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Two-Dimensional Chemistry towards New Graphene Derivatives

Periodic Reporting for period 2 - 2D-CHEM (Two-Dimensional Chemistry towards New Graphene Derivatives)

Reporting period: 2017-12-01 to 2019-05-31

The suite of graphene’s unique properties and applications can be enormously enhanced by its functionalization. As non-covalently functionalized graphenes do not target all graphene’s properties and may suffer from limited stability, covalent functionalization represents a promising way for controlling graphene’s properties. To date, only a few well-defined graphene derivatives have been introduced. Among them, fluorographene (FG) stands out as a prominent member because of its easy synthesis and high stability. Being a perfluorinated hydrocarbon, FG was believed to be as unreactive as the two-dimensional counterpart perfluoropolyethylene (Teflon®). However, our recent experiments showed that FG is not chemically inert and can be used as a viable precursor for synthesizing graphene derivatives. This surprising behavior indicates that common textbook grade knowledge cannot blindly be applied to the chemistry of 2D materials. Further, there might be specific rules behind the chemistry of 2D materials, forming a new chemical discipline we tentatively call 2D chemistry. The main aim of the project is to explore, identify and apply the rules of 2D chemistry starting from FG. Using the knowledge gained of 2D chemistry, we will attempt to control the chemistry of various 2D materials aimed at preparing stable graphene derivatives with designed properties, e.g. 1-3 eV band gap, fluorescent properties, sustainable magnetic ordering and dispersability in polar media. The new graphene derivatives will be applied in sensing, imaging, magnetic delivery and catalysis and new emerging applications arising from the synergistic phenomena are expected. We envisage that new applications will be opened up that benefit from the 2D scaffold and tailored properties of the synthesized derivatives. The derivatives will be used for the synthesis of 3D hybrid materials by covalent linking of the 2D sheets joined with other organic and inorganic molecules, nanomaterials or biomacromolecules.

The specific objectives of the projects are:
1. Design and preparation of new stable graphene derivatives with high dispersability in polar environments, controllable band gap and sustainable magnetic ordering from FG and fluorinated graphenes.
2. Identification of the application potential of the as-prepared derivatives and hybrids in (electrochemical and photoluminescence) sensing, magnetic delivery and catalytic applications.
3. Gain a deeper understanding of CF chemistry and identification of (dis)similarities with the chemistry of perfluorinated haloalkanes with the aim of identifying new rules for 2D chemistry, which will help to target the synthesis of 2D materials with desired properties.
Within the framework of the first period we prepared new stable graphene derivatives cyanographene and graphene acid (ACS Nano, 11(3), 2982, 2017). Using Grignard chemistry we prepared allyl, pentyl and anisolyl graphenes (Chem. Mater., 29(3), 926, 2017). We prepared several hydroxofluorographenes CxFy(OH)z by treatment of fluorographene with bases B(OH)x. These materials displayed magnetic ordering depending on their composition (Nat. Commun., 8, 14525, 2017). A different source of magnetism exhibited N-doped graphenes (J. Am. Chem. Soc., 139(8), 3171, 2017). We also overviewed current knowledge about chemistry, properties and applications of fluorographene (Appl. Mater. Today, 9, 60, 2017). Further attention was paid to understanding surface properties of various 2D materials (Appl. Mat. Today, 5, 142, 2016, J. Chem. Theory Comput., 13(3), 1328, 2017, Nanoscale, 9, 19236, 2017) and mapping of chemical functionalization of graphene surface (Nanoscale , 9, 119-127, 2016).

The project continued with the mission also in the second period. We deeply analyzed mechanism of fluorographene (FG) reactivity and showed that defects stand behind the unexpected reactivity of FG (Nanoscale, 10, 4696-4707, 2018). We utilized the gained knowledge in control of FG functionalization by solvent (J. Phys. Chem. Lett., 9(13), 3580–3585, 2018). Via Hirsch-Bingel reaction on FG, we prepared new graphene derivative, which can be used as supercapacitor electrode materials due to its very high specific capacitance (Adv. Funct. Mater., 28(29), 1801111, 2018). Later we synthetized another supercapacitor electrode material and showed fine tuning of its properties by duration of chemical reaction (Chem. Mater., 31, 13, 4698-4709, 2019). We identified new synthetic routes via FG chemistry utilizing Sonogashira C-C cross coupling reaction (Chem. Commun., 55, 1088-1091, 2019) and leading to dual-functionalized graphene (Carbon, 145, 251-258, 2019). Utilizing previously developed graphene derivatives graphene acid and cyanographene, we prepared very efficient catalyst for arene C-H insertion (Carbon, 143, 318-328, 2018) and single-atomic catalyst for oxidative amine coupling (Adv. Mater., 31(17), 1900323, 2019). Full list of publications is available on: http://www.2dchem.org/publications/.
In future we plan to focus the future research on the basis on the experience gained during the project realization. Nonetheless, we do not expect any significant changes in the planned schedule. Concerning the specific goals we already achieved the specific goal i) of WP1, nonetheless, we plan to prepare more graphene derivatives. Within framework of this project we plan to analyze electrochemical properties of new graphene derivatives. It was speculated that the electrochemical properties of vdW materials can also be related to their surface properties. In order to test this hypothesis we employed iGC and characterized surface properties of various MoS2 samples. We have to use this TMD as a prototype system, because inverse gas chromatography experiments require at least a few tens of milligrams of the prepared powder materials and employing fluorographene chemistry we are currently able to synthetize maximally ~ten milligrams of graphene derivatives. However, we plan up-scaling of our synthetic approaches in order to prepare at least one order of magnitude high amount of graphene derivatives. We analyzed surface properties of various bulk as well as single layer MoS2 samples using inverse gas chromatography. All samples displayed significant variations in surface energies and their heterogeneities. The surface energy ranged from 50 to 120 mJ m−2 depending on the sample and surface coverage. We correlated the surface properties and previously reported structural features of MoS2 with their electrochemical activities. We concluded that the observed differences in electrochemistry are caused by the surface properties. This is an important finding with an enormous impact on the whole field of electrocatalysis of layered materials. This work was published in Nanoscale, 9, 19236-19244, 2017.