Amphiphilic Graphene Aerogel with High Oil and Water Adsorption

May 30, 2019 - ... the 3D structure; in other words, EDTMPA is beneficial for graphene self-assembly. ..... The Supporting Information is available fr...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

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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†,∥

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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, Guangdong 528000, China ∥ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Aerogel is a kind of novel material used to create an amphiphilic surface because of its 3D network structure and functional pore feature. Amphiphilic aerogel can be used as an excellent candidate for the supporter of interface reaction. Hydrophilic domains are highly distributed on the hydrophobic surface, which is beneficial for the two phases to come in contact with 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 recyclability. The EDGA is used as a supporter of interface reaction. First, we decorate the nanocatalyst on the hydrophilic domain of the EDGA beneficial for its amphiphilicity. Then, this nanocatalyst-loaded EDGA is used as the supporter of the interface polymerization. This kind of a catalyst shows high efficiency for C−H activation. The synthesis strategy can extend the application scope of the multifunctional graphenebased aerogel. KEYWORDS: graphene aerogel, good amphiphilicity materials, high adsorption capacity, recyclability, high-performance catalyst

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INTRODUCTION Graphene is a type of a flat monolayer carbon material, which has a two-dimensional honeycomb lattice structure.1 Over the last few decades, advanced research has witnessed its tremendous growth in nanoscience as well as other expanding fields since the first description of graphene by Geim and Novoselov.2 It is flexible, has a large specific surface area, and exhibits remarkable mechanical, electrical, and thermal properties,3,4 making it a promising candidate for electronics, conductive nanocomposites, thermal conductors, and sensors. Material surfaces with amphiphilicity have significant applications in our daily lives.5−7 The current method for the fabrication of amphiphilic materials is selective etching of the polymer or solid templates and plasma treatment and selfassembly of block copolymers.8,9 Graphene oxide (GO) not only has a large number of hydrophilic functional groups such as the 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 material that is assembled by reduced GO (rGO),10 consisting of porous networks with different sizes of pores. Graphene aerogel can provide a large specific surface area, light-weight, and fast ion/electron © 2019 American Chemical Society

transport. These features endow it with high performance in catalysis,11,12 nanocomposites,13 and supercapacitors.14,15 The graphene aerogels not only maintain the intrinsic properties of graphene sheets16−19 but also maintain the 3D monolith structure with ultralow density and high porosity.20−22 Thus, graphene aerogels are good candidates for the amphiphilic materials. However, ascribing to the strong inner-sheet π−π attraction between graphene sheets and the removal of oxygencontaining groups, the rGO sheets tend to agglomerate or restack, making it hydrophobic. To our knowledge, most of the reports focused on the ability of organic liquid absorption,23−25 and only a few reports work on good-amphiphilicity materials.26 It is a major challenge to prepare a microstructure containing hydrophilic domains and hydrophobic domains in amphiphilic graphene aerogel. Introducing the covalent modifications or the noncovalent functional hydrophilic group on the material surface is the universal method to modify the surface.27 Herein, we cross-link the graphene Received: April 19, 2019 Accepted: May 30, 2019 Published: May 30, 2019 22794

DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

Research Article

ACS Applied Materials & Interfaces

beam spot is 500 μm. Samples were dried at room temperature in high vacuum overnight, compressed with 75 MPa, and again dried in 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 the contact angle were conducted using an optical contact angle and interface tension meter (Kino, China) by a small volume of liquid (2 μL) on 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.

aerogel with ethylenebis(nitrilodimethylene)tetraphosphonic acid (EDTMPA) to fabricate 3D graphene-based aerogel (EDGA). EDTMPA contains four phosphate groups which can endow hydrophilicity for aerogel. Meanwhile, phosphate groups can “catch” two or more rGO sheets to form the 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 (NPs) can be successfully introduced into 3D graphene architectures to form a graphene-based catalyst by one-step simultaneous reduction. It is interesting in the application of environmental remediation and catalysis.

