## 1 Introduction  
Titanium dioxide (TiO\({}_{2}\)) is a versatile material being investigated for applications ranging from photocatalysis [1; 2; 3; 4; 5], dye-solar cells [6; 7], sensors [8; 9; 10; 11] to electrochromic devices (EC) [12; 13; 14]. In recent years, the use of TiO\({}_{2}\) integrated on electrochromic devices has been increasing exponentially [15; 16; 17; 18], also in association with tungsten oxide (WO\({}_{3}\)), which is the most widely studied electrochromic material [19; 20; 21; 22; 23; 24]. A material is considered electrochromic when a reversible change on its optical properties (transmittance and reflectance) associated with an electrochemically induced oxidation-reduction reaction occurs under an applied voltage or current [25]. In that case, TiO\({}_{2}\) appears as an appealing electro-active material due to its intrinsic characteristics, such as high EC activity, strong oxidation capability and enhanced chemical stability [26]. Moreover, TiO\({}_{2}\) has a wide band gap (3.00 and 3.21 eV for rutile and anatase, respectively [27], and from 3.13 to 3.40 eV [27,28] for brookite) and presents a lattice structure that easily allows cation intercalation. This intercalation leads to additional electronic states in its bandgap causing the change in optical properties which results in light absorption in the visible range [15]. Indeed, TiO\({}_{2}\) in the form of anatase is considered the most promising material for electrochromic devices [14]. The EC materials are largely integrated in smart windows, displays and mirrors, however the demand for flexible devices has been gradually growing in the recent years, especially due to their conformability characteristic to unlike surfaces and the need to reduce production costs [29]. In this sense, electronics on paper appears as an attractive option as cellulose is the most abundant biopolymer on earth, inexpensive and guarantee highly flexibility to the device, despite being environmentally friendly [30]. In fact, paper has been included in several optoelectronic devices, including solar cells, transistors, among others [30,31], and lately paper started to appear as a reliable alternative for EC devices, as previously reported in [30,32,33,34]. The production route of the electro-active materials is a key factor on the final cost of the EC device, moreover the production route selected must be in line with the limitations of the substrates. Several techniques have been reported to produce TiO\({}_{2}\) EC films, including magnetron sputtering [35], spin-coating [36] and sol-gel methods [37], hydrothermal synthesis [14], and microwave synthesis [38]. This latter synthesis route is compatible with flexible substrates [3,39] and paper electronics [40], and it is reliable, inexpensive, simple and rapid. Moreover, the microwave synthesis guarantee precisely the control of synthesis parameters [41], and thus the produced material homogeneity and uniformity. In fact, the structure of the electro-active materials largely influences the final EC device performance, where nanostructured or nanoporous films are desired [13]. The present study reports the synthesis and characterization of TiO\({}_{2}\) nanostructured films using paper as substrate, and produced with a hydrothermal method assisted by microwave irradiation. Low temperature synthesis was employed (80 \({}^{\circ}\)C) and the materials were tested as EC devices. The main goal is to produce TiO\({}_{2}\) electrochromic nanostructured films with low-cost synthesis routes but also the fabrication of highly electro-active, stable, flexible, environmentally friendly, and cost effective EC devices. Moreover, to the best of the author's knowledge, TiO\({}_{2}\) nanostructured films grown with a microwave-assisted hydrothermal method and having gold coated-paper substrates to be employed as EC devices has never been reported before.