Controllable synthesis and tunable luminescence properties of Y 2(WO4)3:Ln3+ (Ln = Eu, Yb/Er, Yb/Tm and Yb/Ho) 3D hierarchical architectures

Article abstract of DOI:10.1039/c2dt30221g

Yttrium tungstate precursors with novel 3D hierarchical architectures assembled from nanosheet building blocks were successfully synthesized by a hydrothermal method with the assistance of sodium dodecyl benzenesulfonate (SDBS). After calcination, the precursors were easily converted to Y ₂(WO₄)₃ without an obvious change in morphology. The as-prepared precursors and Y₂(WO₄) ₃ were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra, respectively. The results reveal that the morphology and dimensions of the as-prepared precursors can be effectively tuned by altering the amounts of organic SDBS and the reaction time, and the possible formation mechanism was also proposed. Upon ultraviolet (UV) excitation, the emission of Y₂(WO₄)₃:x mol% Eu³⁺ microcrystals can be tuned from white to red, and the doping concentration of Eu³⁺ has been optimized. Furthermore, the up-conversion (UC) luminescence properties as well as the emission mechanisms of Y₂(WO₄) ₃:Yb³⁺/Ln³⁺ (Ln = Er, Tm, Ho) microcrystals were systematically investigated, which show green (Er³⁺, ⁴S_3/2, ²H_11/2 → ⁴I_15/2), blue (Tm³⁺, ¹G₄ → ³H₆) and yellow (Ho³⁺, ⁵S₂ → ⁵I₈) luminescence under 980 nm NIR excitation. Moreover, the doping concentration of the Yb³⁺ has been optimized under a fixed concentration of Er³⁺ for the UC emission of Y₂(WO₄)₃:Yb³⁺/Er ³⁺.

Full text of DOI:10.1039/c2dt30221g

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PAPER
Controllable synthesis and tunable luminescence properties of Y2(WO4)3:
Ln3+ (Ln = Eu, Yb/Er, Yb/Tm and Yb/Ho) 3D hierarchical architectures†
Shaohua Huang,a Xiao Zhang,a Liuzhen Wang,a Ling Bai,a Jie Xu,a Chunxia Lib and Piaoping Yang*a
Received 30th January 2012, Accepted 21st February 2012
DOI: 10.1039/c2dt30221g
Yttrium tungstate precursors with novel 3D hierarchical architectures assembled from nanosheet building
blocks were successfully synthesized by a hydrothermal method with the assistance of sodium dodecyl
benzenesulfonate (SDBS). After calcination, the precursors were easily converted to Y2(WO4)3 without
an obvious change in morphology. The as-prepared precursors and Y2(WO4)3 were characterized by
means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), and
photoluminescence (PL) spectra, respectively. The results reveal that the morphology and dimensions of
the as-prepared precursors can be effectively tuned by altering the amounts of organic SDBS and the
reaction time, and the possible formation mechanism was also proposed. Upon ultraviolet (UV)
excitation, the emission of Y2(WO4)3:x mol% Eu3+ microcrystals can be tuned from white to red, and the
doping concentration of Eu3+ has been optimized. Furthermore, the up-conversion (UC) luminescence
properties as well as the emission mechanisms of Y2(WO4)3:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystals
were systematically investigated, which show green (Er3+, 4S3/2, 2H11/2 → 4I15/2), blue (Tm3+, 1G4 → 3H6)
and yellow (Ho3+, 5S2 → 5I8) luminescence under 980 nm NIR excitation. Moreover, the doping
concentration of the Yb3+ has been optimized under a fixed concentration of Er3+ for the UC emission of
Y2(WO4)3:Yb3+/Er3+.
