**3. Properties of titanium dioxide nanoparticles**

**Figure 14** shows the different crystal structures of TiO2 [65]. As it can be seen in this figure, there are three forms (polymorphs) namely anatase, rutile, and brookite, which are classified according to their crystalline arrangements. Thus, rutile is the most stable at higher temperature, whereas anatase is the most stable at lower temperature. Furthermore, at high temperature, anatase and brookite could be transformed into rutile. Brookite in the powder or thin film forms reveals excellent stability and superior photocatalytic activity to that of anatase [66]. In addition, anatase is favored in photocatalysis because of its high photocatalytic activity between all the three polymorphs [19].

**Figure 14.** *Different crystal structures of TiO2 [65].*
