AngewandteInternational Edition
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www.angewandte.orgChemie
Accepted Article
Title: Molecular bridges stabilize graphene oxide membranes in water
Authors: Mengchen Zhang, Yangyang Mao, Guozhen Liu, GongpingLiu, Yiqun Fan, and Wanqin Jin
This manuscript has been accepted after peer review and appears as anAccepted Article online prior to editing, proofing, and formal publicationof the final Version of Record (VoR). This work is currently citable byusing the Digital Object Identifier (DOI) given below. The VoR will bepublished online in Early View as soon as possible and may be differentto this Accepted Article as a result of editing. Readers should obtainthe VoR from the journal website shown below when it is publishedto ensure accuracy of information. The authors are responsible for thecontent of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913010Angew. Chem. 10.1002/ange.201913010
Link to VoR: http://dx.doi.org/10.1002/anie.201913010http://dx.doi.org/10.1002/ange.201913010
RESEARCH ARTICLE
Molecular bridges stabilize graphene oxide membranes in water
Mengchen Zhang, Yangyang Mao, Guozhen Liu, Gongping Liu,* Yiqun Fan and Wanqin Jin*
Abstract: Recent innovations highlight the great potential of two-
dimensional materials, such as graphene oxide (GO) films, in water-
related applications. However, undesirable water-induced effects,
such as the redispersion and peeling of stacked GO laminates, greatly
degrade their established performance and impact their practical
application. It remains a great challenge to stabilize GO membranes
in water. Here, we report on a molecular bridge strategy, in which an
interlaminar short-chain molecular bridge generates a robust GO
laminate that resists the tendency to swell. Furthermore, an interfacial
long-chain molecular bridge adheres the GO laminate onto a porous
substrate to increase the mechanical strength to the membrane. By
rationally creating and tuning the molecular bridges, the stabilized GO
membranes can exhibit outstanding durability in harsh operating
conditions, such as cross-flow, high-pressure, and long-term filtration.
The proposed general and scalable stabilizing approach for GO
membranes opens new opportunities for reliable two-dimensional
laminar films used in aqueous environments.
Introduction
Graphene oxide (GO) has attracted extraordinary interest for
its distinct two-dimensional structure and tunable
physicochemical properties[1]. This material displays fascinating
characteristics as a selective barrier and precise sieve in aqueous
solutions, making GO-based films widely applicable in numerous
important fields, such as water purification[2], desalination[3], anti-
corrosion and anti-fouling coatings[4], and energy storage and
conversion[5]. After nearly a decade of effort, GO film technologies
have matured and are realizing their potential. However, stability,
which is a prerequisite for broad applications, has become the
bottleneck that seriously impedes the use of GO films in water-
based implementations.
In particular, unfavorable swelling, redispersion, and peeling
of GO membranes in water have been reported often[6], and these
unstable characteristics greatly deteriorate the membranes’
established performance[7]. For example, the plentiful oxygenated
functional groups on GO nanosheets can become strongly
hydrated with water molecules and yield extra electrostatic
repulsion against π-π attraction, causing GO membranes to swell
or even disintegrate in water[6,7]. Several attempts, such as cross-
linking by molecules or ions[3a,8], insertion of polyelectrolytes[9], or
blending of graphene-based materials[10], have been proposed to
introduce multiple interactions between the GO laminate layers.
Unfortunately, the weak secondary bonding (e.g., hydrogen
bonding or intermolecular interaction) is insufficient to rigidly
connect neighboring GO nanosheets, while excessive cross-
linking is likely to disturb and block water transport within GO
nanochannels. In addition, GO membranes easily peel off the
underlying substrate due to the inadequate interfacial adhesion
between the GO layer and substrate, and this severely degrades
the mechanical strength and operational durability of GO
membranes. Huang and co-workers discovered that Al3+ released
from an anodized aluminum oxide (AAO) filter can immobilize the
GO layer on the AAO substrate[6a]. Also, chemically modifying the
substrate surface can enhance the interfacial compatibility of
composite membranes[11]. However, these methods are limited to
the specific chemistry of the substrate and fail to control substrate
nanostructures that also significantly affect the adhesion of the
GO layer.
