Synthesis and characterization of nanocrystalline Nd3+-doped gadolinium scandium aluminum garnet powders by a gel-combustion method

Article abstract of DOI:10.1016/j.materresbull.2012.03.039

Nd³⁺-doped gadolinium scandium aluminum garnet (Nd:GSAG) precursor was synthesized by a gel combustion method using metal nitrates and citric acid as raw materials. The structure and morphology of the precursor and the sintered powders were studied by means of X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). The results showed that the precursor transformed into pure GSAG polycrystalline phase at about 800 °C, and the powders sintered at 800-1000 °C were well-dispersed with average particle sizes in the range of 30-80 nm. Optical properties of Nd:GSAG nano-powders were characterized by using photoluminescence spectroscopy. The highest photoluminescence intensity was achieved for the powder sintered at 900 °C.

Full text of DOI:10.1016/j.materresbull.2012.03.039

Materials Research Bulletin 47 (2012) 1709–1712
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of nanocrystalline Nd3+-doped gadolinium
scandium aluminum garnet powders by a gel-combustion method
*
Jing Su , Ju-hong Miao, Lin-hua Xu, Yu-qing Ji, Chu-qin Wang
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
A R T I C L E I N F O
A B S T R A C T
Nd3+-doped gadolinium scandium aluminum garnet (Nd:GSAG) precursor was synthesized by a gel
combustion method using metal nitrates and citric acid as raw materials. The structure and morphology
of the precursor and the sintered powders were studied by means of X-ray diffraction (XRD), infrared
spectroscopy (IR) and transmission electron microscopy (TEM). The results showed that the precursor
transformed into pure GSAG polycrystalline phase at about 800 8C, and the powders sintered at 800–
1000 8C were well-dispersed with average particle sizes in the range of 30–80 nm. Optical properties of
Nd:GSAG nano-powders were characterized by using photoluminescence spectroscopy. The highest
photoluminescence intensity was achieved for the powder sintered at 900 8C.
Article history:
Received 27 September 2011
Received in revised form 14 January 2012
Accepted 20 March 2012
Available online 30 March 2012
Keywords:
A. Nanostructures
B. Chemical synthesis
D. Optical properties
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
difficult to mix the oxides uniformly. Many wet chemical methods,
such as co-precipitation method [11–14], homogeneous precipi-
Garnet crystals doped trivalent rare-earth ions (Re3+) are well
known as active media for solid-state lasers. Since the first Nd3+
tation method [15], combustion method [16], sol–gel method [17]
and hydrothermal method [18], have been reported for prepara-
tion of YAG nano-powders. In our work, a gel-combustion method,
which could be considered as combination of sol–gel process and
combustion process was used to prepare Nd:GSAG nano-powders.
We successfully prepared Nd:GSAG nano-powders by the gel-
combustion method. Our interest is primarily concerned with the
phase transition, structure and morphology and optical properties
of Nd:GSAG powders from sintering the as-burnt precursors at
different temperatures.
-
doped yttrium aluminum garnet (Nd:YAG) ceramic laser was
developed by A. Ikesue et al. in 1995 [1], subsequent reports have
revealed that the optical properties of Nd:YAG ceramics are nearly
equivalent or superior to those of the high-quality Nd:YAG single
crystal fabricated by Czochralski (CZ) method [2–4]. Additionally,
transparent ceramics has several advantages over single crystals,
such as ease of fabrication, less expensive, fabrication of large size
and high dopant concentration, mass production [5]. Therefore,
transparent laser ceramic has attracted much attention and would
be a new-type laser active media.
2. Experimental
Diode-pumped Nd3+-doped gadolinium scandium aluminum
garnet (Nd:GSAG) lasers operating at 942 nm using Nd3+ 4F2/3-4I9/2
transition have been specified to be suitable for a space-born
differential absorption LIDAR (DIAL) for atmospheric water vapor
detection [6–10]. However, most studies have focused on the
growth and optical properties of Nd:GSAG single crystal, the
fabrication and characterization of Nd:GSAG transparent ceramics
are seldom reported. Our interest is the fabrication of highly
transparent Nd:GSAG ceramics, thus fine well-dispersed starting
powders with high-purity are essential. The methods often used to
fabricate well-sinterable powders include solid-reaction method
and wet chemical methods. Generally, solid-reaction method
between oxides needs higher sintering temperature and it is
Gd(NO3)3, Sc(NO3)3 and Nd(NO3)3 solutions were prepared
by dissolving Gd2O3, Sc2O3, Nd2O3 with purity of 99.995% in
dilute HNO3. Al(NO3)3 solution was obtained by dissolving
Al(NO3)3ꢀ9H2O (>99%) in deionized water. The metal nitrate
solutions were mixed with cationic molar ratio of Gd3+: Nd3+: Sc3+
:
Al3+ = 2.97: 0.03: 2: 3. Citric acid (C6H8O7ꢀH2O, >99%) was then
added into the mixed nitrate solution and the molar ratio of citric
acid to the metal ions is equal to 1.5. The complexes solution was
heated at 70–80 8C with constant stirring, and it was adjusted to
pH 7 by adding dilute NH4OH. Then the solution was heated at
300–500 8C for a few minutes. As water evaporated, the solution
formed a very viscous gel. With constant heating, an auto-
combustion process took place and the gel was burnt out
completely to form a loose precursor. Finally, the as-burnt
precursor was ground and then sintered at 800 8C, 900 8C and
1000 8C for 3 h, respectively, in air.
* Corresponding author. Tel.: +86 25 58731031; fax: +86 25 58731174.
E-mail addresses: zlj007@126.com, sj007@nuist.edu.cn (J. Su).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.03.039

