Lanthanide ions-doped calcium molybdate pie-like microstructures: Synthesis, structure characterization, and luminescent properties

Article abstract of DOI:10.1016/j.jallcom.2016.11.355

Lanthanide ions doped CaMoO₄with pie-like microstructures were synthesized by a hydrothermal method using phenol-formaldehyde resin (PF) as an inducer. The results of powder X-ray diffraction showed a tetragonal phase of both undoped and Ln³⁺-doped CaMoO₄. The microscopical characterization technology revealed the formation process of the pie-like microparticles. As a morphology inducer, the PF molecules played an important role in the formation of phase structure. The as-obtained materials were characterized using different spectroscopic techniques including FT-IR, Raman spectrum, and PL. The emission color can be easily turned by adjusting the relative doping concentrations of lanthanide ions.

Full text of DOI:10.1016/j.jallcom.2016.11.355

Journal of Alloys and Compounds 695 (2017) 3018e3023
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Lanthanide ions-doped calcium molybdate pie-like microstructures:
Synthesis, structure characterization, and luminescent properties
,
,
Yuanyuan Han b **, Liyong Wang a *, Dan Wang a, Danyang Liang a, Shiqi Wang a,
Guoxin Lu a, Zhiyu Di a, Guang Jia a
a College of Chemistry and Environmental Science, Key Laboratory of Medicine Chemistry and Molecular Diagnosis, Ministry of Education, Hebei University,
No. 180 Wusi Road, Baoding 071002, PR China
b Medical Experimental Centre of Hebei University, No. 342 Yuhua Road, Baoding 071000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Lanthanide ions doped CaMoO4 with pie-like microstructures were synthesized by a hydrothermal
method using phenol-formaldehyde resin (PF) as an inducer. The results of powder X-ray diffraction
showed a tetragonal phase of both undoped and Ln3þ-doped CaMoO4. The microscopical characterization
technology revealed the formation process of the pie-like microparticles. As a morphology inducer, the
PF molecules played an important role in the formation of phase structure. The as-obtained materials
were characterized using different spectroscopic techniques including FT-IR, Raman spectrum, and PL.
The emission color can be easily turned by adjusting the relative doping concentrations of lanthanide
ions.
Received 10 September 2016
Received in revised form
16 November 2016
Accepted 24 November 2016
Available online 25 November 2016
Keywords:
Calcium molybdate
Hydrothermal synthesis
Phenol-formaldehyde resin
Lanthanide ions
© 2016 Elsevier B.V. All rights reserved.
Luminescence properties
1. Introduction
with good uniformity via a facile hydrothermal method. PF mole-
cules were used as a shape inducer in hydrothermal condition. In
Nowadays, great attention has been paid for the synthesis of
lanthanide ion (Ln3þ) doped nano/micrometer particles due to their
wide applications such as in optoelectronic and biological field
[1e7]. The unique morphologies with self-assembled hierarchical
superstructures also provide subtle complex functions and direct
bridges between the microscale objects and the macroscale world
[8,9]. Calcium molybdate (CaMoO4), as a famous host material with
scheelite structure has been widely studied in scintillators, X-ray
phosphors, laser crystals, and host lattices in electron spin reso-
nance due to its excellent optical properties [10e14].
Lanthanide ions doped CaMoO4 nano or micrometer particles
have been prepared by various synthetic methods by previous re-
ports [15e20]. The results revealed the relations between particles
size or hierarchical morphology and luminescent properties.
Herein, for the first time we report the controllable fabrication
of novel lanthanide ions doped CaMoO4 hierarchical architectures
addition, the possible formation mechanism of the pie-like
microstructure is proposed. Compared to the previously reports,
our method has advantages including cheap chemical reagents,
facile synthetic procedure, and relatively low reaction temperature.
The phase structure, IR, Raman, and PL spectra, and chromatic
properties were also investigated.
2. Experimentals
2.1. Synthesis
All the chemical reagents were of analytical grade and used as
received without further purification.
A low temperature hydrothermal method was used to synthe-
size the micrometer pie-like CaMoO4 particles. In a typical syn-
thesis process, Ca(NO3)2 (5.0 mmol) and different types and
amounts of Ln(NO3)3 (Tb3þ, Dy3þ, and Eu3þ) were dissolved in
15 mL of distilled water. Then, the mixed solution was dropped into
15 mL of Na2MoO4 (5.0 mmol) solution slowly under vigorous
stirring. Afterwards, phenol, formaldehyde and ammonia water
(13.33 mol Lꢀ1) was added into the above solution. After continuous
* Corresponding author.
** Corresponding author.
E-mail addresses: hanyy_8016@163.com (Y. Han), wangly_1@126.com (L. Wang).
http://dx.doi.org/10.1016/j.jallcom.2016.11.355
0925-8388/© 2016 Elsevier B.V. All rights reserved.

