z Materials Science inc. Nanomaterials & Polymers

Ultra-Sonication of ZIF-67 Crystals Results in ZIF-67Nano-FlakesSebastian Beyer,[a] Carsten Prinz,[a] Robin Schurmann,[a] Ines Feldmann,[b] Annett Zimathies,[a]

Anna M. Blocki,[c, d] Ilko Bald,[a] Rudolf J. Schneider,[a] and Franziska Emmerling*[a]

Zeolitic Imidazolate Frameworks (ZIFs) are crystalline materialsthat comprise of metal nodes and Imidazole derivatives aslinkers. ZIF-67 is often used in polymer composite materialse. g. for gas separation membranes. Post-synthesis treatment ofZIF-67 crystals with ultrasound leads to unforeseen plasticitythat resulted in sintered ZIF-67 and ZIF-67 nano-flakes.Consequently, ultrasound increases the external surface area ofZIF-67 which might improve e. g. blending with polymers incomposite materials. These new morphologies of ZIF-67 werecharacterized by transmission electron, scanning electron, andatomic force microscopy. The ultrasound treatment of ZIF-67did not result in the formation of an amorphous framework ora meta-stable crystal structure as indicated by powder x-raydiffraction. In addition, ultra-sonicated ZIF-67 retained the highgas adsorption capacity and pore size compared to synthesizedZIF-67. The morphological changes are hard to detect withstandard analytical methods that are usually utilized for MOFcharacterization. These findings also suggest that sonochemicaltreatment of ZIFs leads to structural effects beyond increasingthe amount of nucleation clusters during sono-chemical syn-thesis, which is currently not addressed in the field.

Zeolitic Imidazolate Frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs).[1] ZIFs are crystalline materials thatare generally understood to have a rather brittle and rigidstructure.

ZIFs have exceptionally high thermal and chemical stabilitywhile exhibiting a high surface area.[1–2] Ultra-sonication of ZIFshas mainly been studied in the scope of sonochemicalsynthesis routes.[3] Ultrasound is understood to increase thenumber of crystallization nuclei of MOFs in wet chemicalsynthesis, leading to smaller particle sizes.[3d] In general ultra-sound can be described as cyclical mechanical vibration wherecavities with exceptional temperatures and pressures occur inliquid medium. During cavitation, temperatures of up to 5000 Kand pressures around 1000 bar occur for a short period oftime[4] within and in the vicinity (~ 200 nm) of the cavities. Incase the cavity occurs near a macroscopic object, micro jets areformed that clean, erode and activate the surfaces. Ultra-sonication is thus a mechanical vibration that leads to input ofkinetic energy into any given system. The kinetic energy that isapplied during e. g. mechanochemical synthesis approachescan lead to bond breakage or the formation of excitedmolecules, leading to further reactions. This is the reason whythe mechanochemical approach of ball-milling metal salts andligands reliably lead to the formation of MOFs[5] and ZIFs.[5a]

However, the post-synthesis input of kinetic energy by ballmilling to ZIF and MOF systems leads to amorphous MOFstructures (aMOFs).[6] This has been shown for ZIF-1, ZIF-3, ZIF-4, CoZIF-4, ZIF-8, ZIF-69 and other MOFs but not for ZIF-67.[6]

The effect of post-synthesis ultrasound on and within MOFs hasnot yet drawn much attention. One earlier study reportedultrasound-induced Ostwald ripening of a ZIF-8 particlesuspension in tetrahydrofuran.[7] The observed Ostwald ripen-ing was explained by surface activation, partial dissolution andre-crystallization. This observation is different from the pre-sumption that ultra-sonication might lead to amorphization ofZIF crystals. Amorphous MOF structures have a stronglydecreased BET surface and show broadened reflections in XRDdiffractions. Notably, in case of ultrasound mediated Ostwaldripening of ZIF-8 particles, the authors found no sign ofamorphization.[7]

Post-synthetic treatment of ZIF-67 with ultrasound resultsin unforeseen plasticity of ZIF-67. This plasticity leads to ZIF-67nano-flakes that are peeled off ZIF-67 particles, while particlesintering is mediated in a time-dependent manner (Figure 1,pictures b to f). These observations are significant for both, thepossible technological application as well as the scientificknowledge on MOF and ZIF plasticity. ZIF-67 nano-flakes orpartially sintered ZIF-67 provide a higher external surface areacompared to synthesized ZIF-67. Interestingly, earlier reportsobserved that gas separation membranes only perform with

[a] Dr. S. Beyer, C. Prinz, R. Sch�rmann, A. Zimathies, Dr. I. Bald,Dr. R. J. Schneider, Dr. F. EmmerlingDepartment of Analytical Chemistry; Reference MaterialsBAM Federal Institute for Materials Research and TestingRichard-Willst�tter-Str. 11, 12489 Berlin, GermanyE-mail: Franziska.Emmerling@bam.de

