A first-principles study on the role of hydrogen in early stage of graphene growth during the CH4 dissociation on Cu(1 1 1) and Ni(1 1 1) surfaces

The influence of hydrogen for CH4 dissociation on Cu(1 1 1) and Ni(1 1 1) surfaces has been investigated by using the density functional theory. The two possible reactions, i.e. H-abstraction reaction (CHx + H → CHx−1 + H2) and direct dehydrogenation reaction (CHx + H → CHx−1 + 2H), are studied. Our results show that H-abstraction reaction has higher energy barrier than direct dehydrogenation reaction on Cu(1 1 1), while for Ni(1 1 1), only the direct dehydrogenation reaction is observed. The microkinetic analysis supports that H-abstraction reaction is less competitive than the direct dehydrogenation reaction at broad coverage of H atom on Cu(1 1 1) surface. The major intermediate changes from CH to CH3 on Cu(1 1 1) and Ni(1 1 1) with the increase of H2 partial pressure. Furthermore, the behavior of free C atoms on both clean and H pre-adsorbed metal surfaces is discussed. The adsorbed H atom hinders the polymerization of the C atoms on Cu(1 1 1), resulting in sufficient time for C relaxed to the most stable site and further lead to a prefect graphene pattern formation, while H atom has little effect on such process for Ni(1 1 1).