Controlled hydrothermal growth and up-conversion emission of NaLnF 4 (Ln = Y, Dy-Yb)

Article abstract of DOI:10.1021/ic070220e

The controlled hydrothermal preparation of NaYF₄ as both cubic and hexagonal phase types with specific associated morphologies, nanospheres and microtubes, respectively, has been achieved in the absence of organic solvents. The hexagonal NaYF₄ compound can be prepared in novel microtubular form and directly co-doped with Yb³⁺/Er³⁺ ions. When excited by infrared light of 980 nm, these hexagonal NaYF₄ microtubes display strong green up-conversion emission, which was much more intense than that of cubic NaYF₄ or hexagonal NaYF₄ nanoparticles. Other related hexagonal-prismatic microtubes of NaLnF₄ (Ln = Dy-Yb) were also synthesized. A growth mechanism for the microtubes is proposed. In general, the diameter of the hexagonal NaLnF₄ microtubes is strongly dependent on the Ln³⁺ size and increases as the rare-earth ionic radius decreases.

Full text of DOI:10.1021/ic070220e

Inorg. Chem. 2007, 46, 5404−5410
Controlled Hydrothermal Growth and Up-Conversion Emission of
NaLnF4 (Ln Y, Dy Yb)
)
−
Jianle Zhuang,† Lifang Liang,† Herman H. Y. Sung,‡ Xianfeng Yang,† Mingmei Wu,*,† Ian D. Williams,‡
Shouhua Feng,§ and Qiang Su†
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of
Optoelectronic Materials and Technologies and School of Chemistry and Chemical Engineering,
Sun Yat-Sen (Zhongshan) UniVersity, Guangzhou 510275, P. R. China, Department of Chemistry,
Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, P. R. China, and State Key Laboratory of Inorganic Synthesis and PreparatiVe
Chemistry, Jilin UniVersity, Changchun 130023, P. R. China
Received February 5, 2007
The controlled hydrothermal preparation of NaYF4 as both cubic and hexagonal phase types with specific associated
morphologies, nanospheres and microtubes, respectively, has been achieved in the absence of organic solvents.
+
The hexagonal NaYF4 compound can be prepared in novel microtubular form and directly co-doped with Yb3+/Er3
ions. When excited by infrared light of 980 nm, these hexagonal NaYF4 microtubes display strong green up-
conversion emission, which was much more intense than that of cubic NaYF4 or hexagonal NaYF4 nanoparticles.
Other related hexagonal-prismatic microtubes of NaLnF4 (Ln
) Dy−Yb) were also synthesized. A growth mechanism
for the microtubes is proposed. In general, the diameter of the hexagonal NaLnF4 microtubes is strongly dependent
+
on the Ln3 size and increases as the rare-earth ionic radius decreases.
Introduction
that hexagonal NaYF4 is the most efficient UC host material
known to date4-7 and has great potential applications in solid-
state lasers, solar cells, and biolabels.3-13 Traditionally,
NaYF4 was prepared via solid-state reaction from stoichio-
metric mixtures of highly purified NaF and YF3 with other
Rare-earth ion-doped fluorides based on ALnF4 (A )
alkaline metal, Ln ) rare-earth element) have been inves-
tigated for several decades due to their electrical and optical
properties and wide applications as solid electrolytes, solid-
state lasers, and up- or down-conversion hosts.1-13 Among
them, most of the attention has been focused on NaYF4
because it is a particularly important host material for green
and blue up-conversion (UC) phosphors. It is well-established
(4) (a) Kramer, K. W.; Biner, D.; Frei, G.; Gudel, H. U.; Hehlen, M. P.;
Luthi, S. R. Chem. Mater. 2004, 16, 1244-1251. (b) Suyver, J. F.;
Grimm, J.; van Veen, M. K.; Biner, D.; Kramer, K. W.; Gudel, H. U.
J. Lumin. 2006, 117, 1-12. (c) Shalav, A.; Richards, B. S.; Trupke,
T.; Kramer, K. W.; Gudel, H. U. Appl. Phys. Lett. 2005, 86, 013505.
(d) Suyver, J. F.; Aebischer, A.; Biner, D.; Gerner, P.; Grimm, J.;
Heer, S.; Kramer, K. W.; Reinhard, C.; Gudel, H. U. Opt. Mater. 2005,
27, 1111-1130. (e) Suyver, J. F.; Grimm, J.; Kramer, K. W.; Gudel,
H. U. J. Lumin. 2005, 114, 53-59.
(5) Heer, S.; Koempe, K.; Guedel, H.-U.; Haase, M. AdV. Mater. 2004,
16, 2102-2105.
(6) Liang, L. F.; Wu, H.; Hu, H. L.; Wu, M. M.; Su, Q. J. Alloys Compd.
2004, 368, 94-100.
