Three-dimensional superhydrophobic porous hybrid monoliths for effective removal of oil droplets from the surface of water

Abstract

An effective, facile approach is reported for constructing three-dimensional (3D) superhydrophobic porous hybrid monoliths by a reduced self-assembly procedure using a graphene oxide (GO) and polystyrene (PS) solution with L-ascorbic acid. Water contact angle (CA) measurements demonstrate that the entire body of the reduced graphene oxide/PS (rGO/PS) monolith is superhydrophobic, with a water CA of 156° for the surface and 152° for the cross-section. A nitrogen adsorption–desorption analysis reveals that there are a large number of pores (153.9 m² g⁻¹, BET) in the monoliths. The superhydrophobic porous hybrid monoliths exhibit a remarkable capability to adsorb a wide range of organic solvents and oil droplets from the surface of water. Furthermore, the highly water repellent character of the monoliths is still kept even under severe conditions (e.g. acidic, basic and saline). Such expedient and low-cost superhydrophobic porous monoliths have a lot of promising potential applications, including self-cleaning, anti-corrosion, anti-sticking, water remediation and oil spillage cleanup.

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1 Introduction

Due to the fascinating characteristics of the “lotus effect”,^1,2 superhydrophobic surfaces with water contact angles (CA) larger than 150° have provoked extensive interest in the realms of academic research and practical application over the past few decades.^3,4 The extreme water-repellency and oil absorption properties give superhydrophobic surfaces great popularity in a wide variety of areas, such as self-cleaning,^5–7 antifouling,⁸ antifogging,⁹ oil and water separation,¹⁰ drag reduction,¹¹ and so forth.^12–14 Accordingly, a considerable number of efforts have been devoted to fabricating such superhydrophobic surfaces, including electrodeposition,¹⁵ lithography,¹⁶ plasma fluorination,¹⁷ template extrusion,¹⁸ dip coating,¹⁹ and sol–gel methods.²⁰ In most cases, these methods are restrained from being put into effect as scalable manufacturing processes in practical and commercial applications due to the high cost materials used, and the severe conditions, complicated techniques and laborious production processes. Recently, a few investigations have revealed that the formation of superhydrophobic surfaces is accompanied by the simultaneous creation of a superhydrophobic structure throughout the entire body via a simple, rapid and economical method.²¹

Recently, frequent occurrences of water pollution arising from oil spills and chemical leakage have resulted in serious environmental damage on a global scale.²² In this context, a great deal of attention has been paid to exploring effective methods for the rapid and selective removal of oils and organic contaminants from the surface of water. The employment of absorptive materials has proved to be one of the most feasible approaches owing to the ready availability of materials and its ease of operation. Although traditional absorbent materials such as activated carbon,²³ zeolites,²⁴ and organoclays²⁵ have shown great advantages in practical applications, the low absorption capacity, poor reusability and lack of selectivity to oil and water seriously affect their absorption efficiencies.²⁶ Recent research has shown that the introduction of superhydrophobicity to synthetic absorbent materials can remarkably improve the selectivity, meaning that they repel water completely but absorb oil.^27–29 In view of this, a variety of superhydrophobic absorption materials have been fabricated for separating oil from water.^30–35 Among them, superhydrophobic 3D porous materials are regarded as desirable absorbents because of their larger surface area and regular pore structures.

Graphene, a single atomic layer of carbon, has aroused tremendous attention owing to its exceptional mechanical, thermal and electrical properties.^36–38 Moreover, as an additional unique feature of graphene, the ultra-large specific surface area (2630 m² g⁻¹) and flat structure,^39,40 endow it with the ideal characteristics to be an excellent absorbent. Up to now, various graphene composites with 3D porous structures have been prepared and utilized as supporting platforms for absorbing all kinds of oils and organic pollutants.^40–46 However, there are only a few studies concerning graphene-based absorptive materials with superhydrophobicity, which plays a key role in improving the selectivity and efficiency of absorbents.

Herein, we present a convenient, effective method to produce 3D superhydrophobic porous hybrid rGO/PS monoliths through a simple self-assembly procedure using a GO and PS solution under the reduction of L-ascorbic acid. The as-prepared monoliths exhibited superhydrophobicity throughout their whole volume and gave a remarkable performance for selectively eliminating oils and organic contaminants from the surface of water. Moreover, the monoliths demonstrated steady water repellency against corrosive liquids such as acidic, salty and basic solutions. The readily available materials, facile preparation process and superior absorption ability render this method a promising candidate for the scalable fabrication of superhydrophobic composites for practical application in cleaning oil spills and chemical leaks.

