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. When moving to epitaxial Gr on metals, the Gr/Ni(111) interface appears much more favorable than other Gr/metal systems, as the limitations imposed by the presence of a moirè corrugation vanish due to the close lattice match of Gr to the Ni(111) surface. Moreover, hydrogenation might be favored by the peculiar reactivity of Gr/Ni(111). These issues motivated the re-investigation of the H interaction with this interface. 
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.
The first stage is illustrated in Figure 1. The bottom panel of  Figure 1ashows the C1s spectra measured after the exposure to 20, 70 and 700 Langmuir (L) (1 L = 1.33 × 10−6mbar·s) of H. The main Cpeak due to unperturbed Gr/Ni(111) is progressively converted into the components A1, A2, Band B(Figures 1a and 1b). By comparing the experimental C1s core level shifts (CLS) with those calculated by DFT for adsorbed clusters of one to seven H atoms (top panel in Fig.1a), Awas assigned to C atoms bonded to H monomers and dimers, Ato C-H bonds in H trimers or larger clusters, whereas B1and Bwere identified as due to C sites that are first neighbor of one or two and three C-H bonds, respectively. In agreement with these results, adsorbed H monomers, dimers and trimers were observed with STM after 20 L of H dose (see Figures 1c and 1d). At saturation of the chemisorbed phase (700 L) the H adatoms appear as small features coexisting with larger clusters, uniformly covering the surface terraces (Figure 1e).  

Figure 1.     a) DFT C1s CLS for Gr/Ni(111) with chemisorbed H clusters (1H-7H; see schemes on the left) compared with the measured C1s spectra for increasing H coverage; b) C1s component intensities vs. H dose; c-e) STM images after H doses of 20 L (c-d) and 0.7 KL (e). In d) the white circles, the green and blue ellipses indicate monomers, dimers and likely trimers, respectively.

At high H doses a new component Cbecomes evident in the C1s spectrum and progressively rises subtracting intensity to all other components (Figures 2a and 2b). It arises from regions where the H atoms have intercalated below Gr and chemisorbed on Ni. In these areas, the H atoms chemisorbed on Gr are easily wiped up by the abstraction route. The STM image taken after a dose of 2.2 KL (Figure 2c) shows fully hydrogenated (blue box) and fully dehydrogenated regions, where the Gr lattice is clearly visible (green box). Further 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. The H2TPD curves in Figure 2f show that the H atoms chemisorbed on Gr desorb around 630 K (filled red; 0.7 KL). 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 Hfrom 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 amounts of H2released by the sample exposed to 2.2, 29 and 34 KL were 0.56, 0.83 and 1.1 MLGr respectively, which exceed the quantity of 1 MLNi that can be adsorbed on the Ni(111) surface. Likely, some intercalated H atoms diffuse into the bulk of the Ni substrate. Then Gr, besides stabilizing the H atoms bonded to Ni, might enable at RT hydrogen diffusion in the Ni bulk. 

Figure 2.     a ) C1s spectrum (29 KL) and b) C1s component intensities vs. H dose. c-e) STM image after H dose of 2.2 KL with bright (covered by chemisorbed H) and dark (lifted by intercalated H) regions. f) H2TPD curves measured for Gr/Ni(111) exposed to selected H doses. The filled gray curve was measured on the H saturated  Ni(111) surface at 130 K.


This research was conducted by the following research team:

Daniel Lizzit1, Mario I. Trioni2, Luca Bignardi1, Paolo Lacovig1, Silvano Lizzit1, Rocco Martinazzo3, Rosanna Larciprete4


Elettra Sincrotrone Trieste, Trieste, Italy
CNR-Institute of Molecular Science and Technologies (ISTM), Milano, Italy 
Università degli Studi di Milano, Dip. di Chimica, Milano, Italy
CNR-Institute for Complex Systems (ISC), Roma, Italy

Contact persons:

Rosanna Larciprete, email:  
Daniel Lizzit, email: 



Daniel Lizzit, Mario I. Trioni, Luca Bignardi, Paolo Lacovig, Silvano Lizzit, Rocco Martinazzo, and Rosanna Larciprete, “Dual-route hydrogenation of the graphene/Ni interface”, ACS Nano 13, 1828 (2019), DOI: 10.1021/acsnano.8b07996

Last Updated on Friday, 02 August 2019 15:44