Development of Graphene Oxide Framework Membranes via the âfrom

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Development of Graphene Oxide Framework Membranes via “from” and “to” Cross-linking Approach for Ion Selective Separations Sahadevan Rajesh, and Anima Bose ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05465 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

Development of Graphene Oxide Framework Membranes via “from” and “to” Cross-linking Approach for Ion Selective Separations Sahadevan Rajesh and Anima. B. Bose* Department of Engineering Technology and Texas Center for Superconductivity (TcSUH) University of Houston, Houston, TX 77204, USA

*To whom correspondence should be addressed Anima. B. Bose, Ph.D. Associate Professor, Department of Engineering Technology University of Houston, Houston, TX-77025 E-mail: [email protected] Phone: +1-713-743-5765 Fax +1-713-743-4032

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Abstract Graphene oxide (GO) membranes with well-defined nanochannels formed between the stacked GO nanosheets find great interest in molecular separations. However, GO membranes are unstable in aqueous solution environment due to weak interactions between the stacked nanosheets. Herein, we developed a preparation method by diminishing the self-contained oxidized functional groups in GO and subsequent cross-linking to form a GO framework (GOF) membranes with excellent aqueous solution stability. GOF membranes were fabricated by alternate deposition of branched polyethylenimine (BPEI) and a mixed solution of GO and thiourea (TU). Structural elucidation illustrated that TU molecule partially reduced the GO molecules and acted as a “to” cross-linker by bridging adjacent GO nanosheets through in-plane and out-of-plane of interactions. During the GO deposition, BPEI performed the role as a “from” cross-linker by binding the TU linked GO laminates to form a stable GOF membranes with well-defined nanochannels. Morphological studies confirmed the formation of tightly packed structure for BPEI/GO_TU membranes due to the high П-П interactions between the GO nanosheets and bridging effect of TU. The GOF membranes exhibited rejection of 99.5% for anionic dye methyl orange and cationic dye rhodamine B. The BPEI/GO_TU membranes fabricated from 12 bilayers using 0.25 mg/mL of GO solution have a pure water flux of 24 Lm-2h-1 and a Na2SO4 rejection of 94%, this permeability is 2.5 times higher than that of commercial nanofiltration membranes. Moreover, (BPEI/GO_TU)12 GOF membranes exhibited excellent aqueous solution stability in acidic and basic conditions. The excellent separation performance and aqueous solution stability of the BPEI/GO_TU membranes intricately linked to the partial reduction and cross-linking of GO nanosheets in GOF membranes. Thus “from” and “to” cross-linking approach developed in this work can be extended for the fabrication of structurally stable membranes from other 2D materials. Keywords: Graphene oxide membrane, thiourea, cross-linking, TEM, ionic separation, aqueous solution stability. Introduction GO membranes with two-dimensional laminar structure, tunable physicochemical properties and excellent molecular sieving capability find significant interest in the separation industry for desalination,


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water purification, gas separation and molecule/ion separations.1-4 In a stacked GO membrane, water molecules permeate through the laminar nanostructure formed between two adjacent GO nanosheets.5-6 Thus tuning the 2-D laminar structure of the adjacent GO nanosheets has a significant role in controlling the molecular sieving capability of GO membranes.7-10 For instance, in dry state GO nanosheet membranes prepared by vacuum filtration had inter lamellar distance of 0.3 nm, which could impede gas molecules but allows monolayer of water vapor aligned with the nanochannels.5,

11

However, in the

presence of ionic solution interlayer spacing tends to increase to 0.9 nm due to electrostatic interactions between the feed constituents and GO surface functional groups, leading to poor separation efficiency and stability in ionic separations.12-14 The instability of the GO membranes in aqueous solutions poses a significant challenge in implementing these membranes for practical applications.15-18 To improve the molecular sieving efficiency and stability, an ideal GO membranes targeted to use for aqueous solution processing thus should possess a finely tuned 2-D nanochannels smaller than target molecules. It has been well documented in literature that, GO membranes with a narrow inter laminar spacing possess significant stability in aqueous solutions due to the presence of strong П- П interactions.19 In a step to tailor the d-spacing of GO membranes, various methods such as complexation with ions,20 cross-linking,21 thermal annealing,11 intercalating polymers/small molecules,19, 22-25 and partial reduction of GO26 were employed. Among these various methods, cross-linking with amines,21 amino acids,23 acyl chlorides,25 boronic acid19 and complexation with Al3+ were the most effective methods in adjusting the interlayer spacing and stability of GO membranes. Interlaminar spacing and physicochemical properties of the resultant membranes could be finely tuned by carefully selecting linker molecules of desired size and functionality. Such GOF membranes prepared by separating the adjacent GO molecules with desired molecules as linkers/spacer arms help ordered staking of GO membranes for targeted applications. However, change in d-spacing solely depends on the linker molecules used; for instance larger sized nanoparticles result in formation of membranes with larger d-spacing.7,

27

Partial

reduction by minimizing the amount of oxidized regions in a GO membranes is also an effective method for improving both the stability and molecular sieving capability of GO membranes.23 These partially reduced rGO membranes with properties closely resembling that of graphene are ideal barrier structures with a lattice parameter of less than 0.34 nm, which can effectively perform separations based on size



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exclusion. However, direct fabrication of rGO membranes is a serious challenge because of its poor dispersibility, wettability, agglomeration and also rGO sheets crumple in aqueous solutions due to the fewer number of oxygen functional groups. Thus an efficient way of fabricating such partially reduced GO membranes is by depositing the GO sheets on a porous substrate using appropriate linker molecules, which have the capability to cross-link and partially reduce the adjacent GO sheets to tailor the d-spacing. Considering the potential of GOF membranes in aqueous solution processing, in the present study GO based hybrid composite membranes were prepared by cross-linking approach. Environmental friendly fabrication methods such as vacuum filtration and layer-by-layer (LbL) assembly have been employed for the fabrication of GOF membranes.4,

