Fullerene-like rare-earth nanoparticles

Article abstract of DOI:10.1002/anie.200351006

A low-temperature hydrothermal synthesis has been employed to prepare the title materials in the form of their fluorides and hydroxides. Electron microscopy reveals their hollow and closed-cage structures (see HRTEM image). Such nanostructures can be expected to bring forth new opportunities in important research and application fields, such as biological labeling, luminescence, and catalysis.

Full text of DOI:10.1002/anie.200351006

Angewandte
Chemie
synthesis of novel IF nanoparticles remains as a challenging
task for the years to come. Herein we report the synthesis of
rare-earth fluoride and hydroxide IF nanoparticles using a
low-temperature hydrothermal synthetic procedure.
In contrast to nanotubes, the formation of IF particles
requires folding in all directions, and thus involves higher
[
9–10]
elastic strain.
erene-like particles does not need a catalyst, but usually
Generally, the synthesis of inorganic full-
[
9]
requires relatively high temperatures. The techniques by
which the high temperatures have been achieved includes
[
3,5,6,16]
[1]
[14]
furnaces,
laser vaporization, sonolysis, electron and
[
13]
[17]
ion-beam heating, and heating by an STM tip. In this
paper, a rational hydrothermal synthetic procedure has been
successfully developed to prepare rare-earth IF hollow nano-
particles at low temperatures (80–1808C), which may suggest
a route for the future exploration of synthetic procedures to
new IF nanostructures. It is also worthy to note that the
fluorides and hydroxides of IF nanotructures have been
obtained for the first time; previously, this has been limited to
chloride and chalcogenides over the past decade. In addition,
[
18]
the novel properties of rare-earth compounds makes them
rather appealing for practical applications in, for example,
luminescence, catalysis, and biological fields.
Our synthesis is based on a facile hydrothermal method. A
typical synthesis involves the preparation of colloidal fluoride
precipitates and/or mixtures of rare-earth oxides and water,
which is then treated at a designated temperature under
hydrothermal conditions.
Inorganic Fullerene Nanoparticles
The phase purity of the samples has been characterized on
a Bruker D8-Advance X-ray powder diffractometer with
CuKa radiation (l = 1.5418 Š). Under the adopted conditions,
the obtained LaF , PrF , NdF , and SmF IF nanostructures all
Fullerene-Like Rare-Earth Nanoparticles**
3
3
3
3
Xun Wang and Yadong Li*
have hexagonal structures while YF has an orthorhombic
3
structure (Figure 1d). As an example, Figure 1a–c shows
[
1]
The landmark discovery of the fullerenes has led to the
development of thermodynamically stable forms of carbon
typical reflection patterns of LaF IF nanoparticles obtained
3
at a temperature of 120, 140, and 1808C, respectively, which
[
1–2]
whose dimensions are reduced to the nanoscale.
As an
analogue to these novel structures, the concept of inorganic
fullerene-like (IF) nanostructures has been initially described
[
3–6]
and confirmed by Tenne and co-workers.
novel properties of fullerenes,
Stimulated by the
[
7–8]
these IF nanoparticles can
be expected to open new avenues in the solid-state chemistry
of inorganic compounds and offer new opportunities for the
application of such nanostructures in the emerging field of
[
9–12]
nanotechnology, as well as in numerous other areas.
However, up to now, only a limited number of IF nano-
particles have been successfully prepared from their corre-
[
3]
sponding layered inorganic compounds, such as WS ,
2
[
5]
[6]
[13]
[14]
[15]
[16]
MoS2, NiCl2, CdCl2, Tl O, NbS2, and ReS2. The
2
[
*] Prof. Dr. Y. Li, X. Wang
Department of Chemistry
Tsinghua University
Beijing, 100084 (P. R. China)
Fax: (+86)10-6278-8765
E-mail: ydli@tsinghua.edu.cn
Figure 1. XRD patterns of a) LaF IF nanoparticles (1208C,
3
3+
La :NH F=1:3, pH 4–5); b) LaF IF nanoparticles (1408C,
4
3
3+
[
**] This work was supported by the NSFC (20025102, 50028201,
La :NH F=1:3, pH 4–5); c) LaF IF nanoparticles (1808C,
4 3
3+
20151001), the Foundation for the Author of National Excellent
La :NH F=1:3, pH 4–5); d) YF IF nanoparticles (1808C,
4 3
+
3
Doctoral Dissertation of P. R. China, and the State Key Project of
Fundamental Research for nanomaterials and nanostructures.
Y
:NH F=1:3, pH 4–5); e) Nd(OH) nanotubes and IF nanoparticles
4
3
(1408C).
Angew. Chem. Int. Ed. 2003, 42, 3497 –3500
DOI: 10.1002/anie.200351006
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3497

