Step-by-Step Route for the Synthesis of Metal -Organic Frameworks
cradleOsama Shekhah,*,†Hui Wang,†Stefan Kowarik,‡Frank Schreiber,‡Michael Paulus,§Metin Tolan,§Christian Sternemann,§Florian Evers,§Deni Zacher,|Roland A.Fischer,|and Christof Wo ¨ll*,†
Ruhr-Uni V ersita ¨t Bochum,Physikalische Chemie 1,44780Bochum,Germany,Uni V ersita ¨t Tu ¨bingen,Angewandte Physik,72074Tu ¨bingen,Germany,Uni V ersita ¨t Dortmund,Experimentelle Physik I,44227Dortmund,Germany,and
Ruhr-Uni V ersita ¨t Bochum,Organometallics and Materials Chemistry,44780Bochum,Germany
Received August 17,2007;Revid Manuscript Received October 25,2007;E-mail:woell@pc.rub.de;shekhah@pc.rub.de
Supramolecular chemistry holds unique prospects for the fabrica-tion of novel functional materials.Molecular precily defined
subunits (which may already be rather complex lf-asmblies)form even more complex structures that exhibit functionalities not provided by the individual building blocks.The coupling of the covalently bonded subunits by noncovalent interactions is a key requisite for this type of supramolecular asmbly.1In two dimensions,the understanding of such an asmbly of organic molecules (ligands)interacting through hydrogen bonds or ionic interactions has been significantly advanced in recent years.2,3A major reason for this progress is the availability of molecular-resolution microscopic data,which allows following directly the lf-asmbly process taking place after the building blocks are placed on a “tablet”.For appropriate tablets like Au(111)or Ag-(111)surfaces,scanning tunneling microscopy (STM)can be applied in a straightforward fashion to obtain high-resolution images not only of the ordered structures prent after the completion of the lf-asmbly process but also of intermediate,nonperiodic structures.In veral cas it was possible to apply microscopic methods and spectroscopic methods in parallel to study important steps in the lf-asmbly ,the deprotonoation of organic acids and the subquent formation of carboxylates in a step-by-step fashion,by cooling the metal substrates to cryogenic temper-atures.4,5
It is difficult to study lf-asmbly process occurring in three dimensions on the same level of detail.For metal -organic frameworks (MOFs),a class of hybrid porous solid material introduced by Kitagawa,Ferey,and Yaghi about 10years ago,6-8there have been attempts to characterize the formation process by in situ spectroscopic techniques in more detail.9,10Although it was possible to demonstrate that the formation of the highly ordered MOFs occurs first via an asmbly of the primary building blocks to defined condary building blocks (SBUs),and then to the MOF crystallites,9a thorough understanding of the formation process is still lacking.
In order to study the formation of MOFs in a more rational way,we have taken a rather different approach.In contrast to the established synthesis protocols,where the educts (primary building blocks,typically two)are mixed and treated under solvothermal conditions,we combine them in a quential fashion.By using an appropriately functionalized organic surface as a (two-dimensional)nucleation site,we can grow MOF structures in a step-by-step fashion (e Figure 1).This not only allows us to study the kinetics of the individual steps but also provides the potential to fabricate structures possibly not accessible by bulk synthesis (Figure 1).We have chon [Cu 3BTC 2(H 2O)n ](1,HKUST-I)for our study (e
Figure 1in Supporting Information)).The synthesis and structure of this MOF have been described in
detail previously,but the details of its formation are still unknown.
In Figure 2we prent data obtained by surface plasmon resonance (SPR)for the growth of 1on a COOH-terminated SAM surface fabricated by immersing the Au substrate into an ethanolic solution of mercaptohexadecanoic acid (MHDA).The SPR tech-nique,which has not previously been applied to MOF synthesis,allows monitoring the deposition of molecular species on surfaces with submonolayer resolution.The data show that subquently adding copper(II)acetate (CuAc)and 1,3,5-benzenetricarboxylic acid (BTC)leads to step-by-step deposition of multilayers.Data obtained by IR spectroscopy (Figure 2in Supporting Information)fully support this finding.
The deposition of organic layers using such quential process has been reported previously (e ref 17for the ca of multilayers of organosulfur/Cu compounds and ref 18for the deposition of ionic polymers).However,evidence of a three-dimensional long-range ordering of the deposited multilayers with structural features identical to a coordination polymer with the same composition has not yet been prented.In a recent work by us on the quential deposition of Zn/BTC,which has a different structure from HUKST-1,no X-ray diffraction data could be obtained,thus
†Ruhr-Universita ¨t Bochum,Physikalische Chemie.‡Universita ¨t Tu ¨bingen,Angewandte Physik.§U
niversita ¨t Dortmund,Experimentelle Physik I.|Ruhr-Universita ¨t Bochum,Organometallics and Materials
Chemistry.