(7) (a) Wang, L.; Li, Y. Chem. Commun. 2006, 2557-2559. (b) Wang,
L. Y.; Li, Y. D. Nano Lett. 2006, 6, 1645-1649. (c) Zeng, J. H.; Su,
J.; Li, Z. H.; Yan, R. X.; Li, Y. D. AdV. Mater. 2005, 17, 2119-
2123.
(8) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Yue, G.; Yang, W. J.; Chen, D. P.;
Guo, L. H. Nano Lett. 2004, 4, 2191-2196.
(9) (a) Boyer, J.-C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am.
Chem. Soc. 2006, 128, 7444-7445. (b) Boyer, J.-C.; Cuccia, L. A.;
Capobianco, J. A. Nano Lett. 2007, 7, 847-852. (c) Yi, G. S.; Chow,
G. M. AdV. Funct. Mater. 2006, 16, 2324-2329.
(10) 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, 6426-6436.
* To whom correspondence should be addressed. E-mail:
ceswmm@mail.sysu.edu.cn.
† Sun Yat-Sen (Zhongshan) University.
‡ Hong Kong University of Science and Technology.
§ Jilin University.
(1) (a) Tsuboi, T.; Murayama, H.; Shimamura, K. J. Alloys Compd. 2006,
408, 776-779. (b) You, F. T.; Huang, S. H.; Liu, S. M.; Tao, Y. J.
Lumin. 2004, 110, 95-99.
(2) (a) Zakaria, D.; Mahiou, R.; Avignant, D.; Zahir, M. J. Alloys Compd.
1997, 257, 65-68. (b) Wegh, R. T.; Donker, H.; Oskam, K. D.;
Meijerink, A. Science 1999, 283, 663-666. (c) Zakaria, D.; Fournier,
M. T.; Mahiou, R.; Cousseins, J. C. J. Alloys Compd. 1992, 188, 250-
254. (d) Reddy, M. J.; Sreekanth, T.; Rao, U. V. S. Solid State Ionics
1999, 126, 55-63. (e) Wang, M.; Huang, Q. L.; Hong, J. M.; Wu,
W. H.; Yu, Z.; Chen, X. T.; Xue, Z. L. Solid State Commun. 2005,
136, 210-214.
(3) (a) Liu, F.; Ma, E.; Chen, D.; Yu, Y.; Wang, Y. J. Phys. Chem. B
2006, 110, 20843-20846. (b) Chong, K. J.; Hirai, T.; Kawai, T.;
Hashimoto, S.; Ohno, N. J. Lumin. 2007, 122, 149-151.
5404 Inorganic Chemistry, Vol. 46, No. 13, 2007
10.1021/ic070220e CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/31/2007

Hydrothermal Growth of NaLnF4 (Ln ) Y, Dy-Yb)
relevant rare-earth fluoride dopants. The reaction was carried
out in a sealed tube and/or under argon or even fluoride
atmospheres to ensure complete fluorination.14 Recently,
several soft chemistry approaches have been applied to the
synthesis of both cubic and hexagonal NaYF4 with different
morphologies with varying degrees of success.5,7-13 An early
example was the preparation by Haase et al. of colloids of
cubic lanthanide-doped NaYF4 nanocrystals using methanol
as the solvent.5 Solvothermal conditions were employed to
prepare nanorods and nanoparticles of hexagonal NaYF4.7
Hydrothermal synthesis was also carried out by Zeng et al.,7c
but it was stated that the size and morphology were not easily
controlled with poorly shaped resultant microcrystals. Al-
though hydrothermal syntheses of NaYF4 have been
reported,7c,11 detailed and systematic investigations on the
controlled growth and related UC emissions have not been
carried out. Wang reported the hydrothermal growth of
hexagonal NaYF4 as hard microprisms, but their UC emission
has not been reported.11a Well-defined and high-quality
hydrothermally synthesized hexagonal NaYF4 crystals and
their comparably enhanced UC emissions need to be
explored. The growth of high-quality hexagonal sodium rare-
earth fluoride nanocrystals has been achieved by using the
thermal decomposition of lanthanide trifluoroacetate precur-
sors over 300 °C in a mixed solvent of oleic acid/oleylamine/
1-octadecene; however, this approach has the drawback of
the production of toxic byproducts.9,10
oxide,17 nitride,18 chalcogenide,19 chloride,20 silicate,21 and
even organic examples.22 However, there are only limited
reports discussing hollow fluoride materials.23,24 Previously,
we briefly reported the hydrothermal synthesis of rare-earth
metal-based NaHoF4 microtubes and NaSmF4 nanotubes.24
The comparative aqueous solution growth behavior of the
sodium rare-earth tetrafluoride group has yet to be investi-
gated in depth. High-quality and well-defined tubular NaYF4
crystals with strong UC emission are of great importance in
materials science and engineering. Herein, we describe the
controlled growth of either cubic or hexagonal NaYF4
crystallites using mild hydrothermal conditions without any
organic cosolvent and have extended our previous work on
the syntheses for the related hexagonal NaLnF4 (Ln ) Sm-
Yb, Y) crystallites. A new tubular microstructure has been
discovered for hexagonal NaYF4. The optical properties of
its’ Yb3+/Er3+-co-doped phosphor have been investigated.
