*2.1.2. Spray pyrolysis*

ultrasonic-assisted hydrolysis. TiO2

154 Titanium Dioxide - Material for a Sustainable Environment

*2.1.1. Electrophoretic deposition*

tion of a homogeneous layer.

and the properties of the powder are not easy to control.

**Figure 2.** Schematic setup of the electrophoretic deposition cell [16].

films with grain size less than 30 nm and TiO<sup>2</sup>

ticles with sizes less than 10 nm were synthesized by pyrolysis of titanium tetraisopropoxide (TTIP) in a helium/oxygen atmosphere [11]. Thermal plasma synthesis [12] and spray pyrolysis [13] have been used in some studies but they are complex, capital and energy-intensive

This is the most favored method due to several reasons, which include effectiveness in the fabrication of coatings and films from suspensions [14], a short period of time is required for deposition, the ability to deposit a film on a non-uniform surface, it is cost effective, it is easy to control the thickness of the films, ability to utilize suspensions of low solids loadings, homogeneity of resultant coatings, simple apparatus requirements, binder-free process and it

The process involves the movement of charged particles in a suspension medium followed by deposition on a substrate under an applied DC voltage [17, 18] as shown in **Figure 2**. When the voltage is applied to the electrodes, an electric field is created that interacts with the surface charge of the nanoparticles, producing a force that makes the particle move toward the electrode of the opposite charge and their accumulation on this electrode leads to the forma-

Different thickness of the films can be achieved by changing the deposition parameters such as voltage, deposition time, solvent type, zeta potential particle and loading in the suspension [17]. Some researchers have used different combinations in titanium dioxide thin film

is possible to prepare homogeneous coated layers or deposited films [15, 16].

nanopar-

This is a deposition technique that enabled production of a variety of products in the form of fine dispersive porous or dense powders or films. The first reactors for nanoparticles by flame synthesis (FS) started in the 1940s, to produce fumed silica. In 1971, G.D. Ulrich reported the first principles of the FS method. Silica and titanium dioxide were the first materials to be produced by flame synthesis and patented [26] but with time TiO2 became the largest material produced by flame aerosol reactors [9]. The number of researchers using FS has increased in the last years.

Generally, this method can be divided into two groups that are spray pyrolysis synthesis (SPS), which results in powders and spray pyrolysis deposition (SPD), which results in thin films. The method involves the passing of precursor's flux across a direct flame. It can proceed either by supplemental burners that are mounted near the spray nozzle, or by additional feeding of the nozzle by oxidant that could be air or pure oxygen and the combustibles. If an organic solvent is used, it can serve as a flame fuel as well. A nozzle or a nebulizer can be used but the use of a nozzle implies that the diameter of the spray droplets depends on the diameter of the nozzle outlet tip, the surface tension of the respective precursor solution, its viscosity and the pressure difference before and after the spraying [27]. Kozhukharov and Tchaoushev [28] recommended the production of ultrafine dispersive powders by swift rise of the temperature inside the chamber. In this process, the already formed solid particles undergo further splitting, due to mechanical tensions and or phase transitions occurring [28].

Adhesion to the substrate is important for the quality of the deposited film which means SPD could be performed either directly by hot spray, or by cold spray on preliminary heated substrate. Several parameters of the pyrolysis process, such as the size of the spray droplets, chemical composition of the obtained products, their crystal phases and density can be controlled. Selection and precursor preparation is a challenging task when multi-component materials are to be produced. The precursor should be fed into the reactor at low pulsation 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 mixing reactants and oxidizers [29].

leads to the formation of aligned TiO2

a 50 mL solution of 30 wt% H2

molecules in the reaction medium.

, Cl<sup>−</sup> , SO4

formation of TiO2

precipitation [34].

NaX (X = F<sup>−</sup>

applied voltage [31]. In another study, crystallized TiO2

nanorods. The TiO2

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

O2

2−) inorganic salts. Addition of Na2

The sonochemical method has been applied to produce highly photoactive TiO2

high pressures (~1000 atm) are produced by cavitational collapse [31].

ized titanium plate was heat treated at 500°C for 6 h in an oxygen environment [32]. Direct oxidation of the titanium metal with hydrogen peroxide has also been found to lead to the

via mechanism of dissolution precipitation and this phase can be controlled by addition of

of anatase phase and when rutile phase is needed, NaCl can be added during dissolution

Acetone, pure oxygen and a mixture of oxygen and argon can be used as sources of oxygen for oxidation of titanium metal. Acetone is a good source of oxygen and when used at high temperatures, it results in nanorods which are well aligned and highly dense. Use of pure oxygen or a mixture of oxygen and argon results in crystal grain films and morphology of the nanoparticles can be controlled by the diffusion competition of oxygen and titanium [8].

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

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

nanotubes whose diameter is controlled by varying the

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

can be obtained by placing a cleaned Ti metal plate in

at 353 K for 72 h [33]. Formation of crystalline TiO2

SO4

nanotubes were obtained when anod-

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

and NaF results in the formation

occurs

157

nanoparticles

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 systems, and precise temperature control.

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**.
