Extremely permeable porous graphene with high H2/CO2 separation ability achieved by graphene surface rejection

K. Shimizu and T. Ohba *
Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan. E-mail: ohba@chiba-u.jp

Received 17th May 2017 , Accepted 16th June 2017

First published on 4th July 2017


Fabrication of a graphene separation sheet is difficult because of the necessity for leakage-free graphene transfer onto a substrate. In this study, porous graphene sheets with thicknesses of one, two, and four layers were directly fabricated on stainless-steel mesh substrates and demonstrated to display high separation ability for H2, CO2, and CH4. The single-layer graphene sample exhibited higher permeance for these molecules than double- and four-layer graphene and displayed similar high selectivity to that of other porous materials. Permeance was proportional to molecular velocity and inversely proportional to interaction strength with graphene; molecular size-dependent permeance was not seen. Molecules that interacted strongly with graphene were attracted to the graphene surface, which hindered permeation. Such graphene surface rejection allowed graphene containing larger pores than the molecular size to provide both high molecular permeance and selectivity. The relationship between the permeance of porous graphene for H2 and H2/CO2 with selectivity suggested that its permeance was higher than that of other materials with high separation performance. Therefore, the porous graphene samples separated molecules with extremely high permeance by graphene surface rejection.


Introduction

Graphene and graphene oxides have unique properties including high transparency, electronic and thermal conductivities, and flexibility, which facilitate various applications in touch panels, transistors, and solar batteries.1,2 Ideal graphene is impermeable to all atoms, molecules, and ions. Bunch and co-workers demonstrated that concave and convex graphene samples were impermeable by observing the pressure difference of molecules across graphene sheets fabricated on silica.3 A cavity containing captured H2, which was sealed by graphene, released H2 upon etching of graphene; that is, upon pore formation in graphene.4 Porous graphene can be permeable to He, and its quantum fluctuation and tunneling decreased and increased helium permeation, respectively.5,6 Porous graphene is thus attractive as a molecular separation sheet with high mechanical and chemical stability.

Molecular dynamics (MD) simulations of various molecules permeating through porous graphene and graphene-related materials have suggested that the pore size and shape of graphene dominate molecular permeation through it.7–9 Porous graphene was estimated to have higher separation performance for CO2, H2S, and N2 than the upper limit of polymers.10 Jiang and co-workers first proposed that narrow graphene pores could efficiently separate H2 and CH4 molecules without any external field.9 In the separation of H2 and N2 using porous graphene models, N2 permeation had no correlation with the pore area, while H2 had a linear relationship with the pore area, which resulted from the stronger interaction between N2 and graphene than that between H2 and graphene.7 Graphene with 0.34 nm pores perfectly separated CO2 and N2.8 Xue and co-workers reported the selective interaction of CO2 with surface functional groups on graphene edges rather than N2.11,12 The results of a series of molecular permeance experiments using narrow graphene pores reflected the kinetic diameter of the molecules.8 However, non-permeating molecules reduced permeation of any molecules through graphene pores in mixtures of molecules.13 Liquid water flow was considerably depressed through graphene with 0.75 nm pores, whereas fast water flow was observed through graphene with 2.75 nm pores.14 In contrast, water vapor effectively permeated through graphene pores with an effective diameter of 0.4 nm.15 Graphene pores were also able to desalinate NaCl aqueous solution through selective ion permeation.16,17 Hydrophilic pores in graphene increased water flow, but decreased the selectivity between ions and water.16