shape, size, and dimension of the materials, self-assembly driven
by various interactions is a fascinating and promising strategy to
1. Introduction
In recent years, the preparation of nano-/micro-materials with
novel and controllable morphologies has become an important
research issue due to not only the fundamental scientific interest
but also for their potential applications in various fields, such as
catalysis,1 optics,2 magnetism,3 optoelectronics devices,4 solar
cells,5 and biology.6–8 To date, many efforts have been devoted
to design and synthesize nano-/micro-materials with diverse
morphologies and sizes, such as nanofibers,9 nanowires,10 nano-
rods,11 nanocubes,12 microplates,13 and hierarchical architectures
due to their potential applications in a wide range of fields.14
Among these morphologies, constructing materials with hier-
archical three-dimensional (3D) architectures, self-assembled
from basic building blocks (such as 0D nanoparticles, 1D nano-
rods, and 2D nanoplates or nanosheets) with controllable super-
structures have attracted considerable interest for their potential
technology applications.15–17 As an effective way to control the
design 3D ordered hierarchical structures due to the variation of
the arrangements of the building blocks.18,19 Up to now, a large
number of synthetic techniques have been developed to prepare
a wide variety of inorganic 3D hierarchical architectures. For
example, sandwich-like La2O2CO3 were synthesized through a
homogeneous precipitation method,14 SnO2 flowers were pre-
pared by a chemical vapor deposition (CVD) method,20 and an
electrochemical process was employed for the synthesis of
FeOOH star-shaped structures.21 Although some progress has
been made in the self-assembly synthesis of hierarchical archi-
tectures, exploration of rational synthetic routes for controlled
construction of complex 3D architectures of other inorganic
functional materials via a chemical self-assembly route is still an
intensive and hot research topic.22 Furthermore, as a typical sol-
ution-based route, a hydrothermal process with great facility and
flexibility has been proved to be one of the most effective and
convenient approaches in achieving a variety of controllable
sizes and morphologies due to its mild reaction conditions, ease
of operation, tunable reaction parameters, and abundant organic
surfactants.23–26 In particular, various types of surfactants have
been widely used in the strategy to achieve morphology and size
control, because the functional groups of the organic additives
can preferentially adsorb on certain crystal facets and effectively
change the surface energies of the crystals, resulting in modulat-
ing the kinetics of the crystal growth.27
aKey Laboratory of Superlight Materials and Surface Technology,
Ministry of Education, Harbin Engineering University, Harbin, China.
E-mail: yangpiaoping@hrbeu.edu.cn
bState Key Laboratory of Rare Earth Resource Utilization, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30221g
5634 | Dalton Trans., 2012, 41, 5634–5642
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2.2. Preparation of drug storage/release system
As a series of important functional inorganic materials, rare
earth (RE) compounds have been extensively studied and
employed in various fields, including phosphors,28 polarisers,29
laser host materials,30 catalysis,31 solar and other functional
materials based on their electronic, optical, and chemical charac-
teristics,32 which originate from electron transitions within the 4f
shell and are highly sensitive to the compositions and structures
of the rare earth compounds.33 So far, a wide variety of rare
earth compounds, such as metal oxides,22 vanadates,34 phos-
phates,32 borates,35 molybdates,36 and tungstates, have been
extensively reported.17 Among these compounds, intensive
efforts have been concentrated on tungstate materials due to their
outstanding properties, which have promising applications in the
areas of negative thermal expansion materials37 and quantum
electronics and lasers.38 Recently, stimulated by both the promis-
ing applications and the interesting properties, much attention
has been paid to the photoluminescence properties of
Tb2(WO4)3 and Gd2(WO4)3.33,39 However, the preparation and
luminescent properties of Y2(WO4)3:Ln with 3D hierarchical
structures have received little attention, while intensive efforts
have been made in the fabrication of negative thermal expansion
materials of Y2(WO4)3 using the solid state method.40 As an
important organic surfactant, sodium dodecyl benzenesulfonate
(SDBS) has been extensively employed as a structure-directing
agent to kinetically control the growth rates of different crystal-
line faces by interacting with these faces through adsorption and
desorption.41–44 To the best of our knowledge, there has been no
systematic work about the effect of SDBS on the shape control-
lable synthesis of Y2(WO4)3 with uniform size and morphology,
and it has essential meaning to extend the synthetic strategy to
controllable fabrication of other inorganic compounds and
improve the performance in various fields.
In this paper, we report the morphology controllable synthesis
of yttrium tungstate precursors with novel and uniform 3D hier-
archical structures via a facile and mild hydrothermal route in the
presence of SDBS, and the corresponding Y2(WO4)3 3D archi-
tectures were obtained by a further heat treatment of the precur-
sors at 800 °C for 2 h. To investigate the formation mechanism
of the hierarchical superstructures, a series of time-dependent
experiments were carried out. In addition, the down-conversion
(DC) PL properties of the as-prepared precursors could be modu-
lated by altering the doping concentration of Eu3+, while the UC
emission properties of Y2(WO4)3:Ln3+ (Ln = Yb/Er) could be
optimized by tuning the doping concentration of Yb3+.
Especially, it is the first time to investigate the UC luminescence
properties of Y2(WO4)3 with a novel 3D hierarchical shape.
Y(NO3)3 was prepared by dissolving Y2O3 in dilute nitric acid,
then evaporating the water in the solutions by heating. In a
typical process for the synthesis of precursor, 0.5 mmol SDBS
was first dissolved in 35 mL deionized water to obtain a transpar-
ent solution. Then 1 mmol of Y(NO3)3 was added into the above
solution to form a white colloidal precipitate. After vigorous stir-
ring for 15 min, 1 mmol Na2WO4 was introduced to above reac-
tion system. The pH value of the mixed solution was
subsequently adjusted to a value of 5 by adding HNO3 (1 mol
L
−1) or NaOH (1 mol L−1). After additional agitation for
30 min, the as-obtained white colloidal precipitate was trans-
ferred into a 50 mL autoclave, sealed, and heated at 200 °C for
24 h. As the autoclave was naturally cooled to room temperature,
the precipitates were separated by centrifugation, washed with
deionized water and ethanol in sequence, and then dried in a
vacuum oven 60 °C for 12 h. The final product of Y2(WO4)3
was obtained followed by further calcination at 800 °C for 2 h.
