Amphiphilic Graphene Aerogel with High Oil and Water Adsorption

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Applications of Polymer, Composite, and Coating Materials

Amphiphilic Graphene Aerogel with High Oil and Water Adsorption Capacity and high contact area for Interface Reaction Yu Zhao, Tongbing Sun, Weidong Liao, Yanqing Wang, Jiali Yu, Meng Zhang, Zhen-Qiang Yu, Bo Yang, Dayong Gui, Caizhen Zhu, and Jian Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06506 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Amphiphilic Graphene Aerogel with High Oil and Water Adsorption Capacity and high contact area for Interface Reaction Yu Zhao†, ‡, Tongbing Sun†, ‡, Weidong Liao†, Yanqing Wang§, #, Jiali Yu†, Meng Zhang†, Zhenqiang Yu†, Bo Yang†, Dayong Gui†, Caizhen Zhu†* and Jian Xu†，‡ † Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong, 518060, P. R. China. § School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan # School of Materials Science and Energy Engineering, Foshan University, Jiangwan First Road, Foshan 528000, Guangdong, China ‡ Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China Yu Zhao and Tongbing Sun contributed equally to this work and should be considered co-first authors. Corresponding Author * E-mail: [email protected]

Keywords: Graphene aerogel, good-amphiphilicity materials, high adsorption capacity, recyclability, high-performance catalyst.

ABSTRACT. Aerogel is a kind of novel materials to create an amphiphilic surface, due to its 3D network structure and functional pore feature. Amphiphilic aerogel can be

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used as an excellent candidate for the supporter of interface reaction. Hydrophilic domains highly distribute on hydrophobic surface, which is beneficial for the two phases contact each other. In the present work, an amphiphilic graphene-based aerogel (EDGA) is cross-linked by Ethylenebis(nitrilodimethylene)tetraphosphonic acid. The EDGA shows high mass adsorption capacity of both oil and water and recyclability. The EDGA is used as supporter of interface reaction. Firstly, we decorate the nanocatalyst on the hydrophilic domain of the EDGA beneficial for its amphiphilicity. Then, this nano-catalyst loaded EDGA is used as the supporter of the interface polymerization. This kind of catalyst shows a high efficiency for C-H activation. The synthesis strategy can extend the application scope of the multifunctional graphene-based aerogel.

Introduction

Graphene is a type of a flat monolayer carbon materials, which is showed a twodimensional (2D) honeycomb lattice structure1. Over last decades, advanced research has witnessed its tremendous growth in nanoscience as well as other expanding fields since graphene firstly described by Geim and Novoselov2. It is flexible, has a large specific surface area, and exhibits remarkable mechanical, electrical and thermal properties3,

4,

making it a promising candidate for electronics, conductive

nanocomposites, thermal conductors and sensors. Materials surfaces with amphiphilicity have a significant application in our daily lives5-7. The current method for the fabrication of amphiphilic materials are selective etching of polymer or solid templates, plasma treatment and self-assembly of block


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copolymers8, 9. Graphene oxide (GO) not only has a large number of hydrophilic functional groups such as epoxy group and carboxyl group, but also contains the hydrophobic domains. Therefore, the special structure of GO can be used for amphiphilic materials. Graphene aerogel is a kind of 3D structure graphene materials that assembled by reduced GO (rGO)10, consisting of porous networks with different size of pores. Graphene aerogel can provide a large specific surface area, light-weight, and fast ion/electron transport. These features endow its high performance in catalysis11, 12,

nanocomposites13 and supercapacitors14, 15. The graphene aerogels not only maintain

