Fabrication of metal-organic framework and infinite coordination polymer nanosheets by the spray technique

Article abstract of DOI:10.1021/acs.langmuir.6b04353

We have developed a rapid and convenient method for fabricating metal-organic framework (MOF) and infinite coordination polymer (ICP) nanosheets by spraying the atomized solution of metal ions onto the organic ligand solution. Nanosheet formation could be attributed to the anisotropic diffusion of metal ions in the ligand solution, which may give rise to a lateral interface of metal ions and organic ligands, where the crystals tend to grow laterally in the form of nanosheets. Three kinds of Zn- and Cu-based MOF nanosheets and two kinds of Co-based ICP nanosheets have been successfully obtained by spraying under mild conditions. The two-dimensional structures of nanosheets with a nanometer thickness and a homogeneous size can be evidenced by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Brunauer-Emmett-Teller, and Fourier transform infrared spectroscopy measurements. Furthermore, the fabricated ICP nanosheets have exhibited efficient catalytic performance for the conversion of CO₂ to high-value-added chemicals. This spray technique simplifies the nanosheet production process by industrialized means and enhances its controllability by the fast liquid-liquid interfacial fabrication, thus allowing access to the industrialized fabrication of MOF and ICP nanosheets. (Figure Presented).

Full text of DOI:10.1021/acs.langmuir.6b04353

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Article
Fabrication of Metal-Organic Framework and Infinite
Coordination Polymer Nanosheets by the Spray Technique
Yunong Li, Sha Wang, Yuan Zhou, Xiaojue Bai, Guoshuai Song,
Xueying Zhao, Tieqiang Wang, Xuan Qi, Xuemin Zhang, and Yu Fu
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04353 • Publication Date (Web): 09 Jan 2017
Downloaded from http://pubs.acs.org on January 14, 2017
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Fabrication of MetalꢀOrganic Framework and Infinite
Coordination Polymer Nanosheets by the Spray Technique
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Yu-Nong Li,‡* Sha Wang,‡ Yuan Zhou, Xiao-Jue Bai, Guo-Shuai Song, Xue-Ying Zhao, Tie-
Qiang Wang, Xuan Qi, Xue-Min Zhang and Yu Fu*
College of Sciences, Northeastern University, Shenyang 110819, P. R. China
ABSTRACT: This manuscript has developed a rapid and convenient method to fabricate MOF
and ICP nanosheets by spraying the atomized solution of metal ions onto the organic ligands
solution. The nanosheets formation could be attributed to the anisotropic diffusion of metal ions
in the ligands solution, which may give rise to a lateral interface of metal ions and organic
ligands, where the crystals tend to grow laterally as the form of nanosheets. Three kinds of Zn
and Cuꢀbased MOF nanosheets and two kinds of Coꢀbased ICP nanosheets have been
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successfully obtained by spray in mild conditions. These twoꢀdimensional structure of
nanosheets with nanometer thickness and homogeneous size can be evidenced by the SEM,
AFM, XRD, BET and FTꢀIR measurements. Furthermore, the fabricated ICP nanosheets have
shown efficiently catalytic performance for the conversion of CO2 to high valueꢀadded
chemicals. This spray technique simplifies the nanosheets production process by the
industrialized means and enhance its controllability by the fast liquidꢀliquid interfacial
fabrication, thus opening up access to the industrialized fabrication for MOF and ICP nanosheets.
