Supporting Information

Transcription

1 Supporting Information Ultrafast Nanofiltration through Large-scale Single-layered Graphene Membranes Yanzhe Qin, Yongyou Hu*, Stephan Koehler, Liheng Cai, Junjie Wen, Xiaojun Tan, Weiwei L. Xu, Qian Sheng, Xu Hou, Jianming Xue, Miao Yu*, David Weitz *correspondence S-1

2 This PDF file includes: Materials and Methods Figures. S1 to S6 Materials and Methods Graphene transfer. We purchase a high-quality CVD graphene sheet on a flat copper substrate from Hefei Vigon Material Technology Co., LTD. We prepare a Polythehersulfone (PES) N-Methyl-2-pyrrolidone (NMP) solution with isopropanol as additive and take precautions to prevent exposure to humidity in air. This solution is cast onto the exposed graphene surface with resulting thickness of about 100 μm (the membrane thickness can be controlled by changing the solution drainage time after casting and before coagulation in water. In our process, we keep the CVD graphene upright for a certain time on top of paper towel to get rid of excess solution. The longer we allow the solution to flow down, the thinner the membrane is). Slide the coated graphene structure into tap water with PES solution side facing up, making a 30 angle with horizontal, at a constant speed and change water after 20 minutes. PES forms smooth surface during phase inversion process which is well suited for supporting graphene. Additionally, as an engineering polymer, PES has good chemistry/mechanical/thermal stability: it is tolerant of most of chemicals, can be used for a long term between C and has excellent dimension thermal stability. It is widely used in membrane industry and known to be able to support high pressure. To simulate industry fabrication conditions, PES resin is sourced from Solvay group, TX, USA and tap water is used for coagulation. We float the graphene structure on persulfate ammonia until the copper substrate is fully etched. The resulting membrane needs to be properly stored in water below 60 C, totally drying out or higher temperature causes the collapse of finger-like pores in supporting layer and cracks in graphene. Membrane quality inspection. A well transferred graphene membrane appears grey while the polymer substrate is white. Big missing piece can be told from comparison of grey graphene and white substrate. This only happens when content of casting solution has changed (e.g. moisture dissolves in.) To examine quality of graphene transfer and fingerlike pore density, sample is sputter with 5 nm platinum and SEM is conducted at 5 KV for surface feature (Fig. 2a) and 10 KV for underlying structure. Graphene coverage area looks blur and whiter part is graphene covered PES and darker area is valid finger-like pore that underneath graphene. The crack/defect looks completely dark if no PES support or PES support will be exposed. The Raman spectroscopy is conducted for graphene on copper, graphene transferred to polymer and PES raw material at 532 nm and power 140 mw at maximum. Labspec 6 is used for analysis and baseline adjustment. Defect sealing. We punch a hole on nitrile rubber sheet and cut a square in a 3M double stick tape of similar size. To stabilize the graphene membrane on rubber, we press the double stick tape against graphene around the edge of the hollow square, and then peel off the liner and place the graphene membrane to the bottom of the pore on nitrile rubber sheet with graphene side facing up. We place the membrane in an Amicon Stirred Cells 8400 and filter an aqueous polystyrene particle suspension with diameters ranging from S-2

