Adsorption Helmholtz layer

The effect of surface deformation in the Helmholtz layer should also be involved in Eq. (35). In consideration of specific adsorption of anions, such effects can be expressed by the potential gradient Lj y, z, as follows,... 

Hence, in Eq. (36), which sign, positive or negative, should be chosen depends on the adsorption state of ionic species in the Helmholtz layer if any kind of specific adsorption is neglected or such adsorption is not so intense, the positive sign can be adopted because there is no inversion of the signs of the electric potentials, as depicted in Fig. 23. This means that the sign of the potential difference in the Helmholtz layer is the same as that of the potential difference in the diffuse layer, i.e.,... 

In the potential region where nonequilibrium fluctuations are kept stable, subsequent pitting dissolution of the metal is kept to a minimum. In this case, the passive metal apparently can be treated as an ideally polarized electrode. Then, the passive film is thought to repeat more or less stochastically, rupturing and repairing all over the surface. So it can be assumed that the passive film itself (at least at the initial stage of dissolution) behaves just like an adsorption film dynamically formed by adsorbants. This assumption allows us to employ the usual double-layer theory including a diffuse layer and a Helmholtz layer. 

Figure 3.15 The change in ftee adsorption energy as a function of the electric field. At the top of the figure, we estimate the corresponding potential change by assuming that the potential drops over a Helmholtz layer of thickness 3 A.

For a long time, the electric double layer was compared to a capacitor with two plates, one of which was the charged metal and the other, the ions in the solution. In the absence of specific adsorption, the two plates were viewed as separated only by a layer of solvent. This model was later modified by Stem, who took into account the existence of the diffuse layer. He combined both concepts, postulating that the double layer consists of a rigid part called the inner—or Helmholtz—layer, and a diffuse layer of ions extending from the outer Helmholtz plane into the bulk of the solution. Accordingly, the potential drop between the metal and the bulk consists of two parts ... 

Ionic contact adsorption on metallic electrodes alters the potential profile across the compact layer at constant electrode potential. If anions are adsorbed on the metal electrode at positive potentials, the adsorption-induced dipole generates a potential across the inner Helmholtz layer (IHL) as illustrated in Fig. 5-29. The electric field in the outer part (OHL) of the compact layer, as a result, becomes dififerent fi om and frequently opposite to that in the inner part (IHL) of the compact layer. 

In general, the contact adsorption of anions creates an electric field of intensity Sto in the inner Helmholtz layer, which may be greater than the average field intensity of. B,v = ( in +. Eout)/2, where aat is the field intensity in the outer Helmholtz layer. The rate of field increase rj may be derived from electrostatics as shown in Eqn. 5-50 [Liu, 1983] ... 

Since the electron transfer of the interfacial redox reaction, + cm = H.a> on electrodes takes place between the iimer Helmholtz plane (adsorption plane at distance d ) and the electrode metal, the ratio of adsorption coverages 0h,j/ in electron transfer equilibrium (hence, the charge transfer coefficient, 6z) is given in Eqn. 5-58 as a function of the potential vid /diOMn across the inner Helmholtz layer ... 

Cations and anions with a strong solvation shell retain their solvation shell and thus interact with the electrode surface only through electrostatic forces. Since the interaction is exclusively electrostatic, the amount of these ions at the interface is defined by the electrostatic bias between the sample and the counter electrodes and independent from the chemical properties of the electrode surface non-specific adsorption. Considering the size effect of their hydration shell, these ions are able to approach the electrode to a distance limited by the size of the solvation shell of the ion. The center of these ions at a distance of closest approach defined by the size of the solvation shell is called the outer Helmholtz layer. The electrode surface and the outer Helmholtz layer have charges of equal magnitude but opposite sign, resulting in the formation of an equivalent of a plate condenser on a scale of a molecular layer. Helmholtz proposed such a plate condenser on such a molecular scale for the first time in the middle of the nineteenth century. 

Ions with a weak solvation shell, anions in general, lose a part of or the complete solvation shell in the double layer and form a chemical bond to the metal surface. The adsorption is termed specific since the interaction occurs only for certain ions or molecules and is not related to the charge on the ion. The plane where the center of these ions are located is called the inner Helmholtz layer. In the specific adsorption, ions are chemically bound to the surface and the interaction has a covalent nature. In the case of non-specific adsorption, in which an electrostatic force binds ions to the surface, the coverage of ions is below 0.1 -0.2 ML due to electrostatic repulsion between the ions. In contrast, the coverage of specifically adsorbed ions exceeds this value, and a close-packed layer of specifically adsorbed ions is often observed. Specifically adsorbed ions are easily observed by STM [22], indicating that the junction between the electrode surface and the inner Helmholtz layer is highly... 

The term A GE represents the electrical work done in moving an ion of charge ze0 and water molecules with dipole moments fi between the outer Helmholtz layer and the inner Helmholtz layer in the electric field, X, arising from the charge of the metal (Section 6.8.2.1). Thus, it can split into AGE - AG i + AG w. If some transfer of charge (Section 6.8.2.1) occurs during the adsorption process, Eq. (6.210) can be written as... 

Inorganic Ions. Because of electrostatic attraction, positive ions are attracted to negatively charged surfaces and have a higher concentration near the surface than in the bulk. Negative ions are repelled from the negative surface and have a lower concentration near that surface. Ions which are very strongly bound (/zADS > kT) are in the Stem layer, whereas those that can move into and out of the ionic atmosphere (nADS < kT) are in the Helmholtz layer. The effect of ionic attraction or repulsion from the surface is to enhance or reduce the nonionic adsorption coefficient ... 

