Ionic liquid-based synthesis-a low-temperature route to nanophosphates

Article abstract of DOI:10.1002/cssc.201100095

Endless Phossibilities: Microwave synthesis, with phosphate ionic liquids acting as reaction partner, solvent, and in-situ stabilizer all-in-one, allows for a fast, facile, and mild access to nanometer-sized orange-red emitting phosphors. The phosphors show excellent photophysical performance.

Full text of DOI:10.1002/cssc.201100095

DOI: 10.1002/cssc.201100095
Ionic Liquid-based Synthesis—A Low-Temperature Route to
Nanophosphates
[
a, b]
[a]
[b]
[a]
Joanna Cybinska,
Chantal Lorbeer, Eugeniusz Zych, and Anja-Verena Mudring*
Nanotechnology has become a very important part of our lives
waves. This leads to high heating rates, which results in a high
formation rate of nuclei. In consequence, not only are reaction
times drastically reduced in comparison to classical wet-chemi-
cal routes but smaller particle sizes can be achieved, also.
During recent years the exploration of ILs for the synthesis
of different nanomaterials that can be used as host structures
for phosphor materials, such as fluorides, oxides, or phos-
[
1]
in recent years. Manufacturing nanomaterials with a defined
architecture, morphology, and particle size and desired proper-
ties is the major challenge that scientists encounter in this
area. Aside from well-known quantum-size effects, the small
size and the resulting large ratio of surface atoms to “bulk”
atoms leads to very large numbers of weakly bound and unsa-
turated surface atoms. As a results, a number of physical phe-
nomena and physico-chemical properties are different from
[5,6,8]
phates, has started.
Whereas for nanofluorides ILs based
on complex fluorides have been already been used as a re-
agent, solvent, and stabilizer all-in-one, for phosphates ILs
have been only used as additives to enhance microwave sus-
[
2]
those of bulk materials. For example, a lowering of the melt-
ing point and an increased chemical reactivity are typically ob-
served. Especially for phosphor materials, the transition to the
nanometer scale is a tightrope walk between achieving the de-
sirable features of small particles, such as reduced light scatter-
ing, and avoiding unwanted drawbacks, such as a high surface
defect concentration, which is likely to result in luminescence
quenching. From a spectroscopic point of view the large sur-
face of nanoparticles (NPs) is without a doubt critical, and fun-
damental luminescence properties such as decay times and
nonradiative as well as radiative relaxation paths are strongly
[9]
ceptibility (MAIL=microwave-assisted ionic liquid synthesis).
Nanophosphates are of tremendous interest as optical materi-
als as they are known to combine a high energy-absorption
edge (band gap) with excellent chemical and mechanical sta-
bility, which renders them appropriate as host lattices for a
wide range of optical materials such as fast scintillator materi-
[10]
als and vacuum ultraviolet (VUV) phosphors.
In addition,
many phosphates provide host lattices that are able to accom-
modate a high degree of optically active luminescent centres
(lanthanide ions) without strongly suffering from concentration
quenching of fluorescence, because of the relatively large dis-
[
3]
affected. For these reasons, extreme care must be taken in
order to find ways to fabricate nanometer-scale phosphor ma-
terials with good optical properties.
[11]
tance between the lanthanide ions in the host matrix. Nano-
meter-scale lanthanide monophosphates have been obtained
by different synthesis routes such as hydrothermal, co-precipi-
In the last few years the distinctive advantages that ionic liq-
uids (ILs) offer in the synthesis of nanomaterials have become
obvious: they allow to combine unique sets of properties, so
that they can beneficially serve as solvents, reagents, and sur-
[12]
tation, or sol–gel methods. However, in a first step these
wet-chemical routes typically lead to hydrous phosphates. Be-
cause these are poor optical materials, high-temperature post-
reaction treatment is necessary to remove water. Unfortunate-
ly, this usually results in strong particle agglomeration, which
reduces the performance of the material (more straylight, re-
[
4]
face-modifying agents—all in one! Truly, ILs—commonly de-
fined as salts with a melting point below 1008C, but many of
them being liquid at room temperature—open new perspec-
tives in synthetic routes to design nanomaterials. The advan-
tages of ILs in nanoparticle synthesis (high nucleation rate, ex-
cellent electrosteric NP stabilization, morphology control, tune-
[
13]
duced luminescence efficiency).
