Coprecipitation synthesis, structure and photoluminescence properties of Eu3+-doped sodium barium borate

doi:10.1016/j.jallcom.2014.08.050

Journal of Alloys and Compounds

Volume 617, 25 December 2014, Pages 946-951

https://doi.org/10.1016/j.jallcom.2014.08.050 Get rights and content

Highlights

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Micro-sized red-emitting phosphor of NaBaBO₃:Eu³⁺ was developed.
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Regular plate-like particles are obtained by the modified coprecipitation synthesis.
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High absolute quantum efficiency of 53% can be reached.
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Excellent thermal stability was realized with thermal activation energy of 0.213 eV.

Abstract

Micro-sized phosphors of Eu³⁺-doped NaBaBO₃ (0.5–12 mol%) were prepared by the modified co-precipitation synthesis. The samples were characterized by scanning electron micrograph (SEM) images and electron dispersive X-ray (EDX). The polycrystalline samples are well developed to the regular plate-like particles with the uniform morphology. The single crystal-phase NaBaBO₃ of the Eu³⁺-doped sample was confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The optical properties are evaluated by photoluminescence (PL) excitation and emission spectra, and decay curves. The absolute luminescence quantum efficiency (QE) and the activation energy (ΔE) are reported. Eu³⁺-doped NaBaBO₃ presents a bright red-emitting color under the excitation of near-UV light around 400 nm. The maximum absolute QE of NaBaBO₃:Eu³⁺ is estimated to be 53% at 300 K. The phosphor shows an excellent thermal stability on temperature quenching effects. The results were discussed on the relationship between the structure and the luminescence properties.

Introduction

Borates with MNBO₃ formula (M and N are mono- and divalent cations, respectively) form a large family of inorganic compounds. Its crystal structure shows a strong dependence on the relative size of M to N ions [1], [2], [3]. For example, LiMgBO₃ with small M and N cations crystallizes in monoclinic form C2/c. [1]. LiCaBO₃ belongs to an orthorhombic system with space group of Pbca [4]. While the corresponding borates LiNBO₃ (N = Sr, Ba) with big N cation shows a typical monoclinic space group P2₁/n. Borates with NaNBO₃ can crystallize in different crystal structures depending on the size of N in the lattices such as orthorhombic space group of Pmmn (N = Ca [1]), monoclinic P21/c (N = Sr [5], [6]) and monoclinic C12/m1 (N = Ba [3], [7]).

In MNBO₃ borates, there are different possibilities for the coordination number (CN) of M and N ions. The rich structure types make it possible to tailor optical properties of rare earth ions (RE) activated MNBO₃. This is due to the fact that luminescence of RE ions usually shows a strong dependence on the microstructures. In recent years, luminescence materials of RE-doped MNBO₃ have been extensively investigated because of its properties such as the large band gap, moderate phonon energy, high thermal and chemical stability, and exceptional optical damage threshold. Phosphors based on MNBO₃ have been widely reported as potential materials in solid state lighting [8], [9]. NaCaBO₃:Ce³⁺ [10] and NaSrBO₃:Ce³⁺ [11] can be efficiently excited by near ultraviolet (NUV) light, and emits bright broadband blue light. In MNBO₃ host the energy transfers can take place from Ce³⁺ to Mn²⁺ ions in LiCaBO₃:Ce³⁺/Mn²⁺ [12], NaCaBO₃:Ce³⁺/Mn²⁺ [13], NaCaBO₃:Ce³⁺/Tb³⁺/Mn²⁺ [14], and NaSrBO₃:Ce³⁺/Mn²⁺ [15]. Orange–red luminescence is observed in LiSrBO₃:Sm³⁺ [16], NaCaBO₃:Sm³⁺ [17], and NaSrBO₃:Sm³⁺ [18] phosphors. Luminescence properties of NaSrBO₃:Tb³⁺ [19] KCaBO₃:Er³⁺/Yb³⁺ [20], KCaBO_3:Dy³⁺/Eu³⁺ [21] and LiMgBO₃:Dy³⁺ [22] have been reported to have possible applications for lighting or radiation dosimetric measurement. Eu³⁺ ion with the 4f⁶ electronic configuration usually presents groups of sharp lines assigned to the transitions of ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3, 4) levels. The ⁵D₀ → ⁷F₂ transition is electric dipole-allowed hypersensitive in the local crystal field [23]. The local structures such as CNs and defects influence significantly on the luminescence of Eu³⁺ ions in a host [24].

As the most important emitter in the red region of the visible spectrum, Eu³⁺ has gained considerable interest and been widely investigated to be the efficient red-emitting phosphors [25], [26], [27], [28], [29], [30], [31]. In MNBO₃ borates, the luminescence properties of Eu³⁺ ions doped in LiMgBO₃ [32] NaSrBO₃ [6], [33] LiCaBO₃ [34] have been reported. There have been reported the red-emitting phosphors prepared by solid state reaction or combustion synthesis methods together with luminescence properties for possible applications in near-UV excited white-LEDs as in the references.

In this work, Eu³⁺ ion is selected as an activator doped in NaBaBO₃ host. A modified co-precipitation is applied to prepare this phosphor. The crystalline structure was examined with X-ray powder diffraction (XRD) and refinements. The thermal stability of the green luminescence is evaluated by the luminescence decays as a function of temperature. The absolute luminescence quantum efficiency (QE) and the activation energy (ΔE) are reported.

