Low-Density, Mechanical Compressible, Water-Induced Self

Jun 15, 2017 - It is found that the cross-linked GAs (xGAs) show an interesting water-induced self-recovery ability, which can recover to their origin...
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Low-Density, Mechanical Compressible, Water-Induced SelfRecoverable Graphene Aerogels for Water Treatment Shibing Ye,† Yue Liu,† and Jiachun Feng* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Graphene aerogels (GAs) have demonstrated great promise in water treatment, acting as separation and sorbent materials, because of their high porosity, large surface area, and high hydrophobicity. In this work, we have fabricated a new series of compressible, lightweight (3.3 mg cm−3) GAs through simple crosslinking of graphene oxide (GO) and poly(vinyl alcohol) (PVA) with glutaraldehyde. It is found that the cross-linked GAs (xGAs) show an interesting water-induced self-recovery ability, which can recover to their original volume even under extremely high compression strain or after vacuum-/air drying. Importantly, the amphiphilicity of xGAs can be adjusted facilely by changing the feeding ratio of GO and PVA and it exhibits affinity from polar water to nonpolar organic liquids depended on its amphiphilicity. The hydrophobic xGAs with low feeding ratio of PVA and GO can be used as adsorbent for organic liquid, while the hydrophilic xGAs with high feeding ratio of PVA and GO can be used as the filter material to remove some water-soluble dye in the wastewater. Because of the convenience of our approach in adjusting the amphiphilicity by simply changing the PVA/GO ratio and excellent properties of the resulting xGAs, such as low density, compressive, and water-induced self-recovery, this work suggests a promising technique to prepare GAs-based materials for the water treatment in different environment with high recyclability and long life. KEYWORDS: graphene aerogel, compressible, recoverable, adjustable amphiphilicity, adsorption, water treatment

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INTRODUCTION Recent advances in the controllable assembly of graphene building blocks have enabled the successful fabrication of graphene-base macroscopic architectures, such as two-dimensional graphene film and three-dimensional (3D) graphene aerogels (GAs). Because of the high porosity, large surface area, and hydrophobic properties, GAs have demonstrated great promise in purifying wastewater with oil contamination, acting as separation and sorbent materials.1−5 In most contemporary reports, 6−13 GAs have been widely explored for the sequestration and removal of organic pollutants, which are expected to overcome the increasing environmental problems arising from the oil spills. Up to now, most GAs are derived from graphene hydrogels (GHs) fabricated by chemistry assembly,6−8 polymer-induced gelation,9−11 and 3D printing.12,13 To realize the absorption capacity, the freeze-drying or supercritical drying are typically performed to convert GHs into GAs. The abundant pores inside the dried GAs can be filled with external liquid depended on its affinity with the liquid. In the practical liquid adsorption application, the liquid absorbates in these liquid-filled GAs (LfGAs), which behave more like GHs, are always removed by heat and vacuum treatment.14 Compared with these methods involving sophisticated facilities or demanding situations, releasing absorbates © 2017 American Chemical Society

by simple squeezing the LfGAs seems to be an attractive alternative approach. However, the LfGAs derive from common GAs are generally not strong enough to sustain mechanical pressures, resulting in irreversible collapse or deformation during squeezing process, which affects the recyclability and long-life of the sorbent materials. Therefore, to endow the LfGAs with shape recovery ability should be a more competitive option for the restoration of their adsorption capacity in view of the recyclability, low cost, and easy processing. For treatment wastewater with organic pollutants, such as oilspills, by above-mentioned adsorption mechanism, excellent hydrophobicity is necessary for GAs to achieve high adsorption capacity and selectivity for the oil or other organic liquid. However, for wastewater containing water-soluble dyes or heavy metals, which is also one of the most grievous threats to our environment nowadays, treating water by a filter mechanism is a common used way to purify plenty of wastewater. When the wastewater flow through the filter material, the pollutants are absorbed in it due to the strong Received: March 30, 2017 Accepted: June 15, 2017 Published: June 15, 2017 22456

