Surface reconstruction and transformation

FIGURE 2.17. A series of microstructures on stainless steel fabricated by the electrochemical micromachining technique [15]. 

The absence of part of the nearest neighbors that are present in the bulk no doubt affects the bonding as well as the structure of the atoms in the topmost layer. Even if their lateral configuration is identical to that of the corresponding bulk plane, the spacing between the first and the second layer will typically be slightly reduced, while that between the second and the third layer will be somewhat expended (relaxation). As outlined in Section 1.2, the energy of the adsorption bond is comparable to that between 

An example for the modification of the interlayer spacing by adsorption is shown in Fig. 2.18 for the system H/Ni(l 10) at 0 = 1 [17]. The adsorbed H atoms are located in threefold sites formed by atoms from the first and second Ni layers, where the spacing between the first and the second layer is compressed to 1.18 A and that between the second and the third layer is expanded to 1.32 A, while the bulk value is 1.25 A 

True reconstruction implies a variation in the atomic density in the topmost layer of the substrate if compared with the corresponding bulk plane. Such a situation is found not only with numerous clean semiconductor surfaces but also with the (10 0) and (11 0) surfaces of the fee 5d metals [18]. 

As an example, the situation for Pt(l 0 0) will be illustrated. The stable clean surface exhibits a quasi-hexagonal ( hex or 5 x 20) 

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Adsorbate-induced surface reconstruction and dissociative chemisorption are merely natural extremes of this delicate balance. In each case, strong surface-adsorbate interactions direct the course of the transformation, either breaking up bonding in the surface, so that it reconstructs, or disrupting the adsorbed molecule.59 An incisive discussion of the latter situation for the case of acetylene on iron and vanadium surfaces was provided by A. B. Anderson.60... 

The dual-state behaviour of RU-AI2O3 catalysts may also arise from metal-support interaction. In the oxidized state, the catalyst was more selective for nitrogen formation in NO reduction than when in the reduced state. It was also active for the water-gas shift reaction whereas the reduced form was rather inactive and differences were also observed for ammonia decomposition and the CO-H2 reaction. The more active form does not appear to contain ruthenium oxide the reduced catalyst may have been de-activated by reaction with the support and its transformation to the more active form by oxidation may involve surface reconstruction and/or destruction of the metal-support interaction. 

In the past several years, we have already succeeded in demonstrating that the technique can be used to monitor molecular adsorption and desorption at a variety of interfaces, to probe the spectrum of submonolayers of molecules adsorbed on surfaces,3 to measure the orientation and distribution of adsorbed molecules,to study surface reconstruction and phase transformation of semiconductors,5 and so on. In this paper, we describe a few additional experiments that we have recently carried out in our laboratory to further explore the applicability of surface SHG. 

An interesting phenomenon that nicely illustrates the consequence of the dynamic surface events is shown in Fig. 2.36, which shows the structure of an Ir-surface, after exposure at 1000 °C to an oxidizing mixture of methane giving CO, CO2 and H2O. Within half an hour the initial surface, which was flat, is transformed into a moimtainous landscape with altitude differences on the order of 1 /xm. Kramer has estimated that in processes that result in these transformations each surface atom can jump at a rate of 10 /sec, whereas the elementary reaction rates that occur under these conditions occur at a rate of 10 -10 /sec. The most adequate picture of this reactive surface was suggested to be that of a dynamically changing surface that reacts with a quasi-static ensemble of adsorbed molecules or molecular fragments. The surface etching process is the result of the balance between momentous stable surface reconstruction and destabilization of... 

Carbon may bind to a metal surface and induce a surface reconstruction whereby a more active metal plane is transformed to one with a lower activity.28... 

Fig. S-2. Activation energy both ibr reconstruction of the surface (100) plane of platinum crystals in vacuum and for im-reconstruction of the reconstructed surface due to adsorption of C 0 (1x1) (5x20) is surface lattice transformation (reconstruction and un-reconstruc-tion). 6 = adsorption coverage. [From Ertl, 1985.]...

The tungsten (110) surface is one of the best studied of all surfaces, especially in field emission and field ion microscopy for many reasons. It is a very stable surface without surface reconstruction or phase transformation. It is also inert to contaminations. For the study of adatom-adatom interactions, it is a very smooth plane with the largest density of adsorption sites available of any W surface. Lesser restrictions are imposed on the adatom-adatom separation. As the surface is structurally very smooth, wave mechanical interference effects are least affected by the surface atomic structure. 

All these observed features underline the fundamental importance of the adsorbate-induced surface structural transformations and can be qualitatively rationalized in terms of the outlined mechanism. For example, in order to obtain oscillations at given T and p0, the CO pressure (causing the respective CO coverage) must be high enough in order to lift the surface reconstruction, but, on the other hand, also sufficiently low to permit subsequently the reactive removal of the CO adlayer by oxygen. 

From the microscopic point of view, again the CO-induced 1 x 2- 1 x 1 structural transformation of the Pt(l 10) surface (as also underlying the mechanism of temporal oscillations) is of crucial importance for the development of facets, as becomes evident from the fact that this effect is restricted to conditions of high stationary CO coverages. Simply speaking, CO adsorption lifts the 1 x 2 reconstruction and simultaneously creates... 

The mechanism of the kinetic oscillations occurring with the CO + 07 reaction on clean Pt( 100) and Pt( 110) surfaces was based on the reversible transformation of the surface structure by the presence of adsorbed CO and by an associated variation of the oxygen sticking coefficient that increased upon CO-induced lifting of the reconstruction of the clean surface. The most densely packed Pt(lll) surface is not reconstructed and its structure is also not affected by CO adsorption. Accordingly, kinetic oscillations with a clean Pt(lll) surface (i.e., for partial pressure <10 3 torr) could never be observed (13, 26, 27, 38). Again no reconstruction... 

Under the influence of the adsorbate, the surface structure may switch periodically between more (=non-reconstructed) and less reactive (=reconstructed) state, whereby the driving forces are the difference in surface free energy of the clean planes on the one hand and the difference in CO adsorption energy on the other. In other words, the reconstructed phase adsorbs CO more rapidly than it is reacted. Thus, the CO coverage increases beyond its critical value for nucleation of the structural transformation into the non-reconstructed state. The latter exhibits an increased oxygen coefficient so that CO is removed more rapidly from the surface. As a result, the CO coverage drops and the surface transforms back to reconstructed states. 

The clean Pt (100) and (110) surfaces are reconstructed and then construction is lifted if a critical CO coverage is reacted. Both modifications of the respective planes exhibit different sticking coefficients so that as a net result, the surface switches between the states of high and low reactivities. Thus, the rate of catalytic CO oxidation on defined Pt (100) and Pt (110) surfaces at low pressure under isothermal condition exhibits temporal oscillations which are coupled with periodic transformation of the surface structures between reconstructed and non-reconstructed phases [66],... 

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