Spectroscopic properties of Eu3+ doped YBO3 nanophosphors synthesized by modified co-precipitation method

JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011, P. 1142

Spectroscopic properties of Eu3+ doped YBO3 nanophosphors synthesized by modified co-precipitation method A. Szczeszak1, S. Lis1, V. Nagirnyi2 (1. Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland; 2. Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia) Received 25 August 2011; revised 2 September 2011

Abstract: Y1–xEuxBO3 nanophosphors were synthesized by a modified co-precipitation method. The structure of the obtained nanocrystals was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average crystallite size was calculated from the full-width at half-maximum (FWHM) of the diffraction peaks by the Scherrer equation. The average particles size was 25±10 nm. The spectroscopic properties of the Y1–xEuxBO3 nanoborates were characterized by excitation and emission spectra under UV and VUV excitation. In order to improve colour purity, the chromaticity coordinates were also calculated. Keywords: YBO3:Eu3+; co-precipitation; nanophosphor; luminescence; VUV; rare earths

In recent years, much attention has been paid to the yttrium orthoborate because of its efficient luminescent properties, VUV transparency and exceptional optical damage threshold, which allows them to withstand the harsh conditions present in vacuum discharge lamps and screens. Additionally, YBO3 doped with Eu3+ or Tb3+ ions exhibits highly efficient luminescence under VUV excitation. Thus, nanophosphors based on YBO3:Eu3+ or Tb3+ are demanded for the development of plasma display panels (PDP) and Hg-free fluorescent lamps[1,2]. Rare earth orthoborates, REBO3 (RE = lanthanide and yttrium), crystallize mainly in three structures, i.e., the aragonite, calcite and pseudovaterite, determined as the YBO3-type[3]. Until now, there are many different suggestions of the space groups and site symmetry determination for the YBO3-type crystal structure. For example, Newnham et al.[4] distinguished two probable structure models, a disordered (P63/mmc) and an ordered hexagonal (P63/mcm) structure. The first one includes only one D3d point symmetry and the second structure confirms two sorts of point symmetry sites D3 and D3d for the rare earth. Afterwards Bradley et al.[5] proposed the P 6c 2 space group to determine crystalline structure with one D3 point of symmetry. However, Denning et al.[6] concluded the existence of four coordinate boron and trimeric rings with D3h symmetry in the rare earth borates. Chadeyron et al.[7] proved that yttrium atom are eightfold coordinated and have two types of environments. Furthermore, Holsa[8] described two different sites with D3d and T point symmetries for the Ln3+ ions in the

vaterite structure. Recently, Lin et al.[9] suggested the space group C2/c for the vaterite type orthoborate. And, Jia et al.[10] described in details various space groups for the YBO3 doped with Eu3+ ions, based on careful analysis of XRD and luminescence spectra. The characteristic emission spectrum of YBO3:Eu3+ presents nearly equal bands from 5D0ĺ7F1 and 5D0ĺ7F2 transitions. As a result an orange-red instead of red emission is observed. Different applications require red emission originating from 5D0ĺ 7 F2, which is hypersensitive to the symmetry of the crystal field and relatively intense when the symmetry of the environment is low. Therefore, the aim is to reduce the crystal filed symmetry and thus, to improve the chromaticity of the sample[11]. Various methods of synthesis have been developed to obtain YBO3:Eu3+ including hydrothermal method[12], thermal decomposition[13], sol-gel synthesis[14] and co-precipitation method[15]. In the present work, the morphology, structure and exceptional spectroscopic properties of the obtained nanosized YBO3:Eu3+ were investigated. In order to obtain desired product the modified co-precipitation method with organic ligands as a stabilizing factors was used. The presence of organic compound inhibited growth of the nanocrystals by covering their surface.

1 Experimental 1.1 Synthesis The nanophosphors Y1–xEuxBO3 (0”x”0.2) were success-

Foundation item: Project supported by the Polish Ministry of Science and Higher Education (N N204 089838), the Estonian Science Foundation (grant 8893) and the European Community Research Infrastructure Action under the FP6 Structuring the European Research Area Programme (through the Integrated Infrastructure Initiative Integrating Activity on Synchrotron and Free Electron Laser Science) Corresponding author: S. Lis (E-mail: [email protected]; Tel.: +48 61 829 1345) DOI: 10.1016/S1002-0721(10)60613-8

