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Bio-inspired Assembly of Carbon Nanotube into Graphene Aerogel with “Cabbage-Like” Hierarchical Porous Structure for Highly Efficient Organic Pollutants Cleanup Wenwei Zhan, Siruo Yu, Liang Gao, Feng Wang, Xue Fu, Gang Sui, and Xiaoping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15322 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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ACS Applied Materials & Interfaces

Bio-inspired Assembly of Carbon Nanotube into Graphene Aerogel with “Cabbage-Like” Hierarchical Porous Structure for Highly Efficient Organic Pollutants Cleanup Wenwei Zhan, †, ‡ Siruo Yu, †, ‡ Liang Gao, †, § Feng Wang, †Xue Fu, † Gang Sui,*, †and Xiaoping Yang† †

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

§

AVIC Composite Corporation LTD, Beijing 101300, China

ABSTRACT: Nowadays, physical absorption has become a feasible method offering an efficient and green route to remove organic pollutants from the industrial wastewater. Inspired by polydopamine (PDA) chemistry, one dimensional PDA functionalized multi-walled carbon nanotubes (MWCNT-PDA) were creatively introduced into graphene aerogel framework to synthesis a robust graphene/MWCNT-PDA composite aerogel (GCPCA). The whole forming process needed no additional reducing agents, significantly reducing the contamination emissions to the environment. The GCPCA exhibited outstanding repeatable compressibility, ultralightweight as well as hydrophobic nature, which were crucial for highly efficient organic pollution absorption. The MWCNTs in moderate amounts can provide the composite aerogels with desirable structure stability and extra specific surface area. Meanwhile, the eventual absorption performance of GCPCAs can be improved by optimizing the microporous structure. 1 ACS Paragon Plus Environment

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In particular, a novel “cabbage-like” hierarchical porous structure was obtained as the prefreezing temperature was decreased to -80 °C. The miniaturization of pore size around the periphery of GCPCA enhanced the capillary flow in aerogel channels and the super absorption capacity for organic solvents was up to 501 times (chloroform) of its own mass. Besides, the GCPCAs exhibited excellent reusable performance in absorption-squeezing, absorptioncombustion and absorption-distillation cycles according to the characteristic of different organic solvents. Due to the viable synthesis method, the resulting GCPCAs with unique performance possess broad and important application prospects, such as oil pollutions cleanup and treatment of chemical industrial wastewater. KEYWORDS: Graphene Aerogel, Hierarchical Porous Structure, Organic Solvent Absorption, Carbon Nanotubes, Recyclability.

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1. INTRODUCTION Over the past few decades, the increasing water contaminants arising from industrial wastewater and leakage of toxic organic solvents or oil products have been seriously endangering the human health and the ecological balance. Up till now, various methods have been taken for the removal of aforementioned pollutants from water, including absorption, 1 in situ burning, 2 bioremediation 3 and photodecomposition, 4 etc. Among these techniques, physical absorption is regarded as the most attractive one owe to its high efficiency, ease of operation and low energycost. Traditional absorbents, 5-9 such as expanded perlite, zeolites, activated carbon, sawdust, clay and wool fiber, have been investigated because of their microporosity. However, these conventional absorbents present drawbacks of relative low efficiency, poor separation ability and secondary pollution. Therefore, it is an urgent demand for developing ideal absorbents with excellent absorption selectivity, high absorption capacity, proper recyclability and low cost. Recently, the use of three-dimensional (3D) porous materials, such as carbon-based or oleophilic polymer-based aerogel,

10-11

foam

12-13

and sponge,

14-15

has attracted growing attentions. 3D

interconnected monolithic architecture with low density and high porosity has been proved more efficient 16-17 than common 2D membrane materials. Graphene aerogel (GA), as an emerging porous macroscopic material, exhibits unique properties such as intrinsic hydrophobicity, high internal surface area and good chemical stability, thereby enabling it a promising potential candidate for the application in advanced absorption field. 18-20 However, pure GA is generally brittle due to the formation of huge channels during ice 3 ACS Paragon Plus Environment


crystal growth,

21-22

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which inevitably affects the performance during pollutants adoption and

aerogel absorbents recycle process. At present, combining the mechanical strength of one dimensional carbon nanotubes (CNTs) and large surface area of GAs has been taken into account for widespread applications23-28, for instance, solar steam generation, high-performance capacitive deionization, selective oil/water separation and shape memory composite manufacturing. The introduction of CNTs provided aerogels with an ability to maintain the structural integrity under deformation induced from external forces. Meanwhile, CNTs can also improve surface roughness

