Solvothermal-assisted liquid-phase exfoliation of graphite in a mixed solvent of toluene and oleylamine
© Dang and Kim; licensee Springer. 2015
Received: 25 September 2014
Accepted: 30 December 2014
Published: 21 January 2015
We report an effective method for producing graphene sheets using solvothermal-assisted exfoliation of graphite in a mixed solvent of toluene and oleylamine. The mixed solvent of toluene and oleylamine produces higher yield of graphene than its constituents, oleylamine and toluene. The oleylamine molecules with its long chain enwrap the graphene sheets efficiently, while toluene helps the oleylamine molecules become more flexible and easily intercalate into the edge of graphite. The prepared graphene sheets have a high quality, and the concentration of graphene in the dispersion is as high as 0.128 mg mL−1. The high-quality graphene sheets obtained in this work make them suitable for application in many fields such as energy-storage materials and polymer composites.
KeywordsGraphene Solvothermal-assisted exfoliation Mixed solvent Oleylamine Toluene
Graphene, a one-atom thick, two-dimensional monolayer of sp2-bonded carbon, has worldwidely attracted attention of researchers due to its tremendously large surface area, good chemical stability, outstanding electrical conductivity, and high mechanical strength [1,2].
It is noticeable that, however, few existing graphene preparation methods can achieve the defect-free and large-scale production of large-size graphene. For instance, the scotch tape cleavage approach produces high-quality graphene sheets, but it suffers from a main drawback of low yield and throughput .
To overcome the low yield and throughput of the graphene sheets in the graphene production, chemical avenues have been developed including graphene oxide reduction [3-6], sonication-supported exfoliation [7-9], and graphite intercalation [10-12]. Among them, graphene oxide reduction is the most popular method to produce large-size graphene sheets in large quantities. However, the graphene sheets prepared by this method are highly defective, which restricts their application in electronic devices . On the other hand, sonication-supported exfoliation of graphite produces high quality and nearly defect-free graphene sheets, but a major issue of low graphene concentration needs to be addressed. Since defect-free graphene is required for application in a variety of electronic devices, a cost-effective large-scale fabrication method to produce defect-free graphene needs to be developed.
Recently, the sonication-supported exfoliation of graphite in a mixed solvent environment has emerged as a potential method for producing high-quality graphene sheets [14,15]. The selection of effective solvents for graphene dispersion is based on the Hildebrand solubility parameters, the Hansen solubility parameters, and surface free energy. It is understood that the van der Waals attractive interaction between the graphene sheets needs to be weaker than the interaction between graphene and solvent.
Previous studies have reported that highly polar solvents such as 1-methyl-2-pyrrolidone (NMP) are capable of readily dispersing graphene and carbon nanotubes . Similarly, aliphatic amines are also good solvents for dispersing carbon nanotubes, especially acid-treated carbon nanotubes [16-19], because they have strong interactions with the sp2 carbon lattice network of the carbon nanotubes. Among these aliphatic amines, oleylamine with its long chain might also be a useful solvent for graphene dispersion. A solvothermal route has been employed to synthesize graphene sheets and reduced graphene oxide at a higher temperature and pressure [13,20]. The graphite may be exfoliated more effectively under these intense conditions than under mild conditions .
Benefiting from co-synergistic effect of mixed solvent of oleylamine and toluene, herein, we report an effective method to enhance both the efficiency of the liquid-phase exfoliation of graphite and the quality of graphene sheets. By employing solvothermal-assisted exfoliation in a mixed solvent to control the interactions between solvent molecules and sp2 carbon atoms of graphite, we propose a new approach to producing graphene dispersion with high-yield and high-quality graphene sheets.
Expandable graphite (Grade 1721) was kindly provided by Asbury Carbon (Asbury, NJ, USA). Oleylamine, toluene, and 1-methyl-2-pyrrolidinone (NMP) were purchased from Aldrich Chemical Inc (Sigma-Aldrich, St. Louis, MO, USA). Hydrochloric acid and ethanol were purchased from DaeJung Chemicals and Metals Co. Ltd (Shiheung, Korea). All reagents and solvents were used without further purification.
Firstly, 3 g of expandable graphite was heated in a microwave for 3 min to form expanded graphite . The expanded graphite was treated with hydrochloric acid by stirring for 2 days to obtain pre-intercalated graphite. Afterwards, pre-intercalated graphite (90 mg) was added to a mixed solvent (30 mL) of oleylamine and toluene with different volume ratios and was heated in an autoclave at 180°C for 24 h. After solvothermal exfoliation, the graphene solution (30 mL) was gently sonicated using a low-power sonicator bath (JEIOTECH UC-10; JEIO TECH Inc., Seoul, Korea) for 90 min to produce graphene dispersion. The resultant dispersion was centrifuged at 1,000 rpm for 30 min using a centrifuge (GYROZEN-1236MGR; GYROZEN, Daejeon, Korea). After centrifugation, the supernatant of the dispersion (15 mL) was pipetted off and then sufficiently washed with toluene/ethanol to obtain the graphene product.
