Cationic surfactant mediated exfoliation of graphite into graphene flakes
Introduction
The recent discovery of free standing graphene in 2004 [1] has attracted worldwide attention because of its interesting properties such as ballistic transport on submicron scales and massless Dirac fermion charge carrier abilities [2], [3], [4]. Graphene is the name given to a monolayer of carbon atoms closely packed in a honeycomb lattice and is the basic unit of all the three dimensional (3D) carbon nanostructures. It can be rolled into zero dimensional (0D) fullerenes, one dimensional (1D) nanotubes and can be stacked to form 3D graphite [4]. Graphene or its derivatives have been widely used to fabricate a number of electronic devices [5], [6], [7], [8], [9] and also as reinforcement filler in polymer composites [10].
The main challenge in the production of graphene is to reduce the aggregation by re-stacking. Lack of efficient methods to produce processable single layer graphene flakes in significant quantities limits its applications. Exfoliation of the graphitic structure into individual layers is a challenging task due to the strong interlayer attractive forces. Micromechanical cleavage of graphite is the standard procedure used for the production of graphene [5]. Even though, this method gives best samples with the highest charge mobility reported so far [11], [12], mass production of graphene is very tedious and time consuming. Alternatively, methods such as exfoliation of graphite by metal intercalation followed by microwave heating [13], growing graphene by chemical vapor deposition of hydrocarbons on metal substrates [14] and thermal reduction of SiC were also reported [15], [16]. All these methods require very high temperatures and obtaining a uniform graphene layer still remains as a challenge.
Recently, many solution based processes for the exfoliation of graphite were reported which mainly involve chemical oxidation of graphite to exfoliated graphene oxide (GO), followed by reduction in presence of stabilizers such as amphiphilic polymers [17], hydrazine [18], pyrene derivatives [19], phenyl isocyanates [20] and octadecyl amine [21]. But chemical oxidation of graphite disrupts the electronic structure and introduces many carbonyl (–CO) groups, hydroxyl (–OH) groups and epoxides in the sheets. Even though, reduction in presence of stabilizers can remove most of the functionalizations, it still leaves significant number of defects which alters the electronic properties. Very recently, sonication assisted dispersion of graphene in N-methyl pyrrolidone (NMP) and N,N-dimethyl formamide (DMF) [22], [23] and ionic-liquid assisted electrochemical method [24] for the production of graphene from graphite were reported. Graphene sheets produced via these methods show agglomeration which hinders potential applications of graphene. Hence a simple, solution phase approach for the production of unoxidized but stabilized graphene sheets is needed to explore its potential applications.
We have combined the effect of ultrasonication and non-covalent functionalization for the exfoliation and dispersion of graphene nanoflakes. We outline a simple method for preparing a few layered graphene flakes directly from graphite without any oxidative treatment, using cationic cetyltrimethylammonium bromide (CTAB) as a stabilizer. Mild ultrasonication of highly ordered pyrolytic graphite (HOPG) flakes in the presence of CTAB and acetic acid led to the exfoliation of HOPG into graphene flakes. The cationic surfactant molecules adsorbed on the surface of graphene flakes prevent them from re-stacking. Graphene flakes thus obtained were characterized using atomic force microscopy (AFM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), scanning tunneling microscopy (STM), Raman spectroscopy and field emission measurements.
Section snippets
Materials and methods
HOPG was purchased from Good Fellow Ltd., USA. CTAB was purchased from Sigma–Aldrich. Glacial acetic acid and DMF were purchased from Sigma–Aldrich and were used without further purification. Milli-Q water was used for washings.
Preparation of processable graphene nanoflakes
In brief, 100 mg of HOPG flakes were sonicated with (Elma E-30H ultrasonicator) with 0.5 M CTAB solution in glacial acetic acid for 4 h. The resultant solution was then refluxed for 48 h under nitrogen atmosphere. Reaction mixture was left to stand overnight to allow
Results and discussion
Ultrasonication is a powerful tool for extracting nanomaterials from bulk [25], [26]. Sonication of HOPG in CTAB–acetic acid mixture facilitates the exfoliation of graphite without any oxidation. It can be explained by the effect of acoustic cavitation of high frequency ultrasound in the formation, growth and collapse of microbubbles in solution [27], which induces shock waves on the surface of the bulk material, causing exfoliation. Even though, surface energy of acetic acid is not very close
Conclusions
A simple, solution phase method for producing graphene flakes directly from graphite using sonication and CTAB as a stabilizer has been demonstrated. The flakes could be dispersed in common organic solvents such as DMF. Characterization of the flakes by UV–visible spectroscopy, SEM, TEM, AFM and Raman spectroscopy showed the successful exfoliation into graphene flakes of average ∼1.18 nm thicknesses. Field emission measurements showed a turn on voltage of 7.5 V/μm and emission current densities
Acknowledgements
Authors thank the National Research Foundation (NRF), Singapore for the research funding, NUS Nanoscience and Nanotechnology Initiative (NUSNNI) for the graduate scholarship, and Department of Chemistry, National University of Singapore (NUS) for technical support. Dr. Zhang Yongping (Singapore Millennium Foundation Postdoctoral Fellow, NUS) is acknowledged for helping with the STM measurements.
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