Preparation and optical properties of afterglow Sr2MgSi2O7:Eu2+, Dy3+ electrospun nanofibers

Journal of Luminescence 172 (2016) 317–322

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Preparation and optical properties of afterglow Sr2MgSi2O7:Eu2 þ , Dy3 þ electrospun nanofibers Ling He n, Baolan Jia, Lianyu Che, Wensheng Li, Weimin Sun State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 August 2015 Received in revised form 19 November 2015 Accepted 1 December 2015 Available online 23 December 2015

One-dimensional Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers were synthesized by a simple and cost-effective electrospinning technique. The samples were characterized by XRD, FT-IR, TG-DTA, SEM, TEM and PL. The XRD and SEM results show single crystal nanofibers of Sr2MgSi2O7:Eu2 þ , Dy3 þ with an average diameter of 200 nm were obtained by electrospinning preparation and heat treatment at 1150 °C for 5 h. The single crystal structure of Sr2MgSi2O7:Eu2 þ nanofiber was discussed. The luminescent properties, long lasting performance and formation mechanism are also investigated. & 2015 Elsevier B.V. All rights reserved.

Keywords: Eletrospinning Sr2MgSi2O7:Eu2 þ Dy3þ nanofibers

1. Introduction Long-afterglow phosphors have attracted much attention due to their characteristic of long lasting phosphorescence and the glowing at night, and can be apply in photoluminescence, lighting, emergency warning signboards, lasing, sensors luminous ceramic, crafts, biological markers, etc. [1–2]. Rare earth doped aluminatebased phosphors [3–5] with high brightness and long afterglow duration have been developed for more than 10 years and already got practical application [6]. However, most of these materials have some serious shortcomings with poor water resistance [7] and single color variety [8], their application is limited. In order to overcome these drawbacks, a new luminescent material named silicate long-afterglow phosphors is studied by some researchers [9–10]. The silicate phosphor shows some features of lower cost, lower sintering temperature, thermostability, excellent weather resistance and light-emitting colors variety, which can make up for the deficiency of aluminates materials. Sr2MgSi2O7 as one of the silicates material, owing to its excellent characteristics and easy processing, great efforts have been focused on the growth and optical properties of rare earth doped Sr2MgSi2O7 based phosphors. Although the silicates long-lasting luminescent materials have many superior properties, these materials are not widely applied at present due to a low afterglow brightness and short afterglow time comparing with the aluminates materials. One dimensional single crystal materials would solve those problems, if materials n

Corresponding author. E-mail address: [email protected] (L. He).

http://dx.doi.org/10.1016/j.jlumin.2015.12.012 0022-2313/& 2015 Elsevier B.V. All rights reserved.

were fabricated in a form of one-dimensional single crystal nanostructure, most noteworthy are single crystal nanofibers, due to their one-dimensional orientation and confinement effect of electrons, uniformity, anisotropy, symmetry, and have gained much attention for their potential applications [11] in optoelectronic nano-devices, such as nano-lasers, nano-waveguides, polarized luminescence. Different types of Sr2MgSi2O7 based phosphors materials as nanoparticles and nanotubes have been prepared successfully by many methods, for example, hydrothermal synthesis [12], sol–gel method [13], and high temperature solid-state method [14]. Using these methods can only obtain polycrystalline materials, it is still difficult to synthetize one dimensional single crystal materials. In recent years, one dimensional nanostructured nanofibers have gained more and more interesting on account of their special physical and chemical properties, unique anisotropic structure, high length/diameter aspect ratio [15]. So a new method was adopted to prepare Sr2MgSi2O7 based luminescent nanofibers. Electrospinning is one of the most effective, convenient and straightforward techniques to produce nanofibers, which has been studied by many scholars recently because of their dimensional, directional, and compositional flexibility [16–18]. Hundreds of inorganic nanofibers have been prepared successfully by electrospinning [19–21]. However, there have been few reports on synthesis of Sr2MgSi2O7 based long-lasting luminescent one dimensional nanofibers. Therefore, it is significant to synthesize this nanofibers with large scale, pure, single crystalline. In this paper, blue-emitting long afterglow Sr2MgSi2O7:Eu2 þ , Dy3 þ phosphor nanofiber was synthesized firstly by electrospining method. The influence of electrospinning technology parameter, phase structure, nanofibers surface morphology, and the

L. He et al. / Journal of Luminescence 172 (2016) 317–322

luminescence properties of Sr2MgSi2O7:Eu2 þ , Dy3 þ phosphor nanofibers are discussed.