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and hydrophobicity

30

of GAs, consequently improved oil/water

separation efficiency or organic pollution absorption performance. Wan 31 et al. embedded CNTs into GA network by a one-step hydrothermal redox reaction. The obtained graphene–CNT hybrid aerogels exhibited good absorption capacity of oil up to 270 g g-1 as well as excellent recycling performance after absorption–combustion experiments. Lee

32

et al. prepared CNT-bonded

graphene hybrid aerogel by growing CNTs on a GA surface in the presence of nickel catalyst. Both anionic and cationic dyes can be effectively removed from water by using as-prepared hybrid aerogel. Wang 33 et al. synthesized CNT/graphene hybrid aerogel using GO as precursor at first and employed it as a template for further in-situ growth of nanotubes. The hierarchical structure endowed resulting aerogels selective absorption of organics and oils from water. According to the relevant literatures, 34-37 three methods are mainly adopted for fabricating CNTgraphene composite aerogel, including chemical vapor deposition (CVD), hydrothermal reduction and chemical reduction process. CVD method needs expensive precursors and complex 4 ACS Paragon Plus Environment

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equipment that restricts practical applications. Other two strategies seem more feasible. However, the high energy wastes and the use of large amount of chemical reagents actually run contrary to our eco-friendly and economical vision. In recent years, bio-inspired polydopamine (PDA) chemistry 38-40 is widely concerned and has become one of focuses in the field of material surface modification. Under weak alkaline environment, the dopamine can spontaneously oxidize and self-polymerize to form nanoscale PDA layer on almost all substrates. 41-42 Moreover, the resulting PDA is rich in catechol and amino functional groups, which provide reactive sites 43-45 for further reactions. Song 46 et al. used dopamine as reducing agent to prepare N-doped GAs. They believed that dopamine underwent oxidative polymerization to produce PDA adherent layer and simultaneously served as reducing agent for the reduction of graphene oxide (GO). Li 47 et al. fabricated multifunctional ultra-light GAs with low volume shrinkage through hydrothermal process of GO/dopamine colloidal solutions. The addition of dopamine constructed cross-linking points among the GO sheets and provided aerogel with outstanding fire retardant performance, which kept its skeleton integrity under combustion condition during recycling process. Therefore, our aim is to fabricate robust and recyclable graphene aerogels via green route for highly-efficient organic pollutants cleanup. PDA functionalized multi-walled carbon nanotubes (MWCNT-PDA) were creatively introduced into the framework of graphene aerogels herein. The whole forming process of composite aerogels needed no additional reducing agents and asprepared GO/MWCNT-PDA composite aerogels (GCPCAs) exhibited highly repeatable 5 ACS Paragon Plus Environment


compressibility. In this article, in-depth and systematic studies have been done, which mainly concerned about the relationships among aerogel architecture, absorption mechanism and eventual absorption capacity. It was demonstrated that the volume shrinkage and structural stability of the composite aerogels were determined by the content of MWCNT-PDA. Interestingly, “cabbage-like” porous structure was obtained when the pre-freezing temperature was decreased to -80 °C. Directional alignment of hierarchical porous structure significantly enhanced the capillary flow of liquids in aerogel channels, thereby endowed the as-prepared composite aerogels

with super absorption capacity for organic pollutions up to 501 times

(chloroform) of its own mass. More impressively, three recycling means (absorption-squeezing, absorption-combustion and absorption-distillation) of aerogels can be tailored according to the characteristic of absorbed organic solvents and almost 90 % initial absorption capacity was retained after 10 cycle experiments. The proposed composite aerogels with optimized hierarchical porous structure exhibited a promising prospect for efficient treatment of oil contaminations, toxic dyes and heavy metal ions wastewater. 2. EXPERIMENTAL SECTION 2.1 Materials. Pristine graphite flakes with a mean diameter of 500 µm were provided by Xianfeng Nanomaterials Technology Co. Ltd., Nanjing, China. The carboxyl-functionalized multi-walled carbon nanotube (MWCNT-COOH) (purity, ≥95%; diameter, 10-20 nm) was provided by Chengdu Organic Chemical Co. Ltd., China. Dopamine hydrochloride and tri (hydroxymethyl) aminomethane (Tris) were purchased from Sigma-Aldrich Co.Ltd., USA. 6 ACS Paragon Plus Environment

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Potassium permanganate (KMnO4), sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4, 98 %) and hydrogen peroxide (H2O2, 35 %) were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. 2.2 Preparation of GO. The GO was prepared through modified Hummers method.

