Correlation between Size-Dependent Luminescent Properties and

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Chem. Mater. 2003, 15, 3011-3017

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Correlation between Size-Dependent Luminescent Properties and Local Structure around Eu3+ Ions in YBO3:Eu Nanocrystals: An XAFS Study Zheng-Gui Wei, Ling-Dong Sun,* Xiao-Cheng Jiang, Chun-Sheng Liao, and Chun-Hua Yan* State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry, Peking University, Beijing 100871, China

Ye Tao, Jing Zhang, Tian-Dou Hu, and Ya-Ning Xie Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Beijing 100039, China Received March 21, 2003. Revised Manuscript Received May 9, 2003

X-ray absorption fine structure spectroscopy (XAFS), a powerful tool for the study of the local environment of a particular element, especially the luminescent center, was introduced to investigate the correlation between luminescent properties and local structure around Eu3+ ions in YBO3:Eu nanocrystals, which was prepared by a more facile sol-gel pyrolysis process in our previous work and which exhibited size-dependent chromaticity, that is, a relatively better chromaticity could be obtained in the smaller-sized samples. The YBO3:Eu phosphors were studied at the Eu LIII edge (6977 eV), and XAFS results showed that the higher levels of disorder in the nanocrystals were responsible for the relatively better chromaticity. By analyzing recent studies on the nanosized materials, we considered that the high levels of disorder in the YBO3:Eu nanocrystals resulted from the low levels of crystallization.

Introduction YBO3:Eu is an important material with high vacuum ultraviolet (VUV) transparency, exceptional optical damage threshold, and especially good VUV absorption.1 Although all these outstanding properties meet the demands of an ideal VUV phosphor,2,3 the performance of YBO3:Eu is still unsatisfying due to its chromaticity problem.4 The characteristic emission of bulk YBO3:Eu is composed of almost equal contributions of 5D0-7F1 and 5D0-7F2 transitions, which give rise to an orangered emission instead of a red one; hence, it is harmful for its application. Previous studies showed that good VUV absorption of YBO3:Eu is concerned with its hexagonal vateritetype structure.5 At the same time, a relatively strong 5D -7F transition is associated with a relatively low 0 2 local symmetry of Eu3+ ions.6 Therefore, a phosphor, which maintains the hexagonal vaterite-type structure * Authors to whom correspondence should be addressed. E-mail: [email protected]. (1) Boyer, D.; Bertrand-Chadeyron, G.; Mahiou, R.; Caperaa, C.; Cousseins, J. C. J. Mater. Chem. 1999, 9, 211. (2) Ronda, C. R.; Ju¨stel, T.; Nikol, H. J. Alloy Compd. 1998, 275277, 669. (3) Kim, C. H.; Kwon, I. E.; Park, C. H.; Hwang, Y. J.; Bae, H. S.; Yu, B. Y.; Pyun, C. H.; Hong, G. Y. J. Alloys Compd. 2000, 311, 33. (4) Ren, M.; Lin, J. H.; Dong, Y.; Yang, L. Q.; Su, M. Z.; You, L. P. Chem. Mater. 1999, 11, 1576. (5) Yang, Z.; Ren, M.; Lin, J. H.; Su, M. Z.; Tao, Y.; Wang, W. Chem. J. Chin. U. 2000, 21, 1339. (6) Reisfeld, R.; Jørgensen, C. K. Lasers and Excited States of Rare Earths; Springer-Verlag: Berlin, 1977.

and simultaneously possesses a low crystal field symmetry, is desired. We found that the nanosized YBO3: Eu is just this desirable VUV phosphor, which realized the improvements in both fluorescent intensity and chromaticity in comparison with the bulk YBO3:Eu.7,8 In our preliminary work, electron microscopy and X-ray diffraction line broadening analysis were used to obtain the information on the particle size and the long-range order of the YBO3:Eu nanocrystals.7,8 However, the local structures of the luminescent centers, Eu3+ ions, which actually determined the essential properties of YBO3: Eu phosphors, were still unknown. Because of its sensitivity to the short-range order and atomic species surrounding an absorbing site, X-ray absorption fine structure spectroscopy (XAFS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), has long been recognized as a powerful probe for determining the local structure around an absorbing site in the nanocrystals.9 Furthermore, one can quantitatively obtain the structural parameters of one selected element in a complex material by XAFS. Therefore, XAFS is well-suited for studying the local structure around the (7) Wei, Z. G.; Sun, L. D.; Liao, C. S.; Yan, C. H.; Huang, S. H. Appl. Phys. Lett. 2002, 80, 1447. (8) Wei, Z. G.; Sun, L. D.; Liao, C. S.; Yin, J. L.; Jiang, X. C.; Yan, C. H. J. Phys. Chem. B 2002, 106, 10610. (9) Koningsberger, D. C.; Prins, R. X-ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; Wiley: New York, 1988.

