Sr_2−xBa_xTiO₄:Eu³⁺, Gd³⁺: A Novel Blue Converting Yellow-Emitting Phosphor for White Light-Emitting Diodes

Abstract

A highly intense yellow-emitting phosphor :, peaking at 593–611 nm was synthesized by the sol-gel method. XRD and SEM show that the samples are single phase and have irregular shape. The excitation wavelength matches well with that of the emission of the blue-light-emitting diode. The emission peaks at 593 and 611 nm are attributed to the transitions from the ⁵D₀⁷F₁ and ⁵D₀⁷F₂ of ions, respectively. was used as sensitizer, aiming at increasing the luminous intensity. A certain amount of and is contributed to the intensity of light emission.

1. Introduction

A light revolution is sweeping all over the world due to its advanced properties such as long life time, low energy consumption, high efficiency, environmental friendliness, and the potential applications in indicators, backlights, automobile head-lights, and general illuminations [1, 2]. In the context of energy and environmental protection, the development of LED is taken into consideration. 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]. At present, WLEDs are fabricated by combining a blue LED chip GaN with yellow-emitting phosphor such as Y₃Al₅O₁₂:Ce³⁺ [5]. However, this type of white light has low color rendering because of the deficiency in red region of the sunlight spectrum (above 600 nm). These shortcomings limit its prospects in the field of WLED lighting [6, 7]. Therefore, it is critical to add a red-emitting phosphor to compensate the deficiency [8–10].

Titanate system has excellent properties such as very good physical and chemical stability and being stably present in the epoxy resin or the like silicone encapsulating material; meanwhile rich resources of titanium lead to the clear price advantage compared with other molybdate and tungstate. It provides favorable conditions for the development of rare earth titanate functional materials. Figure 1 is the crystal structure of M₂TiO₄. Titanate M₂TiO₄ (M is generally alkaline earth metal or alkali metal and rare earth ion composition) basic phosphor is layered perovskite compound, among the perovskite layers is SrO layer, and Sr²⁺ ion is located among the perovskite layers, around which there are nine oxygen ligands [11, 12].

Figure 1 

The crystal structure of M₂TiO₄.

In this study, the TiO₄:Eu³⁺, Gd³⁺ phosphors are synthesized by sol-gel method, with Eu³⁺ as activator and Gd³⁺ as sensitizer, and their luminescent properties were investigated. These phosphors were excited by blue chip and emit yellow light. Adjusting the ratio of blue and yellow light, more pure white light can be obtained, and the TiO₄:Eu³⁺, Gd³⁺ phosphors are expected to be widely used in the WLED field [13, 14].

2. Experiment

2.1. Materials Preparation

The phosphor TiO₄:Eu³⁺, Gd³⁺ was synthesized through the sol-gel method. All of the chemical reagents used in this experiment were of analytical grade. An Eu(NO₃)₃ solution was prepared by dissolving Eu₂O₃ into the nitric acid. Eu(NO₃)₃, Sr(NO₃)₂, and Ce(NO₃)₃ solution were mixed in a stoichiometric ratio, followed by the addition of citric acid. Afterwards, tetrabutyl orthotitanate was slowly dropped into the mixture solution under constant stirring after being diluted by alcohol. The resulting mixture is placed into water bath at 80°C under stirring for 1 h until the gelation was completed. Then the yellow-green gel was placed in an oven at 80°C for 4 h. After drying process, a brown fluffy porous xerogel was obtained. Finally, the sample is calcined in a high temperature electronic furnace to obtain the desired phosphor.

2.2. Analysis Methods

The structure of the phosphor was 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). All the photoluminescence properties of the phosphors were measured at room temperature.

3. Results and Discussion

3.1. Phase Characterization and SEM

To determine the phase purity of the samples, XRD measurements for the synthesized products were conducted. Figure 2 shows the XRD patterns of Sr₂TiO₄:Eu³⁺, Gd³⁺; Sr₂TiO₄ crystallizes in a tetragonal structure, with space group of I4mmm (number 139). The lattice parameters were determined to be  Å and  Å, which is in good agreement with JCPDS number 39-1471 ( Å,  Å). The ions radii of dopant element, Eu³⁺ (CN = 12,  pm), are expected to occupy the Sr²⁺ sites in the Sr₂TiO₄ host due to the close radii and identical valence of the ions. Figure 3 shows the SEM images of material calcined at 1100°C for 3 h, from which we can see the samples are single phase and have irregular shape.

Figure 2 

The XRD patterns of the Sr₂TiO₄:Eu³⁺.

Figure 3 

SEM image of the Sr₂TiO₄:Eu³⁺.

3.2. The Luminescent Properties of Sr_2−xBa_xTiO₄:Eu³⁺, Gd³⁺

The excitation spectra and emission spectra of Sr₂TiO₄:Eu³⁺, Gd³⁺ phosphors are given in Figure 4. The excitation spectra were under 611 nm emission and the emission spectra were under 466 nm excitation. The excitation spectrum (curve a) extends at 393 nm and 466 nm, which is due to the transitions ⁷F₀-⁵L₆ and ⁷F₀-⁵D₂ of Eu³⁺, respectively. The transition at 466 nm was the strongest absorption, which is corresponding to the excitation of blue chip. The TiO₄:Eu³⁺, Gd³⁺ phosphor shows a yellow emission band peaking at 593 nm and 611 nm under 466 nm excitation (curve b), which are attributed to Eu³⁺⁵D₀-⁷F₁, ⁵D₀-⁷F₂, respectively.

