Evidence for Site-Specific Reversible Hydrogen Adsorption on Graphene by Sum-Frequency Generation Spectroscopy and Density Functional Theory

We have characterized the C-H stretching vibrations of hydrogenated graphene on gold substrates using vibrational sum-frequency generation (SFG) spectroscopy. These C-H stretches are red-shifted as compared to aliphatic C-H stretches, and SFG signal intensities of the C-H vibrational features increase with increasing hydrogen coverage. By comparison with density-functional theory (DFT) calculations, we conclude that hydrogen atoms covalently attach to the graphene lattice from above the graphene sheet; preferential binding motifs are dimers in the ortho and para positions. SFG spectroscopy is hence not only able to detect hydrogen atoms on graphene at sub-monolayer coverage but it can also serve to obtain relative information about the surface coverage, and in combination with DFT can reveal information about binding motifs.