water separation and dye removal

Journal Pre-proof A multifunctional hierarchical porous SiO2/GO membrane for high efficiency oil/water separation and dye removal Yue Liu, Fengrui Zhang, Wenxia Zhu, Dong Su, Zhiyuan Sang, Xiao Yan, Sheng Li, Ji Liang, Shi Xue Dou PII:

S0008-6223(20)30002-6

DOI:

https://doi.org/10.1016/j.carbon.2020.01.002

Reference:

CARBON 14936

To appear in:

Carbon

Received Date: 21 October 2019 Revised Date:

17 December 2019

Accepted Date: 1 January 2020

Please cite this article as: Y. Liu, F. Zhang, W. Zhu, D. Su, Z. Sang, X. Yan, S. Li, J. Liang, S.X. Dou, A multifunctional hierarchical porous SiO2/GO membrane for high efficiency oil/water separation and dye removal, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.01.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author Contribution Statement Yue Liu: Methodology, Investigation. Fengrui Zhang: Methodology, Investigation. Wenxia Zhu: Methodology, Investigation. Dong Su: Resources, Data Curation, Writing- Original Draft, Supervision, Funding acquisition. Zhiyuan Sang: Methodology, Investigation. Xiao Yan: Conceptualization, Writing-Review&Editing, Formal analysis, Funding acquisition. Sheng Li: Investigation. Ji Liang: Formal analysis, Writing-Review&Editing, Funding acquisition. Shi Xue Dou: Supervision

A Multifunctional Hierarchical Porous SiO2/GO Membrane for High Efficiency Oil/Water Separation and Dye Removal Yue Liu,a Fengrui Zhang,a Wenxia Zhu,a Dong Su,a* Zhiyuan Sang,a Xiao Yan,b, c, d* Sheng Li,b, d Ji Liang,a, c* and Shi Xue Douc a

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, School

of Materials Science and Engineering, Tianjin University, Tianjin, 300350, China b

Guangdong Key Laboratory of Membrane Materials and Membrane Separation, Guangzhou

Institute of Advanced Technology, Chinese Academy of Sciences, Guangzhou, 511458, China c

Institute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials,

University of Wollongong, North Wollongong, NSW 2500, Australia d

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, 518055,

China *Corresponding Authors: Dong Su ([email protected]), Xiao Yan ([email protected]), Ji Liang ([email protected])

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Abstract Removing contaminants from wastewater is critical to secure the global water supply. Membrane technologies for water purification are exceptionally attractive due to their high efficiency and low energy consumption. The traditional porous polymer films, however, are easy to be fouled by the organic pollutants, causing pore blockage and deteriorated separation performance. We herein report the rational design of a porous SiO2/GO hybrid membrane by coupling graphene oxide (GO) nanosheets with SiO2 nanoparticles and using ethylenediamine to crosslink them, for efficient oil/water separation and dye removal. The SiO2 nanoparticles provide an excellent hydrophilicity and underwater superoleophobicity interface, resulting in efficient and antifouling oil/water separation with an outstanding rejection rate over 99.4% for different types of oil; and the hierarchical scaffold, formed from the hydrophilic GO nanosheets embedded with SiO2 nanoparticles, greatly facilitates the rapid permeation of water with a high flux rate of up to 2387 L m−2 h−1 for pure water and 470 L m−2 h−1 for oil/water separation. Moreover, the abundant functional groups on the GO surface also render this membrane with a high removal capability for dye blocking, enabling it to remove soluble pollutants in molecular dimensions as well. This design strategy not only provides an outstanding membrane for water purification but also sheds light on the design of multi-purpose functional membranes for a variety of energy and environment-related applications.

Keywords: GO-based membrane; Hierarchical pore; Hybrid membrane; Emulsion separation; Dye removal;

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1. Introduction Pollutants from industrial manufacturing processes, such as mining, oil and gas production, metal smelting, and fine chemical production, have placed an ever-growing threat on the security of the valuable freshwater resource for almost two-thirds of the global population [1], especially for people in the developing regions. Wastewater treatment technologies to remove multiple pollutants from water resources, are therefore critical for the sustainable water supply as well as sustaining livelihoods, human well-being, and socio-economic development [2]. For this purpose, various water purification technologies have been developed, such as distillation, media filtration, membranes, ion exchange, chemical disinfection, precipitation, and electrosorption [2, 3]. Among them, the membrane-based technology is one of the most facile strategies, due to its combined merits of relatively low energy consumption, simple operation, and high efficiency for the removal of various contaminants from water [4-6]. Commonly, polymeric membranes with precisely controlled pore structures and surface features are widely used [7-9]. These polymer membranes are, however, highly vulnerable to organic fouling and corrosion due to the adsorption/blocking of organic foulants and colloidal substances [4, 10], which causes structural distortion, performance degradation, and increased energy consumption. Therefore, it is essential that alternative membranes with antifouling features should be developed with high structural and chemical stability, high separation capability, and antifouling characteristics to address the challenges in water purification under aggressive conditions. Graphene oxide (GO) membranes have laminated and two-dimensional (2D) channels, through which water molecules can rapidly permeate due to the low-friction water flow and two-dimensional capillaries between GO nanosheets [11, 12]. Hydrophilic oxygen-containing groups of GO 3

membranes enlarge the interlayer spacing of the GO nanosheets, which will help the intercalation and permeation of water through the nanochannels. Therefore, GO membrane can act as a molecular sieve by allowing for fast permeation of water molecules while blocking those larger molecules or ions. In the meantime, they are also able to block other larger molecules/ions or charged species because of the size exclusion [13], electrostatic interactions [14], and/or ion adsorption effects [15], making them exceptionally suitable for water filtration and separation [16-20]. With these unique features, GO membranes have been extensively exploited for waste water treatment, in terms of heavy metal removal [21], dye removal [22], biological separation [23], and desalination [24]. In spite of these merits, some intrinsic shortcomings of pure GO membranes still exist. On the one hand, the initial water flux of the pure GO membrane is commonly in the range of 1.7 to 21.4 L m−2 h−1, which is much lower than the conventional polymer membranes (>50 L m−2 h−1) [14, 25, 26], due to the narrow laminate channels between the close-packed GO nanosheets as well as the interaction of the oxygen-containing functional groups with water molecules [27]. On the other hand, during the long-term water purification, the hydrogen bond between the GO nanosheets will be continuously destroyed due to the hydration of the oxygen-containing groups on the GO surface, which will significantly yet uncontrollably enlarge the distance between the GO nanosheets and make the separation performance quickly decay [28]. Consequently, the pure GO membranes usually suffer from low flux rate during water purification and poor stability as well. To deal with this, both organic (e.g., dopamine [29], isophorone diisocyanate [30] and polyethyleneimine [31]) and inorganic species (e.g., palygorskite nanohybrid [20], halloysite nanotubes [22], sepiolite [32] and C3N4 [33, 34]) have been intercalated into the adjacent GO nanosheets to alleviate the re-stacking of GO nanosheets and expand the channel structures for water 4

