Self-assembly and photoluminescence characterization of CaMo O4: Eu3+, Na+ superstructure via a facile surfactant-free hydrothermal method

Article abstract of DOI:10.1149/1.3530864

The Eu³⁺, Na⁺ -codoped CaMo O₄ microphosphors were successfully synthesized at low temperature via a facile hydrothermal method in a surfactant-free environment. Scanning electron microscopy, field-emission scanning electron microscope, and transmission electron microscopy images of the CaMo O₄ products prepared at 150°C for 6 h revealed three dimensional flake-ball and flake-disk superstructures, composed of densely packed nanoflakes. The formation mechanism of CaMo O₄ microstructures was discussed in detail based on the hydrothermal temperature. Meanwhile, other influencing factors on the morphology were also reported. Room-temperature photoluminescence properties of microsized CaMo O₄: Eu³⁺, Na⁺ phosphors were studied. Its excitation wavelengths ranging from 350 to 530 nm in the ultraviolet and visible regions significantly extend the excitation region of phosphor materials. In addition, the effect of reactional temperature, pH value, and Ca source on the photoluminescence properties was also studied systematically.

Full text of DOI:10.1149/1.3530864

K74
Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
0013-4651/2011/158͑3͒/K74/7/$28.00 © The Electrochemical Society
Self-Assembly and Photoluminescence Characterization of
CaMoO4:Eu3+,Na+ Superstructure via a Facile Surfactant-Free
Hydrothermal Method
Yongqin Zhou,a Jie Liu,a Xuyong Yang,b Xibin Yu,a,z and Litong Wanga
aDepartment of Chemistry, Shanghai Normal University, Shanghai 200234, People’s Republic of China
bSchool of Electrical and Electronic Engineering, Nanyang Technological University,
Nanyang Avenue 639798, Republic of Singapore
The Eu3+,Na+-codoped CaMoO4 microphosphors were successfully synthesized at low temperature via a facile hydrothermal
method in a surfactant-free environment. Scanning electron microscopy, field-emission scanning electron microscope, and trans-
mission electron microscopy images of the CaMoO4 products prepared at 150°C for 6 h revealed three dimensional flake-ball and
flake-disk superstructures, composed of densely packed nanoflakes. The formation mechanism of CaMoO4 microstructures was
discussed in detail based on the hydrothermal temperature. Meanwhile, other influencing factors on the morphology were also
reported. Room-temperature photoluminescence properties of microsized CaMoO4:Eu3+,Na+ phosphors were studied. Its excita-
tion wavelengths ranging from 350 to 530 nm in the ultraviolet and visible regions significantly extend the excitation region of
phosphor materials. In addition, the effect of reactional temperature, pH value, and Ca source on the photoluminescence properties
was also studied systematically.
© 2011 The Electrochemical Society. ͓DOI: 10.1149/1.3530864͔ All rights reserved.
Manuscript submitted October 4, 2010; revised manuscript received December 2, 2010. Published January 13, 2011.
The close ionic radius among Eu3+ ͑0.95 Å͒, Na+ ͑0.97 Å͒, and
Ca3+ ͑0.99 Å͒ makes possible the easy substitution of Na+ and Eu3+
for Ca3+. Meanwhile, adding Na+ into the CaMoO4:Eu3+ scheelite
structure also affects the structure and crystallization and, thereby,
causes wide-ranging effects on the photoluminescence ͑PL͒ proper-
ties of the CaMoO4.16,17
In this paper, we successfully synthesized 3D europium and so-
dium codoped calcium molybdate superstructures via a facile hydro-
thermal method with no surfactant, catalysts, and templates at low
temperature. The structure and luminescent properties of self-
assembly CaMoO4:Eu3+,Na+ microstructures with specific morphol-
ogy and novel physical properties were systematically reported. It
has been proved that the formation of CaMoO4 flake-ball and flake-
disk microstructures is affected by reaction temperature, pH value,
and Ca source. In addition, the photoluminescence spectra of as-
prepared CaMoO4:Eu3+,Na+ microstructures demonstrate bright red
fluorescence, and its excitation wavelength ranging from
350 to 530 nm fits well with commercial near-UV GaN chip
͑380–410 nm͒ and blue light-emitting diode chips ͑450–480 nm͒
based on WLEDs. Moreover, comparison of the emission intensity,
the optimal pH value, and Ca source are made.