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INTRODUCTION
Over the past decades, metalꢀorganic frameworks (MOFs) and infinite coordination polymers
(ICPs) have attracted much attention due to their porous structures, channel functionalities and
structural tailorability, which leads to wide range potential applications.1ꢀ3 In particular, making
these materials into twoꢀdimensional (2D) nanosheets is highly desirable, on account of the
intriguing physical and chemical properties that are rarely present in bulk materials.4ꢀ5 In this
context, nanosheets are usually considered to be promising candidates for gas separation,6ꢀ7
energy storage (e.g. batteries, solar cells),8ꢀ9 chemical sensors,10 catalysis11ꢀ15 and other
application in the field of nanotechnology.16
In the structure of ICPs or MOFs, organic bridging ligands and inorganic connecting points
can build an infinite network extending isotropically in three dimensional space.1 Hence, it is
difficult to confine them in the 2D direction. With regard to ICP, the fabrication of ICP
nanosheets has rarely been reported so far,17ꢀ19 and the preparation of spherical ICP particles
are mostly focused on.20ꢀ22 As for MOF nanosheets, the fabrication approaches have been
developed mainly in two ways, which are topꢀdown exfoliation23ꢀ24 and bottomꢀup growth.25ꢀ26
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The topꢀdown exfoliation relies on the disintegration of bulk layered materials into singleꢀ or
multiꢀlayers with large lateral dimensions. However, the morphological damage, reꢀaggregation
and even the introduction of impurities in the exfoliation process are difficult to be avoided.5ꢀ6, 27
Therefore, plenty of efforts have been spent on the bottomꢀup growth strategy, which normally
produces nanosheets at their genesis by either enhancing the anisotropic crystal growth or
restricting the thermodynamically favored layer stacking.28 Recently, a novel bottomꢀup growth
strategy has been reported to prepare MOF nanosheets by threeꢀlayer method. In this system,
metal ions and organic ligands were respectively dissolved in the topmost and bottom solutions
and separated by the intermediate buffer solvent, forming three vertically arranged liquid layers.
The consequent diffusion of metal ions and linker ligands into the buffer segment caused the
growth of MOF crystals in the horizontal direction, yielding corresponding nanosheets.7
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Inspired by the threeꢀlayer mode, we have developed a rapid and convenient method to
fabricate MOF and ICP nanosheets by spraying the atomized solution of metal ions onto the
organic ligands solution as shown in Scheme 1. The atomization of the metal ion solution could
greatly recede the disturbance of the contact between the two reaction solutions. That may give
rise to a steady interface where the crystal would tend to grow in the lateral direction with
anisotropic diffusion, producing nanosheets without the need of buffer layer. As well known, the
spray is an easyꢀtoꢀhandle technique, and valuable for industrial applications, which always
works in combination with other manipulation as sprayꢀpyrolysis,29 plasmaꢀspray30 and sprayꢀ
drying.31ꢀ34 Uniquely, the nanoscale MOF with spherical hollow superstructures have been
successfully obtained via sprayꢀdrying.31 Herein, the spray technique could simplify the
nanosheets production process by the industrialized means and enhance its controllability by the
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liquidꢀliquid interfacial fabrication, which would pave a practical way for nextꢀstep
modifications and applications.
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Scheme 1. The fabrication of MOF and ICP nanosheets by spray. Droplets, metal ions, ligands
are shown in blue, grey and pink, respectively.
EXPERIMENTAL SECTION
Materials. The feedstocks of (CH3COO)2M.XH2O (M= Cu, Zn, Co), 1,4ꢀ
benzenedicarboxylate acid (BDCA), 1,4ꢀnaphthalenedicarboxylate (NDCA), and 1,4ꢀ
diazabicyclo[2.2.2]octane (DABCO) were purchased from Alfa Aesar or Energy Chemical,
which were used without any further purification.
Preparation. Three kinds of MOF nanosheets including CuBDC (copper 1,4ꢀ
benzenedicarboxylate), Cu(1,4ꢀNDC) (copper 1,4ꢀnaphthalenedicarboxylate), ZnBDC (zinc
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1,4ꢀbenzenedicarboxylate) and two kinds of ICP nanosheets CoBDC (cobalt 1,4ꢀ
benzenedicarboxylate), Co2(BDC)2(DABCO) (BDC = 1,4ꢀbenzenedicarboxylate) can be
fabricated by spray in mild conditions. Generally, the fabrication process started with the
injection of metal ion solution into the spray nozzle by automatic injection pump, and then
underwent the ultrasonic atomization of metal ion solution to microdroplets at a certain rate.
