Graphene Oxide Membranes with Strong Stability in Aqueous

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Graphene Oxide Membranes with Strong Stability in Aqueous Solutions and Controllable Lamellar Spacing Yue-Heng Xi, Jia-Qi Hu, Zhuang Liu, Rui Xie, Xiao-Jie Ju, Wei Wang, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00928 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the fabrication strategy and mechanism of the prGO-doped GO membranes with high stability in aqueous solutions and controllable lamellar spacing. (a, b) The GO sheets with plentiful oxidation regions (a) are partially reduced to the prGO sheets (b). (c, d) The pure GO membranes have large interlayer spacing d1 (c) and more oxidation groups on lamellar sheets, leading to stronger repulsive hydration force making GO membranes disintegrate in aqueous solutions (d). (e, f) The prGO-doped membranes have closer interlayer spacing d2 that creates stronger π-π attraction between GO sheets (e) and weak repulsive hydration force to keep GO membranes stable in aqueous solutions (f). 227x322mm (300 x 300 DPI)

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Figure 2. The AFM images (a, c) and XPS spectra (b, d) of GO (a, b) and prGO (c, d) sheets. The inserted height profiles in AFM images are taken along the red lines. 80x83mm (300 x 300 DPI)


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Figure 3. The photographs (a, c, e, g and i) and cross-sectional SEM images (b, d, f, h and j) of GO@AAO membranes with weight percentage of 0 wt% (a, b), 20 wt% (c, d), 50 wt% (e, f), 80 wt% (g, h), 100 wt% (i, j) prGO. Scale bars are 10 mm in (a, c, e, g and i), and 200 nm in (b, d, f, h and j). 78x191mm (300 x 300 DPI)



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Figure 4. (a-c) XRD patterns of GO@AAO (a), GO@CN-CA (b) and GO@Teflon (c) membranes with different weight percentages of prGO. (d) Effect of the prGO content on the lamellar spacing of GO membranes . 80x79mm (300 x 300 DPI)


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Figure 5. Stability of GO@AAO membranes in water, acid and base solutions. (a, b) GO@AAO membranes prepared with 0 wt% prGO. (c, d) GO@AAO membranes doped with 50 wt% prGO. The photographs in (a) and (c) are taken after the membranes have been statically immersed in solutions for different time periods; while, before taking the photographs in (b) and (d), the membranes have been stirred for several seconds with a mini-shaker after immersing. 145x144mm (300 x 300 DPI)



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Figure 6. Stability of GO@CN-CA membranes in water, acid and base solutions. (a, b) GO@CN-CA membranes prepared with 0 wt% prGO. (c, d) GO@CN-CA membranes doped with 50 wt% prGO. The photographs are taken in the same way as that mentioned in Figure 5. 144x144mm (300 x 300 DPI)


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Figure 7. Stability of GO@Teflon membranes in water, acid and base solutions. (a, b) GO@Teflon membranes prepared with 0 wt% prGO. (c, d) GO@Teflon membranes doped with 50 wt% prGO. The photographs are taken in the same way as that mentioned in Figure 5. 145x146mm (300 x 300 DPI)



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Figure 8 408x511mm (300 x 300 DPI)


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Figure 9 80x128mm (300 x 300 DPI)



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Figure 10 122x96mm (300 x 300 DPI)


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TOC FIGURE 59x55mm (300 x 300 DPI)



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Graphene Oxide Membranes with Strong Stability in Aqueous Solutions and Controllable Lamellar Spacing Yue-Heng Xi,†,§ Jia-Qi Hu,†,§ Zhuang Liu,*,† Rui Xie,† Xiao-Jie Ju,†,‡ Wei Wang,† and Liang-Yin Chu*,†,‡,∥ †

School of Chemical Engineering, Sichuan University, No. 24, Southern 1 Section, Yihuan Road,

Chengdu, Sichuan 610065, P. R. China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan

610065, P. R. China. ∥

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing,

Jiangsu 211816, P. R. China §

These authors contributed equally to this work.


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ABSTRACT Graphene oxide (GO) membranes become emerging efficient filters for molecular or ionic separation due to their well-defined two-dimensional nanochannels formed by closely-spaced GO sheets and tunable physicochemical properties. solutions is a prerequisite for their applications.

The stability of GO membranes in aqueous Here we show a novel and easy strategy for

fabricating GO membranes with strong stability in aqueous solutions and controllable lamellar spacing by simply doping with partially reduced graphene oxide (prGO) sheets.