transport. Among them, the inorganic ones can also alter the surface properties of the resulted membranes [35, 36], such as roughness and wettability, leading to a favorable water/membrane interface with a low oil affinity for oil/water separation [37]. For this purpose, SiO2 nanoparticles are especially promising for their low cost and facileness to fabricate [38-41]. On the one hand, the SiO2 nanoparticles can endow the membrane with high specific surface area and hydrophilicity, benefiting the water flux and selective permeation of the hybrid membrane. On the other hand, however, the dosage of SiO2 inorganic particles may also cause deterioration in mechanical strength due to their rigid nature and the poor interface contact between the SiO2 nanoparticles and GO nanosheets that is unfavorable for long-term stability for practical applications [40]. Consequently, the rational design of the SiO2/GO-based membranes with optimized pore structure, high structural stability, and excellent surface characteristics is essential for their performance enhancement, which has been rarely considered or achieved by far, unfortunately. Based on these considerations and with the purpose of achieving a high-rate, selectivity, and stable permeability for water and excellent retard against other larger species (e.g., large organic molecules and oil droplets), we herein report the design and fabrication of a highly flexible and antifouling SiO2/GO hybrid membrane from GO nanosheets separated by SiO2 nanoparticles and stabilized by cross-linked ethylenediamine (EDA). In this material, the SiO2 nanoparticles provided an additional hierarchical structure to the laminar pores between the GO nanosheets as well as excellent hydrophilicity interface, which can effectively enhance the permeation rate of water and oil/water separation. On the other hand, the EDA acted as a binder that can crosslink the GO nanosheets through covalent bonding and thus significantly enhanced the structural stability of the membrane during water purification. Due to these merits, this SiO2/GO hybrid membrane possesses an 5

outstanding water permeability (up to 2378 L m−2 h−1 for water, 470 L m−2 h−1 for O/W emulsion), excellent separation efficiency (rejection of over 99.4% for oil/water emulsion and rejection of 100% for MB solution), high recoverability, and antifouling capability for oil/water emulsion separation and dye removal.

2. Experimental

2.1 Chemicals Ethylenediamine (EDA, Tianjin Yuanli Chemical Co., China) was used as received. Graphene oxide (GO) nanosheets with flake size of several hundred nanometers (Figure S1) were synthesized by a modified Hummers’ method using natural graphite powder (750-850 mesh, 99.95%, Shanghai Aladdin, China) as described previously [42]. SiO2 nanoparticles (ca. 150 nm) were prepared by the Stöber method, using tetraethylorthosilicate (TEOS, Tianjin Jiangtian Chemical Co., China) as precursor and ammonium hydroxide (NH3·H2O, Tianjin Guangfu Chemical Co., China) as the catalyst [43]. All of the reagents used in the experiment were analytical grade. 2.2 Preparation of SiO2/GO membranes Hierarchical SiO2/GO membranes with different SiO2/GO mass ratios were fabricated from GO nanosheets, SiO2 nanoparticles and EDA, through a vacuum-assisted filtration and self-assembling process. In a typical synthesis, 4 mg SiO2 nanoparticles and 2 mg GO nanosheets were dispersed in 25 mL deionized water, followed by stirring for 30 min at room temperature. Then, 1 µL EDA was added into the as-prepared solution and stirred for 10 min. The solution was vacuum filtered through a cellulose acetate filter paper (Xinya Purification Co., Shanghai, China) to form a SiO2/GO membrane which can be easily peeled off after being dried at 50 °C for 1 h to yield a free-standing 6

and flexible SiO2/GO membrane. Meanwhile, the additional EDA was dissolved in water and removed during the film-assembly process. For direct comparison, the prepared hybrid SiO2/GO membranes were denoted as SiO2/GO-0, SiO2/GO-0.5, SiO2/GO-1, SiO2/GO-2, SiO2/GO-3 and SiO2/GO-5 with mass ratio of SiO2: GO of 0, 0.5, 1, 2, 3 and 5, respectively. 2.3 Microstructure characterization Microstructures of the SiO2/GO membranes were characterized using field emission scanning electron microscopy (SEM, TDCLS-4800, Japan) and transmission electron microscopy (TEM, JEOL-200CX, Japan). A tiny and thin fraction was peeled from the hybrid membrane and transferred onto a copper mesh for TEM analysis. Crystal structures of the SiO2/GO membranes were analyzed using an X-ray diffractometer (XRD, D/Max-2500 Rigaku, Japan) ranging from 5° to 60°. The chemical structures of the membranes were analyzed by Fourier transform infrared spectroscopy (FTIR, TENSOR27, Germany) equipped with an attenuated total reflectance (ATR) accessory. The surface compositions of the membranes were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha, America) conducted with Al Kα at 150 W. The water contact angles were measured on the surface of the membranes by an automatical goniometer/tensiometer 7 (KRUSS, DSA100, Germany) using a sessile drop method, in which 5 µL water droplet was gently placed on the testing surface using a micro-syringe. For underwater oil contact angle, dichloromethane was used as testing droplet. 2.4 Preparation of oil/water emulsions Sodium dodecyl sulfate (SDS, Tianjin Guangfu Chemical Co., China) emulsified O/W emulsions were used to evaluate the oil/water (O/W) emulsion separation performance of the SiO2/GO membranes, concluding diesel oil/water, soybean oil/water, gas oil/water and pump oil/water. In a 7

typical O/W emulsion preparation, oil was added into deionized water with the volume ratio of oil/water of 1:100 under stirring for 15 min, and then SDS as an emulsifier (0.2 mg mL-1) was added into the mixture. The homogeneous emulsion was obtained after stirring for 1 h followed by the removal of redundant floating oil. The size and size distribution of the emulsion droplets was characterized by a laser light scattering system (Zetasizer Nano ZS90, England). 2.5 Permeation flux and oil/water separation The permeation performances of the SiO2/GO membranes were evaluated under vacuum filtration (~0.1 MPa) with effective filtration area of 13.85 cm2, and pre-compacted with deionized water under 0.1 MPa for >10 min to gain a stable flux value. The permeate flux J (L m-2 h-1) was calculated following equation (1), in which the volume of filtered solutions within 2 min was recorded. The permeate flux of each membrane was averaged from three tests to ensure the reproducibility.

