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

Abstract

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-(La_0.95Eu_0.05)VO₄ (λ_ex = 275 nm, λ_em = 620 nm) emits ∼5.3 times as strong as m-(La_0.95Eu_0.05)VO₄ does (λ_ex = 313 nm, λ_em = 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-(La_0.95Eu_0.05)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.