Preparation of porous graphene/carbon nanotube composite and adsorption mechanism of methylene blue
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The graphene oxide (GO) and modified carbon nanotubes (MCNTs) were dispersed in water and mixed with toluene to form homogeneous emulsion, then the porous graphene–carbon nanotubes composites (MCG) were prepared through hydrothermal reaction. The adsorption mechanism of MCG was investigated by adsorption methylene blue. The morphology of MCG was analyzed by scanning electron microscope and transmission electron microscopy. It was found that MCG had rich micropore structures and MCNTs were interlaced on graphene sheets. The pore size of MCG can be effectively controlled by changing the volume ratio of toluene to the GO/MCNTs solution in the emulsion. MCG was characterized by X-ray diffractometer, Raman spectrometer, X ray photoelectric instrument and other analytical instruments. It was found that MCG had more oxygen functional groups. The experimental results show that kinetics can be well-described by pseudo-second-order model. The smaller the pore size, the higher the equilibrium adsorption capacity and the slower the adsorption rate. The adsorption thermodynamic parameters show that the adsorption process is spontaneous and belongs to physical adsorption, and high temperature is beneficial to adsorption. The fitting results of MCG-5 equilibrium adsorption data are matched with the Langmuir model and the saturated adsorption capacity is 232 mg g−1. After five cycles of adsorption–desorption operation, the adsorption capacity of MCG-5 decreases slightly.
KeywordsGraphene Carbon nanotubes Methylene blue Thermodynamics Kinetics
Mathematics Subject Classification92E99
The development of papermaking, printing and dyeing and textile industry has caused a large amount of water pollution, among which dye wastewater has the characteristics of large toxicity, difficult treatment and strong stability [1, 2]. At present, the common treatment methods of dye wastewater include adsorption, photocatalytic degradation, chemical oxidation and membrane separation [3, 4, 5, 6, 7, 8]. These methods can solve the problem of dye wastewater to a certain extent, but at the same time, there are also shortcomings such as harsh operating conditions, secondary pollution, low efficiency and high cost. Adsorption is also an important water treatment method, while finding an efficient adsorbent is the key for its application .
Methylene blue (MB) is an important organic dye widely used in textile, dyeing, printing, pesticide, and coating for paper stock . Because of its aromatic ring, MB is highly toxic and very difficult to degrade [11, 12]. Consequently, MB must be removed from wastewater before discharging.
Two dimensional graphene and graphite oxide (GO) are flaky nanomaterials. Their large surface area, rich adsorption site and high mechanical strength also make them become excellent adsorption materials [13, 14, 15]. However, there will be difficult to recycle the powder materials, and it can cause secondary pollution. In order to solve the problem, assembling two-dimensional materials into three-dimensional materials have been regarded. Ma et al.  prepared graphite oxide aerogels (GO-SA) and graphene aerogels (RGO-GA) from GO and two-dimensional graphene, respectively. It was found that the mechanism of methylene blue (MB) adsorption was different. The negative charge of oxygen containing functional groups on the GO-SA surface were favorable for the adsorption of cationic MB. RGO-SA had aromatic ring structure, and MB molecule produced π–π conjugation with RGO-SA. So, the functional groups on aerogels can influence the adsorption mechanism for MB and modification of the material using different functional groups can be carried out according to need.
Carbon nanotube (CNT) is easily oxidized and modified, and more oxidation functional groups can be formed on the surface to enhance the adsorption capacity . And its larger specific surface area is beneficial to adsorption. Ai et al.  loaded the nano Fe particles on the oxidized multi walled carbon nanotubes (M-MWCNTs), and found that M-MWCNTs has a strong adsorption capacity to methylene blue, reaching 48.06 mg g−1. At the same time, Single-wall carbon nanotubes (CNTs) possess strong tensile strength  and elastic modulus  since the buckling of the sp2-hybridized bonds . It may increase the stability and ductility of 3D graphene-based macrostructures by blending CNTs with GO since the flexibility of GO caused by its sp2-hybridized bonds will be reduced [22, 23].
