Due to its electrical properties graphene (G) has been successfully used as a sensing element for the detection of different gases reaching very high sensitivities (ppm or better), which are ascribed to the doping induced by adsorption. The sensitivity depends critically on the chemical nature of the gas and is lower for CO than for other poisoning species. The nature of the active sites is, however, still unclear. The value of the heat of adsorption determines the sensitivity and the range of temperatures at which the sensor can operate. In order to clarify these issues we investigated experimentally adsorption of CO on G supported on polycrystalline Cu and on Ni(111) by HREELS and XPS.
No signature of adsorbed CO was found after exposure both at RT and at 100 K for G/Cu, while chemisorbed CO was observed after just a few L at 100 K for G supported on Ni(111). This result indicates that the nature of the substrate plays an essential role in the adsorption process. The heat of adsorption is estimated to be ~0.58 eV/molecule at low coverage, so that equilibrium coverage of ~0.1 ML is expected at RT under a CO partial pressure of only 10 mbar. We identify top-bridge graphene as the most reactive configuration.
Doping G/Ni(111) by N2+ ion bombardment allows for the formation of a second, more strongly bound moiety, characterized by a CO stretch frequency of 236 meV and by an initial heat of adsorption (0.85 eV/molecule). The presence of N (in pyridinic or substitutional sites) enhances therefore significantly the chemical reactivity of G/Ni(111) towards CO.
We also investigated the role of isolated defects, which were created by low energy Ne+ ions bombardment on single layer graphene supported on different substrates (polycrystalline Cu and Ni(111)). We find that no CO adsorption occurs for defected Graphene (G*)/Cu, while vibrational signatures of the presence of CO are observed for G*/Ni(111). Two moieties, desorbing just above 350 K, are present under vacuum conditions after exposure at RT. The frequency and the relative intensity of the observed vibrational features indicate that CO chemisorbs at the G/Ni(111) interface close to the vacancies rather than at the defected G layer. The red-shift of the C1s binding energy suggests that in such regions detachment of the G layer from the substrate occurs.
Unfortunately defects of G/Ni(111) are not a good candidate as active sensing site since amending of vacancies occurs, as demonstrated by the reduction of the adsorbed coverage in subsequent CO doses followed by annealing at 380 K. We suggest that a Boudouard reaction involving two intercalated CO molecules take place under graphene cover, producing CO2, which desorbs, and C, which repairs the vacancy.