A. Szczeszak et al., Spectroscopic properties of Eu3+ doped YBO3 nanophosphors synthesized by modified co- …

fully prepared by the modified co-precipitation method. Yttrium oxide Y2O3 and europium oxide Eu2O3 (Stanford Materials 99.99%), nitric acid HNO3 (POCh S.A., ultra-pure), orthoboric acid (POCh S.A., p.a. grade), glycerin (POCh S.A., p.a. grade, 99.5%), citric acid monohydrate (CHEMPUR, p.a. grade), ethylene glycol (CHEMPUR, p.a. grade), ammonia solution (POCh S.A., p.a. grade) were used as precursors in the experiment. Ethanol was used to wash obtained nanocrystals. Firstly, various co-reagents, such as glycerin (5 ml per 1 g of product), citric acid (1 g per 1 g of product), ethylene glycol (5 ml per 1 g of product) or citric acid mixed with glycerin or ethylene glycol, were used in order to choose the most appropriate ligand to obtain high-quality product. For comparison direct precipitation was also made. The best results were observed when glycerin was used; hence this organic compound was added in the further experimental way. The detailed procedure is described below. Required amounts of rare earth nitrate solutions were obtained by dissolving lanthanides oxides in HNO3. An excess amount of acid was removed by evaporating solutions several times. Then, the stoichiometric amounts of lanthanide salts were mixed in deionized water together with glycerin (5 ml per 1g of product) and kept under stirring. In the above solution NH3(aq) was added dropwise in order to reach pH~9. The mixture was again kept stirring for 5 h at 80 °C. The precipitated product was washed several times with ethanol by centrifuging. Thus obtained nanocrystals were calcined at 900 °C for 2 h in an air atmosphere. Product annealed in lower temperature was contaminated and had impurities in the structure.

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with a CCD camera. The second arm of the spectrograph equipped with a Hamamatsu photomultiplier R6358P was used at measuring the excitation spectra of different emission lines. In the latter case, the slits of the spectrograph were set for a spectral interval of 10 nm. The excitation spectra were corrected for the spectral distribution of incident light.

2 Results and discussion Fig. 1 presents XRD patterns of the obtained YBO3 annealed at 900 °C for 2 h synthesized without or with the presence of various co-reagents. It was observed that the precipitation in the presence of citric acid caused synthesis of the yttrium oxide instead of the desired product. However, additional peaks from Y3BO6 phase were observed. When the ethylene glycol was added, the obtained crystals were quite big, which was concluded from the reflection broadening. Direct precipitation reaction gave a one-phase product, but it was similar as in the case with the ethylene glycol that the crystal size was not in the nanoscale. Hence, the glycerin was chosen as the best co-reagent and used in the further experiment. As performed in the picture (Fig. 2), all diffraction peaks of the Y1–xEuxBO3 phases match well with that indexed for the JCPDS No. 74-1929. The obtained nanosized Y1–xEuxBO3 phosphors have a hexagonal pseudovaterite structure and no additional phase was observed for the samples

1.2 Characterization X-ray diffraction patterns (XRD) were obtained using a Bruker AXS D8 Advance diffractometer in Debye-Scherrer geometry, with Cu KĮ1 radiation (0.1541874 nm) in the 2ș range from 6° to 60°. The XRD results were assigned to the Joint Committee on Powder Diffraction Standards (JCPDS) database. Average crystallites sizes were calculated from the Scherrer equation D=0.9Ȝ/ȕcosș, where D is the average grain size, the factor 0.9 is specific for spherical objects, Ȝ is the X-ray wavelength, ș and ȕ are the diffraction angle and full-width at half-maximum of an observed peak[16]. The TEM images were recorded at an FEI Tecnai G2 20 X-TWIN transmission electron microscope, by an accelerating voltage of 200 kV. Luminescence parameters of the synthesized samples were collected at a Hitachi F-7000 fluorescence spectrophotometer at room temperature (300 K) with the 150 W xenon excitation source. Excitation and emission spectra were corrected for the instrumental response. The synchrotron radiation study of powder samples was performed at the SUPERLUMI station of HASYLAB, Hamburg, Germany. The setup has been described in detail elsewhere[17]. Luminescence spectra with resolution 1 nm were recorded with a SpectraPro308i (Acton) spectrograph equipped

Fig. 1 XRD patterns of YBO3 calcined at 900 °C for 2 h obtained without or with the presence of various co-reagents

Fig. 2 XRD patterns of Y1–xEuxBO3 synthesized with the presence of glycerin as a co-reagent and calcined at 900 °C for 2 h