48

Typically, 5 g of pristine graphite flakes and 2.5 g of NaNO3 were mixed with 110 mL of H2SO4 in a 500 mL breaker. The mixture was stirred for 0.5 h in an ice bath. Next, 15 g KMnO4 was carefully added into the suspension under moderate stirring. The mixture was first kept at 0~5 °C and then the temperature was increased to 35 °C followed by continuous stirring for 12 h. After that, 200 mL deionized water was slowly added into the mixture. The diluted suspension was stirred for another 0.5 h before 25 mL H2O2 was added to the mixture. Then the mixture solution was treated under ultrasonication for 3 h and dialyzed for several days until the pH value of the solution became neutral. Eventually, GO dispersion solution was obtained after 3000 rpm centrifugation treatment. The concentration of GO in the suspension was determined by freezedrying small amount of suspension and then weighing the dried sample. 2.3 Preparation of MWCNT-PDA. The MWCNT-PDA was prepared according to our previous work. 40 To start with, 100 mg MWCNT-COOH were dispersed into 200 mL deionized water using an ultrasonicator for 0.5 h. Then 200 mg dopamine hydrochlorides were added into the suspension under stirring. The pH value of the mixture was adjusted to 8.5 using Tris-HCl buffer solution. The suspension was then continuously stirred at 25 °C for 48 h and the resulting MWCNT-PDA was washed several times with deionized water and ethanol solvent, followed by 7 ACS Paragon Plus Environment


3000 rpm centrifugation. Finally, the dialysis treatment was used to completely remove free polydopamine and MWCNT-PDA was eventually obtained after freeze-drying process for 24 h. 2.4 Preparation of GO/MWCNT-PDA composite aerogels. For the preparation of GO/MWCNT-PDA composite aerogels (GCPCAs), the aforementioned GO and MWCNT-PDA with a series of mass ratios (2:1, 4:1, 5:1, 6:1 and 8:1) were dissolved in deionized water by ultrasonic dispersion for 0.5 h, and the concentration of GO was 2 mg mL-1. Next, 25 mL of the mixture solution were placed into a cylindrical vial and maintained at 90 °C in an oil bath for 12 h. After that, the vial was cooled to room temperature, and the as-formed GO/MWCNT-PDA hybrid hydrogel was poured out and dialyzed with deionized water and ethanol solvent. The hydrogel was gradually pre-frozen at -20 °C, -50 °C and -80 °C in the lyophilizer (corresponding aerogels were briefly denoted as GCPCA-20, GCPCA-50 and GCPCA-80). Finally, the ultralight highly compressible GCPCA was obtained after freeze-drying process for 24 h and directly annealing at 750 °C in N2 atmosphere for 3 h. It should be noted that when investigating the effect of pre-freezing temperature, the mass ratio of GO and MWCNT-PDA was consistently set as 4:1. In addition, the composite aerogel with “cabbage-like” hierarchical porous structure can be obtained when the pre-freezing temperature was decreased to -80 °C. 2.5 Characterization. The densities of GCPCAs were calculated by measuring the volume using a digital vernier caliper and the mass using a balance with 0.1 mg accuracy, where the density of the air occupied in the pores was inevitably included. The morphology and microstructure of GCPCAs were characterized on a scanning electron microscope (SEM, Hitachi 8 ACS Paragon Plus Environment

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S-4700) and high resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S). Nano Measurer 1.2 image analysis software was used to count the micro-scale pore size distribution of GCPCAs. High resolution X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was performed to analysis the surface chemical composition and atomic ratio of elements in GCPCAs. Fourier transform infrared spectra (FTIR) of all samples were obtained using a Nicolet 8700 spectrometer with a scan range of 4000-400 cm-1. The phase structure of GCPCAs were characterized through X-ray diffraction (XRD) on a Shimadzu LabX XRD-6000 diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (λ=0.154 nm). Raman spectroscopy was recorded using a Renishaw inVia Raman microscope at an excitation wavelength of 514 nm. The nano-scale pore size distribution of GCPCAs was measured via adsorption and desorption of N2 using a Brunauer–Emmett–Teller (BET, JW-BK200C) surface area measurement system. The specific surface area was determined from the measured isotherms at 77 K through Barrett−Joyner−Halenda (BJH) methods. The contact angles of composite aerogels were measured with 5 µL droplets of water and chloroform using the contact angle measuring system (JC2000D3 POWEREACH) at room temperature. The as-prepared GCPCAs were compressed from 0 % to 50 % of the compressive strain. Stress-strain curves were measured by dynamic thermomechanical

analysis

(DMA,

TA

Q800)

in

a

controlled strain-rate mode with

10 %/min strain rate. Chloroform, n-hexane, n-heptane, n-dodecane and acetone were selected as organic pollution to evaluate the absorption capacity of as-prepared GCPCAs. Firstly, the initial mass of 9 ACS Paragon Plus Environment


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the samples was weighed and recorded as mbefore. Next, the samples were immersed into above organic chemicals until fully saturated and then taken out by tweezers after 5 min. After removing the redundant organic solvents with filter papers, the samples were weighed. When the mass of the samples was unchanged with the increasing time, the weight was recorded as mafter. The absorption capacity (Qwt, g g-1) at saturated state was calculated by the following equation: =

(1)

All absorption experiments were repeated five times to obtain statistically reliable values.


