INDEX surface reconstruction

Besides surface reconstructions induced by heat treatment, potential-induced reconstruction has recently become a topic of interest in electrochemistry. It has been observed that at potentials negative with respect to the potential of zero surface charge, [Kolb, 1996, 2002 Dakkouri, 1997], the reconstructions found under UHV conditions are also stable in contact with an electrolyte. Although aU low index faces of Au and Pt undergo potential-induced reconstruction, it has been particularly well characterized for Au(lOO) (Fig. 5.5). 

The existence of active sites on surfaces has long been postulated, but confidence in the geometric models of kink and step sites has only been attained in recent years by work on high index surfaces. However, even a lattice structure that is unreconstructed will show a number of random defects, such as vacancies and isolated adatoms, purely as a result of statistical considerations. What has been revealed by the modern techniques described in chapter 2 is the extraordinary mobility of surfaces, particularly at the liquid-solid interface. If the metal atoms can be stabilised by coordination, very remarkable atom mobilities across the terraces are found, with reconstruction on Au(100), for example, taking only minutes to complete at room temperature in chloride-containing electrolytes. It is now clear that the... 

The clean siuface of solids sustains not only surface relaxation but also surface reconstruction in which the displacement of surface atoms produces a two-dimensional superlattice overlapped with, but different from, the interior lattice structure. While the lattice planes in crystals are conventionally expressed in terms of Miller indices (e.g. (100) and (110) for low index planes in the face centered cubic lattice), but for the surface of solid crystals, we use an index of the form (1 X 1) to describe a two-dimensional surface lattice which is exactly the same as the interior lattice. An index (5 x 20) is used to express a surface plane in which a surface atom exactly overlaps an interior lattice atom at every five atomic distances in the x direction and at twenty atomic distances in the y direction. 

Surface reconstruction or adsorption can often cause a vicinal surface with a single macroscopic orientation to facet into surfaces with different orientations. Generally the reconstruction occurs on a particular low-index flat face, and lowers its free energy relative to that of an unreconstructed surface with the same orientation. However the same reconstruction that produces the lower free energy for the flat face generally increases the energy of surface distortions such as steps that disturb the reconstmction. Thus reconstmction is often observed only on terraces wider than some critical terrace width Wc. When steps are uniformly distributed initially and if Wc is much greater... 

The studies under ultrahigh vacuum have shown that adsorption and surface charging influence the stability of the reconstructed surfaces. A similar influence has been observed for metal surfaces in contact with electrolyte solutions [336]. In this case, the separation of these two influences is not simple, since the surface charging and adsorption processes are interdependent. Generally, it has been concluded [4] that Au surface reconstruction occurs for negative electrode charges and disappears for positive surface charges. It is noteworthy that as early as in 1984, Kolb and coworkers [339, 340], who carried out systematic study on all three low-index faces Au electrodes, showed that the reconstructed surfaces can be stable in electrolyte solutions. 

In summary, a variety of LEED patterns have been observed for oxygen adsorption on the platinum metals. However, their interpretation is complicated by the uncertainty in the oxygen coverage. A second complication is the uncertainty introduced by oxygen-induced surface reconstruction. The stability of 22 high index Miller planes upon exposure to oxygen has recently been investigated by Blakely and Somorjai (160). 

We call this Pt(100) surface reconstructed. Surface reconstruction is defined as the state of the clean surface when its LEED pattern indicates the presence of a surface unit mesh different from the bulklike (1 x 1) unit mesh that is expected from the projection of the bulk X-ray unit cell. Conversely, an unreconstructed surface has a surface structure and a so-called (1 x 1) diffraction pattern that is expected from the projection of the X-ray unit cell for that particular surface. Such a definition of surface reconstruction does not tell us anything about possible changes in the interlayer distances between the first and the second layers of atoms at the surface. Contraction or expansion in the direction perpendicular to the surface can take place without changing the (1 x 1) two-dimensional surface unit cell size or orientation. Indeed, several low Miller index surfaces of clean monatomic and diatomic solids exhibit unreconstructed surfaces, but the surface structure also exhibits contraction or expansion perpendicular to the surface plane in the first layer of atoms (9b). 

Theoretical work by Tasker 131) confirms that this type of surface, which is charged and possesses a dipole moment perpendicular to the surface, can only be stabilized by substantial reconstruction. Overall, for the simple cubic oxides, it is possible that the lower coordination ions are most likely to be associated with imperfections in the low-index surface planes. 

In contrast, other low Miller index surfaces of Ti02 have not attracted so much attention in terms of adsorbate structure determinations. Only two such studies have been reported, both involving Ti02( 100). Fig. 11 shows simple schematic diagrams of the (1x1) and (1x3) phases of this surface, both of which will be mentioned below. The (1x3) reconstruction is known to consist of (110) microfacets from previous work (see Ref. 79 and Refs, therein), whereas the displayed (1x1) structure is merely that expected on the basis of Tasker s rule [80]. 

Although these simple considerations help to frame in a general logic the behavior of these bimetallic surface, there are at present no such simple models to explain the more complex mesoscopic reconstructions, such as the pyramids observed on Pt3Sn(100) or the hill and valley structure observed on PtsSnCl 10). These phenomena are obviously related to the tendency of the system to relax in-plane stress, in turn resulting from the different atomic radius of the elements involved in the presence of concentration gradients. This relaxation appears to take place on the (111) oriented plane simply by an outward relaxation of the tin atoms. On the other two low index surfaces, instead, it takes a more complex route leading to reconstruction phenomena (pyramids on the (100) and hill and valley on the (110)) which are so far unique to the Pt-Sn system. 

The examination of surface properties of three low-index surfaces of Pt3Ni(/j/ 0 has also been pursued recently by a combination of low-energy electron diffraction (FEED) and LEIS [23]. As summarized in Fig. 3.2, the PtjNi(l 11) surface exhibits a (1 X 1) FEED pattern (Fig. 3.2d) the atomically less dense PtjNi(lOO) surface shows a clear (1 x 5) reconstruction pattern (the so-called hex phase) in both the... 

A superlattice can be caused by adsorbates adopting a different periodicity than the substrate surface, or also by a reconstruction of the clean surface. In figure BI.2I.3 several superlattices that are commonly detected on low-Miller-index surfaces are shown with their Wood notation. 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

Clean, Low-Indexed Surfaces. The structures of clean surfaces are of fundamental interest because they are the basis for more complicated systems offering practical applications. On their own, clean surfaces have importance as quasi-2D systems, which can show special effects like relaxations, reconstructions, phase transitions, surface-specific defects, local mass fluctuations, and roughening transitions. In the following we concentrate on face-centered cubic (fee) metals. The geometry of the three low-indexed fee surfaces is shown in Figure 44. 

In Tables 6-8 we have listed the submonolayer and monolayer stractures of metals on the principal low index surfaces of metal substrates from 2- to 4-fold rotational symmetry. This compilation comprises heteroepitaxial systems only since the structure of homoepitaxial systerrrs is in most cases trivial. For umeconstructed surfaces the bulk stacking is pseudomorphically continued and for reconstructed ones the reconstruction is lifted below the adlayer and at the same time taken on by the adsorbate layer. Only few homoepitaxial cases are worth mentioning since their reconstructions can metastably be lifted, as seen for Au/Au(110)-(lx2) [97Giin], or a reconstruction can be induced at a lower tenqreratrrre by homoepitaxial adsorption, as seen for Pt/Pt( 111) [93Bot]. 

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