Surface roughness induced superhydrophobicity of graphene foam for oil-water separation

Accepted Manuscript Surface roughness induced superhydrophobicity of graphene foam for oil-water separation Sudong Yang, Lin Chen, Chunchun Wang, Masud Rana, Peng-Cheng Ma PII: DOI: Reference:

S0021-9797(17)30967-0 http://dx.doi.org/10.1016/j.jcis.2017.08.061 YJCIS 22708

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

31 May 2017 17 August 2017 18 August 2017

Please cite this article as: S. Yang, L. Chen, C. Wang, M. Rana, P-C. Ma, Surface roughness induced superhydrophobicity of graphene foam for oil-water separation, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.061

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Surface roughness induced superhydrophobicity of graphene foam for oil-water separation Sudong Yang 1, Lin Chen 2, Chunchun Wang 1, 3, Masud Rana 1, Peng-Cheng Ma 1, * 1

Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China

2

Xinjiang Uygur Autonomous Region Product Quality Supervision and Inspection Institute, Urumqi 830011, China

3

School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China1

Abstract Surface free energy and roughness are two predominant factors governing the hydrophilicity/hydrophobicity of materials. This paper reported the surface roughness induced hydrophobicity of graphene foam by incorporating silica nanoparticles onto graphene sheet via a sol-gel method and subsequent modification using silane. Various techniques were employed to characterize the morphology, composition and surface properties of sample. The results showed that the as-prepared graphene foam exhibited a superhydrophobic surface with a high water contact angle of 156o, as well as superoleophilicity with excellent adsorption capacities for a variety of oil compounds. Benefiting from the integration of enhancement on the surface roughness and reduction on the surface free energy of material, the graphene foam developed in this study had the capability to effectively separate oil-water mixture with excellent stability and recyclability. Keywords: Graphene; Superhydrophobicity; Surface roughness; Oil-water separation; Adsorption 1

* Corresponding author：E-mail: [email protected] (P.-C. Ma). 1

1. Introduction In recent years, oil spill and chemical leakage usually impose a series of problems on environmental system [1-3]. Recovery of oil compounds by porous materials possessing high adsorption capacity, high selectivity, excellent reusability and environmental friendliness, is a very promising method to address the spilled oil and chemicals, and has attracted great attention due to the high clean-up efficiency [4,5]. Experimental results revealed that the introduction of bio-inspired superhydrophobicity to adsorptive materials greatly enhanced the selectivity of material for oil adsorption [6,7]. The principle behind such observation was that superhydrophobic and superoleophilic materials allow the penetration of oil, and the inherent immiscibility between oil and water made the material repel to water, thus resulting in the effective separation of oil from the mixture consisting of oil and water [8-10]. Among various superhydrophobic adsorption materials, 3-dimensional (3-D) porous materials were considered as promising adsorbents due to their larger surface area, well-developed porous structures compared with their counterparts in the form of membrane [11,12], fiber and particle [13]. In previous studies [14-16], superhydrophobic 3-D materials were reported to be fast at selectively collecting various oil compounds and organic solvents from water surface, providing a facile approach for the cleanup of oil spills. Graphene foam (GF) is a 3-D macroscopic architecture derivated from graphene nanosheets via building block methodology [17,18]. One of the unique characters of such material is that the pore sizes of the GF are in the range of sub-micrometer to several micrometers, which endow the material ultra-light and high adsorption capacity to organic liquids [19-21]. The macroporous morphology ensures a sufficient contact area and free space for adsorption. Additionally, the inherently hydrophobic nature and freestanding states of graphene in GF make the material unique oil-adsorbing performance, which is attractive in the field of environmental remediation [22-24]. While much progress has been made for the preparation 2

