Electron interfadal

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 ... 

For the hydrogen electrode, the interfadal potential between the electrode metal and the hydrogen gas film is determined by the electron transfer equilibrium and the interfacial potential between the hydrogen gas film and the aqueous... 

The surface potential, Xm> due to the interfadal dipole of the electron tailing away from the metal surface is given as a function of the excess or defidt of metal electrons in Eqn. 5-27 ... 

As described in this section, the distance Xta, to the image plane increases with increasing electron density in the electrode metal. Correspondingly, as shown in Fig. 5-24, the capacity of fbo compact layer, Ch, of sp metal electrodes in aqueous solution increases with increasing electron density in the metal. It thus appears that the interfadal electric double layer is affected significantly by the nature (electron density) of the electrode metal. 

Transformation of interfadal lattice has been observed not only on gold metal electrodes but also on other sp-metal electrodes [Kolb, 1993]. In general, on the electrode interface of sp-metals the interfadal lattice of high atomic density vanishes with anion adsorption in anodic polarization and reappears with increasing electron density at the interface in cathodic polarization as shown in Fig. 5-35. 

Figure 5-41 illustrates the profile of electron level across the interfadal double layer of a semiconductor electrode (A) in the state of band edge level pinning and (B) in the state of Fermi level pinning. In Fig. 5-41 the cathodic polarization... 

Fig. 6-53. Interfadal charges, electron levels and electrostatic potential profile across an electric double layer with contact adsorption of dehydrated ions on semiconductor electrodes ogc = space charge o = charge of surface states = ionic charge due to contact adsorption dsc = thickness of space charge layer da = thickness of compact la3rer.

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...

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.

We fiirther obtain the concentration, n, of interfadal electrons in the conduction band and the concentration, p., of interfacial holes in the valence band as shown in Eqn. 8-52 ... 

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. 

The cathodic current of electron transfer is proportional to the concentration of interfadal electrons, n and the anodic current of hole transfer is proportional to the concentration of interfacial holes, p., in semiconductor electrodes as described in Sec. 8.3. Since the concentration of interfacial electrons or holes depends on the quasi-Fermi level of interfacial electrons or holes in the electrode as shown in Eqn. 10-3 or 10—4 (n, = n + dra and p, =p + 4P ), the transfer current of cathodic electrons or anodic holes under the condition of photoexdtation depends on the quasi-Fermi level of interfadal electrons, nCp, or the quasi-Fermi level of interfadal holes, pEp It also follows from Sec. 8.3 that the anodic current of electron transfer (the ipjection of electrons into the conduction hand) or the cathodic current of hole transfer (the ipjection of holes into the valence band) does not depend on the... 

The point at which the straight line of (tph) versus Eintersects the coordinate of electrode potential represents the flat band potential. Equation 10-15 holds when the reaction rate at the electrode interface is much greater than the rate of the formation of photoexcited electron-liole pairs here, the interfadal reaction is in the state of quasi-equilibrium and the interfadal overvoltage t)j, is dose to zero. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

Fig. 10-32. Polarization curves of cell reaction for photoelectrolytic decomposition of water at a photoexdted n-type anode and at a photoezdted p-type cathode solid curve n-SC s anodic polarization curve of oxygen evolution at photoexdted n Qpe anode (Fermi level versus current curve) dashed curve n-SC = anodic polarization curve of oxygen evolution at dark p>type anode of the same semiconductor as photoexdted n-type anode (equivalent to the curve of current versus quasi-Fermi level of interfadal holes in photoezdted n-type anode) solid curve p-SC = cathodic polarization curve of hydrogen evolution at photoexdted p-type cathode (Fermi level versus current curve) dashed curve n-8Cr = cathodic polarization curve of hydrogen evolution at dark n-type electrode of the same semiconductor as photoezdted p-type cathode (equivalent to the curve of current versus quasi-Fermi level of interfadal electrons in photoexdted p-type cathode) > > = flat band potential of n-type (p-type) electrode nn.sc (v p sc) = inverse overvoltage for generation of photoexdted electrons (holes) in a p-type (n-type) electrode.

Aust. J. Soil Res. 16 215-227 Joint Committee on Powder Diffraction Standards Mineral powder diffraction fde. Data book. Published by the JCPDS International Centre for Diffraction Data, Swarfhmore, Pennsylvania, U SA, pp. 942 Jolivet, J.P. Tronc, E. (1988) Interfadal electron transfer in colloidal spinel iron oxide. Conversion of Fe304 to y- Fe203 in aqueous medium. J. Colloid Interface Sci. 125 688—... 

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

M. R. Hofemann N. S. Lewis, Fluxmatching condition at Ti02 photoelectrodes Is interfadal electron transfer to O2 rate-limiting in the Ti02-catalyzed degradation of organics J. Phys. Chem. 1994, 98, 13385-13395. 

Interfadal Electron Transfer in Molecular and Protein Film Voltammetry... 

Theoretical Notions of Interfadal Chemical and Bioelectrochemical Electron Transfer... 

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. 

Chi, Q.)., Zhang, J.D., Jensen, P.S., Christensen, H.E.M., and Ulstrup, J. (2006) Long-range interfadal electron transfer of metalloproteins based on molecular wiring assemblies. Faraday Discussions, 131, 181-195. 

G. W., Andersen, J.E.T., and Ulstrup, J. (2000) Molecular monolayers and interfadal electron transfer of Pseudomonas aeruginosa azurin on Au(l 11). Journal of the American Chemical Society, 122, 4047-4055. 

H. E.M., NazmudUnov, R.R., and Ulstrup, J. (2010) Approach to interfadal and intramolecular electron transfer of the diheme protein cytochrome c(4) assembled on Au(lll) surfaces. journal of Physical Chemistry B, 114, 5617-5624. 

Wackerbarth, H., Klar, U., Gunther, W., and Hildebrandt, P. (1999) Novel time-resolved surface-enhanced (resonance) Raman spectroscopic technique for smdying the dynamics of interfadal processes application to the electron transfer reaction of cytochrome c at a silver electrode. Applied Spectroscopy, 53, 283-291. 

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