Liquid exfoliation of alkyl-ether functionalised layered metal-organic frameworks to nanosheets

Article abstract of DOI:10.1039/c6cc05154e

We report the synthesis of a 2D-layered metal-organic framework incorporating weakly interacting chains designed to aid exfoliation of the layers into nanosheets. Dispersion of the nanosheets exposes labile metal-sites which are shown to exchange solvent

Full text of DOI:10.1039/c6cc05154e

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Liquid Exfoliation of Alkyl-Ether Functionalised Layered
Metal-Organic Frameworks to Nanosheets
Cite this: DOI: 10.1039/x0xx00000x
Jonathan A. Foster,*a Sebastian Henke,b Andreas Schneemann,b Roland A.
Fischer c and Anthony K. Cheethamd
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report the synthesis of
a 2D-layered metal-organic exfoliation and interacting with solvent molecules to enhance
framework incorporating weakly interacting chains designed to dispersion.
aid exfoliation of the layers into nanosheets. Dispersion of the
Furthermore, exfoliation of the frameworks opens up access
nanosheets exposes labile metal-sites which are shown to to the labile coordination sites at the axial positions of the PWs
exchange solvent molecules allowing the nanosheets to act as which would otherwise be buried within the densely packed
sensors in suspension.
MOF structure. MOFs with labile metal-sites are of particular
interest owing to their potential as catalysts and sensors and 2D
nanosheets are ideal for such applications owing to their large
surface areas and the periodic array of active sites they
present.27 We investigate the effect of different coordinating
solvents on the MOFs structure and calculate the binding
constant of pyridine to a suspension of nanosheets through a
UV-vis spectroscopic titration.
The liquid exfoliation of layered materials into two-dimensional
nanosheets has attracted much recent attention, most
prominently with graphene but also many other layered oxides,
clays and organic polymers.1-4 Metal-organic frameworks
(MOFs) combine the tunability of organic linkers with the
unique properties of metal ions allowing the creation of diverse,
well defined architectures with tailored properties.5-8 This
flexibility has allowed them to be developed for applications as
diverse as ‘smart’ materials and sensors to light harvesting, gas
2,5-Bis(3-methoxypropoxy)-1,4-benzenedicarboxylate,
compound H2(
etherification
1
),28 was synthesised via Williamson
of
dimethyl
2,5-dihydroxy-1,4-
storage and drug delivery.6, Despite the large number of
9
benzenedicarboxylate with 1-bromo-3-methoxypropane.25
layered MOFs in the literature, only a small number of examples
of metal-organic framework nanosheets (MONs) have so far
been reported.10-22
Heating of H2(1) with copper nitrate in DMF in a sealed reaction
vial at 110°C for 18 h resulted in the formation of a green
microcrystalline powder. When the synthesis was repeated
using zinc nitrate in place of copper nitrate, clear cubic crystals
were obtained which were identified as Zn(1)(DMF) by single
crystal X-ray diffraction (Fig. 1). The framework crystallises in
A number of methods for the exfoliation of layered
materials have been developed, most straight forwardly in the
case of neutral layered MOFs by the use of ultrasound to
preferentially break apart weaker interactions between layers
in the structures.15-22 MONs based on the robust metal-paddle
wheel (PW) secondary building unit were some of the first to be
exfoliated to form nanosheets.10, 18, 19 The Fischer group have
previously reported a series of 1,4-benzenedicarboxylate
derived linkers functionalised with a range of different short
chains which can adopt a number of MOF structures.23-26 Here
we report the first in a new series of layered MOFs based on the
PW motif incorporating alkyl ether functional groups which we
hypothesised would be positioned between the layers, aiding
the triclinic space group P1 with the expected PW building units.
Four carboxylate linkers
1 coordinate to the Zn2-paddlewheel
and interconnect these units in the two-dimensional sql
topology29, while DMF coordinates to the axial sites of the PWs.
