Self-assembled β-NaGdF4 microcrystals: Hydrothermal synthesis, morphology evolution, and luminescence properties

Article abstract of DOI:10.1021/ic200155q

A self-assembly process has been designed for the controlled synthesis of β-NaGdF₄ with uniform morphology, dimension, and considerable monodispersity under a gentle hydrothermal condition using sodium citrate as the chelating agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrum (EDS), and up-conversion (UC) photoluminescence spectra were used to characterize the samples. The results indicate that the NaGdF₄ microcrystal can be rationally modified in phase, size, and morphology through tuning the pH value, sodium citrate content, and reaction time. Moreover, the hybrid process of the crystal growth and the self-assembly were thoroughly discussed, and a possible formation mechanism was proposed. Furthermore, the UC luminescence properties as well as the emission mechanisms of β-NaGdF₄:17%Yb³⁺/3%Ln ³⁺ (Ln = Er, Tm, Ho) microcrystals were systematically investigated. It is found that under 980 nm excitation, only limited emission bands were discovered which can be attributed to the energy gap and migration function of the Gd³⁺ ions in the β-NaGdF₄ microcrystals. It is expected that the synthetic strategy can be applied to prepare many other types of micro- and nanocrystals as well.

Full text of DOI:10.1021/ic200155q

ARTICLE
pubs.acs.org/IC
Self-Assembled β-NaGdF Microcrystals: Hydrothermal Synthesis,
4
Morphology Evolution, and Luminescence Properties
Fei He, Piaoping Yang,* Dong Wang, Na Niu, Shili Gai, and Xingbo Li
Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, and College of Material Science and Chemical
Engineering, Harbin Engineering University, Harbin 150001, P.R. China
ABSTRACT: A self-assembly process has been designed for the
controlled synthesis of β-NaGdF with uniform morphology,
4
dimension, and considerable monodispersity under a gentle
hydrothermal condition using sodium citrate as the chelating
agent. X-ray diffraction (XRD), scanning electron microscopy
(
SEM), transmission electron microscopy (TEM), high-resolu-
tion transmission electron microscopy (HRTEM), energy-dis-
persive X-ray spectrum (EDS), and up-conversion (UC)
photoluminescence spectra were used to characterize the
samples. The results indicate that the NaGdF microcrystal
4
can be rationally modified in phase, size, and morphology
through tuning the pH value, sodium citrate content, and
reaction time. Moreover, the hybrid process of the crystal growth and the self-assembly were thoroughly discussed, and a possible
formation mechanism was proposed. Furthermore, the UC luminescence properties as well as the emission mechanisms of
3
þ
3þ
β-NaGdF :17%Yb /3%Ln (Ln = Er, Tm, Ho) microcrystals were systematically investigated. It is found that under 980 nm
4
excitation, only limited emission bands were discovered which can be attributed to the energy gap and migration function of the
3
þ
Gd ions in the β-NaGdF microcrystals. It is expected that the synthetic strategy can be applied to prepare many other types of
4
micro- and nanocrystals as well.
1
. INTRODUCTION
different facets under hydrothermal conditions, resulting in the
1
4ꢀ17
formation of various geometries of the final products.
Inmodern chemistryand materials science, controlled synthesis
Recently, rare earth materials have attracted much attention
because of their unique electronic and optical properties based
of well-defined inorganic micro- and nanocrystals (MCs and NCs)
with uniform dimensions and shapes have become the hot spot of
worldwide research because the electronic structure, bonding,
surface energy, and chemical reactivity are directly related to their
surface morphology. Many methodologies including molten-salt
method, hydrothermal process, template synthesis, and high
boiling solvents method have been employed widely for the
synthesis of a large range of nanostructures.
routes, the hydrothermal method is capable of preparing different
structured materials with high crystallinity and wide dimensions as
long as chemicals and modifiers are correctly selected.
typical solution-based approach, this method has been proven to
be an environmentally acceptable process with relatively high yield
of desired products. Furthermore, it has been widely recognized
that the crystal growth process is not only determined by its
intrinsic structure but also significantly influenced by a series of
external parameters, such as pH value, reaction time, and the
chelating agent. In particular, the addition of special chelating
agent, which has a significant effect on the kinetics of the crystal
growth, has been demonstrated to be an effective strategy to
18ꢀ24
on their 4f electrons.
