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graphene hybrid aerogel and its application for the adsorption of organic compounds
Accepted Manuscript Preparation of carbon nanotubes/graphene hybrid aerogel and its application for the adsorption of organic compounds Chunchun Wang, Sudong Yang, Qing Ma, Xin Jia, Peng-Cheng Ma PII:
S0008-6223(17)30357-3
DOI:
10.1016/j.carbon.2017.04.001
Reference:
CARBON 11905
To appear in:
Carbon
Received Date: 10 February 2017 Revised Date:
29 March 2017
Accepted Date: 1 April 2017
Please cite this article as: C. Wang, S. Yang, Q. Ma, X. Jia, P.-C. Ma, Preparation of carbon nanotubes/ graphene hybrid aerogel and its application for the adsorption of organic compounds, Carbon (2017), doi: 10.1016/j.carbon.2017.04.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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A hybrid aerogel consisting of carbon nanotubes (CNTs) and graphene was prepared. The growth of CNTs on the graphene sheets in the hybrid aerogel endowed the material a hierarchical structure with low density, excellent hydrophobicity and oleophilicity to the organic compounds. The aerogel can adsorb a variety of oily liquids with outstanding
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reusability.
ACCEPTED MANUSCRIPT Preparation of carbon nanotubes/graphene hybrid aerogel and its application for the adsorption of organic compounds
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Chunchun Wang 1, Sudong Yang 2, Qing Ma 2, Xin Jia 1, *, Peng-Cheng Ma 2, *
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of
Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented Chemical
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Engineering of Xinjiang Uygur Autonomous Region, Shihezi University, Shihezi 832003, China. Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute
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of Physics and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China.
Abstract
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Graphene-based materials with aerogel structures were developed in recent years for various adsorption applications. In the present study, a hydrothermal process was developed to prepare graphene aerogel by using graphene oxide as a precursor, and the
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obtained aerogel was employed as a template for the growth of carbon nanotubes (CNTs). Various techniques were employed to study the morphology, surface property and
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composition of aerogel samples. The results showed that CNTs were in-situ grown on the sheet of graphene aerogel, endowing the material a hierarchical structure with enhanced surface area and meso- and micro-scale pores. These improved properties made the hybrid aerogel a superior material for the selective adsorption of a variety of organics and oils from water.
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ACCEPTED MANUSCRIPT Introduction Oil spills and organic pollutants from oil exploitation and chemical leakage lead to serious environmental and ecological problems every year [1, 2]. Various methods, such as physical adsorption, mechanical skimmer, in-situ burning and biological degradation, have been
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developed to process spilled organic compounds to minimize any negative effect to water and ecosystem [3]. Among them, adsorbent materials are thought as a most effective and promising candidate to realize oil-water separation. The principle of this technique is that
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non-polar oil molecules can be physically adsorbed by hydrophobic materials, and generally the high surface area and porous structure of materials facilitate this process. Based on this,
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different materials with porous structures, like wool, cotton, activated carbon, polymer foam, nature cellulose, have been used for oil-water separation [4-8]. However, some of these materials have low oil loading and poor selectivity as in most cases water and oil are adsorbed together. Graphene, a two-dimensional material constitutes one or multi-layers of carbon
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atoms arranged in six-membered rings, exhibit a theoretical surface area of 2630 m2/g and excellent transport properties [9]. The exceptionally large surface area combined with inherently hydrophobic nature of graphene make this material a potential candidate for
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oil-water separation.
Up to date, one of the most promising ways to utilize graphene for this application is to
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construct a porous structure [10-12]. For example, Ruoff and co-workers [13] prepared sponge-like graphene using hydrothermal method, and the material showed high absorption for petroleum products, fats and some toxic solvents like toluene, chloroform, nitrobenzene. A unique advantage of graphene sponge is that the material can be regenerated by heat treatment, yielding the full release of adsorbates. Qu et al. [14] prepared a low density graphene framework architecture (2.1±0.3 mg/cm3) by using nitrogen-doped graphene. The adsorption capacity of prepared graphene for oils and organic solvents is much higher than that of the 2
ACCEPTED MANUSCRIPT best carbonaceous sorbents. Although graphene foams have been prepared using different techniques, there are still many issues to be addressed to optimize the structural, morphological and surface properties of materials which are specifically used for oil adsorption and oil-water separation.
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It is known that the hydrophobicity of material is governed by two factors, i.e., surface free energy and roughness [15]. For a specific material, the surface energy is a material property determined by the element and constitute, whereas the surface roughness can be modulated by
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introducing heterogeneous atoms/structures. In this context, the nano-roughness created by carbon nanotubes (CNTs) on the smooth walls of graphene is expected to improve the
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hydrophobicity and adsorption capacity of graphene foam for organic compounds. For example, graphene-CNT hybrid showed good performance for the removal of methylene blue from aqueous solution with a maximum adsorption capacity of 81.97 mg/g [16]. In another study, Zhou et al [17]. synthesized graphene-CNT aerogel by a hydrothermal redox reaction.
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The prepared aerogel possessed ultra-light densities ranging from 6.2-12.8 mg/cm3 with improved specific surface area, hydrophobic properties and adsorption capacities for various organics. By retrieving the literatures in this field, it can be concluded that the most common
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methods for forming porous CNT/graphene hybrid are based on the solution mixing of graphene oxide (GO) and CNTs followed by a hydrothermal or lyophilization treatment.
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These synthesis techniques, however, were unfavorable for the uniform and controlled distribution of CNTs in graphene foam, as CNTs were wrapped around the backbones of graphene foams and were not distributed within the layers of graphene sheets [18, 19]. Motivated by the above discussions, this paper reported the preparation of CNT/graphene hybrid aerogel, aiming at creating carbon-based nanomaterials with hierarchical structures for the adsorption of organic compounds. In the present study, a hydrothermal process was developed to prepare graphene aerogel by using GO as a precursor, and the obtained aerogel 3
ACCEPTED MANUSCRIPT was employed as a template for the in-situ growth of nanotubes. Various techniques were employed to study the morphology and surface properties of materials, and the adsorption behavior of aerogel for various organic liquids and oily products was evaluated.
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2. Experimental 2.1 Materials
Powder-like graphite (Purity 99%, Sinopharm Chemical Reagent Co., China) was used as a
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precursor to prepare graphene aerogel. Dopamine hydrochloride (DA, Sigma-Aldrich) was employed as an accelerator for the preparation of aerogel. Iron chloride hexahydrate
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(FeCl3·6H2O, Tianjin Baishi Chemical Co., China) was used as pre-catalyst for the growth of CNTs. All chemicals were analytical grade and used as received without further purification.