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3. RESULTS AND DISCUSSION Figure 1 illustrates a general synthesis procedure and formation mechanism of the 3D GCPCA. Firstly, the GO and MWCNT-PDA were adequately mixed and sonicated in water to form a well-dispersed mixture solution. The dispersion stability of suspension was significantly enhanced owing to the existence of catechol groups on the sidewall of MWCNT-PDA. Subsequently, the dispersion was transferred into a cylindrical sample vial and treated at 90 °C for 12 h to form GO/MWCNT-PDA hybrid hydrogel. Herein, bio-inspired PDA simultaneously served as weak reducing agent and bonding agent during the self-assembly of GO nanosheets and MWCNT-PDA, thereby effectively shortening the reaction time and temperature. The whole forming process of GCPCAs needed no additional reducing agents, which greatly reducing the pollution emissions to the environment. Four related chemical reactions, which were listed in Figure 1(a), might occur in the solution system. Finally, the hybrid hydrogel was taken out of the reactor and freeze-dried for 24 h. The density of as-prepared GCPCAs can be further reduced by directly annealing at 750 °C under N2 atmosphere. The variations in appearance of obtained hydrogels and aerogels with different reactant mass ratios were shown in Figure S1†. The ultralight property (2.13-4.27 mg cm-3 in density of samples, Figure S2†) of the GCPCA was demonstrated by standing it on the tip of a fluffy feather without bending the shape, as shown in Figure 2 (a). Compression experiments were performed to evaluate the highly compressibility of as-prepared cylindrical GCPCA. In Figure 2 (b)–(f), the GCPCA exhibited a remarkable ability to restore to its original height under high compression deformation (ԑ was nearly 90%) without 11 ACS Paragon Plus Environment


collapsing, indicating the super flexibility of the composite aerogel. The stress-strain curves in compression tests were shown in Figure S3†.

Figure 1. (a) Some chemistry mechanisms involved in the fabrication of GO/MWCNT-PDA composite aerogel (GCPCA). (b) Schematic illustration of synthetic steps for fabricating ultra-light GCPCA.

Figure 2. (a) Image of the as-prepared GCPCA with an ultra-low density of 2.13 mg cm-3 standing on the lip of a fluffy feather. (b–f) Snapshots demonstrating the super compressibility (ԑ was nearly 90%) of the GCPCA.


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The phase structure and chemical composition of as-prepared GCPCA were then characterized through a variety of methods including FTIR, XRD, Raman spectra and XPS analysis. Firstly, FTIR spectra were used to verify the change of functional groups during the forming process. In Figure 3 (a), FTIR curves of GO exhibited broad and strong peaks at 3428 and 1405 cm-1 for the hydroxyl group (O-H and C-O), 1734 cm-1 for the carbonyl group (C=O), 1626 cm-1 for the aromatic ring (C=C) and 1085 cm-1 for the epoxy group, respectively. In contrast, the transmittance of GCPCA significantly decreased at 3428 cm-1 and the carbonyl and epoxy group were almost disappeared compared to that of GO, which demonstrated the removal of oxygen-containing functional groups during the thermal reduction and PDA chemical reduction process. Meanwhile, the emergence of new peaks at 1630 cm-1 (C=O in amine group) and 1517 cm-1 (N-H), can be associated with catechol and amine functional groups in MWCNTPDA. The FTIR results clearly demonstrated that nitrogen-containing functional groups of PDA successfully carried out the amino modification on the obtained GCPCA. Subsequently, XRD patterns of GO, MWNCT-PDA and GCPCA were presented in Figure 3 (b), which further gave supportive understanding on the structural evolution. It can be seen that MWCNT-PDA showed a strong diffraction peak at 2θ = 25.9° (d-spacing = 0.34 nm). While the characteristic interlayer spacing of the GO layers was indicated by the diffraction peak at around 2θ = 11.9° (d-spacing = 0.74 nm), which was larger than that of natural graphite (~ 0.34 nm) due to the introduction of oxygen-containing functional groups during the oxidation process. Due to the thermal reduction and PDA chemical reduction, the peak at 2θ = 11.9° was largely reduced, 13 ACS Paragon Plus Environment


