Systematic Synthesis and Characterization of Single-Crystal Lanthanide Orthophosphate Nanowires

Article abstract of DOI:10.1021/ja037280d

A simple hydrothermal method has been developed for the systematic synthesis of lanthanide orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure LnPO₄ compounds change structure with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured lanthanide orthophosphate LnPO₄ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike morphology. In contrast, tetragonal LnPO ₄ (Ln = Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same experimental conditions consist of nanoparticles. The obtained hexagonal LnPO₄ (Ln = La → Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 °C in air (Hexagonal DyPO₄ is an exceptional case, it transformed to tetragonal DyPO₄ by calcination), while the tetragonal structure for (Ho→ Lu, Y)PO₄ remains unchanged by calcination. The resulting LnPO₄ (Ln = La → Dy) products consist almost entirely of nanowires/nanorods with diameters of 5-120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium doped LaPO₄ nanowires were also prepared, and their photoluminescent properties were reported. The optical absorption spectrum of CePO₄ nanowires was measured and showed some differences from that of bulk CePO₄ materials. The possible growth mechanism of lanthanide phosphate nanowires was explored in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra, and photoluminescence spectra have been employed to characterize these materials.

Full text of DOI:10.1021/ja037280d

Published on Web 11/26/2003
Systematic Synthesis and Characterization of Single-Crystal
Lanthanide Orthophosphate Nanowires
†
,†
†
†
§
Yue-Ping Fang, An-Wu Xu,* Rui-Qi Song, Hua-Xin Zhang, Li-Ping You,
‡
†
Jimmy C. Yu, and Han-Qin Liu
Contribution from the School of Chemistry and Chemical Engineering, Zhongshan UniVersity,
Guangzhou, 510275, China; Department of Chemistry, The Chinese UniVersity of Hong Kong,
Shatin, New Territories, Hong Kong, China; and Electron Microscopy Laboratory,
Peking UniVersity, Beijing, 100084, China
Received July 15, 2003; E-mail: cedc17@zsu.edu.cn
Abstract: A simple hydrothermal method has been developed for the systematic synthesis of lanthanide
orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure
LnPO compounds change structure with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to
4
Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La
to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured
lanthanide orthophosphate LnPO
morphology. In contrast, tetragonal LnPO
experimental conditions consist of nanoparticles. The obtained hexagonal LnPO
convert to the monoclinic monazite structured products, and their morphologies remained the same after
calcination at 900 °C in air (Hexagonal DyPO is an exceptional case, it transformed to tetragonal DyPO
by calcination), while the tetragonal structure for (Hof Lu, Y)PO remains unchanged by calcination. The
resulting LnPO (Ln ) La f Dy) products consist almost entirely of nanowires/nanorods with diameters of
-120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium
4
(Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike
(Ln ) Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same
(Ln ) La f Tb) can
4
4
4
4
4
4
5
doped LaPO
optical absorption spectrum of CePO
of bulk CePO
4
nanowires were also prepared, and their photoluminescent properties were reported. The
nanowires was measured and showed some differences from that
materials. The possible growth mechanism of lanthanide phosphate nanowires was explored
4
4
in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy,
electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra,
and photoluminescence spectra have been employed to characterize these materials.
8
-10
Introduction
from nanotubes,
a variety of 1D nanowires including
1
1
12
14
oxides, metal and alloys, semiconductors, metal chalco-
genides, and polymers, and so forth have been synthesized
Dimensionality plays an important role in determining the
properties of nanomaterials and in the synthesis of nanostruc-
tured materials with controlled morphology, size, chemical
composition, and crystal structure.1-3 Due to the discovery of
carbon nanotubes, a lot of one-dimensional (1D) nanomaterials
have been extensively investigated during the past few decades
1
3
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†
Zhongshan University.
The Chinese University of Hong Kong.
Peking University.
‡
§
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9
16025

A R T I C L E S
Fang et al.
and their novel properties have also been extensively explored.
These 1D nanostructured materials have been shown to play
an important role as active components or interconnects in
fabricating nanoscale electronic, optical, optoelectronic, elec-
trochemical, and electromechanical devices.16 Notable examples
of demonstrated applications include field-effect transistors
LnPO4 morphologies with the crystal structures. These details
will be presented in this article, where we report a systematic
synthesis and characterization of high-quality LnPO4 and
lanthanide ion doped LnPO4 single-crystal nanowires with
controlled crystal phases obtained by hydrothermal treatment.
The measurements of the photoluminescence emission and
photoluminescence excitation spectra of rare earth activated
LnPO4 nanowires/nanorods were performed. The growth mech-
anism of LnPO4 nanowires is also discussed based on the
inherent crystal structure of these materials.
17
13b
(
FETs), light-emitting diodes (LEDs), single-electron tran-
18
19
20
sistors, biological and chemical sensors, photodetectors,
electron emitters,21 and ultraviolet lasers.6
Although a number of synthetic methodologies have been
2
2
developed to fabricate and assemble 1D nanostructures, they
often suffer from the requirements of high temperature, special
conditions, tedious procedures, and catalysts or templates.
Therefore, the development of practical methods for fabricating
large numbers of 1D nanostructures at low cost is still a great
challenge for future study. Chemical methods, on the other hand,
seem to provide an alternative and intriguing strategy for
generating 1D nanostructures with respect to material diversity,
cost, versatility, synthetic tunability, and potential for large-
volume production. Of the methods employed in the synthesis
of 1D nanomaterials, hydrothermal methods have been regarded
as effective routes to the fabrication of high-quality anisotropic
nanomaterials.23 Several solution-phase procedures have been
demonstrated for generating 1D nanostructures.24 More recently,
some studies have been reported on the synthesis of lanthanide
hydroxide nanotubes and nanowires/nanorods by hydrothermal
processes.25 Conventionally, bulk monazite-type (high-temper-
ature phase) lanthanide phosphates have been prepared by solid-
phase reaction at high temperature.26 LnPO4 and rare earth ion
doped LnPO4 nanoparticles or colloids have also been prepared
by solution precipitation methods, and their photoluminescence
properties have been studied.27 However, previously there have
been no systematic accounts of synthesis, characterization, and
properties of LnPO4 nanowires and the correlation between
Orthophosphates are substances that are composed of isolated
PO tetrahedra, analogous to “orthosilicates”. The most common
4
naturally occurring orthophosphates are apatite [Ca (PO ) (F,-
5
4 3
Cl,OH)] and monazite (LnPO ), where Ln refers to lanthanide
4
elements. Lanthanide phosphates have several polymorphic
forms. They appear in hexagonal, tetragonal, and monoclinic
modifications. The hexagonal structure is the low-temperature
phase, and it can transform into the monoclinic structure, while
the tetragonal maintains its structure after calcination at 900
2
8-31
°C.