21, 23, 28-30

The membranes

prepared by vacuum filtration, GO units are held together by physical forces and thus have the tendency to disperse in water. However in LbL, GO molecules and linkers are held together by chemical interactions such as hydrogen bonding, Van der Waals, ionic and П- П interactions, and thus resultant membranes are more stable.29, 31-32 Many studies in the literature have illustrated that LbL self-assembly is a unique technique in producing GOF membranes with controllable interfacial interactions and more structural control at nanoscale.29,

31, 33

GO is derivative of graphene, with one atom sheet thick and

amphiphilic structure. GO possesses a hydrophobic back bone and a highly hydrophilic oxygen containing functional groups on their edges.34-36 Tailoring the interactions between these oxygencontaining functional groups and linker molecules thus help to tune the physiochemical properties, separation efficiency and large-scale fabrication of GOF membranes. By exploring the potential of GO molecules, in the present study, a new class of GOF membranes were designed through cross-linking approach using branched polyethylenimine (BPEI) and thiourea (TU) as “from” and “to” cross-linkers respectively. In the LbL process of GOF fabrication, BPEI and a mixed solution of GO and TU were used as cationic and anionic components. Asymmetric polyether sulfone (PES) membrane was used as substrate for GOF membrane preparation. The BPEI layers first deposited in each bi-layer formation acted as an anchor platform for the incoming GO nanosheets, and thus, described as “from” cross-linker. The TU molecule in GO solution has the GO reducing power and cross-linking ability and is described as “to” cross-linker.24, 37-38 We hypothesize that TU molecules bridges the partially reduced GO nanosheets via


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in-plane and out-of-plane interactions, and then deposit as multiple layers on to the already deposited BPEI layers. Also, each BPEI molecule has several amine groups and thus it facilitates stabilization of proposed GOF via interpenetrating structure formation between the –NH2 groups and oxidized groups of GO nanosheets.39-41 Alternate deposition of these layers lead to the formation of a BPEI/GO_TU multilayer GOF structure capable to perform molecular separation based on both size and charge. The schematic representation of role of interaction between BPEI and GO_TU on the interlayer spacing of GOF membranes is provided in scheme 1. The GOF membranes have excellent stability in acidic and basic over two months with good separation performance. Morphology and chemical interaction of the GOF membranes prepared from various combinations were studied and correlated with both separation efficiency and aqueous solution stability. Results illustrate that BPEI/GO_TU membranes fabricated are promising candidates for both size and charge based separation applications.

Scheme 1. An illustration of the interlayer nanochannels in (a) BPEI/GO and (b) BEPI/GO_TU membranes prepared by LbL assembly. Pathways for the selective permeation through nanochannels are indicated by blue arrows. In “from” cross-linking approach, -NH2 groups of BPEI form ionic cross-links with oxidized functional groups of GO. In “from” and “to” cross-linking approach, TU molecules first partially reduce GO and then bridges adjacent GO nanosheets; these bridged GO_TU frameworks form ionic cross-links with BPEI to stabilize GOF structure. The interlayer spacing of BPEI/GO_TU membranes is smaller than BPEI/GO membranes.



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Experimental Materials and Methods: Polyether sulfone (Veradel 3000P: Mw, 64,000 g mol-1) used for membrane preparation was obtained as a gift sample from Solvay Specialty Polymers USA, LLC. BPEI (Mw, 25,000 g mol-1) from Sigma Aldrich and polyvinylpyrrolidone (PVP, Mw, 58,000 g mol-1) from Alfa Aesar were the other polymers used. Specifications of all other chemicals used in the present study are provided in the supporting information. Preparation of GO Framework Membranes by LbL Process: Asymmetric membranes used as support (based on PES) was prepared by a non-solvent induced phase separation (NIPS) technique as described in literature.42 BPEI/GO based composite nanofiltration (NF) membranes were prepared by LbL process via sequential deposition of positively charged BPEI and negatively charged GO. Circular pieces of PES membranes were first exposed to BPEI solution (0.2 wt%, pH-10.0) for 15 min. The BPEI treated membranes were then washed with DI water for 2 min twice and dried under compressed air. These dried membranes were then exposed to GO aqueous solution (2 mg/8 mL or 0.25mg/mL, pH-3.5) for 15 minutes. The membranes were washed with DI water twice and dried thoroughly as described in the BPEI deposition step. The above process completed formation of a “BPEI/GO” bilayer, which comprised of one layer of GO deposited over a BPEI layer. This process of bilayer formation was repeated for the desired number of cycles. After the completion of LbL deposition, membranes were heated at 70°C for 30 min to complete the formation BPEI/GO composite membranes. For the preparation of TU cross-linked membranes (BPEI/GO_TU), GO suspension in the BPEI/GO bilayer formation was replaced with GO solution containing TU in a molar ratio of 1:1 (2 mg/8 mL of GO and 2 mg of TU, pH-3.5). Before the deposition process, GO and GO/TU suspensions were sonicated for 1h at 50°C to ensure uniform dispersion and mixing of GO particles. The schematic representation of the various steps involved in the fabrication of GO framework membranes is provided in Scheme S1. For comparative evaluation, BPEI/GO and BPEI/GO_TU membranes were also prepared without heat treatment step. To study the effect of GO concentration on the BPEI/GO and BPEI/GO_TU membrane structure, membranes were prepared from four different GO solution concentrations of 0.031, 0.062, 0.125 and 0.25 mg/mL. Since we used 8 mL of GO aqueous solution for each layer deposition; approximately 0.25, 0.5,


