Synthesis and characterization of MgF2 and KMgF3 nanorods

Article abstract of DOI:10.1016/j.jssc.2004.01.005

MgF₂ nanorods with diameters of 60-100nm were synthesized by a microemulsion method. Subsequent hydrothermal reaction of as-synthesized MgF₂ nanorods and KF at 240°C for 3 days or 140°C for 7 days resulted in KMgF₃ nanorods, which retained the rod-like morphology of the source material MgF₂ in the reaction process. The morphology of as-synthesized MgF₂ strongly depended on the molar ratio between water and the surfactant CTAB and the concentration of CTAB.

Full text of DOI:10.1016/j.jssc.2004.01.005

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 2205–2209
Rapid Communication
Synthesis and characterization of MgF2 and KMgF3 nanorods
Minhua Cao,a Yonghui Wang,a Yanjuan Qi,a Caixin Guo,a and Changwen Hua,b,
*
a Faculty of Chemistry, Institute of Polyoxometalate Chemistry, Northeast Normal University, Changchun 130024, PR China
b Department of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China
Received 20 October 2003; received in revised form 6 January 2004; accepted 14 January 2004
Abstract
MgF2 nanorods with diameters of 60–100 nm were synthesized by a microemulsion method. Subsequent hydrothermal reaction of
as-synthesized MgF2 nanorods and KF at 240ꢀC for 3 days or 140ꢀC for 7 days resulted in KMgF3 nanorods, which retained the
rod-like morphology of the source material MgF2 in the reaction process. The morphology of as-synthesized MgF2 strongly
depended on the molar ratio between water and the surfactant CTAB and the concentration of CTAB.
r 2004 Elsevier Inc. All rights reserved.
Keywords: One-dimensional nanostructures; MgF2; KMgF3; Microemulsion; Hydrothermal reactions
One-dimensional (1D) nanostructures (such as nano-
wires, nanorods, and nanotubes) have attracted con-
siderable attention in recent years due to their potential
applications in constructing nanoscale electronic, optoe-
lectronic, and sensing devices [1]. 1D nanostructures
with different compositions have been synthesized by
using various methods including the vapor-phase trans-
port process, chemical vapor deposition, arc discharge,
laser ablation, solution, and a template-based method
[2–11]. While a large part of this work has focused on
materials, such as elemental carbon, carbon-based
substances, metals, oxides, sulfides, nitrides, consider-
ably fewer studies have been carried out on 1D
nanostructures of fluorides and complex fluorides. It is
thus necessary to study the fluoride system with
interesting optical properties.
Fluorides and complex fluorides have been extensively
studied due to their particular photoluminescence
properties. Magnesium fluoride (MgF2) has been
identified as a positive uniaxial crystal, which possesses
the highest birefringence and a wide range of transpar-
ency in the spectrum region [12]. These properties have
led to the use of MgF2 for infrared transparent windows
[13]. Complex fluoride KMgF3, with a typical cubic
perovskite structure, is an ideal material for searching
for a new solid-state laser because of its several
advantages: good homogenous optics, high thermal
stability, low melting point, anisotropy, and high optical
transparency, etc. [14]. However, due to the corrosive
nature of fluorides, conventional solid-state synthetic
method of complex fluorides suffers from the require-
ments of high temperature, special synthetic apparatus,
or rigid conditions, and has thus limited the study of
complex fluoride chemistry [15]. Recently, Zhao et al.
have developed a hydrothermal route to the synthesis of
complex fluorides, which gives rise to the possibility of
making complex fluorides under mild conditions [16].
Very recently, studies on fluorides nanoparticles have
shown that nanoscale fluorides exhibit enhanced lumi-
nescence and photomagnetic properties [17]. However,
to the best of our knowledge, studies on MgF2 and
KMgF3 materials are still at bulk levels, and no work
has dealt with their 1D nanostructures. Due to the
promising applications of MgF2 and KMgF3 in
optoelectronic devices, it is important to synthesize the
two materials with 1D nanostructures. Here, we report a
hydrothermal microemulsion method for the prepara-
tion of MgF2 nanorods. Furthermore, using the as-
synthesized MgF2 nanorods as the magnesium source
material, KMgF3 nanorods were obtained under hydro-
thermal conditions at 240ꢀC, whereas the preparation of
KMgF3 by solid-state reactions was typically carried out
at about 1000ꢀC [15]. Although the microemulsion
method is well known as a mature method of synthesiz-
ing nanoparticles, it can also be used to fabricate some
*Corresponding author. Faculty of Chemistry, Institute of Poly-
oxometalate Chemistry, Northeast Normal University, Changchun
130024, PR China. Fax: +86-10-8257-1381.
