Ultrasonic Exfoliation of Hydrophobic and Hydrophilic Metal–Organic Frameworks To Form Nanosheets

Article abstract of DOI:10.1002/chem.201803221

The modular structure of metal–organic framework nanosheets (MONs) provides a convenient route to creating two-dimensional materials with readily tuneable surface properties. Here, the liquid exfoliation of two closely related layered metal–organic framew

Full text of DOI:10.1002/chem.201803221

A Journal of
Accepted Article
Title: Ultrasonic exfoliation of hydrophobic and hydrophilic metal-
organic frameworks to form nanosheets
Authors: David J Ashworth, Adam Cooper, Mollie Trueman, Rasha W.
M. Al-Saedi, Smith D Liam, Anthony J. Meijer, and Jonathan
A. Foster
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To be cited as: Chem. Eur. J. 10.1002/chem.201803221
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Ultrasonic exfoliation of hydrophobic and hydrophilic metal-
organic frameworks to form nanosheets
David J. Ashworth, Adam Cooper, Mollie Trueman, Rasha W. M. Al-Saedi, Liam D. Smith, Anthony J.
H. M. Meijer and Jonathan A. Foster*
Abstract: The modular structure of metal-organic nanosheets
(MONs) provides a convenient route to creating two-dimensional
materials with readily tunable surface properties. Here we report the
liquid exfoliation of two closely related layered metal-organic
frameworks functionalised with either methoxy-propyl (1) or pentyl
(2) pendent groups intended to bestow either hydrophilic or
hydrophobic character to the resulting nanosheets. Exfoliation of the
two materials in a range of different solvents highlighted significant
differences in their dispersion properties as well as their molecular
and nanoscopic structures. Exchange or loss of solvent was found to
occur at the labile axial position of the paddle-wheel based MONs
and DFT calculations indicated that intramolecular coordination by
the oxygen of the methoxy-propyl pendant groups may take place.
The nanoscopic dimensions of the MONs were further tuned by
varying the exfoliation conditions and through “cascade
centrifugation”. Aqueous suspensions of the nanosheets were used
as sensors to detect aromatic heterocycles with clear differences in
binding behaviour observed and quantified.
scalable, top-down approach to producing ultrathin nanosheets
from layered materials.[8] In some cases, immersion of layered
MOFs in solvent has been shown to result in spontaneous
exfoliation of the materials into nanosheets.[9] In most cases
however, additional energy is required to overcome interlayer
interactions in order for exfoliation to occur. A variety of different
methods for the liquid exfoliation of MONs have been
10]
investigated including ball milling,[2b,
freeze-thaw[11] and
intercalation,[6c, 12] with sonication[2b, 10, 13] being the most widely
employed approach. In most cases these processes produce a
broad distribution of particle sizes. Samples are therefore left to
sediment or centrifuged in order to separate out bulk material
from the nanosheets. Top-down approaches are particularly
attractive for the study of new systems as the bulk layered
materials are typically easier to characterize which aids
determining the structure of the nanosheets.
The effect of parameters such as solvent, sonication time
and centrifugation time for the liquid exfoliation of other layered
materials have been extensively studied and optimized.[8a-c] To
date, most studies on the liquid exfoliation of MONs have
focussed on investigating a single framework in a single solvent.
Polar solvents such as acetone and alcohols have most
commonly been employed. Peng et al. reported a mixture of
methanol and propanol as being optimal for exfoliation of a
layered ZIF.[2b] They hypothesize that the small methanol
molecules are able to penetrate into layers whilst propanol
adsorbs onto the surface of the nanosheets through its
hydrophobic tail helping to stabilize the exfoliated nanosheets in
suspension. Junggeburth et al. note that their hydrophobic
Introduction
Metal-organic framework nanosheets (MONs) are free-
standing, nominally two-dimensional materials formed by the co-
ordination
of
organic
ligands
to
metal
ions
or
clusters.[1]{Sakamoto, 2016 #167} A key advantage of MONs
over inorganic nanosheets such as graphene, boron nitride and
molybdenum disulfide is that their modular structure allows for
ready tuning of their properties. This tunability, combined with
their large external surface area and high aspect ratio, makes
MONs ideal for a diverse range of applications including
separation,[2] sensing,[3] templation,[4] electronics[5] and
catalysis.[6] As with other nanosheets, understanding how to
form concentrated suspensions of high aspect ratio nanosheets
is an important technological challenge.[7] The modular structure
of MONs potentially provides advantages over simple inorganic
nanosheets in allowing easy modification of surface
functionalities to enable nanosheets to be designed for use in
particular solvents. However, their porosity, flexibility, lability and
potential for structural rearrangements also present additional
challenges in undertaking this type of study.
layered
MOF
showed
decreasing
exfoliation
in
THF>toluene>CHCl3.[14] Poor exfoliation was observed when
using the polar solvents DMF and H2O which was attributed to
an inability of the solvents to efficiently penetrate between the
hydrophobic interlayer space. In contrast, Moorthy and
coworkers investigated exfoliation of a layered MOF in which
there was hydrogen bonding between the layers.[15] They found
a correlation between the Gutmann’s hydrogen-bond-accepting
parameter of the solvent used and the intensity of fluorescence
of nanosheets formed following exfoliation. These studies
highlight the different roles that different solvent molecules can
play in aiding exfoliation of different layered MOFs and
stabilizing the resulting nanosheets.
Liquid exfoliation provides an attractive, simple and
Mr D. J. Ashworth, Mr A. Cooper, Miss M. Trueman, Ms R. Al-Saedi,
Mr L. Smith, Prof. A. J. H. M. Meijer, Dr J. A. Foster
Department of Chemistry
University of Sheffield
Sheffield, S3 7HF
E-mail: jona.foster@sheffiled.ac.uk
Supporting information for this article is given via a link at the end of
the document
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Figure 1. Structure of ligands H21 (a) and H22 (b). c) Paddlewheel SBU, with DMF coordinated in the axial positions. d) X-ray crystal structure showing layered
structure of Zn(1)(DMF).
