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Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2+ phosphors prepared by glycothermal methods
Journal of Luminescence 132 (2012) 64–70
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Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2 þ phosphors prepared by glycothermal methods Keisuke Uegaito, Saburo Hosokawa, Masashi Inoue n Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2011 Received in revised form 8 July 2011 Accepted 11 July 2011 Available online 21 July 2011
Manganese-doped Zn2SiO4 phosphors with different crystal structures and morphologies were synthesized by glycothermal reactions of zinc acetate dihydrate and manganese(II) acetate tetrahydrate with tetraethyl orthosilicate in various glycols at 315 1C. The reactions in 1,3-propanediol and 1,4-butanediol yielded a-Zn2SiO4:Mn2 þ , whereas the reactions in ethylene glycol and 1,5-pentanediol yielded b-Zn2SiO4:Mn2 þ and ZnO, respectively. The samples obtained in 1,4-butanediol and 1,3-propanediol emitted green light (522 nm), and the sample prepared in 1,4-butanediol showed a higher emission intensity. The photoluminescence intensity of the Zn1.96Mn0.04SiO4 phosphor prepared by a glycothermal reaction in 1,4-butanediol and subsequently calcined at 1100 1C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was obtained because the highly homogeneous distribution of Mn2 þ in the a-Zn2SiO4 host synthesized by the glycothermal reaction was maintained during calcination treatment in air. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phosphor Glycothermal method Zinc silicate XAFS analysis
1. Introduction Zinc silicate has been widely used as a phosphor host material due to its chemical stability and transparency. Green-light-emitting Mn-doped Zn2SiO4 has found applications in plasma display panels (PDPs) [1], cathode-ray tubes (CRTs) and electroluminescence (EL) devices [2] due to its high luminescence efficiency and chemical stability. An enhanced emission intensity and a controlled morphology are important in the development of highly efficient PDPs and other electroluminescence devices. Commercial Zn2SiO4:Mn2 þ phosphors have been synthesized primarily by conventional solid-state reactions [1,3]. However, in the solid-state reaction method, calcination at temperatures above 1000 1C for several hours is required to produce phosphors with high crystallinities and high luminescence intensities, and this method consumes a large amount of energy. Moreover, controlling the distribution of the doped metal ions has proven difficult. Consequently, various techniques, such as sol–gel [4–8], polymer precursor [9], spray pyrolysis [10], chemical vapor synthesis [11], hydrothermal [12–20] and solvothermal methods [21,22], have been developed for the synthesis of Zn2SiO4:Mn2 þ . In the glycothermal method, reactions are performed in liquid glycols at moderate temperatures (200–300 1C). A similar method in which reactions are performed in water is called the hydrothermal
n
Corresponding author. Tel.: þ81 75 383 2478; fax: þ 81 75 383 2479. E-mail address: [email protected] (M. Inoue).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.07.020
method. We have found that various mixed oxides can be synthesized by glycothermal reactions [23–25]. Takesada et al. [26] prepared Mn-doped zinc gallate (ZnGa2O4: Mn2 þ ) nanophosphors by a glycothermal method and studied the effects of heat treatments on their photoluminescence intensities. The properties of Zn2SiO4:Mn2 þ phosphors prepared by a glycothermal method, however, have not been reported. In this work, we synthesized Zn2SiO4:Mn2 þ phosphors by glycothermal methods and investigated the luminescence properties of the products. The effects of the solvents on the crystal structure, morphology and photoluminescence properties of the products were investigated. The luminescence properties of the products were compared to those of samples prepared by a conventional solid-state method.
2. Experimental 2.1. Glycothermal reaction Ethylene glycol (EG), 1,3-propanediol (1,3-PG), 1,4-butanediol (1,4-BG), and 1,5-pentanediol (1,5-PeG) were used as solvents. Zinc acetate dihydrate (Zn(OAc)2 2H2O) and manganese(II) acetate tetrahydrate (Mn(OAc)2 4 H2O) with a Mn/(ZnþMn) molar ratio (x) of 0–0.1 were mixed in an agate mortar. After the addition of 70 ml of solvent, the mixture was transferred to a plastic bottle and ultrasonicated for 5 min. The starting materials dissolved in EG and 1,3-PG, but not in 1,4-BG and 1,5-PeG. Therefore, the mixtures in 1,4-BG and 1,5-PeG were further stirred in an 80 1C bath for 1 h.
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
2.2. Characterization X-Ray powder diffraction (XRD) patterns were recorded on a Shimadzu XD-D1 X-ray diffractometer using Cu Ka radiation and a carbon monochromator. Simultaneous thermogravimetric and differential thermal analyses (TG–DTA) were performed with a Rigaku Thermo Plus TG8120 thermal analyzer at a heating rate of 5 1C/min in a 40 mL/min flow of dried air. The morphologies of the products were observed with a Hitachi H-800 transmission electron microscope (TEM) operated at 200 kV. Emission and excitation spectra were recorded on a Shimadzu RF-5300PC photoluminescence spectrometer at room temperature. The Mn K-edge X-ray absorption near edge structure (XANES) spectra were recorded in the fluorescence mode in air at room temperature using the facility of the BL01B1 beam line at SPring-8 of the Japan Synchrotron Radiation Research Institute. A Si (1 1 1) two-crystal monochromator was used to obtain a monochromatic X-ray beam. The data reduction was performed by the REX2000 Ver. 2.5 program (Rigaku). Electronic states of Mn2 þ were investigated by electron paramagnetic resonance (EPR) spectroscopy (JEOL, JES-SRE2X) at room temperature. Diffuse reflectance UV–vis spectroscopy was performed with a JASCO V-650 spectrometer at room temperature. The UV–vis absorption spectrum of BaSO4 in a quartz cell was used as the background, and spectra of the samples were expressed using the Kubelka–Munk function.
3. Results and discussion 3.1. Effects of solvents in the glycothermal reactions The XRD patterns of the products obtained by the reactions in various glycols are shown in Fig. 1. The reaction in 1,4-BG at 315 1C yielded phase-pure a-Zn2SiO4 (JCPDS card No. 37-1485), while the reaction in 1,3-PG afforded a-Zn2SiO4 together with a small amount of ZnO (JCPDS card No. 36-1451). However, the reaction in 1,4-BG at 300 1C resulted in the formation of a mixture of a-Zn2SiO4 and ZnO (data not shown). The reaction in EG at 315 1C yielded poorly crystallized b-Zn2SiO4 (JCPDS card No. 19-1479), whereas the reaction in 1,5-PeG yielded ZnO. b-Zn2SiO4 seems to be formed as a result of the high coordination ability of EG. Since a-Zn2SiO4 was not obtained by the reaction in 1,5-PeG having a low dielectric constant, the polarity of the glycol seems to be an important factor for the formation of a-Zn2SiO4. A similar
α-Zn2SiO4
β-Zn2SiO4
ZnO
1,5-pentanediol
1,4-butanediol
Intensity / a.u.