Graphene and graphene oxides fabricated on substrates could be used as molecular separators.18–21 The selectivities for O2 over N2, CO2, and N2, and their permeances through graphene and graphene oxides exceeded those of polymers.18 Corrugations, wrinkles, ripples, and randomly stacked graphene sheets form pores that allow molecular permeation. Graphene also typically contains grain boundaries and defects that can act as pores. These intrinsic pores are useful for molecular separation even if they are nonselective defects.21 Annealing of graphene oxides lowered their defect content and improved H2/CO2 selectivity.19 Meanwhile, higher epoxy and oxygen concentrations introduced larger pores into reduced graphene oxides.20 Double-layer graphene with a few million uniform pores displayed extremely high H2 permeance and high selectivities for H2 and CO2.22 Graphene and graphene oxides thus have considerable potential for use as molecular separation sheets. However, higher separation performance using graphene needs to be obtained, and the mechanism of molecular permeation through graphene is far from being well understood despite the potential of graphene as a separator. Graphene is typically transferred to a substrate after synthesis. Grain boundaries and defects in graphene are inherently induced by polymer residues and dust present during the transfer process. Cleanliness and high-purity graphene are crucial for successful transfer. Direct synthesis of graphene as a separation sheet is desirable, because accidental defect formation during graphene transfer hinders its performance and structure evaluation. In this study, we directly fabricate single-, double-, and four-layer graphene on stainless-steel mesh substrates and investigate their H2 and CO2 separation performance by permeance tests.

Experimental and simulation procedures

Experimental

Graphene sheets were fabricated using chemical vapor deposition (CVD) of CH4 on a stainless-steel mesh with 0.5 μm holes at 1300 K for 1.0, 1.5, and 2.5 h. H2 and Ar molecules were used for reduction and gas flow, respectively. The fabrication details are summarized in Fig. S1a (ESI). The synthesized graphene was evaluated by Raman spectroscopy using a Nd:YAG laser at a power of 0.1 mW (NRS-3000; JASCO, Tokyo, Japan), energy-dispersive X-ray spectroscopy (EDX) at 15 kV (JSM-6510A; JEOL, Tokyo, Japan), transmission electron microscopy (TEM) and electron diffraction at 120 kV (JEM-2100F; JEOL), and X-ray diffraction (XRD) using CuKα radiation of 40 kV and 30 mA (SmartLab; Rigaku, Tokyo, Japan). Each stainless-steel mesh coated with a graphene sheet was connected to a line in our custom-made mass spectrometer system including a quadrupole mass spectrometer (M-101QA-TDF; Canon Anelva, Kanagawa, Japan) (Fig. S1b, ESI). The permeance of H2, CO2, or CH4 molecules through graphene sheets was tested at ambient temperature using controlled flow rates in the range of 3–6 cm3 s−1. Here, we conducted the permeance tests for a single gas component to simplify the mechanism. Permeance is defined as the intrinsic permeance of a graphene sheet to molecules through the thickness of graphene, which was evaluated from a partial pressure difference and the surface area of the graphene sheet.23,24 Here, the permeances of both the graphene sheet and the stainless-steel mesh used as a substrate were considered. These permeances could be separated by the definition of the resistance to permeant flow; that is, the resistance of graphene to permeant flow could be obtained by subtraction of the resistance of the stainless-steel mesh from the total resistance.23,25

MD simulations

MD simulations of H2, CO2, CH4, or He permeation were performed to evaluate molecular permeation through graphene with 0.3, 0.4, and 0.7 nm pores using the leapfrog Verlet algorithm. The temperature was maintained at 300 ± 1 K using the heat bath coupling method. The total run time obtained by the accumulation of each time step of 0.5 fs was 3.0 ns. The unit cell size was 3.689 × 3.408 × 10.00 nm and two porous graphene sheets were located at z = ±1.0 nm. Here, the porous graphene was assumed to be rigid and had four pores with a diameter of 0.3, 0.4, or 0.7 nm. The number of carbon atoms in the porous graphene sheets with 0.3, 0.4, and 0.7 nm pores was 456, 432, and 384, respectively. A periodic boundary condition was used in all three directions. 50 molecules of H2, CO2, CH4, or He were randomly positioned outside the graphene sheets; that is, at z > 2.0 and z < −2.0 nm. The MD simulations of molecular permeation through graphene were performed using Lennard-Jones and coulomb interaction potentials.26–28 The collision diameters (σX) and interaction potentials (εX/kB) were σH = 0.281 nm and εH/kB = 8.6 K for a H2 molecule; σC = 0.2753 nm, εC/kB = 29.07 K, qC = 0.6466C, σO = 0.3029 nm, εO/kB = 83.2 K, and qO = −0.3233C for a CO2 molecule; σCH4 = 0.3758 nm and εCH4/kB = 148.6 K for a CH4 molecule; σHe = 0.2556 nm and εHe/kB = 10.2 K for a He atom; and σC = 0.34 nm and εC/kB = 28.0 K for a C atom of graphene. The atomic distances between H and H in H2, and C and O in CO2 were 0.074 and 0.1143 nm, respectively. Lorentz–Berthelot mixing rules were used to calculate interactions between different types of atoms. Weak external forces of 0.5 and −0.5 μN were applied to all the molecules at z < 0 and >0 nm, respectively, to control slow molecular flow to the center of the unit cell. The stability of molecules and graphene was calculated from those interactions between molecules and non-porous graphene.

