Author’s Accepted Manuscript

Effect of substrate on formation and nanofiltrationperformance of graphene oxide membranes

Mengchen Zhang, Jiajia Sun, Yangyang Mao,Gongping Liu, Wanqin Jin

PII: S0376-7388(18)33187-9DOI: https://doi.org/10.1016/j.memsci.2018.12.071Reference: MEMSCI16754

To appear in: Journal of Membrane Science

Received date: 15 November 2018Revised date: 24 December 2018Accepted date: 26 December 2018

Cite this article as: Mengchen Zhang, Jiajia Sun, Yangyang Mao, Gongping Liuand Wanqin Jin, Effect of substrate on formation and nanofiltration performanceof graphene oxide membranes, Journal of Membrane Science,https://doi.org/10.1016/j.memsci.2018.12.071

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1

Effect of substrate on formation and nanofiltration performance of

graphene oxide membranes

Mengchen Zhang, Jiajia Sun, Yangyang Mao, Gongping Liu*, Wanqin Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, Nanjing, P R China

*Corresponding authors.

E-mail address: wqjin@njtech.edu.cn (W.Q. Jin), gpliu@njtech.edu.cn (G.P. Liu).

Contact information:

Prof. Wanqin Jin

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, 30 Puzhu South Road, Nanjing 210000, P. R. China

Tel.:+86-25-83172266;

Fax: +86-25-83172292

E-mail: wqjin@njtech.edu.cn

Abstract

Graphene oxide (GO) has been considered as a promising material to develop advanced

nanofiltration membranes benefiting from its extraordinary physicochemical properties. GO

membranes for practical application need porous substrates to provide sufficient mechanical

strength. Compared to extensive studies on the manipulation of GO selective layer, the

influence of substrates on the performance of GO membranes has been received much less

attention. Actually, significant differences in physical and chemical properties of the

substrates should lead to distinct assembly structure of resulting GO membranes with uneven

performance. Therefore, the effect of substrate on GO membranes formation and separation

are worth study and optimization. Herein, we employed typical inorganic ceramic tube and

polymeric polyacrylonitrile (PAN) and polycarbonate (PC) substrates to support GO

2

membranes and studied the effects of their surface morphologies and roughness, surface

chemical composition, as well as bulk pore structure on the formation and nanofiltration

performance of resulting GO membranes. Substrates properties were revealed to have

remarkable impacts on the adhesion and transport property of GO membranes. We found that

the surface morphological and chemical structure of substrates induced GO assembly and

determined GO adhesion, and the bulk pore structure of substrates dominated the whole

transport resistance of GO membrane. Especially, the PAN substrate possessing abundant

oxidized functional groups after simple hydrolysis, contributed to a robust interfacial

adhesion with GO selective layer and enabled GO membrane to withstand harsh stability

measurements including cross-flow, high feed pressure and long-period continuous operation.

Besides, the smooth surface morphology along with the bulk highly porous structure of PAN

substrate offered a favorable platform for GO assembly, resulting in competitive

nanofiltration performance with water permeance of 15.5 Lm-2

h-1

bar-1

and dye rejection of

99.5%. Overall, this work gives new insights of design and fabrication of durable GO

membranes for practical applications.

Keywords: graphene oxide membrane, substrate, ceramic, PAN, nanofiltration

1. Introduction

Membrane-based nanofiltration technology has been considered an alternative to

conventional wastewater treatment owing to its advantages in separation capacity and energy

efficiency [1]. Advanced membranes with adequate separation performance and robust

mechanical property are extensively pursued to meet the urgent water shortage and

environmental issues [2]. Two-dimensional (2D) materials, especially graphene oxide (GO),

have been demonstrated as an emerging kind of candidates for developing nanofiltration

membranes, owing to the unique atomic-thin nanosheets assembled 2D nanochannels

offering unimpeded water permeation meanwhile rejecting small molecules and ions [3-5].

With the extremely thin feature, GO selective layer must be supported by a porous

substrate to form a practical membrane. It has long been commonly considered that the

performance of membranes is determined by the top selective layer, and the function of

3

substrate is merely to provide mechanical support. Thus, till now studies are focused on

manipulating structure and chemistry of GO selective layer to pursue precisely molecular

sieving capability [6-8], but the influence of substrates on the performance of GO membranes

has been received much less attention. In addition, there is a common phenomenon that GO

membranes often suffer from poor structural stability in water [9]. As reported, it is hard to

maintain the integrity of GO membranes even with a gentle finger touch in lab experiment [9],

which is unable to withstand harsh conditions in practical applications. In order to enhance

the stability of GO membranes, studies on preventing the swelling of GO laminates in water

has aroused the extraordinary interest of researchers [10]. Nevertheless, it should be pointed

out that the interfacial adhesion between GO layer and underlying substrate is another

essential issue for reliable membrane filtration, which depends sensitively on substrate

properties, while remains seldom explored. Actually, recent works have discovered an

indispensable role of substrate structure on the formation of the selective layer of thin film

composite (TFC) membranes. They attempted to figure out what kind of substrate properties

(e.g. pore size, porosity, surface roughness and hydrophilicity, etc.) could match perfectly

with an interfacially polymerized selective layer [11-13]. It can be expected an equally

important influence of substrate on GO membrane formation, however, lacking of a

systematic study.

