**1. Introduction**

Depletion of fossil fuels and environmental pollution has reached an alarming situation. New techniques are being searched now to overcome this situation by switching toward sustainable and renewable energy resources [1–4]. Severe pollution threats like global warming demand such materials and devices that are environment-friendly and green. The main idea is to fabricate materials that are not only cost-effective [5] but are also more capable to deal with energy crisis in the world.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Recently, different renewable energy resources like wind, solar, bioenergy and geothermal energy have been deployed for energy production [6]. New materials are being explored and synthesized to harness energy from these alternate energy sources. The goal is to improve the competence of these devices by synthesizing materials that provide maximum energy harvesting and power control. For solar energy harvesting by photovoltaic devices, i.e., direct conversion of sunlight to electricity, different generations have been evolved depending upon materials and technologies used in device fabrication.

Various material types are employed for their use in renewable energy resources like organic, organometallic, metallic, semiconductors, etc. Wide band gap semiconductors have been exploited due to their substantial applications so far in cosmetics [7], drugs, electronics [8], photovoltaic devices [9], energy storage materials [10–12] and catalysis [13, 14] like photodegradation (**Figure 1**). The third-generation photovoltaics utilize nanomaterials that have attracted much attention recently due to their novel electric, photochemical, piezoelectric, mechanical and catalytic properties [15].

In PV devices [17], semiconductor materials are mostly chosen on the basis of many properties like band gap, electronic mobility, mesoporosity [18, 19], toxicity levels, robustness [20, 21] and high surface area. So far, many semiconductor oxides have been prepared and tested. Nanomaterials of transition metal oxides like TiO<sup>2</sup> , SnO<sup>2</sup> , and ZnO are being further explored now because of their properties and applications. Titania [22] is the best material among all having distinguished optoelectronic and photochemical properties.

Titania is a promising material [12, 23] as it has high refractive index, biocompatibility and high dielectric constant. It exhibits redox reactions and has excellent optical transmittance in the visible and near IR regions. It is preferred because of its high performance as a photocatalyst for water splitting, oxidation capability [7] and degradation of organics [24]. **Table 1** highlights some of the properties of Titania.

individually as minerals but only rutile and anatase have been synthesized in their pure form yet [13, 25]. Both anatase and rutile phases have tetragonal, while brookite has orthorhombic

Tetragonal Tetragonal Orthorhombic

3.3 eV (anatase)

(rutile), 3.78 g/cm<sup>3</sup>

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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5

(anatase)

The properties of Titania depend on particle morphology, crystallinity, particle size and surface area. Titania has a band gap from 3.0 to 3.23 eV, which makes it photocatalytic only in ultraviolet radiation region [26]. It is desirable to shift its band gap [27] so that it may absorb radiations of the visible light spectrum to enhance its photocatalytic properties. Titania nanostructures offer a larger surface area for light absorption in solar cells and catalytic properties

Titania nanostructures can be synthesized using various techniques. Some of them are hydrothermal [26, 29–34], solvothermal methods [10, 11, 35], sol-gel synthesis [1, 15, 36], microwave irradiation [8, 24, 37, 38], physical and chemical vapor deposition [4, 32, 39], electrochemical

Generally, anatase Titania shows superior properties to rutile Titania because of slow recombination of electron-hole pairs and higher potential energy of photogenerated electrons [45]. Nano-Titania exists in many forms like nanoparticles [15, 40–42], nanorods [10, 24, 46], nanowires [3, 31, 32, 47], nanotubes [43, 48], nanospheres [49], nanoflowers [8, 32, 46, 50], nanoforests [44, 50], etc. as shown in **Figure 3**. Nanotubes show enhanced charge percolation

and direct electronic transport than nanoparticles because of their 1D structures [51].

crystal system [9] (**Figure 2**).

**Table 1.** Physical properties of Titania.

**Properties of Titanium dioxide**

Crystal structure

Anatase Rutile Brookite

Natural forms Anatase, rutile and brookite

Melting point 1941 K Boiling point 3546 K Thermal conductivity 22Wm−1 K−1 Band gap 3.0 eV (rutile)

Solubility in water Insoluble Color White-yellow Molar mass 79.8 g/mol Density 4.23 g/cm<sup>3</sup>

Flash point Non-flammable

for dye and pollutant degradation [28].

methods [40, 41] and anodization [42–44].

Titanium dioxide also known as Titania exists in many crystalline forms among which rutile, anatase and brookite are of particular importance in nanomaterials. These forms can exist

**Figure 1.** Application of TiO<sup>2</sup> for photodegradation of organic pollutants [16].


Recently, different renewable energy resources like wind, solar, bioenergy and geothermal energy have been deployed for energy production [6]. New materials are being explored and synthesized to harness energy from these alternate energy sources. The goal is to improve the competence of these devices by synthesizing materials that provide maximum energy harvesting and power control. For solar energy harvesting by photovoltaic devices, i.e., direct conversion of sunlight to electricity, different generations have been evolved depending upon

Various material types are employed for their use in renewable energy resources like organic, organometallic, metallic, semiconductors, etc. Wide band gap semiconductors have been exploited due to their substantial applications so far in cosmetics [7], drugs, electronics [8], photovoltaic devices [9], energy storage materials [10–12] and catalysis [13, 14] like photodegradation (**Figure 1**). The third-generation photovoltaics utilize nanomaterials that have attracted much attention recently due to their novel electric, photochemical, piezoelectric,

