Phase transition, structure and luminescence of Eu:YAG nanophosphors by co-precipitation method

Article abstract of DOI:10.1016/j.jallcom.2008.02.045

The methods to synthesize Eu:YAG phosphor by co-precipitation method and solid-state synthesis were compared. The dynamic aspects of the phase transition of Eu:YAG precursor synthesized by co-precipitation method were studied by using X-ray diffraction, i

Full text of DOI:10.1016/j.jallcom.2008.02.045

Journal of Alloys and Compounds 470 (2009) 306–310
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Phase transition, structure and luminescence of Eu:YAG
nanophosphors by co-precipitation method
J. Sua,∗, Q.L. Zhangb, S.F. Shaob, W.P. Liub, S.M. Wanb, S.T. Yinb
a College of Math and Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China
b Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The methods to synthesize Eu:YAG phosphor by co-precipitation method and solid-state synthesis
were compared. The dynamic aspects of the phase transition of Eu:YAG precursor synthesized by co-
precipitation method were studied by using X-ray diffraction, infrared and Raman spectroscopy. The
results showed that the precursor transformed to pure-YAG phase at the sintering temperature of
900 ◦C without intermediate phases present. Transmission electron microscope for the precursor pow-
ders sintered at 900–1200 ◦C showed that the powders were well-dispersed and had average size about
50–100 nm.The fluorescence spectra showed that the Eu:YAG phosphors had strong photoluminescence.
The luminescence properties of the sintered powders depend on the sintering temperature were also
analyzed.
Received 9 December 2007
Accepted 16 February 2008
Available online 2 April 2008
Keywords:
Phosphors
Chemical synthesis
Phase transitions
Optical spectroscopy
Luminescence
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
2.1. Co-precipitation method
Yttrium aluminum garnet Y3A15O12 (YAG) has been widely used
as a host for solid-state laser materials and phosphor. Eu3+-doped
YAG materials are promising phosphor candidates in cathode-ray
tubes (CRTs), field emission display (FED), scintillation, vacuum flu-
orescent displays (VFDs) and electroluminescent (EL) [1–4]. For
phosphor materials, the purity of the host and the homogeneity
of doped ions, which associated with the preparation processes
[5–9], would strongly affect their optical characteristics. There-
fore, it is necessary to study its preparation processes in more
detail.
Y(NO3)3 and Eu(NO3)3 solutions were prepared by dissolving spectroscopic pure
Y2O3, Eu2O3 in diluted nitric acid. Al(NO3)3 solution was obtained by dissolving
(>99%) Al(NO3)3·9H2O in de-ionized water. Aqueous solutions of Y(NO3)3, Al(NO3)3
and Eu(NO3)3 were mixed according to the chemical formula Y2.97Eu0.03Al5O12 , then
the mixed nitrate solution and the aqueous ammonia were dropped simultaneously
to ammonia solution with initial pH 9.5 under vigorous stirring. The dropping rate
was adjusted to keep pH value in the range of 8–9. The precipitate slurry was stirred
sufficiently after the reaction. Then the precipitate was separated and washed with
de-ionized water for several times to remove NH4+, NO3 and OH− etc. After that,
−
the precipitation was dried at 110 ◦C and was then ground and sintered at various
temperatures for 3 h in air.
In this paper, Eu:YAG nanophosphors were synthesized by
co-precipitation method and solid-state reaction process. X-ray
diffraction (XRD), fourier transform infrared spectroscopy (FT-IR)
and Raman spectroscopy were used to analyze the conversion pro-
cedure from the precursor to poly-crystalline Eu:YAG. Additionally,
the structure of Eu:YAG poly-crystalline powders and YAG single
crystal were compared by their molecular vibration spectra. Trans-
mission electron microscopy (TEM) and fluorescence spectroscopy
were used to investigate the morphology and luminescence of
Eu:YAG nanophosphors.
2.2. Solid-state reaction process
Spectroscopic pure Al2O3, Y2O3 and Eu2O3 powders were mixed according to
the chemical formula Y2.97Eu0.03Al5O12 and milled sufficiently, and then the mixture
was sintered at 1500 ◦C for 48 h in air.
2.3. Characterizations
Infrared (IR) spectra were recorded on a fourier transform infrared spectrometer
(Nicolet MAGNA-IR 750, USA). Raman spectra were obtained on a Raman spectrom-
eter (RAMALOG 6, USA) with the 514.5 nm line of Ar+ laser as the excitation resource.
The laser beam was vertical to the (1 1 1) plane of YAG single crystal. XRD patterns
were characterized by a Philips X’pert PRO X-ray diffractometer with Cu K␣ radia-
tion. The morphology and microstructure of the sintered powders were observed by
transmitted electron microscopy (H-800, Japan). The luminescences were measured
by a Jobin-Yvon spectrophotometer (FLUOROLOG 3 TAU, France). All the experiments
were performed at room temperature.
∗
Corresponding author. Tel.: +86 25 58731031; fax: +86 25 58731174.
E-mail address: zlj007@126.com (J. Su).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.02.045

