# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 2 Synthetic Routes and Fabrication Techniques for TiO\({}_{2}\)  
### Sol-Gel  
The sol-gel method is a colloidal chemical technique, extensively used to synthetize large quantities of metal oxide NPs at low temperature (\(<\)100 \({}^{\circ}\)C), with an accurate control of size and shape. In the first step, the precursor monomers (metal oxides and metal chlorides) are converted into a colloidal solution (sol) by hydrolysis; the sol acts as the precursor for gel formation, which forms particles or polymers. The precursors are hydrolys and poly-condensed to obtain colloids [34] (Figure 1). In detail, metal alkoxides contain M-O-R bonds (M is the metal, O is oxygen, and R is the alkyl group). Hydrolysis starts with a nucleophilic attack on the M-O bonds and a consequent nucleophilic substitution reaction, in the presence of water, which replaces the O-R bonds in the O-H groups. The condensation step occurs when the OH\({}^{-}\) groups develop a metal oxide network and build small nuclei [35]; in addition, the possibility to customize the route allows for the design of different shapes of TiO\({}_{2}\), applyingseveral steps in the synthetic process [36]. The addition of chelating ligands, such as carboxylic acids, \(\beta\)-diketones, or acetylacetone, in the sol-gel route permit the acquisition of sols and gels with specific properties. For example, the use of \(\beta\)-diketones leads to the formation of smaller particles; these chemical species act as capping and polymerization agents. One of the disadvantages of sol-gel is the low purity of products that require long post-synthesis treatment [37]. The sol-gel method is a versatile alternative for producing TiO\({}_{2}\) films [38; 39]. As already specified in Section 2.1, the hydrolysis of specific precursors (such as metal alkoxides M(OR)\({}_{4}\)) produces the so-called \"sol\" and then a \"gel\", according to the following reactions of hydrolysis and condensation:  
\[\textbf{Hydrolysis:}\textbf{M}(\text{OR})_{n}+\text{nH}_{2}\textbf{O}\rightarrow \textbf{M}(\text{OH})_{n}+\text{nROH} \tag{1}\]  
\[\textbf{Condensation:}\textbf{M}(\text{OH})_{n}\rightarrow\textbf{MO}_{\frac{ \beta}{2}}+\left(\frac{\textbf{n}}{2}\right)\text{H}_{2}\textbf{O} \tag{2}\]  
It has been observed that the color of TiO\({}_{2}\) in the dark state depends on the precursor used; you will have grey colored states if Titanium tetra-n-butoxide is used and blue colored states by adding acetic acid to the alkoxide precursor before the hydrolysis steps in [40]. Eventually, a gel is obtained when solvents undergo complete evaporation and polymerization takes place. In the case of TiO\({}_{2}\), an amorphous xerogel is obtained, as confirmed by X-ray diffraction analysis; eventually, it may turn into crystalline anatase at about 673 K [40]. Sol-gel is a facile synthetic approach at room temperature, allowing for low-cost deposition techniques, such as spin-coating or dip-coating, leading to the fabrication of a dense, ceramic material. Tetraisopropoxide was reported as a suitable precursor for both mesoporous and nanoporous TiO\({}_{2}\) films [41]. [PERSON] et al. [42] investigated the electrochromic (EC) properties of films fabricated using a sol-gel dipping method. [PERSON] et al. [43] adopted the sol-gel technique to fabricate a solid-state EC device embodying TiO\({}_{2}\) and a solid polyelectrolyte, reporting transmittance modulation of 27.3%. The highest performance was obtained in the infrared region. The highest coloration efficiency (CE) of 79.4 cm\({}^{2}\)/C was measured at 1000 nm; visible modulation  
Figure 1: Schematic representation of the typical sol-gel route. was lower (16% at 560 nm). This work highlights the potential role of TiO\({}_{2}\) as a cathodic EC material especially devoted to infrared modulation. [PERSON] et al. [44] prepared EC TiO\({}_{2}\), starting from a dispersion of NPs of different sizes. Their NPs-based films were deposited on a transparent conductive oxide (TCO) by dip-coating using different process rates (from 1000 to 3000 \(\upmu\)m/s).