Effect of Oxidized-Group-Supported Lamellar Distance on Stability of

Subscriber access provided by - Access paid by the | UCSB Libraries

Materials and Interfaces

Effect of oxidized-group-supported lamellar distance on stability of graphene-based membranes in aqueous solutions Yue-Heng Xi, Zhuang Liu, Qian-Cheng Liao, Rui Xie, XiaoJie Ju, Wei Wang, Yousef Faraj, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01959 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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

Industrial & Engineering Chemistry Research

Effect of oxidized-group-supported lamellar distance on stability of graphene-based membranes in aqueous solutions Yue-Heng Xi†, Zhuang Liu†,‡,*, Qian-Cheng Liao§, Rui Xie†,‡, Xiao-Jie Ju†,‡, Wei Wang†,‡, Yousef Faraj† and Liang-Yin Chu†,‡ ,*

†

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China

‡

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

Sichuan 610065, China §

College of Material Science and Engineering, Sichuan University, Chengdu, Sichuan 610065,

China

KEYWORDS Graphene-based membranes; Graphene oxide nanosheets; Two-dimensional lamellar nanostructures; Lamellar distance; Stability

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Page 2 of 31

ABSTACT

Graphene oxide (GO) membranes with diverse lamellar distances can achieve sieving of molecules of different sizes in aqueous solutions.

Without extra assistance of crosslinking,

the increase of lamellar distance supported by self-contained oxidized groups could diminish the internal π-π attraction force of GO membranes, resulting in the instability of GO membranes.

However, the critical value of oxidized-group-supported lamellar distance of

unstable GO membranes in aqueous solutions, which is crucial in applications for precise sieving

of

molecules,

still

remains

unknown.

In

this

study,

the effect

of

oxidized-group-supported lamellar distance on the stability of graphene-based membranes in aqueous solutions is systematically investigated.

By controlling the content of oxidized

groups via changing reducing condition, different lamellar distances of graphene-based membranes are obtained.

With changing the lamellar distance in aqueous solutions under

different pH conditions, the graphene-based membranes show distinct stability.

There exists

a critical lamellar distance in the aqueous solution to ensure the stability of graphene-based membranes.

If the lamellar distance is less than the critical value, the membranes remain

physically stable for a long time, and still provide a favorable water permeation performance. The critical lamellar distances vary in aqueous solutions depending on pH values.

The

results provide a valuable guidance for the applications of graphene-based membranes in aqueous solutions.

2


Page 3 of 31 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


1. INRTODUTION

Graphene oxide (GO) membranes with excellent sieving properties are of great significance in desalination,1-3 water treatment4-8 and molecule/ion separation,9-15 due to their regular two-dimensional (2D) lamellar nanostructures.

By regulating the lamellar distances,

the GO membranes can precisely sieve the molecules of various sizes.11-16 For instance, when the lamellar distance between GO nanosheets is controlled at 3.5 Å, the membrane does not only provide a high-water permeability, but also a high NaCl rejection.2

As the effective

lamellar distance between GO nanosheets is further increased to ~8 Å, a precise sieving of mono-valent and multi-valent metal ions can be achieved;11 while the distance of 9~10 Å enables the GO membranes to separate ions with large hydrated size, organic molecules and macromolecules.10,17

Compared with lamellar distances adjusted through means of ions or

molecules,18-21 the lamellar distances that are set through their self-contained oxidized groups can maintain regularity of the original 2D nanostructures, which is a crucial element for precise sieving of ions or molecules in aqueous solutions based on the dimension matching.7,11,22 The oxidized groups on GO nanosheets have tendency to create hydration effect with water molecules, which push against 2D interlayer spacing in the aqueous solution.23-27

As a result, the GO membranes can easily disintegrate in aqueous solutions,

which poses heavy limitations on applications of the GO membranes.20,27-28

Therefore, it is

crucial to study the stability of the GO membranes with diverse lamellar distances supported by self-contained oxidized groups in aqueous solutions. Recently, the diverse lamellar distances of GO membranes have been achieved by changing the amount of the oxidized groups on the GO nanosheets via reduction process.2,11,29 By decreasing the number of oxidized groups, the lamellar distances of the GO membranes are decreased.7,22,30

With narrowing the lamellar distance, the π-π attraction force among 3



Page 4 of 31

GO nanosheets is strengthened; simultaneously, the repulsive force between the GO nanosheets that are produced by the hydration effect becomes weaker, as a result the GO membrane gains more stability in water.25

It has been reported that when the lamellar

distance is smaller than ~8 Å in a certain ambient humidity, the GO membranes show an reasonable stability in aqueous solutions due to the existence of strong π-π attraction force and the weak hydration repulsive force.2,11,22

However, when increasing the lamellar

distance for separation of different ions or molecules, the π-π attraction force tends to be weak.12,

31

This implies that the GO membranes are likely to disintegrate in aqueous

solutions by enlarging the lamellar distance.