Results and discussion

Graphene was directly synthesized using CVD on a stainless-steel mesh, as shown in the left images of Fig. S2a (ESI). Graphene fabricated on metal surfaces has been reported; stainless steel can serve as a substrate and a catalyst for graphene growth, although copper is commonly used for this purpose.29–32 The Raman spectra of the graphene sheets shown in Fig. 1a indicate that the sheets contained a few defect structures, as illustrated by the lack of D bands at around 1350 cm−1. All samples exhibited sharp G bands at around 1580 cm−1. D and G bands are mainly the results of disorder and symmetric honeycomb sp2 carbon structures, respectively. Edge sites and pore edges also affect the D-band. The intensity ratio of the D band to the G band is in inverse proportion to the crystallite size of nanographene,33 suggesting that the graphene sheets had highly crystalline structures. The intensity and width of the 2D band strongly depend on the graphene layer number.34–36 The intensity ratios of the 2D band to the G band for the graphene samples synthesized for 1.0, 1.5, and 2.5 h were 2.4, 1.2, and 0.6, respectively. These intensity ratios suggest that the graphene samples synthesized for 1.0, 1.5, and 2.5 h were single-, double-, and multilayer, respectively.37 The 2D peaks for samples synthesized for 1.0, 1.5, and 2.5 h appeared at 2696, 2698, and 2700 cm−1, respectively, and peak widths broadened with a lengthening synthesis time. These results also support the above layer numbers of the samples.38,39 Well-stacked graphene layers obtained by direct synthesis are known to contain a few unexpected molecular pathways through interlayer spaces.19 The XRD patterns of the graphene samples in Fig. S3 (ESI) indicate that all samples possessed a honeycomb structure, as evaluated from the (10) peaks at 43° and 45°. The peak at 44° originates from the stainless steel mesh substrate, and an unassigned peak appeared at 39°. The graphene sample synthesized for 2.5 h had stacked graphene layers with an interlayer distance of 0.335 nm, as determined from the (002) peak at 26.5°. The layer number of multilayer graphene obtained by synthesis for 2.5 h was unable to be definitively determined using Raman spectroscopy, although it was supposed to be 3–5 layers from the intensities in the 2D and G bands.36,40
image file: c7cp03270f-f1.tif
Fig. 1 Graphene layer number evaluated from (a) Raman spectroscopy and (b) the relationship between the layer number and the carbon amount determined using energy-dispersive X-ray spectroscopy.

EDX analysis was therefore conducted to determine the layer number of the graphene samples. The EDX mapping images of the graphene samples on the right of Fig. S2a (ESI) and the peaks as a function of binding energy in Fig. S2b (ESI) suggest that C atoms were uniformly dispersed on the mesh substrate. Only C, O, Fe, and Cr (the latter two are species in the stainless-steel mesh) were observed. The ratio of C to Fe increased with lengthening graphene synthesis time, while the amount of O was small and barely changed during graphene synthesis. Oxygen was inherently included in the stainless-steel mesh and rarely involved in synthesized graphene. Overall, these results indicate that graphene synthesized on the stainless-steel mesh rarely had impurities and a defective structure, even though Fe, which is the main component of stainless steel, is not typically used as a substrate for graphene synthesis.32Fig. 1b shows the layer number of graphene evaluated from the Raman spectra as a function of the C amount over the Fe amount evaluated from the EDX data. The relationship between the layer number of graphene and the C amount was linear, as expected. From this linear relationship, the layer number of graphene synthesized for 2.5 h was thus determined to be four. Hereafter, graphene samples synthesized for 1.0, 1.5, and 2.5 h are referred to as single-, double-, and four-layer graphene, respectively. Four-layer graphene was detached from the stainless steel mesh by ultrasonication for 2 days. The detached four-layer graphene was directly observed by TEM, as shown in Fig. 2. The graphene sheets were of micrometer order in size. Electron diffraction peaks of this sample gave lattice spacings of 0.124, 0.215, and 0.338 nm, corresponding to and perpendicular to the armchair and zigzag directions of a graphene layer, and interlayer distance, respectively.41 These results indicate that high-quality large-scale graphene without noticeable defects was synthesized on stainless steel mesh substrates. However, the inherent pores were introduced on graphene, as described later, although those pores were not seen from the TEM image shown in Fig. 2a.