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J. Su et al. / Materials Research Bulletin 47 (2012) 1709–1712
The infrared (IR) spectra were recorded with
a Fourier
transform infrared spectrometer (Nicolet Magna-IR 750, USA).
The phase development was characterized by Philips X’pert PRO X-
ray diffractometer with Cu Ka1 radiation. The morphology and
microstructure of the sintered powders were observed using
transmission electron microscopy (Hitachi H-800, Japan). The
photoluminescence spectra were measured with a Jobin–Yvon
spectrophotometer (Fluorolog 3 Tau, France). All the experiments
were completed at room temperature.
1at.%Nd:GSAG
o
1000 C, 3h
3. Results and discussion
Fig. 1 shows XRD patterns of the as-burnt precursor sintered at
different temperatures for 3 h. The diffraction patterns of all the
sintered powders are well indexable under Ia3d lattice symmetry
consistent with standard value (JCPDS Card No. 43-0659) and no
other crystalline phase was detected, indicating the precursors
transform to pure cubic GSAG polycrystalline at 800 8C. The
diffraction peaks became stronger and sharper with increasing of
the sintering temperature, which should be due to the crystallite
growth of the Nd:GASG powders. Average grain sizes were
estimated from the XRD patterns using Scherrer formula
900oC, 3h
800oC, 3h
10
20
30
40
50
60
70
80
2 theta(degree)
(d = 0.9l/b cos u) to be 17 nm, 22 nm and 30 nm for the powders
sintered at 800 8C, 900 8C and 1000 8C, respectively.
Fig. 1. XRD patterns of the Nd:GSAG precursor sintered at different temperatures for
Fig. 2 shows the FT-IR spectra in the range 400–4000 cmꢁ1 for
the as-burnt precursor and the powder sintered at 800 8C. The wide
bands at 3440 cmꢁ1 can be assigned to O–H stretch mode and
1640 cmꢁ1 to O–H bend mode of absorbed water. During the
process of sol formation, the citric acid acts as a complexing agent.
The complexes solution was adjusted at pH 7 by ammonia solution
to form stable complex compound, thus the major chemical
reactions can be described as follows:
3 h.
800oC
precursor
H2C COOH
H2C COO
M3+
+
3H+
M +
OH
C
COOH
OH
C
COO
H2C COOH
H2C COO
NO3- + NH4+ →NH 4NO3
500 1000 1500 2000 2500 3000 3500 4000
The combustion process is an oxidation–reduction reaction, in
-1
Wavenumber (cm )
which citric acid acts as a reductant and fuel, nitrate as an oxidant.
For the precursor, the absorption bands at 1380 cmꢁ1 is assigned to
N–O stretching vibration of NO3ꢁ and 1550 cmꢁ1 to the stretching
vibration of COOꢁ groups. For the powder sintered at 800 8C, these
bands disappeared because the pyrolysis of the organic compo-
nent. New bands appeared at 738 cmꢁ1, 679 cmꢁ1, 652 cmꢁ1
608 cmꢁ1, 534 cmꢁ1, 468 cmꢁ1, 430 cmꢁ1, 410 cmꢁ1 are charac-
teristic metal–oxygen (M–O) vibrations and can be attributed to
Fig. 2. FT-IR spectra of the Nd:GSAG precursor and the powder sintered at 800 8C for
3 h.
the stretching mode of the tetrahedral units present in GSAG
structure [19,20]. The result of FT-IR agrees well with XRD analysis.
Fig. 3 shows TEM morphologies of the precursor powders
sintered at 800 8C, 900 8C and 1000 8C for 3 h. It was found for the
,
Fig. 3. TEM morphologies of Nd:GSAG powders sintered at different temperatures for 3 h: (a) 800 8C; (b) 900 8C; and (c) 1000 8C.