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a−d) Schematic showing the formation of PGHs and xGAs. (e) A plausible cross-linking mechanism between PVA and rGO sheets through G molecules. 25% glutaraldehyde, 20 mL of methyl alcohol, 20 mL of acetic acid, 2 mL of sulfuric acid, and 140 mL of deionized water. In a typical synthesis process, 20 mL of PVA solution (5 mg mL−1), 20 mL of GO solution (5 mg mL−1), and 1.9 mL of cross-linker G solution were mixed uniformly and diluted into 100 mL by water under stirring. The mixture was then poured into a Teflon container and maintained at 50 °C for 12 h, followed by being freezed in a refrigerator (−18 °C) for 12 h. The above sample was immersed into a 60 °C HI solution (30 wt %) for 8 h to reduce the GO and washed by alcohol/water mixture to remove residual HI. The resulting precursory PGH was freezing dried (−50 °C, 20 Pa) for 48 h, as a result, the aerogel (P4GO4G0.5-1V) was obtained. For comparison, PGHs were also dried by air or vacuum, and they are named as air-dried and vacuum-dried PGHs, respectively, to distinguish from the freeze-dried PGHs (xGAs). Characterization and Tests. Fourier-transform infrared (FTIR) spectra were collected on a Thermo Nicolet Nexus-6700 spectrometer from 4000 to 400 cm−1. X-ray diffraction (XRD) patterns were performed on a PANalytical X’pert diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) spectrum was conducted on a Thermo Escalab 250XI electron spectrometer with monochromatic 150 W Al Kα radiation. Scanning electron microscope (SEM) images were captured using a Zeiss Ultra 55 field-emission SEM operated at 3 kV. Thermogravimetric analysis (TGA) assays were carried out under nitrogen atmosphere with a PerkinElmer Pyris-1 Thermal Analyzer from 50 to 700 °C at a heating rate of 20 °C/min. For the measurement of the bulk density, the weights of aerogels were determined by the Mettler electronic balance, while the sizes of the aerogels were estimated by screw gauge measurement. The average density was calculated from the weights and volumes. The compressive tests of the aerogels were carried out by using a SANS CMT-6503 universal testing machine fitted with a 50 N load cell. The water contact angles were tested by contact angle goniometer (OCA 15/20, Future Digital Scientific Corp.) at ambient temperature. The volume of the water droplet was fixed at 6.0 μL. In a typical sorption test, a weighted aerogel was immersed into organic liquids and water until it was completely filled with the liquids, followed by second weight measurement. To avoid evaporation of the organic liquid, the weight measurement should be done quickly. The weight of a piece of aerogel before and after sorption was recorded for calculating the weight gain. To determine the absorption capacity of dye, the concentrations of methyl blue (MB) in the solution before and after filtered by a homemade equipment based on xGAs were measured by UV-Abs spectroscopy.

interaction between them. To achieve a high efficiency of water treatment, an appropriate water flux is necessary for the filter material. In this case, the hydrophobic GAs are unsuitable because of its poor water flux14−16 and the appropriate hydrophilicity is needed to ensure a higher water flux and wastewater purification efficiency. Because of the different requirements for various water treatment environments, a convenient and controllable fabrication approach of GAs sorbents with demanded amphiphilicity is highly desired. In this work, we reported a way to prepare a family of lightweight, amphiphilicity-controllable, shape-recoverable cross-linked GAs (xGAs) by cross-linking graphene oxide (GO) and poly(vinyl alcohol) (PVA) with glutaraldehyde (G). The first step is to synthesize free-standing PVA/graphene hydrogels (PGHs) through cross-linking, freezing-thawing, and chemical reduction processes, while the second step is to obtain xGAs by freeze-drying above PGHs. The xGAs exhibit controllable amphiphilicity dependent on the mass ratio of PVA and GO, thus realizing the selective uptake in the oil− water mixture. It is found that the hydrophobic xGAs with low PVA/GO ratio can be used as adsorbent for organic liquid, while the hydrophilic xGAs with high PVA/GO ratio can be used as the filter material to remove water-soluble dye in the wastewater. More interestingly, the liquid-filled xGAs show unique water-induced self-recoverability and a certain of air-/ vacuum-drying tolerance, thus the xGAs have the potential to be used as recyclable and long-life sorbents in water treatment.

