Graphene Oxide Membranes with Conical Nanochannels for Ultrafast

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Graphene Oxide Membranes with conical nanochannels for Ultrafast Water Transport Yu Ma, Yanlei Su, Mingrui He, Benbing Shi, Runnan Zhang, Jianliang Shen, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12868 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Applied Materials & Interfaces

Graphene Oxide Membranes with Conical Nanochannels for Ultrafast Water Transport Yu Ma, Yanlei Su, Mingrui He, Benbing Shi, Runnan Zhang, Jianliang Shen, Zhongyi Jiang

ABSTRACT

Membrane-based separations have been increasingly utilized to address global

energy crisis and water scarcity. However, the separation efficiency often suffers from the trade-off between membrane permeability and selectivity. Although great efforts have been devoted, a membrane with both high permeability and high selectivity remains a distant prospect. Inspired by the hourglass structure and ultrafast water transport in aquaporins, we propose a novel approach to fabricating membranes with conical nanochannels to reduce the mass transfer resistance and to introduce Laplace pressure as internal driving force, which successfully breaks the permeability-selectivity trade-off. First, sulfonated polyaniline (SPANI) nanorods were in-situ synthesized and vertically aligned on sulfonated graphene oxide (SGO) nanosheets, forming Dr. Y. Ma, Prof. Y. Su, M. He, B. Shi, R. Zhang, J. Shen, Prof. Z. Jiang Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology Tianjin University Tianjin 300072, China E-Mail: [email protected] Dr. Y. Ma, Prof. Y. Su, M. He, B. Shi, R. Zhang, J. Shen, Prof. Z. Jiang Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Tianjin 300072, China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SGO-SPANIX composites. Then, the graphene oxide (GO) membranes were fabricated by assembling SGO-SPANIX composites through a pressure-assisted filtration, in which the SPANI nanorods would bend and flatten on the SGO nanosheets under low shear force, forming stripe arrays on SGO nanosheets. The tilted stripe arrays between the adjacent SGO nanosheets form the conical nanochannels inside GO membranes. The conical nanochannels significantly decreased the steric hindrance and enabled the generation of Laplace pressure as internal driving force within membranes. Consequently, the resulting membranes exhibit ultrahigh water permeability of 1222.77 Lm-2h-1bar-1 and high efficiency in dye removal from water with rejection of 90.44% and permeability of 528 Lm-2h-1bar-1.

Key Words: graphene oxide membrane, stripe arrays, conical nanochannels, Laplace pressure, water transport

INTRODUCTION Membrane-based separation technology has played more and more critical role in coping with the global energy, environment and water challenges

1 ,2 ,3 , 4

. However, the trade-off between

permeability and selectivity severely hampers the separation efficiency of most synthetic membrane materials. Through the transport characteristics analysis and comparison between biological membranes and synthetic membranes, some key design criteria for advanced membrane configurations have been proposed, such as precisely sized channels and pores, a thin/ultrathin separation layer , well-defined interactions between permeants and membrane

5,6,7,8

. Meanwhile, a

number of design strategies and approaches for enhanced permeability and selectivity have been


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explored with a focus on advanced membrane materials and structures

6,9,10,11,12,13,14

. In theory,

membrane permeability and selectivity depend on mass transfer rate of permeants through the membranes, and the mass transfer rate depends on the ratio of driving force to mass transfer resistance 5,15,16,17,18,56. Therefore, a membrane which can afford increased driving force and reduced mass transfer resistance during utilization remains to be explored.

In recent years, two-dimensional (2D) materials have emerged as new member in the family of high-performance separation membranes. Due to the highly tunable nanopores and/or nanochannels, these 2D material membranes display very intriguing separation properties. Among the 2D material membranes, graphene-based membranes have been most extensively studied in two distinct directions: (1) Engineering nanochannels in graphene interlayers in graphene basal plane

25,26

19,20,21,22,23,24

. (2) Generating pores

. With the continuous efforts, graphene-based membranes have shown

ultrafast selective permeation performance in a broad range of applications including water purification

and

gas

separation.

Albeit

these

great

advances,

the

fundamental

structure/property/processing relations for graphene-based membranes should be more intensively explored.

Herein, inspired by the hourglass structure and ultrafast water transport in aquaporins, we exploit a novel approach to fabricating membranes with conical nanochannels to break the selectivity/permeability trade-off 27 ,55. In this case, the optimized friction at the interface of fluid-solid wall would significantly decrease steric hindrance and thus mass transfer resistance 27,28, 55

. Furthermore, when liquid-vapor interfaces are generated inside conical nanochannels, the

reduced steric hindrance would facilitate the generation of Laplace pressure, which can act as internal driving force to further elevate mass transfer rate and membrane separation properties


29

.


