Surface substrate restructuring

Note finally that, as mentioned in the Introduction, the corrosion of the substrate may also damage irreversibly a microstructured device under the severe conditions of fuel processing reactions. For example, under water vapor pressure, many detrimental effects can occur, such as surface migration of Ni in stainless-steel alloys, surface oxidation of metals (Fe to Fe203), surface enrichment with trace elements able to alloy/react with the coated catalyst (Sn, Pb, Cl ions) and poison it or surface substrate restructuring. 

Figure 4.9. Constant current images (10.5 x 6.9 nm ) at 41 K. (A) HtBDC double row structure (Vt = 1.070 V, k = 0.45 nA). The trenches in the underlying surface are sketched. (B) The trenches in the surface layers are disclosed after manipulating the molecules aside (Vt = 1 mV, k = 1-82 nA). Atomic resolution along the close-packed direction was obtained in the left part of the image (vertical fast scanning direction), whereas it was lost when the tip scanned the restructured area. (C) Ball model of the double row structure. The substrate atoms are shaded darker the deeper the layers lie, while the molecules are shown on top. Reprinted with permission from M. Schunack, L. Petersen, A. Kiihnle, E. Laegsgaard, I. Stensgaard, I. Johannsen and F. Besenbacher, Physical Review Letters 86, 456 (2001). Copyright (2001) by the American Physical Society.

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

This is different at semiconductor surfaces where the covalent bonds between the substrate atoms are often strongly perturbed by the presence of adsorbates. This can result in a significant surface restructuring. Hence the dynamics of the substrate atoms has to be explicitly taken into account which of course increases the complexity of the modelling of the adsorption/desorption dynamics, as will be shown below for the H2/Si system. 

A more dramatic type of restructuring occurs with the adsorption of alkali metals onto certain fee metal surfaces [39] In this case, multilayer composite surfaces are formed in which the alkali and metal atoms are intermixed in an ordered structure. These structures involve the substitution of alkali atoms into substrate sites, and the details of the structures are found to be coverage-dependent. The structures are influenced by the repulsion between the dipoles formed by neighbouring alkali adsorbates and by the interactions of the alkalis with the substrate itself [40]. 

In conclusion, the investigations carried out on the LB films by SPM methods have demonstrated high mobility of the LB films with a tendency for restructuring mainly in the process of formation. The LB film surface morphology changes drastically with varying number of monolayers, subphase and the type of substrate used. 

Ordered Monolayers and the Reasons for Ordering Adsorbate-Induced Restructuring Atomic Adsorption and Penetration into Substrates Metals on Metals Epitaxial Growth Growth Modes at Metal Surfaces Molecular Adsorption... 

The chemical bonds formed during adsorption between the adsorbate and the substrate lead to epitaxy or may induce surface restructuring. 

Ethylidyne restructures the Rh(l 11) crystal face [30], sulfur restructures the Fe(l 10) face [7], and carbon restructures the Ni(lOO) face [6, 46]. The surface metal atoms move into new equilibrium p>ositions upon chemisorption in different ways, and there is evidence of restructuring even in the second substrate layer under the surface. Review the available data and point out the important electronic and structural parameters that influence the nature and magnitude of chemisorption-induced surface restructuring. 

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