Hydrothermal synthesis and luminescent properties of novel ordered sphere CePO4 hierarchical architectures

Article abstract of DOI:10.1021/ic901829v

The ordered-sphere CePO₄ hierarchical architectures have been successfully synthesized by a simple hydrothermal method through the controlled growth of the CePO₄ nanorods and self-assemble hierarchical structure under various reactio

Full text of DOI:10.1021/ic901829v

Inorg. Chem. 2009, 48, 11559–11565 11559
DOI: 10.1021/ic901829v
Hydrothermal Synthesis and Luminescent Properties of Novel Ordered Sphere
CePO Hierarchical Architectures
4
†
,‡
,†
†,‡
†,‡
†,‡
†,‡
†,‡
Mei Yang, Hongpeng You,* Yuhua Zheng, Kai Liu, Guang Jia, Yanhua Song, Yeju Huang,
†
,‡
,†
Lihui Zhang, and Hongjie Zhang*
†
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese
‡
Academy of Sciences 130022, and Graduate University of the Chinese Academy of Sciences, Beijing 100049,
P. R. China
Received June 21, 2009
The ordered-sphere CePO hierarchical architectures have been successfully synthesized by a simple hydrothermal
4
method through the controlled growth of the CePO nanorods and self-assemble hierarchical structure under various
4
reaction conditions. The evolution of the morphology of the samples has been investigated in detail. It was found that
the coexistence of citric acid and cetaltrimethylammonium bromide in the reaction system plays an important role in the
formation of the spherical CePO hierarchical architectures. A possible mechanism of the formation and growth of the
4
hierarchical structure was suggested according to the experimental results and analysis. The effects of the reaction
time as well as the variation of the morphologies on the luminescent properties of the products were also studied.
1
. Introduction
Recently, muchattention has beenfocusedon the synthesis
and photoluminescence properties of lanthanide orthopho-
In recent years, large-scale hierarchical self-assembly struc-
sphates (LnPO ) and lanthanide (III)-doped lanthanide
4
tures from basic building blocks with specific morphology
and novel properties have attracted considerable interest for
their potential technology applications because their unique
properties are determined by their morphology, size, and
dimensions. Many efforts have thus been made recently to
develop new methods for the fabrication of hierarchical
ordered structures in material chemistry. Although some
progress has been made in the self-assembly of highly
organized building blocks of metals, semiconductors,
copolymers, organic-inorganic hybrid materials, and bio-
materials based on different driving mechanisms, effective
large-scale hierarchical self-assembly of some functional
materials is still a challenge to material scientists.
3
þ
orthophosphates (LnPO :Ln ) because of their potential
4
9
,10
11,12
applications in color displays,
field-effect transistors,
13
14
optoelectronics, medical and biological labels, solar
15,16
cells,
17,18
and light sources.
To date, various lanthanide
orthophosphate nanomaterials, such as zero-dimensional
2
0,21
22,23
1,2
nanoparticles,
one-dimensional (1D) structures,
and
24
core/shell structures, have been successfully synthesized.
3
4,5
(
9) Lee, M. H.; Oh, S. G.; Yi, S. C.; Seo, D. S.; Hong, J. P.; Kim, C.; Yoo,
Y. K.; Yoo, J. S. J. Electrochem. Soc. 2000, 147, 3139.
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*
To whom correspondence should be addressed. Tel.: 86431- 85692798.
Nat. Mater. 2005, 4, 455.
Fax: 86431-85698041. E-mail: hpyou@ciac.jl.cn (H.Y.), hongjie@
ciac.jl.cn (H.Z.).
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14935.
r 2009 American Chemical Society
Published on Web 11/13/2009
pubs.acs.org/IC

1
1560 Inorganic Chemistry, Vol. 48, No. 24, 2009
Yang et al.
However, reports on the hierarchical self-assembly of multi-
2
5
dimensional LnPO are still few, although many different
4
kinds of low-dimensional lanthanide phosphate nanomater-
ials have been reported.
