Self-assembled three-dimensional NaY(WO4)2:Ln 3+ architectures: Hydrothermal synthesis, growth mechanism and luminescence properties

Article abstract of DOI:10.1016/j.jallcom.2012.02.156

Novel three-dimensional (3D) flower-like NaY(WO₄) ₂:Ln³⁺ (Ln = Eu, Yb/Er, Yb/Tm and Yb/Ho) microstructures with uniform shape and dimension have been prepared using Y(OH)CO₃ nanospheres as sacrificial template through a hydrothermal process and followed by a subsequent heat treatment process. The whole process was carried out in aqueous condition without using any organic solvents, surfactant, or catalyst. The phase, morphology, size, and photoluminescence (PL) properties were well characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra, and kinetic decays, respectively. The results reveal that the as-prepared precursor of NaY(WO ₄)₂:Eu³⁺ exhibits interesting white light emission under UV excitation. After annealing, the as-obtained 3D flower-like NaY(WO₄)₂:Eu³⁺ microstructures show exclusively red (Eu³⁺, ⁵D₀ → ⁷F ₂) luminescence. Furthermore, the up-conversion (UC) luminescent properties and the emission mechanisms of NaY(WO₄) ₂:Yb³⁺/Ln³⁺ (Ln = Er, Tm, Ho) microstructures have been systematically studied, which show respective green (Er³⁺, ⁴S_3/2, ²H_11/2 → ⁴I_15/2), blue (Tm³⁺, ¹G₄ → ³H₆) and yellow-green (Ho³⁺, ⁵S₂ → ⁵I₈) luminescence under 980 nm NIR excitation.

Full text of DOI:10.1016/j.jallcom.2012.02.156

Journal of Alloys and Compounds 529 (2012) 140–147
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Self-assembled three-dimensional NaY(WO4)2:Ln3+ architectures: Hydrothermal
synthesis, growth mechanism and luminescence properties
Shaohua Huanga,b, Dong Wanga,b, Yan Wanga,b, Liuzhen Wanga,b, Xiao Zhanga,b, Piaoping Yanga,∗
a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China
b College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Novel three-dimensional (3D) flower-like NaY(WO4)2:Ln3+ (Ln = Eu, Yb/Er, Yb/Tm and Yb/Ho) microstruc-
tures with uniform shape and dimension have been prepared using Y(OH)CO3 nanospheres as sacrificial
template through a hydrothermal process and followed by a subsequent heat treatment process. The
whole process was carried out in aqueous condition without using any organic solvents, surfactant, or
catalyst. The phase, morphology, size, and photoluminescence (PL) properties were well characterized by
means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra,
and kinetic decays, respectively. The results reveal that the as-prepared precursor of NaY(WO4)2:Eu3+
exhibits interesting white light emission under UV excitation. After annealing, the as-obtained 3D flower-
Article history:
Received 9 January 2012
Received in revised form 20 February 2012
Accepted 24 February 2012
Available online 3 March 2012
Keywords:
Three-dimensional
NaY(WO4)2
Hydrothermal
Up-conversion
like NaY(WO4)2:Eu3+ microstructures show exclusively red (Eu3+
the up-conversion (UC) luminescent properties and the emission mechanisms of NaY(WO4)2:Yb3+/Ln3+
(Ln = Er, Tm, Ho) microstructures have been systematically studied, which show respective green (Er3+
4S3/2 2H11/2 4I15/2), blue (Tm3+ 1G4 3H6) and yellow-green (Ho3+ 5S2 5I8) luminescence under
980 nm NIR excitation.
,
5D0
→
7F2) luminescence. Furthermore,
,
,
→
,
→
,
→
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
been developed to effectively fabricate 3D hierarchical struc-
tures, such as chemical vapor deposition (CVD) method, Kirkendall
process, and precursor template synthesis [7–9]. Although some
progress has been achieved on the synthetic approaches of 3D
hierarchical structures, the simplest synthetic route to obtain 3D
nanostructures is still self-assembly, in which ordered aggregates
are formed in a spontaneous process. Therefore, it should be highly
promising to develop reasonable synthetic routes for the syn-
thesis of 3D hierarchical structures via a chemical self-assembly
process.