10.1021/cm0341888 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/18/2003

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luminescent centers, which are always the low-concentration dopants in the host. In the present work, XAFS will be used to study the local environment of luminescent centers in YBO3:Eu nanocrystals, and the correlation between size-dependent luminescent properties and local structures around Eu3+ ions will be uncovered. Experimental Section Sample Preparation. An appropriate amount of Y(NO3)3, Eu(NO3)3, H3BO3, and (NH4)2-EDTA (EDTA ) ethylenediaminetetraacetate) aqueous solutions were mixed together and stirred until a homogeneous solution was formed. The final pH was controlled at 6-7. By slow evaporation of the solvent, complex precursors were obtained and then dried at 80 °C under a vacuum for hours. Aliquots of the precursors were calcined at 650 °C for 100 min in oxygen, 700 and 900 °C for 5 h in air to obtain nanosized YBO3:Eu, and 1100 °C for 5 h in air to obtain a sub-micrometer-sized sample. These samples containing 10 at. % Eu are denoted as a, b, c and d, respectively, in the following description. Since H3BO3-EDTA complex, which may have similar coordination behaviors to rare earth-EDTA complexes, might be responsible for the homogeneous distribution of rare earth and borate ions, it makes the ions migrate much easier and particles crystallize at a relatively lower temperature.7,10 Furthermore, the decomposition of the organic agent releases gases that can prevent the particles from serious aggregation. For comparison, the bulk YBO3:Eu was obtained by direct solid-state reaction from the mixture of Y2O3, Eu2O3, and H3BO3 at 1100 °C for 5 h in air. X-ray Diffraction Patterns and Fluorescence Spectra. X-ray diffraction (XRD) studies were carried out on a Rigaku D/max-2000 X-ray powder diffractometer using Cu KR (λ ) 1.54056 Å) radiation. Fluorescence spectra were recorded on an Hitachi F-4500 spectrophotometer at room temperature. XAFS Measurements. XAFS spectra at the Eu LIII edge were measured in transmission mode by using synchrotron radiation with a Si(111) double-crystal monochromator at the XAFS station (Beam line 4W1B) of Beijing Synchrotron Radiation Facility. The storage ring was run at a typical energy of 2.2 GeV with an electron current of about 80 mA. To suppress the unwanted harmonics, the angle between the monochromator crystal faces was adjusted to mistune the incident beam by 30%. The incident and output beam intensities were monitored and recorded using a nitrogen gas and a 50% argon-doped nitrogen flowing ionization chamber. The spectra were scanned in the range of 6.77-7.56 keV, which covers the LIII edge absorption of europium atoms. Energy resolution was about 1.5 eV for XANES and 3.0 eV for EXAFS. The USTCXAFS 2.0 program was used for data analysis.9,11 The midpoint of the absorption jump was chosen as the energy threshold (6975 eV). The pre-edge absorption background was fitted and subtracted by using the Victroeen formula. The postedge absorption backgrounds were fitted by using the spline function and subtracted from the absorption spectra. The EXAFS functions were normalized by using the absorption edge jump and were Fourier-transformed to R-space with κ2weighting. The scattering amplitudes and phase-shift functions were calculated using the FEFF code.12,13 The referenced structural model was obtained from Lin’s work about GdBO3, which has the same crystal structure type and space group as EuBO3,4 and the atomic coordinates were calculated from the XRD data of EuBO3 (JCPDS, No. 13-485). In the present work, the radial distribution functions (RDF) were obtained from Fourier transform of κ2χ(κ), and then the (10) Kustin, K.; Pizer, R. J. Am. Chem. Soc. 1969, 91, 317. (11) Zhong, W. J.; He, B.; Li Z.; Wei S. Q. J. China Univ. Sci. Tech. 2001, 31, 328. (12) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113, 5135. (13) Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. Rev. Lett. 1992, 69, 3937.