the intrinsic properties of graphene sheets16-19 but also make the 3D monolith structure with ultralow density and high porosity20-22. Thus, graphene aerogels are the good candidate for the amphiphilic materials. However, ascribing to the strong inner-sheet π-π attraction between graphene sheets and the removal of oxygen-containing groups, the reduced GO (rGO) sheets tend to agglomerate or restack, leading it hydrophobic. To our knowledge, most of the reports focused on the ability of organic liquids absorbing 23-25, only a few reports work on good-amphiphilicity materials26. It is a major challenge to prepare a microstructure containing hydrophilic domains and hydrophobic domains in amphiphilic graphene aerogel. Introducing the covalent modifications or non-covalent functional hydrophilic group in the material surface is the universal method to modify the surface27. Herein we cross-link the graphene aerogel with Ethylenebis(nitrilodimethylene)tetraphosphonic acid (EDTMPA) to fabricate 3D graphene-based aerogel (EDGA). EDTMPA contains four phosphate groups which can endow the hydrophilicity for aerogel. Meanwhile, phosphate groups can “catch”



two or more rGO sheets to form 3D structure; in other words, EDTMPA is beneficial for graphene self-assembly. EDGA not only shows good-amphiphilicity, but also shows ultralow density, high porosity, and high solvent adsorption. Furthermore, gold nanoparticles can be successfully introduced into 3D graphene architectures to form graphene-based catalyst by one-step simultaneous reduction. It is interesting in the application of environmental remediation and catalysis.

Experimental Section

Materials. Ethylenebis(nitrilodimethylene)tetraphosphonic acid (EDTMPA, ), nitromethane (>99.0%, GC) and acetonitrile (>99.5%, GC) were purchased from MACKLIN Inc. 2-phenyl-1,2,3,4-tetrahydroisoquinoline (>99.0%) was purchased from Shanghai Qiao chemical science Inc. Graphite powder was purchased from Qingdao Jinrilai Graphite Inc. Graphite powder (density: 0.4g/cm3, size: 0.02mm) was purchased from Qingdao Jinrilai Graphite Inc. (Figure S11).. All of reagents and chemicals were employed without further purification. Preparation of Graphene aerogels induced by EDTMPA (EDGA). Graphene oxide (GO) sheets were synthesized from graphite powder via a modified Hummers’ method28, 29. The aerogel was synthesized using the hydrothermal method30. 10 mL of 3 mg/mL homogeneous GO aqueous dispersion containing 1-8 times EDTMPA, with respect to GO were placed in a 25 mL Teflon-lined autoclave and heated at 180 °C for 12 h. Then the autoclave was left and cooled to room temperature and the aerogel was washed in water. Then it was put on an aluminum dish, which was half dipped in liquid


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nitrogen for directional freezing until it was totally frozen24, followed by freeze-drying for 24 h. Synthesis of gold nanoparticle on EDGA (AuNP@EDGA). 15 mL of 10 mM solution

of

NaOH

aqueous

solution

is

in

a

50

mL

beaker,

1

mL

tetrakis(hydroxymethyl)phosphonium chloride dropped in EDGA (The EDGA prepared using the hydrothermal reaction of GO dispersion 5 mg/mL, GO/EDTMPA mass ratio is 1：5), which used as the reducing agent. Then the EDGA is placed on the

surface of NaOH solution. 10 mL of a 7.5 mM Chloro(triphenylphosphine)gold (Au(PPh3)Cl (Ph=phenyl)) in toluene was allowed to stand in contact with NaOH solution. The mixture stood at room temperature for 48h. Then, EDGA gel freeze-dried for 24 h. Procedure

for

the

synthesis

of

1-(nitromethyl)-2-phenyl-1,2,3,4-

tetrahydroisoquinoline. In this experiment, 2-phenyl-1,2,3,4-tetrahydroisoquinoline (20.9 mg, 0,1 mmol), nitromethane (54 μL, 1.0 mmol), acetonitrile (2 mL) and the catalyst AuNP@EDGA (20 mg, Au NP is 5.75 mg/g) were placed in a glass tube, and the reaction was performed with stirring for 24 h at 80 ℃. All solvents were blended together and evaporated to get crude product. The reaction yield was determined by 1H NMR with 1, 3, 5-trimethoxybenzene as an internal standard. The isolated yield was obtained by Medium-pressure liquid chromatography (PE/EA, 2%-10%).

