A wet bacterial cellulose film self-anchored by phosphotungstic acid: Flexible, quick-response and stable cycling performance for photochromic application

Bacterial cellulose (BC) has been recognized as an ideal supporter owning to its abundant hydroxyl groups on the surface. In this work, Keggin-type phosphotungstic acid (H3PW12O40, PWA) is self-anchored on the three-dimensional network of BC by a simple diffusion method at room-temperature. BC fibers can protect the well-dispersed PWA with high transparency, high mechanical strength, and high stability. When PWA/BC serves as a photochromic material, the coloration time and bleaching time are only 3 min and 30 min respectively. After 6 cycles, the photochromic ability of the PWA/BC composite film almost unchanged. At the same time, we convert the coloration and bleaching degree into quantitative data including a color difference calculation of ΔE2000 and the remaining degree of color (RC%). Within the BC system, PWA exhibits a special linear relationship between RC% and bleaching time (0–30 min). The processes of PWA/BC reduction (W6+→W5+) and oxidation (W5+→W6+) are analysed by photo-electrochromic, XPS and ESR characterizations. The revealed excellent photochromic properties of soft PWA/BC films highlight their potential as the photoresponsive materials.