In our work we seek to design new layered MOFs which
incorporate features intended to enhance their exfoliation and
stabilize the resulting MONs in suspension. We recently
isostructural with the single crystal structure that we have
previously reported for Zn(1)(DMF).[13g] In this structure four
carboxylate linkers are coordinated to the M2-paddlewheel (PW)
while DMF coordinates to the axial sites of the PWs. Importantly,
in this form the weakly interacting 3-methoxypropoxy groups or
pentyl chains are positioned between the layers whilst there is
strong metal-carboxylate bonding within the layers. Small
differences in the unit cell parameters (ESI Table S1) for the
copper complexes are ascribed to the different ligand field
effects and different ionic radii of Zn2+ and Cu2+ and to
substitution of the oxygen for a methylene in case of 2.
Elemental analysis is consistent with the proposed formulas and
IR and TGA analysis confirms the presence of coordinated DMF
in these structures.
communicated
a study reporting the liquid exfoliation of
Cu(1)(DMF), a layered MOF incorporating weakly interacting
methoxy-propyl chains designed to aid exfoliation of the layers
into nanosheets.[13g] The nanosheets are based on the popular
metal-paddlewheel secondary building unit (SBU) which has a
labile, lewis acidic axial coordination site which makes it ideal for
a wide range of sensing, catalytic, electronic, separation and
storage applications.[2c, 3b, 6b, 6c, 6e] We hypothesized that liquid
exfoliation of layered metal-organic frameworks functionalized
with either hydrophobic or hydrophilic functionalities would
produce nanosheets with different concentrations, stabilities and
thicknesses in different solvents. To investigate this, we
compared the liquid exfoliation of the relatively hydrophilic
methoxy-propyl functionalized MOF with an isostructural MOF
incorporating a more hydrophobic pentyl-chain in a wide range
of different solvents. We then investigated the molecular and
nanoscopic structure of the resulting nanosheets in selected
solvents under different conditions in order to understand and
optimize the exfoliation process.
Liquid exfoliation
Exfoliation experiments were undertaken using a bath
sonicator. We undertook preliminary experiments investigating
the effect of different variables on the degree of exfoliation using
DMF and isopropanol as model solvents. Different powers (320
W at 30% and 100%), frequencies (37 kHz, 80 kHz) and
temperature of sonication were investigated. It was found that
high power produced higher concentrations of material in
suspension and high frequency increased concentration and
avoided dissolution of the nanosheets (Figure S4). Sonication
was applied using a sweep mode and samples were rotated
through the bath using an overhead stirrer in order to ensure
samples were irradiated evenly. Sonication is known to be more
effective at lower temperature[17] and the temperature was
maintained over the course of the experiment using a cooling
coil giving a temperature of around 16°C. The set-up for
exfoliation is shown in ESI Figure S2.
The following protocol was therefore established for the
exfoliation of the MOFs which was used unless stated otherwise.
The layered MOFs were weighed into glass vials to which
solvent was added (5 mg in 6 mL) and then exfoliated in a
sonicator bath at a frequency of 80 kHz for 30 minutes at a
temperature of <20°C. The samples were then centrifuged at
1500 rpm for 10 minutes to remove larger particles and care was
taken to avoid redispersion of the sediment during transport. UV-
vis spectra were measured using the top 3 mL of suspension
and highly absorbing samples diluted as required using further
solvent.
Results and Discussion
Synthesis of layered MOFs
Compounds H2(1) and H2(2) were synthesized via Williamson
etherification of dimethyl 2,5-dihydroxy-1,4-benzenedicarboxylic
acid with 1-bromo-3-methoxypropane and 1-bromopentane
respectively. The difference in polarity of the ligands was evident
during deprotection of the ligands. Compound H21 was readily
obtained from the corresponding methyl ester by heating under
reflux in aqueous NaOH solution.[13g] Under the same condition
only partial deprotection of 2 occurred due to poor solubility so
an alternative method involving 1:1 THF/5% KOH(aq) was
employed.[16] Both compounds were achieved in good yields and
the purity of the compounds was established by NMR, mass
spectrometry, and elemental analysis.
Heating of H2(1) or H2(2) with copper nitrate in DMF in a
sealed reaction vial at 110°C for 18 h resulted in the formation of
green microcrystalline powders. Attempts to grow single crystals
of these materials were unsuccessful. However, XRPD analysis
of the microcrystalline powders indicates these structures are
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Figure 2. Cu(1)(DMF) and Cu(2)(DMF) are represented with blue and red data, respectively. a, b) UV-vis spectral traces of MONs in suspension following
exfoliation in DMF (a) and water (b). c) plots of concentrations of nanosheets in suspension, following exfoliation and centrifugation; d, e) normalized
concentrations of MON suspensions plotted against the solvent’s surface tension (d) and Hansen solubility parameter of energy from dipolar intermolecular force
(e).
The solvent that the nanosheets are exfoliated into was
expected to have a large effect on the degree of exfoliation and
the stability of the resulting suspension. An initial screen of 23
different solvents was undertaken. However some solvents had
to be excluded due to their UV-vis cut-off points preventing
analysis or their high viscosity resulting in poor dispersion and
centrifugation (ESI Table S2). A selection of the 11 solvents
to the dicarboxylate ligand was used in all subsequent analysis.
Neither compound was able to form stable dispersions in either
cyclohexane or hexane, nominal values of zero are therefore
used for these solvents in the subsequent analysis.