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1.0 and 2.0 mg of GO were respectively used for concentrations of 0.031, 0.062, 0.125 and 0.25 mg/mL. Detailed procedures followed for GOF membrane characterizations, aqueous solution stability and fouling studies were included in the supporting information. Separation Efficiency of GOF Membranes: Separation performance of GOF membranes was evaluated by measuring the pure water flux (PWF, Jw) and rejection behavior (%R). Filtration experiments were carried out in an Amicon Stirred Cell (model 8050) with an effective filtration area of 15.5 cm2. The GOF membrane was loaded in to the filtration cell with GO layer facing the feed solution. The cell was then filled with DI water and a pressure of 4 bar was applied. The PWF of the GOF membrane was then measured using the equation (1), 𝐽𝑤 = 𝐴

∆𝑉

(1)

× 𝑡

Jw is the pure water flux (in Lm-2h-1), ∆V represents the volume of DI water permeated through the GO membrane area “A” in time “t”. To evaluate the rejection behavior, GOF membrane prepared in various compositions was challenged with dye and salt solutions. Salt solutions of NaCl (500 mg/L) and Na2SO4 (500 mg/L) and dye solutions of methyl orange (MO, anion), methylene blue (MB, cation) and rhodamine B (RB, cation) were used for the rejection studies.25, 30 The dye solutions used had an initial concentration of 10 mg/L. In order to avoid the dye adsorption effect, permeate solutions for the analysis were collected after initial stabilization of 1h at an applied pressure of 4 bar. The feed and permeate concentrations of NaCl and Na2SO4 during filtration was monitored using a conductivity meter (OAKTON, Cond 6+).

The

concentrations of MO, MB and RB were recorded using a UV-Vis spectrophotometer (Cary 50, Agilent Technologies) at a wavelength of 465, 665 and 555 nm respectively. The percentage rejection was calculated based on the equation (2),

(

)

𝑐𝑝

% R = 1 - 𝑐𝑓

(2)

Where, cf and cp are concentrations of feed and permeate salt/dye solutions (mg/L), respectively.



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Results and Discussion Morphology of GOF Membranes The morphology of the membranes prepared from various compositions and preparation conditions were recorded using SEM and TEM. The schematic representation of pure PES, BPEI/GO and BPEI/GO_TU membranes are provided in Figure 1(a3-c3). The corresponding SEM images at lower and higher magnifications are provided in Figure 1(a1-c1) and Figure 1(a2-c2) respectively. The PES membrane used as substrate for composite framework membrane has a typical asymmetric structure as

Figure 1. SEM images of (a1) pure PES, (b1) (BPEI/GO)12 and (c1) (BPEI/GO_TU)12 membranes. (a2), (b2) and (c2) are the higher magnification images of (a1), (b1) and (c1) respectively. (a3), (b3) and (c3) are the schematic representation of pure PES asymmetric structure, BPEI/GO and BPEI/GO_TU membranes respectively. The interlayer spacing d2 < d1 and d4 < d3. 0.2 wt% of BPEI, pH-10.0 and 0.25 mg/mL (2 mg in 8 mL) of GO, pH-3.5 prepared in DI water were used for the membrane fabrication. The membranes (b1&b2) and (c1&c2) were cross-linked by heating at 70°C for 30 min.


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shown in Figure 1 (a1&a2). As seen PES membrane has high porosity with a smooth surface and narrow pore openings approximately in the size range of 0.1-0.3 µm. The SEM images of the GOF prepared in absence and presence of GO reducing agent TU with 12 bilayers are provided in Figure 1 (b1&b2) and (c1&c2) respectively. The membranes made from both compositions have a stacked layered structure typical of GOF membranes.15, 23, 30 However, the (BPEI/GO_TU)12 membrane has a more tightly packed layered structure (thickness of 3.0 µ) compared to the porous layered structure of (BPEI/GO)12 membranes (thickness of 4.0 µ). The TEM morphology presented in Figure 2 (a&b) illustrated that GO nanosheets used for the GOF membrane preparation contain both single and multilayers of GO with thickness varying from 0.3 to 4 nm (this value is in contrast to the manufacturer’s reported thickness of 0.4 to 1.2 nm). Considering the thickness and diameter of GO nanosheets used (thickness of 0.3-4 nm, diameter 1.5-5.5 µm), we should assume that GO nanosheets were packed uniformly along the horizontal direction with BPEI molecules on the PES surface to form the GOF membranes. The (BPEI/GO_TU)12 membranes prepared from various concentrations of GO and bilayer number are provided in Figure 3 (a-c) and Figure 1c2. The thicknesses of the BPEI/GO_TU membranes at 0.031, 0.062, 0.125 and 0.25 mg/mL of GO concentrations are 0.8, 1.1, 1.5 and 3.0 µm respectively. The linear increase in thicknesses of the selective layer observed as a function of GO concentration illustrated significant role GO had in forming a GOF membrane of desired separation efficiency. The surface “wrinkled” morphology of BPEI/GO_TU membranes without any defects in Figure 2d suggesting the strong interfacial interactions with BPEI and GO in GOF membranes. The SEM images of BPEI/GO_TU membranes fabricated from 3, 6, 9 and 12 bilayers using GO concentrations of 0.125 mg/mL have thickness of 0.5, 0.8, 1.1 and 1.5 µm respectively (Figure S1a-d). From the morphology of GOF membrane in Figure 3 (a-c) and Figure S1 (a-d), it is important to note that a fewer BPEI/GO_TU bilayers at higher GO concentrations and more bilayers at lower GO concentrations have same effect on the GOF membrane structure. For instance, GOF membranes prepared from 9 bilayers of BPEI/GO_TU using GO concentration of 0.125 mg/mL have the same thickness as membranes prepared from 12 bilayers from GO solution of 0.062 mg/mL (Figure S1c& Figure 3b). Considering the feasibility to produce desired skin layer thickness with minimum bilayers and permeability behavior of GOF membranes (data shown in later part of the manuscript); GO concentration of 0.25 mg/mL (2 mg/8mL) and 12 BPEI/GO



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bilayers are considered as optimum GO concentration and optimum bilayers respectively for the preparation of GOF membranes.