2.2 Preparation of hybrid aerogel
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GO was prepared from graphite powder through a modified Hummer’s method as reported before [20, 21]. In a typical experiment, 17.0 mg of GO was ultrasonically dispersed in 5.0 mL water for 4 h. Then 0.31 mL of DA solution (16.0 mg/mL) was added into the GO
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dispersion (pH=2.3), and stirred for 10 min. This was followed by the addition of 0.10 mL of FeCl3·6H2O (10.0 mg/mL) under a magnetic stir for 1 h. The mixture was then transferred
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into a Teflon-lined stainless steel autoclave and hydrothermally processed at 120 °C for 12 h, yielding gel-like monolith (RGO hydrogel). The hydrogel was freeze-dried to remove water inside, and the obtained sample was named as RGO aerogel. For the preparation of hybrid aerogel, RGO aerogel was put into a chemical vapor deposition (CVD) facility and heated from room temperature to 800 oC at a heating rate 10 oC/min under a gaseous flow consisting of argon (202.0 cm3/min) and hydrogen (14.0 cm3/min). After keeping the sample at 800 oC for 30 min, acetylene (37.3 cm3/min) was introduced into the 4
ACCEPTED MANUSCRIPT system for another 30 min, and CNT/RGO hybrid aerogel was obtained by cooling down the sample under argon flow (202.0 cm3/min).
2.3 Adsorption capacity of aerogel
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Different organic compounds, such as petroleum products (crude oil, diesel, engine oil, gasoline), commercial oil and chemicals (chloroform, hexane, hexadecane, octane, orthodichlorobenzene-ODCB, peanut oil, polydimethylsiloxane-PDMS) were used as probing
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liquids to evaluate the oil adsorption capacity of prepared samples. The adsorption capacity was defined by the weight change of aerogel before and after oil adsorption. During the
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measurement, cubic sample with length of 1.0 cm was immersed into a specific organic liquid for 10 min and then picked it up using a tweezers. After removing the redundant oil on the surface, the weight of sample was measured as soon as possible on an electronic balance to
2.4 Characterization
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minimize the volatilization of organic compounds.
The morphology of aerogel was observed by scanning electron microscopy (SEM, Zeiss
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Supra55VP). Transmission electron microscopy (TEM) observation was performed on a JEM-2010 (JEOL) operated at 200 kV. Raman spectrum of sample was obtained on a
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confocal Raman spectrometer (Labram-HR800, Horiba Jobin-Yvon). Surface functionalities and elemental composition of aerogel were studied using an X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Scientific). Surface area and porosity of samples were evaluated using a surface area analyzer (ASAP 2020, Micromeritics) based on the Brunauer-Emmett-Teller
(BET)
theory
and
Barrett-Joyner-Halenda
(BJH)
method,
respectively. Static contact angle of aerogel against water was measured on a goniometer
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ACCEPTED MANUSCRIPT (XG-CAMA, Shanghai Xuanyichuangxi) at ambient temperature. The optical images of sample were acquired using a digital camera (D7000, Nikon).
3.1 Morphology and surface properties of samples
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3. Results and discussion
Fig. 1 Photos showing the macro-scale morphology of sample at different processing steps (A: GO dispersion; B: RGO hydrogel; C: RGO aerogel; D: CNT/RGO aerogel).
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For the preparation of hybrid aerogel, experiment was designed to grow CNTs on the skeleton of reduced graphene oxide (RGO). To facilitate the formation of such structure, Fe3+ solution was employed as a precursor for the catalytic growth of CNTs. Fig. 1 shows the changes on
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the color and morphology of samples at different steps. GO exhibits a stable dispersion in
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water with dark brown color due to the oxidization state of graphene as well as the Fe3+ ions in solution (Fig. 1A). After hydrothermal treatment of sample in a close system at 120 oC, a hydrogel suspended in water is obtained. The reason for such observation was that GO contained lots of oxygen-contained functional groups, which coordinated with Fe3+ ions to form cross-linked hydrogel [22]. This process was facilitated by the presence of DA as this chemical can easily initiate the polymerization, providing additional benefits for the stabilization of gel structure. In addition, part of functional groups on GO can be reduced by DA under high temperature [23], resulting in the formation of RGO hydrogel. When the water 6
ACCEPTED MANUSCRIPT in the gel was removed by the air during the freeze-drying process, the obtained RGO (Fig. 1C) showed a typical solid state with a density of 9.7+0.2 mg/cm3, and the calculated porosity was 99.0%. Such low density suggested that the material contained porous networks, and generally a material with density less than 10 mg/cm3 and porosity higher than 98% can be
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regarded as an aerogel. The sample after CVD process exhibited a similar state with RGO aerogel, suggesting the high temperature treatment did not sacrifice the aerogel structure of material. The density of sample is decreased by 50% (4.1+0.3 mg/cm3) due to the removal of
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volatile compounds and functional groups on graphene substrate, consequently a cylindrical
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10 µm
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specimen (diameter 1.7 cm, length 0.9 cm) can be supported easily on a dandelion (Fig. 1D).
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Graphene substrate
Fig. 2 Electronic microscope images showing the structural differences of samples (A-C: SEM images of RGO and CNT/RGO aerogel; D-F: TEM images of RGO and CNT/RGO aerogels).
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ACCEPTED MANUSCRIPT The variation on the morphologies of RGO and CNT/RGO aerogels was studied using electronic microscope. Fig. 2 shows typical SEM and TEM images of two samples, illustrating the morphological differences of samples at micro- and nano-scale. RGO aerogel exhibited a porous structure with interconnected frameworks, and the individual graphene
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sheet was flexible with smooth surface (Fig. 2A). In sharp contrast, tubular structures were observed on the surface of graphene sheets in CNT/RGO sample (Fig. 2B), and a close examination on the sample demonstrated that lots of nanowires with random orientation were
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noted (Fig. 2C), suggesting the growth of CNTs on graphene substrate. The growth and fine structure of CNTs was further confirmed by TEM images: RGO sample showed overlapped
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layers with lots of wrinkles, covering the holes of carbon film in copper grid for TEM characterization (Fig. 2D). Similarly, numerous CNTs can be held on the holes by taking graphene as substrate (Fig. 2E). High resolution TEM images demonstrated the co-existence of CNTs and graphene, and the inner structures of nanotubes were hollow ones comprising of
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multilayered graphite sites, which were disorderly extended from outer to inter like a bamboo structure, and the diameter of CNTs was in the range of 20-30 nm (Fig. 2F). The mechanism behind the successful growth of CNTs on graphene aerogel was that with assistance of
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hydrogen, the Fe3+ attached on GO surface can be reduced to metallic Fe, functioning as a
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catalyst for CNT growth by utilizing acetylene as a carbon source.