Characterization. Morphology and microstructure were observed by a JEOL field



emission scanning electron microscope (FESEM, JSM-7800F, JEOL Co. Ltd., Japan) at 15 kV. Elemental information was recorded by X-ray photoelectron spectroscopy (XPS ， Escalab-250Xi, Thermo Scientific Co. Ltd., USA). X-ray source was an Al standard anode (1486.6 eV) at 15 kV and 180 W, which diameter beam spot is 500 µm. Samples were dried at room temperature in a high vacuum overnight, compressed with 75 MPa, and again dried in a high vacuum overnight. The surface was sputtered with argon for 20 s before measurements. Wide-angle X-ray diffraction (XRD) analyses were carried out on an X-ray diffractometer (D8, Bruker Co. Ltd., Germany). All measurements of contact angle were conducted using an optical contact angle & interface tension meter (Kino, China) by a small volume of liquid (2 μL) onto the surface using a 2 mL micrometer syringe. At least five measurements were performed on each substrate. Both the water contact angle, and the oil contact angle were measured with five different positions on the EDGA.

Results and discussion

The photo of these aerogels prepared with 3 mg/mL GO reveals a typical 3D network (Figure 1a). Just presented in Figure 1a, the size of as-prepared aerogels 15 mm in diameter and 15 mm in height. The surface of EDGA was smooth and no crack was observed. To understand the effects of different amounts of EDTMPA on the morphologies of the EDGA, the mass ratio GO/EDTMPA in various mass ratios were selected for further investigations (Figure S1). Interestingly, the density of the EDGA


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decrease with the increasing of fraction of EDTMPA (Figure S2), indicating much more void space formed as the EDTMPA increases. Intuitively, the porous structures with a size around 5 micrometers appeared in EDGA (Figure 1b), and a larger than 10 micrometers cavity was observed in the EDGA with GO/EDTMPA mass ratio of 1:5 (Figure 1c). These results indicated the high hydrophilic EDTMPA has an effect on the process of self-assemble graphene as follow: 1) the EDTMPA molecule has four H2PO3 groups, which can capture rGO sheets as the cross-linker. EDTMPA molecule can interact with several rGO sheets to form a branched structure. Moreover, it is easy to control the holes structure of aerogel by modifying the EDTMPA concentration; 2) the rich-EDTMPA region leads to large pole size in EDGA, attributed to the hydrophilicity of EDTMPA propped up the stacking of rGO nanosheets. Further, the pore structure of EDGA was observed as isotropic pore structures. Just reported by Yu et al., the graphene aerogel has higher compressibility in the axial direction than the radial direction in the freezing condition.24 Hence, in our case, the as-prepared graphene aerogel via freeing method also present isotropic pore structures, which was attributed to the different compressive in the axial and radial direction. The X-ray diffraction (XRD) patterns are shown in Figure 1d. The strong peak at 2θ = 10°, corresponds to the interlayer spacing of GO about 8.87 Å. This peak disappears in EDGA, but a new wide diffraction peak appears at 2θ = 24.9°, corresponding to an interlayer spacing of 3.65 Å, owing to the removal of the residual oxygen-containing functional groups of GO sheets26. These results suggest the existence of π-π stacking between the graphene sheets in EDGA and the



inhomogeneous graphite-like carbon crystalline state30. When EDGA reduced at 180 oC,

