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Transparent, luminescent, and highly organized monolayers of zeolite L
Pengpeng Cao, Yige Wang, Huanrong Li* and Xiaoyan Yu
Received 5th November 2010, Accepted 13th December 2010
DOI: 10.1039/c0jm03798b
Herein we report on the fabrication of very dense, highly homogenous, well-oriented and highly
organized luminescent monolayers by arranging zeolite L (ZL) microcrystals onto the multi-functional
linker modified quartz plates. ZL microcrystals readily assemble in the form of a c-oriented monolayer
with perfect coverage and close packing on the 2-thenoyltrifluoroacetone (TTA)-functionalized quartz
plates. Additional functionalization through coordination to Eu3+ ions to the TTA moieties leads to ZL
monolayers with very high quality exhibiting strong luminescence. Alternatively, the luminescence
color of the layers is fine tuned by arranging ZL microcrystals with Tb3+ ions and sensitizer loaded in
the nanochannels on the luminescent quartz plates. Transparent, luminescent and uniformly oriented
layers are obtained by coating the polymer of PVA on top of the ZL monolayers in a facile way. SEM
and photoluminescence spectroscopy were employed to characterize the oriented monolayers.
Introduction
Zeolites are crystalline materials with highly regular nanometresized channels or cavities and a tunable composition. Assembly
of zeolite microcrystals into oriented monolayers has attracted
much attention in recent years because they can be further
tailored to be applied as membranes,1 chemical sensors2 and
hosts for supramolecular organization of guest molecules or
nanostructures.3 The first dense monolayers of zeolite crystals in
the microsize range were realized with zeolite A.4 An important
approach to the preparation of an oriented zeolite monolayer
with dense packing was binding zeolite crystals to a substrate
through a molecular linker by covalent bonding,5 ionic linkage6
and hydrogen bonding.7
Zeolite L (ZL) crystals feature one-dimensional and strictly
parallel channels arranged in hexagonal symmetry and has
proven to be an ideal host material for the construction of
advanced functional materials.8 This means oriented ZL monolayers with the nanochannels perpendicular to the substrate
surface display a hierarchy of structure presenting a successive
ordering from the molecular up to the macroscopic scale.
Subsequent insertion of guests into the channels leads to materials with exciting properties, such as the transfer of electronic
excitation energy in one direction only.9 This is highly desirable
for a device where a high degree of supramolecular organization
is important for attaining the desired macroscopic properties.10
Molecular linkers are of high importance in the fabrication of
a uniformly oriented zeolite monolayer as they can bind and
Hebei Provincial Key Lab of Green Chemical Technology and High
Efficient Energy Saving, School of Chemical Engineering and technology,
Hebei University of Technology, Tianjin, 300130, P. R. China. E-mail:
lihuanrong@hebut.edu.cn
This journal is ª The Royal Society of Chemistry 2011
direct the arrangements and organization of zeolite crystals on
the surface of the substrate. Furthermore, further adding functionalities such as luminescent properties to the molecular linker
should open a much wider entry for fabricating chemical devices
as the linkers can transfer electronic excitation energy between
the outside and inside of zeolite crystal nanochannels. With this
background in mind, we have assembled monolayers of ZL by
using luminescent molecules as the covalent linkers; the luminescent features are well-retained in the oriented ZL monolayer.11 Recently, we reported a synthesis strategy of achieving
higher organization and luminescence in the oriented ZL
monolayer by using a functional linker that has the ability to
coordinate and sensitize lanthanide ions Ln3+ and to selfassemble on a surface through hydrogen bonding. Dense, welloriented and highly organized luminescent monolayers were
obtained via this strategy by using triethoxysilylated bipyridine
as the functional molecular linker.12 However, despite our
previous success in using functional molecular linkers to organize
ZL crystals into highly ordered structure, further exploration of
other various molecular linkers with additional functionality is
highly desirable and remains a challenge.
