CO interaction with Graphene /Ni(111): chemisorption at regular sites and intercalation at vacancies
Carraro, E. Celasco, A. Lusuan, M. Smerieri, J. Pal, M. Rocca, L. Savio and L. Vattuone
University of Genova, Italy
Due to its exceptional properties graphene (G) is considered one of the most promising materials for the future. Although its possible use as gas sensor has already been demonstrated  the surface chemistry underlying it is still unclear.
We investigated the reactivity of single layer G on different substrates (polycrystalline Cu and Ni(111) single crystal) by High Resolution Electron Energy Loss Spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and Low Temperature Scanning Tunneling Microscopy (LT-STM).
Non-dissociative chemisorption of CO occurs at cold, pristine graphene grown on Ni(1 1 1) . The CO adlayer remains stable up to 125 K, with some admolecules surviving flashes to 225 K. This unexpected result is explained qualitatively by the modification of the density of states close to the Fermi energy induced by the relatively strong graphene–substrate interaction. Indeed no adsorption is found under the same conditions for G supported on less interacting substrates, like polycrystalline Cu.
We also investigated  the reactivity of defects, created by low energy Ne+ ions bombardment.
No vibrational signatures of adsorbed CO is found for G/Cu while they are promptly observed for G/Ni(111). The vibrational frequencies and the ratio of the intensity of single and double coordinated CO molecules indicate that the latters have chemisorbed at the G/Ni(111) interface close to vacancies. The red-shift of the C1s binding energy in XPS proves that, close to vacancies, detachment of the G layer from the substrate occurs.
Therefore, contrary to theoretical predictions , the edges of single and di-vacancies are chemically inert for all substrates and intercalation occurs only in presence of a reactive substrate.
If CO is dosed again after annealing the CO pre-exposed defected G at 380 K, less CO adsorbs. Since only traces of oxygen remain after annealing at 450 K, it is reasonable that a Boudouard reaction between two intercalated CO molecules has occurred leading to CO2 desorption and leaving C atoms which may amend the vacancy. STM inspection of the annealed CO/G/Ni(111) layer confirms a significant change of the surface morphology: after annealing the film. Vacancies induced by sputtering are no longer present and the surface presents “”scars”” indicating that the original surface morphology is not fully recovered .
These results suggest that neither pristine G sites nor vacancies can be responsible for the sensing activity of Single Layer free standing G.
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