Polycation stabilization of graphene suspensions
© Hasan et al; licensee Springer. 2011
Received: 14 May 2011
Accepted: 16 August 2011
Published: 16 August 2011
Graphene is a leading contender for the next-generation electronic devices. We report a method to produce graphene membranes in the solution phase using polymeric imidazolium salts as a transferring medium. Graphene membranes were reduced from graphene oxides by hydrazine in the presence of the polyelectrolyte which is found to be a stable and homogeneous dispersion for the resulting graphene in the aqueous solution. A simple device with gold contacts on both sides was fabricated in order to observe the electronic properties.
The unique physical, electronic, and optical properties of graphene have been reported many times [1–4] and promise a wide variety of applications. Different methods have been adopted for obtaining graphene, e.g., mechanical exfoliation of graphite , epitaxial growth , and chemical exfoliation in different solutions [3, 7–9]. A very promising route for the bulk production of the graphene sheets can be chemical reduction and dispersion of graphene in aqueous solutions.
Two steps are involved in making water dispersible graphene: (1) first chemical oxidation of graphite to hydrophilic graphite oxide and (2) exfoliating it into graphene oxide (GO) sheets in aqueous solution. GO sheets are graphene sheets having oxygen functional groups. These GO sheets are prevented from agglomeration by electrostatic repulsion alone . The insulating GO can easily be reduced to highly conducting graphene by hydrazine reduction. However, the reduction of GO soon leads to agglomeration, while a stable dispersion is key to the possibility of large-scale processing. Polymeric imidazolium salts can be a good way to form a stable dispersion of graphene.
Organic salts based on the imidazolium moiety are an interesting class of ions. Low molecular weight imidazolium salts can have a low melting point and are then termed ionic liquids (ILs). Thus, ILs are molten salts at the room temperature and consist of bulky organic cations paired with organic or inorganic anions. Imidazolium ionic liquids have many advantageous properties, such as no flammability, a wide electrochemical window, high thermal stability, wide liquid range, and very small vapor pressure . They are also known to interact strongly with the basal plane of graphite and graphene. Polymeric imidazolium salts would therefore be interesting to explore as dispersing agents for graphene.
Graphene oxide was prepared by the modified Hummer's method [12, 13]. The graphite flakes (PN 332461, 4 g; Sigma Aldrich, Sigma-Aldrich Sweden AB,) were first put in H2SO4 (98%, 12 mL) and kept at 80°C for 5 h. The resulting solution was cooled down to room temperature. Mild sonication was performed in a water bath for 2 h to further delaminate graphite into a few micron flakes. Sonication time and power are very critical as they define the size of the resulting graphene oxide sheets. Excessive sonication leads to extremely small flakes. Then, the solution was diluted with 0.5 L deionized (DI) water and left overnight. The solution was filtered by Nylon Millipore™ filters (Billerica, MA 01821). The resulting powder was mixed with KMnO4 and H2SO4 and put in a cooling bath under constant stirring for 1.5 h. The solution was diluted with DI water, and 20 mL H2O2 (30%) was added to it.
The supernatant was collected after 12 h and dispersed in dilute HCl in order to remove the metal ion residue and then recovered by centrifugation [12, 13]. Clean GO was again dispersed in water to make a homogeneous dispersion and was centrifuged at 8,000 rpm for 40 min in order to remove the multilayer fragments. We added a polymeric imidazolium molten salt into the aqueous dispersion of GO at a concentrationof 1 mg mL-1 and strongly shook the solution for a few minutes. The imidazolium salt used by us was polyquaternium 16 (PQ-16) sold under the trade name Luviquat Excellence by BASF (Ludwigshafen, Germany), a copolymer with 95% molar of imidazolium chloride and 5% molar of vinylimidazole. Use of this polymeric salt for graphene dispersion is not found in literature. Then, the solution was reduced by hydrazine monohydrate at 90°C for 1 h to obtain a stable dispersion of graphene in aqueous solution.
Results and discussion
The sample was rinsed with DI water and dried with nitrogen. The dried samples were further treated at 400°C for 2 h in Ar/H2 to further reduce the graphene oxide and also to sublimate the solution residue. The optical microscope images were taken in order to identify graphene . Atomic force microscope measurements were carried out to confirm the presence of single- and few-layer graphene by measuring step height . Graphene shows typical wrinkled structure which is intrinsic to graphene  over relatively large sheet sizes. Very large graphene membranes with sizes around 10 × 10 μm were identified. The size was found to be directly related with sonication power and time. Excessive sonication results in very small graphene sheets, whereas insufficient sonication results in incomplete exfoliation of graphite oxide.
In summary, we report a method to produce and functionalize graphene membranes in the solution phase using polymeric imidazolium molten salts as a transferring medium. Graphene membranes were reduced from graphene oxide by hydrazine in the presence of a polyelectrolyte which was found to be a very stable dispersion for the graphene membranes in the aqueous solution. The reduced GO membranes were transferred to a SiO2/Si substrate by simple drop casting and were further reduced by annealing in H2/Ar. A simple device with gold contacts on both the sides was fabricated in order to observe the electronic properties. We conclude that chemical functionalization is a possible route to modify and improve the electronic properties of graphene.
We acknowledge the help of Amir Karim (Acreo Kista) for his technical support in TEM imaging.
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