**2.7. Electrochemical anodization**

length of the alkoxy group [71]. Their results showed that particle size decreased in the order ethoxide > propoxide ≥ isopropoxide > butoxide, corresponding to the order of decreasing

In general, after nucleation, the particles grow by molecular addition or aggregation and this particle growth is affected by the kinds of solvents used because particle interaction potential is different in each solvent. The increase in the amount and molecular weight of the alcohol was found to increase the size of the particles and the smallest size was obtained when no alcohol was used [71]. This is due to the fact that an increase in both concentration and molecular weight leads to a decrease in the dielectric constant of the solvent resulting in decreased stability and

hydrolysis of titanium tetrachloride in water/n-propanol mixtures was investigated by Park et al. [73]. The study showed that when powders were redispersed in various solvents the degree of aggregation increased in the order: methanol > ethanol > propanol suggesting that colloidal destabilization was the primary mechanism by which these alcohols influenced particle size.

increased with increase in the chain of the solvent used in preparation due to the increase in the content of anatase and a decrease in particle size. They found out that when the solvent was changed from methanol to 2-pentanol, the content of anatase increased from 68 to 91% with a decrease in particle size from 11.6 to 10.5 nm. The increase in alcohol concentration in the sol-gel reaction mixture can slow down the hydrolysis rate and the resulting sol would

Template-assisted synthesis is an easy, cost effective and highly versatile approach to fabricate nanostructures. Whenever microporous or nanoporous materials are utilized as templates, one-dimensional (1D) nanostructures can be integrated by saving a material of decision inside the format's nano-channels. Template-based synthesis is a method in which a nanoporous material is used as the mold and the desired nanotube material is coated on the walls of the mold. There are two approaches for template-based synthesis which are the negative and positive template. The template is then dissolved to get the nanotubes or nanorods. If the material of interest is coated on the inner walls, then it is a negative template-based synthesis. If the material is coated on the outer walls of the template then is a positive template-based synthesis. In the negative template synthesis, if the pores of the mold are entirely filled, then nanorods can be synthesized. The most commonly used template for negative template syn-

nanorods using anodic aluminum oxide as a template [75]. The walls of the template were coated using the hydrolysis method. The anodic aluminum oxide was used as the membrane

solution in C4

H9

nanotubes by immersing anodic aluminum oxide

H9 )4

thickness of the nanotubes walls were controlled by varying the concentration of Ti(OC4

from thermal

gradually

nanotubes and

H9 )4 .

OH during hydrolysis. The

enhanced rate of re-aggregation and larger particle size. The formation of TiO2

In another study by Xu et al. [74], the photocatalytic activity of unsupported TiO2

.

thesis is anodic aluminum oxide. Yuan et al. showed the synthesis of TiO2

reactivity of the alkoxide hence the lower hydrolysis rate.

*2.5.1.6. Solvent type and concentration*

164 Titanium Dioxide - Material for a Sustainable Environment

possess a high content of amorphous TiO2

between deionized water and the Ti(OC4

Jiang et al. reported the synthesis of TiO2

**2.6. Template-based methods**

Electro-anodization is an electrolytic process used to grow the oxide layer on the surface of the metal. When anodizing, the metal to be treated forms an anode electrode of an electric circuit. Anodization alters the microscopic texture of the surface and the crystal structure of the metal close to the surface.

Metal anodization has been greatly used in industry as a surface treatment technique to render materials with resistance against uncontrolled oxidation, abrasion and corrosion. Anodizing increases corrosion resistance of oxide film over a metallic surface and wears resistance, and also improves adhesion for paint primers and glues than bare metal [85]. Although this technique has been developed for a long time, it was until 1990s that researchers discovered that highly ordered nanoporous structures can be achieved by varying anodization conditions including electrolyte composition and concentration, temperature, as well as anodization voltage [86]. The dense and uniform layer of anodized oxide inhibits ionic conductivity, thus they can be regarded as barrier oxides to effectively protect the metal from further corrosion. On top of that, the anodized oxide film can improve the surface properties of metal, such as wearing, galling and dielectric layer.

