PAPER
www.rsc.org/crystengcomm | CrystEngComm
Low-temperature hydrothermal synthesis and structure control of nano-sized
CePO4†
Jinrong Bao,ac Ranbo Yu,*a Jiayun Zhang,a Xiaodan Yang,a Dan Wang,*b Jinxia Deng,a Jun Chena
and Xianran Xinga
Received 20th January 2009, Accepted 1st April 2009
First published as an Advance Article on the web 23rd April 2009
DOI: 10.1039/b901313j
The nanostructured CePO4 with hexagonal and monoclinic phases were controllably synthesized
through a hydrothermal route at a low temperature of 100 C by simply varying the reactant PO4/Ce
molar ratio. By analyzing the synthesis procedure and product structures, the formation mechanism of
the CePO4 nanostructures was proposed. The luminescent properties of CePO4 with different structures
and morphologies have been studied and compared. The obvious blue shift of the strongest excitation
peak of the monoclinic CePO4 compared with the hexagonal CePO4 could be observed in their
luminescence spectra. With the cycling use of phosphoric acid, the low-cost preparation of CePO4 could
be achieved. Furthermore, this synthesis strategy will open a novel approach to rare earth phosphates
with multiple structures.
1. Introduction
One-dimensional (1D) nanostructured materials, including
nanotubes, nanorods and nanowires, have attracted intense
research interest, owing to their novel physical and chemical
properties as a result of their low dimensionality and the
quantum confinement effect.1–3 Rare earth compounds with
a unique 4f shell of their ions showing electronic, optical, and
chemical characteristics have been widely used as high performance luminescent devices, magnets, catalysts, time-resolved
fluorescence labels for biological detection and other functional
materials.4,5
In recent years, much interest has been focused on the
synthesis and luminescence of nano-sized rare earth orthophosphates for their potential application in optoelectronic devices
and biological fluorescence labeling.6 CePO4 : Tb and its solid
solutions can be used in luminescent lamps as a highly efficient
emitter of green light.7,8 Also, a few recent studies on the
synthesis and properties of 1D cerium orthophosphate nanostructured materials have been reported. Hexagonal CePO4
could be easily obtained at low temperature, and the corresponding nanorods/nanowires with a variable size have been
hydrothermally synthesized.8–11 Monoclinic CePO4 generally
a
Department of Physical Chemistry, University of Science and Technology
Beijing, Beijing, 100083, China. E-mail: [email protected];
Fax: +86-10-62332525; Tel: +86-10-62332525
b
Key Laboratory of Multi-Phase and Complex System, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
E-mail: [email protected]; Fax: +86-10-62631141; Tel: +86-1062631141
c
School of Chemistry and Chemical Engineering, University of Inner
Mongolia, Hohhot, 010021, China
† Electronic supplementary information (ESI) available: FTIR spectra of
the CePO4$0.5H2O prepared with the reactant PO4/Ce molar ratios of 10
(FIg. S1); TGA plot of the CePO4$0.5H2O prepared with the reactant
PO4/Ce molar ratios of 10 (Fig. S2); XRD patterns of the products
prepared with different reactant PO4/Ce molar ratios (Fig. S3). See
DOI: 10.1039/b901313j
1630 | CrystEngComm, 2009, 11, 1630–1634
exists as natural monazite, bulk materials of which could be
prepared via the solid state reaction and hydrothermal method at
high temperature.12,13 So far, nano-sized monoclinic CePO4 are
mainly synthesized in a liquid phase under higher temperature.
Nanowires of monoclinic CePO4 were synthesized through
a hydrothermal reaction at 200 C.11 Very recently, Haase et al.14
reported the synthesis of monazite-type CePO4 : Tb nanoparticles controlled by liquid-phase synthesis in high boiling
coordinating solvents at 200 C. Up to now, there is no report
about the synthesis of monoclinic CePO4 at low temperature.
Herein, we present a facile approach to controllably synthesize
CePO4 with various crystalline phases using a hydrothermal
process at temperature as low as 100 C. By only increasing the
reactant PO4/Ce molar ratio, the phase transformation of assynthesized CePO4 from the hexagonal to the monoclinic could
be achieved. Correspondingly, it is interesting to find that the 1D
nano-sized CePO4 prefer to disperse as nanorods in a pure
hexagonal phase, and self-assemble as uniform nanostructures in
the mixed hexagonal and monoclinic phases and pure monoclinic
phase. And the flower-like mixed phase expresses special optical
properties. The growth mechanism of the crystals was also
proposed based on the analysis of their crystal structures and
the reaction process.