牛津一年级英语
Figure 1.Schematic diagram for the step-by-step growth of the MOFs on
the SAM,by repeated immersion cycles,first in solution of metal precursor and subquently in a solution of organic ligand.Here,for simplicity,the scheme simplifies the assumed structural complexity of the carboxylic acid coordination modes.
Figure 2.SPR signal as a function of time recorded in situ during quential injections of CuAc (A),ethanol (B),and BTC (C)in the SPR cell containing a COOH-terminated
SAM.
Published on Web 11/17/2007
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pointing to the prence of rather disordered material.In the prent ca,however,we were able to obtain high-quality XRD data,both for out-of-plane and in-plane conditions.The experiments were carried out using a laboratory (Cu K R )as well as a synchrotron radiation source (DELTA,Dortmund).A typical diffraction scan for a 40cycles Cu/BTC multilayer is shown in Figure 3.This out-of-plane diffraction scan clearly demonstrates the prence of a highly ordered and preferentially oriented crystalline material with a periodicity of 6.5Ånormal to the surface.Together with the in-plane data (e Figure 3,int)this demonstrates unambiguously that the deposited multilayer exhibits the same structure as obrved for the bulk compound [Cu 3BTC 2(H 2O)n ](1).
Of cour the finite number of layers perpendicular to the surface should result in an incread width of the out-of-plane diffraction peaks,which is given by )λ/(Nd cos θ),with λdenoting the wavelength of the X-ray radiation,Nd the length of the unit cells,and θthe diffraction angle.Under our conditions (λ)1.54,N )number of layers,d )13Åfor the (200)reflex peak)the width for a 40-layer film amounts to 0.03°,which is below the experimental resolution ud in the prent ca and is fully consistent with the data shown in Figure 3.
It is interesting to note that on our COOH-terminated surface 1grows with the same orientation as obrved previously by Bein and co-workers,when immersing an organothiol-bad COOH-terminat
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ed SAM into an aged (8days)and filtered mother solution of the solvothermal synthesis of 1at room temperature,but different from that obtained for COOH-terminated silane-bad SAMs.Obviously,the (100)-face of 1matches particularly well with a COOH-terminated MHDA-SAM initiating a highly regular growth
at mild conditions.The gas-loading properties of MOF layers grown by the step-by-step method were studied via NH 3/water exchange experiments.IR and NEXAFS data (e Supporting Information)reveal a nonreversible behavior similar to that en in the bulk.Together with the SEM data shown in Figure 4we can thus conclude that the step-by-step synthesis yields homogeneous,highly crystalline MOF films exhibiting the HKUST-I bulk structure.Note that the immersion method ud in previous work 21-24leads to very heterogeneous,rough MOF coatings consisting of fairly large,single crystallites.A cond advantage of our new method is the lower temperature (room temperature vs 75°C required in the one-step synthesis.21Aside from the possibility to u the novel preparation method to study the kinetics of the film formation in more detail using SPR and to model it using theoretical approaches,the step-by-step method offers the unique opportunity to grow novel MOF-like ordered structures which consist of alternating layers,possibly with nonperiodic combinations of different metal ions and/or different linkers.
Acknowledgment.Part of this work has been funded by the EU (STREP “SURMOF”)and the German “Fonds der Chemischen Industrie”.We thank Prof.T.Bein (Munich)for fruitful discussion and for making ref 20available to us prior to publication.
Supporting Information Available:IRRAS and NEXAFS spectra recorded for samples prepared after cycles of immersion in Cu(Ac)2and BTC solutions.IRRAS spectra monitoring the loading of the surface-deposited MOFs with NH 3.This material is available free of charge via the Internet at pubs.acs.科米课堂
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Figure 3.Out-of-plane XRD data for a Cu 3BTC 2‚x H 2O MOF sample (40cycles)grown on a COOH-terminated SAM,the in-plane spectra are also shown as an int.
Figure 4.Scanning electron micrographs of Cu 3BTC 2‚x H 2O MOF (40cycles)grown on a SAM laterally patterned by microcontact printing (µCP)consisting of COOH-terminated squares and CH 3-t
erminated stripes (left),and Cu 3BTC 2‚x H 2O MOF crystallites grown on a COOH-terminated SAM using the method described in ref 21(right).
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