image file: c7cp03270f-f2.tif
Fig. 2 (a) Transmission electron microscopy image of four-layer graphene (synthesized for 2.5 h). (b) Electron diffraction image of four-layer graphene.

H2, CO2, and CH4 permeation through the fabricated graphene sheets was measured; the results are presented in Fig. 3a. The graphene sheets displayed high permeance to all of these molecules, suggesting that graphene pores were inherently formed during the process of direct graphene synthesis, even though a few defective structures were observed in the D-bands in the Raman spectra and the TEM image. This is because carbons on pore edges and edge sites in large graphene could be negligible amounts in comparison with those on the symmetric honeycomb sp2 carbon structure. The permeances of H2, CO2, and CH4 decreased with increasing graphene layer number. In other words, single-layer graphene displayed higher molecular permeation performance than double- and four-layer graphene. The gas molecules are believed to permeate through pores in the graphene sheets, even though the D bands originating from defects were very weak in the Raman spectra of the graphene samples (Fig. 1a). CO2 and CH4 have been separated by permeation through graphene pores smaller than 0.4 nm, but no separation was achieved using pores larger than 0.5 nm.26 We observed relatively high selectivity for CO2 over CH4 in Fig. 3b,42,43 which indicates that the inherent pores in the graphene samples were smaller than 0.5 nm. With increasing graphene layer number, pores in a graphene layer might tend to be covered by another graphene layer, which would decrease permeance, consistent with the above results.18 In addition, permeance decreased in the order H2 > CH4 > CO2. The selectivity obtained from the different permeances of these molecules to the graphene sheets could allow their separation at ambient temperature, especially the separation of H2 and CO2, as illustrated in Fig. 3b. Selectivity increased with layer number, again suggesting that molecular permeation was restricted by overlapping graphene layers.


image file: c7cp03270f-f3.tif
Fig. 3 (a) H2, CH4, and CO2 permeances of graphene sheets. (b) Selective permeation of H2/CO2, H2/CH4, and CH4/CO2 through graphene sheets. Blue bars: single-layer graphene, red bars: double-layer graphene, and green bars: four-layer graphene.

The dependence of permeance on the molecular size, velocity, and interaction with the graphene sheets was examined, as shown in Fig. 4. The relationship between permeance and the molecular size was not monotonic, while permeance monotonically increased and decreased with increasing velocity and stability of a molecule adsorbed on graphene, respectively. The stability was calculated from the interaction between each molecule and non-porous graphene. Here the interactions were calculated using the simulation models of molecules and graphene without pores described in the simulation procedure.


image file: c7cp03270f-f4.tif
Fig. 4 Dependence of permeance on (a) molecular size, (b) velocity, and (c) stability of a molecule adsorbed on graphene. Blue: single-layer graphene, red: double-layer graphene, and green: four-layer graphene.