Figure 2. TEM images of the GO nanosheets and GOF membranes. (a) and (b) are the images of the edges of multilayer and single layer GO nanosheets respectively, and (a1) in inset is higher magnification image of GO nanosheets showing broken edges. (c) and (d) are the selective layers of BPEI/GO and BPEI/GO_TU membranes prepared from 4 bilayers. The concentration of BPEI (0.2 wt%, pH-10.0) and GO (0.125 mg/mL, pH-3.5) were maintained constant for the preparation of membranes. All membranes were cross-linked by heating at 70°C for 30 min. The unique tight structure and reduced interlayer spacing of BPEI/GO_TU framework membranes compared to BPEI/GO membranes is attributed to the chemical interactions of GO with TU molecules. TU molecule used as intermolecular linker has advantages such as (1) small dimension; (2) low toxicity;37, 43 (3) mild reducing ability; and (4) highly reactive functional groups.37 In presence of TU, amount of oxidized groups in GO nanosheets are decreased due to its reducing activity. The decrease in content of functional groups weakened hydration force between adjacent GO nanosheets and conversely increased the П-П interactions between them.12,

17

This П-П attraction force contributes to the narrow

lamellar distance in the (BPEI/GO_TU)12 framework membranes in Figure 1 (c1&c2). In order to further study the packing density and exact distance between the adjacent layers, selective layers of both


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BPEI/GO and BPEI/GO_TU membranes fabricated from 4 bilayers were analyzed using TEM and results are provided in Figure 2c&d. However, we could not measure the exact interlayer distance due to the interpenetrating layered network structure formed. Also note that selective layer thickness observed in TEM was slightly lower than in SEM may be due to the sample to sample variation and epoxy impregnation. Thus it could be concluded that significantly high П-П interaction force between the GO nanosheets and TU bridging effect leads to reduced lamellar distance in BPEI/GO_TU membranes. Considering the faster layer growth observed both in the BPEI/GO and BPEI/GO_TU membranes, it can be concluded that multiple layers of GO were deposited in each deposition step during the LbL process. The schematic representation of the lamellar spacing effect in both BPEI/GO and BPEI/GO_TU membranes are presented in Figure 1 (b3&c3).

Figure 3. SEM images of the (BPEI/GO_TU)12 membranes from various concentrations of GO @ pH-3.5; (a) 0.031 mg/mL, (b) 0.062 mg/mL and (c) 0.125 mg/mL. The image (d) is the top surface of image (c). The concentration of BPEI (0.2 wt%, pH-10.0) was maintained constant for the preparation of all membranes. The GO:TU composition of 1:1 was used for all membrane compositions. All membranes were cross-linked by heating at 70°C for 30 min. Furthermore, change in microstructure of GOF membranes induced by the doping of TU was studied through XRD analysis (Figure S2). The XRD pattern of the composite GOF membranes developed in this study was entirely different than the characteristic sharp diffraction pattern of graphite and pure GO reported in the literature.44 A broad diffraction pattern starting from 10° to 30° with a peak



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intensity around 17-18° was observed indicating long range disorder or exfoliation of BPEI in GO matrix due the plane-to-plane stacking of GO and BPEI. Since each GO layer was deposited immediately after the deposition of a BPEI layer, exfoliated GOF structure observed for these membranes can be justified.11,

45

It is important to note that in exfoliated GOF membranes the characteristic GO peak was

completely disappeared. Also, GOF membranes prepared from various GO compositions shown similar diffraction pattern. Thus we could not calculate precise d-spacing of membranes prepared from various GO concentrations using XRD. The exfoliated structures of BPEI/GO_TU and BEPI/GO membranes prepared from 0.25 mg/mL of GO have interlayer spacing of 0.49 and 0.52 nm respectively (Figure S2). These d-spacing values agree with the interlayer spacing observed in Figure 1b2&c2. Though the inter lamellar distance (d1 and d3 as indicated in Figure 1b3) of BPEI/GO membranes larger than the TU incorporated membranes (indicated as d2 and d4 in Figure 1c3), the interlayer spacing observed is much smaller than GO membranes prepared by vacuum filtration and other LbL modified GO systems reported in the literature (Figure 1b2).23, 25 The compacted microstructure indicate existence of hydrogen and ionic bonding interactions between the –NH2 groups of BPEI and oxidized groups of GO in BPEI/GO membranes and it is termed as “from cross-linking approach”. Due to partial reduction of GO such planeto-plane interactions between BPEI and GO are likely to decrease in TU incorporated GOF membranes.37 However, BPEI/GO_TU structure is still better stabilized via; (i) stronger П-П attractions between GO nanosheets, (ii) available in-plane interactions between BPEI and GO and (iii) bridging effect of TU with adjacent GO molecules via in-plane and out-of-plane interactions. This stabilization of GOF membranes by BPEI and TU is termed as “from” and “to” cross-linking approach. The variation in inter lamellar distance has significant role in determining the separation efficiency of GOF membranes that are discussed in later part of the discussion. Chemical Composition of GOF Membranes The chemical composition of GOF membranes were analyzed using ATR-FTIR, Raman and contact angle measurements. Figure 4a represents the ATR-FTIR spectra of PES substrate, BPEI/GO and BPEI/GO_TU membranes prepared with cross-linking at 70 °C for 30 min at the end of LbL deposition process. The strong symmetric stretching vibration at 1150 cm-1 of pure PES membranes is


