Rapid synthesis of single-crystalline TbF3 with novel nanostructure via ultrasound irradiation

Article abstract of DOI:10.1016/j.materresbull.2010.11.003

Terbium fluoride (TbF₃) nanopeanut has been successfully synthesized via a mild sonochemical route from an aqueous solution of terbium nitrate and fluoroborate without any template or organic additive. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) spectra were utilized to characterize the synthesized samples. The morphologies and optical properties of the obtained TbF₃ nanopeanut can be tuned by ultrasound irradiation as well as the fluoride source. The prepared TbF₃ nanopeanut shows extraordinarily high room temperature photoluminescence intensity comparing to the products prepared by stirring. The possible formation mechanism is proposed in this paper.

Full text of DOI:10.1016/j.materresbull.2010.11.003

Materials Research Bulletin 46 (2011) 252–257
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Rapid synthesis of single-crystalline TbF3 with novel nanostructure via
ultrasound irradiation
*
Ling Zhu, Yangjia Liu, Xizhi Fan, Daowu Yang, Xueqiang Cao
School of Chemical and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, Hunan, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Terbium fluoride (TbF3) nanopeanut has been successfully synthesized via a mild sonochemical route
from an aqueous solution of terbium nitrate and fluoroborate without any template or organic additive.
X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photolumines-
cence (PL) spectra were utilized to characterize the synthesized samples. The morphologies and optical
properties of the obtained TbF3 nanopeanut can be tuned by ultrasound irradiation as well as the fluoride
source. The prepared TbF3 nanopeanut shows extraordinarily high room temperature photolumines-
cence intensity comparing to the products prepared by stirring. The possible formation mechanism is
proposed in this paper.
Received 4 July 2010
Received in revised form 29 September 2010
Accepted 1 November 2010
Available online 4 November 2010
Keywords:
A. Optical materials
A. Nanostructures
C. Ultrasonic measurements
D. Luminescence
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
spheres [10,11], core/shell particles [12,13], have been success-
fully prepared by this method.
Over the past decades, many methods have been explored to
produce various nanomaterials, such as precipitation, reverse
micelles or microemulsions, sol–gel method, electrochemical
method, solvothermal route, microwave method, and sonochem-
ical method, etc. Among these methods, sonochemical synthesis
has been paid more extensive attention due to its special
potential applications. Currently, the sonochemical method has
been proven to be an effective approach for generating materials
with unusual properties, since it results in particles with a much
smaller size and high surface area than those synthesized by
other methods [1]. The chemical effect of ultrasonic irradiation
arises from the acoustic cavitation, in other words, the formation,
growth and implosive collapse of bubbles in the liquid medium.
The implosive collapse of the bubbles generates the local hot
spots through the shock wave formation within the gas phase of
the collapsing bubble. These local hot spots have been shown to
be about 5000 K with a pressure of 1800 atm and cooling rate
higher than 108 K sꢀ1 [2,3]. The extremely transient high
pressure and temperature provide a unique environment for
the growth of materials with novel structures. The advantages of
this method include a rapid reaction rate, the controllable
reaction condition and the ability to form materials with uniform
shape, narrow size distribution and high purity. Up to now,
nanomaterials with various shapes and structures, such as
nanotubes [4,5], nanowires [6,7], nanorods [8,9], hollowed
More recently, rare earth fluorides have been attracting much
attention due to their unique physical and chemical properties and
potential applications in the fields of luminescent, magnets, catalysts,
biochemical probes, and so on [14–18]. The investigation on the
preparation and properties of rare earth fluorides has attracted a great
deal of attention. A variety of methods have been employed to
fabricate these materials. For example, the fullerene-like lanthanide
fluorides, the bundle-like and rod-like YF3 were synthesized by a
hydrothermal process [19–22]; CeF3 nanoparticles have been pre-
pared using a polyol method [23]. Chen and coworkers have obtained
EuF3 nanocrystals with different morphologies via a stirring method
[24,25]. Reverse micelles and microemulsion techniques were also
developed by several groups and have been widely used for the
preparation of CeF3 nanoparticles [26,27], and YF3 nanoparticles with
controllable sizes and morphologies [28]. However, to the best of our
knowledge, the synthesis of TbF3 with the peanut-like nanostructure
has not yet been reported. Accordingly, in this paper, we report a facile
ultrasonic approach of preparing well-defined TbF3 nanopeanuts at
room temperature without any surfactant or template. It is shown
that the sonochemical method is a very powerful approach to
synthesize inorganic materials with interesting morphologies.
2. Experimental
2.1. Synthesis
All the chemicals were of analytical grade and were used as
received. In a typical synthesis, an appropriate amount of Tb4O7
* Corresponding author. Fax: +86 431 85262285.
E-mail address: xcao@ciac.jl.cn (X. Cao).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.11.003