Particularly, because of their special
advantages such as high light penetration depth in tissues, less
photodamage to living organisms, low background light, and high
sensitivity for detection, the up-conversion (UC) phosphors are
of great potential in biological labeling and imaging technology,
optical telecommunication, laser, medical diagnostics, and infra-
1
2ꢀ7
Among these
25ꢀ31
red quantum counters.
As a famous host material, the
NaREF4 (RE = rare earth) phosphors with some distinct
advantages owing to the very low phonon frequencies have
8ꢀ10
As a
21ꢀ23,32,33
aroused intensive research in the past decades.
One
example for this class of fluoride compounds is NaGdF , which is
4
6
an ideal UC host material. Because the lowest excited level ( P
7
/
3
þ
2)
of Gd is situated in the ultraviolet region, which is far higher
3þ
3þ
than most excited level of Yb and Er involved in the UC
processes, the excitation energy loss through energy transfer
3
þ
3þ
þ
3þ
28,34
from Yb and Er to the 4f level of Gd can be avoided.
3
Moreover, the Gd ions can act as an intermediate through
which the excited energy can migrate over the Gd sublattice
3
þ
11ꢀ13
and consequently facilitate the energy transfer process in fluoride
achieve control over the morphology of final products.
Because of the high thermal stability and ability to form complexes
with other metal ions, the citrate ions (Cit ) has the dynamic
effect asa ligandand shape modifier by adjusting the growth rate of
3ꢀ
Received: January 23, 2011
Published: April 06, 2011
r 2011 American Chemical Society
4116
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|Inorg. Chem. 2011, 50, 4116–4124

Inorganic Chemistry
ARTICLE
3
5
phosphor. Recently, several methods have been developed to
prepare the hexagonal β-phase NaREF by involving the
4
36,37
NaGdF4 as the dopant.
Compared with the cubic
R-phase structure, the hexagonal phase NaREF is more stable
4
and displays approximately one or two orders higher UC
38
efficiency, especially in bulk materials and large-size particles.
Furthermore, the formation energy per atom in hexagonal phase
3
þ
3þ 28
increases by about 0.07 eV when Y is replaced by Gd
,
indicating that NaGdF is more energetically stable than NaYF4
4
in hexagonal phase and has longer luminescence lifetime as UC
host material. However, compared with the NaYF NCs which
4
have been a research focus in the RE fluoride compounds,
relatively less attention were paid in the NaGdF NCs.
4
Up to now, some synthetic routes have been developed to
39
Figure 1. XRD patterns of NaGdF
4
MCs prepared at 180 °C for 24 h
selectively prepare the NaGdF NCs. Boyer et al. and Naccache
4
4
0
and pH values of 3 (a), 7 (b), and 10 (c). All of the samples were
et al. prepared NaGdF through the thermal decomposition
4
3ꢀ
3þ
hydrothermally obtained with Cit /Gd molar ratios of 1:1.
synthesis route. The as-synthesized products are composed of
single crystallites with uniform size distribution. However, de-
manding high temperature condition, low yield ratio, and the
organic solvent involved in this method have limited its further
application. Besides, there is still little work on the synthesis of
Table 1. Unit Cell Lattice Constants and Crystallite Sizes
3
for Orthorhombic Phase GdF and Hexagonal β-NaGdF4
Prepared at the pH Values of 3, 7, and 10, Respectively
NaGdF MCs with well-defined shapes and good uniformity
4
cell volume crystallite size
3,41
relative to the numerous effort on the NaYF MCs.
