# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 4 TiO\({}_{2}\) in the Design of Chromogenic Devices--General Considerations
Modulation in the Near Infrared Region (NIR) region (28% at 900 nm) was found to be higher than in the visible wavelength (16% at 560 nm). CE of 79.4 \(\div\) 82.7 cm\({}^{2}\)/C was found for 80 nm thick layers. coloration and bleaching times were 16.1 s and 2.8 s, respectively. [PERSON] et al. (2019) studied niobium (Nb)-doped TiO\({}_{2}\) colloidal nanocrystals for their wide bandgap and promising plasmonic features. TiO\({}_{2}\) nanocrystals doped with varying amounts of Nb nanoparticles have been used to prepare highly transparent electrodes that can be to be used in NIR-selective EC devices. They fabricated a dual-band EC device with 10% doping concentration of Nb. The device showed complementary switching, according to the external voltage applied. In the 0 \(\div\) 3 V window, modulations of transmittance higher than 64% were measured in the 800-2000 nm range of wavelengths. Upon the application of a higher bias, from 3 V to 4 V, further modulation in the visible region was achieved, driven by lithium intercalation into the TiO\({}_{2}\) anatase structure. The same group (174) reported further results obtained using Vanadium-modified TiO\({}_{2}\) nanocrystals and nanocrystalline WO\({}_{3}\). Such materials were deposited on several electrodes, so as to obtain a typical battery-like device architecture. WO\({}_{3}\) was finely tuned to control NIR transmittance, with low voltages (about 1.6 V for coloration and 0.5 V for bleaching), whereas V-modified TiO\({}_{2}\) was capable of modulating in the visible region, by applying a voltage of 2.2 V.  
A single-step process was proposed to deposit WO\({}_{3}\) nanoplate bundles--by chemical bath deposition--over spray deposited TiO\({}_{2}\) films. Nanoplate bundles were capable of improving the charge transport and to prevent degradation of films. Such films showed a current density of 3 mA/cm\({}^{2}\) and a stable optical modulation (\(\Delta\)T) of 78% after 1000 cycles (166). The recent diffusion of studies regarding TiO\({}_{2}\) as a suitable cathodic EC layer has been reviewed by [PERSON] et al. (1975). As is well known, TiO\({}_{2}\) is a cheap and abundant material, is highly compatible with other materials, and is inert. The doping of TiO\({}_{2}\) nanocrystals with metal ions having valence between 5 and 6 (optimal doping of about 10% of Nb, W, Mo, or Ta) allows for the design of nanoparticle absorption due to plasmon resonance. [PERSON] et al. (2019) modified TiO\({}_{2}\) nanoparticles deposited on fluorine-doped tin oxide (FTO) substrates, obtaining a device with reversible three-state optical transformation. TiO\({}_{2}\) particles, with a size of 5 \(\div\) 10 nm, were obtained by dip-coating, with a lifting speed of 3000 \(\upmu\)m/s. The main figures observed were a contrast of 57%, coloration and bleaching times of 6 s and 20 s, respectively, and stability after 1500 cycles. Recent studies show a tendency to exploit the properties of TiO\({}_{2}\) at the nanoscale, with significant effects on the performance of EC devices. For instance, [PERSON] et al. (2019) reported that oxygen vacancies not only confer good near-infrared transmittance modulation, but also improve lithium diffusion. They obtained TiO\({}_{2}\) nanocrystals capable of modulating the infrared and visible light transmittance independently in three distinct modes, with a modulation of 95.5% at 633 nm and 90.5%at 1200 nm, and fast kinetics. Moreover, they demonstrated that the electrical energy consumed in a coloration/bleaching cycle can be stored in the device in the form of chemical energy; afterwards, such \"recycled energy\" can be released for local reuse or even uploaded to the electrical grid. [PERSON] et al. [177] reported the performance of bifunctional electrode-based nanocomposite film based on polyoxotungstate K\({}_{10}\)P\({}_{2}\)W\({}_{17}\)O\({}_{61}\) and TiO\({}_{2}\) nanowires, resulting in a combination of electrochromism and energy storage features. The films were fabricated via hydrothermal and self-assembly combination methods.