Generally, both CNTs and graphene have good toughness and high strength, but neat graphene and CNT aerogels have weak elasticity. It has been reported that GO and CNTs may be mixed to form composite or aerogels [24, 25]. Recently, the combination of CNTs and graphene to prepare 3D composite aerogels for adsorption has aroused great attention. For example, it reported a “sol-cryo” method for the synthesis of carbon aerogels with a density of 0.16 mg cm−3 and these aerogels exhibited excellent mechanical properties . Also, there are many methods to prepare the graphene aerogels: chemical vapor deposition, hydrothermal reaction, sol–gel method, soft template method and so on.
The oxygen-containing groups of GO are mainly on the edge of lamellar, and the middle is polymerized aromatic ring structure, so it has hydrophilicity and lipophilicity. These two affinity can make GO behave as a molecular and colloidal stabilizer. Therefore, it provides advantage for the preparation of aerogels by using soft template method. On the one hand, as a molecular stabilizer, GO can make the carbon nanotubes stabilized in water. On the other hand, When GO is used as a colloidal stabilizer, the oil in water (O/W) type emulsion [27, 28] can be prepared.
In this paper, GO was used as stabilizer, and the modified multi walled carbon nanotubes were dispersed in the solution. After adding the aromatic oil phase, toluene, a stable O/W emulsion was formed. Porous graphene/modified carbon nanotubes 3D composites (MCG) were prepared after high temperature hydrothermal reaction. As a 3D porous material, MCG is a promising adsorbent for MB. The adsorption properties of MB and their dependence on a variety of parameters as well as adsorption isotherm, kinetic, and thermodynamic characteristics were determined for MCG.
Flake graphite powder and sodium nitrate (NaNO3) were purchased from Aladdin’s Reagent Company. Multi-wall carbon nanotubes (CNTs) was provided by the Tsinghua University. Potassium ermanganate (KMnO4), 98 wt% sulfuric acid (H2SO4), 68 wt% nitric acid (HNO3), 36 wt% hydrochloric acid (HCl), 30 wt% hydrogen peroxide (H2O2), l-ascorbic acid (l-AA), acetone, toluene and methylene blue were purchased from the chemical reagent company of national pharmaceutical chemical group. The deionized water was provided by heavy oil Laboratory of China University of Petroleum (East China).
2.2.1 Preparation of graphite oxide
Graphite Oxide (GO) was prepared through modified Hummers method [29, 30]. 0.5 g flake graphite powder and NaNO3 are added into 23 mL H2SO4 (98 wt%). The mixture was stirred under 10°C and then 3 g KMnO4 was added to the mixture slowly. The temperature was increased to 35 °C and the stirring continued for 1 h, and thus a uniform thick pulp formed. The pulp was warmed up to 95 °C thereafter, 40 mL deionized water was added and stirring was kept for another 0.5 h. Then the solution was moved out, and 100 mL deionized water and 3 mL H2O2 were added into the solution. At this time, the color of the solution is yellow. After being washed to neutral, GO powder can be obtained after being freeze-dried in the lyophilizer (FD-1A-50) for 48 h.
2.2.2 Modification of carbon nanotubes
The carbon nanotubes (CNTs) was modified as follows : CNTs was added into HNO3, keeping the temperature of reaction at 75 °C for 11 h. After filtration, the powder was washed to neutral. Then the modified carbon nanotubes (MCNTs) was obtained through freeze-drying in the lyophilizer (FD-1A-50) for 48 h.
2.2.3 Preparation of graphene/carbon nanotube 3D composites
A certain amount of GO was dissolved in deionized water. 5 mg mL−1 GO solution was prepared through ultrasonic dispersion for 1 h. MCNTs was then added into the GO solution with ultrasonic dispersion continued for another 1 h. The mass ratio between GO and MCNTs (G/C) was 2:1. A certain amount of l-AA was added and the pH value of the solution was adjusted to 2 by 1 mol L−1 HCl. And then a certain volume of toluene was added into the above solution. Homogeneous emulsion  formed after being stirred for 3 min at 15,000 r min−1 by digital display high speed homogenizer (FJ200-S), which then was put in Teflon high-pressure autoclave under 95 °C for 8 h in oven (FCD-3000) and hydrogel was obtained. The hydrogels were washed repeatedly with acetone and deionized water. Graphene/carbon nanotube aerogels (MCG) were obtained through being freeze-dried in the lyophilizer (FD-1A-50) for 48 h. The volume of toluene was adjusted, and the volume ratio between toluene and the GO/MCNTs solution (T/S ratio) was controlled at 2:10, 5:10 and 8:10 and MCG-2, MCG-5 and MCG-8, respectively, were named as abbreviation for convenience.
