Synthesis and characterization of single-crystalline lanthanum fluoride with a ring-like nanostructure

Article abstract of DOI:10.1002/ejic.200900126

LaF₃ ring-like nanostructures with a diameter of less than 2 |im have been fabricated by a facile, effective, and environmentally friendly molten salt synthesis route in which NaNO₃ and KNO₃ (2:1 molar ratio) act as reacti

Full text of DOI:10.1002/ejic.200900126

FULL PAPER
DOI: 10.1002/ejic.200900126
Synthesis and Characterization of Single-Crystalline Lanthanum Fluoride with
a Ring-Like Nanostructure
Yang Tian,*[a] Jing Wen,[a] Bin Liu,[a] Ning Sui,[b] Qionghua Jin,[a] and Xiuling Jiao*[b]
Keywords: Molten salt synthesis / Nanostructures / Crystal growth / Lanthanum / Halides
LaF3 ring-like nanostructures with a diameter of less than
2 µm have been fabricated by a facile, effective, and environ-
mentally friendly molten salt synthesis route in which NaNO3
and KNO3 (2:1 molar ratio) act as reaction media and the
rare-earth nitrate and NaF as precursor. X-ray diffraction,
TEM, HR-TEM, energy dispersive X-ray spectroscopy, and
photoluminescence spectroscopy are all used to characterize
the as-prepared samples. Experiments peformed with dif-
ferent reaction times indicate that a central-etching of the
plates from the inner part towards the edge during nanocrys-
tal growth plays a key role in the formation of LaF3 nanorings
since no other templates/surfactants are present in our sys-
tem. Additionally, the luminescence properties of LaF3 nano-
rings doped with Eu3+ cation have been investigated and-
compared with those of bulk materials and nanoparticles
with a size of approximately 50 nm.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Changes to the amount and properties of the salts used
could have profound effects on crystal growth, which would
be reflected in the morphology and physicochemical char-
acteristics of the products formed. Molten salt synthesis has
been shown to be a promising route for the preparation of
metal-oxide materials, especially for the synthesis of nano-
materials, and many new types of nanomaterials have been
prepared by this method.[6,7] However, there have been few
reports on the fabrication of ring-like nanostructures in
molten salts.[7]
Rare-earth fluorides are a class of materials with great
potential in optical applications, such as phosphors in
lamps and display devices, components in optical telecom-
munications, light-emitting diodes (LEDs), biological label-
ling, and solid-state lasers.[8] The size- and shape-controlled
synthesis of rare-earth fluoride nanomaterials has therefore
attracted a great deal of attention during the past few years
and many methods have been developed for their prepara-
tion. For example, Yan and co-workers have prepared sin-
gle-crystal and monodisperse LaF3 triangular nanoplates
via a La(CF3COO)3 pyrolysis route,[9] and Li and co-
workers have successfully employed hydrothermal routes to
yield fullerene-like nanoparticles and Ln3+-doped, bundle-
like YF3 phosphors.[10] Similarly, Cao et al. have obtained
CeF3 nanocrystals in the form of disks, rods, and dots via
a mild ultrasound-assisted route from an aqueous solution
containing different fluorine sources,[11] and Chen et al.
have reported a hydrothermal route for the preparation of
hollow peanut-like structures of YF3 using the tetrafluo-
roborate complex as the fluoride source.[12] Ritcey et al.
