Solvent-assisted selective synthesis of NaLaF4 and LaF 3 fluorescent nanocrystals via a facile solvothermal approach

Article abstract of DOI:10.1016/j.jallcom.2010.10.101

A facile solvothermal approach was developed to selectively synthesize water-soluble NaLaF₄ or LaF₃ nanocrystals (NCs) by tuning the solvent components. In ethonal/H₂O media, high-quality LaF ₃ NCs were prepared by using NaF and LaCl₃ as the precursors. More interestingly, thermodynamically non-preferred NaLaF ₄ nanorods could be obtained when ethylenediamide (EN) was introduced to the ethonal/H₂O solvent. All of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the crucial effects of EN on morphology and structure of the NCs were studied. Furthermore, by doping different lanthanide ions, these LaF₃ and NaLaF₄ NCs could emit intense upconversion (UC) or downconversion (DC) fluorescence, which showed potential applications in color displays, light-emitting diodes, optical storage and optoelectronics.

Full text of DOI:10.1016/j.jallcom.2010.10.101

Journal of Alloys and Compounds 509 (2011) 1964–1968
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Solvent-assisted selective synthesis of NaLaF4 and LaF3 fluorescent nanocrystals
via a facile solvothermal approach
Zhe Wang, Chenghui Liu∗, Yucong Wang, Zhengping Li
College of Chemistry and Environmental Sciences, Hebei University, Baoding 071002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile solvothermal approach was developed to selectively synthesize water-soluble NaLaF4 or LaF3
nanocrystals (NCs) by tuning the solvent components. In ethonal/H2O media, high-quality LaF3 NCs were
prepared by using NaF and LaCl3 as the precursors. More interestingly, thermodynamically non-preferred
NaLaF4 nanorods could be obtained when ethylenediamide (EN) was introduced to the ethonal/H2O
solvent. All of the products were characterized by X-ray diffraction (XRD) and transmission electron
microscopy (TEM), and the crucial effects of EN on morphology and structure of the NCs were stud-
ied. Furthermore, by doping different lanthanide ions, these LaF3 and NaLaF4 NCs could emit intense
upconversion (UC) or downconversion (DC) fluorescence, which showed potential applications in color
displays, light-emitting diodes, optical storage and optoelectronics.
Received 1 October 2010
Accepted 19 October 2010
Available online 30 October 2010
Keywords:
Nanostructured materials
Optical materials
Chemical synthesis
NaLaF4 nanorod
Luminescence
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
our knowledge, it is still lack of powerful and efficient synthetic
approaches for the preparation of NaLaF4 NCs.
In recent years, the controllable synthesis of rare earth (RE)
fluorides nanocrystals (NCs) has attracted considerable interest.
With low energy phonons, fluorides NCs can serve as excellent host
materials for both upconversion (UC) and downconversion (DC) flu-
orescence processes, and therefore can be applied in the fields of
phosphors, displays, optical amplifiers and biolabeling [1–8].
Among these rare earth fluorides NCs, REF3 and NaREF4 are two
types of important structures which have found extensive applica-
tions [9–11]. Although various methods have been developed for
the synthesis of REF3 or NaREF4 NCs, it is still a big challenge to
synthesize NCs with predictable structures of REF3 or NaREF4 for
a certain type of RE ion. For example, when the precursors of Na+,
F− and Y3+ all existed in a reaction system either through modified
co-precipitation routes or solvothermal methods, NaYF4 could be
prepared [6,12,13]. However, when Na+, F− and La3+ were used as
the reactants, only LaF3 but no NaLaF4 NCs were generally obtained
[3,5,9,14].
Additionally, the cation radius of lanthanum is known to be the
largest in the lanthanides. Therefore, if an effective method can be
established to controllably synthesize NaLaF4 NCs, other NaREF4
with a smaller cation radius can be also prepared using the same
technique.
Herein, we developed a facile solvothermal strategy for the con-
trollable synthesis of hexagonal phase NaLaF4 nanorods with the
assistance of ethylenediamide (EN). In addition, by merely tuning
the components of the solvent, NaLaF4 and LaF3 NCs can be selec-
tively prepared. Moreover, the as-prepared LaF3 and NaLaF4 NCs all
show intense green UC and DC fluorescence by co-doping Yb3+/Er3+
or Ce3+/Tb3+ under the 980 nm or 254 nm irradiations respectively,
showing potential application in multiple optics-related fields.
