Photoluminescence properties and thermal stability of Eu3+-activated La7Ta3W4O30 red-emitting phosphors for near-UV-excited w-LEDs

doi:10.1016/j.jre.2020.07.015

Journal of Rare Earths

Volume 39, Issue 8, August 2021, Pages 905-912

https://doi.org/10.1016/j.jre.2020.07.015 Get rights and content

Abstract

Novel trivalent europium (Eu³⁺)-activated La₇Ta₃W₄O₃₀:xEu³⁺ (x = 0.5 mol%–40 mol%) red-emitting phosphors were synthesized by means of a high-temperature solid-state reaction. The structure, morphology, photoluminescence, thermal-stability properties, lifetime, and color-rendering of the prepared phosphors were investigated in detail. The La₇Ta₃W₄O₃₀:Eu³⁺ phosphors show five emission peaks under near-ultraviolet (n-UV) at 397 nm, and these peaks are ascribed to the transitions of ⁵D₀–⁷F_j (j = 0, 1, 2, 3 and 4) by Eu³⁺ ions. The optimal doping concentration of Eu³⁺ is 20 mol%, and the critical distance of the energy transfer between the Eu³⁺ ions was calculated to be 1.768 nm. The quenching temperature (T_0.5) of La₇Ta₃W₄O₃₀:20 mol%Eu³⁺ is about 440 K. The quantum yield (QY) was measured to be 85.85%. The fabricated white-light-emitting diodes (w-LEDs) possess high color-rendering index (R_a) of 90, and high correlated color temperature (CCT) of 5810 K, respectively. The Commission Internationale de L' Eclairage (CIE) coordinates are (0.311, 0.322). Therefore, the prepared phosphor has a promising application for w-LEDs.

Graphical abstract

La₇Ta₃W₄O₃₀:20 mol%Eu³⁺ red phosphor was used to fabricate the white LED by the combination of BaMgAl₁₀O₁₇:Eu²⁺ (BAM, blue) and (Ba, Sr)₂SiO₄:Eu²⁺ (green) with a 397 nm chip, and the electroluminescence spectrum of the fabricated w-LED is exhibited. The inset shows that the packaged w-LED glows with cool bright white light. Besides, the CIE coordinates of the w-LED were evaluated to be (0.311, 0.322), which are close to the equal energy point (0.333, 0.333).

Introduction

As a new type of light source, white-light-emitting diodes (w-LEDs) revolutionized the illumination technology in the lighting industry because of its low cost, long life, small size, high efficiency, low electricity consumption and environmental friendliness.1, 2, 3, 4, 5, 6 Most of the commercial w-LEDs are composed of blue InGaN LED chip and Y₃Al₅O₁₂:Ce³⁺ yellow luminescent powder.⁷^,⁸ However, the commercial w-LEDs have met the drawbacks with poor color-rendering index and high correlated color temperature due to the shortage of red light.⁹^,¹⁰ Nitride and oxynitride red phosphors have been prepared as candidates to overcome these disadvantages because of their high thermal and chemical stabilities. However, these phosphors require critical reaction conditions, such as high temperature and pressure.¹¹ Another good way to achieve white-light emission is the RGB (red, green, and blue) model, which is a combination of red, green, and blue luminescent materials and near-ultraviolet (n-UV) LED chip. It has high luminescent efficiency and color-rendering index and adjustable emission characteristics.12, 13, 14

Trivalent europium (Eu³⁺) has been widely studied in inorganic matrix materials because of its unique f–f transition. In recent years, tungstate has attracted extensive interest due to its excellent photoelectronic properties, good thermal and chemical stability. It has potential applications in photocatalysis, negative thermal expansion materials, phosphors and lasers.15, 16, 17, 18 The tungstate compounds can adopt different lanthanide ions to achieve various emitting colors, such as NaY(WO₄)₂:Dy³⁺,¹⁹ ZnWO₄:Eu³⁺,²⁰ NaGd(WO₄)₂:Tb³⁺²¹ and LiEu (WO₄) _2−x (MoO₄)_x.²² Recently, columnar-perovskite-type La₇W₄M₃O₃₀ (M = Nb, Ta) tungstate has been found to be a member of the new structure of A₇B₇O₃₀ (A = La, B = Mo) family.²³ The structural characterization of La₇Ta₃W₄O₃₀ indicates its practicability to select La₇Ta₃W₄O₃₀ for host matrix. However, Eu³⁺-activated La₇Ta₃W₄O₃₀ phosphors have not been investigated in detail.

