling between the local variables, and the dimensionality of the system (here 2D). This property is called universality [32, 33, 54]. Systems with identical critical behavior form one universality class. Only two examples have been reported for interfacial electrochemical systems In situ surface X-ray scattering (SXS) [53], chronocoulometry [55], and Monte Carlo (MC) simulations [56, 57] demonstrated... 

Recently, in addition to the in situ STM/AFM, many other surface-analysis techniques such as surface X-ray scattering (SXS) [19, 20] and electrochemical quartz crystal microbalance (EQCM) [21, 22] have also been employed to investigate the electrochemical deposition and dissolution processes at atomic resolution. Atomically controlled electrochemical epitaxial growth and layer-by-layer dissolution... 

Lucas et al. studied the structure of the electrochemical double layer at the interface between a Ag(lll) electrode and 0.1 M KOH electrolyte using in situ surface X-ray scattering (SXS) and proposed an interface structure, at the negatively charged and positively charged surface, as shown in Fig. 15.9. At negative potential E = 1.0 V vs. SCE), the presence of an hydrated cation layer at a... 

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