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Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass
Accepted Manuscript Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass Yang Li, Lili Hu, Bobo Yang, Mingming Shi, Jun Zou PII:
S0925-8388(16)33173-5
DOI:
10.1016/j.jallcom.2016.10.065
Reference:
JALCOM 39232
To appear in:
Journal of Alloys and Compounds
Received Date: 14 August 2016 Revised Date:
6 October 2016
Accepted Date: 8 October 2016
Please cite this article as: Y. Li, L. Hu, B. Yang, M. Shi, J. Zou, Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass Yang Li a,b,c*, Lili Hua*, Bobo Yangd, Mingming Shid, Jun Zoud a
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Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China d School of Science, Shanghai Institute of Technology, Shanghai 201418,PR China
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Abstract Colour conversion glasses were prepared by cosintering borosilicate glass frits and Ce:YAG phosphors at various temperatures in the 600–900 °C range. The effect of hydrogen annealing on the photoluminescence (PL) of the colour conversion glasses was investigated. The relative photoluminescence (PL) intensity of the colour conversion glasses sintered at 600–800 °C was significantly enhanced from 4.87% to 225.61% after hydrogen annealing for 3 h. However,
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changes to the relative PL intensity of the colour conversion glasses sintered above 850 °C after hydrogen annealing were miniscule, because the corrosion of the glass matrix disturbed the YAG lattice. The findings revealed that hydrogen annealing improved the PL intensity of the colour conversion glasses only when the crystal structure of YAG was intact. Keywords: Colour conversion glass; Photoluminescence intensity; Sintering temperature; Hydrogen annealing.
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Introduction
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White light-emitting diodes (WLEDs) have been widely applied in lighting, display backlights, and automotive headlights as a new source of luminescence, because of their high efficiency, long lifetime, and environmentally friendly features [1–4]. The current commercial WLED is commonly created using epoxy resin or silicone as an encapsulating agent and packing Ce:YAG yellow-emitting phosphors onto the surface of a blue LED chip [5, 6]. However, epoxy resin and silicone have low thermal conductivity and thermal stability, which may easily degrade the long-term reliability of the WLED [7, 8]. Consequently, inorganic materials such as phosphor ceramics and colour conversion glasses have recently emerged as practical alternatives to organic polymer binders [9, 10]. Phosphor ceramics, particularly transparent ceramic plates, are synthesised from oxide powder, guaranteeing high luminous efficacy [7]. However, high production cost is an unavoidable challenge to the mass production of transparent ceramics [9]. Colour conversion glasses are produced using two main methods: 1) glass crystallisation, well-controlled crystallisation of precursor glasses * Corresponding authors. Tel: +86 13764586556 (Yang Li).Tel./fax: +86 21 59917846(Lili Hu) E-mail addresses: [email protected] (Yang Li).
[email protected] (Lili Hu)
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that are heated at an optimal temperature; 2) the cosintering of phosphors and matrix glass powders at a temperature below 900 °C [11, 12]. Compared with glass crystallisation, the cosintering route has been studied more widely because of its easy process and promising future in LED applications. When producing Ce:YAG colour conversion glasses, the reaction between Ce:YAG and the glass matrix must be controlled during cosintering [13, 14]. This reaction can be limited if the sintering has two key elements: a low sintering temperature which can prevent the melting of the glass and maintain the crystal structure of YAG and reducing atmosphere which can decrease the oxidation of Ce3+ by the glass matrix. In fact, several types of glasses with low melting temperatures are used as glass matrices in colour conversion glasses, such as borate, phosphate, and tellurite [15–17]. However, the effects of a reducing atmosphere on the luminescence of the colour conversion glasses have not been reported. In this study, borosilicate glasses and Ce:YAG phosphors were cosintered at varying temperatures between 600 °C and 900 °C to prepare colour conversion glasses. All the samples of colour conversion glasses were subsequently annealed at 550 °C for 3 h in hydrogen atmosphere. Herein, we compare and analyse the photoluminescence (PL) properties of the colour conversion glasses before and after annealing. Moreover, we discuss the variation in the crystal structure and Ce3+ content of the conversion glasses before and after annealing. Experiments