The starting materials, however, still did not dissolve and instead formed a suspension. The mixtures (either solution or suspension) were charged in a test tube, and tetraethyl orthosilicate (TEOS) was added. The tubes were placed in a 200-ml autoclave, and an additional 20 ml of the glycol was placed in the gap between the autoclave wall and the tube. The autoclave was completely purged with nitrogen, heated to 315 1C at a rate of 2.3 1C/min, and maintained at that temperature for 2 h. After the assembly was cooled to room temperature, the resulting products were collected by centrifugation. The products were washed with methanol by vigorous mixing and centrifuging, and then air-dried. The product was calcined in a box furnace by heating at a rate of 10 1C/min to the prescribed temperature and maintaining that temperature for 0.5 h, unless otherwise noted. These products were designated as P(abbreviation of the solvent). If necessary, the abbreviation is followed by calcination temperature in Celsius. The as-synthesized product is shown by ‘‘as-syn’’. For example, P(1,4-BG)-as-syn refers to the product obtained by the reaction in 1,4-BG and P(1,4-BG)-1100 refers to the product obtained by the reaction in 1,4-BG that was subsequently calcined at 1100 1C.
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1,3-propanediol
ethylene glycol
10
20
30 40 50 2θ/ degree (CuKα)
60
70
Fig. 1. XRD patterns of the products as-synthesized by the reaction in various glycols.
effect of polarity of the medium was found in the glycothermal synthesis of rare-earth–iron mixed oxide [27]. TEM images of the products are presented in Fig. 2. P(EG) was composed of spherical particles with a mean diameter of approximately 100 nm. The particles of P(1,3-PG) and P(1,4-BG) exhibited a rice-like shape and a spindle shape, respectively. The particles of both products were about 200 nm long and 100 nm wide. The crystallite sizes of P(1,3-PG) and P(1,4-BG) calculated from the 113 diffraction peaks were larger than those calculated from the 300 and 220 diffraction peaks (Table 1). These results indicate that the particles grew along the c axis, which resulted in the formation of elongated shapes. The 113 crystallite size of P(1,4-BG) was about twice as large as that of P(1,3-PG), indicating that P(1,4-BG) grew along the c axis to a greater extent than P(1,3-PG). For P(1,3-PG) and P(1,4-BG), the crystallite size (15–50 nm) was significantly smaller than the particle size (100–200 nm) observed by TEM; the particle sizes, however, agree well with the sizes calculated from the BET surface areas of the products (ca. 30 nm). The higher-magnification TEM images of the P(1,3-PG) and P(1,4-BG) particles indicated that the particles contain numerous voids. The electron diffraction pattern of P(1,4-BG)-as-syn indicated that the spindle-shaped particle was not a single crystal (Fig. 3) but rather the crystallites (15–50 nm) of a-Zn2SiO4 were aggregated and aligned along the c axis. A single crystal diffraction pattern was observed, however, for P(1,4-BG)-900; this result suggests that the sintering of the crystallites took place by calcination, although the spindle-shaped particles were not sintered to each other (Fig. 3). 3.2. Luminescence properties The luminescence spectra of the Zn1.96Mn0.04SiO4 phosphor synthesized by the glycothermal method are shown in Fig. 4. It is well known that a-Zn2SiO4:Mn2 þ gives a green light emission, whereas b-Zn2SiO4:Mn2 þ emits a yellow light [15,17,18,28,29]. P(1,3-PG)-as-syn and P(1,4-BG)-as-syn exhibited a broad green emission centered at 522 nm due to the 4T1(4G)-6A1(6S) transition for the tetrahedrally coordinated Mn2 þ ions in the a-Zn2SiO4 host [28,29]. P(1,4-BG)-as-syn showed a much stronger emission than P(1,3-PG)-as-syn. P(EG)-as-syn exhibited only a weak emission due to the low crystallinity of the b-Zn2SiO4 host. These results indicate
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100 nm
100 nm
100 nm
50 nm
50 nm
50 nm
Fig. 2. TEM images of the samples as-synthesized by the reaction in: (a) EG; (b) 1,3-PG; (c) 1,4-BG.
Table 1 Phases, crystallite sizes and surface areas of the products obtained by the glycothermal reactions under various conditions.
EG 1,3-PG 1,4-BG 1,5-PeG 1-butanol toluene a
Phase
b-Zn2SiO4
a-Zn2SiO4 a-Zn2SiO4 ZnO ZnO ZnO
Crystallite size/nm
Surface area/m2 g 1
(3 0 0)
(2 2 0)
(1 1 3)
– 15 16 – – –
– 16 21 – – –
– 27 47 – – –
4a – – – – –
60 22 31 – – –
emission (λex = 250 nm) Intensity / a.u.
Solvent
excitation (λem = 522 nm)
1,4-BG
1,3-PG
Crystallite size was calculated from the peak at 251 2y.
EG
200
300
400 500 Wavelength / nm
600
Fig. 4. Excitation and emission spectra of the samples as-synthesized by the reaction in EG, 1,3-PG and 1,4-BG.
100 nm
200 nm
Fig. 3. TEM images and electron diffraction patterns of: (a) P(1,4-BG)-as-syn; and (b) P(1,4-BG)-900.
that 1,4-BG is the most suitable solvent for synthesizing the efficient phosphor by a glycothermal reaction. The high luminescence property of P(1,4-BG)-as-syn seems to be due to its high crystallinity. We previously reported the crystallite size of the
glycol derivative of boehmite synthesized by the glycothermal reaction of aluminum alkoxide increased in the following order of the medium: EGo1,3-PGo1,6-hexanediolo1,4-BG. This result was explained by the heterolytic cleavage of the C–O bond of glycoxide (HO(CH2)n–O–Alo) formed as an intermediate. When the carbon number is 4, the cleavage of the C–O bond is accelerated by the participation of the intramolecular hydroxyl group forming tetrahydrofuran [25,30,31]. The high crystallinity of P(1,4-BG)-as-syn can be explained along the same line. Fig. 5 shows the effect of Mn2 þ concentration on the emission intensity for Zn2(1 x)Mn2xSiO4 (x¼0.005–0.1) phosphors assynthesized or calcined at 1100 1C for 3 h. Both P(1,4-BG)-as-syn and P(1,4-BG)-1100 showed the strongest emission at x ¼0.02.