the peak of 21 ° was decrease indicates more oxygen-containing group can be

reduced. Interestingly, while the EDGA is reduced at low temperature (95℃), a weak peak at 10°still appears in the XRD pattern (Figure S3), indicating the existence of oxygen-containing groups on the rGO sheets. The shoulder peak at 43° is a fingerprint peak for graphite, indicating the reformation of the regular microcrystal of graphite due to the reduction of GO. In order to further investigate the chemical composition of EDGA, XPS measurements were performed as shown in Figure 2. The compositional analysis of carbon, oxygen and phosphorus on the EDGA were monitored at different reduction times by XPS. XPS revealed a graphitic C1s peak at around 284.8 eV and a O1s peak at around 532.7 eV (Figure 2a), which confirmed the hydrophilic groups of EDGA, such as epoxy group and the hydroxyl group. Meanwhile, a P 2p peak at around 134.5 eV. XPS analysis is considered qualitative characterization and hence the weak XPS peak of P 2p just indicates the introducing of EDTMPA into EDGA. Figure 2b-d show high-resolution XPS of C, O, P and their deconvolution results. These results suggest the EDTMPA not only introduced into graphene aerogel but also formed the dynamic hydrogen bonds with graphene sheets to increase the interaction with graphene sheets26. Just shown in Figure 2d, the XPS peak of P was divided into the fitted peaks, locating at 131.5 and 133.0 eV respectively. Just reported by Jiang et al,31,32 the peak located at 133.0 eV was results from P-O-H···O-Graphene, which confirmed the dynamic hydrogen bonds producing in our case.


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The thermal stability of EDGA has been studied by TGA measurements. Figure S4a shows the TG curves for graphene aerogel (blank) and EDGA after annealing at 950 ℃. For the blank aerogel and EDGA, the mass loss is only 30% over the range of 100 –800 ℃, indicating an efficient reduction of GO. Moreover, the EDGA shows higher quality retention than that of the blank sample. While the temperature up to 850 ºC, the mass loss of EDGA is larger than that of blank sample. Just shown in Figure S4b, the loss mass of EDTMPA was much larger than EDGA at high temperature and the larger loss mass of EDGA at 850 ºC might be attributed to the decomposition of EDTMPA. The result suggests the EDGA is a kind of thermally stable material. GO sheets carry a large number of the carboxyl group, epoxy and hydroxyl groups, and graphitic domains reside in the plane. After the hydrothermal reduction, the GO sheets are reduced to the rGO sheets, containing the more hydrophobic domain. Thus, the EDGA show hydrophobicity, which can absorb the organic liquid. On the other hand, the large fraction of phosphate groups combined on the rGO sheets endowed the hydrophilicity to the EDGA. Therefore, the unique composition of the hydrophilic functional group and hydrophobic domains of EDGA endow it with amphiphilic properties. Figure 3a shows an EDGA gel that placed on a dish that moved towards a drop of water and a drop of oil, demonstrating its capability of absorbing both water and hydrophobic oil. When pushing the EDGA over drops of oil and water, the drops were rapidly absorbed, leaving the dry dish (movie S1). It is different from the previously 3D graphene aerogel, which was either mono-hydrophobic33, 34 or mono-hydrophilic35.



To further examine the wettability of EDGA, videos of contact angle measurements were acquired. The digital photos of the contact angle with water and oil are shown in Figure 3b, c, illustrating that EDGA possessed both hydrophobicity and hydrophilicity. Evidently, the amphiphilic structure of EDGA endows the aerogel surface with amphiphilicity, while the capillary forces cause a wicking action. Meanwhile, the porous structure of aerogel can provide the path of droplet to penetrate into the aerogel. These two factors are assumed to bring about the observed wetting property of the EDGA. Due to the difference of viscosities, water generally spread faster than oil. These results suggested that the EDGA with hydrophilicity and oleophilicity can be used as an excellent candidate for the supporter of interface reaction. Hydrophilic domains highly distribute on hydrophobic surface, which is beneficial for the two phases contact each other. To evaluate the adsorption capacity of EDGA for water and oil, different ratios of GO to EDMPTA is compared in Figure 4. The water adsorption capacity of EDGA with GO/EDTMPA mass ratios of 1:1 to 1:8 is shown in Figure 4a. The maximum value appeared at 1:5, owing to the hydrophilicity and high porosity. While the mass ratio is larger than 1:5, the graphene skeleton could not support the weight of water, leading to the decrease of the adsorption capacity of EDGA. Meanwhile, the oil adsorption capacity of EDGA reached a platform when the mass ratio is 1:5 (Figure 4b). The results suggest that there are two factors that control the oil adsorption capacity of EDGA: 1) the hydrophobic rGO domain controlled the adsorption capacity of oil after the hydrothermal treatment; 2) much more porous structures were generated on


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the high EDTMPA concentration, which was used to store the oil molecule. The EDGA used for different types of liquids as the absorbent. The EDGA immersed in the commercial liquid and solvent (tetrachloromethane, dichloromethane, chloroform, palm oil, DMF, toluene, THF, xylene, liquid paraffin, ethanol, dodecane, pump oil) are considered as the pollutants and the daily products.

