PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Morphology- and phase-controlled synthesis of monodisperse
lanthanide-doped NaGdF4 nanocrystals with multicolor photoluminescence†
Chenghui Liu, Hui Wang, Xinrong Zhang and Depu Chen*
Received 8th September 2008, Accepted 7th November 2008
First published as an Advance Article on the web 9th December 2008
DOI: 10.1039/b815682d
Monodisperse, regular-shaped and well-crystallized nanocrystals (NCs) of lanthanide-doped NaGdF4
with diverse shapes and structures are synthesized in high boiling organic solvents 1-octadecene and
oleic acid, through a competitive nucleation and growth pathway. The NCs can be manipulated to
different morphologies and phase structures by using controlled variations in the reaction conditions
such as composition of the solvent, temperature or reaction time. The NCs are thoroughly
characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area
electron diffraction (SAED), high resolution TEM (HRTEM), field emission scanning electron
microscopy (FESEM), IR spectroscopy, and photoluminescence. Possible mechanisms of NC
nucleation and growth, size and shape evolution are proposed and tested. With different dopants, the
NCs can show intensive multicolor down-conversion emissions under 254 nm UV excitation or
up-conversion fluorescence under 980 nm NIR excitation, showing great promise in applications such
as multi-analyte biolabels, staining, displays and other optical technologies.
1. Introduction
In recent years, more and more attention has been focused on the
development of new luminescent nanomaterials such as lanthanide-doped nanocrystals (NCs) due to their many potential
applications in optics and optoelectronics. Such lanthanide ionbased luminescent NCs show superior chemical and optical
properties, including low toxicity, large effective Stokes shifts,
multicolor emission, as well as high resistance to photobleaching,
blinking, and photochemical degradation.1 Thus they have been
widely used for numerous applications such as biolabels,1–3 lightemitting devices,4,5 displays,6 optical amplifiers,7 and lowthreshold lasers.8
Amongst various host materials for lanthanide-doped NCs,
fluorides provide some distinct advantages over the conventionally used oxide materials owing to the very low phonon
frequencies of their crystal lattices.9 The quenching of the excited
state of the rare-earth ions will be minimized when lanthanide
ions are doped into fluoride hosts, which will lead to long lifetimes of their excited states and high luminescence quantum
yields even in the case of IR emitting ions. Hence, controlled
synthesis of lanthanide doped fluoride NCs has attracted a lot of
interest and has been a popular topic for several decades.
NaGdF4 NCs, existing either in the a-phase (cubic) or the
b-phase (hexagonal), are of particular interest. On one hand,
Department of Chemistry, Tsinghua University, Beijing, 100084, P. R.
China. E-mail: [email protected]; Fax: +86 10
62782485; Tel: +86 10 62781691
† Electronic supplementary information (ESI) available: EDX spectrum
of NaGdF4:Ce3+,Tb3+ (Fig. S1); TEM images and XRD patterns
corresponding to the NCs obtained under different ratios of F!/RE3+
(Fig. S2); TEM images and XRD results of NCs synthesized under
different ratios of TOP/1-octadecene and oleylamine/1-octadecene
(Fig. S3, Fig. S4); and the discussion of self-assembly properties of the
luminescent NCs (Fig. S5). See DOI: 10.1039/b815682d
This journal is ª The Royal Society of Chemistry 2009
certain lanthanide ion doped NaGdF4 NCs can display downconversion emission under deep UV excitation, which is of
significant technological importance, in particular in the applications of mercury-free fluorescent tubes or plasma display
panels.10,11 On the other hand, lanthanide NCs can show multicolor emissions by varying the dopants. For many cases it is
necessary to excite all these NCs by a single wavelength irradiation. However, it is very difficult to do so since each lanthanide
activator has a special set of energy levels. So the same sensitizing
ion for all the lanthanide activators is usually utilized to solve this
problem. In this case, a single-wavelength irradiation can be used
to excite the sensitizers, followed by energy transfer to the
lanthanide activators and then the fluorescence can be generated.
In particular, the NaGdF4 NCs are known to be one kind of very
efficient host lattice for such luminescent processes12 since Gd3+
can act as an intermediate to allow energy to migrate over the
Gd3+ sublattice and consequently facilitate the energy transfer
process.13
Therefore, many works have been done to obtain such
lanthanide-doped NaGdF4 NCs with controlled sizes, shapes
and phases. Hitherto, many approaches including co-precipitation method, hydrothermal method, reversed micelle method and
solid state reaction have been reported to synthesis lanthanidedoped NaGdF4 crystals. These methods are well documented
and compared in the literature.14 Although crystals produced
using these methods were generally in the nano-scale, they were
either terribly aggregated or ill-shaped. More recently, Capobianco and co-workers reported the synthesis of a-phase
NaGdF4:Ce3+,Tb3+ NCs with a diameter of about 5 nm based on
the thermo-decomposition of corresponding lanthanide trifluoroacetate precursors at high temperature.15 Zhang et al. have
synthesized b-phase NaGdF4 NCs by a hydro/solvothermal
technique using polyethyleneimine as a capping reagent.12 The
NCs obtained in their work have an elongated spherical shape
J. Mater. Chem., 2009, 19, 489–496 | 489
with a wide diameter distribution between 25 nm and 45 nm. It
can be seen that only spherical or irregular-shaped NCs were
generally obtained in these approaches. However, little attention
has been paid to the morphology control of NaGdF4 NCs. So it
still remains a challenge to establish an efficient method for
growing monodisperse, high quality crystalline NCs with welldefined sizes/shapes by a facile and economic strategy.
In this contribution, we report on the one-pot synthesis of
regular-shaped, monodisperse and well-crystallized NaGdF4:Ce(Yb)3+,Ln3+ (Ln ¼ Tb, Eu, Dy, Sm, Er or Tm) multicolor
luminescent NCs through a competitive nucleation and growth
pathway. The NCs obtained are dispersible in nonpolar organic
solvents and the morphologies and phase structures of the
obtained NCs can be easily tuned by changing the experimental
conditions such as the solvent, the temperature or the reaction
time. The NCs were characterized using TEM, HRTEM, SAED,
FESEM, IR spectroscopy, XRD and photoluminescence and
possible mechanisms of the NC nucleation and growth, phase
selection, size and shape evolution are proposed.
2. Experimental
2.1.
Characterization
The as-prepared products were characterized by transmission
electron microscopy (TEM), high-resolution TEM (HRTEM)
and field emission scanning electron microscopy (FE-SEM)
using JEM-1200EX, JEM-2010 and JSM-7401F (JEOL, Japan)
microscopes with accelerating voltages of 100 kV, 120 kV and 3
kV, respectively. Samples for TEM measurements were prepared
by sonicating the precipitate products in hexane for 30 min and
evaporating a drop of the suspension onto a carbon-coated
copper grid. Powder X-ray diffraction (XRD) analysis was performed on a D/max-2500 X-ray diffractometer (Rigaku, Japan)
with Cu Ka radiation at 1.5406 Å. Fourier transform infrared
spectroscopic (FTIR) analysis was carried out by using a Perkin
Elmer spectrometer. The down-conversion luminescence spectra
of the NCs were measured on a FP-6500 fluorescence spectrophotometer (Jasco, Japan), while the up-conversion fluorescence
spectra were measured using a LS-55 luminescence spectrometer
(Perkin-Elmer) with an external 980 nm laser diode (1 W,
continuous wave with 1 m fiber, Beijing Viasho Technology Co.)
as the excitation source. All measurements were performed at
room temperature.
Reagents and materials
Sodium fluoride (NaF) was obtained from Yili Chemical Corp.
(Beijing, China). Rare earth (RE) oxides of SpecPure grade
(Gd2O3, Tb4O7, Eu2O3, Dy2O3, Sm2O3, Er2O3, Tm2O3, 99.99%)
were obtained from Grirem Advanced Materials Co., Ltd.
(Beijing, China). Oleic acid, sodium oleate and Ce(NO3)3$6H2O
were purchased from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China). 1-Octadecene and oleylamine were purchased
from Acros Organics and tri-n-octyl phosphine was from Tokyo
Chemical Industry Co., Ltd. (TCI, Japan). RECl3 were prepared
by dissolving the corresponding rare earth oxides in hydrochloric
acid at elevated temperature followed by evaporating the solvent
under vacuum. RE(oleate)3 complexes were prepared by adopting the literature methods.16,17
2.2.
2.3.
Synthesis of the NaGdF4:Ce(Yb)3+,Ln3+ NCs
The synthesis of NaGdF4:Ce(Yb)3+,Ln3+ NCs was modified from
our previous work for the synthesis of NaYF4 nanoplates. In this
work, the procedures were further simplified by using a one-pot
synthetic approach, where no injection processes were needed.
Take the synthesis of uniform NaGdF4:Ce3+,Ln3+ short
nanorods as an example. Typically, 1 mmol of RE(oleate)3
(Gd3+:Ce3+:Ln3+ ¼ 85:10:5, Ln ¼ Tb3+, Eu3+, Sm3+, Dy3+) and
0.21 g of NaF (5 mmol, F!/RE3+ molar ratio was kept at 5 unless
stated otherwise) were first added to the solvent of 15 mL oleic
acid/15 mL 1-octadecene simultaneously, then degassed under
vacuum for 30 min, and finally, heated rapidly to 280 # C and kept
at this temperature for 5 h under vigorous magnetic stirring in the
presence of nitrogen. Subsequently, the mixture was allowed to
cool to room temperature, and the NCs were precipitated by the
addition of ethanol and isolated via centrifugation.
NCs of different sizes, morphologies and structures were
synthesized following the similar procedures only by varying the
experimental conditions such as reaction temperature, time or
the components of the solvent.
490 | J. Mater. Chem., 2009, 19, 489–496
3. Results and discussion
3.1.
