The influence of hydrogen on transition metal - catalysed graphene nucleation

We demonstrate how hydrogen influences graphene nucleation on two archetypal catalysts, Cu(111) and Ni(111), using first-principles methods. The graphene nucleation mechanism is shown to be the result of the balance between the nature and strength of the carbon – metal interaction, and the influence of the hydrogen chemical potential on adsorbed carbon fragments. While the former drives the formation of ring structures in carbon fragments, the latter promotes the growth of saturated carbon chain structures during the nucleation process. Importantly, our results reveal how the presence of hydrogen dramatically influences the nature of the sp → sp² transition, a key step in the nucleation of both graphene and carbon nanotubes. Increasing the presence of hydrogen during nucleation stabilises smaller ring structures earlier in the nucleation process, in fragments as small as carbon pentagons, which are known to be a key intermediate structure in carbon nanostructure nucleation. Conversely, lower hydrogen chemical potentials lead to the formation of carbon ring structures only in much larger fragments. These results present a new potential route by which hydrogen leads to greater control over CVD-synthesised carbon nanotubes and graphene, i.e. by driving the formation of smaller, more stable ring structures earlier in the growth process.