Exciting H-2 Molecules for Graphene Functionalization

Hydrogen functionalization of graphene by exposure to vibrationally excited H-2 molecules is investigated by combined scanning tunneling microscopy, high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H-2 molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H-2 dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H-H bond. The first reacting H-2 molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in, reduced barriers for dissociative adsorption of subsequent H-2 molecules. This enables functionalization by H-2 molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface, and additionally set the stage for a re-interpretation of previous experimental work involving elevated H-2 background gas pressures in the presence of hot filaments.