Figure 1. XRD patterns of YBO3:Eu samples. 20-, 40-, and 90-nm, sub-micrometer-sized YBO3:Eu0.1 samples and a 40nm YBO3:Eu0.2 sample are obtained at 650 °C for 100 min (a), 700 °C (b), 900 °C (c), 1100 °C (d), and 700 °C for 5 h (b′), respectively, by the sol-gel method. The bulk sample is obtained by direct solid-state reaction at 1100 °C for 5 h. The denotation is the same in all the figures. first-shell contributions in the RDFs were isolated through Fourier filtering. These data were back-transformed to κ-space, and then the curve-fitting procedures were conducted by employing the coordination number, bond length, and DebyeWaller factor as the fitting parameters. In this paper, the following EXAFS formula was used to fit the experimental curves:

Xj ) (S0Nj/κrj2)Fj(π,κ) exp(-2κ2σj2) exp(-2rj/λj) × sin(2κrj + φj) Here, Nj is the number of atoms in the j-shell at the average distances rj away from the absorber. Fj(π,κ) is the backscattering factor. S0 is the reduction factor. λj is the mean free path of the photoelectron. φj is the phase shift. σj2 is the Debye-Waller factor and the subscripts the variance at the distance rj, which contains the contribution from both static disorder and thermally induced vibration displacements within the j-shell atoms. With use of the USTCXAFS 2.0 program for data analysis, the standard deviations of bond length are 5% of their values, and those of the coordination number and Debye-Waller factor are (0.001 and (0.0005 nm, respectively.14

Results and Discussion XRD. Figure 1 shows the XRD patterns of YBO3:Eu samples in which all the peaks could be indexed to the hexagonal phase of YBO3 with vaterite-type structure, and no excessive traces of rare earth oxide were observed. When the Scherrer formula is applied to the full-width at half-maximum of the diffraction peaks, the mean particle size of YBO3:Eu could be calculated as 19.5, 40.8, and 88.6 nm for samples a, b, and c, respectively. Obviously, the size of YBO3:Eu samples increased with annealing temperature and time. The least-squares refined crystallographic unit cell parameters were obtained by using the software “LAPOD”,15 as listed in Table 1. The values of all samples are (14) Wang, X. G.; Yan, W. S.; Zhong, W. J.; Zhang, X. Y.; Wei, S. Q. Chem. J. Chin. U. 2001, 22, 349. (15) Langford, J. I. J. Appl. Crystallogr. 1973, 6, 190.

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Table 1. Mean Particle Size and Least-Squares Refined Unit Cell Parameters for YBO3:Eu Bulk and Nanocrystalline Samplesa sample JCPDS std # 16-277 a b b′ c d bulk

particle size (nm)

19.5 40.4 40.0 90.2 sub-micrometer-sized micrometer-sized

a (Å)

c (Å)

c/a

cell volume (Å3)

3.778

8.81

2.332

108.90

3.807 ( 0.004 3.803 ( 0.004 3.811 ( 0.001 3.799 ( 0.001 3.793 ( 0.005 3.787 ( 0.002

8.817 ( 0.009 8.832 ( 0.010 8.858 ( 0.002 8.838 ( 0.003 8.842 ( 0.011 8.839 ( 0.006

2.316 ( 0.003 2.322 ( 0.004 2.324 ( 0.001 2.327 ( 0.001 2.331 ( 0.004 2.334 ( 0.002

110.68 ( 0.20 110.65 ( 0.21 111.40 ( 0.03 110.46 ( 0.06 110.18 ( 0.24 109.80 ( 0.12

a YBO :Eu containing 10 at. % Eu are obtained at 650 °C for 100 min (a), 700 °C (b), 900 °C (c), and 1100 °C for 5 h (d), respectively. 3 YBO3:Eu containing 20 at. % Eu is obtained at 700 °C for 5 h (b′); the bulk sample is obtained by direct solid-state reaction at 1100 °C for 5 h. The denotation is the same in Table 2.

