*2.2.1. Examples*

*2.1.2. Merits*

The hydrothermal method has got the following advantages:

• Titania formed via this process can range from nanoparticle- to nanoflower-like structures

• By increasing the duration and time in hydrothermal reactor, the crystallization of particles

• If NaOH or other reducing or oxidizing agents are used, they can be removed easily by

• By controlling temperature, one can grow anatase or rutile Titania phase depending on

• Water is used as solvent in this synthesis, so the temperature for crystallization cannot be increased from 100 to 150° C. Otherwise, water will dry out and crystallization may hinder. • Proper time should be given to materials during synthesis as complete etching should be

• Sometimes crystallization times are very prolonged as compared to microwave technique.

To summarize, hydrothermal treatment of various precursors under control parameters of temperature, pressure and duration gives a variety of morphologies in HNSs of Titania. The

HNSs.

• The structures produced are diverse so they can be used in various applications.

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

• This method is comparatively easy.

14 Titanium Dioxide - Material for a Sustainable Environment

depending on temperature.

washing with deionized water.

• Hierarchical morphology can be attained.

The demerits of this method are listed below:

done for desired morphology.

*2.1.4. Summary of hydrothermal synthesis*

• A variety of precursors is used for the production of TiO<sup>2</sup>

So it can be termed as a slow process than microwave.

can be improved.

application.

*2.1.3. Demerits*

Ochanda et al. have presented TiO<sup>2</sup> HNSs by solvothermal synthesis technique. Their method involves mixing of 15 ml of 4 M NaOH solution with 0.5 g TiO<sup>2</sup> fibers. Then, 15 ml ethanol solvent 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 solar cells.


**Table 3.** Hierarchical TiO<sup>2</sup> nanostructures produced via solvothermal method.

**Figure 12.** (a) 1 h solvothermal treatment, showing TiO<sup>2</sup> particle nucleation on nanofiber surface. (b) Hierarchical TiO<sup>2</sup> formed after 6 h showing the inception of flower-like structures. (c) After 12 h showing nanoscale flower-like nanostructures, completely covering the nanofiber surface [10].

crystallinity in the products. **Table 3** gives a summary of the reported work on solvothermal synthesis. The prepared HNSs obtained using this method range in size, from a few microm-

structures grown at 90°C. (a–c) TEM images of 3D hierarchical urchin-

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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

have good crystallinity due to high crystallization temperatures offered by possible use of high boiling solvents. Different solvents can provide different chelating effects and hindrance to control morphology of structures. In addition to the type of solvent used, temperature and time control the growth and crystallization of particles during synthesis. These types of HNSs can be used for various photocatalytic applications like solar cells and organic pollut-

This is relatively a new technique with many advantages. The key feature of this method is to heat the reaction mixture in less time via electromagnetic radiations. The frequency is kept from 800 to 2450 MHz range. Although it is not much explored for the HNS synthesis, much literature is available on microwave synthesis of nanoparticles [61], nanospheres [62],

ence of these radiations and localized "superheating" occurs at ambient pressure. This heat energy provided is used for crystallization of amorphous materials. By this method, reactions

Although dedicated microwave lab reactor is the best equipment, modified/non-modified domestic microwave ovens can also be used for the preparation of HNSs. From a few reports found on HNS synthesis, it appears to be an efficient, quick and cost-effective method of HNS

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.

can be completed in just a couple of minutes as compared to conventional heating.

synthesis. The key characteristics of prepared samples are given in **Table 4**.

One reason being the larger light scattering due to morphology of the HNSs.

g−1. The products

17

. Dipole molecules rotate in the pres-

eters to several nanometers. They possess large surface area up to 94 m<sup>2</sup>

nanorods [63], nanowires [64] and nanotubes [65] of TiO<sup>2</sup>

ant degradation.

*2.3.1. Examples*

**2.3. Microwave synthesis**

**Figure 13.** SEM images (1) nonhierarchical TiO<sup>2</sup>

like structures having 1D nanoneedles [35].

Xiang et al. have prepared HNSs of Titania by solvothermal synthesis method using toluene. First, TiCl<sup>4</sup> is mixed with distilled water in ice water bath. In another assembly, toluene (30 ml) is mixed with tetrabutyltitanate (TBT). Both solutions are then stirred together for 1 h. The solution is placed in Teflon-lined autoclave for 24 h at 150°C. Micron ranged hierarchical Titania needles are obtained [35] as shown in **Figure 13**. Temperature plays an important role in defining the morphology of the products as HNSs are produced only above 120°C. Sea urchin-like HNSs are formed via 'nucleation–self-assembly–dissolution–recrystallization' growth mechanism without adding any surfactant or template. These structures are then tested for photocatalytic degradation of methylene blue and complete degradation study is completed in 240 min.