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Fig. 3 XRD pattern (A) and Raman spectra (B) of GO, RGO and CNT/RGO aerogels.
The crystal structures of different samples were studied and compared by XRD and Raman
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spectrometry. Fig. 3A shows the XRD patterns of GO, RGO and CNT/RGO aerogels. For GO sample, a strong peak appeared at 2θ=9.0o, which was due to oxygen-contained groups on oxidized graphene sheets [24]. However, this peak disappeared in the XRD pattern of RGO aerogel, suggesting the successful reduction of GO, and a new broad peak presented at
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2θ=24.3o, corresponding to the diffraction of (002) planes of graphite carbons [25]. CNT/RGO aerogel also showed a broad peak at around 26.0o, indicating a low degree of
TEM analysis.
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crystallinity for the prepared material, which was consistence with the results arising from
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Fig. 3B shows the Raman spectra of materials, and all three samples exhibit two characteristic peaks, i.e., D- and G-band at 1343 and 1582 cm-1, respectively. The D-band is related to the structural defects in carbon-based materials, whereas the G-band originates from the vibration of sp2-bonded carbon. Herein, we used the intensity ratio between the D- and G-band (ID/IG ratio) to quantitatively evaluate the degree of defects in the sample. An interesting observation was that RGO exhibited a higher ratio (1.21) than its counterparts (GO= 1.01, CNT/RGO=1.14), suggesting the largest amount of disordered carbon in the sample. It was reported that upon the reduction of GO, numerous new graphitic domains were created, which 9
ACCEPTED MANUSCRIPT decreased the average size of graphitic domains in the sample, leading to an increase in the ID/IG ratio compared to that of GO [26]. For CNT/RGO aerogel, the sample was thermally treated at high temperature, thus enhancing the crystallinity of carbon material with decreased
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ID/IG ratio.
Fig. 4 General XPS spectra (A) and deconvoluted C1s (B), N1s (C) curves of samples showing the composition and chemical states of materials.
C
GO
66.67
RGO
77.45
CNT/RGO
98.02
O
N
Fe
Cl
O/C ratio
33.33
/
/
/
0.50
17.20
3.89
0.49
0.97
0.22
1.98
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/
/
0.02
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Sample
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Table 1 Atomic concentration of graphene-based materials.
The elemental composition and surface information of samples were analyzed by XPS (Fig. 4), and the elemental composition of different samples was summarized in Table 1. The result showed that GO had a high concentration of oxygen (32.33%), and after hydrothermal reduction, the concentration of this element decreased to 17.20% with a simultaneous increase on carbon from 66.67% to 77.45%. RGO aerogel contained a trace amount of nitrogen, iron and chlorine, originating from the dopamine and FeCl3 which were polymerized/adsorbed on 10
ACCEPTED MANUSCRIPT graphene surface. CNT/RGO sample exhibited the lowest content of oxygen among three samples (1.98%), revealing the removal of oxygen functionalities in the aerogel. Additionally, the elements of nitrogen, iron and chlorine were absence in this sample due to the facts that: i) The growth of CNTs using CVD can easily remove the compounds containing nitrogen and
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chlorine under high temperature of 800 oC, and ii) XPS is a technique which has the capability to reveal surface information of sample with depth profile less than 10 nm, and under such circumstance, the information obtained from CNT/RGO sample is mainly from CNTs other
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than the graphene substrate.
The C1s spectra of different samples are shown in Fig. 4B. For GO sample, four distinct
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peaks associated with sp2 C=C (284.6 eV), C-O (286.4 eV), C=O (288.1 eV) and O-C=O (288.5 eV) were observed. The treatment of sample under a hydrothermal condition resulted in the decreased intensity of oxygen-contained groups at binding energy at 286.8 eV, indicating an effective reduction of GO in RGO aerogel. Interestingly, carbon associated with
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nitrogen appeared at binding energy of 285.3 eV in RGO sample, this was due to the nitrogen-doping effect arising from DA. The deconvoluted N1s curve (Fig. 4C) showed that this element existed in the states of -N-H (399.7 eV) and -NH2 (402.1 eV) [27] in RGO
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aerogel, and -N-H functionality covered 88.1% content of nitrogen, suggesting the polymerization of DA on graphene surface under acidic condition. The nitrogen peak
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disappeared in CNT/RGO sample, confirming the removal of amine groups in the aerogel, which was in good agreement with the absence of nitrogen in the sample (Table 1). The O/C ratio of sample turned to 0.02, reflecting the high purity of carbon material for the sample. Consequently, CNT/RGO aerogel exhibited a mono-peak at bind energy of 284.6 eV with a small shoulder at 286.4 eV, the latter mainly comes from the adsorbed oxygen and moisture in environment [28].
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ACCEPTED MANUSCRIPT The introduction of nano-scale CNTs into RGO aerogel was expected to rectify the surface area of sample. Fig. 5 shows the nitrogen adsorption-desorption isotherms of two samples and corresponding analysis on porous structures. The results shows that RGO aerogel exhibits a typical type-IV isotherm [29] with a maximum nitrogen adsorption of 56.58 cm3/g when the
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relative pressure reaches 1.0 (Fig. 5A), and the calculated BET surface area is 18.03 m2/g. The diameter of pores in the sample is in the range of several nanometer to 1 µm with a peak pore volume of 0.09 cm3/g at pore diameter of 393.4 nm (Insert in Fig. 5A), suggesting the
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presence of macro-pores (>50 nm) in the sample. The CNT/RGO aerogel also exhibits the type-IV isotherm (Fig. 5B) with a much narrower distribution of pore diameter less than 100
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nm (Insert in Fig. 5B). The calculated BET surface area for CNT/RGO is 90.97 m2/g, more than four times higher than that of RGO aerogel. This enhancement was due to the introduction of CNTs with micro- and nano-scale features, as the volume of pores is always below 0.018 cm3/g, which is nearly one order of magnitude lower than those in RGO aerogel.
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The high surface area and low volume of pores in CNT/RGO aerogel are expected to facilitate the capillary pressure for liquid adsorption.
