Single-Layer Graphene Membranes Withstand Ultrahigh Applied

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Letter pubs.acs.org/NanoLett

Single-Layer Graphene Membranes Withstand Ultrahigh Applied Pressure Luda Wang,* Christopher M. Williams, Michael S. H. Boutilier, Piran R. Kidambi, and Rohit Karnik* Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: High mechanical strength is essential for pressure-driven membrane separations with nanoporous single-layer graphene, but its ability to withstand high pressures remains to be demonstrated. We monitored failure of centimeter-scale singlelayer graphene membranes on porous supports subjected to high pressures. Consistent with theory, the membranes were found to withstand higher pressures when placed on porous supports with smaller pore diameters, but failure occurred over a surprisingly broad range of pressures, attributed to heterogeneous susceptibility to failure at wrinkles, defects, and slack in the suspended graphene. Remarkably, nonwrinkled areas withstood pressure exceeding 100 bar at which many kinds of membrane suffer from compaction. Our study shows that single-layer graphene membranes can sustain ultrahigh pressure especially if the effect of wrinkles is isolated using supports with small pores and suggests the potential for the use of single-layer graphene in high-pressure membrane separations. KEYWORDS: Graphene, membrane, pressure-driven, breaking strength, nanoporous

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pressure of 50−55 bar is required for seawater desalination by reverse osmosis (RO) to overcome the osmotic pressure. However, many kinds of membrane, including polyamide RO membranes and graphene oxide membranes, suffer from compaction and loss of performance at high pressures.27,28 This precludes their use in applications involving high pressures such as desalination of high-salinity water produced from oil wells and also limits the recovery of water in seawater desalination because high recovery requires overcoming the high osmotic pressures of the concentrated brine. The high strength of graphene suggests that it may overcome these limitations and open these challenging applications to membrane separations. Simulations have predicted that suspended nanoporous graphene membranes placed on porous supports can withstand high pressure29 but direct experimental evidence is lacking. Although nanoindentation using atomic force microscopy (AFM) has been used to investigate the mechanical strength of suspended graphene at the nanoscale,1,10,11 confinement of the stress to the vicinity of the sharp AFM tip (typically 10−30

raphene, an atomically thin layer of carbon, has exceptional mechanical properties, including high elastic modulus,1 breaking strength (maximum stress a material can sustain),1 adhesion energy,2−6 and bending stiffness7 in addition to excellent wear resistance.8 The elastic modulus and the breaking strength of pristine graphene, obtained by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG),9 are 1 TPa and 130 GPa, respectively,1 with the breaking strength being the highest measured for any material. Although the elastic modulus and the breaking strength are up to 1 order of magnitude lower for graphene in the presence of nanometer-scale vacancy defects,10 or for graphene obtained by chemical vapor deposition (CVD) that has unavoidable intrinsic defects and grain boundaries,11,12 such defective graphene is still strong enough for many applications.13−18 Because of superior mechanical strength, atomic thickness, and impermeability of pristine graphene to molecules and ions, nanoporous graphene with tailor-made pores in its lattice has been proposed as an ideal membrane for gas separations, water desalination, and other applications.19−24 Nanoporous graphene is expected to have high permeance due to its atomic thickness25 and high selectivity due to molecular sieving through selective pores26 while withstanding high pressures. High-pressure operation can increase production rates and is also essential for certain separation processes, for example, a © XXXX American Chemical Society