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
Emission intensity/ a.u.
cal. 1100 °C
as-syn.
0
0.02
0.04 0.06 Concentration x
0.08
0.1
Fig. 5. Relative emission peak intensity of P(1,4-BG)-as-syn and P(1,4-BG)-1100 as a function of Mn concentration under 250-nm excitation.
excitation (λem = 522 nm)
emission (λex = 250 nm)
Intensity / a.u.
1100 °C as-syn.
500 °C
800 °C
200
300
400 Wavelength / nm
500
600
Fig. 6. Excitation and emission spectra of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100.
3.3. Effect of heat treatment Fig. 6 shows the emission spectra of P(1,4-BG) calcined at various temperatures. The emission intensity of P(1,4-BG)-500 was much lower than that of P(1,4-BG)-as-syn, whereas P(1,4BG)-800 did not exhibit luminescence. However, P(1,4-BG)-1100 showed a stronger emission than P(1,4-BG)-as-syn. Fig. 7 shows the Mn K-edge XANES spectrum of P(1,4-BG) (x¼0.02) together with those of MnO, Mn3O4 and Mn2O3 as reference compounds. The main-edge peaks of the XANES spectra correspond to the 1s-4p transition and provide information for the oxidation state of Mn [32]. The edge positions of P(1,4-BG)-as-syn, P(1,4-BG)-500 and P(1,4-BG)-1100 were essentially identical to that of the MnO reference material, which indicates that the oxidation states of Mn in these samples are close to 2þ. However, the edge position of P(1,4-BG)-800 was quite close to that of the Mn3O4
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reference material, which indicates that calcination of P(1,4-BG) at 800 1C increases the oxidation state of the Mn ions, thus leading to a shift of the absorption edge to the higher-energy side. The pre-edge peaks of the Mn K-edge XANES spectra are located approximately 15–20 eV before the main-edge crest and are related to electronic transitions from the 1s core levels to the empty 3d levels that are partially mixed with 4p wavefunctions. Therefore, the intensity of the pre-edge peak relates to the local symmetry of the Mn coordination [33,34], and the compounds in which the Mn atoms are tetrahedrally coordinated by four oxygen atoms exhibited strong pre-edge peaks [35,36]. The intensities of the pre-edge peaks of all the samples were much higher than that of the Mn3O4 reference material, in which one-third of the Mn ions are located in tetrahedral sites. This result indicates that the Mn ions in all of the samples are predominantly present in a tetrahedral environment in the Zn2SiO4 host. The colors of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100 were white, ash-gray, blue and white, respectively. UV–vis spectra of these samples are shown in Fig. 8. P(1,4-BG)-500 exhibited a broad absorption in the range of 200–800 nm. The TG–DTA profile of P(1,4-BG)-as-syn showed a weight loss in the range of 300–500 1C due to the combustion of organic species (Fig. 9). Therefore, the ash-gray color of P(1,4-BG)-500 is attributed to carbonaceous matter remaining on the grain surfaces. The low emission intensity of P(1,4-BG)-500 can be explained by the absorption of light by the remaining carbonaceous matter. Although the UV–vis spectrum of P(1,4-BG)-as-syn exhibited a peak only at 210 nm, P(1,4-BG)-800, P(1,4-BG)-1000 and P(1,4BG)-1100 exhibited a shoulder peak at approximately 255 nm together with a broad peak near 660 nm. P(1,4-BG)-800 exhibited more intense peaks than P(1,4-BG)-1000, whereas the intensity of the peak at 660 nm drastically decreased for P(1,4-BG)-1100. Milella et al. have reported that Mn2O3 exhibits three bands at ca. 370, 485 and 755 nm; the authors assigned these bands to a O2 Mn3 þ charge-transfer transition, to superimposed 5B1g-5B2g and 5 B1g-5Eg crystal-field d–d transitions, and to a 5B1g-5A1g crystalfield d–d transition, respectively [37]. However, Mn3O4 exhibits two bands at 255 and 320 nm that have been assigned to the chargetransfer transitions of O2 -Mn2 þ and O2 -Mn3 þ , respectively [37,38]. The spectrum of P(1,4-BG)-800 showed no indication of the presence of the Mn2O3 or Mn3O4 phases. The band at ca. 255 nm observed for P(1,4-BG)-800 is attributed to the charge-transfer transition of O2 -Mn3 þ in the tetrahedral sites of the zinc silicate host. Similarly, Zhang et al. assigned the peak at 270 nm to the charge-transfer transition of O2 -Mn3 þ in tetrahedral coordination in the framework of MCM-41 [39]. The band at 255 nm in P(1,4-BG)-1100 is presumably due to Mn3O4 because weak diffraction peaks due to Mn3O4 were observed in the XRD pattern of this sample. However, the amount of the Mn3O4 phase seems to be small because XANES analysis indicated that the average oxidation state of the Mn species in P(1,4-BG)-1100 was 2þ. Because the d–d transitions due to Mn2 þ are, in principle, both spin and orbitally forbidden, the characteristic peak at 660 nm is tentatively assigned to a d–d transition of the tetrahedral Mn3 þ ion. This assignment agrees with the XANES results. The blue color of P(1,4-BG)-800 results from this absorption. P(1,4-BG)-1100 also showed very weak absorption at 660 nm. Fig. 10 shows the EPR spectra of the products. A well-defined sextet signal and a broad signal were observed for P(1,4-BG)-assyn (x¼0.005). The sextet signal has been assigned to isolated Mn2 þ ions inside the particle, and the broad signal is due to locally concentrated Mn2 þ ions [40,41]. For P(1,4-BG)-800, the intensity of the sextet was very weak, which indicates that the predominant Mn species are EPR silent, that is, Mn3 þ . In the EPR spectrum of P(1,4-BG)-1100, a quintet fine structure of the sextet
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1.5 1100 °C
1.5 Normalized absorption
Normalized absorption
as-syn. 1
0.5
800 °C
6536
6526
0 6520
6550 Photon energy / eV
1
Mn2O3
0.5
0 6520
6580
Mn3O4
MnO
6550 Photon energy / eV
6580
Fig. 7. Mn K-edge XANES spectra of (a) P(1,4-BG) (x¼ 0.02) calcined at various temperatures: broken line, P(1,4-BG)-as-syn; dashed line, P(1,4-BG)-800; solid line, P(1,4-BG)-1100, and (b) the Mn reference compounds: MnO, Mn3O4, and Mn2O3.
