Self-Exfoliated Metal-Organic Nanosheets through Hydrolytic Unfolding of Metal-Organic Polyhedra

Article abstract of DOI:10.1002/chem.201700848

Few-layers thick metal-organic nanosheets have been synthesized using water-assisted solid-state transformation through a combined top-down and bottom-up approach. The metal-organic polyhedra (MOPs) convert into metal-organic frameworks (MOFs) which subse

Full text of DOI:10.1002/chem.201700848

A Journal of
Accepted Article
Title: Self-exfoliated metal organic nanosheets via hydrolytic unfolding
of metal organic polyhedra
Authors: Rahul Banerjee, Bikash Garai, Arjit Mallick, Anuja Das, and
Rabibrata Mukherjee
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Self-exfoliated metal organic nanosheets via hydrolytic unfolding
of metal organic polyhedra
[
a,b]
[a,b]
[c]
[c]
[a,b]
Bikash Garai, Arijit Mallick,
Anuja Das, Rabibrata Mukherjee and Rahul Banerjee*
[
6]
Abstract: Few layer thick metal-organic nanosheets have been
synthesized using water-assisted solid state transformation through
a combined top-down and bottom-up approach. The metal-organic
polyhedra (MOPs) convert into metal-organic frameworks (MOFs)
which subsequently self-exfoliate into few layered metal-organic
interfacial growth. In recent years, research on delaminated
[
7]
layered metal-organic framework (MOF) structures has gained
attention because of their promising use in gas-separation,
molecular sieving and isotope separation. The performance of
these materials in terms of separation–efficiency and catalysis
[
8]
nanosheets. These MOP crystals experience
a hydrophobicity
gradient with the inner surface during contact with water because of
the existence of hydrophobic spikes on their outer surface. When the
amount of water available for interaction is higher, the resultant
layers are not stacked to form bulk materials; instead few layered
nanosheets with high uniformity were obtained in high yield. The
phenomenon has resulted high yield production of uniformly
distributed layered metal-organic nanosheets from three different
MOPs, showing its general adaptability.
Introduction
Two-dimensional layered nanostructured materials have
recently gained attention because of their potential for
applications in various emerging fields such as optoelectronics,
energy storage, ultrafiltration, semiconductors, supercapacitors
[
1]
and photovoltaic cells. In general, two-dimensional layered
bulk structures are grown by deposition of individual layers one
by one through non-covalent interactions like hydrogen bonding,
[
2]
π-π stacking, Van-der Waals and dipole-dipole interactions.
Although these interactions are ‘weak’ in comparison to the
covalent bonding between atoms; the energy required to
separate these layers in mono- or a few-layered stacks is
[
2]
significant.
Traditionally, the required energy for such
delamination is supplied in the form of mechanical grinding,
mechanical shearing, ultrasonication, and intercalation assisted
[
3]
peeling. In all these cases, the bulk materials are separated
into discrete layers and thus these methods have been
[
4]
categorised as a ‘top-down approach’. Such few layered
nanostructures could also be synthesised via ‘bottom-up
approach’ where the deposition of subsequent layers is
st
[5]
restricted after the formation of the 1 layer. These approaches
include chemical vapour deposition, templated growth and
[
a]
B. Garai, A. Mallick, Dr. R. Banerjee
Physical and Materials Chemistry Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune-411008, India
E-mail: r.banerjee@ncl.res.in
[
b]
c]
B. Garai, A. Mallick, Prof. Dr. R. Banerjee
Academy of Scientific and Innovative Research (AcSIR)
New Delhi, India
[
A. Das, Prof. Dr. R. Mukherjee
Instability and Soft Patterning Laboratory
Department of Chemical Engineering
Indian Institute of Technology, Kharagpur
Kharagpur, India
Figure 1. (a) Chemical diagram of the isophthalic acid based linkers used for
the synthesis of MOPs. ORTEP diagram for (b) MOP-MIA, (c) MOP-EIA, and
(
d) MOP-PrIA; packing of the individual MOP units (e,f,g) showing their
Supporting information for this article is given via a link at the end of
the document.
discrete nature. Each MOP unit has been represented by individual colour and
the interstitial cavities are marked with white boxes.
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on active surfaces increases drastically when the separated
layers are used over the bulk materials. Routes utilized to
obtain such MOF nanosheets include both ‘top-down’ and
nanosheets with a few nm thickness in high yield and uniform
morphological dispersion.
