IRMOF Thin Films Templated by Oriented Zinc Oxide Nanowires

Communication pubs.acs.org/crystal

IRMOF Thin Films Templated by Oriented Zinc Oxide Nanowires Yashar Abdollahian,† Jesse L. Hauser,† Ian R. Colinas,† Carolyn Agustin,† Andrew S. Ichimura,‡ and Scott R. J. Oliver*,† †

University of California, Santa Cruz, Department of Chemistry and Biochemistry, 1156 High Street, Santa Cruz, CA 95064, United States ‡ Department of Chemistry and Biochemistry, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, United States S Supporting Information *

ABSTRACT: We present a new method in the synthesis of metal−organic framework (MOF) thin films using zinc oxide nanowires as the substrate. This facile method involves growing zinc oxide nanowires on a substrate (glass, transparent conducting oxide glass, Si wafer), followed by immersing the nanowire substrate in an iso-reticular metal−organic framework (IRMOF) precursor solution. The resulting 25 μm thick film is highly crystalline and covers the entire substrate. Growth of the IRMOF on the nanowire substrate allows for the film to be used in potential applications in sensing, membranes, photovoltaics, catalysis, and gas storage. We have also successfully used microwaves to rapidly produce these films with comparable film quality to our original method.

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growth on an alumina support.7,13 Zhang et al. have shown the first example of IRMOF-1 films grown on zinc oxide nanowires.14 The films are composed of bowl−rod hierarchical nanowire arrays that are functionalized with 11-mercaptoundecanoic acid as the nucleation sites for MOF growth. This procedure, although novel, requires a fair amount of workup procedures to produce the nanowire arrays and still utilize SAMs as the binding agent for the MOF. Herein, we report the facile synthesis of four different IRMOF thin films on zinc oxide (ZnO) nanowires that were pregrown on a transparent conductive oxide substrate. The films cover the entire substrate and are the first examples of templating micrometer-thick IRMOF films with a nanowire array without the use of SAMs. Films of IRMOF-1 were also fabricated using microwaves in order to demonstrate growth versatility. The synthesis of the films using these methods paves the way toward inexpensive and potentially large-scale applications of IRMOF thin films. Figure 1 depicts the synthesis methodology for the film growth. The films were prepared by growing ZnO nanowires through published procedures (see the Supporting Information). The nanowires serve as the nucleating layer for the subsequent MOF crystal growth. The nanowire substrates were placed in vials containing the precursor MOF solution using previously reported concentrations for the bulk synthesis of the MOFs. The resulting MOF film shows good resilience against mechanical stress via manual physical abrasion with a probe, as well as from vigorous rinsing with water, acetone, ethanol, and dimethylformamide. Visually, only the nanowire side of the

etal organic frameworks (MOFs) are a widely studied class of materials because of their high porosity and vast structural diversity. This versatility leads to chemical properties that can be tuned by choice of size/shape of the organic linker and/or the metal clusters. Main properties of interest are gas storage,1 catalysis,2 ion exchange,3 and sensing.4 Recently there has been growing interest in MOF thin films.5 Growth or immobilization of a MOF on a substrate would be of great interest because it may lead to new applications in thin film devices such as sensors and membranes.6 The method of MOF thin film growth is dictated by the choice of substrate and the MOF composition. Common substrates for film growth are native oxide surfaces and selfassembled monolayers (SAMs). Reports on the growth of MOF films involve one of five methods: (i) growth/deposition from a solvothermal solution containing the MOF precursors,7 (ii) deposition of MOF colloids,8 (iii) liquid phase epitaxy,9 (iv) electrochemical deposition,10 and (v) gel-layer synthesis.11 Solvothermal synthesis is the simplest route to MOF film growth but has little control over the thickness of the film. Liquid phase epitaxy solves this issue by sequentially exposing a functionalized substrate to separate precursor solutions of the MOF building blocks, resulting in relatively smooth and oriented films of controllable thickness. The main drawback to this method is the long period of time needed to achieve reasonable film thickness. Extensive research has been conducted on iso-reticular metal−organic frameworks (IRMOFs), first developed by Yaghi et al., due to their facile synthesis and potential applications.12 Most reports deal with bulk materials, with very few reports on the fabrication of IRMOF thin films. Films of IRMOF-1 [Zn4O(BDC)3, BDC = benzenedicarboxylate] have been achieved either by growth from the mother liquor solution on a carboxylate-terminated SAM or via solvothermal © 2014 American Chemical Society

