Preparation and characterization of uniform, spherical particles of Y2O2S and Y2O2S:Eu Ligia Delgado da Vila, Elizabeth Berwerth Stucchi* and Marian Rosaly Davolos Departamento de Quý�mica Geral e Inorga�nica, Instituto de Quý�mica, Universidade Estadual Paulista, C.P. 355, CEP 14801-970, Araraquara, SP, Brasil The preparation of spherical Y2O2S and Y2O2S:Eu particles using a solid–gas reaction of monodispersed precursors with elemental sulfur vapor under an argon atmosphere has been investigated.The precursors, undoped and doped yttrium basic carbonates, are synthesized by aging a stock solution containing the respective cation chloride and urea at 82–84 °C. Y2O2S and Y2O2S:Eu were characterized in terms of their composition, crystallinity and morphology by chemical analysis, X-ray powder diVraction (XRD), IR spectroscopy, and scanning electron microscopy (SEM).The Eu-doped oxysulfide was also characterized by atomic absorption spectrophotometry and luminescence spectroscopy. The spherical morphology of oxysulfide products and of basic carbonate precursors suggests a topotatic inter-relationship between both compounds. Rare-earth oxysulfides have been known for a long time as and Hsu8 and Sordelet and Akinc9 was adapted by Santos.10 Sulfidization of the basic carbonate precursor compounds was excellent phosphor host materials.When activated with trivalent europium, Y2O2S becomes an important red phosphor carried out in a horizontal tube furnace following the procedure outlined by Luiz.7 Precursors were loaded into the principal in color TV picture tubes1 because it has high brightness, short decay time, and exhibits long-term stability in poly(vinyl furnace in an alumina crucible.The furnace was completely sealed to ensure an oxygen-free atmosphere. Argon gas was alcohol). Recently, yttrium oxysulfide has attracted a great deal of attention because of its electroluminescent properties.2 flushed through the system during both the reaction and cooling time.Sulfur was heated at 220 °C in an auxiliary In modern technology, the necessity of dispersed powders consisting of uniform particles in size and shape is widely furnace. Then the main furnace temperature was gradually increased to 770 °C (heating rate 2 °C min-1) and sulfur vapor recognized. For example, in ceramic manufacture, there is a significant reduction in the sintering time and temperature if was carried by the argon stream across the system (flow rate 116 cm3 min-1).The end of the reaction was accompanied by monodispersed powders are used as starting materials.3 Phosphors used in both CRT and X-ray screens with uniform bubbling of acid vapor products in alkaline solution until it size distribution of the particles results in the best screen reached constant pH.After the desired reaction time the surfaces.4 In materials science, the particle shape is equally auxiliary furnace was shut oV and the product was heated at important. Spherical morphology is interesting because most 800 °C for at least 2 h. The choice of synthesis temperature theoretical models dealing with fine particle properties and was based on DTA curve data.interactions are based on spherical particles. Furthermore, the size distribution of relatively uniform spherical particles can be determined by optical techniques without altering or Characterization destroying the system.5 Lanthanide content was assayed by EDTA titration using Several methods are known for the preparation of rare-earth xylene orange as indicator. Atomic absorption spectrophoto- oxysulfides.6 However, it seems that few investigations have metry was performed using an INTERLAB AA-1475 atomic been reported in the literature on spherical particle preparation absorption spectrophotometer. X-Ray powder diVractograms of these phases.In a previous paper Luiz et al.7 described the were recorded on an XRD HZG-4B diVractometer equipped preparation process of lanthanum and yttrium oxysulfide with Cu-Ka or Co-Ka radiation (36 kV, 20 mA).IR spectra in powders by using a solid–gas reaction of oxalates with elemen- KBr pellets and Nujol in CsI windows were measured with tal sulfur vapor under an argon atmosphere. This paper reports