Optically active uniform potassium and lithium rare earth fluoride nanocrystals derived from metal trifluroacetate precursors

Article abstract of DOI:10.1039/b909145a

This paper reports the first systematical synthesis of near-monodisperse potassium and lithium rare earth (RE) fluoride (K(Li)REF₄) nanocrystals with diverse shapes (cubic KLaF₄ and KCeF₄ wormlike nanowires, nanocubes and

Full text of DOI:10.1039/b909145a

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Optically active uniform potassium and lithium rare earth fluoride
nanocrystals derived from metal trifluroacetate precursors†
Ya-Ping Du, Ya-Wen Zhang,* Ling-Dong Sun and Chun-Hua Yan*
Received 8th May 2009, Accepted 21st July 2009
First published as an Advance Article on the web 2nd September 2009
DOI: 10.1039/b909145a
This paper reports the first systematical synthesis of near-monodisperse potassium and lithium rare
earth (RE) fluoride (K(Li)REF4) nanocrystals with diverse shapes (cubic KLaF4 and KCeF4 wormlike
nanowires, nanocubes and nanopolyhedra; cubic LiREF4 (RE = Pr to Gd, Y) nanopolyhedra;
tetragonal LiREF4 (RE = Tb to Lu, Y) rhombic nanoplates) via co-thermolysis of Li(CF3COO) or
K(CF3COO) and RE(CF3COO)3 in a hot oleic acid/oleylamine/1-octadecene solution. The effects of
the solvent composition, reaction temperature and time on the crystal phase purity, shape, and size of
the as-prepared nanocrystals have been investigated in detail. The formation of monodisperse
nanocrystals is found to strongly depend upon the nature of both alkali metals from Li to K, and the
rare earth series from La to Lu and Y. Based on the series of experimental results, a controlled-growth
mechanism has also been proposed. In addition, the ease of doping of these as-synthesized host
nanocrystals for designed luminescence properties is assessed. For example, monodisperse and
single-crystalline Eu3+ doped KGdF4, Yb3+ and Er3+ co-doped LiYF4 nanocrystals redispersed in
cyclohexane exhibit visible room-temperature red and green emissions under ultraviolet (UV)
excitation and near infrared (NIR) 980 nm laser excitation, respectively.
doped fluorides based on AREF4 (A = alkali metal, RE =
Introduction
rare earth) have been investigated for several decades due to
their unique luminescent, ferromagnetic, insulating/magnetic,
and piezoelectric properties and wide applications in laser de-
vices, three-dimensional flat-panel displays, and low-intensity
infrared imaging.4 For example, the hexagonal phase of NaYF4
is considered as the most efficient host material for green and
blue upconversion (UC) phosphors until now,4e LiGdF4 is an
outstanding host for downconversion luminescence, for which the
quantum efficiency is close to 200%.5b Rare earth doped single
crystals of KYF4 are excellent solid-state laser materials, and laser
action has been observed for Yb3+, Tm3+, Ho3+ ions in the KYF4
lattice.5a,c
As an important category of rare earth fluoride compounds of
AREF4 material, KREF4 and LiREF4 are particularly suitable
luminescence hosts for doping lanthanide ions due to their low
phonon energies, which reduce the thermal disturbance effects
under strong pumping conditions.5 There have already some
reports on the solution phase synthesis and optical characteri-
zation of nanosized NaREF4.6 To our knowledge, the systematic
study of the synthesis and material properties of monodisperse
colloidal KREF4 and LiREF4 nanocrystals has never been
described until now.7 The current synthesis methods available for
nanosized KREF4 and LiREF4 are mainly based on the liquid
coprecipitation reaction between soluble rare earth salts and alkali
fluorides.7a,df–h
Studies on monodisperse colloidal inorganic nanocrystals have
advanced dramatically in the past few years. The colloidal
nanocrystals with controlled shape, size, composition, phase, and
surface endow such materials with unique optical, electronic,
magnetic, and catalytic properties, which are fundamentally im-
portant and technologically useful.1 Precisely controlled synthesis
allows manipulation of the properties of nanocrystals as desired.
Therefore, up to present, various wet chemical strategies have been
designed and developed for obtaining such nanocrystals, which
not only are recognized for their unique material properties, but
also have already acted as the building blocks for the advanced
nanodevices composed of 2D (two dimensional) and 3D (three
dimensional) assemblies.2 Amongst these, the relatively successful
synthetic methods are based on the nonhydrolytic approach and
its alternatives, with which nanocrystalline products can be grown
with controlled-size, shape, phase, and composition for a given
inorganic compound.1e,g2a–f
Colloidal rare earth (RE) compound nanocrystals such as
oxides, phosphates, fluorides, vanadates, sulfides, and oxyhalides
have become a new focus of research, due to their extraordi-
nary properties arising from the 4f electron configuration and
the potential applications in solid-state lasers, optical amplifies,
lighting and displays, and biolabels.3 Among them, the rare earth
Beijing National Laboratory for Molecular Sciences, State Key Lab-
oratory of Rare Earth Materials Chemistry and Applications, PKU-
HKU Joint Laboratory in Rare Earth Materials and Bioinorganic
Chemistry, Peking University, Beijing, 100871, China. E-mail: yan@pku.
