Dipole layer

In order to understand the tendency to fomi a dipole layer at the surface, imagine a solid that has been cleaved to expose a surface. If the truncated electron distribution originally present within the sample does not relax, this produces a steplike change in the electron density at the newly created surface (figme B1.26.19(A)). 

Figure Bl.26.20. Diagram showing the dipole layer ereated on a surfaee by an eleetropositive adsorbate.

Thus the potential difference at the interface between a metal and electrolyte solution is due to both the charges at the interface (electrostatic potential difference) and the surface dipole layers the latter is referred to as the surface or adsorption potential difference. On the basis of the above considerations it might appear that adsorption at a metal surface with an excess charge is solely due to electrostatic interaction with charged species in the solution, i.e. if the metal surface has an excess negative charge the cations... 

A macroscopic model for regular air/solution interfaces has been proposed by Koczorowski et al 1 The model is based on the Helmholtz formula for dipole layers using macroscopic quantities such as dielectric constants and dipole moments. The model quantitatively reproduces Ax values [Eq. (37)], but it needs improvement to account for lateral interaction effects. 

The extent of the agreement of the theoretical calculations with the experiments is somewhat unexpected since MSA is an approximate theory and the underlying model is rough. In particular, water is not a system of dipolar hard spheres.281 However, the good agreement is an indication of the utility of recent advances in the application of statistical mechanics to the study of the electric dipole layer at metal electrodes. 

The attractive potential due to the positively charged cores is not strong enough to keep the valence electrons inside the metal. As a result, the electrons spill out into the vacuum, i.e. the electron density just outside the surface is not zero. Because the charge of these electrons is not compensated by positive ions, a dipole layer exists at the surface, with the negative end to the outside. An electron traveling from the solid to the vacuum must overcome this barrier of height [Pg.228]

The energy needed to surmount the surface dipole layer is the surface contribution to the work function. It depends very much on the structure of the surface For fee metals the (111) surface is the most densely packed surface, and has the largest work function because the dipole barrier is high. A more open surface such as fee (110) has a smaller work function. Also, when a surface contains many defects, the... 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

As an example, ZnO has Zn exposed at one end of the <0001) axis and O at the other. Therefore, there are oppositely directed molecular dipole layers at the two ends ZnO at one end, and OZn at the other. These differing double layers lead to differing hardnesses between the two surfaces (Cline and Kahn, 1963). The difference for BeO was 1300 vs. 1100kg/mm2, or 18 percent, for indentations on the (0001) compared with the (000-1) face. In the case of ZnO the difference was 274 vs. 238kg/mm2, or 15 percent. 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

As was pointed out in Chapters 2 and 3, a dipole layer exists at the surface of a metal, which gives rise to a concomitant surface dipole potential x- The magnitude of this potential changes in the presence of an external electric field. A field E directed away from the surface induces an excess charge density, o e0e E, where e is the dielectric constant of the medium outside the metal. The field E pushes the electrons into the metal, producing the required excess charge, and decreasing the dipole potential (see Fig. 3.4). This has consequences for the interfacial capacity. 

STM is based on the tunneling of electrons between the surface and a very sharp tip [36,49]. As explained in the Appendix, the cloud of electrons at the surface is not entirely confined to the surface atoms but extends into the vacuum (this effect causes the electric dipole layer at the surface that contributes to the work function). When an extremely fine tip (see Fig. 7.18) approaches the surface to within a few angstroms, the electron clouds of the two start to overlap. A small positive potential... 

The intimate relationship between double layer emersion and parameters fundamental to electrochemical interfaces is shown. The surface dipole layer (xs) of 80SS sat. KC1 electrolyte is measured as the difference in outer potentials of an emersed oxide-coated Au electrode and the electrolyte. The value of +0.050 V compares favorably with previous determinations of g. Emersion of Au is discussed in terms of UHV work function measurements and the relationship between emersed electrodes and absolute half-cell potentials. Results show that either the accepted work function value of Hg in N2 is off by 0.4 eV, or the dipole contribution to the double layer (perhaps the "jellium" surface dipole layer of noble metal electrodes) changes by 0.4 V between solution and UHV. 

Emersion involves fundamental aspects of condensed matter surface science and electrochemistry, and its consideration offers new insight into these fundamentals. For example, when a new solid or liquid surface is made the atcms or molecules may rearrange at the surface to form a surface dipole layer. This certainly happens... 

Figure 3.17. Jelium model of a metal surface. The ion density terminates abmptly at the surface, but the electron density A/giectron extends beyond it. The net charge density — A/giectron gives a dipole layer.

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

The effect of the orientation of water dipoles on the electrode on the properties of the interphase was studied by Macdonald (6) and Mott and Watts-Tobin (7). Bockris et al. (8), in a modification of the Grahame model, considered adsorption of completely hydrated ions at the electrode with the water dipole layer present. 

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