It is worth mentioning that the critical value of

the oxidized-group-supported lamellar distance of unstable GO membranes, which is crucial in applications for precise sieving of ions or molecules, have not been reported yet. In this study, the effect of lamellar distances supported by self-contained oxidized groups on the stability of GO membranes in aqueous solutions is systematically investigated. The content of the oxidized groups on the GO nanosheets is controlled by the reduction degree (Figure 1a-c).

The GO membranes are stacked with different lamellar distances using

vacuum filtration (Figure 1a2-1c2).

When the GO membranes are immersed in aqueous

solutions, the oxidized groups-containing GO nanosheets are hydrated with water molecules.32

The hydration force (FH) is formed to shoulder against interlayer distance of

the membranes,25 which heavily affects the lamellar distance of the membrane.23,25

On the

contrary, the decrease of oxidized groups will weaken the FH and narrow the lamellar distance to make the π-π attraction force (Fπ) stronger between nanosheets, which contributes to the enhanced stability of the GO membranes.2,25

In our hypothesis, the adjustment of

oxidized-group-supported lamellar distance generates different Fπ and FH, which work together to dominantly decide the stability of the membranes when they are immersed in aqueous solutions (Figure 1a3-c3 and a4-c4).

By gradually increasing the lamellar distance,

4


Page 5 of 31 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


the Fπ decreases while the FH increases (Figure 1d).25,31

As the value of ΔF (ΔF=Fπ-FH)

becomes considerably small, both forces (Fπ and FH) acting on the nanosheets within the GO membrane reach an equilibrium state, at which lamellar distance can be considered as a critical value (dc).

When the lamellar distance is less than dc, the GO membranes show

extremely long-term stability due to the Fπ and weak FH.

On the contrary, if the lamellar

distance is greater than dc, the GO membranes are unstable.

When immersing the GO

membranes in the solutions with different pH values, they present diverse critical lamellar distances (dc) based on the different effects. It is expected that the results will provide a valuable guidance for the design and application of graphene-based membranes with long-term stability in aqueous solutions.

5



Figure 1. Schematic illustration of the effect of lamellar distances on the stability of graphene-based membranes. a) GO membrane that stacked by original GO nanosheets with plenty oxidized groups is unstable in aqueous solution due to the weak π-π attraction force (Fπ) and strong hydration repulsive force (FH). b) FRGO membrane that stacked by the FRGO nanosheets with mild reduction is still unstable in aqueous solution although the Fπ increases to some extent. c) FRGO membrane that stacked by the FRGO nanosheets with large reduction degree is stable in aqueous solution due to the increased Fπ and decreased FH. d) The schematic relationship between the lamellar distance and the ΔF (ΔF = Fπ-FH) inside the graphene-based membranes. The grey region represents the unstable membranes; while the yellow region stands for the stable ones. The red dashed line represents the critical lamellar distance (dc).

6


Page 6 of 31

Page 7 of 31 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


2. EXPERIMENTAL SECTION Fabrication and reduction of GO. Hummer Method.33

The GO dispersion was synthesized by modified

For being completely exfoliated, the GO dispersion was pretreated by

ultrasonic for 30 min and centrifuged at 3000 rpm for 20 min to remove the unexfoliated part before further use. For the reduction process, the diluted GO dispersion (0.25 mg/mL, 140 mL) was mixed with 48 µL of hydrazine solution (51.2 wt% in water, Kelong) and 550 µL of ammonia solution (25 wt% in water, Kelong).

The mixture was continuously stirred at room

temperature with different reduction time (1, 2, 3 and 4 hours).

After that, the dispersion

was centrifuged at 20 °C under 15000 rpm for 20 min to remove the surplus reductant, and then it was redispersed in 120 mL ultrapure water.

It is worth pointing out that the total

reduction time of GO mainly includes the reaction time and the centrifugation process time. The facilely reduced product is denoted as FRGO-48-t according to the content of the reductant, and the label t stands for the total reduction time. Similarly, the content of hydrazine in the reduction is changed to prepare the FRGO with different reduction degrees.