Figure 2. Emission spectra of YBO3:Eu samples under 240nm UV irradiation.

roughly matched with YBO3 standard values given in JCPDS (No. 16-277). An increscent trend of cell volume values and a decreasing trend of c/a values could be clearly observed as the particle size decreased. We also analyzed the 40-nm sample with 20 at. % Eu, which is denoted as b′, whose cell parameters were also calculated from XRD patterns and listed in Table 1. As expected, due to the larger ionic radius of Eu3+ in comparison to that of Y3+, doping Eu3+ into YBO3 lattice also led to the increase of cell parameters. However, unlike the decreasing trend of c/a values observed as the particle size decreased, the c/a value of YBO3:Eu0.2 was almost equal to that of YBO3:Eu0.1. The above phenomenon can be well-interpreted if we take the c/a value as a measure of the lattice distortion in the YBO3: Eu, considering that the lattices are more distorted in the nanocrystals than in the bulk, and the degree of distortion increased as the particle size decreased. Size-Dependent Luminescent Properties. Figure 2 displays the emission spectra of YBO3:Eu0.1 under 240nm UV irradiation. All of them were normalized to their maximum. The spectra consist of sharp lines ranging from 580 to 720 nm, which are associated with the transitions from the excited 5D0 level to 7FJ (J ) 1, 2, 3, 4) levels of Eu3+ activators,6 whose major emissions are centered at 591 nm (5D0-7F1) and 610 and 625 nm (5D07F ), corresponding well to orange-red and red color, 2 respectively. Although the major peak positions in the emission spectra are identical to each other, the intensity patterns are much different. For the bulk and submicrometer-sized samples, the 5D0-7F1 transition is the

most intense one.16 But for the nanosized particles, the relative intensity of 5D0-7F2 increased with decreasing particle size. It is obvious that with the particle size decreasing, the red emission coming from 5D0-7F2 increases, and as a result, redder fluorescence in a chromatic sense, that is, the superior chromaticity, can be obtained. The emission spectra of the 40-nm YBO3:Eu0.2 sample under 240-nm UV irradiation are also shown in Figure 2. With doping of more luminescent centers, Eu3+ ions, into a YBO3 lattice, the luminescent intensity of 40-nm YBO3:Eu0.2 sample was much higher than that of the 40-nm YBO3:Eu0.1 sample. However, the intensity patterns of these two samples were similar to each other. In other words, the chromaticity was only dependent on the particle size and independent of the doping concentration of Eu3+ ions. XANES. Figure 3L shows the Eu LIII edge XANES spectra for YBO3:Eux. The principal X-ray absorption peak appearing at the higher energy side of the Eu LIII absorption edge can be assigned as the so-called “white line”. It is well-known that the white lines of rare earths (RE) arise from the electron transition from RE 2p2/3 to the RE 4f5d final state, and their positions are sensitive to the chemical environment and the valence state of the absorption atom.9 Therefore, it can be concluded that the Eu ions in the nanocrystals possess a similar chemical environment to those in the bulk, and the valence state of Eu ions in the nanocrystals is also +3. The oscillation peaks at the higher energy side of the white line in the XANES spectra are the so-called “shape resonances”, which are associated with the multiple scattering of the photoelectron. The intensity and frequency of the shape resonance are always used to determine the levels of disorder of the samples, with the lower intensity and frequency correlated with high levels of disorder. For the bulk, the shape resonance curve exhibits a smooth wide peak (peak A) plus a small shoulder peak (peak B). With decreasing particle sizes, the shoulder peak, peak B, gradually became weaker and finally disappeared while the particle sizes were equal to 20 nm. At the same time, peak A also showed a slight reduction in intensity. This indicates that the levels of disorder in the nanocrystals are relatively high, and the smaller the particle sizes, the higher the levels of disorder. Moreover, it should be mentioned that the levels of disorder in the nanocrystals are independent of the doping concentration of Eu3+ ion, as was shown in the XANES spectra of the 40-nm samples. The second (16) Kim, D. S.; Lee, R. Y. J. Mater. Sci. 2000, 35, 4777.

