Photocatalysis rutile

Rutile and anatase are used in photocatalysis and as catalyst supports and we describe some important structural features later. The titanium-oxygen system plays a key role in understanding stoichiometric variations. The highest oxide, Ti02, is polymorphic and the four known crystal structures are rutile, anatase, brookite and a high-pressure form similar to a-Pb02. 

Ti02 is an important oxide with a broad range of applications in catalysis (as a catalyst or a support) (6), photocatalysis (35, 36), and sensor technology it is also used as a pigment. Of the three titanium dioxide polymorphs (rutile, anatase, and brookite), rutile and anatase have been most widely investigated they are the only ones reviewed here. 

Titanium dioxide (Ti02) is one of the most widely used semiconductors for heterogeneous photocatalysis. This is mainly due to its activity, photostabihty, non-toxicity and commercial availability. It is found in nature and can exist in three crystal modifications rutile, anatase and brookite (Kirk-Othmer, 1996). Its composition is temperature dependent at calcination temperatures above 900 K, the anatase modification is transformed into rutile. Ti02 is insoluble in water and in diluted acids, but it dissolves slowly in hot sulfuric acid (Remy, 1973). It has a high surface activity and corrosion stabihty. The commercial production of this white pigment has been known since the early 1900s. 

Oxides.—Because of its importance in photocatalysis Chung et a/. have made a study of the surface of TiOi (rutile). LEED and AES were used to characterize the surface structure and composition and ELS and UPS to find distributions of electron states. 

Let us finally turn to a brief discussion of the third term, Ti02 (i.e., titanium dioxide). Ti02 has three different crystal structures [18] rutile, anatase, and brookite only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties... 

Titanium oxide Titanium dioxide with the nominal composition Ti02 is a semiconductor with a band gap of 3.2 e V it exists in three different crystalline modifications, two of which (anatase and rutile) are commonly employed in photocatalysis. 

On the basis of the above discussion, in its most simple description photocatalysis implies a catalysed process preceded by absorption of a photon by a material acting as the catalyst. Where the photocatalyst is a semiconductor nanoparticulate system, e.g. TiOi, which is the material treated in most detail in this chapter, absorption of photons of energy greater than 3.2 eV (for anatase 3.0 eV for the rutile polymorph) leads to formation of conduction-band electrons and valence-band holes, which subsequently diffnse to the particle surface in competition with bnlk recombination. 

Because of the photocatalytic effect, Ti02 for sunscreen applications is coated with AI2O3 in order to prevent formation of radicals. Further, the rutile crystal modification is employed in most cases, which shows a lower tendency for photocatalysis than anatase. The inorganic particles as such are water-dispersible, but may be rendered oil-dispersible by adding organic coatings. 

Titanium oxide nanostructures have versatile applications, for example, in photocatalysis, solar-energy conversion, sensors, and ductile ceramics. The synthesis of derivatives with all kinds of size and shape (spherical particles, nanotubes, and nanorods) has been described in numerous studies. Out of the three main titanium polymorphs (anatase, brookite, rutile), research so far has been centred on the synthesis of anatase nanoparticles. However, recently the generation of nanometer-sized rutile has received growing attention due to its promising potential as a photocatalyst and as an electrode material. 

---