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Fig. 5 Nitrogen adsorption-desorption isotherm and pore distribution of RGO (A) and CNT/RGO (B) aerogels. 12
ACCEPTED MANUSCRIPT 3.2 Wettability and adsorption behavior of samples Besides the porous structure and ultra-light density, CNT/RGO aerogel exhibited outstanding hydrophobic property. Fig. 6 shows the wettability of two aerogels probed by water and oily compound. For RGO aerogel, water and hexadecane droplets can penetrate into the sample
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easily within a few seconds (A and B in Fig. 6), indicating both hydrophilic and oleophilic properties of material. In sharp contrast, spherical water droplet can stay on the surface of CNT/RGO aerogel without obvious adsorption (Fig. 6C), and the measured static contact
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angle of aerogel against water is 152.4+2.3° (Inset in Fig. 6C), whereas a drop of hexadecane was adsorbed completely by the sample within a second (Fig. 6D), making it impossible to
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determine the static contact angle. These results suggested that CNT/RGO aerogel exhibited good adsorption selectivity for oil due to the porous structure and inherent hydrophobic nature of carbon material.
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Fig. 6 Wetting behavior of liquid droplets on aerogel samples (A and B: Water and hexadecane on RGO; C and D: Water and hexadecane on CNT/RGO, inset showing the water contact angle on CNT/RGO sample).
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ACCEPTED MANUSCRIPT Such selectivity was further verified by studying the adsorption of oil droplets in water using CNT/RGO sample. The porous structure of aerogel provides large space for the entrance and storage of oil liquids, and the adsorption experiment shows that when a piece of aerogel contacts with hexadecane (Floating on water surface, Fig.7 A) and dichloromethane
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(Precipitated in water, Fig. 7B), the material can eliminate organic compounds in water within a few seconds. The adsorption capacities of RGO and CNT/RGO samples were measured by using various organic liquids, and the results were summarized in Fig. 7C. The capacities of
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CNT/RGO aerogel for all oily liquids are over 100.0 g/g with a maximum value of 322.8+8.3 g/g for ODCB. An interesting observation was that for a specific oily liquid, the adsorption
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capacity of CNT/RGO aerogel was much higher than that of RGO sample. The reason for such improvement was attributed to the higher surface area, multi-scale pores in the structure of aerogel as well as enhanced hydrophobicity/selectivity to water. While the introduction of CNTs filled the free volume (pores) in graphene aerogel, leading to a decreased space for oil the
multi-scale
pores
in
CNT/RGO
sample
facilitated
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compounds,
capillary-effect-dominated adsorption behavior for oil, thus holding more oil compounds in porous structure than RGO sample, whereas the latter exhibited numerous macro-pores (Fig.
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the gravity force.
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2A, Fig. 5) and the adsorbed oil may be detached from porous structure in the sample due to
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Fig. 7 Illustration showing the adsorption process of CNT/RGO aerogel for hexadecane (A)
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and dichloromethane (B), and adsorption capacity of aerogel samples for various organic
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liquids (C).
For a high performance adsorbent, it should have the capability not only to effectively separate organic liquids from water, but also have reasonable recyclability for practical applications. Fig. 8 shows the adsorption performance of CNT/RGO aerogel under the cyclic
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operations. The material has a stable adsorption capacity for hexadecane during the cyclic adsorption/burn operations (Fig. 8A), and remained more than 90% of its original adsorption capacity for hexadecane after 10 runs (Fig. 8B). The adsorbed organic solvents can be easily
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removed using in-situ burning (Fig. 8C), whereas the aerogel remains its porous framework
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without collapse or burned in air, suggesting excellent thermal stability and fire resistance of CNT/RGO aerogel.
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Fig. 8 Recyclability of CNT/RGO for oil adsorption (A: Adsorption recyclability of aerogel
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over ten cycles; B: Remained adsorption capacity of sample; C: Photographs showing the excellent thermal stability and reusability of CNT/RGO aerogel via combustion).
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Conclusions
In summary, a two-step method was established to prepare CNT/graphene hybrid aerogel. The
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hydrothermal treatment of GO with presence of dopamine and FeCl3 led to the formation of graphene aerogel. Taking this aerogel as a template, CNTs were in situ grown in the graphene material, yielding a hybrid aerogel with large surface area and porous structures. Unlike a physical mixture of two carbon-based nanomaterials, CNTs were distributed within the layers of graphene sheets in the hybrid aerogel, making the material low density, excellent hydrophobicity and oleophilicity to the organic compounds. The developed hybrid can adsorb a variety of organic liquids, including PDMS, with a maximum adsorption capacity of 322.8 16
ACCEPTED MANUSCRIPT g/g. In addition, the material exhibited outstanding thermal stability and reusability under cyclic operations for oil-water separation. The findings of this paper also showed potential application of CNT/RGO aerogel as a reinforcement for the preparation of three-dimensional polymer nanocomposites. It is known
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that dispersion and re-agglomeration of CNTs in polymer matrix are two major concerns to develop polymer nanocomposites with multi-functional properties. The hybrid material developed in this study, which can adsorb pre-polymer (for example, 318.2+3.5 g/g for
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uncured PDMS) spontaneously within its structure, is expected to ultimately eliminate the problems associated with the dispersion of nanofillers in polymer matrix. In addition, the
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uniform distribution of CNTs on graphene sheets and excellent stability of aerogel provide an alternative to avoid the re-agglomeration of nanotubes in matrix. Our preliminary results showed that the nanocomposites consisting of CNT/RGO aerogel and PDMS exhibited a high electrical conductivity (σ) of 0.25+0.064 S/m, and the incorporation of PDMS did not
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sacrifice the inherent conductivity of CNT/RGO aerogel (σCNT/RGO=0.50+0.35 S/m). Further evaluation on the morphology and properties of nanocomposites will be the subject of our
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upcoming reports.
Acknowledgements
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This project was supported by the National Natural Science Foundation of China (Project No. 11472294), the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Project No. qn2015bs020), Bingtuan Excellent Young Scholars and Bingtuan Innovation Team in Key Areas (2015BD003), and the Research Fund from the Alliance of Special Fine Chemical Innovation and Industrialization in CAS.
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performance. J Mater Chem A 2016;4:1550-65. [20] Yang S, Chen L, Mu L, Hao B, Chen J, Ma P-C. Graphene foam with hierarchical structures for the removal of organic pollutants from water. RSC Adv 2016;6:4889-98. [21] Tung VC, Chen L-M, Allen MJ, Wassei JK, Nelson K, Kaner RB, et al. Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Lett 2009;9:1949-55.
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ACCEPTED MANUSCRIPT [22] Auletta JT, LeDonne GJ, Gronborg KC, Ladd CD, Liu H, Clark WW, et al. Stimuli-responsive iron-cross-linked hydrogels that undergo redox-driven switching between hard and soft states. Macromolecules 2015;48:1736-47. [23] Li L, Li B, Zhang J. Dopamine-mediated fabrication of ultralight graphene aerogels with
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low volume shrinkage. J Mater Chem A 2016;4:512-8.
