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

Andre R. Muniz1, Tejinder Singh1, Michael J. Behr2, Eray S. Aydil2, and Dimitrios Maroudas3. (1) Department of Chemical Engineering, University of Massachusetts at Amherst, Amherst, MA 01003, (2) Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, (3) Department of Chemical Engineering, University of Massachusetts, 686 N Pleasant Street, Amherst, MA 01003

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.