## 4 Physicochemical Properties of Titanium Dioxide and Its General Applications  
Titanium(IV) oxide (TiO2), often referred to as titanium dioxide or titania, commonly appears in nature in three mineral forms: (a) anatase, (b) rutile, and (c) brookite. Aside from naturally occurring forms, commercially produced forms are also available such as P25, which is a mixture of anatase and rutile (depending on the supplier, the ratio of anatase to rutile ranges between 80:20 and 70:30). Brookite is often overlooked due to lower photo-reactivity than that of other TiO2 polymorphic forms. When used, it is usually prepared as a composite of anatase-brookite [68]. The broadness of TiO2 applications comes from the desirable material properties such as non-toxicity, high photostability, low cost, and photocatalytic potential [55]. Many advantages of photocatalysis with titanium dioxide can be quoted, for example, the ease of its immobilization by coating, long-term chemical stability of the photocatalyst, promising photodegradation efficiency, and hydrophilicity, non-toxicity, low cost, high photo-reactivity. However, some disadvantages of titanium dioxide can also be noted, especially concerning the optical response of titania, mainly in the UV light range, and fast recombination of the generated electron-hole pairs. Both can be avoided by proper modification of the material structure [53; 69]. Modifications of the titania catalyst aim to increase its activity and thus boost its reactivity in visible light. The TiO2 band gap of 3.2 eV only allows the absorption of light with a wavelength below 387 nm, which barely falls within the visible light spectrum [70; 71]. Therefore, various surface developing agents for use with titania as well as immobilization techniques of titania as a catalyst within the active phase of the reactors have been developed [53; 72]. Some attempts have been made to modify TiO2 photocatalysts by doping with metals and non-metals (nitrogen, sulfur) as well as by heterojunction with another semiconductor in order to extend the absorption wavelength toward the visible light spectrum and to limit the recombination of electron-hole pairs [69]. The relevant titanium dioxide is a semiconductor-based material, which after irradiation undergoes a three step photocatalytic reaction. Initially, the titania molecule absorbs a photon with energy higher or equal to the value of 3.2 eV and passes to an excited state in which electrons in the conduction band and holes in the valence band are generated. Furthermore, in order to complete a photochemical reaction, electrons react with electron acceptors (usually in the presence of co-catalyst) in reduction reactions, and donors in oxidation reactions. In this way, when we have reactions of photo-induced electrons and hole pairs with surrounding oxygen and water molecules, reactive oxygen species (ROS) are formed such as superoxide (\(O_{2}\)\({}^{\bullet-}\)) and hydroxyl (\({}^{\bullet}OH\)) radicals [73; 74]. The TiO\({}_{2}\) photocatalytic process proceeds according to the following mechanism (based on [PERSON] et al. [75]):  
\[\text{TiO}_{2}+hv\ \rightarrow\ h^{+}+e^{-} \tag{1}\]  
\[O_{2}+e^{-}\ \rightarrow\ \bullet O_{2}^{-} \tag{2}\]  
\[H_{2}O+h^{+}\rightarrow\ H^{+}+\ \bullet OH \tag{3}\]  
\[OH^{-}+h^{+}\rightarrow\ \bullet OH \tag{4}\]  
\[2\bullet O_{2}^{-}+2H^{+}\rightarrow\ H_{2}O_{2}\ +O_{2} \tag{5}\]  
\[\bullet OH+\ \bullet OH\to H_{2}O_{2} \tag{6}\]  
\[H_{2}O_{2}+\ \bullet O_{2}^{-}\rightarrow\ \bullet OH+OH^{-}+O_{2} \tag{7}\]  
\[Substrate+\ \bullet O_{2}^{-}+\ \bullet OH\rightarrow\ CO_{2}+O_{2}+by- product \tag{8}\]  
After receiving a quantum of energy in the form of photons, the energy of which is higher than the bandgap of TiO\({}_{2}\), the unpaired electrons from the valence band of TiO\({}_{2}\) migrate toward the conduction band (1). This leads to the formation of positively charged holes on the valence band (\(h^{+}\)).