Mech
3
4
samples
a (nm) c (nm)
(nm )
(nm)
4
2
3þ
et al. synthesized the hexagonal NaGdF :Eu with average grain
4
sizes of 20 nm from solution by a coprecipitation method. Zhang
JCPDS 49-1804
0.6571
0.20146
0.20112
0.11302
0.11374
0.11368
4
3
3þ
et al. prepared both cubic and hexagonalNaGdF :Ce as well as
GdF (pH = 3)
0.6568 0.3601
0.6020
27.7
4
3
(
Ce, Gd)F MCs by controlling different molar ratio of reactants
3
JCPDS 27-0699
and synthesis temperatures through the hydrothermal procedure.
Unfortunately, the morphology of as-prepared NCs is not uniform
and thecrystalgrowth processis stillnotgiven. Inorganic MCs with
novel morphologies are of special significance in understanding the
growth behavior and the potential technological applications in
β-NaGdF
4
4
(pH = 7)
0.6018 0.3607
19.3
21.3
β-NaGdF
(pH = 10) 0.6024 0.3606
ꢀ
4
After vigorous stirring for 30 min, 3.125 mmol of NaBF (12.5 mmol F )
was added into the above solution. The pH of the mixture was adjusted to 7
by adding 1 mol/L NaOH. After additional agitation for 15 min, the as-
obtained mixing solution was transferred into a Teflon bottle held in a
stainless steel autoclave, which was sealed and maintained at 180 °Cfor24h.
As the autoclave was naturally cooled to room temperature, the pH value of
the solution was measured to be 3.58. Subsequently, the precipitates at the
bottom were separated by centrifugation, washed with deionized water and
ethanol in sequence, and then dried in air at 80 °C for 12 h. Other samples
microelectronic devices. Therefore, NaGdF phosphors with uni-
4
form novel morphology and high monodispersity synthesized by a
facile and environmental friendly method are still desirable.
In the present work, we describe a template-free, no-seed, and
aqueous-phase process for the synthesis of uniform β-NaGdF
4
3þ
3þ
3þ
3þ
3þ
3þ
MCs co-doped with Yb /Ln (Ln = Er , Tm , Ho
)
with high monodispersity via a facile and mild hydrothermal
process. Sodium citrate was introduced into the reaction system
to govern and adjust the morphology of the products. The effects
3
ꢀ
were prepared by a similar procedure, except for different pH values, Cit
/
/
3
þ
3þ
Gd molar ratio, and reaction time. UC luminescent NaGdF :17% Yb
4
3
þ
3þ
3þ
3þ
3þ
of the molar ratio of sodium citrate to RE , pH value of the
initial solution, and reaction time on the crystal morphology have
been investigated in detail. A possible mechanism of crystal
growth has also been proposed. Additionally, the UC lumines-
4 4
3% Er , NaGdF :17% Yb / 3% Tm , and NaGdF :17% Yb / 3%
3
þ
Ho samples were prepared in a manner similar to that for pure NaGdF
sample, which was prepared with pH value of 7 at 180 °C for 24 h.
4
2.3. Characterization. Powder X-ray diffraction (XRD) measure-
ments were performed on a Rigaku-Dmax 2500 diffractometer at a scanning
rate of 15°/min in the 2θ range from 10° to 60°, with graphite-
monochromatized Cu KR radiation (λ = 0.15405 nm). SEM images were
obtained using a field emission electron microscope (FESEM, S4800,
Hitachi) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL
JXA-840). Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM) were performed on a FEI
3
þ
3þ
3þ
3þ
3þ
3þ
cence properties of Yb /Er , Yb /Tm , and Yb /Ho
have been thoroughly studied in the obtained β-NaGdF MCs.
4
2. EXPERIMENTAL SECTION
2
4
.1. Materials. All materials including analytical grade NaFB ,
trisodium citrate, and NaOH were purchased from Sinopharm Chemical
Reagent Co., Ltd. and used as received without further purification. 0.2
M Rare earth chloride stock solutions were prepared by dissolving the
2
Tecnai G S-Twin instrument with a field emission gun operating at 200 kV.