For example, the FRGO dispersions reduced with reductants of

24 µL for 1 h are denoted as FRGO-24-1.

Fabrication of graphene-based membranes. The dispersions were vacuum filtrated through hydrophilic cellulose nitrate−cellulose acetate (CN-CA) substrate membranes (Hangzhou Xidoumen Membrane Industry). approximately 40 mL as-prepared dispersion.

Each membrane was filtrated from After completely drying, the freestanding

membranes could be peeled from the substrate membranes. graphene-based membranes were ca. 7.23±0.7 mg. 7


The weight of the obtained


The stability test of membranes in aqueous solutions.

Page 8 of 31

The membranes were cut into

0.5 cm × 0.5 cm pieces (in the environment of ~25 °C with RH=45-50%), and they were then completely immersed in ultrapure water (pH=6.6), HCl aqueous solution (pH=1.0) and NaOH aqueous solution (pH=11.2) separately at room temperature (20 °C) in a glass tube.

A sharp

cutter was used to cut the membranes to avoid the origin break on the membranes before immersing.

Before observation, the glass tubes containing membranes were vigorously

stirred for ~3 seconds using an IKA vortex mixer (see Supplementary Movies S1 and S2). Then, a photo was taken to record the stability of the membrane in the glass tube as reported previously.22

The water permeation test of graphene-based membranes.

The water permeability

performance of graphene-based membranes was tested by an evaporation device (Figure S1). The vial was loaded with ~4 mL water−ethanol mixtures with different water contents (100 wt%, 75 wt%, 50 wt% and 25 wt%) and placed in an oven at a constant temperature at 25 °C and a constant relative humidity of 25%. periodically.

The mass loss of the device was measured

According to the content of water in mixture, the vapor partial pressure and

relative humidity in the vial can be calculated by the Raoult’s law (Table S1).

Characterization.

SEM images were taken by a JSM-7610F SEM microscope (JEOL).

AFM images were obtained by a Multimode 8 (Bruker).

XRD patterns were recorded using

a X’Pert Pro MPD X-ray diffractometer (Philips) with Cu Kometer X’Pert (l = 1.54060 Å) at 40 kV and 40 mA, and the relative humidity (RH) of operation environment was under the real-time monitoring.

EA measurements were obtained by an EA3000 element analyzer

8


Page 9 of 31 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


(Euro Vector). (Kratos).

XPS measurements were carried out with a XSAM800 XPS spectrometer

FT-IR measurements were recorded using FT-IR Prestige-21 (Shimadzu).

The

water contact angles were measured by the Contact Angle Meter (FM4000, Krüss, Germany), and the temperature of operation environment was under the real-time monitoring. The mechanical strength tests were carried out using EZ-LX test machine (Shimadzu) with membrane samples of dumbbell shapes (length 35 mm, width 2 mm, and gauge length 12 mm).

3. RESULTS AND DISCUSSION 3.1. Characterization of graphene-based nanosheets and membranes The FRGO nanosheets demonstrate similar lamellate morphologies comparing with the GO nanosheets (Figure 2a, c, e).

The AFM images show that the thicknesses of the FRGO

nanosheets are consistent with that of GO nanosheets, which is about 1 nm.

As increasing

the reduction degree, the content of C-O groups such as hydroxyl and epoxy in FRGO membranes decrease.

The peak height of C-O groups in the XPS spectra decreases with

increasing the amount of reductant (Figure 2b, d, f). The FRGO dispersions gradually turn darker with increasing the amount of reductant (Figure S2), because the chemical constituent of FRGO has been gradually changed due to the facile reduction. Therefore, the FRGO freestanding membranes turn darker while the GO membrane is brown (Figure 3a).

Both the

GO and FRGO membranes that are layer-by-layer stacked by corresponding nanosheets have similar lamellar nanostructures with thickness of ~2 µm, as shown in the SEM images (Figure 3b).

9



Figure 2. (a,c,e) AFM images of graphene nanosheets GO (a), FRGO-24-1 (c) and FRGO-48-1 (e), in which the inserted height profiles are taken along the red lines and the scale bars are 1 µm. (b,d,f) XPS spectra of GO (b), FRGO-24-1 (d) and FRGO-48-1 (f) membranes.