2. Experimental
Synthesis
All chemicals were analytical grade reagents, and used without
further purification. In a typical synthesis, Ce(NO3)3 solution
with 0.6–0.01 mol L1 concentrations were prepared. The cerium
nitrate solution was added slowly to 6 mol L1 of orthophosphoric acid solution while kept under stirring. The as-obtained
solution with a different reactant PO4/Ce molar ratio was
transferred into a stainless steel autoclave with an inner Teflon
vessel (volume, 50 ml). After the autoclave was purged with
argon for 40 min to prevent oxidation of Ce3+ to Ce4+, it was
This journal is ª The Royal Society of Chemistry 2009
sealed and maintained at 100 C for 6 h, and then allowed to
naturally cool to room temperature. The resulting white solid
precipitates were filtered, washed three times with deionized
water and absolute alcohol, and finally dried at 60 C for 8 h.
Characterization
The X-ray powder diffraction (XRD) patterns of all samples
were recorded on a 21 kW extra power X-ray diffractometer
(Model M21XVHF22, MAC science Co., Ltd., Japan) using Cu
Ka radiation (l ¼ 0.1541 nm) in the range of 10–60 at room
temperature. The diffraction profiles were analyzed by PowderX
and Treor programme.15 The infrared spectra of the powders
(FTIR) were recorded in range of 400–4000 cm1 on a Nicolet
NEXUS 670 FT–IR. The thermogravimetric plot (TGA) of the
powders performed up to 600 C at the heating rate of 10 C
min1 under an air flow (TGA instrument, model Q50V20.6
Build 31). The size and morphology of the products were characterized by field-emission scanning electron microscopy (FESEM, LEO1530). A high-resolution transmission microscopy
(HRTEM) image was recorded on a JEOL 2010 microscope with
an accelerating voltage of 200 kV. Room temperature fluorescence spectra of dilute colloidal solutions were recorded in
cuvettes (1 cm path length) on F-4500 FL Spectrophotometer.
The dilute colloidal solutions of the luminescence spectrum were
obtained by dispersing the CePO4 powder in methanol containing ca 0.02 mass% of the CePO4 powder in methanol.
3. Results and discussion
It was found that the crystalline phase and morphology of the
products were greatly affected by the reactant PO4/Ce molar
ratio. To investigate the influence of the reactant PO4/Ce molar
ratio on the products structure and morphology, a contrastive
experiment of keeping the other conditions constant, only the
reactant PO4/Ce molar ratio to change from 10 to 600, was
carried out.
The crystalline phases of the prepared samples were identified
by powder X-ray diffraction (Fig. 1). The typical XRD pattern of
the product prepared at the reactant PO4/Ce molar ratio of 10 is
shown in Fig. 1a. All its reflection peaks agree well with both
hexagonal CePO4 [space group P6222 (180), cell parameters a ¼
7.055(3) Å and b ¼ 6.439(5) Å (JCPDS 34-1380)]. FTIR spectra
and thermogravimetric plot further confirmed the hydrated
nature of the derived reactant PO4/Ce molar ratio of 10. Fig. S1†
presents the FI-IR spectrum of cerium phosphate. Three distinct
IR peaks are observed at 1051, 616, and 542 cm1, which are
assigned to P–O stretching, O]P–O bending, and O–P–O
bending mode of vibration, respectively. The absorption band of
around 3457 cm1 is due to –OH stretch and the peak around
1628 cm1 is attributed to –OH bending mode.16 Dehydration
and formation of cerium phosphate were followed by thermal
analysis data provided in Fig. S2.† The TGA curve shows the
weight loss occurring in two steps. The weight loss between 28
and 110 C corresponds to the removal of adsorbed water. The
dehydration of water of the hydrated cerium phosphate takes
place between 170 and 195 C. This is about 0.5 mol of water per
mol of cerium phosphate.17 The XRD pattern of our product
shows that the (200) peak is the strongest, which indicates
This journal is ª The Royal Society of Chemistry 2009
Fig. 1 XRD patterns of the products prepared with different reactant
PO4/Ce molar ratios of: (a) 10, (b) 140, (c) 290, (d) 600 (H: hexagonal
phase; M: monoclinic phase).
preferential growth in a certain direction in accord with the
reported growth patterns of CePO4 nanorods and nanowires.10
When the reactant PO4/Ce molar ratio increased to 120, the
monoclinic phase appeared (Fig. S3†). With the reactant PO4/Ce
molar ratio increasing, the diffraction intensity of the monoclinic
phase enhanced gradually (Fig. 1b and 1c). While the PO4/Ce
molar ratio is reaching 600, all reflection peaks in Fig. 1d can be
indexed to monoclinic CePO4 [space group P21/n(14), cell
parameters a ¼ 6.800(4) Å, b ¼ 7.023(1) Å, c ¼ 6.472(7) Å, and
b ¼ 103.46(0) (JCPDS 32–0199)], no hexagonal phases could be
observed. The final calculated lattice parameters of two pure
phases are a ¼ 7.054(7) Å, c ¼ 6.456(9) Å for the hexagonal, and
a ¼ 6.831(3) Å, b ¼ 7.054(5) Å, c ¼ 6.487(5) Å and b ¼ 103.89(6)
for the monoclinic, respectively. These parameters all increased
as compared to the values of those recorded in the JCPDS cards.