while a new broadened peak centered at 2θ = 21.8° (d-spacing = 0.41 nm) can be observed in asprepared GCPCA. The decreased interlayer space reflected the elimination of a majority of the oxygen-containing functional groups and partial recovery of the GO to rGO, which matched well with above FTIR analysis. In addition, the broader XRD peaks revealed the inhomogeneous structure of reduced GO along their stacking direction. In fact, it was a porous architecture in which the graphene sheets and MWCNTs randomly overlapped or contacted one another. More detailed information about the microstructure will further be demonstrated by SEM and HRTEM analysis in the following section. Raman spectroscopy was a non-destructive approach to characterize graphitic materials, in particular to determine the ordered and disordered crystal structures of graphene. The characteristic peaks of GCPCAs with different mass ratios of GO and MWCNT-PDA (2:1, 4:1, 5:1, 6:1 and 8:1, respectively) were shown in Figure 3 (c). Two strong peaks were observed at 1357 and 1589 cm-1 corresponding to the prominent D- and G-bands of the composite aerogels. The G-band was associated with the vibration of sp2 carbon atoms in a graphitic 2D hexagonal lattice, while D-band was related to the vibration of sp3 carbon atoms in structure defects and disorder arrangement49-51. From the results of area integral, the intensity ratios (ID/IG) of GCPCAs decreased gradually from 1.433 to 1.129 with the increasing initial MWNCT-PDA concentration, indicating the increment of the sp2 hybridized domains. In addition, the reduced ID/IG ratio also reflected the GO was partially reduced to rGO with the assist of PDA chemical reduction.


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XPS analysis was employed to monitor the changes in element composition of GCPCA. As shown in Figure 3 (d), the C1s (286 eV), O1s (533 eV) and N1s (401 eV) elements can be clearly identified in the XPS spectrum. After annealing treatment at high temperature (750 °C), a significant decrease in O1s and N1s contents were observed in Figure S4† due to simultaneous PDA chemical reduction and graphitization. The deconvolution of N1s spectra in Figure 3 (e)-(f) were resolved into three main component peaks at 398.8 eV, 399.7 eV and 401.3 eV, related to the presence of pyridinic-N, pyrrolic-N and graphitic-N, respectively. It was found that the adsorption peak belonging to pyridinic-N became weakened while the peak intensity of graphiticN significantly increased after thermal annealing process. According to the structural transformation of N1s element, it can be concluded that the GCPCA exhibited a microstructure analogous to N-doped multilayer graphitic planes, which has been proved to be benefit for the improvement of its flame retardance performance. 10, 47



Figure 3. (a) FTIR spectra of GO, MWCNT-PDA and GO/MWCNT-PDA composite aerogel (GCPCA). (b) XRD patterns of GO, MWCNT-PDA and GCPCA. (c) Raman spectra D- and Gband peaks of GCPCAs with different mass ratios of GO and MWCNT-PDA (2:1, 4:1, 5:1, 6:1 and 8:1, respectively). (d) XPS spectra of GO, MWCNT-PDA and GCPCA. (e-f) The deconvolution curves of N1s XPS spectra for the GCPCA before and after 750 °C annealing treatment process, suggesting the presence and ratio change of the pyridinic-N, pyrrolic-N and graphitic-N, respectively.

The micromorphology and architecture of 3D GCPCAs were investigated by SEM and HRTEM observation. As shown in Figure 4, the graphene nanosheets/MWCNTs network exhibited a well-defined and interconnected micro-scale porous structure. According to the previous reports, such a macropore structure could contribute to the ultra-light nature and make for the highly compressible performance.

52-53

It should be worth noting that the pore size

distribution of macropores was strongly depended on the growth patterns of ice crystal 54-56 and assumed to be controlled by changing the pre-freezing temperature during the GCPCA forming