10


Page 10 of 31

Page 11 of 31 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. Optical images (a) and cross-section SEM images (b) of GO (a1, b1), FRGO-24-1 (a2, b2) and FRGO-48-1 (a3, b3) membranes. The scale bars are 1 cm in (a) and 1 µm in (b). Figure 4a shows FT-IR spectra of original GO membrane and FRGO membranes reduced by different amounts of reductant.

As for the FT-IR spectra, the characteristic peaks

of C-O groups, carboxyl groups, and C=C groups can be found at 3200 cm-1, 1715 cm-1 and 1600 cm-1, respectively.

The characteristic peaks at 1050 cm-1 and 1250 cm-1 represent the

existence of C-O-C groups.

From the comparative analyses of FT-IR spectra, both the GO

11



and FRGO membranes contain the same species of characteristic peaks, which is in accordance with the results of XPS spectra.

Merely, the content of C-O groups of FRGO

membranes is decreased by reduction, but such variation does not affect the peak shape of the XRD patterns (Figure 4b), which means the regularity of the FRGO membranes is similar to that of GO membrane.11,30

As the reduction degree increasing, the stress and breaking

strength of FRGO membranes are strengthened due to the increased π-π attraction (Figure 4c,d).

Figure 4. Characterizations of GO, FRGO-24-1 and FRGO-48-1 membranes. a) FT-IR spectra of the membranes. b) XRD patterns of the membranes in dry state at ~20 °C with the ambient humidity in the range of 56~59%. (c,d) Typical strain-stress curves (c) and tensile strengths (d) of the membranes.

12


Page 12 of 31

Page 13 of 31 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


3.2. The C/O ratio, hydrophilicity and lamellar distance of graphene-based membranes To achieve different reducing degrees of the FRGO nanosheets, the content of the reductant and the reduction time are systematically altered (see Experimental Section).

If

the FRGO membrane is prepared with heavily reduced nanosheets, more oxidized groups are lost and the C/O ratio increases.

Figure 5a shows that the C/O ratio increases with

increasing the amount of the reductant.

For the FRGO-48 membranes, the increase of C/O

ratio is also achieved by extending the reaction time.

While, for the FRGO-24 membranes,

the C/O ratio remains almost constant, this could be attributed to the depletion of the reductant within the one-hour reaction.

The contents of the oxidized groups are decreased as

the amount of the reductant and the reaction time increases, thus the lamellar distance of the FRGO membranes decreases (Figure 5b).

Due to the reduction of the oxidized groups, the

water contact angles of FRGO membranes become larger with increasing the amount of the reductant and the reaction time (Figure 5c), which suggests that the hydrophobicity of the FRGO membranes is increased.

13



Figure 5. The C/O ratio (a), lamellar distance (b) and water contact angle (c) of membranes with different reduction time periods. The C/O ratio data are tested by elemental analysis. The data of lamellar distances are obtained at ~20 °C with the ambient humidity of 56~59%. The data of water contact angles are obtained at ~27 °C with the ambient humidity of 40~45%. The dashed line is a guide to the eye.

14


Page 14 of 31

Page 15 of 31 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


After the achievement of different reduction degrees of FRGO membranes, a series of the FRGO membranes with various C/O ratios are obtained.

As shown in Figure 6a, the

lamellar distance is diminished as expected with increasing the C/O ratio, which attributes to the loss of the oxidized groups.

While, the water contact angles of the FRGO membranes

are increased with increasing the C/O ratio.

Accordingly, as the C/O ratio increases, the Fπ

becomes stronger and the FH is further weakened.

In Figure 6b, it can be seen that the

graphene-based membrane with larger lamellar distance has smaller water contact angle. Thus, the lamellar distance is inversely proportional to the ΔF (ΔF=Fπ-FH) of graphene-based membranes, as smaller lamellar distance is an indication of a stronger Fπ.

Moreover, the

lamellar distance is a critical parameter for the separation function of FRGO membranes. Therefore, it is set as the variable factor to investigate the stability of the membranes with different C/O ratios and hydrophilic properties in the subsequent investigations.

15



Figure 6. (a) The dependence of the lamellar distance (left axis) and the water contact angle (right axis) on the C/O ratio of GO and FRGO membranes. (b) The relationship between the water contact angle and the lamellar distance of GO and FRGO membranes. The dashed line is a guide to the eye.

3.3. The stability of graphene-based membranes in aqueous solutions with different pH values The GO and FRGO membranes are immersed in aqueous solutions with different pH values for one month to evaluate the long-term stability.