In recent years, large-scale self-assembled micro- and nanopar-
ticles in phosphors have attracted great attention due to their ease of
processing into devices with high resolution, high screen coverage,
intense emission, and long service life.1 Herein, it is critical to have
fine control over the structures and dimensions of micro- and nano-
particles which largely determine the properties and applications. At
present, the conventional solid-state reaction technique, including
cumbersome grinding and firing steps, normally results in irregular
morphology and large grain size because of the high reaction
temperature.2-4 Therefore, various fabrication methods have been
proposed to reduce the reaction temperature and obtain high-quality
micro- and nanostructures for decades. Among them, the frequent
one is the chemical solution route, which synthesizes the size- and
shape-controlled micro- and nanostructures by using appropriate or-
ganic additives, such as surfactants, templates, and polymers as cap-
ping agents, stabilizing agents, modifying agents, directing agents,
and soft templates.5-11 However, these organic additives are usually
difficult to effective removal and the most important is that the
utilization of the organic additives increases the production cost,
which restrict the popularization of the techniques in the further
research and practical applications. So a facile surfactant-free
method is highly preferred and desired to synthesize one dimen-
sional ͑1D͒, two dimensional ͑2D͒, and three dimensional ͑3D͒
micro- and nanoparticles.
Experimental Section
As an important class of lanthanide inorganic compounds, rare-
earth ions doped molybdates have gained much attention due to their
attractive luminescence and structural properties, supporting various
promising applications as phosphor materials in fields such as white
light-emitting diodes ͑WLEDs͒, optical fibers, biolabel, lasers, and
so on.12,13 The molybdate family have promising trivalent cation
conducting properties and most of the optical properties result from
electron transitions of the 4f shell, which are greatly affected by the
composition and structures of rare-earth compounds.14 Recently,
Eu3+-doped CaMoO4 has been investigated extensively as a red-
Chemicals
.— Sodium molybdate ͑Na2MoO4͒, ammonium mo-
lybdate ͑͑NH4͒6Mo7O24·4H2O͒, europium oxide ͑Eu2O3͒, nitrate
acid ͑HNO3͒, sodium hydroxide ͑Na͑OH͒͒, calcium nitrate
͑Ca͑NO3͒2͒, and calcium chloride ͑CaCl2͒ were purchased from
commercial sources ͑analytical-grade͒. 0.2 M Na2MoO4 solution,
0.12 M Ca͑NO3͒2 solution, and 0.12 M CaCl2 solution were pre-
pared by dissolving the corresponding Na2MoO4·4H2O, Ca͑NO3͒2,
and CaCl2 in distilled water at room temperature, respectively.
0.04 M Eu͑NO3͒3 solution was prepared by dissolving the corre-
sponding Eu2O3 in nitrate acid and excess HNO3 was removed by
evaporation and dilution in distilled water. 1 M HNO3 and 1 M
Na͑OH͒ solutions were prepared by dissolving the corresponding
nitrate acid and Sodium hydroxide in distilled water.
7
emitting phosphor, originating from 5D0 → F2 transition of Eu3+
5
upon near-ultraviolet ͑UV͒ excitation into the L6 state of Eu3+ at
395 nm. Compared with the well-known red phosphors Y2O2S:Eu3+
and CaS:Eu2+, molybdates also exhibit more stable physical and
chemical properties.15 Moreover, a few efforts have been devoted to
improve luminescence efficiency of molybdate phosphors. For in-
stance, Na+ has been introduced into different phosphor host lattices
to act as a coactivator and achieve charge compensation function.
Synthesis procedures
.— The CaMoO4 micro- and nanostructures
were synthesized via a simple hydrothermal method. The required
device is a 50 mL Teflon-lined stainless steel autoclave. The detailed
description of reaction procedure is as follows: The Eu͑NO3͒3
͑0.04 M͒ solution was dropped into 0.12 M Ca͑NO3͒2 solution to
form the precursor Ca͑NO3͒2:Eu3+ solution under vigorous stirring.
