Surface dipole layer

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... 

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... 

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... 

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. 

A1 electrode, the polarization of the Co top electrode dropped to 6% and the barrier height increased to 1.8 eV. Santos et al. rationalized the increase in the barrier height in the absence of the AI2O3 layer to the surface dipole layer that is known to exist at Al/Alqs interfaces [49]. One of the key findings of the Santos study was the observation of a positive TMR value both with and without the AI2O3 layer, as expected based on the known sign of the electrode polarization [17]. 

Now, what will happen if the material phase (e.g., the electrolytic solution) is imagined to be bereft of either surface charge or a surface dipole layer Consider a thought experiment (Fig. 6.44) involving the transport of a test chaige from infinity to a point deep inside the solution phase. The outer, or /, potential will be zero because the solution is uncharged. Similarly, the surface, or %, potential will be zero because there are no surface dipole layers. Hence, the inner, or < ), potential will also be zero, which means that zero electrical work is done with the test charge. 

In the case of a liquid phase, the/ potential is associated with the net preferential orientation of dipoles at the surface. This arrangement is equivalent to a charge separation and a potential difference occurs across the surface dipole layer. The estimation of / remains unsolved [8]. 

Let us assume that we have a facetted surface, or a surface with perfect and defect domains. The potential barrier that an electron has to surmount when leaving the solid through a defect region or through an open facet, for example of (110) orientation, is lower than that when the electrons pass through the (111) surface. The relevant quantity in chemisorption theory is not the averaged macroscopic work function, but rather is the work function of the site where a molecule adsorbs. We therefore define the local work function of a site as the difference between the potential of an electron just outside the surface dipole layer and the... 

The other extreme is the case of adsorption outside the surface dipole layer, whereby the adsorbate interacts only weakly with the surface. A case in point... 

It should be noted immediately that of the anticipated corrections, those due to surface dipole layers are different on different crystal faces—owing to different reconstruction, for example. In fact, photothresbolds can vary by quantities of the order ofan electron volt on different faces. (The energy required to take the electron to infinitcdistancc through different faces cannot differ if both faces are on the same... 

A similar treatment can be used to calculate the electric field and the electric potential in the metal. However, in a metal, both the electric field and the electric potential drop to zero at a very short distance from the semiconductor/metal interface. This occurs because metals do not support electric fields, and all of the excess charge density resides on the surface of the metallic phase. The surface dipole layer is therefore effectively screened from test charges at any finite distance into the metallic phase, and the width of the electric potential gradient is extremely small. Because charge carriers can pass freely through this extremely thin barrier, only the electric field in the semiconductor significantly affects the electrical properties of semiconductor/metal contacts. 

The term cj> — (f) ) is also the potential due to the surface dipole layer. It is a function of the condition of the surface and is not a constant. The problem in elctrochemistry is that the emphasis is on measuring electrical potentials. In addition to the potential due to different values of /i for two electrodes, the... 

Changes in ionization potential do also play a role on metalsurfaces. Because of the electric double layer caused by spill-over of surface electrons to the vacuum, the ionization potential of a metal surface (the workfunction) decreases if one compares the workfunction of a close-packed surface containing surface atoms of high coordination, with a more open surface, containing surface atoms with a lower coordination. As we will discuss, not only changes of the surface-dipole layer occur if one compares different surfaces, but also metal-metal atom distances may change (they usually decrease if one compares dense surfaces with more open surfaces). 

The importance of q.(2.320) is that it indicates that local electrostatic terms play a role in the adsorption energies of adsorbates. An example will be given in the next section. In the weak adsorption limit, q.(2.245) for the chemisorptive bond energy has to be replaced by Eq.(2.321) if surface dipole layer effects and electrostatic consequences of differences in particle size have to be discussed. 

We also mentioned earlier the oscillatory nature of the density as a function of metal-surface distance. If the electron density is high the oscillation decreases by the electron-electron interactions. We will revert to this later. Because of the electron spillover at z > 0, negative charges will not be balanced by the positive background. As a result a surface-dipole layer develops. The potential due to this double layer can be calculated with Poisson s equation ... 

V is the ion volume). A larger contraction in perpendicular direction to the surface of the cation distances than that of the anions creates a highly asymmetric local anion environment, enhancing the local electrostatic field which the anion experiences. This decreases the surface energy because of the term Eq.(4.4). Puckering of the surface gives rise to surface dipole layer. 

If strong illumination produces flat bands at the surface without changing X, that is, without aflfecting the surface dipole layer, then the saturation value of the surface photovoltage measured by the Kelvin method A should be... 

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