Morphology and structure of the synthesized NCs
Following the standard synthesis procedures stated above, the
NaGdF4:Ce3+,Ln3+ NCs obtained in the solvent of oleic acid/1octadecene (15mL/15mL) at 280 # C for 5 h were characterized by
TEM and FESEM, as shown in Fig. 1. The typical TEM images
(Fig. 1a–d, taken with different magnifications) demonstrate that
all the as-obtained NaGdF4 NCs are of single-crystalline nature
and display high crystallite size uniformity. All the NCs display
uniform short nanorod shapes. The average diameter of the NCs,
evaluated from 100 random particles from low resolution TEM
images, is (23.8 $ 1.6) nm (diameter) % (35.8 $ 1.5) nm (length),
which indicates a rather narrow size distribution. FESEM
characterizations (Fig. 1f, g) were also performed to further
illustrate the morphologies of the NCs, which also demonstrate
their size/shape-uniformity and monodispersity. It is notable that
the regular hexagonal shape of the top/bottom surfaces of the
NCs can be identified in Fig. 1e, where a higher concentration of
colloid NCs were dropped onto the TEM grid, because at high
concentration some NCs had the opportunity to stand on end
instead of lying on their sides. The hexagonal-shaped top/bottom
surfaces can also be observed in the FESEM images. The selected
area electron diffraction (SAED) pattern (Fig. 1h) shows the
spotty polycrystalline diffraction rings corresponding to the
specific (100), (110), (111), (201),(102) planes of the hexagonal
NaGdF4 lattice. The hexagonal phase structure of the NCs was
further identified by XRD analysis as shown in Fig. 1i. The peak
positions and intensities agree well with the data reported in the
JCPDS standard card (PDF 27-0699) for hexagonal NaGdF4
crystals. The atomic composition ratios of the obtained NCs
were determined by energy-dispersive X-ray analysis (EDX).
Fig. S1 (see ESI†) shows an EDX spectrum of NaGdF4:Ce3+,Tb3+. The atomic ratio of Gd:Ce:Tb was determined as
85.2:9.7:5.1, which is very close to the calculated value (85:10:5).
This journal is ª The Royal Society of Chemistry 2009
Fig. 1 (a–d) TEM images with different magnifications of the NaGdF4:Ce(Yb)3+,Ln3+ nanorods synthesized in 15 mL oleic acid/15 mL 1-octadecene at
280 # C for 5 h; (e) TEM images obtained by using colloid NCs sample with higher concentration than that of (a–d); (f–g) FESEM images of the
NaGdF4:Ce(Yb)3+,Ln3+ nanorods; (h) SAED pattern of the nanorods corresponding to the hexagonal NaGdF4 lattice. (i) XRD pattern of the
NaGdF4:Ce(Yb)3+,Ln3+ nanorods corresponding to the NCs of above TEM images.
In the present synthesis approach, the general process can be
depicted as follows: an anion-exchange reaction between F!
(provided by NaF), and RCOO! (provided by the RE(oleate)3)
would take place when the reaction system was settled at certain
high temperatures. Due to the strong coordination between RE
cations and carboxyl groups of the oleate anions, F! had to
compete with the RCOO! to form NaGdF4 precipitates during
the nucleation and growth processes. So the reaction was
significantly retarded by the strong chelation among the rare
earth cations and the bulky oleate anions, thereby achieving
a well-maintained balance between nucleation and growth
stages. For the oleate anions, the presence of a carboxylic group
with significant affinity to NCs surfaces together with a long
nonpolar tail group for sterical hindering makes it well bound on
the NC surfaces to prevent NCs from aggregating during the
reaction. As a result, monodisperse NCs can be obtained under
certain conditions and the as-obtained NCs can be well dispersed
in nonpolar solvents such as hexane and toluene.
3.2. Effects of reaction temperature and time on the phase
control and morphology of the NCs
It was found that the reaction temperature and time played
important roles in the synthesis of hexagonal phase NaGdF4:Ce(Yb)3+,Ln3+ NCs. The effects of reaction temperature and time
on the structure, morphology and sizes on the NCs were investigated by using XRD (Fig. 2) and TEM analysis (Fig. 3),
respectively. These results are summarized in Table 1.
It can be seen that high temperatures are preferred to form
b-phase NCs when other experimental conditions are
unchanged. For example, the products obtained at 280 # C
showed pure hexagonal phase even for a rather short reaction
time of 15 min (Fig. 2e), while the NCs synthesized below 260 # C
had dominant a-phase for as long as 5 h.
From the TEM images it can be concluded that with the
extension of the reaction time, the NCs tend to grow
This journal is ª The Royal Society of Chemistry 2009
Fig. 2 XRD patterns of the NCs obtained in the solvent of 15 mL oleic
acid/15 mL 1-octadecene under conditions of (a) 280 # C for 5 h; (b) 260
#
C for 5 h; (c) 230 # C for 5 h; (d) 200 # C for 5 h; (e) 280 # C for 15 min. The
peaks marked with asterisks are indexed to residual NaF. The excess NaF
can be removed by washing the products several times with water.
anisotropically from mainly along the radial directions to along
the axial directions (c axis) which finally results in the rod-shaped
NCs.
As far as we know, selective adhesion of capping ligand onto
specific crystal planes is critical in the epitaxial growth of
NCs.18,19 Since 1-octadecene is a non-coordination reagent, we
speculate that oleic acid may play an important role in the shape
evolution in this work. Since oleic acid is a selective absorption
surfactant,20 it is supposed that during the growth of the NCs,
oleic acid is preferentially adsorbed onto the side surfaces that
are parallel to the c-axis of the growing crystallites, which results
in the epitaxial growth along the <001> directions and results in
nanorods. The HRTEM images of an individual nanorod show
the crystalline structures of its top/bottom surface (Fig. 3i) and
side surface (Fig. 3j, k). The HRTEM images clearly show that
J. Mater. Chem., 2009, 19, 489–496 | 491
Fig. 3 (a–d) TEM images of NaGdF4:Ce(Yb)3+,Ln3+ NCs synthesized in 15 mL oleic acid/15 mL 1-octadecene treated for 5 h at 200 # C, 230 # C, 260 # C,
and 280 # C, respectively; (e–h) TEM images of NaGdF4:Ce(Yb)3+,Ln3+ NCs synthesized in 15 mL oleic acid/15 mL 1-octadecene treated at 280 # C for 2.5
h, 1 h, 15 min, 10 h, respectively; (i) HRTEM image of the top/bottom surface of a single nanorod in image (d). The oleic acid layer (indicated by arrows)
adsorbed round the side surfaces can be clearly observed; (j) HRTEM image of the side surface of the nanorod as in image (i); (k) magnified picture of the
selected area in image (j). The scalar bars stand for 10 nm in images (i) and (j).
Table 1 Effects of reaction temperature and time on NC synthesisa
Temperature/# C
Time/hour
Phase
Shape
Size/nm
cubic
cubic
cubic/hexagonal
very tiny
very tiny
&20–30 nm for the regular-shaped
NCs
diameter % length (or thickness)
(30.1 $ 2.3)nm%(14.0 $ 1.2)nm
diameter % length (or thickness)
(16.8 $ 0.6)nm%(18.7 $ 0.5)nm
diameter % length (or thickness)
(17.5 $ 1.1)nm%(28.3 $ 1.0)nm
diameter % length (or thickness)
(23.8 $ 1.6)nm%(35.8 $ 1.5)nm
diameters among 20–25 nm
200
230
260
5
5
5
280
1/4
hexagonal
cannot be identified
cannot be identified
a few regular-shaped NCs; most are
very tiny particles
nanoplates
280
1
hexagonal
dot-like nanorods
280
2.5
hexagonal
nanorods
280
5
hexagonal
nanorods
280
10
hexagonal
sphere-like particles
a
Other conditions are the same: solvent is 15 mL oleic acid/15 mL 1-octadecene.
the epitaxial growth is along the <001> direction. Meanwhile,
the oleic acid layer round the side surfaces of the nanorods can be
clearly observed in Fig. 3i highlighted by arrows. All these results
492 | J. Mater. Chem., 2009, 19, 489–496
support our hypothesis. Furthermore, the shape evolution of the
NCs from nanoplates to nanorods with the extension of reaction
time also proves our theory.
This journal is ª The Royal Society of Chemistry 2009
However, it is worth noting that when the reaction time
exceeded 10 h, the shape of the NCs changed from rod to spherelike with diameters in the range of 20–25 nm (Fig. 3h). The
reason will be discussed in the section ‘‘Formation and growth
mechanisms of the NCs’’.
3.3.
Effects of the amount of oleic acid in the solvent
As mentioned above, oleic acid was speculated to play an
important role in the NC growth and shape evolution. So the
effect of oleic acid on the NC synthesis was explored with other
experimental conditions (280 # C, 5h) fixed.
The results of XRD analysis show that the NCs obtained
under different ratios of oleic acid/1-octadecene all exhibit pure
hexagonal phase (Fig. 4). But the morphologies of the
NaGdF4:Ce3+,Ln3+ NCs can be strongly affected by the amount
of oleic acid in the solvents. TEM images in Fig. 4 show that with
the ratio of oleic acid/1-octadecene of the solvent increased, NCs
grew more anisotropically and the monodispersity of the NCs
increased remarkably. In pure 1-octadecene without the presence
of oleic acid, only very agglomerated NCs as well as some small
particles were formed (Fig. 4a). Under low ratios of oleic acid/1octadecene (below 3/27), the agglomerated NCs disappeared and
ill-shaped nanoparticles with a rather wide size distribution were
obtained (Fig. 4b, c). When the ratio exceeded 6/24, the NCs
became more and more uniform and regular-shaped (Fig. 4d–g).
