# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 4 TiO\({}_{2}\) in the Design of Chromogenic Devices--General Considerations
TiO\({}_{2}\) films are transparent (as-prepared) and turn into absorbing films if the following, reversible redox reactions are activated:  
\[\text{TiO}_{2}+\text{xH}^{+}+\text{xe}^{-}\leftrightarrow\text{TiO}_{2- \text{x}}(\text{OH})_{\text{x}}\]  
\[\text{TiO}_{2}+\text{xLi}^{+}+\text{xe}^{-}\leftrightarrow\text{Li}_{\text{ x}}\text{TiO}_{2}\]  
This gives rise to a modulation, generally due to an absorbance contrast, rather than reflectance [164]. In this way, electrons and ions can reach the defect positions of the TiO\({}_{2}\) lattices when a suitable voltage is applied, leading to the colored compound. Such absorbance change has reached 90%, in some cases, throughout the visible and infrared ranges as a consequence of lithium intercalation within the anatase lattice [41]. EC devices generally include more superimposed layers [165]: an electrolyte and two EC materials (anodic and cathodic ones) are sandwiched between two conductive transparent substrates in a \"battery-like\" architecture (Figure 7). A systematic study dealing with the EC properties of transition metal oxides was reported by [PERSON] [147]. Though tungsten oxide (WO\({}_{3}\)) is by far the most investigated EC oxide, TiO\({}_{2}\) is also a widely investigated cathodic coloring material, with a relatively larger coloration time and lower performance compared to WO\({}_{3}\)[42]. Nevertheless, TiO\({}_{2}\) is widely considered a promising candidate for smart windows due to its favorable electrochemical stability, optical modulation, reversibility, low cost preparation, and easy scale up. Compared to WO\({}_{3}\) performance, TiO\({}_{2}\) shows limited CE, higher bias, and slower kinetics [166]. It is widely recognized that nanosized materials show increased specific surface areas, porosity, and roughness, giving rise, in several cases, to extraordinary physical, chemical, and functional properties [167]. Some groups have reported the EC properties of nanocrystalline TiO\({}_{2}\) films, modified using viologens and anthraquinones [168]. High surface area (80-100 m\({}^{2}\)/g) TiO\({}_{2}\) (highly transparent in the as-prepared state) reported a reflectance contrast ratio of 5 at 600 nm [169]. Among TiO\({}_{2}\) polymorphs, the monoclinic-phase TiO\({}_{2}\)(B) showed fast lithium intercalation features compared to anatase and rutile due to the more open structure of its crystal lattice. Coloration times of 5 s were observed by [PERSON] et al. [170] in EC devices fabricated using TiO\({}_{2}\)(B) films, with full coloration at  
Figure 7: The five-layer stack of a generic EC device, with a “battery-like” architecture. 2.0 V. Furthermore, a study by [PERSON] et al., investigating one dimensional (1-D) brookite nanoneedles (650 nm long and 30 nm wide) obtained by hot-filament metal-oxide vapor deposition, reported an optical contrast of 67% (171). [PERSON] et al. (172) presented some TiO\({}_{2}\)-doped WO\({}_{3}\) films. Starting from the consideration that mixed WO\({}_{3}\)-TiO\({}_{2}\) thin film had a five times longer lifetime than bare WO\({}_{3}\), they prepared amorphous WO\({}_{3}\) thin films, embedding crystalline anatase TiO\({}_{2}\) by dipcoating. The mixing ratio of the two materials was found to strongly affect structural, morphological, and, eventually, the EC performance of films. EC devices were assembled using two electrodes and a Lithium-based liquid electrolyte. They observed that the nanocrystal-doped films showed enhanced ionic and electronic transporting properties and, eventually, better EC performance (higher CE--68 cm\({}^{2}\)/C--and modulation of \(\Delta\)OD up to 0.65--and coloration/bleaching times of just 1 s). These figures were higher than bare WO\({}_{3}\), which showed \(\Delta\)OD of 0.32, CE of 49 cm\({}^{2}\)/C, and a coloration and bleaching time of up to 6 s and 23 s, respectively. [PERSON] et al. (2019) reported enhanced IR modulation in spin-coated, solgel-based TiO\({}_{2}\) films, assembled with solid hybrid polyelectrolytes.