www.sciencemag.org/content/344/6181/289/suppl/DC1

Supplementary Material for

Ultimate Permeation Across Atomically Thin Porous Graphene

Kemal Celebi, Jakob Buchheim, Roman M. Wyss, Amirhossein Droudian, Patrick Gasser, Ivan Shorubalko, Jeong-Il Kye, Changho Lee, Hyung Gyu Park*

*To whom correspondence should be addressed: parkh@ethz.ch

Published 18 April 2014, Science 344, 289 (2014)

DOI: 10.1126/science.1249097

This PDF file includes:

Materials and Methods

Figs. S1 to S11

Full Reference List

Supplementary Materials for

Ultimate Permeation across Atomically Thin Porous Graphene

Kemal Celebi1†, Jakob Buchheim1†, Roman M. Wyss1, Amirhossein Droudian1, Patrick Gasser1, Ivan Shorubalko2, Jeong-Il Kye3, Changho Lee3 & Hyung Gyu Park1*

1 Nanoscience for Energy Technology and Sustainability, ETH Zurich, Sonneggstrasse 3, CH-8092 Zürich, Switzerland. 2 Laboratory for Electronics/Metrology/Reliability, EMPA (Swiss Federal Laboratories for Materials Science and Technology), Überlandstrasse 129, CH-8600 Dübendorf, Switzerland 3 Materials & Components R&D Laboratory, LG Electronics Advanced Research Institute, 38 Baumoe-ro, Seocho-gu, Seoul 137-724, Korea. * To whom correspondence should be addressed: parkh@ethz.ch † These authors contributed equally to this work.

Materials and Methods

Graphene synthesis

Graphene was grown on copper foils (Alfa Aesar #13382) in a cold-wall chemical vapor

deposition system (Aixtron AG). The samples were annealed for 30 min at 950°C under Ar

(1500 sccm) and H2 flow (100 sccm), followed by a two-step growth with flowing ethylene at

25 sccm for 2 min and 50 sccm for 1 min. All growths were carried out at 4 mbar chamber

pressure.

Graphene Transfer

Graphene was transferred using a spin-coat-and-back-etch method in order to obtain double

layer graphene on the target substrate (36). Our modified method begins with the spinning of

1

poly (methyl methacrylate) PMMA (950k, 2% Anisol) on as-grown graphene at 4000 rpm,

yielding sub-100-nm polymer layers. The PMMA/graphene/copper is then placed on the

surface of (NH4)2S2O8 solution (0.5 M in water) to etch the copper. After 10 min, the sample

is removed from the solution and the backside of the foil is cleaned to remove graphene

remainder. The successive, 90-min-long etch removes the copper entirely, leaving the

PMMA/graphene layer. The floating PMMA/graphene layer is transferred to a DI-water bath

for rinsing. Another as-grown graphene on a copper foil is used to fish out the rinsed sample

and left for air-drying, yielding double layer graphene between copper and PMMA. The

procedure for the copper foil removal is then repeated as described above. The second fish-out

is performed by the holey SiNx frame, followed by air-dry and a subsequent hotplate anneal

for 30 min at 180°C, which relaxes PMMA and promotes the adhesion of graphene on the

target substrate. The PMMA is finally removed in a quartz tube furnace at 400°C for 2 hours,

under 500 sccm H2 : 500 sccm Ar flow.

FIB patterning and membrane characterization

The freestanding double layer graphene was patterned using Focused Ion Beam (FIB) milling.

Two FIB methods were employed. First, for pores diameters between 16 nm and 1000 nm,

Ga+ ion beam (FEI Helios 450) exposure (30 kV, 33 pA) was used. A dose of ~0.5-5×10-5

pA/nm2 yielded well-defined pore size distributions. In the second method, sub-10-nm pores

were drilled by a He+ ion FIB (Zeiss Orion Plus) using a 30 kV, 16 pA beam and an exposure

dose of ~6×10-3 pA/nm2. Sub-100-nm pores are drilled using single pixel exposures at high

dwell times. Too high dosage or increased pore density were avoided, due to the possibility of

tearing of the graphene, connecting nearby pores (Fig. S7). The geometry and edge structures

of graphene pores were investigated by high-resolution transmission electron microscopy (Cs-

corrected HRTEM, JEM ARM 200F, JEOL, Japan) at an accelerating voltage of 200 kV (Fig.

