**2.3. Sonochemical and microwave-assisted methods**

**Figure 3.** Schematic representation of spray pyrolysis system: (a) vertical chambers; (b) horizontal chambers and (c)

rates as sprays are sensitive to oscillations in the liquid fuel supply which can affect nanoparticle growth conditions. When the flow rate of the precursor is increased, the particle diameter also increases but decreases when the dispersion gas flow rate is increased as a result of rapid

Some important advantages of this technique include the possibility to produce uniform and dense films which have desirable crystallinity by multiple repetitions of spraying and or annealing cycles and the ability to fabricate entire multilayer devices by subsequent deposition of different functional layers, in the same chamber [27]. In spray pyrolysis methods, nanoparticles and thin films are produced in a one-step process and there is no need for further purification or excessive drying procedures, which could have a negative impact on the total thermal budget and cost of production of titanium dioxide nanoparticles [30]. Besides the above mentioned advantages, there are some limitations of this method and these include the need to control temperature and the difficulty in obtaining low temperature allotropic forms of the respective products hence in a large scale production, there is a need for cooling

With a lot effort being put in trying to develop the technique, a new spray pyrolysis setup has been designed to overcome limitations of previous systems such as reproducibility, temperature control, gas flow rate and solution rate accuracy. The new system is almost fully computerized. A schematic representation of the spray pyrolysis system is shown in **Figure 3**.

These methods involve the oxidation of titanium metal using oxidants or anodization. Anodization of titanium sheet under a voltage between 10 and 20 V in 0.5% hydrogen fluoride

chambers for film deposition [27].

mixing reactants and oxidizers [29].

156 Titanium Dioxide - Material for a Sustainable Environment

systems, and precise temperature control.

**2.2. Oxidation methods**

The sonochemical method has been applied to produce highly photoactive TiO2 nanoparticles by the hydrolysis of titanium tetraisopropoxide (TTIP) in pure water or in an ethanol/water mixture under ultrasonic radiation [35]. Sonochemistry arises from *acoustic cavitation* which is the formation, growth and collapse of bubbles within a liquid medium. Heat (~5000 K) and high pressures (~1000 atm) are produced by cavitational collapse [31].

In microwave-assisted methods, there is a use of microwaves which are electromagnetic waves with frequencies which range from 0.3 to 300 GHz and with wavelengths between 1 mm and 1 m. According to Zhu and Chen [36], microwave heating involves two main mechanisms namely dipolar polarization and ionic conduction. Any materials that contain mobile electric charges such as polar molecules or conducting ions are generally heat by microwaves. In the microwave, heat is generated by rotation, friction and collision of molecules as polar molecules try to orientate with the rapidly changing alternating electric field. If ions are present in solution, they will move through the solution and constantly changing directions based on the orientation of the electric field resulting in local temperature rise due to friction and collision [37].

Microwave heating is as an alternative heat source for rapid heating with shorter reaction time and higher reaction rate, selectivity and yield as compared to the conventional heating methods [36]. There are two types of microwave heating: pulsed microwave heating and continuous microwave heating. Jacob et al. in 1995 proposed two models of the mechanism for microwaveinduced reaction rate enhancements. The first mechanism assumes that, although the reaction time is heavily shortened for a microwave-induced reaction, the kinetics or mechanism of the chemical reaction is not altered implying that the enhancement of the reaction rate is due to the thermal heating effect [38]. The second proposed mechanism makes an assumption that there are "nonthermal microwave effects" in addition to the thermal effects hence the effects of microwave irradiation in chemical reactions are due to both thermal effects and nonthermal effects [39]. The nonthermal effects are due to direct interaction of microwaves with certain molecules in the reaction medium.

Microwave radiations can also be applied to produce various TiO2 nanomaterials [40]. In 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 reproducibility and excellent control of experimental parameters. The colloidal TiO2 nanoparticles 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 between 200 and 1000 nm were also prepared using this method [33]. TiO2 nanoparticles in the anatase phase were prepared by Baldassari et al. [42] using microwave-assisted hydrolysis of titanium tetrachloride (TiCl4 ) in a dilute acidic aqueous medium. They found out that the 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 H2 SO4 since the sulfate prevented the crystallization of brookite. In another study, they also prepared TiO2 nanoparticles in the rutile phase from TiCl4 by a microwave-hydrothermal process 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 power and reactant concentration.

synthesized. The different approaches in the hydrothermal technique can be broadly classified as temperature-difference technique, temperature-reduction technique and metastablephase technique [52]. Temperature-difference technique is a method in which the autoclave is heated to two temperature zones. The solute dissolves in the hotter zone (lower part) and the saturated solution moves from the lower part to the upper part (at a relatively lower temperature) due to the difference in temperature. The cooler solution in the upper portion descends to the lower part resulting in a counter flow. Eventually, the solution in the upper part becomes supersaturated due to the reduction in temperature and the material starts to crystallize. Temperature-reduction technique is a method in which instead of having the two temperature zones, the autoclave is slowly cooled down with the saturated solution inside it. However, this technique has the disadvantage of difficulty in controlling the growth process. Metastable-phase technique is a method based on the difference in solubility between the crystal growth phase and that serving as the starting material. The solution consists of compounds that are thermodynamically unstable under the growth conditions. The solubility of the compounds that are in the metastable phase is more than that of its stable phase. The

