Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Article abstract of DOI:10.1016/j.jpcs.2008.01.015

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.

Full text of DOI:10.1016/j.jpcs.2008.01.015

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1891–1896
www.elsevier.com/locate/jpcs
Synthesis process and the luminescence properties of rare earth
doped NaLa(WO4)2 nanoparticles
Ã
Ning Xue, Xianping Fan , Zhiyu Wang, Minquan Wang
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
Received 27 March 2007; received in revised form 14 January 2008; accepted 26 January 2008
Abstract
Rare earth doped NaLa(WO4)2 nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray
diffraction (XRD) pattern shows that the Eu3+-doped NaLa(WO4)2 nanoparticles with an average size of 10–30 nm can be obtained via
hydrothermal treatment for different time at 180 1C. The luminescence intensity of Eu3+-doped NaLa(WO4)2 nanoparticles depended on
the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er3+ and 20 mol% Yb3+ codoped NaLa(WO4)2
nanoparticles under 980 nm excitation could also be observed. The Yb3+–Er3+ codoped NaLa(WO4)2 nanoparticles prepared by the
hydrothermal treatment at 180 1C and then heated at 600 1C shows a 20 times stronger upconversion luminescence than those prepared
by hydrothermal treatment at 180 1C or by hydrothermal treatment at 180 1C and then heated at 400 1C.
r 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; B. Chemical synthesis; D. Luminescence
1. Introduction
formula ARe(MO4)2 in which A represents a monovalent
cation (K+, Na+), Re a trivalent cation (La3+, Y3+), and
MO4 a divalent anion (MoO24À, WO42À). Many substitutions
are possible within this structure, and rare earth ions have
been substituted for the Re3+ cation. The ARe(MO4)2
doped with rare earth ions has played an important role in
quantum electronics for more than three decades [15]. Their
structural diversity provides these crystals with numerous
physical properties. In addition to the well-known laser
potential [5,16], it is also possible to take benefit of the
nonlinearities of the host as efficient Stokes converters of
laser radiation [17–22]. While over a long time the
investigations on the ARe(MO4)2 family of compounds
are concentrated on the single crystals to take advantage of
the laser properties. The ARe(MO4)2 powders with small
particle size have received a little attention. Most of these
rare earth doped ARe(MO4)2 powders have been synthe-
sized at high temperature (over 800 1C) in a solid-state
reaction, or with at least one stage at high temperature,
involving complicated procedure and long reaction time.
Under such conditions, well-crystallized samples can be
obtained with large crystallites. Some ARe(MO4)2 single
crystals and powders have also been prepared by other
The rare earth doped materials can find a wide variety of
applications including phosphors for lighting, display, and
X-ray imaging [1–3], scintillators [4], laser [5], and
amplifiers for fiber-optic communication [6], Recently,
the synthesis process and luminescence properties of the
rare earth doped nanoparticles have attracted considerable
interest since they are considered as potentially useful
active components in biological probe [7–10], lamps and
display [11], lasers [12] and new optoelectronic devices
[13,14]. The luminescence properties of the rare earth
doped nanoparticles depend strongly on the dopant rare
earth ions and the host lattice. The host lattice may
significantly influence the radiative and non-radiative
(multiphonon relaxation as well as energy transfer) proper-
ties, lead to entirely different luminescence behavior [7].
Double alkaline rare-earth molybdates and tungstates
form a large family of inorganic compounds with general
Ã
Corresponding author. Tel.: +86 571 87952334;
fax: +86 571 87951234.
E-mail address: fanxp@zju.edu.cn (X. Fan).
0022-3697/$ - see front matter r 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2008.01.015

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1892
processes such as the flux growth [23,24] and hydrothermal
reaction over an extensive period [25,26]. Moreover, the rare
earth doped ARe(MO4)2 nanoparticles with smaller particle
size are necessary for some particular application. For
example, the nanoparticles that have the upconversion
luminescence properties can be used as fluorescence labels
for the sensitive detection of bimolecular [7–10,27,28].