S8). After patterning, the membranes were thoroughly investigated for larger holes, cracks 2

and patterning-induced defects by acquiring high resolution SEM (FEI Helios 450) images of

each patterned window. The pore sizes were subsequently characterized using these SEM

images (5 kV, 25 pA). Pore diameter distributions were determined using an image analysis

software (ImageJ 1.45s).

Characterization of pore-edge chemistry (XPS and TOFSIMS)

Samples of freestanding double-layer graphene in the patterned (2500-5000 10-nm-diameter

pores) and unpatterned (pristine graphene) states were investigated for possible differences in

the oxygen content. Since patterning leaves dangling carbon bonds at the pores formed in

graphene, it is expected that at these positions water and possibly oxygen would react to

saturate these dangling bonds. To assess the oxygen content, patterned and unpatterned

samples were mounted on Au coated Si wafers inside the FIB chamber and later analyzed by

X-ray photoelectron spectroscopy (XPS). The utilized probe was a monochromatized Al Kα

X-ray beam with a diameter of about 8 µm in a Quantum 2000 imaging XPS spectrometer

(Physical Electronics Instruments, Inc.). Imaging analysis was not possible, although the SiNx

support frame of 50×50 µm in size could well be localized. This enabled the area-selective

analysis of oxygen, carbon, silicon and nitrogen through their most intense core level lines

(O1s, C1s, N1s, Si2p).

The data show that the relative amount of oxygen with respect to carbon (i.e., the sensitivity-

corrected intensity ratio O/C) is enhanced in the patterned sample by about 10±5% as

compared to the unpatterned sample. This result corroborates the presence of dangling bonds

in graphene as a consequence of the FIB patterning and environmental exposure. Additional

time-of-flight secondary ion mass spectrometry (TOFSIMS) data comparing the fragments

O+, O-, OH- and CO- from both samples show clearly higher oxygen-containing signals from

the patterned sample. In an analogous way, this finding can be an additional support for the

3

presence of oxidation-passivated pores in the porous graphene membrane. From the acquired

data, we could mention about pore edge shape and edge chemistry as follows.

Edge shape - To fabricate our membranes, we use double-layer graphene formed by double

transferring of CVD-grown, polycrystalline graphene sheets, with crystalline sizes ranging

from sub-µm to a few µm. Thus the physically torn edges are likely to take a random

crystalline direction. Actually, the edges produced by FIB are shown random by transmission

electron microscopy (Fig. S8).

Edge chemistry - From the fact that the FIB-perforated graphene samples are taken out of the

FIB chamber and exposed to a slightly humid laboratory environment, we anticipate that

oxidation might proceed and passivate the pore edges. Passivation by oxygen-containing

chemical moieties via strong oxidation has been observed by other researchers, suggesting

that the pore edges can be terminated by carbonyl, hydroxyl, or carboxyl groups. It is,

therefore, possible that the edges of the graphene pores are also terminated by these oxygen-

containing moieties, although their number density may not be as high as observed by the

above studies. Another possibility is that trace carbon and hydrogen in the FIB chamber can

bind to the edges right after the ion bombardment, providing partial hydrogen termination.

The XPS and TOFSIMS data suggest that there exist oxygen-containing moieties at the pore

edge, but we cannot rule out unambiguously the existence of hydrogen termination. It is likely

that the graphene pore edges are terminated by a variety of chemical moieties from oxygen to

hydrogen containing species.

Effect of edge chemistry to transport - Edges are particularly important for small pores, as the

edge chemistry can alter the flow rates and selectivity. There have been theoretical studies

showing significant edge effects, such as water permeation enhancement by hydrophilic edge

termination (OH) (37), H2/CH4 selectivity reduction by N-functionalization (38), anion

4

blockage by F and N functionalization and cation blocking by H-termination (39), as well as

ion blockage by carboxyl groups (40, 41). These studies use sub-nm pores, where the electron

clouds of the functional group take up a significant portion of the pore area. For our

membranes, on the other hand, the pore sizes are 1-3 orders of magnitude larger than such

functional groups, e.g., the size of a C-O group is ~0.40 nm (considering van der Waals radii

of C (~0.11 nm) and O (~0.15 nm) and the C-O covalent bond length of ~0.14 nm) and the

size of a C-N group is ~0.42 nm (considering van der Waals radii of C and N (~0.16 nm) and

the C-N bond length of ~0.15 nm), and therefore we can safely neglect the effect of the edge

chemistry on the direct permeation .

