A hierarchical system of highly porous nanofibers has been prepared by electrospinning MOF (metal-organic framework) nanoparticles with suitable carrier polymers. Nitrogen adsorption proved the MOF nanoparticles to be fully accessible inside the polymeric fibers. © 2011 The Royal Society of Chemistry

Cravillon, Janosch

Ostermann, Rainer

Smarsly, Bernd M.

Weidmann, Christoph

Wiebcke, Michael

Institutionelles Repositorium der Leibniz Universität Hannover

442 Chem. Commun., 2011, 47, 442–444 This journal is c The Royal Society of Chemistry 2011Metal–organic framework nanoﬁbers via electrospinningwzRainer Ostermann,a Janosch Cravillon,b Christoph Weidmann,a Michael Wiebckeb andBernd M. Smarsly*aReceived 30th June 2010, Accepted 1st September 2010DOI: 10.1039/c0cc02271cA hierarchical system of highly porous nanoﬁbers has beenprepared by electrospinning MOF (metal–organic framework)nanoparticles with suitable carrier polymers. Nitrogen adsorptionproved the MOF nanoparticles to be fully accessible inside thepolymeric ﬁbers.The design of hierarchical nanostructures is a long-sought goalof materials science and there has been extensive research intovarious ‘‘top-down’’ and ‘‘bottom-up’’ methods to create thesenanostructures. Electrospinning is one of the simplest top-down methods that allows for an easy generation of nanoﬁbersfrom a wide variety of materials, especially polymers whichwere proven to be useful for many applications like ﬁltrationor controlled drug-release.1,2However, in spite of various attempts, the preparation ofporous polymeric nanoﬁbers with high surface areas could notbe realized so far.Isotactic polymers like PLLA (poly-L-lactide) and blendsthereof3,4 have been electrospun from solvent mixtures toproduce porous ﬁbers.5 Another approach applicable tovarious polymers is to use a cryogenic liquid to trap some ofthe solvent inside the ﬁbers, followed by an extraction of thesolvent under reduced pressure to yield some porosity.6However, the speciﬁc surface area of such ‘‘porous’’ polymericﬁbers is always quite low, usually in the range of 10–15 m2 g1,corresponding only to an increase by a factor of 2–3 comparedto the corresponding ‘‘non-porous’’ ﬁbers. Only in inorganicor carbonized PAN (polyacrylonitrile) ﬁbers suﬃcient micro-porosity can be found allowing for surface areas of up to 300or 600 m2 g1 respectively.7,8 Highly porous polymers remainspecial cases with a high degree of (hyper)cross-linking likePIMs (polymers of intrinsic porosity)9–11 and have not beenprepared in the form of nanoﬁbers so far.In contrast, metal–organic frameworks (MOF) are crystal-line coordination polymers that are well known for theirextremely high porosity and surface areas.12 Zeolitic imidazolateframework (ZIF) materials constitute a new subclass of MOFsthat combine the properties of porous MOFs with highchemical and thermal stability.13 Various MOFs, includingZIFs, have been very recently used as ﬁllers for the fabricationof mixed matrix membranes.14,15In order to achieve a homogeneous distribution of ﬁllerparticles within an organic polymer matrix it should bebeneﬁcial to use monodisperse nanoparticles. Recently,Cravillon et al. succeeded in preparing nanocrystals of aprototypical ZIF material, ZIF-8, that are 50 nm in size andhave a rather narrow size distribution.16Similar nanocrystals have recently been shown to exhibitdistinct advantages like faster adsorption kinetics for porouscoordination polymers.17In this work we present for the ﬁrst time the synthesis andcharacterization of composite MOF–polymer nanoﬁberscombining the advantages of both types of materials toachieve a novel class of hierarchical nanostructure.The colloidal suspensions of ZIF-8 nanoparticles in methanolwere prepared as described before.16 Brieﬂy, a solution ofappropriate amounts of Zn(NO3)26H2O and 2-methylimidazolein methanol was stirred at room temperature for 1 h, beforeseparating the resulting nanocrystals by centrifugation. TheZIF-8 nanoparticles were redispersed in fresh methanol byvortex mixing and ultrasonic agitation and a part of thesolution was dried at 80 1C under reduced pressure todetermine the concentration to be 3.5–4.5 wt% of ZIF-8. Ina typical electrospinning experiment, 500 mg of a solution of12 wt% PVP (polyvinylpyrrolidone, molecular weight(MW) = 1300 000) in methanol was added to 400–2000 mgof the ZIF-8 dispersion and mixed thoroughly. The solutionwas diluted or concentrated under reduced pressure to yield aﬁnal concentration of 3.5 wt% of PVP. This solution was fedthrough a metallic needle by a syringe pump (KDS scientiﬁc)at the rate of 0.35 ml h1. The needle is placed at a distance of6–8 cm from the aluminium foil that serves as collector and avoltage of 5 kV (Spellman CZE1000R high-voltage powersupply) was applied to produce a non-woven mat(see Fig. 1–3). The composite PVP–ZIF-8 nanoﬁbers and thedried ZIF nanoparticles were characterized by SEM (LEOGemini 982), TEM (Philips CM30-ST), XRD (PanalyticalX’Pert PRO diﬀractometer) and N2 adsorption (QuantachromeAutosorb 1 and 6).The diameter of the nanoﬁbers could be adjusted by thepolymer concentration and was found to be roughly 150–300 nm.The nanoparticle loading could be as high as 1 : 1 byweight ratio of ZIF-8 to PVP. SEM and TEM revealed ahomogeneous distribution of the nanoparticles inside theﬁbers with a smooth polymeric surface.As can be seen from Fig. 3, macroscopic non-wovens can beobtained on the centimetre scale. Thus, the composite ﬁberscan be regarded as a ‘‘MOF textile’’, combining the propertiesof polymeric ﬁbers and MOFs.a Institute of Physical Chemistry, Justus-Liebig-University Giessen,35392 Giessen, Germany.E-mail: bernd.smarsly@phys.chemie.uni-giessen.de;Fax: +49 641 9934509; Tel: +49 641 9934590b Institute of Inorganic Chemistry, Leibniz University Hannover,Callinstrasse 9, D-30167 Hannover, Germanyw This article is part of the ‘Emerging Investigators’ themed issue forChemComm.z Electronic supplementary information (ESI) available: XRD data;SEM images; adsorption kinetics; description of ﬁber generation. SeeDOI: 10.1039/c0cc02271cCOMMUNICATION www.rsc.org/chemcomm | ChemCommPublished on 27 September 2010. Downloaded on 26/10/2017 08:42:05. View Article Online / Journal Homepage / Table of Contents for this issueThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 442–444 443A comparison of XRD patterns of the ZIF-8 nanoﬁbers andthe pure nanoparticles showed the crystal structure to bepreserved (see ESIz). From N2 adsorption measurements(Fig. 4) including micropore analysis, the surface area accordingto the BET model was 960 m2 g1 for the pure ZIF-8, while theﬁbers possessed surface areas of up to 530 m2 g1 for a weightratio of approx. 1 : 1 ZIF-8 to PVP. As can be seen in Fig. 4,the adsorption branch for the ﬁbers is shifted to higherpressure. The two steps on the adsorption isotherms that havebeen ascribed18 to pressure-induced adsorbate reorganizationare less pronounced, but still visible for the composite ﬁbers.For lower loadings of ZIF-8 (see Table 1) the surface areadecreased accordingly, proving the ZIF-8 nanoparticles to befully accessible inside the polymeric nanoﬁbers.The nanoﬁbers are stable in the as-prepared form up to 150 1Cand do not degrade in organic solvents such as hexane, but aresensitive to polar solvents of the polymer such as alcohol orwater, causing the nanoﬁbers to coalesce.Using other polymers like polystyrene (PS) and poly-ethylene oxide (PEO) allows increasing the ﬁbers’ stabilityand widening the range of possible applications.In particular, the PS-ZIF-8 nanoﬁbers are stable inalcohols and water and the surface area was fully accessible(determined by nitrogen sorption), even if the ZIF-8 loadingup to 25 wt% was slightly lower. Higher loadings should bepossible by tuning the synthesis parameter and functionalizingthe surface of the ZIF-8 nanoparticles with less polarmolecules (see ESIz for details of preparation).The adsorption measurements show that the ZIF-8 nano-particles are fully accessible.We further compared the adsorptionkinetics of the nanoﬁbers to bare nano- and micrometer-sizeparticles (prepared by classical solvothermal synthesis andFig. 