(c)
F(R∞)
(a)
0.1
0.5
(d)
F(R∞)
F(R∞)
0.5
(e)
(b)
200
400 600 Wavelength / nm
800
200
400 600 Wavelength / nm
800
200
400 600 Wavelength / nm
800
0
30
1
20
2 0 3 -10 4
-20
5 0
200
400 600 800 Temperature / °C
1000
First derivative intensity/a.u.
1100 °C
10 DTA / μV
Weight loss / %
Fig. 8. UV–vis spectra of: (a) P(1,4-BG)-as-syn; (b) P(1,4-BG)-500; (c) P(1,4-BG)-800; (d) P(1,4-BG)-1000; (e) P(1,4-BG)-1100.
800 °C
500 °C
as-syn.
-30
Fig. 9. TG–DTA profiles of P(1,4-BG)-as-syn.
hyper-fine structure of Mn2 þ was observed [42,43]. The quintet fine structure was probably caused by the high crystallinity of the zinc silicate that was induced by high-temperature calcination. These results indicate that the oxidation state of Mn in P(1,4-BG)as-syn and P(1,4-BG)-1100 is 2þ, whereas P(1,4-BG)-800
200
250
300 350 Magnetic field /mT
400
450
Fig. 10. EPR spectra of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100 (x¼ 0.005).
contains a small amount of Mn2 þ species; calcination of P(1,4BG) at 800 1C caused oxidation of some Mn ions to the 3 þ oxidation state. However, the formation of Mn3 þ in the Zn2SiO4
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
200 nm
200 nm
69
500 nm
Fig. 11. TEM images of P(1,4-BG) calcined at: (a) 800 1C for 0.5 h; (b) 1100 1C for 0.5 h; (c) 1100 1C for 5 h.
Emission intensity / a.u.
glycothermal reaction
impregnation
solid state reaction
0
1
4 2 3 Calcination time / h
5
6
Fig. 12. Relative emission peak intensity of the phosphors (x¼ 0.02) prepared with the glycothermal, the impregnation and the solid-state reaction methods as a function of calcination time at 1100 1C under 250-nm excitation.
crystals leads to a charge imbalance. Two mechanisms can explain the results: (1) One-third of the Mn3 þ ions moved to the particles’ surfaces, where manganese oxides are formed, although the UV–vis spectra showed no evidence for the formation of manganese oxide phases; or (2) Mn2 þ is oxidized by molecular oxygen and the thus-formed Mn3 þ ions remain in the Zn2SiO4 lattice while oxide anions locate in the interstitial sites of the Zn2SiO4 structure. Because the Zn2SiO4 structure contains a sufficient number of large voids to accommodate the oxide anions [44–47], the second explanation seems more plausible. This process would increase the sample weight, and we observed a slight weight increase at 450–600 1C in the TG results (Fig. 9). We could not, however, find unequivocal evidence that the apparent weight increase is not due to the drift of the TG baseline. The theoretical weight increase based on the assumption that all the manganese ions are originally in the 2þ oxidation state and are completely oxidized during calcination is only 0.14%. The fact that P(1,4-BG)-1100 showed a much higher photoluminescence intensity than the sample prepared by impregnation of Mn ions to the glycothermally prepared Zn2SiO4 followed by calcination (vide infra) also supports the second explanation. The TEM images of P(1,4-BG)-800 and P(1,4-BG)-1100 calcined for 0.5 and 5 h are shown in Fig. 11. Coagulated particles were not observed for P(1,4-BG)-800, but voids in the particles were
apparent. The morphology of P(1,4-BG)-900 was discussed in Section 3.1. Calcination at 1100 1C for 0.5 h completely alters the morphology of the particles and smoothes the grain surfaces. The sintering between particles occurred during prolonged calcination and resulted in grains sizes of 500 nm. Fig. 12 shows the effect of calcination time at 1100 1C on the luminescence intensity for the P(1,4-BG) (x¼0.02) phosphors. The emission intensity of the samples increased with increasing calcination time. This result is due to the crystal growth that occurred during prolonged calcination (Fig. 11). For the Zn2SiO4:Mn2 þ phosphor prepared by solid-state reaction, the emission intensity increased with firing time. However, the maximum emission intensity was observed for the sample that was fired for 3 h, and prolonged firing (5 h) decreased the emission intensity, presumably due to sublimation of the Zn component. The emission intensity of the calcined P(1,4-BG) phosphor was twice as high as that of the samples prepared by solid-state reaction. For comparison, Mn2þ ions were impregnated to a a-Zn2SiO4 sample prepared by glycothermal reaction in 1,4-BG and the thus-obtained sample was calcined at 1100 1C for 0.5 and 5 h. The phosphor obtained by the impregnation method exhibited a higher emission intensity than the phosphor obtained by solid-state reaction. This result indicates that the difference in the emission intensity was caused by the difference in the dispersion of Mn2 þ ions in the Zn2SiO4 host.
4. Conclusions
a-Zn2SiO4 was synthesized by glycothermal reactions of zinc acetate dihydrate and tetraethyl orthosilicate in 1,4-BG and 1,3PG, whereas b-Zn2SiO4 was obtained by the reaction in EG. The phosphor synthesized in 1,4-BG exhibited the highest emission intensity of the prepared samples. Because Mn2 þ in the a-Zn2SiO4 host was oxidized to Mn3 þ by calcination at 800 1C, the photoluminescence disappeared. However, Mn3 þ ions occupy tetrahedral environments in the a-Zn2SiO4 host, and calcination at 1100 1C reduced the Mn3 þ species in a-Zn2SiO4 to Mn2 þ . The photoluminescence intensity of the Zn1.96Mn0.04SiO4 phosphor obtained by calcination at 1100 1C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was attributed to the Mn2 þ in the a-Zn2SiO4 host remaining highly dispersed during calcination.
Acknowledgment The XANES experiments have been performed with the approval of SPring-8 (Proposal no. 2010A1271).
