Water‐Assisted Synthesis of Layer‐Controlled CsPbBr₃ Nanoplates Spontaneously Encapsulated in PbBr(OH)

CsPbBr 3 nanoplates (NPs) have shown great potential in diverse optoelectronic applications. Despite the wonderful luminescence properties, the inherent instability of these NPs hinders their use in practical situations. Herein, a facile water‐assisted strategy is reported to synthesize highly stable blue emission CsPbBr 3 NPs encapsulated in PbBr(OH). The introduction of water directly into the reaction mixture is pivotal, as it triggers the transformation of bulk CsPbBr 3 nanocubes into 2‐D CsPbBr 3 NPs within the PbBr(OH) matrix, resulting in the formation of CsPbBr 3 @PbBr(OH) microbricks. Remarkably, the water content in the reaction mixture allows the control of the number of perovskite layers in the NPs, thus enabling the emission color to be tuned from deep blue to green. The unconventional approach presented herein not only offers a cost‐effective pathway to synthesize blue‐emitting perovskites which are highly stable in a wide variety of polar and non‐polar solvents, but also holds immense potential for propelling the advancement of high‐performance LEDs and other optoelectronic devices. The findings underscore the significance of water in dictating the growth dynamics and emission characteristics of perovskite NPs, paving the way for their practical application.