46715-AC10
Search and Discovery of a New Crystalline Hydrocarbon: Graphane and Derived Compounds
Graphane, the fully hydrogenated form of graphene that we have predicted, is a very stable compound with formation energies comparable to mixtures of methane and graphite. However, its formation is hindered by a large formation energy barrier. We have estimated a barrier as high at 0.5 eV per hydrogen molecule for the reaction of hydrogen gas and graphene. Finding reactions and reaction conditions that diminish that barrier are the most promising avenues to synthesize graphane. We have been dedicated to study different methods to achieve a barrier reduction of the direct hydrogenation of graphene or partially fluorinated graphene.
Monocarbon fluoride, CF, has the same structure as graphane and forms readily upon exposure of graphite to fluorine gas at 600 C. The driving force of this reaction is the charge transfer between the graphene planes and fluorine due to the high electronegativity of fluorine (3.98 in the Pauling scale). Hydrogen has much lower electronegativity (2.20 in the Pauling scale) and does not intercalate graphite. Additionally, the hydrogen molecule has 0.9 eV/atom higher binding energy. In a study recently published in ACS Nano (in press) we have demonstrated that we can change the work function of graphene by more than one electron-volt with the use of a gate voltage. This control of the work function enables the tuning of the reaction barrier through an adjustment of the charge transfer. We found that when graphene is deposited on a silicon wafer becomes negatively charged. This charge transfer was confirmed experimentally by the group of Peter Eklund who is a coauthor in the publication mentioned before. Our density functional calculations show that the layer of SiO2 that naturally forms on the Si wafer has filled surface states close to the conduction band edge. These surface states lower the work function of the oxide to the level of facilitating charge transfer to the graphene sheet. We found that charge densities of the order of 1013 e/cm2 are easily obtained with moderate gate voltages. Associated with these charge transfer there is a shift in the chemical potential of the graphene sheet of the order of 1 eV and as a consequence we find a corresponding change in the work function.
We expect that the change in the work function produced by the gate voltage will affect the reaction barrier for the adsorption and dissociation of molecular hydrogen ion the surface. Our preliminary results show that this is the case. The extra charge and the change in work function facilitate the reaction and the formation of graphane.
We are also studying the formation of graphene starting from partially fluorinated graphite. A non‑negligible contribution to the energy barrier of the formation of graphane is the elastic deformation produced by the tetrahedral sp3 bonding in a matrix of sp2 bonds. In order to diminish this contribution we are studying the growth of graphene from a seed produced by regions of CF into the graphene matrix. The carbon atoms at the boundary between CF and graphene are more reactive to the adsorption of hydrogen due to the elastic local strain. We start from a fully formed plane of CF were a ribbon of fluorine atoms are removed. This structure is used to study the barrier for hydrogen dissociation and CH formation.
