Controlled Intercalation and Chemical Exfoliation of Layered Metal-Organic Frameworks Using a Chemically Labile Intercalating Agent

Article abstract of DOI:10.1021/jacs.7b04829

Creating ordered two-dimensional (2D) metal-organic framework (MOF) nanosheets has attracted extensive interest. However, it still remains a great challenge to synthesize ultrathin 2D MOF nanosheets with controlled thickness in high yields. In this work, we demonstrate a novel intercalation and chemical exfoliation approach to obtain MOF nanosheets from intrinsically layered MOF crystals. This approach involves two steps: first, layered porphyrinic MOF crystals are intercalated with 4,4′-dipyridyl disulfide through coordination bonding with the metal nodes; subsequently, selective cleavage of the disulfide bond induces exfoliation of the intercalated MOF crystals, leading to individual freestanding MOF nanosheets. This chemical exfoliation process can proceed efficiently at room temperature to produce ultrathin (～1 nm) 2D MOF nanosheets in ～57% overall yield. The obtained ultrathin nanosheets exhibit efficient and far superior heterogeneous photocatalysis performance compared with the corresponding bulk MOF.

Full text of DOI:10.1021/jacs.7b04829

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Communication
Controlled Intercalation and Chemical Exfoliation of Layered Metal-
Organic Frameworks using A Chemically Labile Intercalating Agent
Yanjun Ding, Ying-Pin Chen, Xinlei Zhang, Liang Chen, Zhaohui
Dong, Hai-Long Jiang, Hangxun Xu, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04829 • Publication Date (Web): 27 Jun 2017
Downloaded from http://pubs.acs.org on June 27, 2017
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Controlled Intercalation and Chemical Exfoliation of Layered Metal-
Organic Frameworks using A Chemically Labile Intercalating Agent
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Yanjun Ding,† YingꢀPin Chen,ǁ Xinlei Zhang,† Liang Chen,† Zhaohui Dong,§ HaiꢀLong Jiang,†,‡,* Hangxun Xu,†,*
and HongꢀCai Zhouǁ,*
†Department of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Techꢀ
nology of China, Hefei, Anhui 230026, China
‡Department of Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology
of China, Hefei, Anhui 230026, China
§Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Shanghai, 201800, China
ǁDepartment of Materials Science and Engineering, Texas A&M University, College Station, Texas 77840, United States
Supporting Information
example, the layered MOFs are diverse sources of ultrathin crystalline
ABSTRACT: Creating ordered twoꢀdimensional (2D) metalꢀorganic
nanosheets for molecular sieving if they can be efficiently exfoliated
while retaining their structure and morphology.2c,2d Recent studies on
exfoliation of MOFs exclusively focus on exfoliating 2D frameworks
held together by interlayer van der Waals interactions or hydrogen
bonding in bulk crystals.5 Insufficient control over the mechanical or
solvent mediated exfoliation process by weakening interlayer interacꢀ
tions in MOF crystals often leads to 2D nanosheets with various thickꢀ
nesses and low yields (typically <15%).2e,5f To circumvent this probꢀ
lem, a more reliable exfoliation route using controllable chemical reacꢀ
tions to regulate the interlayer interactions is highly desired. Exfoliation
of the chemically preꢀintercalated layered inorganic solids is an efficient
method to synthesize ultrathin 2D inorganic nanosheets.6 Unfortunateꢀ
ly, the generally used chemical intercalation method in inorganic solids
is not applicable in exfoliating MOFs into 2D nanosheets.
framework (MOF) nanosheets has attracted extensive interest. Howꢀ
ever, it still remains a great challenge to synthesize ultrathin 2D MOF
nanosheets with controlled thickness in high yields. In this work, we
demonstrate a novel intercalation and chemical exfoliation approach to
obtain MOF nanosheets from intrinsically layered MOF crystals. This
approach involves two steps: firstly, layered porphyrin MOF crystals
are intercalated with 4,4'ꢀdipyridyl disulfide through coordination
bonding with the metal nodes; subsequently, selective cleavage of the
disulfide bond induces the exfoliation of intercalated MOF crystals,
leading to individual, freestanding MOF nanosheets. This chemical
exfoliation process can proceed efficiently at room temperature to
produce ultrathin 2D MOF nanosheets (~1 nm) with ~57% overall
yield. The obtained ultrathin nanosheets exhibit efficient and far supeꢀ
rior heterogeneous photocatalysis performance to the corresponding
bulk MOF.