In this paper, we provide a new example for the controlled
growth of ordered-sphere flower-like CePO hierarchical
4
architectures synthesized using a hydrothermal method with
the addition of cetaltrimethylammonium bromide (CTAB)
as a surfactant and citric acid as a chelating reagent. The
influence of the reaction conditions on the morphology and
the properties of the products were investigated, and a
possible formation mechanism of the architectures was
proposed. The obtained architectures may have potential
applications inthe fields of optoelectronic devices. Moreover,
our method may provide a new possibility to synthesize other
materials with desired architectures.
Figure 1. XRD patterns of S1-S5 obtained at 90 °C for 10 h with
different proportions of ethanol to water.
2
. Experimental Section
.1. Synthesis of CePO
O), CTAB, and (NH
chased from Beijing Chemical Corporation. Ce(NO ) 6H O
2
4
.
Citric acid monohydrate
HPO 12H O were pur-
(
C
6
H
8
O
7
H
2
4
)
2
4
2
3
3
3
3
3
2
was purchased from Wuxi Yiteng Rare-Earth Limited
Corporation. All reagents were analytical grade and were used
as-received without further purification.
In a typical synthesis, 2 mmol of Ce(NO3)3 6H O and 3 mmol
2
3
6 8 7 2
of C H O H O were first dissolved in deionized water. An
3
ethanol-water solution containing 3 mmol of CTAB and 3
mmol of (NH ) HPO 12H O was then added into the obtained
solution. A transparent solution was obtained when 3 mL of
HNO was added into the solution drop by drop. After this
4
2
4
3
2
3
process, the resulting solution was transferred into a Teflon-
lined autoclave and sealed tightly. Then, the autoclave was
maintained at 90 °C for 1-10 h. After cooling to room tem-
perature naturally, the precipitate was collected and washed
with deionized water and ethanol several times. The final
products were dried at 60 °C for 12 h in the air.
The proportions of the ethanol to water (v/v) for the obtained
samples were 1/35, 6/30, 11/25, 16/20, and 21/15, and the
corresponding products were labeled as S1, S2, S3, S4, and
S5, respectively.
Figure 2. FT-IR spectrum of S3 prepared at 90 °C for 10 h.
file 32-0199). No additional peaks of other phases had been
found in the other four samples, indicating that the propor-
tion of ethanol to water has little effect on the phase
formation of the products.
2.2. Characterization. The X-ray diffraction (XRD) pattern
was measured with a Rigaku-D X-Ray Powder Diffractometer
˚
with Cu Ka radiation (λ = 1.5418 A). Fourier-transform
infrared spectroscopy (FT-IR) spectra were measured with a
BRUKER Vertex 70 FTIR using the KBr pellet technique. The
morphology of the samples was inspected using a field emission
scanning electron microscope (HITACHI S-4800). Transmis-
sion electron microscopy (TEM) and high-resolution transmis-
sion electron microscopy (HRTEM) images and selected area
electron diffraction (SAED) pattern were obtained on a JEOL-
Figure 2 presents the typical FT-IR spectrum of S3. The
-
1
bands centeredat 1630 and 3460 cm can be due to the water
adsorbed on surface of the samples. The other bands are
assigned to the phosphate groups. The band centered at 1052
-
1
cm is ascribed to the asymmetry stretching vibration of the
3
-
-1
PO4 groups, and the bands centered at 609 cm and 536
2
100 microscope with an accelerating voltage of 200 kV.
-1
26
cm are attributed to the O-P-O bending vibrations. The
absorptions of N-H, CdO, and COO- are not observed in
the FT-IR spectrum, indicating that the reagents of citric acid
and CTAB remaining on the surface of the final products are
so little that they cannot be detected.
Excitation and emission spectra were recorded with a Hitachi
F-4500 fluorescence spectrophotometer equipped with a 150 W
xenon lamp as the excitation source. All of the measurements
were performed at room temperature.
