Selective synthesis and luminescent properties of monazite- and zircon-type LaVO4:Ln (Ln = Eu, Sm, and Dy) nanocrystals

Pure monazite-type monoclinic (m-) and zircon-type tetragpnal (t-) phased LaVO4:Ln (Ln = Eu, Sm, and Dy) nanocrystals could be obtained by a facile hydrothermal method in a controllable way without the presence of catalysts or templates. High-resolution transmission electron micrographs of the as-obtained samples showed high crystallinity. It was found that tuning the pH of the growth solution was a crucial step for the selective control of the structure and morphology of the LaVO4:Ln nanocrystals. When the pH value was lower than 3.5, only irregular nanoparticles of m-LaVO4:Ln were obtained. Increasing the pH value can induce the polymorph transformation from monazite to zircon type. As the pH value ranged from 4.5 to 6.0, uniform t-LaVO4:Ln nanorods with a diameter of 15 nm and length of about 100 nm were obtained. When the pH value was further increased to higher than 6.0, the as-obtained t-LaVO4:Ln nanocrystals exhibited particle-like morphology, with an average diameter of about 20 nm. The possible mechanism responsible for the phase control and dnisotropic morphology evolution of the LaVO4:Ln nanocrystals was discussed. Their luminescent properties were studied and compared. The results indicated that the challenging transformation from monazite to the metastable zircon structure for LaVO4:Ln resulted in a remarkable improvement of the luminescent properties. This improvement was rationally analyzed, and the impact of the structure and shape on luminescent properties was explored. It suggests that we could obtain the function-improved materials by tailoring the structure and shape of the lanthanide-doped LaVO4 nanostructures.