Controlled synthesis and room-temperature ferromagnetism of CaWO4 nanostructures

Article abstract of DOI:10.1016/j.jallcom.2015.07.221

CaWO₄ nanostructures (nanospheres, nanorods and nanoplates) were synthesized by a hydrothermal process. The morphology, size and composition of these nanostructures were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS). Based on the electron microscope observations, the growth of such nanostructures has been proposed as an Ostwald ripening process followed by self-assembly. Furthermore, magnetic measurement of these CaWO₄ nanostructures indicates as-prepared CaWO₄ nanostructures possess obvious room-temperature ferromagnetism, suggesting the potential of CaWO₄ nanostructures in applications.

Full text of DOI:10.1016/j.jallcom.2015.07.221

Accepted Manuscript
Controlled synthesis and room-temperature ferromagnetism of CaWO
nanostructures
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B. Sun, X.J. Jia, J.H. Wu, P. Chen
PII:
S0925-8388(15)30620-4
10.1016/j.jallcom.2015.07.221
JALCOM 34917
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 3 July 2015
Accepted Date: 23 July 2015
Please cite this article as: B. Sun, X.J. Jia, J.H. Wu, P. Chen, Controlled synthesis and room-
temperature ferromagnetism of CaWO nanostructures, Journal of Alloys and Compounds (2015), doi:
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10.1016/j.jallcom.2015.07.221.
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ACCEPTED MANUSCRIPT
Graphical Abstract:
CaWO4 nanostructures (nanospheres, nanorods and nanoplates) were
synthesized by a hydrothermal process. Furthermore, magnetic measurement of these
CaWO4 nanostructures indicates as-prepared CaWO4 nanostructures possess obvious
room-temperature ferromagnetism.