=

(1)

Here, V is the volume of penetrate flow, A is the membrane effective area, and t is the filtration time. The rejection rate (R) of oil in emulsion was calculated using the following equation (2) using 30 mL O/W emulsion. The oil concentration in the filtrate before and after filtration was measured by a total organic carbon (TOC) analyzer (Shimadzu TOC-VCPH analyzer, Japan).

= 1−

× 100%

(2)

The Cp and Cf (mg/L) represent the oil concentrations of the original solution/emulsion and the collected solution/emulsion after filtration, respectively. 2.6 Antifouling property Antifouling property was conducted on the hybrid membrane of SiO2/GO-2, in which 50 mL 8

soybean oil/water emulsion was separated by the membrane first. Then the fouled membrane was backwashed with deionized water for 40s. The combination of O/W separation process and membrane backwash process was termed as one cycle. The reusability test included 8 cycles. 2.7 Dye removal property Typical dye removal test was conducted on the hybrid membrane of SiO2/GO-2 by10 ppm methyl blue (MB, Tianjin Yuanli, China) aqueous solution with the volume ranging from 20 mL to 100 mL. Moreover, dye removal capability was tested by increasing MB concentration from 10 ppm to 40 ppm (20 mL). The MB concentration in feed solution and permeation solution were determined spectrophotometrically on a UV-vis spectrophotometer (UV2550, Shimadzu, Japan) to characterize the MB rejection rate of the membranes following equation (2).

3. Results and Discussion

The fabrication of the SiO2/GO membranes via the filtration-assisted and self-assembling process is schematically depicted in Figure 1a. Firstly, a homogeneous suspension containing SiO2 nanoparticles, GO nanosheets, and EDA was obtained by ultrasonication. During this process, the color of the suspension turned from brown to black upon the addition of EDA, suggesting that reaction between EDA and GO occurred [28]. Then, the suspension was filtered through a cellulose acetate filter paper to form a SiO2/GO hybrid membrane, in which the SiO2 nanoparticles were embedded in between the GO nanosheets, with EDA molecules anchored between the adjacent GO nanosheets, as shown in the chemical-bonding schematism in Figure 1a. The obtained SiO2/GO membranes can be directly peeled off from the filter paper after drying. The free-standing SiO2/GO membrane is highly flexible and can be bent freely without any damage. As shown in Figure S2, the 9

SiO2/GO-2 membrane showed high tensile strength, including the failure strain of about 5.9%, tensile stress of 3.2 MPa and tensile modulus of 54.2 MPa. For comparison, membranes with different SiO2/GO ratios (X) were fabricated and denoted as SiO2/GO-X (details can be found in the experimental section).

Figure 1. a) Schematic diagram of fabrication of SiO2/GO hybrid membranes via vacuum-assisted filtration self-assembly process; cross-sectional SEM image of b) SiO2/GO-0 and c) SiO2/GO-2 membrane; and d) TEM image of SiO2/GO-2 membrane.

The morphology and microstructure of the SiO2/GO hybrid membranes were first studied under SEM in comparation with the pure GO membrane (i.e., SiO2/GO-0). All the hybrid membranes show obvious layered structure coming from the GO stacking, which is similar to but more loose compared

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with the pure GO membrane, as shown in Figure S3. Even though, the GO nanosheets in the film still form a laminar structure, rather than being isolated to yield penetrating holes, which is essential for simultaneously achieving the high resistance of oil/MB from water and the high flux. The thickness of the SiO2/GO-0 is about 1 µm thick with compactly piled GO nanosheets and no obvious pores (Figure 1b). The thickness of the hybrid membrane increases from 6 to 12 µm with the SiO2/GO from 0.5 to 5, as shown in Figure S3, indicating that SiO2 nanoparticles can significantly alter the membrane’s structure by existing between the GO nanosheets and introducing large macro pores. The layer-structure was gradually interrupted by SiO2 nanoparticles to form macro-porous structure with increasing loading.

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Figure 2. Surface morphology of membranes with various amounts of SiO2. a) SiO2/GO-0; b) SiO2/GO-0.5; c) SiO2/GO-1; d) SiO2/GO-2; e) SiO2/GO-3; f) SiO2/GO-5; and g-i) the high magnification SEM images of the selected areas in b, d, and f, respectively.

The surface morphology of the SiO2/GO membranes also varies with the amount of SiO2 (Figure 2). For the SiO2/GO-0 without SiO2 particles, its surface is generally compact and without visible pores. As the SiO2/GO ratio is increased from 0.5 to 5, more SiO2 nanoparticles can be observed, both on the surface and at the edge of GO nanosheets. This generates a more loosely stacking microstructure, which is favorable for better water permeability, however, at the cost of membrane strength and flexibility. Due to this, the hybrid membranes with high SiO2 content (i.e., SiO2/GO-3 and SiO2/GO-5) tend to fracture when peeled off from the support. N2 adsorption-desorption and mercury intrusion analysis show that the SiO2/GO membrane possesses a hierarchically porous nanostructure (Figure S4), including the small mesopores ranging from several nanometers to tens of nanometers coming from the restacking of GO nanosheets [25] and the macropores up to 400 nm coming from the segregation of SiO2 nanoparticles between the GO nanosheets.

