**2. Structures of TiO2**

Titanium dioxide (TiO2) is an n-type semiconducting material with very interesting properties, such as chemical stability, nontoxicity, low cost, availability, good mechanical flexibility, conductivity and high photocatalytic activity. TiO2 has three different polymorphs (**Figure 1**) anatase, rutile and brookite [4]. Crystallographic data of TiO2 structures are summarized in **Table 1**. The most stable form of TiO2 is rutile. All three polymorphs can be synthesized by many methods.

Anatase is a metastable mineral form of titanium dioxide (TiO2) which crystallize in the tetragonal system with I41/amd space group. Rutile is the most common natural form of TiO2 and crystallizes in the same system of anatase with P42/mnm space group and brookite has an orthorhombic crystalline structure with Pbca space group. In all forms, titanium (Ti4+) atoms are coordinated by six oxygen (O2−) atoms, forming the octahedral TiO6 were titanium atom (Ti4+) is in the center and oxygen atoms (O2−) are at corners.

TiO2 is the most crystalline semiconductor used in photocatalytic process, due to the bandgap energy being relatively wide (Eg = 3.2 eV for anatase; Eg = 3.0 eV

**Figure 1.** *Titanium dioxide structures (a) anatase, (b) rutile and (c) brookite.*


#### **Table 1.**

*Crystallographic data of TiO2 structures.*

for rutile; Eg = 3.1 eV for brookite), the material can only be activated by UV irradiation with λ < 380 nm.
