Fabrication of EuF3-mesocrystals in a gel matrix

Article abstract of DOI:10.1002/zaac.201000098

Europium(III) fluoride mesocrystals were synthesised in an organic matrix. This matrix is a gel formed by Eu³⁺ ions and a polycar-boxylate/ sulfonate copolymer, ACUSOL 588G. In the gel phase, the local amount of europium ions is very high since Eu³⁺ acts as a cross-linker, and crystallisation occurs upon addition of F^-. Nucleated seed crystals in the gel phase grow by further ion attachment and form mesocrystals by mutual orientation of the EuF₃ particles in the gel. We propose a dipole field as reason for this alignment and that the dipolar character of the particles originates from adsorption of the polyelectro-lyte on charged crystal faces.

Full text of DOI:10.1002/zaac.201000098

ARTICLE
DOI: 10.1002/zaac.201000098
Fabrication of EuF3-Mesocrystals in a Gel Matrix
Christine Lausser,[a] Michael U. Kumke,[b] Markus Antonietti,[a] and Helmut Cölfen*[c]
Dedicated to Professor Rüdiger Kniep on the Occasion of His 65th Birthday
Keywords: Mesocrystals; Non-classical crystallisation; Europium; Fluorine; Crystal growth
Abstract. Europium(III) fluoride mesocrystals were synthesised in an crystals in the gel phase grow by further ion attachment and form
organic matrix. This matrix is a gel formed by Eu3+ ions and a polycar- mesocrystals by mutual orientation of the EuF3 particles in the gel. We
boxylate/sulfonate copolymer, ACUSOL 588G. In the gel phase, the propose a dipole field as reason for this alignment and that the dipolar
local amount of europium ions is very high since Eu3+ acts as a cross- character of the particles originates from adsorption of the polyelectro-
linker, and crystallisation occurs upon addition of F–. Nucleated seed lyte on charged crystal faces.
Examples for mesocrystals of functional materials are
Introduction
NH4TiOF3 mesocrystals and their topotactic conversion to ana-
The control over the mutual self-organisation of nanoparti-
tase-mesocrystals [4, 5] as well as superparamagnetic magnet-
cles towards superstructures and well organised arrays is a next
ite mesocrystals [6].
step in nanosciences. Recently, non classical crystallisation
was found to be a tool for controlled nanoparticle organisation
In the present study, mesocrystals of europium(III) fluoride
(EuF3) are described. Compounds based on rare-earth elements
to superstructures or single crystals, thus generating so-called
are among the most common luminescent materials, due to
mesocrystals, superstructures built up by nanocrystals, which
high photochemical stability, characteristic sharp emission
are in mutual alignment [1–3]. Often the crystalline building
lines of lanthanide ions and high quantum efficiencies [7].
blocks are in perfect three-dimensional crystalline registry. The
They are applied for high-performance luminescent displays,
properties of mesocrystal materials, such as single crystalline-
optical communication and biochemical probes to laser materi-
behaviour, high crystallinity and high porosity are very useful
als [8–11]. Therefore, it is desirable to develop bottom up strat-
egies for the design of these materials with a controlled nano-
structure. Wang et al. prepared a variety of EuF3 nanostructures
with different crystalline phases and morphologies via a simple
solution route employing different fluoride sources [12, ,13]
and new morphologies of crystalline europium(III) fluoride
were addressed more and more often in the last few years [14,
15]. The optical properties of EuF3 were analysed in detail
by Jayasankar using fluorescence microscopy. He detected the
for functional materials. Such combination of properties in a
single material is unique compared to single-crystalline, poly-
crystalline and amorphous materials. A number of desirable
characteristics can be found within a mesocrystal that cannot
be obtained for the same material in amorphous or single-crys-
talline form or as a polycrystalline aggregate. For example, it
can be very advantageous to use a catalyst-material in form of
a mesocrystal. It combines high crystallinity with small parti-
cle size, high surface area and high porosity as well as good
handling as the mesocrystal has a size in the micrometer range.
7
typical magnetic dipole transition (5D05→ F1) and the typical
7
7
5
7
electric dipole transitions (5D0→ F2, D0 → F3, D0 → F4)
* Prof Dr. H. Cölfen
[16].