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EXPERIMENTAL SECTION

Materials. GO slurry (49.55% solid content) was purchased from the Sixth Element Materials Technology Co., Ltd. (Changzhou, China). The carbon to oxygen atomic ratio of GO is 1.66, the diameters was about 5 μm and the thickness is about 1 nm, suggesting about 3-layers of GO slice (Figure S1). PVA 1799 with the degree of polymerization of 1700 and the alcoholysis degree of 98−99% was purchased from Aladdin Industrial Corporation (Shanghai, China). The 25% glutaraldehyde aqueous solution was purchased from Sinopharm Co. Ltd. (Shanghai, China). Fabrication of xGAs. A series of xGAs were prepared via crosslinking PVA and GO by glutaraldehyde. The obtained sample was referred to as PxGOyGz-nV, where x, y, and z stand for the feeding weight ratio of PVA, GO, and G, while nV stands for the volume (V) of GO/G/PVA solution against 100 mL. Taking P4GO4G0.5-1V as the example, we here show the fabrication of the precursory PGHs and xGAs. The GO solution was prepared by dispersing GO uniformly in water by bath sonication (40 kHz, 20 min) followed by stirring (1000 rpm, 12 h). The PVA solution was prepared by dissolving in deionized water (95 °C, 12 h). The G solution was prepared by mixing 10 mL of

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RESULTS AND DISCUSSION Formation and Structure of xGAs. For formation of xGAs, we first prepared the precursory PGHs via a modified polymer-induced assembly method. Typically, PVA solution, GO solution, and cross-linker G solution were mixed uniformly and diluted into 100 mL of homogeneous GO/G/PVA solution 22457

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

Research Article

ACS Applied Materials & Interfaces

Figure 2. SEM image of P4GO4G0.5-1V (a) at low magnification showing the honeycomb-like cellular structure and (b) at high magnification showing the corrugated cell walls. (c) SEM image of P4GO4G0.5-2V showing the fiber-like PVA bridges rGO sheets. (d) XRD patterns and (e) FTIR spectra of GO, PVA, and P4GO4G0.5-1V. (f) XPS C 1s spectrum of P4GO4G0.5-1V.

bonds but also bridge the neighboring graphene sheets via the formation of covalent cross-linking. The transformation from GO and PVA to the final xGAs was confirmed by XRD, FTIR, and XPS measurements. Typical diffraction peaks of GO and PVA located at 11° and 20° disappeared in the XRD pattern of xGAs (Figure 2d), indicating the reduction and separation of GO sheets and the amorphous structure of PVA attached to sheets.23 Moreover, Raman and TGA measurement of aerogel before and after reducing by HI were adopt to further confirm the GO reduction. After reduced by HI, G band of sample in the Raman spectra shift from 1583 to 1604 cm−1 and the intensity ratio of D band and G band increased from 0.77 to 1.51 (Figure S3a), suggesting the reduction of GO. In the TGA curve of the resulting xGAs, the weight loss between 100 and 220 °C, which corresponds to the dropping of oxygen groups on GO, disappeared (Figures S3b and S4). All these results of XRD, Raman, and TGA indicate a successful reduction of GO. In the FTIR spectra of xGAs (Figure 2e), the characteristic bands of PVA including 3280, 2927, 1420, and 1142 cm−1 were still found in the FTIR spectra of xGAs. Meanwhile, the appearance of a broad peak in the region between 1000 and 1250 cm−1 implies the cross-linking of GO sheets and PVA by forming ether chemical bonds.21 The XPS C 1s spectrum of the xGAs can be fitted with four peaks at binding energies of 284.8, 286.2, 286.9, and 287.8 eV, which may be assigned to CC sp2 bonds, C−O−C bonds, C−O bonds, and CO bonds, respectively (Figure 2f). Compared with the spectrum of GO (Figure S3c), the decrease of peak at 286.9 and 287.8 eV in the spectrum of xGAs may be caused by the reduction of GO, while the more obvious peak at 286.2 eV corresponding to the C−O−C bonds may be attributed to the formation of ether bridges.21 Both FTIR and XPS results indicate the cross-linking through intermolecular acetalization. Density of xGAs. By changing the solution volume and adjusting the feeding weight ratio of PVA, GO and G, several series of xGAs were prepared. It is generally accepted that the continuous network of aerogels formed by freezing-thawing process is mostly determined by the ice crystal template.24 Figure S5a-d describes the cross-sectional morphology of xGAs, corresponding to P4GO4G0.5-1V, P4GO4G0.5-1.25V, P4GO4G0.51.5V, and P4GO4G0.5-2V, respectively. It is easy found that the

