Increased Electrical Conductivity in a Mesoporous Metal–Organic

Communication Cite This: J. Am. Chem. Soc. 2018, 140, 3871−3875

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Increased Electrical Conductivity in a Mesoporous Metal−Organic Framework Featuring Metallacarboranes Guests Chung-Wei Kung,† Kenichi Otake,† Cassandra T. Buru,† Subhadip Goswami,† Yuexing Cui,† Joseph T. Hupp,†,‡ Alexander M. Spokoyny,§ and Omar K. Farha*,†,∥ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, California 90095, United States ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 15551, Saudi Arabia ‡

S Supporting Information *

electrically conductive MOFs, various strategies have been proposed recently, including the formation of π-stacked pathways within three-dimensional MOFs,9 the design of two-dimensional MOFs with π-conjugation,10 the use of sulfurbased ligands to facilitate charge delocalization,11 the formation of a MOF−polymer composite,12 and the introduction of guest molecules.13 Although the electrical conductivity has been established or imparted in certain MOFs and some conductive MOFs have been utilized in electrochemical applications,8i,10a,14 most of these reported conductive MOFs do not possess mesoporosity, and electrically conductive zirconium-based MOFs are still rare. In our recent study, we reported for the first time an electrically conductive zirconium-based MOF formed by electrochemical incorporation of polythiophene in a MOF thin film.15 However, the MOF can only be prepared as thin films and is only conductive in the potential region where the thiophene units oxidize. In this study, an electrically conductive and mesoporous zirconium-based MOF was synthesized by introducing nickel(IV) bis(dicarbollide) (Ni(C2B9H11)2, NiCB)16 into the mesoporous MOF, NU-1000.17 Constructed from pyrenebased linkers and hexa-zirconium nodes, NU-1000 crystallizes with one-dimensional mesoporous hexagonal channels and microporous triangular channels and exhibits high water stability.5a,17 Additionally, the pyrene units present in the linkers of NU-1000 have been reported to be electron donors.18 On the other hand, the bis(dicarbollide) cages in NiCB can serve as electron-deficient moieties.19 Since the size of NiCB fits well within the triangular channels of NU-1000, and the lowest unoccupied molecular orbital (LUMO) of NiCB locates between the conduction bands and valence bands of both the pyrene-based linker and NU-1000 (see Section S2 in the Supporting Information), we hypothesized that NiCB molecules can be incorporated in these channels as guests, between three pyrenes, to engender donor−acceptor interactions through the whole framework and render the MOF electrically conductive. The introduction of NiCB in NU-1000 was confirmed by single-crystal X-ray diffraction (Figure 1). The obtained framework (NiCB@NU-1000) is still mesoporous,

ABSTRACT: Nickel(IV) bis(dicarbollide) is incorporated in a zirconium-based metal−organic framework (MOF), NU-1000, to create an electrically conductive MOF with mesoporosity. All the nickel bis(dicarbollide) units are located as guest molecules in the microporous channels of NU-1000, which permits the further incorporation of other active species in the remaining mesopores. For demonstration, manganese oxide is installed on the nodes of the electrically conductive MOF. The electrochemically addressable fraction and specific capacitance of the manganese oxide in the conductive framework are more than 10 times higher than those of the manganese oxide in the parent MOF.

M

etal−organic frameworks (MOFs) consist of a class of porous materials constructed from metal-rich nodes and organic linkers.1 Due to their desirable characteristics such as periodic porosity and ultrahigh specific surface area,2 MOFs have attracted great attention in several applications over the past two decades.3 The periodic intra-framework functionality of MOFs may be utilized to install well-separated and accessible active sites within the entire framework, which renders MOFs attractive porous supports for a range of catalytic applications.4 Among all existing MOFs, zirconium-based MOFs are known for their high water stability in a wide range of pH values;5 this feature provides the potential of using these MOFs in applications necessitating aqueous conditions, such as hydrogen evolution catalysis6 and water adsorption from air.7 Since a high density of accessible active sites may be built into MOFs, we reasoned that MOF thin films grown on conductive substrates with proper intra-framework functionality for installing electroactive sites should be excellent supports for electrocatalysis and relevant electrochemical applications. Furthermore, due to their water stability, thin films of zirconium-based MOFs are potential candidates for energyrelated electrochemical applications such as water splitting and CO2 reduction, which usually need to be operated in aqueous solutions. However, due to the electrically insulating nature of most MOFs, the performance of MOF-based materials in electrochemical applications is significantly limited.8 To design © 2018 American Chemical Society

