Production of few-layer graphene by supercritical CO2 exfoliation of graphite
Introduction
In recent years, graphene—a new allotrope of carbon in two-dimensional (2D) form—was discovered and immediately attracted great attention from both the experimental and theoretical scientists [1], [2], [3]. Single-layer graphene is a planar sp2-bonded six-membered-ring sheet of carbon, and is the basic building block for CNTs, buckyballs, and graphite [3]. Single-layer graphene is supposed to be a gapless semiconductor with a linear dispersion relation [4]. The electronic band structure rapidly evolves with the number of layers [5], so single-, double-, and few-layer graphene can be distinguished as three different types of 2D crystals [3]. Therefore few-layer graphene is a bridge between pure 2D and bulk systems, and can be used to study the interesting physics at the crossover regime [6].
This new material is expected to have plenty of unique properties such as high thermal conductivity, mechanical stiffness, and fracture strength [7], and the exceptional electrical mobility due to the extraordinary carrier transport behavior [3]. Although the realization of most graphene-based devices still requires great research effort, the application of graphene as a filler material for polymer nanocomposites can be easily implemented. However, efficient approaches to produce pure and well-separated graphene sheets in large quantities are required.
Mechanical cleavage of graphite is an easy way to obtain pure graphene sheets and is widely used by many researchers [1]. Unfortunately the productivity is too low for any large-scale use. Chemical oxidation of graphite followed by subsequent exfoliation is more suitable for bulk-quantity production of graphene sheets [8]. However, the chemical modification of graphene might be undesirable for many applications. For example, the exfoliated graphene oxide (GO) sheets are electrically insulating unless converted back to graphene by chemical reduction. These chemical conversion steps make the manufacturing process more complicated, time-consuming, and costly. In addition, it is difficult to obtain pure graphene due to the bonded functional groups such as hydroxyl, carboxyl, and epoxy [8], [9], and the residual chemicals in the solution.
Supercritical fluids have been utilized to intercalate and delaminate tightly-stacked layered materials such as clay and graphite [10], [11], [12], [13]. When a substance is maintained above its critical temperature (TC) and critical pressure (PC), it exists in a supercritical fluid phase having both gaseous and liquid properties. It can penetrate many materials like gas due to the low viscosity, zero surface tension, and high diffusivity, and dissolve materials like liquid. Supercritical CO2 is widely used as a nonflammable, nontoxic, environmentally friendly solvent with an easily accessible critical point (TC = 31.1 °C and PC = 73.8 b). Recently, Gulari and Serhatkulu used supercritical CO2 as a processing medium to diffuse a coating agent (polydimethylsiloxane, PDMS) between layered graphite particles [13]. Delaminated graphite particles coated with the coating agent were obtained by catastrophic depressurization of the supercritical CO2. The coating agent prevents the reformation of covalent bonds between the delaminated graphite particles, so these particles may be uniformly dispersed in a polymer to form a graphite–polymer nanocomposite. However, in many applications the presence of foreign molecules or polymers is undesirable, and a way to produce relatively clean graphene sheets while keeping them well separated is needed.
In this work, the authors demonstrate a technique for producing few-layer graphene by supercritical CO2 exfoliation of natural graphite. One major advantage of supercritical CO2 over other solvents is the ease to obtain high purity graphene by simply depressurizing, which allows the supercritical CO2 to return to gas phase and evaporate, leaving no solvent residues. To avoid aggregation through van der Waals interaction, these exfoliated graphene nanosheets are dispersed in a solution of sodium dodecyl sulfate (SDS). The simplicity, high productivity, low cost, and short processing time make this technique suitable for large-scale manufacturing.
Section snippets
Experimental
Commercial powdered natural graphite (from Alfa Aesar) was used as our starting material. The commercial graphite has a particle size of ~ 70 μm with a purity of 99.99995% and a density of 2.25 g/cm3. Fig. 1 shows the schematic diagram of our fabrication apparatus. One gram of graphite was placed in a high-pressure vessel with a heater and a temperature controller. CO2 was then added into the vessel until the pressure reached 100 b. Heat was applied to the vessel so that the temperature of the
Results and discussion
During the 30-min immersion in the supercritical fluid, the supercritical CO2 diffused in between the layers of graphite due to its low viscosity, high diffusivity, and small molecule size. The CO2-intercalated graphite was forced to exfoliate or delaminate by the expansion of the supercritical CO2 disposed interstitially between the layers upon rapid depressurization of the vessel. The depressurization was performed by opening a blow-off valve and the gas was released at a rate of about
Conclusions
The authors demonstrate a supercritical fluid processing technique for producing few-layer graphene. After a treatment with supercritical CO2 at 100 b and 45 °C for 30 min, powdered natural graphite is exfoliated by the instantaneous expansion of CO2 during an abrupt depressurization step. The exfoliated graphene is dispersed in a solution of SDS. The morphology of the few-layer graphene nanosheets is examined with TEM and AFM. The results show that the graphene sheets are well separated and
Acknowledgement
This project is sponsored by the National Science Council of Taiwan under grant no. NSC 97-2221-E-606-001.
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