under stirring (Figure 1a). The mixed solution was poured into a Teflon-lined autoclave and maintained at 50 °C for 12 h to form cross-linked GO/G/PVA gels (Figure 1b). The hydrogel sealed in the autoclave then underwent an ice-template freeze casting process to build the macroporous honeycomb-like structure. After a natural thawing process and a chemical reduction with HI, the constituent GO sheets were converted into reduced GO (rGO) sheets (Figure 1c). Subsequent freezing drying transformed the obtained wet PGHs into dried xGAs (Figure 1d). One of the advantages of our fabrication method is its scalability: the size of xGAs can be easily scaled up from 20 to 400 cm3 just by using a larger autoclave under constant reaction conditions (see the Figure S2a and b). It could be expected that further scale-up should be possible with a larger reaction vessel. It is interesting to find that both PGHs and xGAs exhibit negligible volume shrinkage after assembly and freeze-drying in contrast with the size of autoclave (Figure S2c), which is remarkably different from the large volume shrinkage of the common GAs prepared by chemistry assembly method.6−8 Here, P4GO4G0.5-1V, fabricated by 100 mL solution containing 200 mg of GO, 200 mg of PVA, and 25 mg of G, was chosen to demonstrate the chemical structure of xGAs and interactions between PVA and GO. The xGA monolith exhibits a typical honeycomb-like cellular structure with interconnected pores ranging from tens to hundreds of micrometers by microscopic observation according to the SEM images (Figure 2a), similar to those of previous reported GAs.17−19 These cell walls are made of multilayer, stacked rGO sheets, which are pushed together during the formation of ice crystals (Figure 2b). Detailed observation of another sample fabricated from a solution with lower concentration (P4GO4G0.5-2V) reveals that entangled, fiber-like PVA network acted as binder covering and stitching two or more rGO sheets together (Figure 2c). It was reported that G molecule is an effective cross-linker for both GO sheets and PVA.20−22 In our work, the aldehyde groups of G molecules readily react with the adjacent hydroxyl groups on the surface of GO sheets and the PVA chains through intermolecular acetalization (Figure 1e). Therefore, PVA chains not only attach onto graphene surfaces through hydrogen 22458

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

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Figure 3. Bulk density of xGAs as a function of (a) solution volume and (b) weight ratio of PVA and GO. (c) Comparison of the density of P4GO4G0.5-2V with the reported values of graphene-based aerogels.

Figure 4. Compressive stress−strain curves of the xGAs prepared (a) in different solution volume and (b) with different PVA/GO ratio. (c) Relationship between the compressive stresses at 75% strain and the densities of different xGAs. (d) Digital photographs of a compressed P4GO4G0.5-1V showing the recovering process. The compressive stress−strain curves of (e) P4GO4G0.5-1V and (f) P1GO7G0.5-1V at the maximum strain of 50% for 5 cycles.

ratio increases into 3:5, the density of xGAs decreases to 4.5 ± 0.3 mg cm−3. Further, by fixing PVA/GO weight ratio (4/4) and solution volume (100 mL, 1 V), a series of xGAs with different G content (P4GO4G0.25-1V, P4GO4G0.5-1V, P4GO4G11V, and P4GO4G1.5-1V) were prepared and the density of them was measured to study the effect of G content on the density. It is found that the density of the xGAs with different content of G is similar, which is between 4.3 to 4.6 mg cm−3 (Figure S6a). In addition, we also studied the effect of PVA and GO content on the density respectively by fixing the volume and the content of other components, it can be seen that the density of xGAs increased with increasing the content of both PVA and GO when the content of another one was fixed as well (Figure S6b and c). It is suggested that the density of xGAs can be adjusted by the solution volume and the content of components. However, the decrease of density is restricted by the shrink and collapse of aerogel when the pore network is too sparse, so there is a limit for the minimum density. Among all the samples we studied above, the bulk density of P4GO4G0.5-2V reached the