have used reverse microemulsions to prepare hexagonal and
triangular YF3 nanocrystals,[13] Yao et al. have successfully
prepared YF3 sub-microsized truncated octahedra with
Introduction
The direct fabrication of complex architectures with con-
trolled morphology and dimensionality is always important
because such control is critical in investigating the shape
and structural dependence of many applications. Among
those various architectures, ring-like building blocks have
attracted intense research interest because of their novel
properties and promising applications in the design of com-
plex nanostructures for precise nanofabrication.[1] For ex-
ample, the holes in the nanorings can be used as nanoves-
sels for biomolecular probes and for smaller nanostructures
as multifunctional materials.[2] To date, ring-like structures
have been obtained by electron beam lithography and hard
template-assisted techniques,[3] although the high cost and
complexity associated with the templates limits the practical
applications of these approaches. In recent years, a few ring-
like nanostructures, such as CdS, ZnO, Cd(OH)2, Fe2O3,
Co, Ag2V4O11 and so on, have been successfully synthesized
by wet chemical methods in aqueous and organic solu-
tions.[4] However, the challenge of developing simple “bot-
tom-up” techniques for the fabrication of ring-like architec-
tures in solvents such as ionic liquids and molten salts still
remains.
Molten salts are completely ionized nonaqueous solvents
which have a unique and rich chemical character.[5]
[a] Department of Chemistry, Capital Normal University,
Beijing 100048, P. R. China
Fax: +86-010-68903040
[b] School of Chemistry and Chemical Engineering, Shandong
University,
Jinan 250100, P. R. China
E-mail: tianyang@mail.sdu.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Y. Tian, J. Wen, B. Liu, N. Sui, Q. Jin, X. Jiao
FULL PAPER
Eu3+ doping,[14] and Lin and co-workers have prepared lan- fringes on the surface of the nanocrystal (Figure 2, b) show
thanide fluorides with multiform structures and morpho- its good crystallization and the fast Fourier transform
logies by hydrothermal synthesis.[15] Different EuF3-based pattern (FFT, inset in Figure 2, b) shows its single-crystal-
structures have also been fabricated recently.[16] Despite line nature with a crystallographic orientation [001]. The
these advances, the range of rare-earth fluoride geometries plane spacing of 0.323 nm corresponds to the (111) and
¯
still needs to be expanded to meet the increasing nano- and (121) planes of hexagonal LaF3, and the plane spacing of
micotechnological demands. 0.360 nm is assigned to the (110) planes. The structure of
In the present work we report a convenient molten salt as-prepared LaF3 hexagonal nanorings can therefore be in-
method in which a mixture of NaNO3 and KNO3 (2:1 mo- dexed with top/bottom surfaces of (001) and side surfaces
¯
lar ratio) acts as the reaction medium for the synthesis of of {100} and {110}, as shown schematically in Figure 3.
hexagonal LaF3 rings without the need for a surfactant.
The high-quality crystalline nature of the LaF3 structures
was confirmed by various analytic techniques and the fluo-
rescent properties of LaF3 as a host matrix were charac-
terized by doping with Eu3+ cation.
Results and Discussion
The sample was investigated by powder X-ray diffraction
(XRD) to characterize its phase and crystallization. As
shown in Figure 1, all the peaks of XRD pattern are in
good agreement with the previously reported hexagonal
structure (space group: P63/mcm) for crystalline LaF3
(JCPDS Card 08-0461), with a high degree of crystallinity
and few impurities. The crystal cell parameters of the sam-
ple were calculated to be a = 0.7178 nm and c = 0.7358 nm
by JADE 5. These calculated values are in agreement with
the values given in the JCPDS Card 08-0461 file for LaF3
(a = 0.7184 nm and c = 0.7351 nm).
Figure 2. (a) TEM image of the obtained sample (inset: SEM im-
age). (b) HR-TEM image obtained from the frame marked in (a)
(inset: FFT pattern).
Figure 1. XRD pattern of the as-prepared LaF3 product.
Figure 2 (a) shows a typical TEM image of the sample.
It can be seen that multiple crystals have a ring shape with
an almost hexagonal outer-wall and diameters in the range
obtained LaF3.
