Metal unreconstructed

The surface have been assumed, unrelaxed and unreconstructed. The d band filling has been varied in the range (3 - 4.6)e per atom which includes the BCC transition metals and, in particular, the case of Ta and W. The results are displayed in Fig. 2. As often assumed, we have taken Nd(Z + 1) - Nd(Z) = 1.1. However, as shown in Fig. 2, changing this difference to 1 modifies only slightly the numerical results. 

Kolb and Franke have demonstrated how surface reconstruction phenomena can be studied in situ with the help of potential-induced surface states using electroreflectance (ER) spectroscopy.449,488,543,544 The optical properties of reconstructed and unreconstructed Au(100) have been found to be remarkably different. In recent model calculations it was shown that the accumulation of negative charges at a metal surface favors surface reconstruction because the increased sp-electron density at the surface gives rise to an increased compressive stress between surface atoms, forcing them into a densely packed structure.532... 

All Fe oxide films on Pt have strongly relaxed, unreconstructed bulk-terminated surfaces, but while the Fe304 and Fe203 oxide layers are similar to their respective bulk compounds, the ultrathin FeO layers are true 2D oxide phases that are different from the FeO bulk and stabilized by the metal-oxide interface. 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

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... 

In essence the adstoms simulate the missing half of the substrate. These impurity-stabilized unreconstructed surfaces [e.g., Pt(lOO) and Au(lOO)] have the structure known for the other stable clean unreconstructed metal surfaces. 

ZnO can be prepared as single crystals or microcrystalline powders. The single crystals often have needle-like shapes and preferentially expose prismatic (1010) and (1120) faces. When examined by LEED, these neutral faces appear unreconstructed (393). In contrast the Zn and oxygen-rich positively and negatively charged (0001) faces prepared by cleavage of single crystals show a distinct tendency toward extensive reconstruction and accumulation of metal impurities. 

Highly dispersed powders can be prepared in several ways. When prepared by combustion of metallic zinc, the resulting very pure ZnO (with 10 m2g-1 specific surface area) is constituted of microcrystals characterized by well-defined elongated prismatic habits (Fig. 14) exposing preferentially unreconstructed (1010) and (1120) faces (394). When prepared by decomposition of ZnC03, the resulting high-surface-area powder ( 50 m2g ) is constituted of very small microcrystals with ill-defined shapes. 

The surface coverage of an adsorbate is another important parameter in ordering. We shall use the common definition of coverage where one monolayer corresponds to one adsorbate atom or molecule for each unit cell of the clean, unreconstructed substrate surface. Thus, if an adsorbed undissociated carbon monoxide molecule bonds to alternating top-layer metal atoms exposed at the Ni(100) surface, we have a coverage of a half monolayer. 

A breakdown of the structural results by type of surface shows results for nearly 50 clean, unreconstructed metal surfaces and about 10 alloys and reconstructed metal surfaces. The structures of about 65 atomic overlayers on metal surfaces have been determined, some 40 of these involving chalcogen atoms. Just over 20 molecular structures have been determined for metal surfaces, half of these being overlayers of undissociated carbon monoxide and the others various hydrocarbons. Turning to semiconductors, some 13 clean, usually reconstructed structures were determined, against nearly 10 atomic overlayer structures. In addition, about 15 insulator surface structures have been investigated. 

Up to now we have considered unreconstructed, defect-free low-index surfaces, where all surface atoms have the same geometric environment. In the real world, large defect-free terraces of low-index surfaces are the exception rather than the rule, and in nanometer-sized metal particles (clusters) such as those found in industrial catalysts a significant fraction of all surface atoms sit at steps, edges or corners and therefore have lower coordination than those in the terraces. There are many indications that such sites are more reactive than terraces [59],... 

A second consequence of the surface valence charge depletion relates to surface stress. It seems to now be rather well-established that clean unreconstructed elemental metal surfaces are in a state of tensile stress [60]. This means that the surface atoms would prefer to have a shorter interatomic spacing parallel to the surface. In some cases (such as Au(lll) and Au(lOO) surfaces) this effect can lead to a reconstruction of the surface layer to a (more) close-packed overlayer (e.g. [61]). However, in most metals the surface atoms that are under tensile stress are locked in the periodic potential of the underlying bulk. Substituting some fraction of the atoms in such a surface by... 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

The unreconstructed low Miller index surfaces of fee, bcc and hep metals are illustrated schematically in fig. 1. The primary structural feature associated... 

Structure of unreconstructed metal surfaces, ddu is the relaxation of the first inlerplanar spacing expressed as a percentage of the bulk inlerplanar spacing. A positive value implies an expansion of the first interplanar spacing, a negative value implies a contraction. M23, 8 23 and [Pg.7]

The missing row reconstruction may be induced in ordinarily unreconstructed fee metals by driving electrons into the surface region, either electrochemically or by alkali-metal adsorption. For example, a (1x2) missing row reconstruction of Cu(110) and Pd(110) may be created by K and Cs adsorption (Barnes et al., 1985 Hu et al., 1990). The structural parameters of these surfaces are included in table 2 and show the smaller normal relaxations that are qualitatively similar to the stable missing-row forms of Ir(110). 

Compilation of structural parameters for the surfaces of unreconstructed disordered metallic alloys. Cl—C4 are the percentage of atom type A in the corresponding layer of the bulk alloy AB. Atom type A is the first element listed in the alloy column of the table, ddn is the change in the first intcrplanar spacing expressed as a percentage of the (mean) bulk interlayer spacing of the disordered alloy. 9 3 and 3 4 are the equivalent quantities for deeper layers. 

Relatively little catalytic work has been carried out so far under conditions where the surface of the metal alloys can be regarded as unreconstructed, i.e., where the chemical composition and structure of the surface can be assumed to be in the state characteristic for the freshly quenched material. In principle, such investigations can only be performed at temperatures far below the crystallization temperature of the alloy and require special precaution to eliminate possible contamination of the alloy during its transfer from the fabrication (melt spinning) to the catalytic reactor. 

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