Images were acquired digitally on a Gatan multiple CCD camera. The UC
emission spectra were obtained using a 980 nm laser from an OPO (optical
parametric oscillator, Continuum Surelite, U.S.A.) as the excitation source
and detected by a photomultiplier tube (HAMAMATSU R955) from 400
to 900 nm. All the measurements were performed at room temperature.
2 3 2 3 2 3 2 3 2 3
corresponding Gd O , Yb O , Er O , Tm O , Ho O (99.99%, Changchun
Applied Chemistry Science and Technology Limited, China) in hydro-
3
þ
chloric acid at elevated temperature. All of the doping ratios of Ln are
in mole equivalents in our experiments.
2
.2. Preparation of β-NaGdF . In a typical procedure for the
4
3. RESULTS AND DISCUSSION
synthesis of β-NaGdF
4 3
, 5 mL of GdCl (0.2 mol/L) was added into 10 mL
of aqueous solution containing 1 mmol of trisodium citrate (labeled as
Cit ) to form the metal-Cit complex (1:1 molar ratio for Cit /Gd ).
3.1. Phase, Structure, and Morphology. In the previous
reports for the synthesis of the RE fluoride, some necessary
3ꢀ
3ꢀ
3ꢀ
3þ
4
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Inorganic Chemistry
ARTICLE
conditions have been proposed to investigate the changes of the
crystal nuclei can be summarized as follows:
4
4,45
products in different reaction systems.
Figure 1 exhibits
Gd3 þ Cit3ꢀ f Gd ꢀCit ðcomplexÞ
þ
3þ
3ꢀ
ð1Þ
XRD patterns of the as-synthesized samples using NaBF as
4
fluoride source under various pH conditions. The regularity
indicated from the results is described as follows. For the sample
prepared at pH value of 3 (Figure 1a), the diffraction peaks
ꢀ
ꢀ
BF þ 3H O T 3HF þ F þ H BO
ð2Þ
ð3Þ
ð4Þ
4
2
3
3
2H3BO3 þ Naþ f Na2B2O4 þ 2H2O þ 2Hþ
display the presence of orthorhombic GdF phase (JCPDS 49-
3
1
804), while pure hexagonal β-phase NaGdF indexed to the
4
þ
3þ
3ꢀ
ꢀ
3ꢀ
standard data (JCPDS 27-0699) can be obtained under the pH
value of 7 (Figure 1b) and 10 (Figure 1c). The calculated cell
lattice constants of the samples prepared at different pH values
are summarized in Table 1, the standard data (JCPDS 49-1804)
Na þ Gd ꢀCit þ 4F f NaGdF4 þ Cit
In addition, the diffraction peaks of these samples show some
broadening, which indicates the fine nature of the crystallinity in
those samples. The peak broadening was used to calculated the
crystallite sizes by the Scherrer formula, D = 0.89λ/β cos θ,
where D is the average grain size, λ is the X-ray wavelength
(0.154505 nm), and β and θ are the diffraction angle and full-
width at half-maximum (fwhm), respectively. As shown in
Table 1, the calculated average crystallite sizes for the samples
for orthorhombic GdF and the standard data (JCPDS 27-0699)
3
for hexagonal β-NaGdF are also given for comparison. Ob-
4
viously, the calculated cell lattice constants are consistent with
the standard data, implying the high crystallinities. The main
reason for the formation of different phases is that NaBF can
4
slowly hydrolyze to produce HF and H BO in the aqueous
3
3
46,47
solution, as shown in eq 2.
In an acid environment (pH = 3),
this situation is not favorable for the release of F from the
reaction equilibrium. On the contrary, the molar amounts of
F and Na ions will gradually turn equal with the rising of the
pH value in the reaction system. In this case, Gd released from
prepared at the pH value of 3, 7, and 10 are 27.7 (GdF ), 19.3 and
3
ꢀ
21.3 nm (NaGdF ), respectively.