As can be seen from Fig. 4e, uniform nanoparticles with an
average diameter of 21.8 $ 1.4 nm were obtained for an oleic
acid/1-octadecene ratio of 10/20. Under higher oleic acid/1octadecene ratios such as 15/15 and 20/10, regular-shaped short
nanorods were synthesized. The results clearly indicate that the
morphology control in this approach is closely related to the
application of oleic acid as capping surfactant, since the amount
of oleic acid is the only parameter which was changed in this
study to obtain regular-shaped NCs.
3.4.
Formation and growth mechanisms of the NCs
All the above experimental results suggest that the temperature
mainly determines the phase selection, while the reaction time
and the solvents contribute little to the phase control of the NCs
but play important roles in the anisotropic growth and shape
evolution.
Although the b-phase NCs are the thermodynamically stable
crystalline form compared with the a-phase, relatively high
energy is needed to overcome the dynamical energy barrier of the
a / b phase transition. In this work, it seems that high
temperature such as 280 # C can supply sufficient energy to obtain
hexagonal phase NCs, while the a / b energy barrier cannot be
completely surpassed at 260 # C or lower temperature which will
result in dominant a-phase NCs.
As regards the morphology control, according to previous
reports,21–23 the accelerated crystallization process in solutions
related to the high monomer concentrations is the main driving
force for anisotropic growth of the inorganic nanostructures.
Similarly, a monomer concentration-controlled kinetic model in
this work was proposed and can give reasonable explanations for
the shape evolution of NCs.
When the reaction was carried out in pure 1-octadecene, the
monomer concentration in the solution was very low due to the
insolubility of NaF and the rather slow nucleation and growth
process. Therefore, we cannot obtain monodisperse and regularshaped NCs in pure 1-octadecene.
But when oleic acid was added, the situation was quite
different. First, in the presence of oleic acid, the polarity of the
solvent is enhanced and as a result the solubility of NaF is
enhanced. Since oleic acid is a weak acid, even more F! may be
dissolved in the solvent through a NaF–HF pathway.
Second, after the addition of oleic acid, RE(oleate)3 can be
more easily dissolved in the solvent and their mobility in the
solution is enhanced compared with that in pure 1-octadecene.
All these factors benefit the nucleation and growth rate of the
Fig. 4 Effect of oleic acid/1-octadecene ratios on the morphology and structure of NaGdF4:Ce(Yb)3+, Ln3+ NCs. TEM images of NCs synthesized
under oleic acid/1-octadecene ratios: (mL/mL, total volume of 30 mL) (a) 0/30; (b) 1/29; (c) 3/27; (d) 6/24; (e) 10/20; (f) 15/15; (g) 20/10. (h) XRD results of
the NCs obtained under different ratios of oleic acid/1-octadecene (OA/ODE). Other experimental conditions are the same: 280 # C, 5 h.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 489–496 | 493
NCs. So it can be concluded that the higher oleic acid/1-octadecene ratio leads to a higher concentration of monomers and
thus a higher growth rate of the NaGdF4:Ce(Yb)3+,Ln3+ NCs in
the solution. Under this condition, the one-dimensional (1D)
growth was significantly promoted, and thus the shape evolution
from irregular shapes to nanorods was observed. Moreover,
although the crystallization speed is accelerated in the presence
of oleic acid, due to its long alkyl tail, weak acidity, strong
coordination effect with RE ions and the competitive anion
exchange process between F! and RCOO!, the growth rate of the
NCs remains slow enough to prevent the formation of aggregated products.
Third, the preferred adsorption of oleic acid on the side
surfaces of the NaGdF4:Ce(Yb)3+,Ln3+ makes the NCs growing
more anisotropically along the <001> direction (c-axis direction),
which has been demonstrated above. The FTIR spectra (Fig. 5) of
the nanorods show that no characteristic group absorption peaks
from oleic acid can be seen but two strong bands centered at 1457
cm!1 and 1563 cm!1 are observed, which can be associated with
the asymmetric and symmetric stretching vibrations of carboxylate anions.24 The FTIR results indicate that the adhesion of
oleic acid on the NCs is not a merely physical adsorption, while
strong interaction may exist between the RCOO! and the RE
atoms on the surfaces of the NCs, which well blocked the growth
along the radial directions but accelerated the growth rate along
the axial direction. Furthermore, according to the Le Chatelier’s
principle, increasing the amount of oleic acid in the solvent would
slow the decomposition/desorption of oleate anions from the NCs
surfaces and thus strengthen their interactions, which would
further facilitate the anisotropic growth of the NCs. In addition,
the slowed decomposition of RE-RCOO! with the increasing
oleic acid would make the RCOO! bind more easily on the
surfaces of the growing NCs, which can further protect the NCs
from aggregation.
All the experimental results for the shape evolution can be
reasonably explained by the monomer concentration-controlled
kinetic model and the selective adhesion of oleic acid on different
parts of the NCs surfaces. For example, the shape evolution with
extended reaction time mentioned above can be illustrated in
Fig. 5 FTIR spectrum of the NaGdF4:Ce(Yb)3+,Ln3+ nanorods
synthesized in 15 mL oleic acid/15 mL 1-octadecene at 280 # C for 5 h. The
FTIR spectra of pure oleic acid (OA) and sodium oleate were also
measured for comparison.
494 | J. Mater. Chem., 2009, 19, 489–496
more detail. At the early stage, the monomer concentration was
relatively high, so the selective adhesion of oleic acid on specific
surfaces was the dominant factor, and regular-shaped NCs can
be obtained. With a prolonged time of more than 10 h, the
monomer concentration was gradually decreased to a critical
level. So intra-particle moves of atoms from high-energy facets to
other facets took place, and thus corners and tips were smoothed.
Eventually the transition from rod to sphere-like particles was
observed.21,25
Further experiments were designed to examine the important
role of the monomer concentration. As stated above, only
aggregated NCs were obtained in pure 1-octadecene due to the
low monomer concentration. But when the amount of NaF was
raised to F!/RE molar ratios of 16 or 24, monodisperse NCs were
obtained with nearly hexagonal shapes (Fig. S2a, S2b†).
Although the solubility of NaF is poor in 1-octadecene, high F!/
RE molar ratios will raise the F! concentration, and enhance the
chance of nucleation and growth of the NCs at the solid/liquid
interfaces. So the monomer concentration as well as the crystallization speed are higher than that of the reaction system with
F!/RE molar ratio of 5. Therefore more regular-shaped and
uniform NCs are obtained (b-phase, XRD results see Fig. S2c†).
The results further supported the proposed monomer concentration-controlled kinetic model.
To further prove the crucial effect of oleic acid on the nanocrystal growth and crystallization, two other high boiling point
organic ligands, tri-n-octyl phosphine (TOP) and oleylamine,
mixed with the noncoordinating 1-octadecene were employed as
solvents for comparison with oleic acid.
TOP is a widely used capping reagent especially for the
synthesis of quantum dots.26,27 Although TOP has been used as
a stabilizer for the preparation of nanomaterials, it can not
play the same role as oleic acid in this study. First, TOP is
a Lewis base which cannot facilitate the dissolution of F! and
RE(oleate)3 and hence increase the monomer concentrations
and accelerate the crystallization process. Second, to the best of
our knowledge, TOP was seldom reported as a selective adhesion
surfactant for the anisotropic growth of NCs, which is quite
different from oleic acid. Therefore only spherical nanoparticles
were obtained under different TOP/1-octadecene ratios in this
study (Fig. S3a–c†), indicating the absence of anisotropic
growth. In this study, TOP shows a certain stabilization action
for NCs so that the nanoparticles synthesized are monodisperse
with diameters of 15–17 nm and no obvious size changes are
observed under different ratios of TOP/1-octadecene. The XRD
results (Fig. S3d†) indicate that the nanoparticles synthesized
under different TOP/1-octadecene ratios all exhibit hexagonal
phases.
Oleylamine is another widely used surfactant for NC
synthesis.28,29 Compared with oleic acid, oleylamine is a base, so
the results in oleylamine/1-octadecene are also quite different
from those in oleic acid/1-octadecene. The XRD patterns shown
in Fig. S4† demonstrated that both a-NaGdF4 and b-NaGdF4
were obtained in oleylamine/1-octadecene solvent. With
increasing the ratio of oleylamine/1-octadecene, a-NaGdF4
increased gradually and eventually only a-NaGdF4 was synthesized in pure oleylamine. The same tendency could also be clearly
observed from the TEM images (Fig. S4†). According to the
previously reported literature for the synthesis of NaYF4,29 we
This journal is ª The Royal Society of Chemistry 2009
suppose that the energy barrier between a-NaGdF4 and
b-NaGdF4 may be significantly increased by the addition of
oleylamine, which makes a-NaGdF4 more stable. Therefore,
a-NaGdF4 NCs were hardly transformed into b-NaGdF4
nanorods or plates.
In summary, all these control experiments proved the crucial
role of oleic acid in this study and supported our monomer
concentration-controlled kinetic model for shape evolution of
the NCs.
3.5.