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due to the presence of sulphone (–S=O) group in the polymer backbone. After the LbL modification, two small peaks emerged at 1640 cm-1 and 1725 cm-1 respectively indicate the presence of amine (-NH2) and carbonyl (–C=O) groups within the multilayer structure.45 However for GOF membranes prepared in presence of TU, intensity of amine group is decreased and intensity of carbonyl peak is slightly increased indicating a slightly altered hydrogen bonding/ionic interactions between –NH2 of BPEI and –C=O groups of GO.46-47 As discussed in morphological analysis, this altered interaction may be due to more stabilized GOF structure formed through stronger П-П attractions, bridging effect of TU and thus altered in-plane interactions between BPEI and GO. The possible chemical interactions between the building blocks in GOF membranes are presented in scheme S2 of the supporting Information. In order to further understand the effect of cross-linking on membrane structure, GOF membranes were cross-linked after each LbL deposition cycle (heated at 70 °C for 10 min after each bilayer) and FT-IR spectra is presented in the supporting information (Figure S3). In contrast to the two stretching vibrations observed for single step cross-linked membranes, amine and carbonyl groups were merged and shown as a broader peak at 1650 cm-1. The broad absorption peak indicates a different type of chemical interaction, which facilitates the stabilization of membrane structure. A detailed study is essential to understand these chemical interactions and study of these interactions is future research interest of these authors.

Figure 4. Chemical composition of GOF membranes prepared in this study; (a) ATR-FTIR spectra (b) Raman spectra and (c) contact angle. GOF membranes were prepared via LbL process from aqueous solutions of 0.2 wt% of BPEI, pH-10.0 and 0.25 mg/mL aqueous solution of GO, pH-3.5. All membranes were cross-linked by heating at 70°C for 30 min. Raman spectra provided in Figure 4b was used to evaluate the reducing capability of TU in GOF membrane systems. The graphite has two characteristic peaks called D and G bands approximately at 1312 cm-1 and 1582 cm-1 respectively.37 The D and G bands respectively represent the A1g mode and E2g



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modes of the ordered sp2 carbon atoms.44 For GOF membrane fabricated in our study, D and G bands were broadened compared to pure graphite and located at 1340 cm-1 and 1598cm-1 respectively. The broadening of D band indicate presence of disorder in GO used for multilayer structure formation. It can be seen that intensity ratio (ID/IG) of the D band to G band of BPEI/GO_TU membranes (0.976) are higher than BPEI/GO membranes (0.302). The higher intensity ratio confirms generation of more sp2 domains by the reduction of GO by TU in the BPEI/GO_TU system. The mechanical properties of the GOF membranes were also studied and results are presented in Table S1. The (BPEI/GO_TU)12 membranes have a tensile strength and elongation at break of 2.8 MPa and 10.2% respectively compared to 9.4 MPa and 17.5% of PES support (detailed discussion is provided in the supporting information). The contact angle data presented in Figure 4c was also used to confirm the reduction of GO in BPEI/GO_TU membranes. The pure PES substrate has a contact angle of 73.4° and it decreased to 63.1° after modification with BPEI/GO bilayers, which confirms higher hydrophilicity after LbL modification without TU. However, contact angle of membrane prepared by the incorporation of TU (~70.3°) are higher than BPEI/GO membranes indicating loss of oxygen functional groups during the reduction process.2 The removal of functional groups increased graphitic regions in BPEI/GO_TU membranes, which have significant role in determining the П-П interactions between the GO nanosheets, consequently the separation efficiency and stability of GOF membranes. Chemical Interactions in GOF Membranes To confirm chemical interactions between the building blocks in GOF membranes, XPS analysis was performed and the resultant spectra are provided in Figure 5. The survey scan and high resolution C1s scan of the pure PES, BPEI/GO and BPEI/GO_TU membranes were given in Figure 5 (a-c) and Figure 5 (a1-c1) respectively. The survey scan peak positions at 165, 285, 399.8 and 532.5 eV confirmed presence of S2p, C1s, N1s and O1s respectively in these membranes. However, presence and intensity of characteristic S2p, C1s, N1s and O1s is varied in these membrane structures with various chemical compositions. The N1s peak observed for pure PES membrane is due to the PVP used as pore former during membrane preparation and S2p is due to the presence of sulfone (–S=O) groups. The high resolution C1s of PES membrane at 284.2, 285.4 and 286.5 eV respectively represent –C-C, -C-S and –


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C-O linkages (Figure 5a1). After modification of PES membrane with BPEI/GO bilayers, intensity of N1s and O1s peaks in the survey scan are increased due to the –NH2 of BPEI and oxidized groups of GO. Also, the S2p peak is diminished its intensity due to surface coverage of PES membrane with BPEI/GO multilayers.48 Compared to pure PES membrane high resolution C1s peak positions altered significantly with five total peaks; # 284.6 (-C-C, sp3), # 285.6 (-C-N), # 286.3 (-C-O), # 287.4 (-C=O) and # 288.2 eV ( -O=C-NH). The emergence of –C-N peak at 285.6 eV and -O=C-NH peak at 288.2 eV confirmed that – NH2 groups of BPEI covalently linked to -COOH groups of GO via an amide linkage in BPEI/GO membrane (Figure 5b1).48-49

Figure 5. XPS spectra of GOF membranes; (a-c) Survey scan spectrum of PES substrate, BPEI/GO, and BPEI/GO_TU membranes, respectively. In these spectra, the compositions of O 1s, N 1s, C 1s and S2p are represented by binding energy peaks at binding energy 532.5, 399.8, 285 and 165 eV, respectively. a1, b1 and c1 are the high-resolution scan of C1s of a, b and c respectively. The GOF membrane preparation conditions employed were same as those provided in Figure 4.



