Synthesis of red-luminescent Eu3+-doped lanthanides compounds hollow spheres

Article abstract of DOI:10.1016/j.materresbull.2005.03.007

Eu³⁺-doped Y₂O₃, YOF, La₂O ₃, LaOF hollow spheres have been synthesized by a facile template route. Eu³⁺ were doped into the various host materials to make the hollow sphere red-luminescent. Difference in fluorescence spectra recorded on the hollow spheres were compared in detail and attributed to the different crystal symmetry of host materials. These phosphors might find applications in the fields such as light phosphor powders, advanced flat panel display, or biological labeling.

Full text of DOI:10.1016/j.materresbull.2005.03.007

Materials Research Bulletin 40 (2005) 911–919
www.elsevier.com/locate/matresbu
Synthesis of red-luminescent Eu3+-doped lanthanides
compounds hollow spheres
Haiying Wang, Ruji Wang, Xiaoming Sun,
*
Ruoxue Yan, Yadong Li
Department of Chemistry and the Key Laboratory of Atomic & Molecular Nanosciences
(Ministry of Education, China), Tsinghua University, Beijing 100084, PR China
Received 14 December 2004; received in revised form 14 March 2005; accepted 16 March 2005
Abstract
Eu3+-doped Y2O3, YOF, La2O3, LaOF hollow spheres have been synthesized by a facile template route. Eu3+
were doped into the various host materials to make the hollow sphere red-luminescent. Difference in fluorescence
spectra recorded on the hollow spheres were compared in detail and attributed to the different crystal symmetry of
host materials. These phosphors might find applications in the fields such as light phosphor powders, advanced flat
panel display, or biological labeling.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; A. Nanostructures; D. Luminescence
1. Introduction
In recent years, Eu3+-doped nanocrystals have aroused vast interest due to their excellent red-
luminescent property and particle-size-dependent fluorescence that express the optical properties of
the doped nanocrystals and the relaxation mechanisms of the dopant [1–7]. In particular, nanocrys-
tallites of lanthanide compounds have been the hosts, which have been widely investigated due to its
promising applications in the field of the phosphors for lighting and for cathode ray tubes [8–12].
Usually, the rare-earth oxide and rare-earth oxyhalide phosphors are available in the form of powders
* Corresponding author. Tel.: +86 10 6277 2350; fax: +86 10 6278 8765.
E-mail address: ydli@tsinghua.edu.cn (Y. Li).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.03.007

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[9–11] or film [13] through the sol–gel method. However, the hollow spherical structures of the
lanthanide oxides and lanthanide oxyhalides have not been prepared up to now. As we all know the
large surface areas and hollow characteristics of the hollow colloidal particles make them attractive for
scientific study, for instance, these nanostructures might be used for spatial-confined chemical or
biochemical reaction [14,15]. At the same time, the hollow phosphors might radiate essentially the
same amount of light with raw materials saved due to their hollow interior. The spherical profile
would be helpful in forming a phosphor thin film when utilized in flat panel display. To produce
the monodispersed hollow spheres, a number of fabrication methods had been employed. The most
classic one is the stepwise deposition of the precursor on a template followed by removing the
template [16].
Very recently, our group has developed a new method to prepare the monodispersed carbonaceous
spheres with –OH groups on the surface [17], and consequently a surface-layer-absorption (SLA)
templating technique to prepare monodispersed Ga2O3 and GaN hollow spheres [18]. Spurred by this
evidence, we further extended this templating technique to synthesis of Y2O3, La2O3, YOF and LaOF
hollow spheres doped with Eu3+. The optical characterization on them indicated high emission, implying
their applications in the fields such as light phosphor powders, advanced flat panel display, or biological
labeling.
2. Experimental
The chemical reagents (A.R. grade) described throughout this section are yttrium oxide, lanthanum
oxide, europium oxide, ammonium fluoride, bought from Beijing Chemical Reagent Factory, and used as
received without further purification.
The yttrium nitrate, lanthanum nitrate and europium nitrate (5 mmol/l) aqueous solution were
prepared by dissolving Y2O3, La2O3 and Eu2O3 with dilute nitric acid respectively until the solution
was at pH 2.
The templating carbonaceous spheres were prepared using the method described in the reference [17].
The as prepared spheres were redispersed in 200 ml 5 mmol Y0.98Eu0.02(NO3)3, La0.98Eu0.02(NO3)3
solution, respectively and ultrasonicated for 60 min. Then the suspension was aged for 12 h before five
cycles of centrifugation/wash/redispersion in water. The oven-dried sample was heated to 700 8C
respectively in a tube furnace in air and maintained at the temperature for 3 h.
As to the preparation of YOF:Eu0.02 and LaOF:Eu0.02 hollow spheres, the as prepared spheres were
redispersed in 200 ml 5 mmol Y0.98Eu0.02(NO3)3, La0.98Eu0.02(NO3)3 solution and ultrasonicated for
60 min. And then 6 ml NH4F solution (0.5 mol/l) was dropped in under the strong agitation. Then the
suspension was treated using the method mentioned above.
The phase purity of the products was examined by X-ray diffraction (XRD) by using a Brucker D8-
˚
advance X-ray diffractometer with Cu Ka radiation (l = 1.5418 A), the operation voltage and current
keeping at 40 kVand 40 mA, respectively. The 2u range used was from 10 to 708 in steps of 0.028 with a
count time of 2 s.
The size and morphology of the products were observed by using a Hitachi Model H-800 transmission
electron microscope, with a tungsten filament at an accelerating voltage of 200 kV.
Fluorescent spectra were recorded with a Hitachi F-4500 Fluorescence Spectrophotometer at room
temperature.