E-Mail: Coelfen@mpikg.mpg.de, helmut.coelfen@uni-konstanz.de
[a] Max-Planck-Institute of Colloids and Interfaces
Colloid Chemistry
Following the pioneering work of Kniep on oriented super-
structure synthesis in structured biopolymers, e.g. synthesis
and analysis of a hexagonal-prismatic seed crystal of fluorapa-
tite formed in a gelatin gel [17, 18] we made the current meso-
crystals of EuF3 by using a polymer gel as template. Using a
structured organic matrix is an obvious mechanism for meso-
crystal formation since the crystalline nanoparticles could align
along the organic matrix [2, 19]. There are two possibilities
for the formation of a mesocrystal: either the nanoparticles are
formed first and then attach to the polymer and get oriented,
or they nucleate directly on the organic matrix which could
act as an oriented, heterogeneous nucleation site [20]. A large
Research Campus Golm
Am Mühlenberg
14424 Potsdam, Germany
[b] Institute of Chemistry (Physical Chemistry)
University of Potsdam
Karl-Liebknecht-Str. 24–25
14476 Potsdam, Germany
[c] Present adress: Physical Chemistry
University of Konstanz
Universitätsstr. 10
78464 Konstanz, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201000098 or from the
author.
View this journal online at
wileyonlinelibrary.com
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number of examples of ordered macromolecular structures are
In addition, polarisation microscopy with crossed polarisers
also known in the world of biomacromolecules. Mesocrystal shows a strong, very homogeneous birefringence with uniform
formation along a pre-structured organic matrix seems to be colour over distances > 100 µm, which is a proof for the iso-
especially valid in biomineralisation processes, as could also orientation of the nanoparticle aggregates even on the micron
be demonstrated by biomimetic mineralisation experiments scale. Therefore, the obtained nanoparticle superstructure is a
[21–25]. Typical examples for a matrix-mediated nanoparticle mesocrystal [1, 2, 19].
growth are the formation and growth of bones as well as the
The formation reaction of EuF3 is literature described as fol-
growth of nacre [25–27]. Here we report on the exploitation lows [15]
of such principles for the generation of an EuF3 mesocrystal.
Results and Discussion
As soon as Eu(NO3)3·5H2O solution is added to the Accusol/
NaBF4 solution, nanoparticles composed of polymer and min-
The release of the protons [Equation (2)] and thus the forma-
eral precursors are formed in solution. The particle size is in- tion of EuF3 can therefore be followed via pH-measurements.
creasing in the first 3 h to a value of about 500 nm and then The pH value varied in the experiment from 2.6 at the begin-
drops because of the sedimentation of the formed particles ning of the reaction to 2.2 after reaction of 3 hours. This indi-
when they grew further (Figure 1). This is also expressed in cates continuous formation of EuF3.
the decrease of the intensity, which reaches 0 after 8 hours.
At these pH values, the carboxylic acid part of the polymer
After a reaction time of 3 hours and isolation of crystals, template is clearly protonated, but the strongly acidic sulfonic
platelet-like agglomerates with flat surfaces, expressed angu- acid residues (14.7 mol-%) are still deprotonated. As a conse-
larity, and with different sizes and thicknesses were obtained. quence, a gel is formed via electrostatic interactions of the
They are obviously built up by small primary nanoparticles sulfonated copolymer with Eu3+. Gel formation was demon-
with an average size between 30 and 50 nm, as evidenced by strated by swelling experiments under a light microscope and
SEM (Figure 2). XRD measurements (Figure 3) show that the gravimetric comparison of the swollen and deswollen gel.
primary particles are completely crystalline; the diffraction
To investigate a possible templating mechanism of the Accu-
peaks can be indexed to the hexagonal phase of EuF3 (JCPDS sol-Eu gel, we performed a control experiment. By slowly add-
32–0373). The particle size of 30 nm from Scherrer evaluation ing an aqueous Eu(NO3)3 solution to an aqueous ACUSOL
agrees well with the SEM results. Comparison of the obtained 588G solution, the gel was formed. The mixture became turbid
WAXS data with the reference for hexagonal EuF3 shows that immediately, and colloidal gel particles were formed. The pre-
the peak intensities are very similar. No preferred orientation cipitate was centrifuged. SEM images of dried gel samples
can be deduced from the WAXS data.
showed a primary nanostructure similar to the final euro-
Nevertheless, the crystalline building blocks of the agglom- pium(III) fluoride-mesocrystals. It is built up by small particles
erates are in perfect vectorial alignment with their three crys- with similar average size of 50 nm as the building units of a
tallographic axes pointing in joint directions. The electron dif- layered gel film (Figure 5). This confirms the assumption that
fraction on the big polycrystalline aggregates (Figure 4b) gives the microgel particles serve as templates for the subsequent
a single-crystal spot diffraction pattern, which demonstrates crystallisation. The electron diffraction of the control gel parti-
the very high degree of mutual orientational order of the nano- cles demonstrates the amorphous character of the polymer-
particles.
metal ion adducts.