Communications
can be readily indexed to that of the hexagonal phase of LaF3
JCPDS no. 72–1435) with lattice constants a = 7.163 and c =
.329 Š. With decreasing hydrothermal temperature, the
(
7
peaks have been apparently broadened (Figure 1a), which
indicates smaller crystal sizes. It seems that the formation of
IF nanoparticles is closely related to the hexagonal structures
of the corresponding samples, since comparatively fewer IF
nanoparticles have been observed in the samples of ortho-
rhombic YF3 and no IF nanostructures observed in the
orthorhombic samples of other LnF (Ln = Eu, Gd, Tb, Dy,
3
Ho, Er, Tm, Yb, and Lu) under our experimental conditions.
Similar to the fluorides, most of the hydroxide IF nano-
particles that have been examined possess hexagonal struc-
tures, which also coincides well with that of rare-earth
[
19–20]
hydroxide nanowires and nanotubes.
As an example,
Figure 1e shows the diffraction pattern for Nd(OH) , which
3
can be readily indexed to that of the hexagonal structure of
Nd(OH)3 (JCPDS no. 83-2035) with lattice constants a =
6
.418 and c = 3.743 Š. A thorough XRD characterization
indicates that pure phase of the rare-earth fluorides and
hydroxides could be obtained under the reported experimen-
tal conditions.
Transmission electron microscopy (TEM) provides fur-
ther insight into the morphology and microstructural details
of the rare-earth-based series of IF nanostructures. Figures
2
–4 shows typical images of these rare-earth compounds IF
Figure 3. HRTEM images of a) an individual IF nanoparticle with diam-
eter ꢀ10 nm; b) an individual polyhedral IF nanoparticle; c) an indi-
vidual IF nanoparticle with diameter ꢀ30 nm; d) an individual IF
nanoparticle with diameter ꢀ90 nm; e) Magnification of the square
section shown in d); f) electron diffraction pattern of the IF nanoparti-
cle in d); g) EDS analysis of the IF nanoparticle in d).
nanoparticles, from which it can be seen that most of the
samples dispersed on the copper TEM grids have hollow-
sphere morphologies.
The sizes of these IF nanoparticles are flexible to some
extent; temperature and molar ratio have been found to be
responsible for the size and shape control of these nano-
structures. As shown in Figure 2c and d, the size of the LaF3
Figure 2. TEM images of a) LaF IF nanoparticles (1208C,
3
3+
La :NH F=1:3, pH 4–5); b) LaF nanoparticles (1408C,
4
3
3
+
La :NH F=1:3, pH 4–5); c) LaF nanoparticles (1808C,
4
3
3+
La :NH F=1:3, pH 4–5); d) LaF nanoparticles (1808C,
4
3
3+
La :NH F=1:9, pH 4–5).
4
3
498
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 3497 –3500