In this work, we report the preparation of a luminescent and
uniformly oriented ZL monolayer with a very high degree of
coverage and of dense packing by using europium(III) b-diketonate complexes as a functional linker. The motivation behind the
idea is that europium(III) b-diketonate complexes exhibiting
a high quantum efficiency in combination with narrow band
emission and a high color purity are widely exploited for applications such as materials for flat-panel displays, UV sensors and
as laser materials.13,14 Transparent, very dense, highly homogenous, well-oriented and highly organized luminescent monolayers have been obtained by coating the polymer of PVA on top
of the ZL monolayers in a facile way.
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Results and discussion
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Preparation of ZL-TTA-Si-quartz monolayers
The TTA-tethered quartz plates were prepared by reacting
TTA-Si with quartz plates according to step 1 in Scheme 1. The
reaction is probably preceded by nucleophilic substitution of the
terminal triethoxy groups with OH groups on the quartz plate
surface. One simple and direct method of checking the presence
of TTA-Si on the quartz plates is to contact the TTA-tethered
quartz palate with a solution of EuCl3 in ethanol. In fact, the
quartz plates display strong bright red light upon irradiation with
a UV lamp. Fig. 1 shows the excitation and emission spectra, the
excitation spectrum was obtained by monitoring the 5D0 / 7F2
of Eu3+ ions at 612 nm. The broad band with maximum at
350 nm in the excitation spectrum can be ascribed to the
absorption of TTA-Si. The five prominent emission lines peaking
at 578, 593, 612, 652 and 701 nm in the emission spectrum can be
assigned to the 5D0 / 7FJ (J ¼ 0–4) transition with red emission
for J ¼ 2 as the dominant feature. This proves that the TTA
moiety on the substrate absorbs the excitation energy and
transfers it to the Eu3+ ions.15 Vigorous stirring of the substrate in
ethanol cannot cause any decrease in the emission intensities of
Eu3+, suggesting that Eu3+ is coordinated to TTA tethered on the
substrate.
The transparent TTA-functionalized quartz plates (TTAquartz) turn opaque upon contact with the suspension of ZL in
dry toluene under vigorous sonication as described in the
experimental section. The opaque samples become semitransparent after sonication for approximately 5 s in pure toluene
to remove physisorbed excess of ZL crystals. The scanning
electron microscopy (SEM) images of the semitransparent quartz
plates reveal that the coverage degree and close packing degree of
monolayer are both highly satisfactory although some spots with
less surface coverage can be observed as marked with the circle in
Fig. 2. With exception of some physisorbed crystals, most of the
ZL crystals are assembled with the c-axis perpendicular to the
Scheme 1 The procedure to prepare the oriented monolayers of zeolite L. (1) Binding of TTA-Si to the OH groups of a quartz substrate, (3) additional
stabilization and functionalization by coordination to Eu3+ ions, (2) preparation of c-oriented zeolite L monolayers of ZL-TTA-quartz, (4) of ZL-(EuTTA) quartz, and (5) of (Tb-FBP)/-ZL-(Eu-TTA)-quartz.
