Type Semiconductor Electrodes

Taniguchi I, Aurian-Blajeni B, Bockris O MJ (1984) The reduction of carbon dioxide at illuminated p-type semiconductor electrodes in nonaqueous media. Electrochim Acta 29 923-932... 

The electrons produced in the conduction band as a result of illumination can participate in cathodic reactions. However, since in n-type semiconductors the quasi-Fermi level is just slightly above the Fermi level, the excited electrons participating in a cathodic reaction will almost not increase the energy effect of the reaction. Their concentration close to the actual surface is low hence, it will be advantageous to link the n-type semiconductor electrode to another electrode which is metallic, and not illuminated, and to allow the cathodic reaction to occur at this electrode. It is necessary, then, that the auxiliary metal electrode have good catalytic activity toward the cathodic reaction. 

Finally cells containing a p-type semiconductor electrode should be mentioned. In principle the application of p-type electrodes would be even more favorable because electrons created by light excitation are transferred from the conduction band to the redox system. Stability problems are less severe because most semiconductors do not show cathodic decomposition (see e.g. earlier review article. However, there is only one system, p-InP/(V " /V ), with which a reasonable efficiency was obtained (Table 1) . There are mainly two reasons why p-electrodes were not widely used (i) not many materials are available from which p-type electrodes can be made (ii)... 

Practically more important is the sensitization of the n-type semiconductor electrode (Fig. 5.63). The depicted scheme is virtually equivalent to that in Fig. 5.62 the only exception is that the hole is not created in the valence band but formally in the sensitizer molecule. 

Reduction of Carbon Dioxide at Illuminated p-Type Semiconductor Electrodes... 

The photoelectrochemical reduction of C02 at illuminated p-type semiconductor electrodes is also effective for C02 reduction to highly reduced products. The combination of photocathodes with catalysts for C02 reduction leads to a marked decrease in the apparent overpotential. At present, however, light to chemical energy conversion efficiencies are still very low, and negative in some cases. 

While for a solar water splitting cell, light is directly absorbed by the semiconductor electrode (anode or cathode). The separation of electron-hole pairs is achieved in the built-in electric field near the semiconductor surface. The electric field is formed due to the charge transfer between the semiconductor electrode and the electrolyte as schematically shown in Fig. 17.5(b) [28]. Take an n-type semiconductor electrode for example... 

Fig. 5-44. Space charge layers of n-type semiconductor electrodes (c) an inversion layer, (d) a deep depletion layer.

Figure 5-46 shows the capacity observed for an n-type semiconductor electrode of zinc oxide in which an accumulation layer is formed at potentials more cathodic... 

Fig. 6-46. Differential capacity observed and computed for an n-type semiconductor electrode of zinc oxide (conductivity 0. 59 S cm in an aqueous solution of 1 M KCl at pH 8.5 as a function of electrode potential solid curve s calculated capacity on Fermi distribution fimction dashed curve = calculated capacity on Boltzmann distribution function. [From Dewald, I960.]...

Figure 5-47 shows the Mott-Schottky plot of n-type and p-type semiconductor electrodes of gallium phosphide in an acidic solution. The Mott-Schottl plot can be used to estimate the flat band potential and the effective Debye length I D. . The flat band potential of p-type electrode is more anodic (positive) than that of n-type electrode this difference in the flat band potential between the two types of the same semiconductor electrode is nearly equivalent to the band gap (2.3 eV) of the semiconductor (gallium phosphide). 

Fig. 6-47. Mott-Schottky plot of electrode capacity observed for n-type and p-type semiconductor electrodes of gallium phosphide in a 0.05 M sulfuric add solution. [From Meouning, 1969.]...

Figure 5-49 illustrates the Mott-SchottlQ plot observed for two n-type semiconductor electrodes of zinc oxide in a potential range in which the depletion and... 

Fig. 6-48. Differential capacity of a space charge layer of an n-type semiconductor electrode as a function of electrode potential solid cunre = electronic equilibrium established in the semiconductor electrode dashed curve = electronic equilibrium prevented to be established in the semiconductor electrode AL = accumulation layer DL = depletion layer IL = inversion layer, DDL - deep depletion layer.

Fig. 5-56. Capacity Csc of a space charge layer and capacity Ch of a compact layer calculated for an n-type semiconductor electrode as a function of electrode potential Ct = total capacity of an interfadal double layer (1/Ct = 1/ Csc+ 1/Ch). [From Gerisdier, 1990.]...

Fig. 5-61. Mott-Schottky plot of an n-type semiconductor electrode in presence of a surface state ib = flat band potential with the surface state fully vacant of positive charge Eft, - flat band potential with the surface state fully occupied by positive charge Q = maximum charge of the surface state e, = surface state level, s capacity of the surface state ( Ch ).

Fig. 5-63. Flat band potential of two n-type semiconductor electrodes of zinc oxide in 1 M KCl (pH 8.5) as a function of donor concentration A= surface finished in 85 % H3PO4 B = surface finished in 2 M KOH = donor concentration. [From Dewald, I960.]...

Fig. 8-2S. Aoodic transfer reaction of redox holes with transport of mincnity charge carriers (holes) in an n-type semiconductor electrode il.r - anodic hole transfer current at an interface = limiting hole transport current i i = limiting diSiision current of redox partides.

Fig. 8-26. Cathodic iiyectian of minority charge carriers (holes) followed by recomlmation of minority charge carriers (holes) with majority charge carriers in an n-type semiconductor electrode ipr - cathodic current of hole transfer at an interface - current of electron-...

For n-type semiconductor electrodes in which a redox reaction of cathodic hole iiyection reaches its quasi-equilibrium state at the electrode interface, the recombination current of iiqected holes (minority charge carriers) with electrons (minority charge carriers), w, is given by Eqn. 8-70 [Reineke-Memming, 1992] ... 

Fig. 8-27. Polarization curves for transfer of redox electrons at n-type and p-type semiconductor electrodes solid curve near Egaxa = reaction with the Fermi level of redox electrons dose to the valence band edge dashed curve near F redok = reaction with the Fermi level of redox electrons dose to the conduction band edge dot-dash curve (FLP)= reaction in the state of Fermi level pinning.

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

Fig. 8-82. Energy diagram for a redox electron transfer via the conduction band and via the surface state at an n-type semiconductor electrode X = reorganization energy of redox particles .,= surface state level.

Fig. 9-10. Polarization curves of anodic dissolution and cathodic deposition of n-type and p-type covalent semiconductor electrodes n-SC (p-SC) = n-type (p-type) semiconductor electrode i (i ) = anodic dissolution (cathodic deposition) current Cp = Fermi level.

Fig. 9-11. Polamation curves observed for anodic dissolution of n- pe and p-type semiconductor electrodes of germanium in 0.05 M NaOH solution = current of...

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

Figure 10-3 juxtaposes the Fermi levels of the following redox reactions in aqueous solutions and the quasi-Fermi levels of interfacial electrons and holes in an n-type semiconductor electrode erhjo/Hj) of the hydrogen redox reaction F(0a/H20) of the oxj en redox reaction ersc) of the n- q)e semiconductor and... 

Fig. 10-3. Energy diagrams for an n-type semiconductor electrode (a) in the daik and (b) in a photoexdted state S = aqueous solution = conduction band edge level at an interface cy = valence band edge level at an interface = Fermi level of oxygen...

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