This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 221–223 221

Cite this: Chem. Commun., 2012, 48, 221–223

Advanced fabrication of metal–organic frameworks: template-directed

formation of polystyrene@ZIF-8 core–shell and hollow ZIF-8

microspheresw

Hee Jung Lee, Won Cho and Moonhyun Oh*

Received 6th October 2011, Accepted 7th November 2011

DOI: 10.1039/c1cc16213f

The conjunction of porous ZIF-8 with polystyrene spheres is demon-

strated to induce the formation of polystyrene@ZIF-8 core–shell

structures. A subsequent etching process on polystyrene@ZIF-8

core–shells to remove polystyrene cores results in a unique

hollow ZIF-8.

Porous metal–organic frameworks (MOFs) have been intensely

studied and have received a great deal of attention as a result

of their diverse structural topologies, tunable functionalities

and their many useful applications, such as gas storage,1 gas

separation,2 catalysis3 and recognition.4 More recently, there

has been considerable interest regarding the merging of porous

MOFs with other solid materials in order to induce the

formation of hybrid materials.5–7 The formation of porous

MOF materials that have higher order structures or assemblies

will reinforce the usefulness of MOF materials and expand the

scope of utilization of these materials. In fact, many recent

studies have concentrated on the generation of porous

membranes or thin-films from porous MOFs.6,7 Also, the

outstanding utilization of MOF films in gas separation, optics

and chemical sensors has been well demonstrated.7 However,

the fabrication of porous MOFs in sophisticated forms, for

example core–shell or hollow forms, has been little studied.

On the other hand, zeolite imidazolate frameworks (ZIFs),

considered to be a new subfamily of MOFs, have been

attracting particular attention due to exceptional chemical

and thermal stabilities.8 In particular, ZIF-8, which has a

sodalite topology with a cubic space group (I%43m), is one of

the most studied prototypical ZIFs. Until now, ZIF-8 was

prepared as a type of nano-scaled crystal or thin film as well as

a typical macro-scaled crystalline material for its practical

applications in gas storage, catalysis and gas separation.8,9

Herein, we report significant progress in the conjunction of

porous MOFs with other compositional particles for the

induction of core–shell structure from ZIF-8 and polystyrene

spheres, and a simple approach for the fabrication of hollow

structure from ZIF-8. In addition, we also demonstrate that

ZIF-8 layer thickness within a polystyrene@ZIF-8 core–shell and

hollow ZIF-8 can be prudently controlled by altering the number

of growth cycles during the stepwise ZIF-8 growth process.

In a typical synthesis, polystyrene@ZIF-8 core–shell

particles were prepared by the following stepwise solvothermal

reaction. Carboxylate-terminated polystyrene spheres with a

diameter of 0.87 mm were added to a precursor methanol

solution containing 2-methylimidazole (HMeIM) and

Zn(NO3)2 (Scheme 1). The resulting solution was then heated

at 70 1C for 10 min. During this time, the formation of

polystyrene@ZIF-8 microspheres was achieved. Carboxylate

groups on the surfaces of the polystyrene spheres first interacted

with Zn2+ ions and initiated the growth of ZIF-8 on the

surfaces of the polystyrene spheres. The reaction mixture was

cooled to room temperature, and the resulting product was

collected by centrifugation and washed several times with

methanol. During this isolation process, the desired micro-sized

core–shell particles were easily separated from the unwanted

nano-sized pure ZIF-8 particles (ESIw). To achieve sufficient

ZIF-8 shell thickness within the polystyrene@ZIF-8 core–shell,

the second ZIF-8 growth cycle on the as-prepared core–shell

particles was conducted using a fresh precursor solution

containing fresh Zn(NO3)2 and HMeIM. When the shell was

too thin, collapse of the spherical shape after removal of the

polystyrene core from the polystyrene@ZIF-8 core–shell was

observed.

The formation of quite monodisperse polystyrene@ZIF-8

core–shell microspheres was verified by scanning electron

microscopy (SEM) and transmission electron microscopy

(TEM). The composition and inner-structure of these micro-

spheres were then confirmed by energy dispersive X-ray

(EDX) spectroscopy and powder X-ray diffraction (PXRD)

spectroscopy. The diameter change from 0.87 mm for the

initial polystyrene spheres to 0.97 mm after two cycles of the

Scheme 1 Preparation of polystyrene@ZIF-8 core–shell and hollow

ZIF-8 microspheres from carboxylate-terminated polystyrene spheres.