Milder reaction conditions
[14]
employing, for example, a sonochemical process, a microe-
[
4a]
[15]
[9]
able properties) can be combined with the benefits of other
mulsion technique, and a microwave-accelerated synthesis
[
5]
[6]
synthesis techniques, such as microwave, ultrasound and
have been explored. Nonetheless, the obtained materials were
still not completely anhydrous. Furthermore, surface-active
compounds, employed in the synthesis to control particle size
and morphology and prevent particle aggregation, are likely to
diminish the photophysical performance of the obtained nano-
material as they open up additional pathways for radiationless
energy decay. A “green chemistry” method using microfluidic
platforms was reported by Fang et al., obtaining nanorod-
[
7]
physical vapor deposition synthesis. Especially microwave
synthesis takes advantage of key IL properties: because ILs are
ionic media that comprise large ions with high polarizabilities
and conductivities they are highly susceptible towards micro-
[
a] Dr. J. Cybinska, C. Lorbeer, Prof. Dr. A.-V. Mudring
Anorganische Chemie I—Festkçrperchemie und Materialien
Ruhr-Universitꢀt Bochum
[16]
shaped phosphate materials.
Although the nanorods are
Universitꢀtsstr. 150, 44780 Bochum (Germany)
Fax: (+49)234/32-14951
E-mail: anja.mudring@rub.de
very thin, they are rather long (length of a few hundred nano-
meters), which could become an issue in certain applications.
Here, we present a unique, universal, fast, and facile micro-
wave synthesis process with a task-specific IL acting both as re-
agent and as surface-modifying agent. The method gives
access to completely anhydrous nanophosphates with excel-
lent photophysical properties at low temperatures. Simple lan-
[b] Dr. J. Cybinska, Prof. Dr. E. Zych
Faculty of Chemistry
University of Wroclaw
Joliot-Curie 14, 50383 Wrocław (Poland)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100095.
ChemSusChem 2011, 4, 595 – 598
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
595

thanide acetate hydrates can be converted to lanthanide
single phosphate (LnPO ) with the use of dihydrogen phos-
4
phate ILs such as [choline][H PO ] under microwave treatment
2
4
(10 min, 1408C):
LnðOAcÞ þ½cholineꢀ½H PO ꢀ ! LnPO # þ2 HOAc " þ½cholineꢀ½OAcꢀ
3
2
4
4
ð1Þ
During the reaction, lanthanide phosphate NPs precipitate
and the emerging acetic acid is volatile at 1408C. This strongly
shifts the chemical equilibrium to the product side, resulting in
an almost quantitative reaction yield. The residual acetate
anion (with a choline counter ion) becomes part of the IL,
which can be recycled. In the synthesis the IL is truly multi-
functional: (1) it serves as reaction medium—due to its ionic,
highly polarizable character it guarantees an excellent micro-
wave susceptibility, with the connected benefits; (2) it helps to
control the particle size and morphology; and (3) it acts as a
reaction partner. Remarkably, the anhydrous lanthanide phos-
phates are directly obtained. The absence of water and hy-
drous compounds is backed by IR spectroscopy and powder X-
ray diffraction (see Supporting Information). We also have ex-
plored the synthesis of LnPO (Y, La, Gd) as the host matrix
4
3
+
doped with an optical active lanthanide ion such as Eu .
Doping with one or more lanthanide ions can be achieved by
simply adding the desired amounts of the respective lantha-
nide acetates to the reaction mixture. In that way Ce,Tb-doped
Figure 1. Representative TEM images of synthesized Eu-doped YPO
and LaPO
4 4
, GdPO ,
4
.