Raman measurement of graphene membrane

We conducted two-dimensional Raman scans on both patterned (four 1 μm pores in one 4-

μm-wide window) and unpatterned free standing double layer graphene membranes (Fig. S9).

The excitation wavelength and power were 532 nm and 2 mW, respectively, and the pixel

spacing for the 2D Raman mapping was chosen to be 100 nm. The unpatterned freestanding

graphene membrane shows a high quality graphene with high G peak intensity and uniformly

low D peak intensities. The patterned graphene, on the other hand, shows a reduced G peak

intensity and a significant increase in the D peak intensity. The four 1 μm graphene pores can

be easily identified in the 2D Raman mapping.

Stability of graphene membranes

We experimentally confirmed the temperature stability of our graphene membranes. Baking

the membranes at 250°C in air for more than 2 hours did not change the pore size or lead to

any other failure as shown in (Fig. S10A). Porous graphene is also inert in strong acids, bases

and solvents. We exposed the membranes to acidic conditions in 1 M H2SO4 for 2 h and

5

acetone. We confirmed that none of the treatments causes a change of the pore sizes of the

graphene membrane (Fig. S10B, C).

Gas flow measurement setup

Single and multi-component gas flow measurements were carried out in a constant-volume /

variable-pressure apparatus (Fig. S4A). At the upstream of the setup, the calibrated mass flow

controllers (MKS, Germany) regulated the feed as well as the retentate streams; while at the

downstream, another mass flow controller adjusted the flow rate of Ar as the sweeping gas.

The flow rate and the composition of the permeate stream (i.e., H2, CO2, Ar) were determined

respectively through a mass flowmeter and a calibrated mass spectrometer (Cirrus 2, MKS,

Germany). During the measurements, the membrane was clamped in a 2 × 2 cm2 custom made

fixture made of polyoxymethylene (POM). All the measurements were carried out at room

temperature (25°C).

Single component gas measurement

To maintain the gas purity, the flow setup was evacuated and flushed with pure gases

repeatedly before each measurement. The downstream pressure was kept at atmospheric

pressure while the upstream pressure was changed in the range of 0-400 mbar gauge pressure.

All measurements used only pure gases (99.99%) without use of any carrier gas.

Separation factor characterization

The molar compositions of the gas mixtures were regulated by the mass flow controllers. The

feed flow rate was set high enough (stage cut: Permeate / Feed = 1%) in order to avoid feed

composition change due to the permeation of the feed through the membrane. Therefore, the

measurements were independent of the feed flow rates (42). The back diffusion rate of the

carrier gas (Ar) from the permeate side to the feed side through the membrane was estimated 6

to be less than 0.1 sccm, which is less than 1% of the retentate flow rate. The carrier gas

contribution to the gas data was, hence, neglected from the feed side.

Since the mass spectrometer was calibrated with different gas mixtures, the detected ratios of

the flow rates of the permeated gases are independent of the sweeping gas flow rate. The

sweeping gas flow rate was set 100-fold higher than the permeate flow rate, reducing the

partial pressure of the permeating gases at the permeate side. During the gas mixture test, the

upstream pressure was kept constant at 350 mbar gauge, while the downstream pressure was

kept at atmospheric pressure. The permeate composition was determined when the system

reached to steady state after several hours.