of GF, very limited works were reported on the use of GF-based structures to control surface wettability. Most 3-D GF assemblies are based on the use of graphene oxide (GO) [18], which features both hydrophobic aromatic domains and hydrophilic ones carrying various kinds of oxygen functions. The chemical reduction process can not completely remove all oxygen functional groups in GO. The porous graphene-based monoliths adsorbed both oil and water at the same time, decreasing the separation selectivity and efficiency [22]. Therefore, surface modification/treatment are required to change the wettability of GO from the hydrophilic to the hydrophobic one before obtaining the final GF [25,26]. It is known that the hydrophobicity of material can be enhanced by surface roughness. Undoubtedly, generation of a surface structure with high roughness and low surface energy is a simple, feasible and cost-effective way to get oil sorbents with advantageous properties. In this paper, we employed a surface modification strategy to fabricate superhydrophobic and superoleophilic GF. The material was prepared by creating nanoscale surface roughness on the sheets of GF using silica as coating material and subsequent hydrophobic modification using an organic silane. Various techniques were employed to study the morphology and surface properties of the materials. The feasibility of using this material for the adsorption oily compounds and separation of oil-water mixture was investigated, and its advantages were demonstrated as well. 2. Experimental section 2.1 Materials Powder-like graphite (Purity 99.99%, Sinopharm Chemical Reagent Co., China) was used in this study. The chemicals, including tetraethylorthosilicate (TEOS, Aladdin Reagent, China), dodecyltrimethoxysilane (DTMS, Alfa Aesar, China), cetyltrimethyl-ammonium bromide (CTAB, Shanpu, China), hydrazine hydrate (Beijing Chemical Reagent, China), were analytical grade and used as received without further purification. Gasoline, diesel and bean 3

oil were purchased from a local market. The water used in all experiments was generated through a Millipore Water Purification System (18.2 MΩ cm). 2.2 Preparation of GF GO was synthesized from natural graphite powder using a modified Hummers method [27]. GF was prepared by thermal treatment of GO film. In a typical experiment, 60.0 mg GO was dispersed in 30.0 mL water by water-bath ultrasonication for 1 h. Then the mixture was filtered by a vacuum filter equipped with cellulose-ester membrane (50 mm in diameter, 0.2 µm pore size) to obtain GO film. The thickness of film can be controlled by the concentration and volume of solution. GO film was peeled off from the cellulose membrane by immersing the sample into acetone. The freestanding film was placed into an autoclave with presence of hydrazine hydrate (10.0 ml). GF was obtained by keeping the autoclave at 90 °C for 1 h. 2.3 Synthesis of superhydrophobic GF (SGF) GF-SiO2, the precursor for the preparation of SGF, was prepared at first by a sol-gel approach as follows: as-synthesized GF (30.0 mg) was suspended in a 40.0 mL aqueous solution containing CTAB (960.0 mg) and ethanol (8.0 mL). The pH of suspension was adjusted to 11.5 with ammonium hydroxide. After magnetic stirring for 6 h at 45 °C, TEOS (1.0 mL) was slowly dropped into the above mixture and reacted for 12 h. GF-SiO2 sample was obtained by washing with ethanol and water and drying at 50 °C for 12 h. SGF was prepared by putting the GF-SiO2 into an ethanol solution of DTMS (2 wt%) and hydrolyzed for 1 h. Finally, the sample was cured at 120 °C for 1 h. 2.4 Characterization The morphology of samples was observed with a field-emission scanning electron microscopy (FE-SEM, Zeiss Supra55VP, Germany) using an In-Lens detector operated at an accelerating voltage of 20 kV. Elemental analysis was carried out using energy dispersive spectroscopy (EDS) detector equipped on the SEM. The X-ray photoelectron spectroscopy 4

(XPS, Thermo Escalab 250xi, Perkin Elmer) was tested by using a Al Kα radiation exciting source. Contact angle was measured on a goniometer (XG-CAM Shanghai Xuanyichuangxi Industrial Equipment Co., Ltd., China) at ambient temperature, and the volumes of probing liquids in the measurements were approximately 5.0 μL. The roughness of sample surface was characterized by atomic force microscopy (AFM) under contact mode (Veeco Instruments, US). The optical images and movies were obtained by a digital camera (Canon, 600D). 2.5 Adsorption of oil compounds and separation for oil-water mixture The adsorption capacity (Q) of graphene materials for oil and various organic solvents was calculated according to the following equation:

Q

m1  m0 m0

(1)

where m1 and m0 were the weight of graphene materials before and after immersing into a target liquid for 20 minutes, respectively. For the separation of oil-water mixture, the as-prepared foam was fixed between a conical flask and a glass tube with a diameter of 20.0 mm. Oil-water mixture consisting of 1,2-dichloroethane (5.0 mL, colored by oil red O) and water (5.0 mL) was poured into the upper glass vessel. The separation was driven solely by the gravity, and the separated oil and water were collected from the conical flask and the glass vessel, respectively. 3. Results and discussion 3.1 Synthesis and morphology of material Figure 1 schematically shows the experimental setups for the preparation of SGF and subsequent application for oil-water separation. Firstly, GF with high macroporosity was produced by gaseous reduction in a hydrothermal system. Afterwards, the resulting GF was immersed into a solution containing CTAB, ethanol, NaOH and water. Within this step, the surfactant cation (CTA+) was expected to be electrostatically adsorbed and assembled on the 5

negatively charged surface of GF [28]. This was followed by the addition of TEOS into the above mixture. During this process, TEOS would slowly hydrolyze and result in silica deposition on the surface of GF. After washing with ethanol and water for several times to remove CTAB surfactant and following modification by hydrolyzation using DTMS, SGF was successfully constructed. Water