Y2(WO4)3:Ln3+ (Ln = Eu, Yb/Er, Yb/Tm and Yb/Ho) were pre-
pared in a similar procedure except that Eu2O3 with Y2O3 and
Yb2O3/Er2O3, Yb2O3/Tm2O3, Yb2O3/Ho2O3 with Y2O3 were
the initial materials, respectively.
2.3. Characterization
Powder X-ray diffraction (XRD) measurements were performed
on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15°
min−1 in the 2θ range from 10–50°, with graphite-monochroma-
tized Cu Kα radiation (λ = 0.15405 nm). SEM images were
obtained on a scanning electron microscope (JSM-6480A, Japan
Electronics) equipped with an energy-dispersive X-ray spectrum
(EDS, JEOL JXA-840). Transmission electron microscopy
(TEM) and high-resolution transmission electron microscopy
(HRTEM) were performed on a FEI Tecnai G2 S-Twin instru-
ment with a field emission gun operating at 200 kV. Images were
acquired digitally on a Gatan multiple CCD camera. The down-
conversion (DC) excitation and emission spectra were taken on a
Hitachi F-4500 spectrofluorometer 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 para-
metric oscillator, Continuum Surelite, USA) as the excitation
source and detected by a photomultiplier tube (HAHAMATSU
R955) from 400–900 nm. Luminescence decay curves were
obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope
(1 GHz) using a 250 nm laser (pulse width = 4 ns, gate = 50 ns)
as the excitation source (ContinuumSunlite OPO). The X-ray
photoelectron spectra (XPS) were taken on a VG ESCALAB
MK II electron energy spectrometer using Mg Kα (1253.6 eV)
as the X-ray excitation source. Fourier transform infrared spec-
troscopy (FT-IR) was carried out in an ABB Bomen FTLA2000-
100 spectrometer using KBr pellets. Thermogravimetric analysis
and differential scanning calorimetry data were recorded with a
thermal analysis instrument (SDT 2960, TA Instruments, New
Castle, DE) at the heating rate of 10 °C min−1 in an air flow of
100 mL min−1. All the measurements were performed at room
temperature.
2. Experimental section
2.1. Materials
The raw rare-earth oxides, including Ln2O3 (Ln = Y, Eu, Yb, Er,
Tm, Ho) (99.99%), Na2WO4, sodium dodecyl benzenesulfonate
(SDBS), HNO3 and NaOH were purchased from Sinopharm
Chemical Reagent Co., Ltd. All chemicals are of analytical
grade and used directly without further purification.
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Fig. 1 XRD patterns of (A) the yttrium tungstate precursor, (B) the
Y2(WO4)3 sample after calcination and the standard data for Y2(WO4)3
(JCPDS No. 15-0447) as a reference.
3. Results and discussion
Fig. 2 XPS spectra of the yttrium tungstate precursor: (A) wide scan
spectrum, (B) W 4f, (C) Y 3d and (D) O 1s.
3.1. Phase and morphology
Fig. 1 shows the XRD patterns of the as-prepared yttrium tung-
state precursor and the corresponding product after calcination
treatment, respectively. Compared with all the standard XRD pat-
terns in JCPDS cards, the diffraction peaks of the precursors
cannot be indexed to a certain phase because the hydrothermal-
derived product may contain some hydrous compounds originat-
ing from the solution medium,45 as shown in Fig. 1A. In Fig. 1B
for Y2(WO4)3, it can be seen that all the diffraction peaks of the
sample can be well indexed to the pure phase of Y2(WO4)3
(JCPDS No. 15-0447), indicating that the precursor was comple-
tely converted to Y2(WO4)3 at 800 °C for 2 h. In addition, it
should be pointed out that the XRD pattern presents some differ-
ences in the relative intensities according to the standard JCPDS
card, suggesting the possibility of different preferential orien-
tation growth of the sample.46
To further confirm the surface composition and functional
groups of the yttrium tungstate precursor, the sample was sub-
jected to XPS and FT-IR measurements, respectively. Fig. 2 pre-
sents the survey XPS spectrum and respective elemental XPS
spectra of the as-prepared sample. It can be seen that the binding
energies of O (1s, 536 eV), Y (3d, 157 eV), W (4f, 36 eV) and
C (1s, 284 eV) can be obviously found, which further confirm
the existence of Y, Mo, and O in the sample. In the FT-IR spec-
trum of the precursor (Fig. S1, ESI†), the sharp peaks at
3556 cm−1 and the broad band at 3430 cm−1 are attributed to the
O–H vibration of the coordination water and the surface-
adsorbed water, respectively. The vibration band due to the
–CH2 group (2958, 1430 cm−1) may arise from the starting
material of SDBS. The peaks at about 962 cm−1 can be attribu-
ted to the WvO stretching vibration, while the peaks at 844 and
725 cm−1 are assigned to the W–O stretching vibration. In
addition, the thermal behavior of the as-prepared yttrium tung-
states precursor was investigated. From the TG curve (Fig. S2,
ESI†), it can be found that the total weight loss is about 4.02%
between 30 °C and 800 °C, which corresponds to the loss of all
non-coordinated and coordinated water molecules. On the basis
of the above analysis and the XRD pattern, we can reasonably
presume that the final Y2(WO4)3 sample may originate from the
yttrium tungstate precursor.