In PV devices [17], semiconductor materials are mostly chosen on the basis of many properties like band gap, electronic mobility, mesoporosity [18, 19], toxicity levels, robustness [20, 21] and high surface area. So far, many semiconductor oxides have been prepared and tested. Nanomaterials

their properties and applications. Titania [22] is the best material among all having distinguished

Titania is a promising material [12, 23] as it has high refractive index, biocompatibility and high dielectric constant. It exhibits redox reactions and has excellent optical transmittance in the visible and near IR regions. It is preferred because of its high performance as a photocatalyst for water splitting, oxidation capability [7] and degradation of organics [24]. **Table 1**

Titanium dioxide also known as Titania exists in many crystalline forms among which rutile, anatase and brookite are of particular importance in nanomaterials. These forms can exist

for photodegradation of organic pollutants [16].

, and ZnO are being further explored now because of

, SnO<sup>2</sup>

materials and technologies used in device fabrication.

mechanical and catalytic properties [15].

4 Titanium Dioxide - Material for a Sustainable Environment

optoelectronic and photochemical properties.

highlights some of the properties of Titania.

of transition metal oxides like TiO<sup>2</sup>

**Figure 1.** Application of TiO<sup>2</sup>

**Table 1.** Physical properties of Titania.

individually as minerals but only rutile and anatase have been synthesized in their pure form yet [13, 25]. Both anatase and rutile phases have tetragonal, while brookite has orthorhombic crystal system [9] (**Figure 2**).

The properties of Titania depend on particle morphology, crystallinity, particle size and surface area. Titania has a band gap from 3.0 to 3.23 eV, which makes it photocatalytic only in ultraviolet radiation region [26]. It is desirable to shift its band gap [27] so that it may absorb radiations of the visible light spectrum to enhance its photocatalytic properties. Titania nanostructures offer a larger surface area for light absorption in solar cells and catalytic properties for dye and pollutant degradation [28].

Titania nanostructures can be synthesized using various techniques. Some of them are hydrothermal [26, 29–34], solvothermal methods [10, 11, 35], sol-gel synthesis [1, 15, 36], microwave irradiation [8, 24, 37, 38], physical and chemical vapor deposition [4, 32, 39], electrochemical methods [40, 41] and anodization [42–44].

Generally, anatase Titania shows superior properties to rutile Titania because of slow recombination of electron-hole pairs and higher potential energy of photogenerated electrons [45]. Nano-Titania exists in many forms like nanoparticles [15, 40–42], nanorods [10, 24, 46], nanowires [3, 31, 32, 47], nanotubes [43, 48], nanospheres [49], nanoflowers [8, 32, 46, 50], nanoforests [44, 50], etc. as shown in **Figure 3**. Nanotubes show enhanced charge percolation and direct electronic transport than nanoparticles because of their 1D structures [51].

**1.1. Hierarchical nanostructures**

Hierarchical nanostructures are composed of 3D self-assembly of primary structure (nanoparticle, nanorod, nanotube or nanosheets) in nanoscale. Recently, materials with hierarchical morphology have attracted great attention as compared to spherical nanoparticles. Many experiments have been done in order to perk up the efficiency of nanomaterials by synthesizing hierarchical structures and enhancing the specific surface area and porosity of these structures [8, 38, 40, 53]. These types of structures show utmost light harvesting due to maximum and efficient

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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vide a significant improvement in properties due to enhanced porosity [54, 55] and many devices can be optimized using HNSs [43]. Hierarchical morphology can enhance the photon absorption capability [8] as compared to spherical nanoparticle as there is increased absorption of light due to scattering. The mesoporosity acts as distribution channels increasing adsorption of visible light sensitizers. It also creates an ideal environment for mass transportation [39] of electrons.

This chapter presents a compilation of different synthesis routes and control measures employed for the synthesis of "hierarchical nanostructures of TiO2." A brief overview of each synthesis route is provided. Investigation on the synthesis parameters and the correlation

Various types of surfactants, organic/inorganic titanium salts, high temperatures and pressures may be used for the preparation of hierarchical nanostructures of Titania. Following are

As the name indicates, the method involves heating in aqueous medium. Generally, in this method, sealed Teflon-lined steel autoclaves are used under controlled temperature

HNSs.

nanostructures pro-

7

scattering (hence absorption) of incident light within [40]. Hierarchical TiO<sup>2</sup>

with the characteristic properties of the products are also discussed.

**2. Synthesis of hierarchical nanostructures of TiO2**

the reported methods for the synthesis of TiO<sup>2</sup>

• Hydrothermal method • Solvothermal method • Microwave treatment • Pulsed laser deposition

• Anodization

• Photolithography

• Vapor deposition method

**2.1. Hydrothermal synthesis**

○ Chemical vapor deposition ○ Physical vapor deposition

**Figure 2.** Different crystalline forms of TiO<sup>2</sup> [9].

**Figure 3.** Different forms of TiO<sup>2</sup> : (a) nanocubes [24], (b) nanospheres [24], (c) nanorods [24], (d) nanoparticles [52], (e) nanoflowers [8], (f) nanoforest [48].