Scheme 1. Schematic illustration for stabilizing a GO membrane through hierarchical interlaminar short-chain and interfacial long-chain molecular bridges. An amine
monomer acted as the interlaminar short-chain molecular bridge (short wavy line in red) to interlock the GO laminate through condensation and nucleophilic addition
reactions. Aldehyde-modified chitosan served as an interfacial long-chain molecular bridge (long wavy line in blue) to covalently bond with the amine-interlocked
GO laminate through a Schiff base reaction while firmly adhering to a porous substrate through a sufficient physical van der Waals force (dotted line).
To address the challenging instability issues of GO
membranes in water, we introduced hierarchical molecular
bridges to stabilize the GO membrane. As illustrated in Scheme
1, interlaminar and interfacial interactions of a GO membrane
were respectively reinforced by two types of molecular bridges.
To covalently interlock the GO laminate without sacrificing fast
water permeation, interlaminar short-chain molecular bridges
Interlaminar short-chain
molecular bridge
Interfacial long-chain
molecular bridge
GO nanosheet
Porous substrate
M. Zhang, Y. Mao, G. Liu, G.Liu, Y. Fan and W. Jin
State Key Laboratory of Materials-Oriented Chemical Engineering, College
of Chemical Engineering
Nanjing Tech University
30 Puzhu South Road, Nanjing 210009, P.R. China
E-mail: [email protected] (W.J.); [email protected] (G.L.)
Supporting information for this article is given via a link at the end of the
document.
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RESEARCH ARTICLE
were designed to ensure a robust assembly structure of GO
laminate via condensation and nucleophilic addition reactions that
resist the tendency toward swelling. Meanwhile, to firmly adhere
the GO laminate onto the substrate through sufficient interactions,
interfacial long-chain molecular bridges were introduced that
connect the GO layer and substrate by synergistically coupling
the chemical Schiff base reaction and physical van der Waals
interactions. With rational design of the molecular bridges, the GO
membranes were stabilized and exhibited outstanding durability
that can survive harsh conditions and maintain performance in
cross-flow, high-pressure, and long-term filtrations.
Results and Discussion
Interlaminar short-chain molecular bridge. When a GO
membrane is applied in an aqueous solution, excessive water
molecules are easily captured inside the GO interlayer channels
due to the strong hydration enabled by the plentiful oxygenated
functional groups on GO nanosheets[6.7]. The extra absorbed
water molecules in the GO nanochannels can lead to undesirable
swelling that enlarges the interlayer spacing and disrupts the well-
stacked GO laminates designed for selective water transport.
Hence, we expect that applying an interlaminar short-chain
molecular bridge can interlock the GO laminate against this
swelling tendency (Figure S1). Ethylenediamine (EDA) is
commonly used to covalently bond adjacent GO nanosheets[12].
As confirmed by the emergence of amide (C-N) absorption peaks
in IR spectra (Figure S2), the amine monomer generated
chemical linking with GO nanosheets through condensation
reactions with carboxyl groups and nucleophilic addition reactions
with epoxy groups[8,12]. The d-spacing measured by XRD
demonstrates the size of the GO interlayer channels (Figure 1a).
Compared with a pristine GO membrane that underwent
substantial d-spacing enlargement when wetted, from ~0.80 to
~1.45 nm, the EDA-bridged GO membrane exhibited only mild d-
spacing stretching after soaking in water, from ~0.84 to ~0.99 nm.
This indicated the anti-swelling effect of covalent bonding within
the GO laminate. Analyzing the molecular structure and
configuration aspects, we find that EDA with aliphatic amine
groups that donate electrons forms a strong nucleophile with high
electron density. However, we also speculate that it bears the high
electrostatic repulsion of negative charges from upper and lower
GO nanosheets at a very close distance, due to its short chain
length with only two carbon atoms. Alternatively,
paraphenylenediamine (PPD) featuring a six-carbon aromatic ring
is expected to exhibit high steric hindrance to afford much less
electrostatic repulsion. Interestingly, although a benzene ring in
PPD occupied a large interlayer spacing, the PPD-bridged GO
membrane displayed a d-spacing expansion of ~0.11 nm (from
~0.91 to ~1.02 nm), which was less than that of the EDA-bridged
GO membrane (~0.15 nm) and showed a reduced swelling
tendency. It is noteworthy that the distinct d-spacings of
interlaminar molecular-bridged GO membranes in a dry state are
mainly dependent on the spatial configuration of intercalated
amine monomers, whereas their variable d-spacings in a wet
state are likely determined by the bonding strength of amine
monomers within the GO laminate. On the basis of our
observations, we find that the latter may play a more vital role in
restraining GO swelling, as the swelling is closely related to the
binding energy between the amine monomer and GO nanosheet.
