Malate-aided selective crystallization and luminescence comparison of tetragonal and monoclinic LaVO₄:Eu nanocrystals

With malate (Mal2-) as a new type of chelate, tetragonal (t-) and monoclinic (m-) structured LaVO4:Eu crystals (∼10-60 nm) were selectively crystallized as nanosquares and nanorods via a hydrothermal reaction at 200 °C for 24 h. The effects of the Mal2-:(La,Eu)3+ molar ratio, solution pH and Eu3+ content on the phase structure and crystal morphology were systematically investigated and elucidated. The competition between OH- and Mal2- toward rare earth ions was discussed to play a critical role in phase selection, and the t-phase can only be fabricated at pH ∼ 6-8 with the assistance of Mal2-. The optimal Eu3+ content for luminescence was determined to be ∼5 at% under the VO43- → Eu3+ energy transfer mechanism. Experimental comparison showed that t-(La0.95Eu0.05)VO4 (λex = 275 nm, λem = 620 nm) emits ∼5.3 times as strong as m-(La0.95Eu0.05)VO4 does (λex = 313 nm, λem = 616 nm), while theoretical analysis revealed that the 5D0 level of Eu3+ has a quantum efficiency of ∼80% for the former and ∼70% for the latter. Besides, the t- and m-(La0.95Eu0.05)VO4 nanocrystal phosphors were analyzed to have fluorescence lifetimes of ∼1.53 ± 0.01 and 2.28 ± 0.01 ms for their 620 and 616 nm red emissions, respectively.