*2.1.1. Examples*

and pressure conditions. Sometimes, surfactants are also added to control the growth and

Internal pressure is set up by the amount of temperature and solution used. This process is mainly used for the preparation of small-sized particles for achieving enhanced surface area.

morphology of the particles can be varied by changing crystallization temperature, time and concentration of etching chemicals. **Table 2** shows the various routes by which hierarchical

(TT)

K2 TiO<sup>2</sup> (C<sup>2</sup> O4 )2 and

Anatase 36.93 15–25μm Ti foil Flower-like

(DEG)

Titanium tetrabutoxide

diethylene glycol

and titanium isopropoxide

microspheres

dichloride

Anatase 64.8 1–1.5μm Titanium powder Radial nanoflakes Gas sensing

(TBT) and acetic

acid

(Ti(SO<sup>4</sup> )2 ) and urea

nanostructures produced by various hydrothermal routes.

Anatase 168.3 2–4μm Titanium powder Nanospheres

(CO(NH<sup>2</sup> )2 ) from amorphous one. The

DSSCs

DSSCs

DSSCs

DSSCs

Photocatalysis

Photocatalysis

Gas sensing

Photocatalysis

Lithium sulfur battery

**Precursor materials Morphology Used in application**

1D/3D nanorods DSSCs

Flower-like structures

Nanowire trunk on which nanorods are grown

Nanoparticle-based

HNs

Hierarchical microspheres

structure formed from nanobelts

Flower-like shapes formed from nanosheets

Overlapped subunits of nanoflakes

Nanothorn-like hierarchical structures

composed of nanosheets

Basically, this synthesis is used for preparation of crystalline TiO<sup>2</sup>

structures have been prepared by hydrothermal route.

**Particle size (diameter)**

Rutile 67 1–1.5 μm Tetrabutyl titanate

length) 150 nm diameter

95 nm, NR 5 nm

Anatase — 300–600 nm Dodecylamine

Anatase 27.4 4–6μm Primary Titania

Anatase 170 350 nm Titanocene

Anatase 116.6 3–4μm Tetrabutyl titanate

Anatase — 20–50 nm Titanium sulfate

morphology of target materials.

8 Titanium Dioxide - Material for a Sustainable Environment

**Reference Phase Surface** 

**area (m2 g−1)**

Rutile 75.53 (4.4 μm

Anatase — NW trunk

TiO<sup>2</sup>

Lin et al. [56]

Wang et al. [50]

Qiang et al. [3]

Xiang et al. [57]

Zheng et al. [60]

Shao et al. [58]

Zhu et al. [29]

Wang et al. [32]

Gao et al. [2]

Yang et al. [59]

Min et al. [59]

**Table 2.** Hierarchical TiO<sup>2</sup>

Lin et al. have reported rutile TiO<sup>2</sup> hierarchical flower-like structures via hydrothermal synthesis without using any surfactant. Precursor tetrabutyltitanate (TT) is first mixed with HCl for acidification and is subsequently hydrolyzed using distilled water. To ensure complete hydrolysis, the reaction mixture is stirred for about one and a half hour. Next, the mixture is transferred to a Teflon-based autoclave and is placed in an electric oven for 5 h at 150°C for crystallization. Hierarchical nanoflower-like structures are obtained as an end product. By increasing the HCl concentration, the etching rate of TiO<sup>2</sup> structures is enhanced and symmetric flower-like structures are produced [56] (**Figure 4**). These structures are employed for DSSCs and 8.6% of conversion efficiency is achieved.

Wang et al. have prepared rutile Titania 1D/3D structures via hydrothermal treatment. The precursor Titanium tetrabutoxide (0.9 m) is acidified using HCl (16 ml) and then hydrolyzed using DI water (16 ml). The reaction mixture is subsequently heated to 150°C and kept at this temperature for 10 h for crystallization. The position of FTO substrates is varied to obtain different HNSs. 1D/3D HNSs are produced while the FTO substrate lied flat on the bottom of the reactor with the conductive side facing up. 3D nanorods are produced when the conductive side of substrate is placed downwards.

