English

• 0.   Introduction

  In recent years, much attention has been paid to rare earth doped various phosphors, because it has the advantages of long-term stability, long lifetime and higher luminescent efficiency, etc. And it has been studied extensively for their potential applications in light emitting devices, displays, solid state lasers, optical amplifiers, sensors and optoelectronics devices, etc^[1-2]. Managing the red, green and blue light to get warm white light that makes our eyes feel comfortable. The use of red phosphor has more or less defects in recent years, which directly affects the quality of white luminescence. Therefore, the investigation of red phosphor has an important theoretical significance and practical application value^[3-5].

  In general, phosphors are composed of lumines-cent center and matrix materials. The rare earth Eu³⁺ shows red light under excitation with UV light and visible light and is an ideal red luminescence center. It is well known that due to the f-f forbidden transition of rare earth ions. The absorption efficiency is low, so it is necessary to find some methods to enhance the absorption efficiency. The method commonly used to enhance the absorption efficiency is to dope rare earth ions as an activator to select a suitable matrix. The energy absorbs from the matrix effectively transmits to the activation center (rare earth ion) to improve the luminescent efficiency. This shows that the choice of matrix is crucial. Phosphates have the advantages of charge stabilization, excellent thermal stability, high chemical stability and other excellent properties. So the phosphates have become an important matrix material. Meanwhile, the silicon dioxide have the advantages of good chemical stability, mechanical properties and low expansion coefficient. So it also becomes an excellent matrix material. But the reports about CePO₄, LaPO₄ or SiO₂ as single matrix are very common^{[3, 6-10]}, the reports about CePO₄, LaPO₄ and SiO₂ as composite matrix are unusual. In the paper, a novel red phosphor CePO₄-6LaPO₄@4SiO₂:Eu³⁺ was successfully synthesized by sol gel method and high temperature solid state method. The structure and luminescence properties of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphors also were investigated.

  1.   Experimental

  1.1   Synthesis of samples

  Firstly, the raw solution listed in Table 1 were added to clean beaker, which were stirred for 3 min. Then NH₃·H₂O was added to adjust pH value to 9 and kept stirring about 2 h to get a homogeneous sol. Secondly, the sol was transferred to a dry culture dish. After 2 days, it became a gel at room temperature. Next the samples were annealed at 900 ℃ for 17 hours. Finally, the samples were obtained.

  Table 1

  

  Table 1.  Experiment proportion of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

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  Solution	VTEOS/mL	VEtOH/mL	VLa(NO3)3·6H2O/mL	VCe(NO3)3·6H2O/mL	VEuCl3/mL	VH3PO4/mL
  1	0.76	4.24	9.00	1.50	4.39	10.50
  2	1.13	3.87	9.00	1.50	4.39	10.50
  3	1.50	3.50	9.00	1.50	4.39	10.50
  4	1.89	3.11	9.00	1.50	4.39	10.50

  1.2   Chemicals

  NH₃·H₂O purchased from Tianjin Guangfu Fine Chemical Research Institute (98.0%), which was dissolved in distilled water to obtain La(NO₃)₃ solution. Eu₂O₃ was purchased from Baotou Rare Earth Institute (99.99%), which was dissolved in HCl solution to obtain EuCl₃ (0.16 mol·L^-1). H₃PO₄ was purchased from Tianjin Chemical Reagent Third Factory (85.0%). NH₃·H₂O was purchased from Tianjin Sailboat Chemical Reagent Technology Co., Ltd. (25%~28%). CH₃COOH was purchased from Tianjin Beilian Fine Chemicals Development Co., Ltd (99.7%).

