Rewritable Structurally Colored Paper Based on Hollow SiO₂–Polyurethane Composite Photonic Crystal Film

Rewritable paper, which can be used multiple times as an effective solution for sustainable development and lessen the heavy environment pollution, has received widespread attention. A photonic crystal with dye-free character and tunable structure color has attracted significant interest in this area. Generally, handwriting on the photonic crystal structure containing a responsive polymer or hydrogel ingredient was based on the change of lattice spacing. It is necessary to enrich the diversities of color adjustment mechanism for further application. Herein, a flexible rewritable photonic crystal structurally colored paper with excellent mechanical strength based on the hollow SiO 2 (h-SiO 2 ) particle and polyurethane was developed. Owning to the varied optical response of h-SiO 2 photonic crystal film in different solvents, handwriting on this paper was realized by applying polarity solvents such as EG as colorless ink directly, which could also be erased by resoaking the film in water. Writing and erasing on this paper were totally reversible. The color adjustment mechanism and the realization of handwriting on this paper are totally different from those of the previous reported photonic crystal-based rewritable paper. The combination of quick handwriting and flexibility is significant for potential application as rewritable paper.