Y. Han et al. / Journal of Alloys and Compounds 695 (2017) 3018e3023
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stirring of 5 min, the precursor suspension was transferred into a
50 mL Teflon-lined stainless steel autoclave with a filling capacity of
75%, which was subsequently sealed and heated to 160 ꢁC for 4 h.
Then the autoclave was cooled to room temperature naturally. After
the centrifugation of the resulting solutions, the products were
washed several times with deionized water and ethanol, then air
dried at 100 ꢁC overnight. Then the product was calcinated at
600 ꢁC to remove the PF polymer, followed by grinding to yield the
final lanthanide ions doped CaMoO4 powder.
2.2. Characterization
The crystallinity and phase purity of CaMoO4 samples were
examined by powder X-ray diffraction (XRD) with an X-ray
diffractometer (D8 ADVANCE, Bruker, USA) with graphite mono-
chromatic Cu Ka radiation. The morphology and size of the as-
synthesized products were investigated using a scanning electron
microscope (SEM, JEOL, JSM-7500F, Japan). The FT-IR spectroscopy
(IR, Perkin-Elmer, 580B, USA) was performed to confirm the surface
chemical structure of the products in the wave number range of
400e4000 cmꢀ1 by KBr disk method. Thermogravimetric analysis
(TGA) and Differential Thermal Analysis (DTA) was carried out on a
Simultaneous Thermal Analyzer (Netzsch, STA 449c, Germany)
with a heating rate of 10 ꢁC/min from 50 to 800 ꢁC under air at-
mosphere. Characteristic vibration energy changing of chemical
bond and group were recorded on a Laser Raman microscope
equipment (Xplora, Horiba, France). The photo-luminescent (PL)
excitation and emission spectra were recorded with spectropho-
tometer (PL, Hitachi, F-7000, Japan). The chromaticity coordinates
calculated by the CIE system.
3. Results and discussion
The XRD patterns of CaMoO4:xDy3þ (0e0.10) phosphors are
shown in Fig. 1a. It shows that all the diffraction peaks match well
with the tetragonal phase of CaMoO4 according to the standard
reference of JCPDS card no. 41-1431, no additional peaks for im-
purity phases are observed, indicating that the Dy3þ ions can be
effectively doped into the CaMoO4 host and the dopants do not
cause any significant changes to the host structure. However, the
diffraction peak for (112) plane shifts to higher angles for Dy3þ ion
substitution, suggesting the decrease of the lattice constants by
doping Dy3þ ions [21]. PF polymer template can also affect the
crystal structure. From Fig. 1b, it can be observed that different
amount of phenol adding influences the formation of crystal
structure. Comparing with that of the sample prepared in the
absence of phenol, the main diffraction peaks shift to higher angles.
As it is known that the surface chemistry properties and tem-
plate of the materials can be examined by FTIR spectra. Fig. 2 pre-
sents the FTIR spectra of the as-synthesized products, and Fig. 2a
represents the PF template synthesized by hydrothermal condition.
The absorption band composition is consistent with that of PF re-
ported previously [22,23], such as vibrations of the carboxylate
groups (1632 cmꢀ1), absorption bands of the benzene rings (1612,
1479 cmꢀ1), the CeH outer bending vibrations (1146 cmꢀ1), ab-
sorption of CH2 groups (966, 826 cmꢀ1), absorption of phenolic OH
in-plane deformation (1357 cmꢀ1), absorption of alkyl-phenol CeO
stretch (1229 cmꢀ1) and CH2 out of plane ring deformation
(802 cmꢀ1). Furthermore, Fig. 2b represents PF/CaMoO4 compound
without calcination process. Besides the characteristic peaks of PF,
it can be also observed that characteristic absorption band MoeO at
820, 435 cmꢀ1 for stretching and bending mode respectively, which
Fig. 1. a). XRD patterns of CaMoO4 samples doped with different amount of Dy3þ. (b).
XRD patterns of lanthanide ions doped CaMoO4 (5 at. %) samples obtained with
different amount of PF precursor.
confirms the presence of the composite structure. Eventually,
Fig. 2c shows the as-prepared product after 600 ꢁC calcination, the
absence of the characteristic absorption bands indicates that no
polymer template remaining after calcination process.
The thermal behaviors of the as-prepared hydrothermal product
before calcination were investigated by TG-DTA measurements
(Fig. 3). There is one sharp weight loss in the TG curve accompa-
nying two exothermic peaks in the range of 300e600 ꢁC, which can
be can be assigned to the dehydration and decomposition of PF. It
can also be seen that the mass of compound is stable when the
calcination temperature is higher than 600 ꢁC, resulting in the pure
CaMoO4 product.
The structure order of material at short range and the Raman-
active phonon modes can be measured by the FT-Raman technol-
ogy at room temperature. It is known that 13 modes are Raman
active (3Ag, 5Bg, 5Eg) and 8 mode are IR active among 26 modes of
vibrations (3Ag, 5Au, 5Bg, 3Bu, 5Eg, 5Eu) for Scheelite structure