With our

prGO-doping strategy, the interlayer stabilizing force in GO membranes is enhanced due to the weakened repulsive hydration and enhanced π-π attraction between GO sheets; as a result, the fabricated GO membranes are featured with controllable lamellar spacing and extraordinary stability in water or even strong acid and base solutions as well as strong mechanical properties, which will expand the application scope of GO membranes and provide ever better performances in their applications with aqueous solution environments.

KEYWORDS Graphene oxide; Membranes; Water-stability; Lamellar spacings; Nanochannels


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INTRODUCTION Graphene oxide (GO) membranes with well-defined interconnected nanochannels formed between GO sheets, which are beneficial to minimizing transport resistance and maximizing flux, are tempting for precise and ultrafast molecular separation and desalination.1-5

The stability of

GO membranes in aqueous solutions is crucial for their efficient applications in aqueous environments, such as ion or molecular sieving.6,7

The GO membranes with good stability in

water have been reported,4,8 such GO membranes are fabricated by restacking the GO sheets on the anodized aluminum oxide (AAO) filters.

A lot of Al3+ released from the AAO filters during

the filtration can effectively crosslink the GO sheets and strengthen the stability of resultant GO membranes in water.7

However, it has also been reported that the GO membranes are usually

unstable in water.6,7,9-12

The GO sheets contain plentiful oxidized groups such as hydroxyl and

epoxy on the basal planes as well as carboxyl at the edges could form hydration with water molecules.13

The hydrogen bonding between water and hydroxyl/epoxy groups can provide

strong repulsive hydration forces14,15 to separate the GO sheets from each other.6

Furthermore,

the carboxyl groups become negatively charged on hydration and could afford electrostatic repulsion among GO sheets to make the GO membranes disintegrated in water.7,16

Therefore,

the enhancement of the stability of GO membranes in water is of great significance. Up to now, improved stability of GO membranes in water can be achieved by building interlayer crosslinking between the GO sheets via multivalent ions such as Al3+, Mg2+, Zn2+, Mn2+ and

borate

ions,7,12,17-19

or

molecules6,12

such

as

glutaraldehyde,20

alkylamines,21

1,3,5-benzenetricarbonyl trichloride,22 polyallylamine,23 poly(vinyl alcohol)24 and poly(methyl methacrylate).24

However, the GO membranes crosslinked by cations still suffer from

redispersion in acid or base solutions, because the cation crosslinkers could be re-dissolved in


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acid or base solutions.7

Moreover, the crosslinked GO membranes by cations are highly

susceptible to contaminating, which leads to pollution during the ion transportation or filtration process.7

In addition, the crosslinking by the polymeric crosslinkers is supposed to disturb the

well-defined nanochannels,25,26 which will greatly affect the sieving efficiency of the nanostructure.

Generally, both high stability in aqueous solutions with different pH values and

well-defined lamellar spacing are indispensable to the GO membranes for the efficient sieving applications in aqueous solutions.

Unfortunately, the fabrication of GO membranes featured

with both strong stabilities in aqueous solutions at different pH values and regular controllable lamellar spacings still remains challenging. Here we report a novel and easy strategy to fabricate the GO membranes with both extraordinary stabilities in water, acid and base solutions as well as regularly controllable lamellar spacings by constructing the orderly lamellar nanostructure of GO membranes doped with partially reduced graphene oxide (prGO) sheets, which is schematically illustrated in Figure 1.

The GO sheets are synthesized by using the reported Hummers method,13,27 which are

featured with abundant hydroxyl, epoxy, carbonyl and carboxyl groups (Figure 1a).

The prGO

sheets are synthesized by reducing the partial hydroxyl and epoxy groups on GO sheets using hydrazine according to a reported method15,28 (Figure 1b).

Via vacuum filtration with

commercial membrane filters, both pure GO membranes (Figure 1c,d) and prGO-doped GO membranes (Figure 1e,f) can be prepared with lamellar nanostructures.

Because of the plentiful

oxidized groups on the GO sheets, when the pure GO membranes are immersed in aqueous solutions, the water molecules are apt to form abundant hydrogen bonding with the oxidation groups such as hydroxyl, epoxy and carboxyl units.

Eventually, the GO sheets separate from

each other, and the nanochannels between GO sheets are damaged due to the strong repulsive


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hydration forces15 (Figure 1d).