Received: October 1, 2013 Revised: March 12, 2014 Published: March 17, 2014 1506

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Figure 1. Schematic of the templating methodology of MOF thin film fabrication. Zinc oxide nanowires are first grown on a cleaned substrate (transparent conducting oxide glass or silicon wafer). The resultant nanowire array is then exposed to a MOF precursor solution that contains the metal and ligand components, resulting in complete coverage of the nanowire substrate with interconnected cubic MOF crystals.

substrate is completely coated with single crystals. Attempts at producing the films with only the substrate in the presence of the MOF solution failed, confirming that the nanowires are necessary for MOF growth. Optical micrographs of the films show the complete coverage of the substrate with IRMOF-1 single crystals (see the Supporting Information). Minute amounts of hexagonal paddlewheel-like plates were also observed on the film and have yet to be identified. The progress of film growth was monitored via optical microscopy at specific times during the growth process. After 5 h, discrete cubic single crystals can be observed on the nanowire substrate (Figure S1 of the Supporting Information). Complete coverage of the film is achieved within 20 h (Figure S2 of the Supporting Information). The MOF films were further analyzed by scanning electron microscopy (SEM). Top-down views of the film show excellent coverage of the substrate with a 25 μm thick layer of edgesharing single crystals (Figures 2 and 3). Cross-sectional SEM

Figure 3. Magnified top-down view of the IRMOF-1 crystalline layer.

Figure 4. Cross-sectional SEM showing the interface between ZnO nanowires and MOF crystals (A, FTO glass substrate; B, ZnO seeding layer; C, ZnO nanowires; D, IRMOF-1 layer).

signal at 34.5° (2θ), corresponding to the (002) peak. After growth of the MOF film on the nanowires, the same characteristic peaks of the oriented ZnO nanowires were still present in addition to those of the MOF (Figure 5). The nanowire array was therefore not compromised during the MOF growth procedure. The growth of the films on the nanowires is likely due to the rough nanowire surface that provides the necessary nucleation sites for MOF crystal growth. Whether the zinc in the nanowires plays any role in facilitating the growth of the zinc-based IRMOFs is currently being investigated. Attempts at increasing the thickness of the films either with longer exposure to IRMOF precursor solution, varying temperatures, or different concentrations of starting materials have thus far been unsuccessful. At higher concentration and temperature, we have observed a noticeable amount of free bulk IRMOF crystals forming in solution. We believe that at higher concentration and temperature, there is competing formation of the IRMOF film and the bulk solid in solution limiting the growth of the film. Several other IRMOFs were synthesized to elucidate the versatility of this new synthesis method. IRMOF-3, -8, and -9 were selected as initial candidates due to their varying organic functionality. Films of IRMOF-3, -8, and -9 were successfully synthesized with coverage of the substrate comparable to that of IRMOF-1 (Figure 2). All three films display an average

Figure 2. Top-down SEM images of the IRMOF films on ZnO nanowires (A, IRMOF-1; B, IRMOF-3; C, IRMOF-8; and D, IRMOF9).

images confirm that the ZnO nanowire layer is still present, with the crystals grown on top (Figure 4). In order to gain insight on the pathway of MOF growth on the nanowires, several crystals were dislodged from the substrate. SEM indicates that a thin layer of the dislodged crystal was left behind on the surface, with some of the nanowires protruding through the leftover material (Figure S3 of the Supporting Information). From these SEM images, it is estimated that the MOF crystals grow within the top 100 nm of the nanowires. Grazing incidence X-ray diffraction (GIXRD) spectra of the films corroborates the SEM images (Figure 5). The spectrum of the ZnO nanowire film prior to MOF growth shows a strong 1507