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possibly due to the urea groups which might self-assemble on the
surface via hydrogen-bonding interactions.12,16,17 As a consequence, large quantities of terminal triethoxysilane groups are
available to react with the OH groups on the base of the ZL
crystals.18
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ZL-(Eu-TTA)-quartz monolayers
quartz plates according to the SEM images at different magnifications. The uniform orientation and the high degree of
coverage indicate that the interaction between the crystal base
and the substrate is much stronger than any other interactions.9
This implies that a large number of strong covalent linkages
between the crystals and the substrate are formed, which might
derive from the nucleophilic substitution of the terminal triethoxy groups of TTA on the substrate surface by the surface
hydroxyl groups on the base of the ZL crystals. Although further
study is necessary, we proposed for the moment that considerable amounts of TTA are standing on the substrate surface
A luminescent c-oriented monolayer of ZL can be obtained by
a facile way of using Si-TTA as the covalent linker in the presence
of Eu3+ ions as Si-TTA can sensitize the luminescence of Eu3+ via
a so-called antenna effect.15 Contacting (Eu-TTA)-quartz plates
with ZL crystals under vigorous sonication yields luminescent
oriented ZL monolayers ((Eu-TTA)-quartz-monolayer), the
SEM images of which are shown in Fig. 3 with different
magnifications. It reveals that the degree of coverage and of
dense packing become much higher upon introduction of Eu3+
ions to the substrate surface by comparing Fig. 3 to Fig. 2. The
dense packing is very homogenous over the entire plate as
illustrated in Fig. 3 with different magnifications of the SEM
images. This means the introduction of Eu3+ions is beneficial for
the preparation of homogenous and uniformly oriented monolayer with dense packing and a high coverage degree. It is
reasonable to propose that the additional stabilization of TTA
moieties through the coordination of Eu3+ ions can favor the
standing of Si-TTA on the substrate surface as is suggested by
Scheme 1. Further investigations are, however, desirable to shine
light on and allow a detailed understanding of the structure of
the very perfect layers. The oriented monolayer displays bright
red light upon irradiation under a UV lamp. As shown in Fig. 4,
a broad band ranging from 220 to 350 nm can be observed for the
excitation spectrum, which is also observed in Fig. 1 and can be
assigned to the absorption of TTA moieties in the functional
molecular linker. Excitation into this broad band leads to sharp
emission lines attributed to the 5D0 / 7F0–4 transitions for the
Fig. 2 SEM images of a zeolite L monolayer of ZL-TTA-quartz at
different magnifications.
Fig. 3 SEM images of a zeolite L monolayer of ZL-(TTA-Eu)-quartz at
different magnifications.
Fig. 1 Excitation (dotted line) and emission (solid line) spectra. The
excitation spectrum was obtained by monitoring the 5D0 / 7F2 emission
at 612 nm, and the emission spectrum was obtained upon excitation at
336 nm. Both spectra were measured at room temperature in air.
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Fig. 4 Excitation (dotted line) and emission (solid line) spectra of
a monolayer of ZL-(TTA-Eu)-quartz. The excitation spectrum was
obtained by monitoring the 5D0 / 7F2 emission at 612 nm, and the
emission spectrum was obtained upon excitation at 336 nm. Both spectra
were measured at room temperature in air.
Eu-TTA-quartz monolayer. The dominant emission line corresponds to the 5D0 / 7F2 transition line at 612 nm, which is
responsible for the bright red color.
Similarly, assembling guest-loaded ZL crystals on the (EuTTA)-quartz plate leads to oriented layers with the same high
quality according to Scheme 1. 4-Fluorobenzophenone (FBP)
molecules were firstly inserted into the nanochannels of Tb/ZL
resulting in (Tb-FBP)/ZL, (Tb-FBP)/ZL was then organized on
the (Eu-TTA)-quartz plate to form the luminescent ZL
monolayers, (Tb-FBP)/ZL-(Eu-TTA)-quartz. Fig. 5 shows the
excitation (a) and (b) emission spectra of the (Tb-FBP)/ZL-(EuTTA)-quartz monolayer. The excitation spectra shown in Fig. 5a
were obtained by monitoring the 5D0 / 7F2 transition of Eu3+ at
612 nm and 5D4 / 7F5 transition of Tb3+ at 544 nm, respectively.
A broad band peaking at 350 nm and 290 nm can be observed in
the excitation spectra and no sharp line from the Ln3+ ions can be
observed, which can be ascribed to the absorption of TTA
moieties on the substrate surface and of FBP loaded in the
nanochannels of zeolite crystals of the oriented layers, respectively. The emission spectra excited at different wavelengths
show characteristic sharp bands at 488, 544, 584 and 622 nm
attributed to the f–f transition of Tb3+ (5D4 / 7FJ, J ¼ 6,5,4,3)
with 5D4 / 7F5 transition as the dominant feature that is
responsible for the green color. The bands at 578, 590,612, 650
and 700 nm are ascribed to 5D0 / 7FJ (J ¼ 0–4) transitions of
Eu3+ ions with the 5D0 / 7F2 line as the most intense feature that
is responsible for the red color. As shown in Fig. 5b, the relative
intensities of most bands change when changing the excitation
wavelength, leading to a different color of the plates. The red
component decreases significantly upon increasing the excitation
wavelength from 270 to 350 nm. The FBP molecules absorb most
of the light at short excitation wavelength and transfer the light
to Tb3+ ions while most of the light is absorbed by TTA moieties
on the surface and is transferred to Eu3+ ions when the excitation
is longer than 340 nm. As a consequence, the emission color of
the monolayer can be tuned by varying the ratio of Eu3+ ions
coordinated to the molecular linker and Tb3+ ions loaded in the
nanochannels of zeolite crystals and by changing the excitation
wavelength. The changing of the emission color can be seen in
Fig. 5c.