L. Zhu et al. / Materials Research Bulletin 46 (2011) 252–257
253
was first dissolved in 10% nitric acid, and then mixed with the
solution of KBF4. The final concentration of Tb(NO3)3 and KBF4 was
30 mM and 60 mM, respectively. The resulted solution was
sonicated at ambient temperature for 3 h by a high-intensity
ultrasonic probe (JCS-206 Jining Co., China, Ti-horn, 23 kHz).
During the sonication, the temperature of the suspension rose to
about 70 8C. A white precipitate was centrifuged, washed with
distilled water and absolute ethanol in sequence. The final product
was dried under vacuum at 60 8C for 12 h. For comparison, a
sample of TbF3 product was also synthesized via vigorous stirring
under the same conditions but without the ultrasonic irradiation.
After vigorous stirring for 3 h, the reaction solution was still clear
to naked eyes, and no product was formed. Then, the reaction
system was kept still for 24 h at room temperature, resulting in the
formation of white precipitate.
lized. Furthermore, the sample obtained via stirring shows the
similar pattern, indicating the formation of the orthorhombic TbF3
crystal (Fig. 1b). While, it is worth noting that the intensity of the
diffraction peaks of the product obtained via ultrasonic irradiation
are obviously sharper than those of the sample prepared by
stirring, as the result of better crystallinity. It has been well
documented that the utilization of ultrasonic irradiation in
inorganic solution can improve the crystallinity of the products
[29–31].
The morphology and microstructure details of the as-prepared
TbF3 nanocrystals via ultrasound irradiation were investigated by
SEM, TEM, and SAED, as shown in Fig. 2. The SEM image with a low
magnification (Fig. 2a) reveals that the TbF3 sample consists of a
large quantity of peanut-like crystals with uniform size. The well-
defined peanut-like crystal has good symmetry, with 200 nm in
diameter and 550 nm in length as can be seen from the SEM image
with a higher magnification (Fig. 2b), and this is confirmed by the
TEM observation (Fig. 2e). An enlarged SEM image (Fig. 2c) displays
that the surfaces of the peanut-like particles are very scraggy, just
like the surface of the peanut. The energy dispersive X-ray
spectroscopy analysis (EDXA) confirms that the peanuts are
composed of Tb and F in a ratio of about 1:2.9 (Fig. 2d), and
gives further support to the XRD analysis as discussed above. The
TEM image with a high magnification shows that the peanut-like
structures were actually bundles of numerous nanofibers (Fig. 2f),
which were about 3 nm wide and 550 nm long. A typical SAED
recorded with the electron beam perpendicular to the axis of the
TbF3 nanopeanut is shown in Fig. 2g, showing the single-crystal
nature of the sample. The SAED pattern can be indexed as an
orthorhombic crystal recorded along the [0 0 1] zone axis. The
pattern clearly indicates the orientation alignment among all of the
nanofibers in the bundle along [0 1 0], which shows that the
nanowires are aligned not only in length but also in crystallo-
graphic orientation. In addition, the SAED pattern shows the
symmetrical stripes rather than polycrystalline circles or single-
crystal spots, implying the presence of some ordered arrangement
of crystallites in the bundle, which is in good agreement with TEM
observations. While, the sample which is prepared via vigorous
electric stirring instead of the ultrasound irradiation consists of the
caddice flowerlike agglomerates as shown in the SEM images of
Fig. 3a–c. The individual TbF3 flower seems to be composed of the
crooked nanorods with diameters ranging from 12 to 20 nm, and
each nanorod contains the smaller nanosized particles with sizes
ranging from 10 to 20 nm (Fig. 3c).
2.2. Characterization
The X-ray powder diffraction (XRD) pattern was performed on a
Rigaku D/MAX-2500 diffractometer with Cu
Ka radiation
(
l
= 0.15406 nm) and a scanning rate of 58 minꢀ1. The operation
voltage and current were maintained at 40 kV and 200 mA,
respectively. Scanning electron micrographs (SEM) images were
taken on a XL-30 field-emission scanning electron microscope
(Philips) equipped with energy-dispersive X-ray fluorescence
analysis (EDXA). Samples for SEM observation were prepared by
dropping a diluted suspension of the sample powders on the
silicon substrate. Transmission electron microscopy (TEM), select-
ed area electron diffraction (SAED) were recorded on a JEOL-JEM-
2010 operating at 200 kV (JEOL, Japan). Samples for TEM
observation were prepared by dropping a diluted suspension of
the sample powders onto a standard carbon-coated Formvar film
(20–30 nm) on a copper grid (230 mesh). Photoluminescence (PL)
spectra were recorded with a Hitachi F-4500 spectrophotometer
equipped with a 150 W Xenon lamp as the excitation source. All
the measurements were performed at room temperature.
3. Results and discussion
3.1. Structure and morphology control of the nanocrystal
The crystal structure of the as-prepared TbF3 samples was
characterized by XRD. As shown in Fig. 1a, the XRD pattern of the
sample prepared though ultrasonic irradiation is consistent well
with the orthorhombic TbF3 (JCPDS No. 37-1487). The strong and
3.2. Effect of fluoroborate
s[()TD$FIG] harp diffraction peaks indicate that the product is well crystal-
Further experiments indicate that the use of fluoroborate as the
fluoride source is one of the key factors to control the morphology
of the final product. When NaBF4 was used as the fluoride source,
the product with a similar morphology (peanut-like) and size was
obtained (Fig. 4a) by the identical ultrasonic process. If NaF or NH4F
was used as raw materials, and the other experimental conditions
were unchanged, only the irregular nanopaticles with serious
agglomeration were formed (Fig. 4b and c). Therefore, it is
concluded that the fluoroborate has an important effect on the
morphology of the as-formed product. On one side, the fluor-
oborate serves as the source of the fluoride ion. On the other side,
b
a
3ꢀ
during the hydrolysis of KBF4 in aqueous solution, BO3 ions are
produced in the reaction solution, as shown in Eq. (1) (discussed
later) and have been proved by the other group [32]. It is quite
possible that these BO33- anions are adsorbed on one specific plane
of the crystal seed, which result in the formation of the product
with anisotropic growth. It is well documented that the adsorbed
ions can change the growth kinetics and surface energies of
different crystal faces, which can ultimately lead to anisotropic
JCPDS: 37-1487
10
20
30
40
50
60
70
80
90
2θ /degree
Fig. 1. XRD patterns of TbF3 samples prepared with the ultrasonic irradiation (a) and
with the mechanic stirring (b).