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RESULTS AND DISCUSSION Fabrication of Graphen Oxide Membranes with Conical Nanochannels. As the first step of this study, sulfonated polyaniline (SPANI) nanorods were in-situ synthesized and vertically aligned on sulfonated graphene oxide (SGO) nanosheets

30,31

, forming the SGO-SPANIX

composites (The successfully synthesized SGO-SPANIX composites were characterized in section 2 in Supporting Information, Figure S1). Then graphene oxide (GO) membranes with layered structure were fabricated by assembly SGO-SPANIX composites through a pressure-assisted filtration. Because the vertical alignment of SPANI nanorods are unstable in liquid environment, the shear force caused by flowing water in confined SGO nanosheets galleries would bend and flatten SPANI nanorods on SGO nanosheets to form stripe arrays, which can be also explained by the “nanocarpet effect” (Figure 1)

32

. Since stripe arrays are actually continuous tilting planes on SGO

nanosheet and these nanosheets were densely packed to form membrane with orderly laminated microstructure, the tilted stripe arrays between adjacent SGO nanosheets form conical nanochannels inside GO membranes.

Figure 1. Schematic of the fabrication steps of SGO-SPANIX membrane


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Figure 1 shows the fabrication steps of SGO-SPANI membrane. Firstly, SGO, O-aminobenzenesulfonic acid (ASA) and ammonium persulfate (APS) were mixed in 1 M sulfuric acid (H2SO4) to form water phase and aniline monomers (concentrations of 10 mM, 25 mM and 50 mM, respectively) were dissolved in dichloromethane (CH2Cl2) to form oil phase. Aniline oligomers were then formed at the water-oil phase interface, which consist of 50-60 molecular units and contain both ortho-coupling and para-coupling

33

. These oligomeric products have low

solubility and precipitate into needle-like crystallite offsprings on SGO nanosheet surface due to the electrostatic interaction and hydrogen bonding interaction (heterogeneous nucleation, first row)33, 34. When the pH < 1, some of the oligomers become oxidized and acted as the templates of phenazine-like units with active sites for nucleation of SPANI nanorods

33,35

. In this case, active

sites of phenazine-like units on SGO surface could minimize the interfacial energy barrier between the solid surface and bulk solution, and vertically aligned SPANI nanorods were created and the composites were denoted as SGO-SPANI10, SGO-SPANI25 and SGO-SPANI50, respectively (the second row). In detail, the phenazine-like units have a flat structure and are hydrophobic, then π-π stacking between the phenazine-like units allows them to produce columnar aggregates on the initial oligomeric templates, which are further stablized during the chain growth of PANI counterparts by hydrogen bonding 33. At membrane assembly stage, the SGO-SPANIX composites were separately dispersed in water, ammonium hydroxide and n-methyl-2-pyrrolidone (NMP) mixed solvents. Then membranes with layered structure were successfully fabricated by pressure-assisted filtration method (the third row). During this stage, nanorods would bend and flatten on graphene oxide nanosheets due to “nanocarpet effect” to form stripe arrays 32. In fact, the theory of polyaniline (PANI) growth describes that the nanorods are of hollow structure, the wall of



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the needle-like oligomer crystals similarly act as sites for the adsorption of the phenazine-like entities, and the adsorption is selective due to the obvious anisotropy of the crystallites, which leaving the front surfaces uncoated. Consequently, it is believed that the nucleus of nanorods is produced as a sleeve on an oligomer needle and the size of the crystallite inside determines the inner diameter of nanorodes33. Moreover, the interactions between oligomer micelles are governed by π-π stacking and hydrogen bonding, which are weak enough to allow the SPANI nanorods bend and flatten on the SGO surface under the low shear force due to “nanocarpet effect”

30,32,33,34,35

.

Since membranes were constructed with ordered laminar structure and stripe arrays are actually tilting planes on SGO nanosheets, the conical nanochannels were formed inside membrane. Finally, membranes were dried under vacuum at 60 OC to acquire the robust, flexible SGO-SPANIX membranes (middle lower photograph, Figure 1)

36,37

.

Moreover, the control membranes without conical nanochannels were fabricated for comparisons (section 4 in Supporting Information). In this case, aniline monomer concentrations were tuned to 15 mM, 40 mM and 77 mM, corresponding to the concentrations of 10 mM, 25 mM and 50 mM for fabricating the SGO-SPANIX membranes, respectively. The synthesized blends are denoted as SGO-SPANIbX (SGO-SPANIb15, SGO-SPANIb40 and SGO-SPANIb77, respectively) and the corresponding membranes are denoted as SGO-SPANIbX membrane (SGO-SPANIb15 membrane, SGO-SPANIb40 membrane and SGO-SPANIb77 membrane, respectively). Furthermore, some characterization techniques were utilized to demonstrate the process of membrane fabrication. First, TEM images of SGO-SPANIX composites were collected in Figure S2, which exhibit the formation of arrays of homogenous SPANI nanorods (the black spots) with diameters about 6 nm for all composites. It seems that the diameters were not obviously changed