[24] Liu Y, Ma J, Wu T, Wang X, Huang G, Liu Y, et al. Cost-effective reduced graphene oxide-coated polyurethane sponge as a highly efficient and reusable oil-absorbent. ACS Appl
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Mater Interfaces 2013;5:10018-26.
[25] Ma G, Guo D, Sun K, Peng H, Yang Q, Zhou X, et al. Cotton-based porous activated
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carbon with a large specific surface area as an electrode material for high-performance supercapacitors. RSC Adv 2015;5:64704-10.
[26] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide.
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aerogel: A low-cost, facile prepared direct electrode for H2O2 sensing. Sensor Actuat B- Chem
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S0008-6223(17)30357-3
DOI:
10.1016/j.carbon.2017.04.001
Reference:
CARBON 11905
To appear in:
Carbon
Received Date: 10 February 2017 Revised Date:
29 March 2017
Accepted Date: 1 April 2017
Please cite this article as: C. Wang, S. Yang, Q. Ma, X. Jia, P.-C. Ma, Preparation of carbon nanotubes/ graphene hybrid aerogel and its application for the adsorption of organic compounds, Carbon (2017), doi: 10.1016/j.carbon.2017.04.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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A hybrid aerogel consisting of carbon nanotubes (CNTs) and graphene was prepared. The growth of CNTs on the graphene sheets in the hybrid aerogel endowed the material a hierarchical structure with low density, excellent hydrophobicity and oleophilicity to the organic compounds. The aerogel can adsorb a variety of oily liquids with outstanding
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reusability.
ACCEPTED MANUSCRIPT Preparation of carbon nanotubes/graphene hybrid aerogel and its application for the adsorption of organic compounds
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Chunchun Wang 1, Sudong Yang 2, Qing Ma 2, Xin Jia 1, *, Peng-Cheng Ma 2, *
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of
Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented Chemical
2
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Engineering of Xinjiang Uygur Autonomous Region, Shihezi University, Shihezi 832003, China. Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute
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of Physics and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China.
Abstract
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Graphene-based materials with aerogel structures were developed in recent years for various adsorption applications. In the present study, a hydrothermal process was developed to prepare graphene aerogel by using graphene oxide as a precursor, and the
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obtained aerogel was employed as a template for the growth of carbon nanotubes (CNTs). Various techniques were employed to study the morphology, surface property and
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composition of aerogel samples. The results showed that CNTs were in-situ grown on the sheet of graphene aerogel, endowing the material a hierarchical structure with enhanced surface area and meso- and micro-scale pores. These improved properties made the hybrid aerogel a superior material for the selective adsorption of a variety of organics and oils from water.
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ACCEPTED MANUSCRIPT Introduction Oil spills and organic pollutants from oil exploitation and chemical leakage lead to serious environmental and ecological problems every year [1, 2]. Various methods, such as physical adsorption, mechanical skimmer, in-situ burning and biological degradation, have been
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developed to process spilled organic compounds to minimize any negative effect to water and ecosystem [3]. Among them, adsorbent materials are thought as a most effective and promising candidate to realize oil-water separation. The principle of this technique is that
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non-polar oil molecules can be physically adsorbed by hydrophobic materials, and generally the high surface area and porous structure of materials facilitate this process. Based on this,
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different materials with porous structures, like wool, cotton, activated carbon, polymer foam, nature cellulose, have been used for oil-water separation [4-8]. However, some of these materials have low oil loading and poor selectivity as in most cases water and oil are adsorbed together. Graphene, a two-dimensional material constitutes one or multi-layers of carbon
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atoms arranged in six-membered rings, exhibit a theoretical surface area of 2630 m2/g and excellent transport properties [9]. The exceptionally large surface area combined with inherently hydrophobic nature of graphene make this material a potential candidate for
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oil-water separation.
Up to date, one of the most promising ways to utilize graphene for this application is to
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construct a porous structure [10-12]. For example, Ruoff and co-workers [13] prepared sponge-like graphene using hydrothermal method, and the material showed high absorption for petroleum products, fats and some toxic solvents like toluene, chloroform, nitrobenzene. A unique advantage of graphene sponge is that the material can be regenerated by heat treatment, yielding the full release of adsorbates. Qu et al. [14] prepared a low density graphene framework architecture (2.1±0.3 mg/cm3) by using nitrogen-doped graphene. The adsorption capacity of prepared graphene for oils and organic solvents is much higher than that of the 2
ACCEPTED MANUSCRIPT best carbonaceous sorbents. Although graphene foams have been prepared using different techniques, there are still many issues to be addressed to optimize the structural, morphological and surface properties of materials which are specifically used for oil adsorption and oil-water separation.
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It is known that the hydrophobicity of material is governed by two factors, i.e., surface free energy and roughness [15]. For a specific material, the surface energy is a material property determined by the element and constitute, whereas the surface roughness can be modulated by
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introducing heterogeneous atoms/structures. In this context, the nano-roughness created by carbon nanotubes (CNTs) on the smooth walls of graphene is expected to improve the
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hydrophobicity and adsorption capacity of graphene foam for organic compounds. For example, graphene-CNT hybrid showed good performance for the removal of methylene blue from aqueous solution with a maximum adsorption capacity of 81.97 mg/g [16]. In another study, Zhou et al [17]. synthesized graphene-CNT aerogel by a hydrothermal redox reaction.
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The prepared aerogel possessed ultra-light densities ranging from 6.2-12.8 mg/cm3 with improved specific surface area, hydrophobic properties and adsorption capacities for various organics. By retrieving the literatures in this field, it can be concluded that the most common
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methods for forming porous CNT/graphene hybrid are based on the solution mixing of graphene oxide (GO) and CNTs followed by a hydrothermal or lyophilization treatment.
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These synthesis techniques, however, were unfavorable for the uniform and controlled distribution of CNTs in graphene foam, as CNTs were wrapped around the backbones of graphene foams and were not distributed within the layers of graphene sheets [18, 19]. Motivated by the above discussions, this paper reported the preparation of CNT/graphene hybrid aerogel, aiming at creating carbon-based nanomaterials with hierarchical structures for the adsorption of organic compounds. In the present study, a hydrothermal process was developed to prepare graphene aerogel by using GO as a precursor, and the obtained aerogel 3
ACCEPTED MANUSCRIPT was employed as a template for the in-situ growth of nanotubes. Various techniques were employed to study the morphology and surface properties of materials, and the adsorption behavior of aerogel for various organic liquids and oily products was evaluated.
