Luminescence Performance of Yellow Phosphor SrxBa1−xTiO3: Eu3+, Gd3+ for Blue Chip

Dong, Limin; Li, Jian; Li, Qin; Shan, Lianwei; Han, Zhidong

doi:https://doi.org/10.1155/2015/405846

Journal of Nanomaterials

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Research Article | Open Access

Volume 2015 | Article ID 405846 | https://doi.org/10.1155/2015/405846

Luminescence Performance of Yellow Phosphor Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ for Blue Chip

Limin Dong,^1,2Jian Li,¹Qin Li,¹Lianwei Shan,¹and Zhidong Han^1,2

Academic Editor: Zewei Quan

Received22 Sept 2014

Accepted11 Jan 2015

Published31 Jan 2015

Abstract

The Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphors are synthesized by high temperature solid-phase method. Multiple techniques including X-ray diffraction (XRD), and scanning electron microscopy (SEM) are used to examine the surface morphology and structural properties of Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphors. The optical properties are presented and discussed in terms of photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The as-obtained Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphors show higher PL emission intensity (at 591, 611 nm). The peaks at 591 and 611 nm are attributed to Eu³⁺ ⁵D₀-⁷F₁, ⁵D₀-⁷F₂. Gd³⁺ has a strong sensitization on Eu³⁺. A certain amount of Sr²⁺ and Ba²⁺ is contributed to the intensity of light emission. After being irradiated with blue light, the phosphor samples emit yellow light. This suggests its potential applications in many fields.

1. Introduction

White light-emitting diode (WLED) has been developed rapidly over the past decade, since its advanced properties such as long life time, high efficiency, and being environmentally friendly without use of mercury [1, 2]. Currently, there are three main methods for WLED technology: blue chip and yellow phosphor approach; tricolor LED chip direct mixing method; ultraviolet conversion method [3, 4]. Generally, white LEDs are composed of a blue LED chip GaN and yellow phosphor such as Y₃Al₅O₁₂: Ce³⁺. This system has a low color rendering index because of its deficiency in the red region of the visible spectrum [5, 6]. The other approach to overcome this drawback is to add a red-emitting phosphor to compensate the deficiency [7–10].

Titanate system has excellent properties such as very good physical and chemical stability, its stability in epoxy resin or other silicone encapsulating materials; meanwhile rich resources of titanium lead to the clear price advantage compared with other molybdates and tungstate. It provides favorable conditions for the development of rare earth titanate functional materials. Partial titanate MTiO₃ (M is generally alkaline earth metal or alkali metal and rare earth ion composition) basic phosphor is perovskite compound [11], which is the stoichiometric chemical formula ABO₃. Typically, a cation ionic radius is larger, mainly Ca, Sr, Ba, and alkaline earth metal ions Na, Ce; B ion radius is smaller mainly Ti, Nb, and so forth [12, 13]. Perovskite structure is mainly composed of B site cations and oxygen ions consisting of [BO₆]. Figure 1 shows the crystal structure of MTiO₃. Partial titanate is a face-centered cubic which close-packed with cation of larger radius and O²⁻. The transition metal Ti⁴⁺ of small radius is surrounded by six O²⁻, and locate at the center of oxide octahedral [TiO₆] [14, 15]. Since the research of Diallo et al. for CaTiO₃: Pr³⁺ titanate phosphor, its excellent performance has attracted wide attention [11]. Fu et al. got the CaTiO₃: Eu³⁺ red phosphor by doping Eu³⁺ [16]. In recent years, Yang et al. prepared Ba_0.77Ca_0.23TiO₃: Eu³⁺ phosphors [17]. Due to the small concentrations of Eu³⁺, it was surrounded by matrix like Ti⁴⁺ which formed a cluster. Other than the energy transfer for direct excitation, there are also some transfers between matrix and Eu³⁺. The spectral characteristics of different Eu³⁺ doped position are also studied.