However, very little report in the literature on rare earth
doped ARe(MO4)2 nanoparticles can be found. In this paper,
we reported the hydrothermal synthesis process and lumines-
cence behavior of the Eu3+-doped NaLa(WO4)2 nanoparti-
cles and Yb3+–Er3+ codoped NaLa(WO4)2 nanoparticles.
2. Experimental
The rare earth doped NaLa(WO4)2 nanoparticles were
synthesized by a hydrothermal method. Hydrated sodium
tungstate Na2WO4 Á 2H2O, lanthanum chloride LaCl3 Á 4H2O
and europium chloride EuCl3 Á nH2O (erbium chloride
ErCl3 Á nH2O and ytterbium chloride YbCl3 Á nH2O) were
used as raw materials. EuCl3 Á nH2O, ErCl3 Á nH2O and
YbCl3 Á nH2O were prepared by dissolving corresponding
rare earth oxide (Eu2O3, Er2O3, Yb2O3) in dilute hydrochloric
acid and then drying in order to remove the unreacted
hydrochloric acid. To prepare the Eu3+-doped NaLa(WO4)2
nanoparticles, the total concentration of rare earth ions
(La3++Eu3+) was fixed at 10 mmol/L and an aqueous
solution of Na2WO4 Á 2H2O was introduced into an aqueous
solution of LaCl3 Á 4H2O and EuCl3 Á nH2O with vigorous
stirring. The pH of mixture was adjusted to 7 with dilute
aqueous NaOH solution. Finally, the resulting mixture was
poured into a stainless-steel autoclave (250 ml) and heated at
180 1C for different time. The white precipitates were
separated by centrifugation and washed with deionized water
and ethanol for several times. Then the precipitates were
dried at 50 1C for 30 h and collected for characterization.
Similar procedure was used to synthesis Er3+–Yb3+ codoped
NaLa(WO4)2 nanoparticles. To prepare the nanoparticles
with strong upconversion luminescence intensity, the dried
2mol% Er3+ and 20 mol% Yb3+ codoped NaLa(WO4)2
nanoparticles obtained by hydrothermal treatment for 1 h at
180 1C were heated in a stove for 1 h at 400 and 600 1C,
respectively. The final product was a white powder.
Fig. 1. XRD patterns of 10% Eu3+-doped NaLa(WO4)2 nanoparticles
that were hydrothermal treated for different time at 180 1C.
treated for different time at 180 1C. After hydrothermal
treated for 1 h at 180 1C, the XRD pattern exhibits
prominent peaks well accordant with JCPDS standard
card (79-1118) of tetragonal NaLa(WO4)2 crystal with
no second, which indicated that the 10 mol% Eu3+-doped
NaLa(WO4)2 nanoparticles were isostructural with the
NaLa(WO4)2 crystal. Therefore, the Eu3+ can easily
substituted the La3+ sites in NaLa(WO4)2 nanoparticles.
Moreover, the width of the XRD peaks became sharper
with increasing time, which indicated larger NaLa(WO4)2
nanoparticles can be obtained when hydrothermal treated
for longer time. The size of the nanoparticles could be
calculated from Scherrer’s equation using the measured full
width at half-maximum (FWHM) values of peaks [29]. By
selecting the maximum XRD diffraction peak of (1 1 2)
plane, the size of the 10 mol% Eu3+-doped NaLa(WO4)2
nanoparticles can be calculated to be about 10–30 nm.
Fig. 2 shows the TEM images of 10% Eu3+-doped
NaLa(WO4)2 nanoparticles which were hydrothermal
treated for different time at 180 1C. The average size of
the nanoparticles that were synthesized by hydrothermal
treatment for 1 h at 180 1C is about 10 nm, which is
approximately consistent with the Scherrer-calculated
diameter. The larger average size of the nanoparticles can
be obtained when samples were hydrothermal treated for
longer time, which were also accordant with the Scherrer-
calculated results. From TEM images, however, it could be
found that the size of the nanoparticles did not increase
obviously with increasing hydrothermal treated time when
the time was more than 4 h.
