Photoelectrolytic cells of metal and semiconductor electrodes

In order for the photoelectrolytic decomposition of liquid water to proceed, the Fermi levels of the redox reactions in Eqns. 10-53a and 10- 3b need to be located within the band gap of the n-type semiconductor anode. In Fig. 10-26(a), we have assumed that the Fermi level ep(ac) of the n-type semiconductor anode at the flat band potential is higher than the Fermi level ep(h-/h2) of hydrogen redox reaction we have also assumed that the Fermi level e,(M) of the metallic cathode is lower than ekh /Hj)- Further, we have assiuned that the edge level of the conduction band is higher than the Fermi level of hydrogen redox 

When the cell circuit is closed in the dark, as shown in Fig. 10-25(b), the Fermi level is equilibrated between the metallic cathode and the n-lype semiconductor anode. As a result, a depletion layer of space charge (potential barrier, is formed in the semiconductor anode, thereby shifting the potential of the anode from the flat band potential to a more anodic (more positive) potential (= + ). In the dark, however, the anodic hole transfer 

When the n-type semiconductor anode is photoexcited, as shown in Fig. 10-25(c), the Fermi level of the anode is raised (the potential of the anode is lowered) by an energy equivalent to the photopotential at the same time, the Fermi 

The energy balance in the photoelectrol3iic decomposition of water can be obtained from Pig. 10-25(c) as shown in Eqn. 10-54  

Since the highest possible Fermi level of the photoexcited n-type anode corresponds to the flat band potential of the semiconductor anode, the Fermi level of the metallic cathode short-circuited with the photoexcited n-lype anode can also be raised up to the level equivalent to the flat band potential of the semiconductor anode. In order for the cathodic electron transfer of hydrogen redox reaction to proceed at the metallic cathode, the Fermi level 1 of the cathode needs to be higher than the Fermi level of hydrogen redox reaction. Consequently, in 

---