## 4 Functional Materials
On this subject, a case of study has been a solid solution SnO\({}_{2}\)-TiO\({}_{2}\) mixed oxide (as Ti\({}_{x}\)Sn\({}_{1-x}\)O\({}_{2}\), \(0\leq x\leq 1\)). It resulted that the material with Ti molar ratio of 0.3 was the best material to detect carbon monoxide at concentrations low down to about 200 ppb. This result has been due to extreme low crystallite size of 5.5 nm at the temperature of firing of 650 \({}^{\circ}\)C [81; 82]. For the other cited materials, ZnO, synthesized in different nanoforms (see Figure 6) has shown great ability to detect acetone at sup ppm level [83], WO\({}_{3}\), also as solid solution (W,Sn)O\({}_{3}\) to detect NO\({}_{2}\)[84], TiO\({}_{2}\) as detector of VOCs for medical diagnosis [85]. The semiconductor oxides exhibit conductivity due to stoichiometric defects: in the ones of type \(n\), such defects are oxygen vacancies behaving as donor levels; indeed, remaining the electrons weakly bounded, they easily enter into the conduction band. Such electrons contribute to the building of the Schottky barrier eVs when they are captured by the acceptor surface states (in sensing materials oxygen atoms). In nanocrystalline semiconductors, the mechanism of conduction is thereby controlled by the presence of a huge series of intergranular point contacts at which a surface barrier develops, due to the presence of charged surface states. Conductance therefore is an activated process, since only those electrons with sufficient energy to cross the barrier take part to electrical conductance. The sensing mechanism is based on the variation of the potential barrier height as a result of surface chemical reactions with environmental gases, leading to the electrical conductance modification. Sucha mechanism properly works when the temperature, usually between 200 and 500 \({}^{\circ}\)C, is optimized with respect to both functional material and detecting gas. The metal oxides have been widely investigated also as flexible electronics. It must be highlighted that these devices can undergo thermal treatment not higher than 300 \({}^{\circ}\)C. Nevertheless, in the literature many publications on metal oxide gas sensors printed on flexible substrates are reported. As example, in [48] a tin dioxide sensor was developed onto polyimide foil performing electrical measurements toward carbon monoxide and nitrogen dioxide heating the device at temperatures between 200 and 300 \({}^{\circ}\)C. Moreover, an example of humidity sensor that works at RT is reported in [57]. TiO\({}_{2}\) nanoparticles were deposited by screen printing on a PET substrate with gold electrodes, obtaining the series of sensors showed in Figure 7.  
In Figure 8,the sensors response to humidity levels varying from 0% to 70% (a), its calibration curve (b) and the response and recovery times (c) are reported. The prepared humidity sensor is able to detect the target gas down to low levels. The response and the recovery times are fast in a range of RH between 5% and 40%. This is attributable to the TiO\({}_{2}\) sensing mechanism toward humidity. Figure 6: ZnO nanoforms tested toward acetone for breath gas analysis [83]. Figure 7: (a) Picture of 3 × 3 sensor matrices fabricated on PET substrate; (b) SEM image of the interdigitated electrodes; (c) optical image of the humidity sensors. [57]. Carbon nanotubes_Carbon nanotubes were discovered by [PERSON] in 1991. They can be prepared as single (SWCNT) or multi-walled carbon nanotubes (MWCNT) and are consisting of single or several layers of graphene sheets [86]. The unique geometry, morphology, and material properties attracted the attention of many researchers. In gas sensing, it is extremely interesting the enormous surface-to-volume ratio and the hollow structure, particularly suitable for the adsorption of gas molecules. The CNTs can be prepared as gas sensors using many different technologies. Thereby, also the sensing mechanisms can be different and the variation of the CNTs properties can be detected through various methods.