Rutile Titania 3D flower-like nanorods are grown on 1D nanorods with length in microns [50] (**Figure 5**). These structures are employed as a photo anode material in DSSCs and significant improvement in device performance is seen. 1D structure provides directed pathway for electron percolation and 3D morphology provides large surface area for light scattering and dyeloading. Also, the structures exhibit long life time due to less electron-hole recombination.

Qiang et al. have obtained hierarchical anatase TiO<sup>2</sup> nanowire trunks with short nanorod branch HNSs by facile one-way hydrothermal synthesis on FTO glass without using any surfactant/stabilizing agent. The solution is prepared using precursor K<sup>2</sup> TiO<sup>2</sup> (C<sup>2</sup> O4 )2 (0.002 mol)

**Figure 4.** FESEM images of TiO<sup>2</sup> structures. Change in morphology is produced from changing HCl conc. at (a) 1M (b) 2M (c) 3M (d) 4M (e) 5M (f) 6M (f) 7M (g) 8M [56].

mixed with ethanol (360 ml) and DI water (120 ml) for hydrolysis under vigorous stirring

mixture and white powder of anatase Titania is obtained, which is then washed with water

in different ratios. The solution is transferred to Teflon-lined autoclave, kept at 250°C for 12 h

HNSs made up of symmetrically arranged interconnected spherical nanoparticles are obtained as a result of these syntheses [57]. These structures are employed as a photoanode material in this article. The large spheres can provide maximum scattering of sunlight for light-driven

Teflon-lined autoclave at an inclined angle in 5 M NaOH solution for its reduction. Sodium titanate is formed as a result of this reaction. The temperature is maintained at 220°C for 24 h

with water and ethanol to remove all the acidic content and to maintain its pH at neutral. The

nanoparticles with NaOH results in the formation of Na<sup>2</sup>

TiOx + NaOH ⟶ Na<sup>2</sup> TiO<sup>3</sup> + H<sup>2</sup> O (1)

Na<sup>2</sup> TiO<sup>3</sup> + H<sup>+</sup> /H<sup>2</sup> O ⟶ H<sup>2</sup> TiO<sup>3</sup> + NaOH (2)

**Figure 7.** TEM images of hierarchical TiO<sup>2</sup> spherical structures composed of nanoparticles (a) Agglomerated TiO<sup>2</sup>

ions can be replaced with H+ ions by washing them with deionized water or acid.

ion. After calcination, nanobelts (**Figure 8**) forming nanoflower-like structures

formed during reaction is removed from the reaction

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

flower-like structures on Ti foil by placing it in

nanostructures. The sample is then washed

ions of sodium titanate would get

TiO<sup>3</sup>

, which is a

is added in reaction mixture

11

http://dx.doi.org/10.5772/intechopen.74525

at ambient room temperature. HNO<sup>3</sup>

reactions like photocatalysis (**Figure 7**).

nanoparticles (b) Dispered TiO2 nanoparticles [57].

Shao et al. have prepared hierarchical TiO<sup>2</sup>

for complete reaction of converting Ti foil to TiO<sup>2</sup>

sample is then immersed in HCl solution so that all Na<sup>+</sup>

nanoporous structure. The reaction that takes place is as follows.

to dope Ta particles with TiO<sup>2</sup>

replaced by H<sup>+</sup>

are formed [58].

Reaction of TiO<sup>2</sup>

It can be shown as:

These Na<sup>+</sup>

and ethanol to maintain the pH of particles at 7. After that, TaCl<sup>5</sup>

.

**Figure 5.** (a,d) SEM images of 1D/3D nanorod structures (b) 1D nanorods (c) 3D nanoflower-like structures [50].

**Figure 6.** SEM images of hierarchical anatase nanowire trunk covered by short nanorod branches (a) Nanorods covering the nanotrunks (b) Enlarged image of nanorods (c) Nanotrunks covered by nanorods [3].

and diethylene glycol (DEG) (30 ml) as Titania precursor. About 10 ml H<sup>2</sup> O is used for hydrolysis. The solution is then spin coated on FTO substrate for seeding of TiO<sup>2</sup> structures. The spin-coated substrate is immersed in Teflon-based autoclave, which is kept at 180°C for 1–12 h for crystallization of Titania structures. **Figure 6** shows nanotrunks produced having nanowire-like structures grown on them [3].