4
Figure 2 displays the SEM, TEM, HRTEM images, and EDS
ꢀ
þ
spectrum of the typical NaGdF MCs prepared with pH value of
4
3
þ
þ
3þ
3þ
7 and Cit /Gd molar ratio of 1:1 at 180 °C for 24 h. It can be
seen from Figure 2a that the NaGdF MCs exhibit uniform and
ꢀ
the complexes reacts with abundant F and Na to form the
4
crystal nuclei of NaGdF . As can be seen from the reaction,
corresponding H are produced, and the solution become
acidic, which is in good accordance with the change of pH value
in the Experimental Section. Further growth of the particle and
the self-assembly process will be discussed subsequently. The
possible reaction process for the formation of the NaGdF4
hierarchical oblate microspheres with a mean diameter of
600 nm. These MCs exhibit considerable monodispersity and
narrow size distribution. As can be seen from the high-magnifica-
tion SEM image (Figure 2b), the uniform oblate spheroids with
the thickness of about 150 nm are multilayer and self-assembled
with the unit-particles. The typical TEM image shows the
4
þ
Figure 2. Low-magnification FE-SEM image (a), high-magnification FE-SEM image (b), TEM image (c), EDS (d), and HRTEM image (e) of NaGdF
MCs (Cit /Gd = 1:1) prepared at pH value of 7 and 180 °C for 24 h.
4
3
ꢀ
3þ
4
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Inorganic Chemistry
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Figure 3. SEM images of NaGdF
hydrothermally obtained with Cit /Gd molar ratios of 1:1.
4
MCs prepared at 180 °C for 24 h and pH values of 3 (a, b), 7 (c, d), and 10 (e, f). All of the samples were
3
ꢀ
3þ
obvious spherical microstructure with a uniform size distribution
SEM images of NaGdF MCs synthesized at 180 °C for 24 h with
3
4
ꢀ
3þ
(Figure 2c). It should be noted that the fringe of oblate spheroid
various pH values, while keeping the Cit /Gd molar ratio of
is much thicker than the center (inset in Figure 2c), also
indicating the possible self-assembly process. From the EDS
analysis in Figure 2d, it confirms the presence of Na, Gd, and F
1/1. For the GdF sample prepared at pH value of 3 (Figure 3a,
3b), the self-assembled, hierarchical oblate spheroid structure is
3
discovered. Obviously, the dimension of the GdF MCs is larger
3
(Si from the Si substrate), and the atomic ratio of Na/Gd/F is 1/
than that of the NaGdF microcrystal (Figure 3c, 3d) which can
4
1
.007/4.113, which is in great agreement with the theoretical
be attributed to the character of orthorhombic phase of the GdF3
growth as well as the relative speed between the crystal growth
and self-assembly process. In fact, the differences of pH values
exhibit a significantly effect on the existent form and complexing
atomic Na/Gd/F ratio (1/1/4). A polycrystalline structure can
be clearly found from the HRTEM image (Figure 2e), and the
interplanar distance between adjacent lattice planes (marked by
the arrows) is about 0.52 nm, which corresponds well to the value
3
ꢀ
3þ
3ꢀ
ability of Cit to Gd . In acidic condition, the Cit would
þ
xꢀ3
of the [1010] planes of the hexagonal NaGdF phase.
partly combine H and exists as H Cit , directly decreasing its
4
.2. Influential Factors and Possible Formation Mechanism.
x
3
complexing ability with metal ions to a great degree. Further-
more, the self-assembly process can be promoted by the strong
In the following sections, the effects of pH values in the initial
3
ꢀ
48
solution, Cit content, and reaction time on the crystal struc-
tures and shapes of final products have been investigated in detail.