Figure 3 shows the SEM images of the products obtained
with different proportions of ethanol to water. When the
proportion is 1/35, the aggregation type of the products is
irregular, and most of them consisted of nanorods with
diameters of 0.1-0.15 μm. In addition, there are still some
regular hexagonal prisms with diameters of 0.8-1.6 μm in
the product (Figure 3a). When the proportion is increased to
3
. Results and Discussion
Figure 1 shows the XRD patterns of the five samples
obtained at 90 °C for 10 h. It can be found that the peak
positions of the XRD patterns agree well with those of the
hexagonal phase CePO in JCPDS file 34-1380, except that of
S1, which has weak peaks indicated with “/” (the weak peaks
can be assigned to the monazite phase of CePO in JCPDS
4
4
(
25) Guan, M. Y.; Sun, J. H.; Han, M.; Xu, Z.; Tao, F. F.; Yin, G.; Wei,
(26) Li, L.; Jiang, W. G.; Pan, H. H.; Xu, X. R.; Tang, Y. X.; Ming, J. Z.;
X. W.; Zhu, J. M.; Jiang, X. Q. Nanotechnology 2007, 18, 415602.
Xu, Z. D.; Tang, R. K. J. Phys. Chem. C 2007, 111, 4111.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11561
Figure 3. SEM images of S1-S5 (a-e) obtained at 90 °C for 10 h with
different proportions ofethanol to water. S1, 1/35; S2, 6/30; S3, 11/25; S4,
Figure 4. SEM images of the products obtained with different reaction
times, with other reaction conditionssimilartothoseforS3:a, 1 h;b, 1.5h;
c, 2.5 h; d, 5 h; e, 10 h.
16/20; and S5, 21/15.
6
/30, the products display spherical flower-like hierarchical
architectures with diameters of 5-6 μm (Figure 3b). The
diameters of the prisms and architectures decrease gradually
with the increase of the proportion of ethanol to water
(Figure 3c,d). When the proportion is 21/15, the aggregation
type of the product changes again: the nanoprisms self-
assemble into a fanlike structure, and every fan is composed
of numerous nanoprisms with a diameter of about 0.1 μm
(Figure 3e). These results reveal that the proportion of
ethanol to water has an important effect on the shape of
the products.
In order to investigate the possible growth mechanism, the
influences of reaction time and other reaction conditions
were further investigated.
Figure 4 shows the SEM images of the products obtained
with different reaction times. When the reaction time was 1 h,
one can notice that the product is mainly flower-like micro-
clusters composed of nanorods (Figure 4a). As the reaction
time was increased, the small flower-like microclusters
developed into larger spherical flower-like hierarchical
architectures comprised of hexagonal prisms (Figure 4b).
The diameters and lengths of the hexagonal prisms increased
gradually with the reaction time. At the same time, some new
small prisms in spherical flower-like architectures were
formed. As a result, the comparative loose space among
the prisms of the architectures became tight (Figure 4c-e).
These results reveal that the formation of the spherical
flower-like hierarchical architectures needs a certain time.
The product obtained at 90 °C for 1.5 h is further
characterized by TEM, HRTEM, and SAED. Figure 5a
displays the TEM image of the product, which is of perfect
spherical assembly, confirming the above SEM observation.
Figure 5c shows a typical HRTEM image of the selected
prism (Figure 5b); the clear lattice fringes with a d-spacing of
Figure 5. TEM and HRTEM images and SAED pattern of the product
obtained at 90 °C for 1.5 h, with other reaction conditions similar to those
for S3: a, low-magnification TEM image of the product; b, TEM image of
an individual prism broken down from the hierarchical architecture; c,
HRTEM image of the selected prism; d, SAED pattern taken from the
selected prism.
prism. The indexes of the crystal faces are calculated, and the
results shown in the image (Figure 5d) are consistent with the
corresponding XRD pattern. Both the HRTEM image and
the SAED pattern clearly demonstrate the single-crystalline
nature of the prism.
The amount of HNO added into the reaction solution also
3
has an important effect on the morphologies of the products.