These structures are employed as a photo anode material in DSSCs and impressive power conversion efficiency of 7.34% is achieved. Hierarchical morphology aids in efficient electron transfer but these structures provide additional recombination sites so results are inferior to bare TiO<sup>2</sup> nanowires.

Xiang et al. have reported Ta-doped and -undoped hierarchical TiO<sup>2</sup> nanostructures. Dodecylamine (8 g) and titanium isopropoxide (TIP, 8 g) are used as precursor materials and are mixed with ethanol (360 ml) and DI water (120 ml) for hydrolysis under vigorous stirring at ambient room temperature. HNO<sup>3</sup> formed during reaction is removed from the reaction mixture and white powder of anatase Titania is obtained, which is then washed with water and ethanol to maintain the pH of particles at 7. After that, TaCl<sup>5</sup> is added in reaction mixture in different ratios. The solution is transferred to Teflon-lined autoclave, kept at 250°C for 12 h to dope Ta particles with TiO<sup>2</sup> .

HNSs made up of symmetrically arranged interconnected spherical nanoparticles are obtained as a result of these syntheses [57]. These structures are employed as a photoanode material in this article. The large spheres can provide maximum scattering of sunlight for light-driven reactions like photocatalysis (**Figure 7**).

Shao et al. have prepared hierarchical TiO<sup>2</sup> flower-like structures on Ti foil by placing it in Teflon-lined autoclave at an inclined angle in 5 M NaOH solution for its reduction. Sodium titanate is formed as a result of this reaction. The temperature is maintained at 220°C for 24 h for complete reaction of converting Ti foil to TiO<sup>2</sup> nanostructures. The sample is then washed with water and ethanol to remove all the acidic content and to maintain its pH at neutral. The sample is then immersed in HCl solution so that all Na<sup>+</sup> ions of sodium titanate would get replaced by H<sup>+</sup> ion. After calcination, nanobelts (**Figure 8**) forming nanoflower-like structures are formed [58].

Reaction of TiO<sup>2</sup> nanoparticles with NaOH results in the formation of Na<sup>2</sup> TiO<sup>3</sup> , which is a nanoporous structure. The reaction that takes place is as follows.

$$\text{TiO}\_{\text{x}} + \text{NaOH} \longrightarrow \text{Na}\_{2}\text{TiO}\_{3} + \text{H}\_{2}\text{O} \tag{1}$$

These Na<sup>+</sup> ions can be replaced with H+ ions by washing them with deionized water or acid. It can be shown as:

$$\text{Na}\_2\text{TiO}\_3 + \text{H}^+/\text{H}\_2\text{O} \longrightarrow \text{H}\_2\text{TiO}\_3 + \text{NaOH} \tag{2}$$

and diethylene glycol (DEG) (30 ml) as Titania precursor. About 10 ml H<sup>2</sup>

the nanotrunks (b) Enlarged image of nanorods (c) Nanotrunks covered by nanorods [3].

Xiang et al. have reported Ta-doped and -undoped hierarchical TiO<sup>2</sup>

ire-like structures grown on them [3].

10 Titanium Dioxide - Material for a Sustainable Environment

nanowires.

bare TiO<sup>2</sup>

lysis. The solution is then spin coated on FTO substrate for seeding of TiO<sup>2</sup>

spin-coated substrate is immersed in Teflon-based autoclave, which is kept at 180°C for 1–12 h for crystallization of Titania structures. **Figure 6** shows nanotrunks produced having nanow-

**Figure 6.** SEM images of hierarchical anatase nanowire trunk covered by short nanorod branches (a) Nanorods covering

**Figure 5.** (a,d) SEM images of 1D/3D nanorod structures (b) 1D nanorods (c) 3D nanoflower-like structures [50].

These structures are employed as a photo anode material in DSSCs and impressive power conversion efficiency of 7.34% is achieved. Hierarchical morphology aids in efficient electron transfer but these structures provide additional recombination sites so results are inferior to

lamine (8 g) and titanium isopropoxide (TIP, 8 g) are used as precursor materials and are

O is used for hydro-

nanostructures. Dodecy-

structures. The

**Figure 7.** TEM images of hierarchical TiO<sup>2</sup> spherical structures composed of nanoparticles (a) Agglomerated TiO<sup>2</sup> nanoparticles (b) Dispered TiO2 nanoparticles [57].