2. Experimental 2.1. Materials Analytically pure Sr(NO3)2, Mg(NO3)2  6H2O (all with the purity of 99.00%), Eu2O3, Dy2O3 (all with the purity of 99.99%), tetraethyl orthosilicate Si(OC2H5)4 (A.R.), polyvinylalcohol (PVA) and nitric acid (HNO3). Ethanol C2H5OH (A.R.) and deionized water were used as solvents. 2.2. Preparation Sr2  x  yMgSi2O7:xEu2 þ , yDy3 þ samples were prepared by the electrospinning method. According to the nominal composition of Sr2  x  yMgSi2O7:xEu2 þ , yDy3 þ , a stoichiometric amount of Eu (NO3)3, Dy(NO3)3, PVA, Sr(NO3)2, Mg(NO3)2  6H2O, Si(OC2H5)4 were dissolved in deionised water and stirring at 93 °C temperature in the thermostat water bath to form a homogeneous hybrid sol as the precursor solution for electrospinning. The electrospun parameters were optimized as follows: the applied voltage was 20 kV, the working distance was maintained at 15 cm, the spinning rate was controlled in 0.5 ml/min. After electrospinning,the as-prepared fibers were taken off and heated at 600 °C for 2 h in muffle furnace for removing C, H and then continue heated at 1150 °C for 5 h in tube furnace under reducing atmosphere.

The exothermic peak observed between 900 and 920 °C on the DSC curve indicates the formation of crystalline Sr2MgSi2O7:Eu2 þ , Dy3 þ phase. As shown in Fig. 1, the plateau formed after 999 °C on the curves of TG and DSC indicates the remove of water and organics and the decomposition of nitrate in the Sr2MgSi2O7:Eu2 þ , Dy3 þ / PVA composite fibers. The residuum are inorganic oxide and finally the Sr2MgSi2O7:Eu2 þ , Dy3 þ single crystal nanofibers is formed, the total weight loss ratio is 75% in this process. Fig. 2 shows the XRD patterns of Sr2MgSi2O7 electrospun nanofibers calcined at 950 °C, 1000 °C, 1050 °C, 1100 °C and 1150 °C for 5 h. After calcining at 950 °C and 1100 °C for 5 h, the characteristic diffraction peaks of Sr2MgSi2O7 are observed, which means typical phase of Sr2MgSi2O7 is formed in the temperature ranges from 950 °C to 1100 °C. The peaks of SiO2 are appeared at 950 °C and the peak intensity are reducing with the calcining temperature rising. When calcination temperature is 1150 °C, no other phase is observed in the pattern, and all diffraction peaks are assigned to the tetragonal crystal system Sr2MgSi2O7 phase. The XRD result shows that Sr2MgSi2O7 nanofiber has a tetragonal crystal system, P-421m(No, 113) space group and the lattice constant is a ¼b ¼7.9956 Å and c¼ 5.1521 Å, which are consistent with the characteristic diffraction peaks of Sr2MgSi2O7 prepared with high temperature solid state method. On the other side, no 2 100 0 80

202.1 -2

XRD was performed using a Germany D8/ADVANCE X-ray diffractometer with Cu Kα radiation. TG-DSC data were recorded with Thermal Analysis Instrument with the heating rate of 10 °C/ min in an air flow of 100 ml/min. FT-IR spectra were measured with a Nexus 670 infrared spectrophotometer with the KBr pellet technique. The morphology of electrospun nanofibers was performed on a scanning electron microscope (SEM, JEOL 6700F) Transmission electron microscopy (TME) and selected area electron diffraction patterns (SADP) were obtained with a JSM-2010. Photoluminescence (PL) measurements and lifetime decay curves were recorded with F97 pro spectrophotometer. The thermoluminescence glow curves were measured on an FJ-427A TL meter (Beijing Nuclear Instrument Factory). The samples were first exposed for 15 min by UV light, and then heated from room temperature to 300 °C with a heating rate of 1 °C/s

TG (a.u.)