Received: February 1, 2017 Revised: March 28, 2017

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DOI: 10.1021/acs.nanolett.7b00442 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Burst test of graphene membranes. (a,b) Schematic of AFM nanoindentation test and burst test. (c,d) Stress distributions of membranes in (a,b). Stress is highly concentrated under and near AFM tip for AFM nanoindentation, while radial stress is equally distributed across the membrane for the burst test. (e) Schematic of graphene membrane placed on porous polycarbonate support layer, which is placed on a sintered metal support. Arrow indicates that the majority of the flow is through tears in the graphene layer. (f) Gas flow resistance model. The gas flow resistance of the graphene membrane (Rgr) is a few orders of magnitude higher than the gas flow resistance of the substrate (Rsub). The majority of the pressure drop is across the graphene membrane. (g) Flow rate versus pressure for graphene membranes, bare PCTEM, and sintered porous metal support. The graphene membrane measurement includes both a polycarbonate layer and a sintered porous metal support disc; polycarbonate test includes porous metal support disc. Black, red, and green dots are first, second, and third test of the same graphene membrane; blue, light blue, and pink dots are tests of the same bare polycarbonate layer. Gray dots are the flow rate through the sintered porous metal support only. Inset shows the same plot on a log scale, highlighting the difference in flow rates between the first and subsequent tests of the graphene membrane. (h) Ratio of flow rates through the graphene membrane to that through the bare polycarbonate layer at the same pressure shows failure of some of the graphene during the first test and no failure during subsequent tests. (i) Comparison of the fraction of open polycarbonate pores from SEM imaging and from flow rate tests. SEM images were taken before and after bust test and are compared to the flow rate ratios at Δp = 11 bar (before burst test) and Δp = 101 bar (after burst test). All data are for 200 nm polycarbonate membranes. Error bars represent standard deviation.

nanoporous graphene has the potential for use in high-pressure membrane separations. We investigated the ability of graphene membranes to withstand pressure using the “burst test” that involves measuring the pressure at which a membrane that is suspended across a defined aperture fails. We implemented this test on CVD graphene placed on polycarbonate track-etched membranes (PCTEM) with well-defined and isolated pores with diameters in the range of 30 nm to 3 μm, where graphene suspended across each polycarbonate pore effectively forms a micromembrane. Failure under pressure is measured by monitoring changes in the gas flow rate across the entire membrane (Figure 1a−d). This test closely mimics the conditions expected in actual membrane separations; it leads to uniform radial stress in the graphene (as opposed to localized stress in indentation tests) and is therefore sensitive to the global effect of any defects across the membrane. Because it is difficult to directly measure the pressure drop across each graphene micromembrane, we designed the experiment such that the pressure drop occurs primarily across

nm radius) makes this method insensitive to defects away from the tip. Such defects may govern the behavior of graphene membranes under applied pressure, where the stress is distributed fairly evenly across the entire membrane area.30 For example, specialized tensile tests of graphene with defined crack sizes show that failure occurs at the cracks at lower stress than that in pristine graphene, as governed by its fracture toughness.31 However, without precise knowledge of the behaviors of the different types of defects that may occur in practical CVD graphene membranes, it is difficult to infer the behavior of graphene membranes under applied pressure from these studies. Here, we directly investigate the ability of graphene membranes on porous supports to withstand high applied pressure (Figure 1)30 using electron microscopy, AFM, and gas flow measurements. The results reveal that areas of graphene membranes without wrinkles or cracks can withstand pressure differences of 100 bar, which is considerably higher than the working pressure in most membrane separations. This work shows that, with an excellent ability to withstand pressure, B

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Figure 2. Effect of substrate pore size on the ability to withstand pressure. (a−d) SEM images of graphene membranes after burst tests. Membrane sizes are 200 nm, 400 nm, 1 μm, and 3 μm, respectively. Scale bars are 500 nm, 1 μm, 5 μm, and 5 μm. (e) Failure fraction of membranes. Different colors indicate different support pore sizes. (f) Comparison between experimental data and theoretical prediction. Green dashed line is the theoretical calculation of burst pressure for pristine defect free graphene; pink dashed line is the theoretical calculation for nanoporous graphene, assuming E = 0.25 TPa and σ = 13 GPa.10 Blue dashed line is the theoretical calculation for nanoporous graphene, assuming E = 0.25 TPa and σ = 3.5 GPa. Blue dots denote experimentally measured burst pressures at 15% failure fraction, while red dots represent burst pressure at 75% failure fraction. (g) Cumulative distribution of the estimated breaking strength of the graphene membranes.