800–1600 nm. The inset in Figure 2 (a) shows the SEM im-
Figure 3. Schematic diagram showing a single nanoring of the as-
age, which confirms that the morphology and size are in
Different mechanisms for the formation of ring-like
agreement with those observed by TEM. The inductively structures in various synthetic systems have been reported,
coupled plasma mass spectrum (ICP-MS) and the energy such as templates,[17] the Kirkendall effect,[18] the self-as-
dispersive X-ray (EDX) spectrum (Figure S1 in the Sup- sembly of primary nanoparticles,[19] central-etching of
porting Information) of the as-obtained LaF3 show the ex- disks,[20] and self-coiling of nanobelts.[4f] Based on our ex-
pected element signals and stoichiometry, within experi- periments, we believe that a central-etching of disks during
mental error. Moreover, the high-resolution TEM (HR- crystal growth, based on the well-known Ostwald ripening
TEM, Figure 2, b) image demonstrates the intrinsic crystal- process, from the inner toward the outer surface plays a key
lography of the as-obtained LaF3 nanorings. The lattice role in the formation of our LaF3 nanorings since no other
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Eur. J. Inorg. Chem. 2009, 2383–2387

Lanthanum Fluoride with a Ring-Like Nanostructure
templates/surfactants are present in the system. However, it
is well known that the edges of a crystal are less stable than
the center as the edges possess higher energy, therefore it
was not initially clear why etching occurs at the center of
the disks rather than at the edges to form the ring struc-
tures.
To investigate the driving force behind the formation of
these ring-like structures in molten salt solution, we decided
to monitor the reaction progress by performing time-de-
pendent experiments. Although we are aware that the reac-
tion does not stop immediately after the crucible is removed
from the heater owing to heat-transfer reasons, we believe
that the products formed at that time represent certain
stages in the formation process. The TEM images obtained
for these time-dependent experiments are shown in Fig-
ure 4, which shows the morphological evolution of the re-
sulting LaF3 nanorings. Figure 4 (a) shows the TEM image
of the product obtained from the reaction after 10 min. It
can clearly be seen that the morphology of most nanocrys-
tals is that of an asymmetric near-hexagonal plate with the
size of around 800 nm. The SAED pattern (Figure 4, d) of
one of these crystals indicates its single-crystalline nature
with an axis of [001]. On the basis of the SAED pattern, the
side surfaces should be prismatic {–110} or {100} planes of
hexagonal LaF3, which are the same as those in the final
LaF3 nanoring. Furthermore, the XRD pattern (Figure S2
in the Supporting Information) also illustrates that LaF3
nanocrystals are already well crystallized after only 10 min
of reaction. Moreover, it can clearly be seen from Figure 4
(a) that there are many cavities, with a diameter of about
5 nm, on the top/bottom surfaces of the obtained plates. It
can therefore be concluded that these LaF3 nanoplates with
cavities crystallize rapidly under these reaction conditions.
Similar structures have also been observed by Cheng et al.
from a reaction in aqueous solution.[21] The exact mecha-
nism of cavity formation remains unclear, although we pro-
pose that their formation may be related to the growth habit
and speed of crystallization.
Figure 4. TEM images of products obtained from the reaction for
10 min (a), 1 h (b), and 3 h (c), and the SAED pattern (d) corre-
sponding to the crystal in image (a).
attached. In this regard, the etching mechanism is also a
dissolution-recrystallization process, with dissolution of the
inner and growth of the outer surface of the plates. How-
ever, this growth is not discernible due to the variety of
outer diameters of the product.
The LaF3 ring-formation process can therefore be de-
scribed as shown schematically in Figure 5. Dissolution of
La(NO3)3 in the hot molten salts leads to its dissociation
–
into La3+ and NO3 ions, and La3+ tends to react with F–
rapidly to give LaF3 nuclei. Due to their high surface en-
ergy, these nuclei aggregate rapidly, probably driven by the
oriented attachment process during the growth process, to
form plates with many cavities in their upper/lower surfaces.
The plates are then etched by the intensely polar reaction
medium from these cavities outwards. Eventually, the cavi-
ties in one plate grow until they form one big hole. The
etching is not uniform from plate to plate, or even within a
single plate, which is why the large holes appear irregular.
