**3. Conclusions and future prospects**

CVD of TTIP was done in Pyrex reactor. Pure argon was passed through liquid TTIP at 293 K into the tube where the powder formed by former process was already deposited. After depo-

**Figure 28.** (a) TEM images of structures formed by CVD and (b) worm-like agglomerated 3D TiO<sup>2</sup>

decomposed completely for 12 h and finally the powder was treated in dry air at 393 K for 2 h. Spherical agglomerated structures were produced as a result of this synthesis (**Figure 28**).

• If laser is used to heat the precursor material, the deposition becomes selective to the path

• High temperature is required in case of CVD to initiate chemical reactions. CVD runs at

• Substances that cannot tolerate high temperatures, which decompose or sublime, cannot

, which passed through water at 293 K. TTIP was

structures grown by

sition for 24 h, the gas was switched to N<sup>2</sup>

**Figure 27.** (a and b) SEM images of multistacked nanoforest [76].

32 Titanium Dioxide - Material for a Sustainable Environment

• There is less wastage of chemicals and substrate.

*2.7.2. Merits and demerits of CVD*

much higher temperature.

be deposited by CVD.

of laser.

CVD [77].

In conclusion, this chapter gives an overview of synthesis of HNSs of Titania via different routes. The chemistry and different parameters affecting the properties of HNSs are also briefly discussed. It can be seen that the employed techniques are very powerful in synthesizing TiO<sup>2</sup> HNSs in the form of agglomerated nanoparticles, nanospheres, nanoflakes or 1D/3D heterostructures. In hydrothermal synthesis, by changing parameters of temperature, concentration of precursors, etching reagents and time, the morphology of TiO<sup>2</sup> particles can be changed to 3D HNSs. By providing prolonged time for crystallization, the morphology of particles changes. However, in case of solvothermal synthesis, different solvents provide different structures. By using solvents that provide maximum steric hindrance, the morphology of structures can be controlled. Also, solvents with high boiling points can be used. In microwave synthesis, irradiation time, temperature and solvents are key factors in controlling morphology. This method provides short time for crystallization in the presence of radiations and more nucleation sites are formed. In pulsed laser deposition process, nanotree- and nanoforest-like structures are grown from agglomeration of nanoparticles. Pressure plays an important role in controlling the morphology. By increasing voltage in anodization technique, when the energy provided to target material is increased, the diameter and length of structures formed are increased leading to formation of 3D hierarchical-like structures as end products. In photolithography, the structures engraving are much easier and microfabrication can be done by these structures. Vapor deposition processes are new and very little work has been done for TiO<sup>2</sup> HNSs preparation. These processes can be used to grow very thin films of materials and morphologies can be opted by varying parameters like pressure, temperature, precursors (in case of CVD) and mean free path.

Hence, TiO<sup>2</sup> HNSs with different morphologies can be obtained via different synthetic pathways. These structures can help to achieve maximum scattering with high specific surface area for sunlight entrapment. Hierarchical morphology further helps in better absorption of light and efficient electron-hole pair generation can be achieved. Also, reduced recombination rates are being observed by these structures. These mesoporous structures can help in maximum adsorption of dye molecules. So, these properties shown by TiO<sup>2</sup> hierarchical structures increase the efficiency of phenomena taking place at the interfaces and hence efficient results are seen. These facts make HNSs promising candidates for photovoltaic and photocatalytic applications as can be seen in much of the reported work. These structures can be also employed and exploited in future, for increasing efficiency of various devices like:

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Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications


Hierarchical nanostructures can be mixed with nanoparticles to enhance surface area for photocatalytic reactions. These structures can also be doped, codoped or their hybrid structures can be made to increase efficiency of prepared products.