[
L. Zhu et al. / Materials Research Bulletin 46 (2011) 252–257
Fig. 2. SEM images of the as-prepared TbF3 peanuts with different magnifications (a–c), EDX of peanuts (d), TEM image (e), TEM image with large magnification (f), and SEAD
of TbF3 peanut-like particles (g).
growth of low symmetry nanostructures [33–37]. For example,
Pileni and coworkers have demonstrated that the chloride and
bromide ions which could be selectively adsorb on the (0 0 1) and
(1 1 1) faces to favor either crystal growth on the [1 1 0] direction
or to form cubes of copper nanostructures in a micelle system [34].
Liu and coworkers used a seed growth procedure to obtain a
complex hexagonal ZnO nanostructure with capping of the citric
acid, which adsorbs preferably on the (0 0 1) surface of ZnO and
slows down the crystal growth along the c-axis [36]. Hence,
fluoroborate might play a key role in the formation of TbF3
nanopeanuts in the adopted reaction system.
acoustic cavitation. Such cavitation behavior leads to many unique
properties in the irradiated solution and it has been used
extensively to generate materials with interesting morphologies.
Therefore, the ultrasound appears to be particularly effective as a
means of inducing nucleation and may influence crystallization,
which is another key factor for the formation of final peanut-like
TbF3 product. In aqueous solution KBF4 was slowly hydrolyzed to
produce Fꢀ; these Fꢀ anions react with Tb3+ cations to form
terbium fluoride shown in the following equations.
ÞÞÞÞÞÞÞÞÞÞÞÞ
BF4ꢀ þ 3H2O ꢀ! H3BO3 þ 3HF þ Fꢀ ðK ¼ 2:5 ꢁ 10ꢀ10
Þ
(1)
u
Tb3þ þ 3FꢀÞÞÞÞꢀÞÞ!ÞÞÞÞÞÞTbF3 #
(2)
3.3. Effect of ultrasound irradiation and the possible growth
mechanism
The equilibrium-constant of the hydrolysis reaction (Eq. (1)) is very
small, which will keep the low concentration of Fꢀ ions in the
reaction solution, and consequently leads to the slow crystalliza-
It has been well established that the ultrasonic irradiation
introduces unusual physical and chemical effects deriving from the