2.3. Characterization 60

-4

918.8

40

-6

20 0

200

400

600

800

DSC (a.u.)

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1000

-8 1200

temperature Fig. 1. The TG–DSC curves of precursor fibers.

3. Results and discussion Fig. 1 shows the TG–DSC curves of the precursor Sr2MgSi2O7: Eu2 þ , Dy3 þ fibers from room temperature to 1150 °C. Four stages of weight loss are shown in the TG curve, a major weight loss stage from 165 °C up to 918 °C and a minor weight loss step at about 918 °C, no further weight loss was noticed up to 918 °C. Meanwhile, three endothermic peaks and one exothermic peak are showed in the DSC curve as follows: 165 °C, 202 °C, 623 °C and 918 °C, respectively, which correspond to four stages of weight loss. The first endothermic peak observed from 30 °C up to 185 °C is related to the release of adsorbed water from the sample surface and combustion of ethanol in the as-spun Sr2MgSi2O7:Eu2 þ , Dy3 þ /PVA composite fibers. Because of inter-molecular dehydration of PVA, the second endothermic peak is observed at 185–266 °C. The third endothermic peak noticed from 558 to 627 °C is attributed to the decomposition of nitrate and combustion process of organic PVA.

15

20

25

30

35

40

45

Fig. 2. The XRD patterns of Sr2MgSi2O7 calcined at different temperatures.

L. He et al. / Journal of Luminescence 172 (2016) 317–322

impurity phase has been observed when small concentrations of Eu2 þ and Dy3 þ co-doped Sr2MgSi2O7, which indicates that the Eu2 þ , Dy3 þ ions are not built into the Sr2MgSi2O7 lattice and these elements are inserted in place of Sr2 þ , the Sr2MgSi2O7 crystal lattice does not have any obvious lattice distortions. The FT-IR spectra of Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers is shown in Fig. 3. The broad absorption band at 3450 cm  1 is assigned to the symmetric stretching vibration of hydration hydroxyl groups [22]. The bands peaking at 1011 cm  1, 624 cm  1 and in the patterns are due to the asymmetric and symmetric stretching vibration of Si–OT–Si (OT represents the bridge oxygen) group and Si–OB–Si (OB represents the non-bridge oxygen) group, the absorption band at 972 cm  1 represents the symmetrical stretching vibrations of OB–Si–OB group, the absorption bands at 930 cm  1 and 837 cm  1 represent the asymmetrical stretching vibrations of OT–Si–OB group. The band peaking at 566 cm  1 arise from the bending vibration of Si–OB, and the other obvious absorption band at 471 cm  1 is based on the stretching vibrations of Mg–O groups. The FT-IR spectra prove that Sr2MgSi2O7:Eu2 þ , Dy3 þ crystalline-phase has been well formed after annealing at 1150 ºC, agreeing well with the results of XRD and TG–DSC patterns. Fig. 4 shows the SEM images of as-formed fibers and the samples annealed at 1150 °C for 5 h. It can be found that the 100

ransmittance (a.u.)

80

3450

60

566 40

624 837 972 1011

20

930 4000

3500

3000

2500

2000

1500

1000

-1

wavenumber (cm)

Fig. 3. The FT-IR spectra of Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers.

