Facile morphology-controllable hydrothermal synthesis and color tunable luminescence properties of NaGd(MoO₄)₂: Eu³⁺,Tb³⁺ microcrystals

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Well-crystallized and uniform NaGd(MoO₄)₂ microcrystals with morphologies of bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates were selectively synthesized via a facile hydrothermal method without any additives under mild conditions. The effects of Na₂MoO₄/Gd(NO₃)₃ molar ratios and pH values of precursor solutions on the phase and morphology of the as-synthesized microcrystals were systematically investigated. The molar ratios and pH values played key roles in the selective synthesis of pure phase NaGd(MoO₄)₂ microcrystals with regular morphology. With increasing Na₂MoO₄/Gd(NO₃)₃ molar ratios in the appropriate pH range, the morphology of the products changed from bipyramids, truncated bipyramids, quasi-cubes to tetragonal plates, namely, the morphological truncation degree increased gradually. NaGd(MoO₄)₂ tetragonal microplates could be synthesized at the molar ratio of 9 : 1. A possible morphological formation mechanism of NaGd(MoO₄)₂ tetragonal microplates was proposed, i.e. nucleation-Ostwald ripening growth process. Color tunable photoluminescence properties of NaGd(MoO₄)₂:Eu³⁺,Tb³⁺ microcrystals were studied in detail. Among the different morphologies of NaGd(MoO₄)₂:5% Eu³⁺ microcrystals, tetragonal microplates had a broadened, red-shifted and enhanced charge transfer band in the excitation spectrum. What’s more, the full-width at half-maximum for the charge transfer band of the tetragonal plates was highest (58 nm), which favored efficient excitation and absorption in the ultraviolet region. The introduction of a small amount of Tb³⁺ into NaGd(MoO₄)₂ microplates doped with Eu³⁺ would enhance the ⁵D₄ → ⁷F_J transition of Eu³⁺ at 616 nm due to the energy transfer process of cross-relaxation from Tb³⁺ to Eu³⁺. Multicolor tunable luminescence from deep red, red, orange, yellow to green under 291 nm ultraviolet excitation and from red, reddish orange to pink under 380 nm near-ultraviolet excitation could be obtained in NaGd(MoO₄)₂ tetragonal microplates by simply adjusting the doping concentrations of Eu³⁺ and Tb³⁺, suggesting NaGd(MoO₄)₂:Eu³⁺, Tb³⁺ microcrystals might have practical application in optoelectronic devices, such as light emitting diodes and color display systems. This facile morphology-controlled hydrothermal synthesis strategy was simple, low-cost and environment-friendly, and might be extended to other inorganic materials.