Section snippets

Experimental

MNBO₃:xEu³⁺ (x = 0.05–0.12) phosphors were synthesized by a modified co-precipitation method at low temperatures. The starting materials were NaNO₃, Ba(NO₃)₂, Eu(NO₃)₃·6H₂O and HBO₃ (analytical grade, Aldrich Co., Ltd.). The synthesis procedure can be introduced as of four steps: (1) The stoichiometric amount of NaNO₃, Ba(NO₃)₂, Eu(NO₃)₃·6H₂O were dissolved in water forming a clear solution labeled as “A”. The solution “B” was prepared by dissolving HBO₃ in ammonia under constant stirring. (2)

The crystal phase formation

Fig. 1 shows the representative experimental and calculated results from the XRD structural refinement of NaBaBO₃:0.09Eu³⁺ carried out by using GSAS program [35]. The refined crystallographic data and the atomic coordinate parameters of 9.0 mol% Eu³⁺-doped NaBaBO₃ are listed in Table 1, Table 2, respectively. The results indicate that the diffraction peaks of the samples are in good agreement with the ICSD standard pattern, indicating that the obtained NaBaBO₃ samples are single phased. No

Conclusion

Eu³⁺-doped NaBaBO₃ (0.5–12 mol%) were prepared by the modified coprecipitation synthesis. XRD structural refinement was completed in Eu³⁺-doped NaBaBO₃. NaBaBO₃:Eu³⁺ crystallizes in micro-sized particles with tabular shapes. The phosphor shows the efficient excitation around 400 nm (f–f transition of Eu³⁺ ions), which is much stronger than the absorption in the CT region. Under the excitation of near UV light, NaBaBO₃:Eu³⁺ presents bright red color with CIE values of (x = 0.663, y = 0.341) and the

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2009154). This work was also supported by Jiangsu Provincial Natural Science Foundation of China (No. BK2012635).

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When the Eu3+ ions doped into the Na3Ba2B6O12F lattice, there are three cationic sites Ba2+, Na2+ and B3+ for them to substitute for. However, taking their respective ionic radii and coordination number into consideration, the Ba2+ sites will be replaced by Eu3+ ions in the Na3Ba2B6O12F lattice [26]. When Eu3+ ions replaced the Ba2+ sites, some negative charge defects are produced in order to keep charge neutral.
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As can be seen, the as-synthesized phosphors had the same structure with β-BBO (JCPDS No. 80-1489), only all of peaks had redshift. As we know that the covalent radius of Ba2+, Eu3+ and Tb3+ are 1.47 Å, 1.12 Å and 1.095 Å [19,20], respectively. The lattice deformation will occur when the smaller RE3+ (such as Eu3+ or Tb3+) is doped in β-BBO, this results in all of the redshifted diffraction peaks accordingly, and the amount of redshift increased with the RE3+ doped level.
Vacuum ultraviolet (VUV) phosphors are indispensable material to the development of a new generation white-LED, visual display and etc. Searching an ideal host of phosphors from NLO materials is one of effective approaches. Employing excellent NLO crystal β-BaB₂O₄ (β-BBO) as matrix material, we have synthesized lanthanide ions (Eu³⁺ or Tb³⁺)-doped β-BBO phosphors by solid-state reaction, and have investigated their luminescent properties, the optimal doping amount of RE³⁺ (Eu³⁺ and Tb³⁺) by ultraviolet wave excitation, decay curves and lifetimes, respectively. For red phosphor, the measured dominating emission peak (610 nm) was attributed to ⁵D₀ $\to$ ⁷F₂ transition of Eu³⁺. For green phosphor, β-BBO:Tb³⁺ emits bright green at 542 nm with 278 nm excited light. The concentration quenching curve showed that the optimal concentration of Eu³⁺ and Tb³⁺ in β-BBO were about 22.00 mol% and 10.00 mol% (theoretical value), corresponding to the effective value of 20.60 mol% and 9.20 mol% measured by ICP, respectively. The variety of lattice parameters of β-BBO:RE³⁺ (RE³⁺ = Eu³⁺, Tb³⁺), by contrast to β-BBO, has been calculated according to the XRD data. The luminescence mechanism of RE³⁺-doped β-BBO (Eu³⁺ and Tb³⁺) were analyzed. Chromaticity coordinates, color temperature of β-BBO:RE³⁺ (Eu³⁺ and Tb³⁺) were also calculated separately according to the emission spectra, the value of CIE coordinates indicated that the synthesized phosphors have good color purity. The impact of flux-grown synthetic method on the luminescence intensity of as-prepared β-BBO:RE³⁺ (RE³⁺ = Eu³⁺, Tb³⁺) was also investigated.

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Coprecipitation synthesis, structure and photoluminescence properties of Eu3+-doped sodium barium borate

Highlights

Abstract

Introduction

Section snippets

Experimental

The crystal phase formation

Conclusion

Acknowledgments

J. Solid State Chem.

Opt. Laser Technol.

Mater. Res. Bull.

J. Lumin.

Mater. Res. Bull.

J. Alloys. Comp.

J. Rare Earths

Chem. Phys. Lett.

Opt.-Int. J. Light Electr. Opt.

J. Phys. Chem. Solids

Handbook Phys. Chem. Rare Earths

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Lumin.

J. Lumin.

J. Rare Earths

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Alloys. Comp.

J. Solid State Chem.

Coprecipitation synthesis, structure and photoluminescence properties of Eu³⁺-doped sodium barium borate