X-ray diffraction (XRD) patterns were obtained using a
Philips XD98 with Cu Ka radiation. Transmission electronic
microscopy (TEM) images were taken on a JEM-200CX
microscopy operating at 160 kV. The emission spectra in the
visible region were recorded on a Hitachi F-4500 fluorescence
spectrophotometer. The upconversion emission spectra were
measured with Hitachi F-4500 fluorescence spectrophot-
ometer with an external near-infrared laser diode
(lex=980 nm) as the excitation source. All measurements
were performed at room temperature.
Fig. 3 shows the emission spectra (excited at 395 nm) of
10% Eu3+-doped NaLa(WO4)2 nanoparticles which were
hydrothermal treated for different time at 180 1C. Similar
emission spectra can be observed. The emission spectrum
contains only the 5D0-7FJ transitions. The 5D0-7F2 forced
electric dipole transition (the overwhelming predominant
3. Results and discussion
Fig. 1 shows the XRD patterns of 10% Eu3+-doped
NaLa(WO4)2 nanoparticles which were hydrothermal

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Fig. 2. TEM images of 10% Eu3+-doped NaLa(WO4)2 nanoparticles that were hydrothermal treated at 180 1C for (a)1 h, (b) 4 h, (c) 12 h, (d) 24 h.
emission line) can be found to be approximately six times
5
hydrothermal treated time, the luminescence intensity did
not increase obviously with increasing hydrothermal
treated time when the time was more than 4 h, which
indicated that luminescence intensity of Eu3+-doped
NaLa(WO4)2 nanoparticles depended on the size of the
nanoparticles. The smaller the nanoparticles was, the
weaker the luminescence intensity of nanoparticles was.
Surface defects have been blamed for the decreasing
luminescence intensity. The atomic coordination of the
surface that could absorb different chemical species
differed from that in the bulk. The synthesis process of
Eu3+-doped NaLa(WO4)2 nanoparticles was achieved in
water and ethanol and the surface of the nanoparticles
might be covered by a large number of hydroxyl species
[34]. The vibration of the OH would result in the
luminescence quenching. At the same time, the surface
area increased largely with the decreasing size of the
nanoparticles. The surface defects might act as non-
radiative recombination centers [35] and, therefore, might
be responsible for the decrease of the luminescence
intensity.
stronger than the D0-7F1 magnetic dipole transition and
results in a highly red emission, which implies the loss of
inversion symmetry of Eu3+ ions in the NaLa(WO4)2
nanoparticle [30,31]. Two space groups have been proposed
for the tetragonal double tungstate, like NaBi(MoO4)2,
assumed isostructural to NaLa(WO4)2. The centrosymmetric
¯
I41/a and the non-centrosymmetric I4 [32]. In any case, the
structure is built-up with WO4 tetrahedral groups and NaO8
and LaO8 distorted square antiprisms. In the centrosym-
metric model Na+ and La3+ ions occupy statistically a
unique position, with S4 or C2 local site symmetry. In the
non-centrosymmetric model, however, these cations
share two non-equivalent lattice sites with local symmetry
S4 [33]. Therefore, the symmetry S4 of the point site was
occupied by Eu3+ when it replaced La3+ in the Eu3+-doped
NaLa(WO4)2 nanoparticles.
The inset of Fig. 3 shows the dependence of lumines-
cence intensity of 10% Eu3+-doped NaLa(WO4)2 nano-
particles on the hydrothermal treated time. Similar to
relationship between the size of the nanoparticles and

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1894
Fig. 4. XRD patterns of 2 mol% Er3+ and 20 mol% Yb3+ codoped
NaLa(WO4)2 nanoparticles that were hydrothermal treated for 1 h at
180 1C and then heated for 1 h at 400 and 600 1C, respectively.