Moreover, the possible formation mechanism for β-NaGdF4
MCs with various morphologies is presented.
hydrogen-bond in acidic solution. So the speed of forming
GdF nuclei as well as the self-assembly process is higher than
3
that of crystal growth, resulting in the larger crystal dimension.
Furthermore, the inhomogeneous grain diameter of the product
prepared at pH value of 3 implies a lack of equilibrium and
equivalent speed of the self-assembly process, because of the
lower pH value. When prepared at pH = 10 (Figure 3e, 3f), the
Effect of pH Values. To investigate effect of pH values on the
morphology of NaGdF MCs, a series of contrast experiments
were conducted. Figure 3 shows the low and high-magnification
4
4
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Inorganic Chemistry
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3
ꢀ
3þ
Figure 4. SEM images of NaGdF MCs prepared with different Cit /Gd molar ratios. 0 (without sodium citrate) (a), 1:1 (b), 2:1 (c), 4:1 (d), and
4
8:1 (e). All of the samples were hydrothermally obtained at 180 °C for 24 h with the pH value of the initial solution kept at 7.
component monomers of the NaGdF MCs exhibit larger size
compared with the sample prepared at the pH value of 7
the mechanism for the change of crystal morphology grown with
50,51
4
and without sodium citrate has been discussed before,
and
(
Figure 3c, 3d, discussed previously). Owing to the strengthen-
the relative rates of growth along different crystallographic
3þ
3ꢀ
ing repulsive electrostatic interaction of the Gd ions with the
increasing pH value, the self-assembly process is restrained;
directions controlled by the Cit ions are always introduced
49
52,53
to explain the different outlook of the crystallite.
However,
thus, the components can grow further.
Effect of Sodium Citrate Content. The SEM images (Figure 4)
this mechanism has been only demonstrated in the systems with
a single process of crystal growth. In the present system, crystal
3ꢀ
provide the size and shapes of the NaGdF MCs synthesized with
growth and self-assembly coexist. So the effect of Cit ions on
the crystal morphology should be illustrated from both sides. In
4
3
ꢀ
3þ
different Cit /Gd molar ratio. All of the samples were
3
þ
3ꢀ
hydrothermally obtained at 180 °C for 24 h with the pH value
of the initial solution kept at 7. For β-NaGdF obtained without
the initial solution, a stable Gd ꢀCit complex was formed.
Then the hydrolysis of the complex produced colloidal sol under
4
sodium citrate (Figure 4a), the crystals tend to grow into
irregular structures and aggregate in the form of disorder.
However, once sodium citrate is introduced into the reaction
system, significant change takes place in the morphology of
crystals. Figure 4b shows the SEM image of the sample synthe-
sized with Cit /Gd molar ratio of 1/1. It can be seen that the
crystals were assembled with uniform components and exhibit
regular oblate spheroid morphology and considerable monodis-
persity. This indicates that the citrate anions have a significant
high temperature and pressure, where the NaGdF nuclei were
4
3
ꢀ
covered with Cit , and these molecules adsorbed on the surface
of the particles formed a protective layer to hold back the further
growth of particles. However, because of the anisotropy of the
β-phase NaGdF , the inhibitory action is heterogeneous to dif-
4
3
ꢀ
3þ
ferent planes and, consequently, the nearly oval structure (pH = 7)
as well as the laminar structure (pH = 10) were discovered. As for
3
ꢀ
self-assembly process, the Cit plays an important role in
providing the H-bonding and cross-linking between the NaGdF
particles. As its content rises (Figure 4c, 4d), the Cit ions acts
4
3
ꢀ
role in the formation of the β-NaGdF MCs structure. Although
4
4
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Inorganic Chemistry
ARTICLE
3
ꢀ
3þ
Figure 5. SEM images of NaGdF MCs (Cit /Gd = 1:1) synthesized at 180 °C for different reaction time. 2 h (a), 6 h (b), 12 h (c), and 24 h (d). All
4
of the samples were obtained with the initial pH value of 10.