I、II III

Separation of oil/water Oil

GF SGF I: Absorption and assembly of CTAB Floating on liquid II: TEOS hydrolysis and CTAB removal III: DTMS modification Oil adsorption

Figure 1 Schematic showing the fabrication of superhydrophobic GF and its application for oil-water separation.

As above-mentioned in the experimental section, silica (SiO2) nanoparticles were obtained by a typical hydrolysis-polycondensation reaction of TEOS. The reaction can be represented by the following (Eq.(2)) [29] nSi(OC2H5)4 + 2nH2O → nSiO2 + 4n C2H5OH

(2)

The fresh hydrolyzed silica nanoparticles contained numerous hydroxyl (-OH) groups on their surface, which were reacted easily with DTMS, resulting in the conversion of -OH groups to the alkyl-ended ones (Si-C12H25) groups. The above reaction can lead to the formation of hydrophobic silica nanoparticles on the GF surface.

6

Figure 2 SEM images showing the morphology of GF samples at different processing steps. (a: Top-view, and b: Cross-section images of pure GF with with corresponding high magnification images; c: Top-view images of GF-SiO2 and d: Corresponding elemental mapping images of carbon, silicon, and oxygen in the selected area; e: Cross-section image of 7

GF-SiO2 and f: Corresponding EDS analysis; g: Top-view and h: Cross-section images of SGF; Insets are the corresponding high magnification SEM images).

The morphologies of GF, GF-SiO2 and SGF were examined by SEM (Figure 2). It can be seen that the original GF surface structure is smooth and flat (Figure 2a, 2b and the corresponding high magnification SEM inset image). Typical top-view SEM images over large area of GF-SiO2 have been shown in Figure 2c. Apparently, a lot of nanoparticles were uniformly distributed on the GF surface. The composition and structure of GF-SiO2 were further illustrated by element-mapping images of carbon, silicon, and oxygen (Figure 2d), demonstrating the uniform distributions of Si and O elements on its surface and successful preparation of GF-SiO2 materials. Figure 2e displays a cross-sectional SEM image of GF-SiO2. It can be seen that GF possess 3-D interconnected frameworks with randomly opened macroporous structure, and the pore walls in the foams are continuously cross-linked and not simply separated between different layers. Such continuously cross-linked structures can effectively overcome the restacking of graphene sheets. The release of gas during the chemical reduction process by hydrazine vapor would lead to the distribution of pores all over the layered film, eventually forming the porous GF [30]. The size of macropores was ranging from a few hundred nanometers to several microns. More interestingly, the SiO2 nanoparticles were uniformly grown on both sides of graphene between the layers in the GF, as shown in the inset image of Figure 2e. To further this, EDS analysis of the cross-section of GF-SiO2 was performed (Figure 2f). The results unravel the homogenous distribution of carbon, silicon, and oxygen in GF-SiO2, in good agreement with element-mapping analysis. Based on above results, we proposed a possible mechanism for the formation of GF-SiO2: partial reduced GF surface still contained numerous oxygen-contained groups with negative charges, when meeting CTA+, it was expected to be electrostatically adsorbed and assembled on the 8

negatively charged surface of GF. This facilitated the heterogeneous nucleation and growth of silica around the surface of the GF rather than homogeneous nucleation and growth in bulk solution [31]. In the process of the gradual hydrolysis of TEOS, the formed species of the deprotonated silanol groups interacted with cationic surfactants by electrostatic interaction [32]. Along with TEOS hydrolysis, the positive charge of the surfactant was balanced by deprotonated silicate species, generating a number of surfactant-silicate composites. During the self-assembly of surfactant-silicate composites and continuous condensation of silicate species in solution, the surfactant-silicate frameworks converted to the nanoparticles. As a result, SiO2 nanoparticles were uniformly deposited on both sides of the 3-D GF. After DTMS modification (Figure 2g and 2h), the morphology of SGF did not change much. The presence of nanoscaled SiO2 largely enhanced surface roughness of GF, which will be helpful for the formation of superhydrophibic due to the air trapped among nanoparticles. The synergic interplay of surface roughness and low surface free energy arising from silane contributes to the transition from the original GF into the coated SGF. In addition, even though the internal surface between different layers of the GF is covered by silica nanoparticles, the internal 3-D interconnected frameworks with randomly opened macroporous structure is not blocked by the silica nanoparticles, ensuring that the intrinsic adsorption capacity of GF was not sacrificed after modification.

