Dual-route hydrogenation of epitaxial graphene

Although the high surface-to-weight ratio would make graphene (Gr) a promising material for hydrogen accumulation, up to now only moderate gravimetric density values of 1-2% have been obtained at room temperature due to high energy barriers for H chemisorption. In this framework, Gr supported on metals has shown an increased reactivity upon H exposure, and in particular the Gr/Ni(111) interface appears much more favorable than other Gr/metal systems.
By combining high-resolution fast X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), thermal programmed desorption (TPD) and scanning tunneling microscopy (STM) measurements performed at the SuperESCA beamline and the CoSMoS facility, with density functional theory (DFT) calculations, we found that the interaction of Gr/Ni(111) with H atoms at room temperature leads to a dual path hydrogenation: at first H atoms chemisorb on Gr and, in parallel, a slow but continuous intercalation takes place, which leads to the binding of H atoms at Ni surface sites.

In the first stage C1s core level spectra show components that DFT calculations assign to C atoms bonded to H monomers, dimers and also larger clusters, or which are first neighbours to C-H bonds.
In agreement with these results, adsorbed H monomers, dimers and trimers were observed with STM after 20 L of H dose. At saturation of the chemisorbed phase (700 L) the H adatoms appear as small features coexisting with larger clusters, uniformly covering the surface terraces.

At high H doses a new component in the C1s spectrum progressively rises subtracting intensity to all other components. It arises from regions where

the H atoms have intercalated below Gr and chemisorbed on Ni. Further H dosing (up to 34 KL) leads to complete lifting of the Gr layer due to H intercalation, as proved by the C K-edge NEXAFS spectra evolution and by XPS and STM measurements. In the areas where Gr is lifted the H desorption process is favoured by the weakened Gr/metal interaction.

Finally, the H₂ TPD curves show how the H atoms chemisorbed on Gr desorb around 630 K, while at higher H coverage the release of the intercalated H is observed at ~400 K, i.e. a few tens kelvins above the desorption temperature of H₂ from the bare Ni(111) surface. The amount of H that can be chemisorbed on Gr turned out to be about 0.2 MLGr (1 MLGr= 3.72 × 10¹⁵atoms/cm², which is twice the Ni(111) surface atomic density, 2 MLNi) whereas the total amount of H₂ released by the sample exposed to higher H doses reaches the value of 1.1 MLGr , which is more than twice the quantity of H that can be adsorbed on the Ni(111) surface (1 MLNi ). Likely, some intercalated H atoms diffuse into the bulk of the Ni substrate. These results demonstrate that Gr, besides stabilizing the H atoms bonded to Ni, might enable at RT hydrogen diffusion in the Ni bulk.