A Vapor-Liquid Deposition Strategy to Prepare Superhydrophobic and

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A Vapor-Liquid Deposition Strategy to Prepare Superhydrophobic and Superoleophilic Graphene Aerogel for Oil-Water Separation Sudong Yang, Chengmin Shen, Lin Chen, Chunchun Wang, Masud Rana, and Peng Lv ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00027 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Nano Materials

A Vapor-Liquid Deposition Strategy to Prepare Superhydrophobic and Superoleophilic Graphene Aerogel for Oil-Water Separation Sudong Yang,†* Chengmin Shen §, Lin Chen ‡, Chunchun Wang †, Masud Rana † and Peng Lv † †

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, P. R. China §

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, P. R. China ‡

Xinjiang Uygur Autonomous Region Product Quality Supervision and Inspection Institute,

Urumqi 830011, P. R. China

KEYWORDS: graphene aerogel, superhydrophobicity, superoleophilicity, wettability, oil-water separation

ABSTRACT: The problems of high temperature and volume shrinkage are the frequently encountered in the process of fabricating superhydrophobic graphene aerogels. We present ultralight graphene aerogels with superhydrophobicity and superoleophilicity by vapor-liquid

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deposition process. The graphene aerogels are prepared by graphene oxide assembled into monolithic graphene hydrogel in dopamine-mediated aqueous solution at mild temperature and sealed reaction, then by directional-freezing and freeze-drying. Subsequent fluorinated alkyl silane is introduced to produce ultralight graphene aerogel by a vapor-liquid deposition treatments, the surface of aerogel has outstanding superhydrophobicity and superoleophilicity with the water and oil contact angles of 160.2° and 0°, respectively. In addition, the resulting graphene aerogels showed super low density, large specific surface area, excellent adsorption capacities, and superior adsorption recyclability. Its adsorption capacity was higher than 109 g/g for the used common oils and organic solvents. Its properties suggest applications in the field of oil-adsorption and oil-water separation. The new strategy may open up a new way to design high-performance superhydrophobic 3D graphene monoliths for water treatment. INTRODUCTION

Graphene aerogels (GAs) not only keep the advantage of the unique structural of graphene sheets, but also possessed ultra-low density, high porosity, excellent mechanical strength, and extraordinary adsorption properties.1-13 Generally, the synthesis of GAs usually used graphene oxide (GO) as precursor due to the GO with simplicity and high availability.14,15 A variety of methods including self-assembly approaches,6,16-20 template-directed processes,21-23 and other methods24-26 have been used to fabricate GAs. Compared with the other methods, self-assembly approache is the frequently used strategy. It should be noted that, even if the assembly process couple with the reduction of GO, many functional groups are still on the partial reduced graphene surface.16,17 These porous graphene-based monoliths adsorbed water and oil at the same time, reduce the selectivity and efficiency of separation.27,28 So the superhydrophobic graphene-based monoliths should be an ideal choice to adsorb oil from water and completely


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repel water. In order to improve the hydrophobic performance, recently there are two main method including rough surface structures and low surface energy material functionalization.29 In view of the fact that the pore sizes of graphene foams are comparable to the water droplets size, the wetting behavior of graphene foam can be described using the Marmur equation.30 This equation means that an increase in roughness Rf or hydrophobicity θ0 of foam walls will be beneficial to realization of superhydrophobicity of graphene foams. So far, however, as the following introduction that only a few superhydrophobic graphene-based materials have been reported.31-35 For example, to enhance the roughness, superhydrophobic graphene/carbon nanotube foams were prepared by introducing two-step chemical vapor deposition (CVD) grown carbon nanotubes on the graphene surface, and revealed good oil-water separation efficiency.31 In addition, spongy graphene modified by the soot with superhydrophobicity and high adsorption capacity was prepared.32 To improve hydrophobicity θ0, Cheng and Koratkar have proved the validity of the coating Teflon (θ0=104°) onto the pore wall of graphene foams, and the resulted foam showed water contact angles of ～163° and ～143° for advancing and receding fronts.33 The approach of these superhydrophobic graphene-based monoliths required high energy consumption due to CVD or thermal treatments for carbonization. As a result, the preparation of superhydrophobic neat GAs through a general chemical method is still an urgent requirement. Herein, we present ultralight GAs with superhydrophobicity and superoleophilicity. In order to suppress the shrinkage of graphene sheets during functionalization in contrast to solution-based method, a strategy of vapor-liquid deposition treatment was designed. The GAs are prepared by GO assembled into monolithic functionalized graphene hydrogel in dopamine-mediated aqueous solution with a mild condition, then by directional-freezing and freeze-drying. Subsequent


