Water desalination using nanoporous single-layer graphene

Transcription

1 Sumedh P. Surwade, 1 Sergei N. Smirnov, 2 Ivan Vlassiouk, 3* Raymond R. Unocic, 4 Gabriel M. Veith, 5 Sheng Dai, 1,6 Shannon M. Mahurin 1 * 1. Estimation of the defect density from Raman spectra SUPPLEMENTARY INFORMATION DOI: /NNANO Water desalination using nanoporous single-layer graphene For the defect density estimation, the ratio of the G and and D lines, I G /I D, obtained from the Raman spectra is usually used. This ratio is expected to depend not only on the defect density, but also on the types of defects, their size and arrangement against each other. Two empirical equations (Eq. S1 and S2), provide rough estimations for the defect density derived from I G /I D ratio when the defect density is not too high: 1,2 ) ) ) (S1) (S2) where E l is the excitation energy in ev, I G and I D are the intensities of G and D bands, respectively. The L a ~ I G /I D dependence should be valid for large L a down to 2 nm. 3 For L a less than 2 nm, L a is proportional to (I G /I D ) -1/2 instead. In this work we used two excitation wavelengths for collecting Raman spectra, 514 nm (2.41eV) for oxygen plasma treatment and 633 nm (1.96eV) for bombardment by Ga + and electrons. A Renishaw 1000 Raman spectrometer with a Leica microscope were used to collect the data. Sample used for STEM imaging (Fig.3 of the main text), had I G /I D ~1, which corresponds to different values of L a from the two equations: L a ~16 nm from Eq. S1 and L a ~ 5 nm from Eq. S2. Estimated from STEM images value of L a lies in between the two, L a ~ 10 nm (see Supplementary Fig. 1). Pristine graphene (as synthesized in our atmospheric pressure CVD setup) has I G /I D ratio more than 10 for 633nm excitation, which translates to the defect density less than 1 per 0.25μm 2. The defect density increases by over 3 orders of magnitude in graphene membranes prepared by oxygen plasma treatment (1 per 100nm 2 ). Scanning transmission electron microscopy (STEM). Aberration-corrected STEM imaging was performed using a Nion Ultra STEM equipped with a cold-field emission gun as the electron source and capable of aberration correction of the third and fifth order aberrations. 4 The STEM was operated at 60 kv, which is below the knock-off damage of graphene. Medium angle annular dark field (MAADF) STEM images were acquired with a convergence semi-angle of 30 mrad and a mrad collection semi-angle. Prior to STEM imaging, the graphene specimens were baked for 8 hrs under vacuum at 160ºC in order to remove surface contamination. All STEM images presented here are raw and unprocessed. NATURE NANOTECHNOLOGY 1

2 Supplementary Fig. 1. Large area, single layer graphene STEM image. (A,B)The same sample as shown in Fig. 3 of the main text. Pore density is estimated to be 1 pore per 100 nm2, which roughly translates to La~10nm for average distance between the point defects. Pristine graphene (C,D) prior to plasma treatment does not have any visible pores confirming very large La measured from their Raman spectra. 2

3 A 2.0 B C I D /I G I D /I G Plasma etch time (s) L D (nm) Supplementary Fig. 2. (A) Raman spectra of suspended graphene treated by plasma etch for different times. (B) Variation of I D /I G as a function of time for plasma etching treatment (C) Correlation between of the average distance between the defects, L D, and I D /I G. The I D /I G ratio increases with the plasma etch time due to introduction of additional defects. 3

4 2. Fabrication of graphene membranes The synthesized graphene was transferred onto the SiN microchip device, where a 5 μm diameter hole was drilled by FIB in 300 nm SiN film free standing on Si. Each microchip was visually examined using SEM to ensure lack of ruptures or tears in the graphene. Supplementary Fig. 3 shows the SiN microchip devices that were used to collect the Raman spectra in Supplementary Fig. 1B (and Supplementary Fig. 2). Only the two controls (C1 and C2) show clear ruptures while others are intact with no visual holes. This quality control was performed on all microchip devices prior to and after oxygen plasma treatment. Nanopores were produced by placing a sample (with graphene on a whole in SiN) into a plasma cleaner (Harrick plasma, Ithaca, NY) at 20 W RF power in a vacuum chamber with a flow of pure oxygen at 550 mtorr. The samples were always placed at the same position inside the chamber but for different times. I D /I G SEM image

