Facile preparation of quantum cutting GdF3:Eu3+ nanoparticles from ionic liquids

Article abstract of DOI:10.1039/b919732j

Microwave reaction of Ln(OAc)₃·xH₂O, Ln = Gd, Eu; OAc = acetate) with and in the ionic liquid [C₄mim][BF ₄] (C₄mim = 1-butyl-3-methylimidazolium) allows the fast and efficient synthesis of small, uniform, oxygen-free lanthanide nanofluorides with excellent photophysical behaviour. For GdF₃:Eu³⁺ nanoparticles a quantum efficiency of up to 145% was determined.

Full text of DOI:10.1039/b919732j

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Facile preparation of quantum cutting GdF3 : Eu3+ nanoparticles
from ionic liquidsw
Chantal Lorbeer,a Joanna Cybinskaab and Anja-Verena Mudring*a
Received (in Cambridge, UK) 22nd September 2009, Accepted 18th November 2009
First published as an Advance Article on the web 10th December 2009
DOI: 10.1039/b919732j
Microwave reaction of Ln(OAc)3ÁxH2O, Ln = Gd, Eu; OAc =
acetate) with and in the ionic liquid [C4mim][BF4] (C4mim =
1-butyl-3-methylimidazolium) allows the fast and efficient
synthesis of small, uniform, oxygen-free lanthanide nanofluorides
with excellent photophysical behaviour. For GdF3 : Eu3+ nano-
particles a quantum efficiency of up to 145% was determined.
CFLs competitive, a quantum yield of the used phosphors of
larger than 100% needs to be achieved. The conversion of a
UV or VUV (vacuum ultraviolet) photon into more than one
photon of lower energy was first reported in 1974 for YF3 : Pr3
and YF3 : Tm.4 LiGdF4 : Eu and GdF3 : Eu were the first
materials that yielded a high photon output in the visible
region of light.5 In order to use such compounds in CFLs the
materials need not only to have an excellent photophysical
performance, it is also desired that they have a uniform
and small particle size in order to minimize stray light.
Unfortunately, the preparation of lanthanide fluoride nano-
particles is not an easy task and only a few reports have
appeared in the literature so far. Well-established methods
for the synthesis of lanthanide fluoride nanoparticles are
hydrothermal,6 sonochemistry-assisted,7 microemulsion8 and
solution-phase9 synthesis routes. However, to the best of our
knowledge, oxygen-free nanoparticles with a size below 8 nm
have not been obtained using these time-consuming methods.
Indeed, special care has to be taken to avoid the formation of
oxides and oxide-fluorides because even oxygen impurities can
lead to nonradiative relaxation via europium–oxygen charge
transfer states followed by emission from these levels. Here we
report a fast and efficient synthesis route to oxygen-free, single
phase doped lanthanide nanofluorides with good quantum
cutting behaviour by microwave treatment of simple lantha-
nide precursors (Ln(OAc)3ÁxH2O, OAc = acetate) in the
task-specific ionic liquid [C4mim][BF4] (C4mim = 1-butyl-3-
methylimidazolium). The optical properties of the obtained
GdF4 : Eu3+ nanofluorides with different active ion concen-
trations (0.5, 5, 10 mol% of Eu3+) were investigated with
respect to their potential as quantum cutting phosphors as well
as the physicooptical properties of pure EuF3.
Energy-efficient lighting is a prevailing topic. It is estimated
that remarkable amounts electricity can be saved when more
energy-efficient light sources are used.1 Various countries
have already or are currently planning to ban conventional
incandescent lamps,2 which suffer from a poor luminous
efficiency since most of the energy is converted to heat, and
have them replaced by more environmentally benign light
sources such as compact fluorescent lamps (CFLs) or light
emitting diodes (LEDs). Although the use of LEDs seems to
be the most environmentally benign way of lightning, white
emitting LEDs still suffer from a poor color rendering index
(CRI) and a comparatively low luminous intensity (especially
when a good CRI is attempted). At the moment they cannot
completely replace conventional light bulbs in many applications
and CFLs are currently the most prevalent replacements. In
commercially available CFLs a mercury discharge is used to
produce UV (ultraviolet) light (lmax = 254 nm) which is
converted by phosphors on the inner side of the light tube to
visible light. The use of mercury has several major drawbacks.
The most important ones being the environmental safety and
health issues that have to be considered during manufacturing
and at the end-of-life of such fluorescent tubes. Many
consumers today are unaware of the potential risks of CFLs.
Furthermore, mercury based fluorescent light tubes need
about 1–3 min of start-up time until they reach their full light
power as mercury in the tube has to be heated and evaporated
first. Thus, they are not suited for use in combination with
motion sensors. An alternative is the replacement of mercury
by noble gases such as xenon in CFLs. The Xe dimer discharge
results in a broad UV emission with a maximum at 172 nm.
