Adsorption on metal electrodes

No "Jilt has so far been assumed that the semiconductor-electrolyte interphase does not contain either ions adsorbed specifically from the electrolyte or electrons corresponding to an additional system of electron levels. These surface states of electrons are formed either through adsorption (the Shockley levels) or through defects in the crystal lattice of the semiconductor (the Tamm levels). In this case—analogously as for specific adsorption on metal electrodes—three capacitors in series cannot be used to characterize the semiconductor-electrolyte interphase system and Eq. (4.5.6) must include a term describing the potential difference for surface states. 

Table 5-2. The standard chemical free enthalpy, of anion adsorption on metal electrodes in aqueous solution at 25°C. [From Bode, 1972.]...

Pig. 5-27. Contact ion adsorption on metal electrodes in aqueous solution IHP = inner Helmholtz plane OHP = outer Helmholtz plane i,d = adsorbed ion ih = hy-dratedion oM = charge on the metal electrode o i = charge of adsorbed ions o i = charge of excess hydrated ions in solution. [From Bockris-Devanathan-MuUer, 1963.]... 

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. 

During the past decades, anionic and cationic adsorption on metal electrodes have been intensively investigated. 

It is important at this point to recall the difference between atom and ion adsorption. Figure 12 shows that chlorine and iodine adsorption become stronger towards negative fields, whereas it is known that halide adsorption on metal electrodes becomes more favorable towards more positive electrode potentials, and hence fields. This apparent contradiction is readily resolved by noting the different reference states involved (i. e., atom and ion), leading to different E, -F slopes as expressed in Eq. (11). 

Adsorption on metal electrodes, which can be cleaned in solution by electrode reactions, is also studied by RAIRS. There is added interest in the effects of the variable electrode potential on the spectra and structures of the adsorbed species. 

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