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Journal of Nanomaterials
Volume 2015, Article ID 405846, 6 pages
http://dx.doi.org/10.1155/2015/405846
Research Article

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

1College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China
2Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150040, China

Received 22 September 2014; Accepted 11 January 2015

Academic Editor: Zewei Quan

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.

Abstract

The SrxBa1−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 SrxBa1−xTiO3: Eu3+, Gd3+ phosphors. The optical properties are presented and discussed in terms of photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The as-obtained SrxBa1−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 [710].

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 Ba0.77Ca0.23TiO3: 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.

Figure 1: The crystal structure of MTiO3.

In this study, the SrxBa1−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 SrxBa1−xTiO3: Eu3+, Gd3+ phosphor is expected to be widely used in the WLED field [18, 19].

2. Materials and Methods

2.1. Materials Preparation

SrxBa1−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.

Figure 2: The XRD patterns of the SrTiO3: Eu3+.
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.

Figure 3: SEM images of SrTiO3: Eu3+, Gd3+ calcined at 1100°C for 3 h.
3.3. Luminescent Properties Analysis

Figure 4 presents the excitation and emission spectra of the SrxBa1−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 SrxBa1−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].

Figure 4: The excitation ( = 611 nm) and emission ( = 466 nm) spectra of the SrTiO3: Eu3+.

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.

Figure 5: Emission spectra of different doping concentrations: (a) Different doping concentrations of Eu3+ and (b) different doping concentrations of Gd3+ under 5% Eu3+ doped.

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 8S7/2 to 6GJ excited state and then reradiation jumps back to 6PJ, through the cross relaxation process that transfers the excitation energy to neighboring Eu. The obtained energy of Eu3+ exactly conforms to the 7FJ5DJ 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 6PJ 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 5DJ(1,2,3) as nonradiative relaxation. Finally it was radiated to 7FJ state. As a result, the energy absorbed by Gd3+ passes through two stages to excite Eu3+ [2628].

Figure 6: Energy level diagram of Eu3+ and Gd3+.
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 SrxBa1−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].

Figure 7: The SrxBa1−xTiO3: Eu3+ (Sr : Ba = 1 : 0, 4 : 1, 3 : 2, 2 : 3, 1 : 4) XRD patterns.

The emission spectrum of SrxBa1−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].

Figure 8: The emission spectrum of SrxBa1−xTiO3: Eu3+.

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 SrxBa1−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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