Unfortunately, the reactivity of amine groups in the aromatic PPD
is relatively low because of the high resonance stability of the
amine lone pair and benzene ring[12]. Considering integration
issues, we were inspired to use polydopamine (PDA) as the
interlaminar short-chain molecular bridge based on its fascinating
properties: it has both aliphatic amine groups with strong binding
reactivity and an appropriate molecular configuration with high
steric hindrance. Compared with its other counterparts (i.e., GO-
EDA, GO-PPD), the PDA-bridged GO membrane exhibited only a
slight 2θ shift in water, corresponding to a minimal d-spacing
expansion from ~0.90 to ~0.96 nm; thus, it has the most attractive
anti-swelling capability in an aqueous environment. Moreover, the
unique self-polymerization characteristics of PDA further offers a
degree of controllable linking within GO laminates[13].
The suppression effect of an interlaminar molecular bridge on
the swelling of the GO laminate was further demonstrated by the
variation in membrane separation performance over operation
time (Figure 1b). As expected, typical swelling-induced
permeation behavior was observed in the pristine GO membrane:
a progressive decline of rejection along with a rise in permeance.
Remarkably, the interlaminar molecular-bridged GO membranes
well maintain the rejection and permeance once reaching steady
state. Here, the variation ratio of the water permeance or rejection
(i.e., the initial value of the normalized difference between the final
and initial values) was calculated to quantify the swelling-caused
variation in transport properties. As displayed in Figure 1c, the
GO-PDA membrane exhibited a minimal swelling tendency with
only a rejection variation of 0.4% and a water permeance variation
of 6.0%, which are significantly less than those of a pristine GO
membrane. The permeation result confirms that strong
interlaminar interactions are able to hinder excessive intercalation
of water molecules into the GO laminate, thereby preserving the
ordered interlayer channels at an appropriate size for selective
water transport.
To further explore the formation and critical role of an
interlaminar molecular bridge within a GO laminate, we
investigated and regulated the linking degree of GO-PDA. We
observed that the yellow-brown GO dispersion turned into a dark-
brown solution exhibiting a visible change in color with increasing
linking time. On basis of observations in UV-vis (Figure S3) and
XRD (Figure S4) meaturements, we envisioned that DA
monomers self-polymerized to form an entwining network of
covalent interlocked GO nanosheets[14], offering significantly
enhanced interlaminar interactions within the GO laminate. The
separation performance of GO-PDA membranes (Figure S5)
showed that the bulge in the water transport channels was
remarkably relieved as the PDA linking time was prolonged.
These results further demonstrate the favorable suppressed GO
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RESEARCH ARTICLE
swelling resulting from the interlaminar molecular bridge that
closely interconnected adjacent GO nanosheets. However,
excessive PDA chains may also cause unfavorable blockage in
transport channels and thus lower the membrane permeance.
Figure 1. Anti-swelling capabilities of interlaminar short-chain molecular-bridged GO membranes. a, XRD patterns (left) of pristine GO and EDA-, PPD-, and PDA-
bridged GO membranes (color coded). Solid line: dry state; dotted line: wet state. Schematic illustrations (right) of GO-EDA, GO-PPD, and GO-PDA laminates.
Amine monomers covalently interlock GO nanosheets through condensation reactions with carboxyl groups and nucleophilic addition reactions with epoxy groups.
Amine monomers with aliphatic amine groups can form a strong nucleophile to generate high binding strength with GO nanosheets. Meanwhile, amine monomers
with large molecular configuration can exhibit high steric hindrance to afford much less electrostatic repulsion from negatively charged GO nanosheets. b, Operation
time dependence of water permeance and rejection of pristine GO and EDA-, PPD-, and PDA-bridged GO membranes (color coded). Solid line: water permeance;
dotted line: rejection rate. c, Relative variation ratios (initial value normalized difference between final value and initial value) of water permeance and rejection of
pristine GO and EDA-, PPD-, and PDA-bridged GO membranes. Blue: permeance variation; Gray: rejection variation.