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when monomer concentration increase. However, as SEM images in Figure 2(A) and Figure S3 demonstrate, the nanorod lengths of SGO-SPANIX composites were markedly affected by the concentrations of aniline monomer

35

. For SGO-SPANI10, SGO-SPANI25 and SGO-SPANI50

composites, the lengths are about 20 nm, 50 nm and 95 nm, respectively. During membrane assembly process, the bent and flattened SPANI nanorods on SGO surface were observed by SEM in Figure 2(B). Meanwhile XRD patterns in Figure S4 show the SGO-SPANIX membranes have two reflection peaks centered at 2θ=20.04° and 23.92°, respectively, which shift to lower angles compared with the reflection peaks of SGO-SPANIX composite (centered at 2θ=20.76° and 25.44°), indicating the lattice deformation behavior and the bending/flattening phenomena of SPANI nanorods.

(B)

(A)

Figure 2. (A) SEM image of SGO-SPANI50 composite; (B) SEM image of SGO-SPANI50 composite during assembly process.

Structure Analysis of Graphene Oxide Membranes with Conical Nanochannels. Membrane cross-sectional morphologies were observed by SEM in Figure 3(A) and Figure S5, indicating membranes were constructed with ordered laminar microstructures. By setting up a filter to eliminate the frequencies of stripe noises38, membrane surface morphologies were observed by atomic force microscope (AFM) in Figure 3(B) and Figure S6, demonstrating the



existence of stripe arrays on membrane surface. In order to investigate morphologies inside membrane, the outermost layer were peeled off with the assistance of NMP-soaked nylon-66 microfiltration membrane (the details were described in the Experimental Section). Based on this method, morphologies inside membrane were observed by AFM images in Figure S7, which show the existence of stripe arrays inside membrane. Furthermore, The shape of stripes in different membranes were characterized by AFM based on the height and length profiles in Figure S8, indicating stripes are actually tilting planes on SGO nanosheets with the heights of about 0.25 nm, 0.40 nm and 0.55 nm for SGO-SPANI10 membrane, SGO-SPANI25 membrane and SGO-SPANI50 membrane, respectively, and about 4 nm wide stripes for all SGO-SPANIX membranes. Moreover, TEM image was collected to show the structure of ordered conical nanochannels. By using the method mentioned in the experimental section, membrane outermost layers were peeled off and collected to investigate its nanostructures. Compared with the monolayer nanosheet in Figure 3 (C) which shows some distinct nanobulge on SGO surfaces, the double layer nanosheets in Figure 3(D) show striated patterns, indicating the periodical change of SPANI thicknesses. Since the periodically changed SPANI thickness between nanosheets gallery reflect the formation of ordered conical nanochannels (Scheme S1), Figure 3(D) could indicate the existence of conical nanochannels inside membrane.Besides, small angle X-ray scattering (SAXS) spectra were utilized to quantitatively investigate the channel sizes inside membrane. The dual peaks in SAXS spectra (Figure 3(D)) occurs due to the periodic contrast in electron density between SPANI and pores39,40,41, which demonstrate the existence of two different channel sizes inside membrane. This result indicates the surfaces of nanosheet inside membrane are heterogeneous, which is consistent with the result in Figure 3(C), indicating the existence of conical nanochannels. Furthermore, As described in Section 3.1 in supporting information, the corrected Bragg equation was used to estimate the


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channel sizes in different SGO-SPANIX membranes. As a result, the sizes were estimated to be 4.11 and 4.45 nm for SGO-SPANI10 membrane, 6.06 nm and 6.61 nm for SGO-SPANI25 membrane, 10.48 and 11.67 nm for SGO-SPANI50 membrane, respectively, and stripe heights are about 0.20 nm, 0.30 nm and 0.60 nm, respectively, which were roughly consistent with the results measured by AFM in Figure S8. Furthermore, combine the height profiles with the 4 nm wide stripes for SGO-SPANIX membranes, the tilting angles of stripe arrays were calculated to be about 3° for SGO-SPANI10 membrane, about 5° for SGO-SPANI25 membrane and about 7° for SGO-SPANI50 membrane, respectively.

(A)

(B)

Stripe arrays

(D)

(C)

Random pattern Striated patterns

SGO-SPANI50Membrane

(E)

SGO-SPANI25Membrane SGO-SPANI10Membrane


(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Figure 3. (A) SEM cross-sectional image of SGO-SAPNI50 membrane; (B) High-pass filtered AFM image of SGO-SPANI50 membrane, white square frame represents stripe arrays inside (Inset: Mean filtered AFM image of SGO-SPANI50 membrane); (C) TEM image of monolayer nanosheet from exfoliated

SGO-SPANI50 membrane: blue circle represents random pattern; (D) TEM image of double layer nanosheets from exfoliated SGO-SPANI50 membrane: blue circles represent striated patterns; (E) Small angle X-ray scattering (SAXS) patterns of SGO-SPANI10, SGO-SPANI25 and SGO-SPANI50 membranes.

