*2.5.1.4. Temperature*

The sol-gel temperature is a critical parameter in controlling the properties of the resulting TiO2 nanoparticles. Vorkapic and Matsoukas [71] studied the effect of the hydrolysis temperature on particle size where they varied the temperature between 0 and 50°C and they found out that low hydrolysis temperatures favored formation of larger particles. When the temperature was increased, the size decreased and reached a minimum in the range 25–50°C. High temperatures increase the thermal energy of the colloid, decreases viscosity and the dielectric constant of the solvent, thus lowering the electrostatic barrier against aggregation resulting

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Vorkapic and Matsoukas investigated the effect of different alkoxides on the size of the TiO<sup>2</sup> nanoparticles and they found out that at 25°C, the final size decreases with increase in the

in larger particles [72].

**Figure 5.** Sol-gel processes with different water contents [65].

*2.5.1.5. Precursor type*

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review http://dx.doi.org/10.5772/intechopen.75425 163

**Figure 5.** Sol-gel processes with different water contents [65].

temperatures increase the thermal energy of the colloid, decreases viscosity and the dielectric constant of the solvent, thus lowering the electrostatic barrier against aggregation resulting in larger particles [72].

#### *2.5.1.5. Precursor type*

reaction mixture. The amount of water should not be too low otherwise the hydrolysis of the alkoxides with water will be incomplete and condensation occurs between the mono-

ide required for particle formation should be greater than 2.5 as deduced from the equation

in the sol-gel process strongly affect the characteristics of the resultant oxides as shown in **Figure 5(a)**. They suggested that the reaction mechanisms for sol-gel conversion depend on Rw used and they came up with three different mechanisms as shown in **Figure 5(b)**. All mechanisms could occur at the same time but there would be a dominant one [65]. Case I: When Rw is less than four (Rw < 4), the hydrolysis between alkoxides and water is incomplete hence condensation reaction occurs between the monomers. Case II: When Rw is increased to between two and four (2 < Rw ≤ 4), stronger nucleophilic reaction between water and alkoxide molecules monomers occurs and a three-dimensional network is formed. Case III: When Rw used is over the critical value the hydrolysis will be complete. The alkoxides will convert to the corresponding metal hydrates. The monomers will react

The pH of the sol-gel system for the preparation of uniform nanoparticles of anatase titania

sol [68]. When the hydrogen ion concentration is high, the particles grow rapidly to form large grains because the hydrogen ions interfere in the reaction and decrease the nucleation rate. The

acidic solution of titanium tetrachloride at elevated temperatures for 6–47 h. The amount of acid (pH) determines not only the size of the nanoparticles but also the stability of the sol [71].

The sol-gel temperature is a critical parameter in controlling the properties of the resulting

 nanoparticles. Vorkapic and Matsoukas [71] studied the effect of the hydrolysis temperature on particle size where they varied the temperature between 0 and 50°C and they found out that low hydrolysis temperatures favored formation of larger particles. When the temperature was increased, the size decreased and reached a minimum in the range 25–50°C. High

gel is a key factor for controlling the final particle size and shape of the

particles generally increases with increase in pH of the

spherical particles of a narrow size by aging a highly

favored by high hydrolysis rates caused by large amounts of water.

According to a study by Yu and Wang [65], the molar ratio of H2

new nucleus has enough time to grow and aggregate into large TiO2

with each other to form particle-like polymers.

product [67]. The grain size of the TiO2

et al. [70] reported the synthesis of TiO2

O]/[TEOT] > 2.5 [66]. The largest R value reported was 7 which gave particles with average size of 300 nm. If the amount of water is increased, a stronger nucleophilic reaction between water and alkoxide molecules occurs resulting in more alkoxyl groups being substituted by OH groups of water. The monomers obtained then interact with each other to form a three-dimensional network structure. When R is over a critical value, the hydrolysis is more complete and more alkoxides convert to the corresponding metal hydrates, M(OH)z which then react with each other to form particle-like polymers [65]. The formation of Ti(OH)4

Ti(OR)4−x [65]. Other researchers reported that the ratio of water to the alkox-

is

O/alkoxides (Rw) used

particles [69]. Matijevic

mers of (OH)x

162 Titanium Dioxide - Material for a Sustainable Environment

R= [H2

*2.5.1.3. pH*

from condensed TiO2

*2.5.1.4. Temperature*

TiO2

Vorkapic and Matsoukas investigated the effect of different alkoxides on the size of the TiO<sup>2</sup> nanoparticles and they found out that at 25°C, the final size decreases with increase in the 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 reactivity of the alkoxide hence the lower hydrolysis rate.

in an aqueous ammonium hexafluorotitanate solution [76]. Michailowski et al. showed the

aluminum oxide as a template [77]. Correspondingly, Liang et al. revealed the synthesis of

The atomic layer deposition approach is a highly conformal coating process, and the thickness can be controlled very precisely. Therefore, in this method, the thickness of the walls of the nano-

face of the template is eliminated by mechanical polishing. Hoyer showed the synthesis of TiO2

methacrylate) nanorods arrays by the electrochemical deposition procedure. The nanorods were

spun poly(l-lactide) fibers as a positive template [79]. Jung et al. confirmed the synthesis of

nanotubes which were fabricated by condensing Ti(Oi─Pr)4

both surfaces of the self-assembled organogel tubes [80]. In comparison to negative templatebased synthesis, the positive-based template has a better control on the smoothness of the inner and outer wall. The advantage of this template-based synthesis is that the dimensions of the nanotubes can be controlled by controlling the pore dimensions of the template. The disadvantages of this technique are that the fabrication process is relatively more complicated, and the nanotube morphology can be destroyed during fabrication steps such as mechanical polishing. Nanoporous media typically used as templates for 1D nanostructure fabrication include track-etch membranes, which are commonly made from polymers including poly(carbonate), poly(ethylene terephthalate) or poly(imide) [81, 82] and anodized aluminum oxide (AAO), which can be purchased commercially or fabricated using a well-known anodization process [81–84]. In addition to nanoporous materials, templates can also be fabricated using lithographic techniques. The use of AAO templates is especially attractive because of its simple,

low cost and highly controllable fabrication method. Vertically orientated CrO2

nificantly applied in ultrahigh-density perpendicular magnetic recording devices.

arrays were obtained through atmospheric-pressure CVD assisted by AAO templates and were etched to form nanotubes. Such ordered nanorods within an AAO template may be sig-

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

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

tube can be controlled very precisely. The entire undesired TiO2

nanotubes using positive template [77]. In this approach, TiO2

anodic aluminum oxide template. Likewise, TiO2

nanotubes via a thermal decomposition process of Ti(Oi─Pr)4

nanotubes through atomic layer deposition [78]. Anodic aluminum oxide was utilized

was used as the precursor for the atomic layer deposition of TiO2

nanotubes. The nanorods were, however, fabricated using an

using anodic

precursors onto

nanorods

deposited layer on the top sur-

was deposited on poly(methyl

nanotubes were synthesized using electro-

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synthesis of TiO2

double wall TiO2

as the template, and TiCl4

selectively etched to form TiO2

**2.7. Electrochemical anodization**

the metal close to the surface.

TiO2