2 Experimental section

2.1 Material

Natural graphite flakes (purity 99.9%) were purchased from Qingdao Kanglong Graphite Co., Ltd. Polystyrene (average MW 250 000 g mol⁻¹) was ordered from J&K Scientific Ltd. L-Ascorbic acid, N,N-dimethyl formamide (DMF) and all other solvents were obtained from Sinopharm Chemical Reagent Co., Ltd (SCRC) and used as received. Graphene oxide was prepared from natural graphite flakes according to the modified Hummer's method.^47,48

2.2 Preparation of the rGO/PS monoliths

A pre-dissolved PS solution (150 mg in 7.5 mL DMF) was added dropwise into the dispersed GO (30 mg in 7.5 mL DMF) and stirred in a 25 mL cylindrical vial for 2 h. After the addition of L-ascorbic acid (150 mg), the mixed solution was held at 95 °C for 12 h without stirring. The as-prepared columned sample was dialyzed against deionized water to remove soluble impurities, followed by freeze-drying to generate the rGO/PS monoliths. Bare 3D rGO aerogel was fabricated according to the same method, but with no integration of PS, for comparison.

2.3 Oil absorption experiments

The oil absorption capacity of the monoliths was determined by weight measurements. Weighed amounts of the monoliths were put into different kinds of oil and organic solvent, and taken out when the absorption was finished. The samples were weighed again after removing any surface oil with a filter paper. The absorption capacity (Q) was calculated with the following equation:

Q = (W_s − W_i)/W_i

where W_i (g) and W_s (g) are the initial and final weight (after saturated absorption) of the monolith samples, respectively.

2.4 Characterization

The contact angles were measured using an OCA 20 contact angle system (Dataphysics Instruments GmbH, Germany) at ambient temperature. Droplets of 4 μL ultrapure water were dropped onto the samples using an SNS 021/011 needle on a microsyringe. Scanning electron microscope (SEM) images were observed on a NOVA NanoSEM 230 microscope (FEI). Atomic force microscopy (AFM) views were obtained using an SII Nanonavi E-sweep microscope in tapping mode. Fourier transform infrared (FTIR) spectra were collected using a Paragon 1000 spectrometer (Perkin-Elmer). Raman measurements were performed on a Senterra R200-L apparatus (Bruker Optics) with an excitation laser beam wavelength of 532 nm. Thermogravimetry analysis (TGA) was carried out under a nitrogen atmosphere with a TGA 2050 instrument (Perkin-Elmer) at 10 °C min⁻¹. X-ray diffraction (XRD) measurements were taken on a D/max-2200/PC diffractometer (Rigaku) using Cu Kα radiation. X-ray photoelectron (XPS) spectra were obtained on an AXIS ULTRA^DLD instrument (Kratos) with Al Kα radiation. The specific surface area was obtained by the Brunauer–Emmett–Teller (BET) method with an ASAP 2020 apparatus.

3 Results and discussion

The self-assembly of GO and PS in DMF solution with the assistance of L-ascorbic acid was demonstrated to be a powerful way to build 3D porous rGO/PS architectures. The exact transformation procedures were verified with different characterization methods. The FTIR spectra of GO, PS, rGO, and the rGO/PS monoliths are displayed in Fig. 1. GO shows the following characteristic features: a carbonyl or carboxyl stretching vibration (at 1728 cm⁻¹), O–H bending of the C–OH (at 1625 cm⁻¹), an O–H vibration of the C–OH groups (at 1384 cm⁻¹), a C–O vibration of the C–OH (at 1220 cm⁻¹), and a C–O–C vibration in the epoxy groups (at 1053 cm⁻¹). After reduction, the peak intensities of the represented oxygen-containing groups in the rGO and rGO/PS composites decrease remarkably in contrast to that of the GO, indicating effective reduction by the L-ascorbic acid. As for the rGO/PS monolith sample, a series of newly emerged peaks at 3023, 2921, 1601, 1493, 1451, 1025, 904, 753 and 696 cm⁻¹ are assigned to the characteristic peaks of benzene rings in the PS skeleton,⁴⁹ signifying the integration of PS into the rGO planes.

Fig. 1 FTIR spectra of GO, rGO, PS and the rGO/PS monoliths.