LnPO materials, used commercially as emitters in fluorescent
spherical particles (4.2(ꢁ0.9) nm). The obtained LaPO nanoma-
4
4
lamps, cathode ray tubes, and plasma display panels, were ob-
terial is well-crystallized and shows relatively sharp reflections
in the powder X-ray diffraction pattern as well as uniformly
3+
3+
tained. Unfortunately, neither Ce nor Tb shows f–f transi-
tions that are especially sensitive to the local environment. For
grown lattice fringes (200) in TEM. Nanoscale GdPO consist of
4
3+
that reason, we choose Eu as a sensitive optical probe for
our detailed studies.
more agglomerated, but still minute particles with a size of
3+
about 2–8 nm (Supporting Information). All Eu -doped nano-
The anhydrous phosphates YPO4 and GdPO4 crystallize in
LnPO materials show a very intense orange-red emission. The
4
[17]
3+
the xenotime and monazite structure types, respectively. For
emission spectra reveal typical f–f transitions of the Eu ion
5
7
LaPO , two modifications are known. In the bulk material the
( D ! F ) (Figure 2a). However, deviations from the emission
4
0
J
low-temperature hexagonal phase of LaPO irreversi-
4
bly undergoes a phase transformation to the mona-
[
12b]
zite structure above 6008C.
Unexpectedly, we ob-
served the direct formation of the thermodynamically
stable monoclinic phase in the microwave reaction
(see Supporting Information for powder XRD data).
To determine the particle size as well as the morphol-
ogy of the synthesized phosphate material, both
transmission electron microscopy (TEM) and scanning
electron microscopy (SEM) were carried out
(Figure 1). Interestingly, different product morpholo-
gies were observed for the different lanthanide ions
[
18]
under the same reaction conditions.
hollow nanotubes with an inner diameter of about
0 nm and an outer diameter of 200–300 nm were
For YPO4
5
obtained. This might be of particular interest for opti-
cal applications as a second phosphor material could
be incorporated, leading to advanced luminescence
materials. We are currently exploring these possibili-
Figure 2. a) Room-temperature emission spectra excited at lex =393 nm of Eu-doped
, LaPO , and GdPO , and b) corresponding color coordinate diagram.
ties in detail. Compared to YPO , LaPO forms small, YPO
4
4
4
4
4
5
96
www.chemsuschem.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2011, 4, 595 – 598

ꢂ1
spectra of the bulk material can be observed for nano-LnPO4.
10 min. With a ramp of 16 Kmin the mixture was then heated to
408C followed by holding at this temperature for ten minutes. Af-
1
The difference was most obvious for nano-GdPO which also
4
terwards, the reaction solution was rapidly cooled to room temper-
ature with pressurized air. The obtained colloidal solution was cen-
trifuged at 2000 rpm and the precipitate washed several times
with ethanol and dichloromethane. After drying at 608C, colorless
powders were obtained.
was the least crystalline sample judging from TEM and PXRD.
5
7
Here, the D ! F transition dominated the nanometer-sized
0
2
5
7
sample where as in the bulk material the D ! F transition is
0
1
the most intense.
In fact, the intensity (I) ratio of the D ! F and D ! F
5
7
5
7
0
2
0
1
transitions are to be regarded as a measure of the asymmetry
3+
[19]
Acknowledgements
of the Eu ion site. The large surface area and the large
3+
amount of coordinatively unsaturated Eu ions in the nano-
A.-V.M. acknowledges support by the ERC (European Research
Council) through a Starting Grant (“EMIL”, contract no. 200475),
and the Fonds der Chemischen Industrie (FCI) for a fellowship
LnPO :Eu material leads to a considerable reduction of symme-
4
try compared to the bulk material, resulting in an increase of
5
7
5
7
5
7
the I( D ! F )/I( D ! F ) ratio. The strong D ! F emission
0
2
0
1
0
2
(
Dozentenstipendium). J.C. and E.Z. acknowledge partial support
also gives rise to the intense orange–red emission of high
color purity, as shown in the color coordinates diagram (Fig-
ure 2b). For all as-prepared nanophosphate materials
under grant POIG.01.01.02–02–006/09 of the Minister of Science
and Higher Education (Poland).