edu.cn, ywzhang@pku.edu.cn; Fax: +86-10-6275-4179; Tel: +86-10-6275-
4179
† Electronic supplementary information (ESI) available: Table S1 and more
results obtained by means of TEM, XRD, EDAX, and FTIR for KREF4
and LiREF4 nanocrystals. See DOI: 10.1039/b909145a
In this article, we demonstrate a general one-step synthesis of
near-monodisperse KREF4 (RE = La to Gd, Y) and LiREF4
(RE = Tb to Lu, and Y) colloidal nanocrystals via the co-
thermolysis of K(CF3COO) or Li(CF3COO) and RE(CF3COO)3
precursors in oleic acid/oleylamine/1-octadecene. The effects of
different experimental conditions on the crystal phase purity,
shape and size of the KREF4 and LiREF4 nanocrystals have
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been investigated. The formation mechanisms of the nanocrystals
have also been proposed. With the developed synthesis method,
monodisperse KGdF4:Eu, LiYF4:Yb,Er nanocrystals have also
been obtained, which show interesting optical properties under
ultraviolet (UV) and near infrared (NIR) laser (980 nm) excita-
tions.
with a slit of 1/2◦ at a scanning rate of 4◦ min-1, using Cu-Ka
˚
radiation (l = 1.5418 A). The lattice parameters were calculated
with the least-squares method. Samples for transmission electron
microscopy (TEM) analysis were prepared by drying a drop of
nanocrystal dispersion in cyclohexane on amorphous carbon-
coated copper grids. Particle sizes and shapes were examined by a
JEOL 200CX-TEM (Japan) operated at 160 kV. High-resolution
TEM (HRTEM) characterization was performed with a Philips
Tecnai F30 FEG-TEM (USA) operated at 300 kV. FTIR spectra
Experimental
Materials
were obtained on a Bruker Vector22 spectrophotometer
(German). Room-temperature fluorescence spectrum of
KGdF4:Eu nanocrystals were recorded on a Hitachi F-4500
spectrophotometer (Japan) equipped with a 150 W Xe arc
lamp at a fixed bandpass of 0.2 nm with the same instrument
parameters (10.0 nm for excitation slit, 10.0 nm for emission
slit, and 700 V for PMT voltage), with the as-dried nanocrystals
redispersed in cyclohexane at a concentration of about 2 wt%.
The photoluminescence quantum yields of the KGdF4:Eu
nanocrystals were determined by comparing the integral emission
intensity of the dilutions of the concentrated nanocrystal
suspension in cyclohexane with that of rhodamine B in absolute
ethanol at an excitation of 275 nm with collection between
550 and 700 nm at room temperature.3l Upconversion emission
spectra of LiYF4:Yb,Er nanocrystals were obtained on a modified
Hitachi F-4500 spectrophotometer with an external tunable 2
W 980 nm laser diode as the excitation source, with the as-dried
nanocrystals redispersed in cyclohexane at a concentration of
about 2 wt%.
The synthesis was carried out using standard oxygen-free pro-
cedures and commercially available reagents. Rare earth oxides,
oleic acid (OA; 90%, Alpha), oleylamine (OM; >80%, Acros),
1-octadecene (ODE; >90%, Acros), trifluoroacetic acid (99%,
Acros), K(CF3COO) (>97%, Acros), absolute ethanol, cyclohex-
ane were used as received. RE(CF3COO)3 and Li(CF3COO) were
prepared with the literature method.8 All the chemicals were of
analytical grade and used as received without further purification.
Synthesis of KREF4 (RE = La to Gd, Y) nanocrystals
A given amount of K(CF3COO) and RE(CF3COO)3 (1 mmol)
were added into 40 mmol of OA/OM/ODE (Table S1, ESI†) in
a three-necked flask at room temperature. Then the slurry was
heated to 120 ◦C to remove water and oxygen, with vigorous
magnetic stirring under vacuum for 30 min in a temperature-
controlled electromantle, and thus forming a transparent solution.
The solution was then heated to a certain temperature in the
◦
range of 250–330 C at a rate of 22 K min-1 and maintained
at the given temperature for 15–120 min under an Ar atmosphere.
When the reaction was completed, an excessive amount of
ethanol was poured into the solution at room temperature. The
resultant mixture was centrifugally separated and the products
were collected. The as-precipitated nanocrystals were washed
several times with ethanol and dried in air at 80 ◦C overnight.
