**1. Introduction**

The wide applications of titanium dioxide, such as pigments, sunscreen, paints, and various commercialize applications correspond to its profound optical refractive ability, chemical stability, and low toxicity. Furthermore, TiO<sup>2</sup> has been extensively investigated as one of the most promising materials in photovoltaic devices, photocatalysis, photodegradation,

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energy storage, and electrochromic devices. This is being established when the photoelectrolysis of water into H<sup>2</sup> and O2 was reported under UV irradiation of the solar spectrum over the surface of TiO<sup>2</sup> photoelectrode in the photoelectrochemical (PEC) cell by Fujishima and Honda. In recent years, 1D-TiO<sup>2</sup> nanostructures have been significantly studied due to its distinctive advantages with regard to less charge carrier recombination rate and unique physical and chemical properties [1, 2]. In general, 1D nanostructures are well-known for their high aspect ratio in which at least one of its dimensions should be in the range of 1–100 nm regardless of their shapes such as rod, wire, belt or tube. The properties of one-dimensional single crystalline Ti-O-based nanomaterials are comparable with titania nanoparticles not only inherit almost all typical features of the nanoparticles, but also illustrate some new properties and improved performance in specific area [1]. It can afford direct transport pathways for charge carriers, decouple the direction for light absorption, and charge carrier transportation. Moreover, 1D semiconductor nanostructures are able to facilitate the light absorption and scattering, which is beneficial for photocatalytic reactions.

However, wide applications of 1D-TiO<sup>2</sup> nanostructures are diminished due to absorption of small portion of solar spectrum, relatively low specific surface area, and presence of single phase that pose certain limitations for it performance [3]. Several published research works have illustrated that well designed 1D-TiO2 nanostructures play a significant role to overcome the above-mentioned problem, preserve the intrinsic characteristics of TiO<sup>2</sup> , and finally the material with some new properties. In recent years, breakthroughs have continually been made in the preparation, modification, and applications of 1D-TiO<sup>2</sup> nanomaterials. In this chapter, we would like to address the crystal structure of TiO<sup>2</sup> and various synthesis process of 1D-TiO<sup>2</sup> nanostructured materials. Then, we will look into some critical structure engineering strategies that give 1D-TiO2 nanostructured materials excellent properties for various applications.

It is worth to mention, amongst all three different phases of TiO<sup>2</sup>

photons with enough energy (λ ≤ 390 nm for anatase TiO<sup>2</sup>

the conduction band (*e*

nanostructure of TiO<sup>2</sup>

absorbing degradation products [13].

mance of TiO<sup>2</sup>

ously reactions occur, including oxidation of dissociative adsorbed H<sup>2</sup>

**Figure 1.** Crystalline structures of titanium dioxide (a) anatase, (b) rutile, and (c) brookite [5].

for better photocatalytic performance in various applications, including photocatalytic degradation of organic dye molecules, photocatalytic water splitting and dye-sensitized solar cells compared to other phases [9, 10]. The photocatalytic reaction usually accelerates when a semiconductor interacts with light with enough energy (or a certain wavelength). Two simultane-

holes and reduction of an electron acceptor by photoexcited electrons. In the photocatalysis process, light energy greater than the bandgap energy is required to transfer photoexcited electrons to the conduction band of semiconductors. **Figure 2** shows that the absorption of

Many parameters such as size, specific surface area, pore volume, pore structure, crystalline phase, and the exposed surface facets can significantly influence the photocatalytic perfor-

[2, 10]. It is well known that the photocatalytic applications of TiO<sup>2</sup>

the nanoscale materials due to the quantum confinement effects in nanoparticles. Therefore, this effect could change the electron and hole transport behavior and shifts the electronic band structures [7, 12]. However, they also show unavoidable disadvantages such as fast recombination rate of electron and holes, slow charge carriers transfer and high recycling cost [3]. It is noteworthy to mention that effective charge separation can be considered as the most important factor to determine the photocatalytic activities. Construction of one-dimensional

charge transportation and promotes charge separation efficiency [1]. The one-dimensional titanium oxide with a large surface area can be used as catalytic carrier and be beneficial in

¯*CB*) and leave a positive hole behind in the valence band (*hVB*

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

http://dx.doi.org/10.5772/intechopen.72976

369

photocatalyst like nanowire, nanorod, and nanotube also facilitates

, anatase is well-known phase

) transfer photoexcited electron to

O by photogenerated

<sup>+</sup> ) [4, 11].

are based on

#### **2. Crystal structure and properties of 1D-TiO2 nanostructure**

TiO2 is typically the *n*-type photocatalyst due to oxygen deficiency. Anatase (tetragonal, a = b = 3.782 Å, c = 9.502 Å), rutile (tetragonal, a = b = 4.584 Å, c = 2.953 Å), and brookite (rhombohedral, a = 5.436 Å, b = 9.166 Å, c = 5.135 Å) are three different crystalline polymorphs of TiO<sup>2</sup> [4]. **Figure 1a**–**c** shows that all these phases are constructed by connecting Ti─O octahedrons through a variable number of shared corners, and/or faces.