Nicolet FT-730 and Impact 400 FTIR spectrometers.The an analogous method, using monodispersed basic carbonates particle morphology was examined in a scanning electron as precursors. The present method is very attractive because microscope JEOL-JSM-T330 A. Powders were pre-coated with the particle morphology of oxysulfide reproduces that of the Au using a cathodic sputtering Edwards S 150 B instrument. basic carbonates.This fact indicates a topotatic inter-relation- Emission spectra of Eu-doped oxysulfide were obtained in a ship between both groups of compounds. Thus, desired mor- Fluorolog Spex 212 I fluorescence spectrometer equipped with phology for the oxysulfides can be achieved through the an R928 Hammamatsu photomultiplier. The samples were morphological control of precursor particles.excited by a 450 W xenon lamp. Experimental Synthesis of doped and undoped yttrium oxysulfides Results and Discussion Yttrium and europium oxides (99.99 and 99.999% pure, EDTA titration and atomic absorption spectrophotometry Aldrich) were used as starting materials. Other chemicals used EDTA titration and atomic absorption spectrophotometry were grade reagents. Monodispersed yttrium basic carbonate results of the oxysulfides are shown in Table 1.As can be seen, particles were produced by heating a stock solution containing the agreement between calculated and obtained results are a cationic chloride and urea at 82–84 °C under continuous stirring for 2 h. This procedure, based on work by Matijevic satisfactory. J. Mater. Chem., 1997, 7(10), 2113–2116 2113Table 1 EDTA titration and atomic absorption spectrophotometry results for yttrium oxysufide particles Ln (mass%) atomic absorption EDTA titration spectrophotometry compound found calc.found calc. Y2O2S 73.97 73.52 — — (Y0.964Eu0.036)2O2S 75.30 74.00 4.26 4.44 X-Ray diVraction The dhkl data of the oxysulfides are shown in Table 2. The precursors were non-crystalline and the products of their thermodecomposition under sulfur and Ar atmosphere yielded an X-ray pattern identified as crystalline Y2O2S.11 The results Fig. 2 FTIR spectra for yttrium oxysulfides in Nujol–CsI windows did not exhibit patterns due to oxysulfate and/or oxide phases (1500 to 220 cm-1 range): (a) yttrium oxysulfide; (b) Eu-doped in the oxysulfide samples. yttrium oxysulfide FTIR spectroscopy precursor.In the low-frequency region, absorptions in the IR spectra of Y2O2S and Y2O2S:Eu, obtained between 4000 range 800–220 cm-1 can be attributed to LnMO or LnMS and 400 cm-1 (high-frequency region) and 1500 to 220 cm-1 vibrations. This assignment is very diYcult because of the (low-frequency region) are shown in Fig. 1 and 2, respectively. proximity of the bands.However, if this sample spectrum is In the high-frequency region, the absence of the SMO stretchcompared with that of yttrium oxide (Fig. 3), oxysulfide phase ing band at 1200 cm-1 attributed to sulfate,12 is noted. This formation is evidenced. The oxysulfide spectrum exhibits less suggests that the oxygen was entirely removed from the broad and strong bands than those observed for the oxide (see reaction system.Moreover, the bands corresponding to the Table 3). This behavior could be understood considering that carbonate group were also missing, suggesting the absence of Table 2 X-Ray diVraction powder data for yttrium oxysulfide this work lit.11 Y2O2S Y2O2S5Eu d/A ° I/Io(%) d/A ° I/Io(%) d/A ° I/Io(%) 3.280 35 3.255 31 3.283 32 2.930 100 2.942 100 2.920 100 2.320 60 2.320 40 2.312 34 2.190 16 — — — — 1.892 65 1.885 38 1.889 34 1.824 45 1.817 23 1.819 32 1.642 30 1.639 16 1.640 16 1.591 25 1.587 18 1.589 15 Fig. 3 FTIR obtained in Nujol–CsI windows (1500 to 220 cm-1 range): (a) yttrium oxysulfide; (b) Eu-doped yttrium oxysulfide; (c) yttrium oxide Table 3 Comparative frequency ranges of yttrium oxysulfide, Eudoped yttrium oxysulfidettrium oxide bands wavenumber/cm-1 Y2O2S Y2O2S5Eu Y2O3 — 235m (sp) 238w (sp) 269m (sh)a 270w (sh) 270w (sh) 290m (sh) — — — — 306s (sp) 310m (sh) 312m (sh) — — — 339s (sp) 391m (sh) 394m (sh) 380s (br) 467s ( br) 462s ( br) 466s (sp) — — 563s ( br) Fig. 1 FTIR spectra for yttrium oxysulfides in KBr pellets (4000 to 400 cm-1 range): (a) yttrium oxysulfide; (b) Eu-doped yttrium aBand features: br=broad, sp=sharp; sh=shoulder. Band intensity; s=strong, m=medium, w=weak.oxysulfide 2114 J. Mater. Chem., 1997, 7(10), 2113–2116there are only contributions from C3v symmetry for Y2O2S,13 while in Y2O3 there are contributions from C2v and S6 symmetry. 