Figure 1. Time dependent dynamic light scattering data of forming EuF3 in presence of Accusol. Left: Hydrodynamic radius rH and right:
Intensity.
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Fabrication of EuF3-Mesocrystals in a Gel Matrix
Figure 2. SEM images after a reaction time of 3 hours. a) Agglomerates different in size and form occur. b) and c) the agglomerates are built
up by small particles with an average size of 50 nm.
Figure 3. 1: XRD pattern of the sample isolated after a reaction time
of 3 hours. The primary crystallite size was estimated to be 30 nm
via the Scherrer-equation. 2: The reference pattern of hexagonal EuF3
Figure 5. a) and b) SEM-images of the Eu-ACUSOL-gel. c) TEM-
measurement and electron diffraction of the amorphous gel, which is
built up by small particles.
obtained from the JCPDS file 32–0373. (triangles).
Figure 4. a) TEM image of the sample after a reaction time of 3 hours. It is obvious that the agglomerates are built up by small particles. b)
The electron diffraction shows diffraction spots, which indicate a high mutual order in the agglomerates. c) Polarisation microscope image with
crossed polarisers showing the long range order of the particles inside the sample. (see Figure SI-1 for a colour image).
Upon addition of NaBF4 aqueous solution to the as formed in the reference experiment. Therefore, under the standard re-
reference particles, the gel dissolved after three days. After action conditions, an Eu3+ excess with respect to the amount
further two days, a transformation to a crystalline precipitate needed to crosslink the gel was mandatory for mesostructure
took place and could clearly be followed by naked eye. This formation.
shows that the europium ions that were initially bound by the
In another control, the experiment was repeated under the
polymer reacted with the fluoroborate ions, which weakens the same conditions except the removal of the supernatant reaction
gel structure, while EuF3 formed. As only those Eu3+ ions, solution. In that case the Europium-polymer complex was not
which cross-linked the gel structure, were available for EuF3 dissolved due to the Eu3+ excess. This shows that the fluorobo-
formation, the gel was destroyed, and no templating took place rate ions react primarily with the europium ions in the solution.
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However, since light scattering (Figure 1) shows that the parti- minescence spectra of the samples (reaction time 3 h and 16
cles sediment after 3 h when they have reached a radius of 500
nm. It is likely that these larger particles are the centres for
further growth to the structures several tens of micrometer in
size as observed by SEM (Figure 2). The polymer and euro-
pium ions needed for this growth are supplied from the solu-
tion phase, and the gel can act as a template for the formation
of the EuF3 mesocrystal.
h) and of a reference EuF3 sample are compared.
Increasing the reaction time from 3 h to 16 h led to some
different structures. Round and disc shaped structures with
rough surfaces and an average size of 500 nm were obtained
as it is typical for a reaction of NaBF4 and Eu(NO3)3 without
any additive (Figure 6a, b) [12]. The crystallinity of the parti-
cles was proven by XRD (data not shown) and the diffraction
peaks can be indexed to the hexagonal phase of EuF3.
Figure 7. Comparison of the Eu-luminescence spectra of the samples
at 3 h and 16 h. The samples were measured in suspension at room
temperature (λex = 395 nm) in a time-gate detection mode at Δt = 20
5
μs after the laser excitation pulse. Inset: Shown is the D0–7F0 transi-
tion of europium for the three samples.
A clear difference in the spectral intensity distribution was
observed for the samples isolated after 3 h and 16 h: i) for the
sample isolated after 3 h the luminescence transition corre-
5
sponding to the D0–7F0 transition is well pronounced in con-
trast to the sample isolated after 16 h. The luminescence spec-
trum of the sample isolated after 16 h is very similar to a EuF3
reference sample. The spectral intensity distribution of the eu-
ropium luminescence is mainly determined by the symmetry
of the local coordination sphere and the charge density as well
as the covalency of the bonds. Whereas in a highly symmetric
Figure 6. a) and b) The SEM images show a network of round and
disc shaped particles after a reaction time of 16h. c) TEM shows that
these particles are polycrystalline and exhibit a short range order.