Fig. 3. The emission spectra of 10% Eu3+-doped NaLa(WO4)2 nano-
particles that were hydrothermal treated for different time at 180 1C. Inset:
dependence of luminescence intensity of 10% Eu3+-doped NaLa(WO4)2
nanoparticles on the hydrothermal treated time.
respectively. The XRD peak positions and intensities
agreed well with JCPDS standard card (79-1118) of
tetragonal NaLa(WO4)2 crystal. Fig. 5 shows the TEM
images of the 2 mol% Er3+ and 20 mol% Yb3+ codoped
NaLa(WO4)2 nanoparticles that were prepared by the
hydrothermal treatment for 1 h at 180 1C and then heated
for 1 h at 400 and 600 1C, respectively. After heated at
600 1C the size of the crystalline particles increased
significantly. Fig. 6 shows the upconversion luminescence
spectra of the 2 mol% Er3+ and 20 mol% Yb3+ codoped
NaLa(WO4)2 nanoparticles that were prepared by the
hydrothermal treatment for 1 h at 180 1C, as well as by the
hydrothermal treatment for 1 h at 180 1C and then heated
for 1 h at 400 and 600 1C, respectively. The green emission
Upconversion luminescence process, which emit higher-
energy photons when excited by lower energy photons, has
attracted considerable interest in the fields of lasers [36],
infrared quantum counters [37], next-generation lighting or
displays [38] and biological nanolabels [39] since the
development of IR laser diodes. The principal strategies
for obtaining new upconversion compounds involve: (1)
variation of the dopant metal ions, and (2) variation of the
host lattice [7]. Er3+ is extensively used as dopant metal
ions in upconversion compounds. However, the lower
absorption cross-section of Er3+ at 980 nm, a wavelength
that matches the emission wavelength of currently avail-
able diode lasers, resulted in weaker upconversion lumi-
nescence intensity of Er3+-doped compounds. The Yb3+ is
an efficient sensitive ion for Er3+ due to strong absorption
at 980 nm, which has a 10 times higher absorption cross-
bands centered at 525 and 550nm can be assigned to 2H11/2
-
4I15/2 and S3/2-4I15/2 transitions, respectively. The
red emission band centered at 655 nm can be assigned to
4F9/2-4I15/2 transitions [40,49]. Auzel has described the
upconversion process of Yb3+–Er3+ codoped compounds
[50]. Near IR photons are absorbed by Yb3+ ions and
4
section and a much broader peak at 980 nm than Er3+
.
At this wavelength, the Yb3+ ions can be pumped
efficiently by these powerful laser diodes, thus allowing
for an efficient energy transfer from Yb3+ (2F5/2) to Er3+
(4I11/2) due to the near-perfect resonance between the Yb3+
2F5/2|1S and Er3+ 4I11/2|0S states (mismatch ꢀ4 cmÀ1). On
the other hand, changing the host lattice may dramatically
influence the radiative and non-radiative properties, lead-
ing to entirely different upconversion luminescence beha-
vior. Upconversion process of Yb3+–Er3+ codoped oxide
and fluorides have been investigated extensively [7,40–48].
Fig. 4 shows the XRD patterns of the 2 mol% Er3+ and
20 mol% Yb3+ codoped NaLa(WO4)2 nanoparticles that
were prepared by the hydrothermal treatment for 1 h at
180 1C and then heated for 1 h at 400 and 600 1C,
rapidly transferred to resonant metastable I11/2 level of
4
an unexcited Er3+ ion through the energy transfer
process 2F5/2(Yb3+)+4I15/2(Er3+)-2F7/2(Yb3+)+4I11/2
(Er3+). Before this level can de-excite, additional energy
is resonantly transferred to the Er3+ ion causing a stepwise
promotion of energy within its level. One possibility is
the upward transition from 4I11/2 to the 4F7/2 level through
the process 2F5/2(Yb3+)+4I11/2(Er3+)-2F7/2(Yb3+)+
4F7/2(Er3+), which rapidly de-excite non-radiatively to
4
4H11/2 and S3/2 emitting level and produce green lumines-
4
cence (2H11/2-4I15/2 and S3/2-4I15/2 transitions). An
alternate possibility is a partial non-radiative de-excitation
4
from the I11/2 to the I13/2 level, from where another
4
4
photon energy is transferred up to the emitting F9/2 level

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Fig. 6. Upconversion luminescence spectra of 2 mol% Er3+ and 20 mol%
Yb3+ codoped NaLa(WO4)2 nanoparticles that were hydrothermal
treated for 1 h at 180 1C, hydrothermal treated for 1 h at 180 1C and then
heated for 1 h at 400 1C, and hydrothermal treated for 1 h at 180 1C and
then heated for 1 h at 600 1C.