The extinction coefficient for the compounds in each
solvent was estimated by dilution of a suspension containing a
known mass of each compound. Values ranged from 1892 -
representing
a
diverse range of polarities and chemical
6693 mol-1 dm3 cm-1 for Cu(1)(DMF) to 2467
-
4489
functionalities were selected for further investigation: water,
dimethylsulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP),
mol-1 dm3 cm-1 for Cu(2)(DMF). These differences in spectra are
attributed to exchange of the coordinated DMF, variations in
ligand geometry in the different solvents and differences in
particle size which are discussed in detail later in this article.
Clear differences were observed in the concentration of
exfoliated material in suspension following sonication and
centrifugation of Cu(1 or 2)(DMF) in different solvents. Figure 2
shows a plot of the concentration in mM of Cu(1)(DMF) [blue] or
Cu(2)(DMF) [red] suspended in different solvents listed in order
of increasing polarity (left to right) as measured by UV-vis
spectroscopy. Data shown are the average of four repeats.
dimethylacetamide
(DMA),
dimethylformamide
(DMF),
acetonitrile (MeCN), isopropanol (IPA), tetrahydrofuran (THF),
diethylether (Et2O), cyclohexane and hexane.
Both compounds typically show a single major absorption
band the λmax of which ranged between 271-303 nm depending
on the solvent used (ESI Figure S5). Absorption bands were
generally broader and less well defined for Cu(1)(DMF),
particularly in poorly coordinating solvents such as diethylether,
THF and acetonitrile. In acetonitrile, a second local maxima was
observed at 361 nm and 304 nm for Cu(1)(DMF) and
Cu(2)(DMF) respectively. The MLCT band was typically too
weak and broad to be distinguished so the major peak attributed
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Figure 3. Experimental powder diffraction patterns of Cu(1)(DMF) and Cu(2)(DMF) as-synthesised (dark blue), and of post-exfoliation solids recollected through
centrifugation for 1 hr at 1500 rpm, in DMF, NMP, MeCN, diethyl ether and water.
functionalized MOF indicate that they are only “relatively”
At either extreme, the more hydrophilic Cu(1)(DMF)
showed a significantly higher degrees of dispersion in water than
Cu(2)(DMF) whilst the opposite is true in diethyl ether where the
more hydrophobic Cu(2)(DMF) is present at significantly higher
concentrations. Higher concentrations are observed for
Cu(1)(DMF) in all solvents except diethyl ether and DMA. DMSO
and NMP give the highest concentrations of both materials and
significantly higher than DMA and DMF which have very similar
polarities. Samples of both compounds exfoliated into
cyclohexane and hexane showed negligible absorbance
following centrifugation whilst only Cu(2)(DMF) showed any
absorbance following exfoliation into toluene.
hydrophobic and hydrophilic. It should also be noted that this
experiment provides a comparison of the concentration of
material in suspension following exfoliation in different solvents,
not necessarily the suitability of the solvents to form nanosheets.
A detailed discussion of the nanoscopic dimensions of the
materials produced following exfoliation in different solvents is
presented in the section entitled nanoscopic analysis later in the
paper. First, the differences in UV-vis spectrum observed for the
materials in different solvents also led us to question the
composition of the exfoliated material which we discuss in the
following section.
In studies of other nanosheets formed by liquid exfoliation,
a wide range of solubility pararmeters have been put forward as
Structural Analysis
The relatively labile nature of coordination bonds and the high
surface area of the nanosheets mean that it cannot be assumed
that the MOF structure is unchanged following liquid exfoliation.
In particular, the axial site on the copper paddlewheel is known
to be highly labile, allowing for the possibility of loss or exchange
of the coordinated DMF molecules with those of the exfoliation
solvent. We previously observed differences in the XRPD
patterns of Cu(1)(DMF) following exfoliation in different
solvents.[13g] Here we undertake a more detailed study to probe
the structure of nanosheets of Cu(1)(DMF) and Cu(2)(DMF)
following exfoliation in selected solvents (water, DMF,
acetonitrile, NMP and diethylether) representing a range of
polarities. The as-synthesised MOF (5 mg in 6 mL of solvent)
was sonicated for 12 h at 80 kHz before centrifugation at 1500
rpm for 1 h and the resulting sediment collected for analysis by
using XRPD, IR, TGA and NMR spectroscopy.
being important for determining the concentration of exfoliated
8c]
material in suspension.[8b,
We plotted the concentration of
material in suspension against a range of parameters including
polarity, surface tension and Hansen solubility parameters
(Figure
2 c-e) as well as Kamlet-Taft, Gutman, Swain,
Reichardt's polarity parameters and viscosity (ESI Section 3.3)).
The data in these plots is normalized relative to the highest
concentration solvent in order to allow easier visual comparison.
In line with similar studies of other nanomaterials, no single
parameter by itself was
a reliable determinant of the
concentration of material left in suspension following exfoliation
for either material.[8b] In many cases, solvents with similar
solubility parameters to the best performing solvents showed low
concentrations of dispersed materials. For example, the
concentration of Cu(2)(DMF) exfoliated in DMA is only 20% of
that in NMP even though they have similar surface tensions (γL)
36.70 and 40.21 mNm-1 respectively. Conversely, water and
isopropanol have very different polar Hansen solubility
parameters (δp), 16 and 6.1 respectively, but suspensions of
Cu(2)(DMF) with very similar concentrations are formed. It
should be highlighted that the fact that exfoliation of the pentyl
functionalized MOF produces stable suspension in water at all,
The XRPD pattern for Cu(1)(DMF) following exfoliation into
DMF matches the as-synthesised compound indicating no
structural change occurred. In contrast to this, material analysed
following exfoliation in water showed a distinct, new XRPD
pattern. For this sample, no nitrogen was observed by elemental
analysis while TGA showed a 1.4 % mass loss at 66-94 °C.