micropores become sparser and the size of the pores increases as the solution volume increases. Therefore, it can be expected that the bulk density of xGAs can be easily tuned by varying the solution volume. As shown in Figure 3a, the density for the sample prepared from 100 mL solution (P4GO4G0.5-1V) was 4.5 ± 0.2 mg cm−3 and the density decreased with increasing the solution volume when fixing the feeding weight ratio of PVA to GO. When diluting the solution into 200 mL (P4GO4G0.5-2V), the bulk density can be adjusted to as low as 3.3 ± 0.3 mg cm−3. Notably, the density could not be further decreased by increasing the solution volume because the xGAs with high porosity tend to shrink during freeze-drying. We also studied the content effect of each component on the bulk density of xGAs. First, a series of xGAs, P7GO1G0.5-1V, P5GO3G0.5-1V, P4GO4G0.5-1V, and P3GO5G0.5-1V, were prepared by fixing the solution volume (100 mL, 1 V) and total mass of PVA and GO (400 mg) but changing the feeding ratio of PVA to GO. As shown in Figure 3b, the density exhibits a slight decrease trend. The density of xGAs with PVA/GO ratio of 7:1 is 5.7 ± 0.2 mg cm−3, while when the PVA/GO feeding 22459

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

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ACS Applied Materials & Interfaces

Figure 5. Digital photographs showing (a) the squeezing of PGHs and water-induced self-recovery process of the squashed PGHs and (b) the waterinduced self-recovery process of the vacuum-dried PGHs.

minimum value of 3.3 mg cm−3, when 200 mL solution contains 200 mg of PVA, and 200 mg of GO, which is among the lowest density of the graphene-based porous materials ever reported (Figure 3c).25−29 Mechanical Properties of xGAs. Compressive tests were performed to study the compressibility and strength of xGAs. Figure 4a and 4b shows the compressive stress−strain curves of xGAs up to 75% strain at a loading rate of 10 mm min−1. It is found that there are no detectable collapses occurring during the compression process of xGAs (Figure S7). It can be seen that the compressive stress−strain curves show three regions, including the elastic region (ε < 10%), the plateau region (10% < ε < 60%) and the densification region (ε > 60%), corresponding to the elastic bending of cell walls, the elastic buckling of cell walls and the continuously decreasing pore volume, respectively.15,17,18 The compressive properties are influenced by the pore network structure, which is related closely with the solution concentration (volume). As shown in Figure 4a, the compressive strength of P4GO4G0.5-nV was 3.3, 4.6, 5.5, and 10.9 kPa, respectively, when n = 2, 1.5, 1.25, and 1. The result demonstrates that the compressive strength of xGA increases as the volume of the original solutions decreases, which is mainly caused by the densification of the denser cells.30 We believe that xGAs from higher starting concentrations of GO and PVA will have a larger proportion of chemically cross-linked parts and thus they will exhibit higher stress at the same compressive strain compared to those prepared from low starting concentrations. For xGAs with different PVA/GO ratio when fixing the total content of GO and PVA, the compressive behavior was also different. The resulting xGAs exhibit improved compressive strength as decreasing PVA/GO ratio (Figure 4b). Because of the higher modulus and strength of graphene sheets than those of PVA, bending or damage of the graphene macropores is more difficult than the PVA aerogels. In this case, the constituent rGO sheets can enhance the robustness of the cell walls of aerogels.31 For the xGAs with fixed PVA/GO weight ratio and solution volume, the compressive strength only changes a little with varying the feeding mass of cross-linker (shown in Figure S6d). The excellent mechanical properties and structural integrity of the xGAs were attributed to the coupling of the cross-linking networks with the chemical identities of the PVA and rGO sheets.

We plotted the compressive strength at 75% strain for all above xGAs with different bulk densities. As shown in Figure 4c, a clear trend could be found that the value of compressive strength is nearly proportional to the density of xGAs. Since that the higher the bulk density of xGAs, the denser the pores in xGAs and the more serious densification of porous structure, in which the walls of the macropores start to touch each other and become the loading-bearing portion. It was found that some of our xGAs could be manually compressed to ∼50% strain and recovers to its most original height when the loading was removed. As shown in Figure 4d and Movie S1, the cylindrical P4GO4G0.5-1V was rapidly squeezed and bounced back at a high speed. The cyclic strain− stress curves at 50% strain shown in Figure 4e-f quantificationally estimate the compression-resilience property and cycling performance. During the recovery process, the stress decreases to zero before the strain reaches zero, suggesting the xGAs cannot completely recover to its original height instantaneously. The P4GO4G0.5-1V showed a large irreversible deformation of nearly 25% while the P1GO7G0.5-1V only experienced 8% reduction in height, which indicates that the compression-resilience property of xGAs decreases with increasing the PVA content. Water-Induced Self-Recovery of Compressed and Dried PGHs. As precursory hydrogels, our PGHs have the capability of expansion/contraction upon absorption/desorption of the solvent. As shown in photographs (Figure 5a) and movies (Movies S2 and S3), after squeezing out water, a cylindrical PGH with a height of 4.5 cm and a diameter of 2 cm was strongly compressed (ε>90%) into a thin pellet with a height of ∼4 mm and a diameter of 2 cm. When most of the water was squeezed out, the height or the volume of compressed PGHs cannot recover. Upon being reimmerged into water, interestingly, the squashed pellet was rapidly saturated with water and almost recovered its initial shape without any fracture with only 15 s, indicating an attractive water-induced self-recoverable property. This interesting phenomenon means that the pores in compressed PGHs cannot be occupied by air when the water was driven off but can be restored by water.15 This expansion/contraction process of our PGHs enabled by squeezing and absorbing water could be repeated more than 100 times without breaking the hydrogels. The repeated water-induced self-recovery of the 22460