repeated measurements, the flow rates across graphene on PCTEM increased from the first test to the second but did not change thereafter (Figure 1g). The ratio of the flow rate through the graphene membrane to that through the bare PCTEM is indicative of the fraction of PCTEM pores that are open to gas flow, given that the PCTEM pores are largely isolated from each other. This flow rate ratio increased during the first burst test on the graphene membrane, suggesting failure of some of the graphene micromembranes. Its stability during the second and third tests indicates that there was no further damage to the graphene (Figure 1h). Furthermore, the measured flow rate ratio was consistent with the fraction of PCTEM pores that were not covered by intact graphene, as enumerated by scanning electron microscopy (SEM) (Figure 1i). This agreement shows that the majority of the flow occurs through tears and uncovered areas and that the gas flow resistance across graphene micromembranes is much larger than the resistance through the underlying PCTEM pores. These results validate the burst test for investigating the ability of graphene to withstand pressure and also show that the majority of the graphene micromembranes survive the high applied pressure.

the graphene. From a simple resistance-in-series model (Figure 1f), this constraint can be satisfied by guaranteeing that the gas flow resistance of the graphene membrane is significantly higher than the resistance of any supporting layers. The gas flow resistance of graphene, even with nanometer-scale defects, is estimated to be 2−4 orders of magnitude greater than that of PCTEM32 (see Supporting Information (SI) Section IX). Furthermore, the largely isolated PCTEM pores separate the gas flow paths through each graphene micromembrane, preventing flow through burst micromembranes from affecting the pressure drop across intact neighbors. The graphene on PCTEM was further supported on a sintered metal disc (Figure 1e) selected such that its gas flow resistance is much smaller than that across the PCTEM. Experimentally, flow rates across graphene on PCTEM with 200 nm pores were significantly lower than those across a bare PCTEM, which in turn were lower than those through the sintered metal support (Figure 1g). For each of these samples, we repeated the burst test three times by varying the applied pressure difference from 0 to 100 bar, while recording the pressures and gas flow rates. While the flow rates through the bare PCTEM without graphene remained unchanged in C

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Figure 3. Membrane failure is associated with intrinsic defects. (a) Zoomed-in high-resolution SEM images of graphene membranes before burst test (left). Edge detection better reveals the wrinkles in the graphene (right). (b) SEM image of the same graphene membranes after burst test at Δp = 30 bar. Scale bars in (a,b) are 500 nm. (c) Almost all graphene micromembranes with defects visible before burst test fail during the test, whereas only a small fraction of membranes without defects fail. Data are for 1 μm PCTEM support pores and 30 bar pressure difference. (d) Estimated burst pressure versus crack length for graphene on PCTEM with 200 nm, 400 nm, 1 μm, and 3 μm pores.

S3a). The results clearly indicate that graphene on smaller support pores can sustain higher pressure, which agrees qualitatively with eq 1. However, in contrast to a specific burst pressure predicted by eq 1 for graphene micromembranes of a given size, we observe that failure of the graphene micromembranes occurs over a wide range of applied pressures, the reasons for which are discussed later. Regardless, if we plot the burst pressure at a given failure fraction, we find that the burst pressure indeed decreases inversely with the membrane diameter, which is in agreement with theory. For example, at 75% failure fraction, that is, when 75% of the micromembranes have failed, the measured burst pressure is consistent with the pressure predicted by eq 1 using values of the breaking strength (13 GPa) and elastic modulus (250 GPa) of graphene with nanometer-scale defects as measured by AFM nanoindentation10 (Figure 2f). When the first 15% of micromembranes have failed (15% failure fraction), the corresponding burst pressures are lower, indicating a lower breaking strength. The burst pressure is affected not only by the support pore size, but also by the breaking strength and elastic modulus of graphene, which should remain constant across the different membranes, provided that the graphene has a similar level of defects. Given the burst pressure of the graphene membranes (Figure 2e), we can estimate the breaking strength of the graphene using eq 1 (assuming E = 250 GPa and ν = 0.117). The extracted breaking strength distributions for graphene micromembranes on different support pore diameters approximately collapse onto a single curve (Figure 2g), demonstrating that the scaling predicted by eq 1 holds even though failure occurs over a range of applied pressures. The wide range of breaking strength observed in our experiments contrasts with the narrow distribution of breaking strength of defect-free pristine graphene measured in AFM nanoindentation.1,10 Varying slack in the graphene micromembranes can cause deviations from the predictions of eq 1, but its magnitude (∼5 nm) is insufficient to explain the wide