albeit at
a lower concentration than the methoxy-propyl
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Figure 4. DFT calculations showing optimised structures for (a) Cu2(1**)4(DMF)2, (b) Cu2(1**)4(H2O)2, (c) Cu2(1**)4 where 1** is 2,5-Bis(3-
methoxypropoxy)benzoate
Furthermore, the IR pattern shows a loss of the DMF
carbonyl peak at 1670 cm-1 and a small new peak 3604 cm-1. All
these results are consistent with substitution of the axial DMF for
H2O, giving Cu(1)(H2O). Material exfoliated in acetonitrile, diethyl
ether and NMP all showed correlating peaks in their XRPD
patterns corresponding to a third, new phase. In the diethyl ether
samples this was accompanied by coincidences with the pattern
assigned to Cu(1)(H2O) indicating a mixture of the desolvated
and hydrated phases. In acetonitrile and diethyl ether, negligible
weight loss was observed in TGA below the decomposition
temperature around 300°C and elemental analysis showed no
nitrogen was present. The same analysis on Cu(1)(DMF)
exfoliated in NMP shows a mass loss of 4.2 % at 83-205 °C, and
small quantities of nitrogen (0.72 wt %) indicating a small
amount of non-coordinated solvent is present. We suggest this
new material (formed in acetonitrile, diethyl ether and NMP) is
caused by the loss of axial DMF to give a desolvated phase with
the structure Cu(1). This matches previous findings following
exfoliation in acetone and methanol.[13g]
NMR of the digested samples confirmed the presence of
residual DMF, and ruled out substitution by NMP. The two large,
but poorly resolved peaks around 8° in the powder pattern are
consistent with the formation of the sql topology and the
distortions are presumed to be due to partial desolvation.
DFT Modelling
In order to gain further insights into the structure of the different
phases, we undertook DFT modelling to visualize the structure
of the MONs and confirm the phase assignments. Structures of
1 and 2 were initially modelled using a single PW formed using
model monocarboxylate ligands functionalized with only a single
methoxy propyl- or pentyl- chain to speed up the calculation (1*
and 2*). Previous studies by us of PW MOFs have shown that
using isolated unit-cells produces very comparable results to
calculations performed on extended structures[18]. Coordinates
from the known crystal structure of Zn(1)(DMF) were used to
generate starting coordinates. The structure was then modified,
replacing DMF with water and acetonitrile. The fourth iteration
removed any solvent from the axial position. In this final iteration
we manipulated the arms, so that the ether functionality could
conceivably coordinate in the axial position. For 2 the same
procedure was followed. The functional used was B3LYP][19] with
dispersion-corrections due to Grimme (GD3-BJ). Structures of 1*
were subsequently remodeled with both methoxy-propyl chains
(1**) resulting in slight improvements in the correlation between
the calculated and experimental data, but showed no
substantive differences. For further details, please see the
supporting information.
Figure 4 a-c shows images of the relaxed structures for the
three different phases obtained with 1** in which DMF, water
and no-solvent are coordinated at the axial position respectively.
Similar images are shown for the other derivatives in ESI Figure
S54. The corresponding calculated IR patterns for these
structures were compared with the experimental patterns (ESI
Figures S55-57). Whilst there are some significant shifts in peak
position and intensity between the calculated and experimental
patterns particularly in the fingerprint region, the presence or
absence of characteristic solvent peaks could be used to assign
the phases. In particular, characteristic peaks corresponding to
the carbonyl of the coordinated DMF molecules at 1706 cm-1
and of water around 3500 cm-1 were observed in the
corresponding calculated and experimental patterns for material
Samples of Cu(2)(DMF) exfoliated into DMF generated
XRPD data correlating with the pattern produced from the parent
MOF. Exfoliation in water produced
a
powder pattern
corresponding to a distinct phase. This fact, along with the
absence of nitrogen in the elemental analysis and mass loss of
4.6 % at 23-107 °C shown by TGA, is consistent with the
formation of Cu(2)(H2O). In a divergence from the behavior
shown by Cu(1)(DMF), exfoliation of Cu(2)(DMF) into diethyl
ether, acetonitrile and NMP gave materials which showed weak
correlation in peak positions between the resulting XRPD
patterns (Figure 3). Exfoliation into diethyl ether gave rise to a
pattern in which each peak could be assigned to either
Cu(2)(DMF), or to the phase assigned to Cu(2)(H2O). Elemental
analysis concluded a value of 0.59 wt % nitrogen (in comparison
to 2.99 wt % calculated for Cu(2)(DMF), which is consistent with
incomplete removal of DMF and partial substitution by trace
quantities of water. In contrast to this, elemental analysis of the
sample produced through exfoliation in acetonitrile showed no
detectable nitrogen and TGA showed no mass loss. We
therefore assign this powder pattern as corresponding to that of
the desolvated materials. Elemental analysis of the Cu(2)(DMF)
exfoliated into NMP indicates significant levels of nitrogen
present (2.21 %) and a drop in mass at around 105°C consistent
with loss of co-ordinated solvent, on heating the sample. Proton
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exfoliated in DMF and water respectively. Experimental patterns
for material exfoliated in acetonitrile lacked the calculated peaks
for acetonitrile at 2200 cm-1 as well as those for water and DMF
and provided closer matches to the calculated structure with no
solvent coordinated. This data therefore supports the
assignments given in the previous section.
Coordination of two acetonitrile, ethanol, acetone, DMF
and water molecules to Cu(1**) have a binding energy of and 58,
66, 72, 87 and 119 kJ/mol, respectively, relative to three
infinitely separated molecules. This broadly corroborates what is
observed experimentally in that more weakly bound solvents
such as acetonitrile are lost whilst more strongly coordinating
solvents such as DMF and water are retained. However, it
should be noted that these values are based on gas phase
calculations and so do not take into account solvent-solvent
interactions. This may account for discrepancies such as our
previous observation that Cu(1)(DMF) is the observed structure
in 10% DMF in water mixtures.
