**1.4. Titanium dioxide**

The semiconductor of choice for making dye-sensitized solar cells (DSSCs) due to its good properties is titanium dioxide, TiO<sup>2</sup> , also known as titania. This material is a cheap and harmless water-insoluble inorganic material that is related to the family of transition metal oxide [9]. Although TiO<sup>2</sup> is a chemically inert substance and it does not promote chemical reactions in the absence of light, particular safety measures are needed when handling nanosized titania [10]. Moreover, TiO2 has a strong ionic character and can be noticed as being constituted of TiIV+ and O2– ions. The conduction band is formed by the 3d orbital of titanium, and the valence band is obtained by the 2p orbital of oxygen. TiO<sup>2</sup> is commercially available and widely used in industrial applications [11].

The most common form of this oxide in nature is rutile. Another form is brookite. A third form of this oxide is anatase, which is the thermodynamically stable crystalline form at high pressure. In all of them, each titanium is surrounded by six oxygen atoms, leading to more or less distorted TiO<sup>6</sup> 2− octahedrons. Crystal structures differ by the distortion of each octahedron and by the assembly patterns of the TiO6 2− units [12]. For the rutile structure, each octahedron is connected with 10 neighbors (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while for the anatase and brookite structures, every moiety is in contact with eight neighbors (four sharing an edge and four sharing a corner) as shown in **Figure 2**. Threedimensional TiO2 network is obtained from a mixture of corner-sharing and edge-sharing octahedral units [13].

**Figure 2.** Rutile, anatase, and brookite unit cells, all showing octahedral titanium coordination. Gray and red atoms correspond to Ti4+ and O2−, respectively. Reproduced from Ref. [14] with permission from American chemical society 2010.

The most stable phase for titania is anatase below the particle size of 11 nm [15]. The structural parameters lead to differences in mass density and electronic band structure, inducing inherent properties to each polymorph. Then, it is widely confirmed that anatase is the most photoactive TiO2 phase, although a mixture of anatase and rutile is preferred for photocatalytic applications [16].

titanium alkoxide in the reaction content [22]. Polymeric skeletons with three-dimensional struc-

**Figure 3.** (a) An illustration of the mechanisms of sol-gel processes: (A) acidic conditions, (B) alkaline conditions, and (C) P123-templated, containing weak alkaline conditions in this. (b) SEM images of the surface of the tubular titania

Controlling the Microstructure and Properties of Titanium Dioxide for Efficient Solar Cells

http://dx.doi.org/ 10.5772/intechopen.72494

ferred to be obtained with high hydrolysis rates for a medium amount of water. The existence of a large quantity of Ti─OH and scanty development of three-dimensional polymeric skeletons resulted in freely packed first-order particles. Polymeric Ti─O─Ti chains are improved in the presence of a big amount of water [23]. The different sizes and shapes of highly crystalline ana-

Hydrothermal synthesis includes the different techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures, whereas solvothermal method includes crystallization of materials in alcohol-based solvent such as ethanol, methanol, n-propanol, and n-butanol. The hydrothermal method has been used by many groups to

ticles [28]. The precipitate materials were fabricated by adding a 0.5 M isopropanol solution of

for 1 h in the presence of tetra alkyl ammonium hydroxides (peptizer). After that, filtration and treatment have been done at 240°C for 2 h; then, the as-obtained powders were washed with deionized water and absolute ethanol and then dried at 60°C. With the same amount of peptizer, the particle size decreased with increasing alkyl chain length. The morphology of the particles is affected by the peptizers and their concentrations. In another example, TiO<sup>2</sup> nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic

tates of a titanium precursor with water is considered to be a good source of TiO<sup>2</sup>

nanoparticles could be obtained by the polycondensation of titanium alkoxide in the

nanoparticles [26, 27]. For instance, hydrothermal treatment of peptized precipi-

nanoparticles, TiO2

O]/[Ti]) [29], and then they were peptized at 70°C

nanorods have also been syn-

nanorods by treating a dilute

is pre-

355

nanopar-

ture show close packing result from the enhancement of Ti─O─Ti chains. The Ti(OH)<sup>4</sup>

membrane. Reproduced from Ref. [25] with permission from The Royal Society of Chemistry.

presence of tetra methyl ammonium hydroxide [24].

*2.1.2. Hydrothermal and solvothermal methods*

titanium butoxide into deionized water ([H<sup>2</sup>

ethanol-water solution [30]. Besides TiO<sup>2</sup>

thesized with the hydrothermal method [31] that obtained TiO<sup>2</sup>

tase TiO2

obtain TiO<sup>2</sup>