Fig. 3 (A, B, C) SEM images with different magnifications and (D, E,
F) TEM images with different magnifications of yttrium tungstate
precursor.
The morphology, dimension and size of the as-synthesized
yttrium tungstate precursor were investigated by SEM and TEM.
The low-magnification SEM image (Fig. 3A) displays a repre-
sentative overview of the sample, which indicates that the
sample consists of 3D bowknot-like hierarchical microstructures
with relatively uniform size distribution. From Fig. 3B, we can
see that the as-obtained 3D hierarchical structures have a typical
length of about 8 μm with a narrow size distribution. The high-
magnification SEM image (Fig. 3C) shows that the bowknot-like
microstructure is made up of numerous smooth rectangle 2D an-
isotropic nanosheet building blocks with a length of about 8 μm
and a width of about 2.5 μm. These nanosheets are self-
assembled in a separate and cross-linked way to construct the 3D
bowknot-like hierarchical architectures, which compactly bond
to each other in the middle and split in both sides of the struc-
tures. The typical TEM images (Fig. 3D and E) also indicate that
numerous nanosheets with very high density radially arrange
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Fig. 5 (A, B, C) SEM images with different magnifications, (D, E)
TEM images with different magnifications, (F) EDS spectrum, (G)
HRTEM image of Y2(WO4)3 sample annealed at 800 °C for 2 h.
Fig. 5 shows typical SEM, TEM, HRTEM images and EDS of
Y2(WO4)3 sample obtained at 800 °C for 2 h. In Fig. 5A, it is
obvious that the sample is composed of uniform bowknot-like
hierarchical microstructures in high yield, which is similar to the
precursor. Fig. 5B indicates that the surface of the annealed
sample is slightly rough in comparison with that of the precursor
due to the gradual elimination of H2O and CO2 during the calci-
nation processes. The magnified SEM image (Fig. 5C) shows
that Y2(WO4)3 has bowknot-like microstructures, which are self-
assembled from 2D anisotropic nanosheet building blocks.
Obviously, conversion did not lead to the change in the shape of
the final product due to the higher activation energies needed for
the collapse of these structures.47,48 The TEM image (Fig. 5D)
of Y2(WO4)3 indicates that the nanosheets are assembled in a
separate and cross-linked form which bond to each other in the
middle and split in the side of the structure. Fig. 5E displays the
TEM image of a horizontal bowknot-like sample, which also
reveals that the bowknot-like structure is self-assembled from
many 2D rectangle nanosheets building blocks with a slightly
rough surface. In the typical HRTEM image (Fig. 5E), the inter-
planar spacing (0.695 nm) of the lattice fringes corresponds to
the (020) plane of the orthorhombic Y2(WO4)3 phase. The EDS
spectrum (Fig. 5F) indicates the presence of Y, O, W elements in
the final product after annealing, which is consistent with the
XRD pattern. Fig. S3 (ESI†) presents SEM images of the precur-
sors prepared with 0.75 and 1.25 mmol SDBS and their corre-
sponding SEM images after annealing at 800 °C for 2 h. We can
see that the calcined products also inherit the morphologies of
their corresponding precursors whereas their surfaces are slightly
rougher than those of the precursors.
Fig. 4 SEM images with different magnification of yttrium tungstate
precursors prepared with SDBS amounts of (A, B) 0.5 mmol, (C, D)
0.75 mmol and (E, F) 1.25 mmol at 200 °C for 24 h.
from the center of the bowknot-like structures, which is consist-
ent with the SEM images. In the high-magnification TEM image
(Fig. 3F), it can be clearly found that the thickness of the
nanosheet building blocks is about 60 nm.