5
complex the D0–7F0 transition (around λem = 579 nm) is for-
bidden and therefore should not be observed in the lumines-
cence spectrum, distortion of the symmetry lower the forbid-
dance and subsequently, this transition can be observed in the
luminescence spectrum. In case of the reference sample and
These structures are also polycrystalline, and electron dif-
fraction shows again a mutual order as it is typical for meso-
crystals (Figure 6c). However, the order is lower than in the
case of the mesocrystals formed in the polymer gel template
(Figure 4). A time dependent observation of the formed struc-
tures (3 h, 5 h, 7 h, 9 h, 11 h and 16 h) shows that the gel
structure controls structure formation up to 7 h (Figure SI 2).
After 9 h, the particles with the rough surface begin to form
on cost of the polymer gel since the gel gets depleted in the
areas where particles are observed (Figure SI 1). After 16 h,
this process has advanced to a stage, where the entire gel was
consumed and only the secondary species were left. We as-
sume that with increasing time Eu3+ gets depleted, and also the
Eu3+ cross-linking the polymer is consumed. This leads to the
observed dissolution of the gel structure and formation of the
round and disk shaped mesocrystals.
5
the sample isolated after 16 h, the intensity of the D0–7F0
transition is very weak, indicating a high degree of symmetry
in crystals (the minor intensity may be due to matrix vibra-
tions, which lower the symmetry and consequently also the
forbiddance of the 5D0–7F0 transition) (see Figure 7, Inset). For
the sample isolated after 3 h a much higher intensity of the
5D0–7F0 transition is observed, which indicates a relaxed euro-
pium symmetry in the presence of the polymer gel template.
A similar trend is also found for the asymmetry ratio (intensity
5
5
ratio of the D0–7F1 and the so-called hypersensitive D0–7F2
transition at λem = 595 nm and 615 nm, respectively, see also
Figure 8). Furthermore, the differences in the europium envi-
ronment are also reflected in the small spectral shift of the non-
degenerate D0–7F0 transition (see Inset of Figure 7), which is
5
The influence of reaction time on the sample properties is
also found in the luminescence properties. In Figure 7 the lu- representative for a europium species [28].
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Fabrication of EuF3-Mesocrystals in a Gel Matrix
of small nanoparticles, which in addition are able to interact
unperturbed and to arrange. Furthermore, the reduced diffusion
in the gel ensures sufficiently slow growth [31]. The level of
supersaturation of Eu3+ ions along the europium-ACUSOL-gel
is very high and nucleation occurs since the Accusol gel acts
as an “ion sponge”. Small EuF3 nanoparticles develop in the
polymer gel, which then form the mesocrystals. Since EuF3
has a hexagonal crystal structure, it has by symmetry no dipole
moment, which could generate the mutual alignment of the
nanocrystals in the mesocrystal. However, EuF3 has polar fa-
ces, namely {001}, {110} and {011} (see Figures SI 3–5),
which can be Europium-terminated and therefore strongly pos-
itive. This is favourable for the adsorption of the negatively
charged copolymer. If a particle is nucleated in the vicinity of
the polymer, the polymer can adsorb on one of these charged
faces and block them from further growth. However, adsorp-
tion can – for energetic reasons – only occur on one side of a
particle, thus creating two oppositely charged faces, as was
already discussed earlier for CaCO3 [12] and BaCrO4 [32].
This leads to the formation of a strong dipole, resulting in the
mutual alignment of adjacent nanoparticles. We propose this
mechanism also to hold true for the formation of the EuF3
mesocrystals in the copolymer gel. It is interesting to note that
our case is different from the fluoroapatite mesocrystals
formed in a gelatin gel [33]. Gelatin triple helices carry a di-
pole moment, which generates the intrinsic electric field align-
ing the nanoparticles. In the present example, this cannot be
the case, and dipole fields can only be generated by unsymmet-
rical polymer adsorption.
Figure 8. Luminescence decay at λem = 395 nm (left) and time-re-
solved asymmetry ratio (right) of the different samples (λex = 395 nm)
in water suspension. The asymmetry ratio is calculated as the intensity
5
5
ratio of the D0–7F1 and the D0–7F2 transition at λem = 595 nm and
615 nm, respectively.
In Figure 8 the time-resolved luminescence decays at λem
=
595 nm and the asymmetry ratios are shown, which further
support the results of the luminescence spectra analysis.