Interfacial long-chain molecular bridge. Although the
interlaminar short-chain molecular bridge efficiently averts the
undesired redispersion of GO nanosheets in water, a GO laminate
could easily peel away from the substrate during the separation
process due to inadequate adhesion[12,14]. Intrinsically, flexible GO
nanosheets with a unique 2D structure tend to attach only to the
surface of the substrate, and the internal elastic strain energy from
bending further induces GO laminate peeling and freestanding[15].
Moreover, it is difficult to create a highly adhesive platform for a
porous substrate with a micrometer-scale rough and inert surface
using conventional surface modifications with short-chain
agents[16]. Here, we proposed to establish an interfacial long-
chain molecular bridge, which not only provided large contact
areas with the porous substrate layer but also generated chemical
binding with the GO laminate layer. Aldehyde (glutaraldehyde or
maleic anhydride)-modified chitosan (O=CS), with a moderate
molecular weight and abundant functional groups, was designed
as the interfacial long-chain molecular bridge. We expect that a
properly controlled molecular weight would enable O=CS to form
an ultrathin and uniform covering on a porous substrate with
appropriate pore penetration (Figure S6), thus offering sufficient
physical interaction with the underlying substrate[17]. Furthermore,
plentiful aldehyde groups of O=CS would be beneficial for
generating chemical bindings with the amine groups of the upper
GO-PDA laminate[18].
The AFM images (Figure 2a and b) show typical surface
topographies for a porous ceramic tube substrate before and after
O=CS modification, and its roughness remarkably dropped from
282.4 nm to 67.7 nm. This is due to the decoration of O=CS
molecular bridges on the substrate filling in the low-lying regions.
Meanwhile, the skeletons of the underlying substrate remained
clearly identified due to the ultrathin O=CS decoration. These
morphologies agreed well with SEM results (Figure 2c and d, left),
demonstrating an ultrathin decoration of O=CS molecular bridges
uniformly covering the surface of a ceramic substrate. In the
cross-sectional EDX mapping (Figure 2c and d, right),
overlapping of the N elements with the CS and Zr elements from
the ceramic suggested a transition layer formed by partial
penetration of the O=CS molecular bridge into the ceramic
substrate, which significantly enlarged the contact areas to yield
substantial physical van der Waals interactions between them.
We further assembled the GO-PDA laminate on the O=CS-
decorated ceramic substrate and performed XPS depth analysis
(Figure 2e) to distinguish the individual layers of GO-PDA/O=CS
/ceramic according to the contents of N elements (from the GO-
0
5
10
15
20
25 Permeance variation
Rela
tive v
ariation r
atio (
%)
GO-EDA GO-PPD GO-PDAGO
0.0
0.5
1.0
1.5
2.0
Rejection variation
Rela
tive v
ariation r
atio (
%)
0 1 2 3 4 5 63
6
9
12
GO
Perm
eance (
Lm
-2h
-1bar-1
)
Operation time (h)
GO-PDAGO-PPDGO-EDA
GO
GO-EDAGO-PDAGO-PPD
90
92
94
96
98
100
Re
jection
(%
)
5 6 7 8 9 10 11 12
Inte
nsity (
a.u
.)
2 (degree)
GO-PDA
GO-PPD
GO-EDA
Wet state
GO
Dry state d-spacing (nm)
0.99 0.84
0.96 0.90
1.02 0.91
1.45 0.80
GO-EDA
GO-PPD
GO-PDA
b c
a
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PDA laminate and O=CS chains) and Zr elements (from the
ceramic substrate) varying with cross-sectional depth[19]. The
transition region with crisscrossing N and Zr content at 140–160
nm depths further demonstrated the partial pore penetration of CS
molecular bridges into the porous ceramic substrate with
sufficient interaction contact areas. In addition to physically
interacting with the underlying ceramic substrate, the interfacial
O=CS molecular bridges formed chemical bindings with the upper
GO-PDA laminate as confirmed in the C1s XPS spectra (Figure
S7). A new peak arose at ~288.0 eV that can be assigned to C=N
bonding, suggesting the formation of a covalent connection
between PDA and O=CS[20]. In a further IR analysis (Figure S8),
the C=N stretching vibration at ~1690 cm-1 again indicated the
reaction of extra free aldehyde groups of O=CS with amine
groups from GO-PDA through a Schiff base reaction[21].
