Oxygen-assisted monodisperse transition-metal-atom-induced graphite phase transformation to diamond: a first-principles calculation study

Abstract

The low-pressure phase transformation of graphite to diamond has been confirmed with the help of a tantalum, hydrogen, and oxygen composited atmosphere in chemical vapor deposition. Subsequent theoretical calculations revealed the phase-transformation mechanism induced by a monodisperse transition-metal (TM) with an appropriate atomic radius (1.36–1.60 Å) and unfilled d-orbitals (d²s²–d⁷s²) on an H-terminated diamond substrate. However, the specific role of oxygen in TM-induced phase transformation remains unclear. Herein, we employed first-principles calculations to elucidate how oxygen assists the monodisperse TM in inducing ordinary-pressure phase transformation of graphite to diamond. Results show that the adsorption energy of the TM on the O-terminated diamond substrate was lower and the diffusion barrier was higher than that for its hydrogen counterpart, thereby effectively promoting their existence in a monodisperse state. These monodisperse TM atoms reduce the energy barrier of the phase transformation. This is attributed to the hybridization of unpaired electrons in the d-orbitals of the TM with the p_z-orbitals of carbon, resulting in charge transfer within graphene from a horizontal to a vertical direction, leading to a transition from sp²-C to sp³-C. Specifically, TMs with the electronic configuration of d⁹s¹, such as Au, have a weak role in phase transition on H-terminated diamond, while on O-terminated diamond, oxygen takes away the electrons of Au to generate more unpaired electrons that can bond to carbon, significantly promoting the phase transformation. For TMs with electronic configurations of d⁷s² to d³s², the bonding of their unpaired electrons to carbon is largely unaffected by oxygen; thus, oxidation has little influence on the phase transition. In the case of d²s², oxygen takes away all the unpaired d-orbital electrons, thus preventing the TMs from bonding to carbon and providing no contribution to the phase transformation. Our findings offer guidance for the preparation of diamond based on graphite at ordinary pressure.