# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 2 Synthetic Routes and Fabrication Techniques for TiO\({}_{2}\)
### Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD)  
The vapor deposition technique is based on the vaporization of a certain material from a source, assisted by high temperature, vacuum, gaseous plasma. Eventually, the vapor condenses onto the substrate to generate solid thin films. The process can be chemical (CVD) or physical (PVD) [48]. CVD is a deposition route widely used to achieve thin films made of crystalline or amorphous compounds, metal alloys, and semiconductors. In the CVD process, a volatile precursor, in the presence of some carrier gases (NH\({}_{3}\), H\({}_{2}\)), induces the solid material formation at the atomic level in a reaction chamber, which finally is formed on a specific substrate. There are two factors that lead to different grain size and shape formation: substrate temperature and vapor supersaturation (Figure 2a). These two parameters impact the rate of the film formation and the nucleation rate, respectively [49; 50]. In particular, the growth of a single crystal on a substrate is promoted by high temperature and low gas supersaturation. On the contrary, low substrate temperatures and high gas supersaturation boost the formation of amorphous film development as well as the polycrystalline formation [51]. There are some variant of CVD, such as Atomic Layer Deposition (ALD), in which the precursors are pulsed alternately, and they are introduced into the reaction chamber in order to obtain the material by chemical surface reactions [52]. Small TiO\({}_{2}\) NPs (\(<\)10 nm) were achieved using a helium/oxygen atmosphere mixed to TTIP by pyrolysis. In addition, TiO\({}_{2}\) films (30 nm) can also be obtained with the same conditions, as demonstrated by [PERSON] et al. [53]. TiO\({}_{2}\) nanorods (50-100 nm) were obtained by metal organic CVD (MOCVD) using TTIP as a precursor [54]. TiO\({}_{2}\) nanorods can be synthetized on silica substrates by the MOCVD route using titanium acetylacetonate (Ti(C\({}_{10}\)H\({}_{14}\)O\({}_{5}\))). The latter is vaporized in the low-temperature area of a furnace (200-230 \({}^{\circ}\)C) before being moved to the high-temperature zone, at 500-700 \({}^{\circ}\)C, by the gas carrier flow; finally, TiO\({}_{2}\) nanorods can be obtained directly on the substrates [55]. The thermal plasma synthesis permits the acquisition of highly pure products without the use of high vacuum and capping agents. [PERSON] et al. [56] obtained different sizes of pure TiO\({}_{2}\) NPs using different arc currents. The size ranged from 25 nm to 30 nm when the current was set at 80 A and from 30 nm to 42 nm with a current of 120 A. In a CVD typical process, Ti powder is located on a quarts substrate in a tube furnace, and the temperature is increased to 850 \({}^{\circ}\)C in vacuum under argon flow, obtaining TiO\({}_{2}\) nanowires after 3 h [57; 58]. A different CVD method was developed by [PERSON] et al. [59] in order to obtain pure anatase-phase TiO\({}_{2}\) films that were resistant and stable at high temperatures. The precursor was represented by TTIP, which was dissolved in ethanol at a concentration of 0.10 mol/L. The precursor was moved to a reaction chamber, where the substrate was heated at 400 \({}^{\circ}\)C using carrier gas and compressed air followed by a post-annealing treatment using high temperatures (600 to 1100 \({}^{\circ}\)C). The small crystalline anatase TiO\({}_{2}\) showed high thermal stability with a sheet-like grain structure. Differently from the CVD route, in the PVD processes the precursor is not a gas but a solid material that can be deposited by its vaporization due to a high-energy source, such as an electron beam or sputtering [60]. The vapor generated is moved to a low-pressure area by gas (oxygen, nitrogen, or methane) or vacuum for its further condensation on the substrate, developing a thin film with a nanoscale thickness. The main PVD routes are: thermal evaporation, electron beam evaporation, sputtering, and ion plating [61].