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To prepare precursor glass with the composition “B2O3-SiO2-ZnO-BaO-Na2O”, a conventional melting–quenching method was employed in air at 1100 °C for 1 h, and the product was quickly poured into a cold copper mould and cooled to room temperature. The glass was milled to powder using a ball grinder, and then mixed with 3 wt% YAG (Y3Al5O12:Ce; YAP4454-L, RayPower, China) phosphors thoroughly. Subsequently, the mixture was sintered in a platinum crucible at a temperature of 600–900 °C for 20 min in ambient atmosphere. The sintered body was then polished and cut into a sample with 1 mm thickness. Finally, all the colour conversion glass samples were annealed at 550 °C for 3 h in hydrogen atmosphere. The crystal structure of the colour conversion glass was observed through X-ray diffraction (XRD; Rigaku, Ultima IV, Japan) with Cu Ka radiation (k = 0.154178 nm) over a 10°–70° 2θ range, at a scanning rate of 0.02 °/step and 4 °/min. The photoluminescence (PL) and excitation (PLE) spectra were measured by Edinburgh Instruments, using a Xenon lamp as the light source. The concentrations of Ce3+and Ce4+ were tested using an X-ray photoelectron spectrum analyser (Escalab 250Xi, ThermoFisher), of which the minimum resolution was 0.1 eV. 3 Results and discussion
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Fig.1 PLE and PL spectra of the colour conversion glasses sintered at different temperatures: (a) PLE spectra before annealing; (b) PL spectra before annealing; (c) PLE spectra after annealing; (d) PL spectra after annealing Fig. 1a–d exhibits the sintering temperature dependence of the PLE spectra, PLE intensity, PL spectra, and PL intensity of the colour conversion glasses before and after hydrogen annealing.
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When the sintering temperature was below 650 °C, the PLE and PL intensity decreased, because of the poor transparency of the colour conversion glasses [18]. The maximum PLE and maximum PL intensity were observed at a sintering temperature of 700 ℃ both before and after annealing. All excitation and emission peaks disappeared when the sintering temperature increased beyond
(a)
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850 °C. Furthermore, excessively high sintering temperatures clearly weakened the PL intensity of the colour conversion glasses. (b)
Fig.2 Increasing ratio of spectral peak intensity of the colour conversion glasses sintering at
ACCEPTED MANUSCRIPT different temperatures after annealing: (a) Increasing ratio of PLE spectral peak intensity; (b) Increasing ratio of PL spectral peak intensity. Fig. 2a–b displays the increasing ratio of the maximum intensity of the PLE and PL spectra of the colour conversion glasses sintering at various temperatures after annealing. As shown in Fig. 2a, the relative PLE spectral peak intensity of the colour conversion glasses sintered at
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600–800 °C was significantly enhanced from 13.33% to 219.79% after hydrogen annealing for 3 h. Additionally, Fig. 2b illustrates that the PL spectral peak intensity of the colour conversion glasses
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sintered at 600–800 °C increased from 4.87% to 225.61% after hydrogen annealing for 3 h. However, when the sintering temperature was above 850 °C, hydrogen annealing did not improve the PLE and PL intensity of the colour conversion glasses.
Fig.3 XPS patterns of C1s (b)
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Fig.4 XPS spectra and curve-fittings of Ce3d in the colour conversion glasses sintered at 750
ACCEPTED MANUSCRIPT and 850 before and after hydrogen annealing: (a) glass sintered at 750 before annealing; (b) glass sintered at 850 before annealing; (c) glass sintered at 750 after annealing; (d) glass sintered at 850 after annealing. 3+
Table 1 Relative percentage of Ce in the glass sintered at 750
before annealing
FWHM/eV
Area /(a.u)
Sum/(a.u.)