Syntheses for the related hexagonal NaLnF4 microtubes are
also described for the heavy rare earths (Ln ) Dy-Yb), in
which the radius of Ln3+ is generally smaller than that of
Y3+.
Experimental Section
Hydrothermal Synthesis. Aqueous solutions of sodium fluoride
(NaF, AR grade) and ammonium hydrogen fluoride (NH4HF2, AR
grade) with a molar ratio of 1.0/2.0 were mixed in a Teflon beaker
to form a clear solution A. Another mixed solution B of Ln(NO3)3
(Ln ) Y, La, Pr, Nd, Sm-Yb) and ethylenediamine tetraacetic
acid (EDTA) was prepared in a Teflon cup. A milky suspension C
was first formed when the clear solution A was added into solution
B under stirring. The molar ratio of the initial reactant mixture for
hydrothermal synthesis is 6.0 NH4HF2/3.0 NaF/1.0 Ln(NO)3/0.2
EDTA/700 H2O. The initial pH value was around 3.0. Suspension
C was 70% filled in a Teflon-lined stainless steel autoclave (23
mL, Parr type) and sealed for hydrothermal crystallization at
different temperatures and time periods. After the autoclave was
cooled, the final powder products were washed with distilled water
and dried in a desiccator at ambient temperature. Hexagonal NaYF4
nanoparticles were prepared using ethanol as the solvent with the
same mole ratio of initial reactants except with the absence of
EDTA.
The discovery of single-wall carbon nanotubes has stimu-
lated considerable interest in one-dimensional (1-D) struc-
tures with hollow interiors due to their unusual mechanical
properties, porosity, electronic transfer, optical emission, and
wide potential applications in mechanical engineering,
chemical catalysis, gas storage and separation, and opto-
electronic devices.15 To date, many materials with hollow
tubular structures have been synthesized, with elemental,16
(11) (a) Wang, Z. J.; Tao, F.; Yao, L. Z.; Cai, W. L.; Li, X. G. J. Cryst.
Growth 2006, 290, 296-300. (b) Yang, K. S.; Yu, C. Y.; Lu, L. P.;
Li, Y.; Ye, C. H.; Zhang, X. Y. J. Rare Earths 2006, 24, 757-760.
(c) Wang, D. W.; Huang, S. H.; You, F. T.; Qi, S. Q.; Fu, Y. B.;
Zhang, G. B.; Xu, J. H.; Huang, Y. J. Lumin. 2007, 122-123, 450-
452.
(12) (a) Wei, Y.; Lu, F. Q.; Zhang, X. R.; Chen, D. P. Chem. Mater. 2006,
18, 5733-5737. (b) Wei, Y.; Lu, F. Q.; Zhang, X. R.; Chen, D. P. J.
Alloys Compd. 2007, 427, 333-340. (c) Li, Z. Q.; Zhang, Y. Angew.
Chem. Int. Ed. 2006, 45, 7732-7735. (d) Wang, F.; Chatterjee, D.
K.; Li, Z. Q.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology
2006, 17, 5786-5791.
(13) Zeng, J. H.; Li, Z. H.; Su, J.; Wang, L. Y.; Yan, R. O.; Li, Y. D.
Nanotechnology 2006, 17, 3549-3555.
(14) (a) Thoma, R. E.; Hebert, G. M.; Insley, H.; Weaver, C. F. Inorg.
Chem. 1963, 2, 1005-1012. (b) Reddy, C. G.; Pandaraiah, N.; Reddy,
K. N. J. Mater. Sci. Lett. 1988, 7, 1225-1228. (c) Joubert, M. F.;
Linares, C.; Jacquier, B.; Cassanho, A.; Jenssen, H. P. J. Lumin. 1992,
51, 175-87.
(15) (a) Iijima. S. Nature 1991, 354, 56-58. (b) Tremel, W. Angew. Chem.
Int. Ed. 1999, 38, 2175-2179. (c) Ajayan, P. M. Chem. ReV. 1999,
99, 1787-1799. (d) Tenne, R. Angew. Chem. Int. Ed. 2003, 42, 5124-
5132. (e) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem.
ReV. 2006, 106, 1105-1136. (f) Xiong, Y. J.; Mayers, B. T.; Xia, Y.
N. Chem. Commun. 2005, 5013-5022. (g) Baughman, R. H.;
Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792.
(16) Mayers, B.; Xia, Y. N. AdV. Mater. 2002, 14, 279-282.