E-mail address: huchw@nenu.edu.cn (C. Hu).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.01.005

ARTICLE IN PRESS
M. Cao et al. / Journal of Solid State Chemistry 177 (2004) 2205–2209
2206
1D nanostructures under certain conditions. In fact,
many 1D nanostructures, such as ZnO, BaCO3, BaSO4,
CdS, BaWO4, K3[PMo12O40] ꢁ nH2O, BaF2 and so forth
[18–24], were all fabricated via similar microemulsion
methods. In addition, the hydrothermal method has
been widely utilized in synthesizing nanoscale materials
because of its several advantages, resulting from its
unique reaction environment. It has been shown that
when the hydrothermal method is used to synthesize
nanomaterials, it cannot only evidently decrease reac-
tion temperature of systems, but also improve the
crystallinity of the products.
First, MgF2 nanorods were prepared via a quaternary
microemulsion,
cetyltrimethylammonium
bromide
(CTAB)/water/cyclohexane/n-pentanol, under hydro-
thermal conditions, in which the molar ratio of water
to CTAB was w=[H2O]:[CTAB]=10 and the concen-
tration of CTAB was [CTAB]=0.1 M. In this process,
CTAB (4 g) was dissolved in 100 mL of cyclohexane and
5 mL of n-pentanol. Two such solutions were stirred for
30 min and became transparent. Then 2 mL of aqueous
MgCl2 solution (1 M) and 2 mL of 10% HF aqueous
solution were added to the above solutions, respectively,
both forming colorless suspensions of microemulsion
droplets. Afterward, the two microemulsion solutions
were mixed and stirred for another 10 min. The resulting
microemulsion solution was then transferred into
stainless Teflon-lined autoclaves and heated at 120ꢀC
for 12 h. Finally, a white precipitate (MgF2 nanorods)
was collected by centrifuging, washed several times with
absolute ethanol and distilled water, and dried in a
vacuum oven at 50ꢀC for 5 h. KMgF3 nanorods were
synthesized under hydrothermal conditions, which is
similar to the preparation of KMgF3 particles developed
by Zhao et al. [16]. The above-prepared MgF2 (0.31 g)
and KF (0.29 g) were added to 9 mL deionized water, to
which 0.04 mL HF (40 mass%, A. R.) was added with
stirring. Here HF was used as a mineralizer. The
reaction mixture (pH ca. 6) was sealed in a Teflon-lined
stainless-steel autoclave and heated at 240ꢀC for 3 days
or 140ꢀC for 7 days. After the reaction was completed,
the resulting white product (KMgF3 nanorods) was
collected, washed several times using deionized water,
centrifuged, and dried under vacuum at room tempera-
ture.
Fig. 1. XRD patterns of (a) MgF2 and (b) KMgF3.
pattern of KMgF3 nanorods obtained at 240ꢀC for 3
days. All of the peaks can be indexed to a pure cubic
phase (space group: Pm3m [221]) of KMgF3 with lattice
˚
constant a ¼ 3:980 A (JCPDS 75-0307). These results
are in good agreement with those of MgF2 and KMgF3
bulk crystals, respectively. Particularly for KMgF3, the
XRD pattern indicates that pure KMgF3 phases can be
obtained under mild hydrothermal conditions.
Fig. 2a shows the transmission electron microscopy
(TEM) image of the sample prepared in CTAB
microemulsions at w ¼ 10: As shown in Fig. 2a, the
samples display a rod-like morphology with diameters
of 60–100 nm and lengths up to 2 mm. Fig. 2b shows the
TEM image of two MgF2 nanorods, indicating the
uniformity in diameter along the nanorod and the
straightness along the longitudinal axis of the nanorod.
In addition, all nanorods have two cuspate-shaped ends.