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Y. Han et al. / Journal of Alloys and Compounds 695 (2017) 3018e3023
[24e26]. Fig. 4 presents the Raman spectra of the as-synthesized
CaMoO4 and Dy3þ ions doped CaMoO4 (7%), it can be observed
that both Raman spectra exhibit intense and sharp band due to
strong interaction between the OeCaeO and OeMoeO in the
CaMoO4 matrix. The strong intense bands at 878, 844, and
791 cmꢀ1 are due to Ag, Bg, and Eg modes, furthermore, the bands at
109, 141, 200, 321, and 390 cmꢀ1 are due to Eg, Eg, Bg, Ag þ Eg and Ag
modes. It can be observed that Dy3þ doping did not generate
obvious changing for Raman spectra except band intensity, it may
be ascribe to local disorder and defects in the scheelite lattice of
CaMoO4 after doping of Dy3þ. Since Mo6þ ion located at the site B in
the CaMoO4 matrix, the replacement of Ca2þ ions by Dy3þ ions at
site A, which can barely impact the atom arrangement after doping.
Therefore, Dy3þ doping cannot modify stretching, torsion, and
bending vibration modes of MoeO bonds [27].
The effect of reaction time on the morphology evolution of the
CaMoO4:Dy3þ microstructure was investigated in detail at 160 ꢁC
for a given time. Fig. 5aed shows SEM images of CaMoO4:Dy3þ
prepared with different reaction time and the polymer precursors
were 100 mg phenol, 0.3 mL ammonium hydroxide (25%), and
0.2 mL formaldehyde. And it can be observed that when the hy-
drothermal reaction proceeded for 0.5 h, the product was mainly
composed of regular nanospheres with a typical diameter rang of
100e150 nm, and most of the nanospheres have obvious boundary
and narrow particle size distribution. However, some of them are
interconnecting together and merge into larger particles. When the
reaction time increased to 1 h, larger nano-rods were formed via
the Ostwald ripening mechanism to form three-dimensional mi-
crospheres through oriented-attachment and self-assembly
Fig. 2. FTIR spectra of the PF template, as-prepared and 600 ꢁC calcinated CaMoO4
samples (a: PF, b: CaMoO4/PF, c: CaMoO4).
[28,29]. As the reaction time was extended to
2 h, the
morphology and size of microspheres were relatively uniform
compared to those of the products processed for 1 h, and the pie-
like morphology appeared with rough surfaces. With the increase
of reaction time to 4 h, the micro-pies had little changes in thick-
ness and length but with a uniform morphology. Furthermore, the
polymer additive was also an important controlling factor in
shaping the product. It can be observed that (from Fig. 4e) the
irregular morphology appeared when the polymer precursor was
excessively surplus.
The whole evolution process for the as-prepared pie-like
CaMoO4 hierarchical structures is thereby illustrated in Fig. 6.
Fig. 6 shows the schematic illustration of the possible formation
process for the CaMoO4 samples. CaMoO4 nanocrystals were
immediately formed after blending of Ca2þ with MoO42- ions. The
mixed solution of ammonia water, formaldehyde, and phenol could
be the precursor of PF (phenol formaldehyde resin) under hydro-
thermal condition. PF coated on CaMoO4 nanocrystals may act as
anti-growing and assembling agent, thus the compound of poly-
mer/CaMoO4 showed uniform particles size and narrow distribu-
tion. After the thermal decomposition of PF molecules into small
inorganic molecules such as NOx, H2O and CO2 in calcination pro-
cess, the pure micrometer pie-like CaMoO4 was subsequently
obtained.
Fig. 3. TG/DTA isotherms of compound of 5 at% doped CaMoO4:Dy3þ and PF before
calcination process.
Fig. 7 shows the excitation spectra of Dy3þ (5 at. %) doped
CaMoO4 by monitoring the emission wavelength at 573 nm. It can
be observed that a broad band with a maximum at 274 nm due to
the MoO24- groups from 225 to 320 nm. Meanwhile, some weak
lines due to the fef transitions of Dy3þ can be observed in the
wavelength ranging of 350e450 nm. And these weak lines at 352,
366, 388, 399, 428, and 450 nm are assigned to the transitions from
the 6H15/2 ground state to the different excited states of Dy3þ, i. e.,
6F7/2 (352), 6F9/2 (366), 4I13/2 (388), 4F7/2 (399), 4G11/2 (428), and 4I15/
2 (450), respectively. These were assigned to the combination of the
ligand to metal charge transfer (O2-/Mo6þ) and charge transfer
band (CTB) from the completely filled 2p orbitals of O2- to the
Fig. 4. FT-Raman spectra of the as-prepared CaMoO4: xDy3þ (a) x ¼ 0%, (b) x ¼ 5% pie-
like microparticles.