**Figure 1a** reveals that tetragonal structure of anatase is the result of corner (vertices) sharing octahedron which form (001) planes. In rutile structure, sharing edges of octahedrons at (001) planes give the tetragonal structure as shown in **Figure 1b**. While an orthorhombic structure of Brookite is the result of sharing both edges and corners of octahedrons [5, 6]. Rutile is the most stable crystal phase of TiO<sup>2</sup> while anatase is a metastable phase, which can be transformed to thermodynamically stable rutile over calcination temperature exceeding ~600°C [7, 8].

**Figure 1.** Crystalline structures of titanium dioxide (a) anatase, (b) rutile, and (c) brookite [5].

energy storage, and electrochromic devices. This is being established when the photo-

studied due to its distinctive advantages with regard to less charge carrier recombination rate and unique physical and chemical properties [1, 2]. In general, 1D nanostructures are well-known for their high aspect ratio in which at least one of its dimensions should be in the range of 1–100 nm regardless of their shapes such as rod, wire, belt or tube. The properties of one-dimensional single crystalline Ti-O-based nanomaterials are comparable with titania nanoparticles not only inherit almost all typical features of the nanoparticles, but also illustrate some new properties and improved performance in specific area [1]. It can afford direct transport pathways for charge carriers, decouple the direction for light absorption, and charge carrier transportation. Moreover, 1D semiconductor nanostructures are able to facilitate the light absorption and scattering, which is beneficial for pho-

small portion of solar spectrum, relatively low specific surface area, and presence of single phase that pose certain limitations for it performance [3]. Several published research works

material with some new properties. In recent years, breakthroughs have continually been

nanostructured materials. Then, we will look into some critical structure engi-

 is typically the *n*-type photocatalyst due to oxygen deficiency. Anatase (tetragonal, a = b = 3.782 Å, c = 9.502 Å), rutile (tetragonal, a = b = 4.584 Å, c = 2.953 Å), and brookite (rhombohedral, a = 5.436 Å, b = 9.166 Å, c = 5.135 Å) are three different crystalline polymorphs of TiO<sup>2</sup> [4]. **Figure 1a**–**c** shows that all these phases are constructed by connecting Ti─O octahedrons

**Figure 1a** reveals that tetragonal structure of anatase is the result of corner (vertices) sharing octahedron which form (001) planes. In rutile structure, sharing edges of octahedrons at (001) planes give the tetragonal structure as shown in **Figure 1b**. While an orthorhombic structure of Brookite is the result of sharing both edges and corners of octahedrons [5, 6]. Rutile is the most

to thermodynamically stable rutile over calcination temperature exceeding ~600°C [7, 8].

the above-mentioned problem, preserve the intrinsic characteristics of TiO<sup>2</sup>

made in the preparation, modification, and applications of 1D-TiO<sup>2</sup>

chapter, we would like to address the crystal structure of TiO<sup>2</sup>

**2. Crystal structure and properties of 1D-TiO2**

through a variable number of shared corners, and/or faces.

was reported under UV irradiation of the solar spec-

nanostructures are diminished due to absorption of

nanostructures play a significant role to overcome

nanostructured materials excellent properties for various

 **nanostructure**

while anatase is a metastable phase, which can be transformed

, and finally the

nanomaterials. In this

and various synthesis process

nanostructures have been significantly

photoelectrode in the photoelectrochemical (PEC) cell by

and O2

Fujishima and Honda. In recent years, 1D-TiO<sup>2</sup>

electrolysis of water into H<sup>2</sup>

tocatalytic reactions.

of 1D-TiO<sup>2</sup>

applications.

