Abstract
Compared with photoelectrons in metals, photo-excited electrons and holes in semiconductors are relatively stable so that the photoeffect on electrode reactions manifests itself more distinctly with semiconductor electrodes than with metal electrodes. In semiconductor electrodes, photo excitation significantly increases the concentration of minority charge carriers, but influences little the concentration of majority charge carriers. Charge transfer reactions on semiconductor electrodes proceed under the condition of anodic and cathodic polarizations in which the Fermi level is different either from the Fermi level of redox electron transfer reactions or from the equivalent Fermi level of ion transfer reactions. In semiconductor electrodes, the cathodic current of electron transfer is proportional to the concentration of interfacial electrons, ns, and the anodic current of hole transfer is proportional to the concentration of interfacial holes, ps. A photo electrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. It decomposes water to produce gaseous hydrogen and oxygen molecules. The photo electrolytic cell can be composed of both a photo-excited n-type anode and a photo-excited p-type cathode.
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