[b] I. FeldmannDepartment of Materials and the EnvironmentBAM Federal Institute for Materials Research and Testing12205 Berlin

[c] Dr. A. M. BlockiInstitute of Biomaterial Science and Berlin-Brandenburg Centre for Re-generative TherapiesHelmholtz-Zentrum Geesthacht14513 Teltow, Germany

[d] Dr. A. M. BlockiBerlin-BrandenburgSchool for Regenerative TherapiesCharit� Universit�tsmedizin13353 Berlin, Germany

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/slct.201601513

CommunicationsDOI: 10.1002/slct.201601513

5905ChemistrySelect 2016, 1, 5905 – 5908 � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

the desired efficiency when the ZIF particles were ultra-sonicated together with a solution of the membrane-formingpolymer.[7] The here observed ultra-sound induced increase inexternal surface area complements existing explanation for thisobservation.

Ultra-sonication is a commonly employed method to re-disperse ZIF-67 in organic solvents. Indeed, ultra-sonication fora short period of time (e. g. 20 seconds) did not lead to anyvisible change in particle morphology (Figure 1b & 1 g).However, extended exposure of ZIF-67 particles demonstrateda strong effect on the particle morphology. First, particle

surfaces become rough and bumpy after about 30 minutes ofultra-sonication (Figure 1c). These bumps develop into fringes(Figure 1d, yellow arrow) and into nano-flakes (Figure 1e) thatpeel off the particle at 60 and 90 minutes of continuous ultra-sonication, respectively. Peeling off the nano-flakes leadstowards rounded, more spherical particle cores (Figure 1e and1 f). Most interestingly, the peeling of nano-flakes is not anirreversible process. ZIF-67 particles were observed to fusetogether, forming cord-like connections (Figure 1 f, 1 h - yellowarrow) made of ZIF-67 nano-flakes that re-attach to neighbour-ing particles. In addition, these ZIF-67 nano-flakes were foundto decorate the surfaces of ZIF-67 particles (Figure 1i – yellowarrow) and were also dispersed in the solvent phase. Single ZIF-67 sheets (Figure 1e -yellow arrow) had little contrast inTransmission Electron Microscopy (TEM), and were obscured bythe sputtered gold layer in Scanning Electron Microscopy(SEM). Additional images are provided as Supporting Informa-tion, Figure S3 to Figure S31. From Figure 1e we were able toconclude the ZIF-67 nano-flakes to be rather thin and to have asize around 150 nm x 150 nm. Atomic Force Microscopy wasemployed to further investigate the height of ZIF-67 nanoflakes (Figure 2). The height of the ZIF-67 nano-flakes wasfound to be arbitrary and primarily below 30 nm.

The morphological appearance of the ZIF-67 after extensiveultra-sonication suggested a transformation from crystalline toamorphous structures. Powder diffraction patterns of aMOFsare characterized by broadened reflections. This broadened

Figure 1. Schematic treatment of ZIF-67 with ultrasound (a), transmissionelectron micrographs (picture b to f) and scanning electron micrographs(picture g to i) of ZIF-67 particles. ZIF-67 particles were ultra-sonicated forvaried periods of time: b) no sonication, c) 30 minutes, d) 60 minutes, e) 90minutes and f) 120 minutes. Scanning electron micrographs g), h) and i)depict particles that were ultra-sonicated for 0, 90 and 120 minutes,respectively. The yellow arrows point to regions of interest.

Figure 2. Micrographs of ZIF-67 nano-flakes that were adsorbed on a silicawafer and imaged by Atomic Force Microscopy (a). The insert shows a zoom-in area with a single nano-flake (b) of which a height profile was recorded (c).

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5906ChemistrySelect 2016, 1, 5905 – 5908 � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

reflection could not be detected in PXRD analysis of ultra-sonicated ZIF-67 material (Figure 3a). Instead the diffractogram

of ZIF-67 material that was ultra-sonicated for 120 minuteslargely resembled that of as-synthesized ZIF-67 (Figure 3). Thisobservation precluded the permanent transition into anothercrystal conformation. Such transitions are known, for example,from ZIF�L of cobalt to ZIF-67.[8] ZIF�L is usually synthesized bya wet chemical approach using 2-methylimidazole and Zn2 + orCo2 + salts in aqueous environment forming thin, leaf-likestructures.[9] ZIF�L of cobalt transforms into ZIF-67 whendispersed in organic solvents at elevated temperature.[8a]

Combination of in-situ X-ray spectroscopic methods[10] withhigh-intensity ultrasound is challenging. Therefore, the ques-tion whether ZIF-67 nano-flakes transiently adopt a differentmetastable crystal structure during the detachment andadsorption phase of ZIF-67 nano-flakes remains to be inves-tigated. ZIF-67 crystals did not show any permanent trans-

formation into amorphous or metastable crystal structures suchas that of ZIF�L of cobalt.[8b]

Notably, the plasticized particles and nano-flakes of ZIF-67material retained their high gas adsorption capacity (Figure 3b)and specific surface area. The isotherms of the nitrogenphysisorption measurement for ZIF-67 showed a typical type Iisotherm according to the IUPAC isotherm classification system(supplemental Data, Figure S2).

