Chemical cross-linking effect of WO₃ quantum dots in WO₃/methyl cellulose composite films for fast switchable photochromism

Photochromism, the color change of a material upon exposure to light, has attracted attention among researchers and practitioners. The photochromic (PC) properties of WO₃ have been extensively studied owing to its superior PC modulation (transmittance gap between the colored and bleached states > 70 %). However, the slow PC switching kinetics of WO₃, particularly during the bleaching process, delay its commercialization. Here, we demonstrate the chemical cross-linking effect of WO₃ quantum dots (WQDs) in WO₃/methyl cellulose composite films (WQD/WO₃), which enables faster PC switching kinetics compared to bare WO₃. The WQDs form chemical interactions corresponding to the sp² C−C bond with a methyl group in methyl cellulose. These chemical linkages not only provide additional electron transfer channels but also introduce defect levels between the valence and conduction bands. These defect levels preferentially trap photogenerated electrons in the conduction band, accelerating non-radiative recombination instead of trapping them in the near-interface trap sites of methyl cellulose. Therefore, WQD/WO₃ shortened the bleaching time by 60.5 min compared to the bare WO₃. These results provide a new strategy for improved switching kinetics for WO₃-based PC devices.