Morphing graphene for environmental and hydrogen-related applications: in silico studies

Deformed and defective graphene sheets. Epitaxial graphene such as the monolayer obtained evaporating Si from from SiC hexagonal faces, or the quasi free-standing obtained intercalating H or metals, is almost ideal, but still show either regular patterns of corrugation (the former) or almost periodic distributions of localized electronic states (the latter) due to the interaction with the substrate. Our simulations revealed that while physisorption is favored in concanvities, chemisorption is favored onto convexities (video), in agreement with experiments. These properties can be exploited for H-storage devices and for chemical functionalization aimed at building layered 3D structures stabilized by chemical pillars. We are currently also exploring the modulation of reactivity of strained graphene obtained suspending sheets or depositing flakes onto mechanically active substrates.

Disordered 2D and 3D graphenes. 3D nanoporous disordered graphene can be obtained by condensation of inks of graphene flakes esfoliated from graphite. These structures have variable porosity, density and accessible surface. We recently designed and implemented an algorithm to generate model system with given structural properties. We created a large database of these structures with large variability and tested them on their ability to adsorb hydrogen. With the aid of machine learning we extracted quantitative structure-adsorption relationships that can be used to optimize these materials for H-storage. We are also working on polycrystalline 2D graphene, obtained sealing flakes onto a surface. In this case, the hot spots of reactivity interesting for H chemisorption are located on the grain boundaries, rich in topological defects. These, however, generate also other unespected fundamental features, such as pseudo-magnetic fields due to non uniform local strain.

Fullerenes and nanotubes. Graphene sheets can bend to form tubes and closed shells (“fullerenes”) with nano-sized cavities and high curvature. These features are ideal to exploit the enhanced chemical reactivity (on the external surfaces) and enhanced physisorption capability (on the internal cavity). Going beyond the H-storage application, we recently considered these material for an unconventional use related to the detection of the relic post-big bang neutrinos in the cosmic background. Because neutrinos can be detected by their interaction with tritium only if this is kept in atomic form, and at the same time in high volume concentration, we are currently exploring the possibility of encapsulating it into nanotubes or fullerenes, to prevent dimerisation to molecules. These should have the smallest possible diameter, so that the strong concavity prevents the chemisorption, an effect possibly enhanced by external chemical passivation, and subsequently arranged in solid state form, to increase the tritium volumetric density. These studies are performed within the PTOLEMY collaboration on cosmic neutrinos.