Department of Chemistry, Yonsei University, 134 Shinchon-dong,Seodaemun-gu, Seoul 120-749, Korea. E-mail: moh@yonsei.ac.kr;Fax: +82-2-364-7050; Tel: +82-2-2123-5637w Electronic supplementary information (ESI) available: Experimentaldetails, EDX, PXRD, SEM and TEM images. See DOI: 10.1039/c1cc16213f

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222 Chem. Commun., 2012, 48, 221–223 This journal is c The Royal Society of Chemistry 2012

solvothermal ZIF-8 growth reaction indicated the generation

of ZIF-8 shells with thicknesses of ca. 50 nm surrounding the

polystyrene core (Fig. 1). SEM and TEM images revealed the

formation of ZIF-8 shells consisting of numerous nano-sized

ZIF-8 crystals (Fig. 1c and d). The composition of the resulting

microspheres was subsequently analyzed by EDX spectro-

scopy. The observation of zinc atoms in the EDX spectrum

of these microspheres supported the generation of new materials

containing zinc atoms on the surfaces of the polystyrene

particles (Fig. S1, ESIw). In addition, the EDX spectrum

profile scanning obviously validated the production of the

core–shell structure, as evident from the dominance of zinc

atoms at the edge of the microsphere, which is the typical

pattern for core–shell structure (inset in Fig. 1d). Finally, the

topological information of the resulting shell was obtained

from the PXRD pattern, which clearly showed the sodalite

topology of ZIF-8 (Fig. 2), as evidenced by good agreement

with the PXRD patterns obtained from pure ZIF-8 nano-

crystals10 and simulated from crystal structure data of ZIF-8.8

The as-prepared polystyrene@ZIF-8 core–shell microspheres

were then immersed in N,N0-dimethylformamide (DMF) to

remove the polystyrene cores11 and to induce the formation of

hollow ZIF-8 microspheres (Scheme 1). The spherical shape

was properly maintained after removing the polystyrene core,

as shown in Fig. 1e, and the TEM images (Fig. 1f) clearly

showed the formation of hollow ZIF-8 spheres. The hollow

ZIF-8 microspheres were composed of many nano-crystals of

ZIF-8, as shown in the high-magnification SEM and TEM

images (insets in Fig. 1e and f). The uniformity of the hollow

ZIF-8 in diameter and thickness was confirmed by TEM and

SEM (Fig. 1e and f). The chemical composition of the hollow

ZIF-8 spheres was characterized by the EDX spectrum, revealing

a typical pure ZIF-8 composition (ESIw). Finally, the PXRD

pattern of the hollow ZIF-8 spheres revealed that the sodalite

topology of Zn(MeIM)2 in the polystyrene@ZIF-8 core–shell

did not change during the etching process of the polystyrene

cores (Fig. 2). The chemical composition changes during these

two processes clearly indicate material conversion from the

initial polystyrene to the polystyrene@ZIF-8 core–shell and

then finally to hollow ZIF-8. In the EDX spectra (Fig. S1, ESIw),the detection of zinc atoms after the creation of the ZIF-8 shell

portion, and the decrease in carbon atoms as a result of the

elimination of the polystyrene cores and the formation of

hollow ZIF-8 microspheres are in good agreement with the

compositional changes occurring during the processes. As

many hollow structures are useful in many applications such

as catalysts, chemical sensors, chemical reactors and drug

delivery,12 the resulting hollow ZIF-8 can potentially be used

in such applications.

The method for controlling the ZIF-8 shell thickness was

investigated. Eventually, ZIF-8 shell thickness can be controlled

by altering the number of growth cycles during the stepwise

ZIF-8 growth process, just as the thickness of MOF films can

be managed using a stepwise growth method.7a First, the

polystyrene@ZIF-8 core–shell spheres with thin ZIF-8 shells

were obtained through the conduction of only one ZIF-8

growth cycle (Fig. 3a). Smaller nano-crystals of ZIF-8 existing

in the resulting core–shell spheres can be compared with the

relatively larger ZIF-8 nano-crystals produced by conducting

two cycles of ZIF-8 growth (Fig. 1c and 3a and ESIw). After

removing the polystyrene cores, products akin to flat balloons

were obtained (Fig. 3b). The perfect spherical shape was not

maintained due to the thin layer of the hollow structure.