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2. Experimental 2.1 Materials
Powder-like graphite (Purity 99%, Sinopharm Chemical Reagent Co., China) was used as a
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precursor to prepare graphene aerogel. Dopamine hydrochloride (DA, Sigma-Aldrich) was employed as an accelerator for the preparation of aerogel. Iron chloride hexahydrate
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(FeCl3·6H2O, Tianjin Baishi Chemical Co., China) was used as pre-catalyst for the growth of CNTs. All chemicals were analytical grade and used as received without further purification.
2.2 Preparation of hybrid aerogel
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GO was prepared from graphite powder through a modified Hummer’s method as reported before [20, 21]. In a typical experiment, 17.0 mg of GO was ultrasonically dispersed in 5.0 mL water for 4 h. Then 0.31 mL of DA solution (16.0 mg/mL) was added into the GO
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dispersion (pH=2.3), and stirred for 10 min. This was followed by the addition of 0.10 mL of FeCl3·6H2O (10.0 mg/mL) under a magnetic stir for 1 h. The mixture was then transferred
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into a Teflon-lined stainless steel autoclave and hydrothermally processed at 120 °C for 12 h, yielding gel-like monolith (RGO hydrogel). The hydrogel was freeze-dried to remove water inside, and the obtained sample was named as RGO aerogel. For the preparation of hybrid aerogel, RGO aerogel was put into a chemical vapor deposition (CVD) facility and heated from room temperature to 800 oC at a heating rate 10 oC/min under a gaseous flow consisting of argon (202.0 cm3/min) and hydrogen (14.0 cm3/min). After keeping the sample at 800 oC for 30 min, acetylene (37.3 cm3/min) was introduced into the 4
ACCEPTED MANUSCRIPT system for another 30 min, and CNT/RGO hybrid aerogel was obtained by cooling down the sample under argon flow (202.0 cm3/min).
2.3 Adsorption capacity of aerogel
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Different organic compounds, such as petroleum products (crude oil, diesel, engine oil, gasoline), commercial oil and chemicals (chloroform, hexane, hexadecane, octane, orthodichlorobenzene-ODCB, peanut oil, polydimethylsiloxane-PDMS) were used as probing
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liquids to evaluate the oil adsorption capacity of prepared samples. The adsorption capacity was defined by the weight change of aerogel before and after oil adsorption. During the
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measurement, cubic sample with length of 1.0 cm was immersed into a specific organic liquid for 10 min and then picked it up using a tweezers. After removing the redundant oil on the surface, the weight of sample was measured as soon as possible on an electronic balance to
2.4 Characterization
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minimize the volatilization of organic compounds.
The morphology of aerogel was observed by scanning electron microscopy (SEM, Zeiss
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Supra55VP). Transmission electron microscopy (TEM) observation was performed on a JEM-2010 (JEOL) operated at 200 kV. Raman spectrum of sample was obtained on a
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confocal Raman spectrometer (Labram-HR800, Horiba Jobin-Yvon). Surface functionalities and elemental composition of aerogel were studied using an X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Scientific). Surface area and porosity of samples were evaluated using a surface area analyzer (ASAP 2020, Micromeritics) based on the Brunauer-Emmett-Teller
(BET)
theory
and
Barrett-Joyner-Halenda
(BJH)
method,
respectively. Static contact angle of aerogel against water was measured on a goniometer
5
ACCEPTED MANUSCRIPT (XG-CAMA, Shanghai Xuanyichuangxi) at ambient temperature. The optical images of sample were acquired using a digital camera (D7000, Nikon).
3.1 Morphology and surface properties of samples
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3. Results and discussion
Fig. 1 Photos showing the macro-scale morphology of sample at different processing steps (A: GO dispersion; B: RGO hydrogel; C: RGO aerogel; D: CNT/RGO aerogel).
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For the preparation of hybrid aerogel, experiment was designed to grow CNTs on the skeleton of reduced graphene oxide (RGO). To facilitate the formation of such structure, Fe3+ solution was employed as a precursor for the catalytic growth of CNTs. Fig. 1 shows the changes on
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the color and morphology of samples at different steps. GO exhibits a stable dispersion in
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water with dark brown color due to the oxidization state of graphene as well as the Fe3+ ions in solution (Fig. 1A). After hydrothermal treatment of sample in a close system at 120 oC, a hydrogel suspended in water is obtained. The reason for such observation was that GO contained lots of oxygen-contained functional groups, which coordinated with Fe3+ ions to form cross-linked hydrogel [22]. This process was facilitated by the presence of DA as this chemical can easily initiate the polymerization, providing additional benefits for the stabilization of gel structure. In addition, part of functional groups on GO can be reduced by DA under high temperature [23], resulting in the formation of RGO hydrogel. When the water 6
ACCEPTED MANUSCRIPT in the gel was removed by the air during the freeze-drying process, the obtained RGO (Fig. 1C) showed a typical solid state with a density of 9.7+0.2 mg/cm3, and the calculated porosity was 99.0%. Such low density suggested that the material contained porous networks, and generally a material with density less than 10 mg/cm3 and porosity higher than 98% can be
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regarded as an aerogel. The sample after CVD process exhibited a similar state with RGO aerogel, suggesting the high temperature treatment did not sacrifice the aerogel structure of material. The density of sample is decreased by 50% (4.1+0.3 mg/cm3) due to the removal of
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volatile compounds and functional groups on graphene substrate, consequently a cylindrical
B
10 µm
10 µm
C
0.2 µm
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A
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specimen (diameter 1.7 cm, length 0.9 cm) can be supported easily on a dandelion (Fig. 1D).
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Graphene substrate
Fig. 2 Electronic microscope images showing the structural differences of samples (A-C: SEM images of RGO and CNT/RGO aerogel; D-F: TEM images of RGO and CNT/RGO aerogels).