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fluorinated alkyl silane is introduced to produce ultralight graphene aerogel with excellent superhydrophobicity and superoleophilicity. This study of superhydrophobic graphene aerogels (SGAs) has the following advantages compared with the previously reported GAs. (1) Vaporliquid deposition method for SGAs, unlike the solution processes and high-temperature treatment approaches, it can keep the shape and structure and does not result in a morphological shrinkage after processing. (2) The SGAs have ultra-low density, excellent superhydrophobicity and superoleophilicity, and high adsorption capacity for oils and organic solvents. (3) The fabrication method of SGAs is simple, low-cost, and easy for large-scale preparation compared to hydrothermal or solvothermal methods and carbonization process. The findings illustrate a new strategy to fabricating superhydrophobic 3D graphene materials for oil-water separation. EXPERIMENTAL SECTION

Materials. Powder-like graphite was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and ascorbic acid was purchased from Aladdin Reagent Co., Ltd. (China). Dopamine hydrochloride and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOES) were purchased from Sigma-Aldrich. Preparation of SGAs. GO used in this work was prepared from natural graphite by a modified Hummers’ method.36 In a typical procedure for preparing the graphene hydrogel, GO aqueous solution with an appropriate amount was dispersed in water and sonicated for 2 h for dilute and thoroughly strip the GO sheets. Then dopamine (5 mg) was added into GO dispersion with vigorous stirring for 10 min to form a uniform solution. L-ascorbic acid (15 mg) was added into the mixture with vigorous magnetic stirring until completely dissolving. Subsequently, the mixture was sealed in a glass vessel and heated at 95 °C for 10 h to transform the brown aqueous solution into a black graphene hydrogel. Then the hydrogel was placed on a metal plate, which in


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turn rested in a pool of liquid nitrogen after it was dialysis in water to remove soluble species. The hydrogel was totally frozen by directional freezing from the metal/hydrogel interface to the top surface. Finally, the aerogel was obtained from the frozen hydrogel by freeze drying. The dry aerogel was placed in a glass vessel filled with an ethanolic solution of (PFOES) (2 wt.%) with no direct contact between the liquid and the aerogel. Subsequently, the sealed glass vessel was heated at 70 °C for 8 h. The SGAs resulted after thorough drying in air. Characterization. The morphology of the materials was observed with field-emission scanning electron microscopy (FE-SEM, Zeiss Supra55VP). Elemental analysis was carried out by energy dispersive spectroscopy (EDS), and the EDS spectra were obtained through a Zeiss Supra55VP SEM instrument equipped with an EDS detector. Thermal stability of materials was characterized on a thermogravimetric analyzer (TGA, STA 449F3, Netzsch) with a heating rate of 10 °C/min from 30 to 800 °C in nitrogen condition. Functional groups in sample were analyzed through a Fourier-transform infrared spectrometer (FT-IR, NICOLET 5700). The surface area and pore size distribution of materials were obtained by using the nitrogen adsorption-desorption isotherms on a surface area analyzer (AutosorbiQ-MP, Quantachrome). Surface functionalities and elemental composition of samples were determined by an X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250xi, Perkin Elmer). The contact angles (CAs) were measured on five positions for each sample by XG-CAM contact angle meter (Shanghai Xuanyichuangxi Industrial Equipment Co., Ltd., China) at ambient temperature, the volume of measured liquids was about 3 µL. The optical images and movies were obtained using digital SLR camera (Canon, 600D). The compressive properties of aerogel were determined on a universal testing machine (C43-104, MTS) with a constant cross-head speed of 0.5 mm/min. Bulk density of the solid sample was evaluated from the physical dimension and weight of each


