Structure of Carbon Nanocrystals Nucleated by Hydrogen-Induced Intershell C-C Bonding in Multi-Walled Carbon Nanotubes

 

                It has been observed that the exposure of multi-walled carbon nanotubes (MWCNTs) to atomic hydrogen from a hydrogen plasma results in the formation of nanocrystalline carbon phases (e.g., cubic diamond nanocrystals) embedded into the nanotubes. Motivated by these experimental findings, in this presentation, we investigate the formation of inter-shell sp³ C–C bonds in MWCNTs induced by atomic hydrogen and carry out a comprehensive theoretical analysis of the crystalline structures that can be nucleated through such C-C bonding.

In this context, we report (i) pathways of H interactions with the graphene walls of the MWCNTs that induce inter-shell C-C bonding, (ii) a detailed analysis of the atomic and electronic structure of the resulting nanocrystal nuclei, and (iii) a systematic study of the nucleated nanocrystal structure as a function of the chiralities and relative alignment of the adjacent graphene walls of MWCNTs that are bridged by the inter-shell C-C bonding. The analysis is based on a synergistic combination of classical molecular-dynamics (MD) simulations with first-principles density functional theory (DFT) calculations. The MD simulations are used for large-scale structural relaxation in the isothermal-isobaric ensemble. In the MD simulations, the interatomic interactions are described according to the Adaptive Interatomic Reactive Empirical Bond Order (AIREBO) potential. The DFT calculations are performed within the generalized gradient approximation (GGA) and employ plane-wave basis sets, ultrasoft pseudopotentials, and supercell models. They are used for atomic and electronic structure computations, as well as for analysis of optimal reaction pathways for the interactions of H with the MWCNT graphene walls in conjunction with the climbing-image nudged elastic band method.

We show that the resulting MWCNT structures with inter-shell C-C bonds are stable and demonstrate that the atomic clusters in the vicinity of this inter-shell C-C bonding provide seeds for the nucleation of crystalline carbon phases embedded into the MWCNTs. We demonstrate that various such crystalline phases can be generated, including cubic and hexagonal diamond, as well as new tetragonal and monoclinic carbon phases. The structure of these crystalline seeds depends on the chiralities and on the relative alignment between the adjacent concentric walls of the MWCNTs; this relationship is explored systematically and quantified.