Angewandte
Chemie
nanoparicles shrinks from approximately 100 to 30 nm when
of an individual Eu(OH) IF nanoparticle. The interlayer
distances of the IF particles are approximately 0.364 nm
(close to the lattice constant along the c axis of the Eu(OH)3
3
3
+
À
the molar ratio (La :F ) changes from 1:9 to 1:3. Temper-
ature also plays an important role in determining the sizes of
these IF nanostructures. Figure 2a shows that the average
structure). This value is in good agreement with that of the
[
20]
diameters of the LaF obtained at 1208C are approximately
nanotubes obtained by another precipitation method.
3
1
0 nm, while at higher temperatures (1408C and 1808C), the
The hollow-sphere nature of these samples has been
further confirmed through Brunauer–Emmett–Teller (BET)
analysis. Nitrogen adsorption/desorption isotherms and cor-
responding Barrett–Joyner–Halenda (BJH) pore-size distri-
average sizes increase to 20–40 and 30–50 nm, respectively.
Similar IF structures and size-control phenomena have
been observed in the samples of PrF , NdF , and SmF ,
3
3
3
however, comparatively fewer IF nanoparticles have been
bution plots for samples of PrF IF nanoparticles are shown in
3
observed in the samples of YF , perhaps due to its ortho-
rhombic structure.
High-resolution (HR)TEM analysis provides more
detailed structural information on these IF nanostructures,
Figure 5. The isotherms can be categorized as type IV with a
distinct hysteresis loop observed in the range of 0.6–1.0 P/P0
(Figure 5a) for the samples at 1208C, and 0.8–1.0 P/P0
(Figure 5b) for the samples at 1808C. The BET surface area
for samples prepared at 1808C (average diameters: 30–
50 nm) and 1208C (average diameters: about 10 nm) were
3
and a series of HRTEM images of LaF nanoparticles with
3
diameters ranging from 10 to 100 nm have been provided as
further evidence for the formation of rare-earth fluoride IF
nanostructures. Figure 3d shows an individual close-caged
particle with a diameter of approximately 90 nm. Higher
magnification shows that its wall contains about forty layers
2
À1
23.56 and 58.03 m g , respectively; these values are much
larger than the calculated value for monodisperse PrF3
spherical nanoparticles (43.92, 21.96, 14,64, 10.98, and
2
À1
8.78 m g for spherical particles with diameters of 10, 20,
(
Figure 3e). The interlayer distance is calculated to be
30, 40, and 50 nm, respectively; density of a single crystal of
À3
approximately 0.36 nm (the separation between (002)
planes). Electron diffraction (ED) analysis (Figure 3 f)
taken from this hollow nanoparticle has revealed its single-
crystalline nature, and indicates that the nanoparticles have
hexagonal symmetry. Energy-dispersive spectroscopy (EDS)
of this nanoparticle indicates that it contains only La and F
PrF = 6.83 gcm ) that are not hollow. The BJH calculations
3
for the pore-size distribution based on desorption data reveal
(
Figure 3 g). IF nanostructures of LaF with different shapes
3
and sizes have been shown in Figure 3a–c, from which the
close-caged structures of these nanoparticles can be clearly
discerned.
IF nanostructures have also been discovered in our rare-
earth hydroxide samples. In contrast to the fluoride IF
nanoparticles, the hydroxide nanoparticles usually coexist
with nanotubes in the final product (for example in, Y(OH)3,
Sm(OH)3, Eu(OH)3, Gd(OH)3, Tb(OH)3, Dy(OH)3,
Ho(OH) , and Er(OH) ). Figure 4 shows HRTEM images
3
3
Figure 5. a) N adsorption/desorption isotherm of PrF IF nanoparti-
2
3
3
+
À
cles (1208C, Pr :F =1:3); inset, pore-size distribution curve obtained
from the desorption data; b) N adsorption/desorption isotherm of
2
3+
À
Figure 4. HRTEM image of an individual Eu(OH) IF nanoparticle with
PrF IF nanoparticles (1808C, Pr :F =1:3); inset, pore-size distribu-
3
3
diameter ꢀ45nm.
tion curve obtained from the desorption data.
Angew. Chem. Int. Ed. 2003, 42, 3497 –3500
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3499