Ce4+3d5/2 Ce4+3d3/2
886.812 899.095
5.56 7.08
51267.754 74921.807
126189.561
45.65
Ce3+3d5/2 Ce3+3d3/2
890.529 904.396
3.94 5.58
52977.376 97254.166
150231.542
54.35
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Table 2 Relative percentage of Ce in the glass sintered at 850 FWHM/eV
Ce4+3d5/2 Ce4+3d3/2
887.278 901.407
6.84 8.04
Ce3+3d5/2 Ce3+3d3/2
890.068 906.052
4.92 4.55
Area /(a.u)
Sum/(a.u.)
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Peak/eV
Relative percentage/%
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Peak/eV
before annealing Relative percentage/%
47792.376 126463.781
174256.157
75.40
32176.475 24691.438
56867.913
24.60
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Table 3 Relative percentage of Ce in the glass sintered at 750
884.572 898.805
Ce3+3d5/2 Ce3+3d3/2
889.524 903.557
Area /(a.u)
Sum/(a.u.)
Relative percentage/%
4.42 4.91
16577.368 36721.430
53298.798
22.24
5.59 7.12
71314.339 115051.004
186365.343
77.76
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Ce4+3d5/2 Ce4+3d3/2
FWHM/eV
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Peak/eV
after annealing
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Table 4 Relative percentage of Ce in the glass sintered at 850
after annealing
Peak/eV
FWHM/eV
Area /(a.u)
Sum/(a.u.)
Relative percentage/%
Ce4+3d5/2 Ce4+3d3/2
885.983 900.476
6.19 5.74
99553.763 152156.463
251710.226
51.11
Ce3+3d5/2 Ce3+3d3/2
890.726 904.846
5.10 5.43
87713.836 153039.824
240753.660
48.89
As displayed in Fig. 3, the maximum binding energy of the C1s was 284.8 eV, signifying that the equipment was correctly calibrated. Based on the standard database of XPS spectra, the 884, 902, 881.6, and 899.9 eV binding energies correspond to the Ce3+3d5/2 ,Ce3+3d3/2, Ce4+3d5/2, and Ce4+3d3/2 components, respectively. When the crystal field around the Ce atoms changes, the
ACCEPTED MANUSCRIPT maximum binding energy may be offset by a maximum of 17.9 eV [19]. Fig. 4 presents the XPS
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spectra and curve fittings of the Ce3d in the colour conversion glasses sintered at 750 °C and 850 °C before and after hydrogen annealing. All spectral fitting results, namely maximum binding energy, FWHM, peak area, and the relative percentage of Ce3+ (ratio of peak area[Ce3+]/peak area [Ce3++Ce4+]) of the four types of colour conversion glasses are presented in Tables 1–4. The spectral fitting results indicated that the Ce3+content of the glasses decreased with increasing sintering temperature, because Ce3+ was oxidised to Ce4+. As shown in Table 1 and Table 2, before annealing, the Ce3+ content of the glasses decreased from 54.35% to 24.60%; we consider that the decreased Ce3+ content was the reason why the colour conversion glass sintered above
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850 °C exhibited the poorest luminescence performance (Fig. 1). According to the luminescence principle of Ce:YAG [20,21], only one electron in the 4f layer of Ce3+ can transit to the upper level, 5d, to gain luminescence. If Ce3+ is oxidised to Ce4+, then no electrons remain in the 4f layer to achieve excitation and emission between the 4f and 5d levels. Therefore, the Ce3+ concentration would directly affect the PL intensity of the colour conversion glasses. Additionally, as illustrated in Table 3, the concentration of Ce3+ increased from 54.35% to 77.76% after the hydrogen
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annealing of the colour conversion glasses sintered at 750 °C, because a large portion of Ce4+ ions were reduced to Ce3+ ions. Thus, the relative PL intensity of the colour conversion glasses sintered at 750 °C was significantly enhanced to 225.61% as hydrogen reduction continually increased the concentration of Ce3+. Low sintering temperature and hydrogen annealing were therefore beneficial for increasing the concentration of Ce3+ ions and thereby increasing the PL intensity of the colour conversion glasses. (b)