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sample. Oil adsorption experiments. The weight measurements before and after oil-adsorption were used to evaluate the adsorption capacity of SGAs for different kinds of oils or organic solvents. The original weight of the sample was weighed and recorded as Mi. Then the sample was placed into various oils and organic solvents for adsorption. The sample was weighed when with the increase of adsorption time its weight unchanged, this weight was recorded as Mt. The adsorption capacity of materials (Q) for oil and various organic solvents was calculated according to the following equation:

Q=

Mt − Mi Mi

To regenerate the SGAs, the sample was placed into organic solvents to remove it and heated to evaporate the adsorbed organic solvents. Finally, the dry SGAs can be used in the next adsorption-drying process. RESULTS AND DISCUSSION The SGAs were fabricated by vapor-liquid deposition method. The synthetic routes of SGAs are illustrated in Figure 1, including (1) GO assembling into monolithic graphene hydrogel, (2) freezing and freeze drying, and (3) PFOES vapor-liquid deposition and surface modification. By this method, the L-ascorbic acid serves as reductant and dopamine serves as modifier, GO assembled into monolithic graphene hydrogel at mild temperature and sealed reaction in step (1). Dopamine could form crosslinking points with the GO sheets and eventually formed polydopamine thin through self-polymerization, which reduced overlapping of the GO sheets and decreased volume shrinkage of hydrogel.37,38 As a result, a combination of L-ascorbic acid and dopamine during the preparation process endowed the hydrogel with small volume shrinkage.


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Then the graphene hydrogel was by directional-freezing and freeze-drying in step (2) to produce GAs. Finally, the low-energy PFOES layer coat on the GAs to obtain SGAs with intrinsically chemical hydrophobicity in step (3). After being placed into the mixture of the PFOES and ethanol solution, with abundant catechol of polydopamine thin and residual hydroxyl groups by the incomplete reduction of GO, the hydroxyl groups in GAs serves as an active surface for reaction with the alkoxysilane group of PFOES to create perfluorinated SGAs. Compared with solution-processed coating methods, vapor-liquid deposition avoids the unmanageable solvent removal process, and as a result a more stable shape and structure can be maintained. Importantly, different from the frequently used GAs prepared high-temperature treatment approaches,37 the resultant SGAs exhibited negligible volume shrinkage after assembly, in contrast with the initial monolithic GAs.

Figure 1. The schematic illustration of fabrication process and digital image of SGAs.


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The obtained SGAs possess an ultralow density of 4.8±0.3 mg/cm3, comparable to that made so far in all reports of 3D graphene structures with superhydrophobicity by chemical vapor deposition, such as graphene foam (～5 mg/cm3),33 and much lighter than graphene aerogel (~10 mg/cm3).34 The digital image of the cylindrical SGAs is shown in Figure 1. As can be seen that a piece of 2.4 cm3 SGAs material can stably stand on the top of a dandelion without destroying its fluffs at all (Figure 1), indicating that the SGAs is very light. In general, it is considered an ultralight material when the density of the material is less than 10 mg/cm3,39 so the results show that the obtained SGAs is ultra-light. Morphology and structural characterization of SGAs The superhydrophobic performance of the obtained SGAs was caused by both the surface morphological structure and the chemical composition. Figure 2a and 2b showed SEM images of the surface of GAs in different magnifications. The SGAs exhibited 3D interconnected hierarchical porous network structure and the pore sizes distribute in a random order in the scope of submicrometers to ten micrometers. The porous network structure of GAs was fabricated by the crumpled and curly graphene sheets. However, the surface of the graphene was smooth at the magnified scale. Energy dispersive spectroscopy (EDS) analysis had been performed to examine the chemical composition of GAs. As shown in Figure 2c and 2d, only peaks of C, O, and N were detected on the GAs, and no other impurities could be observed. After the in situ vaporliquid deposition process in step (3), on the one hand, the SGAs kept the porous structure with interconnected frameworks (Figure 2e), which lead to formation of the microscale roughness. On the other hand, the graphene surface formed a compact coating with random distribution of many nanoscale granules (Figure 2f), which showed that the accomplishment of covalent interaction on the surface of GAs. The nanoparticle morphology provided the nanoscale roughness to