Separation Performances of Graphen Oxide Membranes with Conical Nanochannels. In order to elucidate the effects of conical nanochannels on membrane separation properties, the control membranes without conical nanochannels were fabricated (section 4 in Supporting Information), which indicates that conical nanchannels can significantly enhance membrane permeability and selectivity, i.e., break the permeability-selectivity trade-off..

In this study, with the increase of aniline monomer concentration, pure water permeabilities of


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SGO-SPANIX membranes were monotonously increased. In detail, compared with the pure water permeabilities of 33.83, 160.19 and 241.13 L⋅m-2⋅h-1⋅bar-1 for SGO-SPANIb15, SGO-SPANIb40 and SGO-SPANIb77 membranes, the pure water permeabilities increased to 84.72, 452.74 and 600.19 L⋅m-2⋅h-1⋅bar-1 for SGO-SPANI10, SGO-SPANI25 and SGO-SPANI50 membranes, respectively, at 25

(Figure 4). Furthermore, when liquid-vapor interfaces are generated inside nanochannels, the

pure water permeabilities of SGO-SPANI10 (Laplace pressure), SGO-SPANI25 (Laplace pressure) and SGO-SPANI50 (Laplace pressure) membranes were increased to 111.32, 793.24 and 1222.77 L⋅m-2⋅h-1⋅bar-1, respectively, at 25

(2 times higher than the pure water permeability of

SGO-SPANIX membranes, Figure 4). Moreover, inappreciable permeabilities variation in Figure S14 demonstrates that Laplace pressure can not be generated within control membranes because of too large steric hindrance.

SGO-SPANIb15 membrane SGO-SPANI10 membrane SGO-SPANI10 (Laplace pressure) membrane

300

200

-2

-1

Flux (L m h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

0 0.0

0.5

1.0

1.5

2.0

2.5

Pressure (bar)


3.0


2000

SGO-SPANI

b 40

membrane

SGO-SPANI25 membrane

Flux (L m -2 h -1)

1500

SGO-SPANI25 (Laplace pressure) membrane

1000

500

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pressure (bar)

b

SGO-SPANI

2500

77

membrane

SGO-SPANI50 membrane SGO-SPANI50 (Laplace pressure) membrane

2000

Flux (L m -2 h -1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1500

1000

500

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pressure (bar)

Figure 4. Pure water fluxes of SGO-SPANIbX membranes, SGO-SPANIX membranes and SGO-SPANIX (Laplace pressure) membranes at different vacuum pressure.The line is the calculated data using Equation S13.

Figure 5 and Table S4 indicate that the SGO-SPANIX (Laplace pressure) membranes can achieve as high as 5.60 times permeability increase (from 241.13 to 1222 L⋅m-2⋅h-1⋅bar-1 for pure water and from 173 to 832 L⋅m-2⋅h-1⋅bar-1 for methyl blue solution) and 3.26 times selectivity increase (from 30.34 % to 99.01 %) compared with SGO-SAPNIbX membranes at 25

. In detail,

the separation performance of Methyl Blue (MB)/water in Figure 5 indicated that the permeabilities ACS Paragon Plus Environment

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of SGO-SPANIbX membranes were 9, 104 and 173 L⋅m-2⋅h-1⋅bar-1, corresponding to the rejections of 77.63 %, 87.32 % and 60.34 %, respectively, at 1 bar, and 30.34 %, 75.83 % and 55.43 %, respectively, at 0.5 bar (Table S3). When SGO-SPANIX (Laplace pressure) membranes were utilized, the separation performances were significantly increased to 72, 528 and 832 L⋅m-2⋅h-1⋅bar-1, corresponding to the rejections of 99.04 %, 90.44 % and 67.83 %, respectively, at 0.9 bar, and 99.01 %, 90.56 % and 67.07 %, respectively, at 0.5 bar (Table S4). Figure 5 and Table S5 indicate the separation performance of SGO-SPANIX (Laplace pressure) membranes are superior to most previously reported data, which can be a successful example of surmounting the selectivity/permeability trade-off.

100

SGO-SPANI10 SGO-SPANIbX (MB)

Nickel hydroxide (DY)

90

separation factor, R (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoS2 (EB)

SGO-SPANIX (MB)

SGO-SPANI25

SGO-SPANIb40

GO (EB)

80

SGO-SPANIb15

PVDF/ nanoclay/ chitosan (MB)

70 Corrugated CCG (DY)

SGO-SPANI50

60 SGO-SPANIb77

Nickel hydroxide (DY)

GO/TiO2 (MO)

PEI/PAA/PVA/GA (MB)

SWCNT-intercalated GO (Cc)

50

CMCNa/PP (MB)

Nematic liquid crystal GO (MLB)

ZIF-8/PSS (MB)

