Materials Research Bulletin, Vol. 32, No. 5, pp. 501-506,1997 Copyright 0 1997 Elsevier Science Ltd FYinted in the USA. All rights resewed 0025-5408/97 $17.00 +.OO
COMBUSTION SYNTHESIS AND PHOTOLUMINESCENCE OF NANOCRYSTALLINE YIOJ:EUPHOSPHORS
Tao Ye’*, Zhao Guiwen’, Zhang Weiping’and Xia Shangda’ 1Department of Applied Chemistry, ’ Department of Physics University of Science and Technology of China, Hefei, Anhui 230026, China (Refereed) (Received August 14, 1996; Accepted October 15, 1996)
ABSTRACT Nanoscale Y203:Eu phosphors have been prepared by glycine-nitrate solution combustion synthesis. The particle size of resultant powders is much related to the combustion flame temperature, which can be controlled by adjusting the glycine-to-nitrate ratio. The IR spectra., the excitation and the diffuse reflection spectra show the particle-size dependence of optical properties. The quenching concentration of nanostructured phosphors is increased greatly, compared with phosphors synthesized by conventional SyMheSiS method. [email protected]
1997 EISW~Wscme Lrd KEYWORDS: A. nanostructure, A. oxides, B. chemical synthesis, D. luminescence, D. optical properties INTRODUCTION Eu3+activated Y203 phosphors have been studied for a long time because of their efficient luminescence under UV, cathode-ray excitation (l-5). The commercial phosphors of this type is generally synthesized by solid reaction or precipitation method, the particle size of which is in the range of pm scale. In view of the special properties concerning the nanoscale materials, it is interesting to know what differences in properties are between the commercial and nanoscale phosphors of Y203:Eu. Submicron Y203:Eu has been synthesized
*To whom correspendence should be addressed. Current address is Synchrotron Radiation Laboratory, Institute of High Energy Physics, PO Box 918, Beijing 100039, China. 501
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by sol-gel process (6). As to our best knowledge, the preparation of nanocrystalline Yz03:Eu has not been reported. By improving the glycine-nitrate solution combustion synthesis method (7,8), we prepared the nanocrystalline Y203:Eu and observed size and interface effects of this novel material. EXPERIMENTAL Analytical grade rare-earth nitrate (Y(NO,), . 6Hz0, Eu(NOx), 3 6H20) and glycine were dissolved in distilled water and mixed in an appropriate ratio to form the precursor solution. The precursor solution was concentrated by heating it in a porcelain crucible until excess free water evaporated and spontaneous ignition occurred. Within 10 s, the combustion was finished with the resultant ash of about 1.1 g (0.005 mol Y203:Eu) filling the 200 ml container. XRD was conducted with a Rigaku D/max 7A rotating anode diffractometer with Cu Kcx radiation. IR spectra were recorded using a Shimadzu IR-440 spectrometer with KBr pellets. UV-Vis fluorescence spectra were measured on a Hitachi 850 fluorescence spectrometer and the diffuse reflectivity spectra on a Shimadzu UV-240 UV-Vis spectrophotometer in the wavelength range from 200 to 800 nm. The spectra measurements were carried out at room temperature. Metal ratios in the products were determined by PE 6500 ICP-AES following dissolution in 1 M nitric acid. The flame temperature was measured by a WGG2-201 disappearing-filament optical pyrometer (Shanghai Automatic Instrument Plant). RESULTS ANL) DISCUSSION Glycine serves as fuel for the combustion reaction, being oxidized by the nitrate ions. Stoichiometrically balanced (9), the exothermic reaction can be expressed as: 3 M(NO& + 5 NHzCHZCOOH + 9 0: + 3/2 b&O3 + 5/2 NZ + 9 NOz + 10 COz + 2512 Hz0 (M = Y, Eu) The reaction temperature has a great influence on the particle size of the product. Adjusting the ratio of glycine to nitrate, we can control the combustion flame temperature. It has been reported that the smallest particle size can be obtained when the glycine-to-nitrate ratio is adjusted stoichiometically to yield the highest flame temperature (7). However, our results indicate that, at least for the preparation of rare-earth oxide, lower temperature leads to smaller particle size (10). The glycine-to-nitrate (G/N) ratio in the precursor solution was adjusted to be oxidant-rich (G/N: 1.O, 1.3) and stoichiometric (G/N: 1.7). The most vigorous combustion was observed for 1.7 G/N ratio, which gave a temperature of 1450 + 20°C The temperature for 1.3 and 1.O G/N ratio is about 1320°C and lOOO”C,respectively. The original cation stoichiometry of metal nitrate-giycine precursor was maintained in aH resultant products within the uncertainty of measurement by ICP-AES. The XRD patterns of all resultant products, (Yo.sEu~1)203, were indexed to Y203 cubic phase. The IR spectra, however, indicated the existence of residual N03- in the resultant ash of oxidant-rich ratio of 1.O and 1.3, so the samples were calcined at 500°C for 1 h to decompose the residual NO,- and the products were named nl and n2, respectively. The
