**2.3. Microwave synthesis**

Xiang et al. have prepared HNSs of Titania by solvothermal synthesis method using tolu-

formed after 6 h showing the inception of flower-like structures. (c) After 12 h showing nanoscale flower-like

ene (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

• Better controlled morphologies due to control on temperature and treatment duration.

• Sometimes solvents are difficult to separate from the materials produced due to high boil-

The solvothermal method in addition to the advantages offered by hydrothermal method allows the use of higher temperatures. This is sometimes advantageous in producing better

• Different solvents will have different effect on growth of target materials.

is mixed with distilled water in ice water bath. In another assembly, tolu-

particle nucleation on nanofiber surface. (b) Hierarchical

ene. First, TiCl<sup>4</sup>

TiO<sup>2</sup>

is completed in 240 min.

Following are some of the advantages of this technique:

• Reliable method because of better reproducibility.

• High boiling organic solvents can be used.

**Figure 12.** (a) 1 h solvothermal treatment, showing TiO<sup>2</sup>

16 Titanium Dioxide - Material for a Sustainable Environment

nanostructures, completely covering the nanofiber surface [10].

The method has the following demerits:

*2.2.4. Summary of solvothermal synthesis*

ing points of the solvents.

*2.2.2. Merits*

*2.2.3. Demerits*

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], nanorods [63], nanowires [64] and nanotubes [65] of TiO<sup>2</sup> . Dipole molecules rotate in the presence 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 can be completed in just a couple of minutes as compared to conventional heating.

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