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Compared with the interlayer distance in the pure GO

membrane (d1) (Figure 1c), that of the proposed prGO-doped GO membrane (d2) (Figure 1e) is slightly shorter due to the decrease of oxidation groups on the prGO sheets that act as supporters1, which results in stronger π-π attraction between adjacent sheets in the prGO-doped GO membrane.10,12,15

Therefore, both the weakened repulsive hydration forces and the dominant

π-π attraction between GO sheets contribute to the high stability of GO membranes in aqueous solutions.10,12,15

Importantly, the interlayer stabilizing force generated by our strategy is not

implicated with the pH values of aqueous solutions; as a result, the prepared GO membranes are also highly stable in acid and base solutions.

The extraordinary stabilities of resultant GO

membranes are demonstrated in aqueous solutions for a long time, e.g., one month, or even under strong stirring condition.

Furthermore, by varying the content of prGO in the GO membranes,

the lamellar spacing between GO sheets can be easily adjusted due to the variation of the oxidation groups on GO sheets as interlayer supporters.1

Such GO membranes with

extraordinary stability and controllable lamellar spacing could provide ever better performances in the applications for ion or molecular sieving and separation.

EXPERIMENTAL SECTION Synthesis of GO and prGO GO was synthesized by the modified Hummers method as reported13,27 and dialyzed against pure water for one week to remove the ions from the synthesis process.

The GO was exfoliated by

ultrasonic for 30 min and centrifuged at 3,000 rpm for 20 min to remove the unexfoliated GO sheets.

Then, 30 μL of hydrazine solution (51.2 wt% in water, Kelong) and 550 μL of

ammonia solution (25 wt% in water, Kelong) were added to the prepared homogenous dispersion


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(140 mL, 0.25 mg/mL) to synthesize prGO, with continuously stirring in a 60 °C water batch for 1 h.

Fabrication of GO Membranes The GO and prGO were mixed with the weight percentages of prGO being 0 wt%, 20 wt%, 50 wt%, 80 wt% and 100 wt%, for preparing GO membranes with different contents of prGO. Then, each mixture was filtrated through different filter membranes (Anodisc AAO, Whatman; CN-CA, Xidoumen Membrane Industry; Teflon, Millipore) via vacuum filtration.

The resultant

GO membranes are denoted as GO@AAO, GO@CN-CA and GO@Teflon, respectively.

The

effective diameters of the filter membranes are 47 mm, and the average membrane pore sizes are 0.2 μm.

It took several hours to filtrate each sample of mixture dispersion (100 ml, average

sheets concentration is ~0.2 mg/mL), and the freestanding membranes could be peeled from the filter membranes after drying in air.

Stability Tests The as-prepared membranes were cut into 1cm×1cm square pieces, and then statically immersed in pure water (pH=6.8), HCl aqueous solution (pH=1.2) and NaOH aqueous solution (pH=10.8) respectively at room temperature.

The stability of membranes in different solutions was

recorded after the membranes have been immersed for certain time periods.

Similarly, the

membranes were cut into 0.5cm×0.5cm square pieces, and soaked in pure water (pH=6.8), HCl aqueous solution (pH=1.2) and NaOH aqueous solution (pH=10.8) respectively in glass tubes at room temperature.

After have been immersed for a certain time period, the membranes in the

glass tubes were stirred for 3~5 sec using an IKA vortex mixer (Supplementary Movies S1-S9).


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Permeability and Separation Tests The water permeability performances of GO membranes with various prGO contents were measured by a water evaporation device.

A GO@CN-CA or CO@AAO membrane on a copper

foil containing a hole with a diameter of 6 mm was sealed to a glass vial that filled with 2 mL deionized water.

The vial was placed in an oven with constant temperature at 25 oC and

constant relative humidity of 25%. mass of the vial for 12 h.

Water loss was measured by periodically monitoring the

Inspired by the excellent water-ethanol separation performance of

GO membranes,1 the water-ethanol separation performances of our GO@CN-CA membranes were measured using water-ethanol solutions with different ethanol contents under different relative humidity conditions at 25 oC.

Characterization SEM images were taken by a JSM-7500F SEM microscope (JEOL).

XRD patterns were

recorded using an X’Pert Pro MPD X-ray diffractometer (Philips) with Cu Kometer X’Pert (λ=1.54060 Å) at 40 kV and 40 mA.