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Fig.5 XRD patterns of Ce:YAG phosphors and colour conversion glasses under different sintering temperatures: (a) XRD of phosphors;(b) XRD of the colour conversion glasses Fig. 5 presents the results of the XRD analyses of Ce:YAG phosphors and the four types of colour conversion glasses sintered at different temperatures. As shown in Fig. 5a, no obvious change in the diffraction peaks of YAG occurred as the sintering temperature increased. However, some XRD peaks of YAG disappeared, as displayed in Fig. 5b. When the sintering temperature increased beyond 850 °C, the lattice structure of YAG became damaged, and thus the diffraction peaks entirely disappeared. The damage of the lattice structure of YAG in the colour conversion glasses engendered the missing excitation and emission peaks for sintering temperatures above 850 °C (Fig. 1). Although the concentration of the Ce3+ ions in the glasses sintered at 850 °C
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Summary
The effect of hydrogen annealing on the colour conversion glasses sintered at 600–900 °C was examined. Hydrogen annealing significantly increased the maximum PL intensity of the
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colour conversion glasses sintered below 800 °C from 4.87% to 225.61%, owing to the increase in the concentration of Ce3+ ions through hydrogen reduction. However, when the sintering temperature was above 850 °C, the colour conversion glasses did not exhibit notable PL performance levels, which can be attributed to the breaking of the crystal structure surrounding the Ce3+ ions. The experimental results demonstrate that hydrogen annealing can increase Ce3+ concentration and PL intensity of colour conversion glasses when the sintering temperature is
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below 800 °C.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51302171), Science and Technology Commission of Shanghai Municipality (14500503300), Natural Science Foundation of Shanghai (12ZR1430900), and Shanghai Municipal Alliance Programme (LM201547).
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[6] C.Yang, X.J.Liang, X.X.Di, P.Z.Li, G.C.Hu, R.Cao, W.D.Xiang, Facile fabrication and luminescence characteristics of Ce:YAG phosphor glass thick films coated on a glass substrate for white LEDs, Ceram. Int. 42(2016)14526–14532. [7] N. Wei, T. C. Lu, F. Li, W. Zhang, B.Y.Ma, Z. W. Lu, J. Q. Qi, Transparent Ce:Y3Al5O12 ceramic phosphors for white light-emitting diodes, Appl. Phys. Lett. 101(2012)061902-061902-4. [8] K.Han, S.H.Lee, Y.G.Choi, W.B.Im,W.J.Chung, Improved color rendering index and
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ACCEPTED MANUSCRIPT Highlights: 1. The color conversion glass was prepared by a co-sintering method. 2. All glass samples were annealed in the hydrogen atmosphere.
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3. Hydrogen annealing was beneficial to improve photoluminescence intensity.
S0925-8388(16)33173-5
DOI:
10.1016/j.jallcom.2016.10.065
Reference:
JALCOM 39232
To appear in:
Journal of Alloys and Compounds
Received Date: 14 August 2016 Revised Date:
6 October 2016
Accepted Date: 8 October 2016
Please cite this article as: Y. Li, L. Hu, B. Yang, M. Shi, J. Zou, Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Effect of hydrogen annealing on the photoluminescence properties of colour conversion glass in borosilicate glass Yang Li a,b,c*, Lili Hua*, Bobo Yangd, Mingming Shid, Jun Zoud a
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Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China d School of Science, Shanghai Institute of Technology, Shanghai 201418,PR China
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Abstract Colour conversion glasses were prepared by cosintering borosilicate glass frits and Ce:YAG phosphors at various temperatures in the 600–900 °C range. The effect of hydrogen annealing on the photoluminescence (PL) of the colour conversion glasses was investigated. The relative photoluminescence (PL) intensity of the colour conversion glasses sintered at 600–800 °C was significantly enhanced from 4.87% to 225.61% after hydrogen annealing for 3 h. However,
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changes to the relative PL intensity of the colour conversion glasses sintered above 850 °C after hydrogen annealing were miniscule, because the corrosion of the glass matrix disturbed the YAG lattice. The findings revealed that hydrogen annealing improved the PL intensity of the colour conversion glasses only when the crystal structure of YAG was intact. Keywords: Colour conversion glass; Photoluminescence intensity; Sintering temperature; Hydrogen annealing.
