**2.5. Anodization**

It is an electrochemical process in which metal sheets are decorated and electrolytic passivation<sup>1</sup> takes place. The thickness of oxide layer is increased in this process. The surface is decorated and finished with more durable and corrosion-resistant surface. The reactions that occur during anodization for oxidation of metals are [70]:

$$2\mathrm{H}\_{2}\mathrm{O} \rightarrow 4\mathrm{H}^{+} + \mathrm{O}\_{2} + 4\mathrm{e}^{-} \tag{7}$$

*2.5.1. Examples*

**Table 6.** Hierarchical TiO<sup>2</sup>

**Reference Phase Surface area** 

**(m2 g−1)**

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

Ali et al. [73] Anatase — 75–90 — Nanotube arrays Water splitting Zhang et al. [42] Anatase — 200 — Nanotubes Photoelectrocatalytic

Tang et al. [40] Anatase 33.4 65 33.3 Microspheres Photocatalysis Smith et al. [72] Anatase — 120–170 — Nanotubes Photoelectrocatalytic

Ali et al. [43] Anatase — 100 40 Nanotubes —

nanostructures produced via anodization.

0.2 wt% DI H<sup>2</sup>

Ali et al. have prepared hierarchical Titania structures (**Figure 21**) by using Ti foil, ammonium fluoride and ethylene glycol as precursor. DI water has been used in electrolyte preparation. Ti foils are being anodized in ethylene glycol electrolyte containing 0.5 wt% NH<sup>4</sup>

trode and platinum foil is used as counter electrode. Both electrodes are placed 10 mm apart. Voltage is maintained at 60 V for 24 h using DC power source and Titania nanotubes are prepared. The sample is being annealed at 450°C for 2 h [43]. These structures can provide highly

Ali et al. have prepared hierarchical structures of Titania by anodization technique. Ti foil is used as precursor and glycerol with ammonium fluoride is used as electrolyte. Anodization of

The voltage has been set at 30 V against the Pt counter electrode for 4 h. The nanotubes are

mesoporous structures for various photocatalytic applications.

Ti foils has been done in glycerol containing 10 wt% H<sup>2</sup>

**Figure 21.** (a) SEM images of hierarchical TiO<sup>2</sup>

view [43].

O for oxidation of titanium. In this process, Ti foil is taken as a working elec-

**Crystallite size (nm)**

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

**Morphology Application**

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decomposition

decomposition

O and 0.25 M NH<sup>4</sup>

prepared via anodization of TNT in F- residue and (b) cross-sectional

F and

25

F electrolyte solution.

$$\text{Metal} + \text{O}\_2 \rightarrow \text{MO}\_x \tag{8}$$

And for titanium,

$$\text{Ti} \star \text{O}\_2 \rightarrow \text{TiO}\_2 \tag{9}$$

TiO<sup>2</sup> is formed on titanium surface. The fluoride ion (F− ) present in solution causes dissolution of oxide layers and etching in nanopits starts to prepare nanotubes. Water is the main source of oxidation in anodization. Hydroxyl ions from electrolyte are injected into the body of anodic oxide layer [71]. These anions impede ion transport, which is necessary for movement of metal-ion interface into metal. The anodization also depends on solution diffusion rate and local electric field supplied to a specific area [72]. **Table 6** shows features of structures prepared via dot pattering technique.

<sup>1</sup> It is a process of coating of protective coating on substance.


**Table 6.** Hierarchical TiO<sup>2</sup> nanostructures produced via anodization.

#### *2.5.1. Examples*

*2.4.3. Demerits*

• This technique is difficult. • High pressure is required.

24 Titanium Dioxide - Material for a Sustainable Environment

desired morphology.

*2.4.4. Summary of PLD*

**2.5. Anodization**

And for titanium,

ation<sup>1</sup>

TiO<sup>2</sup>

1

• It is an expensive process to synthesize materials.

occur during anodization for oxidation of metals are [70]:

is formed on titanium surface. The fluoride ion (F−

prepared via dot pattering technique.

It is a process of coating of protective coating on substance.