Transmission electron microscopy (TEM) images were taken on a JEM-2100 (JEOL Ltd., Seoul, Korea) with an operating voltage of 200 kV. Holey carbon films on 200 mesh copper grids (HC200-Cu; EM Systems Support Ltd., Macclesfields, UK) were employed for TEM measurements. X-ray diffraction (XRD) patterns were recorded on a Rigaku RAD-3C diffractometer (35 kV, 20 mA; Rigaku, Shibuya-ku, Japan) with Cu Ka radiation (λ = 1.548 Å) at a scan rate of 2°/min, in the 2θ angles ranging from 10° to 60°. Surface morphologies and topologies of the graphene sheets were measured by atomic force microscopy (AFM) using a multimode V (Veeco, Plainview, NY, USA) with silicon cantilevers.
UV-visible spectra were recorded on a UV-visible spectrophotometer (SPECORD 210 PLUS-223F1107; Analytik Jena AG, Jena, Germany) using a quartz cell with a 1-cm optical path. The obtained graphene product was dispersed in NMP (30 mL) for UV-visible measurements. By measuring the absorbance of graphene dispersion at 660 nm, the concentration of the exfoliated graphene dispersion was determined from the Lambert-Beer law, using A/l = α 660 C, where A is the absorbance, l [m] is the length of the optical path, α 660 [=2,460 mLmg−1 m−1] is the absorption coefficient, and C is the concentration of graphene [8,23]. Raman spectra were taken on a confocal Raman microscope (alpha 300S; WITec, Ulm, Germany) using an incident laser light. The power and the wavelength of the laser are 6 mW and 532 nm, respectively. The aperture and the lens used for the Raman measurements are 25 μm slit and 500×, respectively.
Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrometer (KBr disk method; NICOLET 380; Thermo Fisher Scientific, Waltham, MA, USA) at wavenumbers of 400 to 4,000 cm−1. Thermogravimetric analysis (TGA) was conducted in nitrogen atmosphere at a heating rate of 10°C/min using a TA Hi-Res TGA 2950 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA). X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with Al Kα X-ray radiation as the X-ray source for excitation.
Results and discussion
Concentration of graphene product
Due to the strong ionic interactions between strong acid and oleylamine, the oleylamine molecules readily intercalate into the graphite layers at high temperature and pressure in the solvothermal process. Moreover, with its long chain, oleylamine can prevent the re-aggregation of the graphene sheets effectively, making the graphene suspension more stable.
For successful exfoliation of graphite, it is necessary to overcome the van der Waals attractions between adjacent graphite layers. One method to reduce the strength of the van der Waals attractions is liquid immersion. By matching the refractive indices of the material and the solvents, the potential energy between adjacent layers which is given by the dispersive London interactions may approach 0. Based on this criterion, solvents with a surface tension of 40 to 50 mJm−2 were suggested for the dispersion of carbon nanotubes and graphene [26,27]. The surface tension of mixed oleylamine/toluene solvents is measured as 27 to 31 mJm−2, which is lower that the suggested surface tension for the exfoliation of graphite. In view of surface tension, oleylamine and toluene do not seem to be good solvents for the exfoliation of graphite. However, under solvothermal conditions, the oleylamine molecules readily intercalate into the graphite layers, which facilitates the exfoliation of graphite. The addition of toluene into oleylamine makes the oleylamine molecule more flexible, thus increasing the effectiveness of the exfoliation process. Because toluene alone can poorly disperse graphene, the graphene concentration decreases as the toluene content in the mixed solvent increases, i.e., the volume ratio of oleylamine to toluene decreases.
In conclusion, we have demonstrated an effective and efficient approach to preparing high-concentration graphene dispersion by solvothermal exfoliation of graphite in a mixed solvent of toluene and oleylamine. The prepared graphene dispersion contains mono or few-layer graphene sheets with a low amount of defects. The graphene concentration in the dispersion is as high as 0.128 mg mL−1 at a volume ratio of oleylamine and toluene of 5. This result is comparable to or higher than previously reported values. An oleylamine molecule with its long chain can intercalate into the edge of graphite during solvothermal process and also wrap the graphene sheets. The addition of toluene, a poor solvent for graphene dispersion, into oleylamine helps oleylamine become more flexible and thus significantly improves the production yield of graphene. High-quality graphene sheets prepared in this work may find their application in many areas from energy-storage materials to polymer composites.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0013914).
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