Communication pubs.acs.org/crystal
Nanostructured Zeolitic Imidazolate Frameworks Derived from Nanosized Zinc Oxide Precursors Yanfeng Yue,† Zhen-An Qiao,‡ Xufan Li,§ Andrew J. Binder,‡ Eric Formo,ξ Zhengwei Pan,§ Chengcheng Tian,‡ Zhonghe Bi,† and Sheng Dai*,†,‡ †
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States § College of Engineering & Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, United States ξ Center for Nanophase Materials Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡
S Supporting Information *
ABSTRACT: A facile method for the large scale transformation of ZnO nanocrystals into the corresponding nanocrystals of a zeolitic imidazolate framework was demonstrated. The methodology based on this nanoscale-facilitated transformation can be adapted to synthesize zeolitic imidazolate framework films on versatile substrates through the transformation from ZnO nanoscopic films derived from chemical vapor deposition (CVD) or solution growth.
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precursors. Because of their small sizes, nanoscopic precursors can undergo facile chemical transformation from one chemical species to another through solid-state reactions without involvement of conventional dissolution processes. This unique transformation has been exploited in making a number of nanomaterials. Notably, Son et al. have successfully utilized this synthesis strategy to make various sulfide nanocrystals via ionexchange processes.6 Intermetallic nanoparticles have been successfully synthesized via the low-temperature nanometallurgy.7 We have recently shown that mesoporous oxyfluorides can be derived from the direct fluorination of mesoporous oxides made of oxide nanocrystals.8 This strategy for synthesis of ZIFs results in highly uniform ZIF nanocrystals and can be used in solution with a large scale. This transformation can be viewed as a nanoscale-facilitated anion exchange that can also be conducted on thin films of nanoscopic ZnO, resulting in the corresponding ZIF-8 nanocrystalline thin films. The ZnO thin film precursors can be derived from both solution and chemical vapor deposition (CVD) syntheses. There are several reports on the direct use of bulk ZnO precursors to synthesize bulk ZIF-8 [Zn(MIM)2, MIM = 2-methylimidazolate] through mechanochemical or hydrothermal syntheses.5e,f,9 To our knowledge, no nanoscopic ZnO precursors have been explored for the synthesis of nanosized ZIF-8.
etal−organic frameworks (MOFs) are hybrid inorganic−organic crystalline solids formed by the linking of single metal ions or metal clusters with tunable oligotopic organic ligands.1−3 The spatial organization of metal ions and imidazole derivatives leads to an interesting class of MOFs with microporous channels and cavities, analogous to those found in zeolites. Therefore, these types of MOFs have been termed zeolitic imidazolate frameworks (ZIFs), featuring large surface areas, high thermal stabilities, and unique gas-adsorption properties.4 Recently, nanometer-sized ZIFs have attracted considerable attention because of the potential applications of these nanocrystals in drug delivery, catalysis, separation, and sensing.5 Typically, ZIF nanocrystals have been fabricated from the methodologies of controlled nucleation and growth of molecular precursors in homogeneous solutions.5b,c However, these methodologies for synthesis of ZIF nanocrystals based on molecular precursors suffer a few key drawbacks. For example, in order to synthesize nanocrystals of ZIFs with uniform size and shape, the synthesis conditions must be tightly controlled and the precursors must be diluted, thereby resulting in very low yields. These synthesis protocols based on nucleation and growth of dilute solution precursors also are difficult to adapt for the synthesis of other nanoscopic materials, such as thin films and membranes, which limits the practical applications of ZIFs. Herein, we report an alternative strategy toward the synthesis of both ZIF nanocrystals and ZIF thin films, through the transformation of the corresponding ZnO nanoscopic © XXXX American Chemical Society
Received: February 10, 2013
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the existence of textural meso/macropores formed by packing the nanosized particles.5c In order to fully understand the details of the transformation process, we carefully examined the reaction conditions, such as solvents, reaction temperature, and reactant ratios. While keeping the reaction time constant, we found that the molar ratio of the reactants was the most important factor to control the particle size, shape, and phase purity of the final products. When the molar ratio of HMIM to ZnO was below 3:4, the presence of ZIF-8 and unreacted ZnO nanoparticles were confirmed by powder XRD (Figures S3 and S4b of the Supporting Information). However, the nanospheres of ZIF-8 did not appear until the molar ratio was above 1:1 (Figure S4, panels c and d, of the Supporting Information). When the molar ratio of HMIM to ZnO was increased to 15:8, hexagonally faceted unconsolidated ZIF-8 nanocrystals were formed (Figure 2a and Figure S4e of the Supporting
To illustrate the potential of our methodology based on the nanoscale-facilitated transformation, commercial zinc oxide nanoparticles (Figure 1a) were used as a zinc source to
Figure 1. (a) SEM image of commercial ZnO nanoparticles from Sigma-Aldrich. (b) SEM image of the as-synthesized ZIF-8 particles synthesized in the current work. TEM image of the ZIF-8 particles (inset). (c) XRD patterns of ZIF-8 nanocrystals: as-synthesized and simulated from single crystal X-ray diffraction data. (d) BrunauerEmmett-Teller (BET) surface area analysis of the as-synthesized ZIF-8 nanocrystals using liquid nitrogen at 77 K. Figure 2. (a) TEM image of unconsolidated ZIF-8 particles with some remaining ZnO particles. (b) Schematic illustration of the formation and aggregation of ZIF-8 particles (I − IV, with the molar ratio of HMIM to ZnO being 3:4, 1:1, 15:8, and 5:2, respectively).