Page 16 of 27

For BPEI/GO_TU membranes, those five peaks shifted significantly with new positions at 284.1, 284.7, 285.7, 286.6 and 288.5 eV respectively represent –C-C (sp2), -C-C (sp3), -C-N, -C-S and –O=CNH groups (Figure 5c1). The sp2 –C-C domain which should have been observed at 284.4 eV was observed 284.1 eV possibly due to broad nature of the peak. The peak of –C-C sp2 domains in TU modified membranes corroborated GO reduction observed in Raman and contact angle results. Meanwhile, recurrence of the –C-S peak illustrates linking of adjacent GO nanosheets by TU through thio linkages in BPEI/GO_TU membranes. It is also very important to note that intensity of –O=C-NH group at 288.5 eV decreased significantly compared to BPEI/GO multilayers which further confirms the reduction of –COOH groups of GO in presence of TU. Considering the structural interactions extracted from XPS spectra, we propose that in BPEI/GO_TU membranes, GOF structure is stabilized through two types of interactions. Thiourea used as linker is a unique molecule due to its small size, three active functional groups (two –NH2 groups and a –C=S group) and mild reducing ability.24 TU molecules first linked the adjacent GO nanosheets through thio linkages and then, formed plane-to-plane interactions which helped to deposit multiple layers in each deposition cycle. When the GO layers comes in contact with already deposited BPEI layers, entire multilayer structure is stabilized through interactions between the –NH2 groups of BPEI and available oxidized groups of GO. The above interactions suggest that “to’ crosslinking between the GO nanosheets and “from” cross-linking between BPEI and GO nanosheets resulted in formation of GOF membranes with layered structure as shown in scheme 1 and scheme S2. However due to the partial reduction of functional groups in GO, the BPEI/GO_TU membranes have higher content of graphitic regions and thus have shorter interlayer distance compared to BPEI/GO membranes. The unique structure of BPEI/GO_TU membranes with stacked graphitic regions provides greater stability in aqueous solutions through stronger П-П interactions. Meanwhile the expanded hydrophilic regions facilitate the frictionless flow of water through available interlamellar space. The greater balance between the stacked graphitic regions and expanded hydrophilic regions provide efficient molecular sieving in the removal of solute molecules with hydration radii comparable to the size of the nanochannels.50 Thus, BPEI/GO_TU membranes likely to have greater separation efficiency and stability in aqueous solutions that are discussed in the subsequent sections.


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Separation Performance of GOF Membranes

Figure 6. Pure water flux of the (BPEI/GO)12 and (BPEI/GO_TU)12 membranes prepared with various GO concentration during LbL process. All membranes were cross-linked by heating at 70°C for 30 min. Amount of GO provided is the total loading in 8 mL of aqueous solution per deposition cycle. PWF measurement was carried out at an applied pressure of 4 bar. Pure PES membrane used as a substrate for GOF membrane fabrication had a water flux of (450 ± 20 L m-2 h-1). Error bar represents a standard deviation (n=3). The PWF and rejection performance of BPEI/GO and BPEI/GO_TU membranes prepared with varies GO compositions are measured at an applied pressure of 4 bar. The pure PES membrane used as support for GOF membrane fabrication had a water flux of 450 L m-2 h-1. As shown in the Figure 6 and Figure S4, PWF of GOF membranes prepared with 12 bilayers in presence and absence of TU decreased as GO content in the multilayer increased. When GO content increased from 0.25 mg to 2.0 mg, PWF of the (BPEI/GO)12 and (BPEI/GO_TU)12 membranes prepared with cross-linking at 70°C for 30 min deceased from 111 to 36 L m-2 h-1 and 72 to 23 L m-2 h-1 respectively. The GOF membranes fabricated without the heat treatment step (without cross-linking) follows the same trend with a linear decline in water flux with increase in GO content (Figure S4). The PWF of (BPEI/GO)12 and (BPEI/GO_TU)12 membranes prepared without heat induced cross-linking decreased from 180 to 70 L m-2 h-1 and 110 to 38 L m-2 h-1 for change in GO concentration from 0.25 mg to 2.0 mg. This decrease in water flux is related to increase in thickness of the GOF membranes selective layer observed as a function of GO concentration in morphological analysis. The GOF membranes prepared from BPEI/GO compositions have higher water flux than BPEI/GO_TU compositions because of the availability of more defects in the former by presence of GO with excess oxidized groups. However, with the partial reduction of GO nanosheets in the BPEI/GO_TU membranes, existence of such defects was considerably decreased and thus has lower water flux. The role of defects in facilitating water transport in GO membranes is well documented in the



literature.5,

36

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From the PWF behavior, we can also suggest that the partial reduction of GO and

subsequent cross-linking of the multilayer structure with heat treatment is essential to produce tight structure with narrow nanochannels for selective molecular transport.

Figure 7. Dye rejection performance of the (BPEI/GO)12 and (BPEI/GO_TU)12 membranes prepared with various GO concentration during LbL process. All membranes were cross-linked by heating at 70°C for 30 min. Amount of GO provided is the total loading in 8 mL of aqueous solution per deposition cycle. Dye solutions (MO, MB and RB) with an initial concentration of 10 mg/L were used as feed solution. Rejection experiments were carried out at an applied pressure of 4 bar. Error bar represents a standard deviation (n=3). Negatively charged MO, positively charged MB and RB were used for the dye rejection studies (chemical structures are provided in scheme S3). The PES membranes used as substrate for GOF preparation have less than 15% rejection for these dye molecules. The dye rejection performance of GOF membranes prepared from both BPEI/GO and BPEI/GO_TU compositions are given in Figure 7. In dye separation experiments, membranes of all compositions were saturated with respective dye adsorption with 30-40 minutes of filtration as evidenced in Figure S5. As expected, BPEI/GO_TU membranes have higher rejection than BPEI/GO membranes and % rejection also increased with increase in GO content. The BPEI/GO and BPEI/GO_TU membranes prepared from GO concentration of 0.5 mg have a MO rejection of 89.5% and 94.6% respectively. Similarly, BPEI/GO and BPEI/GO_TU membranes prepared from same GO concentration have a MB rejection of 83.8 % and 91.2% respectively. GOF membranes prepared from GO concentration of 2.0 mg have more than 99% rejection for all dye molecules under study. As seen in Figure S6, rejection performance of BPEI/GO and BPEI/GO_TU membranes prepared without heat induced cross-linking followed the same trend with a increase in %rejection with an increase in GO content. As could be seen in Figure 7 and Figure S6, rejection of MO is higher than MB for any membrane composition irrespective of the GO content. The higher rejection of anionic dye illustrated that