3.2 Chemical composition of foam XPS measurement was used to further investigate the surface composition of foam. Only carbon, oxygen and nitrogen species were detected in GF, and signals representing silicon were noticed in GF-SiO2 and SGF samples (Figure 3a). After the deposition process and hydrophobic modification, new peaks with binding energies of 102.97 eV and 154.65 eV appeared in the sample, which were attributed to the signals arising from Si2p and Si2s. The elemental composition of different samples was summarized in Table 1. The nitrogen, 9

covering 3.19 % in the GF sample, originated from the N2H4 and/or ammonia during the hydrothermal reaction [33]. After the incorporation of silica nanoparticles on GF, the concentration of nitrogen element decreased to 1.4%, and new element of silicon was detected. GF-SiO2 contained 12.57 % of silicon originating from the SiO2, indicating that the SiO2 nanoparticles successfully deposited on surface of the GF. Additionally, after the hydrophobic DTMS modification process, the nitrogen content in the SGF sample decrease obviously, but the silicon content increase from 12.57 % to 17.31%. The result verify that the siloxane have been successfully modified on the surface of the as prepared SGF.

(a)

O1s

(b)

C1s SGF

Si2p Si2s

N1s

GF-SiO2

GF

(c)

Figure 3 XPS survey spectra of GF, GF-SiO2 and SGF (a), the higher resolution curves of Si2p area of GF-SiO2 (b) and SGF (c). The detailed deconvolution of Si2p spectra of different samples are shown in Figure 3b and 3c. For GF-SiO2 sample, only one peak at 102.8 eV ascribed to Si-O bonds was observed. 10

Deconvoluted Si2p scan of the SGF presents new peaks related to the Si-C covalent bonds centered at about 103.3 eV corresponding to Si-C bonding. The two peaks at different binding energies indicated that there were two chemical states of Si. These results indicated that the silane are chemically bound on the surface of GF-SiO2 through the hydrolysis/condensation reaction. DTMS has a low surface energy due to its high content of long alkyl groups. Accordingly, the combination of the low surface energy and the hierarchical roughness is expected to modify the surface properties of SGF.

Table 1 Atomic concentration of graphene-based materials. Sample

C

O

N

Si

GF

76.24

20.57

3.19

/

GF-SiO2

55.52

30.51

1.40

12.57

SGF

49.27

32.51

0.9

17.31

Figure 4 AFM images of pure GF (a) and SGF (b).

The wettability of a surface is dependent not only on its chemical composition, but also on the topography. To verify this, we used AFM to determine the average roughness of samples. Figure 4 shows the typical 3-D morphologies of GF and SGF materials. The surface of GF was locally smooth, and the average mean square roughness (Ra) was 14.1 nm within the area of 0.25 μm2. In sharp contrast, the surface of SGF exhibited many dispersed islands on sample 11

surface, and the majority of these islands were protruding in the upward direction (Figure 4b). The determined Ra was 23.5 nm, more than 60% higher than its counterpart within the same area investigated. The enhanced surface roughness of SGF is expected to improve the hydrophobicity of material, which will be discussed in the following section.

3.3 Wettability of foam

(a)

(b)

(c)

Figure 5 (a) Water droplets on the SGF; (b) The immersion of SGF in water by an external force; (c) Water contact angle of different materials.