Figure 2. Adhesion characterizations of interfacial long-chain molecular-bridged GO membranes. AFM images with roughness data for a, the ceramic substrate
surface and b, the O=CS/ceramic substrate surface. c, Cross-sectional SEM images of O=CS-decorated ceramic substrate (left) with EDX mappings of selected
region (right). d, Surface view SEM images of O=CS-decorated ceramic substrate (left) with EDX mappings of selected region (right). Gray points: Zr signal; red
points: N signal. e, XPS depth analysis results of GO-PDA/O=CS/ceramic membrane. Red triangle: N element content from GO-PDA laminate and O=CS chains;
blue square: Zr element content from ceramic substrate. Depth 0–80 nm: GO-PDA laminate; depth 80–100 nm: transition region; depth 100–140 nm: O=CS
decoration; depth 140–160 nm: transition region; depth 160–200 nm: ceramic substrate. f, Cross-sectional SEM images of GO/ceramic tube with GO layer peeling
off the substrate. g, Cross-sectional SEM images of GO-PDA/O=CS/ceramic membrane with GO layer firmly adhering to the substrate. Insert: local enlarged image
of GO membrane layer with thickness of ~150 nm. h, Schematic illustration of nanoscratch technique. i, Results of nanoscratch measurements of GO/ceramic tube
and GO-PDA/O=CS/ceramic membrane. Red line: scratching depth signal of GO-PDA/O=CS/ceramic membrane with critical load of 46.3 mN; blue line: scratching
depth signal of GO/ceramic membrane with critical load of 20.9 mN; black line: scratching load.
The resulting GO-PDA/O=CS/ceramic membrane exhibited
excellent integrity without incurring any visible damage under
finger touch, whether in a dry or wet state, whereas the pristine
GO laminate easily peeled off the ceramic substrate. Further SEM
characterizations showed that the pristine GO/ceramic membrane
yielded an evident boundary with a huge crack between the GO
laminate and ceramic substrate (Figure 2f); this was caused by
only a slight vibration during sample preparation. Such severe
detachment of a GO laminate is due to its weak combination with
the substrate. In contrast, with the aid of O=CS as an interfacial
molecular bridge, the GO-PDA laminate firmly adhered to the
ceramic substrate without any gaps (Figure 2g).
We further employed the nanoscratch technique to in situ
quantify the membrane interfacial binding force in order to monitor
and record the scratch distance relationships with the dynamic
load (Figure 2h)[22]. The membrane layer with strong interfacial
adhesion with the substrate is expected to display greater
mechanical strength and endure a higher scratch load. The critical
load at failure is considered to be the interfacial binding force of
the membrane onto the substrate. As shown in the results of
nanoscratch measurements (Figure 2i), the GO-
PDA/O=CS/ceramic membrane had a critical load (46.3 mN)
more than twice that of the pristine GO/ceramic membrane (20.9
mN). Such excellent mechanical properties of the GO-
PDA/O=CS/ceramic membrane again proved that the created and
tuned interlaminar short-chain (PDA) and interfacial long-chain
(O=CS) molecular bridges significantly consolidated the
integrated GO membrane.
