Interlayer Carbon Bond Formation Induced by Hydrogen Adsorption in Few-Layer Supported Graphene

We report on the hydrogen adsorption induced phase transition of a few layer graphene (1 to 4 layers) to a diamondlike structure on Pt(111) based on core level x-ray spectroscopy, temperature programed desorption, infrared spectroscopy, and density functional theory total energy calculations. The surface adsorption of hydrogen induces a hybridization change of carbon from the $s{p}^2$ to the $s{p}^3$ bond symmetry, which propagates through the graphene layers, resulting in interlayer carbon bond formation. The structure is stabilized through the termination of interfacial $s{p}^3$ carbon atoms by the substrate. The structural transformation occurs as a consequence of high adsorption energy.

Figure 1

TPD spectra of $m^{+}/e=4\mathrm{a}.\mathrm{m}.\mathrm{u}$. (${\mathrm{D}}_2$) from H-FLG and H-SLG. (Inset) IR absorption spectra of the C-D vibrational mode displayed for H-FLG and H-SLG.

Figure 2

(a) C $1s$ XPS spectra ($h{v}_{\mathrm{in}}=400\mathrm{eV}$) of H-FLG (4 ML), H-SLG, pristine SLG, and 4 ML FLG (normalized per carbon atom). Spectra were deconvoluted using Gaussian-broadened Doniach-Šunjić functions. Assignments of C-Pt, C-$s{p}^2$, C-$s{p}^3$, and defect components are made based on Pt $4f_{7/2}$ XPS, XAS and $D_2$ TPD measurements. Inset: background normalized C 1s XPS spectra of pristine SLG and FLG (2, 3, and 4 ML nominal thickness). (b) Pt $4f_{7/2}$ XPS spectra ($h{v}_{\mathrm{in}}=165\mathrm{eV}$) comparing hydrogenated and pristine 4 ML FLG and SLG. Pt $4f_{7/2}$ XPS spectra of FLG and SLG were normalized to the peak heights to indicate the similarities.

Figure 3

XES-XAS of pristine and hydrogenated FLG (4 ML) showing $\pi$ states. The $\pi$ component of the DOS below the Fermi level was extracted by subtraction of the half normal emission spectra from grazing emission spectra and area normalized assuming that the occupation is the same. Note that scaling between the XAS and the XES spectra are arbitrary. The C $1s$ binding energy position of the $s{p}^2$ hybridized state provides the Fermi level of the systems.

Figure 4

Computed adsorption energy of hydrogen on SLG/Pt(111) and FLG/Pt(111) for varying number of graphene layers. Empirical fit of the adsorption energies is indicated as solid line. The dashed line represents unadsorbed atomic hydrogen and graphite chosen as the zero reference. The adsorption energy of hydrogen on graphite (freestanding 7 layer graphene) is also indicated (dashed-dotted line). The solid curve would converge to the dashed-dotted line beyond a certain critical thickness of FLG.