(17) (a) Yada, M.; Mihara, M.; Mouri, S.; Kuroki, M.; Kijima, T. AdV.
Mater. 2002, 14, 309-313. (b) Sun, Y.; Fuge, G. M.; Fox, N. A.;
Riley, D. J.; Ashfold, M. N. R. AdV. Mater. 2005, 17, 2477-2481.
(c) Tian, Z. R. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; Xu, H. F. J.
Am. Chem. Soc. 2003, 125, 12384-2385. (d) Chueh, Y. L.; Chou, L.
J.; Wang, Z. L. Angew. Chem. Int. Ed. 2006, 45, 7773-7778.
(18) (a) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen,
M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966-967. (b) Wu,
Q.; Hu, Z.; Wang, X. Z.; Lu, Y. N.; Chen, X.; Xu, H.; Chen, Y. J.
Am. Chem. Soc. 2003, 125, 10176-10177.
(19) (a) Remskar, M.; Mrzel, A.; Skraba, Z.; Jesih, A.; Ceh, M.; Demsar,
J.; Stadelmann, P.; Levy, F.; Mihailovic, D. Science 2001, 292, 479-
481. (b) Rosentsveig, R.; Margolin, A.; Feldman, Y.; Popovitz-Biro,
R.; Tenne, R. Chem. Mater. 2002, 14, 471-473.
(20) Hacohen, Y. R.; Grunbaum, E.; Tenne, R.; Sloan, J.; Hutchison, J. L.
Nature 1998, 395, 336-337.
(21) Wang, X.; Zhuang, J.; Chen, J.; Zhou, K. B.; Li, Y. D. Angew. Chem.
Int. Ed. 2004, 43, 2017-2020.
(22) Yan, D. Y.; Zhou, Y. F.; Hou, J. Science 2004, 303, 65-67.
(23) (a) Wang, X.; Li, Y. D. Angew. Chem. Int. Ed. 2003, 42, 3497-3500.
(b) Liang, X.; Wang, X.; Wang, L. Y.; Yan, R. X.; Peng, Q; Li, Y. D.
Eur. J. Inorg. Chem. 2006, 11, 2186-2191.
(24) Liang, L. F.; Xu, H. F.; Su, Q.; Konishi, H.; Jiang, Y. B.; Wu, M. M.;
Wang, Y. F.; Xia, D. Y. Inorg. Chem. 2004, 43, 1594-1596.
Inorganic Chemistry, Vol. 46, No. 13, 2007 5405

Zhuang et al.
Figure 1. PXRD patterns of NaYF4 synthesized (A) at 60 °C for (a) 3.0 h, (b) 24 h, and (c) 48 h; (B) at 140 °C for (a) 18 h, (b) 24 h, (c) 36 h, and (d)
48 h; (C) at 220 °C for (a) 1.0 h, (b) 3.0 h, and (c) 48 h.
Characterization. The products were characterized by using
powder X-ray diffraction (PXRD) with a Philips Model PW1830
diffractometer using Cu KR radiation (λ ) 0.1541 nm) and a
graphite monochromator operating at 40 kV and 30 mA with a 2θ
scanning rate of 0.05° s-1. The sizes and shapes of particles were
obtained by using a JEOL JSM-6330F field emission gun scanning
electron microscope. Dried samples for scanning electron micros-
copy (SEM) were placed on glass-slide surfaces and then sputter-
coated with platinum. Transmission electron microscopy (TEM)
and selected-area electron diffraction (SAED) analyses were
performed on a JEOL 2010 high-resolution transmission electron
microscope equipped with an Oxford Instrument EDS system.
Infrared-to-visible UC spectra of NaYF4:Yb3+/Er3+ were recorded
by exciting the sample with an unfocused beam from a 980 nm
diode laser. The laser power for excitation was fixed at 45 mW for
each sample for comparison of their emission intensities. The visible
emissions were collected and detected by a fluorescence spectro-
photometer F-4500 (Hitachi) equipped with a mechanically ruled
concave diffraction grating and a R3788 photomultiplier tube.
Results and Discussion
Phase-Selective Hydrothermal Preparation of NaYF4.
Both cubic and hexagonal forms of NaYF4 can be readily
formed phase-pure under appropriate conditions. The PXRD
patterns of NaYF4 products synthesized at 60, 140, and 220
°C for periods between 1 and 48 h are shown in Figure 1.
Subhydrothermally at 60 °C, cubic NaYF4 with fluorite-
related structure (JCPDF No. 77-2042) remains as the phase-
pure product for reactions even up to 48 h (Figure 1A).