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process. In this study, three pre-freezing temperatures (-20 °C, -50 °C and -80 °C) were adopted to verify the viewpoint. From the Figure 4 (a)-(c), it was counted that the average dimensions of micro-scale pore decreased with the reduction of the pre-freezing temperature, and the asprepared GCPCA-80 presented an interesting “cabbage-like” hierarchical porous structure when the pre-freezing temperature was decreased to -80 °C. As shown in Figure 4 (c)-(f), the mean value (µ) and standard deviation (σ) in the pore size distribution histograms were fitted using the GaussAmp peak function. It can be explained that when the pre-freezing temperatures were set as -20 °C and -50 °C, the size dimension of the micro-scale pores was homogeneous due to the relatively uniform heat conduction. However, as the pre-freezing temperature was further reduced to -80 °C, the tiny ice crystals around the edge of aerogel were rapidly formed while those located in the inner core were lagging behind. Therefore, the unique directional alignment of pore size was induced due to the discrepancy in growth rate and different dimension of inside and outside ice crystals. As a result, the smaller pores brought about more capillary forces for organic solvents absorption, while the larger porous structure could maintain excellent mechanical performance of composite aerogel and provide sufficient interspace for absorption of solvent. The images presented in Figure 4 (f)-(i) revealed that a large amount of MWCNTs uniformly covered on the sidewall of graphene layers and some of MWCNTs could act like “bridge” to connect and support two graphene sheets. The various combining forms of graphene nanosheets and MWCNTs could not only suppress the excessively stacking of graphene but also increase the surface roughness51, 57-58, which were benefit for the 17 ACS Paragon Plus Environment


structural stability of as-prepared GCPCAs and the organic pollution absorption performance. Water content angles of GCPCA with different mass ratios of GO and MWCNT-PDA were shown in Figure S5†. Except for aforementioned micro-scale pores, the nano-scale pores formed by intrinsic hollow structure of MWCNTs as well as the lapping of graphene sheets and MWCNTs were further confirmed by BET analysis. The adsorption−desorption isotherm curves in Figure S6† exhibited a typical hysteresis loop, which suggested the existence of mesopores in the framework of obtained GCPCAs. By fitting the isotherm curves with BJH model, it was found that the specific surface area (SSA) of GCPCAs increased with the increasing contents of the MWCNT-PDA (as shown in Figure S6†). In this case, both of the micro-scale and nano-scale porous structures were considered as critical channels for the capillary flow of liquids in GCPCA during the organic pollution absorption process.


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Figure 4. (a-c) SEM images and pore size distributions of cross-section of GCPCAs with different pre-freezing temperatures (-20 °C, -50 °C and -80 °C, respectively). (d-f) SEM images of directional alignment of hierarchical porous structure in GCPCA-80 at various magnifications. (g-h) SEM and HRTEM images of MWCNTs uniformly covered on the sidewall of graphene layers (blue arrows represented well-dispersed MWCNTs). (i) HRTEM image of MWCNTs act like “bridge” to connect and support two graphene sheets.

Water pollution treatment now has attracted immense academic and commercial interest. Combining the ultra-lightness, high porosity and excellent mechanical robustness, the obtained GCPCA was regarded as an ideal candidate for the absorption of organic contaminants from water. Figure S7† exhibited the wettability of the GCPCA to water and organic solvent (exemplified via chloroform). As shown in Figure 5, the GCPCA sample was immersed into chloroform (dyed with Sudan III)/water mixture and could rapidly and selectively absorbed 19 ACS Paragon Plus Environment


chloroform. The whole absorption procedure was completed within a few seconds (Movie S1†). Moreover, due to its low density and hydrophobicity, the GCPCA fully absorbed with organic solvent was always floating on the water, which not only facilitated the collection process but also eased for further GCPCA recycling process. Therefore, the porous GCPCA showed great potential for the facile removal of organic pollutants from the contaminated water59-65. In order to quantitatively understand the absorption efficiency of as-prepared GCPCA, a series of organic solvents were investigated, including n-hexane, n-heptane, n-dodecane, acetone and chloroform. These chemicals were common organic pollutants in our daily lives as well as from industries. In Figure 6 (a), the GCPCA showed an outstanding absorption performance for all of the abovementioned organic solvents, and the absorption capacity can be improved by altering the mass ratio of GO and MWCNT-PDA. It should be noted that pre-freezing temperature was consistently set as -50 °C herein. Combining with the experimental results in Figure S2†, it was demonstrated that the GCPCA with lower density possessed larger absorption capacity. From the Figure 6 (b), the absorption capacities of obtained GCPCAs for various organic solvents ranged from 125 up to 533 g g-1, which was comparable to most of existing carbon-based or polymer-based 3D monolithic aerogels and sponge10, 25, 33, 66-72. It can be observed that the maximum absorption capacity of the GCPCA was achieved using chloroform, which was attributed to its highest density among all of the above organic liquids.


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Figure 5. (a-f) The absorption process of organic solvent (chloroform) using as-prepared GCPCA. Chloroform was dyed by Sudan III for clear observation and the whole absorption process completed within a few seconds.

Figure 6. (a) Super absorption capacity (Qwt, g g-1) of GCPCA for various organic solvents (including n-hexane, n-dodecane, n-heptane, acetone and chloroform) with different mass ratios of GO and MWCNT-PDA (2:1, 4:1, 5:1, 6:1 and 8:1, respectively). Herein pre-freezing temperature was set as -50 °C. (b) Comparison of maximum absorption capacity (g g-1) with experimental results in current studies for various carbon-based or polymer-based three dimensional monolithic aerogels and sponges. 21 ACS Paragon Plus Environment


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The obtained GCPCA exhibited excellent absorption performance, owing to the low density and high porosity, and thereby organic solvents can be stored in the interconnected pore structure. As it is known, the absorption of organic liquids was mainly a physical rather chemical process. 73

Since the GCPCA was a porous architecture, it possessed not only a huge surface area but also

a large amount of capillary channels. Therefore, the capillary effect was considered as the driving force for absorption and transmission of organic solvents into the aerogel. In Figure 7, the capillary effect of organic solvents in GCPCA was investigated by adjusting the micro-scale pore structure. Capillary action referred to the ability of a liquid to flow in narrow spaces without the assistance of external forces. It occurred because of intermolecular forces between the organic solvents and surrounding aerogel surfaces. The height h of the organic liquid column was given as follow:

h=

(2)

where γ was the organic liquid-air surface tension, θ was the contact angle, ρ was the density of organic liquid, g was the local gravitational acceleration and r was the radius of aerogel channel. According to the above formula, the thinner the aerogel channel in which organic solvent could travel, the higher the liquid climbed. Combining with SEM observation in Figure 4, altering the pre-freezing temperature could promote the decrease of average dimension of micro-scale pores. Such an optimizing process made the micro-scale porous architecture more connected and forceful to penetration of organic solvents inside the aerogel. It could be found that the “cabbagelike” hierarchical porous structure obtained from -80 °C significantly strengthened the capillary 22 ACS Paragon Plus Environment

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flow of absorption channels, thereby endowed the GCPCA-80 super absorption capacity for organic pollutions up to 501 times (chloroform) of its own mass. Moreover, GCPCA-80 was the rapidest one to reach the absorption equilibrium comparing to the GCPCA-20 and GCPCA-50, as shown in Movie S2†.

Figure 7. Super absorption capacity (Qwt, g g-1) of GCPCA (GO: MWCNT-PDA=4:1) for various organic solvents (including chloroform, n-hexane, n-dodecane, n-heptane and acetone) at three different pre-freezing temperatures (-20 °C, -50 °C and -80 °C). And the organic solvent absorption mechanism model presented the directional alignment of capillary flow in GCPCA-80 with “cabbage-like” hierarchical porous channels.

The recyclability of absorption materials was also an important criterion for the practical pollution cleanup and environmental protection, because most of the pollutants were either precious raw materials or toxic.

11, 74

Recycling process can provide absorption materials 23

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technical advantages, including saving energy cost and reducing generation of waste. Three common recycling means75-79 (including

absorption-squeezing,

absorption-combustion

and

absorption-distillation) can be tailored according to the characteristic of as-given organic solvents. Inspired by the highly repeatable compressibility of GCPCA, squeezing was an attractive method for low-viscosity or nonflammable organic solvents with high boiling point (such as n-dodecane, Figure 8 (a)-(d)). After compressing the GCPCA filled with n-dodecane to a controlled 50 % strain, it can release the absorbed organic solvent and then recover to nearly 100 % in height. Besides, for those flammable and unusable pollutants (such as n-hexane, Figure 8 (e)-(h)), combustion method sounded like a direct and effective choice. After being lighted, the GCPCA still maintained its original shape, showing good flame retardance performance under combustion process. A more reasonable explanation was that the high porosity of GCPCA helped to quickly release the heat generated during the combustion process, and the existence of MWCNTs can also reinforce the aerogel structure. However, the combustion of organic solvents still might contaminate the environment and discharge pollutants. Distillation process would be more economic and eco-friendly than just burning. In general, distillation was suitable for removal of expensive pollutants or those with low boiling point (such as chloroform, Figure 8 (i)-(l)). Each kind of absorption-desorption process was repeated 10 times to characterize the recyclability of GCPCA, and almost 90 % initial absorption capacity can be retained. The slight decrease in absorption capacity was mainly attributed to the residual contaminants inside the porous channel and possible shape shrinking of GCPCA (as shown in Figure S8†). These results demonstrated 24 ACS Paragon Plus Environment

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that the GCPCA can be employed as promising reusable absorbents for the organic pollutions in different physical characteristics.

Figure 8. (a-c) Snapshots demonstrated the process of recycling GCPCA via squeezing method. (d) Recyclability of GCPCA for absorption of n-dodecane under absorption-squeezing cycles. (e-g) Snapshots demonstrated the process of recycling GCPCA via combustion method. When the inner organic solvents burned out, the GCPCA could still maintain its original appearance. (h) Recyclability of GCPCA for absorption of n-hexane under absorption-combustion cycles. (i-k) Snapshots demonstrated the process of recycling GCPCA via distillation method. Inset photograph: GCPCAs were placed into a distilling apparatus waiting to be recycled. (l) Recyclability of GCPCA for absorption of chloroform under absorption-distillation cycles.

4. CONCLUSION In this study, the ultra-lightweight and robust GCPCAs were synthesized through a feasible green route and used for highly-efficient organic pollutants absorption. No additional reducing 25 ACS Paragon Plus Environment


agents were needed during the forming process, thereby greatly reducing the pollution emissions to the environment. The obtained porous aerogel architectures were highly compressible and hydrophobic, which provided superiority in organics absorption field. After combining with MWCNTs, both structural stability and specific surface area of the GCPCAs were significantly enhanced. Interestingly, the average dimension of micro-scale pores can be adjusted by changing the pre-freezing temperature of the GCPCAs. Due to the discrepancy in growth rate and resulting dimension of the ice crystals between inside and outside of GCPCAs, a novel “cabbage-like” hierarchical porous structure was obtained when the pre-freezing temperature was decreased to -80 °C. Directional alignment of capillary channels in GCPCA-80 made itself more connected and forceful for the absorption and transmission of organic solvents into the aerogel. The super absorption capacity of GCPCA-80 for organic pollutions was up to 501 times (chloroform) of its own mass, which was higher than the majority of reported absorption materials. Moreover, the recycling methods of GCPCAs can be tailored in terms of the characteristic of absorbed organic solvents and almost 90 % initial absorption capacity was retained after 10 absorption-desorption cycles. The use of GCPCAs would be rapidly extended to applications including treatment of oil contaminations, toxic dyes as well as heavy metal ions wastewater.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The appearance and initial densities of GCPCAs with different mass ratios (2:1, 4:1, 5:1, 6:1 26 ACS Paragon Plus Environment

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and 8:1, respectively) of GO and MWCNT-PDA; detailed descriptions of cycling compressing test, XPS analysis, water contact angle measurement and BET analysis. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] Tel: (86) 10-64427698; Fax: (86) 10-64412084. Author Contributions ‡ Wen-Wei Zhan and ‡ Si-Ruo Yu contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (No.U1362205 and No.U1664251). REFERENCES 1. Ge, J.; Zhao, H. Y.; Zhu, H. W.; Huang, J.; Shi, L. A.; Yu, S. H., Advanced Sorbents for Oil-Spill Cleanup: Recent Advances and Future Perspectives. Advanced materials 2016, 28 (47), 10459. 2. Gelderen, L. V.; Brogaard, N. L.; Sørensen, M. X.; Fritt-Rasmussen, J.; Rangwala, A. S.; Jomaas, G., Importance of the slick thickness for effective in-situ burning of crude oil. Fire Safety Journal 2015, 78, 1-9. 3. Megharaj, M.; Ramakrishnan, B.; Venkateswarlu, K.; Sethunathan, N.; Naidu, R., Bioremediation approaches for organic pollutants: a critical perspective. Environment International 2011, 37 (8), 1362-1375. 4. Wu, Y.; Xu, M.; Chen, X.; Yang, S.; Wu, H.; Pan, J.; Xiong, X., CTAB-assisted synthesis of novel ultrathin MoSe2 nanosheets perpendicular to graphene for the adsorption and photodegradation of organic dyes under visible light. Nanoscale 2015, 8 (1), 440. 27 ACS Paragon Plus Environment


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Table of Contents (TOC)

Bio-inspired Assembly of Carbon Nanotube into Graphene Aerogel with “Cabbage-Like” Hierarchical Porous Structure for Highly Efficient Organic Pollutants Cleanup Wenwei Zhan, †, ‡ Siruo Yu, †, ‡ Liang Gao, †, § Feng Wang, †Xue Fu, † Gang Sui,*, †and Xiaoping Yang† †

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

§

AVIC Composite Corporation LTD, Beijing 101300, China Corresponding author: * E-mail: [email protected] Tel: (86) 10-64427698; Fax: (86) 10-64412084. ‡ These authors contributed equally to the work and share first authorship.

Inspired by polydopamine chemistry, robust and recyclable graphene/MWCNT-PDA composite aerogels were prepared via green route for highly-efficient organic pollutants cleanup.


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Bioinspired Assembly of Carbon Nanotube into ... - ACS Publications

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