LnPO :Eu(5%), the decay times under the 270 nm excitation
4
monitored at the most-intense transition were determined
Keywords: ionic liquids · nanostructures · phosphorus ·
photophysics · synthesis design
5
7
5
7
(
D ! F for GdPO and LaPO , and D ! F for YPO ). The ob-
0
1
4
4
0
2
4
tained values varied from 1.20 to 1.82 to 2.04 ms with increas-
ing relative mass of the europium ion in the host lattice. The
achieved results are in good agreement with data on nano-
crystalline single phosphates published earlier, in which for
[1] a) H.-G. Rubahn: Basics of Nanotechnology, Wiley-VCH, Weinheim, 2008;
b) L. Cademartiri, G. A. Ozin, J.-M. Lehn, Concepts of Nanochemistry,
Wiley-VCH, Weinheim, 2009; c) G. A. Ozin, A. C. Arsenault, L. Cademar-
tiri, Nanochemistry: A Chemical Approach to Nanomaterials, Royal Soci-
ety of Chemistry, London, 2008.
3+
Eu doped YPO depending on the synthesis method (solid
4
state, co-precipation, or sol–gel) and grain size, decay times of
[2] H. Goesmann, C. Feldmann, Angew. Chem. 2010, 122, 1402–1437;
Angew. Chem. Int. Ed. 2010, 49, 1362–1395.
[3] K. A. Gschneider, J. Bꢁnzli, V. K. Pecharsky, Handbook on the Physics and
Chemistry of Rare Earths, Vol. 37, Elsevier, Amsterdam 2007.
[4] a) M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, Angew. Chem. 2004, 116,
5096; Angew. Chem. Int. Ed. 2004, 43, 4988; b) H. Kaper, F. Endres, I.
Djerdj, M. Antonietti, B. M. Smarsly, J. Maier, Y.-S. Hu, Small 2007, 3,
[
20]
2
.7–3.4 ms were obtained. For the nanoscale GdPO :Eu vary-
4
[
21]
ing decay times of 1–7 ms have been reported, while for
LaPO4 nanowires and nanorods values between 1.70–
[
22]
[23]
2.40 ms
and 3.96–1.98 ms , respectively, have been ob-
tained (as a function of active ion concentration). The calculat-
ed quantum yields were 34% for YPO :Eu, 47% for LaPO :Eu,
1753; c) Z. Li, Z. Jia, Y. Luan, T. Mu, Curr. Opin. Solid State Mater. Sci.
4
4
2
008, 12, 1.
and 57% for GdPO :Eu. These results are in good agreement
4
[5] C. Lorbeer, J. Cybinska, A.-V. Mudring, Chem. Commun. 2010, 46, 571.
[6] a) T. Alammar, A. Birkner, A.-V. Mudring, Eur. J. Inorg. Chem. 2009, 2765;
b) T. Alammar, A.-V. Mudring, Mater. Lett. 2009, 63, 732.
with previous results on quantum efficiencies for LaPO :Eu
4
[24]
nanoparticles (38%) and LaPO :Eu nanowires (59%).
For
4
[
7] a) N. von Prondzinski, J. Cybinska, A.-V. Mudring, Chem. Commun. 2010,
GdPO :Eu nanoneedles with 200–600 nm length and 10–20 nm
4
4
1
6, 4393; b) K. Richter, A. Birkner, A.-V. Mudring, Angew. Chem. 2010,
22, 2481; Angew. Chem. Int. Ed. 2010, 49, 2431.
[
17]
width, a quantum yield of 19% was published, which is sig-
nificantly lower than the efficiency obtained in our case.
In summary, the novel, fast, and facile low temperature
method reported here leads to nanometer-size orange–red
emitting phosphors with excellent performance. The particles
are free from water (even from surface-bound water). This is
ascribed to the stabilizing role of the ionic liquid. An interest-
ing change in morphology depending on the used lanthanide
ion is reported.
[8] a) J.-L. Lemyre, A. M. Ritcey, Chem. Mater. 2005, 17, 3040; b) N. O.
Nunez, M. Ocana, Nanotechnology 2007, 18, 455606; c) D. S. Jacob, L.
Bitton, J. Grinblat, I. Felner, Y. Koltypin, A. Gedanken, Chem. Mater. 2006,
1
8, 3162; d) G. Bꢁhler, M. C. Stay, M. C. Feldmann, Green Chem. 2007, 9,
924.
[9] G. Bꢁhler, C. Feldmann, Angew. Chem. 2006, 118, 4982; Angew. Chem.
Int. Ed. 2006, 45, 4864.
