*2.3.1. Examples*

Javed et al. have prepared 3D-HNSs by microwave irradiation of anatase nanopowder in 10 M NaOH solution at 1 atm without any surfactant. Submicron-sized flower-like HNSs are produced as shown in **Figure 14** [8]. The crystalline phase is anatase with a decrease in surface area as microwave treatment duration increases from 5 to 20 min. The product is applied as photoanode in DSSCs, whereby the use of HNSs has two-fold improved the device efficiency. One reason being the larger light scattering due to morphology of the HNSs.


**Table 4.** Hierarchical TiO<sup>2</sup> nanostructures produced via microwave treatment.

**Figure 14.** TEM images of hierarchical structures produced via microwave irradiation (a) after 5 min and (b) after 20 min [8].

Calatayud et al. prepared hierarchical crystalline TiO<sup>2</sup> via microwaves from amorphous powder using titanium (IV) tetrabutoxide (Ti(OBut)<sup>4</sup> ) and anhydrous ethanol as precursor materials. The solution is stirred for 6.5 h for complete hydrolysis and replacement of -butoxide group from -hydroxyl groups. Then, the powder is dried under atmospheric conditions. The powder obtained is washed with water and ethanol and irradiated in microwaves for different time durations. This microwave treatment provides the crystallization temperature and time for conversion of amorphous Titania to crystalline one. Anatase TiO<sup>2</sup> spherical HNSs (**Figure 15**) are produced having size from 1 to 2 μm [38].

These HNSs are employed for photodegradation of methyl orange (MO), which is completed in 6 h. By increasing microwave irradiation up to 10 min, clear agglomerated structures are produced, but as crystallization time is increased under microwaves, surface area decreases as particle size increases.

crystallized in microwaves and TiO<sup>2</sup>

aggregation occur during crystallization.

In the presence of microwave, TiO<sup>2</sup>

**Figure 16.** SEM images of 3D TiO<sup>2</sup>

**Figure 15.** SEM images of TiO<sup>2</sup>

15 min [38].

reported.

nanoagglomerates with particle size up to 10 nm are

nanoparticles begin to nucleate and then their growth and

spherical structures after microwave treatment: (a and b) after 7 min, (c and d) after

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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19

prepared in spherical geometry [37]. The crystallite size is measured to be as small as 10 nm.

agglomerates [37].

So, microwave heating provides quick crystallization of particles as particles begin to cluster within 3–5 min. Mesoporous structures are produced as a result of this synthesis and they are employed as a photoanode material in DSSCs. A maximum conversion efficiency of 7.64% is

Wang et al. have synthesized HNSs by microwave irradiation using TiCl<sup>4</sup> (0.5 ml) in ethanol (14 ml) shown in **Figure 16**. After stirring for an hour, the reaction mixture is subjected to microwave irradiation for 10 min at 150°C under pressure of 300 Pa. TiCl<sup>4</sup> is hydrolyzed and Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications http://dx.doi.org/10.5772/intechopen.74525 19

**Figure 15.** SEM images of TiO<sup>2</sup> spherical structures after microwave treatment: (a and b) after 7 min, (c and d) after 15 min [38].

**Figure 16.** SEM images of 3D TiO<sup>2</sup> agglomerates [37].

Calatayud et al. prepared hierarchical crystalline TiO<sup>2</sup>

(**Figure 15**) are produced having size from 1 to 2 μm [38].

rials. The solution is stirred for 6.5 h for complete hydrolysis and replacement of -butoxide group from -hydroxyl groups. Then, the powder is dried under atmospheric conditions. The powder obtained is washed with water and ethanol and irradiated in microwaves for different time durations. This microwave treatment provides the crystallization temperature and

**Figure 14.** TEM images of hierarchical structures produced via microwave irradiation (a) after 5 min and (b) after 20 min [8].

These HNSs are employed for photodegradation of methyl orange (MO), which is completed in 6 h. By increasing microwave irradiation up to 10 min, clear agglomerated structures are produced, but as crystallization time is increased under microwaves, surface area decreases

(14 ml) shown in **Figure 16**. After stirring for an hour, the reaction mixture is subjected to

time for conversion of amorphous Titania to crystalline one. Anatase TiO<sup>2</sup>

Wang et al. have synthesized HNSs by microwave irradiation using TiCl<sup>4</sup>

microwave irradiation for 10 min at 150°C under pressure of 300 Pa. TiCl<sup>4</sup>

der using titanium (IV) tetrabutoxide (Ti(OBut)<sup>4</sup>

**Reference Phase Surface area** 

18 Titanium Dioxide - Material for a Sustainable Environment

Calatayud et al.

**Table 4.** Hierarchical TiO<sup>2</sup>

[38]

**(m2 g−1)**

Wang el al. [37] Anatase 86.90 500 nm 10 TiO<sup>2</sup>

**Particle size (diameter)**

Anatase 113 1–2μm — Nanoparticle

Javed et al. [8] Anatase 18 500 nm 5.6 Nanoflower-like

Rahal et al. [66] Anatase 47 200–300 nm — Flower-shaped

Martínez et al. [67] Anatase 73 500 nm 56 Cauliflower-like

nanostructures produced via microwave treatment.

