Surfaces close-packed

Figure Bl.21.1 shows a number of other clean umeconstnicted low-Miller-index surfaces. Most surfaces studied in surface science have low Miller indices, like (111), (110) and (100). These planes correspond to relatively close-packed surfaces that are atomically rather smooth. With fee materials, the (111) surface is the densest and smoothest, followed by the (100) surface the (110) surface is somewhat more open , in the sense that an additional atom with the same or smaller diameter can bond directly to an atom in the second substrate layer. For the hexagonal close-packed (licp) materials, the (0001) surface is very similar to the fee (111) surface the difference only occurs deeper into the surface, namely in the fashion of stacking of the hexagonal close-packed monolayers onto each other (ABABAB.. . versus ABCABC.. ., in the convenient layerstacking notation). The hep (1010) surface resembles the fee (110) surface to some extent, in that it also...

Reconstruction of the Cu(l 11) close-packed surface at room temperature upon oxygen adsorption has been reported by Niehus.590 This result is in good agreement with data on cyclic voltammetry and second-harmonic generation591 it has been concluded that oxygen-containing spe-... 

For metals, the close-packed surfaces have, in general, the smallest surface free energy and therefore these surfaces dominate on small particles, e.g. the (111) surfaces for the fee and hep metals, and the (110) surface for the bcc metals, although on iron particles the (100) surface is abundantly present. Surface free energies have been tabulated [L. Vitos, A. Ruban, H. Shriver and J. Kollar, Surf. Sci. 411 (1998) 186], To give an idea of how the values depend on crystal face we list some values for palladium ... 

All these questions can be answered if we consider the transition states for the dissociation reactions, which are all very similar. The transition state structure for a given substrate geometry is essentially independent of the type of molecule and substrate. Thus the close packed surfaces as well as the stepped surfaces considered in Fig. 6.42 each form a group. Dissociation is furthermore characterized by a late transition state, in which the two atoms have already separated to a large extent and... 

Pfniir H, Held G, Lindroos M, Menzel D. 1989. Oxygen induced reconstruction of a close-packed surface A LEED IV study on Ru(001)-p(2 x 1)0. Surf Sci 220 43. 

For niobium and cobalt clusters structures have been proposed based upon the elements behavi or (71). Niobium s specific inertness has been associated with structures that are analogous to close-packed surface of W(110) which also has an activation barrier for hydrogen chemisorption. Since the IPs are also expected to be higher for closed packed structures these two sets of observations are in agreement. This model at its current stage of development requires different structures for each system and as yet has not been useful in making predictions. 

Thus, the spectroscopic work proved that the two forms of adsorbed hydrogen are very different. The weakly adsorbed form is definitely adsorbed above the Pt surface plane and so able to interact with the solution. In contrast, the strongly adsorbed form cannot interact with the solution, lying in the plane of the surface Pt atoms with its electron in the conduction band of the metal. This conclusion was supported by work that showed the strongly adsorbed form was not formed on close-packed surfaces. 

Figure 2.11 Thermal desorption spectra of silver from the close-packed surface of ruthenium for different initial Ag coverages. Desorption from the second layer of silver occurs at lower temperatures, indicating that Ag-Ag bonds are weaker than Ag-Ru bonds. Note the exponential increase of the low temperature sides of the peaks, indicating that the desorption follows zero-order kinetics (from Niemantsverdriet et al. [18]).

The surfaces are labeled according to the lattice plane that is exposed. The (111), (100) and (110) surfaces are perpendicular to the < 111 >, <100> and <1I0> directions in the crystal. The close-packed surface of the hep lattice, the (001) plane, [or strictly speaking the (0001) plane, because four coordinates are used for hexagonal lattices], has the same structure as the fee (111) plane [11]. 

Fig. 6-1. TVo-dimensional atomic structure on the (100) plane of platinum crystals (1x1) = cubic close-packed surface plane identical with the (100) plane (5 x 20) = hexagonal dose-packed surface plane reconstructed finm the original (100) plane. [From Kolb, 1993.]...

The problem of the variation in the surface energies of various crystal facets can be attacked from several points of view. Bravais first noted that those planes of a crystal which were most densely packed and were also separated most distantly from the neighbouring parallel plane were those which appeared most frequently in crystals he noted also that a closely packed surface was usually associated with a wide interplanar distance and vice versa. Later Willard Gibbs indicated that the most stable planes on a growing crystal were those possessing the least interfacial surface energy. 

To explain these differences we must assume a different surface valency for copper and silver to that for platinum and gold. The simplest assumption to make is that the copper and silver possess a variable valency on each face as assumed for rock salt whilst the metals forming stable octahedra are those in which a surface atom possesses the same valency irrespective of its position on the crystal surfece the most closely packed surface would then be most 1 V2 V3... 

Fig. 6.5. Close-packed surface with tetragonal symmetry, (a) The square lattice in real space. There is an atom on each lattice point, (b) The reciprocal space.

Using these equations and the conductance distribution functions listed in Table 6.1, the corrugation amplitudes for a tetragonal close-packed surface with different tip states and sample states can be obtained. For example, for a Is state, using Eq. (6.32), we have... 

Fig. 6.9. Corrugation amplitudes of a hexagonal close-packed surface. Solid curve, theoretical corrugation amplitude for an s and a d,- tip state, on a close-packed metal surface with a=2.88 A and 4>=3.5 eV. The orbitals on each metal atom on the sample is assumed to be 1 i-type. Measured STM corrugation amplitudes are from the data of Wintterlin et al. (1989). The first-principle calculation of Al(lll) is taken from Mednick and Kleinman (1980). The corrugation amplitude for a 4-wave tip state is more than one order of magnitude smaller then the experimental corrugation. (Reproduced from Chen, 1991, with permission.)...

As for the close packed surface layer it is assumed that the micelles do not contain any water, i.e. x + x = 1. Assuming ideal mixing of the two surfactant types in the mixed micelle, i.e. f. = f2 = 1, Equation 23 shows that the cmc is the arithmetic mean or the two single surfactant erne s with the surfactant composition in the micelle as the variable. 

We note in Figure 4.24 that the geometrical effect discussed for N2 dissociation holds for all the adsorbates considered. This means that CO, NO, and 02 dissociation should also be much faster at steps than at the most close-packed surface [81-83]. As noted above this is in agreement with a growing body of experimental evidence [83-85]. 

Table II shows that at temperatures between 450 and 550 K all the close-packed surfaces investigated, and additionally Cu(100), Pd(100), and Fe(110), give similar spectra. These have latterly been attibuted to a surface species that we designate a(CCH). The spectrum has bands at ca. 3000 cm 1 (m), r CH 1360-1300 cm-1 (m, bd), vCC and 870-730 cm-1 (s), 5CH or yCH. We designate this species a(CCH) to distinguish it from others given the same formula but with notably different spectra that we discuss later. A particularly good example of an a(CCH) spectrum was observed for the species on Pd(lll) (24), whereby it was clearly distinguished from a spectrum from adsorbed benzene, which also has a strong absorption (at 730 cm-1) in the low-wavenumber region. Another example is provided by the species on Ni(lll) at 550 K (14).

---