Illuminated n-type semiconductor

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

Interaction of illuminated n-type semiconductor oxides with O9 and NO... 

Figure 2.2 Energy band schematic for an illuminated n-type semiconductor under short circuit condition for example, operating at the maximum contact potential difference between semiconductor and redox electrolyte Jl, light-induced current (see text).

Figure 2.7 Schematic representation of the quasi Fermi levels of an illuminated n-type semiconductor with high absorption and low minority carrier diffusion length a denotes the absorption iength after 3or, the light intensity at the surface, /o, is reduced to...

These two groups of excited carriers are not in equilibrium with each other. Each of them corresponds to a particular value of electrochemical potential we shall call these values pf and Often, these levels are called the quasi-Fermi levels of excited electrons and holes. The quasilevel of the electrons is located between the (dark) Fermi level and the bottom of the conduction band, and the quasilevel of the holes is located between the Fermi level and the top of the valence band. The higher the relative concentration of excited carriers, the closer to the corresponding band will be the quasilevel. In n-type semiconductors, where the concentration of elec-ttons in the conduction band is high even without illumination, the quasilevel of the excited electrons is just slightly above the Fermi level, while the quasilevel of the excited holes, p , is located considerably lower than the Fermi level. 

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. 

Thus, although the potential required for polarization would be much larger at n-type semiconductors than at illuminated p-type semiconductors and despite the fact that not all n-type semiconductors can be used because of corrosion (or reduction) of semiconductor materials themselves, the use of n-type semiconductors to examine C02 reduction seems to be indicated because the cathodic current is much larger (the electron is the major carrier for n-type semiconductors), approaching that of metal electrodes, compared to the photocurrent obtained at illuminated p-type semiconductors,... 

Direct splitting of water can be accomplished by illuminating two interconnected photoelectrodes, a photoanode, and a photocathode as shown in Figure 7.6. Here, Eg(n) and Eg(p) are, respectively, the bandgaps of the n- and p-type semiconductors and AEp(n) and AEF(p) are, respectively, the differences between the Fermi energies and the conduction band-minimum of the n-type semiconductor bulk and valence band-maximum of the p-type semiconductor bulk. lifb(p) and Utb(n) are, respectively, the flat-band potentials of the p- and n-type semiconductors with the electrolyte. In this case, the sum of the potentials of the electron-hole pairs generated in the two photoelectrodes can be approximated by the following expression ... 

Scheme I. Semiconductor-based photoelectrochemical cell. Energy output may be in the form of electricity by putting a load in series in the external circuit, or the output can be in the form of chemical energy as redox products formed at the electrodes. N-type semiconductors effect uphill oxidations upon illumination, and p-type semiconductors effect uphill reductions under illumination. Either or both electrodes in the cell can be a photoelectrode.

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

Figure 7.9 Current-potential characteristics for an n-type semiconductor in the dark and under illumination. The difference between the two curves is the photocurrent.

Figures 4 and 5 show an n-type semiconductor with a surface depletion layer, the energy bands bending upwards as the surface is approached from the interior. Under illumination (Figure 5) the band bending is diminished because of the photo-generation...

Figure 5. n-Type semiconductor—electrolyte solution interface with a surface depletion layer, in the dark and with two intensities of illumination. Symbols as in Figure 3 and 4 with Ec and E the band edges of the conduction and valence bands, respectively, under illumination, and Ef(H2) Ef(Om) abbreviations for Ef(H20/h2) and Ep(02/H20)y respectively. The quasi-Fermi levels Ei> and pEp are at different positions in the surface region than in the bulk as a result of the limited penetration of light into the interior. Fermi levels in solution as in Figures 3 and 4(13). 

Sakata T, Hashimoto K, Kawai T (1984) Catalytic properties of ruthenium oxide on n-type semiconductors under illumination. J Phys Chem 88 5214-5221... 

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