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qualitatively it does seem to show some preferred orientation along the (220) direction. All four IRMOFs have the same overall morphology yet differ in their degree of preferred orientation. Microwave synthesis was utilized to explore the conditions that could expedite the film growth process. Microwave-assisted MOF growth is well-established,19 and MOF thin film growth with microwaves has been explored as well.20 We utilized microwaves in the synthesis of a homogeneous IRMOF-1 film for 10 min. GIXRD analysis of the film shows peaks corresponding to highly crystalline IRMOF-1 as well as the presence of the oriented ZnO nanowires (Figure S9 of the Supporting Information). The peaks were lower in intensity relative to the standard synthesis method due to the smaller film thickness. SEM images of the film corroborate these results (Figure 6 and Figure S10 of the Supporting Information). A

Figure 5. GIXRD spectra of IRMOF films on ZnO nanowires. Red bars at the bottom indicate the theoretical pattern for each IRMOF and asterisks indicate zinc oxide nanowire peaks.

Figure 6. Top-down SEM image of microwave-synthesized IRMOF-1 film on ZnO nanowires.

thickness of 25 μm and exhibit the same morphology to that of IRMOF-1. The films were characterized by GIXRD (Figure 5). The spectrum of IRMOF-3 showed the poorest crystallinity but generally matches the theoretical spectra.12 Lack of crystallinity could be due to film drying before or during GIXRD analysis, leading to film degradation. Attempts to improve crystallinity of IRMOF-3 have thus far been unsuccessful. IRMOF-8 exhibited good crystallinity and closely matches the theoretical spectra of interpenetrated IRMOF-8.15 This is expected because the elevated temperatures used to generate this film likely induced the formation of the interpenetrated moiety.16 IRMOF-9 also displayed good crystallinity and seems to reasonably match the calculated spectra.12 A few peaks that do not match the calculated spectrum are likely due to solvent molecules still residing within the pores of the MOF, which has been shown to shift peaks due to structural deformations.17 The differences in experimental versus calculated data could also be a result of film degradation or structural transformations from drying before or during data collection. The crystallographic preferred orientation (CPO) indexing method was used to quantify the degree of orientation of the films (Figures S5 to S8 of the Supporting Information).18 IRMOF-1 displayed a (220) out-of-plane orientation with a CPO220/200 index of approximately 26. IRMOF-9 showed little preferred orientation along the (111) plane with a CPO111/100 of ∼15, and IRMOF-8 had little to no preferred orientation with a CPO110̅ /110 of ∼1.62. The CPO index of IRMOF-3 could not be quantified due to the low crystallinity, though

film (∼ 1 μm thick) of IRMOF-1 was produced along with larger discrete crystals on top of the film that averaged 2 μm in size. The larger crystals are attributed to secondary growth of the IRMOF on top of the initial IRMOF layer. Closer inspection of the film shows several nanowires protruding through the IRMOF film. This observation indicates that the nanowires are serving as the initial nucleation site for the IRMOF film (Figure S10 of the Supporting Information). In conclusion, we have developed a procedure using nanowire arrays for the synthesis of IRMOF films. When the nanowire template is present, MOF films of high crystallinity and excellent coverage of the substrate were produced. We have also used microwaves for the rapid fabrication of these films. Recently, IRMOF-1 has been shown to be semiconducting, with the organic benzenedicarboxylate ligands acting as photoantennae and the zinc oxide clusters as semiconducting quantum dots.21 The band gap of the material can be raised or lowered by introducing pendant groups or higher conjugation to the organic linkers, respectively.22 In addition, new fabrication techniques have emerged that form hybrid MOF films consisting of more than one type of MOF, allowing the films to serve as possible semiconducting multijunctions.23 Coupling the electronic properties of an IRMOF with a conductive substrate may give rise to a new class of photovoltaic and resistance-based sensing devices. We showed four initial examples of MOFs that can be grown by this 1508