The yields of all the obtained nanocrystals without size-selection
were 60–70%. All these as-prepared nanocrystals could be easily
dispersed in various nonpolar organic solvents (e.g., cyclohexane).
Results and discussion
Characterizations of KREF4 and LiREF4 nanocrystals
Cubic KLaF4 and KCeF4 wormlike nanowires. The X-ray
diffraction (XRD) patterns shown in Fig. 1a reveal that cubic
KLaF4 and KCeF4 were formed. The peaks are matched with the
cubic fluorite type crystal structure (CaF2 type, space group: Fm-
˚
3m). The calculated lattice constants are as follows: a = 5.928 A
˚
for KLaF4 (JCPDS: 75–0248) and a = 5.901 A for KCeF4 (JCPDS:
75–2021). The broadening of the diffraction peaks distinctly
indicates the nanocrystalline nature of the samples. Meanwhile,
the atomic ratios of metals in the nanocrystals were determined
by energy-dispersive X-ray analysis (EDAX), confirming the
formation of stoichiometric KREF4 (Fig. S1, ESI†). The TEM
images shown in Fig. 1b and Fig. S2a (see ESI†) demonstrate the
as-obtained KLaF4 and KCeF4 nanocrystals take on the wormlike
wire shape with a uniform diameter of 8.2 0.5 nm and 7.8
0.4 nm, respectively. The inserted HRTEM images indicate that the
as-obtained nanowires are single-crystalline, and the preferred 1D
(one dimensional) growth direction is along the <111> direction.
To our knowledge, such a 1D nanostructure has not been reported
for cubic KLaF4 and KCeF4 before.7a,fg
Synthesis of LiREF4 (RE = Tb to Lu, Y) nanocrystals
The synthetic procedure was the same as that used to synthesize
KREF4 nanocrystals, except that a certain amount of as-prepared
Li(CF3COO) and RE(CF3COO)3 were added into a mixture of
40 mmol of OA/ODE (Table S1, ESI†) in a three-necked flask at
room temperature.
Synthesis of KGdF4:5%Eu and LiYF4:20%Yb,2%Er nanocrystals
The synthetic procedure was the same as that used to syn-
thesize KREF4 nanocrystals, except that 1 mmol K(CF3COO)
or Li(CF3COO) and a suitable proportion of Gd(CF3COO)3,
Eu(CF3COO)3, Y(CF3COO)3, Yb(CF3COO)3, and Er(CF3COO)3
were added into a mixture of OM and/or OA and ODE (total: 40
mmol) in a three-necked flask at room temperature.
Cubic KREF4 (RE = Pr to Gd, Y) nanopolyhedra. Fig. 2a
shows that the peaks for each sample of KREF4 (RE = Pr to Gd,
Y) nanocrystals can be readily indexed to a pure cubic phase (CaF2
type, space group: Fm-3m). From Fig. 2a, the calculated lattice
Instrumentation
˚
˚
constants are as follows: a = 5.891 A for KPrF4, a = 5.872 A for
˚
˚
Powder X-ray diffraction (PXRD) patterns of the dried powders
were recorded on a Rigaku D/MAX-2000 diffractometer (Japan)
KNdF4, a = 5.851 A for KSmF4, a = 5.829 A for KEuF4, a =
˚
˚
5.808 A for KGdF4, and a = 5.625 A for KYF4. The broadening
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KNdF4, 18.9 3.8 nm for KSmF4, 19.5 4.1 nm for KEuF4, 21.2
6.1nm for KGdF4, except for KYF4 in a size of 13.2–25.9 nm. The
HRTEM images insets in Fig. 2b–d and Fig. S2b–d (see ESI†)
show that all the KREF4 nanocrystals are of single-crystalline
nature, and are mainly enclosed by the (111) and (200) planes.