Subsequently, the droplets containing the metal ions fell on the surface of the organic ligand
solution in the culture dish by spraying as shown in Scheme 1. Taking the CuBDC as an example,
30 mg of (CH3COO)2Cu was dissolved separately in a mixture of 10 mL of N,Nꢀdimethyl
formamide (DMF) and 5 mL of acetonitrile (CH3CN), and this (CH3COO)2Cu solution was
sprayed to the 1,4ꢀbenzenedicarboxylate acid (BDCA) solution (2 mg/mL, DMF:CH3CN = 1:2)
in the culture dish with the flow rate of 1 ꢀL/sec for 110 seconds. CH3CN worked as a suitable
miscible coꢀsolvent. Then, the instantly appeared precipitates could be distinctly observed at the
interface, which were simultaneously assembled to crystalline nanosheets. Subsequently, the
precipitates were directly collected by the glass substrate, followed by washing with DMF (1 mL)
and dichloromethane (1 mL) to afford the desired product. The optimum structures of CuBDC,
Cu(1,4ꢀNDC), ZnBDC, CoBDC and Co2(BDC)2(DABCO) nanosheets with nanometer thickness
and homogeneous sizes were obtained in the above method, also with the adjustion of the ions
concentrations. Thus, the optimum concentrations of feedstocks were respectively 2 mg/mL
(CH3COO)2Cu with 2 mg/mL BDCA for synthesizing CuBDC nanosheets, 0.9 mg/mL
(CH3COO)2Cu with 0.75 mg/mL NDCA for synthesizing Cu(1,4ꢀNDC) nanosheets, 2.2 mg/mL
(CH3COO)2Zn and 1.1 mg/mL BDCA for synthesizing ZnBDC nanosheets, 52.3 mg/mL
(CH3COO)2Co.4H2O with 34.9 mg/mL BDCA for synthesizing CoBDC nanosheets, and 52.3
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(CH3COO)2Co.4H2O, 34.9 mg/mL BDCA with 23.4 mg/mL DABCO for synthesizing
Co2(BDC)2(DABCO) nanosheets.
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Characterization. Nanosheets with nanometer thickness and homogeneous size have been
evidenced by the Scanning Electron Microscopy (SEM), Atomicꢀforce Microscopy (AFM), Xꢀ
ray diffraction (XRD), Brunauer Emmett Teller (BET) and Fourier Transform Infrared
Spectroscopy (FTꢀIR) measurements. The SEM images were taken with a JEOLJSMꢀ6700F
field emission scanning electron microscope (15 kV). The AFM images were collected using a
Bruker Dimension Icon equipped with ScanasystꢀAir peak force tapping mode AFM tips from
Bruker. The XRD patterns were recorded using a diffractometer (model D/maxꢀIIB, Rigaku
Co., Tokyo, Japan) in the 2θ range 20–60° using CuKα1 (λ= 1.5405 Å) radiation. The surface
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area was calculated using a multipoint BET model. The total pore volume was estimated at a
relative pressure of 0.99, assuming full surface saturation with nitrogen. The IR spectra were
recorded on a Bruker Tensor 27 FTꢀIR spectrophoꢀtometer with KBr pellets.
RESULTS AND DISCUSSION
Using the spray technique described above, CuBDC nanosheets have been synthesized smoothly,
which are wellꢀdispersed and homogeneously distributed in small square shapes, demonstrated in
SEM image (Fig. 1a). In the AFM image, the square lamellae of CuBDC exhibits lateral
dimensions in the range of 220ꢀ440 nm with the thickness of 5 nm approximately (Fig. 1c and
1d). The CuBDC can be proven to be formed via the coordinated synthesis from FTꢀIR
spectroscopy (Fig. 1e). Characteristic peaks at 3051 and 1507 cmꢀ1 refer to CꢀH stretching
vibration absorption and ring vibration in beneze, respectively. The absorptions of 1629 and
1387 cmꢀ1 indicate the COOꢀ group.35ꢀ36 The XRD pattern of CuBDC gives only two reflections,
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the 2θ at 16.8o and 34.1o (Fig. 1b), being indexed as the (ꢀ201) and (ꢀ402) crystallographic planes
of the CuBDC structure, which can be detected from the peaks of CuBDCꢀMOF crytals.7 Besides,
no additional Bragg diffractions of the bulk counterpart are observed, showing agreements with
standards of 2D MOF structures. Through the cryogenic nitrogen physisorption/desorption and
BET analysis, CuBDC feature a specific surface area (SBET) of 130.1 m2 g-1 and an average
micropore size of 6.5ꢀ7 Å (Fig. 1f). The rapid N2 uptake observed at low relative pressure also
indicates the CuBDC to be the microporous MOF material. There is a H1ꢀtype hysteresis loop at
the high relative pressure (P/P0>0.8), probably owing to the interlaminar porosity between the
nanosheets, which is usually absent in the bulk solid.7
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Figure 1. Characterizations of CuBDCꢀMOF nanosheets fabricated by spray. (a) SEM image; (b)
XRD pattern; (c) AFM image; (d) The height profiles in colourꢀcoded red and blue tracks in
AFM image; (e) FTꢀIR spectrum; (f) N2 sorption isotherms and BET analysis.
