Self-accelerating photocharge separation in BiOBr ultrathin nanosheets for boosting photoreversible color switching

Photoreversible color switching materials (PCSMs) are promising for applications in optical information storage and security features. However, conventional inorganic PCSMs such as semiconductor oxides generally suffer from slow color switching speed and limited reversibility. Here we report a fast light-responsive color switching and high reversible inorganic PCSM based on oxygen vacancies BiOBr ultrathin nanosheets. Owing to the unique 2D ultrathin structure and oxygen vacancies, the BiOBr ultrathin nanosheets possess high photoreductive activity and fast charge separation and transfer efficiency under ultraviolet (UV) illumination to initiate color changing from light yellow to brown black. Importantly, UV illumination induced oxygen vacancies during color switching process act as self-accelerating charge separation centers to further boost the photoreversible color switching. By integrating BiOBr ultrathin nanosheets with agarose as matrix, the freestanding BiOBr ultrathin nanosheets/agarose nanocomposite film exhibit outstanding photoreversible color switching properties, including coloration in 1 min under UV illumination, decoloration in 15 min in ambient air, and high reversibility (≥100 cycles). Moreover, we demonstrate the application of nanocomposite films in time-sensitive, self-erasing rewritable paper with long-awaited positive imaging mode and black color. This work presents the possibility of developing high-efficient inorganic PCSMs that could be significantly broad their practical applications.