# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 1 Introduction  
Titanium dioxide (TiO\({}_{2}\)) is a well-known photocatalyst and anti-microbial metal oxide material, which is widely used in many commercial products [1]. A yearly production of 10,000 metric tons has been estimated [2], mainly for nanosized TiO\({}_{2}\). TiO\({}_{2}\) nanoparticles (NPs) are used in a wide range of applications due to their strong photocatalytic activity [3]. In addition, their white color is particularly suitable for enhancing opacity in paints, plastics, food additives, and cosmetics [4; 5; 6]. The ability of TiO\({}_{2}\) to filter UV-A wavelength radiation justifies its use in sunscreens [7]. Other types of applications for TiO\({}_{2}\) are water treatment, due to its purification capability [8], and self-cleaning coatings for surfaces [9]. Its use in chromogenic devices and materials will be investigated in this work,  
TiO\({}_{2}\) NPs exhibit excellent catalytic behavior; the electronic energy levels, in a semiconductor, constitute energy bands, hereafter designated as Valence Band (VB) and Conduction Band (CB), unconnected by a band gap. When an electron in the VB captures an energy amount greater than the band gap, it can be promoted to the CB, leaving a positive hole in the VB. When the electron hole (e\({}^{-}\) h\({}^{+}\)) pair is produced, it can move towards the semiconductor surface, triggering several redox reactions with chemical species adsorbed on the semiconductor surface, contributing to the photocatalytic reaction [5; 10]. The thermodynamic properties of its band structure are the basis of its photocatalytic performance [11]. When surfaces are exposed to UV light, TiO\({}_{2}\) produces Reactive Oxygen Species (ROS) such as hydroxyl radicals and super oxide ions. When the size decreases at the nanoscale, the catalytic activity is dramatically enhanced [12]. TiO\({}_{2}\) mainly exists in three widely studied crystalline forms; namely, anatase (tetragonal with a bandgap of 3.3 eV), rutile (tetragonal with a bandgap 3.1 eV), andbrookite (orthorhombic); each one shows different photocatalytic activity due to distinct electronic band structures and mass density [13]. Anatase and rutile forms are constituted by chains of distorted TiO\({}_{6}\) octahedra, where each Titanium (Ti) atom is surrounded by six oxygen atoms [14]. The synthesis of brookite is difficult; for this reason, its application is limited. In general, the initial crystalline phase is anatase, which is quickly converted into the rutile phase after calcination (600 \({}^{\circ}\)C); the process is irreversible due to the bonds breaking and reforming. The brookite form can be transformed into the rutile phase by heating it at 800 \({}^{\circ}\)C. Ratile is the most chemically stable of the three phases, with a melting point between 1830 \({}^{\circ}\)C and 1850 \({}^{\circ}\)C [15], in accordance with thermodynamic calculations [16]. The most investigated TiO\({}_{2}\) phase forms for photocatalytic applications are anatase and rutile; they present a bandgap equal to 3.20 eV and 3.02 eV, respectively. Anatase shows larger photo-catalytic activity due to its more negative conduction band edge potential (higher potential energy of photo generated electrons) and high specific area and photochemical stability [17]. When TiO\({}_{2}\) is exposed to UV radiation (\(\lambda\) = 384 \(\div\) 410 nm), e\({}^{-}\) h\({}^{+}\) carriers are generated and redox reactions can occur with chemical elements, such as water, Nitric Oxide (NOx), Sulfur Oxide (SOx), Ammonia (NH\({}_{3}\)), Hydrogen Sulphide (H\({}_{2}\)S), Hydroxide (OH\({}^{-}\)) ions, Carbon monoxide (CO), and Carbon dioxide (CO\({}_{2}\)), that are absorbed on the TiO\({}_{2}\) surface [16]. The photocatalytic behavior of TiO\({}_{2}\) has been understood since the early 19 th century, and it was not long after, in the 1970s, that [PERSON] and [PERSON] proposed the use of TiO\({}_{2}\) powders for the photoelectrolysis of water [18].