demonstrate both the basic principles of the concept and the ease with which the corresponding ZIF nanoparticles can be prepared. Briefly, we prepared large-scale (grams) nanoscale ZIF-8 through the direct heterogeneous reaction of the ZnO nanoparticles with 2-methylimidazole (HMIM) in methanol at room temperature (see the Supporting Information). The morphology and the microstructure of the obtained samples were characterized by scanning electron microscopy (SEM). The average diameter of the ZnO nanoparticles is about 35 nm (Figure 1a). After reaction with HMIM at room temperature for 48 h, uniform ZIF-8 nanocrystals with sharp hexagonal facets were obtained. As shown in Figure 1b, the as-prepared ZIF-8 nanocrystals derived from the ZnO nanoparticles have a narrow size distribution in the range of 100−120 nm. No residue ZnO nanoparticles were found by both powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), indicating that the precursor ZnO nanoparticles were completely used up (Figure 1b, inset). The exact match of the powder XRD pattern of the resultant ZIF-8 nanocrystals to that simulated, based on the published ZIF-8 structure data, unequivocally demonstrates that the product is a pure ZIF-8 phase (Figure 1c). Further, the ZIF-8 nanocrystals can readily be dispersed in ethanol and form stable colloidal suspensions (Figure S1 of the Supporting Information), and thermal gravimetric analysis (TGA) revealed that these particles have high thermal stability (Figure S2 of the Supporting Information). The specific surface area of the ZIF-8 nanocrystals was determined by a N2 adsorption−desorption measurement (Figure 1d), which shows a type I isotherm according to IUPAC classifications. The Brunauer−Emmett−Teller (BET) surface area (1236 m2/g) of our sample is higher than those of most micro and nanoscale ZIF-8 samples obtained via other methods.5b,d,9 The rapidly increasing adsorption volume at very low relative pressure indicates the presence of micropores in ZIF-8, and the second uptake at high relative pressure is due to
Information), with a slight amount of unreacted ZnO particles because of the ligand deficiency. After the molar ratio of HMIM to ZnO reached to 5:2, only phase pure ZIF-8 with sharp hexagonal facets was obtained as all nanoscopic ZnO nanoparticles were completely consumed in the reaction (Figure 1b). On the basis of our observation, the proposed crystallization process entails the surface neutralization reaction of the HMIM ligand with ZnO and subsequent restructuring on the parent ZnO particles (Figure 2b, I and II). At a low molar ratio of the HMIM ligand to the ZnO particle, the growth of the ZIF-8 particles was terminated due to the ligand deficiency. However, when a higher molar ratio of the ligand to ZnO is utilized, unconsolidated hexagonally faceted particles and spheres were obtained (Figure 2b, III). With an excessive amount of the HMIM ligand in the system, hexagonally faceted particles were prepared with smooth surfaces and fairly uniform sizes (Figure 2b, IV). These results clearly indicate that the formation of ZIF8 from a nanoscopic precursor experienced ion exchange process around ZnO particles, which is quite different from the homogeneous nucleation process of soluble Zn(II) salts and ligand in a solution. The facile transformation from ZnO nanoparticles to ZIF-8 nanocrystals is clearly facilitated by the nanoscale morphology of ZnO and can be further used to make nanoscopic ZIF-8 thin films from the corresponding ZnO thin film precursors. The fabrication of crystalline porous films of metal−organic frameworks is one of the practical applications for membranebased separations, membrane reactors, and other advanced applications.10,11 However, one major obstacle to growing B
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MOF films on substrates is the organic linkers, which typically do not bear additional linkage groups that can form bonds with the surface of the supports. Up to now, two approaches have been intensively studied for the MOF thin films growth. One is a step-by-step approach in liquid phase using a solution of structural components and functionalized substrates. For example, a pillared-layer type MOF [Fe(pz)[Pt(CN)4], pz = pyrazine] thin film on a metal substrate was prepared by a stepby-step approach based on a liquid-phase epitaxy with a Au/ Cr/Si substrate functionalized in a solution of 4-mercaptopyridine.12 Another method is a combination of the Langmuir− Blodgett and layer-by-layer methods (LB−LbL).13 These two methods are the two key protocols for generation of thin films that are only applicable to pillared-layer type MOFs. Because ZIF materials are constructed by angular imidazole derivatives, the use of these two methodologies to synthesize ZIF thin films can be problematic. In a sharp contrast, the methodologies for preparing nanoscopic ZnO thin films are highly mature. They can readily be prepared on different substrates by both solutionbased growth and CVD methods. These thin films consist of nanoscopic ZnO objects (nanorods or nanoparticles), which possess the same reactivity as that of ZnO nanocrystals. Accordingly, they can be easily converted into the corresponding ZIF-8 nanoscopic objects through our nanoscale-facilitated transformation. To demonstrate the feasibility of our nanoscale-facilitated transformation in the synthesis of ZIF thin films, a ZnO thin film consisting of ZnO nanorods on a single crystalline Si(100) substrate was used as a precursor. This precursor ZnO thin film was synthesized by a low-temperature oxidization method in a tube furnace system (Figure 3a and Figure S5 of the Supporting