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FIG. 1 XRD patterns of samples nl, n2, and n3 corresponding to 8,40, and 70 mn, respectively.
resultant ash of stoichiometric ratio was named n3. For comparison, n3 was calcined at 1lOO’C for 2 h to obtain sample n4. XRD patterns are shown in Figure 1. Particle size was calculated from the half-width of the peaks of the X-ray powder patterns by using Scherrer’s equation. The value of particle sizes obtained for each peak is listed in Table 1. The average particle size of nl, n2, n3 and n4 is 8,40,70 and 160 run respectively. IR spectra (Fig. 2) show no evidence of residual C and N03-, except for a peak at 3450 cm-’ and a broad band below 700 cm-’ assigned to -OH group of Hz0 and a metal-oxide (Y-O) mode, respectively. It is clear that Y-G absorption bands broaden as particle sizes decrease. As to the cause of broadening, the effects upon characteristics of IR spectra are essentially due to the polarization charge induced at the particle surface by the external electromagnetic field (11,12). It is assumed that with decreasing particle sizes, surface effects will be enhanced so that damping of surface mode absorption will increase and Y-G absorption bands broaden. TABLE 1 The Value of Particle Sizes Obtained for Each Peak of the XRD Patterns Sample nl n2 n3 n4
43 78 165
38 66 154
39 63 150
42 76 170
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FIG. 2 IR spectra of samples n 1, n2, and n3. Excitation spectra show the particle-size dependence of the excitation peak position and the excitation bandwidth. The excitation peak is shifted toward the red from 239 to 250 nm with the decreasing particle size, while the halfwidth of excitation bands becomes narrower. The bandwidth of nl was about 10 run narrower than that of n3. More interesting, a new band at about 220 nm is detected in nl spectra (Fig. 3). The peaks of the samples with particle size larger than those of sample n3 are still located at 239 nm. However, all of the strongest emission peaks are located at 6 12 nm, corresponding to the typical 4f transition ‘Do+‘F2. We know the excitation of Eu” belongs to the charge-transfer state transition, which is related to stability of the electron of the surrounding O*-. Obviously, the position of the excitation peak depends on the nature of surrounding O*- ions. In a crystal lattice, an O*ion is stabilized by surrounding positive ions. However, as particle size decreases, the surface-to-volume ratio of atoms increases and the degree of disorder of the nanostructured system increases. In that case, the O*- is less stable. As a result, it requires less energy to remove an electron from an O*- ion; therefore, the charge-transfer state band is shifted toward lower energy. At the present time, there is no appropriate explanation for the particle-size dependence of bandwidth and the new excitation band other than they are a manifestation of surface effects. When the wavelength of incident light comes close to short wavelength UV range, the excited fluorescence disturbs the measurement of the diffuse reflection spectra, so the reflection spectra are given in the range of 250-800 nm (Fig. 4). Due to small size and interface effects, the spectra show the dependence of the reflectance on particle size from 600 to 800 nm: the smaller the particle size, the stronger the absorption. Compared with the reflection spectra curves for the other samples (Fig. 4), the curve for nl spectra shows the strongest absorption. Especially in the range of 330 to 250 nm, the curve of nl shows steep descent, corresponding to enormously enhanced absorption. The curve for n4 is different from those of the other samples. Sample n4 was fired at 1100°C and its emission intensity is stronger than that of the other samples. It could be that the tail of the fluorescence of the n4 sample may have influenced the spectrum measurement.