Burst pressure directly depends on the support pore size that also defines the size of the graphene micromembranes. The residual stress is negligible since the stress generated by high pressure dominates. The graphene micromembranes are treated as thin membranes because the maximum deflection and the diameter of the membrane are much larger than the thickness of graphene.3 Furthermore, the effect of bending stiffness is not expected to be significant under the large deflection conditions encountered at high pressures in our experiments.7,33 For a thin elastic membrane, the burst pressure is given by29,30 Δp =

t D

96·(1.026 − 0.793ν − 0.233ν 2)σ 3 E

(1)

where Δp is the burst pressure, ν is the Poisson’s ratio, E is the Young’s modulus, σ is the breaking strength, t is the thickness, and D is the membrane diameter. For a given membrane material and thickness, eq 1 predicts that the burst pressure is inversely proportional to the membrane size. To investigate whether this scaling holds for graphene membranes, we performed the burst test for graphene on PCTEM with different pore diameters. Because the flow rate ratio includes flow through burst graphene micromembranes as well as through PCTEM pores that were not covered with graphene at the beginning of the burst test, we subtract out the initial flow rate to capture the failure of initially intact graphene micromembranes, which we define as the failure fraction. At an applied pressure of 100 bar, the failure fraction increased with PCTEM pore size and was approximately 20%, 50%, and 80% for graphene on 200 nm, 400 nm, and 1 μm diameter PCTEM, respectively (Figure 2e; “applied pressure” implies “applied pressure difference” henceforth). By contrast, the failure fraction reached 90% for graphene on 3 μm diameter PCTEM at an applied pressure of only 30 bar. Although the pressure difference in Figure 2e is the total pressure drop measured across the graphene, PCTEM, and metal support, the estimated lower bound of the pressure drop across graphene is comparable to the applied pressure difference (see SI Figure D

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Figure 4. Effect of ion irradiation and etching on ability to withstand pressure. (a) Effect of ion fluence on failure fraction indicates that the strength of graphene is compromised if the fluence exceeds 3.6 × 1013 ions/cm2. (b) Failure fraction of graphene membranes with different treatments. Acid etch treatments is for ion bombardment at 6 × 1012 ions/cm2 and acid etch for 60 min (c) High-resolution AFM image of graphene membrane with ion bombardment and 60 min acid etch treatment. The image was taken after burst test at 100 bar, showing that failed micromembranes are aligned with wrinkles. (d) Most graphene micromembranes lying on visible wrinkles fail, whereas those that do not lie on wrinkles survive. Data are obtained for the same test conditions and images as in (c).

nm to a few nanometers (Figure 3d), which further suggests that defects could account for the observed behavior. Furthermore, SEM images (see SI Section VII) reveal that the majority of failed micromembranes are aligned along wrinkles, which suggests that the defects responsible for failure are associated with wrinkles, provided that each micromembrane fails independently of the others. Preferential failure along wrinkles and defects suggests that use of supports with small pore diameters is beneficial to sustaining high pressure; for a given distribution of defects and wrinkles, the fraction of micromembranes that contain those defects decreases with the micromembrane size. In addition, eq 1 predicts a higher burst pressure for smaller support pore diameters. Figure 2g shows that the estimated burst strength distribution curves for larger support pores have a broad and uniform distribution, whereas those for smaller pore diameters have a tendency to level off, which may be because smaller micromembranes have a smaller chance of having defects and therefore would exhibit much higher breaking strength compared to those with defects. Taking this reasoning one step further, we performed the burst test on graphene placed on PCTEM with 30 nm pore diameter. Although this PCTEM exhibited deformation under applied pressure, likely due to the bullet-shaped pores, failure of the graphene membranes was undetectable using SEM imaging after application of 100 bar pressure (see SI Section IV). Most separation applications using single-layer graphene membranes require nanoporous graphene containing selective subnanometer or nanometer-scale pores. A high density of these selective pores is beneficial to enabling high permeance, but it should not be so large as to compromise its ability to withstand the required pressure. Molecular dynamics simu-

