Kinks atomic

The data presented in Table 1.3 illustrate the dependence of the activation energy of methane on the edge or corner (kink) atom position of some transition-metal surfaces. 

Fig. 4.22 On the surface of a solid, there are a wide variety of atomic processes. A formation of a surface vacancy-adatom pair, or their recombination B association or dissociation of adatoms with an atomic cluster and cluster diffusion C diffusion of a surface vacancy, especially toward the lattice step D falling off a lattice step of an adatom E diffusion of a substitutional or interstitial impurity atom and its interaction with an adatom F diffusion of an adatom and its long range interactions with other adatoms G diffusion, dissociation and activation of a ledge atom H dissociation and activation of a kink atom into an adatom, a ledge atom, or an adatom on the layer above.

Figure 4.9. Projected densities of states onto the d states of the surface atoms for different Pt surfaces with decreasing atom density The hexagonally reconstructed (100) surface, the close-packed (111) surface, the step atoms on a (211) surface and the kink atoms on a (11 8 5) surface. Adapted from Ref. [19].

Fig. 7.134. The half-crystal (or kink) position. The figure shows the binding energy of an atom in a kink position and how the name half-crystal position has been derived. A represents the missing part of the bulk crystal above the surface plane, fithe missing half of the surface plane, and Cthe missing half of the atomic row along the step in front of the kink atom. Together they add the halfcrystal to a bulk crystal. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, 18, copyright 1996, John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.)...

The ability of the STM to achieve atom-resolved real-space images of localized regions of a surface and to directly resolve the local atomic-scale structure has provided essential insight into the active sites on catalysts and emphasized the importance of edges, kinks, atom vacancies, and other defects, which often are difficult to detect with other techniques (46-49). It is evident, however, that STM cannot be used to image real catalysts supported on high-surface-area, porous oxide carriers. 

Fig. 3. LEED patterns and schematic representations of the surface configurations of platinum single-crystal surfaces, (a) Pt(Ill) containing less than 1012 defects/cm2, (b) Pt(557) face containing 2.5 x 1014 step atoms/cm2 with an average spacing between steps of 6 atoms, and (c) Pt(679) containing 2.3 x 10 4 step atoms/cm2 and 7 x 1014 kink atoms/cm2 with an average spacing between steps of 7 atoms and between kinks of 3 atoms.

Deposition of Me on a 3D native bulk phase proceeds by incorporation of atoms in kink site" positions as a final step of the overall reaction (1.1) (cf. Section 2.1). In contrast. Me dissolution can also take place at all sites where lattice atoms are more loosely bound to the crystal than kink atoms. Usually dissolution starts, in addition to the kink sites, at crystal edges and corners, or at surface defects and inhomogeneities. Therefore, the Me deposition-dissolution processes are not necessarily symmetric. At the Nernstian equilibrium potential, equality... 

The salient feature of the kink position is the fact that if an atom is removed fi om such a position, the next atom, in the row of the step edge atoms, will find itself in the same position. The removal of a kink atom fi om the step edge does not change the structure of the surface and is, hence, a repeatable step. A sufficiently large crystal can be disintegrated (evaporated or dissolved) by consecutive removal steps (detachment... 

The second remarkable feature of an atom in a kink position is that it is bonded to the crystal with exactly one half of the bonds of one bulk atom. This is schematically illustrated in Fig. 2.9 for a crystal with a simple cubic lattice. The parts A, B, and C represent exactly the missing half of the infinitely large crystal. After adding A, B, and C, the half ciystal position is changed to a bulk position. For a crystal with a hep or fee (bulk) lattice, a kink atom or an atom in a half crystal position has 6 first neighbors, that is, half of the coordination number of a bulk atom. 

The third veiy important feature of a kink atom is the fact that it represents the final stage of the transfer of an atom from the ambient phase to the crystal. Only after integration of an atom in a kink position can the atom be considered as belonging to the bulk crystal. Therefore, a kink site can be considered as the site of growth or dissolution of the crystal. 

According to Volmer [2.14] the chemical potential,, of kink atoms is related to the separation work of an atom from a kink position, ink. by the equation... 

From viewpoint of the process of deposition or growth, it would be of interest to make an assessment of the step roughness as defined by the density of kink atoms per unit step length, because it would give an important kinetic parameter of growth. The step roughness can be given as the reciprocal of a normalized mean distance parameter Aink /do,Me, where ink is the mean distance between kink atoms and do,Me is the atomic diameter. For the most dense step [110] on a cubic (100) face of a fee crystal, the mean distance parameter is [2.1, 2.15]... 

It has been mentioned already that the exchange of kink atoms with the ambient phase can be used for the definition of the equilibrium. This statement is based on the fact that the separation of kink atoms requires the same Gibbs energy as the average disintegration energy per atom of an infinitely large crystal, which is related to the chemical potential of the atoms in the bulk phase. 

The detachment frequency is proportional to the exponent of the work of separation (or bond energy) of the particular atom at the particular site. For a kink atom, as already discussed, the work of separation is equal to the sublimation enthalpy per atom (cf. eq. (2.2)). 

The equilibrium potential of a crystal is unambiguously determined by the exchange frequency of kink atoms. The concentration of kink sites has no effect on the potential, but the presence of kink atoms is essential for the establishment of the equilibrium potential. 

The equilibrium condition for kink atoms requires an equality between the rates of deposition and dissolution, thus forming an exchange current density of kink atoms. From (2.14) for E = follows ... 

According to eq. (2.21), the exchange current density of kink atoms, io,kink. depends on the concentration of kink atoms kink- As long as the surface profile of a crystal face, which depends on the pretreatment and includes an arbitrary number of steps populated by kinks, the exchange current density of kink atoms is not a constant quantity, but depends on step density L or on the surface topography. It cannot be considered as a system property. 

If, however, for any reason, the electrode potential deviates from the equilibrium potential and the balancing equilibrium with kink atoms is blocked, as, e.g., in the absence of steps and kink sites (cf. also Section 2.3), the adatom concentration may change until a dynamic equilibrium between deposition and dissolution is reached diss,free ads dep.free > free Th adatom concentration, ads, or Cads, becomes a function of electrode potential, E, or overpotential rj = E- E y z+, so that... 

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