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3. Results and discussion
3.1. Synthesis of La2O3:Eu, Y2O3:Eu, LaOF:Eu, YOF:Eu hollow spheres
The XRD patterns of the final phases of Y2O3:Eu, La2O3:Eu, YOF:Eu, LaOF:Eu phosphors calcined at
the temperature of 700 8C are shown in Fig. 1, respectively. As can be seen from this figure the peaks of
the Y2O3:Eu, YOF:Eu, La2O3:Eu and LaOF:Eu phosphors correspond to the Y2O3, YOF, La2O3 and
LaOF phase, respectively (JCPDS 79–1257, 71–2100, 73–2141, 75–1174).
3.2. Structural investigations
TEM micrographs of the Y2O3:Eu(0.02), YOF:Eu(0.02), La2O3:Eu(0.02), LaOF:Eu(0.02) hollow spheres
are presented in Fig. 2, respectively. As we can see from the picture, the size of the hollow sphere is
between 400 and 500 nm, which is smaller than the size of carbon sphere template. All this because the
surface of the carbon spheres is hydrophilic with –OH and –C O groups distributing on. When the
carbon spheres were dispersed in metal salts solutions, the cations would be adsorbed on the surface. And
in the following calcination process, the transition metal in the shell densified and transformed into oxide
hollow spheres, a replica of carbon spheres with reduced size [17,18]. The large shrinkage ratio is
essentially different with traditional sacrificial core synthesis methods. In those methods, the hollow
spheres prepared were nearly the same as the template. It is believed that the large shrinkage was caused
Fig. 1. X-ray diffraction pattern of (a) Y2O3:Eu (0.02); (b) YOF:Eu (0.02); (c) La2O3:Eu (0.02); (d) LaOF:Eu (0.02) phosphors
at 700 8C for 3 h.

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Fig. 2. TEM micrograph of hollow spheres: (a) Y2O3:Eu (0.02); (b) YOF:Eu (0.02); (c) La2O3:Eu (0.02); (d) LaOF:Eu (0.02)
phosphors at the temperature of 700 8C for 3 h.
by the further dehydration of loosely cross-linked structure of starting carbon spheres and it would lead to
densification of mental ions in surface layer. Consequent cross-linking and crystallization of lanthanide
oxide would take place when carbon was removed in air in the calcinations process and thus resulted in
oxide hollow spheres.