energy is transferred to the lattice vibrations, which
depends on the phonon spectrum of the host). The
synthesis process of Yb3+–Er3+ codoped NaLa(WO4)2
nanoparticles was achieved in water and ethanol and the
surface of the nanoparticles may be covered by a large
number of hydroxyl species. The vibration of the OH can
act to enhance non-radiative decay. After the nanoparticles
were heated at 600 1C, the OH could be eliminated
efficiently [51]. At the same time, the increase of the
temperature resulted notably in sintering of the material
and increasing size of the nanoparticles. The larger
nanoparticles have a lower proportion of surface ions to
bulk ions than do the smaller nanoparticles. It is known
that ions on the surface do not have a full coordination
sphere [52]. The incomplete coordination of the surface
ions would enhance non-radiative decay. Therefore, the
increase of the temperature could eliminate efficiently OH
and increase size of the nanoparticles, which resulted in
significant increase of upconversion luminescence intensity.
Fig. 5. TEM images of 2 mol% Er3+ and 20 mol% Yb3+ codoped
NaLa(WO4)2 nanoparticles that were hydrothermal treated for 1 h at
180 1C and then heated for 1 h at 400 1C (a) and 600 1C (b), respectively.
2
through the process F5/2(Yb3+)+4I13/2(Er3+)-2F7/2
(Yb3+)+4F9/2(Er3+) and produce red luminescence (4F9/2
4I15/2 transitions) [48].
-
The green plus red emissions of the 2 mol% Er3+ and
20 mol% Yb3+ codoped NaLa(WO4)2 nanoparticles pre-
pared by the hydrothermal treatment at 180 1C and then
heated at 600 1C were found to be 20 times stronger than
those for nanoparticles prepared by hydrothermal treat-
ment at 180 1C or by hydrothermal treatment at 180 1C and
then heated at 400 1C. The green plus red luminescence of
the nanoparticles prepared by the hydrothermal treatment
at 180 1C and then heated at 600 1C was so bright that be
visible for the naked eye when an unfocused 980 nm laser
beam of typically 50 mW was used. The luminescence of
rare earth ions is generally quenched in two ways, viz. ether
by cross relaxation (the excited energy is transferred to
other rare earth ions, which depends on the concentration
of rare earth ions) or by multiphonon relaxation (the
4. Conclusions
Rare earth doped NaLa(WO4)2 nanoparticles can be
prepared by a simply hydrothermal synthesis procedure.
The Eu3+-doped NaLa(WO4)2 nanoparticles with an
average size of 10–30 nm can be obtained via hydrothermal
treatment for different time at 180 1C. The luminescence
intensity of Eu3+-doped NaLa(WO4)2 nanoparticles de-
pended on the size of the nanoparticles. The bright
upconversion luminescence of the 2 mol% Er3+ and
20 mol% Yb3+ codoped NaLa(WO4)2 nanoparticles under
980 nm excitation could also be observed. The heated
temperature has significant influence on the upconversion

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1896
luminescence intensity. The upconversion luminescence of
2 mol% Er3+ and 20 mol% Yb3+ codoped NaLa(WO4)2
nanoparticles prepared by the hydrothermal treatment at
180 1C and then heated at 600 1C were found to be 20 times
stronger than that of nanoparticles prepared by hydro-
thermal treatment at 180 1C or by hydrothermal treatment
at 180 1C and then heated at 400 1C.
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Acknowledgments
The authors gratefully acknowledge support for this
research from the National Nature Science Foundation of
China (no. 50472062), the Science and Technology
Department of Zhejiang Province (2006C14010) and
Program for Changjiang Scholars and Innovative Research
Team in University.
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