Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination

Abstract

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW₁₂) intercalated reduced graphene oxide (rGO) membrane (rGO-PW₁₂) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW₁₂ result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW₁₂ membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW₁₂ membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW₁₂ membranes with a thickness of 90 nm not only exhibit a high-water permeance of up to 0.11790 L m^-2 h^-1 bar^-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW₁₂ and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.

Keywords: desalination; ion sieving; membranes; polyoxometalates; reduced graphene oxide.

Figure 1

a) The scheme of the fabrication of rGO‐PW₁₂ membranes through a PW₁₂ assisted in situ photoreduction strategy. b) SEM image of the top view of the rGO‐PW₁₂ membranes. (inset: digital photo) c–f) SEM images of the cross‐section of rGO‐PW₁₂ membranes with different thicknesses. g) HRTEM image of highly ordered nanochannels of rGO‐PW₁₂ membranes with one orientation. The yellow dashed arrows were eye‐guiding lines indicating the orientations of rGO. h) HRTEM image of PW₁₂ cluster in rGO‐PW₁₂ nanosheets. i) HAADF‐STEM image of rGO‐PW₁₂ nanosheets. j) Magnified HAADF‐STEM images of PW₁₂ cluster indicated by the dashed circle in Figure 1i. (inset: corresponding EDX line scanning of PW₁₂ cluster and the polyhedron of PW₁₂ cluster).

Figure 2

a) Raman spectra of GO and rGO‐PW₁₂ membranes. b) C 1s high‐resolution XPS spectra of GO and rGO‐PW₁₂ membranes. c) W high‐resolution XPS spectra of rGO‐PW₁₂ membrane and PW₁₂. d) FT‐IR spectra of rGO‐PW₁₂ membrane and PW₁₂. e) The electron density difference of rGO‐PW₁₂. The yellow and blue regions indicated the charge accumulation and depletion, respectively. (isosurface value: 0.02 e Å⁻³) The curved arrows indicated the electronic transfer from rGO to PW₁₂. f) Isosurface and g) scatter graph of independent gradient model (IGM) for unveiling the interaction between rGO and PW₁₂, C‐cyan, H‐white, O‐red, W‐pink, P‐olive drab. (inset: the schematic diagram showed that PW₁₂ was tightly bound to rGO nanosheets).

Figure 3

a) Schematic diagram for GO and rGO‐PW₁₂ membranes structures. b) XRD patterns of pristine GO and rGO‐PW₁₂ membranes in dry and wet states. c) Membrane stability test. d) The water contact angle of GO and rGO‐PW₁₂ membranes.

Figure 4

Ion permeation tests through GO and rGO‐PW₁₂ membranes Feed solutions of 0.1 m KCl, NaCl, LiCl, MgCl₂, CaCl₂, and AlCl₃ were used in draw side, while DI water filled the permeate side, respectively. a) Scheme of rGO‐PW₁₂ membranes how to realize controllable ion transport. b,c) Comparison of the permeation rate of ions through GO and rGO‐PW₁₂ membranes. d) The dependence of permeation rate on the mass ratio of rGO:PW₁₂ for NaCl and MgCl₂. MD simulation snapshots at 0 and 35 ns for two sets of Na⁺ and Cl⁻ permeation systems: untreated GO (e) and rGO‐PW₁₂ (f) membranes. g) The number of ions that passed through the membranes as a function of simulation time.

Figure 5

a) Cycles of desalination/drying performance of rGO‐PW₁₂ membranes. b) Long‐term Na⁺ permeation rate through GO and rGO‐PW₁₂ membranes. (Inset: accumulated permeated Na⁺ along with testing time.) c) Comparison of permeation rate of ions in synthetic seawater through GO and rGO‐PW₁₂ membranes. The synthetic seawater was a mixture of KCl, 0.0093 m; NaCl, 0.42 m; Na₂SO₄, 0.029 m; CaCl₂·2H₂O 0.011M  and MgCl₂·2H₂O, 0.056 m. (Inset: Rejection rate for each cation.) d) Chlorine resistance of GO and rGO‐PW₁₂ membranes. Na⁺ permeation rate and rejections (inset) through the GO and rGO‐PW₁₂ membranes before and after NaClO (200 ppm) treatment for 48 h. e) Salt permeation rate and water flux of GO and rGO‐PW₁₂ membranes with different thicknesses under osmotically driven pressure. f) Water permeance and NaCl rejection performance of different 2D laminar membranes using osmotically driven pressure. The detailed data are listed in Table S3 (Supporting Information).

Figure 6

Schematic representation of efficient ion sieving (K⁺ or Mg²⁺) by the 2D rGO‐PW₁₂ membranes.