The morphology and microstructure of the as-synthesized
products were investigated using scanning electronic microscopy
(SEM). Fig. 2 shows the images of products prepared at
a different reactant PO4/Ce molar ratio. When the reaction
proceeded at the reactant PO4/Ce molar ratio of 10, the product
was composed of nanorods with a diameter of 20–30 nm and
length of 200–300 nm (Fig. 2a). The TEM image further
demonstrating that the obtained product has rod-like
morphology (Fig. 2b). A high-resolution HRTEM image
(Fig. 2b) shows that the hexagonal CePO4 nanorods grow along
the c axis [001], which is in good agreement with the anisotropic
character of the (200) peak in the XRD pattern of hexagonal
CePO4 nanorods. When the reactant PO4/Ce molar ratio was
higher than 120, interesting uniform flower-like nanostructures
were formed (Fig. 2c, 2d). The high-magnification image shows
that the flower-like nanostructures are actually composed of
a self-assembly of the oriented nanorods with a diameter ca 50
nm and a length ca 1 mm, which radiated outwards from the
centers and formed uniform flower-like aggregates. The reactant
PO4/Ce molar ratio increased to 520, the majority of the
CrystEngComm, 2009, 11, 1630–1634 | 1631
Fig. 3 View of the product structures in: (a) the hexagonal phase and (b)
the monoclinic phase, showing the connection of the cerium atom to the
PO4 tetrahedron.
Fig. 2 SEM images of the products prepared with different reactant
PO4/Ce molar ratios of: (a) 10, (b) 10 (TEM image and HRTEM image of
a nanorods). (c) 140, inset: high-magnification image, (d) 290, inset: highmagnification image, (e) 520, (f) 600.
morphologies were uniform bundle-like nanostructures (Fig. 2e).
Similar flower-like morphologies of ZnO, a-MnS, and b-NiS
have also been reported.18–20 However, to the best of our
knowledge, the uniform CePO4 flower-like nanostructure has not
yet been reported. When pure monoclinic CePO4 crystallized at
the reactant PO4/Ce molar ratio of 600, the corresponding
morphology dramatically appears as uniform bundle-like nanostructures composed of nanorods witha diameter of 60–70 nm
(Fig. 2f).
To understand the relation between the structure and the
corresponding morphology, it is necessary to investigate
the interaction of PO43, Ce3+ and the synthesized CePO4 in the
reaction system. It was reported recently that inorganic species
were involved in controlling the shape of the nanoparticles.21–24
For example, Yan et al. suggested that the presence of phosphate
ions is a crucial factor that induces the formation of an iron oxide
tubular structure, which results from the selective adsorption of
the phosphate ions on the surface of hematite particles and their
ability to coordinate with ferric ions.24 In our current reactions,
the reactant PO4/Ce molar ratio was changed between 10 to 600.
The excessive PO43 anions might be responsible for the
morphologies formation of the prepared nanostructured CePO4.
According to the crystal structures of the hexagonal and
monoclinic CePO4 (Fig. 3a, 3b), it could be found that the Ce
atoms in the two structures are surrounded by a different number
of PO4 tetrahedrons.25 In the hexagonal CePO4, the Ce atom
connects six tetrahedral PO4, while the Ce atom connects seven
tetrahedral PO4 in the monoclinic CePO4. Obviously, to form
monoclinic CePO4, the Ce atom need connecting with more
1632 | CrystEngComm, 2009, 11, 1630–1634
PO43, and high PO43 concentration might be beneficial for its
crystallization. Therefore, a mechanism for the structure and
morphology transformation in the present reaction system was
proposed, and the corresponding schematic illustration is presented in Fig. 4. Usually, the Ce3+ ion and the PO43 react to form
hexagonal CePO4 nanorods at low temperature.10,12 When the
phosphoric acid is excessive, the supersaturation of the solution
increased. It is quite possible that excessive PO43 are absorbed
on the surface of the initially formed tiny hexagonal CePO4
particles at the early stage of the reactions, due to the strong
interactions between the Ce3+ and the PO43 on the particle
surface.22,26 When phosphoric acid is excessive, the electrostatic
potential on the crystal surfaces of initial hexagonal CePO4
particles will increase.27,28 In order to reduce the surface energy,
the atoms of the crystal surfaces will rearrange. It is quite
possible that excessive anion PO43 are absorbed on the surface
of the initially formed hexagonal CePO4 particles around the
Ce3+ cation, which might result in the growth of initial monoclinic CePO4 particles on the surface of hexagonal CePO4, and
the aggregation of nanorods. Besides, interactions such as van
der Waals forces, phosphorylation of aggregation and intermolecular hydrogen bonds would also help to induce the assembly
of nanorods.
