Sub-surface layer

Fig.l. Total energy (lmRy=0.314 kcal/mole) per unit cell for a five-layer W(OOl) slab as a function of lateral displacement, 5, and spacing between the surface and sub-surface layers, A the latter is given relative to the bulk spacing of 1.581 A (after Ref. 26). 

Rutherford Backscattering Spectrometry (RBS) is a non-destructive (sub)-surface analysis technique for solid systems. In principle atomic composition and depth distribution can be obtained for a sub-surface layer of a few microns. 

Catalysts such as the platinum group metals can be used in dispersed or monolithic solid form. The catalyst can be deposited on the surface of a membrane (dense or porous) or, in the cases of catalyst particles, dispersed in the sub>surface layer or throughout the matrix of a porous membrane. 

Fig. 16 Electron transfers per bonds introduced in the modelization of an SrTi03(001) surface (left pannel) and of an MgO(lll) surface (right pannel). On SrTi03(001), specific transfers A(p q and Agj o (represented by thick arrows) are introduced inside the surface layer and in between the surface and sub-surface layers for Ti-0 bonds and Sr-0 bonds involving surface atoms. On MgO(lll), only the first inter-plane transfer is assumed to be modified, (from Ref. 4).

By following the discussed experiments, a model for the Ir-Cu(lOO) surface has been suggested and is presented in Fig. 14. As a matter of fact, a two dimensional epitaxial sub-surface alloy has developed and consists of adjacent chains of Ir and Cu atoms along the [Oil] directions to form an ordered (2x1) periodicity. The Ir-Cu sub-surface layer happens to be buried under a monolayer of copper. Remarkably enough, although the surface crystallography of Cu(lOO) expresses four-fold symmetry, a two fold symmetric pattern is showing up for the chains of subsurface Ir to resemble the (2x1) superstructure. 

An example of a different kind is afforded by the catalytic oxidation of CO on nickel oxide. Oxygen is taken up by the NiO to form a deficit semi-conductor the addition of an oxygen atom to an anion lattice site creates an adjacent vacancy in the cation lattice and draws electrons from the valence band of the lattice to produce an 0 ion. The cation vacancy traps a positive hole in its field and can move through the immediate sub-surface layers of the lattice by a vacancy mechanism. 

Fig. 15. Local density of states on W(0(H)(V xv/2)R45D (Singh and Krakauer, 1988). (s) Surface layer (s - 1) sub-surface layer and (c) bulk-like layer.

The sub-surface layer formed beneath the mixed scales after short oxidation times in air and Ar/02 clearly differ. For Ti50Al no clear depletion layer can be observed after air oxidation for 24 h (Fig. 6). Instead, optical metallograhpy clearly revealed the presence of a very thin,Ti-rich nitride layer beneath the oxide (Fig. 6). Similar observations were made for Ti48Al whereas Ti45AI formed after this oxidation time already a clear depletion layer similar to that in Ar/Oz. After lOOh oxidation in air, also for Ti50Al the two-phase depletion layer was found, whereas for Ti48AI, still a thin nitride layer was observed (Fig. 7). 

An interesting result is that the absolute rate of desorption for the desorbing protein is the same in the mixed film as in the pure monolayer providing the area occupied is over 30% of the total surface area ( ). Under these conditions, the concentration of protein in the sub-surface layer is evidently determined by the surface pressure and not the surface density. This result has implications for systems where proteins are transported across Interfaces or membranes. 

It forms a Ni-enriched top-surface above the sub-surface layer of Mgp2 that is followed by the ordinal phase of Mg2NiH4 by a fluorination treatment and the Mgp2 surface works effectively in hydrolysis to break the B—H bond to generate hydrogen (BH4" + 2H2O 4H2 + B02 ). 

Zr. x O2 with X values between 0.6 and 0.7 exhibited four times the OSC capacity." Interestingly, all the samples in this range had the fluorite structure. If one combines these results with surface area data, an important conclusion emerges. Whereas with ceria the OSC properties are limited to the surface, with ceria-zirconia at least one sub-surface layer contributes. 

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