Room temperature synthesis and characterization of NaEuF4 nanorods and Na5Eu9F32 nanospheres

Article abstract of DOI:10.1016/j.ssc.2005.07.031

A simple solution-based route was used to selectively synthesize two ternary metal fluoride NaEuF₄ nanorods with hexagonal structure and Na₅Eu₉F₃₂ nanospheres with cubic structure at room temperature. It is found that NaEuF₄ nanorods can be transformed into Na₅Eu₉F₃₂ nanospheres with the increasing the reaction time in the presence of excessive NaF. The luminescence properties of these two metal fluorides were investigated and the possible formation mechanism was discussed.

Full text of DOI:10.1016/j.ssc.2005.07.031

Solid State Communications 136 (2005) 210–214
www.elsevier.com/locate/ssc
Room temperature synthesis and characterization of NaEuF4
nanorods and Na5Eu9F32 nanospheres
Miao Wanga, Qing-Li Huanga, Jian-Ming Hongb, Wen-Han Wua, Zhi Yua,
d
Xue-Tai Chena,c, , Zi-Ling Xue
*
aState Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, Nanjing 210093,
People’s Republic of China
bAnalytic Center of Nanjing University, Jiangsu, Nanjing 210093, People’s Republic of China
cState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002,
People’s Republic of China
dDepartment of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600, USA
Received 15 June 2005; accepted 22 July 2005 by C.N.R. Rao
Available online 11 August 2005
Abstract
A simple solution-based route was used to selectively synthesize two ternary metal fluoride NaEuF4 nanorods with hexagonal
structure and Na5Eu9F32 nanospheres with cubic structure at room temperature. It is found that NaEuF4 nanorods can be
transformed into Na5Eu9F32 nanospheres with the increasing the reaction time in the presence of excessive NaF. The
luminescence properties of these two metal fluorides were investigated and the possible formation mechanism was discussed.
q 2005 Elsevier Ltd. All rights reserved.
PACS: 61.46.Cw; 78.55.Km; 81.05.Ys; 81.10.Dn
Keywords: A. Nanostructures; A. Metal fluoride; B. Chemical synthesis; E. Luminescence
1. Introduction
fluorides such as NaEuF4, NaYF4 and LiYF4 were
traditionally prepared by high-temperature solid state
reactions [3,5,6]. It is highly desired to develop mild and
easily controlled preparative route for these important
materials. Recently some relative mild chemical approaches
including hydrothermal [7–10], sol–gel [11], and co-
precipitation [12] routes have been developed to prepare
ternary rare earth fluoride powders or bulk samples.
Recently, rare earth fluorides have been attracting much
attention due to their many potential applications in the
fabrication of electronic devices and sensors, because they
are expected to have unique properties such as luminescence
[1–3] and magnetic properties [4]. Ternary metal fluorides
with general formula ABF4 (AZLi, Na; BZtrivalent
transition metals and rare-earth elements) are important
host matrix for optical materials [1–3]. Ternary rare earth
It is well known that nanosized materials will exhibit
different chemical and physical properties from those of the
bulk. Many effective solution routes including hydrother-
mal, co-precipitation, emulsion routes have been success-
fully developed to synthesize various kinds of nanosized
materials [13]. However, only limited work has been
directed toward to the preparation of nanosized metal
fluorides especially the ternary rare earth metal fluorides
*
Corresponding author. Address: State Key Laboratory of
Coordination Chemistry, Coordination Chemistry Institute, Nanjing
University, Nanjing 210093, People’s Republic of China. Tel.: C86
25 83597147; fax: C86 25 83314502.
E-mail address: xtchen@netra.nju.edu.cn (X.-T. Chen).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2005.07.031