Meanwhile, the given amounts of diluted nitric acid ͑1 M HNO3͒
z E-mail: xibinyu@shnu.edu.cn
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Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
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Figure 2. ͑a͒ and ͑b͒ SEM images and ͑c͒ and d͒ FE-SEM images of as-
prepared typical CaMoO4 microstructures at different magnifications.
Figure 1. The XRD pattern of as-synthesized sample at pH 1.5 and 150°C
for 6 h.
of 5 ␮m. As shown in Fig. 2c and 2d, from FESEM images of
individual flake-ball and flake-disk, it can be concluded that such
superstructures are composed of densely packed nanoflakes with an
average 400 nm width and about 40 nm thickness ͑inset of Fig. 2c
and 2d͒.
and sodium hydroxide solution ͑1 M Na͑OH͒͒ were added to regu-
late the pH value. Then, the Na2MoO4 solution was slowly dropped
into Ca͑NO3͒2:Eu3+ solution under vigorous stirring. The vigorous
stirring was continued for 45 min. Finally, the above solution was
transferred into a 50 mL Teflon-lined stainless steel autoclave,
which was subsequently sealed and maintained at 60–200°C for 6 h.
Later, the autoclave was naturally cooled to room temperature. The
products were collected by centrifugation and washing several times
with distilled water and absolute ethanol. Then, these samples were
obtained after being dried at 80°C for 6 h. The Eu3+-doped CaMoO4
synthesized by replacing Na2MoO4 with ͑NH4͒6Mo7O24·4H2O and
the synthesis procedure where CaCl2 was used as Ca source was
exactly the CaNO3 at pH 1.5 and 150°C for 6 h.
The microstructure of as-prepared CaMoO4:Eu3+,Na+ samples
was further investigated with TEM and selected-area electron dif-
fraction ͑SAED͒. As shown in Fig. 3a-3d, TEM micrograph of a
single flake-disk with a diameter of about 5 ␮m. Observing the edge
of the flake-disk, the nanoflakes are very thin and, therefore, rela-
tively transparent to the electron beam ͑inset of Fig. 3a͒. The high-
resolution TEM ͑HRTEM͒ image and SAED pattern, taken from the
edge of a randomly chosen nanoflake ͑Fig. 4͒ of CaMoO4 micro-
structure after prolonged ultrasonic treatment, are shown in Fig. 3b
and 3c, respectively. HRTEM image shows well-defined lattice
fringes with an interplanar spacing of 0.2532 nm for the ͑200͒ face
of tetragonal CaMoO4 crystals, indicating the single-crystal nature
of nanoflake. The corresponding SAED pattern ͑Fig. 3c͒ with regu-
lar dotted lines can be indexed to the tetragonal CaMoO4 structure
and confirms the nanocrystals grow preferentially along the ͑100͒
face.18,19 Here, this can be explained from the viewpoint of the
intrinsic structure feature of the CaMoO4. If we view the ͑100͒ face
Characterization.— The phase and purity of the samples were
characterized by X-ray diffraction ͑XRD͒ ͓Rigaku DMAX 2000
powder diffractometer equipped with Cu target radiation resource
͑␭ = 0.15405 Å͔͒. The morphology and structure of the products
were characterized by scanning electron microscopy ͑SEM, JEOL
JEM-6460͒, field-emission scanning electron microscopy ͑FESEM,
Hitachi S-4800͒ with an X-ray energy dispersive spectrometer
͑EDS͒, and transmission electron microscopy ͑TEM, JEOL JEM-
2100, using an accelerating voltage of 200 kV͒. The luminescent
properties were recorded on a Varian Cary-Eclipse 500 fluorescence
spectrophotometer equipped with a 60 W xenon lamp as the excita-
tion source.
Results and Discussion
Structural characterization of calcium molybdate micro-
crystal.— The phase and purity of as-synthesized samples were de-
termined by the XRD patterns. Figure 1 shows the XRD pattern of
the product obtained by hydrothermal method at pH 1.5 and 150°C
for 6 h. All diffraction peaks can be perfectly indexed to the stan-
dard JCPDS no. 29-0351 of CaMoO4 tetragonal phase, belonging to
space group I41/a, and with lattice parameters of a = 5.226 Å and
c = 11.430 Å. The strong and sharp diffraction peaks prove good
crystallinity in the as-synthesized product. No additional peaks can
be found in this diffractogram, indicating that the hydrothermal
products with high phase purity can be easily obtained under the
current synthetic conditions.