• Temperature and pressure relation on particle growth should be clearly known to attain

So, by controlling parameters like pressure, temperature and time, nanoforest-type structures were grown via pulsed laser technique. By using intermediate pressure, columnar structures were grown and density of structures decreases by increasing pressure [69]. The particle size was in nanometer ranges. Pulsed laser was used to ablate Titania target on which structures were grown. This technique can be used to synthesize different materials by using different target materials. Hence, the prepared structures can be employed in different applications like

It is an electrochemical process in which metal sheets are decorated and electrolytic passiv-

2H<sup>2</sup> O → 4H<sup>+</sup> + O<sup>2</sup> + 4e<sup>−</sup> (7)

Metal + O<sup>2</sup> → MOx (8)

Ti + O<sup>2</sup> → TiO<sup>2</sup> (9)

tion of oxide layers and etching in nanopits starts to prepare nanotubes. Water is the main source of oxidation in anodization. Hydroxyl ions from electrolyte are injected into the body of anodic oxide layer [71]. These anions impede ion transport, which is necessary for movement of metal-ion interface into metal. The anodization also depends on solution diffusion rate and local electric field supplied to a specific area [72]. **Table 6** shows features of structures

) present in solution causes dissolu-

 takes place. The thickness of oxide layer is increased in this process. The surface is decorated and finished with more durable and corrosion-resistant surface. The reactions that

dye sensitized solar cells, perovskite solar cells and photodegradation of pollutants.

Ali et al. have prepared hierarchical Titania structures (**Figure 21**) by using Ti foil, ammonium fluoride and ethylene glycol as precursor. DI water has been used in electrolyte preparation. Ti foils are being anodized in ethylene glycol electrolyte containing 0.5 wt% NH<sup>4</sup> F and 0.2 wt% DI H<sup>2</sup> O for oxidation of titanium. In this process, Ti foil is taken as a working electrode and platinum foil is used as counter electrode. Both electrodes are placed 10 mm apart. Voltage is maintained at 60 V for 24 h using DC power source and Titania nanotubes are prepared. The sample is being annealed at 450°C for 2 h [43]. These structures can provide highly mesoporous structures for various photocatalytic applications.

Ali et al. have prepared hierarchical structures of Titania by anodization technique. Ti foil is used as precursor and glycerol with ammonium fluoride is used as electrolyte. Anodization of Ti foils has been done in glycerol containing 10 wt% H<sup>2</sup> O and 0.25 M NH<sup>4</sup> F electrolyte solution. The voltage has been set at 30 V against the Pt counter electrode for 4 h. The nanotubes are

**Figure 21.** (a) SEM images of hierarchical TiO<sup>2</sup> prepared via anodization of TNT in F- residue and (b) cross-sectional view [43].

then dried under a stream of N<sup>2</sup> gas. The sample is annealed at 400°C for 2 h for crystallization of Titania nanotube arrays (TNTAs). The sample obtained is then immersed in 80 mM TiCl<sup>4</sup> for different time intervals and then sintered at 400°C to produce photoelectrodes. Nanorods (**Figure 22**) having flower-like structures are produced after treatment at 120°C with TiCl<sup>4</sup> [73]. These structures are then utilized for water-splitting application. H<sup>2</sup> can also be produced from these structures and can be stored for energy applications.

Zhang et al. prepared hierarchical nanotube-like structures via anodization [42]. The titanium foil is sonicated in ethanol and cold distilled water to remove impurities. After that, N<sup>2</sup> stream is used to dry the foil. Anodization is done using Ti as anode and Pt as cathode material. NH<sup>4</sup> F is added in ethylene glycol (EG), which is present in 2 vol% distilled water as electrolyte solution. This electrolyte is used for oxidation of Titanium sheet. Anodization is performed at room temperature in two steps. In step 1, Ti sheet is anodized for 10 min at 50 V and grown nanotubes are removed ultrasonically in DI water. In step 2, the same sheet is anodized under the same condition for 30 min. The powder obtained is cleaned with distilled water under N<sup>2</sup> atmosphere.

Anodized TiO<sup>2</sup> nanotubes are annealed in air at 450°C for 1 h with a heating rate of 5°C/min for crystallization of pure anatase phase as shown in **Figure 23**. These structures are used for photodegradation of organic pollutants. These types of structures show improved photocatalytic activity in degradation of organic dye. The enhanced surface area facilitated the reaction rate.