[
9]
[12]
‘
bottom-up’ approaches but both these processes have their
[
10]
individual limitations.
In the top-down approach, the
Results and Discussion
nanosheets are obtained with a good yield but it is difficult to
control their thickness and therefore the generated
nanostructures are non-uniform. On the other hand, nanosheets
with uniform and controllable thickness are generated through
bottom-up approaches, but the processes, in general, require
difficult experimental set up. Thus, the synthesis of metal-
organic nanosheets in high yield with uniform and controllable
thickness in an efficient way requires a combination of both top-
down and bottom-up approaches. However, because of their
divergent nature, till date it has not been possible to combine
both these approaches into a single unified approach. Herein,
we have made an effort to combine both these approaches
under one synthetic domain to convert metal-organic polyhedra
Blue-colored crystals of the MOPs (MOP-MIA, MOP-EIA,
MOP-PrIA) are readily synthesized from the solvothermal
reaction between copper (II) nitrate and the corresponding
organic linkers, 5-methoxy isophthalic acid (5-MIA), 5-ethoxy
isophthalic acid (5-EIA) and 5-(n-propyloxy) isophthalic acid (5-
PrIA), respectively in mixed DMF solvent at 90 C (figure 1).
MOP-MIA is obtained as orthorhombic crystals while MOP-EIA
and MOP-PrIA are separated as polyhedral crystals (section S2
in ESI) after 48 h. The polyhedral architecture in all these MOPs
is constructed from the co-ordination between metal ions and
the organic linkers through the formation of a Cu-Cu paddle
wheel SBU. These MOP units exist as discrete ones, describing
their zero-dimensional architecture. The edges of each
polyhedral architecture is defined by 6 square shaped faces and
[
11]
(MOPs)
directly
into
two-dimensional
metal-organic
Figure 2. Monitoring of the hydrolytic conversion through time dependent PXRD for (a) MOP-MIA, (b) MOP-EIA, and (c) MOP-PrIA. (d) The mechanism for the
conversion of alkyloxy decorated MOPs into MOFs. Scanning electron microscopic images for monitoring the conversion of (e) MOP-MIA, (f) MOP-EIA, and (g)
MOP-PrIA.
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8
triangular faces. The alkyl groups from the ligands are located
on the centre of the edges and are pointed outwards (figure 1).
MOP-MIA was crystallized in the P-1 space group with 2 MOP
units per unit cell, whereas MOP-EIA and MOP-PrIA were
crystallized in Pa-3 space group having 4 MOP units in the unit
cell.
When these polyhedral MOP crystals are treated with a
measured amount of water (50 L/mg), small sized [0.2 x 0.2 x
3
0
.1 mm ] MOF crystals start forming after 10 min. The as-
synthesized crystals of the MOPs turn opaque after the initial
contact with water. With time, the opacity of the MOP crystals
increases and they finally split into crystals of the corresponding
bulk MOF materials (figure 2). This conversion proceeds through
an amorphous phase (figure 2a, 2b, 2c) observed during the
opacity loss of the parent MOP crystals. Notably, this whole
conversion occurs through an all-solid-state pathway, where no
component of the starting materials dissolves in water, as
evidenced from the UV-vis spectra of the collected filtrate (figure
S13 in ESI). As the amount of water added to the system is
increased, the thickness of the resulting MOF crystals starts
decreasing and beyond a certain water concentration (1.5
mL/mg), no solid particles are visually observed. However, the
Tyndall effect (figure 4b) from the obtained liquid indicates the
presence of solid particles as dispersion in the aqueous medium.
Although the nanometer scale size of these particles makes their
structural characterization challenging, the respective successor
MOF crystals were analyzed structurally through single crystal
X-ray diffraction. MOF-MIA crystallizes in the P4/n space group
Figure 3. N
per BET equation is 450 m g .