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

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ACS Applied Materials & Interfaces

Figure 6. (a) Absorption capacity of the P4GO4G0.5-1V for various organic liquids. (b) Absorption capacity of different xGAs for the water and cyclohexane.

amphiphilic properties (Figure S8). It is found that the water contact angle of P1GO7G0.5-1V maintained a constant 103.5° after 60 s, suggesting that P1GO7G0.5-1V was hydrophobic. On the contrary, for all the other three aerogels, water was rapidly absorbed by the aerogels in a few seconds, which means that they are more hydrophilic. However, there are still some differences among the three hydrophilic aerogels (P3GO5G0.51V, P4GO4G0.5-1V, and P5GO3G0.5-1V): water droplet can remain on the surface of P3GO5G0.5-1V about 5 s and shrank quickly within 16 s, while water began to be adsorbed as soon as it touched the surfaced of P4GO4G0.5-1V and disappeared in 4 s; as to P5GO3G0.5-1V, water droplet was adsorbed in only 1 s and it can be noted that the aerogel expanded obviously. These differences can reveal that the order of hydrophilicity is P5GO3G0.5-1V, P4GO4G0.5-1V, and P3GO5G0.5-1V, consistent with increasing the weight ratio of PVA and GO. In conclusion, for our fabricating method, in which the xGAs are prepared through cross-linking PVA and GO by G in this work, the amphiphilicity of xGAs can be adjusted simply via changing the feeding ratio of PVA and GO. It is well-known that the adsorb capacity of aerogels is strongly associated with their amphiphilicity. We chose the sample with a median PVA/GO ratio (P4GO4G0.5-1V, the PVA/GO in which is 1:1) as an example, to determine its absorption capacity for various liquid chemicals. Figure 6a shows that ∼170 mg of methylbenzene, ∼130 mg of petroleum ether, ∼148 mg of octane, ∼159 mg of cyclohexane, ∼162 mg of ethanol, ∼149 mg of pump oil, and ∼274 mg of chloroform can be absorbed per milligram of P4GO4G0.5-1V (on the first absorption cycle). That is to say P4GO4G0.5-1V can absorb the organic liquids at 130−274 times its own weight, Moreover, because P4GO4G0.5-1V contains nearly equal content of hydrophilic PVA and hydrophobic rGO, it not only shows a high adsorption ability for organic liquid, but can also adsorb water at ∼178 times its own weight (Figure 6a). Using amphiphilic P4GO4G0.5-1V as a reference, it can be expected that the xGAs with lower PVA/GO ratio will be more hydrophobic and have stronger capability for oil adsorption, while the xGAs with higher PVA/GO ratio will be more hydrophilic and more suitable for adsorption of water. Figure 6b shows the absorption capacity for polar water and nonpolar cyclohexane of the xGAs with different PVA/GO ratio. With increasing PVA/GO ratio from 1:7 to 5:3, the absorption capacity for water gradually increases from 12 ± 3 to 226 ± 9 mg/mg. On the contrary, the absorption capacity for cyclohexane rises at first and goes down later, producing a

