(Invited) Synthesis of Graphene and Oxo-Functionalized Graphene Derivatives

Graphene (G1) is a popular nanomaterial and a single layer of graphite. Researchers in chemistry, physics and materials science constantly develop novel functionalization concepts for G1 to discover new fascinating properties. The wet-chemical synthesis of pure and the control of the polydispersity of flakes of G1 remain challenging.1 One method to yield G1 involves the oxidative treatment of graphite what leads to single layers of graphene oxide (GO), a long known compound. However, the extend of the density of defects within the carbon framework dominates its properties. Thus, G1 yielded from GO after reduction bears defects on the several % scale and therefore, properties of this type of G1, such as the mobility of charge carriers is very low.2, 3
We developed an oxidative functionalization route that avoids the formation of defects and single layers of oxo-functionalized graphene (oxo-G1) are yielded. The yielded G1 bears a density of defects, as low as 0.01% for the best quality of flakes after reduction (mobility of charge carriers up to 2000 cm2/Vs).2 For an average density of defects of 0.3% mobility values are about 250 cm2/Vs. Here we show a further enhanced method to oxidatively functionalize G1 and minimizing the average density of defects of the carbon framework, which can be as low as 0.04% in average, an unprecedented low average density of defects.
We use Raman spectroscopy to statistically characterize the quality of G1 derived from oxo-G1.4 Recently, we compared the efficiency of reducing agents for oxo-G1 and derived a general mechanism for the reduction of oxo-G1.5 Furthermore, it could be demonstrated that the chemical reduction of oxo-G1 is highly efficient using a strong acid and an electron donor. Furthermore, we conducted high resolution transmission electron microscopy on oxo-G1 and visualize the carbon framework. The results confirm that G1 bears few defects as a consequence of preventing CO2 formation during the synthesis. This low amount of defects of oxo-G1 allows the develop of chemical reactions to alter functional groups attached to the carbon framework of G1 and defects play a minor role for the first time.6-8
The stability of the carbon framework of oxo-functionalized G1 could be evaluated, which is stable up to 100 °C in contrast to conventionally prepared GO.6, 9 Functional groups of oxo-G1, mainly hydroxyl groups, epoxy groups and organosulfate groups (about 1 organosulfate on 30 carbon atoms), are less stable than the carbon framework.10 Consequently, it should be discriminated between the stability of functional groups and the stability of the carbon framework when talking about the stability of oxo-functionalized G1 or graphene oxide. The carbon framework is also stable enough to make controlled chemical reactions such as the substitution of organosulfate by hydroxide or azide.7

1.    Eigler, S.; Hirsch, A., Chemistry with graphene and graphene oxide-challenges for synthetic chemists. Angew. Chem. Int. Ed. 2014, 53, 7720-7738.
2.    Eigler, S.; Enzelberger-Heim, M.; Grimm, S.; Hofmann, P.; Kroener, W.; Geworski, A.; Dotzer, C.; Rockert, M.; Xiao, J.; Papp, C., et al., Wet Chemical Synthesis of Graphene. Adv. Mater. 2013, 25, 3583-3587.
3.    Eigler, S.; Grimm, S.; Enzelberger-Heim, M.; Müller, P.; Hirsch, A., Graphene Oxide: Efficiency of Reducing Agents. Chem. Commun. 2013, 49, 7391-7393.
4.    Eigler, S.; Hof, F.; Enzelberger-Heim, M.; Grimm, S.; Müller, P.; Hirsch, A., Statistical-Raman-Microscopy and Atomic-Force-Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide. J. Phys. Chem. C 2014, 118, 7698-7704.
5.    Eigler, S., Mechanistic insights into the reduction of graphene oxide addressing its surfaces. Phys. Chem. Chem. Phys. 2014, 16, 19832-5.
6.    Eigler, S.; Grimm, S.; Hirsch, A., Investigation of the Thermal Stability of the Carbon Framework of Graphene Oxide. Chem. Eur. J. 2014, 20, 984-989.
7.    Eigler, S.; Hu, Y.; Ishii, Y.; Hirsch, A., Controlled functionalization of graphene oxide with sodium azide. Nanoscale 2013, 5, 12136-9.
8.    Eigler, S.; Grimm, S.; Hof, F.; Hirsch, A., Graphene Oxide: A Stable Carbon Framework for Functionalization. J. Mater. Chem. A 2013, 1, 11559-11562.
9.    Eigler, S.; Dotzer, C.; Hirsch, A.; Enzelberger, M.; Müller, P., Formation and Decomposition of CO2 Intercalated Graphene Oxide. Chem. Mater. 2012, 24, 1276-1282.
10.    Eigler, S.; Dotzer, C.; Hof, F.; Bauer, W.; Hirsch, A., Sulfur Species in Graphene Oxide. Chem. Eur. J. 2013, 19, 9490-9496.