Received: January 16, 2018 Published: March 4, 2018 3871

DOI: 10.1021/jacs.8b00605 J. Am. Chem. Soc. 2018, 140, 3871−3875

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Figure 2. (a) XRD patterns of NU-1000 and NiCB@NU-1000, compared with the simulated pattern of NU-1000.17 (b) SEM image of NiCB@NU-1000. (c) Nitrogen adsorption−desorption isotherms and (d) DFT pore size distributions of NU-1000 and NiCB@NU-1000.

compared with those of pristine NU-1000, are shown in Figure 2c,d. The Brunauer−Emmett−Teller (BET) surface area of NU-1000 decreases from 2215 m2/g to 1260 m2/g after installation of NiCB, but the resulting NiCB@NU-1000 still exhibits mesoporosity. To elucidate the structural changes during the addition of NiCB, large NU-1000 crystals grown from diethylformamide17 for single-crystal XRD measurements were also infiltrated with NiCB. In the solved crystal structure, NiCB was present in all the triangular channels of NU-1000 at 100% occupancy, as illustrated in Figure 1 (see details in Section S4 in the Supporting Information). To investigate the conductivity of NiCB@NU-1000, thin films of NU-1000, NiCB, and NiCB@NU-1000 were deposited on interdigitated electrodes (IDEs) by drop-casting (see experimental details in Supporting Information). Current− voltage (I−V) curves of the obtained IDEs were measured in air and compared with that of the bare IDE. As shown in Figure 3, the electrical conductivity of both the pristine NU-1000 and bulk NiCB is not measurable by the IDE, confirming the insulating nature of both. However, a remarkable slope appears in the I−V curve of the NiCB@NU-1000/IDE, which suggests that the NiCB@NU-1000 is electrically conductive. To

Figure 1. Crystal structure of NiCB@NU-1000 (100 K; only one orientation of NiCB in the triangular channels is shown for clarity). Triangular channels of NiCB@NU-1000 are also shown in space-filling model to present the close interaction between NiCB and three surrounding pyrenes. For simplicity, hydrogen atoms are not shown in the structure of NiCB@NU-1000. The structure of NiCB is also presented.

with accessible aqua and hydroxyl ligands on hexa-zirconium nodes, allowing further installation of electrochemically active species by either atomic layer deposition in MOFs (AIM) 4c,8d,17,20 or solvothermal deposition in MOFs (SIM).4c,20,21 As a demonstration, manganese oxide was deposited via AIM in NiCB@NU-1000. The manganese oxide installed in the conductive NiCB@NU-1000 exhibits a much higher capacitance than when installed in pristine NU1000. Procedures for the synthesis of NU-1000 and NiCB have been reported previously.19c,22 NiCB@NU-1000 was prepared by immersing NU-1000 crystals in a solution of NiCB in DMF for 3 days, followed by subsequent washing and drying (see experimental details in Supporting Information). Both the crystallinity and hexagonal rod-like morphology of NU-1000 are preserved after the installation of NiCB, as revealed in the powder X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) image (Figure 2a,b). From energydispersive X-ray spectroscopy (EDS) elemental line scan, the overlapping distributions of Ni and Zr across a crystal of NiCB@NU-1000 indicate that NiCB was installed uniformly in the NU-1000 crystals (Figure S2).23 Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements of the digested NiCB@NU-1000 samples suggest a loading of 0.74 ± 0.07 NiCB per Zr6 node. Nitrogen adsorption−desorption experiments were utilized to investigate the porosity of NiCB@NU-1000; the obtained isotherm and density functional theory (DFT) pore size distribution,