In order to reveal the hydrophobic and lipophilic effect of material, the wettability of water and oil on the surface of SGF was studied, and the results are shown in Figure 5. It can be seen that all water droplets are ball-shaped (Figure 5a), similar to those on the surface of a lotus leaf [34], and the water droplets can easily roll on the SGF surface (Movie S1 in the ESI†), suggesting the hydrophobic property of material. Additionally, a bright and reflective surface were observed when the material was immersed in the water by external force (Figure 5b), which was a signature of trapped air and the establishment of a composite solid-liquid-air interface, and the corresponding wetting regime was in the Cassie-Baxter state [35]. Most of the area beneath water was the liquid/air interface and the ratio of liquid/solid interface was quite small. After the SGF was taken out of water, its surface remained completely dry, exhibiting excellent repellency to water. 12

The role of SiO2 nanoparticles in generating hydrophobic surface was confirmed by comparing the contact angle of material against water. The original GF exhibits surface hydrophobicity with a contact angle (θ) of 78o. After modified with SiO2 nanoparticles, the surface changed from hydrophobic to hydrophilic with θwater around 0o. This was due to the presence of hydrophilic -OH groups on the surface of SiO2 particles. For comparison, the GF modified with DTMS (without SiO2 particles) showed a hydrophobicity with θwater=105o, but not in the superhydrophobic state (θwater>150o). The SGF exhibited θwater=158o, which is much higher than original GF, indicating that the treatment effectively improves the hydrophobic property of GF. The above results indicated that the hydrophobicity of the SGF was enhanced due to the introduction of SiO2 nanoparticles into GF, which could increases the surface roughness, as confirmed by SEM and AFM results. The particulate morphology of silica provided roughness at the nanoscale to complement the microscale roughness inherent in the GF, and such hierarchically micro- and nanoscale roughness of played an important role to obtain the superhydrophobicity in the SGF. To further illustrate the wetting behavior of water and oil on the foam, we have recorded the contact process between the substrates and water and oil droplets, and the results are shown in Figure 6. Firstly, the water-repellent behavior of SGF was examined. The water droplet (2 μL) contacted on the surface can be hardly pushed into the sample even when the droplet was squeezed (Figure 6a), suggesting excellent hydrophobicity of SGF with low water adhesion. Interestingly, when oily liquid contacted with SGF, continuous spread and penetration of droplet was noticed on the sample, making it impossible to get the reliable value on contact angles. Such excellent oleophilic performance was represented by a series of consecutive photographs using two organic solvents with different viscosities (η), octane (η=0.50 cP) and oleic acid (η=26.0 cP). As shown in Figure 6b, an octane droplet can fully permeate into the porous structure of SGF within 0.10 s, and a highly viscous oleic acid droplet can be fully 13

sucked into the pores rapidly as well within 3 s (Figure 6c), demonstrating the superoleophilicity of SGF material. This is because the surface tension of oil is commonly much smaller than that of water. When the surface tension of the solid substrate lies between those of water and oil, hydrophobicity and oleophilicity can be realized. Apparently the adsorption rates were associated to the viscosity of organic liquids, and a lower viscosity of adsorbate led to a faster adsorption. The unique wettability of SGF towards water and oil indicated the great potential application of material for oil/water separation [36].

(a)

(b)

(c)

0.10 s

0.05 s

0.10 s

1.0 s

3.0 s

Figure 6 Wetting behavior of water (a), octane (b), and oleic acid (c) droplets on SGF surface.

3.4 Adsorption behavior of foam to organic liquids As mentioned, the modified GF with 3-D porous structure showed simultaneous superhydrophobicity and superoleophilicity, which made it very promising as the material for the removal of organic pollutants such as oils and solvents from water. Two organic solvents with different densities, hexadecane and chloroform, were chosen as model adsorbates to 14

investigate the oil/water separation performance of SGF. Hexadecane dyed with oil red was dropped on water surface to form a thin layer, and then a piece of SGF was introduced to contact with hexadecane. The organic liquid was fully adsorbed within a few seconds, leaving a transparent region on water surface (Figure 7a). Such fast absorption was attributed to the combination of high porosity, oleophilic nature, and capillary action in the material. In addition, the adsorption of chloroform from water was carried out to verify the use of the foam with organic chemicals that had higher densities than water. When the SGF was inserted into water to approach chloroform, the droplet was immediately sucked up by the SGF (Figure 7b). Most of the chloroform droplets could be removed by taking out the adsorbed SGF from water.