Hence, the lamellar structure features strong metal-carboxylate
bonding within the layers and weakly interacting 3-
methoxypropoxy groups between the layers.
Analysis (Pawley refinement) of the X-ray powder diffraction
pattern (XRPD) of the copper derivative Cu(
1)(DMF) clearly
indicates the material is isostructural with Zn( )(DMF) (ESI
1
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Fig. 1: Reaction scheme for the synthesis of M(1)(DMF) (with M
= Cu, Zn) showing crystal structure of Zn(1)(DMF) viewed along
the b-axis highlighting the lamellar structure with an interlayer
spacing of ~9.4 Å) (~9.6 Å for Cu(1)(DMF)). C, N and O atoms
Fig. 2: a) Images showing a suspension of Cu(1) nanosheets in
DMF with characteristic Tyndal Scattering of laser light. AFM
images and height profiles corresponding to blue lines for
are shown in grey, blue and red. The coordination polyhedra of
the M ions are displayed in green. H atoms have been omitted
for clarity.
nanosheets formed by exfoliation of b) Zn(
H2O.
1) in DMF c) Cu(1) in
Section S3.3). Small differences in the unit cell parameters (ESI
Table S2) are ascribed to the different ionic radii and different
ligand field effects of Zn2+ and Cu2+. XRPD analysis of a bulk
sample of Zn(1)(DMF) revealed a second phase which was
identified as the previously reported23 3D cubic Zn4O(
framework structure (isoreticular to MOF-5).30
noting that no systematic broadening of the peaks in the XRPD
pattern resulting from the reduced dimensions of the
nanosheets are observed; this may be due to the dominant
contributions of larger crystalline particles or indicate
restacking of the MONs in the concentrated suspension.
The incorporation of alkyl ether linkages into the
frameworks was intended to aid their dispersion into aqueous
1)
3
The frameworks were sonicated in DMF with
a
concentration of 1 mg/mL for 30 minutes and the resulting
suspension was centrifuged at 1500 rpm for 45 minutes to
remove larger particles. The supernatant appeared clear but
showed evidence of Tyndal scattering when irradiated with a
laser (Fig. 2a) indicating the formation of a fine suspension. The
supernatant was deposited onto a quartz crystal and imaged
using atomic force microscopy (AFM) to reveal nanosheets with
thicknesses ranging from 5-30 nm and lateral dimensions
between 100-500 nm (Fig. 2b and ESI Section 4). The crystal
suspensions. The exfoliation of Cu(1)(DMF) into water was
therefore investigated using the protocol previously described.
AFM images of the MONs deposited from aqueous suspensions
onto quartz showed typical thicknesses ranging from 2-30 nm
and lateral dimensions between 50-500 nm (Fig. 2c). XRPD
patterns (Fig. 3e) indicate the formation of a distinct pattern
which could be indexed and refined in the tetragonal space
group I41cd (ESI Section 3.3). The new phase is ascribed to the
exchange of DMF at the axial sites of the PWs against the
structure of Zn(1)(DMF) predicts a single layer thickness of
approx. 14.5 Å and a shorter interlayer spacing of approx. 9.4 Å
(due to interpenetration of the 3-methoxypropoxy groups). This
smaller water molecule, to yield Cu(1)(H2O), which features a
different stacking of the layers. Unfortunately, ab initio
structure solution from the XRPD patterns was unsuccessful due
to the complexity of the organic linker with flexible
substituents. However, space group symmetry and unit cell
dimensions clearly indicate that the 2D sql topology of the
framework is maintained. The space group further suggests the
presence of four layers in the unit cell, which are arranged in a
staggered arrangement (ESI Fig. S20). Consequently the
indicates
a
minimum thickness of the nanosheets
corresponding to approximately 5 MOF layers, comparable to
those observed for related PW based MONs.19
The MONs were isolated by centrifugation of the
suspensions at 4000 rpm for 30 minutes after which no Tyndal
Scattering was observed in the remaining solutions. XRPD
patterns of concentrated suspensions of the nanosheets
measured using Kapton capillaries showed that the material
matched that of the bulk material indicating no structural
rearrangements had taken place (Fig. 3b and c). It is worth
interlayer distance is only ~7.8 Å for Cu(
~9.6 Å for Cu( )(DMF).