Yang et al. prepared hierarchical Titania structures by hydrothermal synthesis. Titanium sul-

tion was complete and white powder was obtained, which was then moved in Teflon-lined autoclave at 180°C for 10 h for its complete crystallization. The powder formed was washed with DI water and ethanol to maintain pH at neutral and annealed to recrystallize. Reactions

+ 3H<sup>2</sup> O ⟶ 2NH<sup>4</sup>

[Ti(H<sup>2</sup> O)(edta)] ⟶ Ti4+ + EDTA + H<sup>2</sup> O (4)

Mesoporous nanothorn-like structures are produced as a result of this process [59] shown in **Figure 10**. These structures are employed for gas sensing application of acetone. This hierarchical morphology provides much higher sensitivity and fast response time with minimum

Min et al. prepared hierarchical Titania structures by hydrothermal synthesis. About 1 g of Titanium powder was being dissolved in (0.5 M) 30 ml HF for its oxidation as it is a power-

Teflon-lined autoclave at 150°C for crystallization for 5 h. The powder obtained was washed via centrifugation and was then dried at 80°C. Nanosheets (**Figure 11**) are being produced as

These structures are employed as a photocatalyst material for photodegradation of organic dye. Methylene blue is used as a model dye in this article. Complete photodegradation of

acetic acid (EDTA) disodium salt was added in them as the chelating agent for TiO<sup>2</sup>

(3 mmol), urea (24 mmol) and EDTA (3 mmol) were mixed and NH<sup>4</sup>

) were used as precursor materials. Ethylene diaminetetra

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

TiO<sup>3</sup>

O)(edta)] with F−1 ions. The reaction mixture was

formation.

13

F (9 mmol) was

. After stirring for 3 h, the reac-

http://dx.doi.org/10.5772/intechopen.74525

<sup>+</sup> + 2OH− + CO<sup>2</sup> (3)

.H<sup>2</sup> O + 8H<sup>+</sup> (5)

. This solution was moved to

.H<sup>2</sup> O ⟶ 2TiO<sup>2</sup> + 2H<sup>2</sup> O (6)

fate (Ti(SO<sup>4</sup>

Ti(SO<sup>4</sup> )2 )2

that took place were:

recovery speed.

a result of this process [52].

organic compound is observed in just 60 min.

) and urea (CO(NH<sup>2</sup>

added in the mixture for attaching [Ti(H<sup>2</sup>

O〓C (NH2)

H<sup>2</sup> Ti2 O<sup>5</sup>

) 2

2

dissolved into 60 ml of deionized water for formation of H<sup>2</sup>

2Ti4+ + 6H<sup>2</sup> O ⟶ H<sup>2</sup> Ti2 O<sup>5</sup>

ful oxidizing agent. The solution was then mixed with 3 ml NH<sup>3</sup>

**Figure 10.** FESEM images of hydrothermal synthesis at (a) 5 min, (b) 1 h, (c) 3 h and (d) 5 h [59].

**Figure 8.** SEM images of nanoflower-like hierarchical TiO<sup>2</sup> structures produced after annealing at 500°C [58].

**Figure 9.** SEM images of hierarchical TiO<sup>2</sup> prepared by hydrothermal heating: (A) after 1 h, (B) after 2 h and (C) after 12 h. (D, E) Annealed powder of sample B [29].

By reduction of nanoparticles and increasing time duration of crystallization, nanoflower-like structures can be grown.

Zhu et al. have prepared HNSs of TiO<sup>2</sup> using titanocene dichloride (Ti(Cp)<sup>2</sup> Cl<sup>2</sup> ) (20 mg) as precursor. DI water (10 ml) is added for hydrolysis, and ethylene diamine (EDA) (2 drops) acts as chelating agent. This results in the production of TiO<sup>2</sup> nanocrystals. The mixture then after sonication is placed in an autoclave at 120°C for 1–12 h. The powder obtained is then washed with water and ethanol and is annealed at 400°C for 2 h. Flower-like HNSs (**Figure 9**) are formed in this process [29].