M. Wang et al. / Solid State Communications 136 (2005) 210–214
[14–16]. Zhang et al. [14] reported mechano-chemical
211
synthesis of nanosized complex fluorides ARF4 (AZLi,
Na, K, RZrare earth) from the reaction of AF and RF3
powders. Wu et al. [15] have prepared tubular NaHoF4 and
NaSmF4 by hydrothermal approach. Yi et al. [16] have used
co-precipitation method to prepare NaYF4:Yb,Er nanoparti-
cles. Herein, we present a solution-based route to synthesize
the ternary metal fluoride NaEuF4 nanorods with hexagonal
structure and Na5Eu9F32 nanospheres of cubic phase by
simplyadjusting the reaction conditions atroomtemperature.
2. Experimental procedures
The ternary metal fluorides NaEuF4 and Na5Eu9F32 were
prepared by the reaction between Eu(NO3)3$6H2O and NaF
in water. In a typical synthesis, 1.0 mmol of Eu(NO3)3$6-
H2O and a certain amount of NaF were dissolved in 50 ml of
distilled water and the solution was stirred at room
temperature. The white solid products were separated by
centrifugation and washed in an ultrasonic bath several
times with distilled water and ethanol. The final products
were dried at 70 8C for 3 h. The products obtained in
different reaction conditions were denoted as S1–S8. A
summary of the reaction conditions is listed in Table 1.
XRD analyses were carried out on a Shimadzu XD-3A
powder X-ray diffractometer with Cu Ka radiation lZ
Fig. 1. XRD patterns of the as-prepared samples.
molar ratio of 1:4 for 4 and 60 h, respectively. A typical
XRD pattern for S3 is shown in Fig. 1. All the peaks can be
indexed to NaEuF4 with hexagonal structure with the
˚
˚
calculated value aZ6.03 A and cZ3.60 A, in good
˚
agreement with JCPDS Card File, 28-1085 (aZ6.044 A,
˚
cZ3.613 A). Pure NaEuF4 (S5 and S6) were prepared with
excessive NaF (Eu3C/NaFZ1:10) with the reaction times of
2 and 12 h, respectively, (Table 1). Longer reactions (60 h)
led to the new product Na5Eu9F32 (S8) from a reaction
mixture containing the 1:10 molar ratio of Eu3C/NaF. The
XRD pattern of S8 shown in Fig. 1 can be readily indexed to
Na5Eu9F32 of cubic structure with lattice constant aZ
˚
1.5418 A. The sizes and morphologies of the products were
studied by transmission electron microscopy (TEM) on a
JEOL JEM-200CX microscope operated at 200 kV. High-
resolution transmission electron microscope (HRTEM) and
energy-disperse X-ray analysis (EDXA) were studied on a
JEOL JEM-2010F operates at 200 kV. The room tempera-
ture luminescent spectra were performed on a photolumi-
nescence spectrometer (SLM instruments).
˚
5.61 A, consistent with JCPDS Card File, 27-0777 (aZ
˚
5.627 A). It can be concluded that two ternary fluorides
NaEuF4 and Na5Eu9F32 can be selectively prepared by
adjusting the reaction parameters, i.e. molar ratio of Eu3C
/
NaF and reaction time.
The morphologies of the as-prepared NaEuF4 and
Na5Eu9F32 were revealed by TEM measurements.
Fig. 2(a)–(c) shows the TEM images of NaEuF4 powders
(S3, S6, S5), from which one can observe the rod-like
morphology. S3 appears to consist of short nanorods with
diameter of 20–30 nm and length of 60–100 nm, which are
aligned into one-dimensional assembly. The TEM images of
3. Results and discussion
It was found that pure NaEuF4 (S3, S4) can be prepared
by the reaction between Eu(NO3)3$6H2O and NaF with
Table 1
Summary of the reaction conditions, phase and morphologies of the products
Sample
Molar ratio Eu3C/NaF
Reaction time (h)
Products
Crystal structure
Morphology
S1
S2
1:3
1:4
2
2
EuF3
Orthorhombic
Mixture
Nanobundle
Nanorod
NaEuF4
EuF3
Nanobundle
Nanorod
S3
S4
S5
S6
S7
1:4
4
60
2
NaEuF4
NaEuF4
NaEuF4
NaEuF4
NaEuF4
Na5Eu9F32
Na5Eu9F32
Hexagonal
Hexagonal
Hexagonal
Hexagonal
Mixture
1:4
Nanorod
1:10
1:10
1:10
Nanorod
12
24
Nanorod
Nanorod
Nanosphere
Nanosphere
S8
1:10
60
Cubic

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M. Wang et al. / Solid State Communications 136 (2005) 210–214
Fig. 2. (a) TEM image of NaEuF4 nanorods (S3); (b) TEM image of NaEuF4 nanorods (S6); (c) TEM image of NaEuF4 nanorods (S5); (d)
HRTEM of nanorod (S5); (e) TEM of one nanorod of S5; (f) TEM image of EuF3 nanorods (S1); (g) TEM image of S2.
S5 and S6 show that the nanorods have similar sizes with
diameters of 200–250 nm and lengths of 1.0–1.5 mm. It
appears that the size and morphology of the NaEuF4
nanorods depend on the Eu3C/NaF molar ratios in the
starting materials. A HRTEM image (Fig. 2(d)) recorded on
a part of a S5 nanorod reveals clear lattice fringe, suggesting
that the NaEuF4 nanorod is well-crystallized. However, it
was found that the as-prepared NaEuF4 was not stable under
the electron irradiation. The surface of the NaEuF4 nanorods
became rough and some tiny particles appeared after