AFM images were obtained by a Multimode Nanoscope

V scanning probe microscope (Bruker). XPS spectrometer (Kratos).

XPS measurements were carried out on a XSAM800

The mechanical properties were tested using an EZ-LX

commercial test machine (Shimadzu) with the membrane samples being cut into dumbbell shapes (length 35 mm, width 2 mm, and gauge length 12 mm).

The permeability and

separation tests were measured by an electronic balance (Mettler Toledo AX205) with precision of 0.00001 g.


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RESULTS AND DISCUSSION Morphological and Compositional Analyses of GO-Based Sheets and Membranes The synthesized GO sheets using the Hummers method are featured with a thickness of ~1 nm and abundant hydroxyl, epoxy and carboxyl groups, as expected (Figure 2a, b).

The prGO

sheets are chemically converted from GO sheets with similar lamellar structures by reducing the partial oxidation regions on GO sheets (Figure 2c, d), which can also remain well dispersed in water for a long time (Figure S1).

It is common for GO sheets prepared by the Hummers

method to retain a C/O ratio of approximately 1.5-4,29 while the C/O ratio of chemically converted graphene (CCG)28 or reduced graphene oxide (RGO)30 sheets is considered to be 8-246.31

The C/O ratio of the as-prepared prGO, which is determined by XPS, is about 4.2

(Figure S2).

The result implies that the synthesized prGO sheets are different from the reported

CCG or RGO sheets,28-31 because partial hydroxyl and epoxy groups still remain on the prGO sheets to keep the spacing between GO laminates for building interconnected nanochannels.1,4 The surviving carboxyl groups being negatively charged in water are hardly changed, which results in no effect on the zeta potential of the mixture of GO and prGO (Figure S3). Our GO membranes are prepared by vacuum filtration of mixtures of GO and prGO dispersions with different prGO contents using different filter membranes including AAO, hydrophilic cellulose nitrate-cellulose acetate (CN-CA) and hydrophobic Teflon membranes. The resultant GO membranes are well freestanding and with very similar and pronounced lamellar nanostructures (Figure 3 and Figures S4).1,8

With the increasing prGO content, the

C/O atomic ratio of as-prepared GO membrane simply increases (Figure S2), which implies that the oxidation regions on GO sheets as the supporters to form the empty spaces decrease.1

The

change of average interlayer spacing between the GO laminates caused by doping prGO with


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different contents is confirmed by the X-ray diffraction (XRD) patterns (Figure 4). interlayer spacing decreases with increasing the prGO content in the GO membranes.

The

For the

GO@CN-CA and GO@Teflon membranes without prGO, the average interlayer spacing is ~9.0 Å; while, that of GO@CN-CA and GO@Teflon membranes with 100 wt% prGO sheets drops to ~7.0 Å (Figure 4d), which indicates that our strategy offers an adjustment of the interlayer distance between GO sheets to nearly 2 Å range.

The above results confirm that, the GO

membranes doping with prGO sheets are prepared with lamellar architecture and adjustable spacing.

The lamellar spacing as the channel for mass transfer is crucial to the separation

performance of the GO-based membranes,1,4 and previous works have confirmed the importance of lamellar spacing for ion separation clearly.4

Stability of GO@AAO, GO@CN-CA and GO@Teflon Membranes in Aqueous Solutions Typically, the GO@AAO membranes without any prGO are quite stable in water,4,7,8 and remain intact even after one-month immersion in water or strong stirring after long-time soaking, because the water-stability of GO@AAO membranes can be strengthened by the Al3+-crosslinking.7

However, the Al3+-crosslinked GO@AAO membranes are instable in acid

and base solutions as shown in Figure 5a and b.

Because aluminum is amphoteric,32 Al3+ can be

dissolved in both acid and base solutions, i.e., the Al3+-crosslinking in the GO@AAO membranes can be damaged in acid and base solutions; as a result, the GO@AAO membranes are instable in acid and base solutions.

Excitingly, the GO@AAO membranes doped with 50

wt% prGO are extremely stable not only in water but also in acid and base solutions for a long time, e.g., one month (Figure 5c).

Although the Al3+-crosslinking could be destroyed in acid or

base environment, the addition of prGO sheets into the GO membranes strengthens the stability


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in aqueous solutions.