3 30 nm to 2 μm at 5 bar for 30 minutes. The particles fill the holes and if transferred graphene is of high quality, then the flux is negligible. Ionic conductance measurement. We first stabilize the graphene membrane on nitrilerubber in a similar way as describe in defect sealing section above. The membrane is placed in Amicon Stirred Cells 8400 and filter 1 M potassium chloride at 5 bar until seeing liquid at outlet. We then place the membrane in a side-to-side homemade cell which is filled by 1 M potassium chloride as conducting solution. Two Ag/AgCl electrodes with enough area are prepared and placed in different side of the cell. Apply bias within 300 mv using open circuit voltage as center voltage to avoid breakage of graphene. The slope of the resulting curve should be highly linear and the resistance is calculated around the voltage point where current is zero. Atomic pore creation. We bombard the graphene by a focused Ga + ion beam (FIB) at 1 kv with dosage of 7 x /CM 2 at incident angle of 0 using 4.2 na. To enlarge the lattice defects and create atomic-scale pores, we etch the membrane in acidic potassium permanganate (1.875 mm KMnO 4 in 6.25% H 2 SO 4 ) which creates ~ 3.89 x /CM 2 nanopores, of which 4% is actually water permeable (pore size > nm). Rejection test. Sterlitech HP 4750 Stirred Cell is used for rejection test and nitrogen cylinder is used as pressure source, as shown in Fig. S4. The membrane and stir bar are positioned inside stirred cell on top of a working stirring plate that set to ~700rpm. The graphene side is facing solution and contact with stainless steel holder or sharp things should be avoided. The permeate is collected from outlet of the stir cell after stabilization; for conductance measurements, pure water is firstly used as feed to thoroughly wash the feed tank and permeate line until the permeate conductance is lower than 20μs cm -1. And then pure water will be replaced by 200mL MgSO 4 or NaCl solution with conductance more than 1000μs cm -1 as feed. To test graphene s selectivity, we use Shimadzu TOC-V ws total organic carbon analyser for organics (Dextran, sucrose, β- cyclodextrin, TOC~200mg/L), VWR symphony SB880PC conductance meter with K=1.0 for MgSO 4 or NaCl, and Agilent Cary 60 UV-Vis spectrometer for K 3 Fe (CN) 6. Water flux is test by counting the time for each drop, at least 4 close data points in a row are collected for each flux data at certain pressure. Bacterial adhesion to filter membranes. A 5 x 5mm graphene composite membrane and a Dows NF 270 polymer filter is positioned in petri dishes and sterilized on both sides by exposing to UV for 30 mins. A 3 µl 10 7 CFU/mL E. coli bacterial aqueous suspension is pipetted onto the membrane surface and incubated for 5h at 37 C. A gentle washing by pipetting 1 ml water onto the membrane surfaces is repeated 10 times. To dissociate the bacteria adhered to the membrane surfaces, we position membranes in centrifuge tubes and add 2 ml normal saline followed by vortex oscillation. 0.1 ml of liquid is retrieved, added to a standard agar culture medium and incubated for 24 hours at 37 C. To detect bacterial adhered to the membrane surfaces using SEM imaging, we repeat the sterilization process and immerse the membranes in 20 ml of the bacteria suspension and incubate for 48h. Sterilized water is used to wash off the suspended bacteria by pipetting 1 ml water onto the membrane surfaces 10 times. To fix the bacterial adhered to the S-3

4 membranes for SEM imaging, a 2.5% glutaraldehyde is used. The membranes are cleaned three times by a 0.2 M phosphate buffer and to dehydrate the bacteria are successively immersed in 30%, 50%, 70%, 85%, 95% ethanol for 10 minutes each. After that, the membranes are immersed twice in pure ethanol for 20 minutes each, freeze dried and sputtered with gold for SEM imaging. S-4

5 Figure. S1. Phase inversion process. (a) The schematic of phase inversion method. (b) Three phase diagram theory of formation of membrane structure during phase inversion. S-5

6 Figure. S2. Graphene and PES supporting substrate characterization. (a) SEM image of the graphene-pes membrane for 300 µm width area. (b) SEM image of a single finger-like pore covered by integrated graphene on the composite membrane; a 20nm width carbon nanotube can be observed which indicates image resolution is higher than 20nm. (c) Raman spectrum of the PES raw material, graphene on copper and the PES - graphene membrane. (d) Pore size distribution of the graphene-pes membrane for 35% void graphene composite membrane. S-6

7 Figure. S3. Two morphologies of intersection of peeling PES composite membrane from graphite after coagulation. (a) The dominant morphology after peeling off PES composite membrane. (b) In << 1% area net-like layer still stay on PES supporting layer. The net-like PES layer binds very strongly to honey-comb lattice of carbon, which is evidenced by tearing that occurs within the PES substrate rather than at the graphite-pes interface when the polymer is cured and peeled off the graphite surface. S-7

8 Figure. S4. Set-up for nanoparticle sealing and high pressure filtration. (a) The photograph of nanoparticle sealing set-up. (b) The photograph of high-pressure water filtration set-up. (c) The photograph of side-to-side cell for ionic conductance measurement in solution. S-8

9 Figure. S5. SEM image for graphene after high pressure filtration. The SEM image of the graphene membrane after filtration of K 3 Fe(CN) 6 at 50 bars, no breakage is observed under SEM. S-9

10 Figure. S6. SEM images of PES supporting layer bottom surface before and after removal of the microporous skin layer. (a) The SEM image of membrane bottom surface before removal of the skin layer. (b) The SEM images of membrane bottom surface after removal of the skin layer. S-10