By increasing the duration of hydrothermal synthesis, the nanoparticles have attained more flower-like mesoporous morphology due to increased time provided for etching. These structures have proved to be better photocatalytic agents as compared to nanocrystals of TiO<sup>2</sup> as they can provide maximum enhanced surface area, and due to their mesoporosity, maximum dye can be loaded on them. Hence, these structures can be used with perspective of various solar cells and photocatalysis for efficient light-driven reactions.

Yang et al. prepared hierarchical Titania structures by hydrothermal synthesis. Titanium sulfate (Ti(SO<sup>4</sup> ) 2 ) and urea (CO(NH<sup>2</sup> )2 ) were used as precursor materials. Ethylene diaminetetra acetic acid (EDTA) disodium salt was added in them as the chelating agent for TiO<sup>2</sup> formation. Ti(SO<sup>4</sup> )2 (3 mmol), urea (24 mmol) and EDTA (3 mmol) were mixed and NH<sup>4</sup> F (9 mmol) was added in the mixture for attaching [Ti(H<sup>2</sup> O)(edta)] with F−1 ions. The reaction mixture was dissolved into 60 ml of deionized water for formation of H<sup>2</sup> TiO<sup>3</sup> . After stirring for 3 h, the reaction was complete and white powder was obtained, which was then moved in Teflon-lined autoclave at 180°C for 10 h for its complete crystallization. The powder formed was washed with DI water and ethanol to maintain pH at neutral and annealed to recrystallize. Reactions that took place were:

$$\text{O=C (NH}\_2\text{)}\_2 + 3\text{H}\_2\text{O} \longrightarrow 2\text{NH}\_4^+ + 2\text{OH}^- + \text{CO}\_2 \tag{3}$$

$$\text{[Ti(H}\_2\text{O)(edta)]} \longrightarrow \text{Ti}^{4+} + \text{EDTA} + \text{H}\_2\text{O} \tag{4}$$

$$2\text{Ti}^{\ast} \star \text{6H}\_{2}\text{O} \longrightarrow \text{H}\_{2}\text{Ti}\_{2}\text{O}\_{3}\text{H}\_{2}\text{O} + 8\text{H}^{\ast}\tag{5}$$

$$\text{H}\_{\text{z}}\text{Ti}\_{\text{z}}\text{O}\_{\text{y}}\text{H}\_{\text{z}}\text{O} \longrightarrow 2\text{TiO}\_{\text{z}} + 2\text{H}\_{\text{z}}\text{O} \tag{6}$$

Mesoporous nanothorn-like structures are produced as a result of this process [59] shown in **Figure 10**. These structures are employed for gas sensing application of acetone. This hierarchical morphology provides much higher sensitivity and fast response time with minimum recovery speed.

Min et al. prepared hierarchical Titania structures by hydrothermal synthesis. About 1 g of Titanium powder was being dissolved in (0.5 M) 30 ml HF for its oxidation as it is a powerful oxidizing agent. The solution was then mixed with 3 ml NH<sup>3</sup> . This solution was moved to Teflon-lined autoclave at 150°C for crystallization for 5 h. The powder obtained was washed via centrifugation and was then dried at 80°C. Nanosheets (**Figure 11**) are being produced as a result of this process [52].

These structures are employed as a photocatalyst material for photodegradation of organic dye. Methylene blue is used as a model dye in this article. Complete photodegradation of organic compound is observed in just 60 min.