Ti + 2H2 O → TiO2 + 4e<sup>−</sup> + 4H+ Oxidation (4)

This process usually takes place in a two electrode setup, where titanium film serves as anode and platinum as cathode. The anode Ti is immersed into electrolyte solution, ionic species such

field-aided transport of mobile ions through the oxide. Depending on the speed of the ionic species (Ti4+ and O2−), the growth of the new oxide proceeds at the interface between the metal and oxide or at the interface between oxide and electrolyte (compact anodic oxide layers may grow at either interfaces, but under most conditions oxide grows at the metal-oxide interface). The growth of an anodic oxide layer is determined by the field-aided transport of mobile ions

The presence of fluorides in the electrolyte strongly affects the anodization process, as fluo-

are ejected at the oxide-electrolyte interface [after migration through the oxide film; Eq. (6)]:

Depending on the fluoride concentration, three very different electrochemical characteristics can be obtained [87]. If the fluoride content is very low (**≤**0.05 wt%), a characteristic as in the fluoride ion-free case is observed; that is, after anodizing the material to a voltage above up, a stable compact oxide layer is formed. If the fluoride concentration is high (~1 wt%), no oxide formation can be observed, as all the Ti4+ formed immediately reacts with the abundant fluo-

For intermediate fluoride concentrations, a competition between oxide formation and Ti4+ solvatization takes place and porous oxide or nanotube formation can be observed. A typical I–t curve for conditions that leads to nanotubes formation. The curve shows three stages: in the initial stage of anodization, the curve essentially follows the fluoride-free case and if samples are removed from the electrolyte, a compact oxide layer is present. In stage II, a current increase occurs, and irregular nanoscale pores are initially formed that penetrate the initial compact oxide (the current increases as the reactive area increases). In stage III, the current drops again as a regular nanoporous structure or nanotube layer forms. The penetrated compact oxide (random pore layer and initiation layer) often remains as remnants that are frequently found after anodization on the tube tops [87, 88]. Various efforts have been directed toward removing these layers [89, 90], as they interfere with any transport process into the tubes or hamper the exploitation of tube size effects [91, 92]. After self-organization occurred, the tube growth continues at steady current densities. For extended anodization, the growth may be determined by diffusional effects [93] and thus agitation (and gravity effects) and viscosity may become important. The length of the TNTs is determined by the dynamic equilibrium between the oxide formation and chemical dissolution of the oxide. Water oxidation at the anode results

2− (the reaction may then be controlled by diffusion of [TiF<sup>6</sup>

are adsorbed on the surface. The growth of anodic oxide layer depends on the

]2− species. On one hand, complexation occurs with Ti4+ ions that

Eq. (5).

<sup>2</sup><sup>−</sup> + 2H2 O Dissolution (5)

http://dx.doi.org/10.5772/intechopen.75425

167

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

<sup>2</sup><sup>−</sup> (6)

]2− from

TiO2 + 6F<sup>−</sup> + 6H+ → [TiF6]

Ti4+ + 6F<sup>−</sup> → [TiF6]

]

the surface, and electro polishing of the sample occurs).

And on the other hand by chemical attack of the formed TiO<sup>2</sup>

as OH<sup>−</sup>

and F<sup>−</sup>

through the oxide.

rides form water soluble [TiF6

ride to form soluble [TiF6

Among all metals that can be anodized, aluminum and titanium can be anodized into nanoporous structures with well controlled depth, diameter and pitch. The working metal is set as anode to oxidize during the electrochemical process and cathode can be any electronic conductor mainly platinum, which would not react in the anodizing bath. The electrolyte is chosen according to its higher growing rate of the oxide compared to its dissolution rate or by the insolubility with the anodized metal. As the reaction progresses the metal at positive terminal withdraws electrons, by so doing allowing the ions on the metal surface to react with water in the electrolyte to form a dense oxide layer. Later the electrons return back to the electrolyte where they react with hydrogen ions to form hydrogen gas. When metals are exposed to sufficient anodic voltage supply in an electrochemical configuration, an oxidation reaction takes place (M → Mn+ + ne<sup>−</sup> ). Depending mainly on the electrolyte and the anodization parameters, there are essentially three possibilities for reactions to occur:


Many properties of these transition metal anodic oxides become more and more important when the possibilities on a micro or nano scale are explored. Anodic oxide films, with the inherent nanoporous structure, exhibit high surface area, and short solid-state diffusion path for catalysis, electrochemical devices and energy applications. The other advantage of the anodic oxide micro/nano structures is that, the fabrications are achieved by chemical or electrochemical processes that are relatively simple and cost effective. The morphologies of anodic oxides include powders, rods, wires and tubes. Materials ranging from Al, Ti, Zr, W to Ta have all been investigated to form self-assembly nanotubes structures [87]. Among these transition metals oxide nanotubes, anodic titanium oxide (ATO) and anodic aluminum oxide (AAO) are among the most widely studied for their functional properties.

#### *2.7.1. Mechanism of growth of the TiO<sup>2</sup> nanotubes*

This section reviews the growth mechanism of titanium dioxide nanotubes (TNTs) by electrochemical anodization technique in fluoride-containing electrolyte solution. The formation of TNTs in acidic electrolyte conditions containing fluoride ions are generally agreed to form in the field assisted oxidation and dissolution of formed titanium oxide surface. This process involves two vital steps that occur simultaneously, the formation of titanium oxide and dissolution of the oxide. The process can be explained by the following reactions:

$$\text{Ti} + 2\text{H}\_2\text{O} \rightarrow \text{TiO}\_2 + 4\text{e}^- + 4\text{H}^+ \text{Oxidation} \tag{4}$$

$$\rm{TiO}\_2 + 6F^- + 6H^+ \rightarrow \ \rm{[TiF}\_6\text{]}^{2-} + 2H\_2O \text{ Discsolution} \tag{5}$$

This process usually takes place in a two electrode setup, where titanium film serves as anode and platinum as cathode. The anode Ti is immersed into electrolyte solution, ionic species such as OH<sup>−</sup> and F<sup>−</sup> are adsorbed on the surface. The growth of anodic oxide layer depends on the field-aided transport of mobile ions through the oxide. Depending on the speed of the ionic species (Ti4+ and O2−), the growth of the new oxide proceeds at the interface between the metal and oxide or at the interface between oxide and electrolyte (compact anodic oxide layers may grow at either interfaces, but under most conditions oxide grows at the metal-oxide interface). The growth of an anodic oxide layer is determined by the field-aided transport of mobile ions through the oxide.

The presence of fluorides in the electrolyte strongly affects the anodization process, as fluorides form water soluble [TiF6 ]2− species. On one hand, complexation occurs with Ti4+ ions that are ejected at the oxide-electrolyte interface [after migration through the oxide film; Eq. (6)]:

$$\rm Ti^{4+} + 6F^{-} \rightarrow \ [TiF\_{\epsilon}]^{2-} \tag{6}$$

And on the other hand by chemical attack of the formed TiO<sup>2</sup> Eq. (5).

highly ordered nanoporous structures can be achieved by varying anodization conditions including electrolyte composition and concentration, temperature, as well as anodization voltage [86]. The dense and uniform layer of anodized oxide inhibits ionic conductivity, thus they can be regarded as barrier oxides to effectively protect the metal from further corrosion. On top of that, the anodized oxide film can improve the surface properties of metal, such as