Figure 4 

The excitation ( = 611 nm) and emission ( = 466 nm) spectra of the Sr₂TiO₄:Eu³⁺ sample

In titanate phosphor, Eu³⁺ replaces the sites of Sr²⁺ as luminescent centers in matrix. When the amount of Eu³⁺ changes, the shape of emission spectra has no difference, but the intensity is changing. When the Eu³⁺ concentration increased, the emission intensity increased with the adding of emission center, especially notable for ⁵D₀-⁷F₂ transition. However, when the doping content of Eu³⁺ continues to increase, the distance of activation centers is decreasing and nonradiative energy transfer between the matrix and the light-emitting center has become increasingly evident, leading to concentration quenching [15]. Thus the emission intensity decreased. Figure 5 is the emission spectra with the different doping concentrations of Eu³⁺. The spectra indicate that the optimum doping concentration is 7%, corresponding to the strongest emission intensity.

Figure 5 

Emission spectra of different concentrations of Eu³⁺ doping.

The emission spectra with different Gd³⁺ doping amount are shown in Figure 6. As can be seen, the sample was under 466 nm excitation, and all the main emission peaks are attributed to Eu³⁺, and the strongest peak is located at 611 nm. Compared to the single doped Eu³⁺, the intensity of the emission peak at 611 nm has a significant enhancement compared to that at 593 nm, which is due to the addition of the symmetry of Gd³⁺ in crystal structure. When the doping amount of Gd³⁺ was in a low level, the luminescence intensity enhances with the increase of the concentration of Gd³⁺. Since the concentration increases to a certain value, energy transfer between the substrate and the sensitizer ions is then released in the form of heat or scattered in the form of nonradiation, thus leading to the reduction of the emission intensity. From Figure 6, it can be found that the optimum doping concentration is 5% corresponding to the strongest emission intensity.

Figure 6 

Emission spectra of different Gd³⁺ doping amount.

3.3. Impact of Alkaline Earth Metal Doping

Crystal structure of the matrix material has a huge impact on the luminescent properties. With the change of the doping content of Ba²⁺, the substitution of Ba²⁺ for Sr²⁺ has an impact on the phase. Figure 7 shows the XRD patterns of TiO₄:Eu³⁺ (Sr : Ba = 1 : 0, 4 : 1, 3 : 2, 2 : 3, 1 : 4), the standard Ba₂TiO₄ (38-1481), and Sr₂TiO₄ (39-1471). As can be seen, with the increasing of Ba²⁺ concentration, the crystalline phase of luminescent materials has a huge change. Since atomic radii of Ba²⁺ (1.35 Å) and Sr²⁺ (1.18 Å) are different, the substitution of Ba²⁺ for Sr²⁺ has impact on crystal structure; then the corresponding XRD patterns will change inevitably. With the increasing of Ba²⁺, the diffraction peak owing to Sr₂TiO₄ becomes low and there are some miscellaneous peaks in the diffraction spectra. When the proportion of Sr and Ba reaches 2 : 3 and 3 : 2, the significant phase of Ba₂TiO₄ can be seen, that is, the phases of Sr₂TiO₄ and Ba₂TiO₄ coexistence. When the concentration of Ba²⁺ continues to increase, the phase of Ba₂TiO₄ is dominant, some weak peaks owing to Sr₂TiO₄ become acute again and even disappear, and the incorporation of rare earth Eu³⁺ into this lattice will result in various fluorescence characteristics depending on the different sites [16, 17].

Figure 7 

The XRD patterns of TiO₄:Eu³⁺ (Sr : Ba = 1 : 0, 4 : 1, 3 : 2, 2 : 3, 1 : 4).

Figure 8 shows the emission spectrum of TiO₄:Eu³⁺, Gd³⁺. The luminescence properties of two crystal phases are quite different with the content of the rare earth Eu³⁺. We can see that, with the increasing concentration of Ba²⁺, the emission intensity varies linearly convexly, that is, because the addition of Ba²⁺ composed composite structure has a certain degree of enhancement of luminescence. However, when the crystalline phase of Ba₂TiO₄ dominates the emission intensity declined, this is mainly due to pure orthorhombic Ba₂TiO₄ which has no advantage on the luminescent of Eu³⁺; there is also a significant blue-shift phenomenon in pure orthorhombic Ba₂TiO₄, which is consistent with the results reported in the literature [18]. When the ratio of Sr²⁺ and Ba²⁺ is 3 : 2, the optimum fluorescence property is obtained.

Figure 8 

The emission spectrum of TiO₄:Eu³⁺.

From Figure 8, we can see that the corresponding relative intensity of peaks at 593 and 611 nm has some changes. It is well known that the emission peak located at 593 nm and 611 nm is attributed to the transition of Eu³⁺⁵D₀-⁷F₁ which is magnetic dipole transitions and ⁵D₀-⁷F₂ which is electrical dipole transition, respectively. When the crystalline phase of Sr₂TiO₄ is dominant and the transition of ⁵D₀-⁷F₁ is dominant, the lattice remains symmetric. However, with the increasing content of Ba, the symmetry of lattice structure has a great change. When Eu³⁺ substituted the position of Sr²⁺ or Ba²⁺, the transition of ⁵D₀-⁷F₂ is prominent. By adjusting the ratio of Sr and Ba combined, blue chip emitting can achieve desired white light.

4. Conclusions

A series of TiO₄:Eu³⁺, Gd³⁺ was synthesized through the sol-gel method. The luminescent color ranges from yellow to red, and Eu³⁺ was used as activator and Gd³⁺ was used as sensitizer, aiming at increasing the luminous intensity. As the fluorescence properties are the best, the optimum doping concentration of Eu³⁺ and Gd³⁺ is 5% and 3%, respectively, and the optimum ratio of Sr and Ba is 3 : 2. When the phosphor combined a blue LED chip, 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 the University of Heilongjiang (2013TD008).