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GOF membranes fabricated by the deposition of BPEI and GO nanosheets have negative surface charge. Such a negative surface charge is justifiable for an LbL fabricated membranes terminated with GO layer due to the presence of carboxylic and other oxidized functional groups.22,

25

Thus it is

reasonable to conclude that Donnan exclusion and sieving effect leads to higher rejection of MO by the GOF membranes. Also as mentioned in PWF results, increase in dye rejection with an increase in GO content is due to the increase in thickness of the BPEI/GO selective layer. The rejection of cationic dyes for any composition of GOF membranes, RB has higher rejection than MB due to its large size (scheme S3). Since cationic dyes and GOF membranes have same charge, we speculate that rejection mechanism leads to separation of cationic dyes should be purely based on size exclusion mechanism. Thus we can concluded that, both BPEI/GO and BPEI/GO_TU membranes prepared in our study have molecular weight cut-off (MWCO) value approximately in the range of 320 g/mol.

Figure 8. Salt rejection performance of the (a) (BPEI/GO)12 and (b) (BPEI/GO_TU)12 membranes prepared with various GO concentration. (a1) and (b1) are the schematic representation of the separation behavior of (a) and (b) respectively. All membranes were cross-linked by heating at 70°C for 30 min. Amount of GO provided is the total loading in 8 mL of aqueous solution per deposition cycle. Salt



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solutions of Na2SO4, MgCl2 and NaCl with an initial concentration of 500 mg/L were used as feed solution. Rejection experiments were carried out at an applied pressure of 4 bar. Error bar represents a standard deviation (n=3). The ability of GOF membranes in the separation of ionic solutes from aqueous solution was evaluated by challenging GOF membranes with Na2SO4, MgCl2 and NaCl. The PES substrate membrane did not show any rejection for these salts. As shown in the Figure 8 (a&b), BPEI/GO and BPEI/GO_TU membranes prepared from a GO concentration of 2.0 mg have Na2SO4 rejection of 82% and 94% respectively. Similarly these membranes have a MgCl2 rejection of 75% & 86% respectively and NaCl rejection of 32% & 43% respectively. From the rejection results, it could be suggested that interlamellar distance, thickness of the selective layer, and net charge effect of the BPEI/GO multilayer had a significant role in determining the separation performance of these salts. According to many studies in the literature, interlamellar space, charge of the membranes and size of the targeted solute molecules are important factors in determining the separation efficiency of GOF membranes.18,

25, 27

Since the last

parameters are almost similar, better rejection efficiency of BPEI/GO_TU membranes could be explained by the shorter interlamellar distance observed in the morphological analysis. However, comparable rejection performance of smaller ionic solutes to the larger dye molecules signifies importance of Donnan exclusion in determining the ionic solute rejection efficiency in both types of GOF membranes. Thus both the BPEI/GO and BPEI/GO_TU membranes with strong negative surface charge rejected more divalent anion (SO42-) than monovalent chloride (Cl-). The rejection behavior of GOF membranes observed here illustrates the importance of tuning the interlamellar distance in GOF structure by adjusting the hydrophobic/hydrophilic balance and tuning the surface charge of composite GOF membranes (Figure 8 a1&b1). The comparable rejection of Mg2+ (in comparison to SO42-) by multilayer membrane terminated with GO layer (with negatively charged –COOH groups) indicate the effect of localized positive charge within the BPEI/GO multilayer, which prevented permeation of mg2+ by acting as an electrostatic barrier. The effect of surface effect and localized charge on the separation of ionic solutes requires separation experiments with solutes of various ionic strengths and exact quantification of the surface charge and these studies are part of our future research interests. According to Boukhvalov et al, in a multilayer GO membrane water flows through the interconnected nanochannels, preferably through the hydrophobic nonoxidized sp2 regions formed


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between the adjacent nanosheets rather than oxidized domains.6-7 In normal scenario, when GO molecule with high amount of oxidized functional groups are assembled for membrane formation via either vacuum filtration or LbL assembly, available hydrophobic regions in these membranes are insufficient to form well defined nanochannels. Extensive presence of these oxidized functional groups in GO membranes leads to poor separation performance and stability in aqueous solution. However, complete removal of the functional groups leads to formation of graphene membranes with narrow interlayer spacing, which is impermeable even to gases. Thus, there is tremendous interest in the fabrication GOF membranes from partially reduced GO, since such a membrane can effectively function as

a

separation

medium

even

in

aqueous

conditions

because

of

the

well

balanced

hydrophobic/hydrophilic balance. Results of our separation studies indicate that partial reduction and cross-linking employed in our GOF membrane fabrication was effective in maintaining a well-defined nanochannels. Comparing “from” and “from” & “to” cross-linking approach, it is worth important to mention the role TU played in creating shorter interlamellar distance in GOF membranes.37 During “to” crosslinking step (or sonication step), TU first reduced GO and then links the adjacent GO nanosheets through via –in plane and out-of-plane interactions. In the subsequent “from” cross-linking approach (during the LbL and heating steps), these GO nanosheets cross-link with BPEI through interactions with –NH2 and – COOH to form either amide linkage or ionic cross-links. However, in “from” cross-linking approach only amide formation between BPEI and GO were available. Though both these approaches formed welldefined nanochannels, better П=П interactions existed between the partially oxidized GO nanosheets in BPEI/GO_TU membranes help to narrow the interlamellar spacing and thus showed a better separation performance. We have compared separation performance of GOF membranes fabricated in this study with commercial NF membranes and GO membranes prepared via either vacuum filtration or LbL process reported in the literature (Table S2). The PWF of the newly developed BPEI/GO_TU membranes was higher than commercial NF membranes (NF 90, Dow Filmtec) and some of the GO membranes reported in the literature.30,

