Hydrogenation Effects On the Structure and Morphology of Graphene and Single-Walled Carbon Nanotubes

 

Chemical functionalization can be used for the modification and control of chemical, mechanical, and electronic properties of graphene layers and single-walled carbon nanotubes (SWCNTs). One example is hydrogenation, achieved by the exposure of these materials to a source of atomic hydrogen (e.g., a H₂ plasma). This process has been considered for hydrogen storage purposes and for the control of the band gap of these materials for applications in carbon-based electronics. Hydrogen atoms are chemisorbed on the surface of these materials, introducing sp³-hybridized C-C bonds in a structure originally formed by delocalized sp² C-C bonding. This locally induced sp²-to-sp³ bonding transition causes outward displacements of carbon atoms, resulting in structural and morphological changes on the graphene layers/walls. For practical applications of this hydrogenation process, a fundamental understanding of these structural transformations is of major importance. Toward this end, in this presentation, we report results of a computational analysis of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene and SWCNTs. The analysis is based on classical molecular-dynamics (MD) and Monte Carlo (MC) simulations of structural and compositional relaxation, as well as first-principles density functional theory (DFT) calculations that complement and validate the classical simulation predictions.

The results demonstrate that carbon nanotubes swell upon hydrogenation, as observed in experiments reported in the literature; this SWCNT swelling depends strongly on the hydrogen surface coverage. At low surface coverages, where sp²-hybridized C atoms are predominant, the strain levels associated with swelling are negligible; a critical H coverage (around 40-50%) is required, beyond which the sp³-hybridized C atoms prevail and the corresponding strain levels start increasing linearly with H coverage. Our compositional relaxation procedure generates structures whose arrangements of H atoms are in excellent agreement with experimental observations. Detailed structural analysis of the relaxed hydrogenated surfaces demonstrates the tendency for clustering of hydrogenated and non-hydrogenated sites; this leads to surface morphologies characterized by ripples, containing mostly hydrogenated sites, surrounded by valleys, formed by long chains of non-hydrogenated sites. These features introduce surface roughness, which depends on the degree of hydrogenation and reaches its maximum levels at intermediate values of H coverage. Our findings are used to discuss the limitations on the maximum H storage capacity of these carbon-based materials upon their exposure to an atomic H flux and to provide explanations for experimental results reported in the literature.