The glass tubes containing

graphene-based membranes are vigorously stirred for seconds before observation (Movies S1 and S2). Figure 7 shows the stability results of FRGO membranes with different lamellar 16


Page 16 of 31

Page 17 of 31 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


distances in neutral ultrapure water (pH=6.6) kept for 1 month. The pristine GO membrane as a control sample entirely disintegrated in water after immersing for only 1 day due to its high hydrophilicity.20

Treated by the gradual reduction, the FRGO membranes with smaller

lamellar distances and supported by self-contained oxidized groups show different degree of stability in water, compared with the pristine GO membrane.

When the lamellar distances of

FRGO membranes within the range of approximately 8.16~8.35 Å, the membranes remain unstable in water due to the weak reduction as Figure 1b illustrated.

While if the lamellar

distance is controlled to be smaller than approximately ~7.91 Å, the FRGO membranes show extreme stability in water, even for over 1 month due to the existence of a remarkable strength of the Fπ and the distinct weakening of the FH. With regard to the lamellar distance between 8.16~7.91 Å, it is an uncertain region and quite difficult to judge the stability of the FRGO membranes.

Figure 7. The photographs of graphene-based membranes with diverse lamellar distances in water (pH=6.6) for 1 day, 1 week and 1month.

17



Page 18 of 31

When the GO and FRGO membranes are immersed in acidic solutions (pH=1.0), all the membranes except the GO show extraordinary stability for one-month test (Figure 8).

The

results are quite different from the stability results in water at pH=6.6 (Figure 7), which may be attributed to the protonation of carboxyl groups on FRGO nanosheets.6,34-35

In acid

solutions, the H+ in solutions will protonate the negatively charged carboxyl groups, which will form the additional hydration force to assist the attraction force for stabilizing the graphene-based membranes.6,34,36

Thus, the FH of the membranes are greatly decreased.

Figure 8 shows that all the FRGO membranes with different lamellar distances of range 7.55~8.35 Å are stable in the acidic solutions due to the above two effects.

Figure 8. The photographs of graphene-based membranes with diverse lamellar distances in acidic solutions (pH=1.0) for 1 day, 1 week and 1month.

18


Page 19 of 31 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


In the case of the FRGO membranes in a basic solution (pH=11.2), the situation is different.

As shown in Figure 9, only the graphene-based membranes with lamellar

distances less than approximately 7.71 Å can stay with long-term stability.

Although the

FRGO membranes with lamellar distances of approximately 7.71~7.91 Å are stable in water for 1 month (Figure 7), they will disintegrate in basic solutions quickly after 1 day (Figure 9). It is because the carboxyl groups on graphene-based nanosheets are deprotonated by OH- in basic solutions.

The carboxyl groups would be negatively charged, which produce the

electrostatic repulsion between nanosheets.6,25

That is to say, graphene-based membranes

suffer from the extra electrostatic repulsion in the basic solution, which causes the instability of graphene-based membranes, compared with that of the membranes in water (pH=6.6) and acid solution (pH=1.0). The graphene-based membranes are stable in basic solutions, only when the Fπ is strong enough such as when the lamellar distance is less than 7.71 Å. Interestingly, when immersed in a strong basic solution (pH=13.0), all the membranes including the pure GO membranes with the high hydrophilicity show an outstanding stability. This could be due to the strong reduction effect of concentrated NaOH solution towards graphene-based membranes,37 which makes the brownish GO membranes turn into darker color gradually, as shown in Figure S3.

19



Figure 9. The photographs of graphene-based membranes with diverse lamellar distances in basic solutions (pH=11.2) for 1 day, 1 week and 1month. Figure 10 shows the systematical results of the stability of graphene-based membranes with diverse lamellar distances in different aqueous solutions.

The critical lamellar distances

(dc) can be achieved to estimate the stability of the graphene-based membranes in aqueous solutions.

The dc values of graphene-based membranes with oxidized-group-supported

lamellar distances are speculated to be approximately 8.4 Å, 8.0 Å and 7.8 Å in an acidic solution, neutral water and basic solution, respectively.

The membranes with lamellar

distances (d) greater than the critical lamellar distance (dc) gradually tend to disintegrate in corresponding solutions, while the membranes with d less than dc presented a long-term stability in the tests.