Even under the strong stirring operation after soaking, which is

conducive to disintegrating the GO membranes, the GO@AAO membranes prepared by our strategy are still intact in all water, acid and base solutions (Figure 5d, Supplementary Movies S1-S3). In order to exclude the interference of Al3+, we have also prepared Al3+-free GO membranes with prGO using hydrophilic CN-CA and hydrophobic Teflon membranes as filters.

The

GO@CN-CA membranes without any prGO are seriously damaged in aqueous solutions such as pure water, acid and base solutions (Figure 6a), which even completely disintegrate under short-time stirring (Figure 6b).

Just as expected, the GO@CN-CA membranes doped with 50

wt% prGO are extremely stable in all water, acid and base solutions after being soaked statically for one month (Figure 6c).

Even under the strong stirring, the GO@CN-CA membranes doped

with 50 wt% prGO still remain intact in water, acid and base solutions (Figure 6d, Supplementary Movies S4-S6).

The GO@Teflon membranes are also featured with strong

stability by doping with 50 wt% prGO (Figure 7, Supplementary Movies S7-S9).

Additionally,

to eliminate the scrupulosity of crosslinking by the ammonium hydroxide from the synthetic process of prGO,30 the stability of GO@CN-CA membranes without any prGO in ammonium hydroxide solution has been tested.

The results show that the membranes disintegrated after

being soaked statically for only one day (Figure S5a) or even for only 10 min with short time stirring (Figure S5b). water.

So, neither NH4+ nor NH3·H2O can make the GO membranes stable in

The results imply that our strategy of doping prGO in the GO membranes remarkably

improves the stability of GO@CN-CA and GO@Teflon membranes without any Al3+ or other multivalent metal cations introduced intentionally7 or unintentionally18.


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To provide systematical information of the stability of GO membranes prepared with prGO-doped strategy, the stabilities of GO@AAO, GO@CN-CA and GO@Teflon membranes with various contents of prGO in water, acid and base solutions are comprehensively investigated (Figures S6-S14).

As illustrated in Figure 8, under conditions with both statically

immersing only or short-time stirring after various-time soaking, the stabilities of all the GO membranes in water, acid and base solutions are strengthened with increasing the prGO content. The higher the prGO content in GO membranes is, the stronger the interlayer stabilizing force between the laminates; as a result, the more stable the GO membranes in aqueous solutions. The GO membranes doped with 50 wt% or more prGO are always extremely stable in aqueous solutions at different pH values; while the GO membranes with 20 wt% or less prGO could disintegrate in aqueous solutions after being soaked for a long time, excepting that the GO@AAO membranes in water are stable due to the Al3+-crosslinking.

In short, our strategy

enables to simply and controllably fabricate highly stable GO membranes for applications in aqueous solutions by just doping with certain amount of prGO sheets in GO membranes. The stabilities of the prGO-doped GO membranes in aqueous solutions are strengthened depending on the decrease of the hydroxyl and epoxy groups on prGO sheets to decrease the interlayer distance and reduce the repulsive hydration force between graphene sheets and water. Once the interlayer spacing becomes compact, the π-π attraction between graphene sheets is enhanced,10,12,15 resulting in improved mechanical strengths of GO membranes.

To confirm that,

the mechanical properties of the pure GO membranes and prGO-doped GO membranes are tested and the results are shown in Figure 9.

The GO membranes doping with 50 wt% prGO

are stronger than those without any prGO.

For examples, the average tensile strength of

GO@CN-CA membranes doped with 50 wt% prGO is 234.6±37.5 MPa, which is significantly


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higher than that of GO@CN-CA membranes without any prGO (172.2±32.6 MPa). The prGO incorporation in GO membranes results in a significant enhancement of the interlayer stabilizing force in GO membranes, so the GO membranes doped with prGO exhibit strong stability in aqueous solutions.

Permeability and Separation Performances of GO Membranes Doping with prGO The doping of prGO sheets in GO membranes results in the decline of interlayer distance between adjacent sheets.

However, the interlayer spacing is still sufficient enough to

accommodate a monolayer of water.33,34

The water permeation and separation performances of

GO membranes with different prGO contents are investigated with a home-made device (Figure 10a). The membrane is sealed on the top of a glass bottle, which is loaded with water or water/ethanol mixture and placed in a constant temperature/humidity chamber.

The

temperature is fixed at 25 oC and the relative humidity in the chamber (RHc) is varied.

The

permeation flux of water across the GO@CN-CA membrane decreases slightly with increasing the prGO content in the GO membrane.