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Figure 3. Eu LIII edge XANES spectra (L) and the second derivative of XANES (R) for YBO3:Eu samples.

Figure 4. Eu LIII edge EXAFS spectra (κ2-weighted) for YBO3:Eu samples.

derivative of XANES spectra was plotted and shown in Figure 3R, which further identified the size dependence of XANES. EXAFS. Figure 4 illustrates the κ2-weighted EXAFS signal κ2χ(κ) for YBO3:Eux samples. For the 20-nm sample, sample a, only a weak noise line could be observed beyond 70 nm-1, but when the particle size was equal to 40 nm, the oscillation peaks appeared, and the magnitudes of oscillations increased with increasing particle sizes. The radial distribution functions (RDF) were obtained from Fourier transform of κ2χ(κ) and are shown in Figure 5. The first peaks corresponding to the Eu-O coordination shell appeared at 0.19 nm, while the second, third, and fourth peaks appeared at 0.33, 0.36, and 0.41 nm, respectively. With decreasing particle size, the intensities of the first peaks exhibited only a slightly decreasing trend, while the second, third, and fourth peaks were all strongly attenuated, and as a result, the intensity ratios of the second, third, and fourth peaks to the first one rapidly decreased. These phenomena also resulted from the relatively high levels of disorder in the nanocrystals, coinciding with the XANES results.

The experimental and the fitting curves are shown in Figure 6, and the fitting results are listed in Table 2. Although many efforts have been devoted to the structure of hexagonal vaterite-type rare earth orthoborate (REBO3), there are still considerable controversies, especially for the eight oxygen atoms located at the first shell surrounding the rare earth central atom. Only a single RE-O distance was proposed in an earlier work.17 However, recent works on REBO3 all suggested a trigonal bicapped antiprism structure in which 6 oxygen atoms are located at the corners of a bicapped trigonal prism and 2 oxygen atoms located above the triangular faces.18,4 Although both were trigonal bicapped antiprism structures, the structure proposed by Chadeyron et al.18 gave two distinct RE-O distances, one for the 6 corner oxygen atoms and the other for the 2 cap oxygen atoms, whereas the structure proposed by Lin et al. gave 8 distinct RE-O distances. Only one Eu-O distance (17) Newnham, R. E.; Redman, M. J.; Santoro, R. P. J. Am. Ceram. Soc. 1963, 46, 253. (18) Chadeyron, G.; ElGhozzi, M.; Mahiou, R.; Arbus, A.; Cousseins J. C. J. Solid State Chem. 1997, 128, 261.

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Figure 5. Eu LIII edge radical distribution functions for YBO3:Eu samples.

Figure 6. EXAFS fitting results for YBO3:Eu samples. Solid lines present the experiment data. Dotted lines present the fitting data. Table 2. EXAFS Results for YBO3:Eu Samples 102σ2

sample

particle size (nm)

RD (nm)

CN

(nm2)

a b b′ c d bulk

19.5 40.4 40.0 90.2 sub-micrometer-sized micrometer-sized

0.245 0.244 0.245 0.241 0.239 0.239

8.6 8.3 8.3 8.0 7.9 7.8

0.0319 0.0245 0.0248 0.0230 0.0193 0.0193

was refined for all of the YBO3:Eu samples in our case. Our work differed from the EXAFS fitting results of vaterite LuBO3 by Boyer et al.,19 which gave two distinct RE-O distances at the first shell. This difference might result from the low resolution of our XAFS measurements. Through our measurements, only one average (19) Boyer, D.; Leroux, F.; Bertrand, G.; Mahiou R. J. Non-Cryst. Solids 2002, 306, 110.

RE-O distance could be obtained. Since, on average, the above works are consistent with each other, we chose Lin’s structural model as the referenced structural model to calculate the scattering amplitudes and phaseshift functions. According to Lin’s structural model, vaterite-type GdBO3 in fact possesses a rhombohedral R32 structure, while the hexagonal cell of YBO3 is only a subcell of the rhombohedral structure, and the relationship between the rhombohedral and the hexagonal cell can be expressed as ar ) x3ah and cr ) 3ch.4 For the first shell around Eu3+ ions, there are 6 oxygen atoms located at the corners of a bicapped trigonal prism and 2 oxygen atoms located above the triangular faces. The capping oxygen atoms are away from the pseudo-3-fold axis, and as a result, the Eu3+ ions are in a C1 symmetry coordination. The average distance of the Eu-O bond is 0.241 nm.