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RESULTS AND DISCUSSION The photograph of these aerogels prepared with 3 mg/mL GO reveals a typical 3D network (Figure 1a). As shown in Figure 1a, the as-prepared aerogels are 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, GO/EDTMPA in various mass ratios were selected for further investigations (Figure S1). Interestingly, the density of the EDGA decreases with the increasing fraction of EDTMPA (Figure S2), indicating that more void space is formed as the EDTMPA increases. Intuitively, the porous structures with a size of around 5 μm appeared in EDGA (Figure 1b), and a cavity larger than 10 μm was observed in the EDGA with a GO/ EDTMPA mass ratio of 1:5 (Figure 1c). These results indicated that high hydrophilic EDTMPA has an effect on the process of self-assembled graphene described as follows: (1) the EDTMPA molecule has four H2PO3 groups, which can capture rGO sheets as the cross-linker. The EDTMPA molecule can interact with several rGO sheets to form a branched structure. Moreover, it is easy to control the hole 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 that propped up the stacking of rGO nanosheets. Further, the pore structure of EDGA was observed as isotropic pore structures. As reported by Yu et al., the graphene aerogel has higher compressibility in the axial direction than the radial direction in freezing conditions.24 Hence, in our case, the as-prepared graphene aerogel via freezing method also presents isotropic pore structures, which attributed to the different compressions in the axial and radial direction. The XRD patterns are shown in Figure 1d. The strong peak at 2θ = 10° corresponds to the interlayer spacing of GO at 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 sheets.26 These results suggest the existence of π−π stacking between the graphene sheets in EDGA and the inhomogeneous graphite-like carbon crystalline state.30 When EDGA reduced at 180 °C, the peak of 21° decreased, indicating that more oxygen-containing groups can be reduced. Interestingly, while the EDGA is reduced at low temperature (95 °C), 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

EXPERIMENTAL SECTION

Materials. EDTMPA, nitromethane (>99.0%, GC), and acetonitrile (>99.5%, GC) were purchased from Macklin Inc. 2-Phenyl1,2,3,4-tetrahydroisoquinoline (>99.0%) was purchased from Shanghai Qiao Chemical Science Inc. Graphite powder was purchased from Qingdao Jinrilai Graphite Inc. Graphite powder (density: 0.4 g/cm3, size: 0.02 mm) was purchased from Qingdao Jinrilai Graphite Inc. (Figure S11). All reagents and chemicals were employed without further purification. Preparation of Graphene Aerogels Induced by EDTMPA (EDGA). GO sheets were synthesized from graphite powder via a modified Hummers’ method.28,29 The aerogel was synthesized using the hydrothermal method.30 Homogeneous GO aqueous dispersion (10 mL) (3 mg/mL) containing 1−8 times EDTMPA with respect to GO was placed in a 25 mL Teflon-lined autoclave and heated at 180 °C for 12 h. Then, the autoclave was allowed to cool down to room temperature, and the aerogel was washed in water. Then, it was placed on an aluminum dish, which was half-dipped in liquid nitrogen for directional freezing until it was totally frozen,24 followed by freezedrying for 24 h. Synthesis of Gold NPs on EDGA. NaOH aqueous solution (15 mL) (10 mM) is taken in a 50 mL beaker and tetrakis(hydroxymethyl)phosphonium chloride (1 mL) was added dropwise in EDGA (the EDGA prepared using the hydrothermal reaction of GO dispersion (5 mg/mL), GO/EDTMPA mass ratio is 1:5), which was used as the reducing agent. Then, the EDGA is placed on the surface of NaOH solution. Chloro(triphenylphosphine)gold [Au(PPh3)Cl (Ph = phenyl)] (10 mL) (7.5 mM) in toluene was allowed to stand in contact with NaOH solution. The mixture stood at room temperature for 48 h. Then, EDGA gel freeze-dried for 24 h. Procedure for the Synthesis of 1-(Nitromethyl)-2-phenyl1,2,3,4-tetrahydroisoquinoline. In this experiment, 2-phenyl1,2,3,4-tetrahydroisoquinoline (20.9 mg, 0.1 mmol), nitromethane (54 μL, 1.0 mmol), acetonitrile (2 mL), and the catalyst gold NP on EDGA (AuNP@EDGA) (20 mg, AuNP is 5.75 mg/g) were placed in a glass tube, and the reaction was performed with stirring for 24 h at 80 °C. All solvents were blended together and evaporated to get the 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 (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). The X-ray source was an Al standard anode (1486.6 eV) at 15 kV and 180 W, whose diameter 22795

DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

Research Article

ACS Applied Materials & Interfaces

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.