In our work, the synthesis and luminescent properties of La₇Ta₃W₄O₃₀:xEu³⁺ (x = 0.5 mol%–40 mol%) red-emitting phosphor under n-UV excitation were investigated. The phase purity, surface morphology, luminescence properties, thermal quenching, and CIE coordinates of La₇Ta₃W₄O₃₀:xEu³⁺ (x = 0.5 mol%–40 mol%) phosphors were studied systematically.

Section snippets

Synthesis

The La_7(1−x)Eu_7xTa₃W₄O₃₀ (x = 0.5 mol%, 1 mol%, 2 mol%, 5 mol%, 10 mol%, 15 mol%, 20 mol%, 30 mol%, and 40 mol%) samples were achieved by means of solid state reaction in air. The oxides of La₂O₃ (99.99%), Ta₂O₅ (99.5%), Eu₂O₃ (99.99%), and WO₃ (99.99%) were fully mixed and ground in an agate mortar. The mixed materials were heated in a furnace at 700 °C for 2 h. The sample was then reground and heated at 1350 °C for 10 h. After that, the furnace was naturally cooled to room temperature. The

Results and discussion

Fig. 1(a) shows the crystal structure of La₇Ta₃W₄O₃₀. It belongs to the trigonal crystal system, with R-3 (No. 148) space group. The compound has the same structure as La₇Mo₇O₃₀ and belongs to A₇B₇O₃₀ family. This type was a columnar-perovskite-type arrangement, which is formed by the accumulation of isolated hexagonal perovskite building units.²³ The parameters of the unit cell are a = b = 1.70701 nm, c = 0.68851 nm, V = 1.73745 nm³, and Z = 3. As shown in Fig. 1(b), the La(1) and La(2)

Conclusions

Novel red-emitting La₇Ta₃W₄O₃₀:xEu³⁺ (x = 0.5 mol%–40 mol%) phosphors were successfully synthesized via high-temperature solid-state reaction. The synthesized samples are all trigonal phase, and the introduction of Eu³⁺ dopant does not change their crystal phase structure. Under 397 nm excitation, La₇Ta₃W₄O₃₀:20 mol%Eu³⁺ presents five emission peaks at 579, 595, 619, 656 and 702 nm, which are attributed to ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3, 4) transitions of Eu³⁺ ions, respectively. The optimum doping

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^☆: Foundation item: Project supported by the Fundamental Research Funds for the Central Universities (2452019076), Undergraduate Innovation Fund of Northwest A&F University, China (201910712037), Hunan Provincial Key Laboratory of Xiangnan Rare-Precious Metals Compounds and Applications (2019XGJSKFJJ01), the Construction Program of the Key Discipline in Hunan Province, the Projects of the Education Department of Hunan Province (18A465), and Science and Technology Plan Project of Chenzhou City (jsyf2017014).

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Photoluminescence properties and thermal stability of Eu3+-activated La7Ta3W4O30 red-emitting phosphors for near-UV-excited w-LEDs☆

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis

Results and discussion

Conclusions

J Rare Earths

Mater Lett

Dyes Pigm

J Rare Earths

Electrochim Acta

J Rare Earths

Electrochim Acta

Tetrahedron Lett

J Rare Earths

Electrochim Acta

Mater Res Bull

J Lumin

J Solid State Chem

Mater Res Bull

Phys Lett A

Spectrochim Acta A

Ceram Int

J Lumin

J Lumin

J Alloys Compd

Mater Lett

Recent developments in the new inorganic solid-state LED phosphors

Dalton Trans

Photoluminescence properties and thermal stability of Eu³⁺-activated La₇Ta₃W₄O₃₀ red-emitting phosphors for near-UV-excited w-LEDs☆