**2. General structure and properties of TiO2**

TiO2 belongs to the transition metal oxide family. There are four different polymorphs of TiO<sup>2</sup> found in nature such as anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic) [6], the most important of which are anatase and rutile. With calcination at high temperatures exceeding ~600°C, the brookite and anatase polymorphs will transform into the thermodynamically stable rutile polymorph [5].

The tetragonal anatase bulk unit cell has dimensions of a = b = 0.3733 nm and c = 0.9370 nm, and the rutile bulk unit cell has dimensions of a = b = 0.4584 nm, and c = 0.2953 nm (**Table 1**). In both structures, the octahedral distortions create the basic building units [7, 8]. The lengths and angles of octahedral coordinated Ti atoms, therefore, dictate stacking in both structures, as shown in **Figure 1**.

pressure at room temperature in solution and also the phase-selective reduction between ana-

phase changed into black color. In the case of P25 TiO2

. Copyright (2003), Elsevier [18].

in **Table 2**. These results show that a slight change occurred along the a and b directions, but there was significant expansion in the c direction, and as a result, the unit cell volume

phase was not changed,

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crystals (right). The black color corresponds to the visual

, the blue colored

are shown

phases. They showed that the white anatase TiO<sup>2</sup>

appeared as a result of the combination of white and black colors (**Figure 2**) [9].

The unit cell parameters and nanocrystalline size profiles of white P25 and blue TiO2

(white P25) (left) and blue TiO<sup>2</sup>

. Copyright (2016), Royal Society of Chemistry [9].

tase and rutile TiO2

TiO2

while the rutile TiO2

**Figure 2.** Schematics of TiO<sup>2</sup>

color of the reduced rutile TiO<sup>2</sup>

expanded significantly as well [10].

**Figure 1.** Bulk structures of anatase and rutile TiO<sup>2</sup>
