# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 2 Synthetic Routes and Fabrication Techniques for TiO\({}_{2}\)
### Hydrothermal Method  
The hydrothermal route is a powerful tool to achieve stable nanomaterials, especially of transition-metal compounds. This method is based on the crystallization phenomenon of a certain material, caused by high vapor pressure and moderate temperature (about 300 \({}^{\circ}\)C) using an aqueous solution of the material. The process occurs in an autoclave at a pressure of about 10 bar.  
In general, the solvent is water, and a metal hydroxide (e.g., NaOH) is added as a mineralizer. The metal salts or metal alkoxides are used as a source of metal ions. The first step is nucleation, followed by particle growth in order to obtain a specific particle-size distribution. The same procedure is required for the solvothermal method, differing only in the wetting liquid, which is usually an organic solvent [80].  
The hydrothermal method is particularly suitable for synthesizing anatase, rutile, and mixtures of the rutile-anatase phases. Through this route, NPs with high purity and crystallinity can be obtained, but some factors, such as temperature, reaction time, and the medium influence the crystallization step. [PERSON] et al. [81] showed the synthesis of pure anatase and pure TiO\({}_{2}\), controlling the reaction conditions during the hydrothermal synthesis from TiCl\({}_{4}\). They obtained pure anatase (5-15 nm) at 120 \({}^{\circ}\)C and pure rutile (15-25 nm) at 250 \({}^{\circ}\)C, without additives. Despite the great advantages, including the possibility to obtain a nanotube morphology, this synthetic process requires long processing times,  
Figure 3: (**a**) Schematic representation of microwave reaction to grow thin titanium dioxide (TiO\({}_{2}\)) film. (**b**) XRD spectra of TiO\({}_{2}\)-indium tin oxide (ITO)-coated glass. The strong peaks were associated with the anatase phase, in particular the (101) peak. (**c**) Microwave-grown TiO\({}_{2}\) film on ITO-coated glass. (**d**) Microwave-grown TiO\({}_{2}\) film on Alluminium-coated glass. (**e**) Microwave-grown TiO\({}_{2}\) film on ITO-coated Polyethylene terephthalate (PET). Adapted from reference [79].  
high NaOH concentrations, and expensive equipment; in addition, the control of crystal growth is very hard. When the temperature is in the range between 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, the crystallinity of TiO\({}_{2}\) nanotubes increases, but a prolonged time of reaction promotes morphological NPs alteration. On the other hand, the solvothermal route employs non-aqueous solvents and the reaction temperature can be enhanced, overcoming the limit of the specific boiling point characterizing liquid solvents. [PERSON] et al. [82] used titanium isopropoxide (TIP) as a precursor to be decomposed by high temperature in the surfactant-dissolved toluene solution. The solution was heated at 250 \({}^{\circ}\)C for 20 h in autoclave in order to obtain small TiO\({}_{2}\) NPs (about 6 nm). When a greater amount of precursor was added, the shape of the nanomaterials turned in rods. [PERSON] et al. [83] used a TiCl\({}_{4}\) solution saturated with NaCl at 160 \({}^{\circ}\)C for 2 h in order to obtain TiO\({}_{2}\) nanorods. The solvothermal method allows control of the shape, size, and crystallinity better than the hydrothermal method due to the higher control of titanium precursors, temperature, duration, and solvents [84]. Recently, pure anatase TiO\({}_{2}\) NPs with a small size (7.5 nm) were achieved using TIP and isopropanol by the hydrothermal method. The product was obtained after high temperature treatment (200 \({}^{\circ}\)C) in autoclave and following annealing at 350 \({}^{\circ}\)C [85]. [PERSON] et al. [86] reported a continuous hydrothermal flow route in order to obtain rutile TiO\({}_{2}\) nanorods with a temperature dependent tunable size (from 35 to 60 nm in length). Consequently, the bandgap ranged from 3.2 to 3.5 eV.