# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 2 Synthetic Routes and Fabrication Techniques for TiO\({}_{2}\)
### Sonochemical and Microwave-Assisted Methods  
The sonochemical methods are used to develop TiO\({}_{2}\) NPs with high photocatalytic properties by the hydrolysis of TTIP in water or in a mix of ethanol/water under ultrasonic waves. Sonochemistry is based on the use of ultrasound with frequencies between 20 kHz and 2 MHz to induce physical, chemical, and thermal effects in solution [69]. This method includes three steps: formation, growth, and implosive collapse of microcavities in a liquid [70]. Cavitation collapse triggers intense local heating (~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (~109 K/s) reaching temperatures in the order of 5000 K [71].  
The cavitation step can be generated by different physical phenomena such as high velocity rotation, Venturi, and high-pressure nozzles. The energy generated by these is converted into friction, waves, and cavitation [72].  
Microwave-assisted synthesis is a green route suitable to the preparation of different product microspheres, gel beads, tablets, and thin films [73]. The microwaves are electromagnetic radiations with wavelength ranges from 0.01 to 1 m, which are particularly effective to induce nucleation or growth of material. The electromagnetic radiations induce an excitation of polar molecules by rotational and vibrational motion producing heat. The process is characterized by low energy consumption with respect to the electric furnaces widely used in CVD processes. In addition, the microwaves permit the acquisition of nanoproducts with a higher yield in a shorter time, with a controllable particle size and purity with respect to the other processes mentioned above [74].  
In the microwave-assisted methods, the synthesis of TiO\({}_{2}\) NPs occurs by irradiation of microwaves with frequencies between 0.3 and 300 GHz. In general, this method is based on two steps, namely dipolar polarization and ionic conduction [75]. Due to the fast heat transfer, this route can be applied to synthesize TiO\({}_{2}\) NPs with a good size control and reproducibility in a short time process (from 5 min to 1 h). However, the decrease in reaction time and temperature induces limitations in the particle growth. In a work by [PERSON] et al. [76], anatase NPs were prepared in 30 min using titanium tetrachloride (TiCl\({}_{4}\)) in H\({}_{2}\)SO\({}_{4}\) aqueous solution, which was used to prevent the crystallization of brookite. [PERSON] et al. [77] proposed the fast synthesis of colloidal anatase by the microwave hydrothermal method, based on a sulphate esterification reaction and its application to the oxygen reduction reaction. The authors obtained a TiO\({}_{2}\) film on porous graphite substrates. Their catalytic activity was measured by means of cyclic voltammetry. The synthesis of anatase, brookite, and rutile TiO\({}_{2}\) NPs were obtained by a microwave-assisted hydrothermal method using amorphous TiO\({}_{2}\) as a starting material at different concentrations (0.05 M, 0.1 M, and 0.2 M) in HCl. The nanomaterials were obtained under hydrothermal conditions at 140 \({}^{\circ}\)C, 160 \({}^{\circ}\)C, 180 \({}^{\circ}\)C, and 200 \({}^{\circ}\)C and dried at room temperature in order to have a final product after vacuum treatment [78]. [PERSON] et al. [79] synthesized crystalline anatase TiO\({}_{2}\) thin films on indium tin oxide (ITO)-coated glass substrates that were immersed in a solution containing tetraethylene glycol (TEG) and a Ti-based sol-gel precursor. The mix was further heated at 150 \({}^{\circ}\)C in a microwave reactor to create a nucleation site for the TiO\({}_{2}\) film to grow on (Figure 3).