The structural changes of GO, rGO and the rGO/PS composites were characterized by Raman spectroscopy. As shown in Fig. 2, the D band and G band of GO are located at 1354 cm⁻¹ and 1582 cm⁻¹ respectively. Meanwhile the rGO displays the D band at 1351 cm⁻¹ and the G band at 1591 cm⁻¹, with the D/G intensity ratio changing from 2.3 to 1.6. This implies an increase of the average size of the crystalline graphene domains after the reduction.⁵⁰ With respect to the rGO/PS composites, the D band is situated at 1342 cm⁻¹ and G band at 1591 cm⁻¹, meaning that the D/G intensity ratio (1.7) is slightly higher than that of the rGO. This could be ascribed to the non-covalent π–π interactions between the PS scaffold and the basal planes of the rGO.

Fig. 2 Raman spectra of GO, rGO and the rGO/PS monoliths.

Fig. 3 exhibits the XRD patterns of GO, PS, rGO and the rGO/PS monolith. The GO shows a characteristic peak at 10.5°, corresponding to an interlayer spacing of 0.84 nm. Whereas the rGO presents a weak and broad diffraction peak at 24.2° with a reduced d-spacing of 0.37 nm, probably owing to the elimination of substantial oxygen-containing groups. The XRD curve of PS displays two typical peaks. The polymerization peak at 9.6° originates from the intermolecular scaffold relevance and the size of side group, corresponding to the arrangement of the molecular chains in an approximately hexagonal order. The other peak at 19.2° belongs to the amorphous halo and associates with the van der Waals distance.^51,52 For the rGO/PS composite, the two peaks become weaker and broader, which could be related to the fact that the peak at 24.2° of the rGO causes the peak at 19.2° of the composite to broaden in the range of 15–30°. Moreover, the d-spacing of 0.45 nm for the rGO/PS monolith is larger than that of rGO, illustrating that the PS chains attached on the rGO nanosheets interfere the van der Waals interactions and extend the interlayer distance of the nanosheets.

Fig. 3 XRD profiles of GO, PS, rGO and rGO/PS monoliths.

The thermal stability of GO, PS, rGO and the rGO/PS monolith was examined with TGA. As exhibited in Fig. 4, the curve of GO displays ∼5% weight loss below 100 °C because of the removal of the absorbed water. The major weight loss (∼20%) at around 200 °C can presumably be assigned to the pyrolysis of the labile oxygen-containing functional groups. There is also a minor weight loss (∼10%) at about 250 °C for the rGO, indicating that the thermal stability of the rGO is improved upon chemical reduction. As for the pristine PS, the initial decomposition temperature occurs at 350 °C, and it degrades completely at 440 °C, which is ascribed to the thermal decomposition of the main chains.⁵³ In the case of the rGO/PS composite, the thermal decomposition temperature shifts toward a higher temperature range in comparison to that of the net PS. This is a result of the strong interactions between the polymer and the rGO hampering the movement of the polymer chains. The slight weight loss in the range of 200–350 °C derives from the elimination of a certain amount of the oxygen-containing groups of the rGO in the rGO/PS composite. This suggests that the reduction degree is somewhat influenced by the incorporation of the PS.⁵⁴

Fig. 4 TGA curves of GO, PS, rGO and the rGO/PS monoliths.

The C 1s XPS spectra of GO, rGO, rGO/PS and general spectra are listed in Fig. 5 and the ESI (Fig. S1†). For GO (Fig. S1a†), four different peaks centered at binding energies of 284.6, 286.6, 287.8 and 289.0 eV are assigned to C=C, C–O, C=O and O–C=O, respectively.⁵⁵ Meanwhile the peak intensity of the oxygen-containing groups in rGO (Fig. S1b†) shows a remarkable decrease, indicating that most of the oxygen-containing groups were effectively removed by chemical reduction. The newly emerged peak at 285.7 eV, assigned to the carbon bound to nitrogen, presumably stems from the residual DMF molecules in the sample. With regard to the rGO/PS composite, a weak peak of oxygen-containing groups as well as a wider peak for the higher C 1s concentration is observed (Fig. 5a). The increased carbon content is closely related to the presence of PS. Moreover, the C/O atomic ratios calculated from the XPS spectra are 2.3, 5.7 and 20.5 for GO, rGO and the rGO/PS composite (Fig. 5b), respectively. The higher C/O atomic ratio for the rGO/PS composite results from the introduction of PS, also demonstrating the incorporation of PS into the rGO nanosheets.