To investigate the growth mechanism of the uniform flowerlike nanostructures, the products subjected to different reaction
time stages were studied by SEM (Fig. 5). The products were
obtained from solution with a reactant PO4/Ce molar ratio of 140
after a hydrothermal treatment at 100 C for 0.5, 2, 4, and 6 h.
Under the present synthetic conditions, Fig. 5a clearly shows that
the rod-like particles with a random size distribution agglomerate together by treatment for 0.5 h. Fig. 5b exhibits the image
of the product obtained by a reaction for 2 h. A large number of
half-bundles with a length of about 1 mm were observed in the
product. The half-bundles were gradually organized into large
flower-like bundles when the reaction time was extended to 4 h
(Fig. 5c). Fig. 4d shows that uniform flower-like nanostructures
finally formed after 6 h of hydrothermal treatment. On the basis
of the above SEM observation, a possible growth process is
proposed. With the extension of the reaction time, the action
between the anion PO43 and the cerium anions in the surface of
the particles enhanced. The hexagonal phase of the CePO4
crystal surface of a certain plane grows in the direction of the
growth monoclinic, and forms half-bundle like nanorods. And
then, the half-bundles are aggregated by weak van der Waals
interactions to flower-like nanorods, and the nanorods radiate in
This journal is ª The Royal Society of Chemistry 2009
Fig. 4 Schematic illustration showing the formation mechanism of the flower-like CePO4 nanostructure with mixed hexagonal and monoclinic phases.
Fig. 5 Time-dependent evolution of the CePO4 flower-like nanorods
obtained from the reactant PO4/Ce molar ratio of 140 at different growth
stages: (a) 0.5 h, (b) 2 h, (c) 4 h, (d) 6 h.
different directions leading to the formation of the uniform
flower-like CePO4 nanorods.
The room-temperature excitation and emission spectra were
recorded for a dilute colloidal solution of CePO4 with different
structures (Fig. 6). The excitation peaks (Fig. 6a) centered at 235,
272, and 296 nm were observed, which could contribute to the
transitions from the cerium ground state 2F5/2(4f1) to the 2D5/2(5d1)
and 2D3/2(5d1), respectively.10 Although the positions of the peaks
in the excitation spectra are identical in these samples, the intensity patterns are different, and the peaks less than those of the
reported references, in which five crystal field split levels are
detected. It can be seen that the strongest peak is found at 272 nm
in the hexagonal CePO4, but at 235 nm in the monoclinic and the
mixed hexagonal and monoclinic CePO4. The emission spectra
(Fig. 6b) show a rather broad emission between about 300 and 400
nm, which corresponds to the 5d-4f transitions of the Ce3+ ions.29
The mixed hexagonal and monoclinic CePO4 with the reactant
PO4/Ce molar ratio of 290 exhibit strong emission intensity. The
difference in luminescence properties is possibly ascribed to the
absorption of the PO43 anion on the surface of the CePO4 in the
synthesis procedure with excessive phosphoric acid, which might
make the as-synthesized CePO4 to show a solvent effect in
methanol. And the luminescence properties are largely affected by
factors such as the different morphologies, sizes and crystal
structure.30–33
4. Conclusions
In summary, a simple hydrothermal process was employed to
synthesize cerium orthophosphate nanostructures. A high reactant PO4/Ce molar ratio would result in the formation of
monoclinic CePO4 at 100 C, which proved an effective lowtemperature route for monoclinic CePO4. The formation mechanism of the flower-like nanostructures is estimated in relation to
the interaction of excessive PO43 to the surface Ce atom of the
initial CePO4 nanorods. In addition, the luminescent property of
CePO4 nanostructure with different crystal phases has been
demonstrated to be susceptible to the synthesis procedure as well
as the crystal structure of cerium orthophosphate. The present
work describes a method for synthesizing nanostructured CePO4
starting from cerium nitrate and excessive orthophosphoric acid
by a hydrothermal process at low temperature. With the cycling
use of phosphoric acid, a low-cost preparation of CePO4 could be
achieved.
Acknowledgements
Fig. 6 Luminescence spectra of the products with different structures at
room temperature: (a) excitation spectra, (b) emission spectra (H:
hexagonal CePO4; H + M: mixed hexagonal and monoclinic CePO4; M:
monoclinic CePO4).
This journal is ª The Royal Society of Chemistry 2009
This work was financially supported by the National Natural
Science Foundation of China (No. 20871015, 20401015),
‘‘Program for New Century Excellent Talents in University’’
CrystEngComm, 2009, 11, 1630–1634 | 1633
(NCET), and Beijing Natural Science Foundation (No. 2082022,
2092019), and PCSIRT (No. IRT0708).
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