*2.1.1. Sol-gel method*

TiO2 nanomaterials have been synthesized by the sol-gel method from hydrolysis of a titanium precursor. This process normally happened through an acid-catalyzed hydrolysis step of titanium (IV) alkoxide followed by condensation as shown in **Figure 3**. The improvement of Ti─O─Ti bonds is developed with small amount of water, low hydrolysis rates, and excess amount of Controlling the Microstructure and Properties of Titanium Dioxide for Efficient Solar Cells http://dx.doi.org/ 10.5772/intechopen.72494 355

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

titanium alkoxide in the reaction content [22]. Polymeric skeletons with three-dimensional structure show close packing result from the enhancement of Ti─O─Ti chains. The Ti(OH)<sup>4</sup> is preferred 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 anatase TiO2 nanoparticles could be obtained by the polycondensation of titanium alkoxide in the presence of tetra methyl ammonium hydroxide [24].

#### *2.1.2. Hydrothermal and solvothermal methods*

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

**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.

An exponential growth of research activities has been noticed in nanoscience and nanotechnology in the past decades [17, 18]. When the size of the material becomes smaller and smaller, down to the nanometer scale, new physical and chemical properties are obtained. Properties also change by the changing in the morphologies of the shrinking nanomaterials. One of the best features of these materials is the swimming of electrons and holes in semiconductor nanomaterials, which are first organized using the well-known quantum confinement, and the moving features owned to phonons and photons are greatly related to the size and geometry of the materials [19]. The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases [20, 21]. Continuously, breakthroughs have been made in the preparation,

 nanomaterials, including nanoparticles, nanorods (NRs), nanowires (NWs), and nanotubes (NTs), are primarily categorized with the preparation techniques. For detailed instructions on each synthesis, the readers are referred to the corresponding literature related to the common major four preparation methods as well as the obtained shapes of titanium dioxide nanopowders.

 nanomaterials have been synthesized by the sol-gel method from hydrolysis of a titanium precursor. This process normally happened through an acid-catalyzed hydrolysis step of titanium (IV) alkoxide followed by condensation as shown in **Figure 3**. The improvement of Ti─O─Ti bonds is developed with small amount of water, low hydrolysis rates, and excess amount of

phase, although a mixture of anatase and rutile is preferred for photocata-

nanomaterials. Here, we focus on recent progress in the

nanomaterials. The syntheses of

photoactive TiO2

TiO2

TiO2

*2.1.1. Sol-gel method*

lytic applications [16].

**2. Synthesis of titania nanoparticles**

354 Titanium Dioxide - Material for a Sustainable Environment

modification, and applications of TiO<sup>2</sup>

synthesis, properties, modifications, and applications of TiO<sup>2</sup>

**2.1. Synthetic methods for titanium dioxide nanostructures**

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 obtain TiO<sup>2</sup> nanoparticles [26, 27]. For instance, hydrothermal treatment of peptized precipitates of a titanium precursor with water is considered to be a good source of TiO<sup>2</sup> nanoparticles [28]. The precipitate materials were fabricated by adding a 0.5 M isopropanol solution of titanium butoxide into deionized water ([H<sup>2</sup> O]/[Ti]) [29], and then they were peptized at 70°C 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 ethanol-water solution [30]. Besides TiO<sup>2</sup> nanoparticles, TiO2 nanorods have also been synthesized with the hydrothermal method [31] that obtained TiO<sup>2</sup> nanorods by treating a dilute TiCl<sup>4</sup> solution at 333–423 K for 12 h in the presence of acid or inorganic salts [32]. **Figure 4** illustrates the fabrication process of the titania nanorods (TNRs) film on FTO substrate via hydrothermal method as well as the SEM image of TiO<sup>2</sup> nanoparticles.

The solvothermal method has been found to be a versatile method for the synthesis of a variety of nanoparticles with narrow size distribution and disparity [33]. TiO2 nanoparticles and nanorods with narrow size distributions can also be developed with the solvothermal method [34]. For example, in a typical synthesis from [35], the suspension of TiO2 powder has been done by keeping 5 M NaOH water-ethanol solution at 170–200°C for 24 h in an autoclave and then cooled to room temperature naturally. After that, the obtained sample is washed with a dilute HCl aqueous solution and dried at 60°C for 12 h in air. Then, TiO<sup>2</sup> nanowires are obtained. The crystal morphology determination is highly dependent on the solvent used.

