Hydrogen uptake by graphene and nucleation of graphane

Reactions of hydrogen with electronic materials are important for the operation of related devices. Here we use first-principles density-functional theory calculations to describe hydrogen reactions on pristine and defective graphene. We show that small hydrogen clusters on defect-free graphene are unstable against emission of hydrogen molecules and that the associated reaction energies and barriers have a subtle dependence on the type of the clusters. In contrast, chemisorption of hydrogen in the vicinity of graphene vacancies leads to progressively larger clusters of adatoms and, eventually, to formation of graphane. The results are relevant to the optimization of graphene- and graphane-based devices, as well to the creation of graphene-graphane hybrid systems.