1
Introduction
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White light-emitting diodes (WLEDs) have been widely applied in lighting, display backlights, and automotive headlights as a new source of luminescence, because of their high efficiency, long lifetime, and environmentally friendly features [1–4]. The current commercial WLED is commonly created using epoxy resin or silicone as an encapsulating agent and packing Ce:YAG yellow-emitting phosphors onto the surface of a blue LED chip [5, 6]. However, epoxy resin and silicone have low thermal conductivity and thermal stability, which may easily degrade the long-term reliability of the WLED [7, 8]. Consequently, inorganic materials such as phosphor ceramics and colour conversion glasses have recently emerged as practical alternatives to organic polymer binders [9, 10]. Phosphor ceramics, particularly transparent ceramic plates, are synthesised from oxide powder, guaranteeing high luminous efficacy [7]. However, high production cost is an unavoidable challenge to the mass production of transparent ceramics [9]. Colour conversion glasses are produced using two main methods: 1) glass crystallisation, well-controlled crystallisation of precursor glasses * Corresponding authors. Tel: +86 13764586556 (Yang Li).Tel./fax: +86 21 59917846(Lili Hu) E-mail addresses: [email protected] (Yang Li).
[email protected] (Lili Hu)
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that are heated at an optimal temperature; 2) the cosintering of phosphors and matrix glass powders at a temperature below 900 °C [11, 12]. Compared with glass crystallisation, the cosintering route has been studied more widely because of its easy process and promising future in LED applications. When producing Ce:YAG colour conversion glasses, the reaction between Ce:YAG and the glass matrix must be controlled during cosintering [13, 14]. This reaction can be limited if the sintering has two key elements: a low sintering temperature which can prevent the melting of the glass and maintain the crystal structure of YAG and reducing atmosphere which can decrease the oxidation of Ce3+ by the glass matrix. In fact, several types of glasses with low melting temperatures are used as glass matrices in colour conversion glasses, such as borate, phosphate, and tellurite [15–17]. However, the effects of a reducing atmosphere on the luminescence of the colour conversion glasses have not been reported. In this study, borosilicate glasses and Ce:YAG phosphors were cosintered at varying temperatures between 600 °C and 900 °C to prepare colour conversion glasses. All the samples of colour conversion glasses were subsequently annealed at 550 °C for 3 h in hydrogen atmosphere. Herein, we compare and analyse the photoluminescence (PL) properties of the colour conversion glasses before and after annealing. Moreover, we discuss the variation in the crystal structure and Ce3+ content of the conversion glasses before and after annealing. Experiments
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To prepare precursor glass with the composition “B2O3-SiO2-ZnO-BaO-Na2O”, a conventional melting–quenching method was employed in air at 1100 °C for 1 h, and the product was quickly poured into a cold copper mould and cooled to room temperature. The glass was milled to powder using a ball grinder, and then mixed with 3 wt% YAG (Y3Al5O12:Ce; YAP4454-L, RayPower, China) phosphors thoroughly. Subsequently, the mixture was sintered in a platinum crucible at a temperature of 600–900 °C for 20 min in ambient atmosphere. The sintered body was then polished and cut into a sample with 1 mm thickness. Finally, all the colour conversion glass samples were annealed at 550 °C for 3 h in hydrogen atmosphere. The crystal structure of the colour conversion glass was observed through X-ray diffraction (XRD; Rigaku, Ultima IV, Japan) with Cu Ka radiation (k = 0.154178 nm) over a 10°–70° 2θ range, at a scanning rate of 0.02 °/step and 4 °/min. The photoluminescence (PL) and excitation (PLE) spectra were measured by Edinburgh Instruments, using a Xenon lamp as the light source. The concentrations of Ce3+and Ce4+ were tested using an X-ray photoelectron spectrum analyser (Escalab 250Xi, ThermoFisher), of which the minimum resolution was 0.1 eV. 3 Results and discussion
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Fig.1 PLE and PL spectra of the colour conversion glasses sintered at different temperatures: (a) PLE spectra before annealing; (b) PL spectra before annealing; (c) PLE spectra after annealing; (d) PL spectra after annealing Fig. 1a–d exhibits the sintering temperature dependence of the PLE spectra, PLE intensity, PL spectra, and PL intensity of the colour conversion glasses before and after hydrogen annealing.