Membrane stability and strategy generality. Cross-flow, high-
pressure, and continuous filtration separation processes were
0 40 80 120 160 200
0
2
20
25
30 N element
Zr element
Ato
mic
(%
)
Etch depth (nm)
0 100 200 300 400
0
20
40
60
80
100
120
Scra
tch
ing
lo
ad
(m
N)
Displacement (m)
20.9 mN
46.3 mN
GO/Ceramic tube
Scra
tch
ing
de
pth
sig
na
l GO-PDA/O=CS/Ceramic tube
1 µm
Ra=282.4 nm
Rq=313.6 nm
Ceramic tube
1 µm
Ra=67.7 nm
Rq=89.5 nm
O=CS/Ceramic tube
500 nm
O=CS/Ceramic tube
1 µm
O=CS/Ceramic tube
c
d
ea
b
10 µm
10 µm
GO/Ceramic tube
~150 nm
hf
g
i
Stylus
GO laminate on substrate
Progressive scratch
Applied load
Displacement
GO-PDA laminate O=CS
decoration
Ceramic
substrate
GO-PDA/O=CS/Ceramic tube
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performed on GO membranes to validate their stability and
durability in water. As shown in Figure 3a, the GO-
PDA/O=CS/ceramic membrane exhibited remarkable integrity
while maintaining excellent rejection and stable permeance when
subjected to a cross-flow feed at 50 mL/min for 300 min. Figure
3b demonstrated that the water flux showed an approximately
linear increase with no slowdown, and the rejection rate displayed
a steady high value as the feed pressure increased to 10 bar. This
suggested that the PDA interlaminar molecular bridge not only
suppressed the swelling of the GO laminate, but also enhanced
the rigidity of GO nanochannels so they could withstand high
transmembrane pressure. The results of the long-term continuous
filtration operation in Figure 3c demonstrated excellent stability of
the GO-PDA/O=CS/ceramic membrane over a duration of 25
days. In comparison, the pristine GO/ceramic membrane was
incapable of withstanding shear stress[10], and fragmented GO
pieces were easily carried away when applying the above cross-
flow condition.
Figure 3. Stability and durability of GO-PDA/O=CS/ceramic membrane. Nanofiltration performance of GO-PDA/O=CS/ceramic membrane under a, cross-flow up
to 300 min, b, transmembrane pressure up to 10 bar, and c, continuous operation up to 25 days. Blue: water permeance; gray: rejection rate. d, Photos of GO/ceramic,
GO-PDA/ceramic, and GO-PDA/O=CS/ceramic membranes after high-power sonication for 30 min in water. e, Comparison of long-term operation times for state-
of-the-art two-dimensional material membranes applied in water separation[10,11,24-27]. More details about the membrane and operation conditions are provided in
Table S1.
Cleaning processes involving vibration and sonication are
often adopted when reusing membranes in practical separation
processes[23]. To further evaluate the structural stability under
cleaning treatment, the pristine GO/ceramic, GO-PDA/ceramic,
and GO-PDA/O=CS/ceramic membranes were subjected to high-
power (40 kHz frequency) sonication for 30 min in water. As
revealed in Figure 3d, the violent vibration from sonication
crumbled the pristine GO membrane and caused exfoliation of
GO stacks into small pieces. The GO-PDA membrane maintained
the integrity of the GO laminate, implying that the PDA
interlaminar molecular bridge greatly enhanced the interlaminar
interactions of the GO laminate to prevent its disassembly.
Unfortunately, the layer entirely peeled off and disclosed the
underlying white ceramic substrate because of insufficient
interfacial interaction between the laminate and substrate. In
contrast, by simultaneously stabilizing the GO layer and
GO/substrate interface, the integrated GO-PDA/O=CS/ceramic
membrane remained intact without any visible gaps, cracks, or
damages.
We summarized representative results in the recent literature
involving stability evaluations of two-dimensional material
membranes, including membranes based on GO[6a, 7, 9b, 10, 11, 15, 20,
24], covalent-organic-frameworks (COF)[25], g-C3N4[26], and
MXene[27] (Table S1), and illustrated their long-term operation
time in water-based separations, such as desalination,
nanofiltration, and pervaporation (Figure 3e). Promisingly, the
superior stability of our GO-PDA/O=CS/ceramic membrane offers
a greater competitive advantage over other counterparts,
especially in long-term operations that demand much more
stringent requirements for membrane stability.
Additionally, it is well known that membrane stability is
correlated with the physicochemical properties of GO materials[28].
