Enhanced Stability of Laminated Graphene Oxide Membranes for

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Enhanced Stability of Laminated Graphene Oxide Membranes for Nanofiltration via Interstitial Amide Bonding Yoon Tae Nam, Junghoon Choi, Kyoung Min Kang, Dae Woo Kim, and Hee-Tae Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09912 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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

ACS Applied Materials & Interfaces

Enhanced Stability of Laminated Graphene Oxide Membranes for Nanofiltration via Interstitial Amide Bonding Yoon Tae Nam, Junghoon Choi, Kyoung Min Kang, Dae Woo Kim* and Hee-Tae Jung*

KAIST Institute for Nanocentury & National Laboratory for Organic Opto-Electronic Materials, Department of Chemical and Biomolecular Eng. (BK-21 plus), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea KEYWORDS: graphene oxide, amide bonding, stability, cross-linking, membranes, nanofiltration.

ABSTRACT Laminated graphene oxide (GO) has promising use as a membrane because of its high permeance, chemical and mechanical stability, as well as the molecular sieving effect of its interlayers. However, the hydrophilic surface of GO, which is highly decorated with oxygen groups, easily induces delamination of stacked GO films in aqueous media, thereby limiting the practical application. To stabilize GO films in aqueous media, we functionalized a polymer support with branched polyethylene-imine (BPEI). BPEI adsorbed intercalated into the stacked GO sheets via

ACS Paragon Plus Environment

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

Page 2 of 31

diffusion during filtration. The GO/BPEI membrane obtained exhibits high stability during sonication (>1 h duration, 40 kHz frequency) in water within a broad pH range (2–12). In contrast, the GO film spontaneously delaminated upon sonication. Furthermore, BPEI treatment did not affect the filtration performance of the GO film, as evidenced by the high rejection rates (> 90%) for the dye molecules methylene blue, rose bengal, and brilliant blue and by their permeation rates of ca. 124%, 34.8%, 12.2%, and 5.1%, respectively, relative to those of a typical GO membrane. INTRODUCTION With the growing demand for next-generation membranes, the utilization of graphene oxide (GO) as a building block for highly selective films1 for gas separation2,3,4, nanofiltration5,6,7, pervaporation8, and desalination9 has also increased. The increasing use of GO is due to its high robustness under harsh chemical and mechanical conditions. In addition, its nanofilms can be thinner than several tens of nanometers and show extremely high permeance and high selectivity that can be modified by controlling the interlayer spacing of stacked graphene sheets1, thus holding promise for various applications. However, the ultra-thinness and highly charged surfaces of GO membranes largely hinder their practical applications, especially in those involving polar solutions, because GO films can be easily re-exfoliated and detached during the filtration and cleaning operation of membranes10,11,12. Various approaches have been used to improve the stability of laminated GO films. Chemical cross-linking using multivalent ions (Mg2+, Ca2+, and Al3+)11,13, borate14, and polyfunctional molecules (e.g. 1,3,5-benzenetricarbonyl trichloride9, polyallylamine15, polyetheramine16, dopamine17, glutaraldehyde18, benzimidazole19, diamine20, dicarboxylic acid, polyols21, and poly(N-isopropylacrylamide)22) is one of the most widely employed methods for enhancing the stability of GO films. Also, increasing π–π interactions between GO and additives such as


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


porphyrin23 and reduced GO24 have been reported to be effective. Chemical or physical methods have significantly improved the adhesion between GO sheets, resulting in the enhancement of both mechanical properties and stabilities of GO films in aqueous media11. However, introducing an excess of the additives into the interlayers of GO sheets or harsh thermal treatments using hydrothermal reaction can increase the interlayer spacing from below one nanometer to several nanometers11,13-15,23,25, possibly reducing the performance in separating target molecules to the scale of sub-nanometers. In addition, an excess of intercalated additives often hinders the flow of water molecules into the nanochannels of GO laminates, resulting in low permeation through GO membranes23. Hence, there is an urgent need for a new method that stabilizes GO membranes in aqueous media without significantly modifying the interlayer spacing and thus maximizes its permeability, selectivity, and stability. Herein, we developed a novel method to enhance the stability of GO membranes with ultrathinness (on the scale of several tens of nanometers) in aqueous solution as maintaining the high nanofiltration performance by cross-linking GO sheets with amide bonds and by cross-linking GO and the polymer support with urethane bonds. First, polycarbonate (PC) polymer supports functionalized with branched polyethylenimine (BPEI) were coated with GO sheets by vacuum filtration, and then BPEI molecules adsorbed on PC were intercalated into GO sheets during vacuum filtration to form amide bonds. Because we avoided direct mixing of additives and GO sheets, using only BPEI adsorbed on the PC filter for the cross-linking of GO sheets, we avoided excessive intercalation and thus obtained GO/BPEI films that retain interlayer spacing of GO (9 Å). The prepared GO/BPEI membranes exhibit high stability under high-energy sonication (>1 h duration, 40 kHz frequency) in aqueous solution within a broad pH range (2–12). They retain high rejection rates for methylene blue (MnB; hydrated radius: 5.04 Å), rose bengal (RosB; 5.88 Å),