Multicolor photoluminescence
Fig. 6a–d show the room-temperature excitation and emission
spectra of the NaGdF4:Ce3+, Ln3+ (Ln ¼ Tb, Eu, Dy, Sm)
nanorods, which are dispersed as a 0.1 wt% (1 mg mL!1) colloid
in hexane. It can be seen that under a single irradiation of 254 nm
in the ultraviolet region, the NaGdF4 NCs doped with different
lanthanide ions show intensive multicolor visible emissions. Take
NaGdF4:Ce3+,Tb3+ as an example, electronic transitions within
4fn configurations of Tb3+ are strongly forbidden.9 But the
emission efficiency of Tb3+ can be greatly improved by exciting
a different ion (sensitizer, i.e., Ce3+ in this study) with an allowed
electronic transition which transfers the excitation energy to the
activator (i.e., Tb3+). The excitation spectra of all the samples are
all investigated and all show broad bands at around 250 nm
which is ascribed to the characteristic 4f–5d (or 2F5/2–5d) transition of Ce3+. During the fluorescence process, the excitation
energy is first absorbed by the 4f–5d transition of Ce3+ and
transferred to the Gd3+ and migrates over the Gd3+ lattice to the
Tb3+, where the energy is released as fluorescent emissions.12 For
example, the emission peaks of NaGdF4:Ce3+,Tb3+ are generated
from the 4f–4f transition of Tb3+ ions of 5D4–7F6 (488 nm),
5
D4–7F5 (541 nm), 5D4–7F4 (583 nm) and 5D4–7F3 (619 nm),
respectively. Moreover, by co-doping Yb-Er or Yb-Tm, the
NaGdF4 NCs can emit bright green or blue/violet up-conversion
fluorescence under a 980 nm NIR laser excitation, as shown in
Fig. 6e–f. Yb3+ acts as the sensitizer for the up-conversion
processes. Briefly, under the 980 nm excitation, an electron of
Yb3+ could be exited from the 2F7/2 to the 2F5/2 level. The energy
could be transferred to the activator ion (Er3+ or Tm3+) nonradiatively to excite it to the corresponding excited level through
multi-photon processes and then the visible up-conversion
luminescence can be observed. Possible up-conversion mechanisms for the Yb-Er and Yb-Tm co-doped nanomaterials have
been demonstrated in detail in our previous works.17,30,31
In this work, both the up-conversion and down-conversion
fluorescence are realized by using sensitizer–activator pairs.
Taking NaGdF4:Yb3+,Er3+ as an example, a relatively high Yb3+
doping level is required to obtain enough irradiation energy and
transfer it to Er3+ and to excite electrons of Er3+ to higher energy
levels. This energy transfer pathway requires a relatively higher
Yb3+ doping level than that of Er3+. In addition, if the doping
level of Er3+ is too high, cross-relaxation will happen and
concentration quenching will occur, which would decrease the
fluorescence intensity.31,32 The situations are similar for the
NaGdF4:Ce3+,Ln3+ NCs. That is why all observed activator
doping levels are generally very low and the sensitizer doping
levels are relatively high.
The multicolor up-/down-conversion luminescence of the
NaGdF4:Ce(Yb)3+,Ln3+ NCs shows their great promise in
applications in biolabels and optical technologies. More interestingly, the as-synthesized NCs with regular shapes such as
hexagonal plates or nanorods tend to align in an orderly manner
to form superstructures via self-assembly, which was observed
Fig. 6 Room-temperature excitation (dashed line) and emission (solid line) spectra of 0.1wt% (a) NaGdF4:10%Ce3+,5%Tb3+; (b) NaGdF4:10%Ce3+,5%Eu3+; (c) NaGdF4:10%Ce3+,5%Dy3+; (d) NaGdF4:10%Ce3+,5%Sm3+ nanorods dispersed in hexane. All the emission spectra were recorded with
an excitation wavelength of 250 nm. (e–f) Room-temperature up-conversion fluorescence spectra of NaGdF4:18%Yb3+,2%Er3+ and NaGdF4:18%Yb3+,2%Tm3+ nanorods powders. (insets) Corresponding luminescence photographs of colloid NaGdF4:10%Ce3+,5%Ln3+ NCs excited at 254 nm
with a hand-held UV lamp and the NaGdF4:18%Yb3+,2%Er(Tm)3+ powder under irradiation with a 980 nm laser.
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J. Mater. Chem., 2009, 19, 489–496 | 495
during the FESEM test (see Fig. S5 in the ESI†). It is well recognized that the construction of highly ordered and densely packed
NCs arrays is very important for many technological applications.33,34 Combined with their multicolor photoluminescence,
the self-assembly properties of the highly luminescent NCs make
them especially promising for the construction of nanoarrays or
superlattices for applications in optical technologies.
4. Conclusions
To sum up, a one-pot synthesis approach was developed for
lanthanide-doped NaGdF4 NCs in this study. Multiple factors
including temperature, reaction time, and components of the
solvent all affected the formation and growth of the NaGdF4:Ce(Yb)3+,Ln3+ NCs and therefore NCs with diverse phase
structures and morphologies can be controllably synthesized by
controlling the appropriate experimental conditions.
It is evident that lower reaction temperatures and the presence
of oleylamine in the solvent tend to result in the formation of
NCs with cubic phase, while higher temperatures will generate
hexagonal phase NCs. On the other hand, in regard to the
morphology control, hexagonal-shaped nanoplates, nanorods,
and spherical particles of b-phase NaGdF4:Ce(Yb)3+,Ln3+ as well
as small a-phase particles can be controllably formed by varying
the conditions of the solvents and the reaction time under certain
temperature. Based on the experimental results, the crucial effect
of oleic acid was illustrated in detail and a monomer concentration-controlled kinetic model for the formation of the NCs
was proposed and examined by a series of further control
experiments.
Furthermore, by co-doping different lanthanide ions, the
obtained NaGdF4 NCs can exhibit intensive multicolor down-/
up-conversion luminescence in the visible range under excitation
with irradiation of 254 nm or 980 nm, which is promising for
applications in multiple biolabels, staining and displays.
Acknowledgements
Financial support from National Natural Science Foundation of
China (NSFC NO. 30671925 and 20535020) and Key Project
of Science and Technology of Beijing Municipal Commission of
Education (KZ200810005004) is gratefully acknowledged.
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This journal is ª The Royal Society of Chemistry 2009
LETTERS
Ultra-large-scale syntheses of
monodisperse nanocrystals
JONGNAM PARK1, KWANGJIN AN1, YOSUN HWANG2, JE-GEUN PARK2, HAN-JIN NOH3,
JAE-YOUNG KIM3, JAE-HOON PARK3, NONG-MOON HWANG4 AND TAEGHWAN HYEON1*
National Creative Research Center for Oxide Nanocrystalline Materials and School of Chemical Engineering, Seoul National University, Seoul 151-744, Korea
Department of Physics and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Korea
3
Department of Physics and Pohang Light Source, Pohang University of Science and Technology, Pohang 790-784, Korea
4
School of Materials Science & Engineering and Nano-Systems Institute (NSI-NCRC), Seoul National University, Seoul 151-744, Korea
*e-mail: [email protected]
1
2
Published online: 28 November 2004; doi:10.1038/nmat1251
T
he development of nanocrystals has been intensively pursued,
not only for their fundamental scientific interest, but also
for many technological applications1–3. The synthesis of
monodisperse nanocrystals (size variation <5%) is of key importance,
because the properties of these nanocrystals depend strongly on their
dimensions. For example, the colour sharpness of semiconductor
nanocrystal-based optical devices is strongly dependent on the
uniformity of the nanocrystals3–6, and monodisperse magnetic
nanocrystals are critical for the next-generation multi-terabit
magnetic storage media7–9. For these monodisperse nanocrystals
to be used, an economical mass-production method needs to be
developed. Unfortunately, however, in most syntheses reported so
far, only sub-gram quantities of monodisperse nanocrystals were
produced. Uniform-sized nanocrystals of CdSe (refs 10,11) and Au
(refs 12,13) have been produced using colloidal chemical synthetic
procedures. In addition, monodisperse magnetic nanocrystals
such as Fe (refs 14,15), Co (refs 16–18), γ-Fe2O3 (refs 19,20), and
Fe3O4 (refs 21,22) have been synthesized by using various synthetic
methods23. Here, we report on the ultra-large-scale synthesis of
monodisperse nanocrystals using inexpensive and non-toxic
metal salts as reactants. We were able to synthesize as much as
40 g of monodisperse nanocrystals in a single reaction, without a
size-sorting process. Moreover, the particle size could be controlled
simply by varying the experimental conditions. The current
synthetic procedure is very general and nanocrystals of many
transition metal oxides were successfully synthesized using a very
similar procedure.
The process conditions required for the synthesis of monodisperse
particles of micrometre size24 are relatively well established, and a
similar principle could be applied to the synthesis of uniform-sized
nanocrystals. The inhibition of additional nucleation during growth,
in other words, the complete separation of nucleation and growth,
is critical for the successful synthesis of monodisperse nanocrystals.
Our research group developed new procedures for the synthesis of
monodisperse nanocrystals of metals19,23,25, metal oxides19,26,27, and
metal sulphides28 without a laborious size-sorting process. In particular,
Metal
+ Na–oleate
chloride
Metal–oleate
+ NaCl
complex
Monodisperse
nanocrystals
Metal–oleate
complex
Thermal decomposition
in high boiling solvent
20 nm
Figure 1 The overall scheme for the ultra-large-scale synthesis of
monodisperse nanocrystals. Metal–oleate precursors were prepared from the
reaction of metal chlorides and sodium oleate. The thermal decomposition of the
metal–oleate precursors in high boiling solvent produced monodisperse nanocrystals.
during the direct synthesis of monodisperse iron and iron oxide
nanocrystals19,23, we were able to ascertain that the iron–oleate complex,
which is generated in situ from the reaction of iron pentacarbonyl and
oleic acid, is decomposed and acts effectively as a growth source in
synthesizing monodisperse nanocrystals with increased particle size.
