Rapid photochromism and water-driven ultra-fast fading of cellulose-based composite hydrogels enabled by sodium tungstate

The potential ability of photochromic hydrogel as smart response material is determined by its coloration and fading speed. However, it is still a huge challenge to improve both the coloration and fading speed simultaneously. This paper presents the successful development of robust hydrogels with fast photoresponse and water-driven ultra-fast fading speed through the implementation of a dual-network preparation strategy. In the dual-network hydrogel, sodium tungstate contributed to its discoloration and crosslink. The obtained hydrogel exhibited fast photochromic response within 20 s under visible light irradiation. In addition, the photochromic response was achieved in 30 s under actual sunlight irradiation. Particularly, rapid fading of photochromic hydrogels was achieved by applying deionized water while leaving it naturally for 90 s. Furthermore, the simulated house experiments showed that the use of this hydrogel as smart glass could decrease the indoor temperature by about 5 °C. The photochromic hydrogel in this paper possesses excellent reversible repeatability, flexibility and foldability, and it exhibits promising potential for use in flexible information storage devices, visual displays and artificial intelligence systems, smart windows and other applications.