Concentration Dependence of Luminescent Properties for Sr_𝟐TiO_𝟒:Eu^𝟑+ Red Phosphor and Its Charge Compensation

Abstract

Sr₂TiO₄:Eu³⁺ phosphors using M⁺ (M = Li⁺, Na⁺, and K⁺) as charge compensators were prepared by the solid-state reaction. The powders were investigated by powder X-ray diffraction (XRD) and photoluminescence spectra (PL) to study the phase composition, structure, and luminescent properties. The results showed that Li⁺ ion was the best charge compensator. The phase was Sr₂TiO₄ when the doping concentration was small (𝑥≤10.0%). When 𝑥 reached 15.0%, the phase turned into Sr₃Ti₃O₇ because of the structure damage. The phosphor could be effectively excited by ultraviolet (365, 395 nm) and blue light (465 nm), and thenitemitted intense red light that peaked at around 620 nm (⁵D₀→⁷F₂). In addition, the emission of 700 nm (⁵D₀→⁷F₄) enhanced the red light color purity. The CIE chromaticity coordinates of samples with the higher red emission were between (0.650, 0.344) and (0.635, 0.352). Doped layered titanate Sr₂TiO₄:Eu³⁺ is a promising candidate red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.

1. Introduction

White light-emitting diodes (LEDs) are considered to be next-generation lighting devices. They have many advantages such as energy saving, environment friendliness, and small size [1–3]. Phosphor conversion method is a principal method in all the technologies to achieve white light owing to its easy achievement, high efficiency, and low cost.

The combination of blue chip and yellow phosphors has already been developed and is commercially available, but the lack of red emitting makes the color rendering index (CRI) lower [4]. The other tricolor white LEDs consisting of red, green, and blue (RGB) phosphors excited with a UV chip emitting 400 nm also have challenges. The luminescence intensity of commercial red phosphors is much weaker than green and blue phosphors. For excellent color render index, both methods need efficient red phosphors that should have the excitation wavelength matching with the emission of the blue LEDs (𝜆_em = 440–470 nm) or the UV LEDs (𝜆_em = 350–410 nm). Therefore, the development of a red phosphor with high luminance and satisfactory chemical stability is a key technology for achieving warm white LEDs.

As a result, a kind of red phosphor with perovskite structure attracts much attention [5–7]. Perovskite structure MTiO₃:Pr³⁺ (M = Ca, Sr, and Ba) phosphors emitted intense red light at 610 nm when excited by UV-light [8, 9]. Its intensity was greatly enhanced by adding charge compensation agents such as Al³⁺, Li⁺, and Na⁺ [10–12]. The optimized excitation wavelength of CaTiO₃:Eu³⁺ was 400 nm, suitable for near-ultraviolet (N-UV) LED chip and emitted red light at 618 nm [13, 14]. However, the intensity is not so high that more-efficient red phosphors are needed to achieve an acceptable efficiency for white LEDs. Layered perovskite compounds have longer distance between layers so that they have bigger doping concentration. Therefore, layered perovskite compounds are good host materials for phosphors. Sr₂TiO₄ is a typical layered perovskite compound. According to these, trivalent Europium ion-activated Sr₂TiO₄ phosphor is prepared by solid-state reaction, and its luminescence properties are investigated to see whether it has the potential to be a red phosphor for N-UV or blue light LED chip.

2. Experimental

2.1. Sample Preparation

All powder samples were synthesized through the solid-state reaction technique. High-purity SrCO₃, TiO₂ (analytical grade) and Eu₂O₃ (>99.99%) were mixed thoroughly in alcohol by ball milling and then dried. Appropriate amounts of Li₂CO₃, Na₂CO₃, or K₂CO₃ (analytical grade) were added as the charge compensators. The synthesis was performed at 1100°C for 2 h under air atmosphere in electric tube furnace. Series of (Sr_1−2xEu_xM_x)₂TiO₄ (M = Li⁺, Na⁺, K⁺; x (mol%) = 5.0, 10.0, 15.0, 20.0, 25.0) powders were prepared. SrCO₃ and Eu₂O₃ come from Sinopharm Chemical Reagent Co., Ltd., land Li₂CO₃, Na₂CO₃, K₂CO₃ and TiO₂ come from Shanghai Lingfeng Chemical Reagent CO., LTD.

2.2. Characterization

The crystalline phases of synthesized powders were determined by X-ray diffraction (XRD, D/Max2500, Rigaku, Japan) using Cu Kα radiation (𝜆=1.5406 Å) in the range of 5^∘–80^∘ with a step size of 0.02°. The photoluminescence spectra of the phosphors were measured using a fluorescent spectrophotometer (FL3-221, HOROBA, Jobin Yvon, France) at room temperature.

3. Results and Discussion

3.1. The Choice of Charge Compensators

Eu³⁺ ions are expected to replace Sr²⁺ ions. It would be difficult to keep charge balance in the lattice. Therefore, Eu³⁺ ions may not be fully introduced into Sr²⁺ sites in order to keep charge balance. Eu³⁺ may exist in Eu₂O₃ state, and it would lead to the decrease of emission intensity. This problem can be solved by adding charge compensators. Li⁺, Na⁺, and K⁺ ions are always chosen as charge compensators for phosphors [15, 16]. Because the radii of Li⁺, Na⁺, and K⁺ are small which are easy to enter into lattice, and they are all of +1 valence which is convenient for charge compensation. Therefore, Li⁺, Na⁺, and K⁺ were added in the same molar weight as Eu³⁺ to act as the charge compensators to improve the luminescence intensity of Sr₂TiO₄:Eu³⁺.

Figure 1 shows the excitation and emission spectra of (Sr_0.8Eu_0.1M_0.1)₂TiO₄ phosphor with different charge compensators. The shape and positions were similar in the PL and PLE spectra for all the samples. Excitation and emission intensities were enhanced obviously after adding charge compensators. Before adding charge compensators, Eu³⁺ could not substitute Sr²⁺ sites totally in order to maintain chemically neutral Sr₂TiO₄:Eu³⁺ phosphor. Some Eu³⁺ could not act as activated ions, and the impurity phase could interdict the energy transfer between matrix and activated ions and restrain the Sr₂TiO₄ grains growth during the sintering process [17]. After adding charge compensators, Eu³⁺  ions were able to be fully introduced into Sr²⁺ sites, and charge compensators and vacancy could substitute Sr²⁺ sites so that SrO and Eu₂O₃ could react with TiO₂ to a great extent. In addition, Li₂CO₃, Na₂CO₃, and K₂CO₃ can act as flux agents to promote the formation of luminescence materials polycrystal. Compared with Na⁺ and K⁺, the intensity was enhanced greatly when adding Li⁺ as charge compensator. These phenomena are assigned to the fact that the ionic radius of Li⁺ (0.92 Å) to the one of Sr²⁺ (1.31 Å) is the smallest, the one of Na⁺ (1.24 Å) being second, and the one of K⁺ (1.55 Å) is the biggest so that it develops the distortion grade different, which impacts on the luminous properties and crystal structures [18]. Thus, Li⁺ is selected as charge compensator in the following experiments.

Figure 1 

PL (𝜆ex=465 nm) and PLE (𝜆em=615 nm) spectra of (Sr0.8Eu0.1 M0.1)2TiO4 phosphor with different charge compensation agents: (a) M = Li (b) M = Na, (c) M = K, and (d) without agents.

3.2. XRD of (Sr_1−2xEu_xLi_x)₂TiO₄ (x (mol%) = 0.0, 5.0, 10.0, 15.0, 20.0) Powders

Figure 2 shows the XRD patterns of Eu³⁺ ions doped Sr₂TiO₄ phosphors with different Li⁺ concentration as charge compensator. At first, when the doping concentration was small (𝑥=5.0%), all the peaks were indexed by Sr₂TiO₄ without any impurity. After 𝑥 reached 10.0%, there appeared some Sr₃Ti₂O₇ peaks, but Sr₂TiO₄ was the major phase. When 𝑥=15.0%, Sr₃Ti₂O₇ turned to be the major phase, because the structure of Sr₂TiO₄ was damaged with the increase of Eu³⁺ and Li⁺. Until 𝑥 came to 20.0%, the phase composition consisted of Sr₃Ti₂O₇ and some impurities, because the content of Sr²⁺ was too little and the radius of Li⁺ was too small to support the whole structure. After doping Eu³⁺, peaks shifted to higher 2θ values relatively, which indicated that Eu³⁺ had occupied the sites of Sr²⁺, because the radius of Eu³⁺ is 1.12 Å (CN = 9), smaller than that of Sr²⁺ (1.31 Å, CN = 9). When Eu³⁺ occupied the site of Sr²⁺, the lattice would shrink so that the diffraction peaks would shift to high diffraction angles compared to pure Sr₂TiO₄ and Sr₃Ti₂O₇ [19].

Figure 2 

XRD patterns of (Sr1−2xEuxLix)2TiO4 powders (x (mol%) = 0.0, 5.0, 10.0, 15.0, 20.0).

3.3. Luminescent Properties of (Sr_1−xEu_xLi_x)₂TiO₄ (x (mol%) = 5.0, 10.0, 15.0, 20.0) Phosphors

Figure 3(a) presents the excitation spectra of (Sr_1−xEu_xLi_x)₂TiO₄ under different Eu³⁺ concentrations (𝜆_em = 615 nm). The excitation spectra consisted of a wide excitation band from 325 to 425 nm and some sharp line peaks of characteristic transitions of Eu³⁺. The broad band was a charge transfer band (CTB) which was caused by several charge transition. Figure 3(b) shows the results of Gaussian peak fitting of the PLE spectra of (Sr_0.7Eu_0.15Li_0.15)₂TiO₄. Peak 1 (350 nm) was ascribed to the O→Ti of phase Sr₃Ti₂O₇, peak 2 (380 nm) was ascribed to the O→Ti of phase Sr₂TiO₄, and peak 3 (362 nm) was ascribed to the O→Eu. The sum of deconvoluted curves (red dash line) was almost fitted with observed line (black solid line). The other intense 395, 465 and weak 415 nm excitation peaks related the intra-4f transitions from ground sate ⁷F₀ to ⁵L₆, ⁵D₂, and ⁵D₃, respectively. The shape and position were similar except that the intensity of absorption varied with the increase of Eu³⁺ concentration. What interests us is that the intensity of CTB is stronger than that of intra-4f transitions at lower Eu³⁺ concentration (𝑥≤10.0%), and it mainly performs wide band absorption of matrix. Then the intensity of CTB decreases with the increase of Eu³⁺ doping concentration. However, characteristic sharp line spectra increase continuously until 𝑥=15.0%, and it performs 395 and 465 nm intense linear excitation peaks. With the increase of Eu³⁺ and Li⁺, the phase composition changes from Sr₂TiO₄ to Sr₃Ti₂O₇ to some impurities, and impurity is bad for luminescence intensity. However, both intensity of CTB and intra-4f transitions are high when Eu³⁺ doped in layered titanate, and there is reason to believe that Sr₂TiO₄:Eu³⁺ and even Sr₃Ti₂O₇:Eu³⁺ are promising red phosphors for white LEDs.

(a)

(b)

Figure 3 

(a) Excitation spectra (𝜆em=615 nm) of (Sr1−2xEuxLix)2TiO4: (a) 𝑥=5.0%, (b) 𝑥=10.0%, (c) 𝑥=15.0% (d) 𝑥=20.0%, (e) 𝑥=25.0%, and (b) Gaussian peak fitting of the (Sr0.7Eu0.15Li0.15)2TiO4 phosphor powder.

Figures 4(a), 4(b), and 4(c) are the emission spectra of (Sr_1−xEu_xLi_x)₂TiO₄ excited by 365, 395, and 465 nm, respectively. The shape and position of the emission spectra were similar, and they all emitted dominated intense 620 nm (⁵D₀→⁷F₂) red light and weak 578 nm (⁵D₀→⁷F₁), 700 nm (⁵D₀→⁷F₄) light. ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄ are electric dipole transitions of Eu³⁺, which are very sensitive to the local environment around the Eu³⁺, and depend on the symmetry of the crystal field. As we know, the transitions are forbidden when Eu³⁺ occupies centrosymmetric sites. The structure of Sr₂TiO₄ is tetragonal K₂NiF₄ type, and perovskite layers are interleaved with SrO layers. In Sr₂TiO₄, all Sr ions occupy positions between the perovskite layers (9-oxygen-ion-coordinated sites with 𝐶_4v symmetry) [20]. Therefore, it indicates that Eu³⁺ has occupied the noncentersymmetrical sites of Sr²⁺. Compared with usual luminescence of Eu³⁺ (594, 615 nm) [21], the luminescence of Eu³⁺ in layered perovskite has some red shift which performs better red light color purity and higher luminescent intensity. In addition, the emission of 700 nm in some degree enhances red light color purity, too. Those are more beneficial to make it become a red light compensation material for white LEDs.

(a)

(b)

(c)

Figure 4 

Emission spectra (a) 𝜆ex=365 nm, (b) 𝜆ex=395 nm, and (c) 𝜆ex=465 nm of (Sr1−xEuxLix)2TiO4 (a) 𝑥=5.0%, (b) 𝑥=10.0%, (c) 𝑥=15.0%, (d) 𝑥=20.0%, (e) 𝑥=25.0%.

The emission intensity was found to increase with the increase of Eu³⁺ concentration up to 10.0 mol% (𝜆_ex=365 nm). While excited at 395 and 465 nm, the best doping concentration was 15.0 mol%. Because, when 𝑥=10.0%, the phase is Sr₂TiO₄ and TiO₄⁴⁻ group has strong absorption at UV light region [22], it has the strongest luminescence intensity when excited at 365 nm. However, when 𝑥=15.0%, the phase is Sr₃Ti₂O₇. The crystal system of Sr₃Ti₂O₇ is tetragonal, and double perovskite layers are interleaved with SrO layers. In Sr₃Ti₂O₇, some Sr ions occupy positions between the perovskite layer (9-oxygen-ion-coordinated sites with 𝐶_4v symmetry), and the other Sr ions occupy positions in the center of the perovskite layer (12-oxygen-ion-coordinated sites with D_4h symmetry) [20]. In spite of the 12-oxygen-ion-coordinated sites in Sr₃Ti₂O₇, 9-oxygen-ion-coordinated sites are vast majority; therefore, it emits characteristic intense red light of Eu³⁺ which is sensitive to the surrounding symmetry of the crystal field. Usually, Eu³⁺ doping concentration is less than 5.0% in normal phosphor. However, the doping concentration of Eu³⁺ is up to or even over 10.0% in Sr₂TiO₄ and Sr₃Ti₂O₇ host. The main reason is that layered perovskite has SrO layers interleaved in perovskite layers, and it makes bigger space between layers so that they have bigger doping concentration which leads to the higher luminescence intensity. In conclusion, layered titanate is a good matrix for phosphors, and Eu³⁺ doped layered titanate is a good red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.

The samples that are emitting most intense red light respectively excited at 365, 395, and 465 nm are chosen to calculate their CIE chromaticity coordinates. The corresponding CIE chromaticity coordinates using symbol “𝑥” to indicate their positions are shown in Figure 5, and they change between (0.650, 0.344) and (0.635, 0.352), which is due to the variability of the relative intensities of 620 nm and 700 nm mainly. They are all close to coordinates of the “ideal red” which is (0.67, 0.33) [23]. In addition, their CIE chromaticity coordinates are rather close the edge of CIE diagram, indicating that this kind of phosphors shows better color purity in solid-state lighting.

Figure 5 

The CIE chromaticity coordinates of red phosphors: (1) (Sr0.8Eu0.1Li0.1)2TiO4 (𝜆ex = 365 nm), (2) (Sr0.7Eu0.15Li0.15)2TiO4 (𝜆ex = 395 nm), (3) (Sr0.7Eu0.15Li0.15)2TiO4 (𝜆ex = 465 nm).

3.4. SEM of (Sr_1−xEu_xLi_x)₂TiO₄ (x (mol%) = 5.0, 10.0, 15.0, 20.0) Phosphor Powders

It is well known that crystallinity and surface morphology of phosphors has a strong effect on the luminescence properties. Figure 6 shows the SEM morphology of (Sr_1−xEu_xLi_x)₂TiO₄ with different Eu³⁺ concentration. The particles enlarged with the increase of Eu³⁺ and Li⁺ concentration, and all presented irregular polygon. The particle size was about 1 μm when 𝑥=5.0% and kept around 5 μm when 𝑥 reached 10.0%, 15.0%, and 20.0% which corresponded with the requirements of particle size to phosphor. It has been established that Li⁺ was conducive to the formation of crystalline phase, and it is mainly because of the function of Li₂CO₃ flux [24]. However, the special doping concentration of active Eu³⁺ also has some influence on the particle size [25].

(a)

(b)

(c)

(d)

Figure 6 

SEM micrographs of (Sr1−xEuxLix)2TiO4 phosphor powders: (a) 𝑥=5.0%, (b) 𝑥=10.0%, (c) 𝑥=15.0%, and (d) 𝑥=20.0%.

4. Conclusions

In this paper, Sr₂TiO₄:Eu³⁺, M⁺ (M = Li⁺, Na⁺, and K⁺) phosphors were synthesized through the solid-state reaction. M⁺ as charge compensators led to the increase of emission intensity, and Li⁺ was confirmed to be the best charge compensator. The excitation spectra consisted of a wide excitation band and some sharp line peaks of characteristic transitions of Eu³⁺ when Eu³⁺ doping concentration is low (𝑥≤10.0%), the phase is Sr₂TiO₄, and the intensity excited by CTB (365 nm) is stronger than that of intra-4f transitions. After 𝑥 reached 15.0%, the structure damaged and the phase became Sr₃Ti₂O₇. The intensity excited by CTB became weaker and characteristic emission peaks of Eu³⁺ stronger when excited at 395 and 465 nm. Because Sr₂TiO₄ has strong absorption at UV light region, while Sr₃Ti₂O₇ has two layers and there are more noncentersymmetrical Sr²⁺ sites for substituting. Those phosphors all exhibited intense 620 nm (⁵D₀→⁷F₂) red light and weak 578 nm (⁵D₀→⁷F₁), 700 nm (⁵D₀→⁷F₄) light. The CIE chromaticity coordinates of three samples that are emitting most intense red light change between (0.650, 0.344) and (0.635, 0.352), close to coordinates of the “ideal red” which is (0.67, 0.33). The particle size meets the demands of phosphor. Doped layered titanate Sr₂TiO₄:Eu³⁺ is a promising candidate red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.

Acknowledgments

The authors would like to thank the generous financial support from National Defense Fundamental Research of China (6134502), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Research and Innovation Program for College Graduates of Jiangsu Province (CXZZ12_0410), and National Natural Science Foundation of China (51202111).