Quantification of leakage flow through the double layer graphene membrane

Graphene lattice is impermeable to gases because the diameter of the geometric opening of

the honeycomb carbon lattice (0.064 nm) is far smaller than the van der Waals diameter of He

atoms (0.28 nm) (5). We can consider leakage as a measure for the existence of defect-

originated pores. Bunch et al. measured a He leak rate of 105-106 atoms/s through a 2×2 µm

freestanding graphene (at ~1 bar ΔP). This area is about two orders of magnitude smaller than

our membrane area. Thus we can expect a maximum leak rate of 108 atoms/s through our

freestanding graphene before perforation. This rate is only valid for a perfect graphene lattice

and it may be claimed that our leakage rate can be larger due to carbon vacancies, grain

boundaries or other defects that can form sub-nm pores. To probe this claim, we measured our

leakage rate before the pore drilling and found it to be 3×1011 molecules/s (this is the

maximum leakage value, limited by the noise of our mass spectrometer). Although this value

is three orders of magnitude larger than the prediction above, it is still 5-6 orders of magnitude

less than our measured N2 flow rates through FIB-drilled pores (in the range of 1016-1017

molecules/s, depending on the membrane and ΔP). Therefore, we believe that the mass-

transport contribution by inherent sub-nm atomic defects of CVD-grown graphene is 7

negligible, thus the influence of graphenic repulsion on the overall permeation can also be

neglected.

Characterization of the membrane clogging during the liquid flow measurement

According to a recent report (20), graphene exposed to water can be torn apart along the grain

defects and peel away from the substrate. During the water permeation test, we also found that

the graphene peeled off and blocked the pores by either covering the membrane (Fig. S11A)

or clogging the pores in an agglomerated shape (Fig. S11B), thereby leading to gradual flux

reduction. We believe that the issue of the graphene pore clogging remains to be further

investigated. In this study, we report the water permeance based on the initial flow rates

assuming that all pores are open. This data agrees with the permeance value estimations using

the final flow rate and reduced pore area (accounting for the N2 flux reduction).

Mechanical deformation of the graphene membrane and feed pressure

The deformation of the double layer graphene can be calculated using the membrane shell

theory. The basic assumption is that the membrane structure cannot support any bending

moment. This assumption leads to a very simple situation where only in-plane stresses apply,

being constant over the cross section of the structure. The bulging (deflection h from the flat

membrane) caused by a uniform pressure load on the membrane can be calculated as ℎ =

�𝑝𝑝𝑅𝑅4(1−𝜈𝜈)8𝐸𝐸𝐸𝐸

�13 (assuming 𝑡𝑡 ≪ 𝑅𝑅 and ℎ ≪ 𝑅𝑅), where p is the applied pressure, R the membrane

radius, Et the two dimensional (2D) elastic modulus with t being the membrane thickness, and

ν the poisson ratio of graphene (43).

Inserting typical values for our double layer of polycrystalline CVD graphene membrane

(𝐸𝐸𝑡𝑡 = 2 × 55 𝑁𝑁/𝑚𝑚, 𝜈𝜈 = 0.3, 𝑝𝑝 = 40𝑘𝑘𝑘𝑘𝑘𝑘, 𝑅𝑅 = 2𝜇𝜇𝑚𝑚) a maximal deflection ℎ ≈ 115 𝑛𝑛𝑚𝑚 at

the center of the 4µm diameter free-standing graphene membrane is obtained (44). This 8

calculated deflection is slightly higher than the deflection of single-crystalline graphene flakes

measured at low pressures by AFM (45), which is consistent with the reported effect of

softened elastic response of multi crystalline CVD graphene (44). Note that the patterned

pores further weaken the elastic response of the graphene membrane. This effect can be easily

accounted for by including the notion of membrane porosity 𝜎𝜎 to reduce the 2D elastic

modulus: 𝐸𝐸𝑡𝑡𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = (1 − 𝜎𝜎)𝐸𝐸𝑡𝑡 . The graphene membranes reported here have a typical

porosity of 3-4%, and therefore the changes of the elastic modulus would be minor.

The calculated membrane deflection is very small causing only very little change of the

curvature of the 4-μm diameter membrane. Since the membrane is operated in steady state

conditions and the estimated curvature caused by the bulging is quite small, we do not believe

that the permeation mechanism would be altered significantly. Another concern is the

corresponding in plane strain in the graphene sheet, which could potentially increase the pore

size. However, the calculated strain 𝜖𝜖 ≈ 2.2 × 10−3 is not sufficient to significantly increase

the membrane pore size. Both assumptions are supported by the gas permeation data obtained

from our membranes. The gas flux is linear in the applied pressure drop over the range of 0 to

2 bar indicating that even at high membrane pressures there is no noticeable pore size

increase. Hence we conclude that the bulging of the graphene membrane in our experimental

condition is not strong enough to have a significant effect on the permeation.