  1.3   Characterization

  The X-ray diffraction (XRD) pattern of powder samples were measured on a Rigaku-Dmax 2500 X diffractometer equipped with Cu Kα radiation (λ=0.154 05 nm, U=40 kV, I=30 mA, 2θ=10°~80°); A 200 keV JEOL 2010 (HRTEM) micros-cope was used to record Transmission Electron Micros-copy (TEM) images; IR spectra was acquired with using a Hitachi U-4100 UV-VIS-NIR spectrometer with the KBr pellet technique; The excitation and emission spectra were measured with using a F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

  2.   Results and discussion

  2.1   XRD study

  XRD patterns of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphors were reported in Fig. 1. For SiO₂ particles, no diffraction peak was observed, excepted for a very small broad peak at 2θ=22°, characteristic of amorphous silica^[11-12]. In addition, all the diffraction peaks of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor were agreed with the standard card (PDF No.32-0199 and No.46-1326). The indicated that a low amount of SiO₂ almost did not affect the crystal structure of phosphate. The above results showed that the sample was made up of crystalline state CePO₄, LaPO₄ and amorphous state SiO₂. Note that the intensity diffraction peaks of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phos-phor were a little improved than those of CePO₄-6LaPO₄:Eu³⁺ phosphor, suggesting that the size of the crystalline grains of phosphate on the surfaces of the silica particles in the composite was smaller than that in the pure phosphate. At the same time, no impurity peak from the third phase was detected, which indicated Eu³⁺ entered lattice in the matrix and occupied the site of La³⁺ and Ce³⁺ in the matrix to form a stable solid solution without affecting the structure of host lattice.

  Figure 1

  

  Figure 1.  XRD patterns of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺

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  According to previous reports^{[10, 12]}, to form a new solid solution, the radius in the percentage difference (D_r) between the doped ions and the possibly substi-tuted ion should be less than 15%, see the following formula:

  ${D_{\rm{r}}} = \frac{{{R_1}\left( {{\rm{CN}}} \right) - {R_2}\left( {{\rm{CN}}} \right)}}{{{R_1}\left({{\rm{CN}}} \right)}} \times 100 $

  Where R₁(CN) is the radius of the matrix cation and R₂(CN) is the radius of the doped ions. Herein, the values of R₁(CN) (Ce³⁺ and La³⁺) and R₂(CN) (Eu³⁺) are respectively 0.103, 0.106 and 0.095. According to this formula, the D_r value respectively was 7.8% and 10.4%, and less than 15%, further confirmed that the Eu³⁺ occupied the site of the La³⁺ and Ce³⁺ in the matrix to form a stable solid solution without affecting the structure of the CePO₄ and LaPO₄ host lattice.

  2.2   TEM study

  TEM image of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phos-phor was reported in Fig. 2(a). It showed that the grain morphology was irregular shape. The grain size was measured by Nano Measure App and was 0.38 μm. According to the XRD data, the grain size was calculated by Scherrer Formula and was 0.31μm. Both results were consistent.

  Figure 2

  

  Figure 2.  TEM images (a, b) and HRTEM image (c) of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  Fig. 2(b) showed TEM image of CePO₄-6LaPO₄@ 4SiO₂:Eu³⁺ phosphor, CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phos-phor was further sub-jected to TEM analysis to deter-mine the core-shell structure. The core-shell structure of the sample could be seen clearly due to the different electron penetrability of the core and shell. The core could be seen as a black irregular sphere and the shells had a gray color.

  The HRTEM image of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor was reported in Fig. 2(c). It clearly showed the CePO₄-6LaPO₄@4SiO₂ lattice and lattice fringes with an inter-planar spacing of 0.191 18, 0.215 28 and 0.312 30 nm, which had good match with those of the standard pattern of PDF card (LaPO₄/CePO₄), i.e., spacing for (231) 0.191 18 nm, (311) 0.215 28 nm and (120) 0.312 30 nm.

  2.3   EDS study

  The EDS spectrum of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor was reported in Fig. 3. The elements of O, Si, P, La, Ce and Eu were observed in the spectrum, and the ratio of La, Ce and Si elements was 6:1:4, which clearly indicated the formation of CePO₄-6LaPO₄@ 4SiO₂:Eu³⁺ phosphor.