Lanthanide compounds have been extensively used as
high performance luminescent devices, magnets, catalysts, time-
resolved fluorescence labels for biological detection, and other
functional materials based on the electronic, optical, and
32
chemical characteristics resulting from the 4f shell of their ions.
Lanthanide orthophosphates (LnPO ) have a variety of poten-
4
tially beneficial properties, including very low solubility in water
-25
-27 33
(their solubility products are on the level of 10
to 10 ),
high thermal stability (the melting points of LnPO are around
4
34
2300 °C), high index of refraction, and, in the cases of Nd,
3
5
Eu, etc., high concentrations of lasing ions. These properties
provide the basis for interest in their use in a wide range of
applications such as phosphors, sensors, proton conductors,
ceramic materials, catalysts, and heat-resistant materials.3
Very recently, lanthanide phosphates have been of interest for
use as inert matrixes for Pu fuel and as multilayered (with Al2O3
6,37
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16026 J. AM. CHEM. SOC.
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VOL. 125, NO. 51, 2003

Lanthanide Orthophosphate Nanowires
A R T I C L E S
properties that make them of interest as scintillators for X-ray
and γ-ray detection in medical imaging applications, as ther-
mophosphors for remote temperature measurements, and as
hosts for microlasers.39 Recently, ceramic LaPO4 doped with
Characterization. The X-ray powder diffraction (XRD) patterns of
all samples were performed on a Rigaku/Max-3A X-ray diffractometer
with Cu KR radiation (λ ) 1.5418 Å). The operation voltage and current
were maintained at 40 kV and 40 mA, respectively. Field-emission
scanning electron microscopic (FE-SEM) images were obtained with
a JEOL JSM-6330F operated at a beam energy of 10.0 kV. Transmis-
sion electron microscopic (TEM) images, high-resolution transmission
electron microscopic (HRTEM) images, and the selected area electron
diffraction (SAED) patterns were obtained on a JEOL-2010 microscope
with an accelerating voltage of 200 kV. Energy-dispersive X-ray
spectroscopy (EDS) was attached to the JEOL 2010. Sample grids were
prepared by sonicating powdered samples in ethanol for 20 min and
evaporating one drop of the suspension onto a carbon-coated, holey
film supported on a copper grid for TEM measurements. A Shimadzu
spectrophotometer (model 2501 PC) equipped with an integrating sphere
was used to record the UV-vis diffuse reflectance spectra of the
samples. The excitation and emission spectra were recorded with an
F-4500 spectrophotometer equipped with a 150 W xenon lamp as the
excitation source. All the measurements were performed at room
temperature. Fourier transform infrared spectroscopy (FTIR) analysis
1
-5 mol % Sr has been found to exhibit proton conduction in
a wet atmosphere at high temperature. Such solid-state protonic
conductors are candidates as electrolytes in fuel cells, hydrogen
separation membranes, and sensors.4 Lanthanide phosphates
have been shown to be a useful host lattice for lanthanide ions
0
2
7,41
3+
to produce phosphors emitting a variety of colors.
Ce and
3+
Tb coactivated bulk LaPO4 is a highly efficient and com-
mercially applied green ( D4- F5 of Tb at 543 nm) phosphor
5
7
3+
in fluorescent lamps.2
7a,42
In compact lamps, the wall temper-
ature is in the range of 150-200 °C. At these temperatures,
most lamp phosphors degrade, but LaPO4/Ce/Tb phosphor is
stable at high temperatures.42 Nanoscaled phosphors could have
some potential advantages over conventional micron-sized
phosphors.43 The properties of lanthanide compounds depend
strongly on the compositions and structure, which are sensitive
to the bonding states of lanthanide ions. If lanthanide compounds
were fabricated in the form of a 1D nanostructure (e.g.,
nanowires) with a different crystal structure, they would have
new functionalities as a result of their marked specific shape.
They would also act as electrically, magnetically, or optically
functional host materials for rare earth ions in phosphor displays,
-1
was carried out using KBr disks in the region of 4000-400 cm by
using FTIR-Bruker-EQUINOX-55 at ambient conditions. X-ray pho-
toelectron spectroscopy (XPS) measurements were performed in a VG
Scientific ESCALAB Mark II spectrometer equipped with two ultra-
high-vacuum (UHV) chambers. All binding energies were referenced
to the C1s peak at 284.8 eV of the surface adventitious carbon.
Results and Discussion
waveguide devices, fluorescence labels for biological detection,
and the active material in lasers.44
I. Synthsis and Characterization of Hexagonal (La f Dy)-
PO4 Nanowires and Tetragonal (Ho f Lu, Y)PO4 Crystals.
Under identical synthetic conditions (at 150 °C for 12 h, pH
Experimental Section
Preparation. Hexagonal (La f Dy)PO
and tetragonal (Ho f Lu, Y)PO nanoparticles were prepared by a
simple hydrothermal method. In a typical synthesis, 10 mL of NH
PO (0.4 M) aqueous solution were added into 10 mL of Ln(NO
4
nanowires (or nanorods)
1
-2), two different types of crystal structure of the obtained
4
LnPO4 products have been identified by X-ray diffraction (XRD)
analyses. XRD patterns of the obtained hexagonal LnPO4 (Ln
4
H
2
-
4
3
)
3
)
La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) nanowires/nanorods
(
Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and
nonlanthanide Y) aqueous solution (0.4 M) under vigorous stirring for
0 min. The pH was adjusted to a specific value using aqueous ammonia
27 wt %) solution. The resulting suspension was poured into a Teflon-
and tetragonal (Ho f Lu, Y)PO4 nanoparticles are shown in
Figures S1 and S2 (Supporting Information), respectively. All
the peaks of the XRD patterns for each sample in Figure S1
can be readily indexed to a pure hexagonal phase [space
group: P3 21 (152)] of (La f Dy)PO , in good agreement with
2
(
lined stainless steel autoclave. The autoclave was sealed and maintained
at 150 °C for 12 h and then air cooled to room temperature. The
resulting LnPO products were filtered, washed with deionized water
4
and absolute alcohol to remove ions possibly remaining in the final
1
4
the data in the JCPDS cards. XRD results show that all nine
LnPO4 (Ln ) La f Dy) crystals are isostructural. Moreover,
the intensity of the (200) peak is much stronger than those of
the other peaks, which is different from bulk hexagonal LnPO4
products, and finally dried at 80 °C in air for further characterization.