Tetragonal LiREF4 (RE = Tb to Lu, Y) nanocrystals. The
diffraction peaks shown in Fig. 3a reveal the formation of
tetragonal LiREF4 (RE = Tb to Lu, Y) (scheelite structure, space
group: I41/a). From Fig. 3a, the calculated lattice constants are
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
as follows: a = b = 5.180 A, and c = 10.870 A for LiTbF4 (JCPDS:
27–1262), a = b = 5.170 A, and c = 10.801 A for LiDyF4 (JCPDS:
27–1233), a = b = 5.150 A, and c = 10.751 A for LiHoF4 (JCPDS:
˚
27–1243), a = b = 5.151 A, and c = 10.672 A for LiErF4 (JCPDS:
˚
27–1235), a = b = 5.131 A, and c = 10.602 A for LiTmF4 (JCPDS:
˚
27–1265), a = b = 5.128 A, and c = 10.581 A for LiYbF4 (JCPDS:
˚
23–0371), a = b = 5.119 A, and c = 10.541 A for LiLuF4 (JCPDS:
˚
˚
27–1251), and a = b = 5.171 A, and c = 10.741 A for LiYF4
(JCPDS: 81–1940). As expected from the lanthanide contraction,
the tetragonal lattice volume has a decreasing tendency from
LiTbF4 to LiLuF4 (Fig. S3, ESI†). TEM characterization has told
us that all the as-obtained tetragonal LiREF4 (RE = Tb to Lu, Y)
nanocrystals display a rhombic plate-like shape. The panels b–d
of Fig. 3 and Fig. S4 (see ESI†) depict the rhombic nanoplates
of LiREF4 (RE = Tb to Lu, Y), whose top surfaces are in the
size of (27.1 2.4) nm ¥ (25.8 4.3) nm for LiTbF4, (55.1
3.1) nm ¥ (49.9 3.7) nm for LiDyF4, (53.1 3.4) nm ¥ (49.8
2.3) nm for LiHoF4, (51.1 2.3) nm ¥ (50.9 3.0) nm for LiErF4,
(56.1 2.3) nm ¥ (54.9 3.0) nm for LiTmF4, (40.3 5.9) nm ¥
(36.9 4.3) nm for LiYbF4, (91.5 2.3) nm ¥ (89.3 3.2) nm
for LiLuF4 and (29.2 3.9) nm ¥ (23.2 3.0) nm for LiYF4.
From the HRTEM images inserted in Fig. 3b–d and Fig. S4 (see
ESI†), the (101) and (100) facets are identified on the top surface
Fig. 1 (a) XRD patterns of cubic KLaF4 and KCeF4 wormlike nanowires.
(b) TEM and HRTEM (inset) images of KLaF4 wormlike nanowires.
of the diffraction peaks reveals the nanocrystalline nature of the
samples. The plots of unit cell volume per KREF4 (RE = La to Gd,
Y) molecule along the rare earth series shown in Fig. S3 (see ESI†)
are in agreement with the lanthanide contraction. The panels b–d
of Fig. 2 and Fig. S2 (see ESI†) show the TEM images of the
as-obtained KREF4 (RE = Pr to Gd, Y) nanocrystals.
Fig. 2 (a) XRD patterns of as-obtained cubic KREF4 nanopolyhedra.
TEM and HRTEM (inset) images of (b) KPrF4, (c) KEuF4, and (d) KYF4
nanopolyhedra.
Fig. 3 (a) XRD patterns of as-obtained tetragonal LiREF4 nanoplates.
TEM and HRTEM (inset) images of (b) LiTbF4, (c) LiErF4, and (d) LiYF4
nanoplates.
The nanocrystals are of polyhedron shape, and have a near-
monodisperse size of 18.2 3.6 nm for KPrF4, 16.4 4.2 nm for
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of the nanoplates. From Fig. 3b–d and Fig. S4 (see ESI†), it is also
noted that most of the LiREF4 nanocrystals are separated, and
exhibit a partially 2D (two dimensional)-ordered arrangement,
indicative of the presence of capping ligands on the surfaces of
nanocrystals.3k,l6d
The surfactant bound on the nanocrystals surfaces was con-
firmed by FTIR spectroscopy, the FTIR spectrum of the as-
synthesized LiYF4 nanocrystals is shown in Fig. S5 (see ESI†),
the presence of strong acyclic C–H stretching at 2931 cm-1 and
weak carbonyl peaks at 1712 cm-1 indicated the existence of a
large amount of free oleic acid in solution, the broad peaks at 1449
and 1645cm-1 could be assigned to a carboxylate (COO-) stretch,
implying a chemical bonding of OA molecules on the nanocrystal
surfaces.9g
Synthesis of KREF4 and LiREF4 nanocrystals
Normally, KREF4 has two polymorphs at room temperature
and ambient pressure: orthorhombic and hexagonal, except that
KLaF4 and KCeF4 possess a cubic phase. Light and middle
KREF4 (RE = Ce to Gd) appear in orthorhombic form, whereas,
the heavy one (RE = Tb to Lu, Y) exists in only hexagonal
form.7a,bd,h On the other hand, LiREF4 presents only one tetrag-
onal phase, which is stable in bulk at room temperature.7c,e In
addition for most of the AREF4 (A = alkali metal), a high-
temperature modification exists, which can be seen as the cubic
CaF2 type structure with statistical distribution of the cations A+
and RE3+ on the Ca2+ sites.7d,fg In our work, however, all the as-
prepared nanosized KREF4 from La to Gd including Y only take
on a cubic structure, while those from Tb to Lu prefer to form
KF and REF3 instead of producing KREF4. The stoichiometric
LiREF4 nanocrystals from Dy to Lu including Y only display
the tetragonal phase, and LiF and REOF are formed in terms of
lighter LiREF4 ones.
