Regulating synthesis and photochromic behavior via interfacial Eu³⁺/Eu²⁺-Pb⁰/Pb²⁺ redox of the CsPbCl_1.5Br_1.5@ Ca_0.9Eu_0.1MoO₄ porous composites

Halogen vacancies are regarded to play a vital role in photo-induced phase segregation and the resulting switchable emission colors in the soft mixed-halide perovskites; however, its control strategy via the balanced Pb⁰ defects remains a big challenge. The research reports the regulation of synthesis and photochromic behavior via interfacial Eu³⁺/Eu²⁺-Pb⁰/Pb²⁺ redox in composites of porous Ca_0.9Eu_0.1MoO₄ and nominal mixed-halide perovskite CsPbCl_1.5Br_1.5. The composite takes full advantage of Eu³⁺ ions with the concerns of its luminescence and variable valences. It provides an additional emission color besides the halide perovskite, manipulates the Pb⁰ defects and the resulting Br-rich domain via interfacial redox reaction in the composites. The more contents of surfaced Eu³⁺ caused by substituting the unequivalent Ca²⁺ ions and the high volume-to-surface ratio of the porous Ca_0.9Eu_0.1MoO₄ guarantees the interfacial access for the Eu³⁺ and the halides. The research provides some perspectives on the regulation of ionic valence and photoluminescence of halide perovskites with the use of lanthanide ions. The composites may find potential applications in the anti-counterfeiting field.