the lattice oxide anions in nanocrystals with the imidazolate anions. Other ZnO thin films can also be utilized as precursors for synthesis of ZIF-8 thin films. A nanoscopic ZnO thin film on a glass slide was synthesized at low temperature in an aqueous solution containing zinc nitrate and hexamethylenetetramine.15 This thin film can also undergo the same nanoscale-facilitated transformation to the corresponding ZIF-8 thin film (Figure S7 of the Supporting Information). Although both size and shape of ZIF-8 nanocrystals on the film are quite different from those of the precursor ZnO nanocrystals, a phase pure ZIF-8 film was obtained, which was confirmed by the powder XRD (Figure S8 of the Supporting Information). This simple two-step strategy for preparation of phase pure ZIF-8 films is versatile and can be easily extended to prepare ZIF-8 thin films on any unfunctionalized bare supports. The advantage of this strategy is that ZnO nanosized particles can easily be deposited on different substrates and as mentioned above, the transformation to ZIF-8 films can be conducted under the HMIM atmosphere, which avoids the problem of directly growing ZIF-8 films on substrates. In conclusion, we have demonstrated a facile and effective method to induce the transformation of ZnO nanocrystals to a zeolitic imidazolate framework of nanoscopic materials. This facile synthetic method results in highly uniform ZIF-8 nanocrystals and can be conducted in solution on a large scale. This methodology based on the nanoscale-facilitated transformation can easily be adapted to synthesize ZIF-8 films through the use of the corresponding nanoscale ZnO thin film templates attached on versatile substrates. The fabrication of crystalline porous films of metal−organic frameworks is highly important for membrane-based separations, membrane reactors, and sensing devices. The simplicity of our synthesis technique may lead to a wide variety of crystalline films of other MOFs, through the simple transformation of metal oxide precursors on substrates under the atmosphere of organic ligands.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental and measurement details, TGA measurements of as-made ZIF-8, photograph of a stable suspension of ZIF-8 nanocrystals in ethanol, PXRD patterns for ZIF-8 nanocrystals obtained with different HMIM/ZnO ratios, TEM images of ZIF-8 nanocrystals obtained with different HMIM/ZnO ratios, SEM images of ZnO nanorods on a single crystalline Si substrate, powder XRD patterns for ZIF-8 film on a Si substrate, photograph of ZIF-8 film on a microscopic glass slide, and powder XRD patterns for ZIF-8 film on a microscopic glass slide. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. (a) SEM image of the nanorods on a Si substrate. (b) SEM images of ZIF-8 film on a Si wafer. A photograph of ZIF-8 film on a Si substrate (inset).
Information).14 The transformation from the precursor film to the ZIF-8 film was carried out in a Teflon-lined autoclave under the atmosphere of HMIM at 180 °C for 48 h. The resulting ZIF-8 film consists of nanosized ZIF-8 crystals (Figure 3b and photo image). The powder XRD pattern given in Figure S6 of the Supporting Information reveals the successful transformation of the ZnO thin-film precursor to the corresponding ZIF-8 thin film with no distortion of the precursor thin film pattern (Figure S6 of the Supporting Information). The ZIF-8 film was formed only on the location where the ZnO thin film was originally present. This observation rules out the possibility that the transformation process involves the conventional dissolution of ZnO and homogeneous growth. Otherwise, the ZIF-8 thin film would nonselectively deposit on the Si substrate. Hence, the above transformation process can be viewed as an anion-exchange process involving displacement of
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DeC
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(13) (a) Motoyama, S.; Makiura, R.; Sakata, O.; Kitagawa, H. J. Am. Chem. Soc. 2011, 133, 5640−5643. (b) Makiura, R.; Motoyama, S.; Umemura, Y.; Yamanaka, H.; Sakata, O.; Kitagawa, H. Nat. Mater. 2010, 10, 565−571. (14) Gu, Z. J.; Paranthaman, M. P.; Xu, J.; Pan, Z. W. ACS Nano 2009, 3, 273−278. (15) (a) Vayssieres, L.; Keis, K.; Lindquist, S.-E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350−3352. (b) Yang, L. L.; Zhao, Q. X.; Willander, M.; Yang, J. H. J. Cryst. Grow. 2009, 311, 1046−1050.
AC05-00OR22725 with Oak Ridge National Laboratory managed and operated by UT-Battelle, LLC. Z.W.P. acknowledges funding by the U.S. NSF (CAREER DMR-0955908).
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