Liquid‐solid‐Interpenetration Surface Reconstruction of Monodisperse Water Stabilized CsPbBr3/CsPb2Br5 Core/Shell Perovskite Nanocrystals

All‐inorganic lead halide perovskite nanocrystals (NCs) have shown great potential in various optoelectronic device applications due to their excellent optoelectronic properties, but their poor stability remains a key obstacle to widespread application. The preparation of core/shell NCs is developed to improve their stability, but most of the coating for single NCs is incomplete, and the crystal orientation and interface binding of core/shell structures are still unclear. Here, a surface reconstruction strategy to synthesize CsPbBr3/CsPb2Br5 core/shell NCs with uniform coating structure is reported. The concentration of [PbBr8]6− monomer in the system is regulated by creating local low H2O concentration conditions to balance the erosion of perovskite by water and the overall heterogeneous nucleation reaction system, resulting in single encapsulated and well‐edged perovskite core/shell NCs. More importantly, advanced characterization tools are used to reveal the core/shell structural features of CsPbBr3/CsPb2Br5 core/shell NCs at the atomic scale, which has not been reported before, and the interface matching of the two‐phase binding is simulated by density functional theory calculations, which confirm that the CsPb2Br5 (116) facet can bind to the CsPbBr3 (100) facet with the lowest binding energy. The prepared core/shell perovskite NCs can still retain photoluminescence emission after >30 days in water. Surface reconstruction of the pristine CsPbBr3 nanocrystals (NCs) using a common liquid‐solid‐interpenetration method results in CsPbBr3/CsPb2Br5 core/shell NCs with uniform coating structure. The prepared core/shell NCs are characterized by a series of advanced characterizations demonstrating a complete and uniformly encapsulated structure, and the core/shell interface relationship is investigated for the first time by density functional theory calculations.