The photocatalytic effects of TiO2 have resulted in a wide range of its environmentally friendly applications (alternative to photocatalytic water purification and the generation of solar hydrogen fuel), including [1,126]: •
• Antiseptic Coatings for Sanitary Areas. The antiseptic properties of TiO2 coatings are related to the light-induced bactericidal effect. The antiseptic coating can be applied in sanitary areas and hospitals.
• Antifogging Coatings on Glasses. Antifogging activity is related to the superhydrophilicity of TiO2- coated surfaces.
• Deodorizing Coatings. Deodorizing effects are related to the strong oxidation power of TiO2 leading to destruction of airborne organic compounds.
• Biological Effects. These include reactions of TiO2 with bacteria, viruses, fungi, and cancer cells. Blake et al.  reported that carcinoma tumor cells are killed by exposure to a mercury arc lamp (500 W) in the presence of TiO2 (P25). An epidemiological study of industrial workers did not show any significant effect of TiO2 dust on the respiratory system, however, some pathologic changes, such as pulmonary fibrosis and skin necrosis may be associated with direct exposure to large quantities of TiO2 particles .
• Gas Sensing. The electrical properties of TiO2 are sensitive to the presence of several gases in the atmosphere, such as oxygen, water vapor, hydrogen, alcohol vapor, CO, and NOx.
Consequently, the electrical properties may be used as sensing functions for measurements of the concentration of these gases in air.
8.6 CONCLUDING REMARKS
Oxide semiconductors in general and TiO2-based semiconductors in particular are the emerging materials in energy conversion applications. Especially, the interest in TiO2 is growing because of its application in solar energy conversion, including photoelectrochemical water splitting and photocatalytic water purification. These two applications are closely related to the reactivity of TiO2 with water. The associated water oxidation may be considered in terms of the following competitive routes:
• Water Purification Route. This reaction consists in photocatalytic partial oxidation of water, which is associated with the removal of two electrons from two water molecules. The formed active radical species (hydroxyl radicals, superoxide radicals, and hydrogen peroxide) have the capacity to oxidize microbial cells as it is represented in Figure 8.30.
• Solar Fuel Route. This reaction leading to total water oxidation is associated with the removal of four electrons from two water molecules (resulting in the formation of one oxygen molecule) as it is schematically represented in Figure 8.14.
These two water oxidation pathways, which are competitive, are schematically represented in Figure 8.44 in terms of both anodic and cathodic reactions. The critical part of the system is the photoanode (anodic site). The initial step of water oxidation is adsorption of water molecules on the surface active sites of photocatalyst (photoelectrode) leading to the formation of active complexes. Removal of electrons from the active complex results in its destabilization and subsequent decomposition into either active radicals and protons (partial oxidation) or oxygen gas and protons
(total oxidation). The intrinsic defects in the TiO2 lattice, which may act as acceptor – type surface active sites, are titanium vacancies. Their oxidation power depends on the position and the local structural environment.
Total oxidation of water, which is associated with coordinated multielectron charge transfer, requires strong acceptor-type active sites, such as titanium vacancies in the outermost surface layer. Alternatively, partial oxidation may be achieved by imposition of weaker acceptors sites, such as titanium vacancies that are located in the sublayer.
Identification of the local active surface sites, responsible for the two reactivity pathways that are shown in Figure 8.44, is essential in the development of photocata – lysts/photoelectrodes with controlled reactivity/photoreactivity.
Enhanced water oxidation may be achieved by reduction of the band gap, enhanced charge transport and optimized flat band potential. However, these key performance- related properties are expected to have little effect on selectivity. The latter may be modified by surface properties. Therefore, the progress in photocatalysis critically depends on better understanding of the effect of surface properties on selective oxidation of water.
There is a common perception that the properties of oxide semiconductors for solar energy conversion are determined by the crystalline structure and phase composition [122-125]. It becomes apparent, however, that the performance-related properties of oxides, such as semiconducting properties and reactivity, are profoundly affected by crystal imperfections (point defects). The imperfections even have an effect on lattice parameter, which is the basic structure-related property.
The extent of defect-related effects and their impact on performance of solar energy conversion systems (photoelectrodes and photocatalysts) indicates the need to consider defect chemistry as an important framework in the formation of systems with enhanced performance. So far, little is known on the specific properties,
Solar fuel route
which are responsible for the mechanism of water oxidation. Better understanding in this matter is urgently needed in order to develop the systems with desired performance.
The photocatalytic water purification is extensively discussed in several overview papers [1,81-83,92]. The progress of research in the area of photoelectrochemical water splitting is the subject of the book recently edited by Vayssieres .
The breakthrough in most recent development of high efficiency (90%) solar cells has been recently reported by Kotter et al. . These cells are able to convert the sunlight-induced heat into electricity using nanoantenna.