Adatom charge transfer

Lastly, we come to the adatom charge transfer, which is obtained from... 

Table 5.1. Adatom charge transfer Aq and chemisorption energy AE for atomic hydrogen on Ni film of (n + l)-layers thickness on ZnO support.

Fig. 5.4. Dependence of hydrogen chemisorption energy AE (solid line) and adatom charge transfer Aq (dashed line) of 2-layer Ni film on interaction parameter 7. Reprinted from Davison et al (1988) with permission from Elsevier.

Figure 2.14 Equivalent electric circuit simulating the metal deposition process on a stepped surface according to [2.321. Cdi, double layer capacitance Cads, adatom pseudo-capacitance Ret. adatom charge transfer resistance ggd. adatom surface diffusion resistance R e, adatom incorporation resistance Rdt, resistance of the direct transfer reaction 4tep. step half-distance.

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

Finally, another possibility is an initial physisorption (adsorption without charge transfer) of the adatom when the electrode is put in contact with the solution, the phy-sisorbed species remaining on the surface after rinsing, and the reduction taking place after the first negative scan of the potential. 

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. 

The incorporation of the adatoms at the steps should be fast because no charge transfer is involved hence the adatom concentration should attain its equilibrium value ... 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

This picture of chemisorbed atoms on jellium, although much too simple, illustrates a few important aspects of chemisorption. First, the electron levels of adsorbed atoms broaden due to the interaction with the s-electron band of the metal. This is generally the case in chemisorption. Second, the relative position of the broadened adsorbate levels with respect to the Fermi level of the substrate metal determines whether charge transfer between metal and adatom takes place and in which direction. 

Charge transfer Aq to the adatom, for the case cs = cb, is displayed in Fig. 6.4(a). As is evident, the dependence of Aq on cb is virtually linear throughout the range of concentrations. On the other hand, when surface segregation is present (Fig. 6.4b), Aq is lowered and becomes closer to the value for pure Cu (0.05), for all concentrations. Hence, in the segregated case, Aq, like AE, reflects the greater concentration of Cu in the surface layer. 

M) solutions when (na+) and (na ) are equal (unequal). As always, these solutions must be found numerically, and some typical curves of (nacalculated self-consistently, the charge transfer to the adatom is once again given by (4.102). 

Once a particular surface structure has been determined to be stable (i.e., autocompensated), the primary factor determining the nature of the surface reconstruction is the energy that can be gained by rehybridizing the surface dangling bond charge density in response to the reduced coordination at the surface. As a consequence, a charge transfer between the atoms at the surface takes place and this results in the formation of new bonds between surface atoms adsorbed to the surface (also known as adatoms). The formation of new bonds on the surface leads to different chemical and physical properties at the surface.11... 

Recently, we have studied the effect of the surface density of states on the charge-transfer probability, in the case where the surface possesses localized states created by surface perturbations or the presence of adatoms. For the tight-binding linear chain these perturbations or adatoms are taken into account by changing the electronic energy of the end atom of the chain to a, which differs from the energy a of the other atoms in the chain. This difference can lead to the formation of a localized surface state, whose energy is... 

The opposite case, i.e., when the band width is much larger than the hopping matrix element, can be seen in Figure 2.5 for the unoccupied As states of adsorbed on Ag. This has been measured using XAS of Ar adsorbed on Ag, since Ar using the Z + 1 approximation becomes as an effect of the final core hole state [28]. We can directly see that the As level has become a broad asymmetric resonance. The adatom resonance has a tail towards lower energies with clear cut-off at the Fermi level. The 45 level mainly interacts with the delocalized unoccupied Ag sp electrons. Most of the 45 resonance is unoccupied which indicates that charge transfer has occurred from the adatom to the substrate. 

Although Si(100) and Ge(100) undergo similar dimer reconstructions, the Ge(l 11) surface reconstructions differ from those of Si(lll). As described above, Si(lll) reconstructs into a (7 x 7) structure that contains 49 surface atoms in the new unit cell. Ge(lll) is found in various reconstructed forms depending on surface preparation, but the most common reconstruction under vacuum is Ge(lll)-c(2 x 8) [51-53]. This structure involves charge transfer from adatoms to restatoms [5]. On the other hand, most of the passivation and functionalization studies reviewed here lead to the Ge(lll)-1 x 1 surface structure. This structure, in which the surface Ge atoms retain their bulk positions, can be achieved by hydrogen, chlorine, or alkyl termination of the surface (discussed below). The structure is analogous to that for H-terminated Si(lll). 

The theoretical model generally used for predicting the overvoltage-current function for metal/metal ion systems is based on the quasi-thermo-dynamic arguments of transition state theory. The anodic charge transfer process is considered to involve the rupture of the bond between an adatom - i.e. a metal atom in a favourable surface site - and the metal, followed by, or coincident with, the formation of electrostatic bonds between the newly formed ion and solvent or other complexing molecules. The cathodic charge transfer follows this mechanism in reverse ... 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

Electrolytic metal deposition ( electroplating ) is an empirical art widely in use to cover corrosion-sensitive surfaces with a thin protecting metal layer, e.g. of tin, nickel, zinc, etc. The complete plating process comprises several partial processes such as mass transport, charge transfer, adsorption of adatoms, surface diffusion of adatoms, and finally nucleation and crystal growth. 

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