Figure 2. XPS spectrum of EDGA (a) and high-resolution XPS spectra of O 1s (b), C 1s (c), and P 2p (d) of the EDGA.

respectively. As reported by Jiang et al.,31,32 the peak located at 133.0 eV was the result of P−O−H···O−graphene, which confirmed the dynamic hydrogen bonds produced in our case. The thermal stability of EDGA has been studied by thermogravimetry (TG) analysis measurements. Figure S4a shows the TG curves for graphene aerogel (blank) and EDGA after annealing at 950 °C. For the blank aerogel and EDGA, the mass loss is only 30% over the range of 100−800 °C, indicating an efficient reduction of GO. Moreover, the EDGA shows higher quality retention than that of the blank sample. While the temperature increased up to 850 °C, the mass loss of EDGA is larger than that of blank sample. As shown in Figure S4b, the mass loss of EDTMPA was much larger than EDGA at high temperature and the larger loss mass of EDGA at 850 °C might be attributed to the decomposition of EDTMPA.

phosphorus on the EDGA were monitored at different reduction times by XPS. XPS revealed a graphitic C 1s peak at around 284.8 eV and a O 1s peak at around 532.7 eV (Figure 2a), which confirmed the hydrophilic groups of EDGA, such as the epoxy group and the hydroxyl group. Meanwhile, a P 2p peak occurred at around 134.5 eV. XPS analysis is considered qualitative characterization and hence the weak XPS peak of P 2p indicates the introduction of EDTMPA into EDGA. Figure 2b−d shows high-resolution XPS of C, O, P, and their deconvolution results. These results suggest that EDTMPA was not only introduced into graphene aerogel but also formed the dynamic hydrogen bonds with graphene sheets to increase the interaction with graphene sheets.26 As shown in Figure 2d, the XPS peak of P was divided into the fitted peaks, locating at 131.5 and 133.0 eV, 22796

DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

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ACS Applied Materials & Interfaces

beneficial for the two phases to come into contact with each other. To evaluate the adsorption capacity of EDGA for water and oil, different ratios of GO to EDMPTA are 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 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 in high EDTMPA concentration, which was used to store the oil molecule. The EDGA is used for different types of liquids as the absorbent. The EDGA immersed in the commercial liquid and solvents (tetrachloromethane, dichloromethane, chloroform, palm oil, N,N-dimethyl formamide (DMF), toluene, tetrahydrofuran (THF), xylene, liquid paraffin, ethanol, dodecane, and pump oil) is considered as pollutants and daily products. They were used to evaluate the adsorption capacity of EDGA. The adsorption capacity of EDGA toward various kinds of liquids is shown in Figure 5. The results revealed that the mass

The result suggests that EDGA is a kind of thermally stable material. GO sheets carry a large number of the carboxyl, epoxy, and hydroxyl groups, and graphitic domains reside in the plane. After hydrothermal reduction, the GO sheets are reduced to 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 EDGA with hydrophilicity. Therefore, the unique composition of the hydrophilic functional group and hydrophobic domains of EDGA endow it with amphiphilic properties. Figure 3a shows that an EDGA gel placed on a dish moved toward a drop of water and a drop of oil, demonstrating its

Figure 3. (a) Sequential adsorption of oil and water by the EDGA. Photo of water (b) and oil (c) contact angles on EDGA.

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 dish to dry (Movie S1). It is different from the previous 3D graphene aerogel, which was either monohydrophobic33,34 or monohydrophilic.35 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 for the droplet to penetrate into the aerogel. These two factors are assumed to bring about the observed wetting property of the EDGA. Because of 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 the hydrophobic surface, which is

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

adsorption capacities for water and various organic solvents or oil liquids were 3000% higher than its own weight, indicating that the EDGA exhibited very efficient space utilization toward various kinds of liquids including water, which is larger than the previous reports of good-amphiphilic aerogels.36 The

Figure 4. Mass-based adsorption capacities of the EDGA for water (a) and oil (b). 22797

DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

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Figure 6. (a) Illustration of the preparation of the AuNP@EDGA. XPS spectrum of AuNP@EDGA. The Au+ was dissolved in toluene; EDGA was put in the interface of NaOH solution and toluene. Because of the amphiphilicity of EDGA, Au+ was reduced to Au0 NP in EDGA. (b) Highresolution XPS spectra of Au 4f (c) of the Au NPs@EDGA. (d) Yield of C−H bond functionalization reaction with Au NPs@EDGA and blank.

improve the catalytic activity.43,44 Commonly, the catalyst such as gold particles are used 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 NP (AuNP@EDGA) based on the amphiphilicity of EDGA. Gold NPs undergoes in situ reduction in the liquid−liquid interface of EDGA (Figure 6a). EDGA not only improves the electron conductivity but also provides a heterogeneous supporter. XPS spectra for the AuNP@EDGA are shown in Figure 6b. Figure 6c shows Au 4f7/2 (84.2 eV) and Au 4f5/2 (87.9 eV), which are consistent with metallic gold.45 The results revealed that Au+ was reduced to Au0 in EDGA. Scanning electron microscopy (SEM)− energy-dispersive X-ray spectroscopy 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 NPs, which was spherical (Figure S7a). The high-resolution transmission electron microscope image exhibited an atomic lattice fringe spacing of 0.205 nm corresponding well to the (200) plane of Au (Figure S7b). Here, the AuNP@EDGA was used for catalytic C−H bond functionalization reaction. The yield of 1-(nitromethyl)-2phenyl-1,2,3,4-tetrahydroisoquinoline is 94%, which is calculated based on the NMR result (Figures 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 electrons, further improving the activity of the catalyst. The aerogel can be recycled from the reaction, which endow the AuNP@EDGA catalyst with good recyclability. The catalyst was recycled five times without significant activity loss (Figure S10). The result indicates the stability of the catalyst in chemical reaction. Therefore, the demonstrated amphiphilicity of AuNP@EDGA has a potential application in the field of organic synthesis.

interconnected porous structure supplied the void space to store the liquid, leading to this high mass adsorption capacity. Meanwhile, the difference of mass-based adsorption capacity toward various kinds of liquids is observed. EDGA can uptake these liquids at 30−70 times its own weight. The result indicates that 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 carbons,37 wool-based nonwoven,38 nanowire membranes,39 carbon aerogels26,34,40 and silica aerogels.41 Hence, EDGA could be employed as excellent absorbents for water and various kinds of organic solvents. The EDGA showed high mass adsorption capacity for both oil and water, indicating that the amphiphilic EDGA promote the two phases to come into contact with each other for interface reaction. To optimize the recyclability and the recoverability of amphiphilic graphene aerogel, a recyclability test was performed among 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 to form the EDGA. All liquid can be driven out form the aerogel over 80% after the 10 times absorption− desorption cycle. These results suggested that 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. First, we decorate the nanocatalyst on the hydrophilic domain of the EDGA beneficial for its amphiphilicity. Then, this nanocatalyst-loaded EDGA is used as the supporter of the interface polymerization. Metal NPs 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 catalysis.42 Owing to the excellent properties of graphene, graphene-based catalysts have been proved that they can 22798

DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800

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CONCLUSIONS In summary, we designed a kind of amphiphilic graphene aerogel with EDTMPA. Because of 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06506. Characterization and recyclability of the aerogel, SEM and TEM images of the aerogel catalyst, and NMR information of the reaction product (PDF) Absorption of oil and water by EDGA (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenqiang Yu: 0000-0002-0862-9415 Bo Yang: 0000-0001-6739-4997 Caizhen Zhu: 0000-0002-7330-8300 Jian Xu: 0000-0002-9370-4829 Author Contributions ⊥

Y.Z. and T.S. contributed equally to this work and should be considered co-first authors.

Funding

This work was financially supported by the National Natural Science Foundation of China (51673117, 51574166), the Science and Technology Innovation Commission of Shenzhen (JCYJ20150529164656097, JSGG20160226201833790, JCYJ20160520163535684, JCYJ20160422144936457, JCYJ20170818093832350, JCYJ20170818112409808, and JSGG20170824112840518), and China Postdoctoral Science Foundation (2017M622786, 2017M622787). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.9b06506 ACS Appl. Mater. Interfaces 2019, 11, 22794−22800