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

Anming Li,Dekang Xu,Hao Lin,Shenghong Yang,Yuanzhi Shao,Yueli Zhang,Zhenqiang Chen

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 5D0 → 7FJ 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.