Fig. 2c shows a high-magnification TEM image of an
individual MgF2 nanorod, showing the cuspate-shaped
morphology of the end of the nanorod. The inset
selected area electron diffraction (SAED) pattern was
taken from a single MgF2 nanorod and reveals the
single-crystalline nature of the sample. Moreover,
SAED patterns taken from different parts of the
nanorod show exactly an identical pattern without
further tilting the nanorod, indicating the single crystal-
linity of the whole nanorod. A high-resolution TEM
(HRTEM) shows that the nanorod is structurally
uniform with an interplanar spacing of about
0.325 nm, which corresponds to the (110) plane of
tetragonal MgF2. KMgF3 nanorods were obtained by
the hydrothermal reaction of the as-synthesized MgF2
nanorods and KF at 240ꢀC for 3 days or 140ꢀC for 7
days, as clearly shown in Figs. 3a and b, respectively. It
can be seen that the morphology of the products
obtained under different conditions is almost the same,
and both KMgF3 nanorods have an average diameter of
The phase purity of the products was examined by X-
ray diffraction (XRD) measurement performed on a
Rigaku X-ray diffractometer with CuKa radiation. All
of the peaks of the XRD pattern in Fig. 1a can be
perfectly indexed to a pure tetragonal phase (space
group: P42/mnm [136]) of MgF2 with lattice constants
˚
˚
a ¼ 4:620 A and b ¼ 3:050 A (JCPDS 72-2231). Since
the XRD pattern of KMgF3 nanorods obtained at
140ꢀC for 7 days is very similar to that of samples
obtained at 240ꢀC for 3 days, here we only show the
XRD pattern of the latter. Fig. 1b shows the XRD

ARTICLE IN PRESS
M. Cao et al. / Journal of Solid State Chemistry 177 (2004) 2205–2209
2207
(a)
(b)
(a)
(b)
(d)
(c)
(c)
(d)
Fig. 3. (a) TEM image of KMgF3 nanorods obtained at 140ꢀC for 7
days. (b) TEM image of KMgF3 nanorods obtained at 240ꢀC for 3
days. (c) TEM image of a single KMgF3 nanorod taken from (3b). The
inset shows the corresponding SAED pattern. (d) TEM image of
KMgF3 obtained from a solid-state reaction at 1000ꢀC.
Fig. 2. (a) TEM image of MgF2 nanorods. (b) TEM image of two
nanorods, indicating the uniformity in diameter along the nanorods
and the straightness along the longitudinal axis of the nanorods. (c)
High-magnification TEM image of a single nanorod, showing the
cuspate-shaped morphology of the end of the nanorod. The inset
shows the corresponding SAED pattern. (d) HRTEM image of a single
nanorod (spacing=0.325 nm).
about 80 nm and lengths up to 1 mm, clearly shorter than
that of MgF2 nanorods. The SAED (inset in Fig. 3c)
pattern taken from a single nanorod (Fig. 3c) can be
indexed as a cubic KMgF3 single crystal. Previously,
Zhao et al. reported the synthesis of KMgF3 using a
similar procedure but with bulk MgF2 as magnesium
source material, except that their synthesized KMgF3 is
in the form of particles. It can thus be concluded that in
the formation of KMgF3 it still retained the rod-like
morphology of the source material MgF2. For compar-
ison, using the same as-synthesized MgF2 nanorods as
magnesium source material we have also repeated the
synthesis but using solid-state reactions at a high
temperature of 1000ꢀC. Pure KMgF3 were obtained as
expected. But most of the samples were large irregular
aggregates of roughly spherical particles instead of
uniform nanorods, and only a few considerably shorter
nanorods were observed, as shown in Fig. 3d. The
reasons may be that MgF2 nanorods partly sintered in
the process of high-temperature reactions, resulting
in the agglomeration of the product. Thus, it can be
seen that the hydrothermal reaction of MgF2 nano-
rods and KF cannot only remarkably decrease the
temperature of reaction systems, but also yield uni-
form nanorod product, which the high-temperature
solid-state reactions cannot obtain. In addition, it is
particularly worth noting that this hydrothermal reac-
tion also achieved the phase transition from tetragonal
phase MgF2 to cubic KMgF3 under mild conditions,
which was typically carried out under high-temperature
conditions.
