Research ArticlesMATERIALS SCIENCE

Ultrafast selective transport of alkali metal ions in metal organic frameworks with subnanometer pores

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Science Advances  09 Feb 2018:
Vol. 4, no. 2, eaaq0066
DOI: 10.1126/sciadv.aaq0066
  • Fig. 1 Ion transport through the ZIF-8/GO/AAO membranes.

    (A) Schematic illustration of ion transport through a ZIF-8/GO/AAO membrane with ~3.4 Å pore windows for ion selectivity and ~11.6 Å pore cavities for fast ion transport (drawing not to scale). The inset indicates the crystal structure of ZIF-8. (B) SEM images the hybrid ZIF-8/GO nanosheet seeds coated on the AAO support. (C) SEM image of the plasma-treated nanoporous ZIF-8/GO seeds. (D) SEM images of the ZIF-8/GO/AAO membrane surface. (E) SEM images of the membrane cross section reveal that a ~446-nm-thick ZIF-8/GO layer is densely grown on the top of the AAO support. (F) XRD patterns of the AAO support, the seeding layer, the plasma-treated seeding layer, the ZIF-8/GO/AAO membrane, and simulated ZIF-8 structure. a.u., arbitrary units.

  • Fig. 2 Current-voltage (I-V) characteristics of an AAO support before and after growth of the ZIF-8/GO layer to make the ZIF-8/GO/AAO membrane.

    (A) I-V curves of the AAO support measured with different ions. (B) I-V curves of ZIF-8/GO/AAO membranes measured with different ions. (C) Ion conductance values of the AAO support with and without the ZIF-8/GO membrane. (D) Schematic of ion transport through a pore with a diameter much larger than the hydrated ionic diameter (dPore >> dH-ion, such as the 200-nm porous AAO support). Ions transport in a hydrated state. (E) Schematic of ion transport through a simplified subnanometer ZIF-8 pore with 3.4-Å-diameter windows, which is smaller than the hydrated ionic diameters but larger than the dehydrated ionic diameters (dH-ion > dWindow > dIon), and a 11.6-Å-diameter cavity, which is larger than the hydrated ionic diameters (dCavity > dH-ion). Ions should undergo a dehydration process when they enter the window and a hydration process when they exit the window. As a result, ions passing through the subnanometer ZIF-8 pore should undergo multiple dehydration-hydration processes.

  • Fig. 3 MD simulations of ion transport in ZIF-8.

    (A) The simulation cartoon shows the ZIF-8 cavities filled with water molecules (green spheres), and they are connected via narrow windows. For clarity, ZIF-8 is shown as a wireframe. The apparent empty spaces are actually occupied by atoms of ZIF-8. K+ and Cl ions are represented by orange and purple spheres, respectively. (B) The normalized mobility of K+, Li+, and Cl ions in ZIF-8 and in water. The mobility of Cl in 1 M aqueous solution is taken as the reference. The Li+ mobility is enhanced in ZIF-8 compared with that in water, whereas the opposite trend is observed for K+. Consequently, Li+ in ZIF-8 has a higher mobility than K+, which is consistent with experiments. (C) Radial distribution function of water molecules around Li+ and K+ in bulk solutions and in ZIF-8. Owing to the confinement effect, the water density in the first hydration shell of Li+ is significantly reduced, and the second hydration shell nearly disappears. Similar (but less) trends can be observed for K+. The diameter of the partly hydrated Li+ appears to be smaller than that of the partly hydrated K+ in ZIF-8, which might explain its higher mobility.

  • Fig. 4 Ion selectivity of synthetic MOF membranes.

    (A) Alkali metal ion selectivity of AAO supports, ZIF-8/GO/AAO membranes, and GO/AAO membranes. (B) Window structures of MOF pores: six-ring ZIF-8 window of ~3.4 Å in diameter, six-ring ZIF-7 window of ~2.9 Å in diameter, and triangular UiO-66 window of ~6.0 Å in diameter (see fig. S11 for crystal structures of ZIF-7 and UiO-66). (C) Dependence of ion selectivity on the pore widow diameter of different MOFs and the pore diameter of nanoporous membranes. At the angstrom scale, the alkali metal ion selectivity of the MOF membranes decreases with increasing window diameter. However, all membranes with pore diameters >1 nm do not have alkali metal ion selectivity.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaaq0066/DC1

    note S1. Gas permeation tests.

    note S2. Fabrication of bullet-shaped single-nanochannel PET membranes.

    note S3. Fabrication of single-nanochannel supported ZIF-8/PET membrane.

    note S4. MD simulations.

    note S5. Fabrication of single-nanochannel supported ZIF-7/PET membrane.

    note S6. Fabrication of single-nanochannel supported UiO-66/PET membrane.

    fig. S1. Structures of a biological ion channel and subnanometer ZIF-8 pores.

    fig. S2. Fabrication process of the ZIF-8/GO/AAO membrane.