Hydrothermal reaction at 140 °C for shorter time periods
also gives cubic NaYF4, but after 48 h a mixture of cubic
and hexagonal phases is observed (Figure 1B). At 220 °C,
the cubic form is found after 1 h but is replaced by phase-
pure hexagonal NaYF4 after only 3 h of reaction time. The
above results indicate that there is a “phase transformation”
from cubic to hexagonal NaYF4 under hydrothermal condi-
tions and that the cubic phase is a kinetic product. This is
consistent with previous data that indicates that the cubic
phase is thermally meta-stable up to 691 °C with respect to
the hexagonal one.14a,25 Hydrothermal synthesis with suf-
ficiently high temperature or aging time is thus a simple,
fast, and economic way to synthesize the phase-pure
hexagonal form of NaYF4.
Figure 2. SEM images of NaYF4 hydrothermally synthesized in the
presence of EDTA at 140 °C for (a) 15 h, (b) 24 h, (c) 48 h, (d) 72 h, at
220 °C for (e) 48 h, and (f) in the absence of EDTA at 220 °C for 48 h;
and SEM images of (g) NaErF4 and (h) NaYbF4 hydrothermally synthesized
in the presence of EDTA at 220 °C for 48 h.
Hexagonal-Prismatic Microtubes and Morphological
Changes of NaYF4. Morphological characterization of the
hydrothermally prepared NaYF4 products was carried out by
SEM, and selected results are shown in Figure 2. The cubic
NaYF4 form is found as nanospheres with an average size
of around 200 nm, and this morphology persists no matter
what the aging conditions. An example is shown in Figure
2a. The hexagonal NaYF4 is found as hollow hexagonal-
prismatic microtubes both at 140 and 220 °C (Figure 2b-
f). The outer diameters of these tubes on average are around
3.0 µm, and their lengths are in the range of 10-30 µm.
The use of the EDTA additive can lead these microtubes
to have better defined hexagonal tubular shape (Figure 2c-
e), due to its complexing to rare-earth ions and, consequently,
its modulation of the crystal growth. In the absence of EDTA,
each tube of these complex fluorides became less defined
(25) Thoma, R. E.; Insley, H.; Hebert, G. M. Inorg. Chem. 1966, 5, 1222-
1229.
5406 Inorganic Chemistry, Vol. 46, No. 13, 2007

Hydrothermal Growth of NaLnF4 (Ln ) Y, Dy-Yb)
in shape and some crevices appeared along the growth
directions as indicated in Figure 2f.
The crystalline structures of both nanospheres and micro-
tubes were further characterized by electron microscopy
(Supporting Information, Figure 1). The bright-field TEM
images show that the cubic NaYF4 product consisted of
granular particles (Supporting Information, Figure 1a,b). The
high-resolution TEM (HRTEM) image with the lattice fringes
of 0.19 nm corresponding to (220) lattice planes of cubic
NaYF4 (Supporting Information, Figure 1c) and the SAED
patters taken from different zone axes (Supporting Informa-
tion, Figure 1d-f) indicate that these nanospheres are
attributed to cubic NaYF4. Whereas the HRTEM image with
the lattice fringes of 0.51 nm (Supporting Information, Figure
1h) corresponding to (100) lattice planes of hexagonal NaYF4
and the related SAED pattern (Supporting Information,
Figure 1i) reveals the growth of the hexagonal NaYF4 tube
is along the c direction. Thus, it can be confirmed that the
spherical crystals belong to cubic NaYF4 and 1-D tubular
microcrystals belong to hexagonal NaYF4, even in a mixed
phase (Supporting Information, Figure 1g,h). The diffraction
results indicated that no mixed cubic-hexagonal crystalline
zones were ever co-present. This implies that cubic NaYF4
crystals with spherical shape are formed at the initial stage
of the hydrothermal synthesis. Further hydrothermal aging
then causes these spherical NaYF4 crystals to dissolve and
then reprecipitate separately as pure hexagonal microtubular
NaYF4.
Figure 3. UC emission spectra of NaYF4:Yb3+/Er3+ excited by a 980 nm
laser: (a) cubic NaY0.94Yb0.05Er0.01F4, (b) hexagonal NaY0.94Yb0.05Er0.01F4,
(c) hexagonal NaY0.79Yb0.20Er0.01F4, (d) hexagonal NaY0.78Yb0.20Er0.02F4,
and (e) hexagonal NaY0.48Yb0.50Er0.02F4.
for the hexagonal form compared with the cubic form
measured under conditions of identical laser power density.
The variable increase of the UC efficiency of hexagonal as
compared with the cubic phase found in various publications
may result from the different growth conditions and crystal
sizes.
The change of doping concentrations of an activator and/
or a sensitizer can modulate the emission intensity of each
color. At low concentrations of Er3+, we find that the green
emission is dominant; however, the red emission becomes
increasingly significant as concentrations of Er3+ and/or Yb3+
go up (Figure 3b-e). Much higher dopant concentrations
result in much weaker green emission due to concentration
quenching. The optimal composition for green emission from
our hydrothermal-synthesized hexagonal-NaYF4 host is
NaY0.79-0.78Yb0.20Er0.01-0.02F4 (Figure 3). This result is quali-
tatively similar to those of some other reports.5,7c Importantly,
our luminescent results confirm that an efficient co-doping
of Yb3+ and Er3+ into the NaYF4 host can be carried out by
using a facile hydrothermal route.