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When the sintering temperature was below 650 °C, the PLE and PL intensity decreased, because of the poor transparency of the colour conversion glasses [18]. The maximum PLE and maximum PL intensity were observed at a sintering temperature of 700 ℃ both before and after annealing. All excitation and emission peaks disappeared when the sintering temperature increased beyond
(a)
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850 °C. Furthermore, excessively high sintering temperatures clearly weakened the PL intensity of the colour conversion glasses. (b)
Fig.2 Increasing ratio of spectral peak intensity of the colour conversion glasses sintering at
ACCEPTED MANUSCRIPT different temperatures after annealing: (a) Increasing ratio of PLE spectral peak intensity; (b) Increasing ratio of PL spectral peak intensity. Fig. 2a–b displays the increasing ratio of the maximum intensity of the PLE and PL spectra of the colour conversion glasses sintering at various temperatures after annealing. As shown in Fig. 2a, the relative PLE spectral peak intensity of the colour conversion glasses sintered at
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600–800 °C was significantly enhanced from 13.33% to 219.79% after hydrogen annealing for 3 h. Additionally, Fig. 2b illustrates that the PL spectral peak intensity of the colour conversion glasses
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sintered at 600–800 °C increased from 4.87% to 225.61% after hydrogen annealing for 3 h. However, when the sintering temperature was above 850 °C, hydrogen annealing did not improve the PLE and PL intensity of the colour conversion glasses.
Fig.3 XPS patterns of C1s (b)
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Fig.4 XPS spectra and curve-fittings of Ce3d in the colour conversion glasses sintered at 750
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Table 1 Relative percentage of Ce in the glass sintered at 750
before annealing
FWHM/eV
Area /(a.u)
Sum/(a.u.)
Ce4+3d5/2 Ce4+3d3/2
886.812 899.095
5.56 7.08
51267.754 74921.807
126189.561
45.65
Ce3+3d5/2 Ce3+3d3/2
890.529 904.396
3.94 5.58
52977.376 97254.166
150231.542
54.35
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Table 2 Relative percentage of Ce in the glass sintered at 850 FWHM/eV
Ce4+3d5/2 Ce4+3d3/2
887.278 901.407
6.84 8.04
Ce3+3d5/2 Ce3+3d3/2
890.068 906.052
4.92 4.55
Area /(a.u)
Sum/(a.u.)
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Peak/eV
Relative percentage/%
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Peak/eV
before annealing Relative percentage/%
47792.376 126463.781
174256.157
75.40
32176.475 24691.438
56867.913
24.60
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Table 3 Relative percentage of Ce in the glass sintered at 750
884.572 898.805
Ce3+3d5/2 Ce3+3d3/2
889.524 903.557
Area /(a.u)
Sum/(a.u.)
Relative percentage/%
4.42 4.91
16577.368 36721.430
53298.798
22.24
5.59 7.12
71314.339 115051.004
186365.343
77.76
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Ce4+3d5/2 Ce4+3d3/2
FWHM/eV
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Peak/eV
after annealing
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Table 4 Relative percentage of Ce in the glass sintered at 850
after annealing
Peak/eV
FWHM/eV
Area /(a.u)
Sum/(a.u.)