The morphology of as-synthesized MgF2 strongly
depended on the molar ratio between the water and
surfactant CTAB, in which the concentration of CTAB
was kept at a constant of 0.1 M. Fig. 4 shows the TEM
images of as-synthesized products obtained under the
same conditions but with w ¼ 5 and 20, respectively.
Only uniform particles were obtained in the product
when the molar ratio of w ¼ 5 was used. The TEM
image (Fig. 4a) shows that the sample was composed of
MgF2 nanoparticles with an average diameter of 60 nm.
When the molar ratio was increased to 20, relatively
shorter nanorods, 50–80 nm wide and 80–200 nm long,
were observed (Fig. 4b). Compared with the products
synthesized at different molar ratios, we can conclude
that the molar ratio of w ¼ 10 is most favorable for the
formation of MgF2 nanorods. In addition, the concen-
tration of surfactant CTAB also has significant effects on
the shape of MgF2. Neither lower nor higher concentra-
tions of CTAB induce the formation of MgF2 nanorods.
At a low micellar concentration ([CTAB]=0.04 M),
particles with a mean diameter of 80 nm were observed

ARTICLE IN PRESS
M. Cao et al. / Journal of Solid State Chemistry 177 (2004) 2205–2209
2208
1.5 M, particles were also observed, most of the particles
(a)
were square (Fig. 5b), which is different from the
products formed in microemulsions at a lower surfactant
concentration. These square-shaped particles have a
mean edge length of 80 nm. The reason for producing
square-shaped particles may be that excess surfactant
CTAB may act as a capping agent to confine the growth
of particles [25]. On the basis of these facts, it can be
deduced that lower or higher surfactant concentrations
may destroy the dynamic equilibrium of microemulsions,
resulting in the loss of microemulsion function.
Very recently, we have reported the preparation of
PbO2, Pb3O4, Cu, Cu2O, CuO, and BaF2 1D nanos-
tructures, in which BaF2 whiskers were obtained via this
hydrothermal microemulsion method under mild hydro-
thermal conditions [24]. Although MgF2 and BaF2 both
belong to group II fluorides and have structural
similarity, by using the identical synthesis method,
MgF2 nanorods were obtained instead of whiskers.
Previously, we have expounded that BaF2 whiskers were
formed via a directed aggregation growth process. In the
case of MgF2, based on our experimental results, we also
prefer the directed aggregation mechanism in the
nanorod formation process. Since MgF2 nanorods with
higher aspect ratios (450) were obtained in the
microemulsions, according to the micellar size and
shape, it is not likely to lead to the formation of such
a long water nanochannel in oils in such a low water
content environment. However, different morphologies
were obtained in the two cases. This may be accountable
for different crystal habits of inorganic materials. As
among the group II fluorides, MgF2 and BaF2 have the
smallest and the largest size of the unit cell, respectively.
Furthermore, they possess different crystal structures.
All these facts may result in the formation of 1D MgF2
and BaF2 nanostructures with different aspect ratios.
In conclusion, MgF2 nanorods with uniform dia-
meters have been first synthesized by the hydrothermal
microemulsion method. Furthermore, using the MgF2
nanorods as magnesium source material, KMgF3
nanorods have also been successfully fabricated under
mild hydrothermal conditions at 240ꢀC for 3 days or
140ꢀC for 7 days. It has been confirmed that the as-
synthesized KMgF3 retained the rod-like morphology of
MgF2. We expect that this methodology will be a
general method to synthesize other 1D fluorides and
complex fluorides.
(b)
Fig. 4. (a) TEM image of the sample produced at a molar ratio of
water to CTAB (w) of w ¼ 5: (b) TEM image of the sample produced
at the molar ratio of water to CTAB of w ¼ 20:
(a)
(b)
This work was supported by NSFC (No. 20071007
and 20271007) and SRFDP (No. 20030007014).
Fig. 5. (a) TEM image of the sample produced at a lower CTAB
concentration ([CTAB]=0.04 M) (b) TEM image of the sample
produced at a higher CTAB concentration ([CTAB]=1.5 M).