In the past several decades, rare-earth compounds have been
extensively studied due to their interesting optical characteris-
tics, such as large Stokes shift, long lifetime, sharp and abundant
emission colors deriving from their unique intra 4f transitions,
which are highly potential in the fields of display systems and
other optoelectronic devices [10–12]. Especially, the white light
emitting diodes (LEDs) have many excellent properties such as
low power consumption, long operating time, and environmen-
tal benefit. The realization of strong white emission requires the
generation and an adequate combination of the three fundamental
red, green, and blue (RGB) light colors, which greatly challenge peo-
ple’s ability of material design including host composition and the
suitable combination of sensitizers and activator ions. Therefore,
white LEDs are expected to be new light sources in the illumina-
tion field and have attracted increasing attention in recent years
Recently, much attention has been paid to the design and syn-
thesis of three-dimensional (3D) hierarchical architectures with
specific morphology and novel properties via a self-assembly pro-
cess using low-dimensional materials as building blocks, which
are due to their the potential applications in diverse fields such
as optics, superconductors, optoelectronics, solar cells, and catal-
ysis [1–4]. The composition, morphology, dimension and size of
these materials are important factors that influence the charac-
teristic properties of the materials, which depend on not only
their chemical composition, but also their structures. Tang et al.
have found that the luminescence properties of the GdVO4:Eu3+
samples change with the shape under the same ultraviolet exci-
tation [5]. Sun et al. also reported that the photostimulated
luminescence spectra of the as-prepared SrSO4 crystals depend
on the crystallographic morphology [6]. Therefore, 3D hierar-
chical structures assembled from inorganic building blocks into
have attracted intensive research interest, because the variation
of building blocks and manners provides a method to tune the
properties of the materials. Up to now, several strategies have
∗
Corresponding author. Tel.: +86 451 82569890; fax: +86 451 82533026.
E-mail addresses: yangpiaoping@hrbeu.edu.cn, piaoping@ciac.jl.cn (P. Yang).
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.02.156

S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
141
[13–18]. Till now, a wide variety of inorganic materials, includ-
ing hydrate, borate, molybdate and tungstate have been prepared
with complex 3D hierarchical shapes [19–22]. As one kind of
important solid-state materials, rare-earth tungstates have gained
much attention because of their remarkable properties in the areas
of catalysts, quantum electronics and lasers [23]. Among them,
scheelite-like alkali rare earth double tungstates with the gen-
eral formula of ARE(WO4)2 (A represents alkali metal ions; RE
represents rare earth ions) are interesting phosphors for visual
display as good laser crystal materials [24,25]. Singly doped and
Tm3+/Ho3+ co-doped NaY(WO4)2 as laser materials were synthe-
sized successfully by Sun et al. [26]. Kasprowicz et al. presented
the investigations of KGd(WO4)2 single crystals triply-doped with
Yb/Tm/Ln (Ln = Pr, Ho, Er) ions by Raman spectroscopy [27]. How-
ever, previous works about alkali rare earth tungstates mainly focus
on the preparation of solid materials by the Czochralski method
or using top seeded solution growth method [26,27], and little
has been done on the fabrication of tungstates with uniform 3D
hierarchical structures. Therefore, it remains a significant chal-
lenge to develop reasonable synthetic methods to synthesize novel
self-assembly 3D hierarchical structures. Compared with the meth-
ods above, as a typical solution-based approach, the hydrothermal
method has been proven to be an effective and facile process
in preparing various inorganic materials with low cost and mild
reaction conditions [21,28–31]. In addition, a promising chemical
conversion route has drawn attention for the fabrication of various
well-defined microstructures [32–35]. Nevertheless, to the best our
knowledge, there are no reports about 3D NaY(WO4)2 hierarchical
structures fabricated by a facile and general chemical conversion
method without using any surfactant or catalyst. Particularly, the
UC luminescence of NaY(WO4)2:Ln has never been reported so far.