The morphology and structure of the products were investigated
by SEM and FESEM images. Figures 2a and 2b show the SEM
images of CaMoO4 samples prepared by hydrothermal method at
pH 1.5 and 150°C for 6 h. We can clearly observe that there are two
types of flake-ball and flake-disk superstructures in the CaMoO4
product and the particles are monodisperse with an average diameter
Figure 3. ͑a͒ TEM image of a single flake-disk microstructure. ͑b͒ and ͑c͒
HRTEM image and SAED pattern of single nanoflake. ͑d͒ EDS spectrum of
the CaMoO4:Eu3+,Na+ superstructure.
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Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
Figure 4. TEM image of a single nanoflake of CaMoO4 superstructure after
prolonged ultrasonic treatment.
of CaMoO4 structural along the a-axis, it is known that all the nega-
tive ͓MoO24−͔ chains lied on the ͑100͒ face are exposed outwardly,
while all the positive Ca2+ chains are shielded. In particular, the
͑100͒ face is perpendicular to the a-axis with the highest chemical
potential among all the facets, such as ͑001͒ and ͑010͒. This struc-
tural feature makes the ͑100͒ face very active to the surrounding
growth conditions, so the nanocrystals grow preferentially along the
͑100͒ plane.20
Figure 5. SEM images and XRD patterns of CaMoO4:Eu3+,Na+ phosphors
synthesized at ͑a͒ 60°C, ͑b͒ 60°C, ͑c͒ 100°C, ͑d͒ 150°C, and ͑e͒ 200°C for
6 h.
The chemical compositions of flake-ball and flake-disk micro-
structures were determined by EDS. The EDS analysis ͑Fig. 3d͒
shows strong peaks from Ca, Mo, and O elements and weak peaks
from Na and Eu elements. No impurities were observed from the
spectra, revealing the high purity of these microstructures. Quanti-
tative analysis shows that the atom ratio of Ca:Mo:O:Na:Eu is about
aggregation and self-assemble layer by layer into compacted flake-
ball and flake-disk morphologies ͑Fig. 5c͒. While the reaction tem-
perature was held at 150°C, the plumpy and round microstructures
indicate a large development of the flake-ball and flake-disk mor-
phologies, with an average diameter of 5 ␮m ͑Fig. 5d͒. When the
hydrothermal temperature was increased to 200°C, the nanoflakes of
the CaMoO4 superstructures seem to melt into one phase and the
products become 3D slippery microparticles ͑Fig. 5e͒.
Figure 5f is the corresponding XRD patterns of
CaMoO4:Eu3+,Na+ crystals obtained at different reaction tempera-
tures for 6 h; all diffraction peaks of the products can be indexed to
pure CaMoO4 phase, according to JCPDS no. 29-0351. With the
increase in hydrothermal temperatures, the intensity of the diffrac-
tion peaks is gradually enhanced, showing the improvement in crys-
tallinity of as-prepared phosphors.
0.8:1:4:0.1:0.1
which
agrees
with
the
formula
Ca0.8MoO4:0.1Eu3+,0.1Na+ in which the concentration of Na+ is
equal to that of Eu3+ to achieve charge compensation.
Growth mechanism of the superstructure.— The
formation
mechanism of the superstructures was discussed on the basis of
temperature-dependent experiments. As shown in Fig. 5, these
samples prepared at different temperatures ͑60–200°C͒ for 6 h ͑the
pH value maintained 1.5͒ display the interesting morphologies of
nanoflakes, self-assembly flake-ball and flake-disk, and slippery mi-
croparticles, respectively. When the reaction was carried out at
60°C, the product is composed of 2D nanoflakes with an average
9 ␮m width and about 20 nm thickness ͑Fig. 5a͒. At 80°C, it con-
sists of some short flakelike aggregation and parallel and inter-
crossed morphologies coexist in the sample ͑Fig. 5b͒. When the
hydrothermal temperature was 100°C, the nanoflakes began directed
In general, the crystal growth is controlled by the intrinsic and
extrinsic factors, such as the degree of supersaturation, solution pa-
rameters, diffusion of the reaction, surface energy, crystal structure,
etc.8 We believe crystal nucleation is very fast and subsequent crys-
tal growth might be better described by Ostwald-ripening process
Scheme 1. Formation mechanism of CaMoO4 superstructures.