Thus, we also tried to stabilize GO membranes by tuning the
physical and chemical characteristics of GO nanosheets (Figure
S9-S12). Due to the inevitable water swelling, it is not surprising
that the d-spacings of wet GO membranes were remarkably larger
than those of dry GO membranes, regardless of the lateral size
and oxidation degree of GO (Figure 4a and Figure S13). The GO
0 200 400 600
Long-term operation time (h)
GO-PDA/O=CS/ceramic
COF@CNF/PAN
GO/PDA-ceramic
GO-PDA/PES
PDI-GOF/ceramic
Cation-GO/PAN
COF-LZU1/ceramic
Polycation-GO/PAN
Fe(OH)3/g-C
3N
4/AAO
MXene/AAO
MXene/nylon
rGO-MWCNT/PC
This work
60 90 120 150 180 210 240 270 3000
2
4
6
8
Pe
rme
an
ce
(L
m-2h
-1b
ar-1
)
Operation time (min)
90
92
94
96
98
100R
eje
ctio
n (
%)
0 5 10 15 20 250
2
4
6
8
Perm
eance (Lm
-2h
-1bar-1
)
Operation time (day)
90
92
94
96
98
100
Reje
ction (
%)
0 2 4 6 8 100
10
20
30
40
50
Flu
x (Lm
-2h
-1)
Operation pressure (bar)
90
92
94
96
98
100
Re
jectio
n (
%)
a cb
e
GO/Ceramic GO-PDA/Ceramic GO-PDA/O=CS/Ceramic
d After sonication in water
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membrane derived from a small lateral size (SGO) suffered the
most severe swelling, with d-spacing stretching from 0.80 nm to
1.52 nm, and the highly reduced, large-lateral-sized GO
membrane (rLGO-3) yielded the slightest d-spacing stretching
from 0.69 nm to 0.97 nm. The lowest d-spacing expansion (0.28
nm) was achieved by increasing the lateral size and reducing the
oxidation degree of GO; however, this was more than 400% larger
than that of the GO-PDA membrane (merely 0.06 nm). These
results clearly demonstrated that the nonlinked GO membranes,
even those with the fewest hydrophilic groups and narrowed
interlayer spacing (rLGO-3) after careful tuning, endured a greater
swelling tendency than the GO-PDA membrane. The above
control experiments highlighted the indispensable role of creating
and tuning molecular bridges in order to stabilize GO membranes
with desirable anti-swelling capability in an aqueous environment.
Figure 4. Generality and scalability of the molecular-bridge stabilization strategy
for GO membranes. a, D-spacing expansion of GO membranes with different
lateral sizes and oxygenated functionality in dry and wet states as calculated
from their XRD results. Gray: dry state; blue: wet state. Cross-sectional SEM
images of b, GO-PDA/O=CS/nylon and c, GO-PDA/O=CS/MCE membranes.
Inserts are photographs of each membrane. d, Photograph of scaled inner
surface GO-PDA/O=CS/ceramic membrane with 40 cm length.
The generality of the proposed molecular-bridge stabilization
strategy for GO membranes was demonstrated earlier by utilizing
various amines (e.g., EDA, PPD, and PDA) for interlaminar short-
chain molecular bridges, as well as alterable O=CS derived from
glutaraldehyde or maleic anhydride for interfacial long-chain
molecular bridges. More importantly, this methodology is
applicable to other porous substrates beyond the ceramics used
above. We took commonly used nylon and mixed- cellulose
acetate (MCE) substrates with a nominal pore size of ~200 nm as
examples (Figure S14 and S15). As shown in Figure 4b and c,
both photographs and SEM images revealed a GO-PDA laminate
that was closely immobilized upon the O=CS-bridged substrate
with outstanding integrity. The separation performance and
stability in water of the as-prepared GO-PDA/O=CS/nylon and
GO-PDA/O=CS/MCE membranes were tested under a cross-flow
feed condition, and they exhibited comparable water permeance
of 7.6 and 8.1 L m-2 h-1 bar-1 with remarkable dye rejection of
98.9% and 98.3%, respectively.
Scale-up is another key step for implementation of GO
membranes[29]. Compared with polymers, ceramics show
advantages in structural, chemical, and thermal stability[17]. The
scalability of a ceramic-supported membrane can be further
enhanced if overcoming an inner-surface membrane technology
that offers better protection of the separation layer as well as
higher packing density if using multi-channels[24d]. In this work, we
successfully scaled up our stabilized GO membrane supported on
the inner surface of a ceramic tube with length from 7 cm to 40
cm (Figure 4d), using PDA as an interlaminar molecular bridge
and O=CS as an interfacial molecular bridge. The large-area
(~100 cm2) stabilized GO membrane exhibited very promising
nanofiltration performance, with water permeance of 3.6 L m-2 h-1
bar-1 and dye rejection of 97.2%.