They were used to

evaluate the adsorption capacity of EDGA. The adsorption capacity of EDGA towards various kinds of liquids is shown in Figure 5. The results revealed that the mass adsorption capacities for water and various organic solvents or oil liquids were 3000% higher than its own weight, indicating the EDGA exhibited very efficient space utilization towards various kinds of liquids including water, which is larger than the previous reports of good-amphiphilic aerogels36. The interconnected porous structure supplied the void space to store the liquid, leading to this high mass adsorption capacity. Meanwhile, it is obviously found that the difference of mass-based adsorption capacity towards the various kinds of liquid. EDGA can uptake these liquids at 30 to 70 times its own weight. The result indicates the mass adsorption capacity depends on the densities of the liquids, which was stored in the void space. Importantly, our EDGA shows much higher absorption capacity than many previously reported sorbents such as activated carbons37, wool-based nonwoven38, nanowire membrane39, carbon aerogels26,

34, 40

and silica aerogels41. Hence, EDGA could be

employed as excellent absorbents for water and various kinds of organic solvents. The EDGA showed the high mass adsorption capacity of both oil and water, indicating that the amphiphilic EDGA promote two phases contact each other for



interface reaction. To optimize the recyclability and the recoverability of amphiphilic graphene aerogel, a recyclability test was performed between the EDGA, water, ethanol, and DMF (Figure S5). Direct drying was applied to remove the water from the EDGA. Direct combustion in air was applied to remove ethanol and DMF form the EDGA. All the liquid can be driven out form the aerogel over 80% after 10 times absorptiondesorption cycle. These results suggested the stability of the aerogel meets the requirement of an excellent recyclable supporter of interface reaction. The amphiphilic EDGA was used as a supporter of liquid/liquid interface reaction. Firstly, we decorate the nano-catalyst on the hydrophilic domain of the EDGA beneficial for its amphiphilicity. Then, this nano-catalyst loaded EDGA is used as the supporter of the interface polymerization. Metal nanoparticles such as Au, Ag and Pt have been prepared by a variety of methods including the thermal decomposition of precursor compounds and the controlled reduction of metal salts in constrained environments, which have recently attracted attention because of their novel functions in catalysis42. Owing to the excellent properties of graphene, graphene-based catalysts have been proved that can improve the catalytic activity43, 44. Commonly, the catalyst used such as gold particles only for one phase catalysis, but they do not interact well with water-oil interface reaction unless modified in several chemical steps. Here we showed a kind of EDGA supported gold nanoparticle (AuNP@EDGA) based the amphiphilicity of EDGA. Gold nanoparticles in-situ reduction in the liquid-liquid interface of EDGA (Figure 6a). EDGA not only


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improves the electrons conductivity, but also provides a heterogeneous supporter. XPS spectra for the AuNP@EDGA is shown in Figure 6b. Figure 6c shows the Au 4f7/2 (84.2 eV) and Au 4f5/2 (87.9eV), which are consistent with metallic gold45. The results revealed the Au+ was reduced to Au0 in EDGA. SEM-EDS analysis (Figure S6) confirmed that the relatively light areas on the EDGA were composed of gold and phosphorus. Transmission electron microscopy (TEM) was used to further investigate the morphology of Au nanoparticles, which showed a spherical (Figure S7a). And the high-resolution transmission electron microscope (HRTEM) image exhibited an atomic lattice fringe spacing of 0.205 nm corresponding well to the (200) plane of the Au (Figure S7b). Here the AuNP@EDGA was used for catalytic C–H bond functionalization

reaction.

