Photocatalysts oxide semiconductor-based

Semiconductor-based heterogeneous photocatalysts have been interested by a large number of scientists. Titanium dioxide (T1O2), whieh is inexpensive, nontoxic, resistant to photo-corrosion, and has high oxidative power, is the most widely used... 

Domen et al. studied the photocatalytic activities of several laya-ed metal oxide semiconductors, mainly K4Nb60n, for water cleavage by bandgap excitation (272). Motivated by the success, a wide variety of photocatalysts based on the layered transition metal oxides have been investigated (273-280). The unique structure has successfiilly been utilized for efficient photocatalytic reactions. In some cases, metal or oxide clusters have been deposited on K4Nb60i7. 

The above features were all demonstrated in thermal catalysis using oxides. However, they can be transposed to photocatalysis, at room temperature, where the simultaneous formation of photoelectrons and holes occurs. Therefore, metal oxide semiconductors have also been widely used as photocatalysts. Under these conditions, the redox reactions are based on photoinduced electronic processes + hi/ -> e -I- h (hv > Eq bandgap). By these bandgap excitations,... 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

These processes, included in a special class of oxidation techniques defined as advanced oxidation processes (AOPs), are based on the irradiation of a semiconductor photocatalyst with UV light that leads to the formation of highly reactive hydroxyl radicals. 

Thin-lilm photoelectrodes are needed in photoelectrocatalytic systems to apply a bias potential, either for the photoelectrode characterization or to facilitate the photocatalytic reactions. However, to be able to present a more comprehensive view on the performance of different materials, our subsequent discussions will focus on particulate semiconductor photocatalysts since the latter have been much more extensively investigated. Their electronic band structure (i.e., both the bandgap energy and the positions of CB and VB) is the key factor to determine whether or not a semiconductor material is suitable for a specific photocatalytic reaction, as will be demonstrated by reviewing a number of selected metal oxides and cou-pled/composite materials based on various semiconductors. 

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