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2 þ phosphors prepared by glycothermal methods Keisuke Uegaito, Saburo Hosokawa, Masashi Inoue n Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2011 Received in revised form 8 July 2011 Accepted 11 July 2011 Available online 21 July 2011
Manganese-doped Zn2SiO4 phosphors with different crystal structures and morphologies were synthesized by glycothermal reactions of zinc acetate dihydrate and manganese(II) acetate tetrahydrate with tetraethyl orthosilicate in various glycols at 315 1C. The reactions in 1,3-propanediol and 1,4-butanediol yielded a-Zn2SiO4:Mn2 þ , whereas the reactions in ethylene glycol and 1,5-pentanediol yielded b-Zn2SiO4:Mn2 þ and ZnO, respectively. The samples obtained in 1,4-butanediol and 1,3-propanediol emitted green light (522 nm), and the sample prepared in 1,4-butanediol showed a higher emission intensity. The photoluminescence intensity of the Zn1.96Mn0.04SiO4 phosphor prepared by a glycothermal reaction in 1,4-butanediol and subsequently calcined at 1100 1C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was obtained because the highly homogeneous distribution of Mn2 þ in the a-Zn2SiO4 host synthesized by the glycothermal reaction was maintained during calcination treatment in air. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phosphor Glycothermal method Zinc silicate XAFS analysis
1. Introduction Zinc silicate has been widely used as a phosphor host material due to its chemical stability and transparency. Green-light-emitting Mn-doped Zn2SiO4 has found applications in plasma display panels (PDPs) [1], cathode-ray tubes (CRTs) and electroluminescence (EL) devices [2] due to its high luminescence efficiency and chemical stability. An enhanced emission intensity and a controlled morphology are important in the development of highly efficient PDPs and other electroluminescence devices. Commercial Zn2SiO4:Mn2 þ phosphors have been synthesized primarily by conventional solid-state reactions [1,3]. However, in the solid-state reaction method, calcination at temperatures above 1000 1C for several hours is required to produce phosphors with high crystallinities and high luminescence intensities, and this method consumes a large amount of energy. Moreover, controlling the distribution of the doped metal ions has proven difficult. Consequently, various techniques, such as sol–gel [4–8], polymer precursor [9], spray pyrolysis [10], chemical vapor synthesis [11], hydrothermal [12–20] and solvothermal methods [21,22], have been developed for the synthesis of Zn2SiO4:Mn2 þ . In the glycothermal method, reactions are performed in liquid glycols at moderate temperatures (200–300 1C). A similar method in which reactions are performed in water is called the hydrothermal
n
Corresponding author. Tel.: þ81 75 383 2478; fax: þ 81 75 383 2479. E-mail address: [email protected] (M. Inoue).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.07.020
method. We have found that various mixed oxides can be synthesized by glycothermal reactions [23–25]. Takesada et al. [26] prepared Mn-doped zinc gallate (ZnGa2O4: Mn2 þ ) nanophosphors by a glycothermal method and studied the effects of heat treatments on their photoluminescence intensities. The properties of Zn2SiO4:Mn2 þ phosphors prepared by a glycothermal method, however, have not been reported. In this work, we synthesized Zn2SiO4:Mn2 þ phosphors by glycothermal methods and investigated the luminescence properties of the products. The effects of the solvents on the crystal structure, morphology and photoluminescence properties of the products were investigated. The luminescence properties of the products were compared to those of samples prepared by a conventional solid-state method.
2. Experimental 2.1. Glycothermal reaction Ethylene glycol (EG), 1,3-propanediol (1,3-PG), 1,4-butanediol (1,4-BG), and 1,5-pentanediol (1,5-PeG) were used as solvents. Zinc acetate dihydrate (Zn(OAc)2 2H2O) and manganese(II) acetate tetrahydrate (Mn(OAc)2 4 H2O) with a Mn/(ZnþMn) molar ratio (x) of 0–0.1 were mixed in an agate mortar. After the addition of 70 ml of solvent, the mixture was transferred to a plastic bottle and ultrasonicated for 5 min. The starting materials dissolved in EG and 1,3-PG, but not in 1,4-BG and 1,5-PeG. Therefore, the mixtures in 1,4-BG and 1,5-PeG were further stirred in an 80 1C bath for 1 h.
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
2.2. Characterization X-Ray powder diffraction (XRD) patterns were recorded on a Shimadzu XD-D1 X-ray diffractometer using Cu Ka radiation and a carbon monochromator. Simultaneous thermogravimetric and differential thermal analyses (TG–DTA) were performed with a Rigaku Thermo Plus TG8120 thermal analyzer at a heating rate of 5 1C/min in a 40 mL/min flow of dried air. The morphologies of the products were observed with a Hitachi H-800 transmission electron microscope (TEM) operated at 200 kV. Emission and excitation spectra were recorded on a Shimadzu RF-5300PC photoluminescence spectrometer at room temperature. The Mn K-edge X-ray absorption near edge structure (XANES) spectra were recorded in the fluorescence mode in air at room temperature using the facility of the BL01B1 beam line at SPring-8 of the Japan Synchrotron Radiation Research Institute. A Si (1 1 1) two-crystal monochromator was used to obtain a monochromatic X-ray beam. The data reduction was performed by the REX2000 Ver. 2.5 program (Rigaku). Electronic states of Mn2 þ were investigated by electron paramagnetic resonance (EPR) spectroscopy (JEOL, JES-SRE2X) at room temperature. Diffuse reflectance UV–vis spectroscopy was performed with a JASCO V-650 spectrometer at room temperature. The UV–vis absorption spectrum of BaSO4 in a quartz cell was used as the background, and spectra of the samples were expressed using the Kubelka–Munk function.
3. Results and discussion 3.1. Effects of solvents in the glycothermal reactions The XRD patterns of the products obtained by the reactions in various glycols are shown in Fig. 1. The reaction in 1,4-BG at 315 1C yielded phase-pure a-Zn2SiO4 (JCPDS card No. 37-1485), while the reaction in 1,3-PG afforded a-Zn2SiO4 together with a small amount of ZnO (JCPDS card No. 36-1451). However, the reaction in 1,4-BG at 300 1C resulted in the formation of a mixture of a-Zn2SiO4 and ZnO (data not shown). The reaction in EG at 315 1C yielded poorly crystallized b-Zn2SiO4 (JCPDS card No. 19-1479), whereas the reaction in 1,5-PeG yielded ZnO. b-Zn2SiO4 seems to be formed as a result of the high coordination ability of EG. Since a-Zn2SiO4 was not obtained by the reaction in 1,5-PeG having a low dielectric constant, the polarity of the glycol seems to be an important factor for the formation of a-Zn2SiO4. A similar
α-Zn2SiO4
β-Zn2SiO4
ZnO
1,5-pentanediol
1,4-butanediol
Intensity / a.u.