JOURNAL OF ALLOYS AND COMPOUNDS
ACCEPTED MANUSCRIPT
Controlled synthesis and room-temperature
ferromagnetism of CaWO4 nanostructures
B. Sun, X. J. Jia, J. H. Wu, and P. Chen*
School of Physics Science and Technology, Southwest University, Chongqing 400715, China
ABSTRACT
CaWO4 nanostructures (nanospheres, nanorods and nanoplates) were synthesized
by a hydrothermal process. The morphology, size and composition of these
nanostructures were characterized by X-ray powder diffraction (XRD), field emission
scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and
X-ray energy dispersive spectroscopy (EDS). Based on the electron microscope
observations, the growth of such nanostructures has been proposed as an Ostwald
ripening process followed by self-assembly. Furthermore, magnetic measurement of
these CaWO4 nanostructures indicates as-prepared CaWO4 nanostructures possess
obvious room-temperature ferromagnetism, suggesting the potential of CaWO4
nanostructures in applications.
Keywords: Controlled synthesis; Ferromagnetism; CaWO4 nanostructures; Nanorods;
Nanoplates
* Corresponding author: Peng Chen
E-mail: pchen@swu.edu.cn
Tel: +86-23-68367015
Fax: +86-23-68254608
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1. Introduction
In the past few decades, the preparation of inorganic nano-materials with special
morphology structures and exceptive properties has aroused extensive attention in material
chemistry and device physics due to their potential applications in nano-devices [1–3]. At
present, it is undeniable that much interest has been focused on preparation of novel
nanostructures materials [4–7], because the physical and chemical properties of
nanomaterials are strongly correlated with their nanostructures, including nano-size,
morphology and dimensionality [8]. Therefore, a variety of methods have been developed to
controllably synthesize novel nanostructures materials, which are special significance and
have become intensive research topics.
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In recent synthetic methods of nanomaterials, it is no doubt that the hydrothermal
method is an effective and simple route to prepare various nanomaterials [9-12]. Moreover,
the reliance of future technologies on exploring facile and economic methods for the
fabrication of nanostructures has spurred intense and rapid development in the field of
materials science and materials application [13-16], including efficient catalysts, photonic
building blocks and drug delayed releasing agents [17-21].
As one of the most promising materials in the nano-electronic and nano-device areas,
metal tungstates, which possess important properties and potential technological applications
in various fields, including photoluminescence, microwave applications, optical fibers,
scintillator materials, humidity sensors, magnetic properties and catalysts, have been
extensively studied over the past few years [22-32]. Among metal tungstates, CaWO4 has
attracted particular interest in practical applications as laser host materials in quantum
electronics and scintillators in medical devices [33-37]. At the same time, various methods,
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such as hydrothermal reaction, template method, microemulsion-mediated route and sol–gel
techniques, have been developed for the synthesis of CaWO4 [38-44].
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In this work, novel CaWO4 nanostructures have been controllably synthesized by a facile
hydrothermal process using Ca(CH3COO)2.·H2O and Na2WO4·2H2O as precursors. It was
found that the reaction conditions (reaction times and reaction temperatures) highly
influenced the morphology and nano-size of these CaWO4 nanostructures. Moreover,
microstructural analyses reveal that the as-prepared product exhibits different
nanostructures, including nanospheres, nanorods and nanoplates. The possible formation
mechanism and growth mechanism of the CaWO4 nanostructures were proposed based on
the observation of intermediate products obtained at different reaction conditions.
Furthermore, the magnetic properties of different CaWO4 nanostructures are reported for the
first time. The CaWO4 nanomaterials show ferromagetism at room temperature.
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2. Experimental section
2.1 Preparation of CaWO4 nanostructures
In our experiments, Ca(CH3COO)2·H2O, Na2WO4·2H2O and cetyl trimethylammonium bromide
(CTAB) were purchased from Sigma. All chemicals were directly used as received without further
purification. In a typical procedure, a certain amount of mmol of Ca(CH3COO)2·H2O was dissolved
in 40 ml of deionized water, and the solution was stirred for 30 min. After that, required amount of
mmol of Na2WO4·2H2O was added and vigorously mixed by stirring. Next, a certain amount of
cationic surfactant cetyltrimethylammonium bromide (CTAB) was added and stirred for 2.0 hours.
Finally, the mixed solution was transferred into a Teflon-lined stainless steel autoclave (50 ml
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capacity) for hydrothermal treatment at 180 C for 12 h. After the autoclave was allowed to cool
down to room temperature, the white precipitate was collected and washed several times under
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sonication with water and ethanol, then dried under vacuum at 60 C for characterization. As to
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other two experiments, the hydrothermal conditions are 180 C for 48 h and 180 C for 96 h
respectively, other process were the same with above.
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2.2 Characterizations
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Field emission scanning electron microscopy (FESEM) measurements were taken with a
Hitachi S4800 scanning electron microscope. Transmission electron microscopy (TEM)
measurements and selected area electron diffraction (SAED) patterns were performed on a JEOL
JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. Energy-
dispersive spectra (EDS) were examined on a JEM-2010F high-resolution transmission electron
microscope equipped with EDS detector working at an accelerating voltage of 200 kV. The phase
structure and phase purity of the as-synthesized products were examined on a Japan Shimadzu
XRD-6000 diffractometer with Cu-Ka radiation (λ = 0.15418 nm). Magnetic properties were
characterized by a vibrating sample magnetometer (VSM) from ADE Corporation (USA).
2.3 Magnetic measurement
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Magnetic properties were characterized by a vibrating sample magnetometer (VSM) from ADE
Corporation in USA. For the saturation magnetization measurement, the sample with mass 0.1 g
packed in small glass box was measured.
3. Results and discussion
The representative field emission scanning electron microscope (FESEM) images,
transmission electron microscope (TEM) images and selected area electron diffraction
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(SAED) patterns of as-prepared products obtained at 180 C for 12 h, 180 C for 48 h and
180 oC for 96 h are shown in Fig. 1. In the Fig. 1(a), the product obtained at 180 oC for 12 h
is composed of a large number of nanospheres. And the size of nanospheres is about 25~35
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nm from the inset of Fig. 1(a). As the reaction time was increased to 48 h, the nanocrystals