Y. Han et al. / Journal of Alloys and Compounds 695 (2017) 3018e3023
3021
Fig. 5. SEM images of the as-prepared CaMoO4 with different hydrothermal reaction time (a: 0.5 h, b: 1 h, c: 2 h, and d: 4 h) and e (redundant polymer additive).
Fig. 6. Schematic illustration of the possible formation process.
Fig. 7. Excitation and emission spectra of CaMoO4:xDy3þ (x ¼ 5 at. %) microparticles.

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Y. Han et al. / Journal of Alloys and Compounds 695 (2017) 3018e3023
detected because of their weak intensity. Upon excitation into the
MoO24- at 273 nm, the emission spectrum contains the characteristic
emission of Tb3þ with 5D4e7F5 green emission (543 nm) as the
most prominent group. Other emissions are located at 488, 586,
and 621 nm, which can be ascribed to the transitions of 5D4e7F6,
5D4e7F4, and 5D4e7F3, respectively [30].
Fig. 9 shows the emission spectra of CaMoO4:xTb3þ (x ¼ 5%)
microparticles with different amount of phenol. It can be seen that
the fluorescence intensities change slightly, therefore, PF pre-
cursors formation in hydrothermal condition and degradation
during calcination process have little effect on the fluorescence
emission.
The tunable emission properties of the CaMoO4 phosphors can
be investigated by co-doping different lanthanide ions. Fig. 10 a and
b show the emission spectra of the co-doped CaMoO4 with
different relative concentrations of Dy3þ, Eu3þ, and Tb3þ (total
concentration: 5 at. %) under excitation at 273 nm, respectively. It
can be seen that the as-obtained lanthanide ions co-doped CaMoO4
phosphors showed strong different emissions. When Eu3þ ions
were co-doped into the CaMoO4 host lattices, besides the Dy3þ
emissions the characteristic emissions of the Eu3þ could be also
observed. Moreover, the emission intensity of Dy3þ decreased
gradually by increasing the relative concentration ratio of Eu3þ. It
can be also seen that the emission colors can be turned from green,
through yellow or yellow-red, to red by adjusting the relative
doping concentrations of the Tb3þ and Eu3þ ions (Fig. 10b). The
result indicates that the pie-like phosphors show multicolor
emissions when excited under UV light, which makes them act as
potential phosphors for light powders and advanced flat panel
display devices [31].
Fig. 8. Excitation and emission spectra of CaMoO4:xTb3þ (x ¼ 5 at. %) microparticles.
partially filled fef orbitals of the Dy3þ ions (O2-/Dy3þ) [10]. In the
emission spectra, the transition band at 4F9/2e6H15/2 (487 nm) is
4
assigned to the magnetic dipole transition, whereas that at F9/
2e6H13/2 (573 nm) assigns to electric dipole transition [10]. The