From these results, we reasoned that a metal–surfactant complex would
make an effective growth source for the synthesis of monodisperse
nanocrystals. The overall synthetic procedure is depicted in Fig. 1 and
the detailed experimental procedures are described in the Methods
section. Instead of using toxic and expensive organometallic
compounds such as iron pentacarbonyl, we prepared the metal–
oleate complex by reacting inexpensive and environmentally friendly
compounds, namely metal chlorides and sodium oleate. The Fourier
transform infrared spectrum of the iron–oleate complex, which was
prepared by reacting iron chloride (FeCl3·6H2O) and sodium oleate,
shows a C=O stretching peak at 1,700 cm–1, which is a characteristic
peak for a metal–oleate complex (see Supplementary Information,
Fig. S1). The iron–oleate complex in 1-octadecene was slowly heated
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LETTERS
a
f
5 nm
20 nm
b
g
5 nm
20 nm
c
h
0.2 µm
Figure 2 12-nm magnetite nanocrystals. The TEM image clearly demonstrates
that the nanocrystals are highly uniform in particle-size distribution. Inset is a
photograph showing a Petri dish containing 40 g of the monodisperse magnetite
nanocrystals, and a US one-cent coin for comparison.
to 320 °C, and was aged at that temperature for 30 min, generating
iron oxide nanocrystals. The amount of the separated nanocrystals
produced was as large as 40 g with a yield of >95%. The nanocrystals
could easily be re-dispersed in various organic solvents including
hexane and toluene.
To understand the mechanism of monodisperse nanoparticle
formation, we obtained transmission electron microscope (TEM;
JEOL EM-2010) images and conducted in situ infrared spectroscopy
on the reaction mixture after heating at various temperatures and
for various times. We also investigated the thermal decomposition
behaviour of the solid-state iron–oleate precursor using
thermogravimetric analysis (TGA), differential scanning calorimetry
(DSC), and temperature-programmed infrared spectroscopy (see
Supplementary Information). The TGA/DSC patterns and infrared
spectra revealed that one oleate ligand dissociates from the precursor
at 200–240 °C and the remaining two oleate ligands dissociate
at ~300 °C by a CO2 elimination pathway. The TEM image of the
sample taken at 310 °C without aging showed that nanoparticles
were not produced, whereas the TEM image taken at 320 °C revealed
the formation of relatively uniform nanoparticles with sizes ranging
from 8 nm to 11 nm. All the TEM images taken after aging at
320 °C for 10, 20 and 30 min showed monodisperse 12 nm
nanoparticles. Aging at 260 °C for one day produced polydisperse
and poorly crystalline 9 nm nanoparticles, and aging at the
same temperature for three days generated monodisperse 12 nm
nanocrystals. When aged at 240 °C for one day, no nanoparticles
were formed, and aging at 240 °C for three days produced highly
polydisperse ~14 nm nanoparticles. When aged at 200 °C for three
days, no nanoparticles were formed.
From these TEM, DSC/TGA and infrared data, we could propose
the following mechanism for the nanocrystal formation. Nucleation
occurs at 200–240 °C triggered by the dissociation of one oleate
ligand from the Fe(oleate)3 precursor by CO2 elimination. The
major growth occurs at ~300 °C initiated by the dissociation of the
remaining two oleate ligands from the iron–oleate species, although
5 nm
20 nm
d
i
5 nm
20 nm
e
j
20 nm
5 nm
Figure 3 TEM images (a–e) and HRTEM images (f–j) of monodisperse iron oxide
nanocrystals. (a, f) 5 nm; (b, g) 9 nm; (c, h) 12 nm; (d, i) 16 nm; and (e, j) 22 nm
nanocrystals. TEM images showed the highly monodisperse particle size distributions
and HRTEM images revealed the highly crystalline nature of the nanocrystals.
slow growth seems to occur at <250 °C. Consequently, we were able
to synthesize monodisperse nanocrystals from the separation of
nucleation and growth processes, which tend to take place at different
temperatures. Because the growth process is time-dependent, when
the precursor was aged at a low temperature of 240 °C (close to the
nucleation temperature) for three days, we obtained polydisperse
nanocrystals. These results imply that the current successful
synthesis of monodisperse nanocrystals can be attributed to the
effective separation of nucleation and growth processes, which result
from the different temperature dependence of nucleation and
growth kinetics.
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LETTERS
a
b
XAS
MCD
∆ρ = ρ+ – ρ–
Fe2+
XMCD
ρ+
ρ–
22 nm
L2
5 nm
B
725
A
710
C
720
730
Fe3O4
Intensity (Arb.units)
Intensity (Arb.units)
720
Fe3O4
22 nm
16 nm
22 nm
16 nm
14 nm
14 nm
12 nm
12 nm
9 nm
9 nm
5 nm
L3
L2
710
5 nm
γ –Fe2O3
γ –Fe2O3
720
Photon energy (eV)
710
730
730
d
c
300
25
1.2
5 nm
9 nm
12 nm
16 nm
22 nm
1.0
20
0.6
0.4
200
15
TB (K)
K (105 erg cm–3)
0.8
M /M (TB)
720
Photon energy (eV)
10
K
TB
0.2
100
5
0.0
–50
0
50
100
150
200
250
300
350
0
400
0
10
20
0
30
Diameter (nm)
Temperature (K)
Figure 4 Characterization of monodisperse iron oxide nanocrystals. a, Fe L2,3-edge XAS and b, XMCD spectra of iron oxide nanocrystals in comparison with those of
reference bulk materials, γ-Fe2O3 and Fe3O4. The magnified L2 region XAS spectra and the XMCD spectra of the 5 nm and 22 nm nanocrystals are shown in the insets of a
and b, respectively. In the inset of b, ρ+ and ρ– represent the absorption coefficients for the photon helicity vector parallel and antiparallel to the magnetization direction of
the nanocrystals, respectively. c, Temperature dependence of magnetization measured after zero-field cooling (ZFC) using 100 Oe. For the sake of presentation, we have
normalized the magnetization data with respect to the value at the maximum of ZFC magnetization, M(TB), for individual samples. d, Size dependence of TB, obtained from
M(T) shown in c.
The nanocrystals were characterized using TEM, X-ray diffraction
(XRD; Rigaku D/Max-3C)), X-ray absorption spectroscopy (XAS),
and X-ray magnetic circular dichroism spectroscopy (XMCD). The
TEM image of the iron oxide nanocrystals synthesized at the 40-g
scale in a single reaction using 200 g of solvent, shown in Fig. 2,
exhibited an extensive 2D assembly of uniform 12-nm nanocrystals,
demonstrating their monodisperse particle size distribution (size
variation = 2.3%, see Supplementary Information). A photograph
of a Petri dish containing 40 g of the nanocrystals is shown in the
inset of Fig. 2. The particle size of the iron oxide nanocrystals could
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LETTERS
Figure 5 TEM images, HRTEM images and electron diffraction patterns of
monodisperse nanocrystals. a, MnO, b, CoO and c, Fe. TEM images show the
highly uniform characteristics of the nanocrystals in terms of both particle size and
particle shape. HRTEM images and electron diffraction patterns revealed the highly
crystalline nature of the nanocrystals.
a
(511)
(422)
(420)
(331)
(400)
(222)
(311)
(220)
(200)
(111)
20 µm
50 nm
11 >
<1
d 2.56
5 nm
b
(112)
(103)
(110)
(102)
(101)
(002)
(100)
20 µm
16 nm
10
nm
(001)
{011}
{010}
–
(001)
100 nm
8 nm
2
<00
<100 >
d 2.6
d 2.78
>
5 nm
5 nm
c
20 µm
>
10100>
<<2
d 1.43
d 2.02
100 nm
5 nm
be controlled by using various solvents with different boiling points.
As shown in Fig. 3, 5 nm (a, TEM; and f, high-resolution TEM),
9 nm (b and g), 12 nm (c and h), 16 nm (d and i), and 22 nm (e and j)
iron oxide nanocrystals were synthesized using 1-hexadecene (b.p.
274 °C), octyl ether (b.p. 287 °C), 1-octadecene (b.p. 317 °C),
1-eicosene (b.p. 330 °C) and trioctylamine (b.p. 365 °C), respectively.
All the nanocrystals are highly monodisperse in particle-size
distribution (size variation <4.1%, see Supplementary Information).
As the boiling point of the solvent increased, the diameter of the
iron oxide nanocrystals increased. This result can be explained by
the higher reactivity of the iron–oleate complex in the solvent with
a higher boiling point. High-resolution TEM (HRTEM) images of
these iron oxide nanocrystals showed distinct lattice fringe patterns,
indicating the highly crystalline nature of the nanocrystals (Fig. 3f–j).
The size of the iron oxide nanocrystals can be further fine-tuned by
varying the concentration of oleic acid. For example, 11 nm, 12 nm
and 14 nm iron oxide nanocrystals were synthesized using solutions
with oleic acid concentrations of 1.5 mM, 3 mM and 4.5 mM,
respectively (see Supplementary Information).
The XRD pattern of the 12-nm iron oxide nanocrystals
revealed a cubic spinel structure of magnetite (see Supplementary
Information). For the quantitative identification of the compositions
of the iron oxide nanocrystals, XAS and XMCD measurements
at the Fe L2,3-edges were carried out at the EPU6 beamline at the
Pohang Light Source. Figure 4a,b shows XAS spectra and XMCD
results of the iron oxide nanocrystals with diameters of 5 nm, 9 nm,
12 nm, 16 nm and 22 nm, in comparison with those of two reference
materials, bulk γ-Fe2O3 (maghemite) and bulk Fe3O4 (magnetite),
which have nearly the same spinel crystal structure with only
~1% difference in the cubic lattice constant. Both the XAS and
MCD spectra of the 5 nm nanoparticles are very similar to those
of γ-Fe2O3, which contains only Fe3+. From the XAS and XMCD
results, we made a quantitative estimation of the compositions for
the iron oxide nanocrystals in the form of (γ-Fe2O3)1–x(Fe3O4)x.
The estimations are x = 0.20, 0.57, 0.68, 0.86 and 1.00 for the 5,
9, 12, 16 and 22 nm nanocrystals, respectively. Therefore, γ-Fe2O3
is the dominant phase of the small 5-nm iron oxide nanocrystals,
whereas the proportion of the Fe3O4 component gradually increases
on increasing the particle size.