Second, polystyrene@ZIF-8 core–shell spheres with thicker

ZIF-8 shells were generated when three cycles of the ZIF-8

growth process were performed (Fig. 3c). The sizes of the ZIF-8

nano-crystals within this product were obviously increased

Fig. 1 (a) SEM and (b) TEM images of carboxylate-terminated poly-

styrene spheres with an average diameter of 0.87 � 0.01 mm. (c) SEM

and (d) TEM images of polystyrene@ZIF-8 core–shell microspheres

with an average diameter of 0.97� 0.02 mm obtained by conducting two

cycles of the ZIF-8 growth process. The inset in (d) is the EDX spectrum

profile scanning of the core–shell microspheres. (e) SEM and (f) TEM

images of hollow ZIF-8 microspheres. The insets in (c), (e) and (f) are

high-magnification images. The average diameters were obtained from

SEM images (s.d., n= 100). White and black scale bars represent 1 mm.

Fig. 2 PXRDpatterns of ZIF-8 nano-crystals10 (top), polystyrene@ZIF-8

core–shell microspheres (middle) and hollow ZIF-8 microspheres (bottom).

All three PXRD patterns are nearly identical.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 221–223 223

compared to those of the products formed by conducting one

or two cycles of the ZIF-8 growth process (ESIw). As shown in

Fig. 3d, hollow ZIF-8 with thicker layers was produced by

removing the polystyrene cores. The noticeable increase in

layer thickness was confirmed in a series of TEM images

(Fig. 3e–g). The layer thicknesses of the hollow spheres

obtained by performing two and three cycles of the ZIF-8

growth process were ca. 50 and 100 nm, respectively. The EDX

spectra and PXRD patterns of these thickness-controlled

core–shell and hollow products confirmed again the formation

of the sodalite ZIF-8 (ESIw).In summary, this communication demonstrates the conjunction

of porous crystalline MOF with other compositional particles

resulting in the formation of a novel core–shell type hybrid

material based on functional ZIF-8. The stepwise ZIF-8

growth approach allows for the rational control of ZIF-8 layer

thickness. Furthermore, we have described the preparation

of uniform hollow ZIF-8 structure through an etching process

on the polystyrene@ZIF-8 core–shell for the removal of

polystyrene cores. To the best of our knowledge, this is the

first successful synthesis of uniform hollow structure from

porous crystalline MOFs. This methodology for the fabrication

of various types of porous MOFs would reinforce the usefulness

of many functional MOFs. In addition, due to the fact that the

compositions and properties of the core and shell portions can

be purposefully altered, these results may provide a vital path

for the preparation of multifunctional hybrid materials based

on porous MOFs and functional core particles, and they can

be utilized in many useful applications, such as catalysis,

separations, gas storage and optics.

This work was supported by several grants (no. 2011-

0028321, 2009-0079545 and R32-2008-000-10217-0) through

NRF grant funded by the MEST. H.J.L. and W.C. acknowledge

the fellowships from the BK21 Program.

Notes and references

1 (a) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi,A. O. Yazaydin, R. Q. Snurr, M. O’Keeffe, J. Kim and O.M. Yaghi,Science, 2010, 329, 424; (b) B. Zheng, J. Bai, J. Duan, L. Wojtas andM. J. Zaworotko, J. Am. Chem. Soc., 2011, 133, 748; (c) J. An andN. L. Rosi, J. Am. Chem. Soc., 2010, 132, 5578; (d) T. K. Maji,R. Matsuda and S. Kitagawa, Nat. Mater., 2007, 6, 142;(e) Y.-M. Jeon, G. S. Armatas, J. Heo, M. G. Kanatzidis andC. A. Mirkin, Adv. Mater., 2008, 20, 2105; (f) W. Cho, H. J. Lee andM. Oh, J. Am. Chem. Soc., 2008, 130, 16943.

2 B. Chen, C. Liang, J. Yang, D. S. Contreras, Y. L. Clancy,E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int.Ed., 2006, 45, 1390.

3 (a) K. K. Tanabe and S. M. Cohen, Angew. Chem., Int. Ed., 2009,48, 7424; (b) F. Song, C. Wang, J. M. Falkowski, L. Ma andW. Lin, J. Am. Chem. Soc., 2010, 132, 15390; (c) K. H. Park,K. Jang, S. U. Son and D. A. Sweigart, J. Am. Chem. Soc., 2006,128, 8740.

4 (a) B. Chen, S. Xiang and G. Qian, Acc. Chem. Res., 2010,43, 1115; (b) S. Xiang, W. Zhou, Z. Zhang, M. A. Green, Y. Liuand B. Chen, Angew. Chem., Int. Ed., 2010, 49, 4615.