  Figure 3

  

  Figure 3.  EDS energy spectrum of the CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  2.4   IR study

  IR spectra of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor were reported in Fig. 4. The absorption bands from 1 600 cm^-1 to 400 cm^-1 were observed, which had different infrared absorption peaks. In the spectra of CePO₄-6LaPO₄ as the matrix, the three peaks at 1 089, 1 063 and 1 016 cm^-1 were obtained by cleaving at 992 cm^-1 and corresponded the asymmetric stretching vibration peak of P-O, which was the characteristic of monoclinic phosphates^[17]. In the spectra of CePO₄-6LaPO₄@4SiO₂ as the matrix, the absorption peaks at 1 107 cm^-1 belonged to anti-stretching vibration peak of Si-O-Si. The absorption peaks located at 992, 952 cm^-1 were observed in the region of the stretching vibration of PO₄^3- group. The absorption peak at 797 cm^-1 was stretching vibration peak of Si-O-Si. The band located at 621 cm^-1 was observed in the region of O=P-O bending vibrations (it compared with the CePO₄-6LaPO₄ as the matrix, the peak position occurred blue shift). The bands at 575, 559 and 537 cm^-1 were the bending vibrations of O-P-O^[13-14] (it compared with CePO₄-6LaPO₄ as the matrix, the peak at 537 cm^-1 occurred red shift). The absorption peak at 466 cm^-1 was bending vibration peak of Si-O-Si (it compared with CePO₄-6LaPO₄ as matrix, the peak position occurred red shift). According to the above analysis, it showed that the sample was made up of phosphate and silicon dioxide, which was consistent with the result of XRD study.

  Figure 4

  

  Figure 4.  IR spectra of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  The polarization of Eu³⁺ was bigger than La³⁺/Ce³⁺, Eu³⁺ replaced La³⁺/Ce³⁺, which caused a slight change in the shape of PO₄^3- electron cloud. So the infrared vibration peak of PO₄^3- caused red shift and blue shift in different degrees.

  2.5   Luminescence study

  2.5.1   Excitation and emission spectrum of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

  The excitation spectra of core-shell structure CePO₄-6LaPO₄@xSiO₂:Eu³⁺ were reported in Fig. 5. The excitation spectrum consisted of two sharp peaks at 395 and 466 nm, and the peaks corresponded to the transitions from ⁷F₀ to ⁵L₆ (395 nm) and ⁵D₂ (466 nm). The emission intensity was strongest at 466 nm, so 466 nm was selected as the optimum excitation wavelength.

  Figure 5

  

  Figure 5.  Excitation spectra of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

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  The emission spectra of core-shell structure CePO₄-6LaPO₄@xSiO₂:Eu³⁺ phosphor were reported in Fig. 6. The emission peaks of 588 and 615 nm corresponded to the transitions from ⁵D₀ to ⁷F₁ and ⁷F₂, respectively. The core shell CePO₄-6LaPO₄@xSiO₂:Eu³⁺ had a luminous intensity that increased first and then decreased with the increased of SiO₂ in the matrix, and there is an optimal core-shell ratio. The shell thickness was measured by the molar ratio of core to the element contained in the shell^[18], when the core shell ratio was 1:4, the sample had the brightest red light at 615 nm. Because a large amount of Eu³⁺ was surrounded by SiO₂ and even diffuses into the SiO₂ network structure, greatly reducing the possibility of Eu³⁺ being in the surface state. That is to say, the appearance of the core-shell structure increased the distance between Eu³⁺ and the surface, thus changing the surrounding matrix environment of Eu³⁺ near the original surface (from unevenness to relatively uniform), which hindered the energy transfer of the original Eu³⁺ and surface states, so that improved luminous intensity of the sample. When the core shell ratio was more than 1:4, the luminescence intensity gradually decreased. As the thickness of the shell continued to increase, the thicker shell provided a uniform matrix environment. But the loss of light through the more, which led to the excitation light could be finally utilized became less, so that causing the emitted light to weaken. The above results indicated that the optimum ratio of phosphate to SiO₂ was 1:4.

