Defibrillation of soft porous metal-organic frameworks with electric fields

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Science  20 Oct 2017:
Vol. 358, Issue 6361, pp. 347-351
DOI: 10.1126/science.aal2456
  • Fig. 1 Design of the permeation cell and SEM of the ZIF-8 layer.

    (A and B) The permeation cell for in situ E-field–controlled gas permeation measurements. (C and D) Cross-sectional SEM images of the ZIF-8 layer on a supporting ceramic. In (C), ZIF-8 of 1-μm thickness is grown on top of an Al2O3 supporting layer; in (D), ZIF-8 of 20-μm thickness is grown on top of a TiO2 support. Scale bars, 2 μm (C), 10 μm (D). Color code of EDXS: blue, Zn L-α1,2; red, Ti K-α1; green, Al K-α1.

  • Fig. 2 Single-gas permeation with an in situ applied E-field of 500 V/mm.

    In (A), the in situ measured single-gas permeances for H2, CO2, and CH4 are shown before applying an E-field, upon application of an E-field, and after relaxation. For all gases, the permeances decrease when an E-field of 500 V/mm is applied. When the E-field is switched off, after a relaxation time of ~100 min, the permeance instantaneously increases. (B) The ideal selectivities α (quotient of the ideal permeances P1/P2) were calculated from these single-gas permeances. The size selectivity decreases for smaller gases after polarization. (C and E) A demonstration of the in situ switching of CO2 is shown in a plot of permeance versus time for the 20-μm membrane (C) and the 1-μm membrane (E). The CO2 permeance decreases when the E-field is applied, remains in this state for approximately 100 min after shutdown of the E-field, and then recovers; the permeance of only the thin membrane recovers completely. (D) The single-gas permeances through the HKUST-1 membrane show no response to the E-field. (F) For propene-propane mixtures in ZIF-8, the real separation factor α (quotient of the real permeances Ppropene/Ppropane) increases upon E-field poling.

  • Fig. 3 Simulated lattice; dielectric and 2H NMR spectroscopy of ZIF-8.

    (A) Lattice overlay of the undisturbed ZIF-8 gate (blue) with the same lattice distorted by an external potential U of the electric field strength (red), |U| = 0.02 eV Å−1 e−1 in the 〈111〉 direction. See (25) for further experimental and theoretical XRD data. (B) Dielectric spectra of the imaginary part of the permittivity from 473 to 373 K, in steps of 20 K, over the frequency range 100 to 105 Hz with a voltage amplitude of 0.1 V. ZIF-8 shows a moment that responds to an AC E-field, as investigated by dielectric relaxation spectroscopy. (C) 2H NMR T1 (squares) and T2 (circles) relaxation curves of ZIF-8 as a function of temperature for the C-D group of the deuterated 2-mIM linker in ZIF-8; a very slow motion is observed in the region marked in yellow. The numerical simulation results are shown as solid lines. (D) The same data on an expanded temperature scale. (E) The evolution of T1 and T2 relaxation times with temperature, showing the presence of three main motions: (i) fast (τa ~ 1 ps) but relatively small-amplitude librations (ϕa = ±30° at ~500 K) of the linker about the ZIF-8 window plane (22) and two much slower modes, (ii) and (iii). (F) A representation of the primitive cells of the ZIF-8 gate as a space-filling model of the different simulated polymorphs. Through deformation of the unit cell, the imidazolate linkers align (along with stiffening the network) and thereby avoid steric energy barriers of the bent unit cells.

  • Fig. 4 Ferro- and piezoelectric testing of a ZIF-8 single crystal.

    (A) Wiring diagram of the ZIF-8 single-crystal polarization measurement. Scale bar, 200 μm. (B) SEM image of the crystal at 20 kV between segments of copper adhesive tape. The scale bar and the giant ZIF-8 crystal are 200 μm. (C) The electrical conductivity in the range of 1 to 107 Hz. The results show a nearly linear curve, indicating the crystal to be almost an ideal insulator without a piezo- or ferroelectric response. (D) The polarization curve is measured at 20 kHz over the range of –775 to 775 V/mm. ZIF-8 shows a broad polarization curve characteristic of soft-material deformation, but without ferroelectric hysteresis.

Supplementary Materials

  • Defibrillation of soft porous metal-organic frameworks with electric fields

    A. Knebel, B. Geppert, K. Volgmann, D. I. Kolokolov, A. G. Stepanov, J. Twiefel, P. Heitjans, D. Volkmer, J. Caro

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    • Materials and Methods 
    • Supplementary Text 
    • Figs. S1 to S8 
    • Tables S1 and S2 
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