9

Supplementary Figures

Fig. S1. SEM image of successfully transferred, ultraclean graphene double layers on a SiNx

micromesh frame having 49 holes.

10 µm

10

Fig. S2. N2 gas fluxes with respect to applied pressure difference across the membrane

showing linear relationship.

Fig. S3. N2 gas flow rate with respect to applied pressure difference across 50-nm-pore

graphene membrane at higher pressures up to 2 bar.

0 100 200 300 400 5000

2

4

6

8

7.6 nm 100 nm 400 nm 1000 nm

Flux

(109 s

ccm

m-2)

Pressure difference (mbar)

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

1.0

1.2

N2 f

low

rate

(scc

m)

Pressure difference (mbar)

11

Fig. S4. (A) Schematic of the gas flow measurement setup. Mass flow controllers are used to

mix gases at desired molar ratios and to set flow rates of the retentate and carrier gases. A

high precision mass flow meter allows precise determination of the permeate flux. The

permeate side is connected to a calibrated mass spectroscope. (B) Separation factor of our

graphene membrane (50-nm pore diameter) for varied H2/CO2 molar ratios at the feed.

Retentate

Pressure Regulator

MFCMFC

MFC

Ar (Sweeping Gas)

MFC

MFM

MS

Vent (1 bar)

Pres

sure

G

auge

CO

2

H2

Membrane Holder

Permeate

A B

12

Fig. S5. Water vapor transmission through a 400-nm-pore graphene membrane. Raw data of

measured weight loss over time in the upright cup evaporation experiment. Red dots are

measured values, and the black line is the linear fit of the weight loss rate. The measurement

was carried out at 35% relative humidity at 25oC.

Fig. S6. Raw data of measured DI water permeation through a 200-nm-pore graphene

membrane. Applied membrane feed pressure is 250 mbar. Red dots are measured values and

black line is the linear fit of the permeate flow rate after flow stablilization.

0 500 10000.0

0.5

1.0

1.5

2.0

2.5

Wei

ght l

oss

(mg)

Time (min)

0 5 10 150.0

0.5

1.0

1.5

2.0

2.5

Perm

eate

d vo

lum

e (m

m3 )

time (min)

13

Fig. S7. (A) SEM image showing successful FIB perforation of a freestanding graphene

double layer, seen at an inclined angle. (B) An effect of overcurrent during FIB perforation of

freestanding graphene. Excessive ion bombardment tears graphene apart following the

crystalline dislocation, or grain boundaries, leaving two 1-μm-wide pores connected to each

other.

Fig. S8. Transmission electron microscope image of an FIB-drilled, 15-nm pore on

freestanding double layer graphene.

1 µm 1 µm

A B

14

Fig. S9. Two-dimensional Raman mapping of graphene membrane (532 nm, 2 mW power,

100 nm spacing). (A) SEM image of membrane scanned. (B) G peak (1600 cm-1) of

unpatterned (left) and patterned (right) double layer graphene membrane. (C) D peak (1350

cm-1) of unpatterned (left) and patterned (right) double layer graphene membrane.

2μm

D peak (1350 cm-1) 1.0

0

G peak (1600 cm-1)1.0

0

A

B

C

15

Fig. S10. (A) SEM image before and after baking a 50-nm-pore graphene membrane in

oxidizing environment for 2 hours at 250°C. (B) SEM image before and after immersing a 50-

nm-pore graphene membrane into 1 M H2SO4 for >2 hours. (C) SEM image before and after

immersing a 50-nm-pore graphene membrane in acetone for >2 hours.

1μm

Before acetone exposure

1μm

After acetone exposure

After H2SO4 exposure

1μm

Before H2SO4 exposure

After baking in air

1μm

Before baking in air

1μm

A

B

C

16

Fig. S11. (A) Peeled graphene covering a large portion of the membrane area. (B) A close-up

SEM image of 50-nm-wide graphene pores clogged after the water permeation experiment.

200nm500nm

A B

17

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