  Figure 6

  

  Figure 6.  Emission spectra of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

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  2.5.2   Luminescence study of CePO₄-6LaPO₄@4SiO₂: Eu³⁺ and CePO₄-6LaPO₄:Eu³⁺ phosphors

  The emission spectra of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ were reported in Fig. 7. Before the addition of SiO₂, the main peak of CePO₄-6LaPO₄:Eu³⁺ phosphor was located at 588 nm. After the addition of SiO₂, the main peak of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor was located at 615 nm. The showed that the addition of SiO₂ effectively increased red light intensity of the sample.

  Figure 7

  

  Figure 7.  Emission spectra of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺

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  The sample emitted orange light and red light, which was closely related to the crystallographic position of Eu^{3+ [15-16]}. The transition ⁵D₀→⁷F₁ was dominant when Eu³⁺ occupied the lattice site with inversion center, and the sample emitted orange light (the main peak was located at 588 nm). The transition ⁵D₀→⁷F₂ was dominant when Eu³⁺ occupied the lattice site without inversion center, and the sample emitted red light (the main peak was located at 615 nm). In the experiment, the Eu³⁺ of CePO₄-6LaPO₄:Eu³⁺ phosphor occupied the lattice site with inversion center, so the sample emitted orange light; the Eu³⁺ of CePO₄-6LaPO₄ @4SiO₂:Eu³⁺ phosphor occupied the lattice site without inversion center, so the sample emitted red light. It as shown as Fig. 8. The intensity pattern of the emission lines demonstrate that the Eu³⁺ was successfully doped in the sample. In addition, the formation of core-shell structure was also beneficial to red luminescence^[18-19].

  Figure 8

  

  Figure 8.  Energy level diagram

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  3.   Conclusions

  The CePO₄-6LaPO₄@xSiO₂:Eu³⁺ red phosphors were synthesized by sol gel method and high temperature solid statue method. The results showed that the optimal excitation wavelength of the phosphor was 466 nm and the optimum ratio of phosphate to SiO₂ was 1:4. And the appearance of core-shell structure CePO₄-6LaPO₄@4SiO₂ was more conducive to the doping of Eu³⁺ and improved the red light of the sample. It indi-cated that CePO₄-6LaPO₄@4SiO₂:Eu³⁺ could be a good candidate as red phosphor or it combined with blue and green phosphor to get ideal quality warm WLED.

• 1. [1]

     Du P, Yu J S. J. Lumin., 2016, 179:451-456 doi: 10.1016/j.jlumin.2016.07.045

  2. [2]

     Li K, Lian H Z, Shang M M, et al. Dalton Trans., 2015, 44(47):20542-20550 doi: 10.1039/C5DT03565A

  3. [3]

     Yang J Y, Jia X H, Zeng X D, et al. J. Mater. Sci., 2015, 50(12):4405-4411 doi: 10.1007/s10853-015-8996-y

  4. [4]

     金叶, 秦伟平, 张继森, 等.光谱学与光谱分析, 2008, 28(12):2768 doi: 10.3964/j.issn.1000-0593(2008)12-2768-04JIN Ye, QIN Wei-Ping, ZHANG Ji-Sen, et al. Spectrosc. Spect. Anal., 2008, 28(12):2768 doi: 10.3964/j.issn.1000-0593(2008)12-2768-04

  5. [5]

     Gupta S K, Ghosh P S, Sahu M, et al. RSC Adv., 2015, 5(72):58832-58842 doi: 10.1039/C5RA09076H

  6. [6]

     王喜贵, 吴红英, 谢大弢, 等.中国稀土学报, 2002, 20(3):172-176 http://www.cnki.com.cn/Article/CJFDTotal-YXTB200203002.htmWANG Xi-Gui, WU Hong-Ying, XIE Da-Tao, et al. J. Rare Earth, 2002, 20(3):172-176 http://www.cnki.com.cn/Article/CJFDTotal-YXTB200203002.htm

  7. [7]

     吴雪艳, 尤洪鹏, 曾小青, 等.高等学校化学学报, 2003, 24(1):1-4 doi: 10.3321/j.issn:0251-0790.2003.01.001WU Xue-Yan, YOU Hong-Peng, ZENG Xiao-Qing, et al. Chem. J. Chinese Universities, 2003, 24(1):1-4 doi: 10.3321/j.issn:0251-0790.2003.01.001

