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