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1 In the format provided by the authors and unedited. DOI: /NNANO Effective NaCl and dye rejection of hybrid graphene oxide/graphene layered membranes Aaron Morelos-Gomez, 1 Rodolfo Cruz-Silva, 1 Hiroyuki Muramatsu, 2 Josue Ortiz-Medina, 1 Takumi Araki, 1,3 Tomoyuki Fukuyo, 4 Syogo Tejima, 1,3 Kenji Takeuchi, 1 Takuya Hayashi, 2 Mauricio Terrones, 2,5 Morinobu Endo 1,2 1 Global Aqua Innovation Center, Shinshu University, Wakasato, Nagano , Japan 2 Institute of Carbon Science and Technology, Shinshu University; Wakasato, Nagano , Japan. 3 Research Organization for Information Science & Technology, , Kitashinagawa, Shinagawa-ku, Tokyo, , Japan 4 Showa Denko K.K., Institute for Advanced and Core Technology, 1-1-1, Ohnodai, Midori-ku, Chiba-shi, Chiba, , Japan 5 Department of Materials Science and Engineering, Department of Chemistry, and Department of Physics, The Pennsylvania State University; University Park, Pennsylvania 16802, USA 1 NATURE NANOTECHNOLOGY 1

2 The sensitivity of XPS to analyse thin layers was used to observe the surface treatment of PSU. An increase in the oxygen atomic content of the surface of PSU suggests a coating of PVA. Table 1. Semiquantive atomic percentage. Measured by XPS for pure PSU and PVA coated PSU (PSU-PVA) PSU PSU-PVA C O S NATURE NANOTECHNOLOGY 2

3 Table 2. NaCl rejection and permeate comparison among GO membranes reported by diverse groups and this work. The NaCl feed solution concentration, NaCl water permeate, NaCl rejection and applied pressure are presented. * The manuscript does not provide the used NaCl concentration for the feed solution. Feed NaCl NaCl Water NaCl Rejection Standard deviation permeate Standard deviation Pure Water permeate Pressure Material % % m 3 m -2 s -1 m 3 m -2 s -1 Mpa GO E GO E GO/MWCNT E GO * E E Reference GO100FLG E E This work GO75FLG E E This work GO60FLG E E This work GO45FLG E E This work GO35FLG E E This work GO25FLG E E This work GO0FLG E E This work GO100FLG E E This work GO75FLG E E This work GO60FLG E E This work GO45FLG E E This work GO35FLG E E This work GO25FLG E E This work GO0FLG E E This work GO100FLG E E This work GO75FLG E E This work GO60FLG E E This work GO45FLG E E This work GO35FLG E E This work GO25FLG E E This work GO0FLG E E This work NATURE NANOTECHNOLOGY 3

4 GO100FLG E E This work GO75FLG E E This work GO60FLG E E This work GO45FLG E E This work GO35FLG E E This work GO25FLG E E This work GO0FLG E E This work GO90DWCNT E E This work GO70DWCNT E E This work GO50DWCNT E E This work GO30DWCNT E E This work NATURE NANOTECHNOLOGY 4

5 Supplementary Figure 1. Diagram of the cross flow system. A feed solution is connected to a water pump. This water pump may also apply pressure to water for the cross-flow cell. The top flow of water is returned to the feed solution and the permeate solution is collected in another vessel. NATURE NANOTECHNOLOGY 5

6 Construction of graphene oxide simulation models We adopted the following method to construct a graphene oxide sheet. At first, several building blocks were prepared: a graphene block with 8 carbon atoms (Supplementary Fig. 1a), a second block with 8 carbon atoms for graphene with two oxygen atoms (Supplementary Fig. 1b), a third block with 8 carbon atoms for graphene with two oxygen (Supplementary Fig. 1c) and one OH and finally another block with 8 carbon atoms for graphene with one oxygen and two OH (Supplementary Fig. 1d). Oxygen atoms were allocated on the bridge site and OH groups on top of graphene. 5 These blocks were randomly assembled and their ratio adjusted in accordance with the C/OH ratio. Then, pores (up to 0.5 nm) were then made by removing carbon atoms from the carbon network and adding randomly oxygen atoms at random positions. Finally, OH and COOH groups were added at the edge of the pores. 6 NATURE NANOTECHNOLOGY 6