4
yellow luminescence color is dominated by the transition at F9/
2e6H13/2 (573 nm) in all the cases.
Fig.
8 shows the excitation and emission spectra of
CaMoO4:Tb3þ. It can be observed that a strong and broad band from
200 to 320 nm with a maximum at about 273 nm is assign to the
charge-transfer transitions within the MoO24- groups. In the wave-
length ranging from 320 to 420 nm, the weak lines ascribed to the f-
f transitions within the Tb3þ 4f8 configuration can be hardly
The CaMoO4:Ln3þ (Ln ¼ Eu, Tb, and Dy) samples exhibit intense
characteristic emissions of lanthanide activator ions under UV
excitation, which can be confirmed by the CIE (Commission Inter-
nationale de I'Eclairage 1931 chromaticity) coordinates for the
emission spectra of the CaMoO4:Ln3þ samples (Fig. 11). The chro-
maticity coordinates of CaMoO4:Ln3þ (Ln ¼ Eu3þ, Tb3þ, and Dy3þ
)
samples are calculated to be x ¼ 0.397, y ¼ 0.318 (point a); x ¼ 0.217,
y ¼ 0.453 (point b); x ¼ 0.344, y ¼ 0.414 (point c). It can be observed
that the PL emission colors of the CaMoO4:Ln3þ samples can be
tuned by doping different lanthanide activator ions.
4. Conclusion
In summary, the novel pie-like architectures of lanthanide ions
doped CaMoO4 phosphors have been successfully prepared via a
facile and efficient hydrothermal process by controlling the
fundamental experimental parameters. The amount of PF pre-
cursor played a key role in the formation of CaMoO4 microstruc-
ture, and the formation mechanism of the structure has been
investigated in detail. Furthermore, the emission colors can be
turned easily by adjusting the relative doping concentrations of
lanthanide ions. The as-obtained uniform and well-dispersed
CaMoO4:Ln3þ phosphors exhibit strong emission, which may
find great potential applications in lighting, displays, and devices
in photochemical fields. Furthermore, this facile strategy
described here may serve as guidance for the design of new well-
defined molybdate materials.
Fig. 9. Emission spectra of CaMoO4:xTb3þ (x ¼ 5 at. %) microparticles with different
amount of phenol.

Y. Han et al. / Journal of Alloys and Compounds 695 (2017) 3018e3023
3023
Fig. 10. Emission spectra of the CaMoO4 co-doped with Dy3þ/Eu3þ (a) and (b) co-doped with Tb3þ/Eu3þ
.
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This study was funded by Project of Medical disciplines con-
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of Hebei education department (Grant No. Z2013053) Students
Research Fund of College of Chemistry and Environmental Science
(Grant No. 2015010), Natural Science Foundation of Hebei Univer-
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