2
adsorption isotherm for MOP-EIA. The available surface area as
2
-1
upto 300 °C with removal of trapped and coordinated solvent
molecules (figure S11 in ESI). However, the thermal stability
gets enhanced to 450 C for the case of bulk MOFs (figure S12
in ESI). Thermal removal of the solvent molecules from the MOP
core results in the gain of readily accessible surface and pores
for incoming guest and other molecules. Adsorption of N by
2
MOP-EIA measures an availability of 450 m g surface as per
the BET equation, one of the highest values reported till date for
2
generating square shaped pores [8 x 8 Å ], while MOF-EIA and
2 -1
MOF-PrIA crystallize in the P321 and P-3 space group,
respectively. For the case of MOF-EIA and MOF-PrIA, two types
of pores were found in their structure; the pore with bigger
[14]
discrete MOPs (figure 3). The available surface areas for the
other two MOPs are less and this variation of the surface area
results from the stability of the MOP units that originated from
their crystal packing. MOP-EIA has a closely packed structure
where the distance between adjacent MOP units is minimal [8.5
2
aperture [11.3 x 4.1 Å ] has hexagonal geometry and contains
all the alkyloxy groups pointed towards the pore centre. The
2
other set of pores are triangular [5.2 x 5.1 Å ] and contains only
coordinated water molecules from the paddle wheel SBUs. All
these MOFs have two dimensional layered structures and the
stacking of the molecular layers generate channels in their
extended form (figure S1-S3 in ESI).
3
x 8.5 x 8.5 Å ]. A similar spacing box bears a dimension of 8.4 x
3
12.9 x 21.0 and 11.0 x 9.0 x 8.6 Å for MOP-MIA and MOP-PrIA,
respectively (figure 1e, 1f and 1g). This close packing reduces
the interstitial voids and minimizes the possibility of structural
collapse during removal of solvent.
The phase purity of all the as-synthesized MOPs and the
resulted MOF crystals has been confirmed by matching the
experimentally observed PXRD patterns with the corresponding
simulated patterns (figure S4 and S5 in ESI). Some peaks in the
observed MOP PXRD pattern remain unindexed because of
higher thermal disorder of the alkyloxy groups and the non co-
As evidenced from the structure of MOPs, the hydrophobic
alkyl functionalities located on the outer surface of the MOPs are
in a disfavoured situation as they directly face the incoming
water molecules. On the other hand, for the less hydrophobic
inner surface, water contact should be a favourable situation.
Thus, a strain gets developed between the two surfaces and in
order to minimize this strain, the system tries to equally
distribute the hydrophobic groups among the two surfaces. This
equalization is achieved through the mechanism explained in
figure 2d.
MOFs are formed and are parallely arranged as per their
packing along the corresponding crystal planes.
It is evident from figure 4a that MOP units from the bulk
crystals first unfold to minimize the surface strain upon
interaction with water through the combination of hydrophobic
repulsion and hydration pressure to generate an amorphous
phase. The neighboring MOP units join along the
[13]
ordinated solvent molecules.
FT-IR spectra of the as-
synthesized MOP crystals reveal that the carboxylic O–H signal
-
1
centred at 2900 cm disappears because of the co-ordination
with metal ions. This co-ordination also causes a red-shift of the
-
1
carboxylic acid C=O peak by 60 cm . No additional signal is
observed in the FT-IR spectra after the conversion of the MOPs
into the MOFs (figure S7-S10 in ESI). This phenomenon
indicates the similarity in the chemical environment and bonding
in the MOPs and the corresponding MOFs. Retention of the
[11]
Thus, from each MOP crystal, several layers of
-
1
peaks for the alkyloxy O–C bond at 1054 and 1375 cm also
suggests that the alkyloxy groups do not get fragmented during
the conversion. All these MOPs are found to be thermally stable
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Figure 4. (a) Schematic representation for the generation of MOF and MOF nanosheets from the MOP crystal under influence of different quantity of water. (b)
Tyndall effect showing existence of nanoscopic particles in the aqueous suspension of MOF nanosheets. (c, d, e) TEM images of the nanosheets derived from
MOP-MIA, -EIA and -PrIA, respectively. Rendered 3D topographic AFM image, obtained in intermittent contact mode showing the cross-sectional profile for (f)
MIA-NS, (g) EIA-NS and (h) PrIA-NS.