Twoꢀdimensional (2D) nanomaterials with atomic or molecular
thickness have received broad interest in recent years due to their
unique dimensionꢀrelated properties and promising applications in
energy storage, separation, catalysis, and nanoelectronics.1 Recently,
2D metalꢀorganic framework (MOF) nanosheets have emerged as a
new class of 2D nanomaterials for molecular sieving, sensing, and caꢀ
talysis.2 Highly ordered ultrathin MOF nanosheets formed via coordiꢀ
nation bonding require the precise control of structure and functionaliꢀ
ty over extended length scale. MOF nanosheets are of significant imꢀ
portance, not only for fundamental structureꢀproperty investigations
but also for technological developments.3 Nonetheless, the rational
synthesis of MOF nanosheets with diverse structures and tailored
properties while keeping them down to atomic thickness is still a great
challenge.
Figure 1. Schematic illustration of the overall process developed to
produce 2D MOF nanosheets via an intercalation and chemical exfoliaꢀ
tion approach.
Here, for the first time, we demonstrate a new strategy for the highꢀ
yield synthesis of 2D MOF nanosheets via chemical exfoliation from
intercalated MOF crystals. The overall fabrication process is schematiꢀ
cally illustrated in Figure 1. In order to obtain chemically responsive
MOFs, we incorporate a chemically labile dipyridyl ligand, 4, 4'ꢀ
dipyridyl disulfide (DPDS), into the layered MOF crystals to form new
intercalated MOFs. Because the interlayer interactions are weakened
between expanded 2D layers after scissoring DPDS by chemical reducꢀ
tion of the disulfide bond using trimethylphosphine (TMP), MOFs
can be easily exfoliated into ultrathin nanosheets (~1 nm) with high
yield (~57%). Furthermore, we demonstrate that the asꢀprepared
Topꢀdown method has been demonstrated to be a formidable apꢀ
proach for efficient and scalable production of various 2D nanomateriꢀ
als.4 Exfoliation of 3D layered MOFs into their 2D constituents is very
attractive if appropriate exfoliation methods can be developed. For
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nanosheets exhibit very high efficiency in singlet oxygen (1O2) generaꢀ
tion for heterogeneous photocatalysis.
ligands (Figure S12) was further verified using synchrotronꢀbased
powder Xꢀray diffraction (PXRD) pattern (Figure S13). Thus, our
results indicate that DPDS can be intercalated into the layered MOF
crystals to form new MOF crystals with expanded interlayer distance. 8
Tetrakis(4ꢀcarboxyphenyl)porphyrin (TCPP) is an extensively used
organic linker to construct a variety of 2D or 3D MOFs.7 Thus, a
known MOF (PPFꢀ1) containing porphyrin sheets with Zn dinuclear
paddlewheel secondary building units (SBUs) was initially used here.8a
The 2D layers can be inserted in the third dimension by coordinating
the metal centers within the paddlewheels and inside the porphyrin
rings.8 As a result, the pyridyl ligands can be inserted into the porphyrin
layers to form a new crystal (Figure S1). Unfortunately, diffractions are
too weak to determine the overall structure. We then tried other metalꢀ
loꢀTCPP (MꢀTCPP, where M is Pd, Ni, and Co) species and obtained
much better crystals by using PdTCPP (Figure S2, S3 and Table S1).
Therefore, the MOF containing PdTCPP is used as model structure for
this proofꢀofꢀconcept study.
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Figure 3. (a) UVꢀVis absorption spectra of PdTCPP, 4ꢀ
mercaptopyridine, and the intercalated MOF crystals after reduction
by TMP. (b) Time course measurement of the exfoliation process by
UVꢀvis absorption spectroscopy. Insert: the absorbance at 341 nm as a
function of exfoliation time.