Recently, hexagonal NaYF4 nanoparticles were fabricated
by Zeng et al. using ethanol as the solvent.7c We similarly
prepared the hexagonal NaYF4:Yb3+/Er3+ nanoparticles
(Supporting Information, Figure 2) to compare their UC
emission intensity to our hexagonal microtubes obtained from
hydrothermal synthesis. The UC emission of these hexagonal
NaYF4 nanoparticles is shown in Figure 4 and is about 4
times weaker than that of similarly doped microtubes. The
result is in contrast with the previous work,7c which indicated
the UC emission intensity of the nanoparticles is just a little
weaker than that of those synthesized hydrothermally. The
marked decrease of the intensity may be due to surface-
quenching nanosize effect. Further studies into the differing
luminescent properties of these nanoparticulate and micro-
tubular fluorides are under investigation.
Infrared-to-Visible UC of Hydrothermally Synthesized
NaYF4:Yb3+/Er3+ Microtubes. Besides the facile phase-
pure preparation of both cubic and hexagonal NaYF4
mentioned above, the hydrothermal method also allows the
ready introduction of other rare-earth dopants to act as
activators or sensitizers for luminescent applications. Thus,
various co-additions of Yb3+ and Er3+ into both cubic and
hexagonal NaYF4 have been successfully accomplished,
through keeping the whole molar ratio of Ln(NO3)3 at 1.00,
where Ln ) Y + Yb + Er.
The mechanism for frequency up-conversion (UC) of
NaYF4:Yb3+/Er3+ has been well established.4-7 The optimum
wavelength of pumping light for UC is 980 nm, which
2
2
matches the F7/2 f F5/2 transition of the Yb3+ ion. The
2
excited state F5/2 of the Yb3+ ion may then transfer the
energy to an Er3+ ion and populate the I11/2 level. Further
4
energy transfer from Yb3+ from a second photon absorption
can then further excite the Er3+ ion from its I11/2 level to
4
4F7/2. Nonradiative decays from Er3+ 4F7/2 to the H11/2 and
2
4S3/2 levels are then believed to occur. Subsequent transitions
from H11/2 and S3/2 to the Er3+ ground state I15/2 yield
bright-green emissions at 545 and 525 nm, respectively,
whereas the red emission around 660 nm is from the Er3+
2
4
4
4
4
electronic transition F9/2 f I15/2 (Figure 3).
Under the pumping of a 980 nm diode laser at 45 mW,
Yb3+ and Er3+ co-doped hexagonal NaYF4 exhibits consider-
ably stronger emission than a similarly co-doped cubic
NaYF4 (Figure 3). The UC emission previously reported for
the hexagonal phase is 5-10 times that of the corresponding
cubic NaYF4.4a In our work we find a ca. 20 times increase
Interestingly, although the addition of EDTA in the
hydrothermal reaction was found to lead to more regular
Inorganic Chemistry, Vol. 46, No. 13, 2007 5407

Zhuang et al.
Figure 4. UC emission spectra of NaYF4:Yb3+/Er3+ excited by a 980 nm
laser: (a) hexagonal nanoparticles synthesized using ethanol as the solvent,
(b) hexagonal microtubes hydrothermally synthesized in the presence of
EDTA, and (c) hexagonal microtubes hydrothermally synthesized in the
absence of EDTA.
Figure 5. PXRD patterns of hydrothermal products in the system of NaF-
LnF3 (Ln ) (a) Pr, (b) Nd, (c) Sm, (d) Eu, (e) Gd, (f) Tb, (g) Dy, and (h)
Ho) at 140 °C for 14 h.
An advantage of using middle lanthanides in the hydro-
thermal synthesis is that phase-pure hexagonal NaLnF4 (Ln
) Sm-Ho) can be prepared relatively easily under mild
conditions (Figure 5c-h), whereas NaYF4 requires more
forcing conditions as described above (Figure 1). This trend
is similar to the work by Zhang and Yan10 but contrasts that
of Li.13 Phase-pure hexagonal NaYF4 can be prepared at a
higher temperature and/or for a longer time, such as at 220
°C for 3.0 h (Figure 1). However, phase-pure hexagonal
NaLnF4 of smaller lanthanides (Ln ) Er, Tm, Yb) cannot
be successfully synthesized with similar condition (Figure
6b). From these PXRD patterns in Figures 1, 5, and 6, it
can be interpreted that there are common properties among
NaHoF4, NaYF4, and NaErF4. Compared with the diffraction
peaks of the cubic 111, the peaks of the hexagonal 110 tend
to be weaker from NaErF4 to NaYbF4 in Figure 6b. This
experimental evidence indicates that a higher temperature
and/or a longer reaction time is required to prepare phase-
pure hexagonal NaLnF4 with a smaller Ln3+ ionic radius.
