Carbon forms a variety of different allotropes, such as cubic diamond, hexagonal diamond, graphite, fullerenes, and carbon nanotubes. These carbon allotropes have numerous technological applications. For example, due to its unique properties (such as extreme hardness, high electrical resistivity, high thermal conductivity, and chemical inertness), diamond is used widely in the fabrication of machine tools, optical coatings, X-ray windows, and light-emitting optoelectronic devices. We examine the synthesis of diamond by treatment of carbon nanotubes and carbon nanofibers with H atoms from a H₂ plasma. We observe that upon H₂ plasma exposure of multi-walled carbon nanotubes (MWCNTs) over the temperature range from 300 K to 1073 K, an amorphous carbon matrix is produced with carbon nanocrystals embedded in it.

In this context, we present a detailed theoretical analysis of the various crystalline phases of C observed upon exposing MWCNTs to a H₂ plasma, as well as the potential role of H in determining the structures of the resulting C nanocrystals. In our studies, we use a synergistic combination of high-resolution transmission electron microscopy (HRTEM), electron diffraction, molecular-dynamics (MD) simulations and first-principles density functional theory (DFT) calculations. Our DFT calculations are carried out within the generalized gradient approximation and employ plane-wave basis sets, ultra-soft pseudopotentials, and supercell models. In our MD simulations, H interactions with MWCNTs and the resulting structural relaxations are modeled using the Adaptive Intermolecular Reactive Empirical Bonding Order (AIREBO) potential.

Structural characterization of the nanocrystals formed yields various distinct crystalline carbon phases consistent with a face-centered cubic (fcc) lattice with lattice parameter a = 0.425 nm, a body-centered cubic (bcc) lattice with a = 0.30 nm, a diamond lattice with a = 0.357 nm, and a hexagonal diamond (lonsdaleite) lattice with lattice parameters a = 0.252 nm and c = 0.412 nm. Our theoretical analysis reveals that structural transformations of MWCNTs are triggered initially by the H-induced formation of inter-shell sp³ C-C bonds. Consequently, a specific alignment of concentric graphene walls results in the formation of inter-shell C-C bonds in the MWCNTs that constitute the seed units, which may nucleate a specific crystalline C phase such as diamond or lonsdaleite (the most abundant ones in our experiments). Furthermore, our results indicate that a new phase of carbon with a body-centered tetragonal lattice with a basis of 4 C atoms (BCT-C4, with lattice parameters a = 0.437 nm and c = 0.252 nm), also referred to as rectangulated carbon, can account for reflections from the electron diffraction that are not matched by the diamond-cubic or lonsdaleite crystalline structures. In addition, we provide other possible interpretations for the experimentally observed crystalline C phases through the incorporation of H atoms, at proper concentrations, in interstitial sites of cubic carbon phases.