(208e) Atomic-Scale Analysis of the Interactions Between Atomic Hydrogen and Multi-Walled Carbon Nanotubes

An atomic-scale analysis of the interactions of atomic
hydrogen with multi-walled carbon nanotubes (MWCNTs) is presented, aiming to
explore the structural changes undergone by MWCNTs when exposed to hydrogen
plasmas.  The analysis is based on a synergistic combination of classical
molecular-dynamics (MD) simulations with first-principles density functional
theory (DFT) calculations.  The Adaptive Interatomic Reactive Empirical Bond
Order (AIREBO) potential is employed in the MD simulations of H-MWCNT
interactions and the resulting structural relaxations.  The DFT calculations
are performed within the generalized gradient approximation (GGA) and employ
plane-wave basis sets, ultrasoft pseudopotentials, and supercell models. 
Parameters that have been varied in our analysis include nanotube diameters and
chiralities, hydrogen concentration, and temperature.

Depending on the H dose and the resulting H coverage,
the chemisorption of hydrogen onto the graphene walls of a MWCNT affects
considerably its structure, leading to the deformation and local amorphization
of the graphene walls to form a structure that consists of both sp²-
and sp³-hybridized carbon atoms.  The diffusion of hydrogen
atoms on and between graphene walls and their reactions with the carbon
structure are analyzed and the results of the analysis provide interpretations
for the formation of nanocrystalline carbon phases (e.g., cubic and hexagonal
diamond), which have been observed in our experiments upon H₂ plasma
exposure of MWCNTs.  In our theoretical study, special emphasis is placed on
investigating systematically the possibility of formation of inter-shell sp³
C-C bonds induced by atomic hydrogen.  These localized inter-shell C-C bonds
can act as a seed for the nucleation of nanocrystalline phases embedded into
the MWCNTs.  We present the results of a comprehensive protocol of DFT and MD
calculations, which show that the resulting structures containing these
inter-shell C-C bonds are stable and that seeds for the nucleation of different
carbon phases (allotropes) can be formed, depending on the alignment between
the graphene walls.