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The ligand and metal center have also been varied to synthesize MOF nanosheets of Cu(1,4ꢀ
NDC) and ZnBDC. The ligand in the MOF may have great influence on the surface profile and
nanosheet distribution. When the ligand is changed to 1,4ꢀnaphthalenedicarboxylate acid,
Cu(1,4ꢀNDC) nanosheets become thick with round and swell edges (Fig. 2a). The appearances of
2θ at 10.14o, 12.26o in XRD measurement can confirm the product to be isostructural to the 2D
structure of Cu(1,4ꢀNDC) (Fig. 2c).7 The FTꢀIR spectrum shows the characteristic peaks of
COOꢀ group in Cu(1,4ꢀNDC) at 1616 and 1370 cm−1. Absorption peaks of naphthalene ring37 can
be observed at 3070, 1489 and 1414 cmꢀ1 (Fig. 2e). Compared with others, Zn(BDC) displays
the thinnest nanosheets and the largest pieces from 2 ꢀm×4.7 ꢀm to 3.6 ꢀm×5.3 ꢀm (Fig. 2b).
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Interestingly, bulk ZnBDC named MOFꢀ5 has limited stability, which can be reversible
transformed into MOFꢀ5W and MOFꢀ5H when exposed to water for different exposure time.38 In
our XRD measurement of ZnBDC (Fig. 2d), the intensity of 2θ at 8.71o corresponds to the
special structure of MOFꢀ5W, and the three reflections 2θ at 8.71o, 17.42o and 26.36o support its
2D framework. In the FTꢀIR spectrum, signals at 1562 and 1390 cmꢀ1 can be assigned to C=O
bands in BDCꢀ group in ZnBDC. The peaks at 2989 and 1507 cmꢀ1 correspond to CꢀH stretching
vibration absorption and ring vibration in beneze, respectively (Fig. 2f). These characterizations
all suggest the successful synthesis of Cuꢀ and Znꢀbased MOF nanosheets.
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Figure 2. Characterizations of Cu(1,4ꢀNDC)ꢀMOF and ZnBDCꢀMOF nanosheets. Cu(1,4ꢀNDC):
SEM image (a), XRD pattern (c), FTꢀIR spectrum (e); ZnBDC: SEM image (b), XRD pattern
(d), FTꢀIR spectrum (f).
Employing the spray technique, we have also obtained Coꢀbased ICP nanosheets.
Remarkably, CoBDC nanosheets can be verified from the regular lamellaes and their thinner
transverse sections (Fig 3a). Adding DABCO into the metal ion solution before the spray step
can also fabricate Co2(BDC)2(DABCO) nanosheets, which closely weave andvertically and
insert into each other (Fig. 3b). Through the XRD analysis, we have found XRD patterns of
CoBDC and Co2(BDC)2(DABCO) nanosheets are totally different from the standards of bulk
MOFs. Nevertheless, both of CoBDC and Co2(BDC)2(DABCO) nanosheets have been proven
to be synthesized via coordination between the Co center and ligands according to the existing
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characteristic absorptions of COOꢀ, beneze and DABCO groups in the FTꢀIR spectra, which
supports their ICP structures.
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Figure 3. SEM images of ICP nanosheets fabricated by spray. (a) CoBDC; (b)
Co2(BDC)2(DABCO).
In order to investigate the role of the spray technique, the direct mixing of the salt
(CH3COO)2Zn or (CH3COO)2Co solution with the ligand BDCA solution without the spraying
process has been carried out as the control experiments. However, the irregular bulks have been
obtained instead of MOFs or ICPs nanosheets, which conforms with the situation and the result
of the omission of the intermediate buffer layer in threeꢀlayer mode7, and further validates the
indispensable role of the spray technique in creating the nanosheet morphology in this work.
Basing on the experimental results and the related references,7, 31, 39ꢀ40 the possible mechanism of
nanosheets formation through spraying was illustrated in Scheme 1. When metal ions solution
was sprayed on the surface of the organic ligands solution, the droplets containing metal ions
were supposed to diffuse in the lateral direction rather than in the vertical one. Consequently,
metal ions would diffuse along with the lateral expansion of droplets and distribute evenly at the
interface. This procedure benefited the rapid interfacial contaction and reaction between metal
ions and organic ligands, thereby tending to generate the MOF or ICP nanometric crystals
quickly as the form of nanosheets in lateral directions. The rapid nanosheets fabrication could
largely rely on the fast reaction by employing the acetate salt (CH3COO)2M as the metal ion
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precursor, which would give high kinetic reaction rate due to the intrinsically protophilic weak
acid anion of CH3COOꢀ. Instead, using M(NO3)2 could not achieve the formation of
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corresponding nanosheets. That is probably because NO3 may result in a hysteretic reaction,
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whose occurrence would miss the opportunity of the lateral diffusion at the interface, yet be
carried out tardily with the followꢀup vertical diffusion, failing to afford the 2D structure.