crystallographic axes through unfolding of the closed structures
and causes the generation of extended nanosheets. When the
amount of water present in the system is limited, the conversion
around each functional moiety remains intact, as evidenced from
the similar FT-IR spectra as that of the corresponding bulk
MOFs. SEM images of the nanostructures clearly indicate their
occurs at controlled rate because of the slower diffusion of water. hexagonal morphology and that with time these structures are
This result in the generation of properly grown MOF layers, well
stacked to produce micrometer sized single crystals. However,
when the water content is higher, the excess amount of water
does not allow the layers to stack upto the full extent which is
required for the growth of the single crystals of the MOFs. As a
result, the metal-organic nanosheets remain dispersed in water
forming an aqueous suspension. The absence of stacking in
their extended form causes the diminishing of the PXRD peak at
getting peeled-off from the bulk MOP materials and finally form
the 2-dimensional sheets with nm scale thickness (figure 2e, 2f
and 2g). TEM imaging provides an input about their lateral
dimension and the nanosheets are found to have aspect of 1.7 x
2
2
1.7 µm for MIA-NS, 2.0 x 2.0 µm form EIA-NS and 1.8 x 1.8
2
µm for PrIA-NS (figure 4c, 4d and 4e). The statistical variation
in the dimension of the nanosheets has been determined using
an AFM, by scanning several samples derived from each of the
sources. The average height of the features are found to be 15.3
± 1.2 nm, 17.2 ± 1.1 nm and 11.8 ± 0.6 nm for sheets obtained
2
 = 5, corresponding to the 100 plane (figure S6 in ESI).
However, their chemical bonding and the chemical environment
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from MOP-MIA, -EIA and -PrIA respectively. The error bar
corresponds to the standard deviation of the data over all the
samples. The aspect of the features were also verified from the
AFM, by taking the average over many samples, and their
values were found to be rather close to that obtained from TEM
analysis (Aspect: 1.63 ± 0.21 m for MIA-NS, 1.96 ± 0.16 m for
EIA-NS and 1.82 ± 0.13 m for PrIA-NS) shown in figures 4c-4e.
The molecular thickness of each layer has been calculated as
present in contact media of MOP during their hydrolytic
conversion into MOFs, control on their growth has been
achieved. Thus, metal-organic nanosheets with uniform size and
thickness were obtained at high yield in a single conversion step.
Additionally, no external energy was supplied for the separation
of the nanosheets from the bulk materials, revealing true ‘self-
exfoliation’.
2.0 nm from the crystal structure. Hence we could clearly say
that each of these nanosheets contain around 6-8 stacks of the
molecular layers. While figure 4f-4h shows the detailed AFM
image of a single sheet, image of larger scan area indeed
verifies that the sheets are of uniform thickness. One such
image showing uniform size and thickness dispersion of PrIA
nanosheets has been shown in figure S19d of ESI. The size of
the nanosheets has again measured from particle size analysis
with Dynamic Light Scattering experiment where the effective
diameter of the nanosheets is found to be 2.1, 1.7 and 3.3 m
for MIA-NS, EIA-NS, and PrIA-NS, respectively (section S11 in
ESI).
PXRD patterns collected during the hydrolytic conversion
of the MOPs indicate that an amorphous phase is first attained
because of the interaction with water. This amorphous phase is
responsible for conversion into metal-organic nanosheets as per
the postulated mechanism. And thus, transformation of the
crystalline MOP units into the amorphous phase is an instance
of the ‘top-down’ approach for nanosheet synthesis. This
amorphous form is then transformed into the nanosheets
following the ‘bottom-up’ approach. Additionally, for the ‘top-
down’ segment of the conversion pathway, no energy was
needed to be supplied in terms of ultrasonication or mechanical
shearing. Rather, the nanosheets were separated from the bulk
amorphous phase by themselves utilizing the interaction with a
pool of water molecules. Hence, the generation of the
nanosheets is best-described as ‘self-exfoliation’. The benefit of
this method has been tested by comparing with the
nanomaterials obtained from two paths of top-down approach,
mechanical grinding and ultrasound treatment to the bulk MOF
crystals (figure S17 in ESI). But in both these cases, the
obtained materials do not posses regular morphology to be
termed as nanosheet.
Acknowledgements
BG and AM acknowledge UGC and CSIR (New Delhi, India) for
SRF. RB acknowledges CSIR (CSC0122 and CSC0102) and
DST (SB/S1/IC-32/2013, INT/SIN/P-05 and SR/NM/NS-
1179/2012G) for funding.
Keywords: metal-organic polyhedra • hydrophobicity • metal-
organic nanosheets • cage compounds • layered materials
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Chemistry - A European Journal
10.1002/chem.201700848
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Two dimensional metal organic
nanosheets with few nanometer
thickness have been synthesized
through a unique combination of top-
down and bottom-up approaches
using water assisted self-exfoliation of
metal organic polyhedra.
Bikash Garai, Arijit Mallick, Anuja Das,
Rabibrata Mukherjeee and Rahul
Banerjee*
Page No. – Page No.
Self-exfoliated two dimensional metal
organic nanosheets via hydrolytic
unfolding of metal organic polyhedra
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