In this work, for a certain compound, the controlled synthesis
of monodisperse KREF4 and LiREF4 nanocrystals have been
carefully conducted. Experimentally, we found that the crystal
phase purity, shape and size of the as-prepared KREF4 and
LiREF4 nanocrystals are mainly affected by conditions, such
as solvent composition, reaction temperature and time (Table
S1, ESI†), the optimal synthetic parameters are screened so as
to achieve the appropriate balance between the nucleation and
growth stages of the nanocrystals to obtain high-quality products.
Fig.
4
TEM images of KLaF4 nanocrystals synthesized in (a)
OA/ODE = 1:1 and (b) OA/OM/ODE = 1:4:5 for 60 min at 280 ◦C. TEM
images of KYF4 nanocrystals synthesized in (c) OA/ODE = 1:1 and
(d) OA/OM/ODE = 1:4:5 for 30 min at 300 ◦C.
monodisperse KLaF4 wormlike nanowires were obtained at an
optimized OA/OM/ODE ratio of 1:1:2 (Fig. 1b), due to a well-
maintained balance between nucleation and growth stages during
synthesis under these conditions. The above case was also similar
to the synthesis of other KREF4 nanocrystals, such as KYF4
nanocrystals (Fig. 4c,d).
Different from the KREF4 (RE = La to Gd, Y) nanocrystals,
as for LiREF4 (RE = Tb to Lu, Y), the presence of a mixed
solvent of OA/ODE is a prerequisite to achieve phase-pure and
monodisperse LiREF4 nanocrystals. The presence of OM ligands
in the mixed solvent might significantly hinder the growth of
LiREF4 nanocrystals, by producing LiF impurity in the nanocrys-
tal growth. Here, we take LiLuF4 as an example, using 1 mmol
Li(CF3COO) and 1 mmol Lu(CF3COO)3 as the precursors, the
reaction under OA/OM/ODE = 4:1:5 at 280 ◦C for 60 min,
produced non-dispersed LiF (space group: Fm-3m) impurities
(Fig. S6, ESI†). The synthesis of the other monodisperse LiREF4
nanocrystals was found to resemble that of LiLuF4 nanocrystals
in OA/ODE (1:1) solvent, such as LiYF4 nanocrystals (Table S1
and Fig. S6, ESI†).
Solvent composition effects. When the ratios of precursor
concentration K(CF3COO) or Li(CF3COO)/RE(CF3COO)3, re-
action temperature and time were fixed, the solvent composition
was found to greatly affect the growth of KREF4 (RE = La to
Gd, Y) and LiREF4 (RE = Tb to Lu, Y) nanocrystals. It seems
that the use of a mixed solvent of OA/OM/ODE is essential for
the preparation of near-monodisperse KREF4 nanocrystals, while
OA/ODE mixture is crucial for phase-pure and monodisperse
LiREF4 ones. For example, for the synthesis of KLaF4 wormlike
nanowires, using 1 mmol K(CF3COO) and 1 mmol La(CF3COO)3
as the precursors, the reaction under OA/ODE = 1:1 at 280 ◦C for
60 min yielded dispersed but severely-agglomerated nanocrystals
(Fig. 4a), implying that the growth of the KLaF4 nanocrystals
became drastic with the addition of more OA, while under
OA/OM/ODE = 1:4:5, the dispersible but ill-shaped nanocrystals
with poor crystallinity were formed (Fig. 4b). Therefore, near-
Reaction temperature and time effects. Except for the solvent
composition, we found that both the reaction temperature and
time also remarkably affected the shapes of the products. For
example, under conditions of 1 mmol K(CF3COO) and 1 mmol
La(CF3COO)3, OA/OM/ODE = 1:1:2 and a reaction time of
60 min, as reaction temperature was decreased from 280 to 250 ◦C,
hardly any solid matter appeared after the precipitation treatment,
indicating deficient energy for the decomposition of the precursor
to form KLaF4. As the reaction temperature was increased to
290 ◦C, some wormlike nanowires coexisted with a large amount
of nanocubes (Fig. 5a). As the reaction temperature was further
increased to 300 ◦C, the as-obtained dispersible KLaF4 wormlike
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the reaction temperature was increased to 300 ◦C, the relatively
near-monodisperse LiLuF4 nanoplates with a much broader size
distribution (41.6 11.8) nm ¥ (39.8 9.9) nm were formed
(Fig. S9a, ESI†). When the reaction temperature was further
raised to 320 ◦C, polygonal aggregated LiLuF4 nanocrystals in
the size range of 16.1–28.2 nm instead of nanoplates were formed
(Fig. S9b, ESI†).
◦
Under a fixed reaction temperature of 280 C, as the reaction
time was shortened from 60 min to 30 min, the co-thermolysis
of 1 mmol K(CF3COO) and La(CF3COO)3 in OA/OM/ODE
(1:1:2) resulted in poorly separated wormlike nanowires (Fig. 6a).