Among all metals that can be anodized, aluminum and titanium can be anodized into nanoporous structures with well controlled depth, diameter and pitch. The working metal is set as anode to oxidize during the electrochemical process and cathode can be any electronic conductor mainly platinum, which would not react in the anodizing bath. The electrolyte is chosen according to its higher growing rate of the oxide compared to its dissolution rate or by the insolubility with the anodized metal. As the reaction progresses the metal at positive terminal withdraws electrons, by so doing allowing the ions on the metal surface to react with water in the electrolyte to form a dense oxide layer. Later the electrons return back to the electrolyte where they react with hydrogen ions to form hydrogen gas. When metals are exposed to sufficient anodic voltage supply in an electrochemical configuration, an oxidation reaction takes

**1.** The Mn+ ions are solvatized in the electrolyte, that is, the metal is continuously dissolved

**3.** After some time during anodization process, completion between solvatization and oxide

Many properties of these transition metal anodic oxides become more and more important when the possibilities on a micro or nano scale are explored. Anodic oxide films, with the inherent nanoporous structure, exhibit high surface area, and short solid-state diffusion path for catalysis, electrochemical devices and energy applications. The other advantage of the anodic oxide micro/nano structures is that, the fabrications are achieved by chemical or electrochemical processes that are relatively simple and cost effective. The morphologies of anodic oxides include powders, rods, wires and tubes. Materials ranging from Al, Ti, Zr, W to Ta have all been investigated to form self-assembly nanotubes structures [87]. Among these transition metals oxide nanotubes, anodic titanium oxide (ATO) and anodic aluminum oxide

). Depending mainly on the electrolyte and the anodization parameters,

O in the electrolyte and form a compact

wearing, galling and dielectric layer.

166 Titanium Dioxide - Material for a Sustainable Environment

there are essentially three possibilities for reactions to occur:

oxide layer (MO), if MO is not stable in the electrolyte.

**2.** The Mn+ ions form react with O2− provided by H2

*2.7.1. Mechanism of growth of the TiO<sup>2</sup>*

and corrosion, or electropolishing of the metal is observed.

formation is established, which results to porous oxide layer.

(AAO) are among the most widely studied for their functional properties.

 *nanotubes*

solution of the oxide. The process can be explained by the following reactions:

This section reviews the growth mechanism of titanium dioxide nanotubes (TNTs) by electrochemical anodization technique in fluoride-containing electrolyte solution. The formation of TNTs in acidic electrolyte conditions containing fluoride ions are generally agreed to form in the field assisted oxidation and dissolution of formed titanium oxide surface. This process involves two vital steps that occur simultaneously, the formation of titanium oxide and dis-

place (M → Mn+ + ne<sup>−</sup>

Depending on the fluoride concentration, three very different electrochemical characteristics can be obtained [87]. If the fluoride content is very low (**≤**0.05 wt%), a characteristic as in the fluoride ion-free case is observed; that is, after anodizing the material to a voltage above up, a stable compact oxide layer is formed. If the fluoride concentration is high (~1 wt%), no oxide formation can be observed, as all the Ti4+ formed immediately reacts with the abundant fluoride to form soluble [TiF6 ] 2− (the reaction may then be controlled by diffusion of [TiF<sup>6</sup> ] 2− from the surface, and electro polishing of the sample occurs).

For intermediate fluoride concentrations, a competition between oxide formation and Ti4+ solvatization takes place and porous oxide or nanotube formation can be observed. A typical I–t curve for conditions that leads to nanotubes formation. The curve shows three stages: in the initial stage of anodization, the curve essentially follows the fluoride-free case and if samples are removed from the electrolyte, a compact oxide layer is present. In stage II, a current increase occurs, and irregular nanoscale pores are initially formed that penetrate the initial compact oxide (the current increases as the reactive area increases). In stage III, the current drops again as a regular nanoporous structure or nanotube layer forms. The penetrated compact oxide (random pore layer and initiation layer) often remains as remnants that are frequently found after anodization on the tube tops [87, 88]. Various efforts have been directed toward removing these layers [89, 90], as they interfere with any transport process into the tubes or hamper the exploitation of tube size effects [91, 92]. After self-organization occurred, the tube growth continues at steady current densities. For extended anodization, the growth may be determined by diffusional effects [93] and thus agitation (and gravity effects) and viscosity may become important. The length of the TNTs is determined by the dynamic equilibrium between the oxide formation and chemical dissolution of the oxide. Water oxidation at the anode results in the formation of oxygen and the hydrogen formation at the cathode as shown in Eq. (7). Meanwhile, a small amount of hydrogen ions from the electrolyte is reduced to form hydrogen bubbles at the cathode shown in Eq. (8). These bubbles can be clearly observed at the cathode during the experiments, while there is no apparent observations of oxygen bubbles flowing up toward the electrolyte surface at the anode electrode. It is possible that there might be only about 2% amount of water volume resulting to no obvious phenomenon.