Presently, polymeric substrates including polyacrylonitrile (PAN) [14], polycarbonate (PC)

[15], polyethersulfone (PES) [16] and polysulfone (PSf) [17] have been generally employed

as supports for GO membranes. The PAN substrate possesses more hydrophilic surface

compared to normal PES and PSf substrates which is expected to have better compatibility

with GO selective layer. In addition, the PC substrate shows extremely smooth surface with

high-density straight-through pores that is supposed to realize minimizing transport resistance

thus boost the water permeance of GO membranes. Alternatively, the stability of GO

membranes can be improved by inorganic ceramic substrates owing to their rigid bulk

structure, high chemical durability and good solvent resistance [18-20]. In addition, preparing

membrane on the inner surface of ceramic tube is more attractive and favorable for

membrane module assembly in industry, because the inner surface membrane layer can be

better protected against mechanical and physical damage during handling operations.

4

Considering the diversity of substrate choices for GO membranes, there are significant

differences in physical and chemical properties of the substrates in terms of surface roughness,

chemical composition and pore structure, which should lead to distinct assembly structure of

resulting GO membranes with uneven performance. Therefore, the effect of substrate on GO

membranes formation and separation are worth study and optimization.

In this paper, we investigated inorganic ceramic tube and polymeric PAN and PC

substrates with distinctive structures and properties as the support for fabricating GO

membranes. Multiple factors of these substrates including surface morphological structure,

surface chemical structure and bulk pore structure that affect the interfacial adhesion and

nanofiltration performance of GO membranes were studied in detail (Fig. 1). We found that

the surface morphological and chemical structure of substrates induced GO assembly and

created extra hydrogen bonding to reinforce connection with GO selective layer, and the bulk

pore structure of substrates dominated the whole transport resistance of GO membrane. Our

findings of substrate effect on formation and transport property of GO membrane would

provide essential basis for development of practically robust GO membranes.

Fig. 1. Schematic illustration of GO selective layer supported on a porous substrate, including some key

factors determining the formation and interfacial adhesion of GO membrane.

2. Experimental

2.1 Materials

GO powders prepared by modified Hummer’s method were purchased from Nanjing

JCNANO Tech Co., Ltd., China. The GO nanosheets have lateral size of 1~5 µm, thickness

of 1~2 nm and single layer ratio of >99%. Ceramic tubes were home-made (Membrane

Science &Technology Research Center of Nanjing Tech University). They are asymmetric

ZrO2/Al2O3 tubular membranes with the inner/outer diameters of 8/12 mm and the length of 7

GO selective layer

Porous substrate

(e.g. ceramic tube, PAN

substrate, PC substrate)

Interfacial adhesionEffects of substrate:

• Surface morphological structure

• Surface chemical structure

• Bulk pore structure

5

cm. The porosities of ceramic tubes are about 35~40%. PAN ultrafiltration membranes with a

nominal pore size of 20 nm were purchased from Shandong Megavision Membrane

Technology & Engineering Co., Ltd., China. PC ultrafiltration membranes with a nominal

pore size of 200 nm were purchased from Merck Millipore, Ltd., Billerica, MA. Rhodamine

B (RhB, C28H31ClN2O3, 479.02 Da), methyl orange (MO, C14H14N3NaO3S, 327.33 Da),

methylene blue (MB, C16H18ClN3S·3H2O, 373.90 Da), and inorganic salts including Na2SO4,

NaCl, MgSO4, MgCl2, were purchased from Sinopharm Chemical Reagent Co., Ltd., China,

and used as received. Deionized water was used in all the preparation and evaluation

processes.

2.2 Membrane preparation

The GO powders were dissolved in deionized water followed by ultrasonication to obtain

the GO dispersion (0.01 mg mL-1

). In order to obtain a relatively homogeneous nanosheets

lateral size, GO dispersion was centrifuged at 12,000 rpm for 20 min to remove small

nanosheets in supernatant, and then redispersed and subjected to centrifugation at 6000 rpm

for 20 min to remove large nanosheets in precipitate. Subsequently, the resultant GO

dispersion was deposited on the substrates under 2 bar pressure, forming the GO membranes

composed of a thin GO selective layer on a porous substrate. The final GO membranes were

dried and solidified overnight at 45 ºC in vacuum before use.

2.3 Characterizations

The surface height profiles with roughness data of substrates were measured by atomic

force microscopy (AFM, XE-100 Park SYSTEMS, Korea) operated in the non-contact mode.

The surface and cross-sectional morphologies of membranes and substrates were imaged by

field emission scanning electron microscope (FESEM, S4800, Hitachi, Japan) at the voltage

of 5 kV and the current of 10 μA. We measured several different regions of each membrane

sample, and selected a representative image among them (see the Supplementary Information

in detail). The surface chemistry structure of substrates was characterized by X-ray

photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA) using monochromatized al

Kα radiation. The mean pore-sizes of the ceramic tubes were determined by the bubble-pore

6

method using pore-size distribution analyzer (PSDA-20, GaoQ Func. Mater. Co., Ltd.,

China).

2.4 Nanofiltration performance evaluation

Membrane performance is tested using nanofiltration process under 2 bar by a

self-designed filtration device at room temperature. The water permeance J (L m-2

h-1

bar-1

)

was measured by weighting the permeated water in a time interval of 10 minutes using an

electronic balance and calculated using the following equation:

𝐽 =𝑉

𝐴 × 𝑡 × 𝑃

where V is the volume of permeate collected (L), A is the membrane effective area (m2), t is

the permeation time (h), P is the operation pressure (bar).