The magnetic properties of these iron oxide nanocrystals were
studied using a commercial superconducting quantum interference
device magnetometer (Quantum Design, MPMS5XL). Figure 4c
shows the temperature dependence of magnetization measured with
an applied magnetic field of 100 Oe from 380 K to 5 K. All of our
nanocrystals show superparamagnetic behaviour at high temperatures.
However, on cooling, the zero-field-cooled magnetization begins to
drop and deviate from the field-cooled magnetization at blocking
temperature, TB. TB is at 40 K for the 5 nm sample, TB increases
continuously as the diameter of the nanocrystals increases: for
example, to 260 K for the 22 nm nanocrystals (Fig. 4d). From the
measured TB, we calculated the magnetic anisotropy constant, K, using
the equation: K = 25kBTB/V, where kB is Boltzmann’s constant and V
is the volume of a single nanocrystal. As is typical of nanocrystals, the
calculated magnetic anisotropy constant was found to increase with
decreasing particle size (Fig. 4d).
The current synthetic procedure turned out to be widely
applicable, and we successfully synthesized nanocrystals of many
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LETTERS
transition metal oxides from the thermolysis of metal–oleate
complexes. For example, when the Mn–oleate complex was
refluxed in a solution containing octyl ether and oleic acid, uniform
12-nm f.c.c. MnO nanocrystals were synthesized (Fig. 5a). For the
Co–oleate complex, short pencil-shaped CoO nanorods (Fig. 5b)
were obtained. The CoO nanorods are uniform in diameter and
form self-assembled superlattices (Fig. 5b). The XRD pattern of the
CoO nanorods revealed an interesting Würtzite structure, similar
to that of ZnO (see Supplementary Information). Furthermore, the
(002) peak is narrower than the other peaks, demonstrating that the
nanocrystals grow preferentially along the c axis. These results were
confirmed by the subsequent HRTEM, which shows the (002) lattice
spacing value of 2.6 Å (Fig. 5b). When the iron–oleate complex
was heated at a higher temperature of 380 °C, novel cube-shaped
20-nm iron nanocrystals were produced. XRD and HRTEM
analyses revealed that the surface of these Fe nanocubes is passivated
by a thin FeO layer (Fig. 5c). Nanocrystals of manganese ferrite
and cobalt ferrite were synthesized from the thermal decomposition
of the reaction mixtures composed of 1:2 molar ratio of the
corresponding metal–oleate complex and iron–oleate complex (see
Supplementary Information).
The synthetic procedures developed in the present study offer
several very important advantageous features over the conventional
methods for the synthesis of monodisperse nanocrystals. First, this
process allows monodisperse nanocrystals to be obtained on an ultralarge scale of 40 g in a single reaction and without a further size-sorting
process. When the reactors are set up in parallel, multi-kilograms
of monodisperse nanocrystals can be readily obtained. Second, the
synthetic process is environmentally friendly and economical, because
it uses non-toxic and inexpensive reagents such as metal chlorides29.
Third, the synthetic method is a generalized process that can be used
to synthesize different kinds of monodisperse nanocrystals.
METHODS
SYNTHESIS OF IRON–OLEATE COMPLEX
The metal–oleate complex was prepared by reacting metal chlorides and sodium oleate. In a typical
synthesis of iron–oleate complex, 10.8 g of iron chloride (FeCl3·6H2O, 40 mmol, Aldrich, 98%) and
36.5 g of sodium oleate (120 mmol, TCI, 95%) was dissolved in a mixture solvent composed of 80 ml
ethanol, 60 ml distilled water and 140 ml hexane. The resulting solution was heated to 70 °C and kept at
that temperature for four hours. When the reaction was completed, the upper organic layer containing
the iron–oleate complex was washed three times with 30 ml distilled water in a separatory funnel. After
washing, hexane was evaporated off, resulting in iron–oleate complex in a waxy solid form.
SYNTHESIS OF IRON OXIDE NANOCRYSTALS
The following is a typical synthetic procedure for monodisperse iron oxide (magnetite) nanocrystals
with a particle size of 12 nm. 36 g (40 mmol) of the iron-oleate complex synthesized as described above
and 5.7 g of oleic acid (20 mmol, Aldrich, 90%) were dissolved in 200 g of 1-octadecene (Aldrich,
90%) at room temperature. The reaction mixture was heated to 320 °C with a constant heating rate of
3.3 °C min–1, and then kept at that temperature for 30 min. When the reaction temperature reached
320 °C, a severe reaction occurred and the initial transparent solution became turbid and brownish
black. The resulting solution containing the nanocrystals was then cooled to room temperature, and
500 ml of ethanol was added to the solution to precipitate the nanocrystals. The nanocrystals were
separated by centrifugation.
Received 11 June 2004; accepted 20 September 2004; published 28 November 2004.
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Acknowledgements
T.H. would like to thank the financial support from the Korean Ministry of Science and Technology
through the National Creative Research Initiative Program. J.G.P. would like to thank the financial
support by the KOSEF through the Center for Strongly Correlated Materials Research at the Seoul
National University. J.H.P. would like to thank the financial support by KISTEP through X-ray/particlebeam Nanocharacterization Program.
Correspondence and requests for materials should be addressed to T. H.
Competing financial interests
The authors declare that they have no competing financial interests.
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Article
Synthesis of Oil-Dispersible Hexagonal-Phase
and Hexagonal-Shaped NaYF:Yb,Er Nanoplates
4
Yang Wei, Fengqi Lu, Xinrong Zhang, and Depu Chen
Chem. Mater., 2006, 18 (24), 5733-5737• DOI: 10.1021/cm0606171 • Publication Date (Web): 25 October 2006
Downloaded from http://pubs.acs.org on May 11, 2009
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Chem. Mater. 2006, 18, 5733-5737
5733
Synthesis of Oil-Dispersible Hexagonal-Phase and Hexagonal-Shaped
NaYF4:Yb,Er Nanoplates
Yang Wei, Fengqi Lu, Xinrong Zhang, and Depu Chen*
Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China
ReceiVed March 14, 2006. ReVised Manuscript ReceiVed July 20, 2006
Oil-dispersible R-NaYF4 spherical nanoparticles and β-NaYF4 hexagonal-shaped nanoplates were
synthesized by the liquid-solid two-phase approach at different reaction temperatures. The TEM and
FE-SEM images reveal that the nanoplates have a relatively narrow size distribution. In comparison with
other methods, pure β-NaYF4 hexagonal-shaped nanoplates were prepared under a relatively mild condition.
The nanoplates grew at the liquid-solid interface with slow crystallization rate, which may be preferable
for achieving β-NaYF4.
Introduction
Upconversion luminescent materials have received great
attention for a few decades.1 Because of their unique antiStokes optical property, upconversion luminescent materials
have a number of potential applications, including lasers,2
three-dimensional displays,3 light emitting devices,4 biological detection,5,6 and many others.7 In recent years, several
research groups have reported the upconversion luminescence
in nanomaterials.8-11 Besides the dried powdered nanocrystalline upconversion phosphors (UCPs), there is a growing
interest to prepare UCPs that have a good dispersibility in
organic solvents.12-17 Spherical cubic-phase NaYF4 nanoparticles that could be dispersed in nonpolar solvent have
been prepared in homogeneous solution13 and by a newly
developed liquid-solid-solution (LSS) process.15 Very
recently, the methods to prepare NaYF4:Yb,Er/Tm nano* To whom correspondence should be addressed. Tel.: +86 10 62781691.
Fax: +86 10 62782485. E-mail: [email protected].
(1) Auzel, F. Chem. ReV. 2004, 104, 139.
(2) Scheps, R. Prog. Quantum Electron. 1996, 20, 271.
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Soc. 2005, 127, 12464.
(5) van de Rijke, F.; Zijlmans, H.; Li, S.; Vail, T.; Raap, A. K.; Niedbala,
R. S.; Tanke, H. J. Nat. Biotechnol. 2001, 19, 273.
(6) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Yue, G.; Yang, W. J.; Chen, D. P.;
Guo, L. H. Nano Lett. 2004, 4, 2191.
(7) Shalav, A.; Richards, B. S.; Trupke, T.; Kramer, K. W.; Gudel, H. U.
Appl. Phys. Lett. 2005, 86, 013505.
(8) Yi, G. S.; Sun, B. Q.; Yang, F. Z.; Chen, D. P.; Zhou, Y. X.; Cheng,
J. Chem. Mater. 2002, 14, 2910.
(9) Matsuura, D. Appl. Phys. Lett. 2002, 81, 4526.
(10) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. J. Phys. Chem. B
2002, 106, 1909.
(11) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli,
M. J. Phys. Chem. B 2003, 107, 1107.
(12) Heer, S.; Lehmann, O.; Haase, M.; Gudel, H. U. Angew. Chem., Int.
Ed. 2003, 42, 3179.
(13) Heer, S.; Kompe, K.; Gudel, H. U.; Haase, M. AdV. Mater. 2004, 16,
2102.
(14) Yi, G. S.; Chow, G. M. J. Mater. Chem. 2005, 15, 4460.
(15) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121.
(16) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.;
Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426.
(17) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am.
Chem. Soc. 2006, 128, 7444.
crystals based on the co-thermolysis of sodium trifluoroacetate and rare earth trifluoroacetate were independently
reported by Yan et al.16 and Capobianco et al.17 NaYF4 exists
in two polymorphs at ambient pressure: cubic R-phase and
hexagonal β-phase. β-NaYF4 has been reported as the most
efficient host material for green and blue UCPs.18 In our
previous research,6 the cubic-to-hexagonal phase transition
process of NaYF4 is an exothermic process. Correspondingly,
the hexagonal-to-cubic phase transition process of NaYF4
has been reported as an endothermic process.18 From the
results provided by differential scanning calorimetric measurement, it is reasonable to conclude that for NaYF4, the
hexagonal phase is more thermodynamically stable than the
cubic phase. However, in most cases for preparation of
NaYF4, R-NaYF4 nanocrystals were obtained. The cubic
phase could transfer into hexagonal phase under heat
treatment. Annealing treatment18 and hydrothermal or solvothermal treatment19,20 were adopted to produce the β-NaYF4.
