A novel ZnO/viologen photochromic composite film with a rapid UV response for rewritable paper, solar UV detection, smart windows and anti-counterfeiting

The development of viologen-based photochromic composite materials with a fast response to ultraviolet light has become a major research focus. In this paper, we fabricated viologen-based composite films with excellent durability and rapid photochromic performance by the solution casting method. The films changed color within one second when exposed to ultraviolet light (3 W, 365 nm). This was because ZnO NPs photogenerated electrons that enhanced the photoreduction of viologens with matched energy bands. The decoloration process of the films could be regulated by different moisture levels while keeping their photochromic properties unaffected. Moreover, the films exhibited less than 15% change in absorbance over 20 repeated coloring and fading cycles. Therefore, the excellent photoresponsiveness, repeatability and environmental friendliness of the films provide them potential for applications in areas such as rewritable paper, solar UV detection, smart windows and anti-counterfeiting.