Adsorbate-Induced Relaxation and Reconstruction

As noted in Section 1.2, accurate determination of adsorbate-induced changes in surface-normal structure, i.e. the Adj2 interplanar spacing between the first and the second atomic layers, can be achieved by measuring the CTRs [1—4, 10, 35]. Previous reviews summarized adsorbate-induced relaxation and reconstruction on well-defined Pt(hkl) and Pt-bimetallic surfaces in aqueous electrolytes at electrode potentials at which a maximum surface coverage of adsorbed species is established [28, 29]. The data revealed that either close to the hydrogen evolu-... 

Upon increasing the coverage, adsorbed H atoms form disordered or, at lower temperatures, ordered surface phases. Adsorption itself can induce relaxation or reconstruction of the substrate surface or may even lift the relaxation or reconstruction of the clean substrate surface, for example Ref [34]. The van der Waals type bonding of physisorbed H2 however is too weak to cause noticeable displacement of substrate atoms. There is experimental and theoretical evidence [34, 36, 57-60]... 

However, adsorbate could induce various kinds of stresses accompanied with versatile patterns of relaxation and reconstruction [59, 81, 82]. The spacing between the first and the second atomic layers expands if an adsorbate such as C, N, and O buckles into space between the atomic layers even if there is contraction of bonds between the adsorbates and the host atoms [81]. For example, H, C, N, O, S, and CO adsorbates on a metal surface could change the surface stress and cause surface reconstmction because of bond making and breaking. Surface adsorption of sodium ions also increases the stiffness of a microcantilever [83]. 

Central to adsorbate-induced changes in a substrate are the two concepts of relaxation and reconstruction, which collectively may be called restructuring. The terms relaxation and reconstruction need to be defined here, since various interpretations are used in the literature, especially for reconstruction. 

The most important features of both the reflectance and the photolumi-nescence spectra have been explained by the preceding model since it is based on ideal surface structures essentially determined by (001) planes. Thus, several likely possibilities, such as the presence of surface defects, impurities, and remaining adsorbates, the relaxation of the planes exposed at the surface, the impurity-induced reconstruction of the surfaces, and changes in the force constants, have been excluded (80). A more detailed model is needed in which the ion pair of the metal cation and oxygen anion can be taken into account on the basis of such experimental evidence as the hydrogen adsorption on MgO obtained by Coluccia and Tench (65) and Ito et al. (90). 

Only one study is reported on the detailed adsorbate-induced structure of bcc (111). The clean surfaces of Fe and Mo(lll) are not reconstructed. The ideal bcc (111) surface is very open, exposing 1st, 2nd and 3rd layer substrate atoms it exhibits large interlayer spacing relaxations that are oscillatory, but the oscillation is not simply antiphase from layer to layer, cf. Table 20. 

Table 23 lists adsorbate-induced structures on Si, Ge and C(lll). Hydrogen affects C(lll) and presumably Si and Ge(lll) in the simplest manner removal of the reconstruction by capping the dangling bonds of the ideal bulk-like termination, with some residual interlayer spacing relaxations that... 

Figure 4.5 Superstructures formed by adsorbates with (a-c) no substrate changes induced (except for some modification of layer relaxation) and (d-e) with different kinds of substrate reconstructions induced. Panel (f) displays schematically the lifting of a reconstruction by the adsorbate and panel (g) an adsorbate-induced reconstruction switch.

However, there is almost always a substantial interaction between adsorbate and substrate, so that the latter s structure is modified and the same holds for the adsorbate (when it is a molecule). The substrate s modification may be only by a change of the multilayer relaxation or, more drastically, by an adsorbate-induced reconstruction. The latter can come, as indicated in panels (d) and (e) of Figure 4.5, simply by induced displacements of substrate atoms or by chemical reactions (including replacements of atoms). Also, the adsorbate can an existing reconstruction of a clean surface or make it switch to another type of reconstraction, as indicated in panels (f) and (g), respectively. In rare cases, it has also been found that the adsorbate is incorporated in deeper surface layers (subsurface). 

Formation of a surface always requires energy. However, the surface free energy can often be minimized by interplanar relaxation or, more severe by reconstruction of atoms at the surface to positions that deviate greatly from those expected from an ideal termination of the bulk lattice. The presence of adsorbates can alter or induce such reconstructions, and adsorbates themselves can form a variety of structures in the adlayer as well. In the past, LEED and other surface-science techniques have been employed to characterize the structure of these reconstructed surfaces and... 

Adsorbates, cf. Table 11, tend to compensate or even overcompensate for the missing half-crystal this is already apparent on the closer-packed surfaces, but becomes much clearer with the more open surfaces like bcc(lOO) and fcc(llO) (see Sect. 4.1.11 for the latter). The reconstructions tend to be removed or replaced by other reconstructions, while the relaxation of the outermost interlayer spacing often turns from a large contraction to a sizable expansion. Adsorption of adatoms occurs primarily within the large hollows of this surface, with frequent bonding to the second metal layer and a large induced buckling of that second metal layer. 

Adsorbates can induce the removal or change of a reconstruction on fcc(llO) surfaces, cf. Table 15. The resulting reconstruction which has been studied most is that of oxygen on Cu, Ag and Ni(l 10), and N on Rh(llO) it consists of one-dimensional chains like -Cu-O-Cu-0-, where the Cu atoms can be most easily viewed as adatoms on a bulk-like (but relaxed) Cu(llO) surface. Another interesting type of induced reconstruction is a simple 1 1 metallic alloy in the top layer, obtained by substitution of half the surface atoms. A number of more complex reconstructions are also known, produced primarily with N, O, S and P, as well as some other metallic adatoms. 

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