LaPO
according to the same method. To prepare CePO
suspension in the autoclave was first purged with argon for 60 min to
4
nanowires doped with 5 mol % europium were prepared
4
nanowires, the
3
0
(
Ln ) La f Dy). This indicates that the obtained LnPO4 (Ln
) La f Dy) nanowires/nanorods grow preferentially along the
001] direction (the c axis). This is further demonstrated below
3
+
4+
prevent oxidation of Ce to Ce before heating. Monoclinic (La f
Tb)PO and tetragonal DyPO nanowires/nanorods were prepared by
calcination of hydrothermally obtained corresponding products at 900
C in air.
[
4
4
by HRTEM and ED measurements. All diffraction peaks shown
in Figure S2 are characteristic of a pure tetragonal phase [space
group: I41/amd (141)] of (Ho f Lu, Y)PO4 (the nonlanthanide
YPO4 also has a tetragonal structure, so it is included in the
present study). It is can be seen that six (Ho f Lu, Y)PO4
samples are isostructural. Moreover, XRD analyses show
systematic shifts in the positions of the diffraction peaks
reflecting the contraction of the ionic radii of the lanthanides,
as clearly seen from Figures S1 and S2. The unit cell dimensions
of all LnPO4 crystals calculated from XRD patterns are
consistent with those reported in the JCPDS cards.
°
(
(
(
39) (a) Moses, W. W.; Weber, M. J.; Derezno, S. E.; Perry, D.; Berdahl, P.;
Boatner, L. A. IEEE Trans. Nucl. Sci. 1998, 45, 462. (b) Allison, S. W.;
Boatner, L. A.; Gillies, G. T. Appl. Opt. 1995, 25, 5624. (c) Rapaport, A.;
David, V.; Bass, M.; Deka, C.; Boatner, L. A. J. Lumin. 1999, 85, 155.
40) (a) Norby, T.; Christiansen, N. Solid State Ionics 1995, 77, 240. (b)
Amezawa, K.; Maekawa, H.; Tomii, Y.; Yamamoto, N. Solid State Ionics
2
001, 145, 233. (c) Tyholdt, F.; Horst, J. A.; Jørgensen, S.; Østvold, T.;
Norby, T. Surf. Interface Anal. 2000, 30, 95.
41) (a) Dexpert-Ghys, J.; Mauricot, R.; Faucher, M. D. J. Lumin. 1996, 69,
2
03. (b) Rambabu, U.; Munirathnam, N. R.; Prakash, T. L.; Buddhudu, S.
Mater. Chem. Phys. 2002, 78, 160. (c) Wu, X.; You, H.; Cui, H.; Zeng,
X.; Hong, G.; Kim, C. H.; Pyun, C. H.; Yu, B. Y.; Park, C. H. Mater. Res.
Bull. 2002, 37, 1531.
(
(
(
42) (a) Kang, Y.; Kim, E. J.; Lee, D. Y.; Park, H. D. J. Alloys Compd. 2002,
The measured lattice parameters a and c, plotted against the
revised crystal (CR) ionic radii of the eight-coordinated three-
3
47, 266. (b) Lenggoro, I. W.; Xia, B.; Mizushima, H.; Okuyama, K.;
Kijima, N. Mater. Lett. 2001, 50, 92.
43) (a) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV.
Lett. 1994, 72, 416. (b) Holmes, J. D.; Ziegler, K. J.; Doty, R. C.; Pell, L.
E.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 3743.
44) (a) Schuetz, P.; Caruso, F. Chem. Mater. 2002, 14, 4509. (b)
45
valent lanthanide cations, are presented in Figure 1. The
(45) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 51, 2003 16027

A R T I C L E S
Fang et al.
Figure 2. SEM images of the as-synthesized LnPO4 nanowires by
hydrothermal treatment. (a) PrPO4, (b) SmPO4, (c) GdPO4, (d) TbPO4.
Figure 1. Plots of the measured lattice constants a (top) and c (bottom) of
the obtained hexagonal and tetragonal LnPO4 against lanthanide ionic radius.
approximately linear plots show the expected effect of the
lanthanide contraction. On the other hand, it is clearly observed
from Figure 1 that La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy are
located at one line and that their phosphates have a hexagonal
structure, while Ho, Er, Tm, Yb, and Lu are located on the other
line and their phosphates have a tetragonal structure. Why does
LnPO4 (Ln ) La f Dy) have a hexagonal structure and (Ho
f Lu, Y)PO4 crystallize in a tetragonal structure under the same
synthetic conditions? It is believed that this could be associated
with the change of lanthanide ion radius, although the exact
reason is not clear at present. In fact, the same phenomena have
also been observed for LnPO4 crystals prepared directly by solid-
phase reaction at high temperature (900-1300 °C). It is found
that LnPO4 (Ln ) La f Dy) crystallize in the monoclinic form,
while (Ho f Lu, Y)PO4 exist in the tetragonal structure under
direct solid-phase reaction conditions. Obviously, the monoclinic
is a high-temperature phase, and the hexagonal is a low-
temperature phase; the structure transition from the hexagonal
to the monoclinic can be observed by calcination at high
temperature. This will be further demonstrated below.
Figure 3. TEM images of the obtained hexagonal LaPO4 (a) and CePO4
(b) nanowires or nanorods.