Tang et al. [40] have prepared nanoflakes/nanoparticles hierarchical structures via anodization technique. In this synthesis, electrochemical spark discharge spallation (ESDS) method is applied in an electrolyte of 10 M NaOH in aqueous solution while using as platinum counter electrode. Titanate hierarchical microspherulite structures are produced as a result of this

sample is then washed to remove the acidic content and to make pH neutral. The samples

Dislocations in the sample decrease as the sample is heated above its crystallization tempera-

difference in morphology can be seen in SEM image in **Figure 24**. By increasing the time for heat treatment, nanosheets were converted into nanoflakes and nanoparticles due to crystal re-growth with larger energy and with minimal stresses. These structures are employed for

Smith et al. have prepared hierarchical nanotubular structures via anodization technique [72]. The synthesis follows basic principle of anodization. Titanium foil is etched in the presence

powder is washed with DI water immediately and is anodized at 60 V for 1 h in presence of

The smooth etched layers on foil cause a uniform electric current and porous nanotubes grow in those areas. The prepared hierarchical structures as shown in **Figure 25** showed better results for photoelectrochemical applications. Hierarchical structures provided large surface

F and ethylene glycol electrolyte. The powder obtained is annealed at 500°C for 2 h. The etching treatment before anodization removed small layers of titanium and provided ripplelike surface. Due to surface roughness, the electric field becomes localized at grains. An initial

and DI water in volumes of 1:3:50 ml for different time intervals. The etched

ions by soaking the prepared sample in HCl. The

nanotubes; Top enlarged image shows the diameter and bottom

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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27

obtained via this synthesis route. The

ions are replaced by H<sup>+</sup>

ture. **Table 6** shows some properties of anatase TiO<sup>2</sup>

photodegradation of organic pollutants.

**Figure 23.** (a) SEM image of top view of porous TiO<sup>2</sup>

enlarged image shows the side walls [42].

area for photocatalytic reactions.

obtained are annealed at different temperatures to crystallize them [40].

oxide layer formed during etching is then oxidized during anodization.

synthesis. The Na<sup>+</sup>

of HF, HNO<sup>3</sup>

NH<sup>4</sup>

**Figure 22.** FESEM images of (a) TNTA, (b) TNTA/TiCl<sup>4</sup> 20 min, (c) TNTA/TiCL<sup>4</sup> 80 min, (d) TNTA/TiCl<sup>4</sup> 120 min [73].

then dried under a stream of N<sup>2</sup>

26 Titanium Dioxide - Material for a Sustainable Environment

NH<sup>4</sup>

rate.

atmosphere.

Anodized TiO<sup>2</sup>

**Figure 22.** FESEM images of (a) TNTA, (b) TNTA/TiCl<sup>4</sup>

gas. The sample is annealed at 400°C for 2 h for crystallization

can also be produced

stream

of Titania nanotube arrays (TNTAs). The sample obtained is then immersed in 80 mM TiCl<sup>4</sup> for different time intervals and then sintered at 400°C to produce photoelectrodes. Nanorods (**Figure 22**) having flower-like structures are produced after treatment at 120°C with TiCl<sup>4</sup>

Zhang et al. prepared hierarchical nanotube-like structures via anodization [42]. The titanium

is used to dry the foil. Anodization is done using Ti as anode and Pt as cathode material.

for crystallization of pure anatase phase as shown in **Figure 23**. These structures are used for photodegradation of organic pollutants. These types of structures show improved photocatalytic activity in degradation of organic dye. The enhanced surface area facilitated the reaction

F is added in ethylene glycol (EG), which is present in 2 vol% distilled water as electrolyte solution. This electrolyte is used for oxidation of Titanium sheet. Anodization is performed at room temperature in two steps. In step 1, Ti sheet is anodized for 10 min at 50 V and grown nanotubes are removed ultrasonically in DI water. In step 2, the same sheet is anodized under the same condition for 30 min. The powder obtained is cleaned with distilled water under N<sup>2</sup>

nanotubes are annealed in air at 450°C for 1 h with a heating rate of 5°C/min

20 min, (c) TNTA/TiCL<sup>4</sup>

80 min, (d) TNTA/TiCl<sup>4</sup>

120 min [73].

foil is sonicated in ethanol and cold distilled water to remove impurities. After that, N<sup>2</sup>

[73]. These structures are then utilized for water-splitting application. H<sup>2</sup>

from these structures and can be stored for energy applications.

**Figure 23.** (a) SEM image of top view of porous TiO<sup>2</sup> nanotubes; Top enlarged image shows the diameter and bottom enlarged image shows the side walls [42].

Tang et al. [40] have prepared nanoflakes/nanoparticles hierarchical structures via anodization technique. In this synthesis, electrochemical spark discharge spallation (ESDS) method is applied in an electrolyte of 10 M NaOH in aqueous solution while using as platinum counter electrode. Titanate hierarchical microspherulite structures are produced as a result of this synthesis. The Na<sup>+</sup> ions are replaced by H<sup>+</sup> ions by soaking the prepared sample in HCl. The sample is then washed to remove the acidic content and to make pH neutral. The samples obtained are annealed at different temperatures to crystallize them [40].