1)(H2O) compared to
1
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Fig. 4: UV-vis titration showing addition of aliquots of a pyridine
solution to an aqueous suspension of Cu( ) nanosheets. Insert
1
shows a plot of change in absorbance against concentration of
pyridine which is fitted to calculate the binding constant for
pyridine to the nanosheets (Ka = 30 ± 8 M-1).
phase (see ESI for details). Samples exfoliated into acetone
showed a mixture of the new phase and the original Cu(1)(DMF)
material. IR patterns for the samples derived from acetone,
ethanol and acetonitrile matched closely and no peaks
corresponding to the relevant solvent molecules could be
identified (ESI Fig. S21). Likewise, thermogravimetric
analysis(TGA) of these three samples showed no significant
weight loss corresponding to solvent removal upon heating the
Fig. 3: a) Calculated XRPD pattern for Zn(
XRPD patterns for b) nanosheets of Cu(
solids collected following exfoliation of Cu(
1
1
)(DMF). Experimental
) exfoliated in DMF*#,
)(DMF) in c) DMF,
1
samples up to 260˚C (ESI Fig. S22). Contrary, for Cu(
1)(DMF) and
d) 10% DMF-water, e) water, f) water, followed by reimersion in
DMF for 4 days*#, g) acetonitrile, h) acetone*, and i) ethanol*.
Patterns marked with an * were measured using Kapton
capillaries to prevent solvent loss, those with a # are background
subtracted patterns, other measurements were collected using
dried powders on flat substrates.
Cu( )(H2O) solvent loss was observed upon heating to 260°C. On
the basis of these observations, we suggest that the phases
derived from acetone, ethanol and acetonitrile are identical and
1
exhibit the formula Cu(1). We propose that in these structures
the methoxy-oxygen atoms of the flexible 3-methoxypropoxy
chains intramolecularly (or intermolecularly) coordinate to the
PW in place of more weakly coordinating, volatile solvent
molecules (MeCN, (Me)2CO or EtOH). Similar side group effects
have been proposed for IRMOFs featuring flexible, methoxy-
terminated substituents.23 High resolution synchrotron data are
required to undertake detailed structural studies on these
materials but it is useful to highlight the strong effect of
exfoliation into different solvents on the MON structure which
has not been documented in other studies on MOF nanosheets.
The photophysical properties and large surface area of the
nanosheets in aqueous suspension were exploited to act as
sensors for the detection of a model guest compound, pyridine.
Related MONs have previously been shown to function as
sensors for ethylamine when coated on a surface,18 but here we
show they can be used in suspension and are able to quantify
the binding interaction. A calibration curve, obtained by serial
Cu(1)(DMF) was exfoliated into DMF-water mixtures (50%,
25%, 10% and 1% DMF by volume). Analysis of the resulting
XRPD patterns shows a gradual transition from the triclinic
Cu(1)(DMF) structure to the tetragonal Cu(1)(H2O) structure
with a mixture of the two phases observed in 10% DMF:water
(Fig. 3d, see ESI for details). Samples initially exfoliated in water,
were collected by centrifugation and redispersed in DMF. XRPD
patterns showed complete conversion back to the DMF
structure over 4 days (Fig. 3f) when the DMF was repeatedly
changed. These results indicate that the lamellar PW structure
is retained in aqueous suspensions with coordinated DMF
molecules at the axial position exchanging with smaller H2O
molecules to give Cu(
The exfoliation of Cu(
solvents was also investigated. XRPD patterns for Cu(
exfoliated into acetonitrile, acetone and ethanol (Fig. 3g-i) could
all be indexed in a triclinic cell similar to that of Cu( )(DMF) but
1
)(H2O).