The second important difference is that the interface potential is present at the (outer) Helmholtz layer of the semiconductor/soiution interface. The interface potential is produced by surface dipoles of surface bonds as well as surface charges due to ionic adsorption equilibria between the semiconductor surface and the solution. If the interface potential can be regulated by a change in the chemical structure of the semiconductor surface, then the semiconductor band energies can be shifted to match the energy levels of the solution species (oxidant or reductant). This is another advantage of the semiconductor system because this enables improvement of the electron transfer rate at the semiconductor/soiution interface and the energy conversion efficiency. 

We assumed in Fig. 4.2 that no surface charge or surface dipole is present in the semiconductor. In general, however, both surface charges and surface dipoles are present in the semiconductor owing to adsorption equilibria for various ions between the electrolyte and the semiconductor surface as well as formation of polar bonds at the semiconductor surface. Such surface charges and surface dipoles change the potential difference in the (outer) Helmholtz layer and thus cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The shifts can be expressed as changes in 0(0) or in the above equations, with the... 

The influence of chemisorbed ions is thus seen in the potential drop across the Helmholtz layer and in the catalytic ability of the surface. Considerable work remains to be done on the chemical pretreatment of surfaces to maximize the catalytic nature of the surface and enhance the adsorption of appropriate ionic species. 

However, the reason for this look back at some of the contents of Chapter 6 is the nature of ions adsorbed at metals. They are of two types. The Gouy diffuse-layer ions simply exist in the solution bulk, but lack any specific chemistry, being either positive or negative, depending on the charge on the electrode and in the first (or Helmholtz) layer. In this latter region, some of the ions turn out to be bound chemically (contact adsorption) to the surface of the electrode itself. 

For the flat band potential situation, i.e. at E = Epb and for eAtpsc = e(Es — Eb) = 0, one obtains an appropriate relation for the Fermi level of the oxide Ep,ox in dependence of pb and the potential drop in the Helmholtz layer Es — Eso (Eq. (23)). The potential drop Aadsorption equilibrium at the oxide surface, i.e. from its isoelectric point. The flat band potential Epb may be determined by extrapolation of the potential dependence of the photocurrent as will be shown in Fig. 40 of Section 6.2 for passivating CU2O on Cu. With these data the positions of the energy bands of Fig. 39 have been determined, however with the assumption of an energy difference of the Fermi level from the conduction or the valence band of 0.25 eV, respectively. For the anodic oxides of Cu, the position of the bands has been determined independently by UPS measurements (Section 6.2). 

If we also consider the role of adsorption on the distribution of products, then we should note that surface concentrations of substrate and intermediate(s) must be taken into account, i.e. their concentrations in the inner Helmholtz layer (cf. for example, Wendt, 1973). One effect of this would possibly be to... 

As already noted (p. 30) effects of surface concentrations are really effects of adsorption, since it is the adsorption properties of the components of the electrolyte solution that influence the structure of the inner Helmholtz layer. 

As with metals, the Helmholtz layer is developed by adsorption of ions or molecules on the semiconductor surface, by oriented dipoles or, especially in the case of oxides, by the formation of surface bonds between the solid surface and species in solution. Recourse to band-edge placement can be sought through differential capacitance measurements on the semiconductor-redox electrolyte interface [29j. 

The properties of the surface layers have a strong effect on the deposition process. The driving force of the electrochemical reaction is the potential difference over the electrochemical double layer. Adsorption of species can change this potential. For example, the additives used in electrodeposition adsorb in the Helmholtz layer. They can change the local potential difference, block active deposition sites, and so on. The thickness of the diffusion layer affects the mass-transfer rate to the electrode. The diffusion layer becomes thinner with increasing flow rate. When the diffusion layer is thicker than the electrode surface profile, local mass-transfer rates are not equal along the electrode surface. This means that under mass-transfer control, metal deposition on electrode surface peaks is faster than in the valleys and a rough deposit will result. 

In all situations discussed so far only one Stern layer capacitance is required. In literature it is however often assumed [7, 8, 22] that diffuse ions can approach the surface up to the Stern plane and that s.a. ions are located at a newly defined adsorption plane, the inner Helmholtz plane. The inner Helmholtz plane is located in between the surface plane and the Stern or outer Helmholtz plane. The double layer model composed of an inner and outer Helmholtz layer plus a diffuse layer is generally called the triple layer (TL) model. 

As already discussed in Section 3.2 the potential across a single solid-liquid interface cannot be measured. One can only measure the potential of an electrode vs. a reference electrode. It has already been shown in Section 3.2 that a certain potential is produced at a metal or semiconductor electrode upon the addition of a redox system, because the redox system equilibriates with the electrons in the electrode, i.e. the Fermi level on both sides of the interface must be equal under equilibrium. It should be emphasized here that the potential caused upon addition of a redox couple to the solution occurs in addition to that already formed by the specific adsorption of, for instance, hydroxyl ions. A variation in the relative concentrations of the oxidized and reduced species of the redox system leads to a corresponding change of the potential across the outer Helmholtz layer, as required by Nernst s law (see Eq. 3.47), which can be detected by measuring the electrode potential vs, a reference electrode. However, there still exists a potential across the inner Helmholtz layer which remains unknown. 

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