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doped in the range of 0”x”0.1. When the concentration of Eu3+ ions was higher than 0.1, excessive traces of Y3BO6 could be observed. Particle size and shape of Y1–xEuxBO3 sintered at 900 °C for 2 h were analysed with the help of TEM image presented in Fig. 3. As shown, the particles are plate-like and tend to agglomerate. The average size of the nanocrystals calculated from the Scherrer formula was 25±10 nm, which is in a good agreement with the TEM picture. Fig. 4 shows the excitation spectra registered for the Y0.9Eu0.1BO3, which presents the highest luminescence intensity, by observing the emission of 5D0ĺ7F1 at 594 nm and 5 D0ĺ7F2 at 613 nm. The most intense and broad bands are characteristic for the charge transfer excitation (CT). The CT band recorded for the 5D0ĺ7F1 transition has a maximum located at a shorter wavelength than that for 5D0ĺ7F2. However, in both cases the maxima of CT bands are shifted in the shorter wavelengths direction, below 250 nm. It is due to reduction of a grain size into nanoscale[11]. The emission spectra of Y0.9Eu0.1BO3 measured for different wavelengths UV excitation are shown in Fig. 5. The intensity ratio of 5D0ĺ7F1 to 5D0ĺ7F2 band varies with the changing of the excitation wavelength. It is related with the existence of two different intrinsic luminescent sites in the Y1–xEuxBO3 structure, and Eu3+ ions could be excited selectively by various excitation sources[11]. However, when the sample was excited with Ȝ=394 nm, the observed emission was composed of almost equal of 5D0ĺ7F1 and 5D0ĺ7F2

JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011

Fig. 5 Emission spectra of Y0.9Eu0.1BO3 calcined at 900 °C for 2 h registered under various wavelength UV excitations

bands. This suggests that both types of Eu3+ ions were excited. Additionally, Wei et al.[11] confirmed the existence of a third environment for Eu3+ in the surface site. It is associated with the transition from macro- to nanoscale. Additionally, measurements in the VUV range at T=10 K were made in order to confirm that YBO3:Eu3+ nanophosphors present strong absorption below 200 nm and emission of visible light under VUV excitation. The excitation spectra measured for Y0.9Eu0.1BO3 upon Eu3+ emission at 590 and 610 nm are shown in Fig. 6. There are two main broad excitation peaks, one is ascribed to absorption of the host lattice that overlapped with CT band of O2–ĺY3+ and the second is assigned to O2–ĺEu3+ charge transfer[2,18–20]. Fig. 7 illustrates the emission spectra of the Y0.9Eu0.1BO3 under various excitation wavelengths. The intense emission sharp peaks, associated with the transitions 5D0ĺ7FJ (J=1, 2, 3, 4) characteristic for Eu3+ ions, are observed. The intensity ratio of 5D0ĺ7F1 to 5D0ĺ7F2 bands varies with the changing of the excitation wavelength. Again, there was similar situation as that mentioned above, where the intensity ratio of 5 D0ĺ7F1 to 5D0ĺ7F2 bands varies with the changing of the excitation wavelength because of the two different luminescent sites in the Y1–xEuxBO3[11]. In order to confirm the earlier described potential applications

Fig. 3 TEM image of Y0,9Eu0,1BO3 synthesized with the presence of glycerin as a co-reagent and calcined at 900 °C for 2 h

Fig. 4 Excitation spectra of Y0.9Eu0.1BO3 calcined at 900 °C for 2 h

Fig. 6 Excitation spectra of Y0.9Eu0.1BO3 calcined at 900 °C for 2 h measured at T=10 K in the range of VUV and UV

A. Szczeszak et al., Spectroscopic properties of Eu3+ doped YBO3 nanophosphors synthesized by modified co- …

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equation. The spectroscopic properties were characterized by excitation, emission spectra registered in the VUV and UV region. Luminescence spectra demonstrated the three most intense 5D0o7F1 (594 nm) and 5D0o7F2 (613 and 627 nm) transition bands, connected to orange-red and red region of the spectrum, respectively. The exceptional luminescence properties such as Vis emission under VUV excitation, as well as chromaticity coordinates of the orthoborates Y1–xEuxBO3 made them potentially useful for optical devices, e.g. display panels or lighting.

References: Fig. 7 Emission spectra of Y0.9Eu0.1BO3 calcined at 900 °C for 2 h registered under VUV and UV excitation

Fig. 8 Chromaticity diagram for Y0.9Eu0.1BO3 calcined at 900 °C for 2 h calculated for Ȝ=160 and 190 nm

of nanosized YBO3 doped with Eu3+ ions, the analysis of color purity of the Y0.9Eu0.1BO3 is performed in Fig. 8. The chromaticity parameters were compared for the different excitation wavelengths. Depending on the type of excited luminescent center the color purity changed and was improved for Ȝ=190 nm, which agrees with appearance of registered emission spectrum. The chromaticity coordinates: x= 0.641, y=0.358 and x=0.647, y=0.353 respectively for Ȝ=160 and 190 nm were calculated, and it was proved that the chromaticity properties of the YBO3 doped with Eu3+ ions make them appropriate for displays[15].

3 Conclusions Hexagonal Y1–xEuxBO3 orthoborates were successfully prepared by a modified co-precipitation method. The glycerin was chosen as the best stabilizing organic compound to synthesize the high-quality nanophosphors. X-ray diffraction confirmed the hexagonal pseudovaterite structure of the obtained nanocrystals at 900 ºC. The transmission electron microscopy corroborated the formation of plates-like nanoborates with the average size of ~25±10 nm, which was in a good agreement with the value calculated from the Scherrer

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