TiO2

However, wide applications of 1D-TiO<sup>2</sup>

have illustrated that well designed 1D-TiO2

neering strategies that give 1D-TiO2

stable crystal phase of TiO<sup>2</sup>

trum over the surface of TiO<sup>2</sup>

368 Titanium Dioxide - Material for a Sustainable Environment

It is worth to mention, amongst all three different phases of TiO<sup>2</sup> , anatase is well-known phase for better photocatalytic performance in various applications, including photocatalytic degradation of organic dye molecules, photocatalytic water splitting and dye-sensitized solar cells compared to other phases [9, 10]. The photocatalytic reaction usually accelerates when a semiconductor interacts with light with enough energy (or a certain wavelength). Two simultaneously reactions occur, including oxidation of dissociative adsorbed H<sup>2</sup> O by photogenerated holes and reduction of an electron acceptor by photoexcited electrons. In the photocatalysis process, light energy greater than the bandgap energy is required to transfer photoexcited electrons to the conduction band of semiconductors. **Figure 2** shows that the absorption of photons with enough energy (λ ≤ 390 nm for anatase TiO<sup>2</sup> ) transfer photoexcited electron to the conduction band (*e* ¯*CB*) and leave a positive hole behind in the valence band (*hVB* <sup>+</sup> ) [4, 11]. Many parameters such as size, specific surface area, pore volume, pore structure, crystalline phase, and the exposed surface facets can significantly influence the photocatalytic performance of TiO<sup>2</sup> [2, 10]. It is well known that the photocatalytic applications of TiO<sup>2</sup> are based on the nanoscale materials due to the quantum confinement effects in nanoparticles. Therefore, this effect could change the electron and hole transport behavior and shifts the electronic band structures [7, 12]. However, they also show unavoidable disadvantages such as fast recombination rate of electron and holes, slow charge carriers transfer and high recycling cost [3]. It is noteworthy to mention that effective charge separation can be considered as the most important factor to determine the photocatalytic activities. Construction of one-dimensional nanostructure of TiO<sup>2</sup> photocatalyst like nanowire, nanorod, and nanotube also facilitates charge transportation and promotes charge separation efficiency [1]. The one-dimensional titanium oxide with a large surface area can be used as catalytic carrier and be beneficial in absorbing degradation products [13].

**Figure 2.** The principle of photocatalytic degradation over TiO<sup>2</sup> photocatalyst [5].

#### **3. Synthesis of 1D-TiO2 nanostructures**

Several methods such as hydrothermal, vapor deposition, sol-gel, and electrospinning, etc. were applied to synthesis various morphologies of 1D-TiO<sup>2</sup> nanostructures like nanotubes, nanorods, nanowires, nanobelts, nanosheets and nanofiber. In this section, we provide the comprehensive information related to hydrothermal method, which is widely used for manufacturing of small particles in the ceramic industry using aqueous or non-aqueous solution.

Although the synthesis process seems simple, the preparation parameters including the choice

and hydrothermal duration), and post washing procedures play important role in the crys-

formed titanium oxide nanotubes with diameter 10 nm in the range of reaction temperature of 100–160°C as illustrated in **Figure 4a** [18]. In addition, the surface area of product also affected by raw materials as surface areas of the produced nanotubes from commercial P-25

mal treatment in the presence of NaOH. **Figure 4b** shows that the product morphology was non-tubular needle-shaped fibers morphology in the presence of NaOH with concentration of 5–15 mol/l at the hydrothermal temperature range of 100–160°C. In addition, Nian et al.

hydrothermal treatment of titanate nanotube suspensions under an acidic environment in the absence of surfactants or templates. They suggested that the transformation of the tube to rode is a result of local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites. Furthermore, the hydrothermal temperature strongly controls the morphologies of products. In addition, the increasing of hydrothermal

initial raw materials such as anatase, rutile, brookite, and amorphous TiO<sup>2</sup>

temperature improves yield, length, and degree of crystallinity of nanotubes.

tal structures and physicochemical properties of 1D-TiO<sup>2</sup>

**Figure 3.** Various morphologies of hydrothermally synthesized 1D*-*TiO2

morphology of the resultant 1D-TiO<sup>2</sup>

nanobelts, (d) nanowires, and (e) nanosheets*.*

and Su [18] reported the effect of various TiO<sup>2</sup>

powder was higher than lab-made anatase TiO<sup>2</sup>

were identified when amorphous TiO<sup>2</sup>

[19] synthesized anatase TiO<sup>2</sup>

precursors, the hydrothermal condition (temperature, the concentration of reactants

nanostructures. Crystalline anatase or rutile or commercial P-25 as the raw materials

nanostructures [17]. The choice of

nanostructures: (a) nanotube, (b) nanorods, (c)

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371

nanostructures but no systematic data is available. Yuan

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

powders were the precursor with similar hydrother-

nanorods with a specific crystal-elongation direction through

precursors on the morphology of produced

. It noteworthy to mention that no nanotubes

may affect the

of TiO<sup>2</sup>

1D-TiO2