**Crystallite size (nm)**

structures

agglomerates

nanoparticle agglomerates

hierarchical structures

as particle size increases.

via microwaves from amorphous pow-

**Morphology Application**

nanoagglomerates DSSCs

DSSCs

Photocatalysis

Photocatalysis

—

spherical HNSs

(0.5 ml) in ethanol

is hydrolyzed and

) and anhydrous ethanol as precursor mate-

crystallized in microwaves and TiO<sup>2</sup> nanoagglomerates with particle size up to 10 nm are prepared in spherical geometry [37]. The crystallite size is measured to be as small as 10 nm. In the presence of microwave, TiO<sup>2</sup> nanoparticles begin to nucleate and then their growth and aggregation occur during crystallization.

So, microwave heating provides quick crystallization of particles as particles begin to cluster within 3–5 min. Mesoporous structures are produced as a result of this synthesis and they are employed as a photoanode material in DSSCs. A maximum conversion efficiency of 7.64% is reported.

Rahal et al. have prepared TiO<sup>2</sup> hierarchical structures by mixing cetyltrimethyl ammonium bromide (CTAB, 11 mmol) and urea (2.4 g, 40 mmol) in 200 ml H<sup>2</sup> O for hydrolysis (**Figure 18**). CTAB is used as a surfactant to control morphology and (NH<sup>2</sup> ) 2 C=O provides steric hindrance. To this reaction mixture, cyclohexane and 1-pentanol are added after stirring of 30 min. Then, TiF<sup>4</sup> (5.94 g, 48 mmol) is added to the solution and whole liquid media is transferred to Teflonlined microwave reactor at 800 W. The mixture is irradiated under microwaves for 5 min at 120°C. The product is then washed thoroughly to remove impurities and other compounds and centrifugation is done to take out less dense particles.

Flower-shaped HNSs made of nanoparticle agglomerates (**Figure 17**) are produced with anatase phase [66]. Microwave treatment even for 5 min provides enough time for crystallization of TiO<sup>2</sup> nanoparticles. The prepared structures are utilized in photodegradation of Rhodamine B dye and complete degradation is observed within 1 h.

Martínez et al. [67] have recently reported TiO<sup>2</sup> HNSs prepared by stirring 3 ml of titanium tetra isopropoxide in 50 ml of H<sup>2</sup> SO<sup>4</sup> and subsequent microwave treatment in a Teflon vessel at 120°C for 2 h. Anatase Titania is formed as a result of this scheme. Concentration of H<sup>2</sup> SO<sup>4</sup> is changed to study the effect on particle size. Cauliflower-shaped HNSs are obtained. The increase in H<sup>2</sup> SO<sup>4</sup> concentration increases the particle size.

*2.3.2. Merits*

(f) 3M-TiO<sup>2</sup>

*2.3.3. Demerits*

rutile.

of Titania is done.

• Shorter crystallization time is required.

crystallization within less time.

**Figure 18.** Hierarchical structures of TiO<sup>2</sup>

[67].

*2.3.4. Summary of microwave synthesis*

• It is a time-saving process.

• Electromagnetic radiations provide much temperature for nucleation and growth during

free-TiO<sup>2</sup>

, (b) 0.5M-TiO<sup>2</sup>

, (c) 1M-TiO<sup>2</sup>

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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, (d) 1.5M-TiO<sup>2</sup>

, (e) 2M-TiO<sup>2</sup>

,

21

• The duration of microwaves should be carefully controlled so that optimum crystallization

to

• Microwave treatment for longer times and high temperature transform anatase TiO<sup>2</sup>

In short, microwave treatment is a quick technique to attain hierarchical morphology in TiO<sup>2</sup> nanostructures. By controlling temperature and exposure duration, HNSs ranging in sizes from micron to nanometers are produced. Localized heating caused by microwaves make this method an energy-efficient one with the possibility of acquiring environment-friendly conditions. The use of NaOH has resulted in submicron-sized HNSs made of radially arranged

• Large material can be synthesized in less time, so a better yield is expected.

 (a) H<sup>2</sup> SO<sup>4</sup>

**Figure 17.** SEM images (a–d) of TiO<sup>2</sup> hierarchical structures having flower-like shapes after microwave irradiations for 5 min [66].

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications http://dx.doi.org/10.5772/intechopen.74525 21

**Figure 18.** Hierarchical structures of TiO<sup>2</sup> (a) H<sup>2</sup> SO<sup>4</sup> free-TiO<sup>2</sup> , (b) 0.5M-TiO<sup>2</sup> , (c) 1M-TiO<sup>2</sup> , (d) 1.5M-TiO<sup>2</sup> , (e) 2M-TiO<sup>2</sup> , (f) 3M-TiO<sup>2</sup> [67].