[
L. Zhu et al. / Materials Research Bulletin 46 (2011) 252–257
255
Fig. 3. SEM images of the as-prepared TbF3 sample with different magnifications prepared via vigorous electric stirring (a–c) and its EDX analysis (d).
tion process. However, the employing of the ultrasonic irradiation
will accelerate the hydrolysis process, which is presumably helpful
to the nucleation and growth of the TbF3 nanocrystals. Further-
more, the cavitation and shock wave created by the ultrasound can
accelerate the formation of solid particles to high velocities, which
leading to the interparticle collision and effective fusion at the
point of collision. With these merits, the fabrication time of
nanomaterial under ultrasound irradiation is significantly reduced.
Inthecaseofthestirringmethod,noproductwasformedafterthe
vigorous stirring for 3 h. The product of the caddice flowerlike
agglomerateswasformedafterthereactionsolutionwaskeptstillfor
24 handonlya littleproductwasobtained.Thisresultfurtherproves
thatthe hydrolysisprocessofKBF4 isveryslowatroomtemperature.
Considering that the CaMoO4 and SrMoO4 particles with similar
shapehavebeenproducedbythesimilarmethod[38]andithasbeen
disclosedthatthemineralizationprocessresultedintheformationof
the complicated structures [38]. Therefore, it is possible that the
caddice flower-like agglomerate was also formed by the mineraliza-
tion process of TbF3 microcrystallites at room temperature.
To further investigate the details of the formation of TbF3
nanopeanuts under ultrasound irradiation, the growth process of
the final product was carefully monitored by time-dependent
experiments. SEM images with different reaction time show an
obvious growth process from the small primary nanoparticles to
the final peanuts (Fig. 5). With ultrasonic treatment for 20 min, the
transparent reaction solution became turbid, which indicates the
formation of nuclei. When reaction up to 30 min, a white
precipitate appeared, showing the formation of disk-like particles
(Fig. 5a). Furthermore, these disk-like particles would like to
further grow in some specific orientations to form peanut-like
structures (Fig. 5b and c). Usually, the growth of the peanut-like
product will complete in 3 h (Fig. 5d). And it is also observed that
the morphology and size of the product have no obviously changed
by further prolonging the sonication time. In this work, the
ultrasound irradiation played an important role in the formation of
the final product. The employing of the ultrasonic irradiation
would accelerate the hydrolysis process, and lead to the formation
of the small primary nanoparticles. With the ongoing of the
[()TD$FIG]
Fig. 4. SEM image and TEM images of the as-prepared TbF3 sample via ultrasound irradiation with the fluoride source of NaBF4 (a), NaF (b) and NH4F (c).