β-NaYF4 could also be formed under drastic conditions
reported by Yan et al.16 In this paper, a liquid-solid twophase approach was used to synthesize β-NaYF4 directly.
The results revealed that not only β-NaYF4 hexagonal-shaped
nanoplates but also R-NaYF4 spherical nanoparticles could
be synthesized by the liquid-solid two-phase approach at
different reaction temperatures. The possible mechanism was
discussed in this paper.
Experimental Section
Materials. Sodium fluoride (NaF), sodium hydroxide (NaOH),
ethanol, and hydrochloric acid (HCl) were obtained from Beijing
Chemical Corp. (Beijing, China). Yttrium oxide (Y2O3, 99.99%),
ytterbium oxide (Yb2O3, 99.99%), and erbium oxide (Er2O3,
99.99%) were obtained from Grirem Advanced Materials Co., Ltd.
(Beijing, China), and were of SpecPure grade. Oleic acid was
(18) Kramer, K. W.; Biner, D.; Frei, G.; Gudel, H. U.; Hehlen, M. P.;
Luthi, S. R. Chem. Mater. 2004, 16, 1244.
(19) Wang, L. Y.; Yan, R. X.; Hao, Z. Y.; Wang, L.; Zeng, J. H.; Bao, H.;
Wang, X; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054.
(20) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. AdV. Mater. 2005,
17, 2119.
10.1021/cm0606171 CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/25/2006
5734 Chem. Mater., Vol. 18, No. 24, 2006
purchased from Beijing Chemical Reagents Co. (Beijing, China).
1-Octadecene was purchased from Acros Organics (NJ). Rare earth
chlorides (RECl3‚6H2O with RE ) Y, Yb, Er) were prepared by
dissolving the corresponding rare earth oxides in hydrochloric acid
at elevated temperature, and then evaporating the solvent in a
vacuum. Sodium oleate was prepared by reacting oleic acid and
sodium hydroxide using ethanol as reaction medium. Sodium
hydroxide was dispersed in ethanol under vigorous stirring, and
then equal molar oleic acid was added dropwise. After the
neutralization reaction was completed, ethanol and water were
evaporated in a vacuum.
Synthesis of Rare Earth Oleate Complexes. A literature method
for the synthesis of iron-oleate complex21 was adopted to prepare
the rare earth oleate complexes. In a typical synthesis of yttriumoleate complex, 20 mmol of yttrium chloride (YCl3‚6H2O) and 60
mmol of sodium oleate were dissolved in a mixture solvent
composed of 40 mL of ethanol, 30 mL of distilled water, and 70
mL of hexane. The resulting solution was added into a 250-mL
round-bottomed flask with a reflux condenser, and then heated to
70 °C and kept at that temperature for 4 h. After the reaction was
completed, the reaction mixture was transferred into a separatory
funnel. The upper organic layer was separated and washed three
times with 30 mL of distilled water. After being washed, yttriumoleate complex was produced in a waxy solid form by evaporating
off the remaining hexane. Ytterbium-oleate complex and erbiumoleate complex were synthesized in the same way.
Synthesis of NaYF4:Yb,Er Nanocrystals. The reaction temperature is the only difference between the synthetic procedures
for preparation of R-NaYF4:Yb,Er spherical nanoparticles and
β-NaYF4:Yb,Er hexagonal-shaped nanoplates. For the synthesis of
β-NaYF4:Yb,Er hexagonal-shaped nanoplates, 0.2 g of NaF solid
powder and 30 mL of 1-octadecene were added into a 100 mL
three-necked flask, and NaF was dispersed in the 1-octadecene with
vigorous magnetic stirring. The mixture was degassed under vacuum
for about 30 min, and flushed periodically with N2. The temperature
of the reaction flask was then stabilized at 260 °C under N2
atmosphere. Next, 0.8 mmol of yttrium-oleate complex, 0.17 mmol
of ytterbium-oleate complex, and 0.03 mmol of erbium-oleate
complex were dissolved in 30 mL of 1-octadecene to form an
optically transparent solution. This solution was bubbled with N2
gas for 10 min, and then was quickly delivered to the vigorously
stirring reaction flask in a single injection with a 50-mL syringe
through a rubber septum. The reaction was kept at 260 °C for 6 h
in N2 atmosphere under vigorous stirring. As the reaction mixture
was cooled to ∼60 °C, it was washed three times with 30 mL of
∼60 °C hot deionized water in a separatory funnel. Next, 100 mL
of ethanol was added to the resulting solution containing the
nanocrystals. The nanocrystals were separated by centrifugation.
The as-prepared nanocrystals could be easily redispersed in various
nonpolar organic solvents, such as hexane and toluene. When the
reaction temperature was lowered to 210 °C, R-NaYF4:Yb,Er
spherical nanoparticles were obtained.
Characterization. Investigations on the size and morphology
of the nanocrystals were performed using a JEM-1200EX transmission electron microscope (TEM) (JEOL, Japan) operating at
accelerating voltages up to 100 kV and a JSM-7401F field emission
scanning electron microscope (FE-SEM) (JEOL, Japan) operating
at accelerating voltages up to 1 kV. Powder X-ray diffraction
patterns were obtained on a D/max-RB X-ray diffractometer
(Rigaku, Japan) and a D/max-2500 X-ray diffractometer (Rigaku,
Japan). Upconversion fluorescent spectra were measured on a LS55 fluorescence spectrophotometer (Perkin-Elmer Corp.) with an
(21) Park, J.; An, K. J.; Hwang, Y. S.; Park, J. G.; Noh, H. J.; Kim, J. Y.;
Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891.
Wei et al.
external 980 nm laser (Beijing Hi-Tech Optoelectronic Co., China)
instead of internal excitation source. A 62.5/125 (core/cladding
dimensions, which are given in micrometers) multimode optical
fiber with the numerical aperture 0.22 was used to conduct the laser
into the spectrophotometer. The distance between the fiber head
and the samples is about 3 mm.
Results and Discussion
Synthesis of NaYF4:Yb, Er Nanocrystals. A liquid-solid
two-phase approach was used to synthesize the NaYF4:Yb,Er nanocrystals. Because of its high boiling point, 1-octadecene was chosen as the solvent for the high-temperature
growth and annealing of NaYF4 crystallites. Rare earth oleate
complexes were dissolved in the organic solvent as the liquid
phase. Because of the insolubility of NaF in 1-octadecene,
NaF was dispersed in the same organic solvent as the solid
phase. NaYF4:Yb,Er nanocrystals grew at the liquid-solid
interface, and oleic acid released from the rare earth oleate
complexes could bind to the growing nanocrystals with the
alkyl chains outward, through which the nanocrystals gained
hydrophobic surfaces.
TEM was used to map the shape and size of the
nanocrystals dispersed on a carbon-coated copper grid from
hexane solutions, and FE-SEM was also used to give the
three-dimensional morphological observation of the nanocrystals. The TEM images of the nanocrystals prepared at
210, 230, and 260 °C are shown in Figure 1A,B and Figure
2, respectively. The TEM images with higher magnification
for these nanocrystals are shown in Figures S1-S3. As
shown in Figure 1A and Figure S1, the nanoparticles obtained
at 210 °C have roughly spherical shapes with the average
size of about 7 nm. Increasing the reaction temperature to
230 °C, much bigger nanocrystals were formed. It could be
found from the TEM images shown in Figure 1B and Figure
S2 that the shapes of these nanocrystals are between round
and hexagonal. In fact, from the FE-SEM image shown in
Figure S5, it is clear that the nanocrystals obtained at 230
°C were mixtures composed of hexagonal-shaped nanoplates
and small nanoparticles glued to the nanoplates. After the
reaction temperature reached 260 °C, pure hexagonal-shaped
nanoplates were obtained. The TEM images of these nanoplates were shown in Figure 2 and Figure S3, and the FESEM images of them were shown in Figure S6. The TEM
images shown in Figure 2 reveal that most of the nanoplates
are lying flat on the face, and a small quantity of the
nanoplates are standing on the edge. The edge lengths of
the nanoplates shown in the TEM images are not equal,
which is probably due to the nanocrystals being tilted by
different angles with respect to the carbon films. The
nanoplates are characterized by ∼35 nm in edge length and
∼20 nm in thickness. The TEM and FE-SEM images reveal
that the nanoplates have a relatively narrow size distribution.
In the liquid-solid two-phase approach, both nucleation and
growth of nanoplates could only occur at the interface of
the two phases. After the nuclei formed at the interface, they
could enter the liquid phase, and thus the nanocrystals
stopped growing. Only if the nanocrystals returned to the
interface could they continue to grow. The bigger size
nanocrystals had, the more slowly they moved. Smaller
Synthesis of Oil-Dispersible NaYF4:Yb,Er Nanoplates
Chem. Mater., Vol. 18, No. 24, 2006 5735
Figure 1. TEM images of NaYF4:Yb,Er nanocrystals obtained at 210 °C (A) and 230 °C (B).
Figure 2. TEM images of β-NaYF4:Yb,Er nanoplates obtained at 260 °C.
nanocrystals could move into the liquid phase and went back
to the interface more quickly than bigger nanocrystals, and
so the smaller nanocrystals had more chance to grow than
the bigger nanocrystals; thus the narrow particle size
distribution was obtained.