That is, the Arrhenius activation energy (Ea) for the phase-
change rate from cubic to hexagonal forms increases.10 This
result is similar to that of ref 10, in which shows that the
growth of a hexagonal NaLnF4 phase is strongly dependent
on the Ln3+ size. However, hexagonal NaLnF4 (Ln ) Dy-
Yb, Y) tends to form tubular crystals in our result but
platelike crystals in ref 10.
The diameters of the NaErF4 and NaYF4 microtubes cannot
be easily distinguished from each other in Figure 2, but they
are larger than those of NaHoF4 illustrated in the Supporting
Information, Figure 3a,b. The diameters of NaHoF4 micro-
tubes (Supporting Information, Figure 3a,b) are generally
greater than those of NaDyF4 (Supporting Information, Figure
4d). Solid nanorods rather than hollow tubules are found for
NaLnF4 (Ln ) Eu, Gd, Tb) systems (Supporting Information,
Figure 4a-c). The nanorods are often jointed with X-shaped
or dendritic nanostructures. We conclude by stating that the
diameters of the 1-D crystals are inversely correlated with
the Ln3+ ionic radii. In general, the microtube diameter
morphology of the hexagonal microtubes, the effect on UC
efficiencies was a substantial decrease in UC emission
(Figure 4). The grown crystals might adsorb some organic
molecules and/or ions on the surface, and hence, their UC
emission intensities are decreased due to faster nonradiative
decay. We find that hexagonal NaYF4 microtubes grown in
the absence of EDTA display more intense UC emission than
those grown in the presence of EDTA, indicating our
synthetic route could make much more efficient hexagonal
NaYF4-based UC crystals.
Related Sodium Rare-Earth Tetrafluorides. While
hexagonal NaLnF4 (Ln ) La-Lu, Y) compounds have been
prepared for all lanthanides by solid-state reactions,14a,25 with
our hydrothermal method, hexagonal NaLnF4 could not be
prepared for the early lanthanide ions in the system of NaF-
LnF3 (Ln ) La-Nd), with only hexagonal LnF3 obtained
instead (Figure 5a,b). Hexagonal NaLnF4 has been synthe-
sized hydrothermally in the present work with Ln ) Sm-
Yb (Figures 5c-h and 6c,d). The failed hydrothermal
synthesis of NaLnF4 (Ln ) La-Nd) may be due to the
relative thermodynamic stability of the hexagonal LnF3 (Ln
) La-Nd), while LnF3 (Ln ) Sm-Yb) occurs in the
orthorhombic form.25 Some reports of the synthesis of early
NaLnF4 phases have appeared; Mai et al. reported that
NaPrF4 and NaNdF4 nanocrystals can be prepared in organic
solvent at much higher temperatures from toxic precursors.10
In addition, preparation of NaCeF4 was previously claimed
from “basic” organic solvent, but no detailed reactant receipts
and synthesis procedures were described and we have been
unsuccessful in duplicating these results.13
PXRD patterns of hexagonal NaLnF4 (Ln ) Sm-Yb)
show almost identical features, indicating that these complex
fluorides are isostructural. The general shift of diffraction
peaks to high 2θ angle from NaSmF4 to NaHoF4 in Figure
5c-h and from NaErF4 to NaYbF4 in Figure 6c,d is due to
the smaller lanthanide ionic radius as the series is traversed.
5408 Inorganic Chemistry, Vol. 46, No. 13, 2007

Hydrothermal Growth of NaLnF4 (Ln ) Y, Dy-Yb)
Figure 6. PXRD patterns of (A) NaErF3, (B) NaTmF4, and (C) NaYbF4 synthesized at 220 °C for various durations: (a) 1.0 h, (b) 3.0 h, (c) 12 h, and (d)
48 h.
increases as the ionic radii of the rare-earth atoms decrease.
These 1-D tubular microstructures and X-shaped nanostruc-
tures are destroyed eventually if these hydrothermal reactions
are performed at much higher temperatures and/or for much
longer times (typical results illustrated in Supporting Infor-
mation Figure 5). In these SEM images, some grains
attributed to hexagonal phases are found. This is due to the
known Ostwald ripening process.
thermal aging results in a pyramidal-end-capped microtube
(Figure 2d,g, arrows). The further hydrothermal aging,
especially performed at higher temperatures, certainly de-
stroys these 1-D and/or hollow structures because of their
intrinsic morphological metastability (Supporting Informa-
tion, Figure 5).