MgSi@RGO/PAN (Chrome blue-black R)

40 0

100

200

300

400

500

600

700

800

900

1000

Water permeability coefficient, Jv (L⋅m-2⋅h-1⋅bar-1) Figure 5. Dyes/water separation performance of SGO-SPANIbX membranes, SGO-SPANIX (Laplace pressure) membranes and previously reported membranes

42,43,44,45,46,47,48,49,50,51,52,53

-1

-1

, where EB: Evans blue,

molecular weight (Mw): 960.81 g⋅⋅mol ; MLB: Methylene blue, Mw: 319.86 g⋅⋅mol ; MB: Methyl blue, Mw:



799.80 g⋅⋅mol-1; DY: Direct yellow 50, Mw: 956.82; MO: Methyl orange, Mw: 327.33 g⋅⋅mol-1;Cc: Cytochrome c, Mw: 884.89 g⋅⋅mol-1; Chrome blue-black R, Mw: 416.38 g⋅⋅mol-1.

Analysis of Mass Transport in Graphene Oxide Membranes with Conical Nanochannels. The analysis relevant to the crossover of permeability/selectivity trade-off was conducted. In theory, when the size of a channel decreases to nanoscale, hydrodynamics at its surface should play more important role, which are mainly controlled by liquid-solid friction at the interface inside nanochannels

54

. In order to minimize the friction and intensify mass transport

(permeability), low energy surfaces (hydrophobic surfaces) with small liquid-solid interactions seems a possible solution 55. However, in this study, the hydrophilic surface is employed to promote membrane separation properties (section 5 in Supporting Information). Particularly, membranes with conical nanochannels was manipulated, which minimize viscous dissipation by the shallow grooves on SGO nanosheets surfaces to minimize liquid-solid friction inside nanochannels (friction would slow down fluid velocity near liquid-solid interface and viscous dissipation is generated) 28.

Since hydrodynamic at liquid-solid interface is basically caused by viscous dissipation and the width of a stripe array is short enough in this study (around 4 nm, Figure 3(B) and S4), we tried to quantify the effect of conical nanochannels on membrane separation properties by the classical Hagen-Poiseulle equation (section 5 in Supporting Information) 27,55. For SGO-SPANIX membranes, the prefactor numeric Cin (Cin reflects nanochannel geometry) are calculated to be 1.83, 1.64 and 1.87, respectively (Equation S13), which are obviously lower than the value of 2.55 in straight nanochannels (Cin positively correlate to hydrodynamic resistance inside nanochannel), indicating conical nanochannels can significantly enhance water flux. Moreover, compared with SGO-SPANI10 and SGO-SPANI50 membranes, SGO-SPANI25 membrane possesses the lowest Cin


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(1.64). Since numerical prefactor is positively correlates with the tilting angles of stripe arrays, we found the angle about 5° (SGO-SPANI25 membrane) acquires the highest permeability enhancement (2.83 times higher than corresponding SGO-SPANIb40 membrane at 25 ℃), which is consistent with the work of Joly et al.55.

Moreover, vacuum-assisted filtration was applied on SGO-SPANIX membranes to enable the generation of Lapalce pressure as internal driving force inside conical nanochannels to further elevate membrane permeability. As Equation S10 and S11 indicate, the channel geometry (numerical prefactor Cin) in Equation S8 is also a key factor to generate Laplace pressure inside nanochannels. In detail, Based on Equation S10, for SGO-SPANI10 (Laplace pressure), SGO-SPANI25 (Laplace pressure) and SGO-SPANI50 (Laplace pressure) membranes, Laplace pressure drops inside nanochannels (channels length lower than 10 nm) are calculated to be 341.23 bar, 249.13 bar and 136.62 bar, respectively (far larger than pressure driving force). Meanwhile, the hindrances are also high enough to hinder Laplace pressure inside nanochannels, which are calculated to be 9.53×1022×Cin, 3.71×1022×Cin and 6.11×1021×Cin Pa⋅S⋅m-3, respectively

54

.

Consequently parameter “Cin” should be a key factor to optimize. As previously reported 27, 29, 55, C can achieve its minimum value when conical structure is utilized. Due to the existence of conical nanochannels, Laplace pressure as internal driving force can be generated inside membranes.