Whereas the reference and the sample isolated after 16 h are
very similar, the luminescence decay as well as the time de-
pendence of the asymmetry ratio of the sample isolated after
3 h are distinctly different: i) the luminescence decays much
faster and ii) the asymmetry ratio continuously changes with
time after the laser flash (in contrast to the sample isolated
after 16 h, which approaches a limiting value similar to the
reference sample (see right side of Figure 8)).
Conclusion
In the luminescence decays, quenching contributions from
We succeeded in fabrication of europium(III) fluoride meso-
ligands are seen. The luminescence decays of all samples are crystals by a simple reaction in an organic matrix. It was dem-
multiexponential, which may be connected to different species onstrated that Eu3+ ions react with the polymer ACUSOL
in the sample (e. g., surface and bulk europium ions interacting 588G to a gel. Adsorption of the polyanion on a charged face
differently with surrounding water molecules, which are strong of a particle nucleated in the vicinity of the polymer leads to
luminescence quenchers due to the effective coupling to OH- oppositely charged faces resulting in a dipole moment. Due to
vibrations). The presence of two species (bulk and surface) can this dipole field, the arrangement of the particles takes place
be seen in the time-dependence of the asymmetry ratio of the according to the particular dipole-dipole-interactions. This is
reference and the sample isolated after 16 h, which both ap- supported by the luminescence results, which confirm the pres-
proach a plateau at long times after the laser flash (right side ence of a modified europium ion surrounding in presence of
of Figure 8, top and middle) [29]. Striking is the fast (com- the polymer gel matrix. The polymer gel has multiple roles in
pared to the other samples) luminescence decay of the meso- this complex forming process. It acts as an ion sponge that
crystal sample isolated after 3 h, which may be attributed to a generates the high supersaturation in the gel phase leading to
strong coupling to the presence of the polymer gel matrix and particle nucleation in the gel only. Secondly, it leads to the
the subsequent efficient luminescence quenching (e. g, via ma- formation of a dipole field by adsorption onto a charged poly-
trix vibrations).
mer face in the vicinity of the gel. In the present case, the
The whole reaction pathway reminds of other, related obser- mesocrystal gel structure is however only transient. With time,
vations of mesocrystal formation via crystallisation in a gel- the gel dissolves again after the crosslinking europium ions
template. One of the best investigated examples of mesocrystal were consumed for particle growth. Our example shows that
development in a gel is the formation of the hexagonal-pris- complex functional mesocrystal structures can be assembled in
matic seed crystal of fluorapatite in gelatin [30]. The crystalli- very simple self-organised reaction schemes
sation in a gel is very suited for the formation of mesocrystals
Nanostructured materials have received extensive attention
as the crystal growth takes place at a high level of supersatura- due to their unusual properties and potential applications. For
tion and in the absence of flow and mechanical distortion [31]. example, europium(III) fluoride is used for high-performance
This results on the one hand in an increase of the nucleation luminescence displays or for laser materials. The here shown
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C. Lausser, M. U. Kumke, M. Antonietti, H. Cölfen
ARTICLE
sions about the chemical structure of the polymer, Peter Cernoch for
DLS-Measurements, and Katlen Brennenstuhl for the measurement of
the TRES. The Max-Planck-Society is acknowledged for financial sup-
port.
mesocrystalline material might be advantageous for these ap-
plications since a mesocrystal combines high crystallinity with
a small particle size.
Experimental Section
Materials: Sodium fluoroborate (NaBF4, Fluka, ≥ 98 %), euro-
pium(III) nitrate pentahydrate (Eu(NO3)3·5H2O, Aldrich, 99.9 %) and
the polymer ACUSOL 588G (Acrylic acid/sulfonic acid copolymer,
likely with 2-acrylamido-2-methylpropanesulfonic acid, AMPS con-
tent from elemental analysis = 14.7 mol-%, Mw = 12000 g·mol–1, a
well known antiscale additive donated by Benckiser, were used without
further purification.
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Supporting Information (see footnote on the first page of this article):
Polarisation microscope image of the EuF3 mesocrystal; time series of
mineralisation; surface cuts through the (001), (110) and (011) faces
of hexagonal EuF3.
Acknowledgement
We thank Rona Pitschke and Heike Runge for SEM- and TEM- meas-
urements as well as Christina Diehl and Ina Dambowsky for discus-
Received: February 19, 2010
Published Online: June 22, 2010
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Z. Anorg. Allg. Chem. 2010, 1925–1930