Direct fluorination of nanographene molecules with fluorine gas

The transformation of graphene into sp3-bonded carbon films through ultra-high pressure, hydrogenation or fluorination has been studied for years. It is considered that only few layers of graphenes on metal surfaces can realize this transformation and the electron orbitals hybridization between metal substrate and graphene layer is thought to be the drving force and stabilize the formed sp3 carbon structure. Here, we use nanographene molecule, hexa-peri-hexabenzocoronene (HBC), as the model compound to exploit the fluorination reaction between graphene and fluorine gas. MALDI-TOF mass, XPS and XRD results show that the direct fluorination realized the intermolecular cross-linking of nanographene molecules without the assistence of mental substrate. Combining with theoretical calculation and EPR test results, we proved that the fluorination of nanographene molecules with F2 follows a free-radical mechanism, and the carbon radicals formed during fluorination caused the covalent cross-linking between graphene molecules which have a tight π-π stacking. Meanwhile, the fluorination of other small nanographene molecules, pyrene and perylene, further confirmed the universality of this free-radical cross-linking mechanism. This would provide a further understanding for the chemical transformation from graphene layers to 2D diamond structure under chemical adsorption process.