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

$$2\text{H}^\* + 2\text{e}^- \rightarrow \text{H}\_2\tag{8}$$

[2] Banfield JF, Zhang H. Nanoparticles in the environment. Reviews in Mineralogy and

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

http://dx.doi.org/10.5772/intechopen.75425

169

[3] Brock T, Groteklaes M, Mischke. European Coatings Handbook. Hannover, Germany:

[4] Beydoun D, Amal R, Low G. Role of nanoparticles in photocatalysis. Journal of Nano-

[5] Adachi M, Murata Y, Okada T, Yoshikawa S. Formation of titania nanotubes and applications for dye-sensitized solar cells. Journal of the Elelectrochemical Society. 2003;**150**(8):

[6] Halary-Wagner E, Wagner F, Hoffmann P. Titanium Dioxide Thin Film Deposition on Polymer Substrates by Light Induced Chemical Vapour Deposition. Submitted to Euro-

[7] Macák M. Growth of anodic self-organazed titanium dioxide nanotube layers [Ph.D.

[8] Chen X, Mao SS. Titanium dioxide nanomaterials: Synthesis, properties, modifications,

[9] Pratsinis SE. History of manufacture of fine particles in high-temperature aerosol reactors. In: Ensor DS, editor. Aerosol Science and Technology: History and Reviews, 1st ed.

[10] Chabra V, Pillai V, Mishra BK, Morrone A, Shah DO. Synthesis, characterization, and

[11] Seifried S, Winterer M, Hahn H. Nanocrystalline Titania films and particles by chemical

via spray hydrolysis of titanium tetraisopropoxide. Journal of Materials Science Letters.

[14] Boccacini A, Zhitomisrky I. Application of electrophoretic and electrolytic deposition techniques in ceramic processing. Current Opinion in Solid State and Material Science.

[15] Besra L, Liu M. A review on fundamentals and applications of electrophoretic deposi-

[16] Nyongesa F, Aduda B. Electrophoretic deposition of titanium dioxide thin films for photocatalytic water purification systems. Advances in Materials. 2017;**6**(4):31-37. DOI:

[17] Narayan MR, Raturi A. Deposition and characterisation of titanium dioxide films formed by electrophoretic deposition. Journal of Materials Engineering Innovation. 2012;**3**(1):17-31

particles. Langmuir. 1995;**11**:3307

nanopowders using RF

powders

Geochemistry. 2001;**44**(1):1-58

particle Research. 1999;**1**:439

CVD 14, Paris, France; 2003

1998;**17**(7):537-539

2002;**6**:257-260

10.11648/j.am.20170604.11

thesis]. University Erlangen-Nürnberg-Germany; 2008

and applications. Chemical Reviews. 2007;**107**:2891-2959

properties of microemulsion-mediated nanophase TiO2

vapor synthesis. Chemical Vapour Deposition. 2000;**6**(5):239-244

[13] Gablenz S, Voltske D, Abicht HP, Neumann-Zoralek J. Preparation of fine TiO<sup>2</sup>

[12] Oh SM, Ishigaki T. Preparation of pure rutile and anatase TiO2

tion (EPD). Progress in Materials Science. 2007;**52**(1):1-61

thermal plasma. Thin Solid Films. 2004;**457**:186-191

RTI International, Research Triangle Park: 2011

Vincent Verlag; 2000

G488-G493