The morphology of the samples was examined with scanning
electron microscopy (SEM) and transmission electron micro-
scopy (TEM). SEM images of the obtained PrPO4, SmPO4,
GdPO4, TbPO4, and DyPO4 nanowires/nanorods are shown in
Figures 2a, b, c, d, and S3e, respectively, and shown in Figure
Figure 4. SEM images of the obtained tetragonal ErPO4 (a) and LuPO4
b) nanoparticles.
(
noting that DyPO4 nanowires aggregate into large bundles, as
clearly shown in Figure S3e (Figure S3, Supporting Informa-
tion).
3
a, b, S3c, and S3d (Figure S3, Supporting Information) are
Under the same synthetic conditions, (Ho f Lu, Y)PO4
crystals with particle morphology can be observed. Figure 4a
and b shows SEM images of the obtained tetragonal structured
ErPO4 and LuPO4 crystals, respectively. YPO4, HoPO4, TmPO4,
and YbPO4 can also be obtained only in the form of irregularly
shaped particles. No rodlike morphology can be observed for
the TEM images of the obtained LaPO4, CePO4, NdPO4, and
EuPO4 nanowires/nanorods, respectively. The as-synthesized
LnPO4 (Ln ) La f Dy) products consist almost entirely of
nanowires/nanorods with diameters of 5-120 nm and lengths
ranging from several hundreds of nanometers to several
micrometers. The high density of nanowires is representative
of the high yields (close to 100%) associated with this new
preparation approach. The results demonstrate that high-quality
hexagonal structured LnPO4 (Ln ) La f Dy) nanowires/
nanorods can be obtained by this simple method. It is worth
(Ho f Lu, Y)PO4 samples.
The morphology and structure of the products were further
characterized by TEM and HRTEM. TEM and HRTEM images
and the selected area electron diffraction (SAED) patterns of
16028 J. AM. CHEM. SOC.
9
VOL. 125, NO. 51, 2003

Lanthanide Orthophosphate Nanowires
A R T I C L E S
calibrated by using C 1s (284.8 eV) as the reference. The binding
energies of the XPS peaks are consistent with that reported for
4
0c,47
bulk LaPO4.
The peak at 133.8 eV (P 2p) suggests that
phosphorus in the products exists as pentavalent oxidation states
5
+ 48
(
P ). The atomic ratio of La to P based on semiquantitative
XPS analysis is about 1:1. The XPS data demonstrate that the
product is pure LaPO4, which agrees with the results of XRD
and FTIR analyses. The XPS results for other lanthanide
phosphates also illustrate that the products were completely
formed in the stoichiometric ratio and that P exists in the P5
state (data not shown here).
+
Controlled experiments were carried out to investigate the
influence of pH and hydrothermal reaction temperature on the
synthesis of LnPO4 samples and the phase evolution. It is found
that pH has some effect on the length and aspect ratio of the
nanowires but little on the morphology of the nanowires. LnPO4
Figure 5. (a) TEM image of a single CePO4 nanowire. (b) HRTEM image
of a single nanowire with the clear lattice fringes of [001] with a spacing
0
.652 nm and [100] with a spacing 0.608 nm. Inset in part b: the
corresponding electron diffraction shows a single crystal recorded from the
010] zone axis.
(
La f Dy) nanowires were always obtained at pH 1-7.
Hexagonal LnPO4 (Ce f Dy) nanowires were obtained at pH
-7 by a hydrothermal process at 150 °C for 12 h. Increasing
[
1
CePO4, GdPO4, and DyPO4 are presented in Figures 5, S4, and
S5 (Figures S4 and S5, Supporting Information), respectively.
Figures 5a, S4a, and S5a show the low magnification TEM
images of the obtained LnPO4 nanowires, further demonstrating
that the obtained products have wirelike morphology. The
HRTEM images (Figures 5b, S4b, and S5c) taken from a single
nanowire show the clearly resolved planes of (001) and (100).
The (001) planes are oriented parallel to the nanowire growth
axis, indicating that the direction of nanowire growth is along
the c axis. The SAED pattern (inset in Figures 5b, S4b, S5b)
taken from a single nanowire can be indexed as a hexagonal
LnPO4 single crystal recorded from the [010] zone axis. The
same results were also obtained for the six other LnPO4
nanowires/nanorods. TEM observations for all wirelike samples
show that each nanowire has a single crystal and that the growth
direction of nine LnPO4 nanowires/nanorods is the same, along
the c axis, in good agreement with the XRD analyses. Energy-
dispersive X-ray spectroscopy (EDS) analysis shows that the
sample contains Ln, P, and O elements (Figure S6, Supporting
Information). The composition of the products as extracted from
the EDS analysis gives an Ln/P/O atomic ratio of ∼1:1:4.2, in
agreement with LnPO4.
the pH of the precursor mixture resulted in an increase in the
lengths and aspect ratios of the nanorods. As for LaPO4, when
the pH of the precursor suspension was increased to 4-5, the
product after a hydrothermal treatment at 150 °C for 12 h had
a monoclinic structure and wirelike morphology (data not
shown). The reaction temperature can cause the phase evolution.
When temperatrue was increased to 220 °C, monoclinic LnPO4
(La f Tb) nanorods/nanowires were obtained at pH 1-7, and
the obtained (Ho f Lu, Y)PO4 nanoparticles have a tetragonal
structure. Below pH 1, no precipitate was obtained. When the
pH was increased over 7, the mixture of LnPO4 and lanthanide
hydroxides was obtained. Our results demonstrate that LnPO4
(La f Dy) nanowires prepared at 150 °C for 12 h, pH 1-2 all
have a hexagonal structure.
II. Structure Transition of LnPO4 from Hexagonal to
Monoclinic after Calcination. After all LnPO4 samples ob-
tained by hydrothermal treatment were calcined at 900 °C in
air for 3 h, it was found that the hexagonal structure for LnPO4
(La f Tb) nanowires/nanorods transforms into the monoclinic
structure for corresponding materials. The hexagonal DyPO4 is
an exceptional case, and it converts to the tetragonal structure.