Figure 3. I−V curves of the bare IDE, NU-1000/IDE, NiCB/IDE, and NiCB@NU-1000/IDE. 3872

DOI: 10.1021/jacs.8b00605 J. Am. Chem. Soc. 2018, 140, 3871−3875

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NU-1000 and Mn-AIM-NiCB@NU-1000 thin films exhibit reversible redox activity (Figure 4b,c). Apparently, the redox behavior of manganese oxide occurring within the same potential window and in the same electrolyte is attributed to the reversible and successive Faradaic reactions between Mn(IV) and Mn(III) along with the surface adsorption− desorption of electrolyte cations.27 Since the capacitances from both the NU-1000 and NiCB@NU-1000 scaffolds are negligible (Figure 4a), the electrochemically addressable fraction of the installed manganese oxide in each thin film can be calculated from the charge estimated from Figure 4b,c and the loading of manganese per square centimeter of the film; the latter was obtained by the previously reported UV−vis approach (also see Section S9 in the Supporting Information).8d,21 As a result, only 1.4% of the entire Mn sites installed in the Mn-AIM-NU-1000 thin film are electrochemically addressable; the poor redox activity of manganese oxide may be attributed to the poor charge transport in the insulating NU1000.8d However, in the Mn-AIM-NiCB@NU-1000 thin film, 16% of the Mn sites are electrochemically addressable, which indicates that the electrical conductivity of the NiCB@NU1000 scaffold significantly improves the redox activity of the installed manganese oxide.28 The chronopotentiometric experiments were also conducted at various charge−discharge current densities, and the specific capacitance of the installed manganese oxide (F/gMnO2) can be calculated by assuming that two oxygens are present on each manganese atom (see the result shown in Figure 4d and Section S9 in the Supporting Information for details). At a charge−discharge current density of 0.02 mA/cm2, the specific capacitance of manganese oxide in the Mn-AIM-NiCB@NU-1000 thin film reaches 276 F/ gMnO2,29,10a,14b which is more than 10 times higher than that in the Mn-AIM-NU-1000 thin film (21 F/gMnO2). A long-term chronopotentiometric experiment suggests that a better approach for film deposition is needed to achieve a better performance retention (see Section S10 in the Supporting Information). It should be noted that although the electrical conductivity of the NiCB@NU-1000 scaffold is on the order of 10−7 S/cm, it is enough to electrochemically address ∼25% of the installed Mn sites within the micrometer-thick film (at 0.02 mA/cm2), which results in an excellent specific capacitance (Table S3). The findings open the opportunity for designing electrically conductive, water-stable MOFs with accessible intraframework functionality for a range of electrochemical applications. In conclusion, nickel bis(dicarbollide) (NiCB) can be installed in a zirconium-based MOF, NU-1000. Single-crystal XRD confirms that all the NiCB units are solely located in the microporous triangular channels of NU-1000, which keeps the mesoporous hexagonal channels of the framework open and allows for further incorporation of other active species. The donor−acceptor interaction between the pyrene-based linker and NiCB imparts electrical conductivity to the resulting mesoporous zirconium-based MOF, with a conductivity of 2.7 × 10−7 S/cm. Manganese oxide can then be deposited in NiCB@NU-1000. With the help of electrical conductivity from the NiCB@NU-1000 scaffold, the installed manganese oxide exhibits a specific capacitance of 276 F/gMnO2 at a charge− discharge current density of 0.02 mA/cm2, which is more than 10 times higher than that of manganese oxide in NU-1000 (21 F/gMnO2). Ongoing work focuses on utilizing the conductive