The starting materials, however, still did not dissolve and instead formed a suspension. The mixtures (either solution or suspension) were charged in a test tube, and tetraethyl orthosilicate (TEOS) was added. The tubes were placed in a 200-ml autoclave, and an additional 20 ml of the glycol was placed in the gap between the autoclave wall and the tube. The autoclave was completely purged with nitrogen, heated to 315 1C at a rate of 2.3 1C/min, and maintained at that temperature for 2 h. After the assembly was cooled to room temperature, the resulting products were collected by centrifugation. The products were washed with methanol by vigorous mixing and centrifuging, and then air-dried. The product was calcined in a box furnace by heating at a rate of 10 1C/min to the prescribed temperature and maintaining that temperature for 0.5 h, unless otherwise noted. These products were designated as P(abbreviation of the solvent). If necessary, the abbreviation is followed by calcination temperature in Celsius. The as-synthesized product is shown by ‘‘as-syn’’. For example, P(1,4-BG)-as-syn refers to the product obtained by the reaction in 1,4-BG and P(1,4-BG)-1100 refers to the product obtained by the reaction in 1,4-BG that was subsequently calcined at 1100 1C.
65
1,3-propanediol
ethylene glycol
10
20
30 40 50 2θ/ degree (CuKα)
60
70
Fig. 1. XRD patterns of the products as-synthesized by the reaction in various glycols.
effect of polarity of the medium was found in the glycothermal synthesis of rare-earth–iron mixed oxide [27]. TEM images of the products are presented in Fig. 2. P(EG) was composed of spherical particles with a mean diameter of approximately 100 nm. The particles of P(1,3-PG) and P(1,4-BG) exhibited a rice-like shape and a spindle shape, respectively. The particles of both products were about 200 nm long and 100 nm wide. The crystallite sizes of P(1,3-PG) and P(1,4-BG) calculated from the 113 diffraction peaks were larger than those calculated from the 300 and 220 diffraction peaks (Table 1). These results indicate that the particles grew along the c axis, which resulted in the formation of elongated shapes. The 113 crystallite size of P(1,4-BG) was about twice as large as that of P(1,3-PG), indicating that P(1,4-BG) grew along the c axis to a greater extent than P(1,3-PG). For P(1,3-PG) and P(1,4-BG), the crystallite size (15–50 nm) was significantly smaller than the particle size (100–200 nm) observed by TEM; the particle sizes, however, agree well with the sizes calculated from the BET surface areas of the products (ca. 30 nm). The higher-magnification TEM images of the P(1,3-PG) and P(1,4-BG) particles indicated that the particles contain numerous voids. The electron diffraction pattern of P(1,4-BG)-as-syn indicated that the spindle-shaped particle was not a single crystal (Fig. 3) but rather the crystallites (15–50 nm) of a-Zn2SiO4 were aggregated and aligned along the c axis. A single crystal diffraction pattern was observed, however, for P(1,4-BG)-900; this result suggests that the sintering of the crystallites took place by calcination, although the spindle-shaped particles were not sintered to each other (Fig. 3). 3.2. Luminescence properties The luminescence spectra of the Zn1.96Mn0.04SiO4 phosphor synthesized by the glycothermal method are shown in Fig. 4. It is well known that a-Zn2SiO4:Mn2 þ gives a green light emission, whereas b-Zn2SiO4:Mn2 þ emits a yellow light [15,17,18,28,29]. P(1,3-PG)-as-syn and P(1,4-BG)-as-syn exhibited a broad green emission centered at 522 nm due to the 4T1(4G)-6A1(6S) transition for the tetrahedrally coordinated Mn2 þ ions in the a-Zn2SiO4 host [28,29]. P(1,4-BG)-as-syn showed a much stronger emission than P(1,3-PG)-as-syn. P(EG)-as-syn exhibited only a weak emission due to the low crystallinity of the b-Zn2SiO4 host. These results indicate
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100 nm
100 nm
100 nm
50 nm
50 nm
50 nm
Fig. 2. TEM images of the samples as-synthesized by the reaction in: (a) EG; (b) 1,3-PG; (c) 1,4-BG.
Table 1 Phases, crystallite sizes and surface areas of the products obtained by the glycothermal reactions under various conditions.
EG 1,3-PG 1,4-BG 1,5-PeG 1-butanol toluene a
Phase
b-Zn2SiO4
a-Zn2SiO4 a-Zn2SiO4 ZnO ZnO ZnO
Crystallite size/nm
Surface area/m2 g 1
(3 0 0)
(2 2 0)
(1 1 3)
– 15 16 – – –
– 16 21 – – –
– 27 47 – – –
4a – – – – –
60 22 31 – – –
emission (λex = 250 nm) Intensity / a.u.
Solvent
excitation (λem = 522 nm)
1,4-BG
1,3-PG
Crystallite size was calculated from the peak at 251 2y.
EG
200
300
400 500 Wavelength / nm
600
Fig. 4. Excitation and emission spectra of the samples as-synthesized by the reaction in EG, 1,3-PG and 1,4-BG.
100 nm
200 nm
Fig. 3. TEM images and electron diffraction patterns of: (a) P(1,4-BG)-as-syn; and (b) P(1,4-BG)-900.
that 1,4-BG is the most suitable solvent for synthesizing the efficient phosphor by a glycothermal reaction. The high luminescence property of P(1,4-BG)-as-syn seems to be due to its high crystallinity. We previously reported the crystallite size of the
glycol derivative of boehmite synthesized by the glycothermal reaction of aluminum alkoxide increased in the following order of the medium: EGo1,3-PGo1,6-hexanediolo1,4-BG. This result was explained by the heterolytic cleavage of the C–O bond of glycoxide (HO(CH2)n–O–Alo) formed as an intermediate. When the carbon number is 4, the cleavage of the C–O bond is accelerated by the participation of the intramolecular hydroxyl group forming tetrahydrofuran [25,30,31]. The high crystallinity of P(1,4-BG)-as-syn can be explained along the same line. Fig. 5 shows the effect of Mn2 þ concentration on the emission intensity for Zn2(1 x)Mn2xSiO4 (x¼0.005–0.1) phosphors assynthesized or calcined at 1100 1C for 3 h. Both P(1,4-BG)-as-syn and P(1,4-BG)-1100 showed the strongest emission at x ¼0.02.