Fig. 5 The curve fit of C 1s spectra for (a) the rGO/PS monoliths and (b) the XPS general spectra.

Digital images of rGO hydrogel and the rGO/PS monoliths are exhibited in Fig. 6. Compared to the net 3D rGO hydrogel, the introduction of PS remarkably increased the volume of the rGO/PS monoliths by generating a rougher surface. After lyophilization, there was almost no apparent volume shrinkage for the dried monolith, which is crucial for the retention of the high cellular structures of the monolith. Surface morphology analysis of the bare rGO aerogel and the rGO/PS monoliths demonstrated that the integration of PS into the 3D rGO skeleton significantly changed the micro–nanostructures and increased the roughness of the graphene surfaces, as shown by scanning electron microscopy (SEM) images of their freeze-dried samples (Fig. 7 and S2†). The surface of the net 3D rGO architecture was composed of large interconnected flat graphene sheets and a porous network with pore sizes in dimensions from dozens of micrometers to one hundred micrometers (Fig S2a and b†). Meanwhile the rGO/PS monoliths displayed a rather coarse surface with densely packed cellular structures at the micrometer scale (Fig. 7a, S3a and b†). The high-magnification SEM image revealed that numerous PS nanospheres are randomly distributed on the surface of the graphene sheet (Fig. 7b). The widespread PS nanospheres, together with the compact porous conformation, resulted in the formation of micro–nanoscale hierarchical structures on the monoliths’ surface, which played an important role in the generation of superhydrophobicity. Moreover, similar appearances were also observed in the cross-sectional SEM images. In contrast to the macroporous construction of the bare 3D rGO skeleton (Fig. S2c and d†), the rGO/PS monoliths exhibited a well-defined and fine porous structure with pore sizes in the range of submicrometer to one micrometer, which represented a decrease of more than an order of magnitude in pore dimension (Fig. 7c, S3c and d†). The reason for the evolution of the inner pore size of the monoliths could originate from the π–π interactions between graphene and PS hindering the graphene sheets from self-restacking into large layer structures. The more closely microporous texture inside the monoliths, in combination with the attached PS nanospheres on the grapheme sheets (Fig. 7d), were beneficial for generating a micro–nano structure throughout the entire volume, which provided the foundation for the formation of superhydrophobicity across the whole body of the monoliths.

Fig. 6 Digital images of the net 3D graphene hydrogel (left), and the rGO/PS monolith before (middle) and after (right) freeze drying.

Fig. 7 SEM images of the rGO/PS monoliths’ surface (a and b) and (c and d) cross-sectional microstructures at different magnifications.

Fig. 8 depicts AFM three-dimensional (3D) and two-dimensional (2D) views of the surface of the monolith. As can be seen, the monolith shows a rough surface at the micro–nanoscale, with large amounts of micro-protrusions. The root mean square (RMS) roughness and average roughness (R_a) values were calculated as approximately 135 nm and 108 nm, respectively. The greater roughness, in combination with the micro–nanoscale hierarchical structures, might have a significant effect on the superhydrophobicity of the monoliths.

Fig. 8 (a) AFM three-dimensional and (b) two-dimensional images of the monoliths’ surface.

To further illustrate the effect of geometric structure and chemical composition on the surface wettability, water CA measurements were carried out to examine the wettability of the rGO/PS monoliths. It was observed that the monoliths exhibited surface superhydrophobicity, with a water CA of 156° (Fig. 9a). Meanwhile, the advancing CA and receding CA of the surface were measured as 151° and 150°, respectively. A water droplet deposited on the surface displayed an almost completely spherical shape. This prominent surface superhydrophobicity should be ascribed to the combination of both the microporous morphological structure and the hydrophobic PS component on the monoliths’ surface. Similar observations were also made in the cross-sectional CA measurements for the monoliths, which displayed surface superhydrophobicity with a water CA of 152° (Fig. 9b). When a monolith was partly or totally submerged in water with an external force, its surface was encompassed by a myriad of air bubbles, appearing as a silver mirror-like surface (Fig. 10). This demonstrated typical Cassie–Baxter nonwetting behavior as a result of a continuous air layer between the superhydrophobic surface and the water.⁵⁶ After removing the external force, the monolith immediately floated on the water surface, without an uptake of water being detected by the subsequent weighing of the monoliths. This evidence implied that the monoliths could be employed on the surface of water for removing oils and organic solvent contaminants. Furthermore, the monoliths also revealed a stable repellency against corrosive liquids, such as acidic, basic, and salt solutions. The static contact angle remained almost unchanged over a wide range of pH values, from 1 to 14, for 12 h. Fig. S4† shows the representative digital image of acidic (left, pH = 1), salt (middle, pH = 7), and basic (right, pH = 14) droplets on the monoliths’ surface and cross section. The water CAs for the surface are 155°, 156° and 156°, corresponding to the acidic, salt, and basic droplet, respectively. In the case of the cross-section, the corresponding water CAs are 151°, 152° and 152°. All these aqueous solution droplets were uniformly situated on the monoliths, presenting perfectly spherical shapes.