grew into nanorods (Fig. 1(c)). And the inset of Fig. 1(c) shows the diameter of nanorods is
50~100 nm nearly. When the reaction time was increased to 72 h, the nanoplates were
formed (Fig. 1(e)). And the thickness of nanoplates is approximately 75 nm from the inset of
Fig. 1(e). It is obvious that the morphology evolution is a function of hydrothermal
treatment conditions (reaction times).
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Fig.1 (a) The FESEM image of as-prepared CaWO4 nanospheres, the inset is TEM image of
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CaWO4 nanospheres. (b) The high resolution TEM image of CaWO4 nanospheres, the inset
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is the corresponding SAED patterns of CaWO4 nanospheres. (c) The TEM image of as-
prepared CaWO4 nanorods, the inset is TEM image of the individual CaWO4 nanorods. (d)
The high resolution TEM image of CaWO4 nanorods, the inset is the corresponding SAED
patterns of CaWO4 nanorods. (e) The FESEM image of as-prepared CaWO4 nanoplates, the
inset is cross section FESEM image of the CaWO4 nanoplates. (f) The high resolution TEM
image of CaWO4 nanoplates, the inset is the corresponding SAED patterns of CaWO4
nanoplates.
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The lattice fringes were measured as about 0.47 nm by high resolution TEM (HRTEM)
from Fig. 1, which corresponds to the (1 0 1) planes of the monoclinic phase CaWO4. In
addition, selected area electron diffraction (SAED) patterns reveal bright polycrystalline
diffraction rings, and lattice spacings derived from the diffraction rings agree with the (1 0 1)
and (1 1 2) plates of CaWO4.
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On the basis of above experimental results, we considered that the formation of the novel
nanostructures is a cooperation effect of Ostwald ripening and self-assembly process [45-47].
The schematic diagram (Fig. 2) clearly illustrated the growth process from CaWO4
nanopheres to nanoplates. In the first stage, tiny crystalline nuclei were generated in the
supersaturated solution and grew into nanoparticles in the hydrothermal reaction. That is to
say, CaWO4 nanoparticles were firstly generated in the precursors of Ca(CH3COO)2.·H2O
and Na2WO4·2H2O. As the reaction time was increased, these nanoparticles were quickly
built and spontaneously aggregated into large spheres to minimize their surface area through
the process known as Ostwald ripening [45]. Then, as the increase of reaction time, the
crystallized particles grew aniscopically along the 1D direction, resulting in the formation of
the nanorods. As the mass diffusion and Ostwald ripening process proceeded, the nanorods
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grew until all the nanoparticles were consumed. And the nanorods grew aniscopically along
the 2D direction, with the further increase of reaction time, CaWO4 nanorods were evolved
into CaWO4 nanoplates.
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Fig. 2 Schematic illustrations of the proposed formation mechanism of the growth process
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from CaWO4 nanopheres to nanoplates.
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The crystal structures of as-prepared CaWO4 nanostructures were studied by XRD
analysis, as shown in Fig. 3(a). All diffraction peaks and positions match well with those
from the JCPDS card (No. 75-0210), which is coincident with that in the reported work [48-
50]. Moreover, the strong and sharp diffraction peaks in the pattern show that the as-
obtained product is well crystallized. In addition, no other impurity peaks were detected,
suggesting the high purity of the as-prepared products.
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The CaWO4 nanostructures powder was further confirmed by elemental analysis carried
out from energy-dispersive X-ray spectra (EDS). The EDS data in Fig. 3(b) confirms that the
elements of nanostructures are Ca, W and O without any other impurities. Moreover, the
molar ratio of Ca:W:O is 1:1.10:4.04 nearly, which is good agreement with the
stoichiometric molar ratio of CaWO4.
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Fig. 3 (a) X-ray diffraction (XRD) patterns of as-prepared different CaWO4 nanostructures powder
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at room temperature. (b) Energy-dispersive X-ray spectrum (EDS) of CaWO4 nanoplate powder.
In order to investigate the magnetic properties of CaWO4 nanostructures, we measured the
magnetic hysteresis loops (M–H) of different CaWO4 nanostructures at room temperature. Fig. 4(a)
shows the M–H loops of the CaWO4 nanospheres, nanprods and nanoplates respectively at room
temperature. All the CaWO4 nanostructures present obvious ferromagnetism, and the saturation
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magnetization is about 9.5×10-3 emu/g for CaWO4 nanospheres, 11×10-3 emu/g for CaWO4
nanorods and 12.5×10-3 emu/g for CaWO4 nanoplates respectively.
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Fig. 4 (a) The M–H hysteresis loops for different CaWO4 nanostructures measured at room
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temperature. (b) The magnification of one segment of M–H hysteresis loops.
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Fig. 4(b) presents the magnification of one segment of M–H hysteresis loops of CaWO4
nanostructures. It is obvious that the coercivity is about 100 Oe, 300 Oe and 500 Oe for CaWO4
nanospheres, nanorods and nanoplates respectively at room temperature. Therefore, the as-prepared
CaWO4 nanostructures possess substantial ferromagnetism at room temperature. The possible
reason is that size-confinement effects could increase magnetization values and enhance the
ferromagnetic interaction in CaWO4 nanoplates.
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Fig. 5 exhibits the magnetic hysteresis loops (M–H) of the CaWO4 nanoplates at room
temperature and 100 K respectively. The CaWO4 nanoplates present enhanced ferromagnetism with
a coercivity of about 1000 Oe at 100 K from the inset of Fig. 5, and the saturation magnetizations of
the CaWO4 nanoplates at 100 K is approximately 18.5×10-3 emu/g.
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Fig. 5 The M–H hysteresis loops for CaWO4 nanoplates measured at room temperature (RT)
and 100 K respectively. The inset is the corresponding magnification of one segment of M–
H hysteresis loops.
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4. Conclusions
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In summary, we report a controllable synthesis of CaWO4 nanostructures by
hydrothermal process. The characterisation results of magnetic hysteresis loops (M–H)
present that the CaWO4 nanostructures hold a substantial ferromagnetism at room
temperatures, indicating the CaWO4 nanomaterials has the potential of applications in
magnetic nano-devices.
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Acknowledgments
This work was supported by the National Nature Science Foundation of China (Grant No.
51372209).
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Highlights:
ꢀ
CaWO4 nanostructures (nanospheres, nanorods and nanoplates) were synthesized.
ꢀ The growth has been proposed as an Ostwald ripening process followed by self-assembly.
ꢀ These CaWO4 nanostructures possess obvious room-temperature ferromagnetism.