The advantages of the hydrothermal method are that it is an easy method to obtain nanotube morphology, variation in the synthesis method can be implemented to enhance the proper-

disadvantages include long synthesis duration, the need for a highly concentrated NaOH

eter of the nanotubes [53]. Furthermore, hydrothermal methods are disadvantaged by the high cost of equipment and the inability to monitor crystals in the process of their growth. Hydrothermal synthesis cannot be affected at both temperatures and pressures below the critical point for a specific solvent above which differences between liquid and vapor disappear, and can only take place under supercritical conditions. The hydrothermal method is affected by alkaline concentration, temperature and reaction time. Temperature is important for promoting growth of crystals and nucleation of nanoparticles. Generally, as temperature increases, the yield, length and degree of crystallinity of nanotubes also increase and with the optimal temperature between 100 and 200°C. As hydrothermal time increases, yield also increases, but prolonged hydrothermal time results in morphological changes of nanopar-

Solvothermal method allows shape, control of size, distribution and crystallinity of TiO2 nanoparticles better than the hydrothermal method. These can be achieved by controlling the following parameters; solvent, addition of surfactants, titanium precursors, reaction temperature and reaction time [55]. Use of organic solvents in the solvothermal method results in a product that is free from foreign anions since organic solvents exhibit low relative permittiv-

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

nanotubes, and it is a feasible method for different applications. However, the

nanotubes with NaOH) and precisely controlling the diam-

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

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

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compounds crystallize due to the decomposition of the metastable phase.

ties of TiO2

solution (contaminating the TiO2

ticles, for example, nanotubes to nanofibers [54].

ity as well as being free from ionic species.

**2.5. Sol-gel methods**

#### **2.4. Hydro/solvothermal methods**

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 solvent, has better control of the properties of TiO<sup>2</sup> and the temperature can be increased 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, specific solvent properties can be obtained. Feng et al. [44] produced TiO2 nanorods by treating titanium tetrachloride solution saturated with sodium chloride at 160°C for 2 h. Kim et al. [45] used the solvothermal method to prepare TiO2 of good quality without the use of surfactants.

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 synthesized. The different approaches in the hydrothermal technique can be broadly classified as temperature-difference technique, temperature-reduction technique and metastablephase technique [52]. Temperature-difference technique is a method in which the autoclave is heated to two temperature zones. The solute dissolves in the hotter zone (lower part) and the saturated solution moves from the lower part to the upper part (at a relatively lower temperature) due to the difference in temperature. The cooler solution in the upper portion descends to the lower part resulting in a counter flow. Eventually, the solution in the upper part becomes supersaturated due to the reduction in temperature and the material starts to crystallize. Temperature-reduction technique is a method in which instead of having the two temperature zones, the autoclave is slowly cooled down with the saturated solution inside it. However, this technique has the disadvantage of difficulty in controlling the growth process. Metastable-phase technique is a method based on the difference in solubility between the crystal growth phase and that serving as the starting material. The solution consists of compounds that are thermodynamically unstable under the growth conditions. The solubility of the compounds that are in the metastable phase is more than that of its stable phase. The compounds crystallize due to the decomposition of the metastable phase.

The advantages of the hydrothermal method are that it is an easy method to obtain nanotube morphology, variation in the synthesis method can be implemented to enhance the properties of TiO2 nanotubes, and it is a feasible method for different applications. However, the disadvantages include long synthesis duration, the need for a highly concentrated NaOH solution (contaminating the TiO2 nanotubes with NaOH) and precisely controlling the diameter of the nanotubes [53]. Furthermore, hydrothermal methods are disadvantaged by the high cost of equipment and the inability to monitor crystals in the process of their growth. Hydrothermal synthesis cannot be affected at both temperatures and pressures below the critical point for a specific solvent above which differences between liquid and vapor disappear, and can only take place under supercritical conditions. The hydrothermal method is affected by alkaline concentration, temperature and reaction time. Temperature is important for promoting growth of crystals and nucleation of nanoparticles. Generally, as temperature increases, the yield, length and degree of crystallinity of nanotubes also increase and with the optimal temperature between 100 and 200°C. As hydrothermal time increases, yield also increases, but prolonged hydrothermal time results in morphological changes of nanoparticles, for example, nanotubes to nanofibers [54].

Solvothermal method allows shape, control of size, distribution and crystallinity of TiO2 nanoparticles better than the hydrothermal method. These can be achieved by controlling the following parameters; solvent, addition of surfactants, titanium precursors, reaction temperature and reaction time [55]. Use of organic solvents in the solvothermal method results in a product that is free from foreign anions since organic solvents exhibit low relative permittivity as well as being free from ionic species.
