Study on optical performance of red-emitting phosphor: SrAl2O4: Eu2+, Dy3+/Sr2MgSi2O7: Eu2+, Dy3+ /light conversion agent for long-lasting luminous fibers


 A novel luminous phosphor SrAl2O4: Eu2+, Dy3+/Sr2MgSi2O7:Eu2+, Dy3+/light conversion agent with both red luminescence characteristics and persistent luminescent performance were prepared. According to the intermediary color law, the color purity of red emission can be controlled by the doping ratio of SrAl2O4: Eu2+, Dy3+ in SrAl2O4: Eu2+, Dy3+/Sr2MgSi2O7: Eu2+, Dy3+ composite luminous material. The anticipated luminescence properties could be realized precisely by adjusting the ratio. The microscopic morphology of the composite material, energy dispersive spectrometer, X-ray diffraction, FT-IR spectra, emission spectrum, CIE chromaticity diagrams and afterglow properties were examined. The brief outlines about some phenomena aspects and detailed physical descriptions as well as manuals were available. From the analyses of CIE chromaticity diagrams and afterglow properties, the doping ratio of SrAl2O4: Eu2+, Dy3+/Sr2MgSi2O7: Eu2+, Dy3+ can influence afterglow brightness and red emission purity. The samples show great optical performance.


Introduction
The rare earth luminous phosphor is a kind of environmentally friendly and sustainable self-luminous functional material, which can absorb the ultraviolet for 10 min and then emit light continuously for more than 10 h. Since scientists founded that the rare earth can activated alkaline earth metal aluminate in 1990s, it has been attracting a lot of interests and further research [1]. The rare earth co-doped longlasting phosphors play important roles in the elds of home decorations, children toys, emergency signs, the in-vivo medical imaging, and many kinds of applied elds because it exhibits excellent photoresistance and a very bright afterglow [2][3] [4]. As for Eu 2+ and Dy 3+ co-doped luminous phosphor, it's characteristics due to the level's transition of Eu ions. After excitation of Eu 2+ from 4f 7 to 4f 6 5d 1 , a trap level released by Eu 2+ is captured by Dy 3+ and forming the afterglow phenomenon when electrons escape from the trap level [5].
In recent years, there has been an increasing interest in the photochromic of the ber which determines the application of the ber. Throughout this paper, the term 'photochromic' will refers to the color of light emitted by the ber itself after excited by ultraviolet light in a non-illuminated environment. As for enrich the color diversity of luminous ber, it is very important to study the red-emitting luminous ber.
Researchers tried a lot to solve this problem. For examples, researchers tried to coat red emitting color convert on the surface of SrAl 2 O 4 : Eu 2+ , Dy 3+ or use Y 2 O 2 S: Eu 3+ , Mg 2+ , Ti 4+ as the luminescent centre, but it's di cult to satisfy requirement of both afterglow and photochromic properties [6][7] [8]. While some studies have carried out on the photochromic of the ber, further research is still required to obtain fullcolor emission and stable materials. SrAl 2 O 4 : Eu 2+ , Dy 3+ and Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ are two kinds of energy-storing materials with excellent optical performance. SrAl 2 O 4 : Eu 2+ , Dy 3+ , which can emit at 525 nm (green light), has more than 30h's afterglow time; Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ , which can emit at 475 nm (blue light), performs water resisting property and chemical stability. Light conversion agent is a kind of oxygen anthracene derivative [9]. The red emission that has the broadest excitation band could be excited about from 200 to 550 nm which means that combining rare earth luminescent materials and the light conversion agent can achieve the purpose of red emission.  BO 3 were weighted and grinded to uniform particles in an agate mortar. After dissolving the samples into suitable ethanol, the homogeneous powder was obtained by using ultrasonic technique. Secondly, the materials were heated in a high temperature tubular furnace of 1400 ℃ for 3 h in a weakly reducing atmosphere. Finally, the sintered products were re-milled and sieved to get the desired samples after the reactor was cooled to a room temperature.  3+ were put into a beaker with an appropriate amount of ethanol in different ratio. Subsequently ethanol, light conversion agent (5 wt.%) and KH560 (1.25 wt.%) were added to the materials and then the mixture was stirring for 10 min before putting it in ultrasonic. After agitating the samples of 70 ℃ for 1h in an enclosed environment, the seal was removed in order to evaporated dry the ethanol. Finally, the sample was dried at 70 ℃ and it was grinded adequately and sieved to get the desired samples. Figure 2 shows the preparation process of

Energy dispersive spectrometer (EDS)
EDS was used to qualitatively analyze the element distribution in the microscopic region of the composite material on Hitachi SU1510.

X-ray diffraction (XRD)
XRD measurement was tested for con rming whether the mixing of the three materials in uence the crystal structure of the composite material. XRD patterns were recorded on a D8 Advance X-ray diffractometer (Bruker AXS, Germany) with Cu Kα radiation (λ = 0.15406 nm) at a voltage of 40 kV and current of 30 mA. The scans versus diffraction angle (2θ) were carried out in the range of 10-70°, with a scan speed of 2(°)/min at room temperature.

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The FT-IR spectra were examined using the Nicolet IS10. The scanning interval of testing conditions was from 4000 cm − 1 to 400 cm − 1 with a resolution of 2 cm − 1 . Before the test, all the sample and KBr were mixed and ground, and then tableted together.