It is interesting to note that in the calculated structures
obtained for Cu(1**), methoxy propyl chains on either side of the
PW are bent over to allow the lone pair of the oxygen to
coordinate intramolecularly to the axial positions of the complex.
This is not observed in the structure for Cu(2*) where the oxygen
is replaced with a methylene group. The binding energy for a
single arm coordinating to Cu(1**) (as calculated through the
difference between the energies of structures with one
coordinated or uncoordinated arm) is 30 kJ/mol. In our
calculations coordination of the second arm only has a binding
energy of 7 kJ/mol. It should be noted that these calculations are
highly dependent on the confirmation around the paddle wheel
and a full conformational search would be required to provide a
better estimate of the true value for the intramolecular binding
which is beyond the scope of this study.
periods: 30 mins and 12 hrs. It was hypothesized that longer
exfoliation times would lead to thinner nanosheets being
produced, and anticipated that the Cu(1)(DMF) would exfoliate
better in H2O than diethyl ether, and the reverse true for
Cu(2)(DMF). After exfoliation, centrifugation at 1500 rpm for 10
mins removed large, unexfoliated material, and AFM was used
to assess nanosheets produced (see Figure 5.)
Both exfoliation procedures resulted in nanosheets with
varying size distributions. In general, more nanosheets with
smaller heights were observed from 12 hr exfoliation than 30
mins, suggesting that longer exposure to ultrasonic waves
results in increased exfoliation. For example Cu(2)(DMF) in
diethyl ether exfoliated for 30 mins and 12 hr resulted in 20-100
and 20-50 nm, respectively. Selected examples of nanosheets
observed using AFM can be found in Figure 5, and additional
figures found in the ESI (Figures S13-19). There are noticeably
large agglomerates and sheet-like particles with heights over
100 nm in many of these images, suggesting that 10 min
centrifugation at 1500 rpm is not effective at removing all larger
particles from the post-sonication suspension.
We therefore suggest that this ability of the methoxy-propyl
chains, but not the propyl chains, to intramolecularly coordinate
to this axial position with values comparable to those of some
solvent molecules may provide at least a partial explanation for
some of the differences observed between the nanosheets. For
example, the co-ordinated methoxy-propyl chains make the
surface of the Cu(1) structures less polar resulting in high
concentrations of nanosheets in apolar solvents than might
otherwise be expected. Similarly, the flexibility of the frameworks
might reduce the impact of the hydrophobic pentyl chains in
polar solvents. These structural insights highlight the challenges
of predicting and understanding the effects of even small
changes in molecular structure on the macroscopic properties of
the nanosheets.
Figure 5. AFM images of Cu(1)(DMF) exfoliated in water for different time
periods: a) 30 mins, b) 12 hrs.. AFM images of Cu(2)(DMF) exfoliated in
different solvents: c) water and d) diethyl ether (c and d, respectively) for 12
hrs. Scale bars are 2, 2, 2 and 1 µm, and height scales are 1000, 200, 50 and
150 nm for a-d, respectively.
In order to compare the effect of solvent on the nanoscopic
dimensions of the nanosheets formed, Cu(1)(DMF) and
Cu(2)(DMF) were exfoliated for 12 hrs in water, DMF, NMP,
acetonitrile and diethyl ether. Samples were centrifuged at
1500 rpm for 1 h as longer/ faster centrifugation times resulted in
insufficient material for analysis in some solvents. Typical AFM
images of observed nanosheets can be found in Figures S27-36.
In general, exfoliation in DMF and NMP resulted in
nanosheets of low quality – lateral dimensions and aspect ratios
Nanoscopic Analysis
In addition to understanding the effect of solvent on the
molecular structure of the nanosheets, we sought to examine
the influence of solvent on the nanoscopic structure of the
resulting material. Exfoliation protocols for other layered
materials have varied significantly, with sonication times ranging
from 20 mins to several days. Here, we first investigated the
exfoliation of the hydrophilic Cu(1)(DMF) and hydrophobic
Cu(2)(DMF) in water and diethyl ether, using two exfoliation time
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Figure 6. Scatter plots of height and lateral dimensions of Cu(1)(DMF) nanosheets observed from exfoliation in MeCN and cascade centrifuged for 1 hr at 1500
rpm (a), then 30 min at 4500 rpm (b), then 4 hrs at 4500 rpm (c). Topographical AFM images of Cu(1)(DMF) (d) and Cu(2)(DMF) (e) exfoliated for 12 hrs, and
Cu(2)(DMF) exfoliated for 30 mins (f) and centrifuged at 4500 rpm for 4 hrs. AFM images of Cu(1)(DMF) exfoliated in water (g) and diethyl ether (h), and
Cu(2)(DMF) exfoliated in water (i) and diethyl ether (j). AFM scale bars are 2 µm, and height scales are 150 nm (d, e, j), 80 nm (f), 40 nm (g), 200 nm (h), and
50 nm (i).
were low, with observed particles having relatively large heights
of > 40 nm. Particles appeared to be rounded in nature, rather
than lamellar, particularly in NMP. This could suggest that the
energetic input upon prolonged exposure times to ultrasound
facilities MON breakdown and dissolution of ligand and Cu into
matching of the solvent and nanosheet properties would lead to
thinner nanosheets. An alternative explanation might be that the
thinner nanosheets formed from Cu(2)(DMF) are the result of
weaker interlayer interactions between the pentyl chains
compared to the methoxy-propyl chains aiding exfoliation during
sonication. Another factor to consider is that poorer interactions
between the nanosheets and solvent may result in more of the
thicker nanosheets produced during sonication being removed
from suspension during centrifugation. This would mean that on
average more thinner nanosheets are observed when there is a
mismatch in solvent and nanosheet properties. Optimising
nanosheet design must therefore balance minimizing inter-layer
solution
– both H21 and H22 are soluble at these low
concentrations in DMF and NMP.