Figure 3. a) XRD spectra and b) FTIR spectra of SiO2/GO membranes with different SiO2/GO mass ratios, GO nanosheets, and SiO2 nanoparticles. 12

More structural information of the material was obtained by XRD, which shows a sharp diffraction peak at about 10.7° and a broad one in the region of 15-25° (Figure 3a). The peak at 10.7° should be assigned for the stacked GO nanosheets [25]; while the other one should be assigned for amorphous SiO2 [44], which also present in pure SiO2 nanoparticles. It is interesting to find that the position of the GO peaks in all the EDA modified samples (i.e., pure GO membrane and hybrid membranes with different SiO2 loading) are slightly shifted to a lower 2θ than that of the pure GO (10.7° vs. 11.6°), corresponding to an increase in the interlayer distance of about 0.08 nm. This should come from the anchoring of EDA molecules between the adjacent GO nanosheets [28]. Compared with the samples in the dry condition, the wet membranes do not show obvious difference in the peak at 10.7°. However, a shift of the silica-related peak from 22 to 28° can be seen (Figure S5), which might be due to the formation of hydrated silica [45-46]. To further confirm this, we then conducted FTIR analysis on the SiO2/GO hybrid membranes (Figure 3b). SiO2/GO-0 has characteristic peaks at 1716 cm-1,1420 cm-1, 1224 cm-1, and 1041 cm-1, which can be attributed to the stretching vibrations of oxygen-containing species of GO, including C=O in carboxyl groups, C-O in carboxyl group, C-O in epoxy group, and C-O in alkoxy group respectively [47]. Compared with the pure GO, the C=O peak at 1716 cm-1 decreases in SiO2/GO-0, suggesting the consumption of carboxyl groups on the GO due to the reaction of EDA. Meanwhile the peak at 1610 cm-1 is also broadened compared with that of pure GO, which could be attributed to the bending of N-H and the stretching of C=O on the secondary amide group [47]. Moreover, a new peak at 1357 cm-1 and an intensified peak at 1181 cm-1 for C-N bonds are observed [48]. These changes arise from the reaction between amine groups of EDA and carboxyl groups of GO, confirming the chemical bonding of EDA with GO nanosheets and the existence of EDA between 13

the GO nanosheets to cause the increased interlayer distance from the XRD analysis. For the SiO2/GO membranes, the characteristic bands of GO (such as C=O, C-O, C-O-C, and C-N) are reserved, and a new strong peak at 1076 cm-1 emerged that can be attributed to Si-O-Si stretching vibrations of the SiO2 particles [43].

Figure 4. Surface chemistry of SiO2/GO-2 membrane in comparation with SiO2/GO-0 membrane. a) SEM image of SiO2/GO-2 and the corresponding elemental mappings; b) XPS survey spectra of the materials; and high resolution; c) O1s, d) C1s, and e) N1s XPS spectra, respectively.

The surface chemistry of the SiO2/GO-2 membrane was further investigated by elemental mapping and XPS (Figure 4). Elemental mappings reveal the uniform distribution of Si, O, C elements, indicating that SiO2 nanoparticles are homogeneously distributed over the membrane (Figure 4a). 14

The XPS survey scan of SiO2/GO-2 (Figure 4b) shows the presence of O, N, C, and extra Si element in comparation with that of SiO2/GO-0 membrane. The O1s spectra of SiO2/GO-0 and SiO2/GO-2 exhibit C-O-C bond at 532.3 eV as the major peak (Figure 4c). Moreover, the O1s spectrum of SiO2/GO-2 also contains peaks at 535.7 eV for Si-O-Si and 534.0 eV for Si-O-C, which should arise from the condensation reaction between hydroxyl groups of GO and SiO2 under the alkaline condition provided by EDA [48]. The N1s spectra can be deconvoluted into two peaks at 399.0, and 400.0 eV (Figure 4c), which can be respectively assigned to corresponding to nitrogen on amine group (C-N) and amide carbonyl group (N-C=O) [49], confirming the covalent bonding between EDA and the GO nanosheets [22]. The C1s spectra of both SiO2/GO-0 and SiO2/GO-2 can be deconvoluted into four peaks at 284.6 eV for C-C, 285.9 eV for C-N, 287.1 eV for C-O, and 288.4 eV for C=O (Figure 4d). The increasing C-N peak and decreasing of C-O peak are observed in SiO2/GO-2 further as well, indicating the formation of C-N bond through acylation reactions between the amino groups of EDA and epoxy/carboxyl groups of GO nanosheets [47]. The chemical bonding between SiO2, EDA and GO can thus enhance the strength, flexibility, and stability of the SiO2/GO hybrid membrane, which is in favor of their application in O/W emulsion separation and dye adsorption. On the basis of these characterizations, the prominent mechanical property and structural stability of the SiO2/GO-2 hybrid membrane with the optimal SiO2 loading should arise from the strong chemical bonding between GO, EDA, and SiO2. Firstly, under the alkaline condition provided by EDA, the hydroxyl groups on the surface of the SiO2 nanoparticles can interact with the oxygenic groups on GO (e.g., carboxyl, epoxy, and hydroxyl groups) through condensation reactions or forming hydrogen bond, providing mechanical strength and flexibility for the hybrid membranes [48]. 15

It seems that there is no obvious difference before and after filtration for the membrane’s surface by SEM, suggesting the stability of SiO2 particles in the membrane (Figure S6a-b). Secondly, the EDA molecule can also crosslink the adjacent GO nanosheets through the acylation reaction between amino groups of EDA and carboxyl groups of GO nanosheets, further leading to enhanced stability in both aqueous or organic solutions (Figure S6c) [50]. On the contrary, the pure GO membrane, without such inter-sheet interactions induced by EDA, suffers from the hydration effect that will destroy the hydrogen bond and enlarge the d-spacing among GO nanosheets [28], leading to the edge cracking in aqueous environments after 15 h (Figure S6d). Therefore, these hybrid SiO2/GO membranes possess the improved mechanical stability that is necessary for a better filtration performance during water purification.

Figure 5. a) Apparent water contact angle of the membranes in air; b) apparent oil contact angle of the membranes under water; c) permeate fluxes for pure water of SiO2/GO membranes with different SiO2/GO mass ratios; and d) comparison of permeation flux on the SiO2/GO membrane and previous 16

GO-based membranes and the percentage in figure is oil rejection of corresponding membrane.

