**2.1. Hydrothermal synthesis**

As the name indicates, the method involves heating in aqueous medium. Generally, in this method, sealed Teflon-lined steel autoclaves are used under controlled temperature and pressure conditions. Sometimes, surfactants are also added to control the growth and morphology of target materials.

*2.1.1. Examples*

Lin et al. have reported rutile TiO<sup>2</sup>

side of substrate is placed downwards.

2M (c) 3M (d) 4M (e) 5M (f) 6M (f) 7M (g) 8M [56].

increasing the HCl concentration, the etching rate of TiO<sup>2</sup>

DSSCs and 8.6% of conversion efficiency is achieved.

Qiang et al. have obtained hierarchical anatase TiO<sup>2</sup>

factant/stabilizing agent. The solution is prepared using precursor K<sup>2</sup>

hierarchical flower-like structures via hydrothermal syn-

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

structures is enhanced and sym-

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

9

nanowire trunks with short nanorod

(0.002 mol)

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

thesis 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

metric flower-like structures are produced [56] (**Figure 4**). These structures are employed for

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

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.

branch HNSs by facile one-way hydrothermal synthesis on FTO glass without using any sur-

**Figure 4.** FESEM images of TiO<sup>2</sup> structures. Change in morphology is produced from changing HCl conc. at (a) 1M (b)

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. Basically, this synthesis is used for preparation of crystalline TiO<sup>2</sup> from amorphous one. The 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 TiO<sup>2</sup> structures have been prepared by hydrothermal route.


**Table 2.** Hierarchical TiO<sup>2</sup> nanostructures produced by various hydrothermal routes.
