Cellulose Nanocrystals/Poly(3,4-ethylenedioxythiophene) Photonic Crystal Composites with Electrochromic Properties for Smart Windows, Displays, and Anticounterfeiting/Encryption Applications

Poly(3,4-ethylenedioxythiophene)-based electrochromic materials with low cost, room-temperature processing, energy conservation, and controllable color switching have attracted wide attention. However, they face challenges such as monotonous color change and slow switching responses, which have limited their applications. Besides, natural compounds have been rarely used in poly(3,4-ethylenedioxythiophene) (PEDOT) composites, making it challenging to produce green and sustainable PEDOT-based electrochromic devices. In this work, a chiroptical, electrochromic photonic crystal coating was fabricated through the co-assembly of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with natural materials such as cellulose nanocrystals (CNCs), oxidized starch (OS), and tannic acid (TA). The hydrogen-bonding-based cross-linking network made by carbohydrate polymers and polyphenols endows the composite coating with good water and acid resistance and strong adhesion to the indium tin oxide (ITO) substrate. After H2SO4 washing, the composite coatings showed enhanced conductivity, achieving a conductivity of 249.4 ± 11.2 S·cm–1 for a PEDOT content of 3.43 wt %. The characterizations based on atomic force microscopy (AFM), Raman spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, and two-dimensional grazing-incidence small-angle X-ray scattering (2D GI-SAXS) confirmed that acid treatment induced conformational and crystalline changes of PEDOT that improved the film’s conductivity. Furthermore, the electrochromic materials based on these composites possess a unique combination of electrochromic and photonic crystal properties, showing colors ranging from green, yellow, orange, and red in the PEDOT-bleached state to dark blue in the PEDOT-colored state under the voltage change from 0.8 to −0.6 V. The electrochromic composite demonstrated a high coloration efficiency (372.4 cm2·C–1), faster switching speed (2.4 s for coloring and 3.5 s for bleaching), and good cycling stability after the 20 000 s test. Moreover, the chiroptical property of the co-assembled CNC structure was maintained in switching. Our study indicates that these composites, mainly composed of natural materials, exhibit a unique combination of electrochromic and chiroptical properties, which makes them an environmentally benign electrochromic material with potential for use in smart windows, displays, chiral sensors, anticounterfeiting, and encryption.