471 500

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precursor fibers have a relatively smooth surface and the diameter is about 1 mm, as shown in Fig. 4(a). After calcined at 1150 °C, the surface of nanofibers becomes rough and the diameter size is about 200 nm due to the decomposition of PVA, evaporation of water and crystallization of silicate during the process of annealing, as shown in Fig. 4(b). The TEM images and selected area electron diffraction patterns (SADP) of Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers are shown in Fig. 5. From Fig. 5(a), the diameter of Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers is about 290 nm and it has a pentagon structure. As shown in Fig. 5 (b), it is confirmed that Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers are the single crystals after annealing and those diffraction spots are parallelogram lattice structure, the interplanar crystal spacing of 0.43 nm, 0.23 nm and 0.34 nm correspond to the (1 0 1), (3 1 1) and (2 1 0) plane of Sr2MgSi2O7:Eu2 þ , Dy3 þ , respectively, which agreeing well with the XRD results. There is directionality existing in eletrospinning process, which is helpful to the formation single crystal nanofibers. By monitoring at 471 nm, the excitation spectra of the Sr2  xMgSi2O7:xEu2 þ (x ¼0.01, 0.02, 0.03, 0.04, 0.05) nanofibers are recorded in Fig. 6. We can find that the broad bands at 300– 450 nm are stronger than that in the region from 200 nm to 300 nm. This character is very useful for the applications of phosphors. Two broad excitation bands centered at 359 nm and 397 nm are observed, which are typical Eu2 þ excitation peaks due to the 4f7–4f65d1 transition, and we can not find the special emission of Eu3 þ in the spectra, indicating that the Eu3 þ has been completely reduced to Eu2 þ . Under the UV excitation of 358 nm from Xe lamp, the emission spectra of the Sr2  xMgSi2O7:xEu2 þ (x ¼0.01, 0.02, 0.03, 0.04, 0.05) nanofiber is shown in Fig. 7. It shows a broad emission band at around 471 nm, corresponding to the typical 4f65d1–4f7transition of the Eu2 þ ions. No special emission peaks of Eu3 þ are found, which indicates that Eu3 þ have been reduced to Eu2 þ completely. The optimal concentration of Eu2 þ is x ¼0.03 and the emission intensity is enhancing gradually with the rising of concentration. When doping concentration of Eu2 þ exceed 0.3, the emission intensity is attenuating with the rising of concentration, it may result in concentration quenching and decreasing of the luminescence effect due to the energy transfer among the activators. Fig. 8 shows the excitation and the emission spectra of Sr1.972þ , yDy3þ (y¼0,0.02, 0.04, 0.06) nanofibers, respecyMgSi2O7:0.03Eu tively. From the figure, we can see that the emission and excitation intensity of the nanofiber co-doped Eu2þ and Dy3þ ions are better than the ones only doped Eu2þ , and the optimal concentration of Dy3þ is y¼ 0.04. When the doping concentration of Dy3þ is 0.02 or 0.06, the intensity of emission and excitation are decreased gradually. The special

Fig. 4. (a) The SEM images of as-formed fibers; (b) The sample annealed at 1150 °C.

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Fig. 5. TEM images for specimens of Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers: (a)TEM image; (b) selected area electron diffraction patterns (SADP).

x=0.03 x=0.04

intensity

x=0.02 x=0.05 x=0.01

250

300

350

400

450

Fig. 6. The excitation spectra of the Sr2  xMgSi2O7:xEu2 þ nanofibers.

x=0.03 x=0.04 x=0.01

x=0.02

intensity

420

440

460

480

500

520

540

Fig. 7. The emission spectra of the Sr2  xMgSi2O7:xEu2 þ nanofibers.

Dy3þ emission and excitation peak [23] are not present in patterns, indicating that Dy3þ ions play the role of the energy transfer and trap center [24], but not serve as the luminescent centers in the host crystal lattice.

