Potential-dependent adsorption

The influence of the surface structure on the anion adsorption. Potential dependence of the adsorption of sulfate ions at various crystal faces of copper as studied by radiotracer technique [101] is shown in Fig. 1. 

Some general features of crystalline surfaces can be pointed out In this case, the adsorption potential depends on both the plane vector t and on the distance from the surface. The force field generated by the crystalline surface has a periodic character. One can observe distinct minima of different depths separated by barriers of different heights. The zone of atomic size located on the minima constitute the adsorption site. These sites are separated from one another by a saddle point, so that an activation energy is required for siuface migration. 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

While Eq. (36) is valid for 9 = 1, a qualitatively similar equation is obtained at any value of 9. Since the condition 9 - 1 is difficult to reach experimentally, the value of AEaB0 (adsorption potential shift) is often estimated by means of extrapolation to 9 = 1. This procedure is very delicate and the result is often misleading. The variation of EOm0 with 9 may be linear or nonlinear, depending on lateral interactions between... 

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

This potential depends on the interfacial tension am of a passivated metal/electrolyte interface shifting to the lower potential side with decreasing am. The lowest film breakdown potential AEj depends on the surface tension of the breakdown site at which the film-free metal surface comes into contact with the electrolyte. A decrease in the surface tension from am = 0.41 J m"2 to nonmetallic inclusions on the metal surface, will cause a shift of the lowest breakdown potential by about 0.3 V in the less noble direction. 

The free energy of adsorption, /16r Ds is potential-dependent and, in the case of an adsorbate which is an uncharged molecule,... 

According to the macroscopic model, the adsorption potential shift is due to the removal of some solvent molecules, s, from the surface region and accommodating there the oriented molecules of adsorbate, B."" Using the assumptions listed in Ref 114, the dependence for A% is of the form... 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

Watanabe, S., Inukai, J. and Ito, M. (1993) Coverage and potential dependent CO adsorption on Pt(llll), (711) and (100) electrode surfaces studied by infrared reflection absorption spectroscopy. Surf. Sci., 293, 1-9. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Had) or more strongly (Oad/OHad) bound. In Fig. 14.7, we illustrate the resulting potential-dependent adlayer formation and replacement processes for anodic (upper part) and cathodic (lower part) scan directions. In the negative-going scan, H pd formed on the Pt islands can react with OHad on neighboring Ru sites and desorb as H2O [equivalent to Reaction (14.1a)]. Spillover of further H pd from the Pt islands to the Ru terraces or direct adsorption of H pd on the Ru areas results in further OHad removal and subsequent replacement by Had. The pronounced shift of peak A from... 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

The XPS results obtained by Kolb and Hansen are reproduced in Fig. 6 and they clearly demonstrate not only that cations as well as anions stay on the surface but also that the amount of ions exhibits the expected potential dependence even in the case of specific adsorption. The preservation of the double layer charge after emersion was also shown by other techniques like charge monitoring [28] and electroreflectance measurements [29],... 

Fig. 4.4. Potential dependence of (a) the coverage degree of platinum by tin and (b) charge transferred during tin adsorption on platinum (according to Eq. 4.1). Tin was adsorbed from Sn(S04)2 ( ) and from SnS04 (O).

Here K is a constant, Te and Tf are the number of empty and filled sites per unit area on the metal surface, , is the adsorption potential, and is the electrostatic potential of the empty site , depends on surface charge. The sum T0 = Te + T/ total number of sites per unit area, depends on the metal, as does fa. 

The potential at which adsorption occurs (if the process is potential-dependent) and that at which stripping commences. 

Corrigan and Weaver employed the PDIR approach to study the potential-dependent adsorption of azide, N , at a silver electrode. The potential was switched between the reference value, —0.97 V vs. SCE (where adsorption is known to be limited) and the working potential every 30-60 scans, i.e. up to a minute per step, to a total of c. 1000 scans. The high number of scans was required in order to obtain the required S/N ratio hence the PDIR technique was employed to minimise instrumental drift. Since the electrochemical process under study was totally reversible on the timescale of the experiment, the PDIR technique was a viable option. 

---