Surface wettability is a critical factor determining the membrane properties because it can affect the water permeate as well as resistance to foulants. To evaluate the wetting behavior of the SiO2/GO hybrid membranes, both apparent water contact angles in air and apparent oil contact angles under water are measured. As shown in Figure 5a, an obvious decrease in the water contact angles of SiO2/GO-0.5 to SiO2/GO-5 can be observed compared with that of SiO2/GO-0. The SiO2/GO-0 membrane has a contact angle of 64°, which is caused by the hydrophilic oxygenic groups on GO nanosheets. In contrast, SiO2/GO-0.5, even with a small amount of SiO2 nanoparticles, shows a significantly decreased contact angle of 32°, indicating that SiO2 nanoparticles can effectively increase the hydrophilicity of the membrane. Furthermore, the contact angles of the hybrid membranes gradually drop to as low as 14° for SiO2/GO-5 with the highest SiO2 loading. This enhanced hydrophilicity should mainly arise from the highly hydrophilic nature of the SiO2 nanoparticles. Firstly, the abundant hydroxyl groups on their surface can enhance the hydration capacity of the membranes. Secondly, the SiO2 nanoparticles can also increase the surface roughness of the membranes, facilitating water contact with surface hydroxyl groups and diffusion through the surface/interface. Afterward, dichloromethane was chosen as a model to further assess the oil contact angle of the SiO2/GO membranes in an underwater environment. On the contrary to the cases for water, the oil contact angles show an increasing trend with the addition of SiO2 nanoparticles, starting from 120° for SiO2/GO-0 to 156° for SiO2/GO-5 (Figure 5b), showing their excellent underwater superoleophobicity capability to repel oil. This strong superoleophobicity could be comprehensively resulted from the strong electrostatic interaction and hydrogen bonding for water molecules of both 17

the hydrophilic GO nanosheets and SiO2 particles that help create a surface hydration layer, which effectively prevents the membranes from being wetted by the oil droplets [20, 22]. It thus clearly demonstrates the superiority of our material’s hierarchical structure and favorable surface chemistry for efficient water purification compared with the other counterparts, especially when considering its unique “polymer-substrate-free” feature that is essential for achieving the antifouling features. Due to these merits, the water permeate fluxes through the SiO2/GO membranes show significant increase with the SiO2 loadings, as shown in Figure 5c. The SiO2/GO-0 shows an extremely low flux of merely 7 L m−2 h−1 for pure water, close to previous reports [10, 51]. This is caused by the restacking of GO nanosheets, which only results in the 2D nanochannels between individual sheets for water permeation. On the contrary, the SiO2/GO hybrid membranes show remarkably higher flux than SiO2/GO-0 membrane. The flux of the SiO2/GO-5 membrane is up to 2387 L m−2 h−1 for water and 470 L m−2 h−1 for soybean O/W emulsion, considerably higher than the previously reported GO-based membranes, as shown in Figure 5d, though it is much thicker (up to 8 µm) than the others (detailed comparison shown in Table S1). The low flux of the membrane for O/W emulsion should be due to the oil droplets with large size might block some permeate channels to limit the rapid permeation of water. According to Hagen-Poiseuille equation (1), the flux will enhance with the increase of pore radius and porosity [52]. J= (Ɛ·r2/8ƞ·ι·l) ·∆P

(1)

in which Ɛ is the porosity, r is the pore radius, ι is the tortuosity ratio of pore, ƞ is the solution viscosity, l is the thickness of membrane, and ∆P is the pressure drop. The high flux of the SiO2/GO hybrid membranes can be attributed to their hierarchical porous nanostructure, which not only preserves the nanochannels between the stacked GO nanosheets, but also provides extra larger pores 18

to shorten the water transport path, leading to enhanced water permeation. The hybrid membranes were further assessed for a range of O/W separation. Four kinds of SDS-emulsified O/W emulsions with the droplet sizes in the range of 160 nm-600 nm were used, including diesel oil/water, soybean oil/water, gas oil/water, and pump oil/water (Figure S7). All the obtained liquid after filtration by the SiO2/GO-2 membrane became very clear (Figure 6a), suggesting the high removal efficiency of different oil droplets from water. As shown in Figure 6b, the SiO2/GO-2 membrane exhibits high fluxes rates of 322-647 L m−2 h−1 as well as high rejection rates over 99.4% for the four types of investigated O/W emulsions. These O/W filtration results clearly show the outstanding separation performance of this hybrid membrane for a wide range of oil types and droplet sizes, even with the droplet size as low as 100 nm. Soybean O/W emulsion with the average droplet size of 320 nm was used to test the filtration performance of the SiO2/GO membranes with different SiO2 loadings for O/W emulsion separation (Figure 6c). The flux of the SiO2/GO membranes are in the range of 72-470 L m−2 h−1, gradually increasing with the higher SiO2 loading; while the oil rejections are all above 99.2%, slightly decreasing with the higher SiO2 loading. This can be attributed to the increased pore size and porosity with SiO2 loading to provide more efficient water path but slightly at the cost of the lower rejection rate.

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Figure 6. a) Digital photos of various oil/water emulsions before (left) and after (right) the separation using SiO2/GO-2 membrane; b) filtration performance of SiO2/GO-2 membrane for different oil/water emulsions; c) permeate fluxes and oil rejection rates for soybean oil/water emulsion of SiO2/GO membranes with different SiO2/GO mass ratios; and d) the reusability performance of SiO2/GO-2 for soybean oil/water emulsion.

The selective penetration of the SiO2/GO membranes should be attributed to membranes’ hydrophilic surface as well as size/aperture sieving effect. The hierarchical porous structure provides a wide variety of planar channels as size-sieving for fast permeation of water molecules while blocking those larger oil molecules [53]. The water molecules, due to the small size and the ability to establish a hydrogen bond with GO surface, are capable of going through the laminar pores rapidly [54]. In this case, the hydrophilic surface should be the key to achieving oil/water separation of the SiO2/GO membranes. The abundant functional groups on the GO surface, as well as the SiO2 nanoparticles can provide excellent hydrophilicity interface, render this membrane with a high 20