Luminous properties
The uorescence spectrophotometer (F-4600 FL Spectrophotometer) with the xenon lamp which set an excitation source at 365 nm excitation light and the slit at 3nm in width was used to test the CIE chromaticity diagrams and afterglow properties of the samples. The afterglow properties of the samples were examined by PR305 afterglow brightness tester, with UV light at 1000 lx for 15 min.

Morphology analysis
The morphologies of the before and after being coating with light conversion agent were observed by  2+ , Dy 3+ has a smaller particle size, a smoother surface and fewer edges and corners. Since both particle size is distributed between 5-20 µm uniformly, it can meet the spinning needs of preparing luminous ber. There are a few particles on the surface of the two materials, which are debris generated during the grinding process after the calcination is completed. The surface of luminescent material becomes rough after coating light conversion agent. This is because silane coupling agent acts as a molecular bridge to tightly coat the light conversion agent on the surface of the material. The hydroxy group formed after the hydrolysis of the alkoxy group on the surface of the silane coupling agent can condense with the hydroxy group on the surface of the inorganic material. It is often used to adjust the interfacial effect of organic and inorganic materials [11]. Therefore, a coating of small particles was formed on the surface which was dense and distributed evenly. As the roughness increases, the particle

Structural analysis
Since the brightness and color of the luminous ber are determined by the activator (Eu 2+ in this study) and the crystal structure, the crystal structure of the rare earth luminescent material has a great in uence on its luminescence performance [12]. In order to explore whether the addition of light conversion agent will affect the phase structure of luminescent materials, the XRD patterns of the rare earth luminescent materials are illustrated in Fig. 4(a) and Fig. 4(b)

FT-IR spectra analysis
The FT-IR spectra of light conversion agent and rare earth luminescent materials is shown in Fig. 5. The broaden absorption band of 3000-3500 cm − 1 for all the samples is assigned to the symmetric stretching vibration of hydration hydroxyl groups, because the samples absorb the water vapor in the moisture air [13]. For light conversion agent, the peak at 1703 cm − 1 and 1640 cm − 1 are attributed to C = O and C = N stretching in ester carbonyl group respectively, and the banding peak at 715 cm − 1 is attributed to C-H bending vibration outside the benzene ring. In the spectra of SrAl 2 O 4 : Eu 2+ , Dy 3+ , it present that peak at Page 7/11 844 cm − 1 and 778 cm − 1 belong to Al-O and Sr-O stretching vibration peak, besides the peak band at 1483 cm − 1 is present symmetric stretching vibrations of the Sr-O-Al bonds [14].  [12]. Besides, the emission peak in the range of 450-550 nm that was weaken re ected that the uorescence resonance energy transfer (FRET) could have occurred. Rare earth luminescent materials are used as luminescent donors, and part of the energy is transferred from the rare earth luminescent materials to the light conversion agent, resulting in the attenuation of the donor's auto uorescence intensity [16]. Meanwhile, the difference in the ratio of luminescent materials has no effect on the degree of red shift of the light conversion agent, indicating that it's decided by addition of light conversion agent. Figure 7 shows the afterglow properties of SrAl2O4: Eu2+, Dy3 + doped of SrAl2O4: Eu2+, Dy3+/ Sr2MgSi2O7: Eu2+, Dy3+/light conversion agent luminous materials. The samples were irradiated by UV light for 15 min, after that they were moved into the brightness meter. The initial afterglow brightness (at 10 s) contrast curves of the ve samples are shown in the inset picture of Fig. 7. The decay curves have been calculated by tting the decay data using the following equation:

Afterglow properties analysis
In this attenuation formula, I stand for afterglow intensity; t stand for the time; R stand for initial brightness; A1, A2, and A3 are constants; λ1, λ2 and λ3 represent the decay time of three different periods of exponential decay for the exponential components which could be obtained by using Origin 8.0. The attenuation of afterglow is divided into three stages, which are the initial fast attenuation stage, the subsequent slow attenuation stage and the nal afterglow brightness stabilized stage. From Fig. 7, it can be observed that the attenuation of the afterglow curve conforms to the exponential decay law. The existence of light conversion agent will affect the partial afterglow performance of the material. This is because the light agent will be coated on the surface of the composite luminous powder and part of the excited light will be absorbed and re ected by the light conversion agent. In addition, the light conversion agent will absorb some of the energy emitted by SrAl 2 O 4 : Eu 2+ , Dy 3+ and Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ , resulting in a loss of energy and afterglow performance.
Besides, as it shown in Fig. 7

Fluorescence characteristics
The color coordinates and color purity are depicted on Fig. 8. Besides, the chromaticity diagram is partitioned by color. The term "purity" for CIE 1931 chromaticity diagram will refers to how close the color of the sample is to the spectral color of the same dominant wavelength. The chromaticity diagram needs to be partitioned because only the purity calculation in a same color area is meaningful. Figure 8  Eu 2+ , Dy 3+ proportion, the purity of the samples will promote at RED area. From 3.3, It can be seen that the difference in the ratio of luminescent materials has no effect on the degree of red shift of the light conversion agent. However, through the analysis of sample purity in Fig. 8, changing the SrAl 2 O 4 : Eu 2+ , Dy 3+ ratio will not change the degree of red shift of the sample emission spectrum, but it will affect the red-light emission purity. The higher the proportion of SrAl 2 O 4 : Eu 2+ , Dy 3+ added, the higher the purity of the red light emitted after excitation.

Conclusion
Herein, a novel red-emitting luminous material was prepared. On the basis of intermediary color law of the Grsassmann color law, two different photochromic luminescent materials and light conversion agent were mixed to obtain a red emission powder, which has both red luminescence characteristics and