Hydrogen interaction with graphene: a neutron spectroscopy approach

The recent development of methods for chemically separating the graphite planes, made possible the synthesis of gram-scale amount of graphene. For the first time, we investigated chemically synthesized graphene samples by inelastic neutron scattering. We focused our attention in studying the interaction between hydrogen and carbon planes, in order to test the potentiality of graphene as a base material for hydrogen storage devices and hence to clarify the physical mechanism underlying the hydrogen storage process. In fact, thanks to the large hydrogen scattering cross section, neutrons are the perfect tools for studying hydrogen in materials and materials for hydrogen storage as well.Chemically synthesized defective graphene samples, either in the pristine form or after thermal treatment in hydrogen flux, were measured on the hot-neutron three axis spectrometer LAGRANGE and the thermal neutron time-of-flight spectrometer IN4 at Institut Laue Langevin, in Grenoble (France). The experimental vDOS of as-prepared graphene shows localized peaks, which can be ascribed to C-H bending modes. When compounds are exposed to hydrogen flux at high temperature, not only this vibrational signature is noticeably enhanced, but also additional features appear in the spectrum. On one side, this reveals extraordinary capability of defects in dissociating the molecule of hydrogen and then trapping atomic hydrogen, up to very high temperatures; on the other side it suggests the existence, in hydrogenated graphene, of different hydrogen populations located at in-plane defects. This hypothesis is supported by abinitio DFT calculations, performed using the VASP code. Chiara Cavallari graduated in Material Science at the University of Parma (Italy) in 2011. Starting in 2012, she is currently working in the TOF-HR group at the Institut Laue Langevin, as PhD student. Her main research interests are carbon nanomaterials for energy storage, neutron scattering and ab-initio calculations.