Reactivity under cover in controlled Near Ambient Pressure conditions: CO on bare and graphene covered NI(111)

The graphene–metal interface, as an interesting graphene-based system, attracts much attention from both the application and the fundamental science points of view. One of the reasons is that Chemical Vapor Deposition (CVD) on metal surfaces is the most promising method to produce large area graphene sheets with nickel and copper as most frequently used substrates. However, Ni has a severe drawback due to the high solubility of carbon at the temperatures required for CVD (>800 K). Dissolved carbon atoms may segregate to the surface of the substrate while cooling down the system, producing unwanted graphene multilayer structures. Careful preparation protocols have therefore to be followed, which leave carbide traces on the surface which can absorb the segregating carbon atoms by transforming into graphene. Another reason is that the graphene properties may be strongly affected by the interaction with the substrate, thus yielding interesting catalytic properties. My thesis is focused on the interaction of CO with Ni(111) which I studied in operando conditions with Near Ambient Pressure by X-Ray Photoemission Spectroscopy (NAP-XPS) at ~ 3 mbar thus extending the so far explored pressure range by nearly one order of magnitude. Under these conditions I observed the detachment of the strongly interacting graphene and its conversion into weakly interacting graphene caused by CO intercalation under it already at 500 K. Intercalation of gases is important since it restores the Dirac cone and thus the exceptionally large carrier mobility of graphene and because chemical reactions may be favored under cover by the constrained volume. Indeed, above 600 K, formation of physisorbed CO2 is observed under the graphene cover, a process I ascribe to the onset of the Boudouard reaction. The latter leads to the formation of additional carbon atoms which transform the residual carbide, present at the surface, into graphene causing the expansion of the graphene islands. Furthermore, my data confirm that CO does not only intercalate, but adsorbs also above the strongly interacting graphene areas forming a weakly bonded species of possible catalytical relevance. The reaction has been observed also after drilling single and double vacancies into the graphene layer by ion bombardment. CO2 tends then to mend the vacancies forming a bridge over their borders.