)(DMF) into other coordinating
)(DMF)
1
1
dilution of
a suspension containing a known mass of
Cu( )(DMF), was used to estimate the concentration of the
1
nanosheets in aqueous suspension as 0.55 mg mL-1 or 1.16 mM
based on the relative molecular mass of the starting material
(ESI Fig. S28).4 This compares with estimations of the
concentration of nanosheets following exfoliation in DMF of
1
with shorter interlayer distance and smaller unit cell volume.
The metrics of the unit cell further give a clear indication of a
slipping of the layers with respect to the original Cu(1)(DMF)
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0.02 mg mL-1 for Cu( )(DMF) and 0.11 mg mL-1 for Zn(
1 1)(DMF)
calculated using the same method.
A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K.
Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science,
2011, 331, 568-571.
Aliquots of a solution of pyridine in the host suspension
(0.22 mM) were titrated into freshly diluted aqueous
suspensions of nanosheets (0.29 mM) and monitored by UV-vis
spectroscopy. Addition of pyridine resulted in a bathochromic
shift in the λmax of the adsorption band of the MONs from 302-
306 nm and a 1.5 fold increase in absorption intensity (Fig. 4).
This data was used to calculate a binding constant for pyridine
to the nanosheets in water as Ka = 30 ± 8 M-1. These changes are
consistent with the expected substitution of water molecules
for pyridine at the axial position of the copper PWs resulting in
5. H. C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673-
674.
6. H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science,
2013, 341.
7. F. A. A. Paz, J. Klinowski, S. M. F. Vilela, J. P. C. Tome, J. A. S.
Cavaleiro and J. Rocha, Chem. Soc. Rev., 2012, 41, 1088-1110.
8. S. M. Cohen, Chem. Rev., 2012, 112, 970-1000.
9. A. Schneemann, S. Henke, I. Schwedler and R. A. Fischer,
Chemphyschem, 2014, 15, 823-839.
10. T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F.
Kapteijn, I. X. F. X. Llabres and J. Gascon, Nat Mater, 2015, 14, 48-
55.
11. C. Marti-Gastaldo, J. E. Warren, K. C. Stylianou, N. L. O. Flack and
M. J. Rosseinsky, Angew. Chem. -Int. Ed., 2012, 51, 11044-11048.
12. S. C. Junggeburth, L. Diehl, S. Werner, V. Duppel, W. Sigle and B.
V. Lotsch, J. Am. Chem. Soc., 2013, 135, 6157-6164.
13. A. Gallego, C. Hermosa, O. Castillo, I. Berlanga, C. J. Gomez-
Garcia, E. Mateo-Marti, J. I. Martinez, F. Flores, C. Gomez-
Navarro, J. Gomez-Herrero, S. Delgado and F. Zamora, Adv.
Mater., 2013, 25, 2141-2146.
14. T. Araki, A. Kondo and K. Maeda, Chem. Commun., 2013, 49, 552-
554.
15. J.-C. Tan, P. J. Saines, E. G. Bithell and A. K. Cheetham, Acs Nano,
2012, 6, 615-621.
changes in the absorption band of the coordinated
31
functionalised dicarboxylate 1.
In summary, we have demonstrated how ultrasound can be
used to exfoliate layered copper and zinc based metal-organic
frameworks functionalised with flexible methoxy-terminated
side chains to produce suspensions of nanosheets in a range of
solvents. Exfoliation of the nanosheets exposes labile metal-
sites which are shown to exchange solvent molecules inducing
photophysical changes which allow for the sensing of pyridine.
This study represents the first in a potentially large series of
metal-organic framework nanosheets in which different
functionalities are introduced to the nanosheets surface to aid
exfoliation and facilitate sensing or catalysis.
16. P. J. Saines, M. Steinmann, J.-C. Tan, H. H. M. Yeung, W. Li, P. T.
Barton and A. K. Cheetham, Inorg. Chem., 2012, 51, 11198-
11209.
