Photoluminescent Films of MoO₄^–2 Exchanged Layered Rare-Earth Hydroxides for Solid-State Lighting

With the advancement of solid-state lighting technology, the design and preparation of fluorescence conversion films have become increasingly important. RE2(OH)5NO3·nH2O is a popular building block in constructing multifunctional films. In this paper, an electrodeposition method is developed to prepare RE2(OH)5NO3·nH2O films within 10 min, avoiding the problems of long-time consumption and low yield of the available four-step method. The study of anion exchange of RE2(OH)5NO3·nH2O found that the higher concentration difference between the anion source Na2MoO4 and RE2(OH)5NO3·nH2O promotes the exchange of interlayer NO3– and ligands (hydroxyl and water) in the [RE2(OH)5(H2O)n]+ main layer with MoO42–, and forms tetragonal-structured NaRE(MoO4)2 film at pH ∼7 and 150 °C. Due to the fluorescence quenching groups in RE2(OH)5NO3·nH2O structure were exchanged by MoO42–, the intensity of Eu3+ emission (5D0 → 7F2) for NaEu(MoO4)2 film is ∼10 times that of Eu2(OH)5NO3·nH2O film under excitation at 394 nm. In addition, the films exhibit multicolor luminescence and long fluorescence lifetime by doping activators (Eu3+ and Tb3+) into the NaLa(MoO4)2 lattice and adjusting the molar ratio of activators, making them promising color-conversion films for solid-state lighting. This work offers a new concept for the design and fabrication of NaRE(MoO4)2 films, and it might be extended to other kinds of functional films.