Facile and scalable one-step production of organically modified graphene oxide by a two-phase extraction
Introduction
While graphite oxide has been known for 150 years [1], only more recently has its delaminated form, graphene oxide (GO), attracted considerable interest as a reinforcing filler in nanocomposites due to its remarkable mechanical [2], [3], [4], [5], [6] and thermal properties [7], [8], [9], [10] as well as an intermediate for the production of graphene [11], [12], [13], [14], [15]. Unfortunately, all well-established synthesis protocols established by Brodie [16], Staudenmaier [17], and Hummers/Offeman [18] require an extremely time-consuming purification step in which the large excess of ionic side products and excess reactants originating from the previous intercalation and oxidation steps are removed. In particular the reduction of ionic strength is indispensible to achieve delamination of GO via osmotic swelling [19]. Therefore GO is usually washed excessively either with diluted hydrochloric acid and water or is dialyzed for several weeks with repeated exchange of the dialysis medium. Both purification methods are exceedingly waste-water-intensive and vastly time-consuming methods, rendering this purification step a bottleneck and hamper large scale industrial fabrication.
Optimization of the performance of nanocomposites additionally requires the adjustment of the surface tension of the filler and the hydrophobic matrix. Hydrophobization of GO can be achieved by covalent bonding of alkyl chains to the hydroxyl- and/or carboxyl-groups on the basal planes or edges, respectively. The reagents for ester- or etherification are, however, extremely water-sensitive. Therefore, prior to covalent modification, GO has to be dried [20] which inevitably will induce aggregation into band-like structures where the huge basal surfaces partially overlap and external surfaces are converted into interlamellar space. Disaggregation of such restacked GO platelets requires, if possible at all, intense ultrasonication, which breaks the platelets and decreases the aspect ratio [21]. Alternatively, the hydrophobization of GO can be done by a cation exchange of internal and external cations with long chain alkylammonium or alkylamines [22], [23], [24], [25]. While most work in this direction focused on the characterization of the intercalation compounds, to our knowledge Liang et al. [25] were the only ones who have applied cation exchange to facilitate suspension of GO in chloroform. In this work, however, lyophilized GO was purified prior to hydrophobization by washing and subsequent dialysis.
To explore the full potential of GO-polymer-nanocomposites, stable suspensions in a variety of organic solvents (polar to non-polar) would be highly desirable for solution blending with various polymer matrices. These suspensions, moreover, should be readily available in larger quantities while the GO should be delaminated to the greatest possible extent to maximize the aspect ratio and the specific interface area [26]. Certainly most ambitious, purification steps should be avoided to minimize waste and make the process as technically benign as possible.
Here, we propose a one-step process that produces highly delaminated organically modified GO suspended in various organic solvents and which spares any additional purification steps. The organically modified GO is instead directly extracted from the reaction mixture with the help of alkylammonium.
Section snippets
Experimental
Graphite flakes (Reinstflocke (RFL) 99.5) were provided by Kropfmühl AG. All chemicals were of analytical grade and used without further purification.
Graphene oxide (GO) was prepared by a modified Hummers/Offeman method [18]. In a typical experiment flake graphite (10 g, 125–250 μm) and sodium nitrate (10 g) were mixed with concentrated sulfuric acid (300 mL, 98%). Subsequently, potassium permanganate (30 g) was interspersed over a period of 3 h and the reaction was kept at room temperature for 12 h.
Results and discussion
Following oxidation of graphite in a modified Hummers/Offeman procedure, the organically modified GO (GO12AM) is directly extracted from this harsh, highly acidic (pH = 0) and concentrated reaction mixture of extremely high ionic strength without prior purification or washing steps (Fig. 1). The GO is organically modified in this reaction mixture by addition of an ethanolic solution of 12AM.
Ethanol not only serves as a solvent for 12AM but it is also capable of suspending both GO as well as
Conclusion
The facile one-step process presented here will render mass-production of organically modified GO technically more benign and environmentally more friendly. The method moreover, produces a superior pure, highly delaminated organically modified GO with a maximized aspect ratio and specific surface area. Stable suspensions in a broad variety of organic solvents may be obtained that will allow for solution compounding with many different polymer matrices.
With 12AM applied here, a Hansen solubility
Acknowledgements
This work was supported by the German Science Foundation (SFB 840). We are indebted to thank A. Fery for making the AFM- and to J. Senker for making NMR-equipment available. We also deeply appreciate the support of D. Hirsemann and D. Gunzelmann with recording solid-state NMR spectra. The authors would like to thank the Kropfmühl AG for providing the flake graphite. Finally, P. Feicht would like to thank Dr. Osamoch.
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