3


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 4 of 31

and brilliant blue (BB; 7.98 Å) molecules (96.4%, 88.8%, and 94.8% respectively). In contrast, a typical GO membrane underwent spontaneous delamination in aqueous solution upon sonication. The approach developed herein is thus very effective for preparing nanofiltration GO membranes that are highly stable in aqueous media and are highly selective toward nano-sized molecules for practical applications in polar solvents. RESULT AND DISCUSSION Preparation of GO Membranes Cross-linked with BPEI Figure 1A–1D show the overall process for fabricating GO membranes cross-linked with BPEI. A commercial track-etched PC filter with a pore size of 200 nm was used as a substrate filter for depositing the graphene film (Figure 1A). First, the PC filter was functionalized with BPEI by immersing it in a 10 wt% BPEI/isopropyl alcohol (IPA) solution at 70 °C for 15 min, which induced a chemical reaction between the BPEI amine groups and PC carbonate groups and thus formed urethane bonds (Figure 1B)26. After the reaction, the PC filter was rinsed several times with DI water to remove unreacted BPEI molecules. Controlling the reaction time was crucial to avoid pore collapse during the chemical reaction of the PC filter. As shown in the field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) images (Figure S1), the BPEI-coated PC filter collapsed and pores shrank after a reaction time of 30 min because of swelling of PC in the BPEI/IPA solution at 70 °C. The N peak in the EDS spectra of BPEI-treated PC is attributed to nitrogen functional groups in BPEI, such as amine and imine, after a reaction time of 15 min. Thus, the optimal reaction time for preparing the amine-functionalized PC filter that retains intact filter pores is 15 min. GO nanosheets prepared by the modified Hummers method were deposited on the BPEI-coated PC filter by vacuum filtration (Figure 1C)27. The GO dispersion was diluted in DI water to a concentration of 0.003 mg mL−1 to allow its


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


uniform deposition on the PC filter. Typically, film deposition required 40 min to allow complete filtration of the GO solution using the PC filter. During filtration of the GO dispersion, BPEI molecules adsorbed on the PC surface diffused and intercalated into the GO sheets, forming crosslinking amide bonds between GO and BPEI-treated PC and between GO molecules. When the DI water was completely filtered, GO/BPEI film on the PC filter was dried at room temperature (Figure 1D). Figure 1E shows a photograph of the as-prepared GO/BPEI membrane on a PC filter with a diameter of 4.7 cm; the brown region represents the area on which GO was deposited. As shown in the SEM images of the fabricated GO/BPEI membranes (Figures 1F and S2), GO was deposited without defects or cracks on the track-etched PC filter. Bright gray regions represent areas on which GO was deposited on the PC region, and dark gray dots with 200 nm diameter represent graphene on pore regions of the PC filter. Hence, BPEI-treated GO membrane without significant defects or cracks for application in water treatment was formed. Furthermore, atomic force microscopy (AFM) images confirmed that the surface morphology of GO and GO/BPEI were quite similar, showing the rippled morphology of multilayer graphene film in Figure S3. Structural Characterization of the GO/BPEI Membrane To verify the cross-linking between GO sheets and BPEI via amide bonds, we performed X-ray photoelectron spectroscopy (XPS) to examine a typical GO film and the GO/BPEI film (Figure 2A and 2B, respectively). The C1s spectrum of GO has characteristic peaks for C–C, C–O, C=O, and C(O)O bonds at 285.4, 287.4, 288.3, and 289.7 eV, respectively28,29 (Figure 2A), while that of GO/BPEI film has peaks for C–C, C–O, C(O)O and additional C–N (286.6 eV), and NC=O (288.5 eV) (Figure 2B). The amounts of carbon related to the amide bonds were 32.5% and 25.7%, corresponding to C–N and NC=O, respectively. The relatively high proportion of C–N and NC=O is partially due to the fact that BPEI adsorbed on the PC filter also reflects the XPS beam;