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complement the microscale roughness inherent in the graphene aerogel. So the surface microscale

and

nanoscale

roughness

were

an

essential

necessity

to

realize

the

superhydrophobicity of SGAs. The chemical composition of SGAs was illustrated by elementmapping images, on the SGAs (Figure 2g), the existence and uniform distribution of the silicon and fluorine can be observed besides carbon, oxygen, and nitrogen. As shown in the overlay image, the signal of C, O, N, F and Si elements coexist over the surface of SGAs. These results show that F and Si element is successful introduction overall the surface of SGAs. The 3D porous and hydrophobic structures of SGAs are highly desirable for oil and organic solvent adsorption.


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(a)

(b)

(d)

(c)

(e)

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(f)

(g)

Figure 2. SEM images and elemental mapping images of GAs and SGAs. (a) low-, (b) highmagnification SEM images of GAs, (c) corresponding elemental mapping images of carbon, oxygen, and nitrogen in the selected area, and (d) corresponding EDS analysis; (e) low-, (f) highmagnification SEM images of SGAs, and (g) corresponding elemental mapping images of carbon, oxygen, nitrogen, fluorine and silicon in the selected area.


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(a)

(b)

Figure 3. (a) Nitrogen adsorption-desorption isotherm and (b) BJH pore size distribution of SGAs. The porous property of SGAs was further investigated through a nitrogen adsorption-desorption experiment. It was found that the typical type-IV isotherm of SGAs with an obviously hysteresis loop, showing that plenty of mesopores was existed in SGAs (Figure 3).40 The surface area of SGAs was 111 m2/g through fitted the isotherms with the Brunauer-Emmett-Teller (BET) model, which was comparable to that of previously reported graphene aerogel.34 In addition, as shown in the pore size distribution curve with Barret-Joyner-Halenda method, it was found that the pores of SGAs with a diameter of 2-80 nm. This kind of broad size distribution indicated that the hierarchical pore structure of SGAs was obtained. Despite the SGAs have high porosity and lowdensity property, it provides good mechanical stability. Compression rebound tests in the axial direction of the SGAs for 5 cycles reveal an excellent resilience when released from compression. The maximum stress decreases from 2.2 to 1.9 kPa (Figure S1) due to the irreversible damage of the cell walls in the first circle prevents it return to its original height. To further verify the formation mechanism of the superhydrophobic surface on GAs, FT-IR and XPS measurements were used to compare the chemical composition of different materials. The surface chemical composition of the GO, GAs, and SGAs was investigated by using XPS


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measurement. The changes in the XPS spectra of GAs compared to GO show a new nitrogen component was detected besides the carbon and oxygen species (Figure 4a). The new peak of N1s appear in GAs should caused by polydopamine on the graphene hydrogel during the process of preparing. After the deposition process, the XPS spectrum of SGAs (Figure 4a) had silicon and fluorine component originating from GAs, but there were no Si2p and F1s peaks for the pristine GAs, indicating that the covalent functionalization of GAs by PFOES successfully occurred. This result was consistent with the results of the elemental mapping. As shown in Figure 4b, c, and d, the detailed deconvolutions of the C1s spectra for GO, GAs, and SGAs were investigated. For GO (Figure 4b), there was a strong peak at high binding energy for the heavily oxygenated carbon species, which was close to those of the previous GO.36 After reducing with ascorbic acid (Figure 4c), the intensity of the peaks was decreased obviously and the peak related to C=C/C-C (∼284.6 eV) becomes more dominant. The atomic ratios of carbon, oxygen, nitrogen, silicon, and fluorine measured by XPS in different materials are shown in Table 1. The initial oxygen content of GO is very high (33.3%). After reduction, the oxygen content decreases gradually, but the content of carbon increases gradually to 81.9%. This result shows that the most oxygen functional groups have been removed. Furthermore, the new peaks corresponding to CNH2 and C-NH- (primary amine and secondary amine) appeared, indicating the reaction of GO with dopamine (Figure 4c).41 The XPS spectra further confirmed the presence of amine groups after polydopamine-mediated assembly. In addition, deconvolutions of the C1s spectrum for SGAs appears new peaks associated with the C-F covalent bonds at about 291.9 eV corresponding to C-F bonding (Figure 4d).42,43 These results indicated fluorine silane are chemically bound on the SGAs surface by the hydrolysis/condensation reaction.