estimate the electrical conductivity of NiCB@NU-1000, electrochemical impedance spectroscopy was utilized with a spin-coated thin film of NiCB@NU-100015 (see Section S5 in the Supporting Information for details); an electrical conductivity of 2.7 × 10−7 S/cm was achieved.24 Both the quenching of emission from the linker and the emergence of charge-transfer band in the diffuse reflectance UV−vis spectra suggest an efficient electron transfer from the excited pyrenebased linkers to NiCB25 (see Section S7 in the Supporting Information); these observations imply that the obtained electrical conductivity of NiCB@NU-1000 may be attributed to the donor−acceptor charge transfer between the linker and NiCB presented uniformly within the entire framework. As revealed in the crystal structure of NiCB@NU-1000, NiCB is highly disordered with various orientations. We speculated that NiCB may not generate a new distinct band but instead behaves as a p-type dopant to render the transport of charge carriers within the framework.26 To demonstrate the potential use of the conductive NiCB@ NU-1000 in electrochemical applications, manganese oxide was deposited in NiCB@NU-1000 to examine its redox activity and capacitance under electrochemical conditions (see experimental details in the Supporting Information); the obtained material is designated as “Mn-AIM-NiCB@NU-1000”. For comparison, manganese oxide was also installed in NU-1000 (Mn-AIM-NU1000) (see detailed characterizations in Section S8 in the Supporting Information). Cyclic voltammetric (CV) curves of the spin-coated NU-1000, NiCB@NU-1000, Mn-AIM-NU1000, and Mn-AIM-NiCB@NU-1000 thin films, measured in an aqueous solution of 0.1 M Na2SO4, are shown in Figure 4a.

Figure 4. (a) CV curves of the spin-coated NU-1000, NiCB@NU1000, Mn-AIM-NU-1000, and Mn-AIM-NiCB@NU-1000 thin films, measured at 25 mV/s. Charge−discharge curves of (b) Mn-AIM-NU1000 and (c) Mn-AIM-NiCB@NU-1000 thin films, measured at 0.08 mA/cm2. (d) Specific capacitance of the installed manganese oxide in Mn-AIM-NU-1000 and Mn-AIM-NiCB@NU-1000 thin films, measured at various charge−discharge current densities.

The enclosed areas in the CV curves of the NU-1000 and NiCB@NU-1000 thin films are barely observable, which indicates that the capacitance originating from NU-1000 or NiCB@NU-1000 is negligible. Reversible redox peaks can be observed in the CV curve of the Mn-AIM-NU-1000 thin film, and the redox activity is significantly enhanced when the MnAIM-NiCB@NU-1000 thin film is used. Chronopotentiometric charge−discharge experiments reveal that both the Mn-AIM3873

DOI: 10.1021/jacs.8b00605 J. Am. Chem. Soc. 2018, 140, 3871−3875

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NiCB@NU-1000 as a platform to incorporate a range of electrochemically active metal oxides or sulfides via SIM or AIM for various electrochemical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b00605. Additional experimental details and data, including Sections S1−S10, Tables S1−S3, and Figures S1−S12 (PDF) X-ray crystallographic data for NiCB@NU-1000 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Chung-Wei Kung: 0000-0002-5739-1503 Cassandra T. Buru: 0000-0001-6142-8252 Subhadip Goswami: 0000-0002-8462-9054 Joseph T. Hupp: 0000-0003-3982-9812 Alexander M. Spokoyny: 0000-0002-5683-6240 Omar K. Farha: 0000-0002-9904-9845 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support of this work from Toyota and the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001059. This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University, and the EPIC facility (NUANCE Center, Northwestern University) supported by the MRSEC program (NSF DMR-1121262), the International Institute for Nanotechnology (IIN), and the State of Illinois, through the IIN. A.M.S. thanks 3M for a nontenured faculty award and the Alfred P. Sloan Foundation for a chemistry research fellowship.

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REFERENCES

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DOI: 10.1021/jacs.8b00605 J. Am. Chem. Soc. 2018, 140, 3871−3875

Communication

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DOI: 10.1021/jacs.8b00605 J. Am. Chem. Soc. 2018, 140, 3871−3875