J. Su et al. / Journal of Alloys and Compounds 470 (2009) 306–310
307
3. Results and discussion
3.1. Phase transition of Eu:YAG precursor
Fig. 1 shows the XRD spectra of the samples sintered at different
temperatures. For the sample prepared by co-precipitation method,
no obvious diffraction peak appeared at 800 ◦C, indicating that
the powder is amorphous. At 900 ◦C, strong characteristic diffrac-
tion peaks of YAG (JCPD Card No. 33-40) were observed because
the precursor converted to YAG poly-crystalline. No other inter-
mediate phases were detected, which also indicates that no other
intermediate phases, such as YAlO3 (YAP, JCPD Card No. 33-41),
Y4Al2O9 (YAM, JCPD Card No. 34-368) occurred. Above 900 ◦C, con-
tinued refinement in peak shapes and intensities were observed
because of the higher crystallization of the poly-crystalline. How-
ever, for the sample prepared by solid-state synthesis, YAP, YAM
and Y2O3 phases still co-existed in YAG phase even if the reac-
tants had been sintered at 1500 ◦C for 48 h. It indicates that
co-precipitation method has advantage of lower synthesis tem-
perature and shorter sintering time compared with solid-state
synthesis.
Fig. 2. FT-IR spectra of the Eu:YAG precursor powders sintered at 900 ◦C and
1500 ◦C.
Fig. 2 shows the FT-IR spectra of the precursor powder sintered
at 900 ◦C and 1500 ◦C, respectively. The wide band at 3444 cm−1
and the weak band at 1639 cm−1 are assigned to O–H stretch and
bend vibrations of the absorbed water. The bands at 787 cm−1
,
and became much sharper because of the higher crystalliza-
tion.
723 cm−1, 690 cm−1, 567 cm−1, 511 cm−1 and 455 cm−1 are char-
acteristic M–O vibrations of garnet crystals [10]. With the increase
of the sintering temperature, the peaks had no apparent shift
Fig. 3a–d shows the Raman spectra of 1 at.%Eu:YAG precursor
and the samples sintered at different temperatures for 3 h. Distinct
changes in spectral shape present for the precursor and the pow-
der sintered at different temperatures. For the precursor (Fig. 3a),
the sharp strong band at 1048 cm−1 is associated with NO3−. The
Raman spectra of the sample sintered at 800 ◦C presented weak
broad Raman bands, corresponding to the amorphous sample. Char-
acteristic garnet phase peaks were observed for the sample sintered
at 1000 ◦C. By increasing the sintering temperature to 1500 ◦C, the
intensities of the peaks increased and no distinct changes in the
modes were observed, which suggested the powder had formed
stable YAG garnet structure. Raman spectra of the powders sintered
at 1000 ◦C (Fig. 3c) and 1500 ◦C (Fig. 3d) and YAG single crystal
(Fig. 3e) were compared, almost all the peaks revealed identical
band frequencies, which indicated that the Eu3+ substituted Y3+ and
the structure of Eu:YAG was equivalent to YAG single crystal. The
band at 478 cm−1 present in the Raman spectra of Eu:YAG nanocrys-
talline powder. However, it was absent in the spectra of YAG single
crystal. According to the theoretical rigid ion model calculation of
the rare earth aluminum garnets (RE3Al5O12) by Papagelis et al.
[11], the band at 478 cm−1 is associated with T2g symmetry, and
it was also absent in their experimental spectra. Additionally, the
relatively intensity of the band at 748 cm−1 associated with Eg
symmetry in Eu:YAG nanocrystalline powder was much stronger
than that in YAG crystal. For the RE3+-doped garnet material, the
chief effect factors for the displacement activity are radius, valence
and electronegativity. The radius of Eu3+ (0.113 nm) is larger than
that of Y3+ (0.106nm), which could cause the lattice distortion. As
a result, the intensity of T2g and Eg modes in Eu:YAG powders
increased.
3.2. Morphology of Eu:YAG nanophosphors
Fig. 4 shows the TEM micrographs of the precursor sintered
at 900 ◦C and 1200 ◦C for 3 h. Both of the samples exhibited good
dispersion. The particles sintered at 1200 ◦C were larger than that
at 900 ◦C, which was mainly due to a higher crystalline after sin-
tering at a higher temperature. It was estimated the diameters of
spherical particles were smaller than 50 nm for the powder sin-
Fig. 1. XRD patterns of the powder synthesized by solid-state reaction method (a)
and the powders sintered at different temperatures by co-precipitation method (b).