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ACCEPTED MANUSCRIPT The variation on the morphologies of RGO and CNT/RGO aerogels was studied using electronic microscope. Fig. 2 shows typical SEM and TEM images of two samples, illustrating the morphological differences of samples at micro- and nano-scale. RGO aerogel exhibited a porous structure with interconnected frameworks, and the individual graphene
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sheet was flexible with smooth surface (Fig. 2A). In sharp contrast, tubular structures were observed on the surface of graphene sheets in CNT/RGO sample (Fig. 2B), and a close examination on the sample demonstrated that lots of nanowires with random orientation were
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noted (Fig. 2C), suggesting the growth of CNTs on graphene substrate. The growth and fine structure of CNTs was further confirmed by TEM images: RGO sample showed overlapped
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layers with lots of wrinkles, covering the holes of carbon film in copper grid for TEM characterization (Fig. 2D). Similarly, numerous CNTs can be held on the holes by taking graphene as substrate (Fig. 2E). High resolution TEM images demonstrated the co-existence of CNTs and graphene, and the inner structures of nanotubes were hollow ones comprising of
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multilayered graphite sites, which were disorderly extended from outer to inter like a bamboo structure, and the diameter of CNTs was in the range of 20-30 nm (Fig. 2F). The mechanism behind the successful growth of CNTs on graphene aerogel was that with assistance of
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hydrogen, the Fe3+ attached on GO surface can be reduced to metallic Fe, functioning as a
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catalyst for CNT growth by utilizing acetylene as a carbon source.
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A
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Fig. 3 XRD pattern (A) and Raman spectra (B) of GO, RGO and CNT/RGO aerogels.
The crystal structures of different samples were studied and compared by XRD and Raman
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spectrometry. Fig. 3A shows the XRD patterns of GO, RGO and CNT/RGO aerogels. For GO sample, a strong peak appeared at 2θ=9.0o, which was due to oxygen-contained groups on oxidized graphene sheets [24]. However, this peak disappeared in the XRD pattern of RGO aerogel, suggesting the successful reduction of GO, and a new broad peak presented at
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2θ=24.3o, corresponding to the diffraction of (002) planes of graphite carbons [25]. CNT/RGO aerogel also showed a broad peak at around 26.0o, indicating a low degree of
TEM analysis.
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crystallinity for the prepared material, which was consistence with the results arising from
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Fig. 3B shows the Raman spectra of materials, and all three samples exhibit two characteristic peaks, i.e., D- and G-band at 1343 and 1582 cm-1, respectively. The D-band is related to the structural defects in carbon-based materials, whereas the G-band originates from the vibration of sp2-bonded carbon. Herein, we used the intensity ratio between the D- and G-band (ID/IG ratio) to quantitatively evaluate the degree of defects in the sample. An interesting observation was that RGO exhibited a higher ratio (1.21) than its counterparts (GO= 1.01, CNT/RGO=1.14), suggesting the largest amount of disordered carbon in the sample. It was reported that upon the reduction of GO, numerous new graphitic domains were created, which 9
ACCEPTED MANUSCRIPT decreased the average size of graphitic domains in the sample, leading to an increase in the ID/IG ratio compared to that of GO [26]. For CNT/RGO aerogel, the sample was thermally treated at high temperature, thus enhancing the crystallinity of carbon material with decreased
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C
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ID/IG ratio.
Fig. 4 General XPS spectra (A) and deconvoluted C1s (B), N1s (C) curves of samples showing the composition and chemical states of materials.
C
GO
66.67
RGO
77.45
CNT/RGO
98.02
O
N
Fe
Cl
O/C ratio
33.33
/
/
/
0.50
17.20
3.89
0.49
0.97
0.22
1.98
/
/
/
0.02
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Sample
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Table 1 Atomic concentration of graphene-based materials.
The elemental composition and surface information of samples were analyzed by XPS (Fig. 4), and the elemental composition of different samples was summarized in Table 1. The result showed that GO had a high concentration of oxygen (32.33%), and after hydrothermal reduction, the concentration of this element decreased to 17.20% with a simultaneous increase on carbon from 66.67% to 77.45%. RGO aerogel contained a trace amount of nitrogen, iron and chlorine, originating from the dopamine and FeCl3 which were polymerized/adsorbed on 10
ACCEPTED MANUSCRIPT graphene surface. CNT/RGO sample exhibited the lowest content of oxygen among three samples (1.98%), revealing the removal of oxygen functionalities in the aerogel. Additionally, the elements of nitrogen, iron and chlorine were absence in this sample due to the facts that: i) The growth of CNTs using CVD can easily remove the compounds containing nitrogen and
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chlorine under high temperature of 800 oC, and ii) XPS is a technique which has the capability to reveal surface information of sample with depth profile less than 10 nm, and under such circumstance, the information obtained from CNT/RGO sample is mainly from CNTs other
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than the graphene substrate.
The C1s spectra of different samples are shown in Fig. 4B. For GO sample, four distinct
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peaks associated with sp2 C=C (284.6 eV), C-O (286.4 eV), C=O (288.1 eV) and O-C=O (288.5 eV) were observed. The treatment of sample under a hydrothermal condition resulted in the decreased intensity of oxygen-contained groups at binding energy at 286.8 eV, indicating an effective reduction of GO in RGO aerogel. Interestingly, carbon associated with
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nitrogen appeared at binding energy of 285.3 eV in RGO sample, this was due to the nitrogen-doping effect arising from DA. The deconvoluted N1s curve (Fig. 4C) showed that this element existed in the states of -N-H (399.7 eV) and -NH2 (402.1 eV) [27] in RGO
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aerogel, and -N-H functionality covered 88.1% content of nitrogen, suggesting the polymerization of DA on graphene surface under acidic condition. The nitrogen peak
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disappeared in CNT/RGO sample, confirming the removal of amine groups in the aerogel, which was in good agreement with the absence of nitrogen in the sample (Table 1). The O/C ratio of sample turned to 0.02, reflecting the high purity of carbon material for the sample. Consequently, CNT/RGO aerogel exhibited a mono-peak at bind energy of 284.6 eV with a small shoulder at 286.4 eV, the latter mainly comes from the adsorbed oxygen and moisture in environment [28].
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ACCEPTED MANUSCRIPT The introduction of nano-scale CNTs into RGO aerogel was expected to rectify the surface area of sample. Fig. 5 shows the nitrogen adsorption-desorption isotherms of two samples and corresponding analysis on porous structures. The results shows that RGO aerogel exhibits a typical type-IV isotherm [29] with a maximum nitrogen adsorption of 56.58 cm3/g when the
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relative pressure reaches 1.0 (Fig. 5A), and the calculated BET surface area is 18.03 m2/g. The diameter of pores in the sample is in the range of several nanometer to 1 µm with a peak pore volume of 0.09 cm3/g at pore diameter of 393.4 nm (Insert in Fig. 5A), suggesting the
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presence of macro-pores (>50 nm) in the sample. The CNT/RGO aerogel also exhibits the type-IV isotherm (Fig. 5B) with a much narrower distribution of pore diameter less than 100
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nm (Insert in Fig. 5B). The calculated BET surface area for CNT/RGO is 90.97 m2/g, more than four times higher than that of RGO aerogel. This enhancement was due to the introduction of CNTs with micro- and nano-scale features, as the volume of pores is always below 0.018 cm3/g, which is nearly one order of magnitude lower than those in RGO aerogel.