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The EXAFS fitting results of Eu3+ ions in all samples are roughly matched with this structural model. Nevertheless, with decreasing particle size, the fitting results showed that the bond length became longer, and the coordination number and the Debye-Waller factor both increased. Furthermore, for the 40-nm samples, doping high-concentration Eu3+ ions into a YBO3 lattice also led to bond distance lengthening. However, under this circumstance, the coordination number and the Debye-Waller factors were almost unchanged. These phenomena also indicated that the levels of disorder in the YBO3:Eu nanocrystals were dependent on the particle size, but independent of the doping concentration of Eu3+ ions. XAFS has long been recognized as a powerful probe for the local environment of a particular element, and many works have been done for the comprehension of the levels of disorder in nanocrystals. However, there seems to be something ambivalent in these works. Earlier authors considered that in nanocrystals there was a higher level of disorder than bulk, either from the highly disordered intergrain regions of materials as proposed by Yuren et al.20 and Deng et al.21 or from the amorphous materials contained in nanocrystalline powders as proposed by Rush et al.22 All their EXAFS results showed a dramatic reduction of the second peak in the RDF and a decrease of the coordination number with decreasing particle size. On the other hand, contradictory results have also been observed. The Sn K edge EXAFS of the pure SnO2 nanocrystals suggested the level of disorder in the crystallites was comparable to that of bulk,23 and the XAFS results about nanophased Cu metal also indicated the grain boundary structure, on the average, is similar to that of conventional polycrystalline Cu.24 Similarly, in Rush’s work, K edge EXAFS spectra of Zr and Y for the dense films of yttriastabilized cubic zirconia with grain sizes of 6, 15, and 240 nm, unlike the powders, showed no major differences with the corresponding spectra of the bulk. By analyzing all the results above, we considered the levels of disorder in the intergrain regions of materials are determined by the degree of crystallization, as has been proposed by Shi and co-workers,25,26 and it was the different degrees of crystallization in different samples that caused the above seeming contradictions. Different nanocrystals obtained from different fabrication methods might have different degrees of crystallization; that is, some fabrication methods might result in the highly crystallized nanocrystals while others might result in the poorly crystallized ones. The intergrain regions of the highly crystallized nanocrystals might exhibit similar levels of disorder to the bulk counterpart. In contrast, the intergrain regions of the poorly crystallized nanocrystals might have higher levels of disorder in (20) Yuren, W.; Kunquan, L.; Dazhi, W.; Zhonghua, W.; Zhengzhi, F. J. Phys.: Condens. Matter 1994, 6, 633. (21) Deng, H.; Qiu, H.; Shi, G. Physica B 1995, 208-209, 591. (22) Rush, G. E.; Chadwick, A. V.; Kosacki, I.; Anderson, H. U. J. Phys. Chem. B 2000, 104, 9597. (23) Davis, S. R.; Chadwick, A. V.; Wright J. D. J. Phys. Chem. B 1997, 101, 9901. (24) Stern, E. A.; Siegel, R. W.; Newville, M.; Sanders, P. G.; Haskel, D. Phys. Rev. Lett. 1995, 75, 3874. (25) Qi, Z. M.; Shi, C. S.; Wei, Y. G.; Wang, Z.; Liu, T.; Hu, T. D.; Zhao, Z. Y.; Li, F. L. J. Phys.: Condens. Matter 2001, 13, 11503. (26) Qi, Z. M.; Shi, C. S.; Wang, Z.; Wei, Y. G.; Xie, Y. N.; Hu, T. D.; Li, F. L. Acta Phys. Sin. 2001, 50, 1318.