When the reaction went on to 120 min, some KLaF4 wormlike
nanowires coexisted with nanocubes (Fig. 6b). The same ten-
dency was observed for the KCeF4 wormlike nanowires. As for
KYF4, when the reaction time was 15 min, polydisperse KYF4
nanocrystals (38.5 5.1) nm with relatively low yields were formed
(Fig. S10a, ESI†). When the reaction time was prolonged to
45 min, the well-crystallized but poorly separated nanocrystals
in improved yields were harvested (Fig. S10b, ESI†). However, as
the reaction time reached 60 min, aggregated KYF4 nanoparticles
were formed (Fig. S10c, ESI†). For the LiREF4 nanocrystals, such
as the synthesis of LiErF4, under a fixed reaction temperature
of 330 ◦C, as the reaction time was shortened from 60 min to
30 min and 15 min, the LiErF4 products appeared as a mixture
of nanoplates and nanowires (Fig. S11a,b, ESI†). When the
reaction went on to 120 min, the LiErF4 aggregated polydisperse
nanoplates were formed (Fig. S11c, ESI†).
Fig. 5 TEM images of KLaF4 nanocrystals synthesized from co-thermol-
ysis of 1 mmol K(CF3COO) and 1 mmol La(CF3COO)3 in OA/OM/ODE
(1:1:2) for 60 min at different reaction temperatures: (a) 290 ◦C, (b) 300 ◦C
and (c) 320 ◦C.
nanowires vanished to form uniform nanocubes ((9.4 0.9) nm)
(Fig. 5b). Further increase in the reaction temperature to 320 ◦C
resulted in monodisperse KLaF4 nanopolyhedra ((9.5 0.5) nm)
(Fig. 5c). A similar observation was achieved in the case of
KCeF4 wormlike nanowires. Another example was the synthesis
of KYF4 nanocrystals, with 1 mmol K(CF3COO) and 1 mmol
Y(CF3COO)3 as precursors, under OA/OM/ODE = 1:1:2 at
300 ◦C for 30 min, poorly crystalline KYF4 nanocrystals were
harvested (Fig. S7a, ESI†). As the reaction time was increased to
60 min, highly-crystalline near-monodisperse KYF4 nanocrystals
were obtained (Fig. 2d), suggesting a long reaction time enhanced
the crystallinity of the nanocrystals. However, further elevating
the temperature to 330 ◦C resulted in nonuniform aggregated
nanocrystals (Fig. S7b, ESI†). The influence of the reaction
temperature on the other KREF4 nanocrystals was found to
resemble that of KYF4.
Fig. 6 TEM images of KLaF4 nanocrystals obtained from the thermoly-
sis of 1 mmol K(CF3COO) and 1 mmol La(CF3COO)3 in OA/OM/ODE
(1:1:2) at 280 ◦C for different reaction times: (a) 30 min, (b) 120 min.
As for LiREF4, the influences of reaction temperature on the
composition and shape evolution of as-synthesized nanocrystals
were observed. Here, taking LiErF4 as an example, at reaction
To sum up, our pure-phase monodisperse KREF4 (RE = La
to Gd, Y) nanocrystals could be obtained in mixed solvents
of OA/OM/ODE (1:1:2), and LiREF4 (RE = Tb to Lu, Y)
nanocrystals could be obtained at a high temperature in OA/ODE
(1:1) with a relatively long reaction time.
