Rapid High-Contrast Photoreversible Coloration of Surface-Functionalized N-Doped TiO₂ Nanocrystals for Rewritable...

Pristine titanium dioxide (TiO₂ ) changes color from white to black when it is reduced from Ti^IV to Ti^III 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 Ti^III 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.