Adsorbate-Induced Surface Relaxation Change

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). 

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-... 

EMIRS has been successfully applied to many systems. Briefly it can be mentioned the study of adsorbates at the electrode surface [10], the detection of adsorbed reaction intermediates for the oxidation of small organic molecules [12], and the determination of the water structure in the double layer [13]. However, the potential modulation in EMIRS is its drawback, since it prevents the study of irreversible processes as the system must return to the same conditions each time the potential is changed. Other important limitations of EMIRS are related to both the electrical and chemical relaxation effects caused by the potential modulation at 12 Hz. The electrical relaxation is due to the high ohmic drop of the electrolyte confined in the thin solution layer required for the in situ measurements. The chemical relaxation is due to ion migration induced by the change in solution composition caused by the electrode potential change. These aspects have been discussed in detail in the following text [14-16] (see Sect. 3.4.2.3). 

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]. 

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). 

Adsorption of the surfactants/polymers and the structure of the adsorbed layer will depend, to a large extent, on the nature of the solid surfaces involved and the interactions between them. Atomic force microscopy can be conveniently used to understand the effect of polymer adsorption on materials such as fibers [13]. It can be seen from fig. 20.2 that such treatment of fibers produces a smooth, uniform, and structurally relaxed surface as compared with the untreated fiber. This indicates that the silicone treatment may be responsible for changes in the microproperties of the fibers, and that it can be used to modify structural properties of fibers to induce smoothness, bounciness, and other such desirable properties. 

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