Conclusion
In conclusion, we presented a facile and efficient
methodology for stabilizing GO membranes by creating and
tuning molecular bridges. A properly designed amine acted as an
interlaminar short-chain molecular bridge to interlock the GO
laminate through condensation and nucleophilic addition
reactions, contributing to a mechanically robust GO assembly
structure with anti-swelling capability. Meanwhile, ultrathin O=CS
served as an interfacial long-chain molecular bridge to covalently
bond with the amine-interlocked GO laminate through the Schiff
base reaction. It firmly adhered to a porous substrate through
sufficient physical attachment areas. The molecular-bridged GO
membrane featured superior stability in a harsh water separation
process, showing excellent durability in cross-flow, high-pressure,
and long-term filtration and the ability to withstand vibration and
sonication treatment. This facile molecular-bridge stabilization
strategy, with additional generality and scalability, paves a new
avenue toward the implementation of new two-dimensional
laminar films for reliable water-related applications, such as water
purification, desalination, anti-corrosion and anti-fouling coatings,
and energy storage and conversion.
Experimental Section
Creating interlaminar molecular bridge. GO powder was dissolved in
deionized water followed by ultrasonication to obtain a 0.01 mg/mL GO
dispersion. 0.4 g tris (hydroxymethyl) aminomethane was dissolved in 200
0.0
0.3
0.6
0.9
1.2
1.5
1.8 Dry state
Wet state
d-s
pacin
g (
nm
)
SGO MGO LGO rLGO-1 rLGO-2 rLGO-3 GO-PDA
0.72 0.70.65
0.61
0.38
0.28 0.06
1 µm
GO-PDA/O=CS/Nylon
1 µm
GO-PDA/O=CS/MCE
GO-based membrane on inner surface
of ceramic tube
40 cm
a
b c
d
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RESEARCH ARTICLE
mL dilute GO dispersion followed by adjusting the pH to 8.5 through adding
1 M HCl dropwise. Then, 0.1 g DA was added in the aforementioned GO
dispersion under stirring at room temperature to obtain PDA linked GO
dispersion. Similarly, 0.1 g diamine monomer (EDA or PPD) was added in
the GO dispersion under stirring to prepare EDA and PPD linked GO
dispersion, respectively.
Creating interfacial molecular bridge. CS solution was prepared by
dissolving 0.05 g CS in a 50 mL 0.5 wt% acetic acid aqueous solution.
Excess GA (25% aqueous solution) or MA (0.1 M) with a high molar ratio
of crosslinkers to CS units (2:1) was added in the CS solution and reacted
under stirring for 2 min. The final O=CS solution was dip-coated on the
surface of ceramic tube and kept still for 5 min before being poured out.
The resultant ceramic tube was rinsed with deionized water and dried at
room temperature.
Preparation of GO membrane on inner surface of ceramic tube.
Ceramic tube was mounted into tube component and sealed by rubber O-
rings on both ends of the substrate. A solution container was connected to
the tube component, and the dilute GO dispersion was drove onto the inner
side of the tube by pressured nitrogen under 2 bar. The flat substrate was
mounted in a self-designed filtration device and sealed by a rubber gasket,
and the dilute GO dispersion was deposited on the substrates driven by
pressured nitrogen under 2 bar as well. It worth noticing that all the GO
solution should be filtrated in order to avoid the redispersion of GO
membrane when relieving the driving pressure. All the as-prepared GO
membranes were dried and solidified overnight at 60 ºC in vacuum before
use.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21490585, 51861135203,
21922805, 21776125), the Innovative Research Team Program
by the Ministry of Education of China (No. IRT_17R54) and the
Topnotch Academic Programs Project of Jiangsu Higher
Education Institutions (TAPP). We thank Prof. Xiaoming Ren for
helpful discussion.
Keywords: graphene oxide • molecular bridge • stability •
membrane • nanofiltration
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Entry for the Table of Contents
A facile stabilization strategy of GO membranes was proposed by creating and tuning
interlaminar short-chain and interfacial long-chain molecular bridges.
Mengchen Zhang, Yangyang Mao,
Guozhen Liu, Gongping Liu,* Yiqun
Fan and Wanqin Jin*
Page No. – Page No.
Molecular bridges stabilize
graphene oxide membranes in
water
Interlaminar short-chain
molecular bridge
Interfacial long-chain
molecular bridge
GO nanosheet
Porous substrate
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