2. Material and methods
Ethylenediamide, anhydrous ethanol and hydrochloric acid were obtained from
Beijing Chemical Corp. (Beijing, China). SpecPure grade lanthanide oxides were
obtained from Grirem Advanced Materials Co., Ltd. (Beijing, China). Lanthanide
chlorides were prepared by dissolving the corresponding rare earth oxides in
hydrochloric acid at elevated temperature, following with evaporation of the solvent
at vacuum.
Take the synthesis of NaLaF4 nanorods for an example, in a typical procedure,
2 mmol lanthanum chlorides was dissolved in 10 mL of DI H2O under sonication and
transferred to a 50 mL Teflon-linked autoclave. Then 20 mL EN was added under
vigorous stirring to form a white mixture. 8 mmol NaF dissolved in 10 mL ethanol
was then injected into the autoclave, and the final mixture was agitated thoroughly
for about 5 min. Finally the autoclave was sealed and solvothermally treated at
190 ◦C for 24 h. The product was collected by centrifugation, washed with anhydrous
ethanol for several times, and dried at 60 ◦C. LaF3 NCs were prepared following the
similar procedures except that the solvent components were changed.
As we known, the cation radius of the RE ions decreases from
3+
3+
˚
˚
1.06 A for La to 0.85 A for Lu while the lattice constants from
the hexagonal phase NaLaF4 to NaLuF4 vary for a0 from 6.157 to
˚
˚
5.967 A and for c0 from 3.822 to 3.528 A. As a result, cations are
difficult to settle into the lattice in compounds with large RE cations
such as NaLaF4, which makes the synthesis of these compounds
more difficult than that of NaYF4 or NaLuF4 [15]. To the best of
∗
Corresponding author. Tel.: +86 312 5079403; fax: +86 312 5079403.
E-mail address: liuch@hbu.edu.cn (C. Liu).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.10.101

Z. Wang et al. / Journal of Alloys and Compounds 509 (2011) 1964–1968
1965
Fig. 1. TEM images of the as-synthesized NCs solvothermally treated at 190 ◦C for 24 h under different solvent conditions. Solvent components: (a) 20 mL H2O/20 mL EN; (b)
10 mL H2O/10 mL ethanol/20 mL EN; (c) 20 mL ethanol/20 mL EN; (d) 40 mL H2O; (e) 20 mL H2O/20 mL ethanol; (f) 40 mL ethanol. Other experimental conditions were the
same as described in Section 2.
A JEM-1200EX transmission electron microscope (TEM) (JEOL, Japan) was used
to investigate the size and morphology of the NCs. Powder X-ray diffraction (XRD)
patterns were obtained on a D/max-2500 X-Ray Diffractometer (Rigaku, Japan) with
Cu K␣ radiation at 1.5406 A. Fluorescence spectra of the NCs were measured on an
F-4500 fluorescence spectrophotometer (Hitachi, Japan).
Fig. 1a showed the TEM images of the NCs prepared in the sol-
vent of 20 mL H2O/20 mL EN. It can be seen that the products were
mainly short nanorods with diameters about 20–30 nm and lengths
up to 200 nm, and also accompanied with a portion of elongated
spherical nanoparticles. XRD patterns (Fig. 2a) demonstrated that
the products were the mixture of hexagonal phase LaF3 (JCPDS PDF
72-1435) and hexagonal phase NaLaF4 (JCPDS PDF 75-1923). It can
be deduced that the nanorods were NaLaF4 and the elongated par-
ticles were LaF3 NCs (Compared with image b and d). However,
when the mixture of 10 mL H2O/10 mL ethanol/20 mL EN was used
as the solvent, the elongated LaF3 nanoparticles were disappeared
and all of the products were short nanorods. Rod-shaped NaLaF4
NCs showed very uniform diameters of 25–30 nm and the lengths
were ranged from 75 to 200 nm. The XRD pattern of the nanorods
(Fig. 2b) showed that the peak positions and intensities agreed well
with the data reported in the JCPDS standard card (75-1923) of pure
hexagonal NaLaF4 crystals.
