**2.5. Sol-gel methods**

Microwave radiations can also be applied to produce various TiO2

ducibility and excellent control of experimental parameters. The colloidal TiO2

between 200 and 1000 nm were also prepared using this method [33]. TiO2

nanoparticles in the rutile phase from TiCl4

of titanium tetrachloride (TiCl4

158 Titanium Dioxide - Material for a Sustainable Environment

power and reactant concentration.

**2.4. Hydro/solvothermal methods**

solvent, has better control of the properties of TiO<sup>2</sup>

used the solvothermal method to prepare TiO2

cific solvent properties can be obtained. Feng et al. [44] produced TiO2

H2 SO4

prepared TiO2

industrial processing, this method has an advantage of rapid heat transfer and selective heating. This technique provides uniform distribution of energy within the sample, better repro-

can be prepared in a short period of time (within 5–60 min) compared to several hours needed for the conventional methods of forced hydrolysis at high temperatures (~195°C) [41]. TiO2 nanotubes which are open-ended and multi-walled with diameters of 8–12 nm and lengths

the anatase phase were prepared by Baldassari et al. [42] using microwave-assisted hydrolysis

product almost completely crystallized in a short reaction time of 30 min under microwavehydrothermal conditions. The acid that they used was to obtain a pure anatase phase was

cess at different temperatures between 100 and 160°C for 5–120 min [42]. The morphology and size of the resulting nanoparticles can be varied by changing the time of reaction, microwave

These are two processes, solvothermal and hydrothermal which are almost similar. The hydrothermal method is a process of crystallizing a substance at a high temperature and high vapor pressure using an aqueous solution of the material [43]. It is commonly depicted as crystal synthesis or crystal growth from substances which are insoluble in customary temperature (100°C) and pressure (<1 atm). The process is carried out in autoclaves under controlled temperature and pressure. It allows the use of temperatures above the boiling point of water/ organic solution. Hydrothermal synthesis is characterized as a concoction response occurring in a dissolvable at temperatures over the dissolvable breaking point and at pressures above bar. Compared to hydrothermal method, the solvothermal method uses a non-aqueous

much higher meaning high boiling point solvents can be used. The hydrothermal strategy exploits that by expanding temperature and pressure the essential properties of water and consequently its capacities as a solvent changes. Important characteristics such as the ionic product density, thermal conductivity, viscosity, heat capacity and the dielectric constant are all highly pressure and temperature dependent and by tuning the synthesis parameters, spe-

titanium tetrachloride solution saturated with sodium chloride at 160°C for 2 h. Kim et al. [45]

The first report of the hydrothermal growth of crystals was by a German geologist Karl Emil in 1845 [46]. The hydrothermal method can be used to synthesize a single crystal of the material depending on the solubility of the material in the solvent. Single crystal growth is done in a high pressure vessel called an autoclave. These are hermitically sealed steel vessels that can withstand high temperatures and pressure for long durations. Also, the vessel must be chemically inert and must not take part in the hydrothermal process. A number of substances such as oxides [47], tungstate [48], molybdates [49], carbonates [50] and silicates [51] can be

since the sulfate prevented the crystallization of brookite. In another study, they also

) in a dilute acidic aqueous medium. They found out that the

nanomaterials [40]. In

nanoparticles

nanoparticles in

by a microwave-hydrothermal pro-

and the temperature can be increased

of good quality without the use of surfactants.

nanorods by treating

Sol-gel process is a wet-chemical technique that is mostly used in the field of materials science and ceramic engineering. It can be defined as the conversion of a precursor solution to an inorganic solid through polymerization reactions induced by water. Hydrolysis forms a sol which is basically a dispersion of colloidal particles in a liquid, and condensation leads in the formation of a gel. Compared to the methods discussed above, the sol-gel process is very promising for synthesis and preparation of inorganic and organic-inorganic hybrid nanomaterials because it allows the use of low processing temperatures (<100°C) and molecular level composition homogeneity [56]. Particle size and shape and are easy to control using the sol-gel method.

The sol-gel process produces fine, spherical powders of uniform size and has been widely used to synthesize TiO2 materials and normally proceeds via an acid-catalyzed step of titanium (IV) alkoxides [57]. One of the most attractive features of the sol-gel process is the possibility to shape the resulting material into desired forms such as fiber, film and monodispersed powder. Several steps and conditions are applied in a sol-gel process to control the final morphology as suggested by Mehrotra and Singh [58] in **Figure 4**.