In this paper, we present an effective route for the prepara-
tion of novel self-assembled 3D NaY(WO4)2 hierarchical structures
through a chemical conversion process followed by further calci-
nation treatment. The structure, shape and formation mechanism
C
B
A
JCPDS No. 48-0886
10
20
40
60
70
302θ (degree) 50
Fig. 1. XRD patterns of (A) the as-prepared YOHCO3:Eu colloid spheres, (B) the as-
prepared precursor prepared by a hydrothermal process at 200 ◦C for 24 h, and (C)
the NaY(WO4)2:Eu3+ microstructure obtained at 800 ◦C for 2 h. The standard data
for NaY(WO4)2 (JCPDS No. 48–0886) are shown as a reference.
of the architectures are systematic investigated. The down-
conversion (DC) photoluminescence properties of the precursors
and NaY(WO4)2:Eu3+ samples, and the UC emission properties
of NaY(WO4)2:Yb3+/Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+) are studied in
detail. And the emission mechanisms of NaY(WO4)2:Yb3+/Ln3+
(Ln = Er, Tm, Ho) microstructures have been proposed. In addition,
it is the first time to investigate the UC luminescence properties of
novel three-dimensional flower-like NaY(WO4)2.
2. Experimental details
2.1. Materials
Ln(NO3)3 (0.5 mol/L) were obtained by dissolving Ln2O3 (99.99%) (Ln = Y,
Eu, Ho, Er, Tm, Yb) in HNO3 solution under heating with agitation. All the
Fig. 2. (A and B) SEM images of the as-prepared Y(OH)CO3 microspheres, (C and D) SEM images of the as-prepared precursor without calcinations.

142
S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
Fig. 3. (A and B) SEM images with different magnification, (C) TEM image, and (D) HRTEM image of the NaY(WO4)2:Eu3+ microstructure.
100
99
98
97
96
95
94
93
other chemicals were of analytic grade and used as received without further
purification.
DSC
0.0
2.2. Synthesis of YCO3OH:Ln colloid spheres
TG
In a typical procedure for the preparation of YCO3OH:10%Eu3+, 1.8 mL of Y(NO3)3
(0.5 mol/L) and 0.2 mL of Eu(NO3)3 (0.5 mol/L) was added into a 50 mL of aqueous
solution. After vigorous stirring for 15 min, 0.1 mol of CO(NH3)2 was added into
the above solution and then distilled water was added to make a total volume of
100 mL, and the beaker was then wrapped with a polyethylene film. After stirred for
15 min, the mixed solution was heated with strong stirring to 85 ◦C and maintained
at this temperature for 3 h. The obtained suspension was separated by centrifugation
and collected after washing with deionized water several times. YCO3OH:3%Yb3+/1%
Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+) samples were prepared in a manner similar to that for
the synthesis of YCO3OH:Eu3+, except that Yb(NO3)3/Ho(NO3)3, Yb(NO3)3/Er(NO3)3,
Yb(NO3)3/Tm(NO3)3 together with Y(NO3)3 were the initial materials, respectively.
-0.2
-0.4
-0.6
630 °C
100 200 300 400 500 600 700 800
Temperature /°C
2.3. Synthesis of NaY(WO4)2:Ln microstructures
Fig. 4. TG-DSC curve of the as-prepared precursor doped with 10 mol % Eu3+
.
In a typical procedure for the preparation of NaY(WO4)2:10%Eu3+, the as-
prepared YCO3OH:Eu3+ was dispersed into 35 ml deionized water and ultrasonic
treatment for 20 min. Then, 2 mmol of Na2WO4 was added into the above solution.
After additional agitation for 30 min, the as-obtained mixing solution was trans-
(HRTEM) were performed on a FEI Tecnai G2 S-Twin instrument with a field emis-
sion gun operating at 200 kV. Images were acquired digitally on a Gatan multiple
CCD camera. The excitation and emission spectra were taken on a Hitachi F4500 flu-
orescent spectrophotometer equipped with a 150 W xenon lamp as the excitation
source. The UC emission spectra were obtained using a 980 nm laser from an OPO
(optical parametric oscillator, Continuum Surelite, USA) as the excitation source
and detected by a photomultiplier tube (HAHAMATSU R955) from 400 to 900 nm.
Luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digi-
tal Oscilloscope (1 GHz) using a 250 nm laser (pulse width = 4 ns, gate = 50 ns) as the
excitation source (ContinuumSunlite OPO). All the measurements were performed
at room temperature.
ferred into a Teflon held stainless steel autoclave, sealed and maintained at 200 ◦
C
for 9 h. As the autoclave was cooled to room temperature naturally, the as-obtained
white precipitates were washed with deionized water, centrifuged, and dried at
60 ◦C in air for 12 h. The NaY(WO4)2:Eu3+ samples were prepared by calcination of
the precursors at 800 ◦C for 2 h in air. The preparation of NaY(WO4)2:3%Yb3+/1%Ln3+
(Ln3+ = Er3+, Tm3+, Ho3+) samples were prepared in a similar approach except that
the as-prepared YCO3OH:3%Yb3+/1%Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+) together with
Na2WO4 were the initial materials, respectively.
2.4. Characterization
3. Results and discussion
Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-
Dmax 2500 diffractometer at a scanning rate of 15◦/min in the 2ꢀ range from 10◦
to 70◦, with graphite-monochromatized Cu K␣ radiation (ꢁ = 0.15405 nm). Ther-
mogravimetric analysis and differential scanning calorimetry (TGA-DSC) data were
recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Cas-
tle, DE) at the heating rate of 10 ◦C min−1 in an air flow of 100 mL/min. SEM images
were obtained using a scanning electron microscope (SEM, JSM-6480), transmission
electron microscopy (TEM) and high-resolution transmission electron microscopy
3.1. Phase, structure and morphology
Fig. 1 shows the XRD patterns of as-prepared YCO3OH:Eu3+
colloid spheres, the precursors prepared through the hydrother-
mal conversion process and the calcined NaY(WO4)2:Eu3+ samples.

S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
143
Fig. 5. SEM images of the precursor prepared at 200 ◦C for reaction times of (A) 1 h, (B) 3 h, (C) 6 h, and (D) 9 h.
In Fig. 1A for YCO3OH:Eu3+, no obvious diffraction peak can be
observed, which can be assigned to the amorphous nature. Com-
pared with all the standard XRD patterns in JCPDS cards, the
precursors cannot be finely indexed to a certain phase, as shown in
Fig. 1B. As for NaY(WO4)2:Eu3+ obtained at 800 ◦C for 2 h (Fig. 1C), all
the diffraction peaks can be well indexed to tetragonal NaY(WO4)2
phase (JCPDS No. 48-0886), and no additional peaks of other phases
can be found, indicating that the precursor was completely con-
verted to tetragonal NaY(WO4)2 at 800 ◦C for 2 h.
Fig. 3A, it can be seen that heat treatment did not cause obvi-
ous change in the shape and size of the product, and the external
shape is almost maintained. Close observation (Fig. 3B) reveals that
the surface of the nanoflake building blocks become rough after
the calcination process, which may be due to the gradual elimi-
nation of CO2 and H2O [28]. Such transformation is common for
decomposing rare earth compounds [36,37]. The detailed shape and
structure of as-annealed NaY(WO4)2:Eu3+ sample were examined
by TEM and HRTEM. In Fig. 3C, TEM image of a part of an individual
microstructure also indicates that the nanoflakes are assembled in
a radial form from the center to the surface of the micro-flower. In
Fig. 3D, the obvious fringe lattices suggest the high crystallinity of
the sample. And the determined interplanar spacing of 0.262 nm
corresponds well to the lattice spacing of (2 0 0) planes for pure
tetragonal NaY(WO4)2.
The thermal behavior of the as-prepared double alkaline rare
earth tungstates precursor was investigated by TG-DSC measure-
ment. From the TG curve (Fig. 4), it can be seen that the total weight
loss is about 5.8% between 300 ◦C and 800 ◦C, corresponding to a
phase transition of forming NaY(WO4)2 at 630 ◦C in the DSC curve.