Fig. 5 The excitation (a) and emission (b) spectra of the (Tb-FBP)/-ZL-(Eu-TTA)-quartz monolayer. The excitation spectrum was obtained by
monitoring the 5D0 / 7F2 emission at 612 nm (dot line), and by monitoring the 5D4 / 7F5 at 544 nm (solid line), respectively. The emission spectra were
obtained after excitation at different wavelengths. (c) Photographs of such a layer upon UV excitation at different wavelengths. The white spots are
reflections from the light source.
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Conclusion
Fig. 6 Photographic images of a ZL-(TTA-Eu)-quartz monolayer. Left:
uncoated monolayer, right: similar sample coated with PVA. The background is the SEM image of the layer.
Transparent oriented monolayers
The c-oriented ZL monolayers exhibit a hierarchy of structure
presenting a successive ordering from the molecular up to the
macroscopic scale. However, such a ZL monolayer is usually
opaque or semitransparent and it therefore strongly scatters light
in the visible region. For some of applications in the development
of optical devices, transparency is highly desirable. Coating the
polymer of CR39 on the monolayer by Calzaferri and his
co-workers makes it transparent due to the index matching
between the CR39 and the ZL crystals,19 transparency and spectroscopy of dye loaded ZL layers were discussed in detail in
chapter 9 of ref. 8a. We herein present an alternative and facile
way to prepare transparent oriented monolayer by directly
immersing the washed ZL monolayer (ZL-(Eu-TTA)-quartz
monolayer) into the aqueous solution of PVA for less than one
minute and then drying the PVA-coated layer in an oven for
10 min. As shown in Fig. 6, the oriented ZL monolayer becomes
completely transparent upon the coating of the polymer of PVA
on top of the ZL monolayer. Furthermore, both the emission
intensity and the shape of the emission spectrum were not obviously changed after the coating of the polymer as revealed in
Fig. 7, indicating that no leaching of Eu3+ coordinated to the TTA
moieties tethered on the quartz surface occurs during the process.
In summary, triethoxysilylated b-diketone (TTA-Si) proves to be
an ideal multifunctional molecule linker that has the ability to
coordinate and sensitize Eu3+ ions and to bind on a quartz plate
for achieving very high organization of ZL microcrystals.
Luminescent, homogenous and uniformly oriented ZL monolayers with very high quality can be realized by organizing ZL
microcrystals onto TTA-Si and Eu3+ ions co-functionalized
quartz plates. The stabilization of TTA-Si on the quartz surface
through coordination to the Eu3+ ions could account for the
formation of the very perfect monolayers. The color of the
luminescence of the layers can be fine tuned by changing
the excitation wavelength. Transparent layers with all the
crystals and therefore all of their channels oriented in the same
direction are obtained by coating the polymer of PVA on the
monolayers. The results are of great interest for application in
different fields of optoelectronics and sensing.