Scheme 1. Schematic Illustration for the Formation Process of NaGdF MCs Prepared at Different Conditions
4
progressively more as the crystal growth depressant and cross-
linking role. The component dimension reveals a trend of
decrease, while the microcrystal structure shows further growth
perpendicular to the orbicular side tendency. By further addition
of the sodium citrate, the chelating effect of the Cit is further
enhanced. Besides, as a surfactant, the sodium citrate can also
largely lower the surface energy of the particles. Therefore, the
formation and growth of the initial nucleus is further reduced,
products become anomalously aggregated, which is probably due
3
ꢀ
to the complex effect of the excessive Cit . Although the exact
3
ꢀ
function of Cit on the formation of the NaGdF MCs in our
4
synthetic system is not yet fully understood, it is clearly that three
3ꢀ
3ꢀ
main roles of the Cit are responsible for the morphology. One
is the strong selective adsorption growth depressant effect of
3ꢀ
Cit on various planes of the particles. The other is the cross-
linked role between the particles with H-bonding. And the last is
the greatly lowered formation and surface energy, which promote
the self-assembly process.
and the self-assembly process of NaGdF crystals is relatively
4
promoted. Moreover, the surfaces of top/bottom have obvious
concave centers, which suggest the low surface energy and the
orientation of the self-assembly process. However, when the
Effect of the Reaction Time. Because the final microcrystal
structure of NaGdF is formed by small components, the self-
4
3
ꢀ
3þ
Cit /Gd molar ratio is increased to 8:1 (Figure 4e), the
assembly process was investigated with a prolonged reaction time
4
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Inorganic Chemistry
ARTICLE
with uniform diameter and considerable monodispersity are
obtained. According to the above process, the growth and
directed attachment of the NaGdF nanoparticles can be dis-
4
tinctly observed, which visually demonstrate the crystal growth
54,55
and self-assembly process.
On the basis of the above analysis, it can be inferred that,
3
ꢀ
besides inherent unit cell structures of nucleated seeds, the Cit
contents, pH values in the initial solution, and reaction times are
all important factors in the phase and morphology evolution of
the final products. Scheme 1 summarizes the possible shape
formation processes of different gadolinium related fluorides
under various experimental conditions.
3.3. Up-Conversion Luminescence Properties of the
Products. To investigate the UC luminescent properties of the
3
þ
3þ
β-NaGdF host, 17% Yb and 3% Ln (Ln = Er, Tm, Ho) were
4
3
þ
3þ
added to form co-doped β-NaGdF :Yb /Ln crystals. Under
4
9
80 nm IR laser excitation, the strong yellowish-green, whitish,
3
þ
3þ
and green luminescence can be observed in the Yb /Er
Yb /Tm , and Yb /Ho ion-pair co-doped β-NaGdF
MCs, respectively. As shown in Figure 6a for β-NaGdF :Yb
Er , the emission bands centered at 522, 541, and 655 nm can
be assigned to H f I , S f I , and F f I
transitions of Er , respectively.
coordinates for the emission spectra of β-NaGdF : Yb /Er
,
4
/
3
þ
3þ
3þ
3þ
3þ
4
3þ
2
4
4
4
4
4
1
1/2
15/2
3/2
36,56,57
15/2
9/2
15/2
3
þ
The corresponding CIE
3
þ
3þ
4
are determined as x = 0.3216, y = 0.5852, located in the green
3
þ
region (point a, Figure 7). In Figure 6b for β-NaGdF :Yb
/
4
3
þ
Tm , the four emission bands centered at 476, 646, 698, and
1
3
1
3
3
3
8
00 nm can be attributed to the G f H , G f F , F f H ,
4 6 4 4 3 6
3 3
3þ 58ꢀ60
and H f H transitions of Tm
.
The corresponding
CIE coordinates for the emission spectra are determined as x =
.3067, y = 0.2879, located in the white region (point b,
4
6
0
3þ
3þ
Figure 7). As for β-NaGdF :Yb /Ho in Figure 6c, predomi-
4
nantly green emission band centered at 542 nm is observed and
5
5
3þ 40,61
corresponding to the S f I transition of Ho
.