#### *2.1.3. Chemical vapor deposition and physical vapor deposition methods*

Materials in a vapor state are condensed to form a solid-phase material is the meaning of vapor deposition operation. These operations are basically used to form coatings to prevent the mechanical, electrical, thermal, optical, corrosion resistance, and wear resistance properties of various substrates. Lately, they have been exceedingly reconnoitered to prepare different shapes of nanomaterials. A vacuum chamber is a place where the vapor deposition process is taken place. In case where no chemical reaction happens, this operation is named physical vapor deposition (PVD); otherwise, it is named chemical vapor deposition (CVD). In CVD processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction. On the other hand, plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate as shown in the schematic diagram in **Figure 5**. Chemical reactions are implicated in the operation, which happened following the creation of a plasma of the reacting gases. The plasma is generally obtained using radio frequency (RF) (alternating current (AC)) or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases. Furthermore, in PVD, materials are firstly evaporated and then condensed to obtain a solid material. The primary

PVD methods contain thermal deposition, ion plating, ion implantation, sputtering, and

consists of a mixture of 80% anastase and 20% rutile with a specific surface area of approximately 50 m2/g. Reproduced

**Figure 5.** Schematic diagram of the plasma-enhanced chemical vapor deposition (PECVD). Reproduced from Ref. [39]

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357

The organic carboxylic acid precursor technique includes the preparation of aqueous solution of required cations and the chelation of cations in solution by addition of carboxylic acid then, raising the temperature of the solution until formation of the precursor. The precursor is

nanowire arrays have been assembled by a simple PVD method or

P-25). It

nanoparticles with an average size of 21 nm obtained from Evonik (Aerosil TiO<sup>2</sup>

laser vaporization. TiO2

**Figure 6.** TEM image of TiO2

thermal deposition [38].

*2.1.4. Organic acid precursor method*

from Ref. [37, 41, 58] with permission from Springer Nature publishing 2017.

with permission from IOPScience publishers.

**Figure 4.** (a) Fabrication process of the TNRs film on FTO substrate via hydrothermal method. (b) SEM photograph of a TiO2 film resulted from P25 and titanium isopropoxide ethanol solution after hydrothermal treatment at 100 °C for 12 h. Reproduced from Ref. [36, 37, 57] with permission from AIMSpress 2016 and Royal Society of Chemistry 2014.

Controlling the Microstructure and Properties of Titanium Dioxide for Efficient Solar Cells http://dx.doi.org/ 10.5772/intechopen.72494 357

**Figure 5.** Schematic diagram of the plasma-enhanced chemical vapor deposition (PECVD). Reproduced from Ref. [39] with permission from IOPScience publishers.

**Figure 6.** TEM image of TiO2 nanoparticles with an average size of 21 nm obtained from Evonik (Aerosil TiO<sup>2</sup> P-25). It consists of a mixture of 80% anastase and 20% rutile with a specific surface area of approximately 50 m2/g. Reproduced from Ref. [37, 41, 58] with permission from Springer Nature publishing 2017.

PVD methods contain thermal deposition, ion plating, ion implantation, sputtering, and laser vaporization. TiO2 nanowire arrays have been assembled by a simple PVD method or thermal deposition [38].

#### *2.1.4. Organic acid precursor method*

TiCl<sup>4</sup>

TiO2

 solution at 333–423 K for 12 h in the presence of acid or inorganic salts [32]. **Figure 4** illustrates the fabrication process of the titania nanorods (TNRs) film on FTO substrate via

The solvothermal method has been found to be a versatile method for the synthesis of a vari-

nanorods with narrow size distributions can also be developed with the solvothermal method

done by keeping 5 M NaOH water-ethanol solution at 170–200°C for 24 h in an autoclave and then cooled to room temperature naturally. After that, the obtained sample is washed with

obtained. The crystal morphology determination is highly dependent on the solvent used.

Materials in a vapor state are condensed to form a solid-phase material is the meaning of vapor deposition operation. These operations are basically used to form coatings to prevent the mechanical, electrical, thermal, optical, corrosion resistance, and wear resistance properties of various substrates. Lately, they have been exceedingly reconnoitered to prepare different shapes of nanomaterials. A vacuum chamber is a place where the vapor deposition process is taken place. In case where no chemical reaction happens, this operation is named physical vapor deposition (PVD); otherwise, it is named chemical vapor deposition (CVD). In CVD processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction. On the other hand, plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate as shown in the schematic diagram in **Figure 5**. Chemical reactions are implicated in the operation, which happened following the creation of a plasma of the reacting gases. The plasma is generally obtained using radio frequency (RF) (alternating current (AC)) or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases. Furthermore, in PVD, materials are firstly evaporated and then condensed to obtain a solid material. The primary