range of burst pressures observed in the experiments (see SI Section VIII). A more plausible explanation is the presence of different defects in the graphene micromembranes with correspondingly different abilities to withstand pressure. To assess whether failure is correlated with defects, we acquired high-resolution SEM images before and after applying pressure and examined whether micromembranes with visible defects had a greater chance of failure than those without any observable defects. Figure 3a,b shows that a 1 μm diameter micromembrane without visible defects remained intact after applying a pressure of 30 bar, but two micromembranes with visible defects, which lay along wrinkles, could not withstand the applied pressure (Figure 3b). Statistically, micromembranes with visible defects all failed at 30 bar, while over 80% of the micromembranes without visible defects remained intact at the same pressure (Figure 3c). This correlation shows that differences in defects in the graphene are a major contributor to the wide distribution of burst pressure of graphene micromembranes of the same size. We used Griffith’s criterion for crack failure to estimate the effect of defects on the breaking strength of graphene by assuming that the defect is a single crack

L=

2ΓE πσ 2

(2)

where L is the crack length, Γ is the fracture toughness, and σ is the burst strength. For CVD graphene, we assume that the Young’s modulus is 250 GPa,10 Poisson’s ratio is 0.117 (ref 29), and the fracture toughness is 15.9 J/m2 (ref 12). The measured burst pressure and estimated breaking strength are consistent with the presence of cracks with lengths ranging from over 100 E

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wrinkles may be generated during the chemical vapor deposition process, or during the transfer of graphene to PCTEM, that is, copper etching, mechanical pressing, membrane rinse, and membrane drying. For example, defects may be generated along wrinkles by chemical attack by the copper etchant used in the transfer due to the formation and collapse of bubbles during the etching process or due to the liquid surface tension during the drying step.39 Furthermore, the high stress at wrinkles may make the graphene susceptible to attack by the acid40−43 used in the pore creation process (ion bombardment and acid etch). Carbon nanotubes are known to get unzipped in the acid etch used in our work,41 which suggests that similar “cutting” may occur along the highly curved edges of graphene wrinkles. Further study is needed to thoroughly understand the effect of wrinkles, which could lead to improvement in the quality of graphene membranes. In addition to wrinkles and defects, slack in the graphene micromembranes may contribute to some of the variation in the burst pressure. Slack may have a significant effect on the burst pressure for small micromembranes (400 and 200 nm), but it is expected to have a negligible effect on the burst pressure for large micromembranes (3 and 1 μm) (see SI Section VIII). AFM measurements indicate slack of ∼5 nm in our graphene micromembranes. On the basis of previously reported properties of graphene,1,10 if graphene has a breaking strength of 5 GPa, the 5 nm slack increases the burst pressure by 29% for 400 nm diameter membranes, but it only increases the burst pressure by 5% for 3 μm-diameter membranes (see SI Section VIII). The maximum breaking strength is estimated from micromembranes with large diameters (3 and 1 μm as shown in Figure 2g), where slack has little effect. Interestingly, we also find that the contribution of slack to the estimated breaking strength is more significant at low burst pressures (see SI Section VIII and eq S6). This effect may be one reason for the rapid increase of failure fraction at low pressures. Our results show that support substrate design is critical for graphene membrane applications under high pressure. Supports with small pores are desirable for three different reasons. First, for a given applied pressure, smaller support pores minimize the stress in graphene (see eq 1). Second, smaller support pores limit the extent of damage due to wrinkles. Third, smaller support pores are also desirable from the perspective of isolation of defects and of presenting an appropriate transport resistance to minimize leakage through defects, as shown elsewhere.32 Scalable production of graphene membranes with high selectivity needs to be achieved before the ability of graphene membranes to withstand high pressure can be practically used. The scalability of graphene membranes depends on the scalability of graphene synthesis, handling, and selective pore creation. By using roll-to-roll methods, Samsung successfully fabricated and transferred 30 in. graphene films in 2010,44 and Sony advanced this technique to generate 100 m long graphene films in 2013.45 However, most of the methods for pore generation, such as using electron beam46 or focused ion beam,25 are not scalable due to high cost, complex operating conditions, or limited membrane size. Oxygen plasma etching47 does not have these restrictions, and may be amenable to integration with roll-to-roll methods to manufacture graphene membranes at the required scales. However, obtaining sufficient selectivity for desalination using large-area graphene has yet to be demonstrated.