20


Page 20 of 31

Page 21 of 31 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 10. Symmetrical results on the stability of graphene-based membranes with diverse oxidized-group-supported lamellar distances in acidic solutions (pH=1.0) (a), water (pH=6.6) (b), and basic solutions (pH=11.2) (c) by stirring test during different time periods. The black circle marks mean that the membranes are intact and the red cross marks mean that the membranes are broken at the observation moment. The blue dotted lines marks the estimated critical lamellar distance of graphene-based membranes for different situation. The yellow regions represent the range of lamellar distances of stable graphene-based membranes in aqueous solutions while the grey regions represent the unstable ones. 21



Page 22 of 31

3.4. The water permeation performance of graphene-based membranes The effect of different lamellar distances on water permeation performance of the graphene-based membranes is investigated by carrying out a water-ethanol separation test. The water-ethanol solution is not used for controlling the water vapor pressures, but in order to show the permeability of the FRGO membrane, which is practical for separation of water-alcohol.26

Concerning that the concentration of the water/ethanol mixture would be

changed during the permeation measurement (Table S2), the water vapor partial pressure and relative humidity inside the device are calculated based on the water content of the mixture (Table S1). Further, the water permeation is normalized by the membrane thickness and the vapor differential pressure.

As shown in Figure 11, for the water-ethanol mixtures with high

water content (>50 wt%), the membranes exhibit very similar water permeation.

When the

water content decreases to less than 50 wt%, the water permeation slightly decreases with the reduce of the lamellar distance, excepting the result of FGRO membrane with interlayer distance of 7.94 Å to permeate 25 wt% water-ethanol solution, which may be on account of the experimental errors.

The results demonstrate that the facile reduction in our work will

not only effectively strengthen the stability of FRGO membranes in aqueous solutions, but also remain the permeation capability as large as the same order of magnitude of that of the pure GO membrane in this work (lamellar distance is 8.57 Å) and in previous work (~10000 g µm h-1 m-2 bar-1).26

22


Page 23 of 31 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 11. Water permeation of graphene-based membranes with different lamellar distances. The membranes are tested with different mixed water-ethanol mixtures under the same environmental conditions.

4. CONCLUSIONS In summary, the stability of the graphene-based membranes with lamellar distances supported by self-contained oxidized groups is systematically investigated.

Using different

reduction degrees, the graphene-based membranes are fabricated with different lamellar distances.

The FRGO membranes show distinct stability properties based on the lamellar

distances in aqueous solutions with different pH values, as the oxidized-group-supported lamellar distance heavily affects the FH and Fπ. With decreasing the lamellar distance, the Fπ is strengthened; while the FH is decreased with reducing the hydrophilicity of FRGO membranes.

Most importantly, based on the results, it was concluded that the value of the

critical lamellar distance can vary depending on the pH values of the aqueous solutions, thus the region within which the graphene-based membrane is stable can shift depending on the pH value of the aqueous solution.

The membranes with lamellar distances smaller than the 23



critical lamellar distances are permanently stable for long-term operation. Moreover, the water permeation capability of FRGO membranes is the same order of magnitude as large as that of the pristine GO membranes.

It is expected that the results can provide a valuable

guidance for the design and application of graphene-based membranes in aqueous solutions.

ASSOCIATED CONTENT

Supporting Information.

The table of the partial pressure and differential pressure of the device; the table of the weight of loss and remaining water of permeation device after different time; the schematic diagram of the device for water permeation test of FRGO membranes; the photographs of the GO dispersion with the reductant after various time periods; the stability of GO membranes in strong basic solution; the video of the unstable membrane during the stability test; the video of the stable membrane during the stability test.

This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors 24


Page 24 of 31

Page 25 of 31 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


* [email protected] (Z.L.).

* [email protected] (L.Y.C.).

Author Contributions

L.-Y.C., Z.L., and Y.-H.X. conceived and designed the study. Y.-H.X. performed the experiments.

All authors discussed the results and contributed to the data interpretation.