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The high surface area and low volume of pores in CNT/RGO aerogel are expected to facilitate the capillary pressure for liquid adsorption.
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Fig. 5 Nitrogen adsorption-desorption isotherm and pore distribution of RGO (A) and CNT/RGO (B) aerogels. 12
ACCEPTED MANUSCRIPT 3.2 Wettability and adsorption behavior of samples Besides the porous structure and ultra-light density, CNT/RGO aerogel exhibited outstanding hydrophobic property. Fig. 6 shows the wettability of two aerogels probed by water and oily compound. For RGO aerogel, water and hexadecane droplets can penetrate into the sample
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easily within a few seconds (A and B in Fig. 6), indicating both hydrophilic and oleophilic properties of material. In sharp contrast, spherical water droplet can stay on the surface of CNT/RGO aerogel without obvious adsorption (Fig. 6C), and the measured static contact
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angle of aerogel against water is 152.4+2.3° (Inset in Fig. 6C), whereas a drop of hexadecane was adsorbed completely by the sample within a second (Fig. 6D), making it impossible to
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determine the static contact angle. These results suggested that CNT/RGO aerogel exhibited good adsorption selectivity for oil due to the porous structure and inherent hydrophobic nature of carbon material.
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Fig. 6 Wetting behavior of liquid droplets on aerogel samples (A and B: Water and hexadecane on RGO; C and D: Water and hexadecane on CNT/RGO, inset showing the water contact angle on CNT/RGO sample).
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ACCEPTED MANUSCRIPT Such selectivity was further verified by studying the adsorption of oil droplets in water using CNT/RGO sample. The porous structure of aerogel provides large space for the entrance and storage of oil liquids, and the adsorption experiment shows that when a piece of aerogel contacts with hexadecane (Floating on water surface, Fig.7 A) and dichloromethane
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(Precipitated in water, Fig. 7B), the material can eliminate organic compounds in water within a few seconds. The adsorption capacities of RGO and CNT/RGO samples were measured by using various organic liquids, and the results were summarized in Fig. 7C. The capacities of
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CNT/RGO aerogel for all oily liquids are over 100.0 g/g with a maximum value of 322.8+8.3 g/g for ODCB. An interesting observation was that for a specific oily liquid, the adsorption
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capacity of CNT/RGO aerogel was much higher than that of RGO sample. The reason for such improvement was attributed to the higher surface area, multi-scale pores in the structure of aerogel as well as enhanced hydrophobicity/selectivity to water. While the introduction of CNTs filled the free volume (pores) in graphene aerogel, leading to a decreased space for oil the
multi-scale
pores
in
CNT/RGO
sample
facilitated
the
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compounds,
capillary-effect-dominated adsorption behavior for oil, thus holding more oil compounds in porous structure than RGO sample, whereas the latter exhibited numerous macro-pores (Fig.
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the gravity force.
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2A, Fig. 5) and the adsorbed oil may be detached from porous structure in the sample due to
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Fig. 7 Illustration showing the adsorption process of CNT/RGO aerogel for hexadecane (A)
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and dichloromethane (B), and adsorption capacity of aerogel samples for various organic
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liquids (C).
For a high performance adsorbent, it should have the capability not only to effectively separate organic liquids from water, but also have reasonable recyclability for practical applications. Fig. 8 shows the adsorption performance of CNT/RGO aerogel under the cyclic
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operations. The material has a stable adsorption capacity for hexadecane during the cyclic adsorption/burn operations (Fig. 8A), and remained more than 90% of its original adsorption capacity for hexadecane after 10 runs (Fig. 8B). The adsorbed organic solvents can be easily
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removed using in-situ burning (Fig. 8C), whereas the aerogel remains its porous framework
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without collapse or burned in air, suggesting excellent thermal stability and fire resistance of CNT/RGO aerogel.
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Fig. 8 Recyclability of CNT/RGO for oil adsorption (A: Adsorption recyclability of aerogel
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over ten cycles; B: Remained adsorption capacity of sample; C: Photographs showing the excellent thermal stability and reusability of CNT/RGO aerogel via combustion).
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Conclusions
In summary, a two-step method was established to prepare CNT/graphene hybrid aerogel. The
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hydrothermal treatment of GO with presence of dopamine and FeCl3 led to the formation of graphene aerogel. Taking this aerogel as a template, CNTs were in situ grown in the graphene material, yielding a hybrid aerogel with large surface area and porous structures. Unlike a physical mixture of two carbon-based nanomaterials, CNTs were distributed within the layers of graphene sheets in the hybrid aerogel, making the material low density, excellent hydrophobicity and oleophilicity to the organic compounds. The developed hybrid can adsorb a variety of organic liquids, including PDMS, with a maximum adsorption capacity of 322.8 16
ACCEPTED MANUSCRIPT g/g. In addition, the material exhibited outstanding thermal stability and reusability under cyclic operations for oil-water separation. The findings of this paper also showed potential application of CNT/RGO aerogel as a reinforcement for the preparation of three-dimensional polymer nanocomposites. It is known
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that dispersion and re-agglomeration of CNTs in polymer matrix are two major concerns to develop polymer nanocomposites with multi-functional properties. The hybrid material developed in this study, which can adsorb pre-polymer (for example, 318.2+3.5 g/g for
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uncured PDMS) spontaneously within its structure, is expected to ultimately eliminate the problems associated with the dispersion of nanofillers in polymer matrix. In addition, the
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uniform distribution of CNTs on graphene sheets and excellent stability of aerogel provide an alternative to avoid the re-agglomeration of nanotubes in matrix. Our preliminary results showed that the nanocomposites consisting of CNT/RGO aerogel and PDMS exhibited a high electrical conductivity (σ) of 0.25+0.064 S/m, and the incorporation of PDMS did not
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sacrifice the inherent conductivity of CNT/RGO aerogel (σCNT/RGO=0.50+0.35 S/m). Further evaluation on the morphology and properties of nanocomposites will be the subject of our
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upcoming reports.
Acknowledgements
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This project was supported by the National Natural Science Foundation of China (Project No. 11472294), the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Project No. qn2015bs020), Bingtuan Excellent Young Scholars and Bingtuan Innovation Team in Key Areas (2015BD003), and the Research Fund from the Alliance of Special Fine Chemical Innovation and Industrialization in CAS.
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