# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 4 TiO\({}_{2}\) in the Design of Chromogenic Devices--General Considerations  
\"Smart windows\" may represent one of the fields in which TiO\({}_{2}\) may be exploited. It has been shown that the use of windows capable of modulating their spectral characteristics can lead to significant energy savings, on an annual basis, both in terms of electricity consumption for air conditioning and for artificial lighting [145]. An intelligent shielding of incident solar radiation also may enhance indoor visual comfort for occupants. In the following pages, the duel use of TiO\({}_{2}\) in the field of \"chromogenics\" is shown. TiO\({}_{2}\) can be used as a cathodic material in EC devices, and in photoelectrochromic (PEC) devices--a relatively recent class of multifunctional devices--it can play the role of a photoanode, with the aim of generating electrons by photovoltaic conversion. In our opinion, this particular versatility of TiO\({}_{2}\) deserves further considerations because this field of application could lead to significant energy savings in several sectors, especially transportation and construction. TiO\({}_{2}\) has caught the attention of several research groups working in the field of chromogenics worldwide, who might offer a relevant contribution for limiting energy consumption. The term \"chromogenic\" indicates the change of optical properties in response to an external stimulus [146]. Such stimuli may be represented by an external bias (in ECs [147]), temperature (in thermochromics [148,149]) or ultraviolet irradiation (in photochromics [150,151]) and oxidizing or reducing gases (in gasochromics [152]). Spectral modulation of optical properties may result in an optimized tuning of the energy flux through glazing, with particular reference to the visible and infrared ranges of the impinging solar radiation. An accurate design and building integration of chromogenics may eventually reduce cooling/heating loads and enhance indoor visual comfort as well [153, 154, 155, 156]. Over the last few decades, a strong impulse towards experimental research dealing with TiO\({}_{2}\) has come via nanoscience. This has disclosed unexpected, emerging properties of the materials due to shape and size, affecting interfacial behavior as well as phonon and photon transport, and inverting the surface-to-volume ratio, as reported previously in this work [157,158, 88]. Different crystal structures have been reported for TiO\({}_{2}\), such  
Figure 6: (a) Representative optical microscopy images of Chinese Hamster Ovary (CHO-K1) cells cultured on TiO\({}_{2}\) thin films annealed at 300, 500, and 800 \({}^{\circ}\)C after 24 h. No significant differences were observed with respect to control. Scale bar = 20 \(\upmu\)m for all the images. (b) Optical density after 24, 48, and 72 h of Triton-X control (borosilicate glass plus Triton-X), control (borosilicate glass), and TiO\({}_{2}\) films in cells. (c) Number of unstained CHO-K1 cells (in millions) on control substrate and on TiO\({}_{2}\) thin films after 24, 48, and 72 h. \({}^{\ast}\) Triton X-100 24 h vs. Triton X-100 48 or 72 h; # 24 h vs. 48 h; and & 24 h vs. 72 h. Reprinted from [144]. as anatase [159], brookite [160; 161], and rutile [162; 163]. In each of them, a TiO\({}_{6}\) octahedron contains Ti\({}^{4+}\) ions, surrounded by six O\({}^{2-}\) atoms. Amorphous TiO\({}_{2}\) has shown a larger bandgap compared to anatase, whereas it was found to be narrower for rutile [147]. TiO\({}_{2}\) has been widely adopted as an EC material because it undergoes color modulation due to an external voltage, activating reversible red/ox reactions. Cathodic TiO\({}_{2}\) films act as intercalation hosts for small cations, (mainly hydrogen or lithium).