5 C1 C2 Supplementary Fig. 3. SEM images of the samples with different defect density used in measurements and presented in Fig. 2B (main manuscript). For each image, the scale bar is 1 μm. 5

6 3. Measurements of the ionic current and of the water flow through nanopores induced by osmotic pressure gradient. The ionic current (type two of the experiments) was measured in the experimental setup shown in Supplementary Fig. 4 which is similar to that reported earlier. 5 In brief, a sample with graphene membrane over 5 μm hole in SiN (5) was mounted in a custom made electrochemical cell with two Ag/AgCl wire electrodes (2) on both sides of the membrane (Supplementary Fig. 4). The ionic current was measured by a Keithley 6487 picoammeter interfaced via Matlab. The cell had two quartz windows (4) allowing observation of the membrane surface by optical microscopy. Inspection of the membrane under an optical microscope was crucial for confirmation of complete wetting by electrolyte, absence of bubbles and identification of any possible debris clogging the pore. Supplementary Fig. 4. Custom made electrochemical cell. (A) Sketch of the experimental electrochemical cell with graphene membrane separating the two PDMS inserts where solutions could be independently exchanged. (B) Disassembled cell: (1) tubing for solution exchange. In the osmosis experiment, tubings were connected to glass capillaries for monitoring the water level difference, (2) Ag/AgCl wire electrode, (3) PDMS insert, (4) quartz window, (5) Si/SiN 6

7 chip and (6) Aluminum body of the cell. (C) Photograph of the assembled cell. Pens are shown for the scale. Supplementary Fig. 5B shows a typical I-V curve for the 5 μm hole in SiN without graphene (squares) and the current calculated as the limiting Hall resistance: 6 DV I (S3) ρ where D is the pore diameter, V is the applied voltage and ρ is the solution resistivity. Our experimental setup allows measurements of the ionic current down to ~10 pa, which translates to 10 GΩ resistance limit at the maximum applied voltages used in this work - 0.1V (Supplementary Fig. 5C). Higher voltages applied across atomically thin graphene membranes often resulted in a peculiar behavior which will be the topic of a separate publication. Supplementary Fig. 5. Measurement of ionic current across graphene membrane. (A) Sketch of the experimental electrochemical cell. (B) Ionic current through 5μm hole in 300 nm thick SiN membrane using 1M KCl solution (squares) and the calculated current corresponding to the Hall resistance described by Eq. S3 (line). (C) Resistances of the graphene membranes vs. I D /I G. C-1 7

8 is a control sample with partially raptured graphene membrane, whereas C-2 is the sample without graphene. I-V curve for similar to C-2 sample is shown on Supplementary Fig. 5B. In the type four of experiments, water flux through nanoporous graphene membranes was measured in the same geometry by connecting two capillaries with 0.8 mm radius to the cell shown in Supplementary Fig. 4. One cell compartment (volume of ~0.5 ml) was filled with 1M KCl while the other one was filled with DI water. The water level difference monitored in a course of 24h, grew in the first ~ 7h but eventually saturated at the final level difference of 1.5 mm which translates to ~3μL of passed water (Supplementary Fig. 6A). Such water flux saturation behavior cannot be explained by concentration polarization effects since the experiment performed with a magnetic stirrer in each of cell compartment yielded similar water flux kinetics. The ionic currents in different electrolytes KCl, NaCl and LiCl measured with the same membrane (I D /I G ~1) appeared almost identical when normalized to the bulk solution conductivities of these electrolytes, as shown in Supplementary Fig 6B. Slight decrease in the conductivity through membrane with increasing size of the hydrated cation (Li + > Na + >K + ) suggests that the pores are very small. However, that difference is within our experimental error. Supplementary Fig. 6. Measurement of the water flux induced by osmotic pressure and ionic current across a nanoporous graphene membrane using different electrolytes. (A). Water flux induced by osmotic pressure (B) Normalized to the bulk conductivities ionic currents across a membrane in different electrolytes. All I-V curves measurements were performed within 30 min after assembly. 4. Measurement of free flow water transport. Water transport measurements in the type one and type three experiments, were performed by attaching the SiN microchip device with graphene onto the lid of a container. After allowing the epoxy to cure, the lid was placed on the container partially filled with DI water (or KCl solution) and sealed. For pure DI water transport measurements (type one experiments), the container was then inverted so that the water was in contact with the graphene. The entire assembly was placed in an oven maintained at 40 C. The mass of the container was measured periodically to determine the mass loss and the water transported through the membrane. Blank SiN microchips 8