M. Wang et al. / Solid State Communications 136 (2005) 210–214
213
exposure to the electron irradiation (Fig. 2(e)). Typical TEM
images for the as-prepared Na5Eu9F32 (S8) are shown in
Fig. 3(a) and (b), and they revealed that the samples are
nanospheres with diameter of ca. 200 nm. The diffraction
spots in ED indicate the samples are of the single
crystallinity in nature, which was further supported by the
clear fringe with a distance of 0.286 nm in the HTREM
image recorded in a region of one nanosphere.
Elemental composition analyses of NaEuF4 nanorods
(S5) and Na5Eu9F32 nanospheres (S8) were performed by
energy-disperse X-ray analysis (EDXA), which revealed
that these nanomaterials contained only Na, Eu and F. The
ratio of Eu3C to NaC is 1:1 and 2:1 for S5 and S9,
respectively. However, only a small amount of F was
detected, which can be attributed to the insensitivity of
EDXA to fluoride ion with the low atomic number.
Some experiments were carried out in order to get insight
into the growth mechanism through varying the reaction
conditions. For the reaction system with the molar ratio
Eu3C/FK of 1:10, a mixture of NaEuF4 and Na5Eu9F32 (S7)
was prepared when the reaction was stopped after 24 h.
However, only pure NaEuF4 (S4) was obtained from the
reaction mixture with the molar ratio Eu3C/FK of 1:4 after
60 h. A comparison of reaction conditions for S5–S8
suggests that, with the excessive NaF in the reaction
mixture, NaEuF4 was formed first and then transformed into
Na5Eu9F32. Similarly, a mixture of EuF3 and NaEuF4 (S2)
was obtained in the reaction system with a molar Eu3C/NaF
ratio of 1:4 in 2 h. Therefore, S2 is likely an immediate
during the transformation from EuF3 to NaEuF4. Pure EuF3
(S1) was isolated when the molar ratio of Eu3C/NaF was
kept at 1:3. The current studies indicate that EuF3 was
formed at the early stage, and then reacted with NaF to
produce NaEuF4, which may be further transformed into the
stable Na5Eu9F32 when excessive NaF is present in solution.
The presence of the intermediate was further supported
by the TEM images of S2 and S7. In TEM image of S7
(Fig. 3(d)), both rod-like and sphere-like particles were
observed, consistent with the existence of NaEuF4 and
Na5Eu9F32. TEM image shows that EuF3 (S1), are
nanobuddles formed from an alignment of nanorods
(Fig. 2(f)). The morphology of S2 (Fig. 2(g)) indicated
that it is a mixture of small nanobuddles and large nanorods,
which are believed to be EuF3 and NaEuF4 according to the
XRD measurements.
On the basis of these experimental observations, a
‘dissolution and precipitation’ mechanism is proposed to
explain the formation of NaEuF4 and Na5Eu9F32 with
different morphologies. EuF3 is first formed, which grows
along one dimension, and aggregated together to exhibit the
bundle-like morphology. During the prolonging of the
reaction, the bundle-like EuF3 nanoparticles react further
with NaF to form NaEuF4 with one-dimensional mor-
phology. This transformation might involve the dissolution
of EuF3 and the EuF3 monomer reacts with NaF to form
NaEuF4. The formation of EuF3 and NaEuF4 nanorods
along one dimension might be attributed to the anisotropic
Fig. 3. (a), (b) TEM image of Na5Eu9F32 nanospheres (S8); (c)
HRTEM and ED patterns of S8; (d) TEM image of a mixture
NaEuF4 and Na5Eu9F32 (S7).

214
M. Wang et al. / Solid State Communications 136 (2005) 210–214
of the immediate products at different reactions stages
suggested that EuF3 was formed first and then reacted as the
precursors to form NaEuF4 and Na5Eu9F32. It is expected
that this simple preparative method can be extended to
prepare the nanosized materials of other ternary metal
fluorides.
Acknowledgements
This work was supported by Natural Science Grant of
Jiangsu Province (BK 2004087), Distinguished Overseas
Young Fund from the Natural Science Foundation of China
(No. 20028101), Natural Science Grant of China
(20001004), US National Science Foundation, Grant of
Instruments of Nanjing University, and Grant of State Key
Laboratory of Structural Chemistry of Fujian Institute.
Fig. 4. The photoluminescent spectra of NaEuF4 nanorods (S5) and
Na5Eu9F32 nanospheres (S8) when excited at 394 nm.
characteristics of their crystal structures. A new product
Na5Eu9F32 with sphere-like morphology was formed with
the prolonging of the reaction. The NaEuF4 nanorod might
be dissolved again to give a new and stable material
Na5Eu9F32 formed. Since, Na5Eu9F32 possesses a cubic
crystal structure, isotropical sphere-like morphology was
formed.
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4. Conclusions
A simple solution route was developed to selectively
prepare two ternary sodium europium fluorides with
different morphologies at room temperature. The isolation
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