Dislocations in the sample decrease as the sample is heated above its crystallization temperature. **Table 6** shows some properties of anatase TiO<sup>2</sup> obtained via this synthesis route. The difference in morphology can be seen in SEM image in **Figure 24**. By increasing the time for heat treatment, nanosheets were converted into nanoflakes and nanoparticles due to crystal re-growth with larger energy and with minimal stresses. These structures are employed for photodegradation of organic pollutants.

Smith et al. have prepared hierarchical nanotubular structures via anodization technique [72]. The synthesis follows basic principle of anodization. Titanium foil is etched in the presence of HF, HNO<sup>3</sup> and DI water in volumes of 1:3:50 ml for different time intervals. The etched powder is washed with DI water immediately and is anodized at 60 V for 1 h in presence of NH<sup>4</sup> F and ethylene glycol electrolyte. The powder obtained is annealed at 500°C for 2 h. The etching treatment before anodization removed small layers of titanium and provided ripplelike surface. Due to surface roughness, the electric field becomes localized at grains. An initial oxide layer formed during etching is then oxidized during anodization.

The smooth etched layers on foil cause a uniform electric current and porous nanotubes grow in those areas. The prepared hierarchical structures as shown in **Figure 25** showed better results for photoelectrochemical applications. Hierarchical structures provided large surface area for photocatalytic reactions.

level [75]. The prepared powder was used in water-splitting application, which can be used

Hierarchical Nanostructures of Titanium Dioxide: Synthesis and Applications

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29

This is a technique that is used in microfabrication to pattern thin films and bulk materials. Light is being used to transfer pattern onto the light-sensitive photoresist. Photoresist basically is a light-sensitive material, which is used to form pattern coating on the surface.

• Positive photoresist: The resist is applied over the area on which the underlying material

• Negative photoresist: The resist is applied over the area on which the area other than resist

Then, etching is done to form patterned hierarchical 3D structures. When photopositive resist coating is done over the substrate, then the growth of structure takes place on the same area where we seed/coat the material. The mask contains the exact copy of resist, which we deposit on the substrate and vice versa. **Table 7** shows features of structures prepared via photoli-

Kim et al. have reported HNSs of Titania by photolithography. A negative PR is prepared on 5 mm Ti foil by baking at 120°C. Upon exposure to UV light, PR gets developed. The foil is etched via reactive ion etching. Titanium foil's area that is not covered with the PR is etched out in this

So, flower-like HNSs are produced via dot patterning technique. The particles are in the micron ranges (diameter). The prepared product is employed in dye-sensitized solar cells and exhibits maximum surface area for dye adsorption. Hence, these show better result for

• If viscosity of photoresist is controlled, we can achieve well-formed structures.

**Particle size (diameter)**

Kim et al. [48] Anatase — 1.5–2μm Ti foil Flower-like structures

nanostructures produced via dot patterning/photolithography.

nanoflowers composed of nanotubes (**Figure 26**) are prepared in this procedure [48].

**Precursor materials**

**Morphology Application**

DSSCs

on nanotubes

for nuclear thermal and solar thermal plants.

Photoresist is of two types, which are as follows:

**2.6. Photolithography**

is to be removed.

is to be removed.

thography technique.

photocatalytic processes.

• Cost-effective process.

**Table 7.** Hierarchical TiO<sup>2</sup>

• No need of specialized equipment.

**(m2 g−1)**

**Reference Phase Surface area** 

*2.6.1. Examples*

process. TiO<sup>2</sup>

*2.6.2. Merits*

**Figure 24.** (a) H-TMS calcinated at different temperatures: (b) 300°C, (c) 400°C, (d) 500°C, (e) 600°C, and (f) 700°C [40].

**Figure 25.** SEM image of foil's surface: (a) After 30 s etching (inset shows anodization after 1 h at 60 V) and (b) after 90 s etching [72].

### *2.5.2. Merits*


#### *2.5.3. Demerits*


#### *2.5.4. Summary of anodization*

So, hierarchical Titania nanostructures were prepared from anodization technique and nanotube-like structures were prepared. The particle diameter was in nanometer ranges. The diameter and length of nanotubes increase by increasing voltage and voltage time up to optimum level [75]. The prepared powder was used in water-splitting application, which can be used for nuclear thermal and solar thermal plants.