Furthermore, Equation S13 was used to calculate the fluxes of SGO-SPANIX (Laplace pressure) membranes at different vacuum pressure. As shown in Figure 4, the calculated permeate fluxes were in agreement with the experimental data. As Scheme S2 and Figure 4 describe, Meniscus radius is dependent of the ratio of ambient pressure (PA) over saturated vapor pressure. When ambient pressure continuously decreasing (vacuum pressure increasing), meniscus generated by liquid-vapor



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interface should gradually flatten. Then the increased contact angle (θ2>θ1) lead to the decrease of Laplace pressure. The phenomenon of non-linear relationship between pressure and pure water flux in Figure 4 should be caused by Laplace pressure decrease. For the separation performances of SGO-SPANIbX, SGO-SPANIX and SGO-SPANIX (Laplace pressure) membranes, we have incorporated the effect of conical nanochannels and Laplace pressure on membrane into modified Donnan-steric-pore (m-DSPM) model

56,57

. As section 5 in

supporting information describes, dyes transport through membranes depends on their diffusion, convection and charge properties their transport efficiency

56

, whereas the channel sizes and electrostatic potential govern

56

. In this study, the charge densities of SGO-SPANIX membranes are

slightly changed (Figure S17(A)), which means dyes transport through membranes is thus merely dependent on steric hindrance (channel sizes). Compared with SGO-SPANIbX membranes, both the selectivity and permeability of SGO-SAPNIX were increased (Figure 5 and Figure S16). The successful crossover of permeability/selectivity trade-off is attributed to conical nanochannels inside membrane (Equation S14, S16 and S17). In fact, membrane selectivity highly depends on Kic and Kid, which hinder the solutes convection and diffusion by channel size (Equation S14). As can be seen in Figure S16, the simulated results indicated that when molecules transport is under an increasing velocity, these hindrances on small molecules (i.e. water molecules) are smaller than those on the large molecules (i.e. dye molecules). In other words, small molecules transport faster than the large molecules 56. Since the velocity ratio of different molecules determine selectivity 15, with permeability increase, membrane selectivity increase accordingly.

CONCLUSION In conclusion, we propose an approach of fabricating membranes with conical nanochannels to


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break the permeability/selectivity trade-off. Sulfonated polyaniline nanorods were in-situ synthesized through interfacial polymerization and vertically aligned on sulfonated graphene oxide nanosheets. The resultant SGO-SPANIX composite nanosheets were fabricated into membranes through pressure–assisted self-assembly. During assembly process, nanorods would bend and flatten on the SGO nanosheets under low shear force to form conical nanochannels. When these membranes with conical nanochannels were utilized for removal of dyes from water through a vacuum-assisted filtration process, the conical nanochannels significantly decreased the steric hindrance and enabled the generation of Laplace pressure at vapor-liquid interface, which rendered internal driving force. Consequently, these membranes not only exhibited ultrafast water penetration (1222.77 L⋅m-2⋅h-1⋅bar-1), but also exhibited high efficiency in removal of dyes from water with rejection of 99.04% and permeability of 528 Lm-2h-1bar-1, the permeability and selectivity were both remarkably increased compared with the control membranes without conical nanochannels. The conical nanochannels can be constructed in many different ways, and the Laplace pressure can be generated under many operating conditions of membrane separation processes. Therefore, our findings that membranes with conical nanochannels to reduce the mass transfer resistance and introduce internal driving force may become a novel strategy to fabricate membranes with both high permeability and selectivity, and meanwhile this study may broaden the vision on how to solve the permeability/selectivity trade-off.



METHODS Preparation of graphene oxide (GO). GO dispersion was synthesized according to the modified Hummers method

58

. Briefly, a mixture of 98 wt% H2SO4 (115 mL), 5 g of graphite powder, and 5 g

of NaNO3 was mechanically stirred at 0 °C in a 1 L three-necked, round-bottomed flask, followed by the slow addition of 15 g of KMnO4. The mixture was stirred at 0 °C for 2 h, and then stirred at 35 °C for 30 min.

After 230 mL of water was added slowly during a period of 30 min, stirring was

maintained at 98 °C for 3 h. Subsequently, the mixture was poured into water, followed by the addition of 30 mL of H2O2. Finally, the mixture was washed with 1 L of HCl (1 M), and then with excess water until the pH reached 7. The GO aqueous dispersion was obtained by dispersing the above product in water using ultrasonic treatment for 1 h. The GO aqueous dispersion was centrifuged at 3000 rpm for 40 min to remove the sedimentary GO (unexfoliated GO or large GO sheets).

Preparation of sulfonated graphene oxide (SGO). The aryl diazonium salt used for sulfonation was prepared as follows: 10 mL NaOH (2 %) and 1 g aminobenzene sulfonic acid (SA) were added to a 100 mL beaker. 0.4 g NaNO2 was then added at room temperature. After dissolution of NaNO2, the solution was added into 20 mL of ice water and 2 mL of concentrated hydrochloric acid under stirring, the temperature was kept at 0

for 15 min and the diazonium salt was formed. Then the

diazonium salt solution was added dropwise into 100 mL of GO (1 mg/mL) solution and the mixed solution was stirred strongly for 4 h in ice water bath. After centrifuging and washing with water for several times, the obtained SGO was dispersed in water and stored at room temperature for use.