◦
temperatures ranging from 280 ◦C to 300 C, the co-thermolysis
of 1 mmol Li(CF3COO) and 1 mmol Er(CF3COO)3 under
OA/ODE = 1:1 for 60 min produced monodisperse ErOF
nanocrystals (23.2 0.4) with few LiF impurities (Fig. S8a,b,
ESI†), indicating the relative low temperature (< 300 ◦C) induced
the formation of LiF and monodisperse REOF nanocrystals,
instead of stoichiometric LiREF4 ones. As the reaction tem-
perature was increased to 320 ◦C, the monodisperse LiErF4
nanoplates were harvested (Fig. 3c), further elevation of the
reaction temperature to 330 ◦C resulted in highly aggregated
LiErF4 nanocrystals (Fig. S8c, ESI†). For another example of
LiLuF4, at 280 ◦C, the co-thermolysis of 1 mmol Li(CF3COO)
and 1 mmol Lu(CF3COO)3 under OA/ODE = 1:1 for 60 min
produced monodisperse LiLuF4 nanoplates (Fig. S4e, ESI†). As
Formation mechansim of KREF4 and LiREF4 nanocrystals
Just like the case of co-thermolysis of Na(CF3COO) and
RE(CF3COO)3 in mixed solvents of OA/OM/ODE,6d as the
reaction solutions containing K(CF3COO) or Li(CF3COO) and
RE(CF3COO)3 were rapidly heated to their thermolysis tempera-
tures under an Ar atmosphere, tiny gas bubbles formed instantly
from the reaction system, indicating a decomposition of the
precursors and a simultaneous formation of KREF4 or LiREF4
nuclei (Scheme 1). As indicated by the experimental results, only
LiREF4 nuclei were generated for the rare earths from Tb to Lu
8578 | Dalton Trans., 2009, 8574–8581
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shape (nanopolyhedra) of nanocrystals at high temperatures of
300 ◦C (Scheme 1). The production of wire-like KLaF4 and
KCeF4 nanocrystals at reduced temperatures (260–280 ◦C) may
be caused by the oriented aggregation of small particles.9h The
wire-like shape was considered to be kinetically stable, since
it was observed that the metastable wormlike nanowires were
transformed to the thermodynamically stable nanocubes and
further nanopolyhedra with increasing reaction temperature and
reaction time. As for LiREF4 (RE = Tb to Lu, Y) nanocrystals,
the as-observed 2D growth modes for the rhombic nanoplates
were probably originated from the crystal structure effect of the
anistropic tetrgonal phase (c/a = 2.1) (Fig. 7b).1e,7f,9
Luminescence properties of lanthanide doped KREF4 and LiREF4
nanocrystals
In order to study the luminescence properties of lanthanide doped
KREF4 and LiREF4 nanocrystals, monodisperse and single-
crystalline (27.2 1.8) nm KGdF4:5%Eu and (14.9 1.4) nm
LiYF4:20%Yb,2%Er, nanocrystals were also prepared using the
current method (Fig. 8a,b).
Scheme 1 Schematic illustration for a controlled growth mechanism of
near-monodisperse KREF4 (RE = La to Gd, Y) and LiREF4 (RE = Tb
to Lu, Y) nanocrystals with diverse shapes.
and Y, while for La to Gd, the LiF and REOF nuclei coexisted.
This result suggests that the tendency for the formation of LiREF4
nuclei was depressed for the light rare earths. On the contrary for
the synthesis of KREF4 nanocrystals, only KREF4 nuclei were
generated for the rare earths from La to Gd and Y, while KF and
REF3 nuclei coexisted for Tb to Lu, indicating the tendency for
the formation of KREF4 nuclei was easier for the light rare earths,
but more difficult for the heavy rare earths. Our research results
on the NaREF4 have demonstrated that the NaREF4 nuclei were
generated for the rare earths from Pr to Lu and Y, while NaF and
REF3 nuclei coexisted for La and Ce.6d,gh These results suggest
that the lighter the alkali metal and rare earth, the easier AF (A =
alkali metal) and REF3 or REOF nuclei formed, owing to the fact
that the basicity of the alkali metal oxide gradually increases along
the alkali metal series, while the basicity of the rare earth oxide
gradually decreases along the rare earth series.
As is well-known, the crystalline phase of the nuclei, the selective
adsorption of the surfactant onto specific crystal planes, and
the balance between the kinetic and the thermodynamic growth
regimes are the three important factors affecting the shape of
nanocrystals.1e,9 For the crystalline seeds with an isotropic unit
cell structure (e.g., cubic), they often experience an isotropic
growth, which results in 0D (zero dimensional) nanostructures.
In our synthesis, KREF4 (RE = La to Gd, Y) have a cubic
structure (Fig. 7a); therefore, as expected, we obtained 0D-growth
Fig. 8 TEM images of lanthanide doped KREF4 and LiREF4 nanocrys-
tals: (a) KGdF4:5%Eu nanopolyhedra (top right is the HRTEM image,
bottom right is the eye-visible luminescence photography of its dispersion)
and (b) LiYF4:20%Yb,2%Er nanopolyhedra (top right is the HRTEM
image, bottom right inset is the eye-visible upconversion luminescence
photography of its dispersion). The luminescent nanocrystal dispersions
are excited by the 254 nm hand-held UV lamp at room temperature.
Fig. 9a describes the emission spectrum of KGdF4:Eu3+
nanocrystals. The emission peaks at 589, 613, 649, and 696 nm
7
can be assigned to the typical 5D0→ FJ emissions (J = 0, 1,
2, 3, 4) of the Eu3+ ions, respectively. The peak located at
5
7
589 nm (corresponding to D0→ F1 magnetic dipole transition)
is weaker than the peak located at 613 nm (corresponding to
7
5D0→ F2 electric dipole transition), suggesting the presence of
a strong surface effect on the red emission of those oleate
capped nanocrystals, as compared with the bulk counterpart.3k,l,11
The photoluminescence quantum yields (QYs) of KGdF4:5%Eu
nanopolyhedra were determined to be 18%. Fig. 9b exhibits the
5
7
luminescence decay curves of D0→ F2 emission at 613 nm for
the KGdF4:5%Eu nanocrystals. These decay curves can be fitted
well into a single exponential function: I = I0 exp(-t/t) (t is the
5
7
lifetime of Eu3+). The lifetime of the D0→ F2 emission of Eu3+
ions (613 nm) is 1.71 ms for the KGdF4:5%Eu nanopolyhedra,
which is close to that (1.72 ms) of 6.3 nm GdOF:5%Eu.3k
Fig. 9c shows the UC fluorescence spectrum of as-
obtained LiYF4:20%Yb,2%Er nanopolyhedra dispersed in
Fig. 7 Crystal structures of (a) cubic KREF4 and (b) tetragonal LiREF4
built by CERIUS2 software (ref.: http://www.accelrys.com/cerius2).
