Liquid-phase exfoliation of graphene in organic solvents with addition of naphthalene
Graphical abstract
Introduction
Graphene is the strongest and toughest material known to man, and it offers extraordinary electronic, thermal and mechanical properties and is expected to find a variety of applications [1], [2], [3]. There are many methods for generating graphene and chemically modified graphene from graphite and derivatives of graphite, each with different advantages and disadvantages [4], [5], [6], [7], [8], [9]. Recently, the most commonly used approach involves the oxidation of graphite to graphene oxide (GO), followed by chemical reduction, but the resultant graphene sheets contain high levels of defects [10], [11]. For many applications, however, pristine graphene (defect-free) are required. Toward this end, the exfoliation of graphene in a liquid phase has shown its superiority for both scaling-up and application of graphene [12], [13], [14], [15], [16]. It is believed that the liquid-phase exfoliation of graphene is potentially important for the production of conducting inks and top-down approaches to electronics. Numerous studies have reported the solution-phase dispersion of pristine, unfunctionalized graphene because there are no defects and oxide groups in the pristine graphene, which is essential for its application in the electronics [17], [18], [19], [20], [21], [22].
Highly polar solvents such as 1-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), 1,2-dichlorobenzene (ODCB), and γ-butyrolactone (GBL) et al. have been chosen for the liquid-phase exfoliation process. The selection of these solvents was based primarily on the trial-and-error experimentation [20], [23], [24], [25], [26]. Solvents that are efficient in dispersing graphene can be selected on the basis of their Hildebrand solubility parameters, Hansen solubility parameters, and surface tensions (or surface free energy). It is known that good solvents for graphite exfoliation have surface tensions in the region of 40–50 mJ/m2 [20], [27]. The graphene–solvent interaction should be sufficiently strong to compensate for the enormous van der Waals attractive interaction between the graphene sheets. This method, however, has a serious drawback of an extremely low yield efficiency of graphene, typically <0.01 mg/mL [20]. This method (to improve the graphene concentration) commonly requires long-time (∼400 h) sonication, which may not be allowed in commercial production. Moreover, the excessive sonication leads to the destruction of graphene. Undoubtedly, how to improve the yield efficiency without destruction of the structure of graphene is a key point in this research area. Many research groups have made a considerable progress in exfoliating graphene with the strategy of mixed solvents [28], [29], [30], solvent exchange [31], solvothermal-assisted exfoliation [32], [33], and addition of intercalants [34], [35], [36], [37], [38], [39].
The representative intercalants are pyrene derivatives [35], [37]. The noncovalent π–π stacking mechanism does not disrupt the graphene crystal structure (its sp2 hybridization) nor degrade the physical and chemical properties of graphene. However, it takes long time and involves complex steps to remove the superfluous pyrene derivatives in the graphene dispersions during the exfoliation process [34]. There have been reports about the adsorption of naphthalene onto multi-walled carbon nanotubes with the π–π interaction [40], [41]. We suggest that similar effects may be exploited between the naphthalene and graphene in organic solvents. Herein, we report a new approach to improving the liquid-phase exfoliation efficiency of graphite in organic solvents. With addition of naphthalene, the graphene yield is significantly increased. Naphthalene intercalates into the graphite and expands the interplanar space between the adjacent graphitic layers during the sonication process.
Section snippets
Materials
All reagents and solvents were purchased from Aldrich Chemical Inc. and used as received. The SEM image of graphite powder (Product Number: 496596-113G, particle size <45 μm, assay ⩾99.99%) is shown in Fig. S1.
Exfoliation
The graphite was dispersed in various solvents (cylindrical vial, 20 mL solvent) at a concentration of 5 mg/mL. Typically, 25 mg of naphthalene powders were added in the graphite dispersion. The mixture was then sonicated in a low-power ultrasonic bath (JEIOTECH UC-10) for 90 min. The
TEM characterization
As shown in Fig. 1, graphite was exfoliated in organic solvents with the addition of naphthalene (see Fig. S1 in the Supplementary material). The exfoliation of graphite using the π–π stacking interaction with naphthalene is depicted in Fig. 1 down: naphthalene acts as a “molecular wedge” to intercalate into the edge of graphite, which facilitates the exfoliation of graphite during sonication significantly improving the production yield of graphene.
Transmission electron microscopy (TEM) study
Conclusions
In summary, we propose an effective and efficient method to prepare high-concentration surfactant-free graphene liquid dispersion by a direct exfoliation of graphite in organic solvents with addition of naphthalene. Our experiments demonstrate that the prepared graphene dispersions contain few-layer graphene flakes without severe defects. The graphene concentration of the dispersion in NMP is as high as 0.15 mg/mL without the need for any surfactant/polymer stabilization. Such a high
Acknowledgment
This work was supported by the 2013 Research Fund of the University of Ulsan.
References (50)
- et al.
Chem. Eng. J.
(2012) - et al.
Chem. Eng. J.
(2011) - et al.
Chem. Phys. Lett.
(2013) - et al.
Carbon
(2012) - et al.
Carbon
(2013) - et al.
Powder Technol.
(2008) - et al.
Nat. Mater.
(2007) - et al.
Science
(2004) - et al.
Chem. Rev.
(2012) - et al.
J. Mater. Chem.
(2012)
Nat. Nanotechnol.
Nano Lett.
J. Mater. Chem.
J. Mater. Chem.
J. Mater. Chem.
Acc. Chem. Res.
J. Mater. Chem.
Rsc Adv.
J. Phys. Chem. C
Chem. Mater.
J. Am. Chem. Soc.
Acs Nano
Small
Nat. Nanotechnol.
J. Phys. Chem. C
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2022, Chemical Physics LettersCitation Excerpt :Various organic solvents have been proved to be able to exfoliate graphene to some extent, including N, N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), etc. [9,17,26–28]. In addition, organic solvents with some additional agents can improve the graphene exfoliation efficiency, such as organic solvent with NaOH salt [29–31], carbon dioxide expanded NMP [32]. Water/surfactant solution is another common solvent for the graphene exfoliation, which is considered as a promising method to obtain high concentration graphene dispersions [33,34].