Preparation And Luminescence Properties Of Eu³⁺ Doped Cubic Nano-CaCO₃

In the past decades, inorganic materials doped with rare earth ions have drawn extensive attentions owing to their potential application in the fields of light phosphor powder, flat panel displays, biology and so on. As a host material, calcium carbonate was utilized to synthesize luminescent materials for their excellent chemical and thermal stability, as well as good absorption for UV light and low cost. Currently, the synthesis of Eu³⁺-doped CaCO₃ has become a research hotspot owing to their excellent properties and their exhibiting of good luminescence properties without any high temperature calcination treatment.In this work, Eu³⁺ doped, Eu³⁺/Gd³⁺ co-doped nano-cubic CaCO₃ and Eu³⁺ doped with hydrophobic nano-cubic CaCO₃ have been successfully synthesized by carbonization method. Some modern test methods were utilized to characterize the samples. The structural and photoluminescence properties of cubic CaCO₃ doped with rare earth ions were systematicly investigated by photoluminescence spectra（PL）. The doped sites of Eu³⁺ in CaCO₃ crystalline were identified by the site-selective spectroscopy.The main results of the research work are as follows:1. CaCO₃:x Eu³⁺cubic nanoparticles were synthesized by carbonization method. The results show that the average size of the Eu³⁺-doped CaCO₃ nanoparticles decreases greatly with the increase of the dopant concentration until the 2.0 mol% Eu³⁺ dosage. The reduction in the crystallite size is attributed to two reasons. One is that Eu³⁺ ions sit in the lattice of the CaCO₃ and act like nuclei/seeds for CaCO₃ growth. The increase in the nucleation sites may lead to the crystallite size decrease. The other reason is as follows: Each substitution of Ca2+ by Eu³⁺ in CaCO₃ requires extra OH- for charge compensation, and the introduction of such OH- ions into the grain surface may induce transient electric dipoles with their negative poles outward. These transient electric dipoles hinder the diffusion of CO32- ions from the solution to the grain surface, thus, retarding the growth of CaCO₃. Under UV light excitation, CaCO₃:Eu³⁺ nanoparticles exhibit strong red emission and the optimum concentration of Eu³⁺ ions is 2.0 mol%. Furthermore, site-selective spectroscopy shows that the Eu³⁺ ions are located at two lattice sites: one is close to the surface of CaCO₃; the other one occupies a site with a higher symmetry, which might be located at the crystal lattice of CaCO₃.2. Hydrophobic and enhanced red-emitting CaCO₃:Eu³⁺ phosphors were in situ prepared via carbonization method in the presence of sodium oleate. The results show that the introduction of sodium oleate can endow the products with hydrophobic surface. When sodium oleate was added to 5.0 wt.%, the relative contact angle reached 99.99°.Under UV light excitation, the modified CaCO₃:Eu³⁺ nanoparticle with sodium oleate exhibit enhanced red emission at 613 nm corresponding to the 5D0â†'7F2 transition of the Eu³⁺ ions in the presence of sodium oleate. The enhancement is attributed to two functions of sodium oleate. One function is that sodium oleate can avoid OH ligands and/or crystal water molecules attached on the surface of CaCO₃ particles; the other function is that Na+ ions can supply effective charge compensation.3. CaCO₃:Eu³⁺ cubic nanoparticles with good luminescence properties were prepared by carbonization method. The emission spectra results show that the optimal doping composition was CaCO₃:2.0mol%Gd³⁺, 2.0mol%Eu³⁺. The energy transfer from Gd³⁺ ions to Eu³⁺ ions had been studied. The mechanism of energy transfer from Gd³⁺ to Eu³⁺ is a resonant transfer, in which electric dipole-dipole interaction plays a leading role. Furthermore, the luminescent decay curves also demonstrate the occurrence of efficient energy from Gd³⁺ to Eu³⁺. The energy transfer efficiency（ηT） reaches 74%, when the concentration of Eu³⁺ was added to 2.0 mol%. It is concluded that the introduction of Na+ can improve the luminescent intensity of Eu³⁺.