Communications
a bimodal distribution for the 1208C samples centered at 8.5
microscope (TEM) and a JEOL JEM-2010F high-resolution trans-
mission electron microscope.
and 12 nm (Figure 5a, inset), and a distribution for the 1808C
samples centered at 30 nm (Figure 5b, inset), which coincides
with the XRD and TEM results of the samples.
Received: January 23, 2003 [Z51006]
Compared with the reported IF nanostructures, rare-earth
fluorides and hydroxides have no typical layer structures.
However, it seems that the closed-cage structures are also
thermodynamically stable for these fluorides and hydroxides.
Based on our experimental results, single-crystal nanotubes of
Keywords: fluorides · fullerenes · hydroxides · nanostructures ·
rare-earth metals
.
[
1] H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, R. E.
Smalley, Nature 1985, 318, 162 – 163.
[
20]
rare-earth hydroxides have been successfully obtained,
which have shown the possibility of hydroxides forming
closed-cage structures. Meanwhile, in the process of studying
the formation mechanism of rare-earth hydroxides nanowires,
[
2] S. Iijima, Nature 1991, 354, 56 – 58.
[3] R. Tenne, L. Margulis, M. Genut, G. Hodes, Nature 1992, 360,
444 – 446.
[4] L. Margulis, G. Salltra, R. Tenne, M. Tallanker, Nature 1993, 365,
[
19]
we have observed the wide existence of lamellar structures.
1
13 – 114.
5] Y. Feldman, E. Wasserman, D. J. Srolovitz, R. Tenne, Science
995, 267, 222 – 225.
Similar two-dimensional (2D) growth phenomena have also
been observed in the formation process of fluoride nano-
structures. Therefore, we believe that the rare-earth fluorides
and hydroxides may not be regarded as typical 3D com-
pounds, and may be the structural intermediates between 3D
and 2D structures. However, a clear explanation of the
structural rational of these IF nanostructures may need a
thorough theoretical and structural investigation.
In this paper, based on a low-temperature hydrothermal
method, a series of rare-earth fluoride and hydroxide IF
nanoparticles have been successfully synthesized, which have
greatly extended the scope of IF nanostructures. Meanwhile,
this facile procedure can be easily adjusted for the large-scale
synthesis of rare-earth IF nanoparticles. Also, the studies have
illustrated the potential of this solution-based route for the
synthesis of IF nanostructures in the low-temperature range,
especially for the layered hydroxides. We believe that the
synthesis and the subsequent properties obtained from these
IF structures will offer many opportunities in a number of
areas.
[
[
1
6] Y. R. Hacohen, E. Grunbaum, R. Tenne, J. Sloan, J. L. Hutch-
ison, Nature 1998, 395, 336 – 337.
[7] M. Prato, M. Maggini, Acc. Chem. Res. 1998, 31, 519 – 526.
[
[
10] R. Tenne, Chem. Eur. J. 2002, 8, 5297 – 5304.
11] G. L. Frey, S. Elani, M. Homyonfer, Y. Feldman, R. Tenne, Phys.
Rev. B 1998, 57, 6668 – 6671.
12] M. Chowalla, G. A. J. Amaratunga, Nature 2000, 407, 164 – 167.
[13] R. Popovitz-Biro, A. Twersky, Y. R. Hacohen, R. Tenne, Isr. J.
Chem. 2001, 41, 7 – 14.
14] S. Avivi, Y. Mastai, A. Gedanken, J. Am. Chem. Soc. 2000, 122,
331 – 4334.
15] C. Schuffenhauer, R. Popovitz-Biro, R. Tenne, J. Mater. Chem.
002, 12, 1587 – 1591.
16] K. S. Coleman, J. Sloan, N. A. Hanson, G. Brown, G. P. Clancy,
M. Terrones, H. Terrones, M. L. H. Green, J. Am. Chem. Soc.
2002, 124, 11580 – 11581.
8] A. L. Balch, M. M. Olmstead, Chem. Rev. 1998, 98, 2123 – 2165.
9] R. Tenne, Prog. Inorg. Chem. 2001, 50, 269 – 315.
[
[
[
[
[
[
4
2
[17] M. Homyonfer, Y. Mastai, M. Hershfinkel, V. Volterra, J. L.
Hutchison, R. Tenne, J. Am. Chem. Soc. 1996, 118, 7804 – 7808.
[
[
18] G. X. Xu, Rare Earths, 2nd ed., Metallurgical Industry Press,
Beijing, 1995.
19] X. Wang, Y. D. Li, Angew. Chem. 2002, 114, 4984 – 4987; Angew.
Chem. Int. Ed. 2002, 41, 4790 – 4793.
[20] “Rare earth compound nanotubes”: X. Wang, Y. D. Li, unpub-
lished results.
Experimental Section
Synthesis of LnF IF nanoparticles: In a typical synthesis, Ln O
3
2
3
(
Y O , La O , Pr O , Nd O , Sm O ; 0.4 g) was first dissolved in 10%
2 3 2 3 2 3 2 3 2 3
nitric acid, then mixed with another solution containing NH F (molar
4
3
+
À
ratio, Ln :F = 1:3) to form colloidal precipitates. 10% KOH (or
NaOH) was then added to adjust the system to pH 4–5. The obtained
colloidal precipitate was transferred to a 40-mL autoclave, sealed, and
maintained at 80–1808C for 12–24 h; the autoclave was then allowed
to cool to room temperature. The precipitate was then filtered,
washed with water to remove ions possibly remnant in the final
products, and dried at 808C in air. Following the above procedures, IF
nanoparticles of LaF , PrF , NdF , SmF and YF3 could be easily
3
3
3
3
obtained. In order to obtain IF nanostructures with different sizes,
factors such as molar ratio and temperature have been varied during
the experimental process. The functionalization process could also be
easily carried out by dissolving the corresponding rare-earth oxides
with target ions or atoms.
Synthesis of Ln(OH) IF nanoparticles: In a typical synthesis,
3
rare-earth oxide powders (La O , Pr O , Nd O , Sm O , Eu O ,
2
3
6
11
2
3
2
3
2
3
Gd O , Tb O , Dy O , Ho O , Er O , Y O ; 0.4 g) were dispersed into
2
3
4
7
2
3
2
3
2
3
2
3
distilled water (30 ml), then sealed in a 40-mL autoclave and
hydrothermally treated at 140–1808C for 24 h.
Characterization: The obtained sample was characterized on a
Bruker D8-Advance X-ray powder diffractometer with CuKa radia-
tion (l = 1.5418 Š). The size and morphology of the IF nanoparticles
were determined at 200 kV by a Hitachi H-800 transmission electron
3
500
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 3497 –3500