51

The higher PWF with comparable %rejection for charged solutes of BPEI/GO_TU

GOF membranes was due to the combined effect of negative surface charge and narrow interlamellar distance. Considering the stable structure, excellent separation efficiency and simple fabrication method,



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chemistry involved in the preparation of BPEI/GO-TU membranes in this study is a useful development for large scale production. For instance, those interested in employing this chemistry in a commercial production unit, LbL formulation developed in this study can be easily extended in to an large scale preparation process such as electrospraying.52 Stability of GOF Membranes The stability of GOF membranes were studied by immersing small pieces of BPEI/GO and BPEI/GO_TU membranes in pH adjusted DI water. The digital photographs of these membranes after two months of immersion are provided in the supporting information Figure S7. As could see, both types of membranes were intact indicating stability of these membranes even in harsh aqueous solution environment. The stability of GOF membranes imply that our strategy to cross-link GOF membranes by BPEI alone and combination of BPEI and TU was effective in developing a well ordered GO nanochannels with optimum П=П interactions. The normalized PWF (which is a ratio of PWF treated membrane to as prepared membrane) measured after 2 months of immersion in pH-2.0 and 11.0 have shown a slight reduction in flux for the BPEI/GO membranes (Figure S8). However, BPEI/GO_TU membrane pure water flux is unchanged. This indicates that at extreme acidity/basicity, intensity of ionic cross-links or amide bond formed between BPEI and GO in BPEI/GO composite membranes is partially broken due to the change in charge density of the building blocks.15,

17

Thus we can conclude that hydration force formed between water and GO nanosheets in

BPEI/GO membrane was strong enough to overcome the energy of ionic cross-links during longer immersion time. However, the remarkable stability of BEPI/GO_TU membranes could be explained by the near to complete removal of oxidized functional groups from GO, which decreased the hydration force between the GO and water.17 As shown in morphological analysis, TU molecules reduce the interlayer spacing between the GO nanosheets through both П=П attractive force and ionic cross-links, which together overcome hydration force formed in BPEI/GO_TU membranes. Thus it is reasonable to conclude that “from” and “to” cross-linking approach using BPEI and TU as building blocks contribute to a remarkable improvement in interlamellar stabilizing force in GOF membranes and thus showed long term stability in aqueous solutions. The fouling propensity of the GOF membranes prepared from various GO


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concentrations also studied using bovine serum albumin (BSA) as model foulant. The flux decline rate (Rfd) calculated after 6 h of BSA filtration are provided in Figure S9. Both BPEI/GO and BPEI/GO_TU membranes prepared from various GO compositions have Rfd values of less than 25%. In addition to surface layer hydrophilicity, compactness of selective layer exhibited significant role in determining the fouling propensity of GOF membranes and detailed discussion is provided in the supporting information. Conclusions In summary, GOF membranes with tightly packed nanochannels were fabricated by LbL process using BPEI, GO and TU molecules as the building blocks. Morphology of BPEI/GO_TU membranes showed a tight GOF structure with narrow interlammellar spacing of 0.49 nm compared to BPEI/GO membranes 0.52 nm. The structural interactions extracted from XPS spectra illustrate that TU molecules first linked the adjacent GO nanosheets through thio linkages and then formed plane-to-plane interactions which helped to deposit multiple layers of GO in each deposition cycle. When the GO layers comes in contact with already deposited BPEI layers, entire multilayer structure is stabilized through interactions between the –NH2 groups of BPEI and available oxidized groups of GO. Separation performance of GOF membranes strongly depends on the amount of GO used in membrane fabrication; (BPEI/GO_TU)12 membranes prepared from 2.0 and 0.5 mg have a MO rejection of 99.6% and 94.5% respectively. Moreover, (BPEI/GO_TU)12 membranes exhibited excellent stability in aqueous solution due to the enhanced П-П interactions between the GO nanosheets and fouling resistance because of the compact selective layer. The excellent separation performance and aqueous solution stability illustrate that “from” and “to” cross-linking approach developed in this study is a feasible method for developing GOF membranes with controlled interlayer spacing for various molecular and ionic separations. Supporting Information (SI): Schematic representation of the LbL process of membrane fabrication (scheme S1), SEM images of BPEI/GO_TU membrane as a function of bilayer number (Figure S1), XRD spectra of GOF membranes (Figure S2), ATR-FTIR spectra of GOF membranes cross-linked after each bilayer deposition (Figure S3), Scheme of chemical interactions in GOF membranes (scheme S2), Structure of dye molecules (scheme S3), pure water flux, dye adsorption Vs time and dye rejection of uncross-linked GOF membranes (Figure S4-S6), aqueous solution stability of GOF membranes (Figure S7), normalized water fluxes and fouling studies of GOF membranes (Figure S8&S9), mechanical properties (Table S1) and comparative evaluation of the performance of GOF membrane in the current study with commercial and recently reported GO membranes (Table S2).



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Author Information: Corresponding author; [email protected] Acknowledgements This research work was supported by The University of Houston startup fund, research seed grant from College of Technology (CoT) and Texas Center for Superconductivity (TcSUH) at The University of Houston. We thank Dr. Dezhi Wang, TcSUH, Dr. Makarenko Boris, Department of Chemistry and Ms. Wenyue Ding, Department of Chemical Engineering, University of Houston for the TEM analysis of GO nanosheets, XPS and mechanical analysis respectively. We also thank Mr. Matthew Meyer, Shared Equipment Authority (SEA), Rice University, Houston for the ultra-microtome and TEM imaging of GOF membranes. Notes The authors declare no competing financial interests. References

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