1 SEM of ZIF-8 nanoparticles in PVP.Fig. 2 TEM of ZIF-8 nanoparticles in PVP.Fig. 3 Digital photograph of a non-woven ZIF-8–PVP ﬁber mat. Thediameter is ca. 1.5 cm.Fig. 4 Nitrogen adsorption isotherms of ZIF-8 nanoparticles in PSand PVP (with diﬀerent loadings of ZIF-8). For clarity the desorptionbranch is omitted and the curves have been scaled to 100% ZIF-8content.Table 1 Nitrogen adsorption dataSampleZIF-8concentrationa(wt%)Microporevolume/cm3 g1Surfaceareab/m2 g1AccessibleZIF-8 fractionb(wt%)ZIF-8 100 0.49 960 100ZIF inPVP56 0.18 530 55.1ZIF inPVP22 0.07 180 19.1ZIF in PS 25 0.07 210 22.7PVP only 0 o0.002 10 n/aa As weighed in. b According to the BET model.Published on 27 September 2010. Downloaded on 26/10/2017 08:42:05. View Article Online444 Chem. Commun., 2011, 47, 442–444 This journal is c The Royal Society of Chemistry 2011referred to as ‘‘microparticles’’) by determining the time toreach equilibrium pressure after dosing a deﬁned amount ofadsorbate into the evacuated sample cell.Although we cannot propose a model yet, it is obvious thatdiﬀusion inside the ZIF-8 is much slower than in the gas phaseand that external surface area and the presence of a polymerlayer determine the kinetics of adsorption.As can be seen in Fig. 5 the nanoparticles attain equilibriumafter 30 s, while the microparticles and the nanoﬁbers require100–120 s.Most of the faster kinetics of the nanoparticles can probablybe accounted for by the larger external surface area being tentimes larger compared to the microparticles, while diﬀusioninside ZIF-8 is fast. The polymer layer around the ﬁbersconstitutes an additional diﬀusion barrier, therefore the nitrogenuptake is also slower than for the pure nanoparticles, but stillas fast as for the microparticles. In fact, a low thickness of thepolymer layer is crucial, as for polymer ﬁlms or coalesced ﬁbermats, the kinetics become too slow to observe signiﬁcantadsorption and accessible surface areas drop below 20 m2 g1.The results from N2 adsorption were corroborated by CO2as adsorbate (see ESIz for isotherms and kinetics data).Further results indicate that the diﬀerent carrier polymersalso aﬀect the adsorption kinetics. It is found that the uptakeis faster for PVP than for PS- and PEO-based nanoﬁbers(see ESIz for SEM pictures, N2 isotherms and kinetics data).In this regard, blends of polymers hold great promise as well.As the carrier polymers and the degree of nanoparticlecoverage by the polymer should also inﬂuence the absorption,this might allow tuning the selectivity of adsorption, especiallywith diﬀerent gases. Therefore we are planning a systematicstudy for more relevant adsorbates like hydrogen, carbondioxide or monoxide. Due to their low resistance to ﬂuid ﬂow,the nanoﬁber mats could ultimately be useful to selectivelyadsorb speciﬁc gases, for example carbon monoxide ingas masks.In conclusion, we could prepare nanoﬁbers of a metal–organic framework for the ﬁrst time by electrospinningpreformed nanoparticles, thereby creating hierarchical nano-ﬁbers with high surface areas and good accessibility. With theanticipated availability of other MOF materials as nano-particles, a broad variety of MOF nanoﬁbers should beaccessible with various applications, for example in gasadsorption and separation.Dr A. Mo¨ller (Institut fu¨r Nichtklassische Chemie, Leipzig,Germany) is acknowledged for CO2 sorption measurements.Notes and references1 D. Li and Y. Xia, Adv. Mater., 2004, 16, 1151–1170.2 A. Greiner and J. Wendorﬀ, Angew. Chem., Int. Ed., 2007, 46,5670–5703.3 M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig,M. Steinhart, A. Greiner and J. H. Wendorﬀ, Adv. Mater., 2001,13, 70–72.4 M. Bognitzki, T. 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Res., 2010, 43, 58–67.14 K. Dı´az, L. Garrido, M. Lo´pez-Gonza´lez, L. F. del Castillo andE. Riande, Macromolecules, 2010, 43, 316–325.15 S. Basu, M. Maes, A. Cano-Odena, L. Alaerts, D. E. De Vos andI. F. Vankelecom, J. Membr. Sci., 2009, 344, 190–198.16 J. Cravillon, S. Muenzer, S. Lohmeier, A. Feldhoﬀ, K. Huber andM. Wiebcke, Chem. Mater., 2009, 21, 1410–1412.17 D. Tanaka, A. Henke, K. Albrecht, M. Moeller, K. Nakagawa,S. Kitagawa and J. Groll, Nat. Chem., 2010, 2, 410–416.18 K. S. Park, Z. Ni, A. P. Coˆte´, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeﬀe and O. M. Yaghi, Proc. Natl.Acad. Sci. U. S. A., 2006, 103, 10186–10191.Fig. 5 N2 adsorption kinetics studied as cell pressure over time.Published on 27 September 2010. Downloaded on 26/10/2017 08:42:05. View Article Online

Metal-organic framework nanofibers via electrospinning

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