Computational Analysis of Structural Transformations in Carbon Nanostructures Induced by Hydrogenation

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 bonding transition induces structural and morphological changes on the graphene layers/walls.  Also, in multi-layered carbon structures, such as multilayer graphene (MLG) and multi-walled carbon nanotubes (MWCNTs), inter-layer or inter-shell C–C bonds can be formed under high temperature and pressure, or due to exposure to ion irradiation or to a hydrogen plasma.

In this presentation, we report results of a comprehensive computational analysis of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene and SWCNTs, as well as of the different nanostructures that can be generated upon formation of inter-shell and inter-layer sp³ C–C bonds in MWCNTs and MLG, respectively.  The analysis is based on a synergistic combination of atomic-scale modeling tools, including first-principles density functional theory (DFT) calculations and classical molecular-dynamics (MD) and Monte Carlo (MC) simulations.

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.  Our MC/MD-based 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 is carried out, providing information, which cannot be extracted easily from conventional experimental techniques.  The 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.  Furthermore, we demonstrate that the resulting structures with inter-shell or inter-layer C-C bonds are stable and provide seeds for the nucleation of crystalline carbon phases embedded into the layers of the MWCNT and MLG structures, respectively.  Various crystalline phases can be generated, including the well-known cubic and hexagonal diamond phases, as well as new stable phases of carbon.  The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent graphene layers/walls in the original structure.  The results of our analysis generate experimentally testable hypotheses regarding different routes for the synthesis of nanostructured carbon materials.  The results also provide explanations for the initial steps in the process of formation of diamond nanocrystals upon exposure of MWCNTs to hydrogen plasmas that has been reported in the literature.