Tetragonal (Ho f Lu, Y)PO4 samples maintain their original
structure after calcination. Moreover, the morphology of nano-
wires was not altered by calcination. Figure S9 (Supporting
Information) shows XRD patterns of the obtained monoclinic
LnPO4 products by calcination, and from the figure it is clear
that all the diffraction peaks for each sample could be indexed
to a pure monoclinic phase [space group: P21/n (14)] of (La
f Tb)PO4, in agreement with the reported data in the JCPDS
cards. As for the hexagonal DyPO4 sample, the tetragonal
structure was obtained after calcination (Figure S10, bottom,
Supporting Information). The XRD patterns of the calcined
YPO4 and HoPO4 products are shown in Figure S10, and from
the figure it is clearly seen that the tetragonal structure in the
Figure S7 (Supporting Information) shows FTIR spectra of
the obtained LnPO4 (Ln ) La f Dy) and (Ho f Lu, Y)PO4
samples. Present FTIR data for lanthanide orthophosphates agree
with reported values46 and show the presence of a discrete PO4
group; no pyrophosphate (P2O7 group, a typical band appearing
-
1
at 1265-1267 cm ) impurity could be detected. There is a
noticeable shift in the absorption frequencies depending on
different lanthanide phosphates. For example, the absorption
-
1
peak (615 or 951 cm for LaPO4) is systematically shifting
-
1
into a higher frequency (626 or 985 cm for DyPO4) from
LaPO4 to DyPO4, as clearly indicated in Figure S7. The
systematic frequency shifts can be associated with the decrease
in ionic radius from La3 to Dy (the effect of the lanthanide
+
3+
(Ho f Lu, Y)PO4 samples remains unchanged (the calcined
contraction).
ErPO4, TmPO4, YbPO4, and LuPO4 smaples all have the
tetragonal structure; data not shown here). TEM images of the
The products obtained were further examined by XPS
analysis. The XPS spectra of hexagonal LaPO4 nanowires shown
in Figure S8 (Supporting Information) give further evidence for
the composition and purity of the products. All the peaks were
(
47) Jørgensen, S.; Horst, J. A.; Dyrlie, O.; Larring, Y.; Ræder, H.; Norby, T.
Surf. Interface Anal. 2002, 34, 306 (XPS).
(
48) (a) Splinter, S. J.; Rofagha, R.; Mcintyre, N. S.; Erb, U. Surf. Interface
Anal. 1996, 26, 471. (b) Yu, J. C.; Zhang, L. Z.; Zheng. Z.; Zhao, J. C.
Chem. Mater. 2003, 15, 2280.
(46) Hezel, A.; Ross, S. D. Spectrochim. Acta 1966, 22, 1949.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 51, 2003 16029

A R T I C L E S
Fang et al.
Figure 6. TEM images of monoclinic LnPO4 nanowires/nanorods obtained
by calcination of as-made corresponding products at 900 °C. (a) LaPO4,
(b) CePO4.
obtained monoclinic LaPO4, CePO4, PrPO4, and tetragonal
DyPO4 nanowires/nanorods are presented in Figures 6a, 6b,
S11c, and S11d (Figure S11, Supporting Information), respec-
tively. It is clearly shown that the morphologies of nanowires/
nanorods have remained the same after calcination at high
temperature, indicating that LnPO4 nanowires/nanorods are very
stable. With close examinations of each nanowire/nanorod by
TEM, we found that the surfaces of nanowires/nanorods become
coarser as compared to those of corresponding hexagonal
materials, an effect which is believed to be caused by calcina-
tion.
Figure 7. Absorption spectra of CePO4 nanowires (red line) and bulk
CePO4 (blue line).
nanowires consists of five peaks with maxima at 215, 240, 257,
2
73, and 320 nm (Figure 7, red line), which correspond to the
2
1
3+
transitions from the ground state F5/2 (4f ) of Ce to the five
crystal field split levels of the Ce
3+ 2
1
D(5d ) excited states. It
can be seen that the intensity of the peak at 273 nm is the
strongest one. On the contrary, the absorption spectrum of bulk
CePO4 has four peaks with maxima appearing at 226, 253, 265,
It is worth noting that DyPO4 is an exceptional case.
According to previous studies,30 it can crystallize in the
hexagonal, monoclinic, or tetragonal form. In fact, all (La f
Dy)PO4 can only crystallize in a monoclinic structure when
and 300 nm (Figure 7, blue line), in agreement with the reported
50a
values in the literature.
The peak at 300 nm shows the
strongest intensity, which is different from that of CePO4
nanowires. Moreover, the broad peak at 320 nm for CePO4
nanowires was not observed for bulk CePO4, which may be
attributable to the formation of low dimensional wirelike CePO4.
This differential spectral behavior observed for CePO4 nanowires
and bulk CePO4 might be ascribed to the distorted lattices. It is
generally considered that the degree of disorder in the nano-
materials is relatively high, thereby a lower crystal field
26a
directly prepared by a solid-phase reaction at high temperature,
while (Ho f Lu, Y)PO4 have the tetragonal structure when
prepared by the same method.49 As compared with LnPO4
prepared by a hydrothermal method, the same phenomena for
crystal phase change can also be oberved for LnPO4 obtained
directly by high-temperature solid-phase synthesis. We have also
prepared DyPO4 crystals by a direct solid-phase reaction and
found that only a monoclinic structure can be observed for
DyPO4. Why does the hexagonal structure for DyPO4 nanowires
obtained by the hydrothermal method convert to the tetragonal
but not the monoclinic after calcination? Figure 1 shows that
DyPO4 exists in the boundary positions between the hexagonal
and the tetragonal, which also indicates that DyPO4 may
crystallize in the tetragonal phase under certain conditions.
Overall, DyPO4 crystals can be obtained either in the tetragonal
phase or in the monoclinic phase depending on the experimental
conditions.