The efficiency of the reaction that occurred at the liquidsolid interface is much lower than that in homogeneous
solution. Several factors, such as the reaction time, stirring
speed, reaction temperature, have strong impacts on the
output of this reaction. Vigorous stirring and enough high
reaction temperature were required. To avoid the thermolysis
of rare earth oleate complexes at high temperature, the
reaction temperature for preparation of hexagonal-shaped
nanoplates was chosen at 260 °C. The output of the reaction
increased greatly with increasing the reaction time. From the
TEM images shown in Figure S4, there were no obvious
difference found in the size and morphology of the nanoplates
obtained from different reaction time. This result could be
good evidence for the reaction mechanism provided above.
The XRD results reveal that the spherical NaYF4 nanoparticles synthesized at 210 °C crystallized in the cubic
R-phase and the hexagonal-shaped NaYF4 nanoplates synthesized at 260 °C crystallized in the hexagonal β-phase.
The crystallographic phase of the initial seed during the
nucleation processes is critical for directing the nanocrystal
shapes due to its characteristic unit cell structure.22 The cubic
R-NaYF4 seeds have isotropic unit cell structures, which
generally induce the isotropic growth of nanocrystals, and
therefore spherical R-NaYF4 nanoparticles were observed.
In contrast, hexagonal β-NaYF4 seeds have anisotropic unit
cell structures, which can induce anisotropic growth along
crystallographically reactive directions, and thus hexagonalshaped nanoplates were obtained.
Structural Characterization of NaYF4:Yb,Er Nanocrystals. X-ray powder diffraction patterns of NaYF4:Yb,(22) Jun, Y. W.; Lee, J. H.; Choi, J. S.; Cheon, J. J. Phys. Chem. B 2005,
109, 14795.
5736 Chem. Mater., Vol. 18, No. 24, 2006
Figure 3. Calculated line pattern (A) and experimental powder XRD data
(B) for the R-NaYF4:Yb,Er nanoparticles obtained at 210 °C.
Figure 4. Calculated line pattern (A) and experimental powder XRD data
for the β-NaYF4:Yb,Er nanoplates obtained at 230 °C (B) and 260 °C (C).
Er nanocrystals obtained at 210, 230, and 260 °C are shown
in Figure 3 and Figure 4. Deduced from the XRD data, all
of the samples were well-crystallized. Except two peaks that
were marked with asterisks, all of the other peaks of the
nanoparticles prepared at 210 °C and of the nanoplates
prepared at 260 °C could be readily indexed to the cubic
R-NaYF4 phase or hexagonal β-NaYF4 phase, respectively.
For the sample synthesized at 230 °C, the β-NaYF4 phase
was the major species; however, a small quantity of R-NaYF4
phase could be detected. The selected-area electron diffraction (SAED) patterns shown in Figure S7, which were taken
from a single nanoplate obtained at 260 °C, demonstrated
the single-crystalline nature of the sample; it could be readily
indexed as hexagonal phase, in good agreement with its XRD
data.
The average particle size estimated by line-broadening was
6.4 nm for the R-NaYF4 spherical nanoparticles. For
β-NaYF4 hexagonal-shaped nanoplates, the particle sizes
estimated by line-broadening from different diffraction peaks
varied from 26.2 to 63.2 nm.
Wei et al.
These two asterisked peaks in the XRD patterns could be
well indexed to the reactant NaF, which means the nanocrystals obtained were mixed with some residual NaF. During
the reaction process, the polar head of oleic acid could absorb
to the NaF particles with the nonpolar tail on the outside,
through which the NaF particles gained hydrophobic surfaces. After the reaction was completed, to remove the
residual NaF, the reaction mixture was washed three times
with deionized water in a separatory funnel; however, it
seemed that this procedure is not effective enough to remove
all of the residual NaF. NaF could be removed completely
from the products; the detailed procedure is described in the
Supporting Information.
For bulk NaYF4, R-phase is the metastable high-temperature phase, while β-phase is the thermodynamically stable
low-temperature phase.16 At temperatures above approximately 700 °C, the β-phase is unstable.18 However, in most
cases for preparation of nanosized NaYF4, it tends to
crystallize in the cubic phase, the less thermodynamically
stable phase, first. In their newly reported work, Yan et al.
demonstrated a general one-step synthesis of NaREF4 (RE
) Pr to Lu, Y) nanocrystals via the co-thermolysis of Na(CF3COO) and RE(CF3COO)3 precursors.16 Pure R-NaYF4
could be obtained at a low temperature (280 °C) and a low
ratio of Na/RE with a relatively short reaction time, while
β-NaYF4 was formed only under drastic conditions (high
Na/RE, 330 °C, and long reaction time).
It is interesting that both pure R-NaYF4 nanocrystals and
β-NaYF4 nanoplates could be obtained in this liquid-solid
two-phase approach, and the only difference between the
synthetic procedures is the reaction temperature. In comparison with the method to prepare the R-NaYF4 spherical
nanoparticles in homogeneous solution,13 the liquid-solidsolution (LSS) process,15 and the thermal decomposition
method,16,17 the reaction rate and crystallization rate are much
slower in the liquid-solid two-phase approach, for both
nucleation and growth of nanocrystals could only occur at
the liquid-solid interface. To overcome the energy barrier
for the formation of β-NaYF4, appropriate high temperature
was needed to prepare β-NaYF4. It is why only R-NaYF4
nanoparticles were obtained at 210 °C and mixtures composed of β-NaYF4 hexagonal-shaped nanoplates and R-NaYF4
spherical nanoparticles were obtained at 230 °C in this
approach. Pure β-NaYF4 hexagonal-shaped nanoplates could
be obtained at 260 °C, a relatively mild condition, in this
liquid-solid two-phase approach. In contrast, drastic conditions were needed for synthesis of β-NaYF4 via the thermal
decomposition method. In addition, in a glycerol-mediated
synthesis of NaYF4:Yb,Er nanoparticles in homogeneous
solution at 260 °C, pure R-NaYF4 was obtained even after 6
h of reaction. However, pure hexagonal phase was detected
in the products obtained after 2 h of reaction via the liquidsolid two-phase approach. (The details are described in the
Supporting Information.)
In the structure of R-NaYF4, the cation sites are occupied
randomly by Na+ and Y3+ cations, while in β-NaYF4 the
cation sites are of three types: a 1-fold site occupied by Y3+,
a 1-fold site occupied randomly by 1/2Na+ and 1/2Y3+, and a
2-fold site occupied randomly by Na+ and vacancies. Thus,
Synthesis of Oil-Dispersible NaYF4:Yb,Er Nanoplates
Chem. Mater., Vol. 18, No. 24, 2006 5737
Figure 5. Upconversion emission spectrum of NaYF4:Yb,Er nanocrystals obtained at 210, 230, and 260 °C (the excitation conditions were the same for
these samples: the laser power is 437 mW, and the emission slit is 3 nm).
for NaYF4, the cubic-to-hexagonal phase transformation is
of a disorder-to-order character with respect to cations.16
From the results described above, we presume that a slow
crystallization process may be preferable for achieving
β-NaYF4, the order phase. A similar phenomenon was found
in the recently reported synthesis of lanthanide fluoride
nanoparticles by a reverse microemulsion method.23 Spherical
amorphous YF3 particles were obtained by the classical
microemulsion method (mixing of two microemulsions
containing fluoride and YCl3, respectively); conversely,
single-crystal particles with regular hexagonal and triangular
shape were obtained by the single microemulsion method
(direct addition of a fluoride solution to a microemulsion
containing YCl3). The particle growth and crystallization
were slower in the single microemulsion method than in the
classical microemulsion method, and, as a result, singlecrystal nanoparticles rather than the amorphous particles were
obtained in the single microemulsion.
Upconversion Fluorescent Properties of β-NaYF4:Yb,Er Nanoplates. Room-temperature upconversion fluorescence spectra of NaYF4:Yb,Er nanocrystals obtained at 210,
230, and 260 °C in the wavelength region of 400-700 nm
are shown in Figure 5. As compared to the fluorescent
intensity of R-NaYF4 nanoparticles prepared at 210 °C, that
of β-NaYF4 nanoplates prepared at 260 °C is enhanced
greatly. This result is in good agreement with the reported
result that β-NaYF4 is the most efficient host material for
green and blue upconversion phosphors known today.18-20
It should be pointed out that the enhancement of the
fluorescent intensity may be partly due to the increase of
the particle size. There are three major bands in the curve,
centered at 522, 542, and 654 nm, respectively. The
mechanisms responsible for the upconversion fluorescence
are shown in the Supporting Information.
The 542 nm-to-654 nm emission ratio of β-NaYF4
nanoplates prepared at 260 °C is about 1.29 to 1. Several
factors such as doping levels, excitation power, and impurities have impacts on the green-to-red emission ratio. In this
paper, the doping levels for Yb3+ and Er3+ were not
optimized. The actual molar ratio of the rare earth metals
(Y:Yb:Er) in the β-NaYF4 nanoplates obtained at 260 °C
examined by ICP-OES was 0.816:0.156:0.028. This result
is in good agreement with the planned molar ratios for Y,
Yb, and Er (0.80:0.17:0.03). Furthermore, the organic
capping groups binding on the surface of the β-NaYF4
nanoplates may increase the multiphonon relaxation rates
between the metastable states, which reduce the green-tored emission ratio.
(23) Lemyre, J. L.; Ritcey, A. M. Chem. Mater. 2005, 17, 3040.
CM0606171
Acknowledgment. Financial support from National Nature
Science Foundation of China (20535020) is gratefully acknowledged.
Supporting Information Available: TEM and FE-SEM images
of NaYF4:Yb,Er nanocrystals, SAED and XRD patterns of β-NaYF4:
Yb,Er nanoplates obtained at 260 °C, procedures for removing
residual NaF from products, brief description for the glycerolmediated synthesis of R-NaYF4:Yb,Er nanoparticles, and mechanisms responsible for the upconversion fluorescence of β-NaYF4:
Yb,Er nanoplates obtained at 260 °C (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.