Interfadal redox electron

Fig. 6-S8. Probability density for the energy level of interfadal redox electrons in adsorbed redox particles of proton-hydrogen and hydroxyl-hydroxide on the electrode interface of semiconductor ADS = adsorption > ost probable...

Next, we consider the interface M/S of a nonpolarizable electrode where electron or ion transfer is in equilibrium between a solid metal M and an aqueous solution S. Here, the interfadal potential is determined by the charge transfer equilibrium. As shown in Fig. 4-9, the electron transfer equilibrium equates the Fermi level, Enn) (= P (M)), of electrons in the metal with the Fermi level, erredox) (= P s)), of redox electrons in hydrated redox particles in the solution this gives rise to the inner and the outer potential differences, and respectively, as shown in Eqn. 4-10 ... 

Fig. 8-8. Energy levels for redox electron transfer reaction at a metal electrode (a) in equilibrium, (b) in anodic polarization with reao tion rate determined by interfadal electron transfer, (c) anodic polarization with reaction rate determined by both interfadal electron transfer and diffusion of hydrated partides. EF0)Eooxj.a= Fenni level of redox electrons at an interface.

Figures 8-16 and 8-17 show the state density ZXe) and the exchange reaction current io( ) as functions of electron energy level in two different cases of the transfer reaction of redox electrons in equilibrium. In one case in which the Fermi level of redox electrons cnxEDax) is close to the conduction band edge (Fig. 8-16), the conduction band mechanism predominates over the valence band mechanism in reaction equilibrium because the Fermi level of electrode ensa (= nREDOK)) at the interface, which is also dose to the conduction band edge, generates a higher concentration of interfadal electrons in the conduction band than interfadal holes in the valence band. In the other case in which the Fermi level of redox electrons is dose to the valence band edge (Fig. 8-17), the valence band mechanism predominates over the conduction band mechanism because the valence band holes cue much more concentrated than the conduction band electrons at the electrode interface.

Next, we consider the anodic reaction current of redox electron transfer via the conduction band, of which the exchange reaction current has been shown in Fig. 8-16. Application of a slight anodic polarization to the electrode lowers the Fermi level of electrode fix>m the equilibrium level (Ep(sc)( n = 0) = eiiOTSDca)) to a polarized level (ep(8C)( n) = ep(REDox)- n)withoutchanging at the electrode interface the electron level relative to the redox electron level (the band edge level pinning) as shown in Fig. 8-20. As a result of anodic polarization, the concentration of interfacial electrons, n, in the conduction band decreases, and the concentration of interfadal holes, Pm, in the valence band increases. Thus, the cathodic transfer current of redox electrons, in, via the conduction band decreases (with the anodic electron im ection current, ii, being constant), and the anodic transfer current of redox holes, (p, via the valence band increases (with the cathodic hole injection... 

As discussed in Sec. 8.3.5, a redox reaction current due to electron or hole transfer depends not only on the concentration of interfadal electrons or holes at the electrode but also on the state density of the redox electrons or redox holes in the range of energy where the electron transfer takes place. Hence, it is important in the kinetics of electron or hole transfer to realize the level of the band edge Cc or Ev of the electrode relative to the most probable level cred or cox of redox electrons or redox holes in the hydrated redox particles. 

If this condition is met there will be no visible response to electron transfer and therefore no measurable amplitude associated with electron transfer kinetics. Quite simply it means that the experimental conditions are such that the interfadal redox equilibrium is not disturbed by a ehange in the interfadal temperature the system was in equilibrium at T and will be in equilibrium at... 

Fig. 10-36. Polarization curves for a redox reaction at an n-type anode and at a p-lype cathode in a photovoltaic cell solid curve n-SC = anodic current at photoexdted n-type anode (Fermi level versus current curve) dashed curve p-SC = anodic current at dark p-type anode (current versus quasi-Fermi level of interfadal holes in photoexdted n-type anode) solid curve p-SC = cathodic current at photoexdted p-type cathode (Fermi level versus current curve) dashed curve n-SC = cathodic current at daric n-type cathode (current versus quasi-Fermi level of interfadal electrons in a photoexdted p- q>e cathode).

AuNPs inserted between the electrode surface and redox metalloproteins therefore both work as effective molecular linkers and exert eflfident electrocatalysis. Recent considerations based on resonance turmeling between the electrode and the molecule via the AuNP as a mechanism for enhanced interfadal ET rates suggest that electronic spillover rather than energetic resonance is a hkely origin of the effects (J. Kleis et al., work in progress). Even slightly enhanced spillover compared with a planar Au(lll) surface is enough to enhance the ET rate by the observed amount over a 10-15 A ET distance. 

The signal-triggered functions of these molecular assemblies have to be first characterized in bulk solution. Then, extensive efforts have been directed to integrate these photoswitchable chemical assemblies with transducers in order to tailor switchable molecular devices. The redox properties of photoisomerizable mono-layers assembled on an electrode surface are employed for controlling interfadal electron transfer [16]. Specifically, electrical transduction of photonic information recorded by photosensitive monolayers on electrode supports can be used in developing monolayer optoelectronic systems [16-19]. Electrodes with receptor sites exhibiting controlled binding of photoisomerizable redox-active substrates from the solution [20] also allow the construction of molecular optoelectronic devices. 

There is a rationale for studying these redox systems, one that originates largely from the work of McCreery and coworkers in recent years with glassy carbon electrodes [139-143]. Redox reactions are generally of two types. One type includes electrode reactions that proceed by simple diffusion of the analyte to the interfadal reaction zone with the electrode serving solely as a source or sink for electrons. In this case, the electrode reaction kinetics are relatively insensitive to factors such as the surface chemistry and microstructure, but very sensitive to the density of electronic states at the formal potential (so-called outer-sphere reaction). The other pathway includes reactions that occur via some specific... 

Interfadal Electron Transfer. There have been several studies of electron transfer reactions[17, 38]and the connection with Marcus s theory[39]. It may be possible to use a Car-Parrinello like scheme on that part of the system directly affecting the electron transfer. There has also been very interesting studies of the ferro-ferri redox couple in solution[40, 41] that address many issues related to electron transfer from an electrode to a hydrated ion. Slow processes can be treated by transition state methods like the ones used in solid state ionic conducdvity[42]. 

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