The
2
8
notably weaker red emission (relative to the green emission)
band discovered at 648 nm is ascribed to the transition from the
5
5
F f I . The corresponding CIE coordinates for the emission
5
8
spectra are determined as x = 0.3588, y = 0.5543, located in the
green region (point c, Figure 7).
The proposed up-conversion mechanism in the β-NaGdF4:
4
Figure 6. NIR-to-visible UC emission spectra of NaGdF :Yb/Er (a),
Yb/Ln MCs is demonstrated in Figure 8. In the β-NaGdF
NaGdF :Yb/Tm (b), and NaGdF :Yb/Ho (c) MCs under 980 nm laser
4
4
4
3
þ
3þ
3þ
crystal lattice, Er , Tm , and Ho do not have an energy level
resonant with 980 nm, which corresponds to the excitation
wavelength of the diode, suggesting that the up-conversion
excitation.
at the same pH value of 10 and hydrothermal temperature
3
þ
2
(
180 °C). Figure 5 shows the SEM images of the NaGdF
process is initiated by the first raising of Yb ion to the F
5/2
62
4
3
ꢀ
3þ
samples prepared with 1:1 Cit /Gd at different reaction time.
At t = 2 h, a typical SEM image (Figure 5a) reveals that the
sample consists of a mass of particles with a particle size of about
excited-state via an incoming pump photon. The subsequent
step involves an energy transfer (ET) process, by which the
3
þ 4
3þ 3
3þ 5
energy levels of Er
excited. Meanwhile, some of the excited ions relax rapidly to the
I11/2, Tm
H , and Ho
I6 were
5
1
60ꢀ180 nm and a small number of hierarchical laminar-
3þ 4
3þ 3
3þ 5
assembled spheroids. The rough surface and the evident bound-
aries of the spheroids indicated that the spheroids are composed
of many thin laminars, and these original laminars are self-
assembled into an integrated structure along the longitudinal
axis direction through face-to-face attachment. When the reac-
tion time is prolonged to 6 and 12 h, the hierarchical oblate
spheroids become the dominant morphology of the products, as
shown in Figure 5b and Figure 5c. Moreover, careful observation
indicates that the tiny components assembly increases gradually
in size with the reaction proceeding. Interestingly, in this period,
the components with grown size and sheet structure are
assembled along the radial direction of spheroids. Further
increasing the reaction time to 24 h, the oblate microcrystals
low-lying levels of the Er I13/2, Tm F4, and Ho I7 states.
Once these states are populated, a subsequent incoming pump
3
þ
photon or a second ET from a neighboring Yb ion may result
4
4
3
5
in the population of the F7 , F , F , F states of these ions.
/2
9/2
2
5
The population of these state may also occur via a nonradiative
2
4
1/2 3/2
3þ 3
3
decay to the low-lying H
S
of the Er , F and F of the
1
2
3
3
þ
5
3þ
Tm as well as I of the Ho . Therefore, the emissions from
4
these populated states to the ground states take place. Moreover,
3
þ
the Gd ions in the ground state cannot absorb 980-nm
photons or the energy in the populated states for the large
energy gap between the first excited state and the ground state.
Thus, superfluous energy consuming is forbidden, and the main
emission discussed above can be enhanced.
4
122
dx.doi.org/10.1021/ic200155q |Inorg. Chem. 2011, 50, 4116–4124

Inorganic Chemistry
ARTICLE
’
AUTHOR INFORMATION
Corresponding Author
*
E-mail: yangpiaoping@hrbeu.edu.cn.
’
ACKNOWLEDGMENT
This project is financially supported the National Natural
Science Foundation of China (NSFC 20871035), China Post-
doctoral Special Science Foundation (200808281), and Harbin
Sci.-Tech. Innovation Foundation (No. 2009RFQXG045).
’
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