7 Supplementary Figure 2. Building blocks to construct a graphene oxide sheet. (a) pure carbon atoms, (b) carbon atoms and 2 pure oxygen atoms, (c) carbon atoms, 2 pure oxygen atoms and 1 OH group, (c) carbon atoms, 1 pure oxygen atom and 2 OH groups. The CNT/GO model membranes were constructed as follows: Two graphene oxide sheets were made using the above-mentioned method. The resulting sheets had 2.72 C/O ratio, 1.48 O/OH ratio and 2.71 C/O ratio and 1.57 O/OH ratio respectively, similar to those values reported in the NATURE NANOTECHNOLOGY 7

8 literature. 7, 8 These sheets were used as the top and bottom sheets of the membrane. CNT models were prepared using either a SWCNT (9,9) or a DWCNT with inner (9,9) and an outer tube (14,14). OH groups were added at the edge of these CNTs. From the MD simulations the diffusion coefficient was calculated according to the following equation: NN DD = 1 NN ( rr ii(tt ii ) rr ii (0) 2 ) 6tt ii ii=1 Here N is the number of water molecules that are in the membrane during a t i time step starting from first entrance into the membrane until the end of the MD simulation (t f ). r i (t i )-r i (0) is the distance between time steps t i and t f. NATURE NANOTECHNOLOGY 8

9 Potential mapping procedure The potential energy between the salt ions and the membrane model was mapped to study the mechanism of salt rejection. The mapping surface is calculated along the membrane curved surface. The membrane surface is considered as the carbon atoms of GO sheet. The sampling grid size was set to 0.5 Å and the Na + and Cl - ions were located at the grid centre. The potential energy was calculated as the sum of the non-bonded interaction, Van der Walls, VdW, and Coulomb interaction using the following equation VV = 4εε iiii [( σσ iiii jj 12 ) rr iiii ( σσ 6 iiii ) rr ii ] + qq iiqq jj 4ππεε 0 rr iiii jj i indexes are the Na or Cl atoms, j indexes are the membrane s atoms. The first term is the VdW interaction considering Lenard-Jones (LJ) potential and the second term corresponds to the coulomb force interaction. LJ parameter εε iiii were 0.13 kcal/mol and 0.10 kcal/mol and σσ iiii, were set to 2.35 A and 4.40 for Na and Cl respectively. Charges of Na and Cl, q i, were +1.0 and Other atom combinations of LJ parameters were calculated by Lorentz-Berthelot combination rules. The potential mapping surface for Na and Cl was calculated in XY plane with Z=0, on GO surface. We also plotted the differential mapping, which was calculated by subtracting the top GO potential map from the entire interaction potential map. The average potential for each model was calculated in the range of to 10 7 J/mol. Potentials above 10 7 J/mol was ignored because it would not allow the passage of Na + and Cl -. For potential mapping, two types of GO sheets were studied, (1) 0.7 nm pore size average and (2) one 2 nm pore. For each model both GO sheets had the same pore sizes. The GO sheets were intercalated with a 4 nm square BLG, SWCNT (one to three SWCNTs) and DWCNT (one and two DWCNTs) for GO pores of 0.7 nm (Supplementary Fig. 16a-i). For a 2 nm pore in GO one 4 nm square BLG, three SWCNTs and two DWCNTs were mixed; these where the pristine models. NATURE NANOTECHNOLOGY 9

10 Then the pristine models where one set was intercalated with Ca 2+ ions and left to optimise their structures under ambient pressure and 1 ns. Those ions outside of the membrane with low interaction with the model were disregarded. The other set with deoxycholate (DOC) had 40 DOC molecules per GO sheets (top and bottom) on both sides, the same manner with the other carbon nanomaterials, and left to optimise their structures under ambient pressure and 1 ns. Later they were combined to make a model with two GO sheets and other carbon nanomaterials in between, all with the DOC previously optimised on each element. NATURE NANOTECHNOLOGY 10

11 Experimental characterization Spray-coated GO/FLG membranes deposited on Si substrates and PSU membranes were characterised by scanning electron microscopy (SEM, JEOL JSM-6335F). GO/FLG membranes on Si substrates were analyzed by X-ray photoelectron spectroscopy (XPS Kratos, UK, Al KR line in an Axis-Ultra, at 10-9 Torr using the MgKa line. The pass energy of the analyzer was set at 160 ev and 20 ev for the wide scan and narrow scans, respectively), attenuated total reflectance FTIR (NICOLET 6700), Raman spectroscopy (Renishaw micro-raman using the 532 nm excitation wavelength). Zeta potential was measured with a SurPASS 3 electrokinetic analyser by Anton Paar. NATURE NANOTECHNOLOGY 11

12 Supplementary Figure 3. Scanning electron microscope images of graphene oxide and fewlayered graphene. SEM images of pure (a and b) GO and pure (c and d) FLG. The GO sheets exhibit an average length of 12 m, whereas the FLG an average of 3 m for large sheets and 0.5 m for small sheets. TEM images of (e and f) FLG and (g and h) and GO. Here it is possible to observe the layers of FLG and on GO pores and the edges can be observed. NATURE NANOTECHNOLOGY 12

13 Supplementary Figure 4. Scanning electron microscope images of the membranes. Samples with (a) 0% (GO0FLG), (b) 15% (GO15FLG) and (c) 100% (GO100FLG) GO content. Top images correspond to the surface with an inset at higher magnification. Bottom images are cross sections with the polysulfone substrate, dashed line indicates interphase between the GO layer and the polysulfone substrate. NATURE NANOTECHNOLOGY 13

14 Each stage was assigned as: (1) pristine after spray coating, (2) 100 o C treatment and (3) CaCl 2 crosslinking followed by rinsing. 10, 11 The ATR-FTIR spectra exhibited typical spectra for GO (Supplementary Fig. 5a) with peaks corresponding to C-O (alkoxy/alkoxide, 1042 cm -1 ), C-O (carboxy, 1410 cm -1 ), C=C (aromatic, 1620 cm -1 ), C=O (carboxy/carbonyl, 1716 cm -1 ) and OH (3300 cm -1 ). Here the C=C peak was used as a reference to compare peak intensities the CaCl 2 treatment the C-O and C=O from the carboxyl group decrease in intensity from 0.97 to 0.77 and 0.70 to 0.51, respectively, caused by the coordination between the carboxylic acid and the Ca 2+ ions. 10 In Raman spectroscopy, the D-band (1350 cm -1 ) and G-band (1600 cm -1 ) were observed (Supplementary Fig. 5b) and the I D /I G ratios corresponded to 0.92, 0.84 and 0.87 for stages (1), (2) and (3), respectively. Thus, indicating that there is a small reduction of GO after the low heat treatment and after the CaCl 2 treatment, a slight disorder is introduced. Regarding the XPS measurements, the well-known spectrum for GO was observed where the deconvoluted peaks were assigned as C=C (284.3 ev), C-O (286 ev) and COO (288 ev) (Supplementary Fig. 5c,d and e). The C=C peak increased in intensity after each stage and the C/O ratio varied as 2.47, 2.64 and 2.32 for stages (1), (2) and (3), respectively. These results are expected for a partial reduction in thermal treatment 12 and alkali metal cross-linking. 10 In addition, only Ca +2 remains within the membrane due to crosslinking with GO (Supplementary Fig. S6). NATURE NANOTECHNOLOGY 14

15 Supplementary Figure 5. Spectroscopy and diffraction measurements of graphene oxide/few-layered graphene membrane. (a) ATR-FTIR of the freshly deposited GO, the same film after drying at 100 o C and after drying and calcium chloride treatment, (b) Raman spectra of the GO, GO-100 o C and GO-100 o C-CaCl 2. XPS C1s core of (c) GO, (d) GO-100 o C and (e) GO-100 o C-CaCl 2. NATURE NANOTECHNOLOGY 15

16 Supplementary Figure 6. X-ray photoelectron spectroscopy of as prepared membranes. XPS spectra of the GO-100 o C-CaCl 2 sample for (a) Cl 2p and (b) Ca 2p core level. NATURE NANOTECHNOLOGY 16

17 Supplementary Figure 7. Water shear resistance photographs. Photographs of pure GO membranes with PVA and (NO PVA) without PVA before cross-flow operation (0 ml/min) and with different cross flows between 200 ml/min and 1000 ml/min. Each photograph was taken after one hour of operation at determined cross flow. NATURE NANOTECHNOLOGY 17

18 Attenuated total reflectance FTIR (ATR-FTIR) was used to study the effect of heat treatment upon the PVA structure. In Supplementary Fig. 8 the peak assignments can be observed for pure PVA and heat treated PVA (PVA-100 o C), with clear change between both samples. The increase in the peak intensity at 1141 cm -1 compared to the peak at 854 cm -1 indicated that PVA had a higher degree of crystallinity after the heat treatment. 13, 14 Supplementary Figure 8. FTIR spectra of pristine PVA and PVA heated at 100 o C for one hour. NATURE NANOTECHNOLOGY 18

19 By X-ray photoelectron spectroscopy (XPS) the PSU substrate surface modification exhibited a higher content in C-O groups with a reduction in sulphur content (Supplementary Fig. 9 and Table S1). Supplementary Figure 9. X-ray photoelectron spectra of polysulfone treated with polyvinyl alcohol. XPS spectra for (a,b and c) PSU and (d,e and f) PSU-PVA of the (left) S 2p, (middle) C 1s and (right) O 1s core levels. NATURE NANOTECHNOLOGY 19

20 Supplementary Figure 10. Desalination performance of the GO/FLG membranes at different GO content and pressure. 100% GO (GO100FLG), 75% GO (GO75FLG), 60% GO (GO60FLG), 45% GO (GO45FLG), 35% (GO35FLG), 25% GO (GO25FLG) and 0% GO (GO00FLG). The membranes were studied at transmembrane pressures between 2 and 5 MPa. For each GO content, three membranes were studied. Permeate against (a) transmembrane pressure and (b) GO content. NaCl rejection against (c) transmembrane pressure and (d) GO content. Vertical lines represent standard deviations calculated from three measurements.. Each data point was taken after more than 20 hours when NaCl rejection and permeate was stable. NATURE NANOTECHNOLOGY 20

21 Transmission electron microscope images (TEM) were taken of the membrane with 90% GO and 10% DWCNT. The low magnification images reveal that the maximum spacing between individual DWCNT or bundles is in the order of 100 nm, and their length is larger than 500 nm (Supplementary Fig. 11a). From the high magnification images, it is possible to observe bundles with up to 4 DWCNT that are observable (Supplementary Fig. 11b). The image with an individual DWCNT exhibits diverse pores in the GO sheet with diameters of ca. 4 nm (Supplementary Fig. 11c). Supplementary Figure 11. Transmission electron microscope images of the GO/DWCNT samples. Membranes with 90% GO. (a) Low and (b) and (c) high magnification. Images show the distribution of DWCNTs within the membrane. 21 NATURE NANOTECHNOLOGY 21

22 Supplementary Figure 12. Zeta potential of pure graphene oxide membranes at different preparation stages. Heat treated at 100 o C (GO-100 o C), GO with DOC heat treated at 100 o C (GODOC-100 o C), pure GO heat treated at 100 o C then treated with CaCl 2 (GO-100 o C-CaCl 2 ) and GO with DOC heat treated at 100 o C then treated with CaCl 2 (GODOC-100 o C-CaCl 2 ). Each curve is an average of two independent measured membranes. NATURE NANOTECHNOLOGY 22

23 Pure GO (GO100FLG) was measured as a reference, and exhibited a typical 001 peak for GO at 2 11 o, corresponding to an interlayer distance of 7.7 Å. When FLG is mixed with the GO the interlayer spacing can increase up to 12 Å, and a peak corresponding to FLG appears at 26.6 o due to an interlayer spacing of 3.34 Å. The low content of GO at 25% could favor the restacking in FLG giving rise to the peak at The shift of the 001 GO peak towards lower 2 values rather than a broadening of the peak centered at 2 11 o suggests that the GO/FLG had a good alternation of both types of layered materials. Supplementary Figure 13. X-ray diffraction patterns of GO/FLG membranes. Samples were supported on Si substrates. GO content where 100% (GO100FLG), 75% (GO75FLG), 60% (GO60FLG), 35% (GO35FLG) and 25% (GO25FLG). NATURE NANOTECHNOLOGY 23

24 Supplementary Figure 14. Desalination with GO/FLG using diverse salts. The prepared membranes contain 25% GO (GO25FLG). The studied salts were Na 2 SO 4, NaCl, MgSO 4 and MgCl 2. The data was collected at 5 MPa after more than 20 hours when NaCl rejection was stable. Vertical lines represent standard deviations calculated from three measurements. NATURE NANOTECHNOLOGY 24

25 Supplementary Figure 15. Salt rejection against NaCl concentration. Here the GO35FLG membrane was studied. Vertical lines represent standard deviations calculated from three measurements. NATURE NANOTECHNOLOGY 25

26 Supplementary Figure 16. ATR-FTIR of GO treated with chlorine. The samples were supported on Si. The curves show the sample before and after exposure to NaOCl (200 ppm) for 24 hours. After exposure, the sample was rinsed with water. The black arrow indicates an increase in oxidation. NATURE NANOTECHNOLOGY 26

27 Supplementary Figure 17. Dye rejection of Acid Blue 9 and Rhodamine B. Using GO35FLG membrane, measured at 0.6 MPa. Vertical lines represent standard deviations calculated from three measurements. NATURE NANOTECHNOLOGY 27

28 DOI: /NNANO Supplementary Figure 18. Molecular dynamics simulations of Graphene oxide/carbon nanomaterial membranes. Water flow entering the membrane at (a) 130 MPa and (b) 300 MPa against simulation time (ns). (c) Final step of the MD simulations with BLG, DWCNT, SWCNT and nothing in-between two layers of graphene oxide, these are the structures with 300 MPa applied above and beneath the membrane after 5 ns of simulation. Only the H2O molecules that penetrate the membrane are shown and for better visualisation, the nanopores are highlighted with green contours. 28 NATURE NANOTECHNOLOGY 28

29 Supplementary Figure 19. Studied models for simulations. (a-f) Have two sheets of GO with pores near 0.7 nm width. (a) Is pure bilayer GO and bilayer GO with carbon nanomaterials between both GO sheets: (b) bilayer graphene (BLG) 4 nm square, (c) one SWCNT, (d) three SWCNTs, (e) one DWCNT and (f) two DWCNTs. (g-i) Consist of two sheets of GO with one 2 nm pore with (g) 4 nm square BLG, (h) three SWCNTs and (i) two DWCNTs. All the shown models were intercalated with (j) Ca 2+ ions and (k) deoxycholic acid; the shown models were selected as examples to visualise the structures. NATURE NANOTECHNOLOGY 29

30 Supplementary Figure 20. Potential mapping of GO/BLG/GO. BLG is 4 nm square and GO has (top) pores of 0.7 nm and (bottom) one 2 nm pore. These were evaluated against (left) Na + and (right) Cl -. The same analysis was done for all the models in Table S3. NATURE NANOTECHNOLOGY 30

31 Table 3. Calculated average potential (Jmol -1 ) for the studied areas in each model against Na + and Cl -. Na+ Cl- Pristine Ca 2+ DOC DOC/Ca 2+ Pristine Ca 2+ DOC DOC/Ca nm pore GO/GO GO/BLG/GO GO/1SWCNT/GO GO/2SWCNT/GO GO/3SWCNT/GO GO/1DWCNT/GO GO/2DWCNT/GO nm pore GO/BLG/GO GO/2DWCNT/GO GO/3SWCNT/GO NATURE NANOTECHNOLOGY 31

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