The disulfide intercalating reagent, DPDS, has an absorption peak at
245 nm (Figure S14). After reduction by TMP, a new peak around 341
nm corresponding to 4ꢀmercaptopyridine appears, suggesting the
DPDS is chemically scissored into 4ꢀmercaptopyridine. Subsequently,
after adding TMP into an ethanol solution containing
Zn2(PdTCPP)(DPDS)2 crystals, new peaks at 341 nm and 425 nm
corresponding to 4ꢀmercaptopyridine and PdTCPP appear (Figure
3a), implying that the dipyridyl intercalating agents are cleaved (Figure
S15). As shown in Figure 3b, upon adding TMP, the absorption peak at
341 nm increases and, after ~10 h, the absorption intensity reaches to
the maximum value. Consequently, exfoliation of the MOF crystals
into ultrathin nanosheets was carried out by adding 20ꢀfold excess
TMP into the crystal solution at room temperature with gentle stirring.
Excess TMP was used to ensure that crystals were exfoliated with the
highest yield. The exfoliation process occurred immediately and was
evidenced by the Tyndall effect upon irradiation with a laser beam
(Figure 4a). The exfoliated nanosheets were characterized by transꢀ
mission
Figure 2. Experimental PXRD patterns of Zn2(PdTCPP) before and
after inserting DPDS along with simulated results considering preꢀ
ferred orientation in (001) direction. The unit cell parameters are obꢀ
tained from singleꢀcrystal Xꢀray crystallography.
As shown in Figure 2, the corresponding unit cell parameter (c, perꢀ
pendicular to porphyrin planes) increases from 19.604 to 45.237 Å
after intercalation, indicating the interlayer distance varies from 9.8 to
22.6 Å which is larger than the longitudinal length of the DPDS ligand
(9.3 Å). This observation indicates that only one pyridinic N from
DPDS ligand coordinates Zn from Zn2(COO)4 SBU and the other is
not coordinated. From the solved structure by single crystal data for
the intercalated MOF, Zn2(PdTCPP)(DPDS)2 (Figure S4 and Table
S1), the PdTCPP layers are clearly observed whereas the inserted
DPDS linkers are incomplete because of the random orientation of the
linkers caused by free rotation of single bonds.7d However, Xꢀray phoꢀ
toelectron spectroscopy (XPS) (Figure S5), elemental analysis, mass
spectra (Figure S6) and energyꢀdispersive Xꢀray spectroscopy (Figure
S7) results indicate the presence of DPDS in the crystals. Powder Xꢀray
diffraction (PXRD) patterns further agrees that one of DPDS ligands
must be coordinated the unsaturated Zn sites (Figure S8), instead of
flowing in the layers. The structure is therefore confirmed by using a
similar ligand, 4ꢀ(phenyldithio) pyridine (PDTP), to replace DPDS for
the intercalation reaction (Figure S9). Moreover, 1H NMR result of the
digested MOF crystals shows a 1:2 ratio between PdTCPP and DPDS
(Figure S10) which is consistent with the theoretical value. After interꢀ
calation, the MOF crystals remained the layered structure (Figure
S11). Taking the preferred orientation into consideration,2e,9 the experꢀ
imental powder Xꢀray diffraction pattern (PXRD) coincides with the
pattern simulated from the structure solved with singleꢀcrystal XRD
data (Figure 2). The proposed structure describing coordinated DPDS
Figure 4. (a) TEM image of the exfoliated MOF nanosheets. Insert:
Tyndall effect before (left) and after (right) exfoliation. (b) TEM imꢀ
age of an individual exfoliated MOF nanosheet. (c) AFM image of the
exfoliated MOF nanosheets with corresponding height profiles. (d)
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Highꢀresolution TEM image of an exfoliated multilayer MOF
nanosheet with the corresponding FFT pattern.
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electron microscopy (TEM) and atomic force microscopy (AFM),
respectively (Figure 4). Both characterization methods indicated that
freestanding nanosheets with sizes up to micrometers were obtained
after exfoliation (Figure S16a). Wrinkled or ruptured sheets can be
observed from TEM (Figure 4a and 4b), suggesting ultrathin nature of
the exfoliated nanosheets. As shown in Figure 4c and S16b, the height
of asꢀexfoliated nanosheets is measured to ~1.0 nm with slight variaꢀ
tions, roughly corresponding to the thickness of singleꢀlayer PdTCPP
nanosheets (Figure S12).
