## 3 Results and Discussion
### Electrochromic and Electrochemical Behavior  
The electrochromic behavior of the TiO\({}_{2}\) nanostructured films grown on Au paper with different acids was investigated by measuring the reflectance variation between 200 to 800 nm. In order to get comparable results, the same active area (1 cm\({}^{2}\)) was defined for all EC devices. Figure 5 shows the data obtained after several coloration/bleaching cycles up a maximum of 1500 in 1 M LiClO\({}_{4}\)-PC and applying \(\pm\)5 V in order to stabilize the redox reactions. The insertion/deinsertion of Li\({}^{+}\) in the TiO\({}_{2}\) nanostructured films is accompanied by a color change. The TiO\({}_{2}\) nanostructured films change from dark blue, at the colored state, to transparent appearing the yellow coloration from the Au contact color, at the bleached state. The optical modulations of TiO\({}_{2}\) films between colored and bleached states were: 57%, 9% and 22%, at 250, 550 and 800 nm, respectively, for the nitric-based material, while the hydrochloric film, it presented 13%, 8% and 20% at 250, 550 and 800 nm, respectively. The acetic-based TiO\({}_{2}\) film, reached optical modulations of 10%, 3%, and 12% at 250, 550 and 800 nm, respectively. The increase of reflectance in the IR region has been reported previously for TiO\({}_{2}\)[59]. The highest optical modulation was achieved for the nitric-based film, demonstrating a good contrast behavior with deep color change, as can be seen on Figure 5a. The enhanced electrochromic performance observed for the TiO\({}_{2}\) film synthesized with nitric acid is expected to be related to the high porosity observed (Figure 2b). The higher EC performance with porous materials have been reported previously [60; 61], and it has been attributed to the increment in charge density incorporated into porous layers (easier electrolyte penetration and Li\({}^{+}\) diffusion) during the oxidation and reduction reactions [60; 61]. The TiO\({}_{2}\) film synthesized using hydrochloric acid also demonstrated the presence of porosity in its structure, however due to its lower thickness, it is expected a poorest EC performance when compared to nitric since thin layers are known to have less active mass [62]. The compact structure of the acetic-based TiO\({}_{2}\) film is thought to be responsible for the worst EC performance observed. The reflectance measurements also demonstrate the high durability of the EC device produced with TiO\({}_{2}\) nanostructured films on paper substrates. As it has been previously said, to the author's best knowledge, it has never been reported a fully flexible EC device using TiO\({}_{2}\) on paper, and Figure 5 shows that this kind of substrates are reliable for at least 1500 cycles using a liquid electrolyte. Figure 5: Reflectance spectra of the TiO\({}_{2}\) nanostructured films for colored and bleached states obtained after 10, 100, 250, 500, 1000 and 1500 coloration/bleaching cycles, applying \(\pm\)5 V. The bleached spectra presented was measured after 250 cycles. The results for TiO\({}_{2}\) nanostructured films synthesized using nitric acid are presented in (**a**), hydrochloric acid in (**b**), and acetic acid in (**c**). It also presented an image of the nitric-based TiO\({}_{2}\) film on the colored and bleached states carried out at room temperature. As previously described, the TiO\({}_{2}\) coloration into dark blue is associated to the intercalation of Li\({}^{+}\) in TiO\({}_{2}\). Thus, it is assumed that Li\({}^{+}\) insertion/extraction is responsible for the gradual optical change of the TiO\({}_{2}\) nanostructured films, and so applying a negative potential causes the rapid accumulation of electrons on the TiO\({}_{2}\), which is compensated by small ions located close to the TiO\({}_{2}\)-electrolyte interface. The proposed mechanism is suggested to be based on the formation of Ti\({}^{3+}\) ions and the presence of the electrons in the conduction band [63].