Elementary processes of H₂ plasma-graphene interaction: A combined molecular dynamics and density functional theory study

Elementary interactions between H atoms and monolayer graphene are investigated using classical molecular dynamics (CMD) and density functional theory (DFT). C-H interatomic potential curves and associated energy barriers are reported depending on the H impact position (top, bridge, hollow, vacancy, or edge sites of graphene nanoribbons). Chemisorption of atomic hydrogen and formation of molecular hydrogen from chemisorbed H states on graphene are examined. The influence of graphene temperature and incident species energy on adsorption, reflection, and penetration mechanisms is also presented. Except for impacts at graphene nanoribbon (GNR) edges or at defect locations, H atoms are shown to experience a repulsive force due to delocalized π-electrons which prevents any species with less than 0.4-0.6 eV to chemisorb on the graphene surface. C-H bond formation requires a local sp²-sp³ rehybridization resulting in structural changes of the graphene sample. Chemisorption sites with deep potential wells and no activation barrier are found on GNR edges, which indicate that H thermal radicals can functionalize GNRs on edges while they cannot do it in the basal plane. The presence of one or more H adsorbates on the graphene surface strongly influences subsequent H adsorption and promotes the formation of energetically favourable H pairs at the para- and ortho-locations. Formation of H₂ molecule via Eley-Rideal recombination of hot radicals [1-1.3 eV] with chemisorbed H atoms is observed.