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Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
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saturation at different temperatures. The freshly formed nuclei have
a strong tendency to grow into larger particles to reduce the surface
energy. Then, the CaMoO4 nuclei gradually grow into flakelike
nanoparticles ͑Scheme 1b͒ through selective adsorption and desorp-
tion on various faces at different rates. This phenomenon may be
explained by the Ostwald ripening process, due to the difference of
surface energy of various particles and crystal faces. Here, reduction
in surface energy is the important driving force for crystal growth
and morphology evolution.19 Subsequently, the nanoflakes tend to
aggregate face to face to reduce the surface energy of the CaMoO4
system. It is worth pointing out that the round morphology is ben-
eficial to reduce the surface energy in the context of
thermodynamics.21,22 So, layer-by-layer self-assembly process, by
the directed aggregation along the specific crystal face, follows a
round growth style that lead to the development of flake-ball and
flake-disk microstructures, which is confirmed by the evolvement of
flake-ball and flake-disk CaMoO4 microcrystals ͑Scheme 1 C
→ D → E͒.
Influencing factors on the morphology and structure.— Trans-
formation of CaMoO4:Eu3ϩ,Naϩ morphology by pH-controlled
surfactant-free route.— The effect of pH values on the structure and
morphology of CaMoO4:Eu3+,Na+ were prepared without any sur-
factant being studied. The CaNO3:Eu3+ solution was adjusted to the
desired pH values by adding HNO3 and NaOH solution, and then
was hydrothermally treated at 150°C for 6 h. Figures 6a-6e show
the morphological evolution of CaMoO4:Eu3+,Na+ samples synthe-
sized at different pH values of the precursor solution. When the pH
value is 1.5, 3D flake-ball and flake-disk superstructures of
CaMoO4:Eu3+,Na+ sample are formed and the nanoflakes serve as
the building units of the CaMoO4:Eu3+,Na+ superstructures, as
shown in Fig. 6a. When the pH value is adjusted to 4.5, the flake-
ball and flake-disk microstructures start the aggregated process ͑Fig.
6b͒. Subsequently, when the pH value of the precursor solution
reaches 6.0, the large masses are obtained ͑Fig. 6c͒. Moreover, with
increasing pH values, the flakelike building units disappear and the
nanoflake self-assembly superstructures gradually disintegrate,
which is clearly shown in Fig. 6d and 6e.
It is obvious in our case that the pH value of the precursor solu-
tion greatly affects the structure and dispersivity of the final product
CaMoO4:Eu3+,Na+. When the reaction occurs at low pH values ͑1–
4.5͒ the concentration of H+ ions is much higher than that of OH−
ions, which restrains the hydrolysis of the Ca2+. Thus, the nucleation
rate of CaMoO4:Eu3+,Na+ is much faster than the rate of crystal
growth due to the existence of a large quantity of Ca2+. Large quan-
tities of CaMoO4:Eu3+,Na+ nuclei tend to aggregate together to
form larger and more thermodynamically stable particles. Subse-
quently, Ostwald-ripening growth and self-assembly take place,
which leads to the formation of CaMoO4:Eu3+,Na+ superstructures
constructed from nanoflake substructures ͑Fig. 6a and 6b͒. With the
Figure 6. SEM images and XRD patterns of the samples synthesized at
different pH values ͑hydrothermally treated at 150°C for 6 h, no surfactant͒:
͑a͒ pH 1.5, ͑b͒ pH 4.5, ͑c͒ pH 6.0, ͑d͒ pH 7.5, and ͑e͒ pH 9.0.
and aggregation mechanism. Ostwald-ripening process expounds the
crystal growth route by growth of larger particles at the cost of
smaller particles according to the well-known Gibbs–Thomson
equation.20 The aggregation mechanism by oriented attachment or
random aggregation involves self-assembly crystal growth process,
which is an efficient and often preferred manner to build micro- and
nanoparticles into ordered multidimensional macroscopic structure.
