Photochromic Cs ₂ Pb(NO ₃ ) ₂ Br ₂ Perovskite Nanosheets as a Novel Anti‐Counterfeiting Material

Abstract Although the photoinduced segregation phenomenon of mixed halide perovskites has application potential in the fields of anti‐counterfeiting and information storage, the easy oxidation of I ‐ into highly volatile I 2 leads to a serious deficiency in material stability. In this study, we addressed this issue by substituting the instable anion of I ‐ with the stable anion of NO 3 ‐ and synthesizing a novel photochromic 2D perovskite of Cs 2 Pb(NO 3 ) 2 Br 2 nanosheets (PNSs). The synthesized functional material exhibited significantly enhanced stability, maintaining stable luminescence under ambient conditions for over a year, far surpassing traditional mixed halide perovskites. Further research revealed that reversible photoluminescence switching between orange and green in Cs 2 Pb(NO 3 ) 2 Br 2 PNSs suspension could be achieved by regulating the UV light power of irradiation and solution concentration, demonstrating the photoinduced segregation phenomenon in non‐halide/halide hybrid perovskites. The photoinduced segregation could be effectively suppressed by doping with Cl ‐ ions. Based on this photochromic property, we designed a novel anti‐counterfeiting label by exploiting the distinct UV‐response characteristics of Cs 2 Pb(NO 3 ) 2 Br 2 and Cs 2 Pb(NO 3 ) 2 Br 2‐x Cl x , offering a new design strategy for stimulus‐responsive perovskite materials.