Graphene pores thus hardly control molecular permeation in this system, because it has been reported that graphene pores considerably restrict molecular permeation when the pore size approaches the size of molecules.4,8,9,13,26 In other words, the pore sizes of the graphene samples were sufficiently larger than the permeating gas molecules. However, high selectivity for H2, CO2, and CH4 was observed despite the large graphene pore size. Permeance was proportional to molecular velocity, suggesting that the permeance depended on the frequency of permeation attempts through each graphene sheet. Conversely, permeance was inversely proportional to the stability of adsorption on graphene. A previous MD simulation of molecular permeation through porous graphene of 0.4–1.0 nm pore size at 298 K assumed that N2 was adsorbed on graphene and reached pores through two-dimensional diffusion, while H2 directly approached pores.7 Accumulation of N2 adsorbed on the graphene surface induced high permeance. Direct flux through a graphene pore was also observed for He and H2 because of their weak interaction potential with graphene. In contrast, surface flux was observed for CH4 and N2, suggesting that graphene pores of 0.4–1.0 nm pore size showed lower total flux to the predicted direct flux to He and H2 than N2.44 These simulation findings and our results are inconsistent. However, permeation flux via graphene pores was in the following order: H2, He, CH4, and N2.44 Frequent trial of molecules to permeate graphene pores and weak interaction with graphene sheets resulted in molecular permeance. The mechanism of molecular permeation through a graphene sheet with relatively large pores involves attempts of a molecule to permeate through the pores in the graphene sheet and the interaction with the graphene sheet preventing its permeation because of attractive forces.

MD simulations of H2, CO2, CH4, and He permeation through various graphene pores were performed to confirm the mechanism of surface rejection involving the interaction of the gas molecules with the graphene sheet. Fig. 5 shows snapshots of H2, CO2, CH4, and He permeation through graphene pores (see also Fig. S4–S7, ESI). The MD simulations indicated that faster molecular permeation occurred through larger graphene pores. Only He atoms smoothly permeated through the 0.3 nm pores, while H2 hardly permeated through the 0.3 nm pores during long calculation periods. In contrast, all the molecules could permeate through the 0.7 nm pores. CO2 hardly permeated through the 0.4 nm pores even though its molecular size in the minor axis direction was smaller than the pore diameter. Molecular permeation numbers through graphene pores were calculated from the snapshots, as depicted in Fig. 6. He was the main permeable species through the porous graphene with 0.3 nm pores, suggesting that molecular separation depended on the graphene pore size. H2 permeation through the 0.4 nm pores was the fastest of the species considered, which might be caused by its highest velocity. CO2 was permeated slightly through graphene with 0.4 nm pores, while CH4 permeated through more readily. 0.4 nm pores were similar in size to those of a CH4 molecule (0.38 nm) and a CO2 molecule (major axis of 0.53 nm and minor axis of 0.30 nm). CO2 was attracted to the walls in the graphene pores because its interaction with graphene was the strongest of the gas species considered, so it was strongly adsorbed on graphene walls, as shown in Fig. S7 (ESI), decreasing permeation via graphene pores. In addition, some CO2 molecules remained in the pores because of the strong interaction between CO2 and graphene pores, blocking the pores and preventing permeation. The porous graphene with 0.7 nm pores allowed permeation of all the gas species. H2 permeation through the 0.4 and 0.7 nm pores occurred most readily, followed by He permeation; CO2 was the least likely to permeate through the 0.4 and 0.7 nm graphene pores. The permeation via 0.4 and 0.7 nm pores was thus in the following order; H2, He, CH4, and CO2, which agreed with the preceding MD simulations, as mentioned above.44 A hydrogen atom has the weakest interaction potential with graphene according to the MD simulations, so H2 rarely interacted with porous graphene at ambient temperature, like the He atoms. CO2 and CH4 molecules were attracted to the graphene surface, which hindered their permeation through the graphene pores. Therefore, molecular velocity and interactions influenced permeation through the 0.4 and 0.7 nm pores, whereas the molecular size-dependent permeation properties were observed for the 0.3 nm pores. The permeation properties of the 0.4 and 0.7 nm pores agreed with the above experimental results.


image file: c7cp03270f-f5.tif
Fig. 5 Snapshots of H2, CO2, CH4, and He permeation through graphene pores with diameters of 0.3, 0.4, and 0.7 nm after 0.3 ns. Each H2, CO2, CH4, and He species is depicted as two yellow spheres, a black and two green spheres, a black and four red spheres, and a blue sphere, respectively. Porous graphene is depicted as a black sheet.

image file: c7cp03270f-f6.tif
Fig. 6 Molecular penetration of H2 (black), CO2 (green), CH4 (red), and He (blue) through (a) 0.3, (b) 0.4 and (c) 0.7 nm pores.