In the formation of the as-prepared CaMoO4:Eu3+,Na+ super-
structures, we believe the self-assembly process plays a key role in
this process. On the basis of the above analysis, a possible arranging
mechanism of the influence of the reaction temperature on the mor-
phology in the hydrothermal process is schematically shown in
Scheme 1. In the heating process, CaMoO4:Eu3+,Na+ nuclei were
generated via the reaction between Ca2+/Eu3+/Na+ and MoO42− when
the CaMoO4:Eu3+,Na+ solution achieves the given degree of super-
Figure 7. SEM image ͑left͒ and XRD pattern ͑right͒ of the prepared sample using CaCl2 as Ca source.
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Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
Figure 10. Emission spectra of as-prepared samples and relative intensity of
the 5D0 → 7F2 transition in the inset at different temperature.
Figure 8. The PLE ͑monitoring under 615 nm͒ and PL ͑under 465 nm ex-
citation͒ spectra of Eu3+-doped and Eu3+,Na+-codoped CaMoO4 samples,
prepared at pH 1.5 and 150°C for 6 h.
diffraction peaks of the XRD pattern shown in Fig. 7 ͑right͒ are in
well accordance with the standard pattern ͑JCPDS no. 29-0351͒ of
CaMoO4.
increasing pH values, the concentration of Ca2+ ions is greatly re-
Photoluminescence properties.— Photoluminescence properties
of calcium molybdate superstructures.— Room-temperature photo-
luminescence properties of the as-obtained samples are studied. Fig-
ure 8 shows the photoluminescent excitation ͑PLE͒ and PL spectra
of Eu3+-doped and Eu3+,Na+-codoped CaMoO4 superstructures pre-
pared at pH 1.5 and 150°C for 6 h via hydrothermal method. Com-
parison with Eu3+-doped sample, Eu3+,Na+-codoped CaMoO4
samples play more stronger photoluminescence intensity, where Na+
acts as a charge compensator. This can be explained by charge com-
duced due to the hydrolysis of the Ca2+
. The rate of
CaMoO4:Eu3+,Na+ nucleation is then depressed, which also slows
down the intrinsic anisotropic growth and the aggregation of
CaMoO4:Eu3+,Na+ nuclei during the hydrothermal process. There-
fore, the nanoflake self-assembly superstructures disintegrate and
gradually disappear, and the flakelike building units are replaced by
nanoparticles ͑Fig. 6d and 6e͒.23 XRD diffraction peaks shown in
Fig. 6f suggest that at different pH values the prepared samples are
all consistent with the tetragonal CaMoO4 phase. Simultaneously,
the crystallinity of CaMoO4 phosphors gradually becomes weaker as
the pH values increase.
pensation in the phosphor system CaMoO4:Eu3+ 16 From Supporting
.
Information Fig. 9, we can observe the undoped CaMoO4 super-
structure sample emits blue light around 480 nm and is excitated
only under UV region. Moreover, from Refs. 16, 17, and 24, it can
be seen that the excitation wavelengths of the bulk crystals of rare-
earth doped CaMoO4 products are generally up to 490 nm. For the
codoped samples in our case, the excitation spectra monitoring
615 nm emission line are composed of an intense broad band and
Using CaCl2 instead of CaNO3.— Keeping the other reaction con-
ditions used before, replacing only CaNO3 with CaCl2 ͑0.2 M aque-
ous solution͒ as the Ca source, it was observed the formation of
ellipselike microstructure, as shown in Fig. 7 ͑left͒. The ellipselike
structure has lengths of about 3 ␮m and diameters of 1 ␮m. All
some sharp lines in the range of 350–530 nm which is assigned to
15
the combination of the f–f transition of O2−–Mo6+ and O2−–Eu3+
.
The effective excitation peaks at about 395 and 465 nm, ascribed to
5
5
the 7F0 → L6 and 7F0 → D2 transitions of Eu3+, respectively,
Figure 9. The PL and PLE spectra of undoped CaMoO4 samples prepared at
pH 1.5 and 150°C, respectively, for 6 h.
