Hollow‐Spheres‐Embedded Heterostructure Enables Rapid and Reversible Fluorescence Switching in Copper‐Based Metal Halides

Metal halide perovskites with high sensitivity toward external stimuli have been deemed as next‐generation responsive materials. However, realizing high‐level reversibility and stability in the halides is still challenging. This study achieves a rapid and reversible fluorescence response triggered by dynamic water vapor in a hollow‐spheres‐embedded heterostructure, which is composed of 0D Cs 3 Cu 2 I 5 metal halide microcrystals and hollow oxide nanospheres. The fluorescence color of a typical MnO 2 ─Cs 3 Cu 2 I 5 heterostructure film shifts from blue to yellow within 3 s under dynamic water vapor, a change driven by the phase transition of Cs 3 Cu 2 I 5 to CsCu 2 I 3 . The response is reversible upon vapor removal and remains stable over 100 cycles. Notably, the fluorescence of the heterostructure remains well‐preserved after 7 months of ambient storage, demonstrating significantly improved long‐term stability compared to the bare Cs 3 Cu 2 I 5 . This strategy can be extended to alternative metal oxide hollow nanospheres, such as CeO 2 , yet it is ineffective for solid nanospheres. Several spatial and temporal encryption‐decryption demonstrations with dynamic water vapor as the key are further developed, highlighting the information‐security‐related application potential of the heterostructure. This work not only provides stable and durable stimuli‐responsive metal halides but also contributes a universal structural design strategy to facilitate their development.