14 Therefore, the IR spectra suggest that the LnMS vibrations are present. This result corroborates the XRD data, showing the formation of the oxysulfide phase.The absorptions at 1460, 1376 and 723 cm-1 are characteristic of Nujol vibrational modes. Scanning electron microscopy Scanning electron micrographs of the basic carbonate precursors and oxysulfides are shown in Fig. 4 and 5, respectively. The precursor and oxysulfide particles were relatively uniform in size and spherical in shape.The average particle size of both undoped basic carbonates and oxysulfides was 0.4 mm, while Eu-doped precursors and oxysulfides showed a medium particle size of 0.2 mm. This suggests that the dopant can influence the size of the particles, but further study is required to confirm this eVect. Fig. 4(b) and 5( b) show that no change in morphology took place during the sulfidization procedure.This suggests the occurrence of a topotatic reaction from basic carbonates to oxysulfides. It can be observed that slight sintering occurs during thermal processing of the precursors. Luminescence spectroscopy Emission spectra of Eu-doped oxysulfide obtained at room temperature (lexc=311 nm) and 77 K (lexc=303 nm) are shown in Fig. 6(a) and (b), respectively. The main signals are found in the region between 580 and 630 nm.The assignments agree Fig. 4 Electron micrographs of: (a) yttrium basic carbonate; (b) yttrium oxysulfide Fig. 6 Emission spectra of Eu-doped yttrium oxysulfide (400–750 nm range): (a) room temperature (lexc=311 nm); ( b) 77 K (lexc=303 nm) Fig. 5 Electron micrographs of: (a) Eu-doped yttrium basic carbonate; Fig. 7 Emission spectra of Eu-doped yttrium oxysulfide (400–750 nm range, room temperature, lexc=265.5 nm) (b) Eu-doped yttrium oxysulfide J.Mater. Chem., 1997, 7(10), 2113–2116 2115p. 315; (b) K. Sowa, M. Tanabe, S. Furukawa, Y. Nakanishi and with that expected for C3v symmetry.13 The dominant emission Y. Hatanaka, Jpn. Appl. Phys., 1993, 32, 12A, 5601. arises from the 5D0�7F2 transition and is observed at 626 nm. 3 Y. Her, E. Matijevic and W. R. Wilcox, J. Mater. Sci. L ett., 1992, According to crystal-field theory, the J=0 state does not split 11, 1629. (A1), and the J=1 state splits into two Stark levels (A2 and 4 L. H. Brixner,Mater. Chem. Phys., 1987, 16, 253. E) resulting in one and two emission bands, respectively. This 5 (a) E. Matijevic, CHEMTECH, 1991, 21, 176; (b) E.Matijevic, Annu. Rev. Mater. Sci., 1985, 15, 483. observed behavior indicates only one emitting Eu3+ symmetry 6 (a) M. Leskela�, Res. Pap., 1980, 64, 39; (b) O. Kanehisa, T. Kano site. The emission spectrum obtained at 265.5 nm excitation, and H. Yamamoto, J. Electrochem. Soc., 1985, 132, 2023. characteristic of Eu3+ in yttrium oxide, is shown in Fig. 7. No 7 J.M. Luiz, E. B. Stucchi and N. Barelli, Eur. J. Solid State Inorg. peak arising from a 5D0�7F2 transition at 611 nm is observed, Chem., 1996, 33, 321. suggesting the absence of oxide impurity. 8 E. Matijevic and W. P. Hsu, J. Colloid Interface Sci., 1987, 118, 506. 9 D. Sordelet and M. Akinc, J. Colloid Interface Sci., 1987, 122, 47. 10 M. F. Santos, Estudo de Precursores paraMateriais L uminescentes: The authors would like to thank to CNPq for a scholarship Hidroxicarbonatos de ý� trio e de Gadolý�nio Dopados, M. Sc. (L.D.V.). Dissertation, Instituto de Quý�mica, UNESP, Araraquara, 1993. 11 JCPDS 24-1424. 12 J. R. Ferraro, L ow-Frequency V ibrations of Inorganic and Coordination Compounds, Plenum Press, New York, 1971. References 13 O. J. Sovers, T. Yoshioka, J. Chem. Phys., 1968, 49, 4945. 14 H. Forest, G. Ban, J. Electrochem. Soc., 1969, 116, 474. 1 P. N. Yocom, US Pat., 3 418 247, 1968. 2 (a) K. Sowa, M. Tanabe, S. Furukawa, Y. Nakanishi and Y. Hatanaka, Electroluminescence, Proc. Int. Workshop 6th, 1992, Paper 7/01540B; Received 4thMarch, 1997 2116 J. Mater. Chem., 1997, 7(10), 211