Photochemical properties, titanium dioxide

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

Transparent titanium dioxide can be manufactured by various wet-chemical and gas-phase processes. The properties of the reaction products can vary greatly, depending on the reaction conditions and starting materials. To reduce the high photochemical activity of untreated transparent titanium dioxides, the pigments are coated with a variety of inorganic oxide combinations (e.g. of silicon, aluminum and zirconium oxides) [5.195]. 

The basic photophysical properties of a dye molecule are maintained upon immobilisation on a semiconductor surface, but the interaction with the semiconductor may open new reactive routes and/or change the rate of particular photochemical processes. An example of the importance of these routes is the fact that some polypyridyl complexes, intrinsically photolabile in solution, become photostable when bound to the semiconductor titanium dioxide, and actually constitute the class of dyes that has enabled some of the most efficient cells constructed to date [4, 5]. 

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