**Figure 1.** *The main photocatalytic process of TiO2 [9].*

TiO2 shows a regular lattice structure. There are three crystalline forms of TiO2 in nature: anatase, rutile, and brookite [6]. Anatase has a tetragonal crystal system. However, it will slowly transform into rutile after heating at about 610°C [7] and completely transform into rutile at 915°C. The latter also has a tetragonal crystal structure, each unit contains six atoms, and its oxygen atoms are densely packed, so rutile shows the highest stability [8]. Compared with anatase, rutile exhibits higher density, hardness, refractive index, and dielectric constant.

TiO2 has excellent physical and chemical properties, however, TiO2 surface has a photocatalytic active site (**Figure 1**). After absorbing ultraviolet light energy, electron–hole pairs (the charge carrier) are generated [10]. The valence band hole (h<sup>+</sup> ) is highly oxidizing while the conduction band electron (e) is highly reducing [11]. The h<sup>+</sup> oxidizes H2O or OH ion to the hydroxyl radical (OH• ), the e reduces adsorbed oxygen (O2) species to superoxide (O2 • ) and then undergoes a series of reactions to give the OH• radical. These radicals will react with surrounding organic substances, resulting in the decomposition of the organic matrix [12]. What's worse, TiO2 particles are easy to agglomerate due to the high special surface area, causing poor dispersion in the organic matrix [13].

To overcome the drawbacks of TiO2 mentioned above, one can use coatings. The coating of TiO2 by inorganic oxides, such as alumina, silica, and zirconia [14], can effectively inhibit the oxidative degradation of the organic matrix, finally improving the light and weather resistance [15, 16]. The poor dispersion of TiO2 can also be effectively solved by coating [17]. Therefore, it is of great social significance and economic value to study the coating of TiO2 to improve the physical stability and dispersion, extending new applications of TiO2. In this chapter, we introduce the modification strategies of TiO2 to the readers. To fully describe the modification mechanism, processes and properties of modified TiO2 will be discussed.