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550 600 650
FIG. 3 (a) Excitation spectra of nl (solid line), n2 (dot-dash chain), and n3 (dashed line) of (YwE~&O~. (b) Emission spectrum of n3 (the relative intensity of peaks is schematic and no quantitative accuracy is intended). The ash by combustion reaction can luminesce directly without further calcination, especially for sample n3, the luminescence intensity of which reaches 80% of that of corresponding commercial phosphors. By optimizing experimental conditions, practical nanoscale red-emitting phosphors may be synthesized. However, it is known that when the activator concentration increases to a limited level, luminescence begins to quench. The quenching concentration of Y20s:Eu phosphors prepared by conventional synthesis is 6% mol Eu (13), but for our samples prepared according to the G/N ratio 1.7 the quenching concentration was apparently 14% mol Eu. Because the quenching concentration is related
FIG. 4 Reflection spectra of nl (dot-dash chain), n2 (dashed line), n3 (solid line), and n4 (dotted line).
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to the energy transfer between the Eu ions, we think that the interface effects of the nanoscale materials may hinder the energy transfer and, as a result, the quenching concentration increases.
CONCLUSION Nanoscale Y203:Eu phosphors have been prepared by glycine-nitrate solution combustion synthesis. The particle size of resultant powders is related to the combustion flame temperature, which can be controlled by adjusting the glycine-to-nitrate ratio. Due to surface effects, the Y-O absorption bands of IR spectra broaden, while the peak of excitation spectra is shifted toward the red and becomes narrower with decreasing particle size. The reflection spectra also show particle-size dependence: the smaller the particle size, the stronger the absorption. The quantitative explanation for particle-size dependence of optical properties requires further research. Because of the interface effects of nanoscale materials, which may hinder energy transfer, the quenching concentration of this type of phosphors is increased greatly, compared with phosphors synthesized by conventional synthesis. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China and the Foundation of National Education Commission of China. REFERENCES 1. N.C. Chang, J. Appl. Phys. 34,350O (1963). 2. H. Forest and G. Ban, J. Electrochem. Sot. 116,474 (1969). 3. Q. Su , C. Barthon, J.P. Denis, F. Pelle and B. Blanzat, J. Lumins. 28, l(1983). 4. Y. Kotera, J. Lumins 3132, 709 (1984). 5. R.G. Pappalardo and R.B. Hunt., Jr, J. Electrochem. Sot. 132, 72 l( 1985). 6. Y.H. Pei and X.R. Liu, Chin. .I. Lumins. 17,52 (1996). 7. L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas and G.J. Exarhos, Mater. Lett. 10,6(1990). 8. L.R. Pederson, G.D. Maupin, W.J. Weber, D.J. McReady and R.W. Stephens, Mater. Lett. 10, 437 (1990). 9. S.R. Jam, K.C. Adiga and V. Pai Vemeker, Cornbust. Flame. 40, 71 (198 1). 10. Y. Tao, C. Wang, Q. Su and G. Zhao, submitted for publication to Nanostr. Mater., 1996. 11. C.J. Serna, M. Ocana and J.E. Iglesias, J. Phys. C 20,473 (1987). 12. H.C. Van de Hulst, Absoption and Scattering ofLight by Small Particles, Wiley, New York, (1957). 13. L. Ozawa, H. Forest, P.M. Jaffe and G. Ban, J. Electrochem Sot. 118,482 (1971).