PAPER www.rsc.org/crystengcomm | CrystEngComm

Low-temperature hydrothermal synthesis and structure control of nano-sizedCePO4†

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 perfor-

mance 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 orthophos-

phates 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 nano-

structured materials have been reported. Hexagonal CePO4could be easily obtained at low temperature, and the corre-

sponding nanorods/nanowires with a variable size have been

hydrothermally synthesized.8–11 Monoclinic CePO4 generally

aDepartment of Physical Chemistry, University of Science and TechnologyBeijing, Beijing, 100083, China. E-mail: ranboyu@metall.ustb.edu.cn;Fax: +86-10-62332525; Tel: +86-10-62332525bKey Laboratory of Multi-Phase and Complex System, Institute of ProcessEngineering, Chinese Academy of Sciences, Beijing, 100190, China.E-mail: danwang@home.ipe.ac.cn; Fax: +86-10-62631141; Tel: +86-10-62631141cSchool of Chemistry and Chemical Engineering, University of InnerMongolia, Hohhot, 010021, China

† Electronic supplementary information (ESI) available: FTIR spectra ofthe 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 reactantPO4/Ce molar ratios of 10 (Fig. S2); XRD patterns of the productsprepared with different reactant PO4/Ce molar ratios (Fig. S3). SeeDOI: 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 nano-

particles 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 as-

synthesized 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 L�1 concentrations were prepared. The cerium

nitrate solution was added slowly to 6 mol L�1 of orthophos-

phoric 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.

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).

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 roomtemperature. 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 cm�1 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

min�1 under an air flow (TGA instrument, model Q50V20.6

Build 31). The size and morphology of the products were char-

acterized by field-emission scanning electron microscopy (FE-

SEM, LEO1530). A high-resolution transmission microscopy

(HRTEM) image was recorded on a JEOL 2010 microscope with

an accelerating voltage of 200 kV. Room temperature fluores-

cence 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 contain-

ing 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 ¼7.055(3) Å and b ¼ 6.439(5) Å (JCPDS 34-1380)]. FTIR spectraand 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 cm�1, 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 cm�1 is due to –OH stretch and the peak around

1628 cm�1 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

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, andb ¼ 103.46(0)� (JCPDS 32–0199)], no hexagonal phases could beobserved. 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, anda¼ 6.831(3) Å, b¼ 7.054(5) Å, c¼ 6.487(5) Å and b¼ 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. 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: high-

magnification image, (e) 520, (f) 600.

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.

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 nano-

structures 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 PO4

3� 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 pre-

sented 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 CePO4particles 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 CePO4particles 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 mono-

clinic 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 intermo-

lecular hydrogen bonds would also help to induce the assembly

of nanorods.

To investigate the growth mechanism of the uniform flower-

like 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 agglom-

erate 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 CePO4crystal 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

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

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(5d

1)

and 2D3/2(5d1), respectively.10 Although the positions of the peaks

in the excitation spectra are identical in these samples, the inten-

sity 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 reac-

tant PO4/Ce molar ratio would result in the formation of

monoclinic CePO4 at 100�C, which proved an effective low-

temperature route for monoclinic CePO4. The formation mech-

anism 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 CePO4starting 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

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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Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...

Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4Electronic supplementary information (ESI) available: FTIR spectra of...