Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

TLDR: In this article, the authors report efficient and fast filtration of organic solutions through Graphene oxide (GO) laminates containing smooth two-dimensional (2D) capillaries made from large (10 to 20) flakes.

Abstract: Highly laminar graphene oxide flakes (10 to 20 μm in diameter) are fabricated. Reducing flake thickness to 10 nm enables water and organic solvent permeation, enabling the flakes to act as a highly effective organic solvent membrane. Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation1,2,3,4,5,6,7,8,9. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents1, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10–20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports1,2,3,4. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

Figures

Figure 2| Molecular sieving and organic solvent nanofiltration through HLGO membranes. (a) Experiments for salt rejection as a function of ion’s hydrated radius (largest ions within the aqueous solutions are plotted). The HLGO membranes are 8 nm thick. The hydrated radii are taken from ref. [5 and 7]. MB- Methylene Blue, RB – Rose Bengal, BB – Brilliant Blue. Inset: MB rejection and water permeance exhibited by the standard GO membrane with different thicknesses (colour coded axes). (b) Permeance of pure organic solvents through an 8 nm HLGO membrane as a function of their inverse viscosity. The used solvents are numbered and named on the right. Inset (top): Methanol permeance as a function of pressure gradient (ΔP). Dotted lines: Best linear fits. (c) Rejection and permeance of several dyes in methanol versus their molecular weight (colour coded axes). The dyes used: Chrysoidine G (CG), Disperse Red (DR), MB, Crystal Violet (CV), BB and RB. Left inset: Photographs of dyes dissolved in methanol before and after filtration through 8 nm HLGO membranes. Right inset:  MB rejection and methanol permeance of CGO membrane with different thicknesses (colour coded axes). Note that even though the dye rejection increases and approaches ~ 90% with increasing the CGO membrane thickness their permeance is

Figure 1| Ultrathin HLGO membrane. (a) SEM image of an 8 nm thick HLGO membrane on an Anodisc alumina support. Scale bar, 1µm. Inset: SEM image of bare alumina support. Scale bar, 500 nm. (b) X-ray diffraction for HLGO and CGO membranes. Inset (left): AFM image of HLGO membrane transferred from an alumina substrate to a silicon wafer. Scale bar, 500 nm. Inset (right): The height profiles along the dotted rectangle.

Figure 3| Probing molecular permeation through HLGO membranes. (a) X-ray diffraction for 70 nm thick HLGO membranes immersed in various organic solvents (colour coded). (b) Thickness dependence of permeance for methanol, hexane, and water through HLGO membranes (colour coded). Red and blue dotted lines are the best exponential fits. The black dotted curve is a guide to the eye. Inset: Water permeance as a function of inverse thickness for HLGO membranes with thicknesses ≥100 nm. Dotted line: best linear fit. The slope of linear fit provides the water permeability as ≈ 32 nm·Lm-2h-1bar-1. The solid line in the main figure shows the detection limit for methanol and hexane in our experiment. All the error bars are standard deviations using at least three different measurements using different samples.

Full text

This is a repository copy of Ultrathin graphene-based membrane with precise molecular
sieving and ultrafast solvent permeation.
White Rose Research Online URL for this paper:
https://eprints.whiterose.ac.uk/131418/
Version: Accepted Version
Article:
Yang, Q, Su, Yang, Chi, C et al. (10 more authors) (2017) Ultrathin graphene-based
membrane with precise molecular sieving and ultrafast solvent permeation. Nature
Materials. pp. 1198-1202. ISSN 1476-1122
https://doi.org/10.1038/nmat5025
eprints@whiterose.ac.uk
https://eprints.whiterose.ac.uk/
Reuse
Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless
indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by
national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of
the full text version. This is indicated by the licence information on the White Rose Research Online record
for the item.
Takedown
If you consider content in White Rose Research Online to be in breach of UK law, please notify us by
emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.

