Photoreversible\nColor-Switching Cu-Doped TiO<sub>2</sub> Nanoparticles for High-Contrast\nRewritable Printing

Light-printable rewritable paper that can be used multiple\ntimes\nhas attracted extensive attention because of its potential benefits\nin reducing environmental pollution and energy consumption. Developing\nrewritable paper with high black-to-colorless contrast, lasting legibility,\nand a fast response is fascinating but challenging. Here, we integrate\nthe redox chemistry of Cu<sup>2+</sup> ions into photoreductive TiO<sub>2</sub> nanoparticles to produce Cu-doped TiO<sub>2</sub> nanoparticles\ncapable of highly photoreversible switching between colorless and\nblack with excellent contrast and color stability. Incorporating such\nnanoparticles into hydroxyethyl cellulose produces a rewritable paper\nwith the same appearance as that of conventional paper. More importantly,\nit demonstrates great features promising for practical applications,\nincluding high black-to-colorless contrast, fast light-printing (<20\ns), long legible time (>3 days), high reversibility (>50 cycles),\nhigh resolution (90 μm), and large scale (A4 size) applicability.