**Figure 4.** (a) Fabrication process of the TNRs film on FTO substrate via hydrothermal method. (b) SEM photograph of a

Reproduced from Ref. [36, 37, 57] with permission from AIMSpress 2016 and Royal Society of Chemistry 2014.

film resulted from P25 and titanium isopropoxide ethanol solution after hydrothermal treatment at 100 °C for 12 h.

ety of nanoparticles with narrow size distribution and disparity [33]. TiO2

[34]. For example, in a typical synthesis from [35], the suspension of TiO2

a dilute HCl aqueous solution and dried at 60°C for 12 h in air. Then, TiO<sup>2</sup>

*2.1.3. Chemical vapor deposition and physical vapor deposition methods*

nanoparticles.

nanoparticles and

powder has been

nanowires are

hydrothermal method as well as the SEM image of TiO<sup>2</sup>

356 Titanium Dioxide - Material for a Sustainable Environment

The organic carboxylic acid precursor technique includes the preparation of aqueous solution of required cations and the chelation of cations in solution by addition of carboxylic acid then, raising the temperature of the solution until formation of the precursor. The precursor is calcined at low temperature. The method is also called combustion method, polymeric precursor method, and acid gel method (oxalate precursor, tartaric acid, lactic acid, and citrate precursor method). The process depends on complexation of metallic salts with aqueous solution of organic acid; the formed complex solution was evaporated at low temperature from 60 to 1000°C until viscous resin is formed; the formed polymer resin was dried and then calcined at low temperature from 200 to 1000°C for 1–4 h [40]. **Figure 6** shows TEM micrographs of titania nanopowders synthesized using organic acid precursor.

> surface sites to guarantee more efficient charge separation [53]. Moreover, NRs can potentially enhance the charge transport in the photoanodes of DSSCs. In addition, NRs offer direct electrical pathways for photo-generated electrons and can enhance the electron transport rate,

photoanodes consisting of the NPs and NRs in the configuration of DSSCs (Kang et al., 2008). From

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359

following two main advantages: (1) confirmation of high surface area directly proportional to the light-harvesting yield (dye uptake) resulted from the NRs synthesized from the neck-

electron transport rate and degraded charge recombination from the decreased intercrystalline contacts between grain boundaries and specific directionality of NRs, bringing about the

The architecture of a dye-sensitized solar cell (DSSC) is discussed in **Figure 8**. The DSSC con-

between two glass substrates. Front and back electrodes are decorated with a transparent con-

FTO at the back electrode is covered with few nanometers of atomic layers of platinum (Pt), which enhance the formation of electrons through the redox reaction with the electrolyte. The front elec-

age particle size of 5–20 nm. Considering a layer thickness of 10 μm, the resulting effective surface

Subsequently, a monolayer of dye molecules (scattering layer) is adsorbed on the surface of the

and is obtained by the two carboxylic groups of the ligand (L = 2,2′-bipyridyl-4,4′-dicarboxylic

. Finally, a liquid redox electrolyte is inserted between the two electrodes.

. The huge nanoporous surface allows for an adsorption of a sufficiently huge number of dye molecules for efficient light harvesting. The most widespread dye molecule employed in DSSC is usually a ruthenium (Ru) metal-organic complex, the so-called N719 [56]. The spectral absorption

rutile, anatase, and brookite. In the DSSC preferably only the anatase modification is used [54].

of the dye lies between 300 nm and 800 nm. Sufficient adsorption of the dye to the TiO<sup>2</sup>

(titanium dioxide) layer and an electrolyte founded in

as a blocking layer which is responsible to prevent

layer. Three modifications of TiO<sup>2</sup>

: F), FTO, is the most widely known. The

NRs showed the

NRs and (2) fast

layer with an aver-

exist:

is critical

which in turn may improve the performance of DSSCs.

Ref. [53]. Reprinted with permission from Royal Society of Chemistry 2016.

improved charge collection efficiency.

**Figure 7.** The TiO2

sists of a dye-covered, nanoporous TiO<sup>2</sup>

trode is covered firstly with thin layer of TiO<sup>2</sup>

(NCS)2

TiO2

acid) of the RuL<sup>2</sup>

ducting oxide (TCO). Fluorine doped tin oxide (SnO<sup>2</sup>

is about 1000 times larger than the dense compact TiO<sup>2</sup>

In the DSSC, the photoanode encompassed of oriented attachment. TiO<sup>2</sup>

ing of truncated NPs by recovering the low surface area of the general TiO<sup>2</sup>

**4. The architecture of the dye-sensitized solar cell (DSSC)**

the holes from reaching the anode and then coated with a nanocrystalline TiO<sup>2</sup>