As the reaction continues conventional Ostwald ripening
occurs. Figure 4 (b) shows a TEM image of the product
obtained after 1 h, which clearly shows that the centers of
nanoplates are gradually disappearing and ring-like struc-
tures are forming. We suggest that the cavities in the nano-
plates increase the surface area to volume ratio of the top/
bottom surfaces of the nanoplates, thereby increasing their
energy. As a result, the centers of nanoplates with cavities
become less stable than their edges, which is why etching
Figure 5. Schematic illustration showing the formation process for
the LaF3 nanorings.
LaF3 has been considered to be an ideal host lattice for
occurs at the center of these nanoplates rather than at their optically active lanthanide ions for use in luminescent appli-
edges. In addition, molten salt syntheses are strongly influ- cations, although different doping modes can lead to quite
enced by the surface and interface energies between the different emission behaviors. To study the luminescence of
constituents and the salts due to their strong polarity and our as-obtained LaF3 nanorings, products doped with
high surface tension, thus making the etching stronger in 15 mol-% Eu3+ were prepared by the same synthetic path-
this case. The molten salts themselves could therefore also way to investigate their optical properties in detail. It
provide an additional driving force for formation of the should be noted that this doping process alters neither the
ring-like structures. Figure 4 (c) shows a TEM image of the crystal structures nor the shapes of the host materials. Fig-
sample obtained from the reaction after 3 h. It can clearly ure 6 shows the emission spectra for bulk LaF3:Eu (15 mol-
be seen that the cavities in the plate have grown so large as %), approx. 50-nm nanoparticles, and nanorings at room
to almost form one big hole with only a few fragments temperature excited at a wavelength of 395 nm. Characteris-
Eur. J. Inorg. Chem. 2009, 2383–2387
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5
tic emission peaks of the D0Ǟ7FJ transitions (J = 0, 1, 2,
3, 4) of the Eu3+ ion can clearly be seen. Monitoring the
intensity of the peaks at around 590 nm due to the
5D0Ǟ7F1 transition (Figure 6) showed that the LaF3:Eu
nanorings emit the highest fluorescence intensity among the
three samples. This may be due to fluorescent quenching in
bulk LaF3:Eu3+ (15 mol-%) as a lower quenching concen-
tration has been reported for a doping level of 3 mol-%.[22]
At the same time, an increase in nonradiative transitions
caused by surface defects as the particle size decreases
means that the emission intensity of the nanorings is much
higher than that of the 50-nm nanoparticles.[23]
Conclusions
In summary, we have developed a simple one-pot molten
salt route for the preparation of high-quality LaF3 nanor-
ings in which NaNO3 and KNO3 (2:1 molar ratio) act as
the reaction medium and the rare-earth nitrate and NaF
the precursor. The reaction mechanism and crystal growth
have been investigated, and a possible formation process for
ring-like LaF3 nanostructures proposed and discussed in
detail. Furthermore, Eu3+-doped LaF3 nanorings show
characteristic emission lines (500–700 nm) corresponding to
transitions from 5D0 to 7FJ, with the 5D0Ǟ7F1 red emission
being the most prominent group. These results not only en-
rich the field of lanthanide fluoride chemistry but also al-
low us to study the growth and formation mechanism of
nano-/microcrystals in molten salts.