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(a)

(b)

GO

GAs SGAs

(c)

(d)

Figure 4. The XPS results. (a) XPS wide-scan spectra of the GO, GAs and SGAs; C1s high resolution spectra of the (b) GO, (c) GAs and (d) SGAs. Table 1. Atomic ratio of GO, GAs and SGAs Sample

C (%)

O (%)

N (%)

Si (%)

F (%)

GO

66.7

33.3

/

/

/

GAs

81.9

16.3

1.8

/

/

SGAs

77.0

17.5

1.9

1.0

2.6


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(a)

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(b)

Figure 5. (a) FT-IR spectra and (b) TGA curves of GO, GAs and SGAs. The FT-IR spectra of GO, GAs and SGAs were shown in Figure 5a. The typical peaks of GO appeared at 1076 cm-1 (C-O-C in epoxide), 1394 cm-1 (C-OH stretching), and 1701 cm-1 (C=O carboxyl stretching vibration). The characteristic bands of the GAs at 1076 cm-1 and 1701 cm-1 almost disappeared after reduction process. In addition, in the spectrum of the GAs, the peak at 1486 cm-1 (N-H of amide group shearing vibration) and 1601 cm-1 (N-H in-plane stretching vibration) appeared, showing the presence of amine groups after polydopamine-mediated assembly. The changes in the FT-IR spectra of GAs compared to GO show that the oxygencontaining groups of GO surface were partially reduced and dopamine was oxidized.37 After surface functionalization of PFOES, new bands at 1105, 1208, and 1384 cm-1 were rocking vibration peaks of C-F bond, and two bands at 2853 and 2927 cm-1 appeared, indicating the stretching of the -CH2 groups from the alkyl chains assigning to silane moieties of PFOESGAs.44 Besides, the bands at 1039 and 996 cm-1 were attributed to the Si-O-Si and Si-O-C bonds, indicating the successful chemical functionalization of GAs by PFOES.45 The TGA curves of GO, GAs and SGAs are shown in Figure 5b. There were three major steps of weight loss for GO TGA curve: below 100 °C, 100-210 °C, 210-800 °C. The weight loss below 100 °C and during 100-210 °C was derived from the desorption of physically adsorbed water (～


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17% weight loss) and decomposition of unstable oxygen-containing groups (～40% weight loss).46 While, the third step of weight loss between 210-800 °C was attributed to desorption of some more stable oxygen function groups. Compared with GO, it was found that GAs reduced by vitamin C is relatively stable and lower mass loss during 30-800 °C because number of oxygen functional groups in GO were removed and some nitrogen functional groups were introduced during the reduction process. After modification with PFOES, the curve of modified SGAs indicates less mass loss than GAs below100 °C, because of the hydrophobic surface is formed after modification with PFOES and making less moisture entrapped inside the modified SGAs. Then the weight loss between 100 to 800 °C was attributed the decomposition of carbon species on the modified SGAs surface.47 In addition, it was found that the residual weight of SGAs was less than that of GAs, which was attributed to the decomposition of PFOES coated on SGAs surface at high temperature. From the above discussion, we could draw a conclusion that the surface of SGAs was modified with PFOES successfully. According to the above results, the formation mechanism of the superhydrophobic surface by vapor-liquid deposition process can be deduced as follows: firstly, there were retained hydroxyl groups and abundant catechol of polydopamine on the surface of GAs. When, the GAs were placed into the mixture of the PFOES and ethanol solution, accompanying with the covalent interaction between hydroxyl groups on GAs surfaces and PFOES, the granules nanostructure was formed, and the surface on GAs became more compact, showing the remarkable increase of the surface roughness. In addtion, compared to the silane coupling agents with long alkyl chain, the perfluoroalkyl groups are superior to the alkyl groups in decreasing the surface energy due to their larger size, which dilutes their physical interactions with water. Therefore, the combination of the hierarchical roughness and low surface energy make the graphene based aerogel has stable