Organic dye solutions of rhodamine B (RhB), methyl orange (MO), methylene blue (MB)

at the feed concentration of 200 ppm, or monovalent and divalent salts solutions (Na2SO4,

NaCl, MgSO4, MgCl2) at the feed concentration of 10 mM were employed to determine the

rejection ratios R (%) by the following equation:

𝑅 = (1 −𝑐𝑝

𝑐𝑓) × 100%

where cp and cf are the concentrations of the permeate and feed solution respectively. UV-Vis

spectrometer (Lambda 950, Perkin Elmer, USA) was used to analyze the concentrations of

the dyes in permeate and feed solutions. Electrical conductivity (DDS-307, Shanghai Leici

Instrument Co., Ltd., China) was used to obtain the conductivities of the monovalent and

divalent salts solutions in permeate and feed solutions.

3. Results and discussion

3.1 Surface morphological structures of substrate

During membrane preparation process, GO nanosheets were deposited on the surface of

substrate and assembled into laminates. The van der Waals interaction tended to bring the GO

nanosheets conformal to the surface of substrates [21]. Therefore, surface morphological

structures of substrate strongly influence the formation of GO selective layer. The surface

7

morphologies and roughness of ceramic tube, PAN and PC substrates are compared and

exhibited in Fig. 2. One can easily identify the totally dissimilar surface features of these

three types of typical substrates attributed to their different materials and preparation methods.

The surface of ceramic tube (Fig. 2a and d) showed ZrO2 and Al2O3 particles agglomerate

together by sintering process [22] and shaped consolidated ceramic surface with average pore

size of ~200 nm. As a result, it displayed a rough “ridge and valley” topography of ceramic

tube with a large surface roughness of 103.4 nm. In contrast, polymeric membranes displayed

flatter surface. The skin layer of PAN substrate (Fig. 2b and e) had a nodular structure formed

in phase-separation process [23] with uniformly distributed micropores with a nominal pore

size of 20 nm. The agglomerated nodules composed a dense surface of PAN substrate

showing an average surface roughness of 11.9 nm. Apparent different surface structure

existed in the case of track-etched PC substrate (Fig. 2c and f) that there is no nodular

structure but large pores throughout the whole surface [24]. It exhibited a relatively low

density of almost perfectly circular straight pores of ~200 nm and a significant smooth

surface with a low surface roughness of 5.1 nm.

Fig. 2. Surface morphologies and roughness of different substrates. AFM images and roughness data of a)

ceramic, b) PAN and c) PC substrate. SEM images of d) ceramic, e) PAN and f) PC substrate.

Considering the extreme flexibility of GO nanosheets, it is reasonable that during the

deposition process, GO nanosheets would wrap and attach on the ridge and tuck into valley

Ra=11.9 nm

Rq=16.4 nm

Ra=5.1 nm

Rq=6.5 nm

a) b) c)

d) e) f)

Ra=103.4 nm

Rq=132.8 nm

8

even fall into interior pores to form GO laminates along the conformal morphologies of the

substrates [25]. As shown in Fig. 3a-c, it can be clearly observed that the substrates were

completely covered by overlapping GO nanosheets and formed a continuous and defect-free

membrane layer. The surface of GO membranes showed small wrinkles and rippled

appearance, which arose from the convoluted and folded structure of GO nanosheets during

membrane assembly [26]. Besides, the skeletons of underlying substrates were faintly visible.

Especially for ceramic-supported GO membrane, a conformal morphology of membrane

surface with apparent fluctuations can be observed. The cross-sectional views of these

membranes (Fig. 3d-f) can give us a clearer perspective. We found GO nanosheets tended to

conform to the surficial structure of substrates, resulting in thin GO selective layers snugly

attached on top of the substrates.

Fig. 3. Surface and cross-sectional morphologies of GO membranes on different substrates. SEM images

of a) and d), GO/ceramic membrane, b) and e), GO/PAN membrane, c) and f), GO/PC membrane.

We further conducted XRD and Raman characterizations of top GO selective layer on

different substrates as shown in Fig. 4. It can be observed that they have similar stacking

structures with d-spacing of ~0.8 nm, and ID/IG of ~0.9, demonstrating that the effect of

substrate has ignorable influence on the stacking structure of top GO selective layer.

a) b) c)

d) e) f)

9

Fig. 4 a) XRD and b) Raman characterizations of top GO selective layer on different substrates.

3.2 Surface chemical structures of substrate

The surface chemical structures of substrates are regarded to provide possible interaction

sites with GO selective layer, which is crucial to construct durable GO membranes with

strong interfacial adhesion [21]. We employed XPS characterization to clarify the chemical

composition and relative content of functional groups on the surface of different substrates

(Fig. 5). The O 1s spectra of ceramic substrate (Fig. 5a) showed contributions arising from

Zr-O, Al-O and O-H at binding energy of 529.8 eV, 531.1 eV and 532.0 eV respectively,

which indicated that the surface of ZrO2/Al2O3 substrate existed a number of hydroxyl groups.

The C 1s spectra of PAN (Fig. 5b) displayed characteristic peaks corresponding to C-C, C-N,

C-O and C=O at binding energy of 284.6 eV, 285.7 eV, 286.6 eV and 288.8 eV respectively,

which signified the surface of PAN substrate contained plenty of nitrile groups and a few

oxidized functional groups. In addition, the C 1s spectra of PC (Fig. 5c) is indicative of C-C,

C-H and C-O at binding energy of 284.5 eV, 285.1 eV and 286.5 eV respectively, which

implied the presence of a substantial amount of methyl groups on the surface of PC substrate.

10.0 10.5 11.0 11.5 12.0 12.5

33 34 35 36 37 38 39 40

Inte

nsity (

a.u

.)