52
symmetry might be induced in such materials. The cell volume
of CePO4 nanowires calculated from the XRD pattern is 280.41
Å, while the cell volume of bulk CePO4 is 267.32 Å. The
increase in the cell volume of CePO4 nanowires indicates that
the lattices are more distorted in the nanowires than in the bulk
and that the crystal field symmetry is lower in the former than
52
in the latter. Lower crystal field symmetry means that it is
3+
easy for the excited Ce 5d degenerate states to be completely
split into five energy levels, and thus five peaks in optical
absorption spectra of CePO4 nanowires can be observed (Figure
7, red line). The lattice deformation may contribute to the change
III. Absorption Spectra of CePO4 Nanowires. Cerium
compounds such as CePO4, CeP2O7, and CeO2 have been known
to have strong absorption for the ultraviolet.50 The optical
absorption spectra of CePO4 nanowires were measured and
compared with those of bulk CePO4. The absorption spectra of
CePO4 nanowires (red line) and bulk CePO4 (blue line) are
shown in Figure 7. The observed absorption peaks are caused
by f-d electron transitions.51 The absorption spectrum of CePO4
of band structure in nanowires, thus giving rise to the absorption
peak shifts as compared with bulk material.
IV. Rare Earth Doped Hexagonal LnPO4 Nanowires
3+
Synthesis and Photoluminescence. Eu doped LaPO4 nano-
wires were prepared by the hydrothermal method, and in this
work, the concentration of dopant Eu ions is 5 mol %, so this
phosphor can be expressed as La0.95Eu0.05PO4. Figure S12
(Supporting Information) shows a representative TEM image
of the obtained La0.95Eu0.05PO4 product, and from the figure it
(
(
(
49) (a) Milligan, W. O.; Mullica, D. F.; Beall, G. W.; Boatner, L. A. Inorg.
Chim. Acta 1982, 60, 39. (b) Milligan, W. O.; Mullica, D. F.; Beall, G.
W.; Boatner, L. A. Acta Crystallogr. 1983, C39, 23.
(52) (a) Wei, Z.; Sun, L.; Liao, C.; Yin, J.; Jiang, X.; Yan, C. J. Phys. Chem.
B 2002, 106, 10610. (b) Tao, Y.; Zhao, G.; Zhang, W.; Xia, S. Mater. Res.
Bull. 1997, 32, 501. (c) Tissue, B. M. Chem. Mater. 1998, 10, 2837. (d)
Williams, D. K.; Bihari, B.; Tissue, B. M.; McHale, J. M. J. Phys. Chem.
B 1998, 102, 916.
50) (a) Imanaka, N.; Masui, T.; Itaya, M. Chem. Lett. 2003, 32, 400. (b)
Imanaka, N.; Masui, T.; Hirai, H.; Adachi, G. Y. Chem. Mater. 2003, 2003,
1
5, 2289.
51) Nakazawa, E.; Shiga, F. Jpn. J. Appl. Phys. 2003, 42, 1642.
16030 J. AM. CHEM. SOC.
9
VOL. 125, NO. 51, 2003

Lanthanide Orthophosphate Nanowires
A R T I C L E S
of the Judd-Ofelt theory.55 The splitting observed in each group
of luminescence lines (Figure 8, red line) is caused by the crystal
3+
field. The above luminescent properties of Eu in the crystalline
LaPO4 nanowires are basically in agreement with those for bulk
LaPO4/Eu reported previously, indicating that Eu3 ions have
been successfully doped into host LaPO4 nanowires. The
+
5
7
transition ( D0- F1) displays more intensity than that of the
5
7
transition ( D0- F2) due to localized energy transfer.
Although the transition energies are the same for nanowires
and bulk particles, the intensity patterns of their luminescence
spectra show small differences. Comparing bulk LaPO4/Eu5
and LaPO4/Eu nanowires, we find that the intensities of the lines
2a
5
7
belonging to the D0- F4 are different in the two cases. A
similar difference can be observed for the components of the
5
7
D0- F2 transition. The intensity of the luminescence lines of
LaPO4/Eu and other lanthanide doped phosphates is known to
Figure 8. Excitation (blue line) and emission (red line) spectra of
as-prepared La0.95Eu0.05PO4 nanowires.
depend on the orientation of the unique crystal axis relative to
5
6
the polarization vector of the incident light. Since our
excitation light was partly polarized, any preferred orientation
of the nanowires in the powders will influence the intensity
pattern of the emission spectra. Meanwhile, the ratio of the
can be seen that the sample consists almost entirely of
nanowires/nanorods with diameters of 5-30 nm and lengths
of several micrometers.
5
7
5
7
intensity of D0- F1 to D0- F2 for LaPO4/Eu nanowires is
5
The excitation spectrum obtained by monitoring the emission
of LaPO4/Eu nanowires is shown in Figure 8 (blue line). The
broad band with a maximum at 260 nm shown in the spectrum
originates from the excitation of the oxygen-to-europium charge-
transfer band (CTB). By comparison with that of bulk LaPO4/
smaller than that of bulk LaPO4/Eu. Considering that the D0-
F2 transition is hypersensitive to the symmetry of the crystal
7
field, and it will be relatively strong if the symmetry of the
crystal field is relatively low. By comparison with bulk LaPO4/
5
7
Eu, the increase of the intensity of the D0- F2 transition for
nanowires is obviously correlated with the distorted lattices. The
above results have demonstrated that the lattices are more
distorted in the LaPO4/Eu nanowires than in the corresponding
bulk material.
5
3
Eu (253 nm), a small shift of the CT band could be observed
for LaPO4/Eu nanowires (260 nm). As reported by previous
studies,5
2a,54
the peak position of the CT band is involved in
the length of Eu-O bond; the longer the Eu-O bond is, the
longer the wavelength of the CT band position will be. In this
study, we have already pointed out that a relatively larger cell
volume can be observed for LaPO4/Eu nanowires compared to
bulk LaPO4/Eu. This indicates that the average Eu-O bond
distance is relatively longer in LaPO4/Eu nanowires, and as a
consequence, a small redshift of the CT band would be observed.
In contrast to semiconductors such as CdSe, CdTe, and ZnO
nanocrystals showing unique absorption and fluorescence
5
7
characteristics due to quantum size effects, the fluorescence
of the LaPO4/Eu nanowires originates from their bulk properties
(i.e., transitions between d and f electron states and their local
symmetry). In fact, particle size effects on the luminescence of
LaPO4/Eu nanowires are expected to be weak, since transitions
of the well-shielded f electrons are mainly affected by the local
symmetry of the crystal site.