PGHs can be explained by the existing covalent cross-linking between GO sheets and PVA. For common GHs, the vacuum- and air-drying induced shrinkages are usually irreversible, which is greatly unfavorable for the repeatable use of hydrogels. Similar to the GHs prepared by other methods,1,2 our PGHs could not tolerate the capillary force of water during vacuum- or air-drying process, resulting in serious shrinkages. Interestingly, vacuum-dried PGHs in our work could recover its original shape, reproducing the saturated hydrogels. As shown in Figure 5b and Movie S4, the small piece of lightweight, vacuum-dried PGH was gradually saturated with water and recovered most of the volume when immersed into water after relatively long periods (∼1 h). In addition, except the vacuum-dried PGH, we also found that the air-dried PGHs and the freeze-dried PGHs (xGAs in this work) under extreme high compression strain (ε>95%) could also recover its original shape after sorption of water. Notably, when xGAs are immersed into water, they adsorb water quickly and behave like the precursory PGH, which exhibits excellent repeated water-induced self-recoverable property. The self-recovery behavior always occurs in the sensitive polymer-based hydrogels,32−34 but has rarely been observed in graphene-based hydrogels and aerogels. From the perspective of chemical structure, the shape-memory property of xGAs and the self-recoverable property of PGHs are mainly attributed to the double network that consisted of the graphene skeleton and the flexible, water-sensitive PVA coating.1 We believe that the covalent cross-linking networks determine the mechanical strength as well as fix the shape of aerogels. In addition, due to the sensibility to water of PVA, water can be adsorbed into pores and form hydrogen bonds with PVA chains easily when the compressed xGAs was reimmersed into water, which helps the cross-linking networks recover to its original shape. In addition, the excellent swelling properties of PVA may be also one of the reasons for self-recovered property. These features of our PGHs and xGAs enable them to be applied in many promising areas and also open a new approach to design vacuum/air drying-tolerant hydrogels or aerogels. Adjustable Amphiphilicity of the xGAs and Water Treatment Application. Considering that PVA has abundant hydrophilic hydroxyls, while rGO sheets is naturally hydrophobic, the amphiphilicity of xGAs may be adjusted simply via changing the feeding ratio of PVA and GO. So the affinity of xGAs can be controlled from polar water to nonpolar organic liquids depended on its amphiphilicity.35 The water contact angle of various xGAs samples was tested to examine the 22461

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

Research Article

ACS Applied Materials & Interfaces highest uptake of 161 ± 8 mg/mg in P4GO4G0.5-1V. The fluctuant oil uptake should be the synergistical result of the changing density and amphiphilicity of xGAs. For the hydrophobic P1GO7G0.5-1V sample, which shows a good selectivity for oil and is suitable for oil−water separation, its absorption capacity for oil is 118 ± 7 mg/mg, which is comparable with those of many previously reported absorbents, such as polymer foam (44−96 times),36 carbon foam (62−102 times),37 CNT sponge (80−180 times),18 carbon fiber aerogel (50−192 times),3 and graphene sponge (120−250 times),15 but lower than those of nitrogen-doped graphene foam (200− 600 times)38 and ultra-fly-weight aerogels (215−743 times).4 Figure 7a and b shows the photographs of xGAs picking up oil or water respectively from the mixture. As shown in Figure