The results reveal that the double alkaline rare-earth tungstates
precursor completely converts to tetragonal-phased NaY(WO4)2
The SEM images of as-prepared YCO3OH:Eu3+ microspheres and
as-obtained double alkaline rare earth tungstates precursor are
shown in Fig. 2. It can be seen from Fig. 2A and B that the prod-
uct consists of uniform and well-dispersed microspheres with a
diameter of about 300–350 nm. The low-magnification SEM image
(Fig. 2C) clearly indicates that the as-prepared precursor possesses
uniform 3D flower-like structures with a diameter of about 2–3 ␮m.
Fig. 2D shows an individual flower-like microstructure composed
of numerous nanoflake building blocks in three dimensional space,
which extends outward from the center and distributes in all direc-
tions. This is a common result of some type of self-assembly.
Fig. 3 shows the typical SEM, TEM and HRTEM images of the
as-annealed NaY(WO4)2:Eu3+ flower-like microstructures. From
Scheme 1. Illustration for the formation process of the double alkaline rare earth tungstates precursor microstructures.

144
S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
after annealing at 800 ◦C for 2 h, which is consistent with XRD anal-
ysis.
In order to make clear the morphology evolution of the 3D
hierarchical microstructures, a series of time-dependent experi-
ments have been employed. Fig. 5 shows the SEM images of the
as-obtained precursors prepared with different reaction times. In
Fig. 5A for the sample prepared with the reaction time of 1 h,
uniform microspheres mixed with some irregular and twisted
building flakes are observed. When the reaction time is increased
to 3 h, some 3D flower-like microstructures with a small amount
of Y(OH)CO3 microspheres are achieved (Fig. 5B). Notably, we can
find that the size of the microspheres is decreased evidently. At
the reaction time of 6 h, the Y(OH)CO3 microspheres disappear
completely and converted to 3D flower-like NaY(WO4)2 precursor
which comprises of a large number of nanoflakes (Fig. 5C). Further
increasing the reaction time to 9 h, more nanoflakes assembled to
form 3D flower-like structures with a diameter of 3 ␮m via a con-
tinual nucleation growth process (Fig. 5D). The time-dependent
experiments clearly demonstrate the shape evolution process from
Y(OH)CO3 microspheres to 3D microflowers.
3.2. Possible growth mechanism
On the basis of the time-dependent evolution results, the con-
version process from Y(OH)CO3 microspheres to 3D flower-like
NaY(WO4)2 precursor can be rationally expressed as a dissolution-
recrystallization mechanism [22,32,38]. At the early stage of the
reaction, the as-prepared Y(OH)CO3 microspheres are gradually
and slowly dissolved and released Y3+ into the reaction system.
With the increase of reaction time, the WO4 ions react with Y3+
2−
and Na+ to form the thin nanoflakes of double alkaline rare earth
tungstates precursor. With prolonging the reaction time, reaction
of the Na+, Y3+ and WO42− ions in the solution keeps reaction on the
surface of the nanoflakes, resulting in the growth of more petals on
the earlier flowers, which further assembly to more compact and
larger 3D hierarchical structures. A schematic illustration of the
possible detailed conversion processes is presented in Scheme 1.
Fig. 6. Excitation (left) and emission (right) spectra of (A) 10 mol% Eu3+ doped pre-
cursor without calcination and (B) the calcinated NaY(WO4)2:Eu3+ sample.
3.3. Luminescence properties
As an efficient luminescent host for doping rare earth ions to dis-
play unique fluorescence, alkali rare earth tungstates doped with
rare earth ions have been widely used in laser active materials
and quantum electronics [39–42]. In the present work, we mainly
focus on the luminescence properties of Eu3+, Yb3+/Er3+, Yb3+/Tm3+
Yb3+/Ho3+ doped samples.
