*2.7.1. Examples*

*2.6.3. Demerits*

edge of the surface steps.

*2.6.4. Summary of photolithography*

30 Titanium Dioxide - Material for a Sustainable Environment

sensing and lithium ion batteries.

**2.7. Vapor deposition method**

• The nature of solvents, sensitizers and additives should be well studied.

**Figure 26.** Hierarchical flower-like structures produced by dot patterning [48].

• Sometimes, the step coverage can be poor, i.e., the ability of photoresist to cover the side-

HNSs of Titania are formed via various techniques and different types of structures are formed by these processes. **Table 7** shows the different characteristics of materials obtained by dot patterning. By changing the reaction conditions like time, temperature and pressure and precursor material, different morphological structures are formed, which have different surface area and porosity for applications like dye-sensitized solar cells, photocatalysis, gas

As the name suggests, this method involves deposition of vapors of the required material. The material is vaporized from the source and condenses on the substrate. It may or may not involve chemical reactions. Vapors can be deposited on the substrates by two main processes:

Flipin et al. [76] grew hierarchical TiO<sup>2</sup> nanotubes by plasma-enhanced chemical vapor deposition technique. Porphyrins and phthalocyanines were used as cost-effective precursor molecules. First of all, seed layer was grown by polycrystalline anatase films for highly dense and homogenous organic nanowires. Physical vacuum deposition was done to grow organic nanowires (ONWs) of phthalocyanine molecules with sublimation temperature of 250°C. As a result, tunable ONWs in the range between 1 and 30 μm and diameters between 50 and 120 nm were produced. Then, PECVD was done to cap ONWs with TiO<sup>2</sup> shells.

Multistacked nanotrees composed of TiO<sup>2</sup> nanowires were grown as a result of this synthesis as shown in **Figure 27**. These nanotubes were grown as 1D structures provide maximum electron percolation as compared to 0D nanoparticles due to less grain boundaries. These structures were then employed for DSSCs to evaluate their efficiency for current production.

Yoshitake et al. [77] prepared hierarchical TiO<sup>2</sup> structures by CVD method. Titanium tetraisopropoxide (TTIP) was used as a precursor material. About 40 g of water was added to 8 g of TTIP with 2.6 g of dodecyl amine at 273 K. The mixture was stirred with 0.1 M HCl and was kept overnight for aging. The reaction mixture was transferred to autoclave at 373 K for 4 days. The powder obtained was washed with methanol and dry ethyl ether.


**Table 8.** Hierarchical TiO<sup>2</sup> nanostructures produced via vapor deposition processes.

*2.7.3. Merits and demerits of PVD*

• Cost of the process is very high.

*2.7.4. Summary of vapor deposition processes*

**3. Conclusions and future prospects**

ing TiO<sup>2</sup>

work has been done for TiO<sup>2</sup>

less usage of heat is required for deposition.

• By process like MBE, we can achieve atomic level growth control.

• Vacuum conditions may be required in some depositions.

vapor deposition processes for growth of hierarchical structures.

• Sputtering does not require the use of specific precursor materials as in CVD.

• Processes like sputtering can initiate and undergo PVD, so less use of energy in terms of

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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

33

These processes can form well-defined structures. Nanocoatings can also be formed as a result of these processes. Temperature, time and target materials need to be well optimized for distinct structure growth. Pressure in the chamber should be controlled in case of physical

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 synthesiz-

heterostructures. In hydrothermal synthesis, by changing parameters of temperature, con-

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

thin films of materials and morphologies can be opted by varying parameters like pressure,

temperature, precursors (in case of CVD) and mean free path.

centration of precursors, etching reagents and time, the morphology of TiO<sup>2</sup>

HNSs in the form of agglomerated nanoparticles, nanospheres, nanoflakes or 1D/3D

HNSs preparation. These processes can be used to grow very

particles can

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

**Figure 28.** (a) TEM images of structures formed by CVD and (b) worm-like agglomerated 3D TiO<sup>2</sup> structures grown by CVD [77].

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 deposition for 24 h, the gas was switched to N<sup>2</sup> , which passed through water at 293 K. TTIP was 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**).

#### *2.7.2. Merits and demerits of CVD*