    fig. S3. SEM characterization of the AAO support before and after growth of ZIF-8/GO membrane.

    fig. S4. Gas permeation and selectivity of the ZIF-8/GO/AAO membrane and N2 adsorption isotherms of ZIF-8 crystals.

    fig. S5. Dependence of the ion conductance of the ZIF-8/GO/AAO membrane on the ionic diameter.

    fig. S6. Ion transport properties of the nanoporous GO/AAO membrane.

    fig. S7. Ion transport in the single-nanochannel supported ZIF-8/PET membrane without GO nanosheets.

    fig. S8. Ion transport mechanism through subnanometer ZIF-8 pores.

    fig. S9. Calculated ion velocities in ZIF-8 pores under an electric field of 0.5 V/Å.

    fig. S10. Radial distribution function g(r) of water molecules around Li+ and K+ in ZIF-8 calculated under different van der Waals forces.

    fig. S11. Cycle performance and stability of the ZIF-8/GO membrane.

    fig. S12. Fabrication and characterization of the ZIF-7/PET and UiO-66/PET membranes.

    fig. S13. Ion transport properties of the ZIF-7/PET and UiO-66/PET membranes.

    fig. S14. Ion transport properties of the multichannel PET membranes with a channel density of 108 cm−2.

    table S1. Conductance values of the AAO supports before and after growth of ZIF-8/GO membranes measured in 0.1 M MCl solutions (M+ = Li+, Na+, K+, and Rb+).

    table S2. Studied ionic species: ionic diameter (d), hydrated ionic diameter (dH), hydration enthalpy, and limited ion conductivity.

    table S3. Calculated ion mobility in bulk solution and in ZIF-8 (in unit of 10−7 m2 V−1 s−1).

    table S4. Ion selectivity ratios of the three reproduced ZIF-8/GO/AAO membranes and three reproduced UiO-66/PET membranes.

    table S5. Dependence of the ion selectivity on the pore diameter of the synthetic membranes.

    table S6. Comparison of the ion selectivity of the ZIF-8 membranes with other synthetic membranes.

    References (3850)

  • Supplementary Materials

    This PDF file includes:

    • note S1. Gas permeation tests.
    • note S2. Fabrication of bullet-shaped single-nanochannel PET membranes.
    • note S3. Fabrication of single-nanochannel supported ZIF-8/PET membrane.
    • note S4. MD simulations.
    • note S5. Fabrication of single-nanochannel supported ZIF-7/PET membrane.
    • note S6. Fabrication of single-nanochannel supported UiO-66/PET membrane.
    • fig. S1. Structures of a biological ion channel and subnanometer ZIF-8 pores.
    • fig. S2. Fabrication process of the ZIF-8/GO/AAO membrane.
    • fig. S3. SEM characterization of the AAO support before and after growth of ZIF-8/GO membrane.
    • fig. S4. Gas permeation and selectivity of the ZIF-8/GO/AAO membrane and N2 adsorption isotherms of ZIF-8 crystals.
    • fig. S5. Dependence of the ion conductance of the ZIF-8/GO/AAO membrane on the ionic diameter.
    • fig. S6. Ion transport properties of the nanoporous GO/AAO membrane.
      fig. S7. Ion transport in the single-nanochannel supported ZIF-8/PET membranewithout GO nanosheets.
    • fig. S8. Ion transport mechanism through subnanometer ZIF-8 pores.
    • fig. S9. Calculated ion velocities in ZIF-8 pores under an electric field of 0.5 V/Å.
    • fig. S10. Radial distribution function g(r) of water molecules around Li+ and K+ in ZIF-8 calculated under different van der Waals forces.
    • fig. S11. Cycle performance and stability of the ZIF-8/GO membrane.
    • fig. S12. Fabrication and characterization of the ZIF-7/PET and UiO-66/PET membranes.
    • fig. S13. Ion transport properties of the ZIF-7/PET and UiO-66/PET membranes.
    • fig. S14. Ion transport properties of the multichannel PET membranes with a channel density of 108 cm−2.
    • table S1. Conductance values of the AAO supports before and after growth of ZIF-8/GO membranes measured in 0.1 M MCl solutions (M+ = Li+, Na+, K+, and Rb+).
    • table S2. Studied ionic species: ionic diameter (d), hydrated ionic diameter (dH), hydration enthalpy, and limited ion conductivity.
    • table S3. Calculated ion mobility in bulk solution and in ZIF-8 (in unit of 10−7 m2 V−1 s−1).
    • table S4. Ion selectivity ratios of the three reproduced ZIF-8/GO/AAO membranes and three reproduced UiO-66/PET membranes.
    • table S5. Dependence of the ion selectivity on the pore diameter of the synthetic membranes.
    • table S6. Comparison of the ion selectivity of the ZIF-8 membranes with other synthetic membranes.
    • References (38–50)

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