,
DC luminescence properties. Fig. 6 presents the excitation and
emission spectra of as-prepared precursor doped with 10 mol %
Eu3+ and the corresponding annealed NaY(WO4)2:Eu3+ samples,
respectively. As shown, the excitation spectrum of the precur-
sor (Fig. 6A, left) monitored with 615 nm consists of a strong and
broad band from 200 to 300 nm with a maximum at about 237 nm,
which corresponds to the charge transfer absorption from the 2p
orbitals of the oxygen ligands to the 5d orbitals of the central
2−
tungsten atoms in the WO4 groups [22,25]. In the longer wave-
Fig. 7. Decay curve for the NaY(WO4)2:10 mol % Eu3+ sample.
length region, some weak peaks ascribed to the f–f transitions of
the Eu3+ ions can also be observed. Upon excitation at 237 nm, the
emission spectrum of as-prepared Eu3+ doped precursor contains
a broad band and some sharp peaks (Fig. 6A, right). The blue broad
band with a maximum at about 455 nm should be caused by the
O–W charge transfer, and the other sharp lines centered at 581,
595 and 615 nm are the characteristic transitions of Eu3+, which
emission. Such mechanism of the formation of the white light of
the as-prepared precursor is consistent with the NaY(WO4)2:Eu3+
precursor reported by Zheng and Lei [22,25]. The corresponding
CIE coordinates (x = 0.317, y = 0.267) for the emission spectrum
of as-prepared Eu3+ doped precursor locate in the white light
region (point a, Fig. 9). For tetragonal NaY(WO4)2:Eu3+ obtained
at 800 ◦C for 2 h, the excitation spectrum (left, Fig. 6B) comprises a
broad band with the maximum at 284 nm and some weak peaks,
which are also assigned to the charge-transfer transitions within
7
can be ascribed to the 5D0 → FJ (J = 0, 1, 2) transitions of the Eu3+
ions, respectively [43–46]. On the basis of the above results, it is
2−
clear that the blue emission of WO4
groups and red emission
of the doped Eu3+ occur simultaneously and yield a white light

S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
145
Fig. 9. CIE chromaticity diagram showing the emission colors of (A) the as-prepared
Eu3+ doped precursor, (B) NaY(WO4)2:Eu, (C) NaY(WO4)2:Yb/Er, (D) NaY(WO4)2:
Yb/Tm, and (E) NaY(WO4)2:Yb/Ho. (For interpretation of references to color in the
text, the reader is referred to the web version of this article.)
In the photoluminescence decay curve of as-annealed 3D
flower-like NaY(WO4)2:Eu3+ microstructure (Fig. 7), the 5D0 → F2
7
transition of Eu3+ ions can be well fitted into a single exponential
function as I = I0 exp(−t/ꢂ), where ꢂ is the decay lifetime and the
lifetime was determined to be 0.955 ms.
UC luminescence properties. Fig. 8 shows the up-conversion
emissions of the NaY(WO4)2:3%Yb3+/1%Ln3+ (Ln = Er, Tm, Ho)
microstructures under 980 nm laser excitation. In the UC emis-
sion spectrum of NaY(WO4)2:Yb3+/Er3+ (Fig. 8A), the peaks at
525 and 545 nm of the green emissions correspond to the transi-
2
4
4
tions of H11/2 → I15/2
,
4S3/2 → I15/2, and the peak at 657 nm of
4
4
the weak red emission is due to the transition of F9/2 → I15/2
[42,47,48]. The corresponding CIE coordinates for the emission
spectrum of NaY(WO4)2:Yb3+/Er3+ are determined as x = 0.242,
y = 0.726, which locate in the green region (point c, Fig. 9). As
for NaY(WO4)2:Yb3+/Tm3+ (Fig. 8B), the peak at 475 nm of strong
blue emission and the peak at 643 nm of weaker red emission are
assigned to the transitions of 1G4 → H6 and 1G4 → F4, respec-
tively [21,49]. The corresponding CIE coordinates for the emission
spectrum of NaY(WO4)2:Yb3+/Tm3+ are determined as x = 0.129,
y = 0.108, which locate in the blue region (point d, Fig. 9). In
Figure for NaY(WO4)2:Yb3+/Ho3+, the peak at 547 nm of green emis-
3
3
Fig. 8. NIR-to-visible UC emission spectra of NaY(WO4)2 doped with (A) Yb/Er, (B)
Yb/Tm, and (C) Yb/Ho under 980 nm laser excitation.
sion and the peak at 658 nm of red emission are ascribed to the
2−
5
the WO4
groups and the characteristic f–f transitions of the
5S2 → I8 and 5F5
→
5I8 transition of Ho3+, respectively [50]. And
Eu3+ ions, respectively. The emission spectrum of NaY(WO4)2:Eu3+
the corresponding CIE coordinates for the emission spectrum of
NaY(WO4)2:Yb3+/Ho3+ are determined as x = 0.335, y = 0.647, locat-
ing in the yellow-green region (point e, Fig. 9).
