Abstract
Electrochromic titanium dioxide (TiO2) nanostructured films were grown on gold coated papers using a microwave-assisted hydrothermal method at low temperature (80 °C). Uniform nanostructured films fully covered the paper substrate, while maintaining its flexibility. Three acids, i.e., acetic, hydrochloric and nitric acids, were tested during syntheses, which determined the final structure of the produced films, and consequently their electrochromic behavior. The structural characteristics of nanostructured films were correlated with electrochemical response and reflectance modulation when immersed in 1 M LiClO4-PC (lithium perchlorate with propylene carbonate) electrolyte, nevertheless the material synthesized with nitric acid resulted in highly porous anatase films with enhanced electrochromic performance. The TiO2 films revealed a notable contrast behavior, reaching for the nitric-based film optical modulations of 57%, 9% and 22% between colored and bleached states, at 250, 550 and 850 nm, respectively in reflectance mode. High cycling stability was also obtained performing up to 1500 cycles without significant loss of the electrochromic behavior for the nitric acid material. The approach developed in this work proves the high stability and durability of such devices, together with the use of paper as substrate that aggregates the environmentally friendly, lightweight, flexibility and recyclability characters of the substrate to the microwave synthesis features, i.e., simplicity, celerity and enhanced efficiency/cost balance.
Highlights
Titanium dioxide (TiO2 ) 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]
The present study reports the synthesis and characterization of TiO2 nanostructured films using paper as substrate, and produced with a hydrothermal method assisted by microwave irradiation
TiO2 nanostructured films were synthesized using a microwave-assisted hydrothermal method and having paper as substrate with gold contacts to be used as electrochromic flexible devices
Summary
Titanium dioxide (TiO2 ) 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]. The EC materials are largely integrated in smart windows, displays and mirrors, 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]. Several techniques have been reported to produce TiO2 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. Structural characterization of the TiO2 nanostructured films has been carried out by scanning electron microscopy (SEM) coupled with focused ion beam (FIB) and X-ray energy dispersive spectroscopy (EDS), and by Raman spectroscopy
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