Figure 11. Relative intensity of 5D0 → 7F2 transition of as-prepared samples
at different pH values: 1.5, 4.5, 6, 7.5, 9.
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Journal of The Electrochemical Society, 158 ͑3͒ K74-K80 ͑2011͒
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intensity. As the pH values increase, the PL intensity is gradually
weaker, which is in accordance with the crystallinity of the as-
prepared phosphors ͑Fig. 6f͒. Hence, luminescence property is
highly sensitive to the morphology changes of CaMoO4 phosphors.
Figure 12 shows the PL spectra under 465 nm excitation using dif-
ferent Ca source ͑the precursor solution fixed pH = 1.5, 150°C for
6 h͒. Obviously, the luminescent intensity of utilization of CaNO3 is
stronger than that of CaCl2. In this case, we can find the nanoflakes
self-assembly flake-ball and flake-disk superstructures prepared by
CaNO3 are different from the ellipselike microstructures prepared
by CaCl2 at morphology, superstructure and particle sizes ͑shown in
Fig. 2 and 7͒. Therefore, these factors together promote the forma-
tion of different emission properties.
Conclusions
In summary, we have provided a convenient, environment
friendly, non-surfactant assisted and low-cost approach for preparing
self-assembly
3D
superstructure.
The
self-assembly
CaMoO4:Eu3+,Na+ superstructures with an average diameter of
5 ␮m, composed of multilayered nanoflakes, are successfully syn-
thesized, presenting specific morphology and physical properties.
The formation mechanism of CaMoO4 superstructures mainly fo-
cuses on the aggregation behavior of CaMoO4 nanoflakes as a func-
tion of the heating temperature. Meanwhile, the pH value and Ca
source also have effect on the structure and dispersivity of the
CaMoO4 products. In the PLE spectra, the excitation wavelengths,
ranging from 350 to 530 nm in the UV and visible region, signifi-
cantly extend the excitation region of photoluminescent materials.
The PL spectra indicate the products emit bright red fluorescence
and have high color purity. In addition, we also conclude the pH
value of 1.5 and CaNO3 as Ca source are the optimal choice for the
best optical properties of these materials.
Figure 12. Emission spectra of as-prepared samples using different Ca pre-
cursors.
agree well with the commercial near-UV and blue light-emitting
diodes.7,16 Under 465 nm excitation, all the PL spectra consist of
two main emission peaks at about 595 and 615 nm in the region of
7
550–650 nm, corresponding to the characteristic 5D0 → F1 and
7
5D0 → F2 transitions of Eu3+, respectively. As it is well known,
CaMoO4 has scheelite structure in which the Mo metal ion is sur-
rounded by four oxygen atoms in tetrahedral symmetry ͑Td͒, and the
large cations ͑Ca2+/Eu3+/Na+͒ are bounded to eight oxygen atoms
from different tetrahedra. Among these peaks, the predominated red
emission at about 615 nm is corresponding to the characteristic
7
5D0 → F2 electric dipole transition of Eu3+ and results from the
Acknowledgment
noncentrosymmetrical sites for the occupying Eu3+. The orange
7
The authors are grateful for financial support from the Innovation
Program of Shanghai Municipal Education Commission ͑10YZ70,
09ZZ136͒, Science Foundation of Shanghai Normal University
͑SK201002͒, Shanghai Science and Technology Development Fund
͑no. 09520500500͒ , and the Key Laboratory of Resource Chemistry
of Ministry of Education of China.
emitting one at about 595 nm is assigned to the 5D0 → F1 mag-
netic dipole transition of Eu3+, which is insensitive to the site
symmetry.17 The lack of centrosymmetry of Eu3+ sites in
5
7
CaMoO4:Eu3+,Na+ implies that the D0 → F2 transition has much
7
stronger emission intensity than the 5D0 → F1 transition, as it is
shown in Fig. 8. The Commission Internationale del’Eclairage ͑CIE͒
chromaticity coordinates under 465 nm excitation are ͑0.65, 0.35͒
with high color purity and are closer to the NTSC standard values
͑0.67, 0.33͒ and Y2O2S:Eu3+ ͑x = 0.63, y = 0.35͒.24
Shanghai Normal University assisted in meeting the publication costs of
this article.
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