To validate the effect of SDBS on the crystal formation of the
products, a series of concentration-dependent experiments were
carried out in our current reaction system. Fig. 4 depicts the
typical SEM images of the samples prepared at 200 °C for 24 h
with different SDBS amount of 0.5, 0.75 and 1.25 mmol,
respectively. As shown in Fig. 4A and B, when 0.5 mmol SDBS
was introduced, uniform bowknot-like microstructures of about
8 μm in length are obtained, and the width of the splitting both
ends is about 2.5 μm. With the increase of the SDBS amount up
to 0.75 mmol, the as-prepared sample still keeps the 3D
bowknot-like hierarchical shape (Fig. 4C). From the high-mag-
nification SEM image (Fig. 4D), it is obvious that the product
obtained with 0.75 mmol SDBS is longer in ends length and has
more densely nanosheets building blocks compacted in the
middle of the structure, in which the average length and the
diameter of the splitting ends is about 8.5 and 6 μm, respectively.
When the SDBS amount is increased to 1.25 mmol, the mor-
phology of the product is converted to 3D hierarchical micro-
flowers with an average diameter of 4 μm (Fig. 5E). An individ-
ual particle (Fig. 5F) clearly reveals that the single 3D micro-
flowers architecture is composed of a large number of 2D
smooth nanoflakes building blocks in a radial way from the
center to the surface. On the basis of the above results, we can
reasonably infer that the SDBS amount plays an important role
in the morphology evolution of the as-prepared yttrium tungstate
precursors.
3.2. Possible growth mechanism
In order to investigate the detailed formation mechanism of the
novel bowknot-like hierarchical architectures precursors, we
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planes with low energy.49,50 When organic surfactants are intro-
duced during the crystal growth process, the relative order of
surface energies can be modified in that the organic molecules of
the surfactants can be selectively adsorbed on the surface of the
growing nanoparticles, and change the corresponding surface
energy of the bound plane, thus the growth rates of different
crystallographic directions are varied and lead to a change in the
final morphology. As an important organic surfactant, SDBS can
tailor the surface energy by capping the surface of nuclei and
control the morphology of the crystals. In addition, as previously
reported, in the SDBS organic molecules, the functional DBS−
group has a strong tendency to coordinate with inorganic metal
cations to form complexes through coordination interaction.51,52
Under the hydrothermal conditions of high temperature and
pressure, inorganic metal cations in the complex will be gradu-
2−
ally released when attacked by the free WO4 ions in the reac-
tion system, which can control the concentration of free
inorganic metal cations in the solution and thus control the
nucleation and growth of the crystals in view of the dynamic
process.16 In summary, on the basis of the present experimental
results and the previous literature, it is believed that SDBS may
have a double function in adjusting the final morphology of the
crystal. On the one hand, as a strong chelating ligand, when Y3+
ions were added in the solution containing SDBS, the functional
group DBS− will form a stable complex with Y3+ ions through
strong chelating interactions, which will slow down the nuclea-
tion and subsequent crystal growth of the Y2(WO4)3 precursor,
resulting in the formation of 3D microstructures with a narrow
size distribution. The Y3+–DBS− complexes can be confirmed
by the fact that a white precipitate is produced after directly
mixing the aqueous solution containing Y3+ and DBS−. Under
the proper hydrothermal conditions, the chelating of Y3+–DBS−
complexes will be weakened and gradually release Y3+ to
Fig. 6 SEM images of yttrium tungstate precursors with 0.5 mmol
SDBS at 200 °C for a reaction time of (A) 1 h, (B) 3 h, (C) 6 h, (D) 9 h,
(E) 12 h, and (F) 24 h.
conducted a series of time-dependent experiments, which are
given in Fig. 6. At the initial stage of 1 h, the as-prepared
sample is composed of numerous spherical nanoparticles
(Fig. 6A). When the reaction time is increased to 3 h, some ir-
regular nanorod-like particles with some aggregation are
obtained (Fig. 6B). With further increasing the reaction time to
6 h, a small amount of bundle-like structures formed by many
crossed nanosheets and a large number of nanosheets are found
(Fig. 6C). Therefore, it is believed that the final perfect bowknot-
like microstructure should originate from these nanosheets build-
ing blocks. As the reaction time is increased to 9 h (Fig. 6D), we
can see that more nanosheets continuously aggregate into the
petal of the crossed bundle-like structure, and the 3D bowknot-
like hierarchical architectures are gradually developed. At a reac-
tion time of 12 h, both ends of the bowknot-like structure
become dense and thick, almost no nanosheet building blocks
can be found (Fig. 6E). After the reaction time is further
increased to 24 h, the integrated bowknot-like microstructures
can be achieved except that the petals are thicker and more compact
than those of the sample prepared at 12 h, as shown in Fig. 6F.