Fig. 9 Water contact angle images of a water droplet on the surface (a) and cross-section (b) of the as-prepared monoliths.

Fig. 10 Digital image of a monolith partly (a) or totally (b) immersed in water by an external force, showing a silver mirror-like surface.

The porous features of the rGO/PS monoliths were investigated with nitrogen adsorption–desorption analysis. The adsorption–desorption isotherm of the monoliths exhibited a typical hysteresis loop (Fig. 11a),^57,58 revealing the existence of a large number of mesopores. The surface area was found to be 153.9 m² g⁻¹ by fitting the isotherm to the Brunauer–Emmett–Teller (BET) model. The pore size distribution curves determined by the Barrett–Joyner–Halenda method show that the majority of the pore volumes of the monoliths possess a pore diameter in the range of 2.0–100 nm (Fig. 11b). The broad size distribution suggests that the monoliths hold hierarchical pore structures.⁵⁷

Fig. 11 (a) Typical nitrogen sorption isotherms. (b) a BJH (Barret–Joyner–Halenda) desorption pore size distribution curve.

Generally, porous materials with simultaneous superhydrophobicity and superoleophilicity are considered as high-capacity absorbents for the removal of oil spills and organic contaminants. Accordingly, the adsorption properties of the monoliths were evaluated for a range of organic solvents and oils. As shown in Fig. 12, when it came into contact with an oil film (dodecane) dispersed on the surface of water, a monolith immediately absorbed the oil layer around it, giving rise to the emergence of a transparent water surface. In the meantime, the oil layer gradually shrunk and completely disappeared in a few minutes. The absorption capacities of the monoliths for different oils and solvents varied from 32 to 50 times their own weight, depending on the density and viscosity of the oils and organic solvents (Fig. 13). Table S1† lists the comparison of the monolith with various materials reported previously. It is clear that the monolith has a superior adsorption ability compared with the other adsorbents. This excellent absorption capability could be attributed to the cellular structure and large superhydrophobic surface area of the graphene/PS monoliths, as depicted in the SEM images. Furthermore, the adsorbed oils in the monoliths could be extracted by submerging the oil-containing monoliths in acetone followed by drying, while absorbed organic solvents were readily removed by heating. Thus, the recovered monoliths could be repeatedly used for oil–water separation for multiple rounds without a significant degradation in the adsorption capacity (Fig. S5†). After the tenth oil–water separation cycle the oils were collected and analyzed with ATR-FTIR to measure the water content. As exhibited in Fig. S6,† the absorption bands at 2850–2960 cm⁻¹, 1460 cm⁻¹, 1377 cm⁻¹ and 720 cm⁻¹ were assigned to the characteristic vibrations of alkanes, without an adsorption band being ascribed to water. This evidence demonstrated the high selectivity and efficiency of the superhydrophobic monoliths for oil–water separation.

Fig. 12 Optical images of the removal of a dodecane film spreading on the surface of water by the monoliths. The dodecane was dyed with oil red for clear observation.

Fig. 13 Absorption capacities of the monoliths for oil and organic solvents.

4 Conclusions

3D superhydrophobic porous rGO/PS monoliths were constructed through a facile and effective synthesis strategy, i.e., a reduced self-assembly process using a GO and PS solution with L-ascorbic acid. The resulting monoliths displayed whole body superhydrophobicity, a high adsorption capacity and stable repellency towards corrosive aqueous solutions. These integrated functionalities enabled them to remove oils and organic contaminants from the surface of water with a high selectivity. On the basis of cost-effective accessibility, simplicity of operation and scalability of fabrication, the present method could be used to construct superhydrophobic graphene composites for oil spillage cleanup and water remediation.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (no. 50973061, 91127047 and 21274089) and the National Basic Research Program (no. 2009CB930400, 2012CB821500, 2013CB834506 and 2014CB643600).