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Monolayers on Au(111). J. Am. Chem. Soc. 2005, 127 (40), 13744− 13745. (8) Spokoyny, A. M.; Kim, D.; Sumrein, A.; Mirkin, C. A. Infinite Coordination Polymer Nano- And Microparticle Structures. Chem. Soc. Rev. 2009, 38 (5), 1218−1227. (9) Shekhah, O.; Wang, H.; Paradinas, M.; Ocal, C.; Schupbach, B.; Terfort, A.; Zacher, D.; Fischer, R. A.; Woll, C. Controlling Interpenetration in Metal-Organic Frameworks by Liquid-Phase Epitaxy. Nat. Mater. 2009, 8 (6), 481−484. (10) Ameloot, R.; Pandey, L.; Auweraer, M. V. d.; Alaerts, L.; Sels, B. F.; De Vos, D. E. Patterned Film Growth of Metal-Organic Frameworks Based on Galvanic Displacement. Chem. Commun. 2010, 46 (21), 3735−3737. (11) Schoedel, A.; Scherb, C.; Bein, T. Oriented Nanoscale Films of Metal−Organic Frameworks By Room-Temperature Gel-Layer Synthesis. Angew. Chem., Int. Ed. 2010, 49 (40), 7225−7228. (12) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295 (5554), 469−472. (13) Yoo, Y.; Lai, Z.; Jeong, H.-K. Fabrication of MOF-5 Membranes Using Microwave-Induced Rapid Seeding and Solvothermal Secondary Growth. Microporous Mesoporous Mater. 2009, 123 (1−3), 100−106. (14) Zhang, Y.; Lan, D.; Wang, Y.; Cao, H.; Jiang, H. MOF-5 Decorated Hierarchical ZnO Nanorod Arrays and Its Photoluminescence. Phys. E 2011, 43 (6), 1219−1223. (15) Yao, Q.; Su, J.; Cheung, O.; Liu, Q.; Hedin, N.; Zou, X. Interpenetrated Metal-Organic Frameworks and Their Uptake of CO2 at Relatively Low Pressures. J. Mater. Chem. 2012, 22 (20), 10345− 10351. (16) Feldblyum, J. I.; Dutta, D.; Wong-Foy, A. G.; Dailly, A.; Imirzian, J.; Gidley, D. W.; Matzger, A. J. Interpenetration, Porosity, and High-Pressure Gas Adsorption in Zn4O(2,6-naphthalene dicarboxylate)3. Langmuir 2013, 29 (25), 8146−8153. (17) Li, H.; Davis, C. E.; Groy, T. L.; Kelley, D. G.; Yaghi, O. M. Coordinatively Unsaturated Metal Centers in the Extended Porous Framework of Zn3(BDC)3·6CH3OH (BDC = 1,4-Benzenedicarboxylate). J. Am. Chem. Soc. 1998, 120 (9), 2186−2187. (18) Bux, H.; Feldhoff, A.; Cravillon, J.; Wiebcke, M.; Li, Y.-S.; Caro, J. Oriented Zeolitic Imidazolate Framework-8 Membrane with Sharp H2/C3H8Molecular Sieve Separation. Chem. Mater. 2011, 23 (8), 2262−2269. (19) Klinowski, J.; Almeida Paz, F. A.; Silva, P.; Rocha, J. MicrowaveAssisted Synthesis of Metal-Organic Frameworks. Dalton Trans. 2011, 40 (2), 321−330. (20) Yoo, Y.; Jeong, H.-K. Rapid Fabrication of Metal Organic Framework Thin Films Using Microwave-Induced Thermal Deposition. Chem. Commun. 2008, 0 (21), 2441−2443. (21) Llabrés i Xamena, F. X.; Corma, A.; Garcia, H. Applications for Metal-Organic Frameworks (MOFs) as Quantum Dot Semiconductors. J. Phys. Chem. C 2006, 111 (1), 80−85. (22) Gascon, J.; Hernández-Alonso, M. D.; Almeida, A. R.; van Klink, G. P. M.; Kapteijn, F.; Mul, G. Isoreticular MOFs as Efficient Photocatalysts with Tunable Band Gap: An Operando FTIR Study of the Photoinduced Oxidation of Propylene. ChemSusChem 2008, 1 (12), 981−983. (23) (a) Yoo, Y.; Jeong, H.-K. Heteroepitaxial Growth of Isoreticular Metal−Organic Frameworks and Their Hybrid Films. Cryst. Growth Des. 2010, 10 (3), 1283−1288. (b) Tu, M.; Fischer, R. A. Heteroepitaxial Growth of Surface Mounted Metal-Organic Framework Thin Films with Hybrid Adsorption Functionality. J. Mater. Chem. A 2014, 2 (7), 2018−2022.