On the basis of the above analysis and time-dependent exper-
imental results, we can reasonably infer that the formation of
such complicated microstructures is achieved via a hierarchical
self-assembly process, which is related to the function of the
anionic surfactant sodium dodecyl benzenesulfonate (SDBS). As
we know, the growth rate of a crystal face is determined by the
relative specific energy of each face. The fastest crystal growth
will occur in the direction perpendicular to the face with the
highest surface energy, resulting in the fast growing planes with
high energy disappearing to leave behind the slowest growing
2−
promote the formation of Y3+–WO4 precursor nuclei. On the
other hand, as a structure-directing reagent, during the crystal
stage, the DBS− molecules with long chains can be selectively
adsorbed on a certain active facet. Therefore, the surface energy
of the certain facet can be reduced and will become more
exposed in areas, while these facets with a higher surface energy
will vanish in the final shape. Then many well-defined
nanosheets with large scale exposed facets are formed via the
subsequent crystallization process, followed by the assembly of
these building blocks into 3D hierarchical architectures. With the
increase of reaction time, more nanosheets grow on the earlier
petals, and make them be more compact and bigger. With
increasing the amount of SDBS, more functional groups (DBS−)
take part in the chelating action of Y3+–DBS− complexes, and
more nanosheets self-assembly to the petal of the hierarchical
microstructure. This should be the reason that the overall mor-
phology changes from sparse to bushy bowknot-like architec-
tures and then to more compact flower-like structures with the
increase of the SDBS amounts. The whole process of the mor-
phology evolution of yttrium tungstate precursors is summarized
in Scheme 1.
3.3. Luminescent properties
As one kind of important solid-state materials, rare-earth tung-
states have been proved to be good host lattices for the
5638 | Dalton Trans., 2012, 41, 5634–5642
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Fig. 8 (A) Emission spectra and (B) CIE chromaticity diagram of
Y2(WO4)3 doped with different Eu3+ concentrations.
corresponds to the charge transfer absorption from the 2p orbitals
of the oxygen ligands to the 5d orbitals of the central tungsten
atoms in the WO42− groups,17 and a number of characteristic f–f
transition lines within the Eu3+ 4f6 configuration in the longer
wavelength region between 350 and 450 nm, which are assigned
Scheme 1 Schematic illustration for the whole formation process of
the yttrium tungstate precursor.
7
to the transitions from the F0 ground state to the Eu3+ excited
5
5
5
5
states of D4 (363 nm), G2 (383 nm), L6 (396 nm), and D3
(417 nm), respectively.55 The presence of the excitation peak of
2−
the WO4 groups in the excitation spectrum of Eu3+ indicates
2−
that there is an energy transfer from the WO4 groups to Eu3+
ions in the Eu3+ doped yttrium tungstate precursor. Upon exci-
2−
tation into the WO4 at 276 nm, the emission spectrum of as-
prepared precursor is composed of a series of sharp lines cen-
tered at 539, 583, 596 and 617 nm, which can be ascribed to the
7
5
7
5D1 → F1 and D0 → FJ (J = 0, 1, 2) transitions of the Eu3+
ions,56–62 and the D0 → F2 transition at 617 nm is the most
5
7
prominent group. Compared with the emission of Eu3+, the
2−
intrinsic blue emission at about 450 nm from WO4 groups is
2−
very weak, revealing an efficient energy transfer from WO4
groups to Eu3+. Fig. 7B shows the excitation and emission
spectra of Y2(WO4)3:7%Eu3+ obtained at 800 °C for 2 h. As
Fig. 7 Excitation and emission spectra of (A) yttrium tungstate precur-
shown, the excitation spectrum monitored by Eu3+ 5D0 → F2 at
7
sor doped with 7 mol% Eu3+, and (B) Y2(WO4)3:7 mol% Eu3+ sample.
618 nm comprises a broad band with the maximum at 264 nm
and some weak peaks, which are also assigned to the charge
luminescence of rare earth ions.33,39,53,54 Nevertheless, to the
best of our knowledge, the optical properties of Y2(WO4)3
doped with rare earth metal ions have rarely been reported.
Herein, the bowknot-like samples prepared with 0.5 mmol
SDBS were used as representative examples to investigate the
luminescence properties of Y2(WO4)3:Ln3+ (Ln = Eu, Yb/Er,
Yb/Tm, Yb/Ho). In addition, to obtain the maximum lumines-
cence intensity of the DC and UC luminescence, the doping con-
centrations of Eu3+ and Yb3+ in the hosts have also been
optimized. Especially, it is the first time investigating the UC
luminescence properties of Y2(WO4)3:Ln with 3D hierarchical
microstructures.