The

yield

of

1-(nitromethyl)-2-phenyl-1,2,3,4-

tetrahydroisoquinoline is 94%, calculated based on NMR result (Figure S8 and S9). This high yield is much higher than previous work without AuNP@EDGA (42%) （Figure 6d）46. Obviously, the use of EDGA as a supporter increases the interfacial area to improve catalytic activity. Meanwhile, attributed to the special structure of graphene, the graphene sheets favor the transport and availability of electron, further improves the activity of the catalyst. The aerogel can be recycled from the reaction, which endow the AuNP@EDGA catalyst good recyclability. The catalyst was recycled 5 times without significant activity loss (Figure S10). The result indicates the stability of catalyst in chemical reaction. Therefore, the demonstrated amphiphilicity of AuNP@EDGA has a potential application in the field of organic synthesis.



Conclusion

In summary, we designed a kind of amphiphilic graphene aerogels with EDTMPA. Due to the synergy effect of highly hydrophilic EDTMPA and highly hydrophobic graphene, the EDGA showed an amphiphilic surface. EDGA can provide four phosphate groups to capture the rGO sheets, and form hydrophilic phosphate nanodomains to improve the hydrophilicity of aerogel. An EDGA supported gold catalyst is obtained at an aqueous-organic interface. This catalyst can readily be recycled. We envision that graphene-based aerogel with amphiphilicity can expand the application of multifunctional materials in heterogeneous catalysis, chemical sensing and biosensing. The approach to preparing graphene-based aerogel should be applicable to other systems with specific properties and functions.

Figure 1. (a) Morphology of the EDGA prepared using the hydrothermal reaction of GO dispersion with EDTMPA. SEM images of the EDGA under different GO/EDTMPA mass ratios of (b) 1:1, (c) 1:5, and (d) XRD pattern of GO and EDGA.


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Figure 2. XPS spectrum of EDGA (a) and high-resolution XPS spectra of the O1s (b), C1s (c) and P2p (d) of the EDGA.

Figure 3. (a) The sequential adsorption of oil and water by the EDGA. Photo of water (b) and oil (c) contact angel on EDGA.



Figure 4. The mass-based adsorption capacities of the EDGA for water (a) and oil (b).

Figure 5. The mass-based adsorption capacity of EDGA for different liquid. The GO/EDTMPA mass ratio is 1:5, where DMF and THF are N,N-dimethyl formamide and tetrahydrofuran;respectively.


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Figure 6. (a) Illustration of the preparation of the Au NP@EDGA. XPS spectrum of Au NP@EDGA. The Au+ was dissolved in toluene, EDGA was put in the interface of NaOH solution and toluene. Due to the amphiphilicity of EDGA, the Au+ was reduced to Au0 nanoparticle in EDGA. (b) and high-resolution XPS spectra of the Au 4f (c) of the Au NPs@EDGA. (d) The yield of C–H bond functionalization reaction with Au NPs@EDGA and blank.

ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. The following files are available free of charge. Characterization and recyclability of the aerogel, SEM and TEM images of aerogel catalyst, NMR information of the reaction product (PDF). AUTHOR INFORMATION



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Corresponding Author * E-mail: [email protected]

Funding Sources This work was financially supported by National Natural Science Foundation of China (51673117, 51574166), the Science and Technology Innovation Commission of Shenzhen

(JCYJ20150529164656097,

JSGG20160226201833790,

JCYJ20160520163535684, JCYJ20160422144936457, JCYJ20170818093832350, JCYJ20170818112409808, JSGG20170824112840518), China Postdoctoral Science Foundation (2017M622786, 2017M622787)

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Table of Contents

The EDTMPA was introduced in graphene aerogel to create amphiphilic materials. The EDGA is used as supporter to create high catalytic efficiency catalyst. The synthesis strategy can extend the application scope of the multifunctional graphene-based aerogel.


Amphiphilic Graphene Aerogel with High Oil and Water Adsorption

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