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The reduction of disulfide bonds is a chemical reaction which can be
quantitatively controlled by varying the amount of TMP used along
with the reaction time. Generally, in order to exfoliate MOF crystals
into 2D nanosheets as much as possible, excess TMP is required. Howꢀ
ever, if the reaction time was shortened to 4 h and the amount of TMP
used was 10ꢀfold higher than that of the disulfide groups present in the
crystals, the majority of obtained products were multilayer nanosheets
with ~4 nm in height (Figure S17), indicating a controllable exfoliation
process. More importantly, highꢀresolution TEM (HRTEM) clearly
shows the lattice fringes of the exfoliated multilayer MOF nanosheets
with an interplanar distance of 1.65 nm, which belongs to the (100)
plane of intercalated crystals (Table S1). Meanwhile, the correspondꢀ
ing fast Fourier transform (FFT) image also displays a fourfold symꢀ
metry (Figure 4d). These results directly prove that the crystalline
structure is still maintained after exfoliation. Thus, our results show
that cleavage of DPDS could efficiently lead to the formation of ulꢀ
trathin MOF nanosheets from intercalated MOFs. Control experiꢀ
ments by adding TMP or 4ꢀmercaptopyridine to the Zn2(PdTCPP)
crystals could not lead to the spontaneous exfoliation process (Figure
S18), confirming exfoliation starts with the reduction of DPDS ligands.
The exfoliation is likely caused by the significantly decreased interlayer
interactions after removing part of the intercalating agents, an extracꢀ
tion process similar to the synthesis of MXenes.6d
Figure 5. (a) UVꢀVis absorption spectra of DPBF in the presence of
chemically exfoliated MOF nanosheets upon visible light irradiation.
(b) Absorbance decay of DPBF by different catalysts. (c) Photooxidaꢀ
tion of DHN in CH3CN catalyzed by chemically exfoliated nanosheets.
(d) Absorbance of juglone (λ=419 nm) as a function of reaction time
by different catalysts.
Ultrathin 2D nanosheets typically exhibit superior photoresponsivꢀ
ity and enhanced photocatalytic activity with more easily accessible
reaction sites.10 Porphyrin derivatives are widely used for 1O2
generation due to their unique photochemistry and highꢀefficiency in
light harvesting.7c,11 We then further investigated the potential
application of asꢀexfoliated nanosheets in 1O2 generation for
heterogeneous photocatalysis. 1, 3ꢀdiphenylisobenzofuran (DPBF)
was used to detect the 1O2 evolution upon visible light irradiation
(λ>420 nm). As shown in Figure 5a, the absorption at 410 nm
decreases in the presence of exfoliated MOF nanosheets, indicating the
1
formation of O2. The chemically exfoliated nanosheets exhibit better
performance in 1O2 generation compared to other samples (Figure 5b).
As a result, the chemically exfoliated 2D MOF nanosheets are more
efficient in photooxidation of 1,5ꢀdihydroxynaphthalene (DHN) to
form the corresponding oxidized product juglone (Figure 5c and 5d).
Recently, Zhang group reported a novel surfactantꢀassisted synthetic
method to prepare ultrathin 2D MOF nanosheets with high yields.2e
This method can conveniently produce multilayer nanosheets (<10
nm) but is difficult to achieve singleꢀlayer nanosheets. Moreover, liquid
exfoliation method was used to exfoliate layered Zn2(PdTCPP) crysꢀ
tals for comparison. Although irregular nanosheets with thickness in
the range of 1.5~3.5 nm can be obtained, the majority of nanosheets
are 30~120 nm in height (Figure S19 and S20). More importantly, the
overall yield is ~10% with a large portion of small fragmented pieces
(Figure S20). Therefore, selective chemical exfoliation from predeꢀ
signed MOFs potentially allows for efficient and controllable formation
of 2D MOF nanosheets.