J. Su et al. / Materials Research Bulletin 47 (2012) 1709–1712
1711
4
4F3/2 I11/2
2H9/2
a
a
b
a
b
c
900oC
4F3/2
2G7/2
+4F5/2
1000oC
800oC
+4G5/2
4S3/2
+4F7/2
2K13/2
+4G7/2
+4G9/2
b
4D3/2
2K15/2
+4D5/2
+4G11/2
+2G9/2
4F9/2
2H11/2
c
c
4
4F3/2 I9/2
4
4F3/2 I13/2
300
400
500
600
700
800
900
900
1000
1100
1200
1300
1400
1500
wavelength(nm)
Wavelength(nm)
Fig. 5. Excitation spectra of 1 at.% Nd:GSAG powders sintered at different
temperatures for 3 h, em = 942 nm: (a) 900 8C; (b) 1000 8C; and (c) 800 8C.
l
Fig. 4. Emission spectra of 1 at.% Nd:GSAG powders sintered at different
temperatures for 3 h, ex = 808 nm: (a) 900 8C; (b) 1000 8C; and (c) 800 8C.
l
somewhat quantum-size effect. Although the powder sintered at
800 8C has the smallest size, the stronger light diffusion due to its
smaller size generally results in a lower intensity.
powders sintered at 800 8C and 900 8C, the particles were well
dispersed, with an average particle sizes of approximately 30 nm
and 50 nm, respectively. The particle sizes increase with increasing
the sintering temperature, which was mainly associated with a
higher crystalline after sintering at a higher temperature. For the
powder sintered at 1000 8C, the spherical particles were somewhat
agglomerated with an average particle size of 80 nm.
The emission spectra of 1 at.% Nd:GSAG powders sintered at
various temperatures under 808 nm excitation are shown in Fig. 4.
The spectra consist of three characteristic emission bands at 850–
950 nm, 1000–1150 nm and 1300–1380 nm, which are attributed
4. Conclusions
Nanocrystalline Nd:GSAG powders were synthesized by a gel-
combustion method with citric acid as fuel and nitrate as oxidant.
The method has the advantages of inexpensive precursors, easy to
operate and a resulting nano-sized, homogeneous, highly reactive
powders. Pure GSAG polycrystalline phase was obtained by
sintering the as-burnt precursor at 800 8C. Nd:GSAG powders
from sintering the precursors at 800–1000 8C were well dispersed
with average particle sizes of 30–80 nm and the powder sintered at
900 8C shows a higher luminescence. These nano-sized, well-
dispersed powders would be suitable for fabrication of Nd:GSAG
transparent ceramics.
4
4
4
to 4F3/2 ! I9/2, 4F3/2 ! I11/2 and 4F3/2 ! I13/2 transitions of Nd3+
,
respectively. It shows the emission peak at 1062 nm is most strong
and the peak at 942 nm is relatively weak. It is well known that in
many Nd3+-doped crystals and glasses, the stimulated emission
cross-section for F3/2 ! I11/2 transition of Nd3+ ion is large and
4
4
the laser emission between the 4F3/2 and 4I11/2 levels of the Nd3+ ion
can be easily obtained. Recently, the 4F3/2 ! I9/2 transition of the
4
Acknowledgments
Nd3+ ion around 940 nm has received a great deal of attention for
lidar detection of water vapor in atmospheric, therefore we
measured the excitation spectra monitored at 942 nm, as shown in
This work is supported by the National Natural Science
Foundation of China (nos. 51002079 and 90922003).
8
6
Fig. 5. The sharp peak located at 274 nm due to the S7/2 ! IJ
transition of Gd3+ [21], indicating that the energy transfer between
Gd3+ and Nd3+ occurs. According to the energy level of Nd3+ ion in
GSAG [22,23], the bands in the wavelength range 320–900 nm are
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