Photo‐Sensitive Bi‐Doped TiO ₂ Nanocrystals with Reversible Single and Multimode Color Transformations for Rewritable Printing

Abstract The integration of intrinsic chromism with catalytic reactions in semiconductor/redox‐driven light‐responsive color switching systems (LRCSs) can realize multimode color transitions, but such systems are unreported due to a lack of synergy and background color effect. Here, polyol‐capped Bi‐doped TiO ₂ nanocatalysts with reversible intrinsic chromism are designed for the catalytic transformation of redox‐sensitive dyes. The photo‐induced transformation of Bi‐doped TiO ₂ nanocrystals leads to instantaneous reduction of Bi ³⁺ /Ti ⁴⁺ to Bi ⁰ /Ti ³⁺ species, resulting in intrinsic chromic transitions that can be reversed by photooxidation. The combination of photo‐sensitive Bi‐doped TiO ₂ nanocrystals with redox‐sensitive dyes and polymer substrates can realize rewritable films with multimode color transitions. Exposure of these films to 365 nm light activates cascaded reactions enabled by polyol molecules, leading to effective carrier separation and partial reduction of Bi ³⁺ /Ti ⁴⁺ species. The reductive electrons react with the dyes culminating in their rapid decoloration. The reverse color transition can be achieved by 808 nm light irradiation in air, which activates the photooxidation process. The rewritable films have good cyclability and reversibility (>30 cycles). This work gives new perspectives into integrating chromism and secondary catalytic reactions for reversible printing for time‐sensitive information storage which can be implemented in light‐activated rewritable media and sensors.