From the TEM and XRD results of sample a and b shown in Fig. 1
and Fig. 2, it seemed that the addition of ethanol to the EN/H2O
was beneficial for the formation of NaLaF4 nanorods. So we specu-
lated at the beginning that ethanol in the solvent might play crucial
roles in the formation of pure NaLaF4 nanorods. However, this pos-
sibility was weakened, because when we employed the solvent
of 20 mL ethanol/20 mL EN, only very tiny and agglomerated LaF3
nanoparticles, instead of NaLaF4, were obtained (Figs. 1c and 2c).
These results indicated that for the proposed method, although
appropriate volume of ethanol in the solvent could promote the
formation and crystallization of NaLaF4 nanorods, it was not the
dominant influence factor in this study. Then we performed a series
of experiments to further study the effects of solvents on the NCs
growth.
˚
3. Results and discussion
3.1. Solvent-assisted selective synthesis of NaLaF4 nanorods and
LaF3 nanoparticles
Following the standard synthesis procedures stated above, the
products synthesized in various solvents consisting of H2O, ethanol,
EN or their mixtures were characterized by TEM, and the images
were shown in Fig. 1. The corresponding XRD patterns were shown
in Fig. 2. As can be seen, when other experimental conditions were
fixed, the morphology and structure of the final products were
greatly influenced by the solvent compositions.
In order to investigate the effect of EN on the formation of NaLaF4
nanorods, the solvents without the addition of EN were used to
prepare samples while other parameters were not changed.
When using H2O, ethanol or their mixture as the solvents with-
out EN, the products were all pure hexagonal phase LaF3 NCs, which
could be deduced from the XRD results shown in Fig. 2. In pure
H2O, monodisperse, elongated spherical LaF3 nanoparticles were
obtained with the diameters of 30–60 nm (Fig. 1d). When the mix-
Fig. 2. XRD patterns of the as-prepared NCs corresponding to the samples (a–e)
shown in the TEM images of Fig. 1.

1966
Z. Wang et al. / Journal of Alloys and Compounds 509 (2011) 1964–1968
Fig. 3. TEM images of samples prepared in the solvent of (a) 1 mL EN/39 mL H2O and (b) 5 mL EN/35 mL H2O. Other experimental conditions were the same as in Fig. 1.
ture of 20 mL H2O and 20 mL ethanol was used, the addition of
ethanol to H2O could further improve the crystallinity of LaF3, thus
more regular spherical-shaped NCs with narrower size distribu-
tions (25–35 nm) and good monodispersity were obtained (Fig. 1e).
These well-crystallized LaF3 NCs were prepared without adding any
size-controlling reagents as usually used in conventional prepa-
ration approaches, and they could be easily dispersed in H2O or
ethanol to form stable colloids under sonication. However, simi-
lar to the preparation of NaLaF4 nanorods, when pure ethanol was
used, only irregular and aggregated LaF3 NCs were synthesized
(Fig. 1f).
However, when EN was added into the solvent, the situation was
quite different. First, it served as the solvent and efficient chelator
for RE ions. In addition, EN was a strong base when dissolved in
H2O. Therefore, in the solvents containing EN and H2O, La3+ cations
would hydrolyze to produce a white precipitate of La(OH)3. Under
the solvothermal conditions, La(OH)3 would slowly re-dissolve to
release La3+, and La–EN complexes would be formed due to the
strong coordination effect of La3+ and EN. Because of the strong
chelation of the EN with the released La3+, F− had to compete with
EN during the nucleation and growth process of the NCs. Therefore,
compared with the solvothermal synthetic routes without using
EN, the hydrolysis of La3+, dissolution of the La(OH)3, as well as the
strong coordination of La3+ and EN made the nucleation and growth
rate of the NCs slow enough to allow the cations settle into the
NaLaF4 lattice, which facilitated the formation of NaLaF4 nanorods.
Furthermore, the decomplexed EN would attach to the surface of
the nanorods to render the products water-soluble. The results of
this contribution also agreed well with our previous work which
had demonstrated that slow nucleation and growth processes could
provide sufficient crystallization time to produce high-quality NCs
[5,18].