[
L. Zhu et al. / Materials Research Bulletin 46 (2011) 252–257
Fig. 5. The time-dependent SEM images of the peanut-like crystals after ultrasound treatment for 0.5 h (a), 1 h (b), 2 h (c) and 3 h (d).
[()TD$FIG]
range from 470 to 630 nm, corresponding to the transitions from
5 7
D - F
4
the excited levels of 5D4 to 7FJ (J = 3–6), with the electronic
5
7
transition 5D4 ꢀ F5 (543 nm) being the most prominent emission
line [39,40]. Furthermore, it is worth noting that the emission
intensity of the TbF3 nanopeanuts is much higher than that of the
flower-like product which was prepared via vigorous stirring. The
crystallinity of TbF3 nanopeanuts prepared via the ultrasonic
irradiation obviously is higher than that of the flower-like TbF3
prepared by stirring (as shown in Fig. 1). The high crystallinity
favors the improvement of the luminescent intensity of products
[41,42]. It is also assumed that the flower-like crystals would
possess more defects, and some of these defects may act as the
nonradiative recombination centers, which will lead to lower
intense emission. These results indicate that the luminescence
properties of TbF3 are largely affected by factors such as the
morphology, the crystallinity, and the structural defects.
a
b
5
7
D - F
4
6
5
7
D - F
4
4
5
7
D - F
4
3
400
450
500
550
600
650
700
Wavelngth / nm
Fig. 6. Room-temperature PL spectra of the as-prepared TbF3 peanuts via
sonochemical method (a) and the TbF3 flowers by vigorous electric stirring (b).
4. Conclusion
reaction, the primary nanoparticles further grow to the nanodisks,
and then the freshly formed nanoparticles will spontaneously
‘‘land’’ on the as-formed disks in some specific orientations to form
the peanut-like structure. Although the exact formation mecha-
nism for this complex nanostructure is not yet very clear, it is
believed that the growth of the peanut-like nanostructures is not
catalyst-assisted or template-directed, because the only material
sources used in our synthesis are pure rare earth oxide crystals and
fluoroborate. In the present case, the role of sonication is not only
to accelerate the reaction between the raw materials but also to
lead to the growth and crystallization of TbF3 due to its extreme
conditions.
In summary, terbium fluoride with novel nanostructure has
been successfully synthesized via a mild sonochemical route. The
ultrasonic irradiation as well as the fluoroborate has a remarkable
effect on the morphology of the final product. This simple and
unique synthetic method has a potential advantage for synthesis of
novel materials or materials with novel morphology. Room
temperature photoluminescence of TbF3 samples formed with
the two methods have similar features with the characteristic
emission of Tb3+ transitions, but the emission intensity of the
peanut-like TbF3 nanocrystals under ultrasound irradiation was
greatly improved comparing with the product prepared via
stirring.
Acknowledgements
3.4. Optical property
Fig. 6 shows the room-temperature emission spectra of TbF3
products obtained in the presence and absence of ultrasound
irradiation. Both the samples were measured under identical
Financial support from the National Natural Science Foundation
of China (21001017), Startup Fund for Doctoral Program, Huxiang
Scholars Program and Hunan Provincial Key Laboratory of
Materials Protection for Electric Power and Transportation
(2011CL02) (Changsha University of Science & Technology) are
gratefully acknowledged.
conditions with the excitation wavelength
lex = 350 nm. The
emission spectra of these two samples show similar patterns and
have a characteristic emission peaks of Tb3+ within the wavelength

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