2.2.4 Adsorption of MB by MCG
The desorption and cyclic adsorption processes are as follows : MCG-5 reaching adsorption equilibrium at 298 K was put into ethanol to wash out the adsorbed MB. After washing several times, the ethanol became colorless, and then MCG-5 was freeze-dried in the lyophilizer (FD-1A-50) for 48 h and then used to adsorb MB at 298 K again, which was repeated for 5 times.
The droplet micromorphology of the emulsion was observed by the Japanese Olympus CX31 optical microscope (OM). The morphology of the MCG samples was analyzed by the German Zeiss Gemini 500 scanning electron microscope (SEM) and the Japanese JEM-2010 transmission electron microscope (TEM). The infrared spectrum was measured by IS10 Thermo Scientific full reflection Fourier transform infrared spectrometer. The Raman spectrum was measured by the DXR Microscope Raman spectrometer. The X-ray diffraction spectrum (XRD) was measured by the X’pert PROMPD X ray diffractometer in Holland. The X-ray photoenergy spectra (XPS) was measured by Escalab 250XI photoelectron. The concentration of MB was measured by the UV-752 ultraviolet spectrophotometer from Shanghai RuoKe co, under the absorption wavelength of 664 nm.
3 Results and discussion
3.1 Characterization of MCG
Figure 2a–c are the SEM images of MCG-2, MCG-5 and MCG-8. It can be seen that the MCGs have rich micro-pores, and with the increase of T/S ratio, the pore size of MCG becomes smaller and the microstructure is more abundant. The stacking between graphene layers for MCG-2 is less significant than that of MCG-8, which indicates that the effect of the soft template, toluene droplets, in MCG forming is weakened with the increase of T/S ratio, and some pores were damaged and pores with smaller size was formed. Figure 2d is a SEM image at high magnification of MCG-5. It shows that many wrinkles exist on the surface of graphene layers in MCG-5, and MCNTs overlaps with each other on the surface of graphene. This structure enhances the roughness of the surface and generates a large number of mesoporous channels. These channels increase the specific surface area and pore volume of MCG, which is beneficial to improve the adsorption capacity of MCG. The TEM of MCG-5 (Fig. 2e) can clearly show the lamellar structure of graphene and tubular structures of MCNTs. MCNTs are interlaced on graphene lamellar, indicating that GO plays an important role in dispersing MCNTs. At the same time, MCNTs enhance the stability of micro-pore structure and reduce the stacking  of graphene lamellar. So, the MCG-5 was used as main adsorbent for MB in later adsorption experiments.
3.2 The effect of MCG types and temperature on the adsorption of MB
The adsorption temperature is also an important factor. In the range of 298–328 K, the adsorption amount of MB on MCG-5 varies with time, as shown in Fig. 6b. The equilibrium adsorption capacity of MCG-5 at 298 K is 199.2 mg g−1; While it increases to 253.1 mg g−1 at 328 K, indicating that the higher the adsorption temperature is, the higher the adsorption capacity is.
3.3 Adsorption equilibrium
The adsorption isothermal model can be used to analyze the distribution of MB molecules in the two phase of solid and liquid during adsorption equilibrium. The adsorption isotherm model is usually related to the nature of adsorbed molecules . There are two common models of isothermal adsorption: Langmuir model and Freundlich model.