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
Emission intensity/ a.u.
cal. 1100 °C
as-syn.
0
0.02
0.04 0.06 Concentration x
0.08
0.1
Fig. 5. Relative emission peak intensity of P(1,4-BG)-as-syn and P(1,4-BG)-1100 as a function of Mn concentration under 250-nm excitation.
excitation (λem = 522 nm)
emission (λex = 250 nm)
Intensity / a.u.
1100 °C as-syn.
500 °C
800 °C
200
300
400 Wavelength / nm
500
600
Fig. 6. Excitation and emission spectra of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100.
3.3. Effect of heat treatment Fig. 6 shows the emission spectra of P(1,4-BG) calcined at various temperatures. The emission intensity of P(1,4-BG)-500 was much lower than that of P(1,4-BG)-as-syn, whereas P(1,4BG)-800 did not exhibit luminescence. However, P(1,4-BG)-1100 showed a stronger emission than P(1,4-BG)-as-syn. Fig. 7 shows the Mn K-edge XANES spectrum of P(1,4-BG) (x¼0.02) together with those of MnO, Mn3O4 and Mn2O3 as reference compounds. The main-edge peaks of the XANES spectra correspond to the 1s-4p transition and provide information for the oxidation state of Mn [32]. The edge positions of P(1,4-BG)-as-syn, P(1,4-BG)-500 and P(1,4-BG)-1100 were essentially identical to that of the MnO reference material, which indicates that the oxidation states of Mn in these samples are close to 2þ. However, the edge position of P(1,4-BG)-800 was quite close to that of the Mn3O4
67
reference material, which indicates that calcination of P(1,4-BG) at 800 1C increases the oxidation state of the Mn ions, thus leading to a shift of the absorption edge to the higher-energy side. The pre-edge peaks of the Mn K-edge XANES spectra are located approximately 15–20 eV before the main-edge crest and are related to electronic transitions from the 1s core levels to the empty 3d levels that are partially mixed with 4p wavefunctions. Therefore, the intensity of the pre-edge peak relates to the local symmetry of the Mn coordination [33,34], and the compounds in which the Mn atoms are tetrahedrally coordinated by four oxygen atoms exhibited strong pre-edge peaks [35,36]. The intensities of the pre-edge peaks of all the samples were much higher than that of the Mn3O4 reference material, in which one-third of the Mn ions are located in tetrahedral sites. This result indicates that the Mn ions in all of the samples are predominantly present in a tetrahedral environment in the Zn2SiO4 host. The colors of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100 were white, ash-gray, blue and white, respectively. UV–vis spectra of these samples are shown in Fig. 8. P(1,4-BG)-500 exhibited a broad absorption in the range of 200–800 nm. The TG–DTA profile of P(1,4-BG)-as-syn showed a weight loss in the range of 300–500 1C due to the combustion of organic species (Fig. 9). Therefore, the ash-gray color of P(1,4-BG)-500 is attributed to carbonaceous matter remaining on the grain surfaces. The low emission intensity of P(1,4-BG)-500 can be explained by the absorption of light by the remaining carbonaceous matter. Although the UV–vis spectrum of P(1,4-BG)-as-syn exhibited a peak only at 210 nm, P(1,4-BG)-800, P(1,4-BG)-1000 and P(1,4BG)-1100 exhibited a shoulder peak at approximately 255 nm together with a broad peak near 660 nm. P(1,4-BG)-800 exhibited more intense peaks than P(1,4-BG)-1000, whereas the intensity of the peak at 660 nm drastically decreased for P(1,4-BG)-1100. Milella et al. have reported that Mn2O3 exhibits three bands at ca. 370, 485 and 755 nm; the authors assigned these bands to a O2 Mn3 þ charge-transfer transition, to superimposed 5B1g-5B2g and 5 B1g-5Eg crystal-field d–d transitions, and to a 5B1g-5A1g crystalfield d–d transition, respectively [37]. However, Mn3O4 exhibits two bands at 255 and 320 nm that have been assigned to the chargetransfer transitions of O2 -Mn2 þ and O2 -Mn3 þ , respectively [37,38]. The spectrum of P(1,4-BG)-800 showed no indication of the presence of the Mn2O3 or Mn3O4 phases. The band at ca. 255 nm observed for P(1,4-BG)-800 is attributed to the charge-transfer transition of O2 -Mn3 þ in the tetrahedral sites of the zinc silicate host. Similarly, Zhang et al. assigned the peak at 270 nm to the charge-transfer transition of O2 -Mn3 þ in tetrahedral coordination in the framework of MCM-41 [39]. The band at 255 nm in P(1,4-BG)-1100 is presumably due to Mn3O4 because weak diffraction peaks due to Mn3O4 were observed in the XRD pattern of this sample. However, the amount of the Mn3O4 phase seems to be small because XANES analysis indicated that the average oxidation state of the Mn species in P(1,4-BG)-1100 was 2þ. Because the d–d transitions due to Mn2 þ are, in principle, both spin and orbitally forbidden, the characteristic peak at 660 nm is tentatively assigned to a d–d transition of the tetrahedral Mn3 þ ion. This assignment agrees with the XANES results. The blue color of P(1,4-BG)-800 results from this absorption. P(1,4-BG)-1100 also showed very weak absorption at 660 nm. Fig. 10 shows the EPR spectra of the products. A well-defined sextet signal and a broad signal were observed for P(1,4-BG)-assyn (x¼0.005). The sextet signal has been assigned to isolated Mn2 þ ions inside the particle, and the broad signal is due to locally concentrated Mn2 þ ions [40,41]. For P(1,4-BG)-800, the intensity of the sextet was very weak, which indicates that the predominant Mn species are EPR silent, that is, Mn3 þ . In the EPR spectrum of P(1,4-BG)-1100, a quintet fine structure of the sextet
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1.5 1100 °C
1.5 Normalized absorption
Normalized absorption
as-syn. 1
0.5
800 °C
6536
6526
0 6520
6550 Photon energy / eV
1
Mn2O3
0.5
0 6520
6580
Mn3O4
MnO
6550 Photon energy / eV
6580
Fig. 7. Mn K-edge XANES spectra of (a) P(1,4-BG) (x¼ 0.02) calcined at various temperatures: broken line, P(1,4-BG)-as-syn; dashed line, P(1,4-BG)-800; solid line, P(1,4-BG)-1100, and (b) the Mn reference compounds: MnO, Mn3O4, and Mn2O3.