Comparison of physisorption isotherms of Nitrogen be-tween (blue) as synthesized and (red) ZIF-67 that wasextensively ultra-sonicated (b)

The relative pressure range for BET analysis of the isothermswas selected by optimizing the correlation coefficient of theregression function within the BET-plot. The same relativepressure range was chosen in order to obtain valid compar-isons for BET surfaces between different samples. This proce-dure is validated through the finding of similar specific surfacesaccording to the BET and Langmuir model. The analysis of theisotherm revealed a BET specific surface area of ABET = 1440 �0.8 m2/g and 1410 � 1.0 m2/g, for “as synthesized” and “ultra-sonicated” ZIF-67, respectively (Table 1). These BET data are

consistent with the calculated Langmuir area of ALangmuir = 1483m2/g and 1456 m2/g, for “as synthesized” and “ultra-sonicated”ZIF-67, respectively. Notably, the external surface area is13.47 m2/g for as synthesized ZIF-67 and 20.48 m2/g for ZIF-67that was ultra-sonicated for 120 minutes. Further informationon nitrogen physisoprtion can be found in the SupportingInformation, Table S1 to Table S6 as well as Figure S1 and S2.The observations from physisorption measurements are con-sistent with the observed change in morphology from smoothuniform ZIF-67 crystals to rough and uneven shaped plasticizedZIF-67 and ZIF-67 nano-flakes. The external surface area ofultra-sonicated ZIF-67 has increased to roughly 150 % while thetotal surface did not change significantly. The microporevolume was as well not significantly influenced by extendedultra-sonication and was found to be 0.6775 cm3/g and0.6663 cm3/g for as synthesized and ultra-sonicated ZIF-67,respectively. In addition the median pore width was deter-mined with the Horvath-Kawazone method that uses a modelaccording to Saito-Foley describing zeolitic frameworks andwas found to be 1.26 nm for as synthesized and ultra-sonicatedZIF-67. Determination of the porosity distribution usingmethods based on density functional theory yielded a porewidth of 1.26 nm in both cases (Table S5).

In summary, ultra-sonication lead to very thin ZIF-67 nano-flakes and partially sintered, fringed ZIF-67 particles while otherproperties such as pore size and the Langmuir or BET area are

Figure 3. Comparison of PXRD patterns between (blue) as synthesized and(red) ZIF-67 that was extensively ultra-sonicated (a).

Table 1. Specific surface area based on BET and Langmuir model.

Sample ABET

m2/gALANGMUIR

m2/gExternal surfacem2/g

ZIF-67 as synthesized 1440 � 0.8 1483 � 4.2 13.47ZIF-67 ultra-sonicated 1410 � 1.0 1456 � 4.6 20.48

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5907ChemistrySelect 2016, 1, 5905 – 5908 � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

not affected. Ultra-sonication of ZIF-67 for 120 minutes lead toan external surface area that was increased by 150 %. Inaddition this study demonstrates that no amorphization occursupon ultra-sonication and that no metastable crystallinematerial is formed that could explain the observed morphology.The effects of sono-chemical treatment on ZIF-67 crystalscannot be detected with PXRD or physisorption studies.Fringed ZIF-67 surfaces or ZIF-67 nano-flakes are obscured inscanning electron microcopy (SEM) by the necessity ofsputtering a gold layer with a thickness of several nanometer.TEM can detect these subtle changes but is rarely performedon ZIFs and MOFs while literature records on AFM studies ofMOFs were found to be even rarer. ZIF-67 nano-flakes areenvisioned to be valuable for nano-structured self-assembledmaterials. In addition ZIF-67 nano-flakes could yield nm-thin N-doped carbon upon pyrolysis, which has interesting propertiesin catalysis and for electrodes in lithium ion batteries.[11] Theobservations on ZIF-67 that was exposed to ultrasound are alsoof significance to interpret and to design future works in thefield of sono-chemical MOF and ZIF synthesis. Ultrasound isknown to introduce functional groups on external surfaces. Acurrent underestimation in the field is the fact that ultrasoundnot only increases the number and mobility of nucleationclusters during sono-chemical synthesis of MOFs but alsochanges the external surface area.

Supporting information contain detailed experimentalprocedures, additional gas physisorption data as well as acollection of TEM and SEM images.

Acknowledgements

This work was funded by BAM, SB appreciates the postdoctoralAdolf Martens Fellowship. AMB appreciates the BCRT postdoctoralfellowship. All authors appreciate the excellent art-work drawingsby Karina Fast (BAM).

Keywords: MOFs · nano-flakes · plasticity · sonochemistry · ZIF-67

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Submitted: October 14, 2016

Accepted: October 28, 2016

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5908ChemistrySelect 2016, 1, 5905 – 5908 � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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