308
J. Su et al. / Journal of Alloys and Compounds 470 (2009) 306–310
Fig. 3. Raman spectra of Eu:YAG precursor (a), the powder sintered at 800 ◦C (b), 1000 ◦C (c), 1500 ◦C (d) and YAG single crystal (e).
tered at 900 ◦C and were about 100 nm for the powder sintered at
1200 ◦C.
The bands in longer wave number region are corresponding to
the f–f transitions within Eu3+ 4f6 configuration, the most promi-
nent group at 394 nm is associated with 7F0–5L6 transition. The
broad band in the lower wave number region is associated with
the charge transfer band (CTB) of Eu3+–O2−. For the powder sin-
tered at 800 ◦C, the CTB of Eu3+ has a half width of 45 nm and a
maximum at 270 nm. For the powders sintered at 1000 ◦C, the CTB
3.3. Luminescence properties
Fig. 5 shows the excitation spectra of the powders sintered
at different temperatures by monitoring the emission at 590 nm.
Fig. 4. TEM morphologies of YAG powders sintered at different temperatures (a) 900 ◦C and (b) 1200 ◦C.

J. Su et al. / Journal of Alloys and Compounds 470 (2009) 306–310
309
band at 590 nm is ascribed to the Eu3+ magnetic dipole tran-
sition 5D0 → F1. The 608 nm attributed to Eu3+ electric dipole
7
transition 5D0 → F2 is relatively weak. However, for the powder
7
7
sintered at 800 ◦C, the Eu3+ electric dipole transition 5D0 → F2 at
612 nm had stronger intensity compared with that of 5D0 → F1
7
at 589 nm. It is due to that the magnetic dipole transition
7
5D0 → F1 is not sensitive to the changes in the neighborhood
of the Eu3+ ion while the electric dipole transition 5D0 → F2
7
is very sensitive to any structural change [12–14]. Addition-
ally, for the powder sintered at 800 ◦C, 5D0 → F0 electric dipole
7
transition at 579 nm was observed, but it was absent for the pow-
ders sintered at 1000 ◦C and 1500 ◦C. XRD and Raman analysis
above suggest that the powder sintered at 800 ◦C is amorphous,
indicating that most of the Eu3+ irons are located at the sites with-
out inversion symmetry and the site symmetry of Eu3+ maybe
Cs, Cn and Cnv [5,15]. As a result, the parity selection rules
7
were broken and 5D0 → F0 electric dipole transition at 579 nm
present.
For the sample sintered at 1000 ◦C and 1500 ◦C, no changes
were observed in the excitation and emission spectra except
for the intensity. It indicated YAG garnet structure formed and
the doped Eu3+ substituted for 8-coordinated Y3+ with site
symmetry D2, and the exact local symmetry is only a small dis-
tortion of the centrosymmetric D2h point symmetry [2], according
with our Raman spectra analysis. As a result, their luminescent
Fig. 5. Excitation spectra of the powders sintered at 800 ◦C, 1000 ◦C and 1500 ◦
(ꢀem = 590 nm).
C
7
intensity was concentrated mainly in the 5D0 → F1 magnetic
of Eu3+ is at 243 nm and has a half width of 19 nm.The spectra of
the powders sintered at 1500 ◦C showed a higher intensity com-
paredwith thatat 1000 ◦C, whichshouldbeassociatedwith ahigher
crystallinity.
The emission spectrum excited upon 394 nm light is shown
in Fig. 6. For the powders sintered at 1000 ◦C and 1500 ◦C
(Fig. 6b), the spectra were composed of several sharp bands,
which belong to the intrinsic emission of Eu3+. The strongest
7
dipole transition rather than 5D0 → F2 forced electric dipole
transition.
For the powder sintered at 800 ◦C, the emission spectra excited
upon 270 nm were shown in Fig. 6c, which had the same profile
as that excited upon 394 (Fig. 6a). The results indicated that phos-
phor could be excited upon CTB of Eu3+–O2− and the f–f transi-
tion.
Fig. 6. Emission spectra of the powders sintered at 800 ◦C (a and c), 1000 ◦C and 1500 ◦C (b).

310
J. Su et al. / Journal of Alloys and Compounds 470 (2009) 306–310
4. Conclusions
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This work is financially supported by a grant from National Nat-
ural Science Foundation of China (No. 50472104).