(c)
F(R∞)
(a)
0.1
0.5
(d)
F(R∞)
F(R∞)
0.5
(e)
(b)
200
400 600 Wavelength / nm
800
200
400 600 Wavelength / nm
800
200
400 600 Wavelength / nm
800
0
30
1
20
2 0 3 -10 4
-20
5 0
200
400 600 800 Temperature / °C
1000
First derivative intensity/a.u.
1100 °C
10 DTA / μV
Weight loss / %
Fig. 8. UV–vis spectra of: (a) P(1,4-BG)-as-syn; (b) P(1,4-BG)-500; (c) P(1,4-BG)-800; (d) P(1,4-BG)-1000; (e) P(1,4-BG)-1100.
800 °C
500 °C
as-syn.
-30
Fig. 9. TG–DTA profiles of P(1,4-BG)-as-syn.
hyper-fine structure of Mn2 þ was observed [42,43]. The quintet fine structure was probably caused by the high crystallinity of the zinc silicate that was induced by high-temperature calcination. These results indicate that the oxidation state of Mn in P(1,4-BG)as-syn and P(1,4-BG)-1100 is 2þ, whereas P(1,4-BG)-800
200
250
300 350 Magnetic field /mT
400
450
Fig. 10. EPR spectra of P(1,4-BG)-as-syn, P(1,4-BG)-500, P(1,4-BG)-800 and P(1,4-BG)-1100 (x¼ 0.005).
contains a small amount of Mn2 þ species; calcination of P(1,4BG) at 800 1C caused oxidation of some Mn ions to the 3 þ oxidation state. However, the formation of Mn3 þ in the Zn2SiO4
K. Uegaito et al. / Journal of Luminescence 132 (2012) 64–70
200 nm
200 nm
69
500 nm
Fig. 11. TEM images of P(1,4-BG) calcined at: (a) 800 1C for 0.5 h; (b) 1100 1C for 0.5 h; (c) 1100 1C for 5 h.
Emission intensity / a.u.
glycothermal reaction
impregnation
solid state reaction
0
1
4 2 3 Calcination time / h
5
6
Fig. 12. Relative emission peak intensity of the phosphors (x¼ 0.02) prepared with the glycothermal, the impregnation and the solid-state reaction methods as a function of calcination time at 1100 1C under 250-nm excitation.
crystals leads to a charge imbalance. Two mechanisms can explain the results: (1) One-third of the Mn3 þ ions moved to the particles’ surfaces, where manganese oxides are formed, although the UV–vis spectra showed no evidence for the formation of manganese oxide phases; or (2) Mn2 þ is oxidized by molecular oxygen and the thus-formed Mn3 þ ions remain in the Zn2SiO4 lattice while oxide anions locate in the interstitial sites of the Zn2SiO4 structure. Because the Zn2SiO4 structure contains a sufficient number of large voids to accommodate the oxide anions [44–47], the second explanation seems more plausible. This process would increase the sample weight, and we observed a slight weight increase at 450–600 1C in the TG results (Fig. 9). We could not, however, find unequivocal evidence that the apparent weight increase is not due to the drift of the TG baseline. The theoretical weight increase based on the assumption that all the manganese ions are originally in the 2þ oxidation state and are completely oxidized during calcination is only 0.14%. The fact that P(1,4-BG)-1100 showed a much higher photoluminescence intensity than the sample prepared by impregnation of Mn ions to the glycothermally prepared Zn2SiO4 followed by calcination (vide infra) also supports the second explanation. The TEM images of P(1,4-BG)-800 and P(1,4-BG)-1100 calcined for 0.5 and 5 h are shown in Fig. 11. Coagulated particles were not observed for P(1,4-BG)-800, but voids in the particles were
apparent. The morphology of P(1,4-BG)-900 was discussed in Section 3.1. Calcination at 1100 1C for 0.5 h completely alters the morphology of the particles and smoothes the grain surfaces. The sintering between particles occurred during prolonged calcination and resulted in grains sizes of 500 nm. Fig. 12 shows the effect of calcination time at 1100 1C on the luminescence intensity for the P(1,4-BG) (x¼0.02) phosphors. The emission intensity of the samples increased with increasing calcination time. This result is due to the crystal growth that occurred during prolonged calcination (Fig. 11). For the Zn2SiO4:Mn2 þ phosphor prepared by solid-state reaction, the emission intensity increased with firing time. However, the maximum emission intensity was observed for the sample that was fired for 3 h, and prolonged firing (5 h) decreased the emission intensity, presumably due to sublimation of the Zn component. The emission intensity of the calcined P(1,4-BG) phosphor was twice as high as that of the samples prepared by solid-state reaction. For comparison, Mn2þ ions were impregnated to a a-Zn2SiO4 sample prepared by glycothermal reaction in 1,4-BG and the thus-obtained sample was calcined at 1100 1C for 0.5 and 5 h. The phosphor obtained by the impregnation method exhibited a higher emission intensity than the phosphor obtained by solid-state reaction. This result indicates that the difference in the emission intensity was caused by the difference in the dispersion of Mn2 þ ions in the Zn2SiO4 host.
4. Conclusions
a-Zn2SiO4 was synthesized by glycothermal reactions of zinc acetate dihydrate and tetraethyl orthosilicate in 1,4-BG and 1,3PG, whereas b-Zn2SiO4 was obtained by the reaction in EG. The phosphor synthesized in 1,4-BG exhibited the highest emission intensity of the prepared samples. Because Mn2 þ in the a-Zn2SiO4 host was oxidized to Mn3 þ by calcination at 800 1C, the photoluminescence disappeared. However, Mn3 þ ions occupy tetrahedral environments in the a-Zn2SiO4 host, and calcination at 1100 1C reduced the Mn3 þ species in a-Zn2SiO4 to Mn2 þ . The photoluminescence intensity of the Zn1.96Mn0.04SiO4 phosphor obtained by calcination at 1100 1C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was attributed to the Mn2 þ in the a-Zn2SiO4 host remaining highly dispersed during calcination.
Acknowledgment The XANES experiments have been performed with the approval of SPring-8 (Proposal no. 2010A1271).
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