Heterogeneous Photochemical Semiconductor Systems

Heterogeneous semiconductor systems involve either suspensions or slurries of larger-sized semiconductor powders, or smaller colloids in solution. In principle, these semiconductor particles may act as tiny photoelectrolysis cells, similar to the photoelectrochemical systems discussed above. However, as many of the materials used for bulk electrodes are also described here in particulate form, both similar and new problems may arise, most notably irreproducibility in particle preparation, stability issues, and low C02 reduction rates. 

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Since photoexcitation induces significant enhancement of the reactivity of electron transfer, photochemical reactions via photoinduced electron transfer have been explored in homogeneous systems [43 52], On the other hand, the term photocatalysis has usually been used in heterogeneous systems involving photoinduced electron transfer across the gas-solid or liquid-solid interface [53-60], Photocatalysis has been extensively studied using a semiconductor particle as a photocatalyst [53-60], Photocatalysis is initiated by the absorption of a band gap photon... 

Photo-Oxidation and Photo-Reduction on the Same Surface and in Pc rticulate System. Photoelectrochemical reactions generally occur in a heterogeneous structure. That is, an n-type semiconductor electrode is connected either to a metal counter-electrode or to a p-type semiconductor. If these connections are made within a monolithic structure (i.e., no separated electrodes and no external wires), the devices are called photochemical diodes (47) (see Figure 21). The heterogeneous structure is required to produce the most favorable energy level scheme for the efficient separation of electrons and holes. Thus, in Figure 21, the hole... 

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