2(degree)

d-spacing: ~ 0.8 nm

GO/ceramic tube

GO/PAN

GO/PC

1000 1200 1400 1600 1800 2000

Inte

nsity (

a.u

.)

Raman shift (cm-1)

D G

ID/I

G ratio: ~ 0.9

GO/PC

GO/PAN

GO/ceramic tube

a) b)

10

Fig. 5. Surface chemical composition and species characterization of different substrates. XPS a) O 1s

spectra of ceramic, b) C 1s spectra of PAN, c) C 1s spectra of PC, and d) C 1s spectra of h-PAN.

GO nanosheets contain many hydroxyl, carboxyl and epoxy groups, which are beneficial

for the generation of hydrogen bonding with substrates to enhance the interfacial adhesion of

GO membranes [27]. Actually, the effectiveness of interfacial adhesion appears to be directly

related to the number of oxidized functional groups available on surface of substrates with the

abundant bonding sites on GO laminates. In order to enhance the oxidation of the substrate,

alkaline hydrolysis of PAN (namely, h-PAN) was employed as a facile approach [28-31]. The

pristine PAN substrate was placed in the sodium hydroxide solution at 55 °C for 2 hours and

underwent obvious color changes during reaction. The C 1s spectra of h-PAN (Fig. 4d)

exhibited weakened peak of C-N at 285.7 eV and strengthened peak of C-O at 286.5 eV,

which evidenced that the nitrile groups decreased and the hydroxyl groups increased after the

hydrolysis process. On basis of the formation theory of hydrogen bonds, we assumed that

hydrogen bonds would form among oxidized functional groups on GO laminates and

substrates through interaction sites in the case of ceramic substrate and h-PAN-supported GO

534 532 530 528 526

Inte

nsity (

a.u

.)

Binding energy (eV)

O-H (532.0 eV)

Al-O (531.1 eV)

Zr-O (529.8 eV)Ceramic tube

O 1s

292 290 288 286 284 282 280

PC

C 1s

Inte

nsity (

a.u

.)

C-H (285.1 eV)

C-O (286.5 eV)

C-C (284.5 eV)

Binding energy (eV)

292 290 288 286 284 282 280

Inte

nsity (

a.u

.)

Binding energy (eV)

h-PAN

C 1s

C-C (284.6 eV)

C-N (285.7 eV)

C-O (286.5 eV)

C=O (289.4 eV)

292 290 288 286 284 282 280

Binding energy (eV)

PAN

C 1s

Inte

nsity (

a.u

.)

C-C (284.6 eV)

C-N (285.7 eV)

C-O (286.6 eV)

C=O (288.8 eV)

a) b)

c) d)

11

membranes, and PC-supported GO membrane joined only through weak Van der Waals force,

as illustrated in Fig. 6. Based on the characterizations of surface chemical structures of three

substrates, it was expected that the ceramic and h-PAN substrates would offer substantial

interfacial adhesion with GO laminates, while the PC-supported GO membrane might be

challenging to maintain the membrane integrity.

Fig. 6. Schematic illustration of the assembly of GO laminates on different substrate based on hydrogen

bond and Van der Waals force formed between functional groups on GO and substrate.

3.3 Bulk pore structures of substrate

Besides the aforementioned surface morphological and chemical structure of substrate that

affected GO assembly, the bulk pore structure is regarded as dominant factor to determine the

mechanical and transport properties of GO membrane [11]. From the cross-sectional views in

Fig. 3 and 6, we found the three different substrates showing no resemblance in appearance

of their bulk pore structures. Ceramic is expected to be a robust substrate with rigid bulk

structure. The main body of ceramic tube was composed of sintered pellets and formed a

compact porous structure, which involved extraordinarily strong mechanical strength but

increased transport resistance as well. Ceramic substrates with various pore-sizes were tested

(Fig. 7a), and the results showed that they possessed narrow pore-size distributions with the

average pore-size of 20 nm, 225 nm and 790 nm, respectively. As observed in Fig. 7b, the

water permeance of these ceramic substrates followed the general sequence that higher water

OH OH CH3 CH3 CH3

COOHO OH COOHO OH COOHO OH

Hydrolysis

Hydrogen bond (strong)

Van der Waals force (weak)

CN CN CN CN CN

OH CN OH OH OHOH

Ceramic tube h-PAN

PAN

PC

12

permeance achieved as pore-size increased. In addition, the PAN substrate with a nominal

pore size of 20 nm had asymmetric structure consisting of a very thin, dense top layer

roughly 1 μm thick and a finger-like cylindrical pore structure bottom layer with large tubular

macrovoids. This feature enabled PAN substrate to provide a flat platform for the assembly

of GO laminates and a considerable number of large channels for water permeation. As a

result, with the same pore size of 20 nm, the PAN substrate generated water permeance of

585 Lm-2

h-1

bar-1

which is more than 5-fold higher than that of ceramic substrate. Moreover,

the PC substrate with a nominal pore size of 200 nm exhibited straight pore channels with

practically no tortuosity through its bulk structure, contributing to its ultrahigh water

permeance of 4575 Lm-2

h-1

bar-1

because of the extremely low transport resistance.

Nevertheless, as one can notice in Fig. 3f, the internal pore channels of PC substrate easily

collapsed. Very careful sample preparation process can cause the serious deformation and

even crack of the bulk pore structure, indicating the poor mechanical property of PC

substrate.

Fig. 7. Pore-size and water permeance of different substrates. a) Pore-size distribution of three ceramic

substrates; b) Water permeance of different substrates.