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cyclohexane under 980 nm excitation. The green emissions
at 521, 541 and 550 nm are attributable to the transitions from
4
4
4
2H11/2 to I15/2 and from S3/2 to I15/2 of Er3+ ions, respectively,
4
and the red emissions at 651 and 667 nm come from the F9/2
to I15/2 transition of Er3+ ions. To determine the number of
4
photons involved in the UC process of the Yb3+, Er3+ doped
LiYF4 nanocrystals, the relationships between laser diode power
density and emission intensities for the green and red emission
are depicted in Fig. 9d. The slopes of different curves are
found to be approximately 2, suggesting the normal two-photon
emission processes are involved in the UC mechanism of the
present nanocrystals.10 Under the 980 nm laser excitation, the
activated Yb3+ ion absorbs one photon and transfers it to the
Er3+ ion, the Er3+ ion receives the energy and its ground state
4
4I15/2 electron is excited to the I11/2 level, and the Yb3+ promotes
4
Er3+ to F7/2 state via nonradiative energy transfer process. The
excited electron decays nonradiatively to 2H11/2, 4S3/2, 4F9/2 levels,
and precedently radiative relaxation to the ground state results
in the emission of 525 nm, 541 and 550 nm, 651 and 667 nm,
respectively. The as-obtained KGdF4:Eu3+ and LiYF4:Yb3+, Er3+
nanocrystals display intense red emission and green emission
under UV excitation and 980 nm laser excitation, respectively.
(The digital camera images of luminescence from KGdF4:Eu3+
and LiYF4:Yb3+, Er3+ nanocrystals are shown in the insets of
Fig. 8a and Fig. 8b, respectively).
Conclusions
Near-monodisperse KREF4 and LiREF4 colloidal nanocrys-
tals with various shapes (cubic KLaF4 and KCeF4 wormlike
nanowires, nanocubes and nanopolyhedra; cubic KREF4 (RE =
Pr to Gd, Y) nanopolyhedra; LiREF4 (RE = Tb to Lu, Y)
rhombic nanoplates) have been synthesized via co-thermolysis
of Li(CF3COO) or K(CF3COO) and RE(CF3COO)3 in hot oleic
acid/oleylamine/1-octadecene solutions. The crystal phase purity,
shape and size of the as-prepared nanocrystals were concluded to
be greatly affected by the synthetic parameters such as solvent
composition, reaction temperature and time. The controlled-
growth of KREF4 and LiREF4 nanocrystals has suggested that
the lighter the alkali metal and rare earth, the easier AF
(A = K, Li) and REF3 or REOF nuclei formed, owing to the fact
that the basicity of the alkali metal oxide gradually increases along
the alkali metal series, while the basicity of the rare earth oxide
gradually decreases along the rare earth series. Contrarily, AREF4
nuclei were more easily formed. With the developed synthetic
strategy, the monodisperse and single-crystalline KGdF4:5%Eu
and LiYF4:20%Yb,2%Er nanocrystals have been prepared. The
KGdF4:5%Eu nanocrystals display intense visible red emission
under UV excitation at room temperature. Under a 980 nm near-
IR laser excitation, the LiYF4:20%Yb,2%Er nanocrystals can emit
UC green light through a two-photon process. We expect the as-
prepared AREF4 nanocrystals to be useful in optical nanodevices
and bioprobes in the future.
Fig. 9 (a) Excitation and emission spectra of KGdF4:5%Eu nanopoly-
hedra dispersed in cyclohexane (2 wt%). (b) Luminescence decay curve of
the 613 nm emission of Eu3+ ions in KGdF4:5%Eu nanopolyhedra (lex
=
Acknowledgements
275 nm). (c) Upconversion spectra of LiYF4:20%Yb,2%Er nanocrystal
dispersion in cyclohexane (2 wt%). (d) Power density dependence of
the upconversion emission intensity for LiYF4:20%Yb,2%Er nanocrystals
under lex = 980 nm.
We gratefully acknowledge the financial support from the MOST
of China (grant no. 2006CB601104), the NSFC (grant nos.
20871006, 20821091, and 20671005).
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