Preparation of SPANI nanorods: The SPANI nanorods were synthesized by an interfacial polymerization method. a typical procedure was as follows: a certain amount of aniline-2-sulfonic


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acid (ASA) was added into mixed solution of 20 mL isopropyl alcohol and 100 mL of 1M H2SO4, then the mixture was sonicated for 15 min to obtain a well-dispersed suspension. Finally, a certain amount of ammonium persulfate (APS) was dissolved in above solution to form water phase. Meanwhile, a certain amount of aniline monomer was dissolved in 100 mL dichloromethane to form oil phase. Water phase and oil phases were then carefully transferred to a 600 mL beaker. The reaction was performed at 0

for 24 h and resulting product was filtered, washed with deionized

water and ethanol for several times. Finally, the product was dried at 60

under vacuum to obtain

the SPANI nanorods.

Preparation of SGO-SPANIX composite. The SGO-SPANIX composite (the SPANI nanorods vertically aligned on SGO surfaces) was synthesized by an interfacial polymerization method. a typical procedure was as follows: SGO (21 mg) and a certain amount of aniline-2-sulfonic acid (ASA) was added into a mixed solution of 20 mL sopropyl alcohol and 100 mL 1M H2SO4, then mixture was sonicated for 15 min to obtain a well-dispersed suspension. Finally, a certain amount of ammonium persulfate (APS) was dissolved in the above solution to form water phase. Meanwhile, a certain amount of aniline monomer was dissolved in 100 mL dichloromethane to form oil phase. Water phase and oil phases were then carefully transferred to a 600 mL beaker. The reaction was performed at 0

for 24 h. The resulting product was filtered, washed with deionized water and

ethanol for several times and dried at 60

under vacuum to obtain SGO-SPANIX composite. (Table

S1 in the Supporting Information). Preparation of SGO-SPANIbX blend. The SGO-SPANIbX blend (no SPANI nanorods on SGO surfaces) was synthesized by an interfacial polymerization method. a typical procedure was as follows: SGO (21 mg) and a certain amount of aniline-2-sulfonic acid (ASA) was added into a



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mixed solution of 20 mL sopropyl alcohol and 100 mL deionized water, then

mixture was

sonicated for 15 min to obtain a well-dispersed suspension. Finally, a certain amount of ammonium persulfate (APS) was dissolved in the above solution to form water phase. Meanwhile, a certain amount of aniline monomer was dissolved in 100 mL dichloromethane to form oil phase. Water phase and oil phases were then carefully transferred to a 600 mL beaker. The reaction was performed at 0

for 24 h. The resulting product was filtered, washed with deionized water and

ethanol for several times and dried at 60

under vacuum to obtain SGO-SPANIX composite (Table

S1 in the Supporting Information).

Preparation of Membranes. Mixed solvent was prepared by mixing 0.6 mL ammonium hydroxide (2.5 wt%), 12 mL n-methyl-2-pyrrolidone (NMP) and 8 mL water in a 50 mL beaker. 10 mg SGO-SPANIX composite was added in mixed solvent and then sonicated for 2 h. After filtration with cotton to remove the trace amount of precipitate, the mixture was carefully poured into a membrane cell equipped with a 0.22 µm porous nylon-66 membrane and filtered under pressure to fabricate membranes under 1 bar. Finally, the membrane was dried at 60

under vacuum. The

control membrane was prepared in the same procedure by using the SGO-SPANIbX blend to replace the SGO-SPANIX composite. The membranes prepared from the SGO-SPANIX composite were designated as SGO-SPANIx membranes, which x represents the concentrations of aniline monomer. In our experiments, the concentrations of aniline monomer for the preparation of SGO-SPANIx membranes were 10 mM, 25 mM and 50 mM, respectively. It should be noted that under vacuum-assisted filitration, SGO-SPANIx membranes enabled the generation of Laplace pressure inside conical nanochannels. In these cases, SGO-SPANIx membranes were designated as SGO-SPANIx (Laplace pressure) membranes. Meanwhile, the control membranes prepared from the SGO-SPANIbX blend were designated as SGO-SPANIbX membranes, which x represents the ACS Paragon Plus Environment

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concentrations of aniline monomer. In our experiments, the concentrations of aniline monomer for the preparation of SGO-SPANIbx membranes were 15 mM, 40 mM and 77 mM, respectively. Moreover, when vacuum-assisted filitration was utilized, SGO-SPANIbx membranes were designated as SGO-SPANIbx (vacuum driving) membranes. It should be noted that the molar concentration ratios of aniline monomer to APS and the molar concentration ratios of aniline monomer to ASA are 1:1 and 4:1 for all the experiments.