lations predict that nanoporous graphene membranes with nanometer-scale defects placed on a support with 1 μm diameter pores can withstand a pressure of 570 bar.29 We first studied the effect of gallium ion bombardment (52° incident angle and 8 kV acceleration voltage), which is known to create vacancy and other defects in graphene,34,35 for graphene placed on 200 nm PCTEM supports. At an applied pressure difference of 100 bar, the fraction of open polycarbonate pores increased from less than 30% for nonbombarded graphene to 67% at a fluence of 3.6 × 1013 ions/cm2, 72% at 9 × 1013 ions/cm2, and to 90% at 3.6 × 1014 ions/cm2. At high ion bombardment densities, graphene eventually becomes amorphous,34 resulting in significantly lower strength than that of the original material.10 Therefore, in this case the suitable ion fluence is below ∼1013 ions/cm2 (Figure 4a) to create pore defects while maintaining the ability of graphene to withstand the applied pressure. Ion irradiation followed by etching is known to create selective, subnanometer pores in graphene.35 Graphene was irradiated by gallium ions with 52° incident angle and 8 kV acceleration voltage at a density of ∼6 × 1012 ions/cm2 and then etched for 60 min in acidic permanganate solution. This process has been shown to result in nanoporous graphene with subnanometer pores with diameters of 0.40 ± 0.24 nm.35 The burst test on graphene with pores created by ion irradiation and etching showed that ∼40% of the graphene membranes survived an applied pressure of 100 bar, although 40% is not as good as in the case of untreated CVD graphene. A significant fraction of graphene micromembranes endured this high pressure, indicating that porous graphene membranes with created defects are still ultrastrong. We used high-resolution AFM imaging to investigate whether the failed nanoporous graphene micromembranes are associated with defects, similar to the case of graphene without created pores. We find that failure of the nanoporous graphene micromembranes is almost exclusively associated with wrinkles. Almost all nanoporous graphene micromembranes that had wrinkles running across them failed, while almost all micromembranes that were not associated with wrinkles survived. This is in contrast to the behavior of untreated CVD graphene, where a significant fraction of micromembranes associated with wrinkles survived the burst test (see SI Section VI). Consequently, the results suggest that ion bombardment and acid etching undermines the strength of micromembranes with wrinkles to a sufficient extent to cause failure under pressure and that wrinkles again constitute the weakest feature of the membranes. The ability of membranes to withstand high pressure is required in many applications.27,36 Our results show the potential of using atomically thin membranes for high-pressure separations. The observed maximum breaking strength is ∼13 GPa, which is comparable with that of the mechanical exfoliated graphene with vacancy defects,10 measured by AFM nanoindentation method. We observed that wrinkles are weak points that lower the burst pressure and breaking strength of graphene micromembranes. Although it is unclear why wrinkles are weak,37 one possible reason is the presence of defects along the wrinkles. Graphene with defects is well-known to be mechanically weaker than pristine graphene.31 Initially, wrinkles are generated from the mismatch of the thermal expansion coefficient of the copper substrate and that of the graphene38 during the cooling step of graphene synthesis. Defects along F