Experimental section
Materials
2-Thenoyltrifluoroacetone (TTA), 4-fluorobenzophenone (FBP),
PVA and 3-isocyanatepropyltriethoxysilane were purchased
from Aldrich. The triethoxysilylated molecule (TTA-Si) was
synthesized and characterized according to the reported
method.20 ZL microcrystals were synthesized and characterized
as described in ref. 21. Quartz plates (10 20 mm) were dipped
into an acid bath consisting of potassium dichromate and
sulfuric acid for 12 h to remove possible organic residue on the
surface. The plates were then washed with deionized water and
dried at 80 C in clean air for 3 h. Solutions of EuCl3 and TbCl3
in ethanol were prepared by dissolving Eu2O3 and Tb4O7,
respectively, in concentrated hydrochloride acid. Tb3+-exchanged
zeolite L (Tb/ZL) samples were prepared by an ion-exchange
method as described in ref. 8c. Tb/ZL samples were degassed and
dried for 2 h at 423 K and then kept in contact with the vapor of
FBP at 120 C overnight. The resulting material ((Tb-FBP)/ZL)
was washed with EtOH in order to remove only
physically adsorbed FBP molecules, and dried at 50 C under
vacuum for 6 h.
Modification of quartz plates with TTA-Si (TTA-quartz)
Typically, the quartz plates were immersed in a solution of TTASi (0.5 mmol) in dry THF (10 mL) in a round-bottomed Schlenk
flask and refluxed for 3 h under a N2 atmosphere, cooled to room
temperature, and washed with copious amounts of THF, the
quartz plates were finally dried for approximately 2 h at 40 C in
air.
Preparation of c-oriented zeolite L monolayers
Fig. 7 Luminescence of the uncoated layer (solid line) and similar layer
coated with PVA (dotted line).
This journal is ª The Royal Society of Chemistry 2011
ZL-TTA-quartz monolayer: an excess of ZL (10 mg) was added to
toluene (10 mL) and sonicated for approximately 30 min, the
pretreated quartz plates (TTA-Quartz) were introduced, and the
mixture was then sonicated for 15 min. The opaque quartz plates
coated with ZL were sonicated in toluene for 5 s to remove
the physisorbed crystals; ZL-(Eu-TTA)-quartz monolayer:
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TTA -quartz plates were immersed into a solution of EuCl3 in
ethanol in a round-bottomed Schlenk flask at room temperature
for 5–10 min, the functionalized quartz plates ((Eu-TTA)-quartz)
were washed with copious amounts of EtOH. An excess of ZL
(10 mg) was added to the toluene (10 mL) in a round-bottomed
Schlenk flask and sonicated for approximately 30 min, (EuTTA)-quartz plates were introduced and the mixture was sonicated for 15 min. The opaque quartz plates coated with zeolite L
crystals were sonicated in a fresh toluene solution for 5 s to
remove the physisorbed crystals. (Tb-FBP)/ZL-(Eu-TTA)quartz monolayers were prepared similarly except that the
(Tb-FBP)/ZL sample was used.
2
3
4
5
Oriented monolayers coated with polymer
Typically, the layers were immersed into the transparent and
viscous aqueous solution of PVA (0.05 g mL1) which was
prepared by heating PVA and water at 150 C and pulled out
quickly, the procedure was repeated twice and the polymer
coated monolayers were dried in the oven at 80 C for 10 min,
and transparent monolayers were obtained.
6
7
8
Physical measurements
Infrared (IR) spectra were obtained on a Bruker Vector
22 spectrometer using KBr pellets for solid samples, from
400–4000 cm1 at a resolution of 4 cm1 (16 scans collected). 1H
NMR spectra were obtained on a Bruker ARX 300 apparatus.
SEM images were obtained using a FE-SEM (Hitachi S-4300) at
an acceleration voltage of 10 kV. The steady-state luminescence
spectra were measured on an Edinburgh Instruments FS920P
spectrometer, with a 450 W xenon lamp as the steady-state
excitation source, a double excitation monochromator
(1800
lines
mm1),
an
emission
monochromator
1
(600 lines mm ), and a Hamamatsu RMP928 photomultiplier
tube.
9
10
11
12
13
14
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 20871040, No. 20901022),
Program for New Century Excellent Talents in University
(NCET-09-0113), Tianjin Natural Science Foundation
(09JCYBJC05700), the Key Project of Chinese Ministry of
Education (208016), Hebei Province Natural Science Foundation for Distinguished young Scholar (No. B2010000034), and
Hebei Province Natural Science Foundation (B2009000013).
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