(581f) 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.