In conclusion, we have successfully demonstrated a versatile apꢀ
proach for synthesizing 2D MOF nanosheets with high yields. This
novel strategy relies on the incorporation of a chemically scissable
intercalating agent, 4, 4'ꢀdipyridyl disulfide, into the intrinsically layꢀ
ered MOF crystals. The cleavage of intercalated disulfide groups occurs
rapidly and, to a certain degree, is capable of controlling the thickness
of exfoliated MOF nanosheets. Considering the vastly available organic
ligands and metal nodes in MOFs, the approach demonstrated here
holds great promise for synthesizing various ultrathin 2D metalꢀorganic
nanosheets with desired structures and properties.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Detailed synthesis, singleꢀcrystal data, additional PXRD, SEM, TEM,
AFM, NMR, and XPS characterization results.
AUTHOR INFORMATION
Corresponding Authors
hxu@ustc.edu.cn (H. X.); jianglab@ustc.edu.cn (H. L. J.);
zhou@chem.tamu.edu (H. C. Z.)
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Ma, R.; Sasaki, T. Acc. Chem. Res. 2015, 48, 136. (d) Naguib, M.;
Gogotsi, Y. Acc. Chem. Res. 2015, 48, 128.
Notes
(7) (a) Gao, W. Y.; Chrzanowski, M.; Ma, S. Chem. Soc. Rev. 2014, 43
,
The authors declare no competing financial interests.
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5841. (b) Morris, W.; Volosskiy, B.; Demir, S.; Gandara, F.; McGrier,
P. L.; Furukawa, H.; Cascio, D.; Stoddart, J. F.; Yaghi, O. M. Inorg.
Chem. 2012, 51, 6443. (c) Son, H. J.; Jin, S.; Patwardhan, S.; Wezenꢀ
berg, S. J.; Jeong, N. C.; So, M.; Wilmer, C. E.; Sarjeant, A. A.; Schatz,
G. C.; Snurr, R. Q.; Farha, O. K.; Wiederrecht, G. P.; Hupp, J. T. J. Am.
Chem. Soc. 2013, 135, 862. (d) Park, J.; Feng, D.; Yuan, S.; Zhou, H.
C. Angew. Chem. Int. Ed. 2015, 54, 430.
(8) (a) Burnett, B. J.; Choe, W. CrystEngComm 2012, 14, 6129.
(b) Burnett, B. J.; Barron, P. M.; Hu, C.; Choe, W. J. Am. Chem. Soc.
2011, 133, 9984. (c) Karagiaridi, O.; Bury, W.; Mondloch, J. E.; Hupp,
J. T.; Farha, O. K. Angew. Chem. Int. Ed. 2014, 53, 4530. (d) Li, T.;
Kozlowski, M. T.; Doud, E. A.; Blakely, M. N.; Rosi, N. L. J. Am. Chem.
Soc. 2013, 135, 11688.
(9) Choi, E. Y.; Barron, P. M.; Novotny, R. W.; Son, H. T; Hu, C. H;
Choe, W. Inorg. Chem., 2009, 48, 426.
(10) (a) Wang, H.; Yang, X.; Shao, W.; Chen, S.; Xie, J.; Zhang, X.;
Wang, J.; Xie, Y. J. Am. Chem. Soc. 2015, 137, 11376. (b) Yang, S.;
Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.;
Ajayan, P. M. Adv. Mater. 2013, 25, 2452.
ACKNOWLEDGMENT
We acknowledge 15U1 beamline at Shanghai Synchrotron Radiation
Facility (SSRF) for the XRD measurements. We also thank Prof. S.
Wang for acquiring the single crystal Xꢀray diffraction data. This work
was supported by the National Key Basic Research Program of China
(2015CB351903, 2014CB931803), the National Natural Science
Foundation of China (51402282, 21474095, 21371162, 21673213 and
21521001), CAS Key Research Program of Frontier Sciences
(QYZDBꢀSSWꢀSLH018). Y.ꢀP. C was funded by Texas A&M Univerꢀ
sity. H.ꢀC. Z. gratefully acknowledges support from the Welch Foundaꢀ
tion (Aꢀ0030).
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