3.4 Interfacial adhesion of GO membranes

The interfacial adhesion between GO laminate and underlying substrate plays a crucial role

in maintaining the stability of GO membrane during practical application [25]. However,

researchers paid more attentions to reinforcing the interaction between GO nanosheets to

overcome the swelling of GO laminate in water [10], but few studies have involved in the

assessment of interfacial adhesion of GO membranes. Unlike reported works that only

0 100 200 700 800 900

0

10

20

30

40

50

Po

re s

ize

dis

trib

utio

n,

f(d

)

Pore diameter (nm)

0

500

1000

5000

475

Perm

eance (

Lm

-2h

-1bar-1

)

Ceramic tube

(20 nm)

Ceramic tube

(225 nm)

Ceramic tube

(790 nm)

PAN

(20 nm)

PC

(200 nm)

110

791

581

4575

a) b)

13

statically soaked the membranes in water, here we evaluated the interfacial adhesion of GO

membranes under cross-flow condition, which is closer to the actual operation process [32].

The GO membranes were potted into a homemade cross-flow testing cell, and a peristaltic

pump was utilized to pump water into the cell. The cross-flow rate was controlled to 500 mL

min−1

for a duration of 10 min by the flowmeter.

Fig. 8. Photos showing the membrane integrity of the GO membranes supported by different substrates

before and after the cross-flow testing.

Fig. 8 compared the local enlarged photos of GO membranes supported by different

substrates before and after the cross-flow testing, and distinct interfacial adhesion

performance can be observed. We found the ceramic-supported GO membrane adhered firmly

on the substrate with no apparent cracks and defects. We attributed the good adhere

performance of GO/ceramic membrane to two main reasons. On one hand, the rough surface

of ceramic tube substrate along with the conformal topography of highly flexible GO

laminates allowed sufficiently large contact area that would induce more intense Van der

Waals interactions [25]. On the other hand, the extra hydroxyl groups on the ceramic

substrate benefited the formation of strong hydrogen bonds to further enhance the adhesion

force. However, some small cracks and slight exfoliation can be observed for GO/PAN

Before

After

GO/Ceramic tube GO/PAN GO/h-PAN GO/PC

Cracks

Peeling off

14

membrane indicating a weak interfacial adhesion. Fortunately, the h-PAN substrate exhibited

the strongest interfacial adhesion to the top GO selective layer without any visible

detachment or damage after subjected to the harsh cross-flow testing. It is reasonable to

explain the remarkably improved adhesion performance that numerous oxidized functional

groups introduced by the hydrolysis of PAN substrate can create sufficient amount of

hydrogen bonds and thus lead to an ultimate stronger interfacial adhesion. Meanwhile, it can

be clearly observed that GO laminates suffered the weakest adhesion to the PC substrate with

the brown GO layer completely peeling off from PC substrate. The severe peeling of GO/PC

membrane is attributed to the ulta-smooth surface of substrate that would make it difficult for

the attachment of GO laminates. Another issue is the lack of strong binding between GO

laminates and PC substrate, only the weak Van der Waals force is insufficient to combine the

two layers.

3.5 Nanofiltration performance of GO membranes

The nanofiltration performance of GO membranes (Fig. 9) revealed that their optimum GO

loading amounts were dependent on the substrate properties. It is a reasonable phenomenon

because GO nanosheets tended to conform the morphological structure of substrates, as

described and confirmed above. Thus, for the ceramic substrate with rough and uneven

surface, GO nanosheets had priority of filling in the valley. After forming a flat platform, GO

nanosheets would continue to assemble into a defect-free laminate. This could explain that

when GO loading amount was less than 1.0 gm-2

, GO nanosheets cannot completely cover the

ceramic substrate and form a well-stacked laminar structure with molecular sieving properties,

leading to the low rejection. In contrast, the PAN and PC substrates with more even surface

can achieve high rejections with less GO loading amount of 0.2~0.5 gm-2

. In addition, we

found the water permeance of GO membranes was positively associated with that of their

substrates. Considering the requirement of practical application, a qualified rejection rate

(>99%) together with the highest possible water permeance would be preferred. Therefore,

we considered the membrane made of GO loading amount of 0.5 gm-2

on PAN substrate

possessed optimized separation properties with water permeance of 15.5 Lm-2

h-1

bar-1

and

RhB rejection of 99.5%. We used it to further investigate the nanofiltration performance for

15

different dyes and salts separation.

Fig. 9. Water permeance and RhB rejection of GO membranes with different loading amounts. a) GO

membranes on ceramic substrate; b) GO membranes on PAN and PC substrates. The optimal condition is

marked as blue bar.

The retention of three typical dyes namely MO (327.33 Da, negatively charged), MB

(373.90 Da, positively charged) and RhB (479.02 Da, electroneutral) were tested. The results

(Fig. 10a) demonstrated that rejection sequence of these organic dyes followed the order of

their molecular weight regardless of their charge properties, suggesting that molecular sieving

effect was the dominant mechanism in dyes removal [33]. We also found a pH-dependent

performance (Fig. 10b) of the GO membrane. It showed low water permeance and high

rejection at pH below 4 because of the narrowed interlayer spacing of GO laminates induced

by the weakened electrostatic repulsion between GO nanosheets as a result of protonation of

carboxylic acid. And within the pH range from 5 to 8, the water permeance and rejection

were overall stable. As the pH exceeds 9, it again exhibited the reduced water permeance

while increased rejection which is attributed to the predominated ionic screening effect

caused by high ion concentration shrinking the nanochannels and forming the osmosis

pressure [34].