However, the discharge efficiency is lower compared to that of
mercury. In addition, conversion of the yet shorter wavelength
radiation to visible light results in an even larger energy loss
compared to Hg based CFLs. In order to make xenon-based
The concept of task-specific ionic liquids (TSILs), hence,
room temperature molten salts designed to fulfil a certain
application, was developed some time ago and has found its
application mainly in separations (especially CO2 capture),
organic synthesis and catalysis.10 However, the concept has
been neglected for applications in materials synthesis although
the advantages of ionic liquids in this area11 have been
realized. Here we employ [C4mim][BF4] as a task-specific ionic
liquid. During synthesis [C4mim][BF4] acts both as a reaction
medium and reaction partner. It is well-known that thermal
decomposition as well as hydrolysis of [BF4]-containing ionic
liquids leads to the release of fluoride. This circumstance has
turned out to be a nuisance for organic synthesis and
catalysis12 but becomes extremely helpful for the microwave
synthesis of fluoridic nanomaterials. As a heating source a
single mode microwave (CEM, Discover. USA) operating at a
frequency of 2455 MHz was used. One of the key advantages
a Anorganische Chemie I-Festkorperchemie und Materialien,
¨
Ruhr-Universitat Bochum, D-44780 Bochum, Germany.
¨
Web: www.anjamudring.de
b Faculty of Chemistry, University of Wroc!aw, Joliot-Curie 14,
PL-50383 Wroc!aw, Poland
w Electronic supplementary information (ESI) available: Experimental
details, characterization and physicochemical properties. See DOI:
10.1039/b919732j
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Fig. 1 TEM micrographs of representative samples of GdF3 : Eu3+
with 5% (left) and 10% (right) doping.
of ILs in microwave synthesis is the presence of large ions with
high polarizability and conductivity which renders them highly
susceptible to microwaves, leading to large heating rates which
results in a high formation rate of nuclei favouring nano-
particle formation.13 Fig. 1 shows TEM (transmission electron
microscopy) photos of typical GdF3 : Eu3+ samples. Analysis
of the particle size distribution yields a range between 2 and
9.5 nm and an average particle size of about 6 nm.
Fig. 3 Emission spectra of EuF3 and GdF3 : Eu3+ 0.5% excited at
393 nm at 77 K. For other doping concentrations see ESI.w
(5D0 - 7F1 transition) could be fitted with a single exponential
function. For EuF3 the decay times are strongly temperature
dependent, they are equal to 0.7 and 1.4 ms at RT and 77 K,
respectively. The decay times for the diluted samples increased
dramatically (B10 ms) and became nearly temperature inde-
pendent (see ESIw).
The excitation spectra of GdF3 : Eu3+ with 0.5, 5, 10 mol%
europium and of pure EuF3 were measured at room and liquid
nitrogen temperature (Fig. 2 and ESIw). Monitoring the
7
emission at 592 nm (5D0 - F1 transition of Eu3+ ion) leads
to a series of narrow absorption lines related to the f–f
transitions of Eu3+ in the case of EuF3 and features char-
In GdF3 : Eu3+ downconversion occurs upon excitation at
the 6GJ levels of a Gd3+ ion and two subsequent energy
transfer processes: first cross-relaxation Gd3+ 6GJ, Eu3+ 7F1 -
Gd3+ 6PJ, Eu3+ 5D0 (thermally activated) yields one photon
acteristic for Gd3+ and Eu3+ for GdF3 : Eu3+
.
The emission spectra of typical samples (Fig. 3) recorded at
room and low temperature under direct excitation of the
5
7
from the D0 - FJ transition and secondly the Gd3+ ion in
the 6PJ state transfers the remaining excitation energy to
a high-energy state of another Eu3+ ion through migration
via the Gd3+ sublattice. After fast nonradiative relaxation
5
7F0 - L6 transition of the Eu3+ ion (lex = 393 nm) show
transitions from the 5D0 excited state to the 7FJ (J = 1, 2, 3, 4)
ground states of the Eu3+ ion.
The point symmetry of the Eu3+ crystal sites estimated by
from this high excited state to the DJ levels, a second visible
5
5
7
5
7
the relative intensity of the magnetic dipole D0 - F1 and
photon is emitted due to the D0,1,2,3 - FJ transition with a
normal branching ratio (for the energy scheme see Fig. 4).3
Consequently, a substantial increase in the relative intensity of
5
7
electric dipole D0 - F2 transitions is in agreement with the
YF3 type of structure of EuF3 (and GdF3).14 In addition to
pure EuF3, the profiles of the fluorescence spectra of GdF3
doped with Eu3+ also show emission transitions from higher
localized excited states, the most intensive corresponding to
5D1 - 7F2 ones at 554 nm. In neat EuF3 these transitions are
quenched by cross-relaxation and ion–ion interaction. The
decay times of all studied nanocrystals were measured at
low and room temperature using two different excitation
5
the D0 emission is expected if quantum cutting through this
two-step energy transfer occurs. When exiting into the 6PJ, 6IJ
or 6DJ levels, a single energy transfer step to Eu3+ occurs and
the normal branching ratio for the Eu3+ 5DJ emission is
observed. Thus, the occurrence of quantum cutting can
be confirmed by comparing the emission spectra for
excitation into the 6GJ levels (B200 nm) and the 6PJ levels
5
energies (393 and 275 nm). The decay curve for the D0 level
(B305 nm) of Gd3+
.
