The structure of single-crystal surfaces

While the structure of metals and metal surfaces belongs to solid state physics, a basic understanding is essential for many electrochemical processes, particularly those involving adsorption. A thorough treatment of this topic is beyond the scope of this book. Many metals that are used in electrochemistry (Au, Ag, Cu, Pt, Pd, Ir) have a face-centered cubic (fee) lattice, so we will consider this case in some detail. For other lattice structures we refer to the pertinent literature [1] and to Problem 1. 

A perfect surface is obtained by cutting the infinite lattice in a plane that contains certain lattice points, a lattice plane. The resulting surface forms a two-dimensional sublattice, and we want to classify the possible surface structures. Parallel lattice planes are equivalent in the sense that they contain identical two-dimensional sublattices, and give the same surface structure. Hence we need only specify the direction of the normal to the surface plane. Since the length of this normal is not important, one commonly specifies a normal vector with simple, integral components, and this uniquely specifies the surface structure. 

For an fee lattice a particularly simple surface structure is obtained by cutting the lattice parallel to the sides of a cube that forms a unit cell (see Fig. 4.6a). The resulting surface plane is perpendicular to the vector (1,0,0) so this is called a (100) surface, and one speaks of Ag(100), Au(100), etc., surfaces, and (100) is called the Miller index. Obviously, (100), (010), (001) surfaces have the same structure, a simple square lattice (see Fig. 4.7a), whose lattice constant is a/ /2. Adsorption of particles often takes place at particular surface sites, and some of them are indicated in the figure The position on top of a lattice site is the atop position, fourfold hollow sites are in the center between the surface atoms, and bridge sites (or twofold hollow sites) are in the center of a line joining two neighboring surface atoms. 

The densest surface structure is obtained by cutting the lattice perpendicular to the [111] direction (see Fig. 4.6b). The resulting (111) surface forms a triangular (or hexagonal) lattice and the lattice con- 

The (110) surface has a lower density than either the (111) or the (100) planes (Fig. 4.6c). It forms a rectangular lattice the two sides of the rectangle are a and a/y/2 (Fig. 4.7c). The resulting structure has characteristic grooves in one direction. 

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Hydrogen adsorption and oxidation of formic acid show a pronounced dependence on the structure of single crystal surfaces. The influence of the terrace and step orientation and step density is reflected in both reactions on step surfaces. The multiple states of hydrogen adsorption can be correlated with the nature of adsorption sites. 

LEED is used to determine the surface structure of single crystal surfaces and the structure of ordered adsorbate layers [18-21], The principle is illustrated in Fig. 6.7 a beam of monoenergetic low energy electrons (50-200 eV, minimum mean free... 

From the perspective of this symposium, analysis of the atomic dynamics and electronic structure of surfaces constitutes an even more exotic topic than surface atomic geometry. In both cases attention has been focused on a small number of model systems, e.g., single crystal transition metal and semiconductor surfaces, using rather specialized experimental facilities. General reviews have appeared for both atomic surface dynamics (21) and spectroscopic measurements of the electronic structure of single-crystal surfaces (, 22). An important emerging trend in the latter area is the use of synchrotron radiation for studying surface electronic structure via photoemission spectroscopy ( 23) Moreover, the use of the very intense synchrotron radiation sources also will enable major improvements in the application of core-level photoemission for surface chemical analysis (13). 

For many studies of single-crystal surfaces, it is sufficient to consider the surface as consisting of a single domain of a unifonn, well ordered atomic structure based on a particular low-Miller-mdex orientation. However, real materials are not so flawless. It is therefore usefril to consider how real surfaces differ from the ideal case, so that the behaviour that is intrinsic to a single domain of the well ordered orientation can be distinguished from tliat caused by defects. 

LEED is the most powerfiil, most widely used, and most developed technique for the investigation of periodic surface structures. It is a standard tool in the surface analysis of single-crystal surfaces. It is used very commonly as a method to check surface order. The evolution of the technique is toward greater use to investigate surface disorder. Progress in atomic-structure determination is focused on improving calculations for complex molecular surface structures. 

In summary, LEED is most often used to verify the structure and quality of single crystal surfaces, to study the structure of ordered adsorbates and to study surface reconstructions. In more sophisticated uses of LEED one also determines exact positions of atoms, the nature of defects and the morphology of steps, as well as Debye temperatures of the surface. 

Prior to the publication in 1980 of Clavilier s historic paper (1) reporting anomalous voltammetry of Pt(lll), there had been a number of studies of the voltammetry of single crystal Pt electrodes, with some using modern methods of surface analysis (e.g., LEED or RHEED) for characterization of the structure of the crystal prior to immersion in electrolyte (2-6). and all were in qualitative agreement with the seminal work (in 1965) on Pt single crystals by Will (7.). 

As shown below, for structure-insensitive reactions the surface characteristics of the single crystal catalysts simulate the activity of supported catalysts in the same reactant environment. This proves to be most fortunate since the advantages of single crystals are retained along with the relevance of the measurements. Moreover, the use of single crystals allows the assessment of the crystallographic dependence of structure-sensitive reactions. 

A particular Pt-skin single-crystal surface, the Pt3Ni (111) face, was reported to exhibit an extraordinary ORR activity after annealing and formation of the Pt skin structure [87]. This facet exceeded the activity of the Pt(lll) single-crystal surface by a factor of 10 x, while it was found to be about 90-fold more active than a state-of-the-art high surface area carbon-supported Pt electrocatalyst. The enhancement... 

Most of the R2Ni2Pb plumbides have been grown in the form of single crystal platelets either via a lead flux or by special annealing procedures (Chinchure et al., 2003). The surfaces of such crystals show terrace-like structures. The single crystals then allow direction dependent magnetic measurements. 

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