removal capability for blocking the soluble pollutants in molecular dimensions. This strong superoleophobicity could be comprehensively resulted from the strong electrostatic interaction and hydrogen bonding for water molecules of both the hydrophilic GO nanosheets. SiO2 particles, on the other hand, would help create pathway with large sizes than the 2D interlayer space between the GO sheets as well as a surface hydration layer, which allows the fast transport of water molecules through the channels but effectively prevents the membranes from being wetted by the oil droplets and to block the MB dye molecules [28]. It is worth noting that an excessive amount of SiO2 particles would possibly bring a change of “penetrating pore” through the film, which is unfavorable for maintaining the high separation efficiency of the film. As a result, the SiO2/GO membrane with well-balanced SiO2 contents would achieve both high selective penetration properties and mechanical strength, benefiting long-term practical applications. In addition, the long-term usability and durability of the GO-based membrane is important for its practical applications. The stability of the SiO2/GO-2 membrane’s O/W permeate fluxes and rejection rate upon cycle numbers was afterward evaluated using soybean oil/water emulsion as the typical pollutant. As shown in Figure 6d, throughout 8 cycles of oil/water fouling-washing tests, the O/W oil rejection rate was over 99.1%, showing the excellent antifouling properties even during long-term filtration process. The outstanding reusability and antifouling performance of the SiO2/GO-2 membrane mainly arise from its excellent underwater superoleophobicity and hydration capacity, which facilitate the formation of a hydration layer on the membrane surface during filtration to avoid oil adhesion and accelerate water permeation. Moreover, with the assistance of EDA, the rigid SiO2 nanoparticles are firmly anchored on the flexible GO nanosheets, which are fairly stable to even survive in the vigorous ultrasonication process during the sample preparation to 21

maintain a high recoverability for water purification.

Figure 7. a) MB dye rejection rate the SiO2/GO membranes tested with different volumes of MB solution; b) optic images of the difference between the SiO2/GO-2 membrane and SiO2/GO-0 membrane for MB removal.

Due to the unique laminated structure, which could act as a highly selective and permeable filtration membrane to adsorb and/or block the large organic compounds, our SiO2/GO hybrid membranes also have the potential for retrieving clean water from solutions contaminated by organic dye molecules with a high flux and rejection rate. The dye removal capability of the SiO2/GO membranes with various SiO2 loadings were measured using 10 ppm MB solutions of different volumes, and their rejection rates are almost 100% for 20 mL MB solution (Figure 7a). When MB solution volume is increased, the SiO2/GO-0.5, 1, 2 samples exhibit better dye removal capability than the other samples (SiO2/GO-3 and 5), and their rejection rates still kept 100% in the 60 mL MB solution test and over 96% in the 100 mL test. The slight decrease in MB rejection with volume may be due to the enhanced concentration gradient of MB on the surface of the membrane. Moreover, at various MB concentrations (10-40 ppm, fixed volume of 20 mL), the SiO2/GO-2 membrane still showed high rejection rates of 95-100% (Figure. S8). The MB removal capability of our SiO2/GO members is superior to that of previously reported GO-based membranes, such as SWCNT/GO 22

(98.6%) [55], D-HNTs/GO/EDA (99%) [10], GO-Isophorone Diisocyanate (97.6%) [30], RGO/PDA/g-C3N4 (97.5%) [33], PVDF/RGO@SiO2/PDA (99.8%) [41] and PDA/RGO/Halloysite nanotubes (99.72%) [22]. It is reasonable to believe that the adsorption and size sieving effect simultaneously play important roles for the filtration of organic compounds with big molecular weight or size (such as MB dye molecule) [56]. On the one hand, the interaction between GO and MB molecules initially enables the GO sheets to capture the MB molecules (i.e., the electrostatic interaction and π-π stacking interaction), which result in the tight binding between the MB molecules and the GO sheets. On the other hand, GO membrane acts as a highly selective sieving to block MB molecules due to the large size of MB molecules. Due to the high flux rate and high rejection capability of this SiO2/GO hybrid membrane, it can be directly used to obtain fresh water from the MB solution, only under the osmotic pressure without any external force. As shown in Figure 7b, the SiO2/GO-2 was fitted as the membrane in the U-shaped tube with 20 mL MB solution on one side while being empty the other side, and the SiO2/GO-0 was tested as for comparison. For the SiO2/GO-2 sample, clear water was obtained in the right tube from the left tube with the MB solution and an equal liquid level was achieved after 3 h, clearly showing its excellent dye separation property and high permeation performance. On the contrary for SiO2/GO-0, the liquid level is almost unchanged even after 7 days, due to the absence of hierarchical porous structures on this membrane.

4. Conclusion

In summary, multifunctional hierarchical SiO2/GO hybrid membranes with excellent separation property for O/W emulsion and dye solution are successfully prepared through a vacuum-assisted 23

filtration and self-assembly process. Introducing SiO2 nanoparticles and EDA into the membranes can form hierarchical nanostructures with enlarged water channels and excellent structural stability, resulting in a drastic increase in permeate fluxes up to 2387 L m−2 h−1 for water, 470 L m−2 h−1 for O/W emulsion and oil rejection as high as over 99.4% for oil/water emulsion. Moreover, owing to the combination of enhanced hydrophilicity and surface roughness from SiO2 nanoparticles, the SiO2/GO hybrid membranes possess the unique merits of underwater superoleophobic and low oil-adhesive water/membrane interfaces, enhancing their antifouling capability and long-term stability. More interestingly and importantly, the high adsorption capability of GO for dye molecules endows the hybrid membranes excellent MB removal capability. Therefore, the excellent separation efficiency and capability of the SiO2/GO membranes for oil/water emulsion and dye solution enlighten the great prospects of GO-based membranes in water treatment.

Acknowledgment We acknowledge the funding supports from National Pre-Research Foundation of China (No. 61409220117), Natural Science Foundation of Tianjin City (No. 19JCYBJC18200), the Science and Technology Service Network (STS) Project of the Chinese Academy of Sciences (No. KFJ-STS-QYZX-043) and Australian Research Council (ARC) through Discovery Early Career Researcher Award (DECRA, No. DE170100871) program. X. Yan would like to thank China Scholarship Council (No. 201804910018) for supporting his visit to University of Wollongong.