H. Wang et al. / Materials Research Bulletin 40 (2005) 911–919
915
3.3. Optical properties
Fig. 3 shows the room-temperature emission spectrum of Y2O3:Eu samples. This figure shows the
5
7
typical emission spectrum of Eu3+ observed due to the D0 ! FJ (J = 1, 2) transitions (from 575 to
5
7
630 nm). At the 0.02 mol Eu3+ concentration, the D0 ! F2 transition (616, 619 nm) prevails
5
7
the D0 ! F1 transition (594 nm), implying the satisfactory color purity. It is known that the
5
7
5
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relative intensities of D0 ! F1 and D0 ! F2 emission, which are typical magnetic and electronic
dipole–dipole transitions, respectively, depend strongly on the local symmetry of the Eu3+ ions. In a
5
7
site with inversion symmetry the D0 ! F1 magnetic dipole transition is dominating, while in a
5
7
site without inversion symmetry the D0 ! F2 electric dipole transition is the strongest. So such
spectrum indicated that Eu3+ mainly occupied the positions without the inversion symmetry in the host
lattice.
Fig. 3. Emission spectrum of Y2O3:Eu (0.02) hollow spheres annealed at the temperature of 700 8C for 3 h.

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From the recorded fluorescence spectra of Eu3+:YOX (X = F, Br, Cl) phosphors, the following five
5
7
7
7
7
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emission transitions have been previously identified [19–21]: D0 ! F4, F3, F2, F1, F0.
5
7
Of these, the D0 ! F2 transition has been found to be the strongest in intensity compared to the
other transitions with little shifts in emission level peak position, depending on the host matrix
5
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composition. The forbidden transition D0 ! F0 has been split into two components uniformly and
7
magnetic dipole transition 5D0 ! F1 has also been split into two components. From Fig. 4, we can see
that the room-temperature emission spectra of YOF:Eu (0.02) hollow spheres annealed at the
temperature of 700 8C is completely in accordance with the records in the previous references.
7
What is more, the sensitive forbidden transition (5D0 ! F0) is also observed in the emission spectrum
of YOF:Eu (0.02) hollow spheres. This emission peak is typical in lanthanide oxyfluoride structure
[22,23] with the tetragonal structure. All this have presented the doping of the Eu3+ and the structure
of the YOF.
The emission spectrum of the La2O3:Eu (0.02) hollow spheres is presented in Fig. 5. In this figure,
7
typical emissions of Eu3+, which can be ascribed to the 5D0 ! FJ (J = 0, 1, 2) transitions (from 570 to
Fig. 4. Emission spectrum of YOF:Eu (0.02) hollow spheres annealed at the temperature of 700 8C for 3 h.

H. Wang et al. / Materials Research Bulletin 40 (2005) 911–919
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Fig. 5. Emission spectrum of La2O3:Eu (0.02) hollow spheres annealed at the temperature of 700 8C for 3 h.
7
650 nm) was observed. And the 5D0 ! F2 transition is the strongest among all the transition peaks. It
can be seen that the spectral positions of the europium transitions were similar for the nanosized hollow
sphere and bulk La2O3:Eu [24].
Fig. 6 shows the spectrum of the LaOF:Eu (0.02) hollow spheres that was annealed at the temperature
of 700 8C for 3 h. The excitation wavelength used was 268 nm, which was efficient to promote the O2-
Eu3+ charge-transfer excitation mechanism. The characteristic luminescence of Eu3+ based on intra-
configurational f–f transitions is observed, indicating the successful doping of Eu3+ ion in the
nanostructured LaOF phosphors. Each emission can be assigned according to theoretical and experi-
mental studies on a crystal field of LaOF having a rhombohedral fluorite-type structure [25–27]. Due to
7
the hypersensitive electric dipole transition, 5D0 ! F2, the emission color was red, which was visible to
the naked eye. The occurrence of the Eu3+ emissions lies in that Eu3+ was successfully doped in the
nanostructured LaOF phosphors. Since the hollow characteristics made the apparent density smaller, and