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In conclusion, we have used burst tests to directly measure the burst pressures of graphene micromembranes. Failure of graphene measured by high-resolution SEM/AFM imaging before and after burst tests was consistent with the failure estimated from flow rate measurements. We observed a wide distribution of burst pressures of graphene micromembranes, attributed primarily to wrinkles and large defects. Experiments with different supports show that graphene on supports with smaller pore diameter (D) tend to withstand higher burst pressures, which is in agreement with the 1/D scaling of pressure predicted by eq 1. Furthermore, smaller graphene micromembranes have better overall quality as a result of defect isolation. In fact, around 70% of graphene membranes with 200 nm diameter survived a Δp of 100 bar, which exceeds the working pressures even for seawater desalination (50−80 bar). Pore creation is essential for membrane separations, and we find that porous graphene membranes with created defects remain ultrastrong, although they have lower strength than membranes without created pores. We demonstrate that CVD graphene at the macroscale is ultrastrong, suggesting the possibility of its use in several applications, including high pressure gas separations and water desalination. Materials and Methods. Graphene Transfer. CVD singlelayer graphene on copper was purchased from Graphenea. The back side of copper foil was pre-etched using ammonium persulfate (APS-100, Transcene) for 3−5 min to remove the graphene. Graphene on the other side was directly transferred to track-etched-polycarbonate-membrane (PCTEM, Sterlitech non-PVP coated, hydrophobic) to avoid any contamination, which occurs in polymer-based transfers.48 The PCTEM had 3 μm, 1 μm, 400 nm, or 200 nm cylindrical pores. The remaining copper foil was completely etched by ammonium persulfate (APS-100), and the graphene/PCTEM composite was rinsed 3−5 times in deionized water. Ethanol was added into the deionized water before drying to reduce the surface tension of the liquid and minimize damage to the graphene membranes. Before transfer of graphene on PCTEM with 30 nm pores (polyvinylpyrrolidone (PVP) coated, hydrophilic), pretreatment is needed to make the substrate more hydrophobic. The PCTEM was rinsed 3−5 times in methanol, followed by atomic layer deposition of hafnia (Savannah ALD 10 cycles at 130 °C at a rate of 0.1 nm/cycle). After PCTEM with 30 nm pores becomes hydrophobic, the transfer procedure is similar to transfer graphene on the other PCTEMs. Characterization. The SEM images were acquired by FESEM Ultra 55, Supra 55VP, or Ultra Plus, the AFM images were taken by Asylum MFP-3D, and the Raman spectrum was acquired using a Horiba Multiline Raman Spectrometer. Figure 3a (right) is processed by imageJ under unsharp mask with radius −5 pixels and mask weight −0.9. Burst Test. The membrane area available to gas flow was defined by a mask with hole in a copper tape (TED PELLA 16072-1), which was placed over the graphene. The graphene/ PCTEM membrane was placed on a porous metal support (Figure 1e). Argon gas was then continuously flowed through the membrane. The upstream pressure was increased slowly from 5 to 105 bar (gauge pressure) to ensure a steady state at each measured pressure, and the downstream pressure was held at 5 bar (gauge pressure) by a backpressure regulator. As shown in Figure 1e, gas flows through graphene, polycarbonate, and the sintered metal support. Additional details are provided in Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b00442.

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Figures S1−S17 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (L.W.) [email protected]. *E-mail: (R. K.) [email protected]. ORCID

Luda Wang: 0000-0001-6222-7807 Rohit Karnik: 0000-0003-0588-9286 Author Contributions

L.W. and R.K. designed the experiments. L.W. performed the experiments. L.W. and C.M.W. built the experimental system. C.M.W., L.W., M.S.H.B., and P.R.K. helped prepare the samples. L.W. and R.K. analyzed the data and wrote the manuscript. All authors contributed to discussions. Notes

The authors declare the following competing financial interest(s): Rohit Karnik discloses financial interest in a company aimed at commercializing graphene membranes.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Steve Sinton and Jacob Swett for useful discussions. This work was supported by MIT Energy Initiative. L.W., M.S.H.B., and P.R.K. were supported in part by the U.S. Department of Energy Office of Basic Energy Sciences award number DE-SC0008059. This work used the facilities at the MRSEC at MIT, supported by the U.S. National Science Foundation (NSF) under award number DMR-1419807, and the facilities at the Center for Nanoscale Systems at Harvard University, a member of the National Nanotechnology Infrastructure Network that is supported by the NSF under award number ECS-0335765.

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DOI: 10.1021/acs.nanolett.7b00442 Nano Lett. XXXX, XXX, XXX−XXX