**Figure 10.** FESEM images of hydrothermal synthesis at (a) 5 min, (b) 1 h, (c) 3 h and (d) 5 h [59].

By reduction of nanoparticles and increasing time duration of crystallization, nanoflower-like

precursor. DI water (10 ml) is added for hydrolysis, and ethylene diamine (EDA) (2 drops)

after sonication is placed in an autoclave at 120°C for 1–12 h. The powder obtained is then washed with water and ethanol and is annealed at 400°C for 2 h. Flower-like HNSs (**Figure 9**)

By increasing the duration of hydrothermal synthesis, the nanoparticles have attained more flower-like mesoporous morphology due to increased time provided for etching. These structures have proved to be better photocatalytic agents as compared to nanocrystals of TiO<sup>2</sup>

they can provide maximum enhanced surface area, and due to their mesoporosity, maximum dye can be loaded on them. Hence, these structures can be used with perspective of various

using titanocene dichloride (Ti(Cp)<sup>2</sup>

prepared by hydrothermal heating: (A) after 1 h, (B) after 2 h and (C) after 12 h.

structures produced after annealing at 500°C [58].

Cl<sup>2</sup>

nanocrystals. The mixture then

) (20 mg) as

as

structures can be grown.

**Figure 9.** SEM images of hierarchical TiO<sup>2</sup>

(D, E) Annealed powder of sample B [29].

are formed in this process [29].

Zhu et al. have prepared HNSs of TiO<sup>2</sup>

**Figure 8.** SEM images of nanoflower-like hierarchical TiO<sup>2</sup>

12 Titanium Dioxide - Material for a Sustainable Environment

acts as chelating agent. This results in the production of TiO<sup>2</sup>

solar cells and photocatalysis for efficient light-driven reactions.

morphology ranges from hierarchical arrangement of nanoparticles to nanorods producing

by increasing duration of hydrothermal synthesis, better etched morphology was obtained. **Table 2** summarizes reported examples of hydrothermal synthesis of HNSs with respect to precursor used for the synthesis and various properties of obtained HNSs together with the application studied. The HNSs obtained from this method varies in size from 20 nm to 4.5

exploited in various applications including solar cells, photocatalysis and sensors, etc. and

In this synthesis, the conditions are the same as for a hydrothermal method but non-aqueous solvent is used instead of water. The process takes place in an autoclave and temperature can be increased because of high boiling of certain organic solvents as compared to water. This method results in uniform particle size distribution and high purity products. Also by changing the temperature, morphology of the grown crystals can be varied. In addition to that, different morphologies result due to differences in steric hindrance offered by different functional groups in various organic solvents. **Table 3** shows various routes adopted to synthesize

vent is added to this solution followed by heating in a 50 ml Teflon-lined autoclave at 150°C for 0.5, 1, 6 and 12 h. The resultant white precipitates are then dried in air. Nanoflower-like structures are grown over the nanorod structures with average crystallite size of 4 nm (**Figure 12**) [10]. These structures are then employed for photocatalytic degradation of methylene blue dye. Complete degradation of organic dye has been achieved in 120 min by these structures. Hence, these HNSs can be used for determining photocatalytic behavior and applications like

> **Precursor materials**

HNSs by solvothermal synthesis technique. Their method

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

 fibers 3D nanoflowers grown on nanofibers

> like structure composed of nanoneedles

TiCl<sup>4</sup> 3D urchin-

fibers. Then, 15 ml ethanol sol-

**Morphology Application**

Photocatalysis

Photocatalysis

have improved respective performance owing to their unique structural properties.

particles, and

15

g−1. These structures are

http://dx.doi.org/10.5772/intechopen.74525

flower-like structures. Increasing temperature increases crystallization of TiO<sup>2</sup>

μm. The surface area of the resultant HNSs ranges from 27 to 170 m<sup>2</sup>

**2.2. Solvothermal synthesis**

HNSs via solvothermal route.

Ochanda et al. have presented TiO<sup>2</sup>

**Reference Phase Surface** 

**area (m2 g−1)**

Anatase 37 2.5–3.0μm

involves mixing of 15 ml of 4 M NaOH solution with 0.5 g TiO<sup>2</sup>

**Particle size (diameter)**

(microspheres), 20–40 nm (nanorods)

nanostructures produced via solvothermal method.

Anatase 94.01 50–70 nm TiO<sup>2</sup>

TiO<sup>2</sup>

*2.2.1. Examples*

solar cells.

Ochanda et al. [10]

Xiang et al. [35]

**Table 3.** Hierarchical TiO<sup>2</sup>

**Figure 11.** FESEM images of Titania hierarchical nanospheres composed of nanosheet-like structures [52].
