Single crystal surfaces, typical structures

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

The great advantage of STM is the ability to do in situ structural studies on electrode surfaces. However, STM does not yield chemical information. STM-derived structural information is confined to a very small area on the electrode, typically 1000 Ax 1000 A, so meaningful correlations between the stmctnral data and electrochemical data can be done only on single-crystal surfaces. The close proximity of the tip to the electrode surface can distort kinetic data acqnired by an STM. 

Cocrystals are often prepared by a traditional solution crystalhsation approach such as solvent evaporation, coohng, or anti-solvent addition. There are a number of reasons for the popularity of the solution-based approach. Solution crystallisation can yield large, well-formed single crystals, from which one may easily evaluate crystal habit and surface features. Analysis of the diffraction pattern of a single crystal is typically the best means of obtaining an absolute crystal structure determination. Further, solution crystalhsation is an established and effective purification step. 

MgO is a particularly well studied oxide the structure of the (100) single crystal surface is extremely flat, clean, and stoichiometric. Recent grazing incident X-ray scattering experiments have shown that both relaxation, -0.56 0.4%, and rumpling, 1.07 0.5%, are extremely small [66]. However, no real crystal surface consists of only idealized terraces. A great effort has been undertaken in recent years to better characterize the MgO surface, in particular for polycrystalline or thin-film forms which in some cases exhibit an heterogeneous surface, due to the presence of various sites. All these sites can be considered as defects. The identification and classification of the defects is of fundamental importance. In fact, the presence of appreciable concentrations of defects can change completely the chemical behavior of the surface. A typical example is that of the reaction of CO on MgO (see 3.1). 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

The activity for the ORR of Pt monolayers, deposited on different single-crystal surfaces, using the Cu UPD technique [14], were investigated in acid and in alkaline electrolytes [7, 8]. Figure 5.1a show the typical ORR curves obtained for pure Pt/C and Pt monolayer on Pd/C nanoparticles, and Fig. 5.1b shows the plot of ORR activity versus Pt d-band center on different surfaces [7]. As can be seen, the ft monolayer electrocatalysts exhibited support-induced tunable activity by arising either by structural and/or electronic effects. It can be observed that the most active of all surfaces is PtML/Pd(lll), and the least active is PtML/Ru(0001). The plots of the kinetic current on the platinum monolayers on various substrates at 0.8 V as a function of the calculated d-hznA center, e, generated a volcano-like curve, with PtML/Pd(lll) showing the maximum activity (Fig. 5.1a). 

Let us concentrate on the structure of these various surfaces, the prototypes of which are shown in figure 7. There are flat surfaces which are low-Miller-index, single-crystal surfaces of cubic materials that have six or four atoms as nearest neighbors in the surface layer. Most surface science studies have been carried out on these flat, low Miller index surfaces. Then there are stepped surfaces which are the typical structure of high Miller index, single-crystal surfaces. Often the steps are also in the direction of high Miller index in which case there are ordered ledges in the steps. 

How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

The single-crystal structure of 31 clearly reflects the presence of an integral fulvene-type Jt-system on the spherical surface. The average bond length for the [5,6]-bond between C1-C2 and C3-C4 is 1.375 A, which is considerably shorter than a typical [5,6]-bond in CgQ. In contrast, the bond between C2 and C3 (1.488 A) is notably... 

The orientation of the atomic planes in the single-crystal wafer is important for the formation of snbsequent layers that are necessary to form the thin, multilayer structures illustrated in Figure 6.99. Typically, these layers are from 0.5 to 20 ptm in thickness, and strict compositional control is necessary during their formation. The process of depositing these thin layers of single-crystal materials on the surface of... 

---