9 without a hole and with graphene not exposed to oxygen plasma were similarly measured to ensure that no water was lost through the epoxy seals or through gap between graphene and SiN membrane. For the type three experiments, the container was filled with a low concentration KCl solution and placed on top of a second empty container to collect transported water. This second container had a hole punched on the side to keep the pressure inside of it in equilibrium with surrounding. The conductivity of the initial solution was measured before the transport measurements and the collected water was carefully removed from the lower container for weighing and conductivity measurement. Supplementary Fig. 7. Schematics and images showing the experimental set-up. Bottom container always had a small hole (~1mm in diameter) to equilibrate the pressure with the atmosphere. The osmotic pressure gradient for the salt concentrations used in this approach was equal to ΔΠ = icrt ~30kPa (~6 mm KCl concentration difference between the top and bottom containers). The overall additional pressure to the solution in the top container had several contributions: a) increased water vapor pressure which was approximately 8 kpa at 40 0 C; b) increased pressure of air above the solution due to a drop of its solubility in water and expansion with temperature rise from 20 o C to 40 o C, ~8 kpa; c) hydrostatic pressure of ~1 kpa for ~10 cm water height. When added together (~17 KPa) and subtracted by the vapor pressure in the lab (~1KPa, 20 C, at 50% humidity), it gives the driving pressure of ~16 kpa which is less than the estimated osmotic pressure gradient. Other contributions are not as easy to assess and include capillary effects for water spreading on the backside of graphene surface. That surface is hydrophilic 7,8 and the driving pressure for its wetting can help to overcome the osmotic pressure. 9

10 Water penetration through the SiN/graphene interface is also a possible water transport channel especially after oxygen plasma treatment which makes graphene hydrophilic 9 but we have demonstrated that its contribution is most probably insignificant (see later). For the control sample with no graphene, the measured water flow rate, ~0.30 g h -1 (or g m -2 s -1 ), was close to that estimated from the Hagen Poiseuille expression (with entrance/exit loss): 3 ΔP R Q = μ L C + 8 (S4) πr where C~1.5 is the loss coefficient, R and L are the SiN pore radius and length, respectively, ΔP is the driving pressure and μ is the water dynamic viscosity. 10 Measuring the water transport at different temperatures (20ºC, 40ºC, 50ºC, 60ºC) suggests that for at least 5μm hole without graphene on top, the water vapor in the top container is a major driving force (Supplementary Fig. 8) and water transport occurs in a liquid form following Hagen-Poiseuille equation (see eq.2 in main text). Water flux (g/h) Water flux (g) Water vapor pressure (KPa) Eq Temperature ( o C) 20 0 ΔP, Water vapor pressure (KPa) Supplementary Fig. 8. Flux of water through a 5μm hole in 300 nm thick SiN membrane. Points are experimental data. Black curve is eq. 2 of the main text, where the pressure difference was taken to be equal the saturated water vapor pressure (red curve). 5. Nanopores hydrophobicity. Nanopores with large aspect ratios (length>>diameter) must be hydrophilic in order for water to fully wet the pores spontaneously. Pores with diameters of 1 nm and modest surface contact angle of 95º require pressure in access of 250 atm, as Laplace-Young equation (S5) suggests. 10