contrast, Figure 7b shows that the P5GO3G0.5-1V selectivity removed the highly polar water from the bottom of the cyclohexane/water mixture within tens of seconds (Movie S6), indicating a good affinity with water. As discussed before, the PGHs and liquid-filled xGAs exhibit a unique water-induced self-recovery ability. Here we found that the compressed PGH (the precursor of P5GO3G0.5-1V) could also selectively adsorb water from the cyclohexane/water mixture during the recovery process (Figure 7c and Movie S7). We believe that the waterinduced self-recovery ability combined with the excellent compressive properties can greatly improve the recyclability of these xGAs as absorbents for removing water from oils. In addition to oil/water separation through an adsorption mechanism, the amphiphilicity-controllable xGAs are also found to be a class of promising absorbents for the treatment of wastewaters containing water-soluble dyes through a filter mechanism. On the one hand, PGHs and xGAs have good affinity toward water, guaranteeing sufficient contact area for the pollutants dissolved in water;34 On the other hand, PGHs and xGAs can capture the aromatic dye molecules through formation of π−π stacking with the constituent graphene sheets.39 We roughly measured the water flux of each xGAs using a homemade equipment based on an injector (inset of Figure 8b). Under gravity effect, the water flux was measured to be 10, 26, and 0.3 mL min−1 for the P1GO7G0.5-1V, P4GO4G0.51V, and P7GO1G0.5-1V, respectively (Figure 8a). The water flux may be affected by two aspects: the channel porosity and hydrophilicity of xGAs. Among the investigated samples, P1GO7G0.5-1V has lower water flux than P4GO4G0.5-1V because the former is more hydrophobic than the latter, while the P7GO1G0.5-1V shows an extremely low water flux due to the swollen PVA seriously blocks the channel pores in the hydrogels. An appropriate water flux is essential for the absorbents or membranes in the water treatment, therefore, the filter material with appropriate hydrophily is more favorable than those extreme hydrophobic carbon-based aerogels. To determine the efficiency of the wastewater purification using xGAs, we insert an aerogel into the injector and used it as a filter for the treatment of the water polluted with MB (inset of Figure 8b). After quick filtration through ∼30 mg of P4GO4G0.5-1V, the original 20 mL of blue MB solution (50 mg L−1) changed to nearly colorless water (Movie S8). Analyzed from UV/vis spectra,11 the concentration of MB in the decontaminated water decreased from 30 to 4 mg L−1, which mean that about 92% MB was removed during filtrated by P4GO4G0.5-1V (Figure 8b), indicating that xGAs has an efficient adsorption capacity for MB. Actually, not only MB, any

Figure 7. Digital photographs showing the process of absorbing (a) chloroform from the chloroform/water mixture with P4GO4G0.5-1V and (b) water from the cyclohexane/water mixture with P5GO3G0.51V, as well as (c) water from the cyclohexane/water mixture with a compressed PGH (the precursor of P5GO3G0.5-1V). Chloroform was stained with Sudan III while the water was stained with MB.

7a, P4GO4G0.5-1V picking up nonpolar chloroform from the chloroform/water mixture within several seconds (Movie S5), where chloroform is denser and sits at the bottom of the vial. In

Figure 8. (a) Measured water flux of different xGAs. (b) Absorption UV spectra of the initial MB solution (50 mg L−1) and the purified solution after filtering through a P4GO4G0.5-1V sample. Inset showing the homemade filtration equipment and the corresponding solutions. 22462

DOI: 10.1021/acsami.7b04536 ACS Appl. Mater. Interfaces 2017, 9, 22456−22464

Research Article

ACS Applied Materials & Interfaces water-soluble pollutant, which has strong interaction with xGAs, can be removed by this method. Considering the convenience of adjusting the amphiphilicity by simply changing the PVA/GO ratio, our method can extend the GAs potential usage range in the wastewater purification.

Movie S6 showing P5GO3G0.5-1V selectivity removing the highly polar water from the bottom of the cyclohexane/water mixture (AVI) Movie S7 showing compressed PGH selectively adsorbing water from the cyclohexane/water mixture during the recovery process (AVI) Movie S8 showing that quick filtration through ∼30 mg of P4GO4G0.5-1Vchanges the blue MB solution to nearly colorless water (AVI)

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CONCLUSIONS In summary, we have fabricated a new kind of compressible, lightweight (3.3 mg cm−3) graphene aerogels through simple cross-linking of GO sheets and PVA by G. The xGAs can recover to their original volume even under extremely high compression strain or after vacuum-/air-drying. Unlike conventional, hydrophilic carbon-based aerogels, the amphiphilicity of xGAs can be adjusted facilely by changing the feeding ratio of GO and PVA and it exhibits affinity from polar water to nonpolar organic liquids depended on its amphiphilicity. Therefore, this method can fulfill the requirements of water treatment in different environment, such as treating wastewater with oil-spills through adsorption mechanism and wastewater with water-soluble dye through filter mechanism. Considering the convenience of adjusting the amphiphilicity by simply changing the PVA/GO ratio and the excellent properties of the resulting xGAs including low-density, compressive and waterinduced recovery properties, our approach is suitable to prepare xGAs for the different wastewater purification situation with high recyclability and long-life.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 65643735. Fax: +86 21 6564 0293. E-mail: [email protected]. ORCID

Jiachun Feng: 0000-0002-9410-7508 Author Contributions †

S.Y. and Y.L. contributed equally to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (51373042) and the Science and Technology Planning Project of Guangdong Province of China (2014B090901009).

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ASSOCIATED CONTENT

* Supporting Information S

REFERENCES

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