2−
transfer transitions within the WO4 groups and the character-
istic f–f transitions of Eu3+ ions, respectively. The emission spec-
2−
trum of Y2(WO4)3:7%Eu3+ obtained by excitation into WO4
groups at 264 nm contains exclusively the characteristic emis-
sions of Eu3+, which are mainly identical to the excitation spec-
trum of the precursor. In summary, it is clear that there are some
difference in the PL spectra of Eu3+ doped yttrium tungstate pre-
cursor and Y2(WO4)3:7%Eu3+ sample. First, the broad band in
the excitation spectra shifts from 276 nm to 264 nm, which may
be due to the conversion from the distorted WO42− groups in the
precursor to refined ones in Y2(WO4)3. Second, the PL intensity
of the Y2(WO4)3:7%Eu3+ is much stronger and the intensity of
2−
the broad WO4 groups can hardly be observed compared with
DC luminescence properties. Fig. 7 presents the PL excitation
and emission spectra of the yttrium tungstate precursor and the
corresponding Y2(WO4)3:7%Eu3+ after annealing at 800 °C for
2 h. In Fig. 7A, the excitation spectrum of yttrium tungstate pre-
cursor is obtained by monitoring the emission of the Eu3+ 5D0
those of the precursor, indicating that the energies transfer from
WO42− groups to Eu3+ ions is more efficient in Y2(WO4)3.
Fig. 8A gives the emission spectra of Y2(WO4)3:x mol% Eu3+
(x = 1–10) under 264 nm excitation. As shown, with an increase
of Eu3+ ions concentration up to 7%, the PL intensity of Eu3+
7
→ F2 transition at 617 nm. It can be seen that the excitation
2−
ions increases while the intensity of WO4 groups decreases
spectrum consists of two main features, a strong and broad band
due to the enhancement of the energy transfer probability from
from 200–350 nm with a maximum at 276 nm, which
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Fig. 9 Decay curves for (A) yttrium tungstate precursor doped 7 mol%
Eu3+, (B) Y2(WO4)3:7 mol% Eu3+ sample.
Fig. 11 (A) NIR-to-visible UC emission spectra of Y2(WO4)3:xYb3+
/
1%Er3+ doped with different Yb3+ concentrations (B) their correspond-
ing CIE chromaticity diagram.
Fig. 10A for Y2(WO4)3:Yb3+/Er3+, the three emission bands at
around 521 nm, 543 nm and 658 nm can be assigned to the ther-
2
4
4
4
4
4
malized H11/2 → I15/2, S3/2 → I15/2, and F9/2 → I15/2 tran-
sitions of Er3+, respectively.63–67 The corresponding CIE
coordinates for the emission spectrum of Y2(WO4)3:Yb3+/Er3+
are determined as x = 0.178, y = 0.766, locating in the green
region (point A, Fig. 10D). As for Y2(WO4)3:Yb3+/Tm3+
(Fig. 10B), the peak centered at 474 nm of blue emission,
643 nm and 693 nm of weaker red emission are ascribed to the
1
3
1
3
3
3
transitions of G4 → H6, G4 → F4 and F3 → H6 transitions
of Tm3+, respectively.68,69 The corresponding CIE coordinates
for the emission spectrum of Y2(WO4)3:Yb3+/Tm3+ are deter-
mined as x = 0.152, y = 0.141, which locates in the blue region
(point B, Fig. 10D). In the case of Y2(WO4)3:Yb3+/Ho3+
(Fig. 10C), the peak centered at 541 nm can be assigned to the
5
5S2 → I8 transition, and the peak at 660 nm may originate from
Fig. 10 NIR-to-visible UC emission spectra of (A) Y2(WO4)3:Yb3+
Er3+ (B) Y2(WO4)3:Yb3+/Tm3+ (C) Y2(WO4)3:Yb3+/Ho3+ under
980 nm NIR excitation, and (D) their corresponding CIE chromaticity
diagram.
/
5
5
the F5 → I8 transition.70 The corresponding CIE coordinates
for the emission spectrum Y2(WO4)3:Yb3+/Ho3+ are determined
as x = 0.397, y = 0.509, locating in the yellow region (point C,
Fig. 10D).
