Semiconductors light-driven water splitting

The bipolar cell of Sharon-Schottky cell has been further developed. Bard and coworkers [21, 22) developed a bipolar semiconductor photoelectrode array (Fig. 10) and studied its application to light-driven water splitting and electrical power generation. They used five -Ti02 electrodes in series. In the subsequent developments [23], they devised a similar bipolar cell (Fig. 11) with CoS/CdSe. In this cell, also a salt bridge is used to complete the electrical circuit. CoS is used to make ohmic contact with... 

It is these quasi-Fermi levels that are important in defining the thermodynamic criteria for light-driven water splitting. In the case of an n-type semiconductor such as rutile, the concentration of electrons in the dark is high (typically >10 cm ), whereas the concentration of holes is vanishingly small since the law of mass action applies. 

Now, we are in a position to define the thermodynamic criteria for light-driven water splitting under short-circuit conditions in a two-electrode photoelectrolysis cell. E needs to lie above the H /H2 redox Fermi level, and pEp needs to lie below the Oj/HjO redox Fermi level. As the electron Fermi level for n-type semiconductors is close to the conduction band, this means in practice that the conduction band energy should lie well above the H" /H2 redox potential. At the same time, the valence band needs to be sufficiently far below the O2/H2O Fermi level to ensure that water oxidation by holes is feasible. In addition to satisfying these thermodynamic criteria, the semiconductor needs to be stable under illumination. Many semiconductors are either oxidized or reduced under illumination as a consequence of the reaction of holes or electrons with the crystal lattice. For example, n-type ZnO is oxidized by photogenerated holes to form Zn " " and oxygen. 

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