On the other hand, EN also had been widely applied as the cap-
ping reagent to synthesize various rod-like nanomaterials [19–21].
According to these literatures, it was reasonable to speculate that
EN also acted as a structure-directing reagent binding to the sur-
face of crystals, directly affecting the growth of different crystal
facets by adjusting the growth rate of different facets, and resulted
in the formation of anisotropically regular rod-shaped NaLaF4 in
this work.
To sum up, in the solvent of H2O, ethanol or their mix-
ture, only LaF3 but no NaLaF4 NCs can be obtained in this
work. Upon the addition of appropriate volume of EN, the
resulted slow nucleation and crystal growth rate, and the
structure-directing effect of EN can stimulate the formation of
well-crystallized NaLaF4 nanorods. Based on the proposed mech-
anism, all the experimental results in this work can be well
explained.
3.2. Reaction mechanism
In this work, products with various morphology and structures
were obtained by using the same reaction precursors but differ-
ent solvent compositions, which meant that the solvent played key
roles in the NCs synthesis.
For NCs preparation, it was general to add ethanol as low dielec-
tric medium to the aqueous solution to alter the dielectric property
of the solvent as well as the resulting particle properties, and addi-
tion of appropriate volume of ethanol to H2O could improve the NCs
quality [16,17], which was consistent with our results. However, it
had been demonstrated above that ethanol was not the dominant
influence factor in this study because high-quality products could
not be obtained when pure ethanol (or ethanol/EN) was used as the
solvent.
Since the possibility of ethanol had been excluded, all of the
experimental results shown above suggested that EN might play a
crucial role for the formation of NaLaF4 nanorods in the presence of
H2O. Based on these phenomena, the roles of EN in the formation
of NaLaF4 nanorods can be postulated as follows.
As we known, the synthesis of NaREF4 NCs is easy for small-
radius RE cations (e.g., Y3+, Lu3+). However, cations are somewhat
difficult to settle into the lattice in compounds with large RE cations
such as NaLaF4 [15]. The cation radius of lanthanum is the largest
in the lanthanides due to the lanthanide contraction, which makes
the solution synthesis of NaLaF4 more difficult than that of NaYF4.
As a result, for most reported wet chemical NCs preparation routes,
LaF3 instead of NaLaF4 were generally obtained although abundant
Na+ ions existed [3,5,9,14].
To further prove our proposed reaction mechanism and the cru-
cial role of EN on the formation of NaLaF4 nanorods, two additional
verification experiments were carried out by employing the sol-
vents of 1 mL EN/39 mL H2O and 5 mL EN/35 mL H2O. It can be seen
from Fig. 3 that when only 1 mL EN was added into the solvent,
the products were all spherical or elongated spherical LaF3 NCs.
However, when the amount of EN increased to 5 mL, some rod-
like crystals appeared along with the dominating spherical LaF3
NCs (Fig. 3b), and some weak characteristic peaks of hexagonal
phase NaLaF4 emerged in the XRD pattern (data not shown). These
In the absence of EN for this proposed method, the NCs were
formed through a co-precipitation reaction-based solvothermal
route and the nucleation and growth rate of the NCs was rapid. As a
result, only LaF3 NCs could be prepared although a large amount of
Na+ existed in the system, which was consistent with the literature
results.

Z. Wang et al. / Journal of Alloys and Compounds 509 (2011) 1964–1968
1967
vator ion) nonradiatively to excite it to the corresponding excited
level through two-photon processes and then the visible upconver-
sion luminescence could be observed. It was worth noting that the
Yb3+/Er3+ ion-pair co-doped NaLaF4 nanorods also demonstrated
the similar optical properties as that of LaF3:Yb3+, Er3+ NCs and
was not shown here for the sake of conciseness.
Fig. 4b showed the fluorescence spectrum of the NaLaF4
nanorods with a doping concentration of 5% Ce3+ and 5% Tb3+ under
the excitation of 254 nm. Ce3+ ion served as sensitizer and Tb3+
as the activator in the fluorescence processes. Ce3+ ion has a rela-
tively broad absorption band centered at ∼250 nm with an allowed
4f–5d transition. After being excited, Ce3+ can transfer their energy
to the Tb3+ ions which emit green fluorescence. The typical emis-
sion peaks of terbium were observed around 486, 543, and 587 nm
assigned to the transitions of 5D4–7FJ (J = 6, 5, 4), respectively.