method, and we anticipate that this approach should be of broader use toward a variety of MOF thin films.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, microscope images, and GIXRD. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web site: http://oliver.chemistry. ucsc.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The XRD data in this work were recorded on an instrument supported by the NSF Major Research Instrumentation (MRI) Program under Grant DMR-1126845. We acknowledge Dr. Tom Yuzvinsky for image acquisition of the microwave synthesized IRMOF-1 film and the W.M. Keck Center for Nanoscale Optofluidics for use of the FEI Quanta 3D Dualbeam microscope.

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REFERENCES

(1) (a) Morris, R.; Wheatley, P. Gas Storage in Nanoporous Materials. Angew. Chem., Int. Ed. 2008, 47 (27), 4966−4981. (b) Ma, S.; Zhou, H.-C. Gas storage in porous metal-organic frameworks for clean energy applications. Chem. Commun. 2010, 46 (1), 44−53. (2) (a) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Commercial Metal-Organic Frameworks As Heterogeneous Catalysts. Chem. Commun. 2012, 48 (92), 11275−11288. (b) Swanson, C. H.; Shaikh, H. A.; Rogow, D. L.; Oliver, A. G.; Campana, C. F.; Oliver, S. R. J. Antimony Oxide Hydroxide Ethanedisulfonate: A Cationic Layered Metal Oxide for Lewis Acid Applications. J. Am. Chem. Soc. 2008, 130 (35), 11737−11741. (c) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. A Homochiral Metal-Organic Porous Material for Enantioselective Separation and Catalysis. Nature 2000, 404 (6781), 982−986. (3) (a) Fei, H.; Oliver, S. R. J. Copper Hydroxide Ethanedisulfonate: A Cationic Inorganic Layered Material for High-Capacity Anion Exchange. Angew. Chem. 2011, 123 (39), 9232−9236. (b) Oliver, S. R. J. Cationic Inorganic Materials for Anionic Pollutant Trapping and Catalysis. Chem. Soc. Rev. 2009, 38 (7), 1868−1881. (4) (a) Lu, G.; Hupp, J. T. Metal-Organic Frameworks as Sensors: A ZIF-8 Based Fabry-Perot Device as a Selective Sensor for Chemical Vapors and Gases. J. Am. Chem. Soc. 2010, 132 (23), 7832−7833. (b) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112 (2), 1105−1125. (5) (a) Zacher, D.; Shekhah, O.; Woll, C.; Fischer, R. A. Thin Films of Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38 (5), 1418− 1429. (b) Bétard, A.; Fischer, R. A. Metal−Organic Framework Thin Films: From Fundamentals to Applications. Chem. Rev. 2011, 112 (2), 1055−1083. (6) (a) Zacher, D.; Schmid, R.; Wöll, C.; Fischer, R. A. Surface Chemistry of Metal−Organic Frameworks at the Liquid−Solid Interface. Angew. Chem., Int. Ed. 2011, 50 (1), 176−199. (b) Liu, B.; Tu, M.; Fischer, R. A. Metal−Organic Framework Thin Films: Crystallite Orientation Dependent Adsorption. Angew. Chem., Int. Ed. 2013, 52 (12), 3402−3405. (7) Hermes, S.; Schröder, F.; Chelmowski, R.; Wöll, C.; Fischer, R. A. Selective Nucleation and Growth of Metal−Organic Open Framework Thin Films on Patterned COOH/CF3-Terminated Self-Assembled 1509

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