Conclusions
Growth Mechanism. HRTEM and SAED techniques do
not indicate the presence of NaLnF4 grains composed of both
cubic and hexagonal structures. This suggests that the “phase
transformation” is not based on an epitaxial or topotaxial
growth from cubic phase to the hexagonal phase. Rather,
the hydrothermal phase transformation is based on the
dissolution of solid (S) crystals of cubic NaYF4, the mass
transfer in liquid (L) solution, and the nucleation and growth
of solid crystals of hexagonal phase via a SLS (solid-
liquid-solid) or a dissolution-recrystallization process;
where the crystal’s size of cubic NaYF4 is smaller than a
critical size, the cubic crystal then dissolves quickly. The
sudden increase of the growth species concentrations in
aqueous solution from the quick dissolution of cubic NaYF4
promotes hexagonal NaYF4 to grow quickly at the initial
stage (Figure 2). The further redissolution of spherical grains
of cubic NaYF4 still serves as the component source for the
maintenance of a high concentration for the further growth
of the hexagonal phase with a 1-D tubular microstructure
along the c axis during hydrothermal aging.
In this paper, we have described a new group of well-
defined prismatic microtubes based on multicomponent
fluorides, typically, sodium yttrium fluoride (NaYF4) by a
simple hydrothermal processing from NaF, NH4HF2, and
lanthanide nitrate. Cubic NaYF4 with fluorite-related struc-
ture, the metastable phase below 691 °C, is formed as the
kinetic product but is replaced by thermodynamically stable
hexagonal NaYF4 under hydrothermal conditions. Higher
aging temperatures and/or longer aging times can promote
this hydrothermal “phase transformation”. Over a wide range
of reaction temperatures and times, the hydrothermal route
can successfully lead to an easily controlled synthesis of both
cubic-phase nanospheres and hexagonal-phase microtubes of
NaYF4. The crystal morphology of NaYF4 can be designed
through consequent phase control via such wet chemistry.
Rare-earth ions such as Yb3+ and Er3+ acting as sensitizers
and activators can be directly doped into both cubic and
hexagonal NaYF4 hosts through hydrothermal synthesis.
These doped materials retain their morphologies related to
their respective hosts. The hexagonal NaYF4:Yb3+/Er3+
microtubes display considerably stronger UC emission than
cubic NaYF4:Yb3+/Er3+ nanospheres and hexagonal NaYF4:
Yb3+/Er3+ nanoparticles. The addition of EDTA decreases
the UC emission, though it makes the perfect hexagonal
form.
In Supporting Information Figure 3b, a neck is found, as
pointed out by a black arrow. The connections from which
the nanorods of NaEuF4, NdGdF4, and NaTbF4 grew from
and yielded X-shaped crystals were clearly observed using
TEM (Supporting Information, Figure 4 (black arrows)). A
bowtie-like crystal was also observed for NaDyF4 (Support-
ing Information, Figure 4d). These findings lead us to
propose that the preferential growth at the circumferential
edges of these 1-D crystals (Figure 2b-h and Supporting
Information Figure 3) facilitates the growth of hollow
interiors in a similar way to the elongated growth of tellurium
nanotubes along the +c and -c directions.16 With the
disappearance of spherical crystals, the growth species
concentration decreases and further growth is restrained. By
the maintenance of solution saturation, the further hydro-
Hydrothermal synthesis of the related sodium rare-earth
fluorides is crucially dependent on the ionic radius of the
rare-earth ions. In addition to NaYF4, tubular NaLnF4 (Ln
) Dy-Yb) can be also synthesized with hollow interiors.
These prismatic hollow structures are synthesized from safe
and simple inorganic reactants. They can be doped with rare-
earth ions hydrothermally and might be promising high-
efficiency luminescence materials. As these complex fluo-
rides generally also possess excellent fluorine-ionic conduc-
tivities, these 1-D microtubes may find wide applications as
Inorganic Chemistry, Vol. 46, No. 13, 2007 5409

Zhuang et al.
microprobes in galvanic chemical sensors in which the
hollow interiors can be designed as either anodes or cathodes.
Chemistry Division of the Argonne National Laboratory for
their helpful discussions and measurements in UC.
Acknowledgment. This work is supported by the NSF
of China (No. 20571087), the NSF of Guangdong (No.
5003273) for the preparation and luminescence properties
of NaYF4, and the State Key Laboratory of Inorganic
Synthesis and Preparative Chemistry in Jilin University for
rare-earth fluorides. We are indebted to Dr. Xiaoxia Zhang
and Prof. Kok Wai Cheah in the Center for Advanced
Luminescent Materials, Department of Physics, Hong Kong
Baptist University, as well as Dr. Guokui Liu in the
Note Added after ASAP Publication. This article was
posted ASAP on May 31, 2007. Revised Supporting Infor-
mation was added and the correct version was posted on
June 7, 2007.
Supporting Information Available: Additional figures in a
PDF file. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC070220E
5410 Inorganic Chemistry, Vol. 46, No. 13, 2007