We investigated the stability of the nanosheets in DMF
over 5 days by UV-vis spectroscopy and found broadening of
the ligand absorption band which was attributed to the formation
of a new peak corresponding to the neutral ligand (ESI Figure
S13a,b). In contrast, material exfoliated in water and diethylether
showed no shift in absorbance maximum over time. Furthermore, interactions with complimenting solvent properties to form stable
the intensity of these bands remained constant over 5 days
indicating that stable suspensions had been formed (ESI Figure
S13c).
dispersions of nanosheets and developing centrifugation
protocols that ensure removal of larger particles.
In order to investigate nanosheet size control, Cu(1)(DMF)
was selected as a test system, and exfoliated in acetonitrile for
12 hrs. Acetonitrile was chosen as we observed good particle
separation and minimal agglomeration upon deposition for AFM
analysis using this solvent, which enabled more accurate sizing
of nanosheets. LCC is a versatile strategy which uses multiple
sequential centrifugation steps of increasing rate or time period,
using the supernatant of the previous step as the suspension for
the next, in order to remove particles of various size from
suspension. We employed LCC using steps of 1500 rpm for 1hr,
4500 rpm for 30 mins then 4500 rpm for 4 hrs. The particle size
distribution of the resulting nanosheets as determined through a
statistical analysis (n= 94-161) can be seen in Figure 6a-c and
Nanosheets of Cu(2)(DMF) exfoliated in water were
angular and typically < 1 μm laterally with heights 10-30 nm.
Some examples of ultrathin flakes of 5 μm x 2 nm were
observed (Figure 6). Nanosheets of Cu(1)(DMF) exfoliated in
H2O were more irregularly shaped and typically 10-40 nm in
height, with lateral dimensions up to 1.5 μm, consistent with our
previous report.[13g] Exfoliation of Cu(1)(DMF) in diethyl ether
produced low concentrations of materials in suspension and the
nanosheets observed have relatively low aspect ratios, typically
50-100 nm in height and < 600 nm laterally. In contrast,
Cu(2)(DMF) exfoliated in diethyl ether produces nanosheets
which were typically <40 nm x <1 μm with examples observed
below 10 nm thickness and with lateral dimensions up to 2 μm
(ESI Figure S37).
the mean (x̄ ) and standard deviation (SD) in particle size are
summarized in Table 1. AFM images used for these analyses
can be found in the ESI (Figures S20-22).
It is interesting to note the more hydrophobic ligand 2
produced nanosheets with higher aspect ratios and more regular
shapes than those of the hydrophilic ligand 1 in both water and
diethylether. This is contrary to our expectation that closer
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Table 1. Statistics calculated from nanosheets produced from the 12 hr
exfoliation of Cu(1)(DMF) and Cu(2)(DMF) in acetonitrile, and cascade
centrifuged.
observed as thicker nanosheets are removed from solution by
centrifugation. The wide distributions of particle sizes that result
from prolonged exposure of the bulk MOF to ultrasonic waves
can be narrowed through LCC and the average particle size
reduced. Controlling the centrifugation rate enables nanosheet
size distribution to be optimized for particular applications. In
some applications having a narrow distribution of ultrathin
nanosheets will be essential, for others having a broader
distribution of thicker nanosheets at a higher concentration could
be more important.
Sample
Cu(1)(DMF)
Cu(2)(DMF)
Centrifugation
Cycle
95
111
161
94
134
n
0.33
0.12
0.09
0.12
0.04
Conc. / mM
x
̄
LD ±
512±
234
347±
154
307±
108
348±
202
367±
155
Sensing
SD / nm
We have previously reported the sensing of the small aromatic
heterocycle pyridine from aqueous solution, using aqueous
suspension of Cu(1)(H2O) nanosheets. Titration of pyridine was
found to bind to the axial position of the Cu2-paddlewheel, with a
Ka of 30 ± 8 M-1. When this experiment was replicated, instead
using Cu(2)(H2O), a drop-off in absorbance at λmax was observed,
as well as the suspension of nanosheets visibly turning cloudy
upon addition of pyridine. This could be attributed to
agglomeration of nanosheets upon addition of pyridine, which
displaces coordinated H2O. This would render the MON surface
increasingly hydrophobic, which may cause agglomeration.
In order to be able to compare the binding strength of
Cu(1) and Cu(2) MONs, imidazole was selected as a more
hydrophilic binding substrate to prevent agglomeration.
Cu(1)(DMF) and Cu(2)(DMF) were exfoliated in water for 12 h
and centrifuged at 1500 rpm for 1 h to give suspensions with
concentrations of 0.65 and 0.2 mM respectively. The samples
were diluted with water and aliquots of the guest substrate (73
mM and 43 mM for Cu(1)(H2O) and Cu(2)(H2O), respectively) in
aqueous host suspension (0.13 mM Cu(1)(H2O) and 0.08 mM
(Cu(2)(H2O)) were titrated into host suspension and monitored
using UV-vis spectroscopy. Addition of imidazole in both cases
resulted in bathochromic shifts of λmax from 301-297 nm and
42 % and 36 % increases, respectively, in the absorption
intensity (ESI Figures S48 and S51). These changes are
consistent with expected substitution of water molecules for
imidazole at the axial positions of the Cu2-paddlewheel, which
would result in changes to the absorption band of the
coordinated dicarboxylate ligands 1 and 2. It is most likely that
imidazole binds to the Cu atoms through the sp2-hybridised N
electron pair donation.