kT 2m

!  Bτ ð2kT m Þ

ð1Þ

  C τ ¼ 1:51 þ 3:0 mg  0:42

ð2Þ

  Bτ ¼ 1:58 þ 4:2 mg  0:42

ð3Þ

E ¼ Cτ

x=0.05

400

The decay curve of Sr1.97  yMgSi2O7:0.03Eu2 þ , yDy3 þ nanofibers is shown in Fig. 9. All of them show both a rapid decaying process and slow-decaying process. The initial rapid decay is due to the short survival time of the electrons in Eu2 þ , the long-lasting afterglow decay is caused by the electrons captured in the deep trap energy level resulting from Dy3 þ . Compared with Sr1.93MgSi2O7:0.03Eu2 þ nanofibers, the Sr1.93MgSi2O7:0.03Eu2 þ , yDy3 þ nanofibers have a great improvement in luminescence intensity and decay times, indicating that Dy3 þ is a good doped rare earth ion. The afterglow intensity of Sr1.93MgSi2O7:0.03Eu2 þ , 0.04Dy3 þ nanofibers is strongest and decay speed is slowest. Consequently, Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofiber is a long-lasting luminescent material with superior properties. The thermoluminescence glow curves of Sr1.97 yMgSi2O7:0.03Eu2þ , yDy3þ nanofibers are shown in Fig. 10. Each curve showed a strong thermoluminescent peak at about 80 °C and the position deviation of peak is very small, but the thermoluminescence intensity has a significant change. The much broader bands and stronger intensity are shown with the increase of Dy3þ doping concentration, which due to the more Dy3þ was doped, the more resulting hole was captured by the trap. It is reasonable to deal with the trap depths of nanofiber using the general order kinetics and peak shape method proposed by McKeever [25] and Chen [26].

τ

where E is the trap depth, Tm is the temperature of the maximum peak, T1 is the temperature of half the peak intensity in low temperature, T2 is the temperature of half the peak intensity in high temperature, k is Boltzmann's constant, mg ¼ δ/ω is the geometrical factor, δ ¼ T2 Tm, ω ¼ T2  T1, τ ¼ Tm  T1. The trap depth was obtained with E¼ 0.61 eV, E¼ 0.64 eV and E¼ 0.60 eV for Sr1.97  yMgSi2O7:0.03Eu2 þ , yDy3 þ (y¼ 0.02, 0.04, 0.06) nanofibers, respectively. The trap depth E value is increasing firstly and then decreasing with the adding of Dy3 þ doping content. On the other hand, the trap depths was influenced by the codoping content of Dy3 þ , the more of doping Dy3 þ , The deeper the trap depths, the greater the number of the capture carrier, which accords well with the decay curves of Sr1.97 yMgSi2O7:0.03Eu2 þ , yDy3 þ nanofiber (Fig. 9).

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Fig. 8. The excitation and emission spectra of Sr1.97  yMgSi2O7:0.03Eu2 þ , yDy3 þ nanofibers.

4. Conclusions Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers with an average diameter of 200 nm were successfully prepared by electrospinning method. The single crystal Sr2MgSi2O7:Eu2 þ , Dy3 þ nanofibers were formed after calcinated at 1150 °C for 5 h. The optimal concentration of codoped Eu and Dy is x¼0.03, y¼0.04 in Sr2  x  yMgSi2O7:xEu2 þ , yDy3 þ electrospun nanofibers, the nanofibers have a blue emission peak at 471 nm attributed to the typical emission of Eu2 þ , which is made of the 4f65d1–4f7 transition. The co-doped Dy3 þ ion plays an important role as play the role of the energy transfer and electron traps and can prolong the persistence time of Sr2MgSi2O7: Eu2 þ , Dy3 þ luminescent nanofibers.

y=0.04

y=0.06 y=0.02

y=0 0

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100

Acknowledgment

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Time/s Fig. 9. The decay curves of Sr1.97  yMgSi2O7:0.03Eu2 þ , yDy3 þ nanofibers.

This work was financially supported by the Natural Science Foundation of Gansu Province of China (1310RJZA044), State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology (No. SKLAB02014010), Scientific Research Foundation of the Higher Education Institutions of Gansu Province (2013B-016) and Key Program for International S&T Cooperation Projects of China (2013DFR50790).

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Fig. 10. The thermoluminescence glow curves of Sr1.97  yMgSi2O7:0.03Eu2 þ , yDy3 þ nanofibers.

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