(184s) 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.