0.5 1.0 1.5 2.0

0

10

20

30

40

GO/Ceramic tube (20)

GO/Ceramic tube (225)

GO/Ceramic tube (790)Pe

rme

an

ce

(Lm

-2h

-1bar-1

)

GO loading amount (gm-2)

50

60

70

80

90

100

Re

jectio

n (

%)

0.0 0.2 0.4 0.6 0.8

0

10

20

30

40

GO/PAN (20)

GO/PC (200)

Pe

rme

an

ce

(Lm

-2h

-1bar-1

)

GO loading amount (gm-2)

50

60

70

80

90

100

Re

jectio

n (

%)

a) b)

16

Fig. 10. Nanofiltration performance of GO/PAN membrane a) for different dyes separation and b) under

different pH conditions (only tested for RhB separation).

In addition, we conducted a set of filtration experiments for monovalent and divalent salts

(Na2SO4, NaCl, MgSO4, MgCl2) in water and investigated the influences of salt concentration

on the separation performance. We found the salt rejections of negatively charged GO

membrane were highly selective to Z+/Z

– (ratio of charge number for the cation and anion)

values (Fig. 11a), following the order of R (Na2SO4)＞R (MgSO4)＞R (MgCl2). It agreed

with the Donnan exclusion theory, indicating that electrostatic repulsion is the dominant

effect [35-38]. The exceptionally low rejection of NaCl due to its small size also reflecting

the influence of sieving effect. Meanwhile, the slightly lower salt solution permeance

compared to pure water permeance might be the consequence of higher osmotic pressure

difference across the membrane and higher viscosity in the salt solution. In addition, it can be

observed that the salt rejections suffer different degrees of decline with the increase of salt

concentrations (Fig. 11b). This is not surprising because the greater charge screening effect

under higher salt concentration would lessen the Donnan exclusion effect, finally causing the

decrease of salt rejections [39,40].

2 4 6 8 10

10

12

14

16

18

20

Pe

rme

an

ce

(Lm

-2h

-1bar-1

)

pH

95

96

97

98

99

100

Re

jectio

n (

%)

0

5

10

15

20

25

Permeance

Rejection

P

erm

ea

nce

(Lm

-2h

-1bar-1

)

90

92

94

96

98

100

Re

jectio

n (

%)

MO RhB MB

a) b)

17

Fig. 11. Nanofiltration performance of GO/PAN membrane a) for different salts separation and b) under

different salt concentrations.

We further studied the stability of GO membranes under high pressure and long-period

operations, which are essential for realizing practical application. We found the water flux

showed a high linearity within the pressure range from 1 bar to 6 bar with negligible decrease

in rejection (Fig. 12). Furthermore, the long-term testing showed that the membrane kept high

and stable performance over long-period continuous operation. These promising results

demonstrated that our GO membrane is robust to withstand high feed pressure up to 6 bar and

operating time up to 120 hours with no defects generated, showing a great advancement for

practical application.

Fig. 12. Nanofiltration performance of GO/PAN membrane a) under high pressure and b) under long

period.

By the comparison of nanofiltration performance of the GO membranes supported by three

different substrates, we found all the membranes displayed high rejection (>96%) for dyes

0

5

10

15

20

25

Permeance

Rejection

Perm

eance (

Lm

-2h

-1bar-1

)

0

20

40

60

80

100

Reje

ction (

%)

Na2SO

4NaCl MgSO

4MgCl

2

50 100 150 200

0

20

40

60

80

100

Reje

ction (

%)

Salt concentration (ppm)

NaCl

Na2SO

4

Na3PO

4

a) b)

0 20 40 60 80 100 120

0

5

10

15

20

25

30

Perm

eance (

Lm

-2h

-1bar-1

)

Operation time (h)

80

85

90

95

100

Reje

ction (

%)

0 1 2 3 4 5 6 7

0

20

40

60

80

100

Flu

x (

Lm

-2h

-1)

Operation pressure (bar)

80

85

90

95

100

Reje

ction (

%)

a) b)

18

separation, implying the formation of well-assembled GO selective layers with intrinsic

molecular sieving capability. The GO/ceramic membrane had a good adhesion performance

as confirmed before. However, we interpreted its low water permeance of 3.4 Lm-2

h-1

bar-1

as

a result of the relatively high transport resistance arose from compact bulk structure of

ceramics. In addition, the GO/PC membrane exhibited an order of magnitude higher water

permeance of 20.2 Lm-2

h-1

bar-1

, benefiting from thin GO layer and straight substrate pores.

But the poor stability of GO/PC membrane severely limited its implementation. Taking each

aspect into consideration, the GO/PAN membrane possessed the optimized performance, with

water permeance of 15.5 Lm-2

h-1

bar-1

and excellent dye rejection of 99.5%, as well as strong

interfacial adhesion and superior stability. Overall, the GO membranes in this work showed

competitive nanofiltration performance compared with relevant GO membranes reported in

literature [17,32,33,41] (Fig. 13).

Fig. 13. Nanofiltration performance comparison of GO membranes in this work with reported relevant

membranes for dyes separation.