References
by the TEM image (Fig. 5a). XRD showed that the
obtained particles were also a pure tetragonal MgF2
phase. While at a higher surfactant concentration of
[1] J. Hu, M. Ouyang, C.M. Lieber, Nature 399 (1999) 48.
[2] Y. Wu, P. Yang, Chem. Mater. 12 (2000) 605.

ARTICLE IN PRESS
M. Cao et al. / Journal of Solid State Chemistry 177 (2004) 2205–2209
2209
[3] C.C. Chen, C.C. Yeh, Adv. Mater. 12 (2000) 738.
[4] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, X.Z. Gai, Q.L. Hang,
G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311.
[5] M. Yazawa, M. Koguchi, A. Muto, M. Ozawa, K. Hiruma, Appl.
Phys. Lett. 61 (1992) 2051.
[15] A.R. West, Solid State Chemistry and its Applications, Wiley,
New York, 1984, p. 338.
[16] C.Y. Zhao, S.H. Feng, Z.C. Chao, C.S. Shi, R.R. Xu, J.Z. Ni,
Chem. Commun. (1996) 1641.
[17] (a) C.M. Bender, J.M. Burlitch, Chem. Mater. 12 (2000) 1969.
(b) J.W. Stouwdam, F.C.J.M. van Veggel, Nano Lett. 2 (2002)
733;
[6] Y.C. Choi, W.S. Kim, Y.S. Park, S.M. Lee, D.J. Bae, Y.H. Lee, G.S.
Park, W.B. Choi, N.S. Lee, J.M. Kim, Adv. Mater. 12 (2000) 746.
[7] (a) X.F. Duan, C.M. Lieber, Adv. Mater. 279 (2000) 208.
(b) A.M. Morales, C.M. Lieber, Science 279 (1998) 208.
[8] T.J. Trentler, K.M. Hickman, S.C. Goel, A.M. Viano, P.C.
Gibbons, W.E. Buhro, Science 270 (1995) 1791.
[9] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287
(2000) 1471.
(c) X.M. Sun, Y.D. Li, Chem. Commun. (2003) 1768.
[18] J. Zhang, L.D. Sun, H.Y. Pan, C.S. Liao, C.H. Yan, New
J. Chem. 26 (2002) 33.
[19] L.M. Qi, J.M. Ma, H.M. Cheng, Z.G. Zhao, J. Phys. Chem. B 101
(1997) 3460.
[20] J.D. Hopwood, S. Mann, Chem. Mater. 101 (1997) 3460.
[21] P. Zhang, L. Gao, Langmuir 19 (2003) 208.
[22] S. Kwan, F. Kim, J. Akana, P.D. Yang, Chem. Commun. (2001)
447.
[10] (a) M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. 12
(2000) 1603.
(b) J. Zhu, S. Fan, J. Mater. Res. 14 (1999) 1175.
[11] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Appl. Phys. Lett. 76
(2000) 2011.
[23] X.H. Zhang, S.Y. Xie, Z.Y. Jiang, L.C. Zhou, Z.X. Xie, R.B.
Huang, L.S. Zheng, Chem. Commun. (2002) 2032.
[24] (a) M.H. Cao, C.W. Hu, G. Peng, Y.J. Qi, E.B. Wang, J. Am.
Chem. Soc. 125 (2003) 4982;
[12] B. Cochet-Muchy, J. Portier, in: P. Hagenmuller (Ed.), Inorganic
Solid Fluorides, Academic Press, Orlando, 1985, p. 565.
[13] E. Carnall, S.E. Hatch, L.S. Ladd, W.F. Parsons, US Patent 3,
294, 878, 1967.
(b) M.H. Cao, C.W. Hu, Y.H. Wang, Y.H. Guo, C.X. Guo, E.B.
Wang, Chem. Commun. (2003) 1884
[14] (a) T. Kobayasi, S. Mroczkowski, J.F. Owen, J. Lumin. 21 (1980)
247.
(b) G. Blasse, A. Bril, Philips Technol. Rev. 31 (1970) 304.
(c) M.H. Cao, C.W. Hu, E.B. Wang, J. Am. Chem. Soc. 125
(2003) 11196.
[25] L.F. Gou, C.J. Murphy, Nano Lett. 3 (2003) 231.