Notes and references

1. W. Barthlott and C. Neinhuis, Planta, 1997, 202, 1.
2. C. Neinhuis and W. Barthlott, Ann. Bot., 1997, 79, 667.
3. H. Li, J. Wang, L. Yang and Y. Song, Adv. Funct. Mater., 2008, 18, 3258.
4. T. Sun, L. Feng, X. Gao and L. Jiang, Acc. Chem. Res., 2005, 38, 644.
5. W. Ming, D. Wu, R. van Benthem and G. de With, Nano Lett., 2005, 5, 2298.
6. X.-M. Li, D. Reinhoudt and M. Crego-Calama, Chem. Soc. Rev., 2007, 36, 1350.
7. V. A. Ganesh, H. K. Raut, A. S. Nair and S. Ramakrishna, J. Mater. Chem., 2011, 21, 16304.
8. J. L. Dalsin, B.-H. Hu, B. P. Lee and P. B. Messersmith, J. Am. Chem. Soc., 2003, 125, 4253.
9. J. A. Howarter and J. P. Youngblood, Adv. Mater., 2007, 19, 3838.
10. Z. Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng and L. Jiang, Adv. Mater., 2011, 23, 4270.
11. F. Shi, J. Niu, J. Liu, F. Liu, Z. Wang, X. Q. Feng and X. Zhang, Adv. Mater., 2007, 19, 2257.
12. B. Bhushan and Y. C. Jung, Prog. Mater. Sci., 2011, 56, 1.
13. X. Yao, Y. Song and L. Jiang, Adv. Mater., 2011, 23, 719.
14. T. Sun, G. Qing, B. Su and L. Jiang, Chem. Soc. Rev., 2011, 40, 2909.
15. X. Zhang, F. Shi, X. Yu, H. Liu, Y. Fu, Z. Wang, L. Jiang and X. Li, J. Am. Chem. Soc., 2004, 126, 3064.
16. D. Öner and T. J. McCarthy, Langmuir, 2000, 16, 7777.
17. D. O. H. Teare, C. G. Spanos, P. Ridley, E. J. Kinmond, V. Roucoules, J. P. S. Badyal, S. A. Brewer, S. Coulson and C. Willis, Chem. Mater., 2002, 14, 4566.
18. L. Feng, S. Li, H. Li, J. Zhai, Y. Song, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2002, 41, 1221.
19. Q. Zhu, Q. Pan and F. Liu, J. Phys. Chem. C, 2011, 115, 17464.
20. J. T. Han, D. H. Lee, C. Y. Ryu and K. Cho, J. Am. Chem. Soc., 2004, 126, 4796.
21. X. Zhu, Z. Zhang, G. Ren, J. Yang, X. Xu, X. Men and X. Zhou, J. Mater. Chem., 2012, 22, 20146.
22. S.-K. Song, Z.-H. Shon, Y.-K. Kim, Y.-H. Kang and K.-H. Kim, Atmos. Environ., 2011, 45, 1312.
23. M. Mohammadi, A. J. Hassani, A. R. Mohamed and G. D. Najafpour, J. Chem. Eng. Data, 2010, 55, 5777.
24. S. Kesraoui-Ouki, C. R. Cheeseman and R. Perry, J. Chem. Technol. Biotechnol., 1994, 59, 121.
25. O. Carmody, R. Frost, Y. Xi and S. Kokot, J. Colloid Interface Sci., 2007, 305, 17.
26. M. O. Adebajo, R. L. Frost, J. T. Kloprogge, O. Carmody and S. Kokot, J. Porous Mater., 2003, 10, 159.
27. C. H. Lee, N. Johnson, J. Drelich and Y. K. Yap, Carbon, 2011, 49, 669.
28. T. Darmanin, M. Nicolas and F. Guittard, Phys. Chem. Chem. Phys., 2008, 10, 4322.
29. L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2004, 43, 2012.
30. C. Yang, U. Kaipa, Q. Z. Mather, X. Wang, V. Nesterov, A. F. Venero and M. A. Omary, J. Am. Chem. Soc., 2011, 133, 18094.
31. Y. Chu and Q. Pan, ACS Appl. Mater. Interfaces, 2012, 4, 2420.
32. A. Li, H.-X. Sun, D.-Z. Tan, W.-J. Fan, S.-H. Wen, X.-J. Qing, G.-X. Li, S.-Y. Li and W.-Q. Deng, Energy Environ. Sci., 2011, 4, 2062.