L.-Y.C., Z.L., Y.-H.X. and Y.F. wrote the manuscript, and all authors commented on the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge support from the National Natural Science Foundation of China (21490582, 21776182), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

25



REFERENCES

(1) Abraham, J.; Vasu, K. S.; Williams, C. D.; Gopinadhan, K.; Su, Y.; Cherian, C. T.; Dix, J.; Prestat, E.; Haigh, S. J.; Grigorieva, I. V.; Carbone, P.; Geim, A. K.; Nair, R. R. Tunable sieving of ions using graphene oxide membranes. Nat. Nanotechnol. 2017, 12, 546−550. (2) Liu, H.; Wang, H.; Zhang, X. Facile fabrication of freestanding ultrathin reduced graphene oxide membranes for water purification. Adv. Mater. 2015, 27, 249−254. (3) Sun, P.; Ma, R.; Deng, H.; Song, Z.; Zhen, Z.; Wang, K.; Sasaki, T.; Xu, Z.; Zhu, H. Intrinsic high water/ion selectivity of graphene oxide lamellar membranes in concentration gradient-driven diffusion. Chem. Sci. 2016, 7, 6988−6994. (4) Sun, P.; Wang, K.; Wei, J.; Zhong, M.; Wu, D.; Zhu, H. Effective recovery of acids from iron-based electrolytes using graphene oxide membrane filters. J. Mater. Chem. A 2014, 2, 7734−7737. (5) Z. Li; Y. Liu; Y. Zhao; X. Zhang; L. Qian; L. Tian; J. Bai; W. Qi; H. Yao; B. Gao; J. Liu; W.Wu; Qiu, H. Selective separation of metal ions via monolayer nanoporous graphene with carboxyl groups. Anal. Chem. 2016, 88, 10002−10010. (6) Huang, H.; Mao, Y.; Ying, Y.; Liu, Y.; Sun, L.; Peng, X. Salt concentration, pH and pressure controlled separation of small molecules through lamellar graphene oxide membranes. Chem. Commun. 2013, 49, 5963−5965. (7) Han, Y.; Xu, Z.; Gao, C. Ultrathin graphene nanofiltration membrane for water purification. Adv. Funct. Mater. 2013, 23, 3693−3700.

26


Page 26 of 31

Page 27 of 31 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


(8) Huang, L.; Chen, J.; Gao, T.; Zhang, M.; Li, Y.; Dai, L.; Qu, L.; Shi, G. Reduced graphene oxide membranes for ultrafast organic solvent nanofiltration. Adv. Mater. 2016, 28, 8669−8674. (9) Hong, S.; Constans, C.; Martins, M. V. S.; Yong, C. S.; Carrió, J. A. G.; Garaj, S. Scalable graphene-based membranes for ionic sieving with ultrahigh charge selectivity. Nano Lett. 2017, 17, 728−732. (10) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and ultrafast molecular sieving through graphene oxide membranes. Science 2014, 343, 752−754. (11) Xi, Y.-H.; Liu, Z.; Ji, J.; Wang, Y.; Faraj, Y.; Zhu, Y.; Xie, R.; Ju, X.-J.; Wang, W.; Lu, X.; Chu, L.-Y. Graphene-based membranes with uniform 2D nanochannels for precise sieving of mono-/multi-valent metal ions. J. Membr. Sci. 2018, 550, 208−218. (12) Liu, Z.; Wang, W.; Ju, X.-J.; Xie, R.; Chu, L.-Y. Graphene-based membranes for molecular and ionic separations in aqueous environments. Chin. J. Chem. Eng. 2017, 25, 1598−1605. (13) Liu, G. P.; Jin, W. Q.; Xu, N. P. Graphene-based membranes. Chem. Soc. Rev. 2015, 44, 5016−5030. (14) Huang, L.; Zhang, M.; Li, C.; Shi, G. Graphene-based membranes for molecular separation. J. Phys. Chem. Lett. 2015, 6, 2806−2815. (15) Sun, P.; Wang, K.; Zhu, H. Recent developments in graphene-based membranes: Structure, mass-transport mechanism and potential applications. Adv. Mater. 2016, 28, 2287−2310. 27