1

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast
solvent permeation
Q. Yang
1,2,†
, Y. Su
1,2,†*
, C. Chi
1,2,†
, C. T. Cherian
1,2
, K. Huang
1,2
, V. G. Kravets
3
, F. C. Wang
4
,
J. C. Zhang
5
, A. Pratt
5
, A. N. Grigorenko
3
, F. Guinea
3,6
, A. K Geim
3
, R. R. Nair
1,2*
1
National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK.
2
School of Chemical Engineering and Analytical Science, University of Manchester,
Manchester, M13 9PL, UK.
3
School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK.
4
Chinese Academy of Sciences Key Laboratory of Mechanical Behavior and Design of
Materials, Department of Modern Mechanics, University of Science and Technology of
China, Hefei, Anhui 230027, China.
5
Department of Physics, University of York, York YO10 5DD, UK.
6
Imdea Nanociencia, Faraday 9, 28015 Madrid, Spain.
†These authors contributed equally to this work.
*yang.su@manchester.ac.uk & rahul@manchester.ac.uk
Graphene oxide (GO) membranes continue to attract intense interest due to their
unique molecular sieving properties combined with fast permeation rates
1-9
. However,
the membranes’ use has been limited mostly to aqueous solutions because GO
membranes appear to be impermeable to organic solvents
1
, a phenomenon not fully
understood yet. Here, we report efficient and fast filtration of organic solutions through
GO laminates containing smooth two-dimensional (2D) capillaries made from flakes
with large sizes of ~ 10-20 µm. Without sacrificing their sieving characteristics, such
membranes can be made exceptionally thin, down to
10 nm, which translates into fast
permeation of not only water but also organic solvents. We attribute the organic solvent
permeation and sieving properties of ultrathin GO laminates to the presence of
randomly distributed pinholes that are interconnected by short graphene channels with
a width of 1 nm. With increasing the membrane thickness, the organic solvent
permeation rates decay exponentially but water continues to permeate fast, in
agreement with previous reports
1-4
. The application potential of our ultrathin laminates
for organic-solvent nanofiltration is demonstrated by showing >99.9% rejection of
various organic dyes with small molecular weights dissolved in methanol. Our work
significantly expands possibilities for the use of GO membranes in purification,
filtration and related technologies.

2

Membrane-based technologies enable efficient and energy-saving separation processes which
could play an important role in human life by purifying water or harvesting green energy
10,11
.
Recently, it was shown that molecular separation processes could benefit from development
of graphene-based membranes
2-4
that show tunability in pore size
8,12-15
and ultimate
permeance
15
defined by their thinness. In particular, GO-based membranes are considered to
be extremely promising for molecular separation and filtration applications due to their
mechanical robustness and realistic prospects for industrial scale production
2-4,7,9
. A
considerable progress in nanofiltration through GO membranes
2-4,16
was achieved mainly for
water (due to its ultrafast permeation
1-4
) while organic-solvent permeation has received
limited attention. This disparity is rather surprising as organic solvent nanofiltration (OSN)
attracts a tremendous interest due to its prospective applications in chemical and
pharmaceutical industries
11,17-20
. The development of novel inorganic membranes for OSN is
particularly vital because of the known instability of many polymer-based membranes in
organic solvents. The possible lack of motivation for exploiting graphene-based membranes
for OSN could have come from the previous reports on impermeability of organic solvents
through sub-micron thick GO membranes that remained highly permeable for water
1,2,21
.
Although some latest studies report the swelling of GO membranes in organic solvents and,
accordingly, indicate permeability of organic molecules even through thick GO
membranes
22,23
, this seems inconsistent with the previous reports
1,2,21
and could be explained
by the presence of extra defects that produce a molecular pathway. In an another work
24
OSN
was performed using a solvated reduced GO-polymer composite membrane and only
achieved a molecular sieve size of ≈ 3.5 nm due to the larger nanochannels in the membrane
than that of pristine GO membranes
1,2,5
. Molecular rejection for the above membranes
involves charge specific separation rather than the physical size cutoff. Membranes with
Angstrom size precise sieving along with high organic solvent permeance are of great
interests for OSN technology, however, such demostration is still lacking. In this report, we
investigate permeability and sieving properties of ultrathin GO membranes with respect to
organic solutions using an improved laminar structure and demonstrate the membranes’
potential for OSN.
The preparation of GO membranes used in our work is described in Methods. Figure 1 shows
the scanning electron microscope (SEM), atomic force microscope (AFM) images and X-ray
diffraction (XRD) of the studied GO membranes. Short duration ultrasonic exfoliation and a
stepwise separation (Methods) were used to obtain large GO flakes (lateral size D of 10 – 20
µm) with a relatively narrow size distribution (supplementary Fig. 1). The membranes
prepared from these large GO flakes are referred to as highly laminated GO (HLGO)
membranes due to their superior laminar structure. They show a narrow XRD peak (full
width at half maximum of 0.4 degree) as compared to 1.6 degree for the standard GO
membranes prepared from smaller flakes (D  0.1 – 0.6 µm). Below the latter are referred to