,
,
In order to further investigate the UC emission properties of
Y2(WO4)3:Yb3+/Er3+ 3D hierarchical microstructures, a series of
experiments with different doping levels of 1%Yb3+/1%Er3+, 2%
Yb3+/1%Er3+, 3%Yb3+/1%Er3+, and 4%Yb3+/1%Er3+ were
carried out to seek the optimal doping levels of Yb3+/Er3+. As
shown in Fig. 11A, the emission spectra have no obvious diver-
sification in shape and the major bands differ only in their inten-
sities as a function of Yb3+ doping concentration. Obviously,
when the doping concentration of Er3+ is fixed at 1%, the emis-
sion intensity increases with the increased Yb3+ concentration
from 1% to 3%, and then decreases with further raise of the
Yb3+ concentration. The results indicates that the Y2(WO4)3:3%
Yb3+/1%Er3+ microstructures have the highest emission intensity
under the same measurement conditions. When Yb3+ concen-
tration is increased from 1% to 3%, more Yb3+ ions become
available to furnish and transfer the energy to the Er3+, resulting
in the higher emission intensity. As the Yb3+ concentration is
further increased to a higher concentration of 4%, the Yb3+ ions
may act as trapping centers and dissipate energy nonradiatively,
because the interatomic distance of Yb3+ between Er3+ decreases
with the enhanced amount of Yb3+, which remarkably enhances
the probability of energy migration to the quenching center
caused by resonance transfer, resulting in the concentration-
dependent quenching.71 The respective CIE coordinates for the
2−
WO4
groups to Eu3+ ions and finally almost completely
convert to the characteristic emission of Eu3+. When the Eu3+
concentration is further increased to 10%, the emission intensity
of the Eu3+ ions is decreased due to the concentration quenching
of the Eu3+ ions. Fig. 8B presents the chromaticity diagram for
Y2(WO4)3:x mol% Eu3+ samples with different Eu3+ concen-
tration, which reveals that the emission colors can be tuned from
blue to white to red only by altering the doping concentration of
the Eu3+ ions. Notably, the CIE coordinates for the
Y2(WO4)3:3%Eu3+ sample are determined to be x = 0.323, y =
0.280, locating in the white light luminescence zone (point B,
2−
Fig. 8B), which may be caused by the blue emission of WO4
groups and red emission of the doped Eu3+ simultaneously.
In the PL decay curves of as-prepared 3D hierarchical yttrium
tungstate precursor and the corresponding Y2(WO4)3:7%Eu3+
(Fig. 9), the 5D0 → 7F2 transitions of Eu3+ ions can be well fitted
into a single exponential function as I = I0exp (−t/τ), where τ is
the decay lifetimes and the lifetimes were determined to be
0.368 and 0.497 ms, respectively.
UC luminescence properties. Fig. 10 presents the UC lumi-
nescent properties of the Y2(WO4)3:3%Yb3+/1%Ln3+ (Ln = Er,
Tm, Ho) microstructures under 980 nm NIR excitation. In
5640 | Dalton Trans., 2012, 41, 5634–5642
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View Article Online
By simply changing the SDBS amounts, we can tune the mor-
phology and dimension of yttrium tungstate precursors from 3D
hierarchical bowknot-like microstructure to flower-like micro-
structures. The Y2(WO4)3 sample can be obtained from the pre-
cursor by a simple annealing process without changing their
original morphologies. The crystal growth mechanism was pro-
posed for the formation of this hierarchical structure. Under UV
excitation, the as-annealed Y2(WO4)3:Eu3+ hierarchical micro-
structures emit white to red emissions by simply altering the
doping Eu3+ concentration. Upon 980 nm NIR laser excitation,
Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ doped Y2(WO4)3 exhibit
strong green, blue and yellow UC luminescence. Moreover, on
the basis of the green up-conversion emission performance, the
Yb3+ concentration doped in the Y2(WO4)3:Yb3+/Er3+ samples
has been optimized. The 3D hierarchical microstructures with a
novel morphology and tunable PL properties might have poten-
tial applications in various fields. We believe that the synthetic
strategy demonstrated here can also be extended to the controlla-
ble synthesis of other inorganic compounds by adjusting the
amount of surfactant.
Fig. 12 The proposed energy transfer mechanisms under 980 nm NIR
excitation in Y2(WO4)3:Yb3+/Er3+ Y2(WO4)3:Yb3+/Tm3+ and
Y2(WO4)3:Yb3+/Ho3+
,
.
UC emission spectra of Y2(WO4)3:Yb3+/Er3+ with different Yb3+
doping concentrations are given in Fig. 11B. The difference of
the CIE chromaticity values is due to the different green to red
intensity ratios for the samples.
The proposed UC mechanism in Y2(WO4)3:Yb3+/Ln3+ (Ln =
Er, Tm, Ho) is described in the energy diagram, as shown in
4
Fig. 12. When excited at 980 nm, the Yb3+ in the F7/2 ground
state can absorb the photon and promote to its excited 2F5/2 state.
Acknowledgements
3+
4
For Y2(WO4)3:Yb3+/Er3+ (Fig. 12, left), the ion from F7/2 to
4F5/2 process promotes a Er3+ ion from ground state 4I15/2 to 4I11/2
by transferring the energy to a Er3+ ion, and then the excited
Research Fund for the Doctoral Program of Higher Education of
China (20112304110021) and the Fundamental Research Funds
for the Central Universities of China (HEUCF201210009) are
greatly acknowledged.
4
Er3+ ion transits to a higher F7/2 level when another Yb3+ ion at
4
excited-state F5/2 continuously transfers the energy to the Er3+
4
ion. The populated F7/2 level decays non-radiatively to the low-
lying 2H11/2 and 4S3/2 states of Er3+ ions, and produces emissions
at 521 and 543 nm with a radiative transition to ground state 4I15/2
,
Notes and references
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5642 | Dalton Trans., 2012, 41, 5634–5642
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