Compared with the water permeation rate of

GO@CN-CA membranes fabricated without any prGO, which shows unimpeded permeation of water,1 that of the GO@CN-CA membrane doped with 50 wt% prGO exhibits a decrease of only ~18.8% (Figure 10b).

Even for the GO@CN-CA membranes fabricated with 100 wt% prGO,

the water permeation rate only decreases ~32.1%

(Figure 10b).

The results verify that,

besides the strong stability in aqueous solutions, the GO@CN-CA membranes doping with prGO still possess good water permeability.

However, the water permeation rate of GO@AAO

membranes is smaller than that of GO@CN-CA membranes (Figure S15).

Especially for the

GO@AAO membrane fabricated with 100 wt% prGO content, the water permeation rate


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decreases over 75%, due to the blocked nanochannels between GO nanosheets by Al3+ ions released from AAO filters.7

In the separation of water and ethanol molecules, the water

permeation flux of GO@CN-CA membranes decreases with increasing the ethanol content (CE). Particularly, the ethanol molecules cannot pass the GO@CN-CA membrane with different prGO contents, as the ethanol permeation flux approximates zero (Figure 10b).

Compared with the

permeation flux of water, the permeation flux of ethanol is considered as zero for the water-ethanol mixture tests.

The reason for this phenomenon exists in that the intercalating

water in the GO-based membranes blocks, or at least impedes, ethanol molecules from passing through the membranes, which has been verified in previously reported literatures.1,35

So, we

can infer that all the mass loss of mixture solution is due to the permeation of water in the separation tests. The humidity outside the membrane affects the water permeation flux heavily (Figure 10c). When the relative humidity in the chamber (RHc) is 10%, the permeation rates of the GO@CN-CA membranes with different prGO contents are much larger compared with the case of RHc being 25%. membrane.

That is, the lower the RHc is, the faster the water permeation rate across the

With increasing humidity difference between the spaces inside and outside the

membrane, the permeability for water transferring across the membrane increases; as a result, the water permeation flux becomes larger based on the sorption-diffusion theory.

This has been

verified by calculating the results of water permeability [g·h-1·m-2·Pa-1] values with different relative humidities through an open aperture and the same aperture covered with GO membranes with different prGO contents (Fig. S16).

Therefore, for effectively separating water molecules

from the water-ethanol solutions, the relative humidity in the chamber is fixed as 10%.


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Three series of water-ethanol solutions with original ethanol contents (CE0) being respectively 20 wt%, 50 wt% and 80 wt% are selected to investigate the water-ethanol molecular separation performances (Figure 10d).

With the GO@CN-CA membrane fabricated with 50 wt% prGO

content, excellent water-ethanol separation performances are observed.

With the separation of

water molecules from the water-ethanol solutions going on, the ethanol content in the water-ethanol solution increases with time.

The results demonstrated that the GO membranes

fabricated with our strategy show excellent molecular separation performances.

CONCLUSION In summary, we have demonstrated a novel and easy strategy for fabricating GO membranes with high stability in aqueous solutions at different pH values by doping prGO sheets in the GO membranes.

The proposed GO membranes are featured with extraordinary stability in water or

even strong acid and base solutions as well as excellent molecular separation performances. Furthermore, our strategy provides a novel methodology to slightly adjust the interlayer spacing of well-defined sieving channels, which enables the GO membranes to promise fertile applications for precise ionic or molecular separations in the future.

The proposed easy and

feasible strategy in this study has tackled the long-time problem of the instability of GO membranes without crosslinking in water or aqueous solutions, and it may open up enormous opportunities for the GO membranes to be employed in myriad applications with aqueous solution environments.

ASSOCIATED CONTENT Supporting Information.


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The dispersing performance of GO and prGO; the zeta potential of the mixtures of GO and prGO; the cross-sectional SEM images of various membranes; the curves of prGO content with C/O ratio; scheme illustration of the interlayer spacing changing with the content of prGO; the stability images of GO membranes with different prGO content at different pH and in different substrate; the effect of prGO content on the water permeation rate; the stability movies of GO membranes with different prGO content at different pH and in different filters; effect of the prGO content on the water permeation rate of GO@AAO membranes. This material is available free of charge via the Internet at http://pubs.acs.org or from the author.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (L.-Y. Chu) * E-mail: [email protected] (Z. Liu). Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors gratefully acknowledge support from the National Natural Science Foundation of China

(21490582)

and

State

Key

Laboratory

of

Polymer

Materials

Engineering

(sklpme2014-1-01).