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Fig. 6. Emission spectrum of LaOF:Eu (0.02) hollow spheres annealed at the temperature of 700 8C for 3 h.
would be much lighter than the normal phosphor, we estimate that the hollow phosphors maybe have
some potential applications in the light phosphor and advance flat.
4. Conclusion
Europium-doped Y2O3, YOF, La2O3, LaOF hollow spheres have been synthesized by a sonochemical
assisted template route, which is completely different from the traditional template method. The hollow
spheres can be produced in a simple procedure, and the size could be adjusted by selecting different sized
templating carbon spheres. The optical properties of the Y2O3, YOF, La2O3, LaOF hollow spheres doped
with europium ion were studied in detail. In particular, the feasibility of inducing red emissions
7
(5D0 ! F2) in the phosphors was demonstrated. And it suggested that these novel nanophosphors were

H. Wang et al. / Materials Research Bulletin 40 (2005) 911–919
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promising candidates for applications such as active components for high-resolution imaging and
displays, and especially fluorescent labels for biomolecules.
Acknowledgements
This work was supported by NSFC (50372030, 20025102, 20131030), the Foundation for the Author
of National Excellent Doctoral Dissertation of PR China and the State Key Project of Fundamental
Research for Nanomaterials and Nanostructures (2003CB716901).
References
[1] Z.Q. Ye, M.Q. Tan, G.L. Wang, et al. J. Mater. Chem. 14 (2004) 851.
[2] J. Dhanaraj, M. Geethalakshmi, R. Jagannathan, Chem. Phys. Lett. 387 (2004) 23.
[3] Y.X. Pan, M.M. Wu, Q. Su, Mater. Res. Bull. 38 (2003) 1537.
[4] P.K. Sharma, M.H. Jilavi, V.K. Varadan, J. Phys. Chem. Sol. 63 (2002) 171.
[5] Y. Suna, L. Qia, M. Leea, B.I. Leea, J. Lumin. 109 (2004) 85.
[6] M.R. Davolos, S. Feliciano, A.M. Pires, J. Sol. Chem. 171 (2003) 268.
[7] D.F. Wallace, M.D. Celso, L.L. Ricardo, J. Lumin. 105 (2003) 97.
[8] S.Y. Zhang, K. Wei, Spectra. Spect. Anal. 24 (2004) 407.
[9] R. Bazzi, M.A. Flores, C. Louis, J. Colloid Interface Sci. 273 (2004) 191.
[10] P.A. Tanner, K.L. Wong, J. Phys. Chem. B 108 (2004) 136.
[11] U. Rambabu, P.K. Khanna, I.C. Rao, Mater. Lett. 34 (1998) 269.
[12] U. Rambabu, A. Mathur, S. Buddhudu, Mater. Chem. Phys. 61 (1999) 156.
[13] S. Fujihara, T. Kato, T. Kimura, J. Mater. Sci. Lett. 20 (2001) 687.
[14] F. Causo, Chem. Eur. J. 6 (2000) 413.
[15] Y.N. Xia, B. Gates, Y.D. Yin, Adv. Mater. 12 (2000) 693.
[16] Y.D. Yin, Y. Lu, B. Gates, et al. Chem. Mater. 13 (2001) 1146.
[17] X.M. Sun, Y.D. Li, Angew. Chem. Int. Ed. 43 (2004) 597.
[18] X.M. Sun, Y.D. Li, Angew. Chem. Int. Ed. 43 (2004) 3827.
[19] U. Rambabu, T. Balaji, S. Buddhudu, Mater. Res. Bull. 30 (1995) 891.
[20] U. Rambabu, A. Mathur, S. Buddhudu, Mater. Chem. Phys. 61 (1999) 156.
[21] U. Rambabu, P.K. Khanna, I.C. Rao, Mater. Lett. 34 (1998) 269.
[22] J. Holsa, M. Karppinen, Eur. J. Solid State Inorg. Chem. 28 (1999) 151.
[23] J. Holsa, Acta Chem. Scand. 45 (1991) 583.
[24] J.K. Park, S.M. Park, C.H. Kim, H.D. Park, J. Mater. Sci. Lett. 20 (2001) 2231.
[25] Q.P. Wang, Y.P. Gao, J. Phys. Chem. Solids 56 (1995) 285.
[26] S. Fujihara, T. Kato, T. Kimura, J. Mater. Sci. 20 (2001) 687.
[27] E. Hosono, S. Fujihara, T. Kimura, Langmuir 20 (2004) 3769.