The general f-f transitions within the Eu3 4f electron
configuration in the longer wavelength region (300-500 nm)
can also be observed (Figure 8, blue line). These peaks
+
6
3+
correspond to the direct excitation of the Eu ground state into
V. Crystal Structure of LnPO4 and the Possible Growth
Mechanism of Hexagonal LnPO4 Nanowires. In general, three
kinds of mechanisms including vapor-liquid-solid (VLS)
7
5
higher levels of the 4f-manifold such as F0- L6 at 397 nm.
Upon excitation into the CTB of Eu3 at 260 nm, the emission
spectrum of powders of hydrothermally prepared LaPO4/Eu
nanowires (Figure 8, red line) consists of sharp lines as expected
for the transitions between europium levels, which couple only
+
5
8
59
growth, solution-liquid-solid (SLS) growth, and template-
6
0
mediated processes have been proposed for the formation of
crystalline nanowires. Clearly, the formation of the nanowires
in our synthetic system would not be dominated by the above-
mentioned three processes. To date, the shape of metal nano-
particles or semiconductor nanocrystals has mainly been con-
trolled by the presence of surfactants, polymers or strong
5
7
very weakly to lattice phonons. Among D0- FJ (J ) 1, 2, 3,
) emission lines of Eu3 shown in Figure 8 (red line), the
+
4
5
7
magnetic-dipole transition D0- F1 (591 nm) is the most
prominent group, which is characterized by orange-red emission.
5
5
3+
Emission from the higher energy levels ( D1, D2) of Eu is
not detected due to multiphoton relaxation based on the vibration
(
55) (a) Judd, B. R. Phys. ReV. 1962, 127, 750. (b) Ofelt, G. S. J. Chem. Phys.
1962, 37, 511.
-1
(56) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Haase, M. AdV.
of phosphate groups (ca. 1067 cm ), which can bridge the gaps
between the higher energy levels ( D1, D2) and the lowest D0
level of Eu effectively. The intensity of transitions between
different J-number levels depends on the symmetry of the local
environment of the europium ions and can be described in terms
Mater. 1999, 11, 840.
5
5
5
(
57) (a) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich,
A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Peng, Z. A.; Peng, X. G. J.
Am. Chem. Soc. 2001, 123, 183. (c) Tang, Z.; Kotov, N. A.; Giersig, M.
Science 2002, 297, 237. (d) Guo, L.; Ji, Y.; Xu, H.; Simon, P.; Wu, Z. J.
Am. Chem. Soc. 2002, 124, 14864.
3
+
(
(
58) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208.
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53) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem. 2003, 13,
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413.
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(54) (a) Hoefdraad, H. E. J. Solid State Chem. 1975, 15, 175. (b) Tao, Y.; Zhao,
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J. AM. CHEM. SOC.
9
VOL. 125, NO. 51, 2003 16031

A R T I C L E S
Fang et al.
chelating ligands. Peng et al. presented a three-stage shape
evolution mechanism based on the spherical diffusion controlled
crystal growth theory, which elucidated the shape evolution of
CdSe nanocrystals well.61 However, this theory has seldom been
used to explain the formation mechanism of 1D nanowires/
nanorods in surfactant-free systems. Yu et al. have recently
reported the general synthesis of metal tungstate nanorods by
the hydrothermal process and found that the formation process
6
2
of the nanorods could fit the spherical diffusion model well.
In our system, the formation of the nanowires could also be
explained by this theory. Here, the strong pH-dependent relation
with the lengths and aspect ratios of the nanowires is due to
the sensitive influence of the pH on the solute concentrations
3
+
3-
(
[Ln ] and [PO4 ]). This observation suggests that the 1D
growth stage was rightly captured by maintaining the solute
concentrations in the correct range, through control of the pH.
The obvious 1D growth stage observed here is also in agreement
with the previous model proposed by Peng et al.61
LnPO4 (Ln ) La f Dy) nanowires that were fabricated have
63
a hexagonal crystal structure, similar to that of ZnO and Ln-
(
OH)3,25 which are well-known to exhibit anisotropic growth.
Studying on the natural chemical character of lanthanide
phosphates can help us to understand the growth mechanism
of lanthanide phosphate nanowires. In this solution phase
process, the morphology of the final product is largely deter-
mined by the anisotropic nature of the building blocks, that is,
the 1D characteristics of the infinite linear chains of hexagonal
structured LnPO4 (Ln ) La f Dy) in the crystalline phase.
The crystal structure of representative hexagonal CePO4 is
shown in Figure 9.64 Each cerous ion is coordinated to eight
oxygen atoms, four at distances of 2.34 Å and four at 2.66 Å.
The shorter distances are to the corners of four different
tetrahedra, linking atoms which are practically coplanar. The
longer distances extend to pairs of oxygens forming the edges
of two tetrahedra, one above and one below the plane of the
four shorter distances. A given phosphate group is coordinated
to six cerous ions, each corner by one short bonding distance,
and opposite edges by longer bonding distances. The cerium-
cerium separation is 4.13 Å. The phosphate ion has been taken
to be a tetrahedral grouping of oxygens about phosphorus, with
the P-O distance fixed at 1.56 Å (Figure 9a). The overall
structure of this compound may be described as columns built
up of alternate cerous and phosphate ions, extending along the
c axis, each column linked to four neighboring columns is such
that open channels run though the structure along the hexagonal
axis (Figure 9b). The packing structure of hexagonal CePO4
viewing along the a axis can be described as infinite linear
chains, parallel to the c axis (Figure 9b). From a structural point
of view, hexagonal CePO4 consists of infinite linear chains
extending along the c axis. From a thermodynamic perspective,
the activation energy for the c axis direction of growth of
Figure 9. (a) Packing view of CePO along the c axis. Each column is
linked to four neighboring columns; open channels run though the structure
along the hexagonal axis (the c axis). Oxygen atoms are omitted for clarity.