11 = (S5) Here ΔP is the pressure difference, γ is the surface tension of water, θ is the contact angle, and D is the diameter of the pores. However, the accuracy of the description of nanopores with the diameters approaching molecular size and the aspect ratio less than one (length<diameter) by eq. S5 is questionable. For example, it is well known that carbon nanotubes with diameters less than 2 nm can be easily wetted 11 despite the predicted by eq. S5 enormous pressure required for their wetting. In any case, if the surface has the contact angle less than 90º, the nanopores of any size will be spontaneously filled with water. Even though the exact structure of the graphene pores generated by oxygen plasma in this work is unknown, treatment by oxygen plasma most likely results in hydrophilic pores. For example, oxygen plasma treatment has been shown to decrease graphene contact angles below 90º at least for graphene on substrates. 7,8 Other treatments also result in hydrophilic termination of the nanopores. For example, there are reports of nanopores in graphene with <10nm in diameter produced by electron beam which are also spontaneously wetted in aqueous solution Statistics for data reproducibility and major difficulties in graphene membrane preparation. We have prepared more than 200 Si/SiN chips in our study. After graphene transfer and thermal annealing >140 chips (70%) had no obvious tears, proved by assessing the membrane integrity by SEM, ionic current values and Raman spectra. To this end, we note that the overall successful yield of membranes which showed water transport was around 20% and varied from batch to batch (dependending on I D /I G and other factors). Membranes which showed water flux, demonstrated consistent water permeation values. The salt concentration of the permeate solution ranged from 11 μs/cm to 65 μs/cm with our best value at less than 11 μs/cm. Besides the I D /I G, other factors responsible for a low yield of functional devices can be attributed to several major factors: 1. Contamination of graphene surface by PMMA residues. It is well known that even after PMMA dissolution in various solvents and thermal annealing, the polymer residues partially remain and contaminate graphene surface. 13 Such contamination is visible in our STEM images. We found that freshly transferred graphene (within several days after PMMA spin coating) resulted in a higher yield of functional devices. This can likely be attributed to polymer aging that causes stronger interactions with the graphene surface over time and thus larger contamination as a result. Substitution of PMMA by polycarbonate did not result in noticeable improvement of functional device yield. During the final stages of manuscript preparation, we have found in the literature that annealing in CO 2 rather than in Ar/H 2 can result in much cleaner graphene surface. 14 Unfortunately, we saw that such treatment resulted in membranes having higher probability of tear formation. We also have found that our oxygen plasma treatment for nanopore creation also results in a cleaner surface compared to other methods of nanopore production, such as irradiation by energetic particles, possibly through etching of the PMMA residue by O2 plasma during nanopore preparation. 11