5 (a) C. Petit and T. J. Bandosz, Adv. Mater., 2009, 21, 4753;(b) I. Imaz, J. Hernando, D. Ruiz-Molina and D. Maspoch,Angew. Chem., Int. Ed., 2009, 48, 2325; (c) H.-L. Jiang, T. Akita,T. Ishida, M. Haruta and Q. Xu, J. Am. Chem. Soc., 2011,133, 1304; (d) S. Hermes, M.-K. Schroter, R. Schmid,L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer andR. A. Fischer, Angew. Chem., Int. Ed., 2005, 44, 6237; (e) C. Jo,H. J. Lee and M. Oh, Adv. Mater., 2011, 23, 1716.

6 (a) C. Scherb, A. Schodel and T. Bein, Angew. Chem., Int. Ed., 2008,47, 5777; (b) O. Shekhah, H. Wang, S. Kowarik, F. Schreiber,M. Paulus, M. Tolan, C. Sternemann, F. Evers, D. Zacher,R. A. Fischer and C. Woll, J. Am. Chem. Soc., 2007, 129, 15118;(c) R. Makiura, S. Motoyama, Y. Umemura, H. Yamanaka,O. Sakata and H. Kitagawa, Nat. Mater., 2010, 9, 565.

7 (a) G. Lu and J. T. Hupp, J. Am. Chem. Soc., 2010, 132, 7832;(b) H. Guo, G. Zhu, I. J. Hewitt and S. Qiu, J. Am. Chem. Soc.,2009, 131, 1646; (c) L. E. Kreno, J. T. Hupp and R. P. Van Duyne,Anal. Chem., 2010, 82, 8042; (d) A. Lan, K. Li, H. Wu,D. H. Olson, T. J. Emge, W. Ki, M. Hong and J. Li, Angew.Chem., Int. Ed., 2009, 48, 2334; (e) Y.-S. Li, F.-Y. Liang, H. Bux,A. Feldhoff, W.-S. Yang and J. Caro, Angew. Chem., Int. Ed.,2010, 49, 548; (f) T.-H. Bae, J. S. Lee, W. Qiu, W. J. Koros,C. W. Jones and S. Nair, Angew. Chem., Int. Ed., 2010, 49, 9863.

8 K. S. Park, Z. Ni, A. P. Cote, 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.

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10 J. Cravillon, S. Munzer, S.-J. Lohmeier, A. Feldhoff, K. Huber andM. Wiebcke, Chem. Mater., 2009, 21, 1410.

11 The polystyrene is presumably leached-out from the core throughdefects or cracks existing between individual nano-size ZIF-8crystals that comprise the shell. (a) Y. Zhao and L. Jiang, Adv.Mater., 2009, 21, 3621; (b) A. Madani, B. Nessark, R. Brayner,H. Elaissari, M. Jouini, C. Mangeney and M. M. Chehimi, Poly-mer, 2010, 51, 2825.

12 (a) S.-W. Kim, M. Kim, W. Y. Lee and T. Hyeon, J. Am. Chem.Soc., 2002, 124, 7642; (b) J. Zhang, X. Liu, S. Wu, M. Xu, X. Guoand S. Wang, J. Mater. Chem., 2010, 20, 6453; (c) J. Li andH. C. Zeng, Angew. Chem., Int. Ed., 2005, 44, 4342; (d) Y. Yin,R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somorjai andA. P. Alivisatos, Science, 2004, 304, 711; (e) K. Cheng, S. Peng,C. Xu and S. Sun, J. Am. Chem. Soc., 2009, 131, 10637.

Fig. 3 (a) SEM and TEM (inset) images of polystyrene@ZIF-8

core–shell microspheres with an average diameter of 0.90 � 0.02 mmobtained by conducting one cycle of the ZIF-8 growth process. (b) TEM

and SEM (inset) images of hollow ZIF-8 obtained by etching the poly-

styrene cores within core–shell microspheres shown in (a). These products

do not maintain perfect spherical shape due to the thin layer thickness.

(c) SEM and TEM (inset) images of polystyrene@ZIF-8 core–shell micro-

spheres with an average diameter of 1.06 � 0.02 mm obtained by

conducting three cycles of the ZIF-8 growth process. (d) TEM image of

hollow ZIF-8 obtained by etching the polystyrene cores within core–shell

microspheres shown in (c). The average diameters were obtained from

SEM images (s.d., n = 100). The high-magnification TEM images

of hollow ZIF-8 obtained by conducting (e) one cycle, (f) two cycles and

(g) three cycles of the ZIF-8 growth process. Scale bars represent 1 mm.

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