Reversible CsPbBr3 ↔ CsPb2Br5 Transformation via Reverse Micellar Aqueous Solution

Lead halide perovskites suffer from water and moisture instability due to the highly ionic nature of the crystal structures, though a few groups took advantage of it for chemical transformation via water-assisted strategy. However, direct exposure of the perovskite to bulk water leads to uncontrolled chemical transformation. Here, we report a controlled chemical transformation of CsPbBr₃ to CsPb₂Br₅ triggered by nanoconfined water by placing CsPbBr₃ in the nonpolar phase within a reverse micelle. The chemical transformation reaction is probed by using steady-state and time-resolved optical spectroscopy. We observe absorption and photoluminescence in the UV region stemming clearly from the CsPb₂Br₅ phase upon interaction with the reverse micellar aqueous solution. Transmission electron microscopy and X-ray diffraction measurements further provided the structure and morphology. Our results direct the formation of CsPbBr₃-CsPb₂Br₅ nanocomposite under dry conditions while the chemically transformed CsPb₂Br₅ phase exists only in moist conditions, which we explain via the CsBr-stripping mechanism.