Hence, this nanosheets fabrication process arises from the anisotropic diffusion, which is
strategically similar to the reported threeꢀlayer diffusion method. However, the spray approach
needs no buffer layer and has more advantages in combining with the industrialization
technology for nanosheets production depending on the fast interfacial synthesis.
With the nanosheets in hand, we also tried to develop their application in catalysis. The
coordination nanosheets with multiple Lewis acid centers, zeoliteꢀlike properties and 2D
structural features represent promising heterogeneous catalysts for some organic reactions.
Particularly, the cycloaddition of CO2 with epoxides to produce fiveꢀmembered cyclic
carbonates41ꢀ42 is of great significance for the utilization of waste gas CO2.43ꢀ44 In the
cycloaddition, the homogeneous catalysts usually have the limitation of hard separation and
o
high reaction temperatures (>100 C).41 Fortunately, using CoBDC nanosheets as the catalyst,
the phenyl epoxide 1 can be successfully converted 95% to selectively produce phenyl cyclic
carbonates 2 in the high yield of 85%, also affording little of byproducts phenylglycol 3
(Scheme 2). It is worth mentioning that the reaction conditions are extremely mild with the
atmospheric pressure of CO2 and the temperature of 70 oC, without any organic solvent, which
is difficult to implement in the previous cycloaddition (details see SI).
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Scheme 2. CoBDC nanosheetsꢀcatalyzed cycloaddition of CO2 with epoxides.
CONCLUSION
In summary, we have developed a fast, economical and versatile spray method for the
fabrication of MOF and ICP nanosheets, which is applicable to the fabrication of a series of Cu,
Znꢀbased MOF nanosheets and Coꢀbased IPC nanosheets with different organic ligands. On the
basis of the anisotropic diffusion in the bottomꢀup strategy, the liquidꢀliquid interfacial synthesis
and the simultaneous lateral assembly of nanosheets have been readily achieved, dispensed with
the need of the buffer layer or the solid substrate during the nanosheets formation. Furthermore,
the fabricated ICP nanosheets also have efficiently catalytic performance for the conversion of
CO2 to high valueꢀadded chemicals under mild conditions. This spray approach opens up an
effective way for the MOF and ICP nanosheets fabrication by industralized means, potentially
beneficial for the further manipulations and applications. The researches on deeper insight of
synthetic mechanism as well as further utilization are ongoing.
ASSOCIATED CONTENT
Supporting Information
Instruments for catalytic conversion of CO2 and the characterization data of products.
AUTHOR INFORMATION
Corresponding Author
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*Eꢀmail: liyunong@mail.neu.edu.cn; fuyu@mail.neu.edu.cn.
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Notes
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The authors declare no competing financial interest.
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‡ These two authors contributed equally.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from the National Natural Science
Foundation of China (Grant no. 21404021, 21503037, 51601032), the Doctoral Scientific
Research Foundation of Liaoning Province (20141013, 201501149), Fundamental Research
Funds for the Central Universities (N130205001, N150504004, N150504005, N140502001,
N142004001, N140503001) and the Open Project of the State Key Laboratory of Supra
molecular Structure and Materials (SKLSSM201609).
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Fabrication of MetalꢀOrganic Framework and Infinite
Coordination Polymer Nanosheets by the Spray Technique
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Yu-Nong Li,‡* Sha Wang,‡ Yuan Zhou, Xiao-Jue Bai, Guo-Shuai Song, Xue-Ying Zhao, Tie-
Qiang Wang, Xuan Qi, Xue-Min Zhang and Yu Fu*
College of Sciences, Northeastern University, Shenyang 110819, P. R. China
A rapid and convenient spray method has been developed to fabricate metalꢀorganic framework
and infinite coordination polymer nanosheets by the fast liquidꢀliquid interfacial fabrication on
the basis of anisotropic diffusion.
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