*2.4.1. Examples*

Fonzo et al. have prepared hierarchically organized nanostructured TiO<sup>2</sup> by ablating Titanium foil with KrF excimer laser pulses (h 248 nm, duration 10–15 ns, energy density 4 J/cm<sup>2</sup> ) in dry air (O<sup>2</sup> ) background with pressure. Thin films of Titania are grown both on silicon and pure titanium substrates. Annealing is done at 400°C for 1 h for crystallization of samples prepared. By changing the pressure of chamber, thickness of the sample is varied from dense columnar structures to tree-like structure [39] (**Figure 19**). This technique can be employed for preparation of HNSs as it provides a stimulating outlook both for photocatalytic and for advanced photovoltaic application, and it also substitutes the time-consuming deposition of different layers and longlasting annealing steps.

This type of assembly hampers electron-hole pair recombination and it facilitates the efficiency if employed in solar cells. This assembly also promotes mass transport of electrons in mesoporous structures. This morphology provided efficient light trapping and high surface area for dye adsorption and efficiency of 5% is obtained. The porosity and surface area can also be optimized for making these structures highly competent for photovoltaic devices.

at (a) 10 Pa, (b) 20 Pa, (c) 40 Pa [39].

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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

23

produced via anodization [68].

• This technique produces better results than anodization.

• Nanotree-like structures can be formed.

**Figure 19.** SEM images of different morphologies of TiO<sup>2</sup>

• Large surface area structures can be achieved.

• Porosity can be controlled by using high pressure.

• The surface area of materials synthesized can be controlled.

**Figure 20.** SEM images of nanoforest-like structures prepared at (a) 20 Pa and (b) 40 Pa [44].

• It is a flexible technique and parameters can be controlled.

• The photocatalysis is better than anatase powder and TiO<sup>2</sup>

• Synthesis can be done without surfactants and chelating agents.

*2.4.2. Merits*

Sauvage et al. have prepared hierarchical TiO<sup>2</sup> structures by PLD. Nanoparticles of TiO<sup>2</sup> are grown directly on FTO substrate by ablating Titanium target in the presence of O<sup>2</sup> background. TiO<sup>2</sup> nanostructure attained the symmetry of tree-like structures. Height and thickness of trees increase with respect to deposition time [44]. By increasing pressure from 10 to 40 Pa, the particles formed nanoforest-like structures. The structures formed are shown in **Figure 20**.


**Table 5.** Hierarchical TiO<sup>2</sup> nanostructures produced via PLD. Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications http://dx.doi.org/10.5772/intechopen.74525 23

**Figure 19.** SEM images of different morphologies of TiO<sup>2</sup> at (a) 10 Pa, (b) 20 Pa, (c) 40 Pa [39].

**Figure 20.** SEM images of nanoforest-like structures prepared at (a) 20 Pa and (b) 40 Pa [44].

This type of assembly hampers electron-hole pair recombination and it facilitates the efficiency if employed in solar cells. This assembly also promotes mass transport of electrons in mesoporous structures. This morphology provided efficient light trapping and high surface area for dye adsorption and efficiency of 5% is obtained. The porosity and surface area can also be optimized for making these structures highly competent for photovoltaic devices.

#### *2.4.2. Merits*

nanosheets, whereas by using other precursor materials, simple microwave treatment produces HNSs made of nanoparticle agglomerates. Hence, flower-like hierarchical morpholo-

This technique is well known for its flexibility to grow variety of materials. Nanotree-like structures can be grown via this synthesis route without using any surfactant and prior treatment. Pressure treatment and laser ablation are used for preparation of hierarchical structures

It is a top-down approach in which pulsed laser is used to decompose the precursor material and then these materials are deposited on substrates. By controlling the parameters like laser power, temperature, chamber geometry and pressure, one can grow structures of different morphologies from columnar structures to dense forest-like [39] structures. The porosity of structures is increased when grown at high pressure because of fast process. Also the surface area can be controlled by controlling inter columnar spacing and thickness. **Table 5** shows

foil with KrF excimer laser pulses (h 248 nm, duration 10–15 ns, energy density 4 J/cm<sup>2</sup>

pure titanium substrates. Annealing is done at 400°C for 1 h for crystallization of samples prepared. By changing the pressure of chamber, thickness of the sample is varied from dense columnar structures to tree-like structure [39] (**Figure 19**). This technique can be employed for preparation of HNSs as it provides a stimulating outlook both for photocatalytic and for advanced photovoltaic application, and it also substitutes the time-consuming deposition of

 nanostructure attained the symmetry of tree-like structures. Height and thickness of trees increase with respect to deposition time [44]. By increasing pressure from 10 to 40 Pa, the particles formed nanoforest-like structures. The structures formed are shown in **Figure 20**.

> **Crystallite size (nm)**

) background with pressure. Thin films of Titania are grown both on silicon and

structures by PLD. Nanoparticles of TiO<sup>2</sup>

nanotrees

nanotree-like structures

**Morphology Application**

by ablating Titanium

) in

are

background.

Photocatalysis

DSSCs

gies are obtained as a result.

by this technique.

*2.4.1. Examples*

dry air (O<sup>2</sup>

TiO<sup>2</sup>

**2.4. Pulsed laser deposition (PLD)**

22 Titanium Dioxide - Material for a Sustainable Environment

features of structures prepared via dot pattering technique.

different layers and longlasting annealing steps.

Sauvage et al. have prepared hierarchical TiO<sup>2</sup>

**area (m2 g−1)**

**Reference Phase Surface** 

**Table 5.** Hierarchical TiO<sup>2</sup>

Fonzo et al. have prepared hierarchically organized nanostructured TiO<sup>2</sup>

grown directly on FTO substrate by ablating Titanium target in the presence of O<sup>2</sup>

**Particle size (diameter) (nm)**

Fonzo et al. [39] Rutile 300 260 — Nanoforest composed of

Sauvage et al. [44] Anatase 86 20 25 Nanoforest composed of

nanostructures produced via PLD.