Characterization. Fourier transform infrared spectroscopy of SGO, SPANI and SGO-SPANIX membranes were recorded by an attenuated total reflectance (ATR) technique with a spectrometer (Thermo Nicolet, Nicolet 6700) in the region of 4000-400 cm-1 and a resolution of 4 cm-1 for 64 scans. Cross-section morphologies of composites and membranes were observed by field emission SEM (Nanosem 430, FEI Co., USA) after being sputtered with a thin layer of platimum. The crystalline properties of SPANI nanorods, SGO-SPANIX and SGO-SPANIbX composites and SGO-SPANIX and SGO-SPANIbX membranes were measured by wide-angle XRD and small angle X-ray scattering (SAXS) using a D/MAX-2500 X-ray diffractometer (Cu Kα). The surface morphologies of SGO-SPANIX, SGO-SPANIbX composites and SGO-SPANIX, SGO-SPANIbX membranes were observed by AFM (CSPM 5000). AFM was operated in air atmosphere in the tapping mode. It worth noting that in order to observe the surface morphology of SGO-SPANIX and SGO-SPANIbX nanosheets inside the membranes, the NMP-soaked nylon-66 microfiltration membrane was stuck onto the SGO-SPANIX or SGO-SPANIbX membranes before they were dried under vacuum, after few seconds, the microfiltration membrane was peeled off to remove the outermost layer of SGO-SPANIX or SGO-SPANIbX membranes and expose the inner layer of membranes, and the stripe arrays of the inner layer of SGO-SPANIX or SGO-SPANIbX membranes was then observed by AFM images. Transmission electron microscope (TEM) images of ACS Paragon Plus Environment


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SGO-SPANIX nanosheet from exfoliated SGO-SPANIX membrane were obtained by Hitachi S-4800 TEM instrument with operating voltage of 200 keV.The experimental processes are described as below: firstly, the NMP-wetted whole carbon coated copper grid was stuck onto SGO-SPANIX membrane surface before the membrane dried under vacuum, then after few seconds, the copper grid was peeled off, and the outermost layers of the membrane were collected on its surfaces. Lastly, the copper grid was dried at 25 OC to derive membrane samples. During TEM measurements, multiple images were collected to pick out the monolayer nanosheet and double layer nanosheets from exfoliated SGO-SPANI50 membrane The static contact angles of SGO-SPANIX and SGO-SPANIbX membranes were measured using a contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China). A water droplet with volume of 5 µL was dropped onto the membrane surface with a microsyringe. At least five contact angles at different locations on one surface were averaged to get a reliable value. The surface Zeta potential was determined from streaming potential measurements by a SurPASS Eletrokinetic Analyzer (Anton Paar KG, Austria). Streaming potential measurements were determined by an adjustable gap cell which could immobilize the selected membranes with a 1 mM KCl (pH=6.3±0.2) solution at 25±0.5

. The

measurement progress was controlled by Visolab for Surpass software. The yield of SPANI was estimated by calculate the weight percentage of SGO from the weight of the SGO-SPANIX composites and SGO-SPANIbX blends (Table S2). The permeation and separation performance evaluation was conducted in a dead-end stirred cell (model 8010, Millipore Co.) filtration system. The system was connected with a nitrogen gas cylinder and a filtration cell with a volume capacity of 10 mL. Pure water permeability of membranes was evaluated by permeation tests with pure water. The rejections of four kinds of dyes including Alcian Blue 8GX, Congo Red, Orange G and Methyl Blue were evaluated by permeation ACS Paragon Plus Environment

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tests with the dye solutions of pH=6.5 after the membranes immersed in the corresponding dye solutions for 24 hours to eliminate the effect of dye adsorption on membrane separation properties. The operating pressure in the system was maintained by nitrogen gas and vacuum. All the experiments were performed at 25±1℃ and a stirring speed of 400 rpm. The water Permeability (Jw) was calculated according to Equation 1: ‫ܬ‬௪ = ܸൗ(‫)ݐ∆ܣ‬

(1)

Where V(L) was the volume of permeation water, A(m2) was the membrane effective area, ∆‫(ݐ‬h) was the operation time.

To determine the rejection properties of membranes, the filter cell was filled with aqueous solutions of dyes or inorganic salt. The rejection ratio (R) was determined using Equation 2: R = ቌ1 −

‫ܥ‬௣ ൘‫ ܥ‬ቍ × 100% ௙

(2)

Where ‫ܥ‬௣ and ‫ܥ‬௙ were the solute concentrations of permeant and feed solutions, respectively. Dye concentrations of Alcian Blue 8GX, Orange G, Congo Red and Methyl Blue solutions were determined through a UV-vis spectrophotometer (Hitach UV-2800, Hitach Co., Japan) at the wavelength of 330, 485, 490 and 600 nm, respectively.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional AFM, SEM images, XRD, FTIR, XPS, Zeta potential and Contact angle results, photo images, comparison with other membranes, additional tables, additional theory descriptions, and additional dye separation performance.



AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Zhongyi Jiang: 0000-0002-2492-4094 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors thank the financial support from National Key Research and Development Program of China (2016YFB0600503), National Natural Science Foundation of China (21621004) and the National Science Fund for Distinguished Young Scholars (21125627).

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Graphical abstract


Graphene Oxide Membranes with Conical Nanochannels for Ultrafast

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