5


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 6 of 31

furthermore, GO films with thickness of several tens of nanometers allow the XPS beam to penetrate the PC filter surface28. On the other hand, the NC=O peaks clearly indicate that GO sheets bonded via amide bonds between the amine and imide groups of BPEI, and the carboxyl and epoxide groups of GO. Figure 2C and 2D shows the distribution of BPEI in the GO/BPEI film on the PC filter, which was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Among the various species detected during the sputtering step (Figure S4), CO3−, NH3−, C−, and O− were analyzed (Figure 2C), since they were generated by the decomposition of GO, PC, and BPEI30. Longer sputtering time resulted in deeper regions on the GO/BPEI membrane surface. Within a sputtering time of 100 s, the amount of CO3− linearly increased and reached a plateau, indicating that the sputtering ion beam reaches the PC filter layer within 100 s. C− and O−, typical constituents of GO and PC, were observed at the beginning of sputtering. Both reached saturation at a sputtering time of 40 s because the sputtering ion beam decomposed GO and then decomposed PC. On the other hand, the intensity of the NH3− species increased from 0 to 50 s and decreased after a sputtering time of 50 s, indicating formation of amine groups at the GO/BPEI film surface and at the top of the functionalized PC filters. Because our method was relied on the diffusion of BPEI during filtration of GO dispersion, BPEI showed gradient distribution with lower concentration of NH3- species at the top surface of GO membrane comparing to the bottom of GO membrane. To understand clearly the distribution of each species in Figure 2C, the distributions of C− (gray), CO3− (red), NH3− (green), and their overlay were examined by three-dimensional mapping (Figure 2D). As expected, CO3− could be observed at the bottom of the cube, and a broad NH3− distribution could be observed at the top region of the cube. Thus, BPEI appears to be broadly distributed from the PC filter surface to the GO film interlayer. Because BPEI was not added


6

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


further during vacuum filtration of the GO solution, the presence of BPEI in the GO film and the cross-linking of GO sheets with BPEI via amide bonds are attributed to diffusion of BPEI molecules from the surface of the BPEI-functionalized PC filter to the GO film during vacuum filtration. This conclusion is similar to that of the diffusion of multivalent ions from alumina oxide supports to the GO layer during vacuum filtration11. By the additional water contact angle investigations for GO and GO/BPEI film, the hydrophilic surface of GO film was not significantly changed by the BPEI treatment, showing highly hydrophilic surface: 38.21° for GO and 34.68° for GO/BPEI film, respectively (Figure S5). Figure 3 shows grazing-incidence X-ray diffraction (GIXRD) spectra of the GO and GO/BPEI films. GIXRD at 0.03° incident angle for each sample was performed to investigate the variation of the GO interlayer spacing with BPEI treatment. Figure 3A and 3B respectively show twodimensional GIXRD images for the GO and GO/BPEI membranes. Both images display clear (002) peaks of GO corresponding to the out-of-plane direction with respect to the graphene film coating, which was applied parallel to the PC substrate during vacuum filtration. The broad, bright blue peak in Figure 3B is attributed to the PC filter and not to the GO film. Figure 3C shows intensity profiles in Figure 3A and 3B similarly obtained in the out-of-plane direction. q values of the (002) peaks of GO for the GO and GO/BPEI membranes are 0.686 and 0.71 Å−1, which correspond to interlayer spacing of 9.15 and 8.84 Å, respectively; these values are within the typical range of interlayer spacing for GO (8 Å ~ 10 Å)1,31. Although the diffusion and intercalation of an additive such as BPEI can affect the interlayer spacing of stacked GO, typically increasing the interlayer spacing by several nanometers5-8, BPEI cross-linking did not significantly change the interlayer spacing of the GO/BPEI film relative to that of a typical GO film. This minimal effect is due to the use of only a very small amount of BPEI in our approach; in contrast, previously reported


7


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 8 of 31

methods, which directly mix additives and GO, use excess additive for cross-linking32. Maintaining the interlayer spacing is crucial to the high performance of the nanofiltration membranes because the size-sieving effect of the interlayer governs rejection by the GO membranes1,5-7. Stability Test of GO/BPEI Membranes in Aqueous Solutions In the membrane operations based on commercial polymer membranes, cleaning processes including flushing, scrubbing, air sparging, vibration and sonication have been widely adopted to reuse the membrane12. Therefore, stability in water should be required for GO membranes to stand during cleaning process. To examine the stability of GO/BPEI membranes in an aqueous solution, GO and GO/BPEI membranes were immersed in DI water and sonicated (Branson B3510DTH Ultrasonic Cleaner, 40 kHz) for a specified time, as shown in Figure 4. Sonication is a powerful method that is widely used in exfoliating and detaching stacked graphene sheets33. Figure 4A and S6 show brown GO and GO/BPEI membranes on the PC filter after vacuum filtration before sonication. The GO film completely detached from the PC filter after sonication for 10 min, as shown by the white region on the PC filter. In contrast, the GO/BPEI remained brown and did not show significant detachment of the GO film even after sonication for 1 h. Figure 4C and 4D show SEM images of the GO and GO/BPEI films after sonication for 30 min at pH 7. After sonication, no GO film remained on the PC filter surface; in contrast, GO films were intact on the BPEItreated PC filter, albeit showing disrupted pores due to the applied mechanical energy of sonication. GO membranes detached under both acidic (pH 2) and alkaline (pH 12) conditions (Figure S6), but the GO/BPEI membranes remained highly stable at both pH values after sonication for 10 min (Figure 4B), being more stable in alkali. Part of the GO/BPEI membrane slightly detached after


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


long sonication (30 min) because of the defects and cracks caused by physical damage of sonication; the film was only ~30 nm thick. Indeed, stacked GO is unstable under both acid and alkaline conditions11,24. GO sheets easily disintegrated because of electrostatic repulsion at pH 12. That is, their oxygen-containing functional groups such as hydroxyl and carboxyl deprotonated in alkali, resulting in negatively charged sheets. Under acidic conditions, the hydration of GO in a strong ionic atmosphere can weaken π–π interactions between GO sheets, causing the GO film to easily detach upon sonication at pH 224,35,36. Thus, the drastically enhanced stability of the GO/BPEI film under both alkaline and acidic conditions suggests the role of amide bonds in stabilizing the GO film in aqueous solution. Figure 4E shows a schematic of the mechanism for the stabilization the GO film via BPEI treatment. The branched structure of BPEI and the high reactivity of amines form strong chemical bonds with the oxygen groups of GO, even when a small amount of cross-linking agent was used. At pH 7 and 12, intercalated BPEI in the interlayer of GO sheets linked each GO sheet via amide bonds, resulting in stabilization in aqueous media. Nevertheless, despite the high stability of GO/BPEI film under acidic conditions, amide bonds between GO and BPEI slightly weakened at pH 2 because of conversion of partial amide bonds into amine and carboxylic acid bonds via hydrolysis in the presence of protonated ions36. Thus, BPEI treatment strongly enhances the stability of the GO film in aqueous media at both pH ranges, especially more effectively so under neutral and alkaline conditions. Filtration Performance of BPEI/GO Membranes To investigate the effect of BPEI treatment on the performance of GO membranes in nanofiltration, four dyes with different hydration radii and charges were filtered using the GO and GO/BPEI


9


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 10 of 31

membranes (Figure 5). Methyl red (MR; electroneutral), methylene blue (MnB; positively charged), rose bengal (RosB; negatively charged), and brilliant blue (BB; negatively charged) have hydration radii of 4.87, 5.04, 5.88, and 7.98 Å respectively. The concentration of feed solution was 10 mg L−1, and the dye solutions were pressurized with nitrogen gas at 5.0 bar. Figure 5A shows the permeance of the GO and GO/BPEI membranes. The GO membrane exhibited permeances of 1.06, 1.55, 1.26, and 1.92 L m−2 h−1 bar−1 for solutions of MR, MnB, RosB, and BB, respectively, whereas the GO/BPEI membrane exhibited permeances of 2.39, 2.09, 1.42, and 2.01 L m−2 h−1 bar−1 respectively. The GO/BPEI membrane thus showed ca. 124%, 34.8%, 12.2%, and 5.1% enhanced permeation of all dyes as compared with that of the GO membrane. Such enhancement in permeation is attributed defect formation in the basal plane of GO during chemical reaction with BPEI, which facilitates penetration of water molecules through the laminated GO film15. The inset of Figure 5A shows a photograph of the dye solutions before and after filtration through the GO and GO/BPEI membranes. Before filtration, the colors of the feed solutions for MR, MnB, RosB, and BB were red-violet, blue, hot pink, and purple, respectively. Permeates of the MnB, RosB, and BB solutions obtained from the GO and GO/BPEI membranes were colorless, and that of the MR solution was pale pink. Figure 5B plots the rejection rate for each dye on the GO and GO/BPEI membranes, which was calculated from ultraviolet–visible absorbance measurement, versus the dye’s hydration radius. The GO membrane exhibited high rejection rates of 98.7% (standard deviation: 0.78), 94.1% (2.02), and 96.3% (2.15) for MnB, RosB, and BB, respectively. GO/BPEI membranes also showed high rejection rates of 96.4% (standard deviation: 1.94), 88.8% (3.76), and 94.8% (0.75), respectively, albeit slightly lower than those of the GO membranes. Especially, rejection rates for MR on both membranes were low, 67.6% and 47.9%, respectively. The interlayer spacings of GO and GO/BPEI membranes as calculated from GIXRD


10

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


measurements were 9.15 and 8.84 Å (Figure 3); therefore, molecules with a hydrated radius of >5 Å, such as MnB (5.04 Å), RosB (5.88 Å), and BB (7.98 Å), were rejected by the molecular sieving effect with additional contribution of Donnan exclusion5,6. In addition, the negatively charged surface of GO strongly contributes to the rejection of positively charged molecules such as MnB in our experiments; that is, negatively charged oxygen groups on the surface adsorb positively charged molecules via electrostatic interaction37. On the other hand, the low rejection rate for MR (4.87 Å radius) on GO (67.6%) and GO/BPEI membranes (47.9%) may be attributed to the radius of MR, which is comparable to the interlayer spacing of GO, which permits the penetration of partial dye molecules through the GO membrane. The above results for membrane tests therefore suggest that the stacked GO film rejects molecules with a hydrated radius of >5 Å. Thus, the BPEI treatment utilized herein effectively enhanced the stability of GO membranes in aqueous solution while maintaining the unique filtration performance of the membranes. CONCLUSION We developed a novel method for fabricating GO membranes that enhances their stability in aqueous solutions and maintains their high performance in nanofiltration. GO sheets cross-linked with BPEI strongly attached to the polymer support and strongly bonded with other GO sheets, resulting in high stability during high-energy loading by sonication in strongly acidic and basic aqueous solutions. Such stabilization is attributed to amide bonds between amine and imide groups of BPEI and oxygen groups of GO, which connect the GO sheets. In addition, the intercalated structures slightly improved the water flux through the GO membranes, as maintained the high rejection rates for specific dyes with hydration radii of >5 Å (>90 %) due to the synergistic effect of molecular sieving and electrostatic interaction. Our approach is facile and simple, and the GO/BPEI membranes show promise in applications such as desalination, water treatment, and


11


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 12 of 31

pharmaceutical separation, which require high durability of membranes during separation and cleaning. EXPERIMENTAL METHODS Preparation of GO Dispersion. GO was synthesized via the modified Hummers method. First, 1.0 g of graphite powder was sufficiently immersed in a sulfuric acid solution (98%) in a beaker. Second, 3.5 g of potassium permanganate was slowly added to the mixture. Third, the mixture was stirred and allowed to react at 35 °C for 2 h in a water bath. The beaker containing the mixture was then placed in an ice bath, 200 mL of DI water was slowly Added, and then 10 mL of hydrogen peroxide was added. Subsequently, the mixture was passed through filter paper, and the residue was washed five times with hydrochloric acid (10%) to remove remaining manganese impurities. The resulting product, GO, was dehydrated by freeze-drying for 2 days. GO was mixed with DI water to form a 0.1 mg mL−1 dispersion, which was then centrifuged at 7000 rpm (Sorvall Stratos) for 30 min to obtain a pure dispersion of completely exfoliated GO at a concentration of 0.1 mg mL–1. Fabrication of the GO/BPEI Membrane. A commercial PC filter was used as the substrate for the membrane. First, BPEI (Mw = 25,000 g mol−1) was dissolved in IPA to a concentration of 10 wt %. The resulting solution was heated at 70 °C with stirring, and the PC filter was immersed in the solution. The optimal reaction time was found to be 15 min. The as-prepared GO dispersion was diluted to 0.003 mg mL−1 for optimum stacking of GO sheets. The diluted dispersion (100 mL) was then vacuum-filtered to obtain the amine-functionalized PC membrane filter. The membrane was dried in a desiccator at room temperature overnight.


12

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


Characterization. SEM images and EDS mapping images of the GO and GO/BPEI membranes were recorded on a Nova230 FE-SEM system. X-ray photoelectron spectra were recorded using a Thermo-VG SIGMA probe. A TOF-SIMS 5 spectrometer (ION-TOF GmbH) was used to obtain depth profiles. The spectrometer uses two interlaced ion sources, namely, a 1 kV Cs+ beam as the sputter beam and a 30 kV Bi+ beam as the analysis beam. Negative secondary ions were analyzed using the TOF mass analyzer. AFM was also used to characterize surface morphologies by Park XE-100 system. We conducted GIXRD studies using a beam size of 200 μm × 500 μm at the Pohang Accelerator Laboratory (PAL, Pohang, Korea), utilizing a Rayonix 2D MAR 165 detector. Stability Test in Aqueous Solution. Stability tests were conducted by analyzing the residue remaining on the GO membrane after immersion in each solution. First, the as-prepared membranes were immersed in DI water (pH 7), a HCl aqueous solution (pH 2), and a NaOH aqueous solution (pH 12) in separate beakers at room temperature. Residues on the GO membranes were examined after the membranes were sonicated for a specified period. Sonication was conducted with a Branson B3510DTH ultrasonic cleaner operated at 40 kHz. Filtration Performance Test. The filtration performance of the GO membrane was evaluated from the permeance of water and from the rejection of the dyes MR, MnB, RosB, and BB at concentrations of 10 mg L−1. The effective area of the membrane at the operating pressure was 4.523 cm2. All experiments were performed in triplicate at room temperature (25 °C) and under nitrogen gas at a pressure of 5.0 bar. The permeance (equation (1)) was calculated by times to obtain 10 mL of permeate solutions, where Vp is the volume of the permeate solution, t is the permeation time, A is the effective area of the membrane, and ΔP is the applied nitrogen gas pressure. To obtain the rejection rate (equation (2)), the dye concentrations in the feed solution (Cf) and in permeate (Cp) were calculated through the Beer–Lambert Law using ultraviolet–visible


13


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

absorbance

measurements

based

on

relevant

peaks

Page 14 of 31

(JASCO

V-570

UV/VIS/NIR

spectrophotometer).

Permeance (L m−2 h−1 bar−1) =

Rejection (%) =

Cf − Cp Cf

Vp t A ∆P

× 100(%)

(1)

(2)

ASSOCIATED CONTENT Supporting Information. Details of the experimental methods and characterization are provided in the Supporting Information. The morphology and chemical variation of the PC filter after BPEI treatment were observed by SEM, EDS and AFM. TOF-SIMS spectra were obtained to establish the depth profile of the GO/BPEI membrane. Hydrophilicity was checked by water contact angle. Further stability tests were conducted by sonication for a specified period in aqueous media at different pH values. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Correspondence and requests for materials should be addressed to Dae Woo Kim (email: [email protected]) and Hee-Tae Jung (email: [email protected]).


14

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


ACKNOWLEDGMENT This study was supported by the Climate Change Research Hub of KAIST (Grant No. N01150139) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1A6A3A04057367), as well as by the Leading Foreign Research Institute Recruitment Program through the NRF funded by the Ministry of Education, Science and Technology (NRF-2015K1A4A3047100). PAL provided technical support for the small-angle X-ray scattering (3C1 SAXS) beamline used in our study.

REFERENCES [1] Mi, B. Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343, 740742. [2] Kim, H. W.; Yoon, H. W.; Yoon, S. M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S.; Choi, J. Y.; Park, B. H. Selective Gas Transport Through Few-Layered Graphene and Graphene Oxide Membranes. Science 2013, 342, 91-95. [3] Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M. Ultrathin, Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342, 95-98. [4] Kim, D.; Kim, D. W.; Lim, H. –K.; Jeon, J.; Kim, H.; Jung, H. –T.; Lee, H. Intercalation of Gas Molecules in Graphene Oxide Interlayer: The Role of Water. J. Phys. Chem. C. 2014, 118, 11142-11148.


15


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 16 of 31

[5] Han, Y.; Xu, Z.; Gao, C. Ultrathin Graphene Nanofiltration Membrane for Water Purification. Adv. Funct. Mater. 2013, 23, 3693-3700. [6] Akbari, A.; Sheath, P.; Martin, S. T.; Shinde, D. B.; Shaibani, M.; Banerjee, P. C.; Tkacz, R.; Bhattacharyya, D.; Majumder, M. Large-Area Graphene-Based Nanofiltration Membranes by Shear Alignment of Discotic Nematic Liquid Crystals of Graphene Oxide. Nat. Commun. 2016, 7, 10891-10902. [7] Han, Y.; Jiang, Y.; Gao, C. High-Flux Graphene Oxide Nanofiltration Membrane Intercalated by Carbon Nanotubes. ACS. Appl. Mater. Interfaces. 2015, 15, 8147-8155. [8] Cao, K.; Jiang, Z.; Zhao, J.; Zhao, C.; Gao, C.; Pan, F.; Wang, B.; Cao, X.; Yang, J. Enhanced Water Permeation through Sodium Alginate Membranes by Incorporating Graphene Oxides. J. Membr. Sci. 2014, 469, 272-283. [9] Hu, M.; Mi, B. Enabling Graphene Oxide Nanosheets as Water Separation Membranes. Environ. Sci. Technol. 2013, 47, 3715-3723. [10] Lim, A. L.; Bai, R. Membrane Fouling and Cleaning in Microfiltration of Activated Sludge Wastewater. J. Membr. Sci. 2003, 216, 279-290. [11] 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. 2015, 7, 166-170. [12] Bruggen, B. V. D.; Manttari, M.; Nystrom, M. Drawbacks of Applying Nanofiltration and How to Avoid them: A Review. Separation and Purification Technology. 2008, 63, 251-263.


16

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


[13] Park, S.; Lee, K. S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical properties via Chemical Cross-Linking. ACS Nano 2008, 2, 572-578. [14] An, Z.; Compton, O. C.; Putz, K. W.; Brinson, L. C.; Nguyen, S. T. Bio-Inspired Borate Cross-Linking in Ultra-Stiff Graphene Oxide Thin Films. Adv. Mater. 2011, 23, 3842-3846. [15] Park, S.; Dikin, D. A.; Nguyen, S. T.; Ruoff, R. S.; Graphene Oxide Sheets Chemically CrossLinked by Polyallylamine. J. Phys. Chem. C. 2009, 113, 15801-15804. [16] Chen, L.; Huang, L.; Zhu, J. Stitching Graphene Oxide Sheets into a Membrane at a Liquid/Liquid Interface. Chem. Commun. 2014, 50, 15944-15947. [17] Tian, Ye.; Cao, Y.; Wang, Y.; Yang, W.; Feng, J. Realizing Ultrahigh Modulus and High Strength of Macroscopic Graphene Oxide Papers Through Crosslinking of Mussel-Inspired Polymers. Adv. Mater. 2013, 25, 2980-2983. [18] Gao, Y.; Liu, L. Q.; Zu, S. Z.; Peng, K.; Zhou, D.; Han, B. H.; Zhang, Z. The Effect of Interlayer Adhesion on the Mechanical Behaviors of Macroscopic Graphene Oxide Papers. ACS Nano 2011, 5, 2134-2141. [19] Cui, Y.; Cheng, Q. Y.; Wu, H.; Wei, Z.; Han, B. H. Graphene Oxide-Based BenzimidazoleCrosslinked Networks for High-Performance Supercapacitors. Nanoscale 2013, 5, 8367-8374. [20] 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.


17


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 18 of 31

[21] Jia, Z.; Wang, Y. Covalently Crosslinked Graphene Oxide Membranes by Esterification Reactions for Ions Separation. J. Mater. Chem. A 2015, 3, 4405-4412. [22] Wang, Y.; Chen, S.; Qui, L.; Wang, K.; Wang, H.; Simon, G. P.; Li, D. Graphene-Directed Supramolecular Assembly of Multifunctional Polymer Hydrogel Membranes. Adv. Funct. Mater. 2015, 25, 126-133. [23] Xu, X. L.; Lin, F. W.; Du, Y.; Zhang, X.; Wu, J.; Xu, Z. K. Graphene Oxide Nanofiltration Membranes Stabilized by Cationic Porphyrin for High Salt Rejection. ACS. Appl. Mater. Interfaces. 2016, 8, 12588−12593. [24] 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. [25] Qui, L.; Zhang, X.; Yang, W.; Wang, Y.; Simon, G. P.; Li, D. Controllable Corrugation of Chemically Converted Graphene Sheets in Water and Potential Application for Nanofiltration. Chem. Commun. 2011, 47, 5810-5812. [26] Jankowski, P.; Ogończyk, D.; Lisowski, W.; Garstecki, P. Polyethyleneimine Coating Renders Polycarbonate Resistant to Organic Solvents. Lab Chip 2012, 12, 2580-2584. [27] Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. [28] Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A. Jr.; Ruoff, R. S. Chemical Analysis of Graphene Oxide Films


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


after Heat and Chemical Treatments by X-ray Photoelectron and Micro-Raman Spectroscopy. Carbon 2009, 47, 145-152. [29] Kim, D. W.; Kim, D.; Min, B. H.; Lee, H.; Jung, H. –T. Sonication-Free Dispersion of LargeArea Graphene Oxide Sheets using Internal Pressure from Release of Intercalated Carbon Dioxide. Carbon 2015, 88, 126-132. [30] Benninghoven, A. Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Angew. Chem. Int. Ed. Engl. 1994, 33, 1023-1043. [31] Dikin, D. A.; Stankovich, S.; Zimney, E. K.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457-460. [32] Zou, J. & Kim, F. Diffusion Driven Layer-by-Layer Assembly of Graphene Oxide Nanosheets into Porous Three-dimensional Macrostructures. Nat. Commun. 2014, 5, 5254-5262. [33] Ciesielski, A.; Samori, P. Graphene via Sonication Assisted Liquid-Phase Exfoliation. Chem. Soc. Rev. 2014, 43, 381-398. [34] 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.


19


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 20 of 31

[35] Shih, C. –J.; Lin, S.; Sharma, R.; Strano, M. S.; Blankschtein, D. Understanding the pHDependent Behavior of Graphene Oxide Aqueous Solutions: A Comparative Experimental and Molecular Dynamics Simulation Study. Langmuir 2012, 28, 235-241. [36] Nishiyama, N.; Suzuki, K.; Yoshida, H.; Teshima, H.; Nemoto, K. Hydrolytic Stability of Methacrylamide in Acidic Aqueous Solution. Biomaterials 2004, 25, 965-969. [37] Ramesha, G. K.; Kumara, V.; Muralidhara, H. B.; Sampath, S. Graphene and Graphene Oxide as Effective Adsorbents toward Anionic and Cationic Dyes. J. Colloid Interface Sci. 2011, 361, 270-277.


20

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 1. (A) – (D) Schematic of the experimental procedures for preparing graphene oxide (GO) membrane cross-linked with branched polyethyleneimine (BPEI). (A) Polycarbonate (PC) filter. (B) PC filter functionalized with BPEI. (C) Vacuum filtration of a graphene oxide solution using the PC/BPEI filter. (D) GO/BPEI membrane on PC. (E) Photograph of the GO/BPEI membrane. (F) SEM image of the GO/BPEI membrane. Inset shows a magnified SEM image of the selected area in (F).


21


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 22 of 31

Figure 2. Chemical structure of GO film cross-linked with BPEI. C1s X-ray photoelectron spectra of (A) GO and (B) GO/BPEI. (C) TOF-SIMS depth profile of the GO/BPEI membrane. A long sputtering time results in a deep region on the membrane surface. (D) Three-dimensional −

−

−

rendered images for CO3 , NH3 , and C , and the corresponding overlay.


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 3. Interlayer spacing of GO via BPEI treatment. (A) and (B) GIXRD images for GO and GO/BPEI, respectively. (C) Intensity profiles from GIXRD data for (A) and (B), obtained in the out-of-plane direction with respect to the PC surface. Incident angle for the GIXRD measurements was 0.03°.


23


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 24 of 31

Figure 4. Stability of GO/BPEI membranes in aqueous solution. (A) Photographs of GO/BPEI and GO membranes subjected to sonication for various durations. GO membrane without BPEI spontaneously detached upon sonication. Membranes were immersed in DI water (pH 7) and sonicated for up to 10 min. (B) Stability of GO/BPEI under acidic (pH 2) and alkaline (pH 12) conditions during sonication for 10 min. (C) and (D) SEM images of GO and GO/BPEI membranes on PC after sonication for 30 min at pH 7. (E) Mechanism of stabilization of the GO/BPEI membrane at different pH values.


24

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


Figure 5. (A) and (B) Permeances of water and rejection rates of dye molecules on GO and BPEI/GO membranes with respect to hydration radius. MR. MnB, RosB, and BB represent methyl red, methylene blue, rose bengal, and brilliant blue, respectively. The charge of each dye is also −1

shown. A 10 mg L solution of each dye was filtered under a pressure of 5 bar.

Table of Contents


25


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 1 96x113mm (300 x 300 DPI)


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


Figure 2 169x146mm (300 x 300 DPI)



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 185x60mm (300 x 300 DPI)


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


Figure 4 181x155mm (300 x 300 DPI)



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 5 212x84mm (300 x 300 DPI)


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


TOC 88x31mm (300 x 300 DPI)


Enhanced Stability of Laminated Graphene Oxide Membranes for

Recommend Documents