59±35
7-157
49±26
8-143
41±19
6-96
20±12
5-64
19±9
4-58
x
̄
H ± SD / nm
H range / nm
100-1050
80-810
120-800
80-830
120-820
LD range / nm
n = number of analysed nanosheets, conc. = concentration, determined
through UV-Vis spectroscopy, x = mean, SD = standard deviation, LD = lateral
̄
dimension (recorded as the largest lateral vector across a nanosheet), H =
height. See SI for AFM images used.
The results of the statistical analyses show that the
average nanosheet thickness and length of Cu(1)(DMF)
decrease sequentially from 59 x 512 nm to 41 x 307 nm
between the first and last steps due to the removal of larger
particles. This correlates with a decrease in the concentration of
material in suspension from 0.33 mM to 0.09 mM. The smallest
nanosheets observed in each case are of a similar size at 6-8
nm. The concentration of Cu(2)(DMF) in suspension following
the final centrifugation step is lower than for Cu(1)(DMF),
however the nanosheets are significantly thinner and larger than
Cu(1)(DMF) with minimum thicknesses of 4 nm and average
dimensions of 19 x 367 nm following the final step.
DLS data were also collected for both systems after each
of the three steps of LCC (ESI Figures S28-29). The trend
observed by DLS is consistent with that observed by AFM in that
LCC lowers the average particle diameter of the MONs by
reducing the number of larger particles remaining in the
supernatant. However, the diameters determined by DLS are
consistently lower (ESI table S9) than those obtained in the AFM
analysis. For example, the mean LD for Cu(1)(DMF) exfoliated in
acetonitrile for 12 h followed by the three steps of LCC is
measured as 106 nm by DLS and 307 nm by AFM. Obtaining
accurate particle size measurements from high aspect ratio
nanosheets using DLS is known to be problematic as the
Stokes-Einstein equation assumes spherical particles[20] and
previous comparisons have also shown DLS produces lower
average particle sizes than AFM.[21]
This data was used to calculate binding constants of Ka =
1370 ± 180 and 1950 ± 140 M-1 for imidazole to Cu(1)(H2O) and
Cu(2)(H2O) respectively. The 43 % increase of Ka observed
between Cu(1) and Cu(2) is consistent with the hypothesis of the
terminal methoxy oxygen of the ligand alkyl-ether arm in 1 being
able to bind to the axial Cu sites, as this would provide an extra
competing species for substrate coordination in Cu(1)(H2O)
which is not present in Cu(2)(H2O), which could explain why
imidazole binds more strongly to Cu(2)(H2O).
Sonicative exfoliation is recognized to be an effective
delaminative technique. For MONs, long exfoliation times at low
temperatures produce more, thinner nanosheets. Solvent choice
is important in determining the thickness and morphology of the
nanosheets obtained and avoiding dissolution over time. Small
differences in ligand too can have a significant impact on the
strength of interlayer interactions. Complimentary solvents may
play a role at weakening interlayer interactions and aiding
exfoliation. However, poor matching of solvent-nanosheet
interactions may also result in thinner nanosheets being
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10.1002/chem.201803221
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were exploited for use as sensors. Addition of pyridine resulted
in aggregation of Cu(2) but not Cu(1) whilst imidazole was
shown to bind significantly stronger to Cu(2) than Cu(1). We
note that the weaker binding seen for Cu(1) may be in part due
to competition from intramolecular binding by the oxygen of the
methoxy-propyl chain.
Overall, this study demonstrates the potential of the
modular structure of MONs in allowing systematic tuning of their
surface properties through isoreticular substitutions. It also
highlights the subtle interplay between ligand, metal cluster,
solvents and exfoliation conditions in determining the molecular,
nanoscopic and macroscopic structure and properties of
nanosheets. Only by better understanding these structure
property relationships will we be able to harness the potential of
MONs for use as sensors, catalysts and for processing into
composite materials for separation and electronics applications.
Conclusions
MONs are an emerging class of two dimensional materials
with significant potential for use in a wide range of applications
thanks to their tunable structure, high surface area and
nanoscopic dimensions.1e Liquid exfoliation using ultrasound is
an appealing route to generating nanosheets from layered
MOFs thanks to its broad applicability to different systems, the
wide availability of ultrasonic baths and scalability of the
approach. However, there have so far been few studies
investigating the impact of ligand design, solvent choice and
exfoliation conditions on the molecular and nanoscopic
structures of the nanosheets formed and their stability in
suspension.
We investigated two layered Cu-PW based MOFs formed
using dicarboxylic acid ligands functionalised with either
methoxy-propyl or pentyl pendant groups intended to bestow
hydrophilic and hydrophobic character respectively. Exfoliation
of Cu(1)(DMF) using an ultrasonic bath produced higher
concentrations of material suspended in water than diethylether
whilst the opposite trend was observed for Cu(2)(DMF).
Cu(1)(DMF) typically showed higher dispersed concentrations
than Cu(2)(DMF) and NMP and DMSO gave the highest overall
concentrations for both compounds. Exfoliation in a wide range
of other solvents showed significant differences in the degree of
exfoliation between the two compounds, however this was not
found to correlate with any single solvent parameter.
The lack of simple correlation was partially explained by
solid state analysis which showed that whilst the two-
dimensional connectivity of the layered MOFs is maintained
following exfoliation, the presence of a labile axial site on the
Cu-PW SBUs mean that the surface functionalization of the
nanosheets can vary depending on the exfoliation solvent. This
effect is not typically observed in simple inorganic nanosheets
but is likely to be common amongst MONs with exchangeable
metal sites. DFT analysis indicated that the oxygen of the
methoxy-propyl ligand 1 is able to coordinate intramolecularly to
the axial position of the copper paddlewheels. This may further
explain the complex dispersion behavior of the MONs.