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superhydrophobicity. Vapor-liquid deposition method for GAs, unlike the solution processes and high-temperature treatment approaches, on the one hand, it can keep the shape and structure, on the other hand, it does not result in a morphological shrinkage of the GAs. Properties of SGAs (a)

(b)

Oil

Water (e)

(c)

(f)

(d)

Water (g)

Oil (h)

Figure 6. Wetting behavior of water and oil droplets. (a) The water contact angle and (b) oil contact angle of the GAs; (c) The water contact angle and (d) oil contact angle of the SGAs; (e) Water droplets on the SGAs; (f) A jet of water bouncing off the SGAs surface; (g) The SGAs immersed in water by an external force; (h) Photograph of the SGAs after placement on the water surface. The surface wettability of GAs and SGAs was one of the most important considerations when selecting separation of oil-water mixture, which was examined through the water contact angle measurement. From Figure 6a, when a water droplet touches to the surface of GAs, it quickly spreads out and CA nearly 0°. When oil replace water obtain the similar result that the oil droplet spreads out and spontaneously penetrates into the porous structure of GAs (Figure 6b). In air, the GAs is superamphiphilicity because its surface with microstructured domains containing


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hydrophilic amine groups and hydrophobic aromatic nanodomains. In contrast, the SGAs exhibit the superoleophilicity with a water contact angle of 160.2°, while the oil drop is adsorbed completely by the SGAs and no contact angle can be found (Figure 6c and d). The reason is that the surface tension of water is generally much larger than that of oil. So the hydrophobicity and oleophilicity of the solid surface can be achieved when the solid substrate surface tension between those of water and oil. Solid surface energy was 75.02 and 0.84 mN/m for the GAs and SGAs, respectively, which was calculated by Owen’s two liquid methods.48 The lower surface energy of the SGAs is crucial for get superhydrophobic and superoleophilic surface in air. Interestingly, as shown in Figure 6e, there is a bright and reflective surface underneath the water droplet, which is feature of trapped air and formation of a composite solid-liquid-air interface. The corresponding wetting state conforms to the state of the Cassie-Baxter state.49,50 Additionally, a flow of water from a pipette can bounce off the surface of SGAs and do not leave a trace, showing the weak interaction between sample surface and water (Figure 6f and Movie S1). The superhydrophobicity of the aerogel is further confirmed by using a water-repellent experiment.51,52 When the aerogel is immersed in water by an external force, the mirror-like is found on the surface of SGAs because of the formation of an air cushion between the SGAs and water

(Figure 6g), similar to the reported superhydrophobic monoliths.51,53 The liquid/air

interface area account for most of the beneath water and the proportion of liquid/solid interface is very small. After the aerogel is removed from the water, the surface of it is still completely dry and the weight remains unchanged before and after immersion in water, indicating that no water is absorbed. As shown in Figure 6h, the water repellency behavior of the superhydrophobic aerogel is further verified by the photographs. The SGAs can float on water without sinking when place it on the surface of water due to its characteristics of superhydrophobicity, light


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weight, and water repellency. All the results mentioned displayed a stable superhydrophobicity of the SGAs. Consequently, this graphene based aerogel with water-repellency can be used to absorb oils and organic solvents from water. (a)

Lift up

Preload

(b)

t=0s

t = 0.01 s

t = 0.05 s

t = 0.12 s

t=0s

t = 0.1 s

t = 1.0 s

t = 2.8 s

(c)