4
(b) Packing view of CePO4 along the a axis. Each column is built up of
alternate cerous (shadow circles) and phosphate ions (open circles) and
extends along the c axis. Oxygen atoms are omitted for clarity.
nanowires/nanorods that grow preferentially along the [001]
direction. This is in very good agreement with the abnormally
strong intensity of the (200) peak in the XRD patterns (Figure
S1, Supporting Information) and HRTEM results (Figures 5,
S4, and S5). Hexagonal LnPO4 (Ln ) La, Pr, Nd, Sm, Eu, Gd,
Tb, and Dy) are isostructural with hexagonal CePO4; therefore,
the formation of other LnPO4 nanowires/nanorods can also be
explained based on its highly anisotropic character along the c
axis, in agreement with HRTEM observation (Figures 5, S4,
and S5). It is this structural feature that plays a key role in the
highly intrinsic preferential growth along the c axis of LnPO4
(Ln ) La f Dy) nanowires.
On the contrary, tetragonal (Ho f Lu, Y)PO4 has no preferred
growth direction in the crystalline phase based on their crystal
4
9
structure. The molecular arrangements of tetragonal LuPO4
viewing along the a, b, and c axis are presented in Figure 10a,
b, and c, respectively. Compound LuPO4 crystallizes in the
tetragonal system conforming to space group I41/amd. The Lu
atom is eight-coordinated to oxygen atoms with two unique
metal-oxygen bond distances. Each of these discrete lengths
are tetrahedrally oriented orthogonal to one another thus forming
a distorted dodecahedron, which is a bisphenoid contains two
interpenetrating distorted tetrahedra distinguished by the bond
lengths Lu-O’s ) 2.262 Å and Lu-O’s ) 2.344 Å. In other
words, the eight vertexes of the dodecahedron are not equivalent
hexagonal LnPO4 is lower than that of growth perpendicular to
the c axis.65 This means a higher growth rate along the c axis
and a lower one perpendicular to the c axis to form LnPO4
(
(
(
61) (a) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389. (b) Peng,
Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343.
62) Yu, S. H.; Liu, B.; Mo, M. S.; Huang, J. H.; Liu, X. M.; Qian, Y. T. AdV.
Funct. Mater. 2003, 13, 639.
63) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.;
Russo, R.; Yang, P. Science 2001, 292, 1897.
(
(
64) Mooney, R. C. L. Acta Crystallogr. 1950, 3, 337.
65) Murphy, K. E.; Altman, M. B.; Wunderlich, B. J. Appl. Phys. 1977, 48,
4
122.
16032 J. AM. CHEM. SOC.
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Lanthanide Orthophosphate Nanowires
A R T I C L E S
Figure 10. (a) Perspective view along the a axis of the molecular packing of LuPO4. (b) Perspective view along the b axis of the molecular packing of
LuPO4. (c) Perspective view along the c axis of the molecular packing of LuPO4.
but are divided into two bisphenoidal sets, and those within
each set are equal. The phosphate group PO4 is a distorted
tetrahedron with variable bond angles. No anisotropic nature
can be observed in the tetragonal structured LuPO4, which is
different from hexagonal structured CePO4. Therefore, the
prepared LuPO4 sample presents particle morphology instead
of nanowires. Tetragonal HoPO4, ErPO4, TmPO4, YbPO4, and
YPO4 are isostructural with tetragonal LuPO4;49 therefore, the
resulting irregularly shaped particle morphology can also be
explained based on their crystal structure.
particles. The structure transition of LnPO4 (Ln ) La f Tb)
nanowires/nanorods from the hexagonal to the monoclinic
monazite can be observed, while their morphology is retained
after calcination at 900 °C in air. In the case of hexagonal DyPO4
nanowires, they are transformed to tetragonal DyPO nanowires
4
by calcination. No phase change can be observed for the
tetragonal structured (Hof Lu, Y)PO after calcination. The
4
as-synthesized LnPO (Ln ) La f Dy) products consist almost
4
entirely of nanowires/nanorods with diameters of 5-120 nm
and lengths ranging from several hundreds of nanometers to
several micrometers. Europium doped LaPO4 nanowires were
also successfully prepared. The absorption spectra of CePO4
nanowires and photoluminescent properties of LaPO4/Eu nano-
wires were reported. Based on the crystal structure of LnPO4,
we suggest that the growth of LnPO4 (Ln ) La f Dy)
nanowires is determined by its highly anisotropic character along
the c axis. The simplicity of the hydrothermal process, cheap-
ness, and availability of raw materials, without the need for
catalysts or templates, are important considerations for industrial
manufacturing. Lanthanide phosphate nanowires could be doped
with different lanthanide elements and used for fluorescent
lamps, new type plasma display panels excited by ultraviolet
radiation, and luminescent labels for biomolecules. We believe
that other LnPO4 phosphors with a wirelike morphology such
Conclusions
This article describes the development of a simple hydro-
thermal process for the systematic preparation of LnPO4 crystals
that have been preformed by a facile hydrothermal process. It
has been found that lanthanide phosphates LnPO4 crystallize
with hexagonal or tetragonal structure depending on the ionic
radius of the lanthanide ions. LnPO4 (Ln ) La f Dy)
nanowires/nanorods crystallize in the hexagonal form, while (Ho
f Lu, Y)LnPO4 materials exist in the hexagonal structure under
low-temperature hydrothermal conditions. The obtained hex-
agonal LnPO4 (Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy)
products have wirelike morphology. On the contrary, tetragonal
LnPO4 (Ln ) Ho, Er, Tm, Yb, Lu, Y) samples prepared under
the same experimental conditions consist of irregularly shaped
J. AM. CHEM. SOC.
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A R T I C L E S
Fang et al.
as LaPO4/Tb, LaPO4/Ce/Tb, and (La, Gd)PO4/Tb can be
prepared by the same method. Metal phosphates, also known
as a component of bones and teeth, have proven to be inert and
safe, with no known side effects to the human body. The features
of lanthanide doped lanthanide phosphates are their high
quantum yield, their high chemical stability, and their expected
low toxicity, making them particularly suitable for biological
labeling applications based on the individual rare earth doped
LnPO4 nanowire.
Acknowledgment. This work was supported by the National
Natural Science Fund of China and the Guangdong Province
“The Tenth Five-Year Plan” Key Projects (2001A1070103).
Supporting Information Available: XRD patterns; TEM and
SEM images; EDS spectrum; FTIR spectra; XPS spectra (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA037280D
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