References [1] M. M. Pendergast, E. M. V. Hoek, A review of water treatment membrane nanotechnologies, Energ. Environ. Sci. 4(6) (2011) 1946-1971. 24

[2] M. M. Mekonnen, A. Y. Hoekstra, Four billion people facing severe water scarcity, Sci. Adv. 2(2) (2016) 1500323. [3] M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas, A. M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301. [4] J. Kim, B. Van der Bruggen, The use of nanoparticles in polymeric and ceramic membrane structures: Review of manufacturing procedures and performance improvement for water treatment, Environ. Pollut. 158(7) (2010) 2335-2349. [5] K. P. Lee, T. C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination-Development to date and future potential, J. Membrane Sci. 370(1) (2011) 1-22. [6] X. Liu, S. Wang, J. Miao, Y. Liu, X. Yan, S. Chen, Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment, Ceram. Int. 42 (2016) 15618-15622 [7] J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment, J. Membrane Sci. 479 (2015) 256-275. [8] S. G. Chaudhri, B. H. Rajai, P. S. Singh, Preparation of ultra-thin poly (vinyl alcohol) membranes supported on polysulfone hollow fiber and their application for production of pure water from seawater, Desalination 367 (2015) 272-284. [9] Q. Ma, H. Cheng, A. G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small 12(16) (2016) 2186-2202. [10] G. Zeng, Y. He, Z. Ye, X. Yang, X. Chen, J. Ma, F. Li, Novel halloysite nanotubes intercalated graphene oxide based composite membranes for multifunctional applications: oil/water separation and dyes removal, Ind. Eng. Chem. Res. 56(37) (2017) 10472-10481. 25

[11] R. R. Nair, H.A. Wu, P. N. Jayaram, I. V. Grigorieva, A. K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335(6067) (2012) 442. [12] R. K. Joshi, P. Carbone, F. C. Wang, V. G. Kravets, Y. Su, I. V. Grigorieva, H. A. Wu, A. K. Geim, R. R. Nair, Precise and ultrafast molecular sieving through graphene oxide membranes, Science 343(6172) (2014) 752. [13] L. Huang, M. Zhang, C. Li, G. Shi, Graphene-based membranes for molecular separation, J. Phys. Chem. Lett. 6(14) (2015) 2806-2815. [14] C. Xu, A. Cui, Y. Xu, X. Fu, Graphene oxide-TiO2 composite filtration membranes and their potential application for water purification, Carbon 62 (2013) 465-471. [15] S. Koushkbaghi, P. Jafari, J. Rabiei, M. Irani, M. Aliabadi, Fabrication of PET/PAN/GO/Fe3O4 nanofibrous membrane for the removal of Pb(II) and Cr(VI) ions, Chem. Eng. J. 301 (2016) 42-50. [16] L. Chen, N. Li, Z. Wen, L. Zhang, Q. Chen, L. Chen, P. Si, J. Feng, Y. Li, J. Lou, L. Ci, Graphene oxide based membrane intercalated by nanoparticles for high performance nanofiltration application, Chem. Eng. J. 347 (2018) 12-18. [17] L. Chen, Y. Li, L. Chen, N. Li, C. Dong, Q. Chen, B. Liu, Q. Ai, P. Si, J. Feng, L. Zhang, J. Suhr, J. Lou, L. Ci, A large-area free-standing graphene oxide multilayer membrane with high stability for nanofiltration applications, Chem. Eng. J. 345 (2018) 536-544. [18] Y. Wei, Y. Zhang, X. Gao, Z. Ma, X. Wang, C. Gao, Multilayered graphene oxide membranes for water treatment: A review, Carbon 139 (2018) 964-981. [19] E. Mahmoudi, L. Y. Ng, M. M. Ba-Abbad, A. W. Mohammad, Novel nanohybrid polysulfone membrane embedded with silver nanoparticles on graphene oxide nanoplates, Chem. Eng. J. 277 26

(2015) 1-10. [20] X. Zhao, Y. Su, Y. Liu, Y. Li, Z. Jiang, Free-standing graphene oxide-palygorskite nanohybrid membrane for oil/water separation, ACS Appl. Mater. Inter. 8(12) (2016) 8247-8256. [21] J. Mao, M. Ge, J. Huang, Y. Lai, C. Lin, K. Zhang, K. Meng, Y. Tang, Constructing multifunctional MOF@rGO hydro-/aerogels by the self-assembly process for customized water remediation, J. Mater. Chem. A 5(23) (2017) 11873-11881. [22] Y. Liu, W. Tu, M. Chen, L. Ma, B. Yang, Q. Liang, Y. Chen, A mussel-induced method to fabricate reduced graphene oxide/halloysite nanotubes membranes for multifunctional applications in water purification and oil/water separation, Chem. Eng. J. 336 (2018) 263-277. [23] L. Yu, Y. Zhang, B. Zhang, J. Liu, H. Zhang, C. Song, Preparation and characterization of HPEI-GO/PES ultrafiltration membrane with antifouling and antibacterial properties, J. Membrane Sci. 447 (2013) 452-462. [24] Y. Qian, C. Zhou, A. Huang, Cross-linking modification with diamine monomers to enhance desalination performance of graphene oxide membranes, Carbon 136 (2018) 28-37. [25] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct. Mater. 23(29) (2013) 3693-3700. [26] P. Sheath, M. Majumder, Flux accentuation and improved rejection in graphene-based filtration membranes produced by capillary-force-assisted self-assembly, Philos. T. R. Soc. A 374(2060) (2016) 20150028. [27] G. Li, X. Wang, L. Tao, Y. Li, K. Quan, Y. Wei, L. Chi, Q. Yuan, Cross-linked graphene membrane for high-performance organics separation of emulsions, J. Membrane Sci. 495 (2015) 439-444. 27

[28] Y. Zhang, S. Zhang, T.-S. Chung, Nanometric graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration, Environ. Sci. Technol. 49(16) (2015) 10235-10242. [29] E. Yang, C.-M. Kim, J.-h. Song, H. Ki, M.-H. Ham, I.S. Kim, Enhanced desalination performance of forward osmosis membranes based on reduced graphene oxide laminates coated with hydrophilic polydopamine, Carbon 117 (2017) 293-300. [30] P. Zhang, J.-L. Gong, G.-M. Zeng, C.-H. Deng, H.-C. Yang, H.-Y. Liu, S.-Y. Huan, Cross-linking to prepare composite graphene oxide-framework membranes with high-flux for dyes and heavy metal ions removal, Chem. Eng. J. 322 (2017) 657-666. [31] D. Qin, Z. Liu, H. Bai, D. D. Sun, X. Song, A new nano-engineered hierarchical membrane for concurrent removal of surfactant and oil from oil-in-water nanoemulsion, Sci. Rep. 6 (2016) 24365. [32] F. Li, R. Gao, T. Wu, Y. Li, Role of layered materials in emulsified oil/water separation and anti-fouling performance of modified cellulose acetate membranes with hierarchical structure, J. Membrane Sci. 543 (2017) 163-171. [33] F. Li, Z. Yu, H. Shi, Q. Yang, Q. Chen, Y. Pan, G. Zeng, L. Yan, A Mussel-inspired method to fabricate reduced graphene oxide/g-C3N4 composites membranes for catalytic decomposition and oil-in-water emulsion separation, Chem. Eng. J. 322 (2017) 33-45. [34] Y. Liu, Y. Su, J. Guan, J. Cao, R. Zhang, M. He, K. Gao, L. Zhou, Z. Jiang, 2D heterostructure membranes with sunlight-driven self-cleaning ability for highly efficient oil-water separation, Adv. Funct. Mater. 28(13) (2018). [35] Y. Boyjoo, M. Wang, V. K. Pareek, J. Liu, M. Jaroniec, Synthesis and applications of porous 28