Figure 6 displays the SEM images of the products obtained
with different amounts of HNO and other reaction condi-
3
0
.61 nm belong to the lattice fringe of the (100) plane.
tions similar to those of S3. It can be noticed that spherical
Figure 5d shows the SAED pattern taken from the selected
flower-like hierarchical architectures remain with an increase

1
1562 Inorganic Chemistry, Vol. 48, No. 24, 2009
Yang et al.
Figure 6. SEM images of the products obtained with different contents
of HNO and other reaction conditions similar to those for S3: a, 3.2 mL;
b, 3.4 mL; c, 3.6 mL; d, 3.8 mL.
Figure 7. SEM images of the products obtained with different contents
of citric acid and other reaction conditions similar to those for S3: a, 0
mmol citric acid; b, 1.5 mmol citric acid; c, 3 mmol citric acid; d, 4.5 mmol
citric acid.
3
in the amount of HNO lower than 3.6 mL, though the
3
composed prisms grew larger gradually (Figure 6a-c). When
the amount of HNO increased to 3.8 mL, the prisms did not
3
assemble together and form spherical flower-like hierarchical
architectures; all of them remain single (Figure 6d). These
results exhibit that the acidity of the solution also has an
effect on the shape of the products.
During the experiment, it is worth noting that the amount
of citric acid plays an important role in controlling the
morphologies of the product. Figure 7 shows the SEM
images of the products obtained with different amounts of
citric acid and other reaction conditions similar to those of
S3. Without the addition of citric acid into the reaction
system, only nanorods were observed in the product
(Figure7a). When the content of citric acid is 1.5 mmol,
dandelion-like architectures with diameters of 4-6 μm were
obtained. Every “dandelion” is comprised of numerous
nanorods originating from the center (Figure 7b). As the
content of citric acid was increased to 3 mmol, spherical
flower-like hierarchical architectures were formed through
the self-assembly of nanorods (Figure 7c). While the content
of citric acid was further increased to 4.5 mmol, their
diameters became larger and their shapes became elliptic
Figure 8. SEM images of the products obtained with different contents
of CTAB and other reaction conditions similar to those for S3: a, 0 mmol
CTAB; b, 1.5 mmol CTAB; c, 3 mmol CTAB; d, 4.5 mmol CTAB.
On the basis of the experimental results and analysis, it is
reasonable to assume that the products may grow following
the crystal splitting theory that was proposed by Tang and
Alivisatos for the formation of the sheaf-like Bi S hierarch-
(Figure 7d). These results indicate that appropriate content
2
3
27
of citric acid is important for the formation of the flower-like
ical structure. At the beginning of the reaction, CePO4
nuclei were produced, which grew the building blocks for the
hierarchical structure. In the following growth process, the
nuclei grew into nanorods under the effects of the crystal
structure and external powers. At the same time, branches
were developed on the surroundings of the nanorods stems,
leading to the splitting. This progress was a fast growth
CePO hierarchical architectures.
4
Figure 8 shows the SEM images of the products obtained
with different amounts of the CTAB and other reaction
conditions similar to those for S3. It can be found that the
architectures obtained without the addition of CTAB into
the reaction system consist of a hexagonal prism in the
middle and many small prisms tightly growing on its
side without any space between them, but the shape of the
surrounding prisms is not complete (Figure 8a, their lower
and higher magnifications are shown in SP). When CTAB
was added into the reaction system, the shape of the
product changed dramatically, and spherical flower-like
hierarchical architectures composed of prisms were formed.
It is noted that the main shape of the product changes little
with the increase of the content of CTAB (Figure 8b-d).
Therefore, the existence of CTAB also plays an important
role in the formation of the spherical flower-like hierarchical
architectures.
process. The growth rate of CePO nuclei and the surfactant
4
adsorbed on the crystal facet should play key roles in the
formation of the final hierarchical architectures. As the
reaction time increased, splitting growth continued, and the
comparative loose space among the prisms of the architec-
tures became tight gradually. The possible growth process of
the product is shown in Figure 9.