  8. [8]

     Hatada N, Nose Y, Kuramitsu A, et al. J. Mater. Chem., 2011, 21(24):8781-8786 doi: 10.1039/c0jm03171b

  9. [9]

     Cao M H, Hu C W, Wu Q Y, et al. Nanotechnology, 2005, 16(2):282-286 http://www.ncbi.nlm.nih.gov/pubmed/21727437

  10. [10]

      Riwotzki K, Meyssamy H, Kornowski A, et al. J. Phys. Chem. B, 2000, 104(13)2824-2828 doi: 10.1021/jp993581r

  11. [11]

      Zhang J M, Zhao D L, Zhang D D, et al. Asian J. Chem., 2014, 26:2211-2214 doi: 10.14233/ajchem.2014.15656

  12. [12]

      Musi S, Filipovi-Vincekovi N, Sekovani L, et al. J. Chem. Eng., 2011, 28:89-94

  13. [13]

      Ray S, Nair G B, Tadge P, et al. J. Lumin., 2018, 194:64-71 doi: 10.1016/j.jlumin.2017.10.015

  14. [14]

      Krishna B L, Jeon Y I, Yu J S. J. Alloy Compd., 2014, 614:443-447 doi: 10.1016/j.jallcom.2014.06.108

  15. [15]

      王喜贵, 薄素玲, 齐霞, 等.无机化学学报, 2009, 25(2):350-353 doi: 10.3321/j.issn:1001-4861.2009.02.028WANG Xi-Gui, BO Su-Ling, QI Xia, et al. Chinese J. Inorg. Chem., 2009, 25(2):350-353 doi: 10.3321/j.issn:1001-4861.2009.02.028

  16. [16]

      曹娇兰, 王喜贵.无机化学学报, 2018, 34(2):325-330 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?file_no=20180215&flag=1CAO Jiao-Lan, WANG Xi-Gui. Chinese J. Inorg. Chem., 2018, 34(2):325-330 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?file_no=20180215&flag=1

  17. [17]

      Zhao M L, Li G S, Li L P. Cryst. Growth Des., 2012, 12:3983-3991 doi: 10.1021/cg300451d

  18. [18]

      Lim M A, Seok Ⅱ S, Ghung W J, et al. Opt. Mater., 2008, 31(2):201-205

  19. [19]

      刘桂霞, 李若兰, 董向婷, 等.发光学报, 2011, 32(5):466-470 http://en.cnki.com.cn/Article_en/CJFDTOTAL-FGXB201003027.htmLIU Gui-Xia, LI Ruo-Lan, DONG Xiang-Ting, et al. Chin. J. Lumin., 2011, 32(5):466-470 http://en.cnki.com.cn/Article_en/CJFDTOTAL-FGXB201003027.htm

• 

  Figure 1  XRD patterns of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺

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  Figure 2  TEM images (a, b) and HRTEM image (c) of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  Figure 3  EDS energy spectrum of the CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  Figure 4  IR spectra of CePO₄-6LaPO₄@4SiO₂:Eu³⁺ phosphor

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  Figure 5  Excitation spectra of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

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  Figure 6  Emission spectra of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

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  Figure 7  Emission spectra of CePO₄-6LaPO₄:Eu³⁺ and CePO₄-6LaPO₄@4SiO₂:Eu³⁺

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  Figure 8  Energy level diagram

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  Table 1.  Experiment proportion of CePO₄-6LaPO₄@xSiO₂:Eu³⁺

  Solution	VTEOS/mL	VEtOH/mL	VLa(NO3)3·6H2O/mL	VCe(NO3)3·6H2O/mL	VEuCl3/mL	VH3PO4/mL
  1	0.76	4.24	9.00	1.50	4.39	10.50
  2	1.13	3.87	9.00	1.50	4.39	10.50
  3	1.50	3.50	9.00	1.50	4.39	10.50
  4	1.89	3.11	9.00	1.50	4.39	10.50

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•