(a)

(b)

(c)

(d)

Figure 7 Oil adsorption performance of different foams (a and b: Illustration showing the adsorption process of SGF for hexadecane and chloroform; c: Adsorption capacity of GF and SGF for various organic liquids; d: Adsorption capacity of SGF for organic liquids in different aqueous solutions). 15

The high porosity and superhydrophobicity make the SGF a perfect candidate for the quick removal of various oils and organic solvents, and the material exhibited excellent adsorption capacities towards a wide range of oils and organic solvents, ranging from 15.0 to 31.0 times of its own weight depending on the density of testing liquid (Figure 7c). It was worth to mention that the adsorption capacity of SGF for a specific liquid was far superior to that of GF (11-22 times). For example, the adsorption capacity of SGF for motor oil was 31.0 g/g, which was 55% higher than its counterpart with an adsorption capacity of 20.0 g/g. The mechanism for the high capacity and selectivity of SGF was ascribed to the following reasons: Oily liquid was driven through the open pores of the foam into its bulk in the presence of a capillary force, while water was completely excluded by the superhydrophobic surface, resulting in separating oils from water with a high efficiency. The capillary flow was further reinforced when the oil molecules spread into the inner pores of foam due to the superoleophilic siloxane decorated on the interconnected skeleton [37]. Consequently the oil was stored in the pores formed by the interconnected skeleton of the graphene foam, exhibiting a high oil adsorption capacity [38]. The versatility of SGF was further demonstrated by measuring the oil uptake under the acidic and basic aqueous conditions. Figure 7d summarizes the adsorption capacity of SGF for three typical organic liquids (motor oil, toluene and octane) in different pH solutions. Marginal changes were found for the performance of sample under the extreme conditions, suggesting the excellent stability of SGF against acid and alkali.

16

(a)

(b)

(c)

Figure 8 a: Oil/water separation studies of the SGF. For clarity, the probe liquid, 1,2-dichloroethane was dyed by Oil Red O; b: WCAs recorded for the SGF after each oil/water separation process; c: Relationship between pH and the WCAs of SGF.

3.5 Capability of foam for oil-water separation Considering the practical application in industry, it is of vital importance to investigate the separation capability and stability of the as-prepared material with selective wettability for oil and water. Thus, the proof-of-concept study was carried out to verify the solely gravity driven oil-water separation capability of material by employing SGF as a membrane. As shown in Figure 8a, the sample was fixed between the glass funnel and conical flask, and as-prepared mixture was poured onto the sample surface. Oil dyed by Oil Red O quickly permeated through the membrane and dropped into the conical flask due to the superoleophilicity. Meanwhile, water was repelled and retained above the membrane due to the superhydrophobicity and low water-adhesion property of material. No external force was 17

employed during the separation process, and only gravity was responsible for such behavior, indicating the ease of operation and low energy consumption by employing the material developed in this study. Additionally, the water content in the collected oil and oil content in the collected water was not detectable, suggesting a very high separation efficiency of SGF and a stable separation capacity. Moreover, Figure 8b recorded the water contact angles (WCAs) of the membrane after each separation process, it was clear that the material exhibited a WCA higher than 150o after separating oil-water mixture for 10 cycles, demonstrating the excellent recyclability and anti-fouling property of material. In addition, SGF can be retrieved after washing with ethanol and drying in oven, and showed good recyclability of adsorption/desorption. In addition, the stability in acidic and alkaline environments of the practical oil-water separation was investigated as well, and the membrane exhibited robust superhydrophobicity towards water with a broad range of pH from 1 to 13 (Figure 8c). The excellent stability with WCAs higher than 150o indicated the material was immune to the corrosive and harsh conditions. These results suggested that the as prepared SGF membrane was a promising candidate for industrial oil-contaminated water treatment and marine spilt oil cleanup, especially from a practical point of view.

4. Conclusions In summary, GF with superhydrophobic and oleophilic properties was prepared by incorporating silica nanoparticles onto graphene sheet via sol-gel method and subsequent hydrophobic modification using hydrolyzed organic silane. The nanoscale SiO2 nanoparticles enhanced the surface roughness of the foam, resulting in the prepared foam a high water contact angle of 156° and low water adhesion property. Significantly, the superhydrophobic foam can adsorb a broad variety of oil liquids with enormous adsorption capacities and excellent recyclability, and can be effectively applied to separate oil-water mixture. We 18

expected that the material developed in this study can be used as a promising candidate for the purification of industrial oil-contaminated water and cleanup of oil spills in marine system.

Acknowledgements This project was supported by the National Natural Science Foundation of China (Project No. 51302308, 11472294), the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Project No. qn2015bs020, qn2015bs028), the Urumqi Science and Technology Plan (Project No. P141010006).

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Water

I、II III

Separation of oil/water Oil

GF SGF I: Absorption and assembly of CTAB Floating on liquid II: TEOS hydrolysis and CTAB removal III: DTMS modification Oil adsorption

24