The main parameters controlling molecular separation are permeance and selectivity. The performance of graphene in separation of H2 and CO2 could thus be assessed from a plot of its selectivity and permeance, as provided in Fig. 7. Here, selectivity was defined as the permeance of H2 over that of CO2. Both high selectivity and permeance are desired for high-performance separation sheets. Polymers have been used extensively for molecular separation. The empirical upper limit relationship obtained from experimental data suggests that there is a trade-off relationship between selectivity and permeance.22,45 The highest selectivity and permeance using polymers achieved to date are of the orders of 1 and 10−6 mol m−2 s−1 Pa−1, respectively.22 To improve the performance of separation membranes, various porous media have been tested.46 Selectivity was improved considerably by using porous media. In contrast, the permeance was similar to that using a polymer, although some silica and metal–organic frameworks have displayed a higher performance of 10−5 mol m−2 s−1 Pa−1.47–49 The single-, double-, and four-layer graphene samples prepared in this study had extremely high H2 permeances of 5.2 × 10−2, 1.4 × 10−2, and 0.7 × 10−2 mol m−2 s−1 Pa−1, respectively, as well as high selectivities of 5–20; the averaged selectivities were 5.8 for single-layer graphene, 6.4 for double-layer graphene, and 8.7 for four-layer graphene. Thus, graphene could be an atomic-thickness sheet that promises ultimate permeance, as mentioned above. Celebi and co-workers experimentally demonstrated that porous double-layer graphene displayed an extremely high permeance of 10−2 mol m−2 s−1 Pa−1 with a high H2/CO2 selectivity of 2–4.22 These values are similar to the above results for double-layer graphene. Graphene with a smaller layer number thus provided higher permeance, as expected. Double- and four-layer graphene exhibited similar permeances to that of porous graphene reported previously,22 while the permeance of single-layer graphene was improved to 0.05 while maintaining high selectivity. Conversely, higher selectivity was observed for the four-layer graphene sample.


image file: c7cp03270f-f7.tif
Fig. 7 Performance of graphene sheets in H2/CO2 separation (colored symbols). Blue: single-layer graphene, red: double-layer graphene, and green: four-layer graphene. Black symbols: reported relationships of H2/CO2 selectivity and H2 permeance for zeolites (filled circles),50–54 silica (filled squares),47,48,55–61 graphene oxide (filled diamonds),18,62 porous graphene (open circles),22 metal–organic frameworks (open squares),49,63 SiC and AlPO4 (open diamonds),64,65 and polymers (grey rectangle).22,45

Conclusions

The present study illustrated the extremely high H2 permeance of graphene sheets as well as their high selectivity for H2 over CO2. High-quality graphene sheets were directly fabricated on stainless-steel mesh substrates using CVD and their performance as molecular separation membranes was evaluated. Despite their atomic-scale thickness, the graphene sheets efficiently separated H2, CO2, and CH4. The selectivity for H2 over CO2 using the graphene sheets was 5–20, which was higher than that achieved by polymers and similar to that of porous materials. The permeance of the graphene sheets to H2 was considerably higher than that to other materials because of their ultimate thinness. The graphene pores were sufficiently larger than the size of gas species to allow their passage. The different dynamic and interaction properties of the gas species with graphene sheets caused their different permeation behavior through the graphene sheets. The interaction of molecules with graphene surfaces containing large pores separated molecules with different intermolecular interactions through graphene surface rejection, allowing molecular separation with extremely high permeance. A further study on controlling of graphene pore sizes is necessary for obtaining the higher separation performance with high permeance.

Acknowledgements

EDX and TEM observations were conducted at the Center for Analytical Instrumentation, Chiba University. This research was supported by the Japan Society for the Promotion of Science KAKENHI (Grant Numbers 26706001 and 15K12261) and the Futaba Electronics Memorial Foundation.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Schematic images of graphene synthesis and detection of gas permeation through graphene, optical and EDX mapping images, EDX spectra of graphene, XRD, and MD snapshots. See DOI: 10.1039/c7cp03270f

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