The range of electrode potential for photoelectrode reactions

Electrochemical thermodynamics predicts that an anodic reaction may proceed only at electrode potentials more positive than the equilibrium potential of the reaction and a cathodic reaction may proceed only at electrode potentials more negative than the equilibriiun potential. In other words, the Fermi level of the electrode must be higher for the cathodic reaction to proceed and must be lower for the anodic reaction to proceed than the Fermi level of the reaction. 

For metal electrodes, the concentrations of both electrons and holes in the electrode are sufficiently high at the Fermi level that the eneigy of electrons and holes, which participate in the electrode reaction, may be represented by the Fermi level namely by the electron level corresponding to the electrode potential. For semiconductor electrodes, in contrast, the electrons and holes that participate in the electrode reaction are not at the Fermi level, but at the levels of the conduction and valence bands different finm the Fermi level of the electrode, i. e. from the electron level corresponding to the electrode potential. For example, as shown in Fig. 10-13, the anodic OJ gen reaction at n-iype semiconductor electrodes proceeds with interfacial holes in the valence band, whose energy — Cy (—Cv = 

With n-type semiconductor electrodes, the anodic oiQ en reaction (Euiodic hole transfer) will not occur in the dark because the concentration of interfacial holes in the valence band is extremely small whereas, the same reaction will occur in the photon irradiation simply because the concentration of interfadal holes in the valence band is increased by photoexcitation and the quasi-Fermi level pEp of interfadal holes becomes lower than the Fermi level the o en redox 

For illustrations, we compare the transfer of anodic holes at metal electrodes and the transfer of anodic photoexdted holes at n-type semiconductor electrodes for the oxygen redox reaction shown in Eqn. 10-16  

For metal electrodes, the anodic 03Q n reaction proceeds at electrode potentials more anodic than the equilibrium potential Bo of the reaction as shown in Fig. 10-14. For n-type semiconductor electrodes, the anodic photoexdted oxygen reaction proceeds at electrode potentials where the potential E of the valence band edge (predsely, the potential pEp of the quasi-Fermi level of interfadal holes, pCp = — CpEp) is more anodic than the equilibrium oxygen potential Eq, even i/the observed electrode potential E is less anodic than the equilibrium oxygen potential E03. The anodic hole transfer of the o Qgen reaction, hence, occurs at photoexdted n-type semiconductor electrodes even in the range of potential less anodic than the equilibriiun potential Eq of the reaction as shown in Fig. 10-14. 

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