The formation of one-dimensional hexagonal prisms
should be the main result of the inner cell structure of
hexagonal CePO . Figure 10 shows the simulated crystal
4
(27) Tang, J.; Alivisatos, A. P. Nano Lett. 2006, 6, 2701.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11563
4
Figure 9. Possible growth process of the CePO sphere flower-like hierarchical architecture.
4
Figure 10. Simulated crystal structures of CePO : a, crystal structure; b, packing view along the c axis; c, packing view along the a axis.
structures of CePO4. The overall structure of hexagonal
CePO can be described as columns built up of alternate
4
cerium and phosphate ions, extending along the c axis, each
column linked to four neighboring columns (Figure 10b).
The packing structure of hexagonal CePO viewed along the
4
a axis can be described as infinite linear chains, parallel to the
28
c axis (Figure 10c). This 1D characteristic of the infinite
linear chains of hexagonal-structured CePO should play
4
crucial roles in determining the one-dimensional hexagonal
29
prisms.
In addition, it is well-known that CTAB is a soft
template which facilitates the growth of the 1D structure,
and many researchers have used it to control the growth of
3
0-32
the 1D structures.
CTAB can ionize completely in
water. The resulting cation is a positively charged tetra-
3
-
hedron with a long hydrophobic tail, while PO4 also has
a tetrahedral geometry but is negatively charged. There-
þ
3-
fore, ion pairs between CTA and PO4 could form due to
electrostatic interaction. When an amount of CTAB is
þ
added to the reaction solution, the resulting CTA ions
adsorb on the surface of the nuclei and serve as a growth
controller and an agglomeration inhibitor, which further
inhibit the increase of the diameter of the prisms as well
as the aggregation of the other nuclei on the formed
3
2
crystals.
(28) Mooney, R. C. L. Acta Crystallogr. 1950, 3, 337.
(29) Fang, Y. P.; Xu, A. W.; Song, R. Q.; Zhang, H. X.; You, L. P.;
Jimmy, C. Y.; Liu, H. Q. J. Am. Chem. Soc. 2003, 125, 16025.
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005, 11, 2183–2195.
31) Li, Y.; Li, Y. D.; Deng, Z. X.; Zhuang, J.; Sun, X. M. Int. J. Inorg.
(
2
(
Figure 11. Excitation (a) and emission (b) spectra of the products
obtained at different reaction times: a, 1 h; b, 1.5 h; c, 10 h (the three
samples are measured by using the same experimental conditions).
Mater. 3 2001, 633–637.
(32) Sun, X. M.; Chen, X.; Deng, Z. X.; Li, Y. D. Mater. Chem. Phys.
2002, 78, 99.

1
1564 Inorganic Chemistry, Vol. 48, No. 24, 2009
Yang et al.
35
as the direction preference of growing nuclei As described
above, when the content of the ethanol increases, the viscidity
of the solution decreases dramatically, which can create more
3þ
3-
chances for the Ce ions and PO4 ions to collide with each
other and form nuclei. Under the cooperation of CTAB and
citric acid, the following crystal splitting process is acceler-
ated, which further induces the formation of the final 3D
hierarchical architecture.
Figure 11 shows the excitation and emission spectra of the
CePO hierarchical architectures obtained at different reac-
4
tion times. The excitation spectra show a broad band, which
2
corresponds to the transitions from the ground F to
5/2
3þ
excited 5d states of the Ce ions. The emission spectra
exhibit a strong ultraviolet broad band centered at 344 nm,
3þ
due to the 5d f 4f transition of the Ce ions. From the
emission spectra, it can be found that the intensity of the
luminescence increases with a prolonging of the reaction time.
In general, radiative mechanisms compete with nonradiative
mechanisms, and the defects in materials can form quenching
centers which lead to nonradiative recombination and lumi-
nescence quenching. Therefore, the difference in emission
intensity should be mainly associated with the defects which
come from the surface states of the prisms since the surface
defects are a most important pathway for nonradiative relaxa-
36
tion in nanomaterials. This result reveals that the crystal
structure is perfected and the defects on the surface decrease
gradually with increases in the reaction time.
Furthermore, the effect of the morphology on the lumi-
nescent properties of the products also has been investigated.
Figure 12 shows the excitation and emission spectra of the
CePO4 with different morphologies. From the emission
3
þ
Figure 12. Excitation (a) and emission (b) spectra of the products with
different morphologies: a, flower-like hierarchical architectures (S3,
Figure 3c); b, prisms (3.8 mL HNO , Figure 6d); c, nanorods (0 g citric
3
acid, Figure 7a). (The three samples are measured by using the same
experimental conditions.)
spectra, one can find that the intensity of the Ce emission
depends on the shape of the products. The intensity of flower-
like hierarchical architectures (S3) is the weakest one, while
that of the prisms obtained with 3.8 mL of HNO added into
3
the reaction solution is the strongest one of the three samples.
One reason for the change in luminescent intensity is that the
remnants of the organic reagents of CTAB and citric acid
absorbed on products have a quenching effect on the lumi-
nescence of the products. Another reason for the variation is
that the different surface defects of the products, which come
from the surface states of the components, affect luminescent
efficiency, because the defects in materials can lead to
luminescence quenching. In addition, the complication of
the morphology of the flower-like hierarchical architectures
reduces the luminescence efficiency by reflecting part of the
luminescence in the inner part of the architectures, which
leads to the absorption of the defects in the flower-like
hierarchical architectures. Therefore, the difference in the
luminescent intensity of the products may be due to the
remnants of the organic reagents of CTAB and citric acid
absorbed on the samples, the defects on the surface, and the
change of the morphology. The detailed mechanism behind
morphology’s influence on the properties of photolumine-
scence still needs further research.
The addition of citric acid also plays an important part in
the formation of the final hierarchical architecture. It is
known that citric acid possesses three carboxylic acid groups
and one hydroxyl functional group, which can provide
chelating ability and selectively bind to specific crystallo-
graphic facets. In the beginning of the reaction, citric acid
3þ
reacts with the Ce ions to form citrate complexes and
3þ
decreases the concentration of the Ce ions in the solution.
After the addition of the phosphorus source, an ion-exchange
3-
reaction between PO4 and citrate ions takes place, leading
to the formation of CePO under hydrothermal conditions.
4
Reaction velocity can be adjusted through the slow release of
the complex. At the same time, the special molecular struc-
ture of citric acid makes them selectively bind sideways to the
33,34
pre-existing prisms,
which further induces the aggrega-
tion of the newly formed nuclei on the preformed prisms and
forms the final flower-like hierarchical structures.
It should be noted that the addition of the ethanol also has
an effect on the final morphology of the product by adjusting
the viscidity and saturated vapor pressure of the solution
under thermal conditions, which further effect the homo-
genization of the reactants in the reaction medium, the amount
of the individual nuclei formed, and the amalgamation as well
4. Conclusion
In summary, spherical flower-like CePO4 hierarchical
architectures composed of numerous hexagonal prisms
(
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1
(

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11565
originating from the same center have been successfully
obtained via a hydrothermal method. The investigation of
synthesis parameters reveals that the coexistence of citric acid
and CTAB in the reaction system should be responsible
for the formation of the spherical flower-like CePO4
hierarchical architectures. It is suggested that the growth
process follows the crystal splitting theory. The luminescent
properties exhibit that the intensity of the luminescence
increases with the reaction time, revealing a decrease of
study may open a novel and facile route for the design and
synthesis of inorganic functional materials with hierarchical
architectures.
Acknowledgment. This work is financially supported
by the National Natural Science Foundation of
China (Grant No. 20771098) and the National Basic
Research Program of China (973 Program, Grant Nos.
2007CB935502 and 2006CB601103).
the surface defects in the CePO hierarchical architectures.
4
3þ
It is also observed that the intensity of the Ce emission
depends on the shape of the products. These unique
hierarchical architectures may have potential applications
in the fields of optoelectronic devices. Moreover, our
Supporting Information Available: Figure showing the lower
and higher magnifications of the product obtained without
CTAB. This material is available free of charge via the Internet
at http://pubs.acs.org.