4. Conclusions

In this work, we studied inorganic ceramic tube and polymeric PAN and PC substrates as

support for formation of GO membranes. It was found that surface morphological and

chemical structure as well as bulk pore structure of the substrates have notable influence on

0 5 10 15 20 25 30

60

70

80

90

100

1-GO/ceramic tube (225 nm): RhB

2-GO/PAN (20 nm): RhB

3-GO/PAN (20 nm): MB

4-GO/PAN (20 nm): MO

5-GO/PC (200 nm): RhB

Reported GO membranes

Re

jectio

n (

%)

Permeance (Lm-2h

-1bar

-1)

543

1

2

19

the interfacial adhesion and nanofiltration performance of GO membranes. Ceramic substrate

with rough surface and rigid structure provided sufficient contact area and mechanical

strength to GO selective layer, which is attractive for industrially adaptable operations. PAN

substrate has abundant oxidized functional groups leading to remarkably enhanced interfacial

adhesion of GO membrane, and its flat skin with highly porous sublayer allows very low

transport resistance resulting in excellent nanofiltration performance of GO membrane. PC

substrate possesses ultrahigh water permeance owing to its circular straight through pores,

but relatively poor adhesion performance due to the lack of surface functionalities. This work

provides fundamental understandings of substrate effect on the formation and transport

property of GO membranes, which potentially offer a guidance on development of durable

GO membranes for practical applications.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China

(21490585, 51861135203, 21728601, 21606123, 21476107), the Natural Science Foundation

of Jiangsu Province (BK20160980), the Innovative Research Team Program by the Ministry

of Education of China (grant No. IRT_17R54) and the Topnotch Academic Programs Project

of Jiangsu Higher Education Institutions (TAPP).

References

[1] M. Pendergast, E. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci.

4 (2011) 1946-1971.

[2] A. Lee, J. Elam, S. Darling, Membrane materials for water purification: Design, development, and

application, Environ. Sci.: Water Res. Technol. 2 (2016) 17-42.

[3] G. Liu, W. Jin, N. Xu, Two-dimensional-material membranes: A new family of high-performance

separation membranes, Angew. Chem. Int. Ed. 55 (2016) 13384-13397.

[4] H. Huang, Y. Ying, X. Peng, Graphene oxide nanosheet: an emerging star material for novel separation

membranes, J. Mater. Chem. A, 2 (2014) 13772-13782.

[5] G. Liu, W. Jin, N. Xu, Graphene-based membranes, Chem. Soc. Rev. 44 (2015) 5016-5030.

20

[6] S. Xia, M. Ni, T. Zhu, Y. Zhao, N. Li, Ultrathin graphene oxide nanosheet membranes with various

d-spacing assembled using the pressure-assisted filtration method for removing natural organic matter,

Desalination, 371 (2015) 78-87.

[7] J. Shen, G. Liu, K. Huang, Z. Chu, W. Jin, N. Xu, Subnanometer two-dimensional graphene oxide

channels for ultrafast gas sieving, ACS Nano. 10 (2016) 3398-3409.

[8] L. Chen, G. Shi, J. Shen, B. Peng, B. Zhang, Y. Wang, F. Bian, J. Wang, D. Li, Z. Qian, G. Xu, G. Liu, J.

Zeng, L. Zhang, Y. Yang, G. Zhou, M. Wu, W. Jin, J. Li, H. Fang, Ion sieving in graphene oxide

membranes via cationic control of interlayer spacing, Nature, 550 (2017) 380-383.

[9] C. Yeh, K. Raidongia, J. Shao, Q. Yang, J. Huang, On the origin of the stability of graphene oxide

membranes in water, Nature Chem. 7 (2015) 166-170.

[10] S. Zheng, Q. Tu, J. Urban, S. Li, B. Mi, Swelling of graphene oxide membranes in aqueous solution:

characterization of interlayer spacing and insight into water transport mechanisms, ACS Nano. 11

(2017) 6440-6450.

[11] W. Lau, A. Ismail, N. Misdan, M. Kassim, A recent progress in thin film composite membrane: a

review, Desalination, 287 (2012) 190-199.

[12] N. Misdan, W. Lau, A. Ismail, T. Matsuura, D. Rana, Study on the thin film composite poly

(piperazine-amide) nanofiltration membrane: Impacts of physicochemical properties of substrate on

interfacial polymerization formation, Desalination, 344 (2014) 198-205.

[13] X. Kong, M. Zhou, C. Lin, J. Wang, B. Zhao, X. Wei, B. Zhu, Polyamide/PVC based composite

hollow fiber nanofiltration membranes: Effect of substrate on properties and performance, J. Membr.

Sci. 505 (2016) 231-240.

[14] J. Wang, P. Zhang, B. Liang, Y. Liu, T. Xu, L. Wang, B. Cao, K. Pan, Graphene oxide as an effective

barrier on a porous nanofibrous membrane for water treatment, ACS Appl. Mater. Interfaces, 8 (2016)

6211-6218.

[15] Y. Zhang, S. Zhang, T. Chung. Nanometric graphene oxide framework membranes with enhanced

heavy metal removal via nanofiltration[J]. Environ. Sci. Technol. 49 (2015) 10235-10242.

[16] L. Yu, Y. Zhang, B. Zhang, J. Liu, H. Zhang, C. Song, Preparation and characterization of

HPEI-GO/PES ultrafiltration membrane with antifouling and antibacterial properties, J. Membr. Sci.

447 (2013) 452-462.

[17] M. Hu, B. Mi, Enabling graphene oxide nanosheets as water separation membranes, Environ. Sci.

Technol. 47 (2013) 3715-3723.

[18] K. Huang, G. Liu, Y. Lou, Z. Dong, J. Shen, W. Jin, A graphene oxide membrane with highly selective

molecular separation of aqueous organic solution, Angew. Chem. Int. Ed., 53 (2014) 6929-6932.

21

[19] K. Huang, G. Liu, J. Shen, Z. Chu, H. Zhou, X. Gu, W. Jin, N. Xu, High-efficiency water-transport

channels using the synergistic effect of a hydrophilic polymer and graphene oxide laminates, Adv.