[
[
10] Q. Y. Zhang, X. Y. Huang, Prog. Mater. Sci. 2010, 55, 353.
11] Phosphor Handbook (Ed.: W. M. Yen, H. Yamamoto), 2 edition CRC
nd
Press, Boca Raton 2007.
[
12] a) E. E. Boakye, P. Mogilevsky, R. S. Hay, G. E. Fair, J. Am. Ceram. Soc.
2
008, 91, 3841; b) M. Ferhi, K. Horchani-Naifer, M. Ferid, J. Lumin. 2008,
128, 1777; c) M. Ferhi, K. Horchani-Naifer, M. Ferid, J. Rare Earths 2009,
7, 182; d) O. Lehmann, H. Meyssamy, K. Kçmpe, H. Schnablegger, M.
Experimental Section
2
The samples were prepared under an argon atmosphere using
common Schlenk and glove-box techniques. Defined amounts of
lantanide (III) acetate hydrates in a small amount of ethylene glycol
were dissolved in a three-fold excess of choline dihydrogen phos-
phate under stirring. The reaction solution was placed in a pres-
sure-proof microwave reaction vessel equipped with a Teflon
septum and heated in a single mode microwave (CEM, Discover)
with a frequency of 2455 MHz. Temperature control during the re-
action was achieved by an IR sensor measuring the temperature
and automatically adjusting the irradiation power. First, the reac-
tion mixture was heated to 608C and kept at this temperature for
Haase, J. Phys. Chem. B 2003, 107, 7449; e) O. Lehmann, K. Kçmpe, M.
Haase, J. Am. Chem. Soc. 2004, 126, 14935; f) N. Yaiphaba, R. S. Ningth-
oujam, N. R. Singh, R. K. Vatsa, Eur. J. Inorg. Chem. 2010, 2682; g) N. Yai-
phaba, R. S. Ningthoujam, N. S. Singh, R. K. Vatsa, N. R. Singh, J. Lumin.
2
010, 130, 174.
13] W. B. L. Cushing, V. L. Kolesnichenko, C. J. O’Connor, J. Chem. Rev. 2004,
04, 3893.
14] C. Yu, M. Yu, C. Li, X. Liu, J. Yang, P. Yang, J. Lin, J. Solid State Chem.
009, 182, 339.
[
[
[
1
2
15] a) Y. Xing, M. Li, S. A. Davis, S. Mann, J. Phys. Chem. B 2006, 110, 1111;
b) C. Zhang, J. Chen, X. Zhu, Y. Yunchun, D. Li, Chem. Mater. 2009, 21,
3570.
ChemSusChem 2011, 4, 595 – 598
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
597

[16] J. Fang, Y. Guo, G. Lu, C. L. Raston, K. S. Iyer, Green Chem. 2011, 13, 817 .
[17] Y.-X. Ni, J. M. Hughes, A. N. Mariano, Am. Mineral. 1995, 80, 21.
[18] C. Lorbeer, J. Cybinska, A.-V. Mudring, Cryst. Growth Des. 2011, 11, 1040
.
[22] L. Yu, H. Song, S. Lu, Z. Liu, L. Yang, Chem. Phys. Lett. 2004, 399, 384.
[23] G. Phaomei, R. S. Ningthoujam, W. R. Singh, N. S. Singh, M. N. Luwang,
R. Tewari, R. K. Vatsa, Opt. Mater. 2010, 32, 616.
[24] L. Yu, H. Liu, J. Nanomater. 2010, 46, 1309.
[19] L. D. Carlos, R. A. S. Ferreira, V. de Zea Bermudez, S. J. L. Ribeiro, Adv.
Mater. 2009, 21, 509.
[
[
20] J. M. Nedelec, D. Avignant, R. Mahiou, Chem. Mater. 2002, 14, 651.
21] N. Yaiphaba, R. S. Ningthoujam, N. Shanta Singh, R. K. Vatsa, N. Rajmu-
hon Singh, S. Dhara, N. L. Misra, R. Tewari, J. Appl. Physics 2010, 107,
Received: February 22, 2011
034301, and references cited therein.
Published online on April 28, 2011
5
98
www.chemsuschem.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2011, 4, 595 – 598