17. Y. Peng, Y. Li, Y. Ban, H. Jin, W. Jiao, X. Liu and W. Yang, Science,
2014, 346, 1356-1359.
We thank the Ras al Khaimah centre for Advanced Materials,
Ramsay Memorial Trust (J.A.F) and Alexander von Humboldt
Foundation (S.H.) for financial support.
18. Z.-Q. Li, L.-G. Qiu, W. Wang, T. Xu, Y. Wu and X. Jiang, Inorg.
Chem. Commun., 2008, 11, 1375-1377.
19. P. Z. Li, Y. Maeda and Q. Xu, Chem. Commun., 2011, 47, 8436-
8438.
20. A. Kondo, C. C. Tiew, F. Moriguchi and K. Maeda, Dalton Trans.,
2013, 42, 15267-15270.
21. P. J. Beldon, S. Tominaka, P. Singh, T. S. Dasgupta, E. G. Bithell
and A. K. Cheetham, Chem. Commun., 2014, 50, 3955-3957.
22. P. Amo-Ochoa, L. Welte, R. Gonzalez-Prieto, P. J. Sanz Miguel, C.
J. Gomez-Garcia, E. Mateo-Marti, S. Delgado, J. Gomez-Herrero
and F. Zamora, Chem. Commun., 2010, 46, 3262-3264.
23. S. Henke, R. Schmid, J.-D. Grunwaldt and R. A. Fischer, Chem. Eur.
J., 2010, 16, 14296-14306.
24. S. Henke, A. Schneemann, S. Kapoor, R. Winter and R. A. Fischer,
J. Mater. Chem., 2012, 22, 909-918.
25. S. Henke, A. Schneemann, A. Wütscher and R. A. Fischer, J. Am.
Chem. Soc., 2012, 134, 9464-9474.
26. I. Schwedler, S. Henke, M. T. Wharmby, S. R. Bajpe, A. K.
Cheetham and R. A. Fischer, Dalton Trans., 2016, 45, 4230-4241.
27. J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C.-Y. Su, Chem. Soc.
Rev., 2014, 43, 6011-6061.
28. S. Henke and R. A. Fischer, J. Am. Chem. Soc., 2011, 133, 2064-
2067.
29. M. O’Keeffe, M. A. Peskov, S. J. Ramsden and O. M. Yaghi, Acc.
Chem. Res., 2008, 41, 1782-1789.
30. H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Nature, 1999,
402, 276-279.
Notes and references
University of Sheffield, Department of Chemistry, Sheffield, S3
a
7HF, UK
b
Lehrstuhl für Anorganische Chemie II, Organometallics &
Materials, Ruhr-Universität Bochum, D-44780 Bochum, Germany
c
Department of Chemistry, Technical University Munich,
Lichtenbergstrasse 4, D-85748 Garching, Germany and Catalysis
Research Centre, Technical University Munich, Ernst-Otto-Fischer
Strasse 1, 85748 Garching, Germany
d
University of Cambridge, Materials Science and Metallurgy, 27
Charles Babbage Road, Cambridge, CB3 0FS, UK
Electronic Supplementary Information (ESI) available: Ligand
and MOF synthesis, description of crystal structure, refinement
and PXRD analysis, additional AFM images, binding isotherms.
CCDC 1460747. For ESI and crystallographic data in CIF or other
electronic format See DOI: 10.1039/x0xx00000x
1. V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N.
Coleman, Science, 2013, 340, 1226419-.
2. R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero and F.
Zamora, Nanoscale, 2011, 3, 20-30.
3. X. Zhang and Y. Xie, Chem. Soc. Rev., 2013, 42, 8187-8199.
4. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan,
K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora,
G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam,
J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty,
31. B. Supronowicz, A. Mavrandonakis and T. Heine, J. Phys. Chem.
C., 2015, 119, 3024-3032.
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