(right, Fig. 6B) excited at 284 nm exhibits red emission, which is
dominated by 5D0 → F2 transition of Eu3+ ions. The correspond-
7
ing CIE coordinates for the emission spectrum of NaY(WO4)2:Eu3+
are determined as x = 0.430, y = 0.299, which locate in the red region
(point b, Fig. 9). From the above analysis, we find that the PL spectra
of NaY(WO4)2:Eu3+ greatly change as follows. First, the broad band
in the excitation spectrum shifts from 237 nm to 284 nm which
may be caused by the conversion from the distorted WO42− groups
in the precursor to refined ones in NaY(WO4)2. Secondly, only the
characteristic emission peaks of the Eu3+ ions are observed in the
emission spectrum of NaY(WO4)2:Eu3+, indicating that the energies
absorbed by the O W charge transfer transition can be transferred
effectively to the Eu3+ ions.
Fig. 10 presents the proposed energy transfer mecha-
nism of NaY(WO4)2:Yb3+/Er3+
,
NaY(WO4)2:Yb3+/Tm3+
,
and
NaY(WO4)2:Yb3+/Ho3+ under 980 nm excitation. Yb3+ ion is
2
2
excited by a 980 nm photon from F7/2 ground state to the F5/2
excited-state. For NaY(WO4)2:Yb3+/Er3+ (Fig. 10, left), Yb3+ ion
2
2
from F7/2 to F5/2 process promotes a Er3+ ion from ground state
4I15/2 to I11/2 by transferring the energy to Er3+ ion, and then
4
excites Er3+ ion to a higher F7/2 level when another Yb3+ ion at
4
excited F5/42 continuously transfers the energy to Er3+ ion. The
2
populated F7/2 level decays non-radiatively to the low-lying
2H11/2 and S3/2 states of Er3+ ions, thus producing emissions at
4

146
S. Huang et al. / Journal of Alloys and Compounds 529 (2012) 140–147
Fig. 10. The proposed energy transfer mechanisms under 980 nm laser excitation in NaY(WO4)2:Yb/Er, NaY(WO4)2:Yb/Tm, and NaY(WO4)2:Yb/Ho.
4
and the Fundamental Research Funds for the Central Universities
of China (HEUCF201210009).
525 and 545 nm with a radiative transition to ground state I15/2
,
respectively. Alternatively, Er3+ ion electron can further relax and
4
4
4
populate the F9/2 level, resulting in a red F9/2 → I15/2 emission
at 657 nm. In the case of NaY(WO4)2:Yb3+/Tm3+ (Fig. 10, middle), a
Tm3+ ion at 3H6 is firstly excited to 3H5 by an initial energy transfer
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4. Conclusions
In summary, we have reported an effective route to fabri-
cate novel self-assembled 3D flower-like NaY(WO4)2:Ln3+ (Ln = Eu,
Yb/Er, Yb/Tm and Yb/Ho) hierarchical microstructures via
a
hydrothermal process followed by further calcination treatment.
The possible mechanism for the conversion process has been
investigated in detail on the basis of a series of time-dependent
experiments. We believe that the synthetic strategy demonstrated
here can also be of much significance in the synthesis of many other
compounds. Furthermore, under UV excitation, the as-prepared
precursor and as-annealed NaY(WO4)2:Eu3+ microstructures emit
white and red emissions, respectively. Upon 980 nm NIR laser
excitation, Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ doped NaY(WO4)2
exhibit strong green, blue and yellow-green UC luminescence. The
as-prepared hierarchical structures with a novel morphology and
interesting optical property might have potential applications in
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This project is financially supported by the National Natural Sci-
ence Foundation of China (NSFC 20871035), Research Fund for the
Doctoral Program of Higher Education of China (20112304110021)

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