Figure 7. Photographs of dynamic measurements of water adhesion and oil permeation on the surface of SGAs. (a) water (b) octane and (c) oleic acid droplets In order to investigate the dynamic wettability of water and oil on the surface of SGAs, we used a high-speed camera system to record the adhesion and permeating process of the liquid droplet. Figure 7a displays the adhesion performance force when a water droplet was drived to completely contact with the surface of SGAs and then lifted up it. From the corresponding photographs of the water droplet, no force drop and no deformation were observed when water leaved the surface of aerogel, thus confirming the extremely low water adhesion for SGAs. Simultaneously, the SGAs behaved as a superior oil-adhesion property. When oily liquid droplet contact with SGAs, it continuous spread and permeate on the material surface with the contact


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angles of 0° (b and c in Figure 7). To examine such excellent oleophilic performance of SGAs, a series of consecutive photographs with two different viscosities organic solvents (octane 0.54 cP and oleic acid 26 cP) was used to compare. The results indicate that SGAs can adsorb the octane and oleic acid droplet within 0.12 s and 2.8 s, respectively. Obviously, the adsorption rates are related with the viscosity of solvents, and a higher viscosity of adsorbate lead to a slower adsorption. This promising superhydrophobic and superoleophilic wettability ensures fast oil permeation and moving through the aerogels and avoids direct contact with water during oilwater separation process, this will greatly improve the separation performance of aerogels materials and keep separation efficiency stable with time. The SGAs was considered as a promising material for the oil-water separation and organic solvents adsorption due to the superhydrophobicity, superoleophilicity, ultra-low density, and large specific surface. The adsorption performance of SGAs was evaluated through a series of adsorption experiments for various oils and organic solvents. The floating oils on water surface (Figure 8a and Movie S2) and heavy oils under water (Figure 8b and Movie S3) can be selective adsorbed by SGAs within a few seconds. As shown in Figure 8a, the SGAs immediately adsorbed the toluene layer (dyed with oil red) around it and leaved a transparent regions on water surface when a piece of it contact with the toluene on water. The toluene was completely absorbed by SGAs within a few seconds can be due to a rapid adsorption process for oils and organic solvents on water. The aerogel saturated with oil remainded floating on water surface can be attributed to its hydrophobicity and ultralow density, then it can be easily removed from water after complete adsorption of oils, offering convenience in collecting and recycling the aerogel in practical applications. In contrast, chloroform sinks to the bottom of water (dyed with oil red) but can also use the SGAs to remove it (Figure 8b). When the SGAs was immersed into


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water and started to contact with chloroform, the droplet of chloroform was immediately sucked into the aerogel. The materials demonstrate excellent adsorption selectivity for water-immiscible oils because of there was no detectable water in the adsorbed aerogel. (a)

(b)

(c)

(d)

Figure 8. Oil adsorption performances of SGAs. (a) Snapshots showing the SGAs adsorbs hexadecane (dyed by oil red) floating on the water; (b) Snapshots showing the SGAs adsorbs chloroform (dyed by oil red) from the bottom of the water; (c) The adsorption capacity of the SGAs for various kinds of organic solvents and oils; (d) The time-dependent sorption behaviour of various oily compounds and water by the SGAs.