In this study, the Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphors are synthesized by high temperature solid-phase method, with Eu³⁺ as activator and Gd³⁺ as sensitizer. The CRI (Color Rendering Index) of the material is improved by adding some Ba. Compared with the traditional YAG: Ce in the missing part of red light, the light of this phosphor is more pure on yellow which solved YAG: Ce partial yellow-green issues. YAG: Ce main emission peak located at 560 nm which belongs to yellow-green band. The optimum excitation wavelength of this phosphor locates at 466 nm position which accords with the excitation wavelength of blue chip. Adjusting the ratio of blue and yellow light can get more pure white light, and the Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphor is expected to be widely used in the WLED field [18, 19].

2. Materials and Methods

2.1. Materials Preparation

Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ was synthesized successfully via a conventional high temperature solid state reaction method. The reactants used in the preparation were Sr(NO₃)₂ (analytical grade, Tianjin Guangfu Fine Chemical Research Institute), Ba(NO₃)₂ (analytical grade, Tianjin Guangfu Fine Chemical Research Institute), TiO₂ (analytical grade, Tianjin Guangfu Fine Chemical Research Institute), Eu₂O₃ (analytical grade, Baotou Research Institute of Rare Earths), and Gd₂O₃ (analytical grade, Baotou Research Institute of Rare Earths). The stoichiometric amount of starting materials was weighed out and then mixed and milled thoroughly for 1 h in an agate mortar. Afterwards, the mixtures were transferred into corundum crucibles and prefired at 500°C for 1 h in a furnace. After cooling down naturally to room temperature, they were ground again for 30 min and finally calcined at 1100°C for 3 h in furnace.

2.2. Analysis Methods

The structures of the phosphor were established by X-ray diffractometer (XRD) (Shimadzu, XRD-6000, Cu Ka target) and the morphology of the particles was observed by field emission scanning electron microscope (FE-SEM) (Sirion 200, Philip). The photoluminescence properties of the phosphors were studied on fluorescence spectrophotometer (Shimadzu, model RF-5301 PC).

3. Result and Discussion

3.1. Structural Analysis

Structurally, SrTiO₃ crystallizes in a cubic structure, space group Pm-3m (no. 227), with a lattice parameter a = 3.905 Å. Figure 2 presents the XRD patterns of SrTiO₃: Eu³⁺, Gd³⁺ phosphors. It is in good agreement with JCPDS Card number 84-0433. The lattice constants of the phosphors are calculated as a = 3.900 Å for SrTiO₃: 0.05Eu³⁺, consistent with the unit-cell parameters reported in the literature. The ionic radii () of Sr²⁺ (CN = 12) are 112 pm. The ions radii of dopant elements, Eu³⁺ (CN = 12, pm) and Gd³⁺ (CN = 12, pm), are expected to occupy the Sr²⁺ sites in the SrTiO₃ host due to the close radii and identical valence of the ions [20]. There are only tiny TiO₂ impurity phases seen at 28–32 degrees which does not have apparent effect on the performance and can be ignored.

3.2. Morphology Analysis

Figure 3 shows the SEM images of SrTiO₃: Eu³⁺, Gd³⁺ calcined at 1100°C for 3 h. From the sample global views and local features in Figure 3, we can see that the morphology of material is close to the sphere, with good homogeneity and smooth surface, and its size is about 0.25 μm. In the LED material encapsulation process, morphology of the fluorescent powder has a greater impact on luminescent properties. It is generally believed that when the morphology of material is nearly sphere and up to the grade of close packing, we can improve the luminous intensity per unit volume [21]. In the experiment, the prepared samples can reach such requirements.

3.3. Luminescent Properties Analysis

Figure 4 presents the excitation and emission spectra of the Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ sample. The excitation spectrum was measured at 611 nm monitored. Emission spectra were measured at 466 nm excited. The excitation spectrum (curve a) extends at 394 nm and 466 nm, due to ⁷F₀-⁵L₆ and ⁷F₀-⁵D₂ transitions of the Eu³⁺. The Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphor shows a yellow emission band peaking at 591 nm and 611 nm under 466 nm excitation (curve b), which are attributed to ⁵D₀-⁷F₁, ⁵D₀-⁷F₂ [22].

In titanate phosphor, Eu³⁺ replaces the sites of Sr²⁺ in matrix. When the amount of incorporation of Eu³⁺ is changed, the number of Eu³⁺ replaces Sr²⁺ as luminescent center is changing. As Eu³⁺ concentration increases, the emission intensity increased with the adding of emission center, especially particular evident of ⁵D₀-⁷F₁ transition. However, when the doping concentration is increased and the distance of luminescent center is decreasing, nonradiative energy transfer between luminescent centers has become increasingly evident, result of concentration quenching [23, 24], thereby reducing the emission intensity. Figure 5(a) shows the emission spectra of different concentrations of Eu³⁺ doping. The insets in Figure 5(a) are the change of intensity with the concentrations of Eu³⁺. The spectra indicate that the optimum doping concentration is 5% corresponding to the strongest emission intensity.

(a)

(b)

Figure 5(b) presents the emission spectra of different Gd³⁺ doping amount and the intensity trend. It can be seen that under 466 nm excited, all characteristic peaks in emission spectra are attributed to Eu³⁺, and the strongest peak locates at 611 nm. Compared to the single doped Eu³⁺, the enhancement on ⁵D₀-⁷F₂ is significantly higher than ⁵D₀-⁷F₁, which is due to the addition of the symmetry of Gd³⁺ in crystal structure. With the changing of concentration, the luminous intensity was reduced after the first enhancement process. Since the concentration increases to a certain value, interaction between the dopant ions within the host lattice, the energy released in the form of heat and energy passed to the luminous center of Eu³⁺ reduction, resulting in nonradiative transitions greater than radiative transitions, reducing the emission intensity of the product [25]. From Figure 5(b) it can be found that the optimum doping concentration is 3% corresponding to the strongest emission intensity.

Figure 6 shows the energy level diagram of Eu³⁺ and Gd³⁺ and the energy transfer process within or between rare earth ion energy levels when codoping Eu³⁺ and Gd³⁺. The energy transfer process of Gd³⁺ to Eu³⁺ includes cross relaxation and direct transmitting. Under the excitation of ultraviolet to blue light, Gd³⁺ is excited and jumped from the ground state ⁸S_7/2 to ⁶G_J excited state and then reradiation jumps back to ⁶P_J, through the cross relaxation process that transfers the excitation energy to neighboring Eu. The obtained energy of Eu³⁺ exactly conforms to the ⁷F_J⁵D_J transition energy. Excited Eu³⁺ returns to the ground state and emits orange light near 600 nm. On the other hand, the Gd³⁺ returning to the ⁶P_J state directly transmits remaining energy to another nearby Gd, and when facing Eu³⁺, Eu³⁺ absorbs energy and was excited to a higher excited state, then quickly radiated to ⁵D_J(1,2,3) as nonradiative relaxation. Finally it was radiated to ⁷F_J state. As a result, the energy absorbed by Gd³⁺ passes through two stages to excite Eu³⁺ [26–28].

3.4. Impact of Alkaline Earth Metal Doping

Crystal structure of the matrix material has a huge impact on luminescent properties. The experiment adding some Ba²⁺ to substitute Sr²⁺ is made to study its impact on the phase. Figure 7 shows the Sr_xBa_1−xTiO₃: Eu³⁺ (Sr : Ba = 1 : 0, 4 : 1, 3 : 2, 2 : 3, 1 : 4) XRD patterns and the standard BaTiO₃ (75-2120) and SrTiO₃ (74-1296) comparison chart. As can be seen, with the change in amount of Ba²⁺ added, the diffraction peak intensity was changed. Since Ba²⁺ (1.35 Å) and Sr²⁺ (1.18 Å) atomic radius have some differences, the substitution of Sr will produce changes in the crystal structure, and the corresponding XRD patterns are inevitably changing. With the increasing of proportion of Ba²⁺, the original representative SrTiO₃ diffraction peak becomes low, and there were many miscellaneous peaks. When Sr²⁺ : Ba²⁺ were 2 : 3 and 3 : 2, the two phases of BaTiO₃ and SrTiO₃ coexist. When Ba²⁺ : Sr²⁺ = 4 : 1, some low peaks again become acute. It can be found through XRD pattern that with the changing of doping percentage XRD patterns are in good agreement with BaTiO₃ (75-2120), indicating that the majority of Ba²⁺ replaces the sites of Sr²⁺; the situation becomes crystal structure of BaTiO₃ structure majority [29, 30].

The emission spectrum of Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ is shown in Figure 8. From the figure we can see that the peaks which located at 591 and 611 nm did not change dramatically with the different proportions of Sr²⁺ and Ba²⁺; with the increasing of Ba, the luminescent intensity of phosphor powder shows a downward trend after the first rising. Since the different radii of Sr²⁺ (1.18 Å) and Ba²⁺ (1.35 Å), the incorporation of Ba²⁺ not only affects the lattice constant but also affects the symmetry of the crystal structure, thereby reducing the crystal field strength. With Ba²⁺ ions being incorporated, the outermost d-orbital is affected by the crystal field greatly; when Ba²⁺ incorporation amount exceeds a certain limit, the performance of the spectral emission is intensely reduced. When the proportion of Sr²⁺ and Ba²⁺ is 4 : 1 the luminous intensity increased, from which we can see that a certain amount of Ba²⁺ is contributed to the intensity of light emission [31, 32].

The spectra at 591 nm and 611 nm were observed; we can found that the two corresponding relative strengths have some changes. The change of Eu³⁺ lattice site results in the different emission spectrum. The luminescence emission peak at 591 nm is corresponding to ⁵D₀-⁷F₁ which is magnetic dipole transitions and 611 nm of ⁵D₀-⁷F₂, which is electrical dipole transition. It is associated with the symmetry of the crystal. When there is no Ba²⁺ substituted Sr²⁺, the transition of ⁵D₀-⁷F₁ is dominant as the symmetrical sites. But with the increasing of Ba²⁺, symmetry has a great change; Eu³⁺ substituted Ba²⁺ or Sr²⁺ which are in asymmetric sites; the transition of ⁵D₀-⁷F₂ is prominent. Adjusting the ratio of blue and yellow can receive desired white light.

4. Conclusions

A series of Sr_xBa_1−xTiO₃: Eu³⁺, Gd³⁺ phosphors were synthesized via the conventional high temperature solid state reaction method. The morphology of material is close to the sphere, and its size is about 0.25 μm. The emission color of them was yellow to red, which was achieved with Eu³⁺ doped. The adding of Gd³⁺ could increase the intensity of Eu³⁺. The energy transfer process of Gd³⁺ to Eu³⁺ includes cross relaxation and direct transmitting of Gd³⁺ and Eu³⁺. It can be found corresponding to the strongest emission intensity the optimum doping concentration of Eu³⁺ and Gd³⁺ is 5% and 3%, respectively. The doping of Eu³⁺ and Gd³⁺ did not change the crystal structures but influence on the crystallinity. However after doping with Ba²⁺, the crystal symmetry and ionic environment are changed; thus the spectral intensity and the spectral structure are transformed. By changing the proportion of the magnetic dipole transitions and electrical dipole transitions controllable switched the spectral structure of the optimum ratio of Sr²⁺ and Ba²⁺ is 4 : 1. According to the configuration coordinates, with blue chips, the sample can be well applied to WLED.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was financially supported by Program for Innovative Research Team in University of Heilongjiang (2013TD008).

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Copyright © 2015 Limin Dong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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