(16) Gao, T.; Huang, L.; Li, C.; Xu, G.; Shi, G. Graphene membranes with tuneable nanochannels by intercalating self-assembled porphyrin molecules for organic solvent nanofiltration. Carbon 2017, 124, 263−270. (17) Nam, Y. T.; Choi, J.; Kang, K. M.; Kim, D. W.; Jung, H. T. Enhanced stability of laminated graphene oxide membranes for nanofiltration via interstitial amide bonding. ACS Appl. Mater. Interfaces 2016, 8, 27376−27382. (18) Sun, P.; Zhu, M.; Wang, K.; Zhong, M.; Wei, J.; Wu, D.; Xu, Z.; Zhu, H. Selective ion penetration of graphene oxide membranes. ACS Nano 2013, 7, 428−437. (19) Sun, P.; Zheng, F.; Zhu, M.; Song, Z.; Wang, K.; Zhong, M.; Wu, D.; Little, R. B.; Xu, Z.; Zhu, H. Selective trans-membrane transport of alkali and alkaline earth cations through graphene oxide membranes based on cation−π interactions. ACS Nano 2014, 8, 850−859. (20) Yeh, C. N.; Raidongia, K.; Shao, J.; Yang, Q. H.; Huang, J. On the origin of the stability of graphene oxide membranes in water. Nat. Chem. 2014, 7, 166–170. (21) Hung, W. S.; Tsou, C. H.; Guzman, M. D.; An, Q. F.; Liu, Y. L.; Zhang, Y. M.; Hu, C. C.; Lee, K. R.; Lai, J. Y. Cross-linking with diamine monomers to prepare composite graphene oxide-framework membranes with varying d-spacing. Chem. Mater. 2014, 26, 2983–2990. (22) Xi, Y.-H.; Hu, J.-Q.; Liu, Z.; Xie, R.; Ju, X.-J.; Wang, W.; Chu, L.-Y. Graphene oxide membranes with strong stability in aqueous solutions and controllable lamellar spacing. ACS Appl. Mater. Interfaces 2016, 8, 15557–15566.

28


Page 28 of 31

Page 29 of 31 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


(23) Huang, L.; Li, Y.; Zhou, Q.; Yuan, W.; Shi, G. Graphene oxide membranes with tunable semipermeability in organic solvents. Adv. Mater. 2015, 27, 3797–3802. (24) Zhang, M.; Wang, Y.; Huang, L.; Xu, Z.; Li, C.; Shi, G. Multifunctional pristine chemically modified graphene films as strong as stainless steel. Adv. Mater. 2015, 27, 6708−6713. (25) Yang, X.; Zhu, J.; Qiu, L.; Li, D. Bioinspired effective prevention of restacking in multilayered graphene films: Towards the next generation of high-performance supercapacitors. Adv. Mater. 2011, 23, 2833−2838. (26) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Unimpeded permeation of water through helium-leak-tight graphene-based membranes. Science 2012, 335, 442−444. (27) Hu, M.; Mi, B. Enabling graphene oxide nanosheets as water separation membranes. Environ. Sci. Technol. 2013, 47, 3715−3723. (28) Liu, P.; Yang, H.; Zhang, X.; Jiang, M.; Duan, Y.; Zhang, J. Controllable lateral contraction and mechanical performance of chemically reduced graphene oxide paper. Carbon 2016, 107, 46−55. (29) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008, 3, 101−105. (30) Chen, J.; Li, Y.; Huang, L.; Jia, N.; Li, C.; Shi, G. Size fractionation of graphene oxide sheets via filtration through track-etched membranes. Adv. Mater. 2015, 27, 3654−3660.

29



(31) Zhang, M.; Guan, K.; Shen, J.; Liu, G.; Fan, Y.; Jin, W. Nanoparticles@RGO membrane enabling highly-enhanced water permeability and structural stability with preserved selectivity. AIChE J. 2017, 63, 5054−5063. (32) Medhekar, N. V.; Ramasubramaniam, A.; Ruoff, R. S.; Shenoy, V. B. Hydrogen bond networks in graphene oxide composite paper: Structure and mechanical properties. ACS Nano 2010, 4, 2300−2306. (33) W. S. Hummers Jr; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (34) Cote, L. J.; Kim, J.; Zhang, Z.; Sun, C.; Huang, J. Tunable assembly of graphene oxide surfactant sheets: Wrinkles, overlaps and impacts on thin film properties. Soft Matter 2010, 6, 6096−6101. (35) Sun, S.; Wang, C.; Chen, M.; Li, M. The mechanism for the stability of graphene oxide membranes in a sodium sulfate solution. Chem. Phys. Lett. 2013, 561, 166−169. (36) Ilmain, F.; Tanaka, T.; Kokufuta, E. Volume transition in a gel driven by hydrogen bonding. Nature 1991, 349, 400−401. (37) Mao, S.; Pu, H.; Chen, J. Graphene oxide and its reduction: Modeling and experimental progress. RSC Adv. 2012, 2, 2643−2662.

30


Page 30 of 31

Page 31 of 31 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


Graphic for TOC

31


Effect of Oxidized-Group-Supported Lamellar Distance on Stability of

Recommend Documents