Fig. 2 Excitation spectra of EuF3 (a) and GdF3 : Eu3+ 5% (b),
Fig. 4 Experimentally determined energy scheme of GdF3 : Eu (a)
emission recorded at 592 nm.
and downconversion pathways in GdF3 : Eu (b).
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Notes and references
1 A. Zukauskas, M. S. Shur and R. Caska, Introduction to
Solid-State Lighting, John Wiley & Sons, New York, 2002.
2 http://ec.europa.eu/energy/efficiency/ecodesign/doc/committee/2008_
12_08_technical_briefing_household_lamps.pdf; http://eur-lex.europa.
eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:076:0003:0016:DE:PDF;
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:
076:0017:0044:DE:PDF; http://www.stuff.co.nz/national/764487;
http://www.environ.ie/en/Environment/Atmosphere/ClimateChange/
News/MainBody,16444,en.htm; http://europa.eu/rapid/pressReleases
Action.do?reference=IP/08/1909;
http://www.telegraph.co.uk/
news/worldnews/europe/3174452/Traditional-lightbulbs-banned-by-
EU.html; http://www.energystar.gov/index.cfm?c=cfls.pr_cfls (last
accessed Nov. 10th 2009).
3 J. L. Sommerdijk, A. Bril and A. W. de Jager, J. Lumin., 1974, 8,
341; W. W. Piper, J. A. DeLuca and F. S. Ham, J. Lumin., 1974, 8,
344.
Fig. 5 Emission spectra of GdF3 : Eu3+ 10% and 5% nanocrystals
upon excitation into the 6PJ and 6GJ Gd3+ levels recorded at RT. The
4 R. Pappalardo, J. Lumin., 1976, 14, 159.
5
7
spectra were normalized for D1 - FJ emission.
5 R. T. Wegh, H. Donker, U. D. Oskam and A. Meijerink, Science,
1999, 283, 663; R. T. Wegh, H. Donker, U. D. Oskam and
A. Meijerink, J. Lumin., 1999, 82, 93.
Fig. 5 presents the emission spectra of 5 and 10 mol%
Eu3+-doped samples excited by high energy synchrotron
radiation. Due to the quantum cutting effect the ratio of the
5D0 - 7FJ/5D1 - 7FJ peaks increases by a factor of up to 1.5.
Judging from the intensity ratios the cross-relaxation probability
is B25% for the 10 mol% Eu3+-doped sample and B45% for
the 5% mol Eu3+-doped sample, yielding a quantum efficiency
of about 125% and 145%, respectively. These values are
astonishingly high when taking the extremely small particle size
into account. While in the bulk materials a quantum yield close
to the theoretical maximum of 200% can be observed (B170%
for GdF3 :Eu3+),5 with smaller particle size and rising surface
area a drastic reduction of the quantum yield due to surface
quenching is expected. Maybe for this reason and the experi-
mental difficulties in synthesizing uniform LnF3 particles, almost
no reports on quantum cutters on the nanoscale exist. However,
for GdF3 :Eu3+ nanorods of substantially larger size than our
particles (length 60–200 nm, diameter 18–20 nm) a quantum
yield close to the bulk material has been suggested.15
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New York, 2008; A. E. Visser, R. P. Swatloski, W. M. Reichert,
R. Mayton, S. Sheff, A. Wierzbicki, J. H. Davis, Jr and
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In conclusion, in this work we describe a fast and facile
access to pure, oxygen free, small size (B6 nm), polycrystalline
GdF3 : Eu3+ nanoparticles via microwave synthesis. Here, the
ionic liquid not only serves as the synthesis medium but also as
a fluoride source, hence, the reaction partner. Quantum yields
as high as 145% can be obtained which renders these materials
of great interest for optical applications.
13 It has to be noted that our synthetic approach is different from the
MAIL (microwave ionic liquid assisted) method which makes use
of adding a small amount of IL to a conventional solvent to
enhance its susceptibility to microwave radiation, cf.: X.-G. Zhu,
Angew. Chem., Int. Ed., 2004, 43, 1410; G. Buhler and
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The authors would like to acknowledge support from the
European Research Council CRC through an ERC Starting
Grant (‘‘EMIL’’, contract no. 2008) and the HASYLAB
(proposal no. II-20090181).
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 571–573 | 573