Relative percentage/%
Ce4+3d5/2 Ce4+3d3/2
885.983 900.476
6.19 5.74
99553.763 152156.463
251710.226
51.11
Ce3+3d5/2 Ce3+3d3/2
890.726 904.846
5.10 5.43
87713.836 153039.824
240753.660
48.89
As displayed in Fig. 3, the maximum binding energy of the C1s was 284.8 eV, signifying that the equipment was correctly calibrated. Based on the standard database of XPS spectra, the 884, 902, 881.6, and 899.9 eV binding energies correspond to the Ce3+3d5/2 ,Ce3+3d3/2, Ce4+3d5/2, and Ce4+3d3/2 components, respectively. When the crystal field around the Ce atoms changes, the
ACCEPTED MANUSCRIPT maximum binding energy may be offset by a maximum of 17.9 eV [19]. Fig. 4 presents the XPS
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spectra and curve fittings of the Ce3d in the colour conversion glasses sintered at 750 °C and 850 °C before and after hydrogen annealing. All spectral fitting results, namely maximum binding energy, FWHM, peak area, and the relative percentage of Ce3+ (ratio of peak area[Ce3+]/peak area [Ce3++Ce4+]) of the four types of colour conversion glasses are presented in Tables 1–4. The spectral fitting results indicated that the Ce3+content of the glasses decreased with increasing sintering temperature, because Ce3+ was oxidised to Ce4+. As shown in Table 1 and Table 2, before annealing, the Ce3+ content of the glasses decreased from 54.35% to 24.60%; we consider that the decreased Ce3+ content was the reason why the colour conversion glass sintered above
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850 °C exhibited the poorest luminescence performance (Fig. 1). According to the luminescence principle of Ce:YAG [20,21], only one electron in the 4f layer of Ce3+ can transit to the upper level, 5d, to gain luminescence. If Ce3+ is oxidised to Ce4+, then no electrons remain in the 4f layer to achieve excitation and emission between the 4f and 5d levels. Therefore, the Ce3+ concentration would directly affect the PL intensity of the colour conversion glasses. Additionally, as illustrated in Table 3, the concentration of Ce3+ increased from 54.35% to 77.76% after the hydrogen
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annealing of the colour conversion glasses sintered at 750 °C, because a large portion of Ce4+ ions were reduced to Ce3+ ions. Thus, the relative PL intensity of the colour conversion glasses sintered at 750 °C was significantly enhanced to 225.61% as hydrogen reduction continually increased the concentration of Ce3+. Low sintering temperature and hydrogen annealing were therefore beneficial for increasing the concentration of Ce3+ ions and thereby increasing the PL intensity of the colour conversion glasses. (b)
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(a)
Fig.5 XRD patterns of Ce:YAG phosphors and colour conversion glasses under different sintering temperatures: (a) XRD of phosphors;(b) XRD of the colour conversion glasses Fig. 5 presents the results of the XRD analyses of Ce:YAG phosphors and the four types of colour conversion glasses sintered at different temperatures. As shown in Fig. 5a, no obvious change in the diffraction peaks of YAG occurred as the sintering temperature increased. However, some XRD peaks of YAG disappeared, as displayed in Fig. 5b. When the sintering temperature increased beyond 850 °C, the lattice structure of YAG became damaged, and thus the diffraction peaks entirely disappeared. The damage of the lattice structure of YAG in the colour conversion glasses engendered the missing excitation and emission peaks for sintering temperatures above 850 °C (Fig. 1). Although the concentration of the Ce3+ ions in the glasses sintered at 850 °C
ACCEPTED MANUSCRIPT increased after hydrogen annealing, the colour conversion glasses did not display PL, because of the broken crystal structure surrounding the Ce3+ ions. 4
Summary
The effect of hydrogen annealing on the colour conversion glasses sintered at 600–900 °C was examined. Hydrogen annealing significantly increased the maximum PL intensity of the
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colour conversion glasses sintered below 800 °C from 4.87% to 225.61%, owing to the increase in the concentration of Ce3+ ions through hydrogen reduction. However, when the sintering temperature was above 850 °C, the colour conversion glasses did not exhibit notable PL performance levels, which can be attributed to the breaking of the crystal structure surrounding the Ce3+ ions. The experimental results demonstrate that hydrogen annealing can increase Ce3+ concentration and PL intensity of colour conversion glasses when the sintering temperature is
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below 800 °C.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51302171), Science and Technology Commission of Shanghai Municipality (14500503300), Natural Science Foundation of Shanghai (12ZR1430900), and Shanghai Municipal Alliance Programme (LM201547).
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ACCEPTED MANUSCRIPT Highlights: 1. The color conversion glass was prepared by a co-sintering method. 2. All glass samples were annealed in the hydrogen atmosphere.
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3. Hydrogen annealing was beneficial to improve photoluminescence intensity.