Few-layer graphene obtained by electrochemical exfoliation of graphite cathode
Graphical abstract
Introduction
Graphene [1], owing to its excellent physical and chemical properties [2], is promising to be used in a wide range of devices such as high speed transistors [3], [4], [5], transparent conducting films [6], [7], lithium ion batteries [8], [9], [10], and supercapacitors [8], [10], [11]. The electrical conductance of graphene, which is sensitive to the content of defects and functional groups [12], affects strongly the performance of these devices. How to prepare graphene with low content of defects and functional groups is an important issue for applications of this material.
Chemical exfoliation by oxidation is one of the most widely used methods to obtain graphene [13], [14], [15]. This method involves oxidation of graphite to produce hydrophilic graphite oxide, which can be exfoliated as individual graphene oxide sheets by sonication in water. The hydrophilicity of graphene oxide comes from the oxygen functional groups such as –OH, C–O–C, and –COOH produced during oxidation of graphite [16], [17]. Although most of these groups are removed in subsequent reduction, the electrostatic repulsion forces between graphene sheets induced by ionization of the remaining –COOH and phenolic –OH in alkaline condition make graphene well dispersed [18]. As a result, when graphite is oxidized vigorously, the graphene product without further thermal treatment has inevitably many oxygen functional groups and structural defects, resulting in poor electrical conduction.
Preparation of graphene by electrochemical exfoliation of graphite anode was first reported by Liu et al. in 2008 [19]. They used a mixed solution containing ionic liquid 1-octyl-3-methyl-imidazolium hexafluorophosphate and water as electrolyte. Through intercalation of ions into the graphite anode assisted by oxidation of graphite edges by hydroxyl radicals, a dispersion containing graphene was obtained. Subsequently, others also obtained successfully graphene using this method by changing the recipe of the electrolyte [20], [21], [22]. Although this method is easy to implement and to prepare graphene, the obtained graphene still has a lot of oxygen functional groups and structural defects due to the oxidation reactions at the graphite anode. On the other hand, it has also been reported that high yield production of few-layer graphene was achieved by intercalation of graphite cathode with Li+/propylene carbonate complex [23]. After brush painting of this graphene onto a commercial paper, a sheet resistance as low as 15 Ω/□ was obtained. Given the reported amount of deposited graphene and assuming less than 50% packing of graphene in the film, this value corresponds to an electrical conductivity of about 7000 S m−1. By adjusting the voltage between working and counter electrodes, Morale et al. controlled the intercalation of hydrogen ions into graphite cathode in perchloric acid electrolyte. Combined with posterior microwave irradiation and sonication, graphene with few defects was successfully prepared [24].
Herewith we report the preparation and characterization of few-layer graphene by electrochemical exfoliation of graphite cathode using an electrolyte containing NaCl, dimethyl sulfoxide (DMSO), thionin acetate, and water. We demonstrate that the intercalation of Na+/DMSO complexes will lead to exfoliation of graphite. These edge-exfoliated graphite particles can be stabilized in situ by thionin acetate and be further exfoliated into few-layer graphene with mild sonication in water.
Section snippets
Preparation of graphene
In our experiment, graphite rods were used as both anode and cathode. The electrolyte contained deionized water, NaCl, DMSO, and thionin acetate salt. Exfoliation of graphite cathode began after a DC voltage of 5 V was applied. The electrolyte containing edge-exfoliated graphite was first filtered using porous nylon membrane. After washing with ethanol and deionized water before ultrasonic treatment, the obtained suspension was shelved overnight to make most of the unexfoliated graphite settle
Preparation of graphene and mechanism of formation
In our method, a mixed solution containing NaCl and DMSO was used as the electrolyte for electrochemical exfoliation of the graphite cathode. It has been reported that the sodium ions tend to combine with four or five DMSO molecules to form Na+/DMSO complexes [25]. After a DC voltage was applied, these complexes would intercalate into the interlayer space of graphite to form ternary graphite intercalation compound (Na+(DMSO)yCn−). The interlayer spacing of this compound was reported to be 1.246
Conclusions
We have studied few-layer graphene produced by exfoliation and electrochemical intercalation of graphite cathode using Na+/DMSO complexes as intercalant and thionin acetate salt as stabilizer. The obtained graphene material has lower content of defects and oxygen functional groups compared with chemically reduced graphene oxide. The graphene paper obtained by filtration shows a forty times higher electrical conductivity than that of thionin-stabilized chemically reduced graphene oxide.
Acknowledgements
Jie Tang wishes to thank financial support from the JST ALCA Program and JSPS Grants-in-Aid for Scientific Research 22310074, Japan. This research is also partially supported by the National Basic Research Program of China (2009CB930801), NSFC (21003145, 11204325), Zhejiang Provincial National Science Foundation (D4080489), Ningbo Municipality (2009B21005), China.
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