Isotherm parameters of MB adsorbed onto MCG-5 at 298 K
KL (L mg−1)
qm (mg g−1)
Saturation adsorption capacities for MB onto various adsorbents
3.4 Adsorption thermodynamics
Thermodynamic parameters in the adsorption process of MB for MCG-5
ΔG0 (kJ mol−1)
ΔH0 (kJ mol−1)
ΔS0 (kJ mol−1 K−1)
3.5 Adsorption kinetics
Parameters of pseudo first- and second-order kinetics for MB on MCG at 298 K
qe,exp (mg g−1)
qe,cal (mg g−1)
qe,cal (mg g−1)
k2 (g mg−1 min−1)
7.93 × 10−5
7.21 × 10−5
6.67 × 10−5
3.6 Desorption and cyclic adsorption
3.7 Adsorption mechanism of methylene blue and Challenges
Because there are many factors that can influence the adsorption process. The adsorption mechanism of methylene blue has been reported widely in many related references. Zhao et al.  reported that MB molecules can transfer from solution to the surface of catalyst and be adsorbed with offset face-to-face orientation via π–π conjugation between MB and aromatic regions of the graphene. In the paper of Ma et al. , it was found that the adsorption mechanism of MB was different on two different adsorbents. The negative charge of oxygen containing functional groups on the GO-SA surface were favorable for the adsorption of cationic MB. RGO-SA had aromatic ring structure, and MB molecule produced π–π conjugation with RGO-SA. Also, Zheng et al.  found that the main mechanism of Pb(II) adsorption on β-CD-GO is surface complexation and electrostatic interaction. And π–π interaction is the main adsorption mechanism of 1-naphthol on β-CD-GO. So, we can know that adsorption is a complex process after all. There are different adsorption mechanism when different adsorbents adsorb different substances. Therefore, not only can it be influenced by the π–π conjugation between MB and aromatic regions of the graphene, but also it is related to the physicochemical properties of carbon-based adsorbents.
In a word, it is difficult to come to the same conclusions due to inconsistent experimental conditions. Therefore, it is necessary to study the effects of the various influencing factors under a uniform experimental condition. Meanwhile, there are huge differences in the chemical composition of natural surface water. There is still a great knowledge gap between the simplified laboratory results and the actual behavior of carbon materials in natural water. We should do more further investigation about this work in the future.
GO was used as emulsion stabilizer, and homogeneous emulsion was obtained after high-speed stirring. Then MCG was prepared through hydrothermal reaction. It was observed that MCG had rich micro-pores from SEM and TEM images. When the T/S ratio increased, the size of the pore size decreased. The MCNTs interlaced on the graphene layer, which enhanced the wrinkle of the graphene lamellar and the nanoscale pores. the XPS and FT-IR analysis shows that the C/O atom ratio of MCG was 5.96 and the oxygen containing functional group was O–C=O/C–O.
The equilibrium adsorption capacity of MCG-2, MCG-5 and MCG-8 is 164.1, 199.2 and 233.8 mg g−1, respectively. The smaller the pore size is, the greater the equilibrium adsorption is. The adsorption thermodynamic parameters such as ΔH0, ΔS0 and ΔG0 show that the adsorption process of MB on MCG is physical adsorption, and the higher the temperature is, the better the adsorption effect is. The adsorption isotherm of MCG-5 is better fitted by Langmuir model. The saturated adsorption capacity is 232 mg g−1, which is higher than that of common adsorbent.
The adsorption process of MB onto different kinds of MCG conforms to the pseudo-second-order kinetics model. The adsorption rate is related to the pore size, and the smaller the pore size is, the slower the adsorption rate is.
Adsorption mechanism of methylene blue is complex. Not only can it be influenced by the π–π conjugation between MB and aromatic regions of the graphene, but also it is related to the physicochemical properties of carbon-based adsorbents.
The results of cyclic adsorption showed that MCG has good property that can still maintain a high adsorption capacity after 5 cycles. So, the MCG can be used as a kind of excellent adsorbent for MB dye and other sewage and waste-water polluted by MB. Also, the further industrial mass production of MCG can be considered in the future.
All the authors of this article thank for the support of the Natural Science Foundation of Shandong Province (ZR2017MB015), Projects of State Key Laboratory of Heavy Oil Processing (SLKZZ-2017002), PetroChina Innovation Foundation (2017D-5007-0601) and the Postgraduate Innovation Project of China University of Petroleum (East China) (YCX2017036).
This study was funded by the Natural Science Foundation of Shandong Province (ZR2017MB015), Projects of State Key Laboratory of Heavy Oil Processing (SLKZZ-2017002), PetroChina Innovation Foundation (2017D-5007-0601) and the Postgraduate Innovation Project of China University of Petroleum (East China) (YCX2017036).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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