Furthermore, the novel yellowish-green UC fluorescence photo
of the LaF3:Yb3+, Er3+ colloid and the bright green DC luminescence
photo of NaLaF4:Ce3+, Tb3+ colloid were respectively presented in
Fig. 4. The naked eye-visible luminescence again confirmed the
unique optical property of the as-synthesized fluoride NCs in this
work.
4. Conclusions
High-quality NaLaF4 and LaF3 NCs were selectively prepared by
controlling the solvent compositions. Generally, NaLaF4 NCs are dif-
ficult to be synthesized through conventional hydrothermal route
by using NaF and La3+ as the precursors. In this contribution, it
was found that the addition of appropriate amount of EN to the
aqueous solvent could finely slow down the NCs nucleation and
growth rate, and thus facilitate the formation of hexagonal phase
NaLaF4 nanorods. Since the cation radius of La3+ is the largest in
the lanthanides, if NaLaF4 NCs can be effectively synthesized, other
NaREF4 with a smaller cation radius can be also prepared using the
same technique. Then we believe that the proposed method may
be extended as a general approach to synthesis well-crystallized
NaREF4 NCs especially for the RE cations with large radius. In addi-
tion, in the solvents without EN, well-crystallized LaF3 NCs can be
prepared without using any additional capping reagents. Effect of
the solvent components on the NCs formation was investigated and
a reasonable reaction mechanism was proposed.
Fig. 4. (a) Room-temperature upconversion fluorescence emission spectrum of
0.2 wt% LaF3:12%Yb3+, 3%Er3+ NCs under irradiation of a 980 nm laser. (b) Room-
temperature fluorescence spectrum of 0.2 wt% NaLaF4:5%Ce3+, 5%Tb3+ nanorods
excited at 254 nm. Insets: Corresponding luminescence photographs of colloid NCs.
results further revealed that the proposed reaction mechanism was
reasonable.
In addition, although EN was crucial for the formation of NaLaF4
nanorods, H2O was also indispensable because the strong basicity
of EN, hydrolysis of La3+ ions, strong chelating effect of La3+–EN all
depended on the presence of H2O. It can be seen from Fig. 1c that
when the solvent of ethanol/EN was used without H2O, no NaLaF4
crystal could be formed. This also supported our speculation.
Furthermore, the as-synthesized NaLaF4 and LaF3 NCs all exhib-
ited strong UC and DC fluorescence by co-doping Yb3+/Er3+ or
Ce3+/Tb3+ ion-pairs, which showed potential applications as color
displays, light-emitting diodes, optical storage and optoelectronics.
Acknowledgments
3.3. Fluorescence properties
This work was supported by the National Natural Science Foun-
dation of China (20905018), the Natural Science Foundation of
Hebei Province (B2010000202), and the China Postdoctoral Science
Foundation (20100470985, 20090450920).
Both LaF3 and NaLaF4 NCs are good host matrixes for lanthanide-
based fluorescence processes. To test the luminescence capabilities
of the products in this work, the synthesis of Yb3+/Er3+ ion-pair or
Ce3+/Tb3+ ion-pair co-doped LaF3 and NaLaF4 NCs were carried out.
It was found that by co-doping Yb3+/Er3+, well-crystallized LaF3
NCs and NaLaF4 nanorods both exhibited bright yellowish-green
upconversion fluorescence under the irradiation of a 980 nm laser.
Fig. 4a showed the emission spectrum of the LaF3: 12%Yb3+, 3%Er3+
NCs (synthesized in 20 mL H2O/20 mL ethanol). There were three
main emission peaks at around 525, 545 and 658 nm, which were
assigned to the 2H11/2 to 4I15/2, 4S3/2 to 4I15/2 and 4F9/2 to 4I15/2 tran-
sitions of erbium, respectively. All of these emissions mentioned
above were based on a two-photon process. The UC mechanisms
for the Yb3+/Er3+ co-doped nanomaterials had been demonstrated
in detail in previous works [5,14,18]. Briefly, under 980 nm excita-
tion, an electron of Yb3+ (sensitizer) could be excited from the 2F7/2
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