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It can be seen that adsorption capacity of SGAs range from 109 up to 236 times their own weight for various oils and organic solvents (Figure 8c). Furthermore, the adsorption capacity of SGAs was roughly proportional to the density of the various solvents, similar to the previously reported.14,26 In this study, the hydrophobic SGAs surface due to the fluorinated functional groups could interact with residues of oxygen-containing functional groups of GAs sheets was conductive to selective removal of the oil in the water. In addition, SGAs with 3D networks structure provided large surface area and high porosity. So there will be a combined or synergistic effect with the strong hydrophobicity and the large surface area of SGAs for removing oils and organic solvents from water. Firstly, oils are entered the open pores of the aerogels into its bulk and spread into the inner pores of aerogels while water is completely blocked due to the superoleophilicity of the surface. Then, the oils were stored in the pores formed by the interconnected sheets of the aerogels. Consequently, the SGAs exhibited high separation efficiency and adsorption capacity for various oils and organic solvents, which were much higher than traditional adsorbent materials such as most previous activated carbon and polymers,54-56 and superhydrophobic graphene-based aerogels reported in the previous literatures.57-60 In addition, the porous structure and high adsorption for oils of SGAs make the material as a promising filler for developing 3D polymer nanocomposites. In addition, we investigated the adsorption kinetics of SGAs to water and four organic liquids (Figure 8d). From the experimental results, the adsorption capacity increase obviously with adsorption time until saturation. For example, for the chloroform, the aerogels reaches sorption equilibrium and a sorption capacity of 233 g/g within less than 2 s. For the diesel, the equilibrium time is about 9 s and saturated sorption capacity is 160 g/g. As shown in Figure 8d, the SGAs can achieve the adsorption saturation within 10 s, which is much faster than


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microporous polymer aerogels (~ 1 min) and CNT sponges (~50 s).61,62 The saturated adsorption time of high-viscosity oils (diesel, η≈3.38 cP) is much longer than that of low-viscosity organic solvents (chloroform, η≈0.55 cP), indicating that the adsorption rate is associated with the viscosity of oils.63,64 This phenomenon can be understood considering the fact that the low viscosity organic solvents is easier to penetrate the 3D network structure of the SGAs.

Figure 9. Cycling adsorption performance of the SGAs using toluene and octane as model. The recyclability and recoverability of the adsorbents for the oils and organic solvents are the key demands in the actual application of oil cleaning. Under our experimental condition, we used toluene and octane as an example by a simple sorption-drying cycle to evaluated the recycling potential and recovering of SGAs for the oils and organic solvents (Figure 9). The process was repeated 10 times to demonstrate recyclability. As a result, without remarkable change in adsorption capacity and structural deformation observed after ten recycling test. So the SGAs could be reused many times with no obvious change in adsorption capacity by taking this simple sorption-drying method over ten repeat times. In addition, a slight drop in oil adsorption capacity may be due to the residual oil entrained in the pores of the aerogels.65 Above results show it is


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easy to restore the activity of SGAs and reobtain absorbates through a simple sorption-drying cycle method. CONCLUSIONS In summary, a controllable and superhydrophobic/superoleophilic GAs was facilely prepared by elaborately devising and fabricating GAs and modification on the 3D substrate. By the combination of controlled solvothermal synthesis, directional-freezing and freeze-drying, an ultra-light GAs were fabricated, providing both substantially high porosity and mechanical stability for further functionalization. Then, the modified coating of hydrophobic PFOES layer that simultaneously with superhydrophobicity and superoleophilicity properties formed through vapor-liquid deposition and surface modification. The resulting SGAs showed ultra-low density, large specific surface area, excellent mechanical property, high adsorption capacities, complete water repellency, and superior adsorption recyclability (109~236 times weight gain). Consequently, this study provides an excellent material, which can be used for oil adsorption and oil-water separation. In addtion, this kind of method may open a new door for controllable devising graphene based aerogels for various applications in the fields of environment remediation, catalyst supports, sensors and energy storage.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Compressive stresse-strain curves of SGAs; movies showing the superhydrophobicity and adsorptive selectivity of the SGAs surface.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the support of the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Nos. qn2015bs020, qn2015bs028), the National Natural Science Foundation of China (Nos. 51302308, 11472294). Chen L. also acknowledge funding from the Program for One Hundred Young Doctor in Xinjiang Uyghur Autonomous Region. ABBREVIATIONS GAs, Graphene aerogels; GO, graphene oxide; SGAs, superhydrophobic graphene aerogels; PFOES, 1H,1H,2H,2H-perfluorooctyltriethoxysilane;

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Table of Contents/Abstract Graphic


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A Vapor-Liquid Deposition Strategy to Prepare Superhydrophobic and

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