12 2. Mechanical strength. Transferred pristine graphene membranes on 5μm holes are quite strong and can withstand large pressure differences up to several atmospheres. Plasma treated graphene is much weaker 15 and some membranes burst during the experiments which contribute to lowering yield of functional devices. 3. Airborne contaminates. To this end, we saw that freshly plasma treated samples had better reproducibility of water fluxes as compared to the samples prepared several days before the experiment. It is well documented that various hydrocarbons readily absorb on the graphene surface and thus contaminate the surface. This may be the origin of the observed irreproducibility of the aged samples. The characteristic time of such uncontrollable hydrocarbon adsorption is around several minutes, 16,17 which is much longer than the time required for cell assembly for water transport and ionic current measurements. 4. Water purity. In all our experiments we have used nanopure DI water with resistivity of no less than 18 MΩ, treated by UV to ensure decomposition of organics and characterized by TOC less than 5 ppb. Nevertheless, we have frequently observed water flux decrease with time most likely due to uncontrollable contamination in water. Filtering solution through 20nm alumina filters did not improve the yield of stable devices. 7. Other means of defect formation in graphene. Bombardment by electrons and Ga ions. For comparison to the plasma etched samples, we also prepared porous graphene using electron and ion bombardment. Single layer graphene was transferred onto a silicon nitride TEM grid with 2.5 μm holes. Bombardment by electrons was done using Zeiss Merlin SEM by scanning the area for desired period of time, probe current and accelerating voltage. Irradiation by ions was done using dual beam FEI instrument by scanning the area for desired period of time. Accelerating voltage of 30kV and 1-10pA current beam was used. Bombardment by electrons and ions did not result in creating nanopore large enough for water molecules despite the broad range of dosage (up to 3100 electrons/nm 2 and 4ions/nm 2 ) corresponding to the density of defects up to 1/nm X-ray photoelectron spectroscopy of graphene X-ray photoelectron measurements were performed using a PHI 3056 XPS spectrometer utilizing an Al X-ray source operated at 350 W and 15 kv and a pass energy of 23.5 ev. Charge calibrations, on the order of 0.1 ev, were made by assigning the first C peak a value of ev. C1s X-ray photoelectron spectroscopy data collected on the pristine, an O 2 plasma processed and an O 2 plasma processed/annealed samples on the SiN windows (without hole) along with a PMMA reference are shown in Supplementary Fig. 9. In all the data, there is clear evidence of N from the SiN window indicating the graphene films are less than 2 nm thick. Given the evidence for SiN it is impossible to determine if Si is within the graphene layer. All samples show evidence of various degrees of C-O chemistry, depending on processing, along with C-C bonding at ev. The C1s data collected for the pristine sample (Supplementary Fig. 9 12

13 lower left) shows a carbon surface with little C-O functionality compared to the samples after O 2 plasma processing. This is consistent with a high purity graphene layer. The presence of this C- O after O 2 plasma treatment chemistry is better observed in the normalized intensity plots (Supplementary Fig. 9 top and middle right) which clearly reveal an increase in the spectral intensity around 286 due to C-O formation. 18 After O 2 plasma processing and annealing, there is a clear growth of another C-O species (Supplementary Fig. 9 middle left and top) attributed to the formation of O-C=O bonds. These species have slightly higher binding energy (289.5) than the PMMA standard (289 ev, Supplementary Fig. 9 lower right). 19 Attempts to analyze the O1s spectra were complicated due to the strong underlying Si-O bonds. Supplementary Fig. 9. C 1s spectra of a pristine, an O 2 plasma processed and an O 2 plasma processed/ annealed samples on the SiN windows. 9. Surface hydrophobization. SiN surface hydrophobization was performed as discussed in Ref. 27 of main text. In brief, SiN surface was treated by oxygen plasma and reacted in 2.5% solution of trimethoxyhexadecylsilane in toluene overnight. After reaction, membranes were washed and cured at C on a hot plate for 1 hour. Such treatment resulted in >90 degree contact angle with water, as discussed previously. 5 A 5 μm pore was drilled by FIB after surface modification insuring hydrophilicity of 13

14 the pore s interior but hydrophobic surface of SiN exposed to graphene. Graphene was transferred on top of hydrophobic SiN using the same procedure as for the rest of experiments. Strong interaction between hydrophobic SiN and graphene and their hydrophobic character should prevent water sliding in between. The results of water flux through plasma treated membranes appeared to be unaffected by this. 10. Kinetics Water flux (g) Time (h) Supplementary Fig. 10. A plot of kinetics of water flux (g) vs time (h) for graphene membrane with Id/Ig ~0.6 at 40 o C in the type 3 experiment with 5 mm KCl as a feed solution. The water flux through graphene is not linear with the delay time due to the water vapor pressure build up inside the feed solution container upon heating. Slowing down at longer times is most likely due to pore blockage by ions, as discussed in the main text. The set-up and experimental procedure are similar to the type 1 experiment except 5mM KCl solution is used as feed solution instead of DI water. The water flux through graphene is not linear and varies with time. For example, as shown in the plot, during the first 4 hours, the water transport is relatively low with little water transport observed. However, as the water vapor pressure builds up inside the feed solution container, the rate at which water transports through the membrane increases linearly and then slightly decreases. References 14

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