## 4 Physicochemical Properties of Titanium Dioxide and Its General Applications
Next, the conduction band electrons travel toward the catalyst's surface where they interact with surface oxygen, leading to the formation of superoxide radicals (\(\bullet O_{2}^{-}\)) (2). Following the conduction band electrons, the positively charged holes (\(h^{+}\)) reach the surface and react with surface water (\(H_{2}O\)) and adsorbed hydroxyl groups (\(OH^{-}\)), leading to the formation of hydroxyl radicals (\(\bullet OH\)) ((3) and (4)). This leads to a cascade of reactions where newly created hydroxyl radicals react with each other, leading to the formation of hydrogen peroxide (\(H_{2}O_{2}\)) ((5) and (6)). Then, the created ROS species (7) interact with substrates, in this case, contaminants, leading to their degradation (8) [75; 76; 77]. In order to enhance the properties of photocatalysts and overcome their inherent limitations such as low light utilization, redox rates, and high recombination rates of electron-hole pairs; semiconductor photocatalysts are modified in different ways, leading to the formation of hybrid materials and composites. In these conjugates, two or more materials with different bandgaps are used, which leads to the formation of heterojunctions of different character. The most common types of these heterojunctions are type-II and Z-scheme while step-scheme (S-scheme) heterojunctions are also slowly becoming more and more common [78]. In the case of type-II heterojunctions (Figure 3), electrons that are situated on higher energetic levels on the conduction band migrate to another photocatalyst with a conduction band that has lower energetic level. This occurs when both photocatalysts in a system are exposed to light with sufficient energy to generate electron-hole pairs. Similarly, the generated holes in both photocatalysts follow a reverse migration, from a lower energetic level valence band to a higher one. This leads to the accumulation of electrons on one photocatalyst (PC II) and holes on the other (PC I), which in such a case act separately as a reductor and oxidizer, respectively. The recombination of electron-hole pairs is effectively suppressed due to this separation, which promotes better energy conversion rates in the photocatalytic system. However, the redox ability of these systems is limited due to the occurrence of respective redox reactions on photocatalysts with lower reduction and oxidation potentials [79; 80]. Another type of heterojunction is the S-scheme heterojunction, which is similar in the mode of action to the aforementioned type-II heterojunction. The key difference between the two heterojunctions lies in the charge-transfer route. In the type-II heterojunction, the charge-transfer occurs within the same conduction and valence bands of the photocatalysts in the system. In the S-scheme heterojunction, the charge-transfer occurs between the conduction band with a lower energetic level of one photocatalyst and the valence band with a higher energetic level appearing on the other. Due to better charge separation, the redox reactions occur on the photocatalysts with either better reduction or better oxidation potential [78]. Traditional \(Z\)-scheme photocatalysts utilize redox pairs, and because of this, their utility is restricted to liquid phase solutions only. In order to overcome this limitation, all-solid-state photocatalysts equipped with an electron mediator (usually a noble metal like Ag, Au, or Pt), and direct Z-scheme photocatalysts without an electron mediator were developed [79]. Titania is mainly used as a white pigment in the production of ceramics, cosmetics, paint, paper, pharmaceuticals, and electronics. TiO\({}_{2}\) has revealed many applications in pharmacy and medicine because of its photocatalytic properties and oxidizing activity. It is known as a European food additive E171 and, because of its high reflective index, has a common role as a pigment for pharmaceutical products. This molecule is used in sunscreens as a UVA and UVB blocker to diminish UV light penetration through the skin [81; 82] (Figure 4).