Typical precursors are metal oxides and metal chlorides. A metal alkoxide consists of an M─O─R linkage where M is the metal, O is oxygen and R is an alkyl group. The polarization that takes place in the M─O bond makes it susceptible to nucleophilic attack. In the presence of water, the alkoxide undergoes a nucleophilic substitution reaction in which the alkoxy groups (OR) are replaced by the hydroxyl groups from water and this process is called hydrolysis. The metal hydroxide groups will link and generate a hydrated metal-oxide network which eventually forms small nuclei and this process is called condensation.

The chemistry, hydrolysis and poly condensation reactions are very convenient to obtain both polymeric and particulate titanium sols:

$$\text{TiOR} + \text{mH}\_2\text{O} \rightarrow \text{Ti} \text{(OR)}\_{4\cdot n} \text{(OH)}\_{\cdot n} + n\text{ROH} \tag{1}$$

There are several advantages of the sol-gel process and these include [60]: (i) use of low temperatures during preparation, (ii) easy and effective control of particle size, shape and properties, (iii) better homogeneity from raw materials, (iv) better purity from the starting materials and it is possible to design the material structure and property through the proper selection of the precursor.

that influence hydrolysis and condensation reactions of the sol-gel process should be controlled. It has been established that some parameters are more important than others. The parameters include pH, nature and concentration of the catalyst, water/precursor molar ratio, reaction temperature, precursor concentration, type of solvent and type of precursor [61, 62].

Particle size increases with increasing precursor concentration due to enhanced coagulation

precursor concentrations [63]. Increase in precursor concentration increases the crystallinity

The amount of water is a crucial parameter in controlling the hydrolysis reaction. Xiaobo [31] reported that the development of Ti─O─Ti chains through alkoxylation is favored when the content of water is low, with low hydrolysis rates and excess titanium alkoxide in the

particles pro-

nuclei generated at high TTIP

particles with desirable properties, the parameters

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

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

161

There are various parameters that influence the size and properties of the TiO<sup>2</sup>

**Figure 4.** Different sol-gel process steps to control the final morphology of the product [58].

*2.5.1. Sol-gel process parameters affecting properties of TiO<sup>2</sup>*

and sintering resulting from the large concentration of TiO2

of the anatase, and enhances transformation from anatase to rutile [64].

duced via the sol-gel process. To get TiO2

*2.5.1.1. Precursor concentration*

*2.5.1.2. Water content*

$$\text{Ti}\text{--OH} + \text{OR} - \text{Ti} \rightarrow \text{Ti}\text{--O} - \text{Ti} + \text{ROH} \tag{2}$$

$$\text{Ti}\text{--OH} + \text{OH} \begin{aligned} \text{Ti} + \text{OH} &\xrightarrow{} \text{Ti} \rightarrow \text{Ti} \rightarrow \text{O} \rightarrow \text{Ti} + \text{H}\_2\text{O} \end{aligned} \tag{3}$$

Polycondensation turns monomers into oligomers and, lastly, polymers. As long as the number of alkoxide groups, is greater than 2, complex random branching may occur finally leading to fractal structures.

Metal alkoxides used for the sol-gel process are generally very reactive and thus there is need for controlling the reactivity in order to obtain sols and gels with desirable properties by using modifiers or addition of chelating ligands such as β-diketones, carboxylic acids or other complex ligands [56]. These modifiers react with alkoxides giving rise to new molecular precursors that can be used in sol-gel processing to provide better control of the hydrolysiscondensation process. These new precursors reduce reactivity and functionality, prevent condensation and lead to formation of species that are smaller. Livage et al. in 1988 investigated the use of acetylacetone to improve the sol-gel processing of metal alkoxides [59].

Modification by modifiers reduces the number of M─OR bonds available for hydrolysis and thus hydrolytic susceptibility. If β-diketones are used, they decrease the nuclearity resulting in small particles since these ligands are surface capping reagents and polymerization lockers. Carboxylate ligands such as acetic acid mostly act as bridging chelating ligands.

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review http://dx.doi.org/10.5772/intechopen.75425 161

**Figure 4.** Different sol-gel process steps to control the final morphology of the product [58].

There are several advantages of the sol-gel process and these include [60]: (i) use of low temperatures during preparation, (ii) easy and effective control of particle size, shape and properties, (iii) better homogeneity from raw materials, (iv) better purity from the starting materials and it is possible to design the material structure and property through the proper selection of the precursor.
