Morphology control and luminescent properties of Eu/Tb-doped calcium carbonate nanoparticles using natural limestone

Highlights: • Calcium carbonate with different morphologies were synthesised from natural limestone. • The microstructure and photoluminescence of Eu/Tb-doped CaCO₃ were studied. • Eu- doped CaCO₃ nanorods exhibit characteristic emission of both Eu²⁺ and Eu³⁺ ions. • Tb-doped CaCO₃ nanospheres exhibit multicolor luminescence from both Eu³⁺ and Tb³⁺ ions. The Eu/Tb-doped CaCO₃ nanocrystals including nanoneedles, nanorods and nanospheres are respectively synthesized from natural limestone via carbonation technique. The doping of Eu/Tb ions can not change the polymorph, but the morphologies and sizes of synthesized CaCO₃ differ significantly depending on the species and concentrations of Eu/Tb ions. The undoped CaCO₃ nanoneedles exhibit a length of 280–920 nm and a diameter of 55–78 nm, while the 6% Eu doped CaCO₃ nanorods exhibit a length of 1.68–2.56 μm and a diameter of 560–680 nm, respectively. The introduction of Tb ions changes the CaCO₃ morphology from nanoneedle to irregular sphere and the diameter of CaCO₃: 8% Tb primary particle is around 110–200 nm. Under the excitation of 318 nm or 384 nm ultraviolet light, CaCO₃: x% Eu nanorods exhibit characteristic emission bands at 360 nm and 420 nm assigned to 4f⁶ 5 d¹(t_2g) →⁸S_7/2 transition of Eu²⁺ ions and sharp peaks at 596 nm from the ⁵D₀ →⁷F₁ transitions of Eu³⁺ ions. XPS data further confirm the coexistence of Eu²⁺ and Eu³⁺ ions. The CaCO₃: y% Tb nanospheres present intense blue-green emission at 424 nm and 545 nm under excitation at 397 nm, which are originating from both ⁵D₃→⁷F_J and ⁵D₄→⁷F_J transitions of Tb³⁺ ions. However, under 343 nm excitation, in addition to the emission of Tb³⁺, Eu³⁺ ions also present intense emissions at 572 nm, 596 nm, 624 nm and 650 nm, which may be caused by the unknown Eu -containing impurities from natural limestone. Whiteemitting and color-tunable photoluminescence performance of CaCO₃ nanostructures are realized by regulating the excitation wavelengths and doping ions.