Funct. Mater. 25 (2015) 5809-5815.

[20] M. Zhang, K. Guan, J. Shen, G. Liu, Y. Fan, W. Jin, Nanoparticles@rGO membrane enabling highly

enhanced water permeability and structural stability with preserved selectivity, AIChE J. 63 (2017)

5054-5063.

[21] W. Gao, R. Huang. Effect of surface roughness on adhesion of graphene membranes, J. Phy. D: Appl.

Phys. 44 (2011) 452001.

[22] B. Nandi, R. Uppaluri, M. Purkait. Preparation and characterization of low cost ceramic membranes

for micro-filtration applications, Appl. Clay Sci. 42 (2008) 102-110.

[23] G. Guillen, Y. Pan, M. Li, E. Hoek, Preparation and characterization of membranes formed by

nonsolvent induced phase separation: A review, Ind. Eng. Chem. R. 50 (2011) 3798-3817.

[24] S.  Yang, I. Ryu, H.  Kim, J.  Kim, S.  Jang, T. Russell, Nanoporous membranes with ultrahigh

selectivity and flux for the filtration of viruses, Adv. Mater. 18 (2006) 709-712.

[25] R. Hu, Y. He, M. Huang, G. Zhao, H. Zhu, Strong adhesion of graphene oxide coating on polymer

separation membranes, Langmuir, 34 (2018) 10569-10579.

[26] K. Guan, D. Zhao, M. Zhang, J. Shen, G. Zhou, G. Liu, W. Jin, 3D nanoporous crystals enabled 2D

channels in graphene membrane with enhanced water purification performance, J. Membr. Sci. 542

(2017) 41-51.

[27] L. Tang, J. Kardos. A review of methods for improving the interfacial adhesion between carbon fiber

and polymer matrix, Polym. Compos. 18 (1997) 100-113.

[28] M. Yang, J. Tong. Loose ultrafiltration of proteins using hydrolyzed polyacrylonitrile hollow fiber, J.

Membr. Sci. 132 (1997) 63-71.

[29] A. Litmanovich, N. Platé. Alkaline hydrolysis of polyacrylonitrile: On the reaction mechanism,

Macromol. Chem. Phys. 201 (2000) 2176-2180.

[30] M. Gupta, B. Gupta, W. Oppermann, G. Hardtmann, Surface modification of polyacrylonitrile staple

fibers via alkaline hydrolysis for superabsorbent applications, J. Appl. Polym. Sci. 91 (2004)

3127-3133.

[31] G. Zhang, H. Meng, S. Ji. Hydrolysis differences of polyacrylonitrile support membrane and its

influences on polyacrylonitrile-based membrane performance, Desalination, 242 (2009): 313-324.

[32] K. Goh, W. Jiang, H. Karahan, S. Zhai, L. Wei, D. Yu, A. Fane, R. Wang, Y. Chen, All-carbon

nanoarchitectures as high-performance separation membranes with superior stability, Adv. Funct.

Mater. 25 (2015) 7348-7359.

22

[33] Y. Han, Z. Xu, C. Gao. Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct.

Mater. 23 (2013) 3693-3700.

[34] H. Huang, Y. Mao, Y. Ying, Y. Liu, L. Sun, X. Peng, Salt concentration, pH and pressure controlled

separation of small molecules through lamellar graphene oxide membranes, Chem. Comm. 49 (2013)

5963-5965.

[35] J. Schaep, B. Bruggen, C. Vandecasteele, D. Wilms, Influence of ion size and charge in nanofiltration,

Sep. Purif. Technol. 14 (1998) 155-162.

[36] G. Liu, H. Ye, A. Li, C. Zhu, H. Jiang, Y. Liu, K. Han, Y. Zhou, Graphene oxide for high-efficiency

separation membranes: Role of electrostatic interactions, Carbon, 110 (2016) 56-61.

[37] J. Wang, P. Zhang, B. Liang, Y. Liu, T. Xu, L. Wang, B. Cao, K. Pan, Graphene oxide as an effective

barrier on a porous nanofibrous membrane for water treatment, ACS Appl. Mater. Interfaces, 8 (2016)

6211-6218.

[38] Y. Yuan, X. Gao, Y. Wei, X. Wang, J. Wang, Y. Zhang, C. Gao, Enhanced desalination performance of

carboxyl functionalized graphene oxide nanofiltration membranes, Desalination, 405 (2017) 29-39.

[39] Y. Cho, H. Kim, H. Lee, J. Shin, B. Yoo, H. Park, Water and ion sorption, diffusion, and transport in

graphene oxide membranes revisited, J. Membr. Sci. 544 (2017) 425-435.

[40] Y. Oh, D. Armstrong, C. Finnerty, S. Zheng, M. Hu, A. Torrents, B. Mi, Understanding the

pH-responsive behavior of graphene oxide membrane in removing ions and organic micropollulants. J.

Membr. Sci. 541 (2017) 235-243.

[41] H. Huang, Z. Song, N. Wei, L. Shi, Y. Mao, Y. Ying, L. Sun, Z. Xu, X. Peng, Ultrafast viscous water

flow through nanostrand-channelled graphene oxide membranes, Nat. Commun. 4 (2013) 2979.

Highlights

Surface morphology and chemical structure of substrates control GO layer formation

Bulk pore structure of substrates dominates transport resistance of GO membrane

Hydrolyzed PAN substrate optimizes formation and NF performance of GO membranes