Experimental Section
Synthesis
Commercial solvents and reagants were used without further
purification. Synthesis of organic liagnds was carried out in dry
glassware with a nitrogen overpressure. Solvothermal synthesis
of MOFs was undertaken using borosilicate vials with Teflon
faced rubber lined caps.
Dimethyl
2,5-dihydroxyterephthalate
and
2,5-Bis(3-
(1) were
methoxypropoxy)-1,4-benzenedicarboxylate
synthesised according to previously reported procedures.[22] 2,5-
Bis(pentoxy)-1,4-benzenedicarboxylate
synthesised, however the hydrolysis of the protected acid
groups was achieved instead through refluxing in THF with aq.
KOH (5 %). See ESI 1.1 for details and full materials
characterisation.
was
similarly
Cu(1)(DMF) was synthesised according to our previous
method.[13g] Cu(2)(DMF) was similarly synthesised. Specifically,
Cu(NO3)2.6H2O and ligand H21 or H22 were dissolved in DMF
and sealed into reaction vials, and heated to 110 °C for 18 hrs,
The nanoscopic dimensions of the exfoliated material were
investigated using AFM and nanosheets with thickness as low
as 2 and 10 nm were observed. Cu(2)(DMF) typically formed
nanosheets which were thinner, had higher aspect ratios and
were more angular than those of Cu(1)(DMF) in both water,
diethylether and acetonitrile. This is hypothesized to be the
result of the apolar pentyl chains resulting in weaker interlayer
interactions than those of the methoxy-propyl chains aiding
exfoliation during sonication, However, as with the dispersion
study, a complex balance of sometimes competing factors will
determine the profile of the nanosheets generated. Longer
exfoliation times typically produced higher concentrations of
thinner nanosheets whilst cascade centrifugation could be used
to remove larger particles and narrow the size distribution.
The ability of the axial position to exchange solvent
molecules and the photophysical properties of the nanosheets
then slow-cooled, resulting in
microcrystalline Cu(2)(DMF).
characterisation including elemental analysis, FTIR, TGA and
PXRD can be found in the ESI.
a
77
%
yield of green,
details and
Synthetic
Exfoliation
MOF and solvent were added to 10 mL reaction vials in the
quantities stated in-text. These were rotated using an adapted
Heidolph RZR 2020 overhead stirrer with a multi sample holder,
in a Fisher brand Elmasonic P 30H ultrasonic bath (2.75 L,
380/350 W, UNSPSC 42281712) filled with water. The ultrasonic
bath was operated at 100 % power, at 80 kHz, and was fitted
with a cooling coil so as to prevent bath heating upon prolonged
exfoliation times.
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10.1002/chem.201803221
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Acknowledgements
Characterisation
We would like to thank the Ramsay Memorial Trust (JAF) and
Royal Society RG170002 (JAF), Engineering and Physical
Sciences Research Council (EP/K503149/1) (DJA), Sheffield
University Research Experience Scheme (LDS) and the Higher
Committee for Education Development (HCED) in Iraq (RWMA)
for Funding. A license for the OpenEye tools, obtained via the
free academic licensing program, is gratefully acknowledged.
There are no conflicts of interest to declare.
NMR spectra were recorded on a Bruker Advance DPX 400
spectrometer. 1H and 13C chemical shifts are reported in ppm on
the δ scale and were referenced to the residual solvent peak. All
coupling constants are reported in Hz. Mass spectra were
collected using an Agilent 6530 QTOF LC-MS in positive
ionization mode. Elemental analyses were obtained on an
Elementar vario MICRO cube. X-Ray powder diffraction patterns
were collected using
a
Bruker D8 Advance powder
diffractometer equipped with a copper kα source (λ=1.5418 Å)
operating at 40 kV and 40 mA. The instrument was fitted with an
energy-dispersive LYNXEYE detector. IR spectroscopy was
Keywords: Metal-organic framework
•
nanosheet
• liquid
exfoliation
chemistry
•
two-dimensional materials
•
supramolecular
performed on
a
Perkin Elmer ATR-FTIR Spectrum 2.
Thermogravimetric analyses were collected using a Perkin
Elmer Pyris 1 TGA from 30-600 ºC at 10 ºC min-1, under a 10
cm3 min-1 flow of nitrogen. UV-vis absorption spectra were
obtained on a Varian Cary 50 UV or Varian Cary 5000 UV-Vis-
NIR spectrophotometer, using standard 1 cm width quartz cells
and Perkin Elmer Spectrum One software. The nanoscopic
morphology of the samples was investigated using a Bruker
Multimode 5 AFM with an equipped Nokia 10x visualising lens,
operating in soft tapping-mode using Bruker OTESPA-R3
cantilever. Samples were prepared by dropping 10 μL (sample
dependant) of suspension onto a freshly cleaved mica substrate.
Images were processed using standard techniques with
Gwyddion software. DLS data were collected using a Malvern
Zetasizer Nano Series particle size analyser equipped with a He-
Ne laser at 633 nm, operating in backscatter mode (173 °).
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This article is protected by copyright. All rights reserved.

10.1002/chem.201803221
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Two metal-organic frameworks
functionalised with either hydrophilic
or hydrophobic alkyl-ether side
chains were synthesised and
exfoliated in a wide range of
solvents. The concentration,
structure and dimensions of the
resulting nanosheets were
investigated.
David .J. Ashworth, Adam. Cooper,
Mollie. Trueman, Rasha W. M. Al-
Saedi, Liam. D. Smith , Anthony. J. H.
M. Meijer and Jonathan. A. Foster*
X. – X+11.
Ultrasonic exfoliation of
hydrophobic and hydrophilic metal-
organic frameworks to form
nanosheets
This article is protected by copyright. All rights reserved.