33. J. Li, L. Shi, Y. Chen, Y. Zhang, Z. Guo, B.-l. Su and W. Liu, J. Mater. Chem., 2012, 22, 9774.
34. P. Calcagnile, D. Fragouli, I. S. Bayer, G. C. Anyfantis, L. Martiradonna, P. D. Cozzoli, R. Cingolani and A. Athanassiou, ACS Nano, 2012, 6, 5413.
35. W. Dai, S. J. Kim, W.-K. Seong, S. H. Kim, K.-R. Lee, H.-Y. Kim and M.-W. Moon, Sci. Rep., 2013, 3, 2524,  DOI:10.1038/srep02524.
36. L. P. Biró, P. Nemes-Incze and P. Lambin, Nanoscale, 2012, 4, 1824.
37. H. Kim, A. A. Abdala and C. W. Macosko, Macromolecules, 2010, 43, 6515.
38. X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666.
39. S. H. Lee, H. W. Kim, J. O. Hwang, W. J. Lee, J. Kwon, C. W. Bielawski, R. S. Ruoff and S. O. Kim, Angew. Chem., Int. Ed., 2010, 49, 10084.
40. S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.
41. H.-P. Cong, X.-C. Ren, P. Wang and S.-H. Yu, ACS Nano, 2012, 6, 2693.
42. J. N. Tiwari, K. Mahesh, N. H. Le, K. C. Kemp, R. Timilsina, R. N. Tiwari and K. S. Kim, Carbon, 2013, 56, 173.
43. Z. Niu, J. Chen, H. H. Hng, J. Ma and X. Chen, Adv. Mater., 2012, 24, 4144.
44. T. Wu, M. Chen, L. Zhang, X. Xu, W. Wang, J. Yan, Y. Liu and J. Gao, J. Mater. Chem. A, 2013, 1, 7612.
45. H. Li, L. Liu and F. Yang, J. Mater. Chem. A, 2013, 1, 3446.
46. X. Dong, J. Chen, Y. Ma, J. Wang, M. B. Chan-Park, X. Liu, L. Wang, W. Huang and P. Chen, Chem. Commun., 2012, 48, 10660.
47. N. I. Kovtyukhova, P. J. Ollivier, B. R. Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzaneva and A. D. Gorchinskiy, Chem. Mater., 1999, 11, 771.
48. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
49. H. Wu, W. Zhao, H. Hu and G. Chen, J. Mater. Chem., 2011, 21, 8626.
50. H. Wang, J. T. Robinson, X. Li and H. Dai, J. Am. Chem. Soc., 2009, 131, 9910.
51. C. Ayyagari, D. Bedrov and G. D. Smith, Macromolecules, 2000, 33, 6194.
52. J. Yang, M. Wu, F. Chen, Z. Fei and M. Zhong, J. Supercrit. Fluids, 2011, 56, 201.
53. H. Hu, X. Wang, J. Wang, L. Wan, F. Liu, H. Zheng, R. Chen and C. Xu, Chem. Phys. Lett., 2010, 484, 247.
54. P. Song, Z. Cao, Y. Cai, L. Zhao, Z. Fang and S. Fu, Polymer, 2011, 52, 4001.
55. J. Gao, F. Liu, Y. Liu, N. Ma, Z. Wang and X. Zhang, Chem. Mater., 2010, 22, 2213.
56. I. A. Larmour, S. E. J. Bell and G. C. Saunders, Angew. Chem., Int. Ed., 2007, 46, 1710.
57. Z. Xu, Y. Zhang, P. Li and C. Gao, ACS Nano, 2012, 6, 7103.
58. H. Huang, P. Chen, X. Zhang, Y. Lu and W. Zhan, Small, 2013, 9, 1397.

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Footnote

† Electronic supplementary information (ESI) available: XPS spectra of GO and rGO, SEM images of a net 3D rGO aerogel and rGO/PS monoliths, absorption recyclability, and ATR-FTIR spectra of adsorbed oil and organic solvents. See DOI: 10.1039/c4ra00047a

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