non-silica metal oxide submicrospheres, Chem. Soc. Rev. 45(21) (2016) 6013-6047. [36] B. Liang, P. Zhang, J. Wang, J. Qu, L. Wang, X. Wang, C. Guan, K. Pan, Membranes with selective laminar nanochannels of modified reduced graphene oxide for water purification, Carbon 103 (2016) 94-100. [37] T. Yang, L. Wei, L. Jing, J. Liang, X. Zhang, M. Tang, M.J. Monteiro, Y.I. Chen, Y. Wang, S. Gu, D. Zhao, H. Yang, J. Liu, G.Q.M. Lu, Dumbbell-shaped bi-component mesoporous janus solid nanoparticles for biphasic interface catalysis, Angew. Chem. Int. Ed. Engl. 56(29) (2017) 8459-8463. [38] F. Zhang, D. Su, J. He, Z. Sang, Y. Liu, J. Cao, Y. Ma, R. Liu, X. Yan, Methyl modified SiO2 aerogel with tailored dual modal pore structure for adsorption of organic solvents, Mater. Lett. 238 (2019) 202-205. [39] M. Ikram, H. A. Qayyum, S. Ali, Z. Tan, M. Ahmad, X. Jin, Hydrothermal-hot press processed SiO2-rGO hybrid with enhanced physical properties, J. Solid State Chem. 265 (2018) 364-371. [40] J. Sun, H. Bi, S. Su, H. Jia, X. Xie, L. Sun, One-step preparation of GO/SiO2 membrane for highly efficient separation of oil-in-water emulsion, J. Membrane Sci. 553 (2018) 131-138. [41] Y. Peng, Z. Yu, F. Li, Q. Chen, D. Yin, X. Min, A novel reduced graphene oxide-based composite membrane prepared via a facile deposition method for multifunctional applications: oil/water separation and cationic dyes removal, Sep. Purif. Technol. 200 (2018) 130-140. [42] P. Miao, J. He, Z. Sang, F. Zhang, J. Guo, D. Su, X. Yan, X. Li, H. Ji, Hydrothermal growth of 3D graphene on nickel foam as a substrate of nickel-cobalt-sulfur for high-performance supercapacitors, J. Alloy. Compd. 732 (2018) 613-623. [43] K. Sinko, Influence of chemical conditions on the nanoporous structure of silicate aerogels, 29

Materials 3 (2010) 704-740. [44] J. He, H. Zhao, X. Li, D. Su, F. Zhang, H. Ji, R. Liu, Superelastic and superhydrophobic bacterial cellulose/silica aerogels with hierarchical cellular structure for oil absorption and recovery, J. Hazard. Mater. 346 (2018) 199-207. [45] E.O. Serqueira, R.F. de Morais, N.O. Dantas, Controlling the spectroscopic parameters of Er3+-doped sodium silicate glass by tuning the Er2O3 and Na2O concentrations, J. Alloy. Compd. 560 (2013) 200-207. [46] R. Subasri, H. Näfe, Phase evolution on heat treatment of sodium silicate water glass, J. Non-Cryst. Solids 354(10) (2008) 896-900. [47] F. Zhou, H. N. Tien, Q. Dong, W. L. Xu, H. Li, S. Li, M. Yu, Ultrathin, ethylenediamine-functionalized graphene oxide membranes on hollow fibers for CO2 capture, J. Membrane Sci. 573 (2019) 184-191. [48] N. Meng, W. Zhao, E. Shamsaei, G. Wang, X. Zeng, X. Lin, T. Xu, H. Wang, X. Zhang, A low-pressure GO nanofiltration membrane crosslinked via ethylenediamine, J. Membrane Sci. 548 (2018) 363-371. [49] O. C. Compton, D. A. Dikin, K. W. Putz, L. C. Brinson, S. T. Nguyen, Electrically conductive “alkylated” graphene paper via chemical reduction of amine-functionalized graphene oxide paper, Adv. Mater. 22(8) (2010) 892-896. [50] B. Ramezanzadeh, Z. Haeri, M. Ramezanzadeh, A facile route of making silica nanoparticles-covered graphene oxide nanohybrids (SiO2-GO); fabrication of SiO2-GO/epoxy composite coating with superior barrier and corrosion protection performance, Chem. Eng. J. 303 (2016) 511-528. 30

[51] L. Chen, J.-H. Moon, X. Ma, L. Zhang, Q. Chen, L. Chen, R. Peng, P. Si, J. Feng, Y. Li, J. Lou, L. Ci, High performance graphene oxide nanofiltration membrane prepared by electrospraying for wastewater purification, Carbon 130 (2018) 487-494. [52] X. Peng, J. Jin, Y. Nakamura, T. Ohno, I. Ichinose, Ultrafast permeation of water through protein-based membranes, Nat. Nanotechnol. 4 (2009) 353. [53] H. Li, Z. Song, X. Zhang, Y. Huang, S. Li, Y. Mao, H. J. Ploehn, Y. Bao, M. Yu, Ultrathin molecular-sieving graphene oxide membranes for selective hydrogen separation. Science 342 (2013) 6154. [54] M. Foroutan, H. Zahedi, E. Soleimani, Investigation of water-oil separation via graphene oxide membranes: A molecular dynamics study. Colloid. Surface. A 555 (2018) 201-208. [55] S. J. Gao, H. Qin, P. Liu, J. Jin, SWCNT-intercalated GO ultrathin films for ultrafast separation of molecules, J. Mater. Chem. A 3(12) (2015) 6649-6654. [56] H. Yan, X. Tao, Z. Yang, K. Li, H. Yang, A. Li, R. Cheng, Effects of the oxidation degree of graphene oxide on the adsorption of methylene blue, J. Hazard. Mater. 268 (2014) 191-198.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: