Mechanism of CO Intercalation through the Graphene/Ni(111) Interface and Effect of Doping

Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.


Computational Details
Density Functional Theory (DFT) calculations were performed using the plane-wave-based Quantum ESPRESSO package (QE). 1,2 The ultrasoft pseudopotentials 3 were adopted to describe the electronion interaction with Ni (3d, 4s), C (2s, 2p), O (2s, 2p), N (2s, 2p), and H (1s), treated as valence electrons. Energy cutoffs of 30 Ry and 240 Ry (for kinetic energy and charge density expansion, respectively) were adopted for all calculations. The Perdew-Burke-Ernzerhof functional (PBE) was used for electron exchange-correlation. 4 To properly describe the Gr/Ni interaction, semiempirical corrections accounting for the van der Waals interactions were included with the DFT-D2 formalism. 5 Spin polarization was always included.
The geometry relaxation of all considered systems was performed only at Γ point, followed by a single self-consistent field (SCF) cycle calculation with a 2 × 2 × 1 Monkhorst-Pack k-points mesh 6 to get more accurate total energies. The Ni(111) surface was modeled by a three-layer slab with a bottom layer fixed to the bulk positions during the geometry relaxation to mimic a semi-infinite solid. To avoid interactions between adjacent periodic images, a vacuum space of about 15 Å in the direction perpendicular to the surface was used.
The Climbing Image−Nudged Elastic Band (CI−NEB) method 7 was employed to simulate the CO diffusion process at the Gr/Ni interface, generating the minimum energy path of the reaction step and an evaluation of the energy barrier.
STM simulations were performed using the Tersoff-Hamann approach, 8 according to which the tunneling current is proportional to the energy-integrated Local Density of States (ILDOS). Ball-andstick models and STM images were rendered with XCrySDen 9 and Gwyddion 10 software, respectively.
The adsorption energy (ΔE ads ), as normalized by the number of CO molecules (n), was calculated as follows: ΔE ads = (E nCO/system -nE CO -E system )/n where E nCO/system is the total energy of the system (Gr/Ni or 4VG-6H or 4VG-3CO or 4VG-6N) with n adsorbed CO molecules, E nCO is the total energy of n isolated CO molecules in the gas phase, and E system is the total energy of the optimized system without any adsorbed CO molecule.

S3
In the following, we define the contributions for the energy decomposition analysis, as shown in Figure 5 and reported in Table 1: where E 4VG-6N,dist , E 4VG-6N-fs,dist , and E Ni,dist are the total energies of Ni-supported 4VG-6N, free-standing 4VG-6N, and Ni substrate, respectively, in the optimized geometry for nCO/4VG-6N, whereas E 4VG-6N , E 4VG-6N-fs , and E Ni are the total energies of Ni-supported 4VG-6N, free-standing 4VG-6N, and Ni substrate, respectively, in the optimized geometry of 4VG-6N; where E nCO,dist and E nCO are the total energies of n CO molecules in the optimized geometry of nCO/4VG-6N and isolated in the gas-phase, respectively; where E nCO/4VG-6N is the total energy of nCO/4VG-6N in its optimized geometry; n is the number of adsorbed CO molecules.

Experimental Details
Pristine Gr and N-Gr layers were prepared in a UHV chamber with a base pressure of ~2 ×10 −10 mbar.
The Ni(111) single crystals were cleaned by several cycles of Ar + sputtering at 1.5 kV at room temperature (RT) and annealing at 700 °C, for a few minutes. For the N-Gr growth, a N-doped Ni(111) crystal was used (for the preparation details see Ref. 11). Standard Gr growth was performed in UHV by low-pressure CVD, using ethylene (C 2 H 4 ) as precursor. Low energy electron diffraction (LEED) and STM characterization was performed in UHV in order to assess the quality and homogeneity of the as-grown Gr sample.
CO reactivity experiments have been carried out in-situ, in a home-made high-pressure cell inside a small chamber, connected to the experimental setup through a gate valve and kept in high vacuum (~10 -9 mbar). STM measurements were performed in UHV at room temperature with an Omicron variable-temperature (VT) STM. All topographic images were acquired in constant-current mode.
STM images were analyzed with the Gwyddion software package 10 , after applying moderate noise filtering. Crystallographic orientation of the images was determined by analyzing the epitaxial structure formed by pristine Gr on the Ni(111) surface, as described in Ref. 12. XPS measurements were performed at a base pressure in the range of 10 −9 mbar. All the spectra were collected at RT in normal emission geometry using a hemispherical electron energy analyser and a conventional Mg S4 K α1,2 (1253.6 eV) X-ray source, with an overall experimental energy resolution of ~0.8 eV. All binding energies were calibrated by measuring the Fermi level. The spectra are normalized to the incident photon flux and analysed by performing a non-linear mean square fit of the data. We used a Shirley background and reproduced the C 1s photoemission peacks using asymmetric Doniach-Sunjic lineshapes.      Table   Table S1 Energy contributions of the energy decomposition analysis for CO adsorption on 4VG-6N of distortion (positive, ΔE dist ), decoupling (positive, ΔE decoup ) and of binding (negative, ΔE bind ) to the total adsorption energy (ΔE ads-tot ) for all CO molecules, at different CO coverages. The energy contributions are calculated using as a reference the optimized 4VG-6N interface and isolated CO molecules in the gas-phase.