Rapid High‐Contrast Photoreversible Coloration of Surface‐Functionalized N‐Doped TiO2Nanocrystals for Rewritable Light‐Printing

Pristine titanium dioxide (TiO2 ) changes color from white to black when it is reduced from TiIV to TiIII by photoexcited electrons. However, the black coloration requires substantial light energy to create, and it vanishes instantaneously upon exposure to air. This work reports the synthesis of surface-functionalized N-doped TiO2 nanocrystals that rapidly change color (i.e., within seconds) from whitish to black under low-power irradiation with excellent color stability in atmospheric conditions. The N-doping plays a critical role in promoting the surface-adsorption of polyol groups to stabilize the TiIII species and accelerate the coloration process. A rewritable paper fabricated using these nanocrystals exhibits excellent writing and erasing reversibility in response to UV irradiation and oxygen exposure. The low-cost, rapid response, excellent reversibility, and good color stability are vital advantages of N-doped TiO2 nanocrystals for color-switching applications.