3

as the conventional GO (CGO) membrane. The narrow X-ray peak for HLGO laminates
suggests the importance of the GO flake size for the alignment process, which can be
attributed to stronger interlayer interaction between larger overlapping areas
25
. The stronger
interactions could further assist to eliminate the occasional wrinkles and corrugation found in
the CGO membranes
2,3
, and this could lead to achieving smoother 2D capillaries in HLGO
membranes.
Figure 1| Ultrathin HLGO membrane. (a) SEM image of an 8 nm thick HLGO membrane
on an Anodisc alumina support. Scale bar, 1µm. Inset: SEM image of bare alumina support.
Scale bar, 500 nm. (b) X-ray diffraction for HLGO and CGO membranes. Inset (left): AFM
image of HLGO membrane transferred from an alumina substrate to a silicon wafer. Scale
bar, 500 nm. Inset (right): The height profiles along the dotted rectangle.
To probe molecular sieving properties of HLGO membranes, we first performed vacuum
filtration of aqueous solutions of several salts and large molecules through HLGO
membranes (Methods). Figure 2a shows the molecular sieving properties of an 8 nm thin
HLGO membrane. Similar to micron-thick GO membranes
5
, HLGO membranes also block
all ions with hydrated radii larger than 4.5 Å. We emphasize that no molecular sieving was
observed in similar experiments but using CGO membranes with thickness of 8-50 nm (Fig.
2a inset). Hence, the ultra-sharp sieving cut-off can be achieved in HLGO membranes that
are more than two orders of magnitude thinner than conventional membranes showing same
sieving properties
5
. This drastic improvement can be attributed to the highly laminated nature
of our HLGO membranes. We failed to observe a cut-off in sieving only for the membranes
thinner than 8 nm, which sets a minimum thickness for HLGO membranes used in this study.
Ultrahigh permeance to fluids may occur in ultrathin membranes due to a decreased
molecular permeation length
6,15
. To further evaluate liquid permeance of HLGO membranes,
we have performed vacuum filtration and dead-end pressure filtration (supplementary
section2) experiments with water and a wide range of organic solvents using only 8 nm thick

4

membranes. All the permeance values were recorded after reaching a steady state condition,
typically achieved within 30 minutes. The liquid flux is found to be linearly proportional the
differential pressure (ΔP) across an HLGO membrane (Fig. 2b inset). The permeance for
various solvents as a function of their inverse viscosity (1/

) is shown in Fig. 2b. In contrast
to much-thicker GO membranes that exhibit ultrafast water permeation and impermeability
for organic solvents
1
, our HLGO membranes are highly permeable to all tested solvents. The
highest permeance is observed for solvents with the lowest viscosity. For example, hexane
shows permeance of ~18 Lm
-2
h
-1
bar
-1
, i.e, a permeability of ~144 nm·Lm
-2
h
-1
bar
-1
, despite
the fact that its kinetic diameter is almost twice larger than that of water
26
. On the contrary, 1-
butanol with a kinetic diameter similar to that of hexane
26
but much higher viscosity exhibits
the lowest permeance of 2.5 Lm
-2
h
-1
bar
-1
. The linear dependence of permeance on 1/

(see
Fig.2b) clearly indicates that the solvent viscosity dictates its permeability and proves the
viscous nature of the solvents’ flow through HLGO membranes.
Figure 2| Molecular sieving and organic solvent nanofiltration through HLGO
membranes. (a) Experiments for salt rejection as a function of ion’s hydrated radius (largest
ions within the aqueous solutions are plotted). The HLGO membranes are 8 nm thick. The
hydrated radii are taken from ref. [5 and 7]. MB- Methylene Blue, RB – Rose Bengal, BB –
Brilliant Blue. Inset: MB rejection and water permeance exhibited by the standard GO
membrane with different thicknesses (colour coded axes). (b) Permeance of pure organic
solvents through an 8 nm HLGO membrane as a function of their inverse viscosity. The used
solvents are numbered and named on the right. Inset (top): Methanol permeance as a function
of pressure gradient (ΔP). Dotted lines: Best linear fits. (c) Rejection and permeance of
several dyes in methanol versus their molecular weight (colour coded axes). The dyes used:
Chrysoidine G (CG), Disperse Red (DR), MB, Crystal Violet (CV), BB and RB. Left inset:
Photographs of dyes dissolved in methanol before and after filtration through 8 nm HLGO
membranes. Right inset: MB rejection and methanol permeance of CGO membrane with
different thicknesses (colour coded axes). Note that even though the dye rejection increases
and approaches ~ 90% with increasing the CGO membrane thickness their permeance is