Experimental Section
Preparation of LaF3 Nanostructures and Eu3+-Doped LaF3: NaNO3,
KNO3, and NaF were purchased from Tianjin Chemical Reagent
Company. La(NO3)3·6H2O and Eu2O3 were purchased from
Shanghai Chemical Reagent Company. All chemicals were analyti-
cal grade and were used without further purification. In a typical
synthesis, 6.7 g of molten salt (mixture of NaNO3 and KNO3; mo-
lar ratio: 1:2) was heated to 480 °C in a crucible in muffle furnace
until it melted to a transparent liquid. La(NO3)3·6H2O (0.001 mol)
and NaF (0.003 mol) were mixed in an agate mortar in the appro-
priate stoichiometric ratio and ground for 10 min. This mixture was
subsequently dissolved in the molten salt and maintained at 350 °C
for 1.0 h. After reacting for 4 h and then cooling to room tempera-
ture, the product was obtained by washing the melt several times
with deionized water to remove the nitrates, and dried at 100 °C in
air for 12 h. The Eu3+-doped LaF3 nanorings were prepared in a
similar manner but with 15 mol-% of the La(NO3)3·6H2O precur-
sor replaced by Eu(NO3)3·6H2O. The bulk LaF3/Eu3+ (15 mol-%)
materials were synthesized by solid synthesis by calcining stoichio-
metric amounts of La(NO3)3·6H2O, Eu(NO3)3·6H2O, and NaF at
700 °C for 5 h. The approximately 50-nm Eu3+-doped LaF3 nano-
particles were prepared according to a literature procedure.[24]
Figure 6. Room-temperature fluorescence emission spectra of LaF3
with Eu3+ doping for (a) bulk, (b) 50-nm nanoparticles, and (c)
nanorings (λ = 395 nm).
Characterization: The X-ray diffraction (XRD) patterns of the sam-
ples were collected using a Rigaku D/Max 2200PC diffractometer
equipped with a Cu-Kα radiation source (λ = 1.5418 Å) and a
graphite monochromator from 20 to 70° at a scan rate of
5.0°min–1. Unit cell dimensions were determined with the JADE 5
program for X-ray diffraction pattern processing, identification,
and quantification. The chemical composition of the as-prepared
nanoplates, which were boiled in distilled water and then dissolved
in HNO3, was determined by ICP-MS (Agilent 7500C). The size
and morphology of the products were determined by TEM
The relative intensity ratio of the 5D0Ǟ7F2 and 5D0Ǟ7F1
transitions (I610/I590) was calculated to be 0.763 and 0.895
for the nanorings and nanoparticles, respectively. It is well-
5
known that the D0Ǟ7F2 (ca. 610 nm) transition is an al-
lowed electric-dipole transition and that its intensity is sig-
nificantly affected by the symmetry of the local environ-
5
ment around the Eu3+ ions. The D0Ǟ7F1 transition (ca.
590 nm), on the other hand, is magnetic-dipole allowed and
it is relatively insensitive to the local environment around (JEM100-CX), selected-area electron diffraction (SAED), and
Eu3+. The I610/I590 ration could therefore be used as an indi-
field-emission scanning electron microscopy (FE-SEM; JEOL JSM
cator of the symmetry variation around the Eu3+ species in
6700F) equipped with an energy dispersive X-ray (EDX, Oxford)
spectrum. HR-TEM ( JEOL-2010) was used to study the intrinsic
the host lattice, with an increase indicating an increase in
crystallography of the obtained samples. The fluorescence proper-
the asymmetry of the coordination polyhedron around
ties of all samples were investigated with an Edinburgh FLS920
Eu3+ in the lattice. Furthermore, the Eu3+ ions at the sur-
fluorescence spectrometer equipped with a 450-W Xe lamp. The
face are subjected to a less symmetric environment than
samples for fluorescence testing were prepared by dispersing the
those in the inner part of the crystals, therefore the rela-
obtained nanoplates in ethanol to form approximately 0.1 wt.-%
tively lower surface area to volume ratio of the as-prepared
clear solutions. The asymmetric ratio was calculated by taking the
Eu3+-doped LaF3 nanorings compared to the 50-nm nano-
ratio of the area under the peaks corresponding to the 5D0Ǟ7F2
5
particles leads to a decrease in the D0Ǟ7F2 transition.
and D0Ǟ7F1 transitions.[25]
5
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Lanthanum Fluoride with a Ring-Like Nanostructure
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Received: February 5, 2009
Published Online: April 23, 2009
Eur. J. Inorg. Chem. 2009, 2383–2387
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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