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comparison with those of the bulk. The nanocrystalline yttrium-stabilized cubic zirconia films synthesized by Rush et al.22 and the above nanosized SnO223 and Cu24 might be highly crystallized, and as a result, the EXAFS spectra of these samples showed no major differences with the corresponding spectra of the bulk. The ZrO2: Y2O3 nanocrystals synthesized by Yuren et al.,20 Deng et al.,21 and Shi and co-workers25,26 might be poorly crystallized, with lower degrees of crystallization for smaller particles, which finally resulted in the higher levels of disorder. In these cases,20,21,25,26 both the degrees of crystallization and the particle sizes were determined by the calcination temperature, and thus a size dependence of the disorder levels could be observed. In our case, the size-dependent chromaticity could be attributed to the latter consideration, that is, there is no direct correlation between size and chromaticity, and this phenomenon is actually caused by the poor crystallization and the resulting high levels of disorder as the particle size decreased. The correlation between the levels of disorder and the chromaticity could be explained by the transition rules. The intensity of the transitions between different J levels depends on the symmetry of the local environment of the Eu3+ activators and can be described in terms of Judd-Ofelt theory.6 According to these transition rules, magnetic dipole transition is permitted and electric dipole transition is forbidden, but for some cases, in which the local symmetry of the activators does not involve an inversion center, the parity forbiddance is partially permitted, such as Eu3+ ions occupying C2 sites in Y2O3:Eu.6 It is well-known that the relative intensity of 5D0-7F1 and 5D -7F transitions depends strongly on the local sym0 2 metry of Eu3+ ions. Subsequently, when Eu3+ ions occupy the sites with inversion centers, the 5D0-7F1 transition will be relatively strong, while the 5D0-7F2 transition is parity forbidden and will be very weak. The abnormal luminescent behavior of the nanosized YBO3: Eu must be correlated to the microstructure. If the YBO3:Eu nanocrystals become more disordered, it means that the lattice around the luminescent centers is distorted, and this trend induces the luminescent centers to deviate from the inversion centers. In other words, the local symmetry of Eu3+ ions will be relatively low, and consequently, the transition probability of 5D07F is increased and the chromaticity will be improved. 2 In contrast, if the YBO3:Eu nanocrystals become ordered, the chromaticity will be relatively inferior. This correlation between chromaticity and crystallinity was further confirmed by the hydrothermal synthesis of well-crystallized YBO3:Eu nanocrystals, whose ratio of 5D -7F to 5D -7F transitions was somewhat lower 0 2 0 1 than the samples prepared by the sol-gel pyrolysis with similar particle sizes, while the luminescence was greatly improved, indicating the variance of chromaticity and luminescence is not a direct result of particle size, but has something to do with the degree of crystallization. The above discussions tell us, in most cases, the performance of the nanocrystals is in fact determined by their degree of crystallization. Since most attention was paid to the quantum efficiency, it was thought before that the high levels of disorder in nanosized

XAFS Study of Eu3+ Ions in YBO3:Eu Nanocrystals

phosphors was harmful to their performance, and many works have been done to improve crystallization. To our knowledge, no one had ever tried to utilize the high levels of disorder. In this paper, the high levels of disorder in nanocrystalline YBO3:Eu, which were considered to be associated with the degrees of crystallization finally, were used to lower the symmetry of Eu3+ ions so as to improve the chromaticity of YBO3:Eu. Conclusion In this work, a more facile sol-gel pyrolysis process was adopted to prepare pure hexagonal phased YBO3: Eu nanocrystals with different particle sizes, and a relatively better chromaticity was obtained in smaller sized samples. On the basis of recent studies and our own analysis of the X-ray absorption fine structure spectroscopy of differently sized YBO3:Eu nanocrystals,

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we logically attributed their varied luminescent performances to the different levels of disorder, which are finally determined by the degree of crystallization. This is helpful for the resolution of some seeming contradictions about the relations between particle size and spectral properties, and what is more important, we made an attempt to utilize the disorder of the poorly crystallized nanocrystals, which once was thought to undermine a phosphor’s potential application, to realize a better chromaticity, as an alternative way for improvement in the performance of nanosized materials. Acknowledgment. This work is supported by the NSFC (20001002, 20013005, and 20221101), MOST (G19980613), MOE (the Foundation for University Key Teacher), and the Founder Foundation of PKU. CM0341888