ABBREVIATIONS GO, graphene oxide; prGO, partially reduced graphene oxide; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction;

AFM, atomic force microscope; SEM, scanning

electron microscope; AAO, anodized aluminum oxide; CN-CA, cellulose nitrate-cellulose acetate.


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FIGURES

Figure 1. Schematic illustration of the fabrication strategy and mechanism of the prGO-doped GO membranes with high stability in aqueous solutions and controllable lamellar spacing. (a, b) The GO sheets with plentiful oxidation regions (a) are partially reduced to the prGO sheets (b). (c, d) The pure GO membranes have large interlayer spacing d1 (c) and more oxidation groups on lamellar sheets, leading to stronger repulsive hydration force making GO membranes disintegrate in aqueous solutions (d). (e, f) The prGO-doped membranes have closer interlayer spacing d2 that creates stronger π-π attraction between GO sheets (e) and weak repulsive hydration force to keep GO membranes stable in aqueous solutions (f).


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Figure 2. The AFM images (a, c) and XPS spectra (b, d) of GO (a, b) and prGO (c, d) sheets. The inserted height profiles in AFM images are taken along the red lines.


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Figure 3. The photographs (a, c, e, g and i) and cross-sectional SEM images (b, d, f, h and j) of GO@AAO membranes with weight percentage of 0 wt% (a, b), 20 wt% (c, d), 50 wt% (e, f), 80 wt% (g, h), 100 wt% (i, j) prGO. Scale bars are 10 mm in (a, c, e, g and i), and 200 nm in (b, d, f, h and j).


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Figure 4. (a-c) XRD patterns of GO@AAO (a), GO@CN-CA (b) and GO@Teflon (c) membranes with different weight percentages of prGO. (d) Effect of the prGO content on the lamellar spacing of GO membranes.


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Figure 5. Stability of GO@AAO membranes in water, acid and base solutions. (a, b) GO@AAO membranes prepared with 0 wt% prGO. (c, d) GO@AAO membranes doped with 50 wt% prGO. The photographs in (a) and (c) are taken after the membranes have been statically immersed in solutions for different time periods; while, before taking the photographs in (b) and (d), the membranes have been stirred for several seconds with a mini-shaker after immersing.


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Figure 6. Stability of GO@CN-CA membranes in water, acid and base solutions. (a, b) GO@CN-CA membranes prepared with 0 wt% prGO. (c, d) GO@CN-CA membranes doped with 50 wt% prGO. The photographs are taken in the same way as that mentioned in Figure 5.


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Figure 7. Stability of GO@Teflon membranes in water, acid and base solutions. (a, b) GO@Teflon membranes prepared with 0 wt% prGO. (c, d) GO@Teflon membranes doped with 50 wt% prGO. The photographs are taken in the same way as that mentioned in Figure 5.


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Figure 8. Stability of GO membranes doped with various contents of prGO in water, acid and base solutions. (a, b) GO@AAO membranes. (c, d) GO@CN-CA membranes. (e, f) GO@Teflon membranes. The circle mark means the membrane is intact, the cross mark in the orange regions means the membrane is broken, and the cross mark in the red regions means the membrane is disintegrated.


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Figure 9. Effect of prGO incorporation on mechanical property of GO@CN-CA and GO@Teflon membranes. (a) Effect of the prGO incorporation on typical strain-stress curves of membranes. (b) Effect of the prGO incorporation on the tensile strength of membranes.


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Figure 10. The permeability and separation performances of GO membranes doped with different prGO contents. (a) The device and system for water-ethanol separation tests. The glass bottle has a 25 mm diameter and a 25 mm height, and is loaded with 2 mL water-ethanol solution. (b) Effects of the prGO content in the GO@CN-CA membrane and the ethanol content in the water-ethanol solution on the permeation rate of water across the membrane. RHc = 25 %. (c) Effect of the relative humidity outside the membrane in the chamber on the permeation rate of water across the GO@CN-CA membrane. CE = 95 wt%. (d) The water-ethanol separation performance of GO@CN-CA membrane with prGO content being 50 wt%, in which CE0 is the initial ethanol content in the water-ethanol solution. RHc = 10 %. All the GO@CN-CA membranes are prepared with the same packing density of 1.26 mg/cm2.


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TOC FIGURE


32

Graphene Oxide Membranes with Strong Stability in Aqueous

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